T. P. O'Brien

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.

Van Allen Probes show that the inner radiation zone contains no MeV electrons: ECT/MagEIS data

We present Van Allen Probe observations of electrons in the inner radiation zone. The measurements were made by the Energetic Particle, Composition, and Thermal Plasma/Magnetic Electron Ion Spectrometer (MagEIS) sensors that were designed to measure electrons with the ability to remove unwanted signals from penetrating protons, providing clean measurements. No electrons >900 keV were observed with equatorial fluxes above background (i.e., >0.1 el/(cm2 s sr keV)) in the inner zone. The observed fluxes are compared to the AE9 model and CRRES observations. Electron fluxes <200 keV exceeded the AE9 model 50% fluxes and were lower than the higher-energy model fluxes. Phase space density radial profiles for 1.3 ≤ L* < 2.5 had mostly positive gradients except near L*~2.1, where the profiles for μ = 20–30 MeV/G were flat or slightly peaked. The major result is that MagEIS data do not show the presence of significant fluxes of MeV electrons in the inner zone while current radiation belt models and previous publications do.

Energetic electron injections deep into the inner magnetosphere associated with substorm activity

From a survey of the first nightside season of NASAs Van Allen Probes mission (December 2012 to September 2013), 47 energetic (tens to hundreds of keV) electron injection events were found at L shells ≤ 4, all of which are deeper than any previously reported substorm-related injections. Preliminary details from these events are presented, including how all occurred shortly after dipolarization signatures and injections were observed at higher L shells, how the deepest observed injection was at L ~ 2.5, and, surprisingly, how L ≤ 4 injections are limited in energy to ≤250 keV. We present a detailed case study of one example event revealing that the injection of electrons down to L ~ 3.5 was different from injections observed at higher L and likely resulted from electrons interacting with a fast magnetosonic wave in the Pi2 frequency range inside the plasmasphere. These observations demonstrate that injections occur at very low L shells and may play an important role for inner zone electrons.

A background correction algorithm for Van Allen Probes MagEIS electron flux measurements

We describe an automated computer algorithm designed to remove background contamination from the Van Allen Probes Magnetic Electron Ion Spectrometer (MagEIS) electron flux measurements. We provide a detailed description of the algorithm with illustrative examples from on-orbit data. We find two primary sources of background contamination in the MagEIS electron data: inner zone protons and bremsstrahlung X-rays generated by energetic electrons interacting with the spacecraft material. Bremsstrahlung X-rays primarily produce contamination in the lower energy MagEIS electron channels (∼30–500 keV) and in regions of geospace where multi-MeV electrons are present. Inner zone protons produce contamination in all MagEIS energy channels at roughly L < 2.5. The background-corrected MagEIS electron data produce a more accurate measurement of the electron radiation belts, as most earlier measurements suffer from unquantifiable and uncorrectable contamination in this harsh region of the near-Earth space environment. These background-corrected data will also be useful for spacecraft engineering purposes, providing ground truth for the near-Earth electron environment and informing the next generation of spacecraft design models (e.g., AE9).

The effects of geomagnetic storms on electrons in Earth's radiation belts

We use Van Allen Probes data to investigate the responses of tens of keV to 2 MeV electrons throughout a broad range of the radiation belts (2.5 ≤ L ≤ 6.0) during 52 geomagnetic storms from the most recent solar maximum. Electron storm time responses are highly dependent on both electron energy and L shell. Tens of keV electrons typically have peak fluxes in the inner belt or near-Earth plasma sheet and fill the inner magnetosphere during storm main phases. Approximately 100 to ~600 keV electrons are enhanced in up to 87% of cases around L~3.7, and their peak flux location moves to lower L shells during storm recovery phases. Relativistic electrons (≥~1 MeV) are nearly equally likely to produce enhancement, depletion, and no-change events in the outer belt. We also show that the L shell of peak flux correlates to storm magnitude only for hundreds of keV electrons.

An empirically observed pitch‐angle diffusion eigenmode in the Earth's electron belt near L* = 5.0

Using data from NASAs Van Allen Probes, we have identified a synchronized exponential decay of electron flux in the outer zone, near L* = 5.0. Exponential decays strongly indicate the presence of a pure eigenmode of a diffusion operator acting in the synchronized dimension(s). The decay has a time scale of about 4 days with no dependence on pitch angle. While flux at nearby energies and L* is also decaying exponentially, the decay time varies in those dimensions. This suggests the primary decay mechanism is elastic pitch angle scattering, which itself depends on energy and L*. We invert the shape of the observed eigenmode to obtain an approximate shape of the pitch angle diffusion coefficient and show excellent agreement with diffusion by plasmaspheric hiss. Our results suggest that empirically derived eigenmodes provide a powerful diagnostic of the dynamic processes behind exponential decays.

Inner zone and slot electron radial diffusion revisited

Using recent data from NASAs Van Allen Probes, we estimate the quiet time radial diffusion coefficients for electrons in the inner radiation belt (L < 3) with energies from ~50 to 750 keV. The observations are consistent with dynamics dominated by pitch angle scattering and radial diffusion. We use a coordinate system in which these two modes of diffusion are separable. Then we integrate phase space density over pitch angle to obtain a “bundle content” that is invariant to pitch angle scattering, except for atmospheric loss. We estimate the effective radial diffusion coefficient from the temporal and radial variation of the bundle content. We show that our diffusion coefficients agree well with previously determined values obtained in the 1960s and 1970s and follow the form one expects for radial diffusion caused by exponentially decaying impulses in the large-scale electrostatic potential.

Outer Electron Belt Specification Model

Data assimilation techniques have already been developed and have shown to provide ldquothe bestrdquo estimate of the state of the electron radiation belts. Data assimilation proceeds in analysis cycles-in each analysis cycle, observations of the current (and possibly past) state of a system are combined with the results from a mathematical model (the forecast) to produce an analysis, which is considered as the best estimate of the current state of the system. In this paper, such an analysis has been performed from January 1990 to December 2006 and has produced full electron radiation belt state every 20 min. Then, based on this 17 year run, a new outer electron belt specification model is developed: we perform a data synthesis to deduce a yearly average electron belt state over a full solar cycle. Lastly, this new specification model for the Earths radiation belt is compared to existing specification models.

Breaking all the invariants: Anomalous electron radiation belt diffusion by pitch angle scattering in the presence of split magnetic drift shells

Relativistic electron observations near geostationary orbit routinely show pitch angle distributions peaked away from 90 degrees. These “butterfly” distributions are consistent with magnetic drift shell splitting combined with a radial flux gradient. During magnetic storms, nature adds pitch angle scattering to split drift shells, breaking all three adiabatic invariants of the particles motion. Therefore, some degree of anomalous radial diffusion is likely, and cross terms between the gyration and drift invariants and between the bounce and drift invariants arise. Using typical assumptions about the pitch angle scattering and the magnetic field topology, we calculate these anomalous diffusion coefficients near geostationary orbit. We show that the anomalous radial diffusion can exceed that due to more traditional drift-resonant wave-particle interactions. We also show that the neglected cross terms, particularly the bounce-drift cross term, can be significant. These results suggest necessary additions to some global electron radiation belt simulations.

The Trapped Proton Environment in Medium Earth Orbit (MEO)

Energetic proton flux maps of the differential flux intensity in the medium-Earth orbit (MEO) regime (altitudes ~ 7000-15,000 km) are developed from measurements taken by detectors aboard the Combined Release and Radiation Effects Satellite (CRRES), HEO-F1, HEO-F3 and ICO satellites. Measurement errors have been estimated by cross-calibrating to a standard sensor aboard the GOES satellite during solar proton events. Spectral inversion techniques were employed to derive differential flux spectra from the HEO and ICO integral channel dosimeters. Two methods for combining the four different satellite data sets on a standard energy and coordinate grid are presented and the ramifications due to limited spatial and temporal coverage are explored. Comparison to the NASA AP-8 models shows the new model median flux maps to be of approximately equivalent or lower magnitude in the slot region while new model 95th percentile maps are always higher. Implications for the proton dose received by MEO satellites are discussed.