M. H. Denton

Energy-dependent dynamics of keV to MeV electrons in the inner zone, outer zone, and slot regions

Abstract We present observations of the radiation belts from the Helium Oxygen Proton Electron and Magnetic Electron Ion Spectrometer particle detectors on the Van Allen Probes satellites that illustrate the energy dependence and L shell dependence of radiation belt enhancements and decays. We survey events in 2013 and analyze an event on 1 March in more detail. The observations show the following: (a) at all L shells, lower energy electrons are enhanced more often than higher energies; (b) events that fill the slot region are more common at lower energies; (c) enhancements of electrons in the inner zone are more common at lower energies; and (d) even when events do not fully fill the slot region, enhancements at lower energies tend to extend to lower L shells than higher energies. During enhancement events the outer zone extends to lower L shells at lower energies while being confined to higher L shells at higher energies. The inner zone shows the opposite with an outer boundary at higher L shells for lower energies. Both boundaries are nearly straight in log(energy) versus L shell space. At energies below a few 100 keV, radiation belt electron penetration through the slot region into the inner zone is commonplace, but the number and frequency of “slot filling” events decreases with increasing energy. The inner zone is enhanced only at energies that penetrate through the slot. Energy‐ and L shell‐dependent losses (that are consistent with whistler hiss interactions) return the belts to more quiescent conditions.

Storm-time plasma signatures observed by IMAGE/MENA and comparison with a global physics-based model

] We present energetic neutral atom (ENA) fluxesmeasured by the medium energy neutral atom (MENA)imager onboard the IMAGE satellite for the geomagneticstorm of 21 October 2001 at energies of 6 and 12 keV. Thefluxes indicate strong low altitude emissions close to theEarth and a nightside peak close to local midnight.The fluxes are compared with theoretical ENA fluxescalculated using the ring current-atmosphere interactionmodel (RAM). We find good quantitative agreementbetween MENA data and RAM results, both of whichindicate a peak in ENA emissions on the nightside, close tolocal midnight which varies in radial location between 2and 5R

An improved empirical model of electron and ion fluxes at geosynchronous orbit based on upstream solar wind conditions

In this study, a new empirical model of the electron fluxes and ion fluxes at geosynchronous orbit (GEO) is introduced, based on observations by Los Alamos National Laboratory (LANL) satellites. The model provides flux predictions in the energy range ~1 eV to ~40 keV, as a function of local time, energy, and the strength of the solar wind electric field (the negative product of the solar wind speed and the z component of the magnetic field). Given appropriate upstream solar wind measurements, the model provides a forecast of the fluxes at GEO with a ~1 h lead time. Model predictions are tested against in-sample observations from LANL satellites and also against out-of-sample observations from the Compact Environmental Anomaly Sensor II detector on the AMC-12 satellite. The model does not reproduce all structure seen in the observations. However, for the intervals studied here (quiet and storm times) the normalized root-mean-square deviation < ~0.3. It is intended that the model will improve forecasting of the spacecraft environment at GEO and also provide improved boundary/input conditions for physical models of the magnetosphere.

Calculation of IMAGE/MENA geometric factors and conversion of images to units of integral and differential flux

The Medium Energy Neutral Atom (MENA) instrument flown on the NASA IMAGE spacecraft is a time-of-flight neutral particle imager designed to image energetic neutral atom emissions from the Earth’s inner magnetosphere over an energy per mass range of 1–60keV∕amu. Images are generated by combining data from three separate heads and have a nominal angular resolution of 4°×4°. Here, we present a first-principles calculation of the geometric factors for each of the start-byte/stop-byte combinations for each of the three heads in the IMAGE/MENA instrument based on a detailed understanding of the its physical construction. The geometric factors are used to compute combined integral flux images and it is demonstrated that they result in head-to-head matching of the data that are both continuous and physically reasonable. We also discuss several issues associated with energy binning as a means for constructing differential flux images and present a powerful and robust approach that solves several critical problems ...

The complex nature of storm‐time ion dynamics: Transport and local acceleration

Data from the Van Allen Probes Helium, Oxygen, Proton, Electron (HOPE) spectrometers reveal hitherto unresolved spatial structure and dynamics in ion populations. Complex regions of O+ dominance, at energies from a few eV to >10 keV, are observed throughout the magnetosphere. Isolated regions on the dayside that are rich in energetic O+ might easily be interpreted as strong energization of ionospheric plasma. We demonstrate, however, that both the energy spectrum and the limited MLT extent of these features can be explained by energy-dependent drift of particles injected on the night side 24 hours earlier. Particle tracing simulations show that the energetic O+ can originate in the magnetotail, not in the ionosphere. Enhanced wave activity is co-located with the heavy-ion rich plasma and we further conclude that the waves were not a source of free energy for accelerating ionospheric plasma but rather the consequence of the arrival of substorm-injected plasma.

The response of the inner magnetosphere to the trailing edges of high‐speed solar‐wind streams

The effects of the leading edge stream interface of high-speed solar-wind streams (HSSs) upon the Earths magnetosphere have been extensively documented. The arrival of HSSs leads to significant changes in the plasmasphere, plasma sheet, ring current, and radiation belts, during the evolution from slow solar wind to persistent fast solar wind. Studies have also documented effects in the lower ionosphere and the neutral atmosphere. However, only cursory attention has been paid to the trailing-edge stream interface during the transition back from fast solar wind to slow solar wind. Here we report on the statistical changes that occur in the plasmasphere, plasma sheet, ring current, and electron radiation belt during the passage of the trailing-edge stream interface of HSSs, when the magnetosphere is in most respects in an extremely quiescent state. Counterintuitively, the peak flux of ~1 MeV electrons is observed to occur at this interface. In contrast, other regions of the magnetosphere demonstrate extremely quiet conditions. As with the leading-edge stream interface, the occurrence of the trailing-edge stream interface has a periodicity of 27 days, and hence, understanding the changes that occur in the magnetosphere during the passage of trailing edges of HSSs can lead to improved forecasting and predictability of the magnetosphere as a system.

Preface: Unsolved problems of magnetospheric physics

Magnetospheric physics, as a separate discipline, has existed for little more than 50 years [Stern, 1989; 1996]. In that time great strides have been made in understanding both the physical processes at work in the region, and also in determining connections and links between these processes. However, for some problems (many of which have been in existence throughout the preceding decades), the community has yet to reach consensus regarding solutions. To address this, a meeting was convened to bring workers in the field together to discuss how we can define (and hopefully solve) the outstanding problems facing the community. The Unsolved Problems of Magnetospheric Physics (UPMP) Workshop was held in September 2015 in Scarborough, UK. In contrast to most other meetings, people were specifically asked not to present and discuss their recent results. Rather, they were asked to bring their opinions and thoughts on unsolved problems to the meeting. Short presentations were encouraged after which the audience would debate and discuss definitions of the problems and how they could be overcome. Were new observations required? New missions? Or simply did the community need to work better together to resolve pertinent and outstanding science questions? Around 50% of the meeting schedule was devoted to discussion sessions on these topics.

On the origin of low-energy electrons in the inner magnetosphere: Fluxes and pitch-angle distributions

Accurate knowledge of the plasma fluxes in the inner magnetosphere is essential for both scientific and programmatic applications. Knowledge of the low-energy electrons (approximately tens to hundreds of eV) in the inner magnetosphere is particularly important since these electrons are acted upon by various physical processes, accelerating the electrons to higher energies, and also causing their loss. However, measurements of low-energy electrons are challenging, and as a result, this population has been somewhat neglected previously. This study concerns observations of low-energy electrons made by the Helium Oxygen Proton Electron instrument on board the Van Allen Probes satellites and also observations from geosynchronous orbit made by the Magnetospheric Plasma Analyzer on board Los Alamos National Laboratory satellites. The fluxes of electrons from ~30 eV to 1 keV are quantified as a function of pitch-angle, McIlwain L parameter, and local time for both quiet and active periods. Results indicate two sources for low-energy electrons in this energy range: the low-energy tail of the electron plasma sheet and the high-energy tail of the dayside ionosphere. These populations are identified primarily as a result of their different pitch-angle distributions. Field-aligned outflows from the dayside ionosphere are observed at all L shells during quiet and active periods. Our results also demonstrate that the dayside electron field-aligned fluxes at ~30 eV are particularly strong between L values of 6 and 7, indicating an enhanced source within the polar ionosphere.

Ring/Shell Ion Distributions at Geosynchronous Orbit

One years worth of plasma observations from geosynchronous orbit is examined for ion distributions that may simultaneously be subject to the ion Bernstein (IB) instability (generating fast magnetosonic waves) and the Alfven cyclotron (AC) instability (generating electromagnetic ion cyclotron waves). Confirming past analyses, distributions with robust ∂fp(v⊥)/∂v⊥>0 near v||=0, which we denote as “ring/shell” distributions, are commonly found primarily on the dayside of the magnetosphere. A new approach to high-fidelity representation of the observed ring/shell distribution functions in a form readily suited to both analytical moments calculation and linear dispersion analysis is presented, which allows statistical analysis of the ring/shell properties. The ring/shell temperature anisotropy is found to have a clear upper limit that depends on the parallel beta of the ring/shell (®||r) in a manner that is diagnostic of the operation of the AC instability. This upper limit is only reached in the post-noon events, which are primarily produced by the energy- and pitch-angle dependent magnetic drifts of substorm-injected ions. Further, it is primarily the leading edge of such injections, where the distribution is strongly ring-like, that the AC instability appears to be operating. By contrast, the ratio of the ring energy to the Alfven energy remains well within the range 0.25-4.0 suitable for IB instability throughout essentially all of the events, except those that occur in denser cold plasma of the outer plasmasphere.

Northern Hemisphere Stratospheric Ozone Depletion Caused by Solar Proton Events: The Role of the Polar Vortex

Ozonesonde data from four sites are analyzed in relation to 191 solar protons events (SPEs) from 1989-2016. Analysis shows ozone depletion (~10-35 km altitude) commencing following the SPEs. Seasonally-corrected ozone data demonstrate that depletions occur only in winter/early-spring above sites where the northern hemisphere polar vortex (PV) can be present. A rapid reduction in stratospheric ozone is observed with the maximum decrease occurring ~10-20 days after SPEs. Ozone levels remain depleted in excess of 30 days. No depletion is observed above sites completely outside the PV. No depletion is observed in relation to 191 random epochs at any site at any time of year. Results point to the role of indirect ozone destruction, most likely via the rapid descent of long-lived NOx species in the PV during the polar winter.