7.4 Relativistic electron acceleration and losses

The most significant hazard to Earth-orbiting satellites is posed by high fluxes of relativistic electrons, which can penetrate the spacecraft systems. These electrons are transported under the same electric and magnetic fields as the ring current ions, but their higher speed makes them less sensitive to the details of the electric field structure.

The relativistic electron intensity variations are driven by the solar wind and interplanetary magnetic field conditions. Early studies found a correlation between the relativistic electron flux enhancements and solar wind high speed streams (Paulikas and Blake, 1979). This result was later augmented by the understanding that those high-speed streams that were coincident with southward interplanetary magnetic field were more efficient in enhancing the electron fluxes (Blake et al., 1997). As southward interplanetary field and high solar wind speed are the main drivers of magnetospheric storm activity, it is not surprising that the average electron flux levels trace geomagnetic activity as shown in Figure 18View Image.

View Image

Figure 21: Magnetic storm on May 2 – 4, 1998. Relativistic electron distribution in the inner magnetosphere. Logarithmic color coding shows the daily values inner magnetosphere relativistic electron flux intensity (#/cm2-s-sr) as a function of time and L shell (in 0.1 bins) giving the equatorial distance from the Earth during April 1 – June 30, 1998. Note the dropout prior to the storm as well as the strong intensification and Earthward intrusion of the electron population after the storm onset (courtesy X. Li/University of Colorado).

Figure 21View Image shows the 2 – 6 MeV relativistic electron fluxes as a function of L (distance from the Earth) and time as measured by the low-Earth orbiting SAMPEX satellite. At storm onset, the electron fluxes often decrease, which has been attributed partially to strong stretching of the field and the spacecraft moving to higher magnetic latitudes, relative to that of the undisturbed field, and hence measuring equatorial fluxes further from the Earth (Reeves, 1998Jump To The Next Citation Point), and partially to real loss of electrons either out from the magnetosphere or into the ionosphere. During this event, the flux dropout at storm onset is clearly seen over a wide range of L-values. Somewhat later, the fluxes were strongly enhanced reaching the usually empty slot region below L = 3. The electron fluxes remained at an elevated level for several weeks after the storm.

However, more detailed analyses revealed that during any individual storm event, the geostationary orbit fluxes can either increase, decrease, or show no effect at storm onset (Reeves, 1998Reeves et al., 2003). Furthermore, although there is a general correlation between storm activity and electron enhancement, there is no one-to-one correlation that would indicate that higher peak intensity would necessarily lead to higher electron flux levels (O’Brien et al., 2001). Finding the relevant processes has proven to be difficult, as the net change in the fluxes is a consequence of a delicate balance of dynamic and adiabatic effects as well as acceleration and loss processes.

The candidate processes that can account for acceleration of the magnetospheric electrons to relativistic energies can be divided into three major categories: radial diffusion, rapid transport by intense electric field pulses, and local heating via wave-particle interactions. The recirculation model assumes that radial diffusion combined with pitch-angle scattering close to the ionosphere as well as in the equatorial plane can lead to inward motion and hence adiabatic energization of the electrons (Fujimoto and Nishida, 1990). The same adiabatic effects that lead to local reduction of the electron fluxes at storm onset can account for an increase during the storm recovery phase as the magnetospheric activity subsides and the inner magnetosphere recovers its quiet-time quasi-dipolar configuration. Substorm-associated injections transport both electrons and ions rapidly and non-adiabatically over a range of L-shells, which can lead to significant energization. If a suitable seed population of electrons with energies in the several hundred keV range is already present in the inner magnetosphere or near-magnetotail, this process can account for acceleration of electrons to the required MeV energies. Rostoker et al. (1998) proposed that long-duration elevated Pc 5 ultra-high frequency (ULF) wave activity can lead to inward transport and adiabatic heating of electrons whose drift frequency is in resonance with the pulsations. Finally, electrons can be heated by cyclotron resonance with whistler mode chorus waves outside the dusk-sector plasmapause (Summers et al., 1998). During any given storm, one or more of these processes may be active in producing the observed electron acceleration (Friedel et al., 2002).

Electron losses are similarly a combination of many processes (Koskinen, 2005): convective losses by electrons drifting to the dayside magnetopause are significant especially at storm onset when the magnetosphere is often rapidly compressed to almost half of its original size. Several wave modes interacting with the electrons at resonant energies can scatter them to the atmospheric loss cone hence leading to increased precipitation and loss of electrons from the magnetosphere. Plasmaspheric hiss is a wave mode confined within the plasmasphere, driven unstable by gyroresonant interaction with energetic electrons. Lightning-induced whistler modes or man-made very low frequency (VLF) signals in the inner magnetosphere interact strongly with relativistic electrons. Electromagnetic ion cyclotron (EMIC) waves in turn are excited near the duskside plasmapause as a result of cyclotron resonance with anisotropic ring current ions, and also interact with the van Allen belt electron population. Again, it is likely that more than one process is active in the inner magnetosphere during any given time.

Results presented in this section highlight the strong coupling between the different plasma and energetic particle populations in the inner magnetosphere: The magnetic field configuration is a key element in determining the adiabatic transport properties of both electrons and ions. The field configuration is determined by the large-scale current systems, in the inner magnetosphere mainly the ring current carried by energetic ions. Hence, the ring current ions affect the relativistic electron population through their influence on the field configuration and through their influence on wave development. Changes in the large-scale convection electric field change the plasmaspheric configuration thus changing the locations where the plasmaspheric hiss (in the nightside, inside the plasmapause), EMIC waves (in the dusk sector, inside the plasmapause), and whistler mode chorus waves (in the morning sector, outside the plasmapause) occur. As these wave modes are key elements for both acceleration and loss of the relativistic electrons, the plasmasphere and its dynamics driven by the large-scale convection electric field play a key role in the relativistic electron problem. Substorms are associated with inductive, localized electric fields, and are effective in transporting both electrons and ions to the inner magnetosphere. The rapid field variations provide a means for pitch-angle scattering as well as inward transport and adiabatic energization of the electrons. The substorm-associated energetic electrons can act as a seed population that, if further energized, can become part of the outer van Allen belt relativistic electron population. In summary, while the relativistic electrons themselves interact relatively weakly with the other plasma populations, it seems that resolving their temporal evolution requires detailed knowledge of the dynamics and coupling of as well the cold plasmasphere, hot ring current, tail plasma sheet as the electromagnetic fields guiding the particle motions.

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