SM23B-1691
The magnetotail total pressure and lobe magnetic field at substorm onsets and their relation with the solar wind during storm-time sawtooth events
Sawtooth events in the Earth's magnetosphere are global, large-amplitude oscillations of energetic plasma particle fluxes at geostationary orbit and represent periodic magnetospheric substorms with a typical period of ~3 hours. Sawtooth events generally occur during intense magnetic storms when the magnetosphere is continuously driven by strong southward IMF and high-speed solar wind stream. It has not been well understood how the magnetotail parameters (the total pressure, the lobe magnetic field, and the tail lobe total magnetic flux) at the onset of sawtooth events are related to the solar wind driver. We have conducted a statistical analysis of the magnetotail parameters measured by the Geotail satellite during sawtooth events over 1998-2006. At the onset of sawtooth events (storm-time substorms), the magnetotail total pressure and lobe magnetic field decrease exponentially with the radial distance and increase with the solar wind pressure and merging electric field, and the total magnetic flux in the tail lobe increases linearly with the merging electric field and interplanetary electric field. Empirical formulas of the relationship of the magnetotail parameters at the sawtooth onset with the tail distance and the solar wind are derived for the first time. We have also made a superposed epoch analysis. The magnetotail total pressure and lobe magnetic field take ~50 min for gradual buildup and then ~30 min for rapid increase before the substorm onset, and they decrease for ~80 min after the onset. We have compared our results with previous studies on quiet-time tail behavior and isolated substorms. The magnetotail total pressure at the sawtooth onset is 2-3 times that of the quiet-time magnetotail, the lobe magnetic field at the sawtooth onset is 8-10 nT higher than the value for isolated substorms, and the total lobe magnetic flux at the sawtooth onset is systematically higher than the flux at isolated substorms. The results imply that the sawtooth onset occurs when the magnetotail reaches a critical state and that the critical state depends on the solar wind parameters. Our finds provide new insight into the storm-time magnetospheric dynamics and important guidance for model simulations.
SM23B-1692
Global properties of magnetotail current sheet flapping: A THEMIS case study
Magnetic field variations with amplitudes of 15 - 30 nT and a time scales of one to several tens of minutes frequently observed in the magnetotail plasma sheet are historically referred to as "flapping" motion of the magnetotail current sheet. Flapping can be either a response to variations of solar wind/IMF parameters or due to internal dynamics of the magnetotail plasma sheet. Relations of flapping to modes of the magnetospheric activity, such as substorms and BBFs, are discussed. THEMIS gives a possibility to track flapping waves on wide range of the geocentric distances in the equatorial magnetosphere with simultaneous monitoring of magnetic and auroral activities by GBOs. We study a sequence of magnetic field oscillations with amplitudes up to 30 nT and a time scales of 10-30 min, detected by four of the five THEMIS spacecraft. The probes P1 and P2 were at X=-15.2 and -12.7 RE, P3 and P4 were at X=-7.9 RE. All four probes were at -6.5>Y>-7.5 RE (major conjunction). Multi-point timing analysis of the magnetic field variations shows that fronts of the oscillations propagated flankward, quasi-perpendicular to the direction of the magnetic maximum variation at velocities of 20 - 30 km/s. The observed anti-correlation between ∂ Bx/∂ t and the Z-component of the bulk velocity make possible to estimate flapping amplitude to be of 1 to 3 RE. A cross-tail wave-length was found to be of about 5 RE Thus the flapping waves are narrow, steep, tail-aligned structures with the lengthwise scale >10 RE, i.e., flapping is a global mode of the energy transfer in the magnetotail. The vortex-like plasma motion, observed during flapping, indicates that the flapping waves were propagating through the ambient plasma. An IMF disturbance was detected by ACE, WIND and Geotail just prior to flapping. Observations of the magnetic field variations by ground-based magnetometers show that the flapping oscillations were observed during the growth phase of a substorm.
SM23B-1693
Investigating the Earth's magnetosphere response to IMF Bz magnitude using SWMF
Observations indicate that the magnitude of southward interplanetary magnetic field (IMF) contributes to different magnetosphere response modes. Using the University of Michigan Space Weather Modeling Framework (SWMF), including a global magnetosphere model, coupled with an inner magnetosphere model and an ionosphere electrodynamics model, we attempt to quantify the role of the IMF Bz in determining the level of activity in the magnetosphere. In separate experiments, the IMF Bz component is set to -2.5 nT, - 5 nT, -10 nT, -15 nT and -20 nT while keeping the other input solar wind parameters constant. We have found that the magnetosphere becomes more and more active as IMF Bz becomes more negative. The average vertical magnetic field at geosynchronous orbit, at all local times, decrease systematically, i.e., the inner magnetosphere becomes more stretched as the IMF Bz becomes more negative. The standard deviation also becomes larger, implying that the magnetosphere shows more variability. When the IMF is weak (-2.5 nT), the magnetic field at local midnight is quasi-steady. The magnetosphere starts to show quasi- periodic (~ 2 hours) dipolarizations when IMF is -5 nT and stronger. However, as the IMF Bz becomes more negative, increased turbulence develops around the inner magnetosphere and appears to disrupt the periodic formation of the night side reconnection.
SM23B-1694
Night-side DP2 Fluctuation Observed MAGDAS/CPMN Network
DP2 caused by IMF southward/northward variations has important information about how the solar wind effects are transferred into the magnetosphere, and ionosphere, and on the ground. Since dayside ground magnetic field variations are significantly enhanced during DP2, dayside DP2 fluctuations have been investigated for ages. But night-side DP2 variations are not investigated enough yet. As a new approach to DP2, in this study we examined night-side magnetic variations when DP2 occurred in dayside. Ground data from MAGnetic Data Acqisition System and Circum-pan Pacific Magnetometer Network (MAGDAS/CPMN) stations were analyzed. To identify dayside DP2 events, we used the records of stations located at the dip equator. The association of DP2 with the solar wind variations, magnetic and velocity data from the ACE satellite were investigated. The obtained results by statistical analysis can be summarized as follows. (1) About half (82/153) of the dayside DP2 events are found to accompany the night-side DP2 with similar wave form. (2) The amplitude of the night-side variation (which correlates with the dayside DP2) shows a good correlation with the intensity of the dawn-to-dusk electric field in the solar wind. In the present paper, we will discuss a generation mechanism of the night-side DP2 for a case study. Acknowledgments. We would like to thank the organization for cooperating MAGDAS/CPMN project; Commodore Rodolfo M. Agaton (Director, CGSD, Coast and Geodetic Survey Department, Mutinlupa), Dr. Mazlan Othman (Director General of National Space Agency, Langkawi), Fr. Daniel McNamara (Director, Manila Observatory, Davao), Dr. David Aranug (Director, Weather Service Office YAP, Yap), Dr. Ronald Woodman Pollitt (Presidente Ejecutivo Instituto Geofisico del Peru, Ancon), Dr. Baylie Damtie (IHY National Coordinator in Ethiopia, Dept. of Physics, Bahir Dar University, Addis Ababa).
SM23B-1695
Tailward flows with northward magnetic field in the Earth's magenetotail
In a series of global magnetohydrodynamic simulations of the interaction of the solar wind with the Earth's magnetosphere we explored magnetospheric convection during prolonged intervals with southward interplanetary magnetic field and found strong tailward flows on closed magnetic field lines. These flows were part of large scale vortices in the magnetosphere. In this study, we will present results from an observational study using plasma flow and magnetic field observations from Geotail, Cluster and THEMIS. We have found several intervals during which prolonged southward IMF was followed by tailward flow coupled with a northward magnetic field in the plasma sheet. These flow observations were coupled with poleward convection in SUPERDARN observations of the ionosphere. A statistical study of the conditions which lead to tailward convection coupled with northward magnetic field in the plasma sheet will be presented.
SM23B-1696
Substorms under northward IMF conditions and their implications on the question of energy availability in the tail
A southward IMF condition is generally considered as the most natural precondition for a substorm to occur. The condition is preferred for effective energy entry from the solar wind into the magnetosphere that is to be released via substorm occurrence. But it has been previously reported by some researchers that substorms sometimes occur under northward IMF conditions as well. Based on the IMAGE WIC aurora observations and the solar wind data from ACE and Geotail, we found that the occurrence of substorms under preceding northward IMF conditions is indeed not uncommon. In this paper, we present details of several such substorms that occurred in 2000 and 2001. In selecting these events, we have imposed a distinguishing condition that an identified substorm is preceded by another substorm, both being under northward IMF conditions, so that the later one is indeed a northward IMF substorm in the sense that it is not affected directly by any earlier southward IMF condition that can easily supply solar wind energy. Surprisingly, the substorms identified this way are major substorms, i.e., those with clear auroral breakup followed by substantial auroral expansion. For some of the events, we find that they occurred during the recovery phase of a magnetic storm (the IMF turned and remained northward). This implies the possibility that the availability of the magnetospheric energy for the northward IMF substorms of this kind is related to the preceding magnetic storm. For other northward IMF substorms, however, they indicated no obvious association with preceding magnetic storm activity. We use various data sets and models to address the fundamental question, i.e., what process can effectively transfer the solar wind energy into the magnetosphere under northward IMF conditions to lead to a major substorm, and also the question whether the tail is left with sufficient energy for a substorm occurrence even after major energy release by an earlier substorm occurrence.
SM23B-1697
Radial profile of the storm-time convection electric field in the equatorial magnetosphere: THEMIS observations
The radial profile of convection electric fields and its temporal evolution in the evening sector of the equatorial magnetosphere during a storm main phase is examined on the basis of the plasma and field data obtained by the THEMIS spacecraft constellation. Before the storm main phase started, the duskward component of the DC electric field enhanced in both the inner magnetosphere (R ~ 5 RE) and the tail plasma sheet of R > 10 RE upon a significant southward IMF. The large (~1-2 mV/m) duskward electric field persists in the inner magnetosphere until the spacecraft enters the electron plasma sheet region, while that in the tail plasma sheet is not as stable, but shows large fluctuations. As moving into the electron plasma sheet, the electric field still has a small but significant (~0.1-0.3 mV/m) duskward component in the plasma sheet at R ~ 6 RE, while that in the plasma sheet at R ~ 10 RE shows no systematic duskward component, even though the storm keeps growing during its main phase. These simultaneous observations in the inner magnetosphere and the plasma sheet down the tail reveal that the temporal evolution of the storm-time convection electric field is not spatially coherent but differs for different regions; a characteristic structure of the convection electric fields associated with different particle regimes develops in the inner magnetosphere during storm times.
SM23B-1698
Solar Wind-Magnetosphere Energy Coupling: Directly Drive or Loading- Unloading
The most direct response of the Earth's magnetosphere and ionosphere to the solar wind energy input is seen in the high latitude ionosphere as increased auroral activity, ionospheric convection and joule heating etc. These reactions are operated by same and/or different mechanisms. Then, interesting questions include with the same solar wind source why the reaction can be so different? What determines the energy transfer budget of each reaction? If one reaction (such as the auroral intensity enhancement) is caused by more than one magnetospheric disturbances (such as subtorms and storms), can we distinguish corresponding energy depositions, respectively? Recently, we have expanded the Natural Orthogonal Component (NOC) method to the separation of the ionospheric electric field (Sun et al., GRL, 2008; Zhou et al., submitted to Adv. Geosci, 2008) based on the AMIE potential data. We have successfully separated nightside ionospheric electric fields that are the Directly Driven (DD) component (a westward electric field, EW) and the Loading-Unloading (UL) component (a southward electric field, ES). The latter represents substorm expansion phase (SEP). In this study, we calculate energy depositions corresponding to the two processes, respectively. The intensity and location of Joule heating are quantitatively depicted in two dimensions. Whether the DD process is a substorm or is more adequate for a magnetospheric convection will be discussed.
SM23B-1699
THEMIS Observations of Steady Magnetospheric Convection at the Dawn and Dusk Terminators
Steady magnetospheric convection (SMC) events are a mode of magnetospheric response, consisting of enhanced, steady convection with no substorms and with duration longer than a typical substorm recovery phase. SMCs are one important mode whose study will aid in understanding the convecting magnetosphere's response to the solar wind. During this mode, large scale convection in the magnetosphere is stable, while on shorter time scales transient activations have been seen. We will utilize data from the THEMIS mission, a set of five spacecraft at varying distances around the Earth, to study flux return to the dayside. THEMIS ground magnetometers will eliminate substorm expansions from our dataset, and the All-Sky Imagers will provide dawn auroral observations. THEMIS experiences conjunctions at the dawn and dusk terminators during the months of November-December (dawn) and April-May (dusk). We use THEMIS observations to examine two SMC events, one at dawn on November 10, 2007 from 12:00-16:00 UT and another at dusk on May 22, 2008 from 0:00-7:30 UT. The behavior of the magnetic and electric fields and plasma flows in these regions will be described.
SM23B-1700
Seeing the Plasma Sheet with ENAs
Energetic neutral atom images (ENAs) from the IMAGE satellite have been used previously to observe features of the plasma sheet. From MENA images, McComas et al. [Geophys. Res. Lett., 29(22), 2079, doi:10.1029/2002GL016153, 2002.] showed that high flux from the plasma sheet is associated with high solar wind density. From HENA images, Brandt et al. [Geophys. Res. Lett., 29(20), 1954, doi:10.1029/2002GL015160, 2002] observed the acceleration of the plasma sheet ions during a substorm. We show here that the plasma sheet can also be observed from bright pixels in the ENA images that result from low altitude emissions caused by charge exchange of plasma sheet ions and ionospheric neutral oxygen. Magnetic field lines from these bright pixels are mapped into the plasma sheet using the Tsyganenko 2005 magnetic field model. Calculations using plasma sheet models, show that the observed flux is consistent with expectations. Thus we are able to observe plasma sheet properties during both CME and CIR geomagnetic storms inferred from geosynchronous observations by Borovsky and Denton [J. Geophys. Res., 111, A07S08, doi:10.1029/2005JA011447, 2006] For example, CME storms result in a plasma sheet that has a longer lasting, higher density, while CIR storms produce a longer lasting, hotter plasma sheet.
SM23B-1701
Dawn-dusk asymmetry in ionospheric return flows: relationship between flows and Birkeland currents
Intense duskside ionospheric flows occurring equatorward of the discrete auroral precipitation zone have been attributed to large electric fields resulting from partial ring current injections. In this picture the driver is the ion injection that feeds the Region 2 currents which in turn cause the electric field. Alternatively, one may take the view that the return flow at dusk is favored energetically by the coupled magnetosphere-ionosphere system. In this energetics view, the upward Region 2 at dawn corresponds to discrete aurora and enhanced conductance on closed field lines on the dawn side, whereas the downward Region 2 at dusk does not contribute as substantially to enhancing the duskside conductance. The energy dissipation associated with a given flow is proportional to the conductance. Thus, it is energetically more favorable for the coupled system to send more return flow to the dayside at dusk than at dawn. In this paper we assess whether there is a persistent dawn dusk asymmetry in the return flow relative to the Birkeland current location and intensity. Birkeland currents derived from the Iridium Satellite constellation, augmented with DMSP F13 magnetic field data are used to place DMSP F13 drift meter data in the context of both the global and local Birkeland currents. Using events from the Iridium stable currents database of Anderson et al. (2008), we identified 59 events with high quality drift observations during southward IMF. The events are divided into two ranges of IMF intensity and five IMF clock angle bins. We order the data in latitude relative to the peak Region 1 current density. There is a persistent suppression of the dawn return flows relative to those at dusk regardless of IMF By or IMF intensity. This result indicates that the return flow predominantly occurs through the evening closed field line region, consistent with the above energetics picture of return flow in the coupled system.
SM23B-1702
A Comparative Study of Magnetotail, Solar Wind and Ground Observations associated with Substorms and SMCs
Convection is the basic response of the magnetosphere to external driving by the solar wind electric field. The supply of solar wind energy into the magnetosphere may lead to different types of magnetospheric response. These include substorms, steady magnetospheric convection (SMC), poleward boundary intensifications, and sawtooth injection events. In a steady state the magnetic flux should be balanced between dayside merging process and tail returning process. However, the magnetic flux moving between the dayside and nightside is seldom in balance and eventually leads to instability in the magnetotail. The imbalance of magnetic flux is one of the main reasons why there are different types of magnetospheric activities. At the present time, the evolution from one mode of geomagnetic activity to another is still not understood. Is it dependent upon solar wind input only or is it controlled by internal magnetospheric processes only? Is it possible that both solar wind and internal magnetospheric processes can influence different dynamic regimes? A clear understanding of this issue would provide us important information concerning how the solar wind couples with the magnetosphere. In this study we have identified several interesting intervals composed of isolated substorms, substorm to SMC and SMC to substorm during THEMIS tail passages. A detailed comparison of solar wind, magnetotail, and ground observations from these events provides a good opportunity to examine the most important processes (quantities) responsible for different modes of activity. Several indices of activity will be estimated and compared such as the auroral electrojet evolution and the associated auroral disturbance. This is particularly interesting because ionospheric dissipation may be the main energy sink for the energy deposited in the magnetosphere and its understanding can reveal important information about how a transition from one mode to another occurs. The high-latitude and mid to low latitude ULF waves will be examined to see whether this is any difference in pulsation excitation. Important tail observations such as lobe magnetic field and plasma flows will be examined to see whether the loading-unloading process is significant. Solar wind observation will be used to estimate the magnetic flux transported from the dayside. The transported flux will then be compared with the tail returning flux to examine how flux imbalance can influence the dynamic mode transition. Finally, we will examine whether there is any difference in the TEC (total electron content) during SMC and substorm time.
SM23B-1703
Detecting Causality In Space Plasmas With Entropy Based Measures
Understanding causal relationships in space plasmas is a key ingredient of modeling, but can often be difficult to establish. Frequently, causality is investigated by looking at the cross-correlation and checking for a shift in the peak as a function of lag time or by examining differences in the forward and backwards directions. Some of the shortcomings of this method can be illustrated by cross-correlations that exhibit multiple peaks that are large in the non-causal direction and causal systems that do not exhibit asymmetries in the cross-correlation. Furthermore, cross-correlations only reveal linear dependency and may not be as useful for a nonlinear storage and release dynamics (such as might be expected for the magnetospheric response to the solar wind). An alternative choice for studying causality is the one-sided transfer entropy which is highly directional and accounts for static internal correlations so that it is possible to examine whether two variables are driven by a common driver or whether they are causally connected. We apply the transfer entropy to several test systems to illustrate its utility for detecting causality and to space data to illustrate causality in space systems with examples from solar wind-magnetosphere coupling.
SM23B-1704
Geomagnetic Storms Driven by CIRs as Seen by the TWINS ENA Imagers
Geomagnetic storms in the Earth's magnetosphere are predominately driven by either interplanetary coronal mass ejections (ICMEs) or corotating interaction regions (CIRs). ICMEs tend to be the dominate driver for storms during solar maximum, while CIRs are the dominate driver during solar minimum. CIRs are formed by the interaction between fast and slow solar wind streams and tend to corotate with the Sun. CIR-driven storms are often identified by their weak Dst perturbations and 27-day occurrence pattern. A series of small CIR-driven storms with an unusual storm sudden commencement (SSC) were identified during the TWINS science mission (June 2008 through September 2008). Observations from the TWINS energetic neutral atom (ENA) imagers are presented, giving a global perspective of storm-time dynamics within the magnetosphere during these CIR-driven storms.
SM23B-1705
An examination of the origin of substorm periodicities in CIR- and CME-driven storms
The differences in magnetospheric response between storms driven by ejecta (CMEs) and coronal hole outflow (CIRs and HSSs) are a topic of current discussion in the literature. CMEs are well known to cause periodic injection signatures in geosynchronous particle data - so called sawtooth events. A number of authors have concluded that these are phenomenologically indistinguishable from isolated substorms, except for their magnitude and local time extent. What determines the timing and periodicity (~2-3h) of these substorms is widely debated. CIRs have also been reported to drive periodic substorms and again the origin of the timing and periodicity (~4h) of these substorms is unresolved. Cited origins include reductions in the motional electric field (northward IMF turnings) and solar wind pressure variations. Using the Minimal Substorm Model (MSM), we model periodic substorm events driven by both CIR (and attendant high-speed stream) and ejecta. We present a possible explanation for the lengthened inter-substorm period under HSS driving, and show that the periodicities and timings of the onsets are consistent with a simple loading- unloading model. Further, the extent to which a simple model such as the MSM can explain substorm magnitudes is discussed, as are the factors that may limit its predictive power.
SM23B-1706
Plasma Flow Channels at Dawn/Dusk Polar cap Boundaries: Momentum Transfer on old Open Field Lines and the Roles of IMF By and Conductivity Gradients
Using DMSP F13 data we investigate the newly discovered channels of enhanced antisunward convection occurring at the dawn (0600-0900 MLT) or dusk (1500-1800 MLT) flanks of the polar cap for the different combinations of IMF By polarity, hemisphere and the dawn/dusk sides. Dawn-side cases occur for the following combinations of hemisphere (NH/SH) and By polarity: NH-dawn/By > 0 and SH-dawn/By < 0 . The dusk-side cases are: NH-dusk/By < 0 and SH-dusk/By > 0 . The flow channels are placed in the context of particle precipitation regimes/boundaries and ionospheric conductivity gradients. They are found to be threaded by "old open field lines" characterized by polar rain precipitation. In the dawn-side cases (NH-dawn/By > 0 and SH-dawn/By < 0 ) the polar rain contains the "solar wind strahl" component. The convection enhancement is attributed to the Pedersen current closure of Birkeland currents poleward of the R1/R2 currents. This is momentum transfer from the solar wind via dynamo action at the high-latitude boundary layer (HBL). The conductivity gradient at the polar cap boundary contributes to establishing the convection channel and the enhancementent of the dawn-dusk convection asymmetry extending beyond the dawn-dusk terminator during intervals of nonzero IMF By component. The HBL - ionosphere coupling via Birkeland currents is a source of dawn-dusk convection asymmetry and Svalgaard - Mansurov effect which comes in addition to the effect of magnetic tension acting on "newly open field lines".
SM23B-1707
Characterizing the Solar Wind at L1
The nature of solar wind-magnetosphere energy transfer plays a big role in understanding the time history and types of global-scale magnetospheric phenomena. However, systematic approaches to quantifying how the specific magnetospheric "modes" (if they can be called that) of substorms, SMCs, sawtooth events, and geomagnetic storms could be controlled by the solar wind are still difficult. We present a fresh approach to characterizing the solar wind and its time history using self-organizing maps. The thrust of this effort is geared towards detecting and classifying solar wind structure on time scales relevant for the magnetospheric responses of interest. Performing this characterization at the L1 point is ideal for uncovering solar wind- magnetosphere relationships. It also provides a very long, contiguous time series that helps us explore these relationships over a complete solar cycle. We present the technique and initial results of solar wind comparisons during and leading up to SMCs and sawtooth events.
SM23B-1708
THEMIS Observations of Current Sheet Flapping
We report on a 4-hour interval of periodic current sheet flapping observed by the three inner THEMIS probes (P3, 4 and 5) and the outermost probe P1 (at X=-24 RE). These probes were located post-midnight in radial alignment. Substorm onsets bounded the interval of flapping. Several substorm-like activations were observed as the current sheet kinks propagated past the spacecraft. The field at the location of the inner probes dipolarized and disrupted the kink structure locally, but did not appear to disrupt the generation of the periodic kinks. This is strong evidence that the periodic kinks were unrelated to the geomagnetic activity occurring in the post-midnight sector. THEMIS probe P2 was located in the near-Earth, pre-midnight sector, and did not observe kinks. This suggests the periodic kinks were generated near the midnight meridian and propagated in one direction, eastward. Using auroral imagery, ground magnetometers, and multi-point magnetospheric measurements, we discuss the source of the kink structures, the relationship of the flapping to geomagnetic activity, and examine the role of external solar wind driving.
SM23B-1709
The magnetotail implications of optical observations of the brightening substorm aurora
Ground-based optical and topside in situ observations have shown conclusively that many substorm auroral onsets occur immediately poleward of the proton aurora brightness peak. This has long been interpreted as placing a rather stringent constraint on the location of whatever magnetospheric process corresponds to the brightening auroral arc, namely that the relevant field lines thread the region of transition between dipole- and tail-like magnetic field topologies. However, this constraint, by itself, presents us with a number of critical questions. For example, we do not know how abrupt the transition between dipole- and tail-like topologies is, nor do we know the location of that transition in the magnetosphere on either an event by event or even a general basis. Not knowing the radial extent of the transition region limits our ability to explore the nature of the onset instability. Not knowing the actual location of the transition limits our ability to optimally utilize the ionospheric observations in conjunction with contemporaneous satellite observations. In this paper we take a step back from event studies and explore broader implications of the location of the onset arc relative to the peak in proton auroral brightness. More specifically, we use pitch-angle resolved FAST ESA ion observations from more than 20000 transits of the auroral zone to explore the latitudinal separation between the peak precipitating ion energy flux (which corresponds to the peak in proton auroral brightness) and the inner edge of the electron and ion plasma sheets. We use these results, in conjunction with magnetic mapping and published observations of the equatorial location of the inner edge of the ion and electron plasma sheets, to explore the range of locations in the magnetotail to which the onset arc is most likely to map, as well as how the relative distance between the transition region and the inner edge of the plasma sheet varies through the substorm cycle.
SM23B-1710
Solar wind energy transfer into the magnetosphere during a substorm event as seen near the polar cusp boundary: a case study
Energy transfer through the high altitude polar cusp-magnetosphere boundary layer during a substorm event on Feb 10, 2008, is studied by examining the properties of energy fluxes which include the convection Poynting flux, wave Poynting flux and the kinetic energy flux. The fluxes are calculated using the electric and magnetic field measurements from the Polar spacecraft. The substorm signatures are detected by Themis spacecraft and the Themis ground based observations. It is found that in the region of the cusp- magnetosphere boundary, the convection Poynting flux is the most significant energy flux among the three. There is a significant normal component of Poynting flux pointing toward the magnetosphere. We suggest that this part of the energy flux plays a more important role in the near earth energy transfer compared to the kinetic energy flux. A comparison between the observed Poynting flux and the energy flux input from the solar wind is made. A dynamo process on open field lines is discussed as a mechanism how the Poynting flux can be transferred into the magnetosphere.
SM23B-1711
Quasi-Stable Storm Reconnection Region
The Lyon-Feddar-Mobarry (LFM) global MHD code for the magnetosphere develops a quasi-steady reconnection region at about 35-45 RE in the tail during storms. One of the features of this structure is that it produces a region of fast sunward flow just inside the magnetopause. In this paper we present evidence from THEMIS that such a flow layer does exist during the main phase of at least one storm, but it was absent during a period of substorm activity. The implication is that the LFM solution to magnetospheric convection and magnetotail topology resulting from a strong, steady southward magnetic field in the solar wind may be physically realistic.
SM23B-1712
Effects of Solar Wind Parameters on the Energization of the Ring Current
The flow of energy into the ring current is regulated by solar wind parameters. It is well known that VBs in the solar wind is a major factor in regulating the transfer of energy into the magnetosphere as measured by the ring current injection rate. Using ten years of data, from 1995 to 2004, we have identified 93 storms. In this study we will examine the relative role of the solar wind speed, V, as compared to the southward component of the IMF components, Bs, in relation to the ring current injection rate, as defined by the Burton equation. We will address the reinterpretation of the Burton equation by Vasyliunas [2006] as well as the role of the saturation of the polar cap potential on these relationships.
SM23B-1713
Mass Loading During SMCs: Pre-Conditioning of Transport in the Magnetosphere
Steady Magnetospheric Convection designates a state of the magnetosphere where substorm activity is absent despite sustained electromagnetic power delivered by the solar wind. FAST and POLAR measurements of a collection of SMCs observed in the early years of these missions show that the magnetosphere is significantly mass-loaded as a result of extended periods of sustained ion outflow from a wide range of latitudes. The increase in the inertia of the magnetospheric plasma can hinder reconnection in the magnetotail, thus inhibiting substorm triggering, or reconnection in the dayside magnetopause, thus inhibiting magnetic flux transfer into the magnetosphere. It is observed that magnetospheric "bubbles" (flux tubes of rarefied plasma sheet) prompt the extraction of ions from the ionosphere and are responsible for a non-negligible fraction of the total ion outflow. Bubbles are tracked using the ionospheric footprint of rarefied flux tubes and the optical signature of their boundaries.
SM23B-1714
Dependency of the polar cap phenomena and magnetospheric dynamics on the intrinsic magnetic field
It is well known that the polar cap phenomena and magnetospheric dynamics in the earthfs magnetosphere are generally controlled by the solar wind and IMF (interplanetary magnetic field) through magnetic reconnection. The cross polar cap potential has a tendency to be saturated under a strong IMF, however it has a different tendency to increase of the solar wind speed. Potential saturation only occurs with the increasing magnitude of southward IMF, however does not with the increasing velocity of the solar wind. On the contrary, how do the polar cap and magnetosphere behave when the magnitude of an intrinsic magnetic field of earth changes? The relationship is not simple because the sizes of magnetosphere and polar cap, magnetic reconnection and induced convection in the magnetosphere and ionosphere are complicatedly involved. We have studied the dependency of the polar cap phenomena and magnetospheric dynamics on the intrinsic magnetic field by using a 3D global MHD simulation of interaction between the solar wind and earthfs magnetosphere. When the magnitude of intrinsic magnetic field, for example, decreases, the size of magnetosphere decreases, on the other hand the size of polar cap expands. Moreover the polar phenomena also depends on the orientation and magnitude of IMF. We will present the relationship from simulation results.
SM23B-1715
An Information Theory Approach to Storm-Substorm Relationship
The storm-substorm relationship is one of the most intriguing and debated aspects of the global magnetospheric response to solar wind changes. Although since the early 90s several studies on the O+ ions of ionospheric origin ions evidenced the relationship between magnetospheric substorms and large magnetic storms, this link is still under investigation. Here, we present the first results on the existence of a clear transfer of information among these two classes of magnetospheric phenomena by approching this problem via the information theory. In detail, the investigation of transfer entropy [Schreiber, Phys. Rev. Lett., 85, 465, 2000] between two proxies of these phenomena (AL-index and Dst-index) clearly suggests the existence of a statistical evidence for the driving of geomagnetic storms from substorms. The characteristic time delay in transfer of information is found to be below 1hr.
SM23B-1716
Prediction of AL and Dst Indices from ACE Measurements Using Hybrid Physics/Black-Box Techniques
ACE measurements of the solar wind velocity, IMF and proton density is used to drive a hybrid Physics/Black- Box model of the nightside magnetosphere. The core physics is contained in a low order nonlinear dynamical model of the nightside magnetosphere called WINDMI. The model is augmented by wavelet based nonlinear mappings between the solar wind quantities and the input into the physics model, followed by further wavelet based mappings of the model output field aligned currents onto the ground based magnetometer measurements of the AL index and Dst index. The black box mappings are introduced at the input stage to account for uncertainties in the way the solar wind quantities are transported from the ACE spacecraft at L1 to the magnetopause. Similar mappings are introduced at the output stage to account for a spatially and temporally varying westward auroral electrojet geometry. The parameters of the model are tuned using a genetic algorithm, and trained using the large geomagnetic storm dataset of October 3-7 2000. It's predictive performance is then evaluated on subsequent storm datasets, in particular the April 15-24 2002 storm. This work is supported by grant NSF 7020201
SM23B-1717
Physical Meaning of the Equinoctial Effect for Seasonal Variation of Geomagnetic Activity
The general tendency for magnetic disturbances to be more stormy at equinoxes than at solstices has been recognised for more than 150 years. To explain the seasonal variation three principal hypotheses have been proposed; the axial hypothesis (Cortie, 1912), the equinoctial hypothesis (Bartels, 1932; McIntosh, 1959), and the Russell and McPherron (RM) hypothesis (Russell and McPherron, 1973). The RM hypothesis, which is based on the recognition that the magnetic field in the solar equatorial plane tends to have the largest southward component in geocentric solar magnetospheric (GSM) coordinates in early April and October, has been largely accepted for many years. However, recent studies have confirmed that the RM effect accounts for only a subordinate proportion of the seasonal variation of geomagnetic activity, and that the larger part of the phenomenon is attributable to the equinoctial effect in which the angle between the solar wind flow and the dipole axis of the Earth plays an essential role (Cliver, Kamide and Ling, 2000; Cliver, Kamide, Ling and Yokoyama, 2001; O'Brien and McPherron, 2002). In this paper physical meaning of the equinoctial effect is investigated based on the data of three-hourly am index and solar wind parameters acquired by the ACE satellite. The am indices are well correlated with BsVxVx, where Bs is the southward component of the interplanetary magnetic field (IMF) and Vx is the solar wind velocity in the sun-earth direction. It is found, however, that the am - BsVxVx relation depends on the range of VxVx: The am in higher ranges of VxVx tends to be larger than am in lower ranges of VxVx for both equinoctial and solstitial epochs for the same value of BsVxVx. Using the data sets of the same VxVx range, it is shown that distribution of points in the am - BsVxVx diagram at the solstitial epochs overlaps with that at the equinoctial epochs and the average am values in each BsVxVx bin in solstitial epochs are almost equal to those in equinoctial epochs, if VxVx for each point at solstices are reduced to VxVx sin (c) where c is the geomagnetic colatitude of the sub-solar point. This finding indicates that the emergence of the geomagnetic disturbance is regulated by the component of the solar wind velocity perpendicular to the dipole axis of the geomagnetic field. The magnitude of the perpendicular velocity component varies seasonally even if the solar wind velocity remains constant. This appears to be the long-missed key factor causing the equinoctial effect. It is interesting to note that both the RM and equinoctial effects are related to seasonal changes in the efficiency of solar wind – magnetosphere coupling caused by changes in the geometric configuration between the sun and the geomagnetic dipole field, one in relation to Bs of the IMF, and the other in relation to the component of solar wind velocity perpendicular to the dipole axis.
SM23B-1718
Seasonal Variation of Geomagnetic Activity and Interhemispheric Currents
The seasonal variation in geomagnetic activity and related events in the Earth's magnetosphere and ionosphere is well known for many years. One of important effects related to seasonal variation is the variation in the correlation between geomagnetic activity indices and solar wind data: The geomagnetic activity indices show good correlation with solar wind data for winter and equinoxes but the correlation is strongly reduced in summer months when polar cap is predominantly sunlit. This reduces significantly the reliability of forecasting of geomagnetic activity and related events in summer months. We investigated the correlation between the AL index (showing substorm activity in Northern hemisphere) and two geomagnetic activity indices, the Polar Cap (PC) index and the recently developed Polar Magnetic (PM) index [Lyatsky and Khazanov (2008), Space Weather, 6, S06002, doi:10.1029/2007SW000382] showing the magnetic disturbances in two polar caps. For the analysis, we used the data for four years. We obtained an unexpected yet important result: Substorm activity in summer months correlates much better with geomagnetic activity not in the nearby polar cap but in the opposite polar cap. A possible cause for this effect may be the interhemispheric field-aligned currents flowing from the summer high-latitude ionosphere and close the ionospheric currents in the opposite auroral zone. These interhemispheric currents are directed opposite to substorm field-aligned currents in the summer hemisphere providing a significant decrease in the total substorm field-aligned currents, which may be the cause for the strong seasonal variations in geomagnetic activity and related events.
SM23B-1719
Singular Spectrum Analysis of Multiple Time Scale Oscillations of Geomagnetic Activity
Geomagnetic activity is driven by solar wind and interplanetary magnetic field variations and is well represented by the temporal variations in the Geomagnetic storm (Dst) and Ap indices. A study of the temporal oscillations in these activity indices can help in understanding the role of solar wind –magnetosphere coupling on space weather at different temporal scales. These oscillations are quasi- periodic and cannot be studied by conventional Fourier techniques. We, therefore, use the technique of singular spectrum analysis in the present study. 50 years data of Ap and Dst indices are subjected to the analysis. Both daily and monthly mean values of the indices are used to bring out essential characteristics of temporal scales ranging from the solar rotation to solar cycle oscillations. The results of the study are interpreted in terms of the linear and non-linear response of the magnetosphere to solar wind forcing.
SM23B-1720
The coupling of tail fast flows to ionospheric flow signatures and their relationship to substorm onset
Earthward convection of the tail plasma sheet is often organized in bursts of fast ion flows restricted in azimuthally narrow channels. It has been shown that Auroral Poleward Boundary Intensifications (PBIs) are often the ionospheric signature of such fast flow channels in the midtail. While PBIs can occur for all IMF orientations and solar wind conditions, they have a clear preference for southward IMF and their occurrence peaks within 1 hour after a substorm onset, with a secondary occurrence peak at 3 hrs after onset. Equatorward flow bursts have been observed in the ionosphere, that are presumably the ionospheric mapping of the tail fast flow channels. We focus on identifying such ionospheric signatures and understanding the physics of this magnetosphere-ionosphere interaction via conjunctions of the THEMIS probes with the Sondrestrom radar. From a number of such conjunctions we find that the onset of a substorm that is soon followed by a PBI has a very distinct signature in the radar data. At onset and expansion the ionospheric flow turns strictly westward. During the PBI, tail fast flows originate in the mid-tail and ionospheric flows turn equatorward. The generality and physical implications of this pattern are explored.
SM23B-1721
The Role of Specific Entropy During Geomagnetic Activity
The method described in Wolf et al. [J. Geophys. Res., 111, A12218, doi: 10.1029/2006JA012010] to obtain the instantaneous pV5/3 (specific entropy) of flux tubes from single-satellite measurements of the magnetic field and plasma pressure. The specific entropy is used to investigate the distinction between substorm expansion onsets, pseudobreakups, and convection bays. Fast, earthward flows are comprised of flux tubes of reduced pV5/3, i.e., "bubbles". Flux tubes injected over the satellite at local onset have values of pV5/3 comparable or below geosynchronous values. In pseudobreakups, pV5/3 returns to its larger pre-onset value within a few minutes, while for substorm expansions, pV5/3 remains at depressed levels through the expansion phase. pV5/3/le 0.08 nPa(RE/nT)5/3 (near Earth) during convection bays. The depressed value of pV5/3≤ 0.1 nPa(RE/nT)5/3 during expansions and convection bays implies that pV5/3 is approximately constant with distance in the inner plasma sheet.