Van Allen Belt Electrons Research Articles

From solar sneezing to killer electrons: outer radiation belt response to solar eruptions.

Electrons in the outer Van Allen (radiation) belt occasionally reach relativistic energies, turning them into a potential hazard for spacecraft operating in geospace. Such electrons have secured the reputation of satellite killers and play a prominent role in space weather. The flux of these electrons can vary over time scales of years (related to the solar cycle) to minutes (related to sudden storm commencements). Electric fields and plasma waves are the main factors regulating the electron transport, acceleration and loss. Both the fields and the plasma waves are driven directly or indirectly by disturbances originating in the Sun, propagating through interplanetary space and impacting the Earth. This paper reviews our current understanding of the response of outer Van Allen belt electrons to solar eruptions and their interplanetary extensions, i.e. interplanetary coronal mass ejections and high-speed solar wind streams and the associated stream interaction regions. This article is part of the theme issue 'Solar eruptions and their space weather impact'.

On the Statistics of Acceleration and Loss of Relativistic Electrons in the Outer Radiation Belt: A Superposed Epoch Analysis

AbstractWe investigate the response of the outer Van Allen belt electrons to various types of solar wind and magnetospheric disturbances. We use electron phase space density calculations as well as concurrent Pc5 and chorus wave activity observations in the outer belt during the Van Allen Probes era to compare 20 electron enhancement and 8 depletion events. Results indicate that the combined effect of magnetopause shadowing and outward diffusion driven by Pc5 waves is present in both groups of events. Furthermore, in the case of enhancement events, the synergy of enhanced seed population levels and chorus activity—due to the enhanced substorm activity—can effectively replenish the losses of relativistic electrons, while inward diffusion can further accelerate them.

A new method to study the time correlation between Van Allen Belt electrons and earthquakes

ABSTRACTA new method to study a possible temporal correlation between hundreds of keV Van Allen Belt electrons and strong earthquakes is proposed. It consists in measuring the electrons pitch angle distribution (PAD), searching for PAD disturbances, and studying the time correlation between these PAD disturbances and strong earthquakes, occurring within a defined time window. The method was applied to measurements of energetic electrons, which were performed with the Energetic Particle, Composition, and Thermal Plasma (ECT)-MagEIS detector on board the Van Allen Probes (VAPs) mission and strong continental earthquakes, with magnitude M 5.0 and hypocenter depth 100 km. We report the correlation studies for electrons with energies of about 350 keV, with which a 3.84 standard deviations correlation peak was found at hour time bin, and about 450 keV with which no correlation peaks above 2.0 standard deviations were found. Our work proves the feasibility of the proposed method and the obtained results add useful and additional information with respect to past studies.

Source and seed populations for relativistic electrons: Their roles in radiation belt changes

AbstractStrong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August–September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13–22 September, initiated by a short‐lived geomagnetic storm and characterized by a long period of primarily northward interplanetary magnetic field (IMF), showed strong depletion of relativistic electrons (including an unprecedented observation of long‐lasting depletion at geostationary orbit) while an immediately preceding, and another immediately subsequent, storm showed strong radiation belt enhancement. We demonstrate with these data that two distinct electron populations resulting from magnetospheric substorm activity are crucial elements in the ultimate acceleration of highly relativistic electrons in the outer belt: the source population (tens of keV) that give rise to VLF wave growth and the seed population (hundreds of keV) that are, in turn, accelerated through VLF wave interactions to much higher energies. ULF waves may also play a role by either inhibiting or enhancing this process through radial diffusion effects. If any components of the inner magnetospheric accelerator happen to be absent, the relativistic radiation belt enhancement fails to materialize.

An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts.

Early observations indicated that the Earth's Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep 'slot' region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth's intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.

Significance of lightning‐generated whistlers to inner radiation belt electron lifetimes

The behavior of high‐energy electrons trapped in the Earth's Van Allen radiation belts has been extensively studied, through both experimental and theoretical techniques. While the evidence for whistler induced electron precipitation (WEP) from the radiation belts is overwhelming, and the mechanisms behind WEP are well understood, the overall significance of WEP on radiation belt loss rates has not been clear. In this paper we investigate the L‐shell variation and significance of WEP‐driven loss of Van Allen belt electrons by combining in situ measurements of electron precipitation, local WEP rates determined from Trimpi perturbations, and global lightning distributions. Our modeling suggests that long‐term WEP driven losses are more significant than all other inner radiation belt loss processes for electron kinetic energies in the range ∼50–150 keV in the L‐shell range L = 2–2.4. These calculated lifetimes are comparable to the observed decay rates of artificially injected high‐energy electrons. The upper energy limit of the WEP significance range increases with decreasing L to ∼225 keV at L = 2. For electron energies above this range manmade VLF transmitters and plasmaspheric hiss should dominate over all other loss processes. However, as our lifetimes are based on rather conservative parameter estimates, these conclusions should represent the lower bounds for the energy ranges over which WEP losses are significant. For lower L‐shells the coupling of lightning activity to the production of WEP events rapidly decreases, such that by L ∼ 1.7 WEP will be unimportant in the overall loss processes.

X rays from Van Allen belt electrons

The x-ray spectrum due to atmospheric absorption of Van Allen radiation belt electrons is calculated, based on the assumption that small-angle atmospheric scattering is the dominant loss mechanism for these electrons. It is concluded that, although the x-ray flux is highly variable, at peak intensity it should be detectable above the cosmic radiation background at balloon altitudes. A detailed series of balloon measurements of the x-ray flux at several locations around the earth, including the Capetown region, could provide quantitative data on the rate at which electrons are lost from the radiation belt and, therefore, information on the Van Allen electron source strength. Such measurements would provide an experimental test to differertiate between the neutron decay origin and the local acceleration origin for the Van Allen belt electrons. Funthermore, it should be possible from such observations to make a definitive statement as to whether the primary loss of outer zone electrons occurs through atmospheric scattering or through rapid dumping during geomagnetic storms. Specific balloon experiments are suggested. (auth)