R. H. W. Friedel

Acceleration and loss of relativistic electrons during small geomagnetic storms

Abstract Past studies of radiation belt relativistic electrons have favored active storm time periods, while the effects of small geomagnetic storms (D s t > −50 nT) have not been statistically characterized. In this timely study, given the current weak solar cycle, we identify 342 small storms from 1989 through 2000 and quantify the corresponding change in relativistic electron flux at geosynchronous orbit. Surprisingly, small storms can be equally as effective as large storms at enhancing and depleting fluxes. Slight differences exist, as small storms are 10% less likely to result in flux enhancement and 10% more likely to result in flux depletion than large storms. Nevertheless, it is clear that neither acceleration nor loss mechanisms scale with storm drivers as would be expected. Small geomagnetic storms play a significant role in radiation belt relativistic electron dynamics and provide opportunities to gain new insights into the complex balance of acceleration and loss processes.

Relativistic electron dynamics in the inner magnetosphere — a review

Abstract The dynamics of relativistic electrons in the inner magnetosphere around the time of geomagnetic disturbances have received considerable attention in recent years. In addition to the environmental impact these electrons have on space-hardware in MEO and GEO orbits, and their obvious impact on space weather, the scientific issues surrounding the transport, acceleration and loss of these particles in the inner magnetosphere have not been fully resolved. One of the prime difficulties in understanding the dynamics of relativistic electrons is their somewhat uncorrelated behavior with regard to the major solar wind drivers of the Earths magnetospheric dynamics (solar wind velocity, density and magnetic field strength/direction) and the major indices representing these dynamics (Dst, Ae, Kp). Relativistic electrons observed at geosynchronous altitude reach their peak several days after the onset of a magnetic storm, and a wide range of responses can occur for seemingly similar geomagnetic disturbances/storms. We give here a review and comparison of the current state of research into relativistic electron dynamics, covering simple diffusion, substorm acceleration, ULF wave acceleration, recirculation by ULF waves or plasmaspheric hiss. We present the results of a recent statistical study which has identified the presence of sufficient ULF wave power for a duration of at least 12 h during a storm as being the most geoeffective indicator of subsequent relativistic electron enhancements at geosynchronous altitudes. For completeness we also briefly examine some of the problems and ideas related to relativistic electron losses.

Which magnetic storms produce relativistic electrons at geosynchronous orbit

Relativistic electrons appear in the geosynchronous environment following some, but not all, geomagnetic storms. The ability to identify which storms produce these electrons would bring us much closer to explaining the mechanism responsible for their appearance, and it would provide the space weather community with a means to anticipate the electron hazard to geosynchronous spacecraft. We apply a recently developed statistical technique to produce an hourly time series of relativistic electron conditions at local noon along geosynchronous orbit using several geosynchronous monitors. We use a cross-correlation analysis to determine what parameters in the solar wind and magnetosphere might influence the flux of relativistic electrons. We then perform a superposed epoch analysis to compare storms with and storms without the appearance of these electrons. We investigate a number of solar wind and magnetospheric parameters for these two sets of storms at 1-hour resolution. In particular, sustained solar wind velocity in excess of 450 km s−1 is a strong external indicator of the subsequent appearance of relativistic electrons. In the magnetosphere, long-duration elevated Pc 5 ULF wave power during the recovery phase of magnetic storms appears to discriminate best between those storms that do and do not produce relativistic electrons.

The relativistic electron response at geosynchronous orbit during the January 1997 magnetic storm

The first geomagnetic storm of 1997 began on January 10. It is of particular interest because it was exceptionally well observed by the full complement of International Solar Terrestrial Physics (ISTP) satellites and because of its possible association with the catastrophic failure of the Telstar 401 telecommunications satellite. Here we report on the energetic electron environment observed by five geosynchronous satellites. In part one of this paper we examine the magnetospheric response to the magnetic cloud. The interval of southward IMF drove strong substorm activity while the interval of northward IMF and high solar wind density strongly compressed the magnetosphere. At energies above a few hundred keV, two distinct electron enhancements were observed at geosynchronous orbit. The first enhancement began and ended suddenly, lasted for approximately 1 day, and is associated with the strong compression of the magnetosphere. The second enhancement showed a more characteristic time delay, peaking on January 15. Both enhancements may be due to transport of electrons from the same initial acceleration event at a location inside geosynchronous orbit but the first enhancement was due to a temporary, quasi-adiabatic transport associated with the compression of the magnetosphere while the second enhancement was due to slower diffusive processes. In the second part of the paper we compare the relativistic electron fluxes measured simultaneously at different local times. We find that the >2-MeV electron fluxes increased first at noon followed by dusk and then dawn and that there can be difference of two orders of magnitude in the fluxes observed at different local times. Finally, we discuss the development of data-driven models of the relativistic electron belts for space weather applications. By interpolating fluxes between satellites we produced a model that gives the >2-MeV electron fluxes at all local times as a function of universal time. In a first application of this model we show that, at least in this case, magnetopause shadowing does not contribute noticeably to relativistic electron dropouts.

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.

Ion composition and pressure changes in storm time and nonstorm substorms in the vicinity of the near-Earth neutral line

[i] Using CLUSTER/CODIF data from close to ∼ 19 Re in the magnetotail, we have performed a superposed epoch analysis of storm time and nonstorm substorms to determine how the ion composition changes during a substorm. We find that the median O + density and pressure in the plasma sheet are a factor of 5 higher during storm times than during nonstorm times. However, we do not observe significant changes in the composition during a substorm that would indicate that ionospheric outflow is playing a dynamic role in loading the plasma sheet or triggering the substorm at this location. There are differences between the storm time and nonstorm substorms, and it is intriguing to consider whether the composition differences play a role. The storm time substorms exhibit more loading and faster unloading than the nonstorm substorms. In addition, we observe differences in the H + and O + behavior at onset in the storm time substorms that we attribute to the different dynamics of the two ion species at the reconnection site and during the field reconfiguration due to their different gyroradii. The H + density and pressure decrease over the whole energy range at substorm onset, while the O + density and pressure decrease less, and the O + temperature increases. That more O + is left after substorm onset indicates that either the O + is more quickly replenished from O + in the lobes and/or that the more energetic O + , due to its larger gyroradius, is not depleted when the field reconfigures and is accelerated in the thin current sheet.

Identifying the radiation belt source region by data assimilation

We describe how assimilation of radiation belt data with a simple radial diusion code can be used to identify and adjust for unknown physics in the model. We study the drop-out and the following enhancement of relativistic electrons during a moderate storm on October 25, 2002. We introduce a technique that uses an ensemble Kalman Filter and the probability distribution of the forecast ensemble to identify if the model is drifting away from the observations and to find inconsistencies between model forecast and observations. We use the method to pinpoint the time periods and locations where most of the disagreement occurs and how much the Kalman Filter has to adjust the model state to match the observations. Although the model does not contain explicit source or loss terms, the Kalman Filter algorithm can implicitly add very localized sources or losses in order to reduce the discrepancy between model and observations. We use this technique with multi-satellite observations to determine when simple radial diusion is inconsistent with the observed phase space densities indicating where additional source (acceleration) or loss (precipitation) processes must be active. We find that the outer boundary estimated by the ensemble Kalman filter is consistent with negative phase space density gradients in the outer electron radiation belt. We also identify that specific regions in the radiation belts (L 5 6 and to a minor extend also L 4)where simple radial diusion fails to adequately capture the variability of the observations, suggesting local acceleration/loss mechanisms.

Whistler anisotropy instabilities as the source of banded chorus: Van Allen Probes observations and particle-in-cell simulations

Magnetospheric banded chorus is enhanced whistler waves with frequencies ωr<Ωe, where Ωe is the electron cyclotron frequency, and a characteristic spectral gap at ωr≃Ωe/2. This paper uses spacecraft observations and two-dimensional particle-in-cell simulations in a magnetized, homogeneous, collisionless plasma to test the hypothesis that banded chorus is due to local linear growth of two branches of the whistler anisotropy instability excited by two distinct, anisotropic electron components of significantly different temperatures. The electron densities and temperatures are derived from Helium, Oxygen, Proton, and Electron instrument measurements on the Van Allen Probes A satellite during a banded chorus event on 1 November 2012. The observations are consistent with a three-component electron model consisting of a cold (a few tens of eV) population, a warm (a few hundred eV) anisotropic population, and a hot (a few keV) anisotropic population. The simulations use plasma and field parameters as measured from the satellite during this event except for two numbers: the anisotropies of the warm and the hot electron components are enhanced over the measured values in order to obtain relatively rapid instability growth. The simulations show that the warm component drives the quasi-electrostatic upper band chorus and that the hot component drives the electromagnetic lower band chorus; the gap at ∼Ωe/2 is a natural consequence of the growth of two whistler modes with different properties.

Plasma sheet access to the inner magnetosphere

We present here plasma data from the Polar HYDRA instrument giving comprehensive coverage of the inner magnetospheric region from L ∼ 2 outward. Data is projected to an equatorial reference plane yielding a global view of the inner extend of the plasma sheet. We determine the inner boundary for plasma sheet electrons and ions in the μ range 0.05 - 50 eV nT -1 and we compare these to the predicted Alfven boundaries as a function of the geomagnetic activity index Kp. In general, the simple conventional drift paradigm is shown to be globally consistent with the averaged data in the inner magnetosphere, with electrons adhering better to the predicted boundaries than ions. The data are further compared to the geosynchronous slice as measured by the Los Alamos Magnetospheric Plasma Analyzer (MPA) which measures the crossing point of the Alfven boundaries at geosynchronous altitudes with much better statistical resolution than Polar. Integral to the drift model used is an assumption about the form of the global electric field. The agreement with data validates the simple corotation and convection electric field used and shows that this model describes well the average transport for a wide range of geomagnetic activity and over a large part of the inner magnetosphere.

Substorm injection of relativistic electrons to geosynchronous orbit during the great magnetic storm of March 24, 1991

The great March 1991 magnetic storm and the immediately preceding solar energetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate here, using data from eight satellites, the very large delayed buildup of relativistic electron flux in the outer zone during a 1.5-day period beginning 2 days after onset of the main phase of this storm. A notable feature of the March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower-energy substorm-injected electron flux and the relativistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and local noon shows evidence that both classes of electrons arrive at geosynchronous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric field transports not only the usual (50–300 keV) substorm electrons but also the relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A simplified calculation of the electron e × B and gradient/curvature drifts indicates that sufficiently strong substorm dipolarization inductive electric fields (≳ 10 mV/m) could achieve this, provided sufficient relativistic electrons are present in the source region. Consistent with this interpretation, we find that the injected relativistic electrons have a pitch angle distribution that is markedly peaked perpendicular to the magnetic field. Furthermore, the equatorial phase space density at geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during this time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a strong addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for this magnetic storm.