SM43A-1685
Substorms and CMEs: A Search for a Common Onset Mechanism
Substorms at Earth and CMEs on the Sun are relatively accessible examples of the explosive energy conversion phenomenon in the heliosphere. This talk reports on a project to determine whether these two examples might have an underlying common cause. There are two generic types of onset mechanisms: microscale instability and macroscale instability. The microscale option takes the form, for example, of a sudden onset of rapid magnetic reconnection and receives considerable attention by the substorm community. The macroscale option takes the form, for example, of a sudden onset of rapid expulsion when the threshold for a non-equilibrium, macro-instability is exceeded. This option appears more commonly in CME research. The two options have clear phenomenological signatures: in the former, rapid reconnection precedes expulsion; in the latter, expulsion precedes rapid reconnection. The latter scenario has been shown to operate in numerical MHD models of CME onset. Here we report on what MHD models say about the relative timing of rapid reconnection and plasmoid expulsion in the geotail.
SM43A-1686
Solar Wind Influence on the Driven Electrojet System and the Magnetotail
A substorm onset at approximately 10 UT on December 12 2007 was well observed from both the THEMIS constellation and ground observatories in eastern Canada, most emplaced to support the THEMIS project. Clear step-like changes in the solar wind parameters on this day aid in relating them to magnetospheric responses. With the large set of ground magnetometers available, quantitative inversion can be done using the Automated Regional Modelling (ARM) technique which includes both auroral zone and subauroral stations. Not only can the substorm current wedge parameters be well determined, but the response of the driven system morning electrojet to southward IMF during the growth phase can be quantified. A clear subauroral Y component magnetic bay observed in eastern North America before onset can be attributed to this driving, as can stretching at GOES 11 (W) and evening sector bays. The onset appears to have been triggered by a northward turning. Analysis of tail response to the onset must take into account the fact that simultaneously the solar wind northward velocity component reversed from approximately -30 km/s to +40 km/s. The THEMIS C and B spacecraft, located 12 and 14 Re downtail, respectively, showed slow magnetic field X component reversals around the time of onset that are most likely mainly related to the solar wind Vz velocity blowing the tail across the spacecraft, as opposed to direct effects of the substorm. This result is confirmed by MHD modeling and studied using an event-oriented mapping approach.
SM43A-1687
First results from the Blackstone HF radar in support of THEMIS
A new HF radar has been constructed and built in Blackstone, VA, as part of the SuperDARN/StormDARN programme specifically with the purpose of providing coverage over the Canadian ground sector in THEMIS. The new radar began operations in February 2008, an optimum time to support the first THEMIS tail campaign. This paper will discuss the importance of having radars at lower latitudes than the auroral zone locations of SuperDARN for substorm studies. Furthermore, the paper will also present the first results from this radar together with other SuperDARN observations during the THEMIS tail period.
SM43A-1688
SuperDARN Observations of Pi2 Electric Field Pulsations during THEMIS Substorms
The NASA THEMIS mission is focused on resolving the time sequence of events that occur in the magnetotail during the onset of the expansion phase of magnetospheric substorms. In support of THEMIS mission goals, the SuperDARN community is using a special camping-beam mode during THEMIS conjunctions over North America to maximize the temporal resolution of measurements of ionospheric convection during THEMIS substorm events. The SuperDARN THEMIS mode provides 8-second resolution on a single camping beam while simultaneously marching through each beam of the normal 2-minute scan. In this paper, we present SuperDARN measurements of ULF waves identified on SuperDARN camping beams during the onsets of several THEMIS substorms. These events clearly demonstrate the gains in temporal resolution that can be achieved using the THEMIS camping-beam mode. The characteristics of the Pi2 oscillations measured by the SuperDARN radars are found to be very similar to those seen by nearby ground-based magnetometers.
SM43A-1689
The Relationship Between Substorm Onset Locations and Nightside Convection Pattern Features
This paper presents a study using the SuperDARN convection database in conjunction with a list of substorm onsets determined from the IMAGE satellite [Frye et al., 2004] to examine the relationship between substorm onset locations and features of the convection pattern. Particular attention is devoted to the latitude of onset locations compared to the latitude of the near-midnight convection reversal boundary (CRB) and the latitude of the peak flow observed equatorward of the CRB. Because of this focus, the set of observations examined is limited to cases where SuperDARN provided sufficient observations in the midnight sector to determine the latitude of the CRB. Distributions of the latitude difference between the onset location and the CRB and flow peak were examined for different times leading up to substorm onset and for a few different conditions. It was found that the onsets occurred equatorward of the CRB in more than 95% of the cases, and that the distribution was approximately centered on the flow velocity peak.
SM43A-1690
PFISR observations of the nightside Region-2 electrodynamics during substorm growth and expansion phases: preliminary results
The nightside Region-2 (R2) electrodynamics, including the Harang reversal, has been shown recently to be closely related with substorm dynamics. The newly available Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) is able to measure the ionospheric convection, as well as electron density and some other important plasma parameters, and thus can provide some new aspects to this problem. In the past one and a half years, more than 60 runs have been conducted when PFISR was on the nightside. By taking advantage of the THEMIS ground based all-sky camera array, substorm events, for which good all-sky camera measurements are available during the runs, have been selected and categorized by the location of the radar at the time of onset relative to that of the auroral onset. Preliminary results show consistency with our previous SuperDARN observations, that is the convection flows at nightside change significantly at onset and the way they evolve highly depends on their location relative to that of substorm onset. The electron density profiles measured by PFISR enable us to study the characteristics of the substorm arc brightening and the associated field-aligned current system. Preliminary results show evidence that the onset arc may be Alfvénic. Other ground based instruments, such as magnetometers and SuperDARN, are also studied as complementary to PFISR. These observations provide a unique ionospheric perspective on substorm process and can contribute to an integral understanding of the substorm dynamics.
SM43A-1691
Subauroral plasma convection during substorm cycle
The time evolution of the subauroral polarization stream (SAPS) during substorms is investigated using the HF radar observations at magnetic latitudes of 60-65 deg S. It is demonstrated that SAPS often exist for two or more substorm cycles. During the growth phase, the SAPS plasma convection is comparable to the background auroral convection. The subauroral plasma convection starts to intensify near substorm onset and peaks during the recovery phase. The response times to substorm onset range from -5 to +40 min and show some dependence on the substorm location with longer delays observed for substorms eastward of the radars' viewing area. It is proposed that the time evolution of the SAPS plasma convection is a two-stage process. During the initial phase, the plasma velocity is mostly controlled by the polarization electric field being established in the subauroral ionosphere after the injection of energetic ions. The time evolution following the initial increase may depend on both the ionospheric conditions through the ionosphere- magnetosphere coupling processes and additional supply of energetic ions during subsequent intensifications.
SM43A-1692
Azimuthal structure of substorm breakup arcs prior to expansive phase onset using ISUAL/FORMOSAT-2
Substorm breakup arcs are investigated for substorm onset that occurs at the location of "Harang discontinuity" at pre-midnight. The auroral breakup arc images are obtained by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard FORMOSAT-2 satellite. We identify that the auroral breakup arc brightening occurs at the same time as the associated negative H-Bay and Pi 2 pulsations. It begins with a brightening on the arc and evolves into clear bead-like structure with approximate equally spaced separation along the arc direction. The enhanced auroral arcs finally break up into several parts. The azimuthal mode number of auroral breakup arcs prior to expansive phase onset ranges from ~200 to ~300. Based on the observations, we suggest that the substorm initial breakup arcs are consistent with kinetic ballooning instability, which is localized at the center of cross tail current sheet at about -(8-10) RE for strong substorms.
SM43A-1693
Equivalent ionospheric currents from the GIMA, Greenland, MACCS, and THEMIS ground magnetometer arrays
With data from the GIMA, Greenland, MACCs, and THEMIS ground magnetometer arrays, we obtain maps of equivalent ionospheric currents (EIC) over North America using the state-of-art technique based on SECS (spherical elementary currents systems) developed by Amm and Viljanen [1999] . The EIC maps can be calculated with the same time resolution as the magnetometer data, which will be 1 sec. The results thus show in detail the dynamic evolution of the currents over the whole THEMIS ground network. The EIC maps can further be compared and quantitatively combined with near simultaneous images of the THEMIS all sky imager mosaics, SuperDARN RADAR data, and THEMIS spacecraft data. For this study we will first demonstrate the validity of the technique when used with the mentioned magnetometer arrays by applying it to a synthetic model case, and then examine the equivalent ionospheric currents associated with substorm events on March 23, 2007 and February 26 2008.
SM43A-1694
Superposed epoch analysis of the ionospheric convection evolution during substorms: onset latitude dependence
Using data from the Super Dual Auroral Radar Network (SuperDARN) we investigate the ionospheric convection response to magnetospheric substorms. Substorms were identified using the Far Ultraviolet (FUV) instrument on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft, and were then grouped according to the magnetic latitude of their onset. A superposed epoch analysis of the ionospheric convection patterns for each latitude group was then performed using radar data for the interval 60 minutes before onset to 90 minutes after. It is found that lower latitude onset substorms are associated with generally more enhanced convection than the higher latitude substorms, although they suffer from the most significant localised suppression of the flow in the midnight sector during the expansion phase. On the other hand, the higher-latitude events are associated with a significant and rapid increase in the nightside convection following substorm onset. These results suggest differences in the electrodynamics associated with substorms occurring at different latitudes.
SM43A-1695
Tail Processes and Auroral Activity at the Substorm Expansion Onset
In the Geotail data set we have found observations of dipolarizations in the vicinity of the magnetic equator of the near-Earth tail (XGSM~ -10 RE) that have concurrent aurora imager data from Polar UVI and VIS cameras. These events (six in total over the ten years) allow us to compare tail processes and auroral activities. Detailed inspection of the imager data in the course of this study have led us to recognize that, in five of them, auroras developed into large-scale activities in a two-stage manner: an initial auroral brightening and the subsequent "second aurora". When the activity started, the former and the latter can be recognized as a spatially and temporally discrete spot in the images; it is not appropriate to consider the emergence of the second spot as a continuous expansion of the first spot. The second aurora occurred ~ 4 min after the first one for three out of the five events, while the second aurora occurred ~ 10 min afterward for the other two events. All the second auroras are located eastward of the first one, suggesting systematic sequence of the auroral brightening process. During these auroral activities at substorm expansion onsets, the Geotail detected various signatures of plasma processes, fast flows, flow reversals, fast mode waves, and ballooning mode waves. We will discuss a possible framework to synthesize these observed features in the context of magnetospheric substorms.
SM43A-1696
A Study of Pseudo Breakup Events in the Aurora Using Polar/UVI
We have analyzed pseudo breakup events in the aurora using 37 second LBHL Polar/UVI data during the
months of November and December 2007 to study the relationship between breakups and pseudo breakups.
Historically, the distinction between pseudo breakups, which expand locally, and normal breakups, which
expand more globally, has been ill defined. We now present an explicit definition based on MLat expanse and
peak auroral power. Often, the aurora will exhibit a series of pseudo breakup events before the
commencement of a normal breakup. Sometimes a normal breakup does not occur. We analyze the
properties of pseudo breakups and parameters that might make a normal breakup more or less likely to
occur after a series of pseudo breakup events. There is no statistical difference in MLat location, peak area,
peak power dissipation, and total energy deposition for individual pseudo breakups leading to a normal
breakup and those that do not. A normal breakup is likely to occur only during the first 100 minutes of a
sequence of pseudo breakups. Pseudo breakup sequences leading to a breakup are much shorter than
those not leading to a breakup. However, distributions of total energy (in electron precipitation) for
sequences leading and not leading to a normal breakup are the same. Although the physical implications of
our results are complicated and unclear, pseudo breakup events should be incorporated in any
magnetospheric substorm model.
http://www.ssl.berkeley.edu/~phiggins/pseudobu.html
SM43A-1697
Multi-scale radar and optical measurements of a substorm expansion
The substorm phases are most often discussed in terms of meso-scale features in the optical aurora, with
limited consideration of the micro-scale physics governing how this visual information is produced. Recently
deployed radio and optical instruments at Poker Flat, AK, are providing new insight into the elemental
processes underlying the global-scale drivers observed in the distant magnetosphere. In this paper, this new
observational information is presented in the context of multi-scale measurements of a substorm that
occurred on 23 March 2007. We take, as the starting point, measurements of the large-scale
electromagnetic perturbations observed by the THEMIS satellites. We then consider the cascade of energy
conversion from the perspective of conjugate, multi-scale, ground-based observations--in particular, the
THEMIS all-sky camera array, the Poker Flat Incoherent Scatter Radar (PFISR), and two colocated narrow-
field spectral imaging system. Observation of coherent wave-packets in the aurora, accompanied by ion-
acoustic turbulence in common-volume ISR measurements, suggests that the substorm energy is dissipated
in Alfven resonant cones which, in turn, appear to form coherently with magnetic pulsations in the Pi2 range.
http://heaviside.bu.edu/semeter
SM43A-1698
Fine structures and dynamics in auroral initial brightenings at substorm onsets
The physics that causes auroral expansion during magnetospheric substorm are still unknown. In this presentation, we show four auroral initial brightenings focusing on fine-scale structures and their dynamics, which were observed at substorm onsets. The observations were carried out using all-sky TV cameras with a sampling rate of 30 Hz at Gillam (56.4N, 265.4E) and Fort Smith (60.0N, 248.1E), Canada during January 2- 15, 2008. Two substorms were observed at (Event 1) ~2148 LT and (Event 2) ~0130 LT on January 8 at Fort Smith. Although the initial brightening might start from beyond the field of view of all-sky cameras, developing edges of initial brightenings showed characteristic dynamics and structures, as follows. Event 1: Auroral expansion phase started after several auroral arcs were generated periodically (period ~15s) and propagated westward with velocities of 4-16 km/s which increase with time. Event 2: Initial brightening had peal-beading structure moving eastward with a velocity of ~5 km/s. These repetition features of aurora at substorm onsets indicate that physical process having same repetition periods derived field-aligned currents in the magnetosphere. Initial brightenings of the other two substorms started just in the field of views of all- sky cameras at (Event 3) ~0041 LT on January 7 at Fort Smith and at (Event 4) ~2155 LT on January 15 at Gillam. Characteristic expansion features in the longitudinal direction are seen, as follows. Event 3: Initial brightening had an arc-like structure. It developed eastward with a velocity of ~23 km/s at the first 10 seconds, and the velocity decreased to ~11 km/s at the next 10 seconds. Event 4: Initial brightening had ray- like structures. It developed westward with a velocity of ~20 km/s at the first 10 seconds, and the velocity decreased to ~12 km/s at the next 10 seconds. The velocity of 20-km/s in the ionosphere is roughly projected to be 400-km/s at 10 Earth Radii on the equatorial plane. This velocity is consistent with the typical velocity of high-speed ion bulk flow that is considered to be associated with substorm. From these four fine-scale structures and dynamics of initial brightenings at substorm onset, we will discuss possible plasma instabilities that triggered auroral expansion.
SM43A-1699
Response of high-energy particle precipitation to substorm onset
The process of extra-terrestrial radiowave absorption by ionospheric electrons is known as cosmic noise absorption (CNA) and is routinely detected by ground-based relative opacity meters or riometers, which are sensitive to high-energy particle precipitation (>10 keV). In this study, we investigate the response of high- energy particle precipitation to substorm onset by employing a 7×7-beam imaging riometer system in Northern Scandinavia, IRIS, and the substorm onset database from the IMAGE satellite. The CNA response is investigated with respect to substorm location relative to IRIS. Instantaneous responses are mostly observed when IRIS is located on the same magnetic parallel as the substorm onset region. We investigate dispersionless injections (DIs) associated with substorm onsets as detected by the imaging riometer. It is shown that an auroral absorption signature is present predominantly for DIs located westward of IRIS, which is consistent with high-energy particles propagating eastward from substorm injection location. Superposed epoch analysis of CNA relative to substorm onset timing shows a strong dependence on the azimuthal sector in which the onset is located relative to IRIS. Rapid responses are observed in 90-135 deg geographic azimuths indicating fast westward and poleward propagation from onset location. Responses to onsets located between -90 to -180 deg show a monotonic decrease in rise time. The results are examined in the context of the geosynchronous LANL and GOES satellite data in order to investigate the two propagation mechanisms: expansion of injection region and particle drift.
SM43A-1700
Physical Mechanism of Substorm Breakup Arcs and Onset
Observations show that Pi 2 waves are excited prior to the appearance of breakup auroral arcs that break up after substorm expansion onset. The Pi2 waves and the breakup arcs are modeled by the Kinetic Ballooning Instability (KBI), which is destabilized by plasma pressure gradient and magnetic field curvature in the high beta magnetic well region in the near-Earth plasma sheet. Our model is based on the theoretical analysis and numerical solutions of the gyrokinetic mode equations for late growth phase 3D magnetospheric equilibria. The results show that the KBI has a real frequency associated with the ion magnetic drift frequency, which is in the Pi2 frequency range, and the most unstable KBI has an azimuthal mode number on the order of 200- 300. The theoretical KBI features are consistent with observational features in both the aurora breakup arcs and the near-Earth plasma sheet. Comparison between our KBI model and substorm breakup arc observations by FORMOSAT-2¡¦s ISUAL and THEMIS All Sky Imagers will be presented.
SM43A-1701
Specific Entropy During Substorms Observed With Themis
We report on changes in the specific entropy of midnight-sector flux tubes during substorms observed with the Themis constellation of satellites. Major violations of the constancy of specific entropy during earthward transport are indicative of the operative substorm mechanism. According to the near-Earth current disruption model, the constancy of specific entropy is violated near-Earth in the vicinity XGSE≈ -10RE. According to the near-Earth neutral-line model, the constancy of specific entropy is violated near the reconnection site, XGSE≈ -20RE. Favorable alignments of two or more of the Themis satellites will allow us to investigate where the specific entropy violations occur.
SM43A-1702
Pressure Change Associated with Dipolarization in the Near-Earth Plasma Sheet
We have studied pressure changes that occur associated with dipolarization in the plasma sheet of the near- Earth magnetotail around substorm onsets. Using Geotail data, we performed a superposed epoch analysis and examined in detail a few individual cases that were observed in the vicinity of the magnetic equator. Here the contribution of high-energy particles to the ion pressure was considered. It is found that, unlike the previously reported results, the ion pressure increases in association with dipolarization within X ~ -10 RE, where the initial dipolarization takes place. This pressure change is largely contributed by high- energy particles. Tailward of this region, the ion pressure does not significantly increase even after the beginning of dipolarization, while it decreases in some cases; the contribution of high-energy particles is not very large in both cases. Furthermore, the ion beta enhances around substorm expansion onset in the close vicinity of the magnetic equator of the initial dipolarization region. These observations suggest that the characteristics of the dipolarization differ between the initial dipolarization and tailward regions. We discuss implications for the current disruption model (the rarefaction wave), pressure balance, and relationship with auroras.
SM43A-1703
Evolution of Inner Plasma Sheet Pressure Associated with Substorm Onset
We have investigated the evolution of plasma pressure associated with substorms onsets within the inner equatorial plasma sheet using the five THEMIS spacecraft data during the 2008 tail season. The central plasma sheet thermal pressure is estimated from the measured off-equatorial plane thermal pressure and magnetic pressure. Based on simple pressure balance, we expect the reduction of cross-tail current associated with current wedge formation to be associated with a reduction of the plasma sheet pressure that initiates at a radial distance of ~-10RE and expands tailward. However, our results show that associated with substorm, the plasma pressure undergoes a large enhancement after onset earthward of ~-11RE. Also, associated with the onset, a gradual precursor pressure increase is found several minutes before the large enhancement, and the precursor appears to be associated with the gradual enhancement of the substorm breakup arc that has been observed a few minutes prior to the dramatic brightening and breakup of that arc. One the other hand, the plasma pressure beyond ~-16RE shows a remarkable decrease after onset. During the recovery phase, the pressure gradually returns to its original level. These results indicate that during the substorm expansion phase, the current wedge connects to the region further tailward than ~-16RE. The pressure increase earthward of ~-11RE could be related to the substorm current system oppositely directed to the substorm current wedge, which has recently been observed equatorward of the substorm onset region in the ionosphere. The relationship between the auroral break up arc, which is believed to map to ~-10RE, and the pressure and current evolution in that region of the plasma sheet near the time of onset becomes an interesting question for future study.
SM43A-1704
Periodic Dipolarizations in the Near-Earth Magnetotail Observed by Cluster
Large-scale, periodic oscillations are frequently observed in the Earth's magnetosphere. In this study we will present observations of periodic dipolarizations in the near-Earth magnetotail by the Cluster spacecraft. The average dipolarization period is 2 - 3 hours, which is much longer than the periods of ULF waves. These dipolarizations are distinguished from the fast (< ~10 minutes) dipolarizations usually observed with fast earthward flows. Ion observations show that there is no clear flow activity associated with the dipolarizations, except for some intermittent bursty bulk flows and significant density drops. Examination of the relationship of the dipolarizations with the solar wind variations shows that out of 18 periodic dipolarizations selected from 2001 to 2005, ten events occurred during fast solar wind Vsw > ~700 km/s). No event shows a clear relationship with the variations of the interplanetary magnetic field. The periodic dipolarizations have been observed through the entire tail crossings from northern to southern lobes and can last over 24 hours. Also, the dipolarizations are simultaneously observed by all four Cluster spacecraft even when the spacecraft are separated more than 1 RE, indicating the dipolarizations are large-scale and possibly occur over the entire near-Earth magnetotail. There exists a close relationship between the periodic dipolarizations and periodic geomagnetic activities though no flow activities are observed in the magnetotail in association with the dipolarizations. We will discuss the dynamics of the near-Earth magnetotail during the periodic dipolarizations and its role in the periodic geomagnetic activities.
SM43A-1705
Observational test of the ideal MHD frozen-in condition for bursty bulk flow /dipolarization events in the near-Earth tail
The ideal MHD frozen-in condition is tested for a number of bursty bulk flow (BBF) events and dipolarization events, as identified using data from the Cluster satellites. The selection of the BBF events were done based on the criteria: 1) The spacecraft location is such that X(GSE) is -5 RE or farther, Y(GSE) is within 10 RE, and Z(GSE) is within 5 RE, and 2) Plasma ¥â is 0.1 or larger, 3) X component of the perpendicular flow is 200km/s or larger. For the selection of dipolarization events, we imposed the condition, in addition to the conditions 1) and 2) above, that the magnetic elevation angle increase is 10 degrees or more in 5 minutes. For each of the selected events, we have first compared Ey directly with -(VxB)y. The comparison indicates that the value of -(VxB)y is larger than the actually measured Ey for most of the BBF and dipolarization events duration, a clear indication of break-down of the frozen-in condition. For the BBF events, this seems to imply that the magnetic transport rate based on the frozen-in condition electric field, -VxB, likely overestimates the true magnetic transport rate toward the Earth. We will present the statistical results of some quantitative estimates of the discrepancy between the ideal MHD and actual transport rates. On the other hand, in order to examine in what aspects the frozen-in condition becomes invalid, we have performed a FFT analysis for each of the dipolarization/BBF events. The analysis for Ey and -(VxB)y during a given event interval indicates some difference, even in low frequency regime, in the spectral power feature between the two quantities. When band-passed filtered, we see some out-of-phase behavior between the waves of Ey and -(VxB)y for times before the onsets of dipolarization/BBF events. The phases begin to agree to a reasonable degree at and after the event onset times, but the difference in magnitude still remains significant to be the major reason for the discrepancy between the two quantities. Therefore the deviation from the frozen-in condition occurs during entire durations of both BBF and dipolarization events.
SM43A-1706
Field Aligned Currents Detected During Fast Flows in the Magnetotail
In this poster we present Cluster observations of two kinds of field aligned currents in the magnetotail. The first kind is the localized current wedge produced by the shear in the flow and the magnetic field at the boundary of a BBF. Cluster enter the flow from the side in the outer central plasma sheet and detect negative excursion in BZ and BY at the flank of the BBF which produce a field aligned current. Inside the flow BZ and EY increase. That does also the temperature, while the density decreases. Thus the BBF has the properties of a depleted flux tube. The second kind is the field aligned currents produced by the divergence of the Hall currents in the ion diffusion region. This result is from a statistical survey of Cluster magnetic field measurements during fast flows. By fitting the magnetic field to a Harris current sheet, a derivative of BY with respect to the Z axis has been used to estimate the X-component of the current density (jX). The analysis reveals the quadrupolar pattern of BY and the corresponding loop-like pattern of jX.
SM43A-1707
Distributions of Charged Particles for Non-storm Time Substorm Events from THEMIS observations
THEMIS mission has provided plentiful observational data including fluxes and angular distributions for both ions and electrons at lower energy (< 30 keV) and higher energy (> 30 keV) from the instrument ESA and SST, respectively. The separation of the five composed satellites at the equatorial plane at different L-shells provides simultaneous observations at multi-points to study the spatial structure and temporal evolution of these charged particles. We will present case studies on analysis of the observed charged particle data for non- storm time substorm events. The evolution of pitch-angle distributions is specifically investigated. The local magnetic field data from the instrument FGM and upstream solar wind data (e.g. from ACE) will also be adopted to identify the magnetospheric environment. This will enhance our understanding on the possible mechanism for charged particles occurred during non-storm time substorms and benefit to future tudies on storm time substorm events.
SM43A-1708
Wavelet Bicoherence Analysis of the Nonlinear Wave-Wave Interaction at the Substorm Onset
The magnetic field fluctuation associated with the current disruption at the substrom onset observed by the THEMIS satellite (P5) near Earth at around XGSM ~ -8RE on 1/29/2008 has been analyzed by using the wavelet bicoherence method. The analysis reveals that the inverse cascade process takes place from the fraction of the ion cyclotron frequency(~0.1Hz) to low frequencies(~ 0.01Hz) corresponding to the time scale of the current disruption via a nonlocal wave-wave interaction. Detailed bispectral analysis of the THEMIS magnetic field data are presented.
SM43A-1709
Identification of Dipolarization-Associated High Frequency Waves That Exhibit an Inverse Cascade Feature
We investigate the origin of waves leading to current disruption and dipolarization observed by a THEMIS satellite at XGSM ~ -8 RE in the magnetotail near a substorm expansion onset. Continuous wavelet transform of the magnetic activity associated with the current disruption shows a clear inverse cascade feature of waves starting at frequencies slightly below the ion cyclotron frequency and gradually evolving to waves at low frequencies that correspond to the time scale of dipolarization. There was no other low frequency wave associated with the time scale of dipolarization. We developed the diagnostics of wave polarization based on cold plasma theory in order to identify the wave propagation angle and the normal mode characteristics. A preliminary theoretical analysis is carried out to account for the initial wave characteristics identified through wavelet and wave polarization procedures. From these results, we propose that the excited waves causing eventual dipolarization in this event may originate from the drift-driven electromagnetic ion cyclotron instability propagating at an oblique angle with respect to the magnetic field and approximately parallel to the cross-field current flow. On the basis of such information we have also solved the two-fluid theory of oblique Alfvén-ion-cyclotron instability driven by the cross-field ion motion.
SM43A-1710
The region-2 current driven model of the substorm
The substorm mechanism is investigated by analyzing the numerical results obtained from the recent more refined magnetohydrodynamic (MHD) simulation. Present simulation results reproduce the observed signatures of substorm onset including the dipolarization, the geosynchronous D deflection, formation of the near earth neutral line (NENL) and earthward flow in the plasma sheet together with the development of the nightside field aligned currents (FACs) and electrojets in the ionosphere. The sudden onset is promoted in the magnetosphere by the abrupt formation of high-pressure region in the inner magnetosphere. This high- pressure region is generated through the feeding of internal energy by the magnetic tension release from the NENL, associated with sudden movement of relatively high-pressure region gradually established around x=- 10 Re during the growth phase to the geosynchronous altitude. Such rapid movement can be realized in the magnetosphere, because magnetospheric motion can be decoupled from ionospheric convection if it is a short time. The position of resulting high-pressure region is more inside from the braking point of flow from the NENL. Primary driver of the substorm current system is the partial ring current and incidental region 2 FAC which is driven through the conversion of internal energy to electromagnetic energy by convection crossing the high-pressure region formed at the geosynchronous altitude. Associated nightside region 1 FAC is not from the traditional current wedge but from the usual cusp dynamo. The region 1 FAC path to the ionosphere is altered from dayside to nightside so as to form a short circuit in the ionosphere with the region 2 FAC from the partial ring current. In the expansion phase, convection proceeds to relax the distorted pressure distribution formed at the onset, accompanying the development in dipolarization and plasma sheet thickening in the magnetosphere and increase of westward and eastward electrojets in the ionosphere. The present model can explain the longstanding question concerning the position of first brightening arc, together with the explosive growth phase and the thinning of mid-tail plasma sheet just after the onset.
SM43A-1711
Analysis of Substorm Recovery Phase Using a Global MHD Simulation and Satellite Data
Substorms have been an important part of magnetospheric studies for the past 40 years. However, the recovery phase has been relatively neglected compared to the much greater analyzed growth and expansion phases. From an observational viewpoint the recovery phase begins with the decay of the electrojet or the substorm current wedge in the near-Earth tail (this being the maximum in the AE index) [Baker et al., 1996]. As put forth by Baker et al. [1999], one of the main questions regarding the recovery phase is, "why does it start when it does?" When the recovery phase begins, there is generally still free energy remaining in the lobes that could conceivably be transported earthward through reconnection at the neutral line; yet some process forces the substorm and energy transport to diminish. In this work, we analyze the Lyon-Fedder- Mobarry MHD simulation of six substorm events to study the processes that govern the substorm recovery phase. We examine the magnetospheric changes from a global perspective during the substorm process by using the simulation to connect activity in the tail with ionospheric signatures as well as large scale energy flow through the system. We compare the simulation output to observations from ground magnetometers and with the Cluster spacecraft to verify that the simulation captures the accurate picture of the substorm process.
SM43A-1712
Understanding Magnetotail Current Sheet Evolution during a Substorm by Using MHD Simulations
We have investigated the dynamics of the magnetotail current sheet during a substorm that occurred on March 1, 2008 by using global magnetohydrodynamic (MHD) simulations driven by Wind solar wind observation. We have validated the simulation results by comparing them with THEMIS observations in the magnetotail, GEOTAIL observations in the magnetosheath, and GOES-11, and GOES-12 observations at synchronous orbit. During this substorm the THEMIS spacecraft were in conjunction in the dusk side magnetotail. This arrangement provided an excellent data set with which to investigate meso-scale structures in the current sheet. Ground auroral observations have been used to identify an initial intensification of the substorm at 0148UT followed by the main onset at 0155UT [Runov et al., 2008]. Earthward and tailward flows as well as flux ropes were found in both the observations and simulations. We will use our simulation results combined with the observations to investigate the global convection system and current sheet structure during this event. We show how meso-scale structures in the MHD simulations and observations are related to the overall magnetotail dynamics.
SM43A-1713
Modeling Substorm Ion Injection in the Near-Earth Magnetotail
On March 23, 2007 the THEMIS satellite constellation provided us with a unique opportunity to study substorm injection in the region beyond geosynchronous orbit. On March 23 all five THEMIS spacecraft observed substorm injection in the near earth tail (X~-7RE). We have used the THEMIS observations to examine magnetic field dipolarization and plasma bulk flow. We have carried out large scale kinetic simulations in which we traced the trajectories of millions of ions in the time dependent magnetic field and electric fields obtained from a global MHD simulation of this substorm. These particles were launched before substorm onset earthward of the X-line with drifting Maxwellian distributions. The results reproduce the dispersionless energetic particle fluxes observed by THEMIS. We will investigate the mechanism by which the particles gain energy during the substorm process. We also will compare the results with Maxwellian distributions with those from launches in which we initialized the particles with kappa distributions.
SM43A-1714
Global observations of injection region evolution during a reconnection initiated substorm
Recent studies have shown that dispersionless injections can be identified in riometer data [Spanswick et al., 2007]. This raises the possibility of using a network of riometers to identify the time when and location (at least as projected in the ionosphere) where an injection began, and of following its subsequent spatio- temporal evolution. In this paper, we present a detailed picture of the evolution of a large substorm injection. The event occurred on August 27, 2001, and has been discussed in numerous publications. The expansion phase in question followed several smaller activations that would reasonably be described as pseudobreakups. Baker et al. [2002] used Cluster in situ and IMAGE FUV observations to argue that near- Earth reconnection preceded the auroral onset. Blake et al. [2005] used Cluster, Polar, and Chandra data to investigate the large-scale expansion of the injection region, subject to the caveat that the injection was well underway when it expanded over the satellites. In this paper, we present riometer and in situ observations of an injection seen at more than 20 separate locations (including 6 satellites: Cluster, Polar, Chandra, and 3 LANL spacecraft). The injection is observed to be dispersionless at 12 of these locations (three of them are observations previously reported by Blake). We are able to identify the time and location at which the injection begins. Combining these observations with information from the GOES-8 geosynchronous satellite we argue that the injection initiated near-geosynchronous orbit and expanded poleward (tailward) and equatorward (earthward) from its beginning. Further, the injection began several minutes after the reconnection identified in the Cluster data, providing concrete evidence that near-Earth reconnection, in at least some events, has little if any ionospheric signature.
SM43A-1715
Global Alfvenic Interaction and Substorm Onset
Substorm onset is a result of Alfvenic interactions in the global current system including the tail and magnetopause current sheets as well as the auroral field-aligned current system. During the growth phase, the solar wind-magnetosphere interaction occurs in multiple regions throughout the magnetopause current sheets and stresses the tail current sheet, leaving it susceptible to the breakdown of the frozen-in condition. Changes in solar wind parameters or intrinsic tail plasma processes lead to a force imbalance that can often generate plasma flows and fast mode wave packets. These wave packets interact with the stressed current sheet and cause the breakdown of the frozen-in condition in multiple active localized regions throughout the tail current sheet. During this process and the further reconfiguration of the plasma sheet, Alfven waves carrying field aligned currents can be generated, which can cause auroral intensification and lead to the subsequent auroral development seen in the expansion phase. Observations predicted by this alternative Alfvenic interaction theory on substorm triggering and timing will be compared with the CD and NENL models.
SM43A-1716
A magnetoseismic investigation of substorm onsets and its validation by satellite observations
We applied the travel-time magnetoseismic technique on the Pi~2 pulsations associated with the January 29, 2008 substorm to estimate the onset time and location in the magnetotail. The arrival time of Pi~2, which refers to the first peak in Pi~2 amplitude, observed by McMAC, THEMIS, and CARISMA ground magnetometers presents a strong function of latitude. Using the Tamao travel time as the forward model, the inversion from the observed Pi~2 arrival time estimates that the onset in the magnetotail started at X = -17.9 Re at 0713:25 UT, or approximately one minute earlier than the Pi~2 onset time identified in ground data. The estimated onset location in the magnetotail is consistent with the in situ observations by the THEMIS satellites, among which P1 and P2, located at X = -30 and -18 Re, observed negative Bz, while P3 detected earthward flows at X = -11 Re. The estimated start time in the magnetotail is at least 30 sec earlier than the auroral brightening seen by THEMIS all-sky imagers. Our magnetoseismic analysis and observations of this event are both consistent with reconnection model of substorm triggering.
SM43A-1717
Lessons Learned from the THEMIS Major Conjunction on January 5, 2008
On January 5 2008, a strong substorm with a double onset occurred during a THEMIS 'major conjunction' in which the spacecraft were radially aligned in the magnetotail. The two distinct onsets, separated by 38 minutes are characterized both by space and ground observations. Pi2 pulsations and mid-latitude H bays provided essential measurements for accurate substorm timing and the determination of onset location. The substorm current wedges and the tail reconnection onset in the two events are located differently. The substorm current wedge is closer to the Earth and expands gradually after the first onset, while the second current wedge is located at higher latitudes and the onset of reconnection appears to be more tailward. The reconnection rate is higher at the second onset as evidenced by the faster depletion of tail lobe flux. This event teaches us more about multiple-onset substorms and demonstrates the important synergy between observations in space and on the ground. Both data types are essential for the correct interpretation of the events.
SM43A-1718
THEMIS Observation of a Substorm Event on 4:35, Feb 22, 2008
We report on THEMIS observations during a substorm between 0430~0450 UT on Feb 22, 2008. The spacecrafts were aligned along the tail from X=-5 RE to -25 RE GSM. The most distant probe P1 (X=-24.5 RE) detected two successive tailward moving bipolar magnetic structures. P2 (X=-18 RE), P3 (X=-11 RE), P4 (X=-10.5 RE) and P5 (X=-6 RE) all captured signatures related to the earthward movement of a magnetic structure. THEMIS ground stations and all-sky imagers also recorded Pi2 pulsations and an aurora sudden brightnening. We perform a detailed timing analysis of spacecraft and ground-based data and reconstruct the time sequence of phenomena during this substorm. The earliest sign of substorm onset was the bipolar perturbation in the Northward component of the magnetic field (interpreted as the result of reconnection onset) at P1 at 04:35:16UT and corresponding magnetic perturbation at P2 at 04:35:14UT. Auroral onset was seen at or before 04:36:18UT, consistent with the onset of Pi2 pulsations at 04:36:10 UT. Earthward flows at P3 and P4 seen at ~04:36:03UT, and dipolarization onset at ~04:36:50UT, were observed slightly after the ground onset signature, implying that near-Earth dipolarization happened in the aftermath of both tail reconnection and auroral intensification. Reconnection in the tail preceded ground onset by 60~80 seconds, and also preceded near-Earth dipolarization (current disruption) by ~2 minutes. Two reconnection pulses were observed and a direct association with two auroral intensifications is possible, suggesting that tail reconnection, like the auroral expansion, advance in steps rather than continuously.
SM43A-1719
Timing and localization of near-Earth tail and ionospheric signatures during a substorm onset.
On 16 February 2008, the THEMIS spacecraft bracketed the near-Earth signatures of a substorm onset as identified in the THEMIS ground-based observatories measuring an AETH index up to 180 nT. The main onset was associated with the formation and tailward release of a plasmoid (a proto-plasmoid) at XGSM = - 18.3 RE, and a dipolarization in the inner part of the plasma sheet at XGSM = -11.0 RE. Using timing analysis between the spacecraft we approximate where and when the main onset occurs. The time history and geometry of the event in the tail is consistent with magnetic reconnection as the cause of the substorm expansion onset process. Two activations of the plasma sheet, evidenced by tailward streaming of energetic ions and southward or bipolar signatures of the magnetic field preceded the main substorm. The first activation was associated with an intensification of an arc observed separately from the substorm arc, while the second with the onset of ULF pulsations at mid- and low-latitude stations. We conclude that near-Earth plasma sheet activity that may also be due to reconnection and may be related to non-substorm arc intensifications can precede substorm onset by several minutes.
SM43A-1720
A Unified Scenario of Near-Earth Substorm Onset: Analysis of THEMIS Events
We propose an alternative scenario for the substorm onset process, based on ideal ballooning stability
analysis of the near-Earth plasma sheet during recent THEMIS substorm events. In this scenario, the
ballooning instability is initiated by the magnetic reconnection in the near-Earth plasma sheet, which in turn
directly contributes to the trigger of a full onset. Using the solar wind data from WIND satellite observation for
the substorm event as an input at dayside, we reconstructed a sequence of global magnetospheric
configurations around the substorm onset by means of OpenGGCM simulation. These simulations have
reproduced most of the salient features, including the onset timing, observed in the THEMIS substorm events
[Raeder et al, 2008]. The ballooning instability criterion and growth rate are evaluated for the near-Earth
plasma sheet region where the configuration satisfies a quasi-static equilibrium condition. Our analysis of the
evolution of the near-Earth magnetotail region during the substorm events reveals a correlation between the
breaching of the ballooning stability condition and the substorm onset in both temporal and spatial domains.
The analysis suggests that the Earthward bulk plasma flow induced by the reconnection event in the near-
Earth plasma sheet, leads to the pressure build-up and creates a favorable condition for the initiation of the
ballooning instability in that same region. This new alternative scenario further elaborates earlier conjectures
on the roles of reconnection and ballooning instability [Bhattacharjee et al, 1998], and has the potential to
integrate both the near-Earth neutral-line model [McPherron et al, 1973] and the near-Earth current-sheet-
disruption model [Lui et al, 1988] into a unified model of the near-Earth substorm onset.
Research supported by U.S. NSF Grant No. ATM-0542954.
http://homepages.cae.wisc.edu/~pzhu/draft/gcm/sub.pdf
SM43A-1721
THEMIS 26-Feb-08 substorm expansion onset intensified by northward turning of IMF
We propose to interpret the THEMIS 26-Feb-08 events as substorm expansion onset intensified by northward turning of IMF. Auroral arcs in the oval including the substorm onset arcs can be produced by enhanced convection driven by the distant-tail X-line during the growth phase. Dipolarization at the expansion onset of the THEMIS substorm events are intensified by the northward turning of IMF. If the dipolarization-induced thinning is intense enough to produce the NEXL (near-Earth X-line), the expanding auroral bulge will be driven by the NEXL during the expansion phase. Otherwise the expansion onset terminates in a pseudo breakup. The one-on-one correspondence between the causal sequence of the substorm events of the model and the timing sequence of the THEMIS substorm events speaks well for the model.
SM43A-1722
Re-evaluation of Observations for the February 26, 2008 Substorm Reported to Show Tail Reconnection Triggering Substorm Expansion Onset
A recent article from the THEMIS program reported that tail reconnection triggered a substorm expansion onset on February 26, 2008. We have re-examined the observations of that substorm with the following findings: (1) the conventional signatures of reconnection at P1 (at Xgsm = -21.5 Re) occurred later by ~1-2 min than the dipolarization activity onset at P4 (at Xgsm = -10.2 Re) and P3 (at Xgsm = -10.9 Re), indicating dipolarization onset in the near-Earth region preceded tail reconnection onset in the mid-tail; (2) the unconventional signatures used to indicate reconnection onset at P1 is subject to an alternative interpretation of plasma sheet thinning; (3) observations at P2 (at Xgsm = -17.2 Re) shortly after the claimed reconnection onset at P1 showed discrepancy from the expected reconnection signatures (positive excursion of Bz accompanied by tailward plasma flows); (4) the observed electron energy flux at P1 at reconnection onset was too weak to be responsible for the observed auroral intensity at Gillam, indicating that reconnection activity at P1 was not directly responsible for the auroral activity on the ground; (5) a substorm expansion started at ~0403 UT on that day and was centered around Iceland located in the midnight sector, suggesting the possibility that auroral activity at Gillam and reconnection onset at P1 might be related to substorm intensification at a later stage of this substorm. This re-evaluation shows that the reported event is not a compelling case of tail reconnection triggering substorm expansion onset. Furthermore, the reported case is more consistent with current disruption than with tail reconnection to be the trigger for the substorm expansion onset.
SM43A-1723
Thin current sheet in the substorm late growth phase: THEMIS observations
The formation of a thin current sheet prior to the expansion onset of a substorm event, which occurred on Feb 26, 2008, is studied in the near-Earth magnetotail based on THEMIS observations. During this time interval, the ion distribution showed mushroom-shaped structures with clear non-gyrotropic features, indicating that the warmer component of the ions were unmagnetized, which becomes possible only if the gyroradii of these ions are comparable with the current sheet thickness. By comparing the observations with the Sitnov (2006) model, which is a generalization of the Harris (1962) model by considering the effect of the meandering ions in the thin current sheet, we reconstruct the current sheet structure in the late growth phase of the substorm, and discuss the role of the ion kinetics in the thin current sheet.
SM43A-1724
Electron acceleration in the near-Earth magnetotail in substorms
We investigate substorm events in the near-Earth magnetotail in order to examine acceleration signatures of electrons using data from the Cluster satellites with separation larger than 1 RE. Thermal electrons detected by the PEACE instrument and the high-energy electron flux from the RAPID instrument are analyzed and compared with simultaneous magnetic field, electric field, and ion observations from FGM, EFW, and CIS instruments, respectively. It is found that electrons with energies up to a few hundreds keV exhibit the hardest spectra in the initial stage of the events. These electrons are associated with fast Earthward ion flows and the enhancement of the dipolar magnetic field and the electric field. Although most of the distributions are isotropic, electrons sometimes show the preferential increase of the perpendicular flux, suggesting the effect of betatron acceleration. These electron signatures last only for about one minute, and after that either the flux quickly decreases or a more isotropic flux is observed. The spectra gradually become softer in the course of substorms, and the spectra are softer than the initial state in some cases. The soft spectra are sometimes associated with the temporal drop of the perpendicular electrons. The larger flux is observed by the satellite closer to the Earth, while the satellite on the tailward side shows a faster response to magnetotail perturbations. We discuss possible acceleration mechanisms and the flux transport in the magnetotail.
SM43A-1725
Ion temperature drop and quasi-electrostatic electric field on edge of the current sheet minutes prior to the local current disruption
Cross-tail current disruption (CD) in the near-Earth plasma sheet has been well recognized as the key process at the substorm expansion phase onset. However, some outstanding questions such as: what is the principal local magnetospheric parameters that control the stableness/unstableness of a thin current sheet (TCS), what is the dominating plasma instability mode leading to the CD, remain to be answered. Recent theoretical studies indicated that the stableness of the TCS is particularly sensitive to the distribution of ion drift velocity across the thickness of the current sheet, and that the quasi-electrostatic electric field developed in an ion-kinetic-scale TCS may play a key role in driving the TCS to instability. This motivates an investigation of the variations of plasma parameters at the edge of the TCS during the final minutes prior to the local CD. On a statistical survey of the near-Earth CD events during substorm intervals collected form the first tail season of the THEMIS mission, we identify a consistent behavior of precipitous drops of the thermal ion temperature at the edge of the current sheet in a couple of minutes before the local CD. Such ion temperature drop is interpreted as associated with the extreme thinning of an embedded TCS structure that leads to its ultimate disruption. The observed ion cooling is always accompanied with significant electric and magnetic field perturbations containing both systematic shift and wave oscillations. In particular we also reveal the concurrent growth of a neutral sheet-pointing quasi-electrostatic electric field minutes prior to the local CD. We discuss and simulate the role of such quasi-electrostatic field in leading to the ion temperature drop and the subsequent CD process. The wave components of the observed electric/magnetic perturbations are interpreted as Alfvenic effects of the kinetic ballooning mode.
SM43A-1726
Electrostatic field prior to local current sheet disruption
We present observation of plasma waves in the current sheet across multiple THEMIS satellites in a substorm event on March 5, 2008. It is shown that the arrival of a kinetic ballooning perturbation interacts with the local current sheet to generate a quasi-electrostatic wave a few minutes before local onset, consistent with the prediction that the current sheet thins after the passage of a rarefaction wave. It is proposed that the presence of the quasi-electrostatic field may change the local stability condition and induce local current disruption. Preliminary theoretical calculation is given to substantiate this idea.
SM43A-1727
Taiwan Ascii and Idl_save Data Archives (AIDA) for THEMIS
THEMIS (Time History of Events and their Macroscopic Interactions during Substorms) is a satellite mission that aims to determine where and how substorms are triggered. The space research team in Taiwan has been involved in data promotion and scientific research. Taiwan Ascii and Idl_save Data Archives (AIDA) for THEMIS is the main work of the data promotion. Taiwan AIDA is developed for those who are not familiar with the Interactive Data Language (IDL) data analysis and visualization software, and those who have some basic IDL concepts and techniques and want more flexibilities in reading and plotting the THEMIS data. Two kinds of data format are stored in Taiwan AIDA: one is ASCII format for most users and the other is IDL SAVE format for IDL users. The public can download THEMIS data in either format through the Taiwan AIDA web site, http://themis.ss.ncu.edu.tw/e_data_download.php. Taiwan AIDA provides (1) plasma data including number density, average temperature, and velocity of ions and electrons, (2) magnetic field data, and (3) state information including the position and velocity of five THEMIS probes. On the Taiwan AIDA web site there are two data-downloading options. The public can download a large amount of data for a particular instrument in the FTP equivalent option; the public can also download all the data for a particular date in the Data Search option.