The Interaction and Relationship of the Plasmasphere and Plasma Sheet With the Radiation Belts II
Presiding: T O'Brien, Aerospace Corporation; M B Moldwin, University of California, Los Angeles
SM31A-01 INVITED 08:30h
Plasmaspheric Influence on Radiation Belts During Major Geomagnetic Storms
We investigate a possible causal relationship between erosion of the plasmasphere and enhancement of the radiation belts during major geomagnetic storms. The Earth's plasmasphere was observed by the IMAGE EUV imager to be drastically reduced in size by the 2003 Halloween geomagnetic storm event. Before the storm the plasmapause was seen at roughly 4 Earth radii (RE) geocentric distance; after the storm, the plasmapause had moved inside 2 RE. This dramatic erosion of the plasmasphere apparently had a profound effect on the global distribution of the Van Allen radiation belts. Wave-particle interactions inside the plasmasphere normally act to remove the radiation belt electrons inside the slot region; however, in the days following the 2003 Halloween storm's drastic plasmasphere erosion, SAMPEX witnessed the formation of an extremely intense new radiation belt in what was formerly the slot region. This chain of events suggests that the plasmasphere configuration can have a primary influence on the global radiation belt distribution. We present preliminary results of the investigation of this possible relationship for three major geomagnetic storms, using global images of the plasmasphere side-by-side with in situ energetic particle data. We discuss likely mechanisms for quickly accelerating electrons to the high levels observed in the slot that normally is devoid of such relativistic electrons.
SM31A-02 INVITED 08:45h
How the Plasmapause Affects Relativistic Electron Variability in the Outer Radiation Belt
Variability of energetic electrons in the outer radiation belt is due to an imbalance between source and loss processes. Source processes include radial diffusion (driven by enhanced ULF waves) and local stochastic acceleration by VLF plasma waves, while losses are primarily due to wave-particle scattering and ultimate loss to the atmosphere. At the onset of a magnetic storm, despite a pronounced increase in the source of acceleration, losses usually dominate causing a net depletion of relativistic outer zone flux. Flux increases can occur during the storm recovery, when losses become less pronounced. The plasmapause location plays a fundamental role in controlling the effectiveness of the source and loss processes. Enhanced ULF waves and VLF chorus emissions are primarily excited outside the plasmapause during disturbed conditions. Both waves contribute to the acceleration source. Intense EMIC waves, excited along the dusk-side plasmapause (and plume) can provide an effective scattering loss for relativistic electrons, especially during the main phase of a storm as ring current protons are injected into the system. Recent theoretical simulations of the effectiveness of potential source and loss processes will be presented, and related to the location of the disturbed plasmapause location.
SM31A-03 INVITED 09:00h
Relationship Between the Plasmasphere and Relativistic Electron Flux Depletions in Earth's Outer Radiation Belt
Decades of electron flux measurements from satellites probing Earth's outer Van Allen belt depict a highly variable radiation environment that researchers have been challenged to explain. The erratic flux changes induced by geomagnetic activity suggest that acceleration processes are often countered by profuse loss. Thus, to predict flux variations both processes must be understood. Some acceleration mechanisms have been proposed and tested. However, the conditions and processes that remove electrons from the magnetosphere are still uncertain. Recent work suggests that scattering into the atmosphere may be the dominant process responsible for some flux depletions [Green et al., 2004]. We use a superposed epoch analysis of multi-satellite data to demonstrate how scattering into the atmosphere contributes to flux depletions in the outer radiation belt. We identify waves responsible for scattering the electrons by comparing the local time of precipitating electrons to the local time of observed electromagnetic ion cyclotron (EMIC) and whistler waves. Finally, we examine how changes in the high density plasmasphere enhance or inhibit the growth of these waves and their ability to interact with high energy electrons, thus, indirectly dictating radiation belt electron flux levels.
SM31A-04 09:15h
Sub-Auroral proton precipitation as a Result of Plasmapause-Ring-Current Interaction
The proton aurora imager SI-12 on the IMAGE spacecraft occasionally observes subauroral proton precipitation in the morning and the afternoon sectors. Sub-auroral Morning Proton Spots (SAMPS) rotate with 80-90% of the Earth's corotation speed, while the detached afternoon proton arcs do not move in local time. Coincident particle measurements by DMSP of both phenomena confirm the source as pure high energy precipitating protons with energy likely above the detector limit of 30 keV. The spots appear after magnetic storms in the recovery phase and last for 1-4 hours in the magnetic local time region of 0600-1200 hours. The subauroral arcs appear after a direction change of the southward or westward components of the interplanetary magnetic field. IMAGE-EUV observations of the plasmasphere indicate a relationship with density gradients in the plasmapause. This is supported by nearby geosynchronous observations of increased cold plasma density. We interpret these phenomena as the result of the interaction of ring current protons with electromagnetic ion-cyclotron (EMIC) waves caused by the dense, cold plasmasphere ions. The spots are therefore a consequence of plasmasphere refilling after geomagnetic storms. In contrast, the afternoon arcs are the result of enhanced EMIC waves during moderately disturbed geomagnetic conditions.
SM31A-05 09:30h
Correlative Study of High- and Low-Altitude Whistler Wave Observations and Radiation Belt Structure
It is known that whistler mode waves permeate the plasmasphere in the form of plasmaspheric hiss, although their origin has been a controversy for many years. Whistler waves are believed to be generated either by storm/substorm-injected energetic electrons interacting with the cold plasmaspheric plasma or by escaping spherics from atmospheric lightning. Because of the apparent connection of plasmaspheric hiss to the loss processes of radiation-belt particles and to regulating the belt structure, the origin and distribution of plasmaspheric hiss remain a topic of high interest, particularly for space weather investigations. In the present study, we investigate the possible relationship between plasmaspheric hiss characteristics observed by the wave instruments on the Dynamics Explorer-1 and IMAGE spacecraft at high altitudes (> 1000 km) and lightning activities at low altitudes or on the ground. Results from this study will be discussed in the context of observed radiation belt structure.
SM31A-06 09:45h
Preliminary X-ray 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 24 2005. In this paper, we present the preliminary results from the MINIS North and South X-ray data. The first and second Southern payloads observed a rarely-seen phenomenon: gamma-ray line emission from nuclear interactions of solar protons in the Earth's atmosphere. When the solar particles abated, there were numerous opportunities for simultaneous observations of MeV precipitation from multiple payloads; we will present the first analysis of these data.