SM34A-01 16:00h
Diffusion of Radiation Belt Electrons in Three Dimensions
A model is developed describing magnetic and electric fields associated with poloidal-mode Pc5 ULF waves. The frequency and L dependence of the ULF wave power is included in this model by incorporating published ground-based magnetometer data. The influence of ultra low frequency (ULF) waves in the Pc5 frequency range on radiation belt electrons in a dipole magnetic field is examined. The three dimensional dynamics of relativistic electrons are simulated using guiding center approximation equations to track the bounce and drift motion of particles. This is the first analysis in three dimensions utilizing model ULF wave electric and magnetic fields on the guiding center trajectories of relativistic electrons. It is demonstrated here that realistic spectral characteristics play a significant role in the rate of diffusion of relativistic electrons via drift resonance with poloidal mode ULF waves. Radial diffusion rates including bounce motion are calculated for $\alpha_{eq} \geq 61^0$ ($\lambda \leq 20^0$). L and energy dependence of diffusion rates are also calculated. A compression is then added to the dipole field model and diffusion rates are compared to equatorial plane results. The diffusion coefficient maximizes for $90^0$ pitch angles when radial and frequency dependence of ULF wave power matches observations. This suggests that simulations of relativistic electron dynamics in the equatorial plane can serve as an upper limit in modeling flux or phase space density evolution.
SM34A-02 16:15h
Radial Diffusion Coefficients of Radiation Belt Electrons Determined from Test-Particle Simulation Using Realistic Field Perturbations
A model of magnetic and electric perturbations in the Ultra Low Frequency (ULF) wave regime is used to simulate in a realistic way the radial diffusion of radiation belt electrons. The field model is based on the assumption that compressional oscillations propagate from the magnetopause into the inner magnetosphere and are reflected at 1 RE. The electrons are traced for various levels of magnetospheric activities. Through the diffusion in the electrons' orbits we determine the radial diffusion coefficients corresponding to different levels of ULF waves; the calculated diffusion coefficients are consistent with theoretical calculations. Finaly, the response of constructed electron fluxes as measured by a virtual satellite to the field perturbations is monitored.
SM34A-03 16:30h
Identifying Loss Mechanisms Responsible for the Rapid Depletion of Outer Radiation Belt Electron Flux
Since the discovery of earth's radiation belts researchers have sought to explain and predict the changing relativistic electron flux levels in the outer belt. This goal has proved a perplexing challenge because, surprisingly, flux levels do not always rise as energy input from the solar wind increases during active periods such as geomagnetic storms [{\it Reeves et al}., 2003;{\it O'Brien et al.}, 2001]. The erratic response of the radiation belt electrons to geomagnetic activity suggests that flux levels are set by a teetering struggle between acceleration and loss. Thus, to predict flux variations, both processes must be understood. Some acceleration mechanisms have been proposed and tested resulting in incremental progress, but still little is known about how relativistic electrons are removed from the magnetosphere. We investigate how relativistic electrons are lost from the outer radiation belt using a superposed epoch analysis of electron flux decrease events identified in multi-satellite data [{\it Onsager et al.}, 2002; {\it Green et al.}, 2004]. More specifically, we test three mechanisms proposed to explain the flux reductions: adiabatic motion in response to a changing magnetic field topology, drift out the magnetopause boundary, and scattering into the atmosphere. The superposed study shows that the magnetic field becomes temporarily stretched at dusk suggesting that adiabatic electron motion might contribute to the initial flux reduction; however, the electron flux does not recover when the magnetic field recovers, indicating that true loss from the magnetosphere occurs. Magnetopause encounters should similarly affect both high energy protons and electrons; however, no concurrent reduction of proton flux is observed implying that this mechanism is not active. Low altitude observations show increased electron flux in the loss cone suggesting that scattering to the atmosphere is the cause the flux depletions. We investigate possible causes of the increased scattering including current sheet scattering and wave particle interactions.
SM34A-04 16:45h
Energy spectral characteristics of Electron Micro-bursts observed by the Korean STSAT-1
Energy spectra of electrons from 100 eV to 20keV and from 170keV to 340keV have been measured in the outer zone by a combination of electrostatic analyzers and solid state telescopes aboard the low-altitude (680km), polar-orbiting STSAT-1 (Science and Technology SATellite) satellite. With high accurate attitude control systems of the satellite and high sampling rate of the instrument, we have clearly identified precipitating electrons responsible for microbursts with durations of less than 1s. The data show that microburst precipitation occurs with uniform pitch angle distributions, and less particle fluxes and lower e-folding energy than those of quasi-trapped electrons. These observations support the idea that microbursts may due to the pitch angle scattering associated with wave-particle interaction in the equatorial region. There is evidence this mechanism is more effective for lower energy electrons.
SM34A-05 17:00h
Average and Post-disturbance Pitch Angle Distributions Versus L From the CRRES Medium Electron A Instrument
The Medium Electron A instrument aboard CRRES (Combined Radiation and Release Experiment Satellite) provides differential flux measurements and pitch angle information for electrons in 16 energy channels ranging from 110 to 1633 keV. We extract pitch angle distributions for the inbound and outbound portion of each orbit, in 0.1 L bins from L=3.0 to 7.0, for individual energy channels using the data prior to March 24, 1991 (approximately half a year). These distributions are used to provide an average picture of electron pitch angle distributions versus L-shell. We use the countrate at 90 degrees and the average of the countrates at 135 and 45 degrees to classify each distribution as butterfly (defined as a $>$ 15% drop from 90 degrees to the average of 45 and 135 degrees), flattop (the count at 90 degrees is within the average of the count at 45 and 135 $\pm$15%) and pancake (all others). We define times of magnetic disturbance based on Dst levels during each orbit (i.e., Dst $<$ -30, -50 nT) and compare the relative contribution of flattop, butterfly, and pancake distributions during and after these orbits, at particular L-shells. Preliminary results show that, compared to the average profile versus L, a sharp increase in the percentage of butterfly distributions at L-shells beyond L=5.5 is observed post-disturbance. Of the non-pancake distributions at L-shells between L=3.0 to 5.0, flattop distributions contribute more than butterfly. In addition, the time evolution of post-disturbance distributions is presented in the form of a superimposed epoch analysis.
SM34A-06 17:15h
Radiation Belt Electron Pitch Angle Measurements from the GOES Satellites
Radiation belt electron pitch angle distributions provide important information regarding the source and loss processes that control the electron flux levels. As the flux levels vary, it is important to understand the extent to which the distinctive pitch angle distributions are formed through specific source and loss processes versus adiabatic drifts. In addition, pitch angle information is critical when mapping electron fluxes from one location to another. In this presentation, we give an analysis of the pitch angle distribution of $>$2 MeV electrons measured at geosynchronous orbit by the GOES satellites. Although the current GOES satellites are three-axis stabilized during normal operation, extensive data coverage is available during on-orbit storage of the satellites when they were spinning. During these times, well resolved pitch angle distributions have been obtained using the simultaneous electron and magnetic field measurements. These measurements are available from late 1998 to the present, allowing us to characterize the radiation belt electron pitch angle distributions as a function of local time, flux level, and geomagnetic activity during key phases of the current solar cycle.
SM34A-07 17:30h
Relativistic electron loss measurements from the LANL GPS energetic particle instruments.
In a recent study Green et al. performed a superposed epoch study of 52 relativistic electron loss events as measured by the LANL geosynchronous satellites and posed a basic question: "Where did all the relativistic electrons go?" Their study covered weakly disturbed times ($DST>-60$) and indicated an initial adiabatic response followed by permanent loss at geosynchronous. Losses as deep as $L=4$ and no proton losses rule out magnetopause shadowing, leaving precipitation losses due to wave particle interactions as a likely candidate. We use here the dense coverage of the LANL GPS energetic particle instrument constellation to perform a superposed epoch study of the same set of events. The GPS satellites cover the region from L=4 outward every three hours. Using two inter-calibrated GPS satellites (ns24 and ns33) enables us to achieve both a high time resolution on the L=dependent loss timescales. Initial results indicate that for higher L and lower energies ($L \sim 5-6$, $E \sim 300$ KeV) losses occur quickly over a period of 3-9 hours, while at lower $L$ ($L \sim 4$) and higher energies ($E \sim 2$ MeV) the rates are longer, 8--24 hours. We will attempt to use the local time information from our constellation to identify in which local time regions these losses occur first or are more severe, which will enable us to test some of the current theories for these losses (EMIC or whistler waves?).
SM34A-08 17:45h
The Detection of a Striking Increase in the Microwave Emission from Jupiter's Radiation Belts in June and July 2003.
Synchrotron emission from energetic electrons in Jupiter's radiation belts has been routinely measured by ground-based radio telescopes for three decades. The NASA-JPL Jupiter Patrol, using NASA's Deep Space Network (DSN) antennas at Goldstone, CA., has reported significant (5 %-to-30 %) variations in Jupiter's flux density near 13-cm wavelength with timescales from a few days to several months. In this paper we report observations of an unusually sudden increase in flux density from 3.8 to 4.3 Jy that occurred between 20 June and 15 July 2003. The rate of increase (approximately 0.6 percent per day) is the steepest increase that we have detected with the exception of the increase in 1994 following the impacts of fragments from comet Shoemaker-Levy 9. More than half of the reported observations were conducted by middle- and high school students from classrooms across the nation. The students and their teachers are participants in the Goldstone-Apple Valley Radio Telescope (GAVRT) science education project, which is a partnership involving NASA, the Jet Propulsion Laboratory and the Lewis Center for Educational Research (LCER) in Apple Valley, CA. Working with the Lewis Center over the Internet, GAVRT students conduct remotely controlled radio astronomy observations using 34-m antennas at Goldstone. We also report preliminary results from a special GAVRT observing campaign conducted in the fall of 2003 before, during and after the controlled impact of the Galileo spacecraft into the Jovian atmosphere. Simultaneous observations were made at 3.5 and 13 cm wavelengths three-to-four days per week. These data are being incorporated into synchrotron emission studies of the state of the radiation belts during the last weeks of the Galileo mission. The JPL contribution to this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.