The Interaction and Relationship of the Plasmasphere and Plasma Sheet With the Radiation Belts I Posters
Presiding: T Onsager, NOAA Space Environment Center; M Spasojevic, Stanford University
SM23C-01 1330h
Electron Precipitation Bands: Possible Causes, Expectations, and Observations
We examine electron precipitation bands: spikes of energetic electron precipitation extending over 1/3-L in low altitude observations by SAMPEX and POES. We demonstrate that MeV precipitation bands occur over a wide range of geomagnetic activity and quiescence, with a typical occurrence probability of about 12% over the L range from 5-7 on the night side, with emphasis on the midnight sector. We also demonstrate that though common, bands do not result in a substantial depletion of the trapped MeV electron population. We consider four possible causes for the precipitation: ECH, EMIC, and Chorus waves as well as field-line-curvature scattering. We use the L, MLT, and energy dependence of the bands as well as the simultaneous proton response to evaluate each of these four possible causes.
SM23C-02 1330h
The correlation between the inner edge of outer radiation belt electrons and the location of plasmapause
During the period of enhanced geomagnetic activity, both the outer radiation belt electrons and plasmapause move toward the Earth. Are these just a natural consequence of enhanced solar wind interaction with the magnetosphere or are they intrinsically related? Recent studies indicate that the location of the plasmapause may determine the location of the peak in the outer belt electron intensity. Our study of long term satellite measurements shows that the inner edge of the outer belt electrons correlates well with the location of plasmapause. Combining data from SAMPEX, CRRES, Polar, IMAGE, ACE and ground magnetometers and modeling efforts, we will address the role of the plasmasphere in radiation belt acceleration and loss processes.
SM23C-03 1330h
Observations and Modeling of Low-Energy Ring Current Particles in the Dawn Sector of the Inner Magnetosphere During Geomagnetic Storms
Four Defense Meteorological Satellite Program (DMSP) satellites detected fluxes of low-energy ions precipitating well equatorward of auroral electrons in the dawn/morning magnetic local time sector during the magnetic storm of October 29-31, 2003. Ion precipitation weakened and vanished after recover began. A survey of the DMSP database reveals that near-dawn ion precipitation is a main-phase characteristic of all large magnetic storms. The CRRES satellite observed similar ion signatures near the magnetospheric equator during the magnetic storm of March 24, 1991. To explain the observations, we calculated drift paths of test-particles in the equatorial plane in the inner magnetosphere using a modified Volland-Stern electric field model. We allowed the driving electric field and the shielding parameter to vary in time, as indicated by the March 1991 storm dynamics. In all cases we follow the drift trajectories of ions and electrons with selected values of magnetic moment Μ above, equal to and below Μc, where Μc is defined as the critical magnetic moment for which the westward gradient-curvature drift exactly matches its eastward corotation drift. The main difference between the storm's dynamic and lull stages is that ions with 0 < Μ < Μc penetrate inside the zero-energy Alfven boundary (ZEAB). Furthermore, low-energy ions with Μ ~ Μc drift to the dawn sector inside the ZEAB. To reconcile DMSP and CRRES observations with allowed ion drift paths, the data suggest two source populations for near-dawn ion precipitation. The lowest-energy ions were initially energized in the evening local-time sector of the plasmasphere and subsequently co-rotated eastward. Higher-energy ions with Μ < Μc originated in the plasma sheet and drifted close to the Earth under the combined influences of time-varying electric fields and azimuthal gradients in the Earth's magnetic field generated by the stormtime ring current. After the electric field was shielded from the inner magnetosphere the newly injected ions also co-rotated into the dawn sector. Ion precipitation in the dawn sector results from a combination of pitch-angle scattering by ambient waves and by the dictates of drift paths that carry ions to very low L-shells.
SM23C-04 1330h
Relativistic electron dropout events as observed concurrently by SAMPEX and GOES sensors: Relationships to plasmaspheric and ionospheric conditions
Recent attention has focused upon abrupt and deep "dropouts" of energetic electron fluxes at geostationary orbit as seen by GOES particle detectors. We have used data from SAMPEX sensors measuring E>2MeV electrons and concurrent data from GOES in a similar energy range to study such dropout events. We have identified 27 major events from 1998 to 2003 where SAMPEX and GOES saw corresponding sharp, deep relativistic electron deceases. In most instances SAMPEX data indicated dropouts extending across a broad swath of L-shells, often down to L~~3.0. We have compared these radiation belt signatures with penetration electric field data from the Millstone Hill radar system and the DMSP spacecraft and with total electron content (TEC) enhancement measurements made using the Global Positioning System (GPS). Such multi-platform data show the intimate relationships that exist between radiation belt, plasmaspheric, and ionospheric properties during strong geomagnetic events.
SM23C-05 1330h
The Brightest Stable Auroral Arc (SAR) Ever Measured
A key role played by the "case study" approach in geoscience is to test mechanisms for their strongest possible effects. In the case of SAR arcs, this means the amount of heat conduction from the ring current/plasmapause region into the sub-auroral thermosphere/ionosphere system. A search of our multi-decade database of SAR arcs observed with an all-sky imager at Millstone Hill revealed that the brightest one occurred on the night of 29 October 1991. A new calibration of that event gives 9336 Rayleighs (R) as the brightness at 01:04 UT. Other regions of the SAR arc saturated the detector and so values above 10 kR probably occurred. A search of the published literature did not result in finding brighter SAR arc events. The surest way we know to check this is simply to claim that the above levels are the brightest ever observed, and then wait for objections! In this paper, we will present the extraordinary set of images obtained on this night, together with modeling results for the required electron temperatures.
SM23C-06 1330h
Preliminary Electric Field Results From A Multiple Balloon Campaign to Study Relativistic Electron Loss
The MINIS balloon campaign was successfully conducted in January 2005 to investigate relativistic electron loss mechanisms. Quantifying and understanding losses is an integral part of understanding the variability of relativistic electrons in the radiation belts. Balloon-based experiments directly measure precipitation and thus provide a method for quantifying losses, while the nearly stationary platform allows for the separation of temporal and spatial variations. A new class of precipitation event, characterized by extremely hard spectra, short durations, and complex temporal structure, occurring in the evening to midnight sector, was discovered by the INTERBOA balloon in 1996 and studied further by the MAXIS balloon in 2000. The MINIS campaign provided the first opportunities for multi-point measurements of electron precipitation up to MeV energies, including simultaneous measurements at different longitudes and at near-conjugate locations. Two balloons, each carrying an X-ray spectrometer for measuring the bremsstrahlung produced as electrons precipitate into the atmosphere, were launched from Churchill, Manitoba at 0850 UT on 21 January 2005 and 0140 UT on 25 January 2005. Four balloons, each carrying an X-ray spectrometer, a Z-axis search coil magnetometer, and a 3-axis electric field instrument providing DC electric field and VLF measurements in 3 frequency bands, were launched from the South African Antarctic Station (SANAE IV). The Southern launches took place at 1400 UT on 17 January, 1309 UT on 19 January, 2115 UT on 20 January, and 0950 UT on 24 January 2005. In this paper, we present the preliminary results from the MINIS South electric field instrumentation. We have good DC and VLF electric field data from all payloads, and the payload rotation mechanism worked in all four as well. The campaign began with two large solar flares. In the post-flare environment, some very magnetospherically active periods are included in our data, with strong and variable electric fields.
SM23C-07 1330h
Observations of VLF Chorus in the Aftermath of Large Geomagnetic Storms
In the aftermath of large geomagnetic storms, the plasmasphere is eroded to low L-shells and the flux of ring current electrons (10-100 keV) is substantially elevated. These conditions are conducive to the generation of intense whistler mode chorus emissions in the inner magnetosphere. There is increasing evidence that resonant interactions between chorus and energetic electrons can result in the acceleration of these electrons up to relativistic energies. We will present observations of broadband VLF wave activity from ground stations located at L=2.5 (Palmer Station, Antarctica) and L=5.1 (Chistochina, Alaska) during several of the geomagnetic superstorms of the past few years including the most recent January 2005 events and the Halloween 2003 event. We will explore the relationship between chorus wave intensity and duration to global enhancements in the radiation belts.
SM23C-08 1330h
Ground-Based Measurements of Magnetospheric Mass Density Using the Tsyganenko '01 Magnetic Field Model
The plasma mass density of the inner magnetosphere can be inferred from resonant frequencies of closed magnetospheric field lines. The field line resonant (FLR) frequency depends on field line length, magnetic field strength, and the mass density along the field line. In many studies, field line length and magnetic field strength are approximated by a dipole. However, during periods of increased geomagnetic activity, the inner-magnetospheric field configuration can differ significantly from a dipole. In order to accurately determine mass density, a better approximation of the magnetic field must be used. In this study, FLR frequencies measured by pairs of stations in the MEASURE array (L=1.7-3.2) are used to infer equatorial plasma mass density. Field strength and field line length are computed from the Tsyganenko '01 empirical magnetic field model. The resulting mass densities are compared to those obtained assuming a pure dipole. Data from all local times, seasons, and activity levels are used. The results show the greatest differences during the most disturbed times, demonstrating that in the plasmasphere (for L < 3.2) the dipole magnetic field approximation is valid except during extreme levels of geomagnetic activity.
http://measure.igpp.ucla.edu/berubeAGUS05/