R. S. Selesnick

On the source location of radiation belt relativistic electrons

Observations from the High Sensitivity Telescope (HIST) on Polar made around January and May 1998 are used to constrain the source location of outer radiation belt relativistic electrons. Phase space densities calculated as a function of the three adiabatic invariants show positive radial gradients for L 4, peaks in the radial dependence of the phase space density are suggestive of a local electron source that may be nonadiabatic acceleration or pitch angle scattering. However, discrepancies in the results obtained with different magnetic field models and at different local times make this a tentative conclusion.

Proton, helium, and electron spectra during the large solar particle events of October-November 2003

The extraordinary period from late October through early November 2003 was marked by more than 40 coronal mass ejections (CME), eight X-class flares, and five large solar energetic particle (SEP) events. Using data from instruments on the ACE, SAMPEX, and GOES-11 spacecraft, the fluences of H, He, O, and electrons have been measured in these five events over the energy interval from ∼0.1 to >100 MeV/nucleon for the ions and ∼0.04 to 8 MeV for electrons. The H, He, and O spectra are found to resemble double power laws, with a break in the spectral index between ∼5 and ∼50 MeV/nucleon which appears to depend on the charge-to-mass ratio of the species. Possible interpretations of the relative location of the H and He breaks are discussed. The electron spectra can also be characterized by double power laws, but incomplete energy coverage prevents an exact determination of where and how the spectra steepen. The proton and electron fluences in the 28 October 2003 SEP event are comparable to the largest observed during the previous solar maximum, and within a factor of 2 or 3 of the largest SEP events observed during the last 50 years. The 2-week period covered by these observations accounted for ∼20% of the high-energy solar-particle fluence over the years from 1997 to 2003. By integrating over the energy spectra, the total energy content of energetic protons, He, and electrons in the interplanetary medium can be estimated. After correcting for the location of the events, it is found that the kinetic energy in energetic particles amounts to a significant fraction of the estimated CME kinetic energy, implying that shock acceleration must be relatively efficient in these events.

The global response of relativistic radiation belt electrons to the January 1997 magnetic cloud

In January 1997 a large fleet of NASA and US military satellites provided the most complete observations to date of the changes in >2 MeV electrons during a geomagnetic storm. Observations at geosynchronous orbit revealed a somewhat unusual two-peaked enhancement in relativistic electron fluxes [ Reeves et al., 1998]. In the heart of the radiation belts at L ≈ 4, however, there was a single enhancement followed by a gradual decay. Radial profiles from the POLAR and GPS satellites revealed three distinct phases. (1) In the acceleration phase electron fluxes increased simultaneously at L ≈ 4–6. (2) During the passage of the cloud the radiation belts were shifted radially outward and then relaxed earthward. (3) For several days after the passage of the cloud the radial gradient of the fluxes flattened, increasing the fluxes at higher L-shells. These observations provide evidence that the acceleration of relativistic electrons takes place within the radiation belts and is rapid. Both magnetospheric compression and radial diffusion can cause a redistribution of electron fluxes within the magnetosphere that make the event profiles appear quite different when viewed at different L-shells.

Atmospheric losses of radiation belt electrons

A numerical model of the low-altitude energetic electron radiation belt, including the effects of pitch angle diffusion into the atmosphere and azimuthal drift, predicts lifetimes and longitude-dependent loss rates as a function of electron energy and diffusion coefficient. It is constrained by high-altitude (�20,000 km) satellite measurements of the energy spectra and pitch angle distributions and then fit to low-altitude (�600 km) data that are sensitive to the longitude dependence of the electron losses. The fits provide estimates of the parameterized diffusion coefficient. The results show that the simple driftdiffusion model can account for the main features of the low-altitude radiation belt inside the plasmasphere during periods of steady decay. The rate of pitch angle diffusion is usually stronger on the dayside than on the nightside, frequently by a factor �10. The average derived lifetimes for loss into the atmosphere of �10 days are comparable to the observed trapped electron decay rates. Considerable variability in the loss rates is positively correlated with geomagnetic activity. The results are generally consistent with electron scattering by plasmaspheric hiss as the primary mechanism for pitch angle diffusion.

A quiescent state of 3 to 8 MeV radiation belt electrons

During a ∼3 month period in mid-1996 outer radiation belt electrons in the energy range from ∼ 3 to 8 MeV were diffusing inward and decaying in intensity with no internal or external source. Measurements from the HIST instrument on POLAR are used to constrain a model for time dependent lossy radial diffusion of these electrons, and to obtain estimates of a parameterized radial diffusion coefficient and lifetime. For lower energy electrons, of ∼ 1 to 3 MeV, a source at L > 6 is apparent throughout most of the same period.

Predicting the L-position of the storm-injected relativistic electron belt

In the paper we confirm earlier work on the dependence of the L-position (Lmax) of the peak intensity of storm-injected relativistic electrons upon the magnetic storm amplitude |Dst|max. Early results for |Dst|max=30–140 nT were reported by Williams et al. (1968). Later studies gave the following formula: |Dst|max= 2.75·104/Lmax4 for |Dst| up to 400 nT (Tverskaya, 1986). This sort of relationship also can be applied to the lowest position of the westward electrojet center during the storm, and the boundaries of both the trapped radiation zone and discrete auroral forms. Data from high-altitude (Polar and 1997-068) satellites, and a low-altitude satellite (SAMPEX) were analyzed and compared to the predictions of the above formula. The congruence between formula and observations is excellent: the discrepancy is not larger than 0.2 Re. There are several reasons why the discrepancies might be expected including: fast radial transport due to sudden impulses; the overlapping in time of multiple storms; and the additional injections onto L>Lmax in the storm recovery phase. There could be difficulty relating high and low altitude measurements due to problems with magnetic field models, but we believe this is not a major factor in our analysis.

Dynamics of the outer radiation belt

Radial profiles of electron intensity and phase space density in the energy range of ∼0.7 to 8 MeV, between L values of 3 and 9, show that particle injections followed by fast radial diffusion lead to the main part of the outer radiation belt near L=4 to 5 during active periods. In quiet times the belt is supplied by inward diffusion from an external source. Various combinations of these two basic configurations are also seen.

Radiation belt electron observations following the January 1997 magnetic cloud event

Relativistic electrons in the outer radiation belt associated with the January 1997 magnetic cloud event were observed by the HIST instrument on POLAR at kinetic energies from 0.7 to 7 MeV and L shells from 3 to 9. The electron enhancement occurred on a time scale of hours or less throughout the outer radiation belt, except for a more gradual rise in the higher energy electrons at the lower L values indicative of local acceleration and inward radial diffusion. At the higher L values, variations on a time scale of several days following the initial injection on January 10 are consistent with data from geosynchronous orbit and may be an adiabatic response.

How efficient are coronal mass ejections at accelerating solar energetic particles

The largest solar energetic particle (SEP) events are thought to be due to particle acceleration at a shock driven by a fast coronal mass ejection (CME). We investigate the efficiency of this process by comparing the total energy content of energetic particles with the kinetic energy of the associated CMEs. The energy content of 23 large SEP events from 1998 through 2003 is estimated based on data from ACE, GOES, and SAMPEX, and interpreted using the results of particle transport simulations and inferred longitude distributions. CME data for these events are obtained from SOHO. When compared to the estimated kinetic energy of the associated coronal mass ejections (CMEs), it is found that large SEP events can extract ~10% or more of the CME kinetic energy. The largest SEP events appear to require massive, very energetic CMEs.

A multi-spacecraft synthesis of relativistic electrons in the inner magnetosphere using LANL, GOES, GPS, SAMPEX, HEO, and POLAR

One of the Brussels Radiation Belt Workshop recommendations was the establishment of a near-real-time data driven model of the inner magnetospheric energetic particle population (L < 8). Although the “ideal” missions and data sets for such a model do not exist at present, more spacecraft than ever before are currently sampling the inner magnetosphere. We attempt here in a case study of the January 10, 1997 magnetic cloud event to construct such a model with the energetic electron data available from 5 geosynchronous and 5 elliptically orbiting satellites. We examine the constraints and difficulties of putting together a large number of datasets which are measured near-simultaneously at very different locations in the inner magnetosphere. First results indicate that we can achieve a time resolution of about 3 hours for a given “snapshot” of the inner magnetosphere, and that large azimuthal asymmetries of the energetic electron population can be observed during large storms.