SM51A-0332 0800h
Model of Ionospheric Ducts Due to HF-heating
Ducts in the Earth's ionosphere and plasmasphere along magnetic field lines, defined by their density gradients from one field line to any neighboring line, have implications to the transmission of ELF radio waves across the globe. Strong HF heating has been shown to create a depletion of electrons at the heating region, and could lead to a pressure perturbation that propagates along the entire magnetic field line. The objective of this paper is to present a numerical model of ionospheric ducts caused by HF heating. Discussed here are results of simulations using the Sami2 1D ionospheric modeling code, which has been modified to include a flexible source of strong HF heating. Some parameters of the source, including intensity, location, and vertical spread of the irradiated region were varied. A non-linear relationship was found between these source parameters with maximum pressure, temperature, and density perturbations, propagation velocity of density perturbations, and characteristic heating and cooling rates of the HF irradiated region. After the ionosphere and plasmasphere pass a transient stage, as the gas nearly settles into equilibrium, pressure and density perturbations were found to achieve a nearly steady state. The density profile shows electron depletion at the heated region, surrounded by density increases in the regions just below and above the heated region. The density perturbations then propagate deep inside the plasmasphere down to the conjugate F2 point. It should be possible to use these perturbations spreading along the field line as an ionospheric duct.
SM51A-0333 0800h
TARANIS: a Tool to investigate Potential Links Between Sprites and Ionospheric and Magnetospheric Plasmas
TARANIS (Tool for the Analysis of RAdiations from lightNIngs and Sprites) is a CNES microsatellite project which will be in phase A in 2005. The main scientific objective is to compare observations of sprites and other optical emissions (blue jets, halos, elves,etc.) with observations of terrestrial gamma and X ray flashes, electromagnetic and electrostatic emissions, and energetic electrons, in order to investigate physical mechanisms allowing impulsive transfers of energy between the neutral atmosphere and the ionospheric and magnetospheric plasmas. The main questions to be addressed for a satellite mission are presented. They include : the triggering factor of the optical emissions, the quasi electrostatic field above thunderstorms, the modification of the electrodynamics of the ionosphere, the detection and the modeling of energetic runaways electron beams, the associated electromagnetic and electrostatic emissions, the presence of the generated electron beams within the magnetosphere and more specifically within the radiation belts. The adequation of the scientific payload to the scientific objectives is discussed.
SM51A-0334 0800h
Comparisons of Thermal Electron Measurements on Two Sounding Rocket Experiments
Thermal electron instruments built by the University of New Hampshire have accompanied standard instrumentation flown on a series of two sounding rocket flights, SIERRA and SERSIO. In 2002, SIERRA was launched from Poker Flat Research Range, Alaska, to 735 km into a modest substorm. The instrument response of the thermal electron detector (TED) has been studied extensively (MacDonald et al., 2004). The TED design featured a pinhole electrostatic analyzer designed to detect the flux and energy distribution of the coldest ambient ionospheric electrons. Despite positive biasing the instrument exhibited formation of a potential barrier restricting access to the thermal core. Using coincident data from other instruments information about the thermal plasma temperature, density, and spacecraft potential can be reconstructed. These data and the theories developed to examine them can be studied in conjunction with the next flight. In 2004, SERSIO was launched from Svalbard, Norway to 790 km in intense cusp ion outflow at the beginning of a geomagnetic storm. This payload contained two different designs for measuring thermal electrons and two identical but orthogonal top-hat thermal ion analyzers. In addition to the TED, another new instrument, the ERPA, was developed for detecting thermal electrons via a retarding potential current collector. On the TED, the bias sweep and coating were altered to improve performance. The SERSIO payload flew into sunlight whereas the previous flight was in darkness which greatly changes the nature of the payload current balance situation. Unfortunately SERSIO data was severely limited by mechanical problems which affected instrument deployment and orientation but it is still useful for this purpose. Extensive ground-based radar observations should prove useful for facilitating quantitative comparisons. The performances of the two TEDs are contrasted with the aim of identifying differences due to changes in internal instrumental parameters versus external environment parameters. Also, the two different thermal electron designs on SERSIO, the TED and the ERPA can be compared. Finally, this payload allows a complete comparison between ground-based thermal parameters and their in-situ electron and ion counterparts. This work should help us to understand more about the true nature of the potential sheath around a rocket, necessary for successful direct measurement of ionospheric thermal electrons.
SM51A-0335 0800h
Two component M-I coupling during auroral substorms
A detailed investigation of the temporal and spatial morphology of the auroral electrojets and its coupling to the magnetosphere during classical auroral substorms has been performed. This is done by superposing all available ground based magnetometers onto Polar VIS Earth Camera images. While the auroral zone ground stations are used to study the auroral electrojets the mid-latitude ground stations are used to study the net field-aligned currents coupling the auroral zone and the magnetosphere. Based on a number of case studies it is found that: 1) part of the wide intense post-midnight westward electrojet is diverted out near midnight; 2) the onset of the westward electrojet pre-midnight and post-midnight components can be shifted by several minutes relative to each other; 3) there is a significant latitudinal shift between the two westward electrojet components with the post-midnight located at a lower latitude.
SM51A-0336 0800h
Interhemispheric Study of SuperDARN HF Radar Echoes : Occurrence, Line of Sight Velocity and Spectral Width of Near Range Echoes Observed With the Hankasalmi and Kerguelen Radars
SuperDARN radars are well suited to study interhemispheric features related to ionosphere-magnetosphere coupling. The behaviour and limits of magnetic conjugacy of auroral zones is not yet fully understood. In this study, we have analysed the occurrence, line of sight velocities and spectral width of near range echoes (60° to 65° of magnetic latitude) for meridional beams of the Hankasalmi and Kerguelen radars, respectively in the northern and southern hemisphere for the whole year 2001. Several events exhibiting a high level of conjugacy for all radar parameters are presented as well as the general features.
SM51A-0337 0800h
Multi-Instrument Mapping of the Small-Scale Flow Dynamics Related to a Cusp Auroral Transient
In this paper we focus on flux transfer events (FTEs) and poleward moving auroral forms (PMAFs) in the cusp region, combining data from the EISCAT Svalbard radar, SuperDARN HF radars, ground-based optics, and three low-altitude polar-orbiting spacecraft. During an interval of southward interplanetary magnetic field the EISCAT Svalbard radar tracked a train of narrow flow channels drifting into the polar cap. One 30-60 km wide flow channel surrounded by flow running in the opposite direction is studied in great detail from when it formed equatorward of the cusp aurora near magnetic noon until it left the field-of-view and disappeared into the polar cap. Satellite data shows that the flow channel was on open field lines. The flow pattern is consistent with field-aligned currents on the sides of the flow channel; with a downward current on the equatorward side, and an upward current on the poleward side. The poleward edge of the flow channel was coincident with a PMAF that separated from the background cusp aurora and drifted into the polar cap. A passage of the DMSP F13 spacecraft confirms that the FTE flow channel was still discernable over 15 minutes after it formed, as the spacecraft revealed a 30-40 km wide region of sunward flow within the anti-sunward background convection. From the dimensions of the flow channel we estimate that the magnetic flux contained in the event was at least 1 MWb. Furthermore, we suggest that the Birkeland current filaments often seen by low-altitude spacecraft in the cusp/mantle are really associated with FTEs in progress, and consequently we suggest that a more precise name for the region 0 or cusp/mantle current would be the FTE current system, as it is rather attributed to a flux tube on the move than a fixed region in space.
SM51A-0338 0800h
Plasma instabilities in auroral ionospheric density gradients from 120-200 km altitude
Recent observations by the Sonderstrom incoherent scatter radar show narrow field aligned filaments of enhanced plasma density extending from ~100 to 400 km in altitude, with widths of order 1 km transverse to B, and with peak density of ~5e11/m3. These features typically appear at the footprint of nightside reconnection where strong, sheared, ionospheric flows exist. We examine the stability of such structures when immersed within plasma convection using a 2-D (plane perpendicular to the magnetic field) hybrid plasma simulation. Initial results indicated the relatively rapid development (order seconds) of gradient drift type instability waves; for parameters near 200 km altitude, and convection velocity of 600 m/s, such plasma columns are found to be unstable to waves with a wavelength of ~10 to 20 m. These results have implications for the interpretation of HF backscatter measurements in such ionospheric regions.
SM51A-0339 0800h
HF Doppler oscillations in the low-latitude ionosphere coherent with equatorial long-period geomagnetic field oscillations
We present observations of HF Doppler (HFD) oscillations in the low-latitude ionosphere seen during global long-period (5-15 min) geomagnetic field oscillation events, which occurred on 21 April 1993 and on 28 February 1998. In both events, daytime polar-equatorial magnetometer data on the ground indicated that the long-period geomagnetic field oscillations at the daytime dip equator were not only considerably enhanced but also highly correlated with those at afternoon high latitudes with no apparent time shift. This earlier finding [Motoba et al., 2002, 2003] strongly suggested that the long-period geomagnetic field oscillations at the daytime dip equator were produced by an extension of polar-originating ionospheric current system associated with high-latitude geomagnetic field oscillations. In the first event on 21 April 1993, the HFD measurement at the post-midnight low latitude detected frequency oscillations coherent with the geomagnetic field oscillations at the afternoon dip equator in the Pc5 range. However, there was no magnetic field change at low-latitude magnetometer stations adjacent to the reception site of the HFD. Therefore, it is reasonable to consider that the HFD oscillations are not effects of directly incoming hydromagnetic waves on the nighttime low-latitude ionosphere. In the second event on 28 February 1998, both the low-latitude HFD and equatorial magnetometer measurements were located in the post sunrise-terminator. Similar to the first event, HFD oscillations were well correlated with long-period geomagnetic field oscillations at the daytime equator, although both variations were in anti-phase. In the same local time sector the corresponding low-latitude magnetic field variations were predominant in the D component rather than the H component, suggesting that the meridional ionospheric currents originating in the polar region make a major contribution for the low-latitude magnetic field oscillations. The two case studies presented here provide strong evidence that the observed low-latitude HFD oscillations arose from fluctuations in the vertical plasma drift due to direct penetration of a polar dawn-dusk electric field.
SM51A-0340 0800h
Meso-Scale Velocity Structure in the High Latitude F Region
At high latitudes in the F region the bulk ion flow is driven predominantly by electric fields originating in the magnetosphere and magnetosheath. During times of southward IMF a large-scale 2-cell convection pattern usually prevails with characteristic spatial scales of 1000 km (10 degrees of latitude). In addition to this large-scale convection feature there exist smaller scale velocity structures with scale sizes ranging from 1001s of km to 101s of meters. At intermediate scales between 10 km and 1000 km the ion velocity structure can have a significant impact on the ion temperature and perhaps the neutral gas velocity. Here we report on a study of the characteristics of ion velocity structure and the corresponding neutral behavior in this scale size range. We examine the characteristics of structure and its influence on neutral particles in the auroral zone and polar cap in the winter and summer hemispheres during times of southward IMF. We also consider relationships between the structure of the ions and the ion bulk flow to determine occurrences probabilities of certain features within the data.
SM51A-0341 0800h
Update on the Development of the Collision Meter
The collision meter will enable precise in-situ measurements of the major neutral gas species in the upper atmosphere (95-200 km). The principle of operation can be summarized as: A beam of charged particles is emitted from from the instrument, perpendicular to the ambient magnetic field. By gyromotion the beam returns to the instrument. From the amplitude of the collected beam, information can be extracted about the neutral density and composition, and about the nature of collisions between charged and neutral species in the upper atmosphere. The current technology readiness level (TRL) of the collision meter is 1, which means that paper study shows the possibility for a useful scientific instrument. We have begun a project which will ultimately advance to TRL 5. The present work reports on work to bring the TRL to 3. We present an update on the development of the collision meter in the laboratory. Using a magnetic field defined by a 3-axis Helmholtz coil, we have tested an initial prototype of the instrument in a controlled environment. By varying the ambient gas mixture and the characteristics of the emitted beam, we have made a preliminary investigation into the response of the instrument. We discuss the implications of this study for the continuing development of the collision meter.
SM51A-0342 0800h
A statistic analysis of ionospheric responses to the severe magnetic storms in Taiwan Sector
This study presents the results of a temporal statistic study of the ionospheric response to the magnetic storm disturbances. The study is based on the F layer critical frequency (foF2) and the minimum virtual height (h-F) from Chung-Li, which is on the equatorial anomaly station. In this paper we examined a pre- 15 day running median of the foF2 and h-F as a reference for identifying abnormal signals during all of 35 Kp-U 6 magnetic storms from 1994 to1999. Results show the negative ionospheric storm phase over equatorial anomaly area, observed in summer and winter and the positive phase in equinox. We also present results of a storm time and general temporal variation associated with the foF2 disturbances. Meanwhile, this study confirms earlier studies that show the seasonal and Local Time effects dependent foF2. Although, the negative ionospheric storm phase over equatorial anomaly area is intensified with the disturbance zonal electric field and the meridional wind.
SM51A-0343 0800h
SuperDARN Observations in the Vicinity of Polarization Jets
Existing radars of the SuperDARN network are not optimized for monitoring fast plasma flows equatorward of the auroral oval, the polarization jets. However, signatures of such flows are detectable in some cases. In this presentation, examples of SuperDARN observations of high-velocity streams similar to the polarization jets are given and differences in their characteristics with other observations are discussed.
SM51A-0344 0800h
Case Study of the Evolution of Global Ionospheric Convection during Substorms
Evolution of global ionospheric convection is investigated for a substorm event on February 2, 2002 with unusually good SuperDARN radar echo coverage. Global magnetometer chains from the polar cap to the dip-equator show the development of the DP2 electric field during the growth phase, which followed a sudden southward turning of the IMF. Three DP2 reductions were observed during the 90 min substorm growth phase, the first two of which were followed by further DP2 enhancement. Electron precipitation measured by the IMAGE FUV/SI13 at 16-17 MLT at 70-75 degrees magnetic latitudes was coherent with these changes of DP2, and the dusk-side proton precipitation measured by the IMAGE FUV/SI12 showed more gradual changes reflecting the overall DP2 changes. The DP2 convection changes were seen clearly in the SuperDARN data. Prior to the second two convection reductions, but not prior to the first, flow shear of the Harang discontinuity was observed to develop in the post-midnight sector. The Harang discontinuity shifted equatorward from 75 to 68 degrees magnetic latitudes as the growth phase developed. The second and the third reductions in convection led to the pseudo break-up and the expansion phase onset, respectively, both of which initiated within the Harang flow-shear region and were associated with large weakening of the Harang flow shear. Our results suggest that both auroral activity on the night-side and auroral precipitation on the afternoon-to-dusk region are strongly controlled by global convection, that reduction in convection leads to substorm expansion or pseudo break-up only under the condition of a well-developed Harang flow shear, and that an enhancement of convection prior to full expansion-phase development leads to termination of the expansion phase (i.e. a pseudo breakup) and reformation of the Harang shear.
SM51A-0345 0800h
Nonlocal Self-Consistent Wave Model for Field Line Resonances Sustained Auroral Arcs
Formation of Alfven wave field line resonances sustained auroral arcs is an intriguing topic of research, despite the lack of abundant data to support the model. The FAST spacecraft data has had an immense impact on the development of theoretical formalisms (such as nonlocal kinetic wave models), in general, to explain the origin of parallel electric field that can accelerate electrons to form an auroral arc. However, the current nonlocal kinetic models of field line resonances (FLRs) sustained auroral arcs, in particular, are restricted to test particle approach only, and do not incorporate the feedback effects of electrons on Alfven waves along the whole geomagnetic field lines. Present work will develop a self-consistent model that will include the feedback effects of electrons on FLRs along the whole closed geomagnetic field line that supports the FLRs. The results will be discussed in the context of available FAST spacecraft data.
SM51A-0346 0800h
Double-Layer Driven Plasma Filaments in the Upward Current Region of the Auroral Plasma
Simulations of double layers (DL) were performed using a 2.5-dimensional particle-in-cell code. The simulations included hot and cold plasmas on the low- and high-potential sides of the DL, respectively. This configuration of plasma and polarity of the potential corresponds to the DL, which accelerate electrons downward and ions upward in the auroral plasma. Both electrons and ions in the simulations are strongly magnetized. A persistent feature of the simulations performed for a variety of combinations of the densities and temperatures of the cold and hot plasmas is that the formation of a planar DL is immediately followed by the formation of thin and magnetic-field aligned plasma filaments. The process of filament formation begins just above the DL in the hot plasma permeated by an ion beam accelerated by the DL itself. A preliminary analysis shows that the beam-driven electrostatic ion-cyclotron (EIC) modes filament the DL and the hot plasma as the driven waves propagate upward with the beam velocity. The filamented structures appear in the plasma density, electron and ion energies as well as in parallel and perpendicular electric fields. When the filamented DL rises sufficiently above the ionospheric cold-plasma boundary, the plasma heating below the DL depletes the plasma creating a density cavity. In response to the cavity formation, the filamented DL moves downward filling the cavity with the hot plasma from the top. During the period of this downward motion, the DL tends to regain its initial laminar (planar) feature. As soon as the laminar DL reforms, the EIC waves are triggered again and the above process involving the filament formation reoccurs. The relevance of the recurring process of filament formation driven by a DL to satellite observations will be discussed.
SM51A-0347 0800h
Boreal winter comparison of auroral images from Polar UVI and IMAGE FUV
Same-scene images made with Polar UVI and IMAGE FUV are compared for the boreal winter of 2000-2001. The results of the comparison are used to determine whether the use of both instruments could lead to a better evaluation of the average precipitation and total energy input than with either one individually. These results are a part of a broader investigation to quantitatively compare conjugate images using both instruments and to correlate observed asymmetries with solar wind and seasonal parameters.
SM51A-0348 0800h
Simultaneous observation of an auroral substorm at Antarctic stations and by the POLAR UVI
In 2003, there were considerable times for the POLAR UVI to observe global auroral activity in the southern hemisphere. We searched such periods that auroral substorms were observed simultaneously both at Japanese Antarctic stations, Syowa and Dome Fuji, and by the POLAR UVI. We analyzed one of such periods, 23-24 May, 2003. Our interest is to elucidate relation between the large scale features observed by the satellite and the small and meso-scale features observed on the ground. Syowa Station is located at auroral latitudes and Dome Fuji is at cusp latitudes. Black and white all-sky TV camera, multi-wavelength all-sky imager, meridian scanning photometer, zenith fixed photometer, and zenith fixed tilting filter proton auroral photometer were in operation at Syowa, and all-sky TV camera and all-sky color digital camera were at Dome Fuji. For the 23-24 May 2003 event, expansion phase onset of the substorm occurred around 23:40 UT on 23 May, and maximum poleward expansion appeared around 00:02 UT on 24 May. During the recovery phase, a discrete auroral form appeared around the poleward boundary, and a large-scale undulation of diffuse aurora (Omega band) appeared at lower latitudes. Within the diffuse region, a pulsating auroral activity was observed. The auroral activity receded into a very quiet state around 01:20 UT on 24 May. In our presentation, we will focus on the evolution of the pulsating auroral activity observed on the ground in the context of the global auroral substorm evolution observed by the POLAR UVI.
SM51A-0349 0800h
Self-consistent Solution for the Electrostatic Potential Distribution Along Auroral Magnetic Field Lines
The distribution of the electrostatic potential along current-carrying auroral magnetic flux tubes is studied analytically, assuming Maxwellian magnetospheric and ionospheric particle sources. Although the current gives the total potential drop from simple orbit theory (Knight's current-voltage relation based on collision-free particle motion conserving energy and momentum), the determination of the potential distribution and electric field requires detailed considerations of (1) particle accessibility, (2) mapping of the source particle distribution functions, and (3) the necessary quasi-neutrality everywhere along the magnetic field lines. In general, a self-consistent solution can only be found if a potential step (electrostatic double layer) is allowed, accounting for a sizable fraction of the total potential drop. For typical parameters this occurs at an altitude of around 1 Re, the potential is nearly constant below the potential step, drops by hundreds of volts or a few kilovolts at the step, and then asymptotically falls of in proportion to the magnetic field intensity.
SM51A-0350 0800h
Parallel Electric Fields and Double Layers in Downward Auroral-Current Regions from Theory and Satellite Data
A method for determining the parallel electric field (E$_{?}$) and the presence of double layers for downward auroral-current regions that includes wave-particle interactions is given. We derive the multi-constituent fluid equations for a weakly inhomogeneous, magnetized plasma where the Vlasov-Maxwell hierarchy is used to treat the particle dynamics and the Fokker-Planck method is used to calculate the momentum (anomalous resistivity) and energy (anomalous heating) transfer rates between the waves (turbulence) and the particles. Two major assumptions are necessary: (1) a renormalized kinetic theory for the turbulence either exists or can be developed; and (2) both the length and frequency scales between the single-particle distributions and the fluctuations are separable. For downward currents, we may approximate the momentum and energy transfer rates by using FAST satellite data for the renormalized spectral density of the fluctuating electric field, the conservation laws, and a scaling assumption for the renormalized dielectric screening function. Using FAST data for the particle velocity moments as a boundary condition, we integrate the fluid equations both upward and downward from the satellite altitude in order to determine the potential and the particle velocity moments as functions of distance along the geomagnetic field line. We analyzed a winter FAST satellite pass near local midnight at $\sim$4130 km which shows a downward current region having a latitudinal width of about 45 km. At each subinterval ($\sim$1.5 km) along the pass, we found a double layer (DL) below the satellite altitude; a transition region (TR) just above the DL where strong electron thermalization and intense ion heating occur; and a long range potential region (LRPR) extending from the top of the TR to several earth radii and beyond. In the LRPR, ion conics are produced and further electron thermalization occurs. The average altitude of the DL/TR is in good agreement with experimental observations. Our analysis suggests that the formation of the DL, the particle dynamics, and the turbulence are intermittent in space and time. We also calculated the anomalous resistivity in the LRPR and showed that it has a very small effect on E$_{?}$ ($<$ few %) and that E$_{?}$ is determined primarily by the velocity-space anisotropy and pressure gradient terms in the momentum balance equation.
SM51A-0351 0800h
Auroral electrodynamics during pseudo-breakups
We present initial results from a study of global and local auroral electrodynamics in pseudo-breakups. These short-lived localized auroral disturbances have a typical lifetime of tens of minutes and a spatial size of hundreds of kilometers. Pseudo-breakups occurring after a quiet period were identified by the use of the Polar VIS Earth Camera. The spatial and temporal morphology of the auroral electrojet configuration is studied by the use of SuperDARN line-of-sight convection measurements and all available ground based magnetometers. In addition FAST measurements are used whenever possible.
SM51A-0352 0800h
Absorption, Electrojet and Energy Flux Changes Related to Auroral Arcs
This study shows all-sky camera (ASC) and IRIS imaging riometer observations on evening sector auroral arcs. We examine the relative locations and intensities of riometer absorption, eastward electrojet (as deduced from the magnetic recordings) and inverted electron energy flux estimated from the all-sky images (with an ASC inversion method). One of the arcs took place during the Finnish optical and EISCAT campaign on 22 Feb 2004 allowing the usage of the radar data. IRIS, ASC and EISCAT data have different spatial resolutions and are affected by precipitation with different initial energies, but show an obvious correlation. Especially absorption and energy flux behave similarly in location and intensity, and they seem to appear in the shear region between the eastward and westward electrojets. The electrojet strength, however, shows mainly anticorrelation with respect to the precipitation intensity.
SM51A-0353 0800h
FAST auroral DC electric field studies using ion data and fields data to provide the full DC E vector
We present an analysis of auroral FAST perpendicular E data using ion distributions in return current regions to study the full DC E vector and potential structures. While the axial boom measurement is available, its interpretation requires careful use and some assumptions. Our new technique provides an independent measure of this axial component. Our new tool extracts two perpendicular components of electric field, using the electric field data from the field instrument for the spin-plane component of E, and the ion drift measurements for the axial DC E. This allows studies of the full perpendicular DC E vector for the first time with FAST data. In addition the new tool transforms from velocity-based coordinates to north-south, east-west coordinates for analyzing the morphology and structure of the auroral return current region more effectively. With more than fifteen return current region crossings collected at FAST altitudes above 3000 km in either the pre-noon dayside or near midnight sector, three quarters of our data show linearly polarized diverging electric field structures. A significant fraction (almost one quarter) show rotational polarity during large field events. For these rotational events, it is probable that the spacecraft was passing through the edge of elongated quasi-static potential structures. They can also be interpreted as a temporal variation. Generally in many orbits, linear and rotational polarity appear together, one followed by the other, which means the potential structure has a wiggled or droopy shape. Statistical comparison shows several differences between these two different polarizations. (1) When a rotational polarity appears, the correspondence between electron characteristic energy and the potential obtained by integrating E weakens. (2) For linear polarization the electric field vector is likely to be almost perpendicular to the magnetic disturbance, while for rotational polarization the E is not perpendicular to delta-B. (3) Both the absolute scale length of the current signature and its size relative to that of the electric field signature are smaller for the linearly polarized cases. With this full DC E vector, we can study various questions including the morphology of auroral return currents and inconsistencies with static return current models. Our tool for extracting this information will be part of the FAST software library.
SM51A-0354 0800h
The Dynamics of a Double-Layer Along an Auroral Field Line: An Improved Model
The auroral field lines represent an important channel through which the ionosphere and the magnetosphere exchange mass, momentum, and energy. When the cold, dense ionospheric plasma interacts with sufficiently warm magnetospheric plasma along the field lines (with upward currents), double layers form with large parallel potential drops. The potential drops accelerate ionospheric ions, which in turn cause ion-beam-driven instabilities. The resulting wave-particle interactions (WPI) further heat the plasma, and hence, influence the behavior of the double layer. Understanding the coupling between these microscale and macroscale processes is crucial in quantifying the ionosphere-magnetosphere (I-M) coupling. Previous theoretical studies addressed the different facets of the problem separately. We developed a particle-in-cell (PIC) model that simulate the behavior of the double layer along auroral field lines, with special emphasis on the effect of the current along filed lines. Moreover, our model includes the effects of ionospheric collision processes, gravity, magnetic mirror force, electrostatic fields, as well as wave instabilities, propagation, and wave-particle interactions. The resulting self-consistent electrodynamics of the plasma in an auroral flux tube with an upward current is presented with emphasis on the formation and evolution of the double layer. In particular, we address questions such as: (1) what is the I-V relationship along the auroral field line, and (2) how the potential drop is distributed along the filed lines. These, and other results, are presented.
SM51A-0355 0800h
Electrostatic Fields in Density Cavities and Nonlinear Energization of Ions
In auroral ionospheric plasmas, intense electrostatic fields in density depleted regions and associated transverse energization of ions are observed [1]. We have been studying, theoretically and computationally, the characteristic features of fields in density cavities and the energization of ions by electrostatic fields. Intense localized electric fields can be generated in regions where plasma resonances exist. An appropriate description of the fields is obtained from Maxwell's equations for a cold plasma with a spatial profile. We will discuss the characteristic features of the fields obtained from such a description. We further study the interactions of ionospheric ions with electrostatic fields to understand the observed transverse energization of ions. The electrostatic fields are assumed to be represented either by a set of plane waves propagating obliquely to the geomagnetic field or localized field structures whose spatial dimensions are small compared to the thermal Larmor radii of the ionospheric ions. We find that for obliquely propagating waves, nonlinear, coherent, transverse energization of low energy ions can occur [2,3], while for localized field structures the phase space of the energized ions is chaotic. The ion dynamics will be compared and contrasted in the two cases using detailed analytical and numerical studies. \noindent This work is supported by NSF Grant No. ATM-98-06328. \noindent [1] K.A.\ Lynch et al., {\it J.\ Geophys.\ Res.} {\bf 104}, 28,515 (1999). \noindent [2] A.K. Ram, A. Bers, and D. Benisti, {\it J.\ Geophys.\ Res.} {\bf 103}, 9431 (1998). \noindent [3] D.J.\ Strozzi, A.K.\ Ram, and A.\ Bers, {\it Physics of Plasmas} {\bf 10}, 2722 (2003).
SM51A-0356 0800h
SERSIO Sounding Rocket Thermal Ion Data
While the auroral phenomena of ion up/outflows in the dayside cusp-cleft region are often recognized, the mechanism creating this drift is not. The SERSIO (Svalbard EISCAT Rocket Study of Ion Outflows) sounding rocket mission was designed to probe possible sources of this energy transfer, such as joule heating, wave-particle interactions, and ambipolar fields. SERSIO was launched January 22, 2004 at 8:57UT from Ny-Alesund, Svalbard, Norway into an event simultaneously observed by the EISCAT radars. It reached an apogee of 790 km. Multiple ground cameras confirmed soft electron precipitation over the length of the trajectory while the radars showed increased ion velocity above 500km and enhanced electron temperature and density. The extensive suite of observations indicates the event was exceptional due to its intensity and 2.5 hr duration. Unfortunately, an attitude control system malfunction compromised much of the in situ data. Particle energy and temperature data were recoverable and compare well with the EISCAT profiles. Our instruments showed enhancements in the tail of the ion velocity distributions that were invisible to the radar. I will discuss the issues that arise when we measure the spacecraft potential with the thermal ion detectors and show that sphere-to-skin potentials are not a good measure of charging. Also of interest is the broader behavior of our thermal detectors including an investigation of a possible instrument energy cutoff and our future avenues of research.
SM51A-0357 0800h
Rocket Observations of Narrowband Structured HF Waves near $f_{pe}$ in the Auroral Ionosphere
The Rocket Auroral Correlator Experiment (RACE) sounding rocket was launched into active aurora from Poker Flat, Alaska on February 6th, 2002, at 9:38:51 UT. The payload reached an apogee of 922km at approximately 550 seconds. We report high resolution measurements of structured HF wave emissions in the overdense plasma ($f_{pe}>f_{ce}$) observed after apogee at about 650 seconds after launch. They are narrowband emissions, and of relatively short duration, that occur near the $f_{pe}$ cut off. They have been observed previously and termed "HF chirps", with amplitudes around 1 mV/m and durations of about 100 ms (McAdams et. al., 1999). These new data provide the opportunity to study HF chirps in greater detail and compile an atlas of their characteristics, since preliminary anlysis reveals that they have diverse features unlike the first observations by McAdams et. al. \bibitem[{\textit{McAdams and LaBelle}(1999)}]{McAdams-99} McAdams, K.~L., and J.~LaBelle (1999), Narrowband structrure in HF waves above the electron plasma frequency in the auroral ionosphere, \textit{Geophys. Res. Lett.}, \textit{26}, 1825.
SM51A-0358 0800h
Plasma Transport in Auroral Arcs and Parallel Electric Field Structures
We investigate the connection between plasma transport along discrete auroral arcs, parallel electric fields and density gradients with help of Cluster E-field and density measurements. A method developed by Karlsson et al. (2004) is used to separate between spatial and temporal variations in the Cluster multipoint measurements. An analysis of Cluster data for strong E-field events reveals that in most cases the E-field structure is dipolar and appears close to a plasma population boundary. It can be shown that a density gradient perpendicular to a dipolar E-field structure causes a net plasma flow along the auroral arc, causing locally a change in the ionospheric convection. The possible consequences of a local decoupling between ionospheric and magnetospheric plasma convection are discussed.