A. J. Halford

Observations of coincident EMIC wave activity and duskside energetic electron precipitation on 18–19 January 2013

Electromagnetic ion cyclotron (EMIC) waves have been suggested to be a cause of radiation belt electron loss to the atmosphere. Here simultaneous, magnetically conjugate measurements are presented of EMIC wave activity, measured at geosynchronous orbit and on the ground, and energetic electron precipitation, seen by the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) campaign, on two consecutive days in January 2013. Multiple bursts of precipitation were observed on the duskside of the magnetosphere at the end of 18 January and again late on 19 January, concurrent with particle injections, substorm activity, and enhanced magnetospheric convection. The structure, timing, and spatial extent of the waves are compared to those of the precipitation during both days to determine when and where EMIC waves cause radiation belt electron precipitation. The conjugate measurements presented here provide observational support of the theoretical picture of duskside interaction of EMIC waves and MeV electrons leading to radiation belt loss.

Interplanetary shocks and the resulting geomagnetically induced currents at the equator

Geomagnetically induced currents (GICs) caused by interplanetary shocks represent a serious space weather threat to modern technological infrastructure. The arrival of interplanetary shocks drives magnetosphere and ionosphere current systems, which then induce electric currents at ground level. The impact of these currents at high latitudes has been extensively researched, but the magnetic equator has been largely overlooked. In this paper, we investigate the potential effects of interplanetary shocks on the equatorial region and demonstrate that their magnetic signature is amplified by the equatorial electrojet. This local amplification substantially increases the regions susceptibility to GICs. Importantly, this result applies to both geomagnetic storms and quiet periods and thus represents a paradigm shift in our understanding of adverse space weather impacts on technological infrastructure.

Global-scale coherence modulation of radiation-belt electron loss from plasmaspheric hiss.

Over 40 years ago it was suggested that electron loss in the region of the radiation belts that overlaps with the region of high plasma density called the plasmasphere, within four to five Earth radii, arises largely from interaction with an electromagnetic plasma wave called plasmaspheric hiss. This interaction strongly influences the evolution of the radiation belts during a geomagnetic storm, and over the course of many hours to days helps to return the radiation-belt structure to its ‘quiet’ pre-storm configuration. Observations have shown that the long-term electron-loss rate is consistent with this theory but the temporal and spatial dynamics of the loss process remain to be directly verified. Here we report simultaneous measurements of structured radiation-belt electron losses and the hiss phenomenon that causes the losses. Losses were observed in the form of bremsstrahlung X-rays generated by hiss-scattered electrons colliding with the Earths atmosphere after removal from the radiation belts. Our results show that changes of up to an order of magnitude in the dynamics of electron loss arising from hiss occur on timescales as short as one to twenty minutes, in association with modulations in plasma density and magnetic field. Furthermore, these loss dynamics are coherent with hiss dynamics on spatial scales comparable to the size of the plasmasphere. This nearly global-scale coherence was not predicted and may affect the short-term evolution of the radiation belts during active times.

EMIC waves and plasmaspheric and plume density: CRRES results

Electromagnetic ion cyclotron (EMIC) waves frequently occur during geomagnetic storms, specifically during the main phase and 3–6 days following the minimum Sym − H value. EMIC waves contribute to the loss of ring current ions and radiation belt MeV electrons. Recent studies have suggested that cold plasma density structures found inside the plasmasphere and plasmaspheric plumes are important for the generation and propagation of EMIC waves. During the CRRES mission, 913 EMIC wave events and 124 geomagnetic storms were identified. In this study we compare the quiet time cold plasma density to the cold plasma density measured during EMIC wave events across different geomagnetic conditions. We found statistically that EMIC waves occurred in regions of enhanced densities. EMIC waves were, on average, not associated with large local negative density gradients.

A summary of the BARREL campaigns: Technique for studying electron precipitation.

Abstract The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) studies the loss of energetic electrons from Earths radiation belts. BARRELs array of slowly drifting balloon payloads was designed to capitalize on magnetic conjunctions with NASAs Van Allen Probes. Two campaigns were conducted from Antarctica in 2013 and 2014. During the first campaign in January and February of 2013, there were three moderate geomagnetic storms with SYM‐H min < −40 nT. Similarly, two minor geomagnetic storms occurred during the second campaign, starting in December of 2013 and continuing on into February of 2014. Throughout the two campaigns, BARREL observed electron precipitation over a wide range of energies and exhibiting temporal structure from hundreds of milliseconds to hours. Relativistic electron precipitation was observed in the dusk to midnight sector, and microburst precipitation was primarily observed near dawn. In this paper we review the two BARREL science campaigns and discuss the data products and analysis techniques as applied to relativistic electron precipitation observed on 19 January 2013.

BARREL observations of an ICME-shock impact with the magnetosphere and the resultant radiation belt electron loss

The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) mission of opportunity working in tandem with the Van Allen Probes was designed to study the loss of radiation belt electrons to the ionosphere and upper atmosphere. BARREL is also sensitive to X-rays from other sources. During the second BARREL campaign, the Sun produced an X-class flare followed by a solar energetic particle event (SEP) associated with the same active region. Two days later on 9 January 2014, the shock generated by the coronal mass ejection (CME) originating from the active region hits the Earth while BARREL was in a close conjunction with the Van Allen Probes. Time History Events and Macroscale Interactions during Substorms (THEMIS) satellite observed the impact of the interplanetary CME (ICME) shock near the magnetopause, and the Geostationary Operational Environmental Satellites (GOES) were on either side of the BARREL/Van Allen Probe array. The solar interplanetary magnetic field was not ideally oriented to cause a significant geomagnetic storm, but compression from the shock impact led to the loss of radiation belt electrons. We propose that an azimuthal electric field impulse generated by magnetopause compression caused inward electron transport and minimal loss. This process also drove chorus waves, which were responsible for most of the precipitation observed outside the plasmapause. Observations of hiss inside the plasmapause explain the absence of loss at this location. ULF waves were found to be correlated with the structure of the precipitation. We demonstrate how BARREL can monitor precipitation following an ICME-shock impact at Earth in a cradle-to-grave view; from flare, to SEP, to electron precipitation.

Dependence of EMIC wave parameters during quiet, geomagnetic storm, and geomagnetic storm phase times

As electromagnetic ion cyclotron (EMIC) waves may play an important role in radiation belt dynamics, there has been a push to better include them into global simulations. How to best include EMIC wave effects is still an open question. Recently many studies have attempted to parameterize EMIC waves and their characteristics by geomagnetic indices. However, this does not fully take into account important physics related to the phase of a geomagnetic storm. In this paper we first consider how EMIC wave occurrence varies with the phase of a geomagnetic storm and the SYM-H, AE, and Kp indices. We show that the storm phase plays an important role in the occurrence probability of EMIC waves. The occurrence rates for a given value of a geomagnetic index change based on the geomagnetic condition. In this study we also describe the typical plasma and wave parameters observed in L and magnetic local time for quiet, storm, and storm phase. These results are given in a tabular format in the supporting information so that more accurate statistics of EMIC wave parameters can be incorporated into modeling efforts.

EMIC waves and associated relativistic electron precipitation on 25‐26 January 2013

Using measurements from the Van Allen Probes and the Balloon Array for RBSP Relativistic Electron Losses (BARREL), we perform a case study of electromagnetic ion cyclotron (EMIC) waves and associated relativistic electron precipitation (REP) observed on 25 – 26 January 2013. Among all the EMIC wave and REP events from the two missions, the pair of the events is the closest both in space and time. The Van Allen Probe-B detected significant EMIC waves at L = 2.1 – 3.9 and MLT = 21.0 – 23.4 for 53.5 minutes from 2353:00 UT, 25 January 2013. Meanwhile, BARREL-1 T observed clear precipitation of relativistic electrons at L = 4.2 – 4.3 and MLT = 20.7 – 20.8 for 10.0 minutes from 2358 UT, 25 January 2013. Local plasma and field conditions for the excitation of the EMIC waves, wave properties, electron minimum resonant energy Emin, and electron pitch angle diffusion coefficient Dαα of a sample EMIC wave packet are examined along with solar wind plasma and interplanetary magnetic field (IMF) parameters, geomagnetic activity, and results from the spectral analysis of the BARREL balloon observations to investigate the two types of events. The events occurred in the early main phase of a moderate storm (min. Dst* = -51.0 nT). The EMIC wave event consists of two parts. Unlike the first part, the second part of the EMIC wave event was locally generated and still in its source region. It is found that the REP event is likely associated with the EMIC wave event.

Simulation of ULF wave‐modulated radiation belt electron precipitation during the 17 March 2013 storm

Balloon-borne instruments detecting radiation belt precipitation frequently observe oscillations in the millihertz frequency range. Balloons measuring electron precipitation near the poles in the 100 keV to 2.5 MeV energy range, including the MAXIS, MINIS, and most recently the Balloon Array for Relativistic Radiation belt Electron Losses balloon experiments, have observed this modulation at ULF wave frequencies. Although ULF waves in the magnetosphere are seldom directly linked to increases in electron precipitation since their oscillation periods are much larger than the gyroperiod and the bounce period of radiation belt electrons, test particle simulations show that this interaction is possible. Three-dimensional simulations of radiation belt electrons were performed to investigate the effect of ULF waves on precipitation. The simulations track the behavior of energetic electrons near the loss cone, using guiding center techniques, coupled with an MHD simulation of the magnetosphere, using the Lyon-Fedder-Mobarry code, during a coronal mass ejection (CME)-shock event on 17 March 2013. Results indicate that ULF modulation of precipitation occurs even without the presence of electromagnetic ion cyclotron waves, which are not resolved in the MHD simulation. The arrival of a strong CME-shock, such as the one simulated, disrupts the electric and magnetic fields in the magnetosphere and causes significant changes in both components of momentum, pitch angle, and L shell of radiation belt electrons, which may cause them to precipitate into the loss cone.

Space physics and policy for contemporary society

Space physics is the study of Earths home in space. Elements of space physics include how the Sun works from its interior to its atmosphere, the environment between the Sun and planets out to the interstellar medium, and the physics of the magnetic barriers surrounding Earth and other planets. Space physics is highly relevant to society. Space weather, with its goal of predicting how Earths technological infrastructure responds to activity on the Sun, is an oft-cited example, but there are many more. Space physics has important impacts in formulating public policy.