Shrikanth G. Kanekal

Rapid local acceleration of relativistic radiation-belt electrons by magnetospheric chorus.

Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density, which are compelling evidence for local electron acceleration in the heart of the outer radiation belt, but are inconsistent with acceleration by inward radial diffusive transport. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth’s outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

Electron Acceleration in the Heart of the Van Allen Radiation Belts

Local Acceleration How the electrons trapped in Earth-encircling Van Allen radiation belts get accelerated has been debated since their discovery in 1958. Reeves et al. (p. 991, published online 25 July) used data from the Van Allen Radiation Belt Storm Probes, launched by NASA on 30 August 2012, to discover that radiation belt electrons are accelerated locally by wave-particle interactions, rather than by radial transport from regions of weaker to stronger magnetic fields. Satellite observations provide evidence for local relativistic electron acceleration in Earth’s radiation belts. The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth’s magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA’s Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process.

A long-lived relativistic electron storage ring embedded in Earth's outer Van Allen belt.

Van Allen Variation The two rings of relativistic particles called Van Allen Belts that encircle Earth were discovered during the space age, and are known to pose risks to satellites in geostationary orbit. NASA launched twin spacecraft, the Van Allen Probes, on 30 August 2012 to measure and characterize Earths radiation belt regions. Baker et al. (p. 186, published online 28 February) have shown that a third, unexpected and temporary, radiation belt formed on 2 September 2012 to disappear 4 weeks later in response to changes in the solar wind. NASA’s Van Allen Probes revealed an additional, dynamic belt of relativistic particles surrounding Earth. Since their discovery more than 50 years ago, Earth’s Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is composed predominantly of megaelectron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days, depending primarily on external forcing by the solar wind. The spatially separated inner zone is composed of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (>2 MeV) electrons that formed on 2 September 2012 and persisted largely unchanged in the geocentric radial range of 3.0 to ~3.5 Earth radii for more than 4 weeks before being disrupted (and virtually annihilated) by a powerful interplanetary shock wave passage.

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.

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

©2015. American Geophysical Union. All Rights Reserved. Strong 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. Key Points Source/seed energy electrons required to produce MeV radiation belt energization Substorm injections lead to VLF wave growth, producing MeV acceleration ULF waves may enhance loss/acceleration due to increased outward/inward diffusion

Observations of the impenetrable barrier, the plasmapause, and the VLF bubble during the 17 March 2015 storm

Van Allen Probes observations during the 17 March 2015 major geomagnetic storm strongly suggest that VLF transmitter-induced waves play an important role in sculpting the earthward extent of outer zone MeV electrons. A magnetically confined bubble of very low frequency (VLF) wave emissions of terrestrial, human-produced origin surrounds the Earth. The outer limit of the VLF bubble closely matches the position of an apparent barrier to the inward extent of multi-MeV radiation belt electrons near 2.8 Earth radii. When the VLF transmitter signals extend beyond the eroded plasmapause, electron loss processes set up near the outer extent of the VLF bubble create an earthward limit to the region of local acceleration near L = 2.8 as MeV electrons are scattered into the atmospheric loss cone.

Artificial Neural Networks for Determining Magnetospheric Conditions

Abstract This chapter presents a neural-network-based technique that allows for the reconstruction of the global, time-varying distribution of some physical quantity Q, that has been sparsely sampled at various locations within the magnetosphere, and at different times. We begin with a general introduction to the problem of prediction and specification, and why it is important and difficult to achieve with existing methods. We then provide a basic introduction to neural networks, and describe our technique using the specific example of reconstructing the electron plasma density in the Earth’s inner magnetosphere on the equatorial plane. We then show more advanced uses of the technique, including 3D reconstruction of the plasma density, specification of chorus and hiss waves, and energetic particle fluxes. We summarize and conclude with a general discussion of how machine learning techniques might be used to advance the state-of-the-art in space weather prediction, and insight discovery.

CIMI simulations with newly developed multiparameter chorus and plasmaspheric hiss wave models

Numerical simulation studies of the Earths radiation belts are important to understand the acceleration and loss of energetic electrons. The Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model considers the effects of the ring current and plasmasphere on the radiation belts to obtain plausible results. The CIMI model incorporates pitch angle, energy, and cross diffusion of electrons, due to chorus and plasmaspheric hiss waves. These parameters are calculated using statistical wave distribution models of chorus and plasmaspheric hiss amplitudes. However, currently, these wave distribution models are based only on a single-parameter, geomagnetic index (AE) and could potentially underestimate the wave amplitudes. Here we incorporate recently developed multiparameter chorus and plasmaspheric hiss wave models based on geomagnetic index and solar wind parameters. We then perform CIMI simulations for two geomagnetic storms and compare the flux enhancement of MeV electrons with data from the Van Allen Probes and Akebono satellites. We show that the relativistic electron fluxes calculated with multiparameter wave models resemble the observations more accurately than the relativistic electron fluxes calculated with single-parameter wave models. This indicates that wave models based on a combination of geomagnetic index and solar wind parameters are more effective as inputs to radiation belt models.

Special Issue “Global data systems for the study of solar-terrestrial variability”

© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. This special issue includes selected papers presented in the “SCOSTEP–WDS Workshop on Global Data Activities for the Research of Solar-Terrestrial Variability,” which was held at the National Institute of Information and Communications Technology (NICT), Tokyo, Japan, on September 28–30, 2015 (http://isds.nict.go.jp/ scostep-wds.2015.org/). This workshop was promoted by the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP) and the World Data System (WDS), both of which are Interdisciplinary Bodies of the International Council of Science (ICSU). The principal objective of the workshop was to stimulate interaction among data providers, data scientists, and data-oriented researchers participating in the SCOSTEP’s current research program VarSITI (Variability of the Sun and Its Terrestrial Impact, http://www.varsiti.org/). The long-term preservation and provision of quality-assessed data and information will be common objectives for both SCOSTEP and WDS. The development of advanced data systems to enable scientists to perform multidisciplinary data analysis will be another common target. Data analysis of selected solarterrestrial events was another important component of the workshop. The principal topics of the workshop were: (1) application of information technologies to data activities; (2) data systems for VarSITI; (3) data analysis of VarSITI Campaign Intervals and others; and (4) dataoriented collaborations between SCOSTEP and WDS. The total number of participants was 71 (53 Japanese and 18 foreign participants). In the workshop, 51 papers were presented (four keynote presentations, 21 papers on the data analysis of solar-terrestrial phenomena, and 26 technical papers on data systems). For topic (1), the technical report by Ritchel et al. (2017) explores the use of a semantic web-based mashup of appropriate data and models to enable interdisciplinary usage of data and information. This approach will be important for the data-oriented study of space weather and solar–climate connections in which multidisciplinary data analysis is inevitable because the majority of data are not well documented and tend to be suitably structured for machine-based combination. For topic (3), five papers are included in this issue. Among them, two papers discuss solar–interplanetary phenomena relating to the intense geomagnetic storm that initiated on March 17, 2015, widely known as the St. Patrick’s Day Event. This geomagnetic storm was associated with a partial halo coronal mass ejection (CME) occurred on March 15, 2015, which was associated with a C9.1/1F flare (S22W25). This storm’s minimum Dst reached − 228 nT (provisional) on March 18, and this was the first super geomagnetic storm of solar cycle 24. This event attracted considerable interest from the VarSITI community because the worldwide network of space weather agencies did not expect such a strong geomagnetic storm to be associated with the relatively minor solar flare (e.g., Kamide and Kusano 2015; Baker et al. 2016). As reported in this issue, Wu et al. (2016a, b), basing on detailed data analysis of solar and interplanetary observations, showed that the storm was caused by subsequent arrivals of an interplanetary shock sheath, carrying the southward interplanetary magnetic field (IMF), and a large magnetic cloud (MC) with a strongly southward IMF. Marubashi et al. (2016) fitted a flux-rope model to the temporal change of IMF near the Earth, and they concluded that the observations are most consistently explained by a toroidal flux rope with the torus plane nearly parallel to the ecliptic plane and that the observations are characterized by the peculiar location of near-Earth spacecraft, staying on the east-side flank of the flux-rope loop throughout its passage. The second strong geomagnetic storm discussed in the workshop was that occurred on June 21–24, 2015, with Open Access

Radiation belt electron enhancements: History and new results from RBSP

Energetic electron data from low-altitude Earth-orbiting spacecraft show both a long historical record of the Van Allen radiation belts and the specific effects of powerful storms such as the 2003 Halloween storms. The fluxes of 2-6 MeV electrons measured by the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) from July 1992 to the current time are presented in this talk. Data demonstrate intense electron acceleration events (associated with high-speed solar wind), for example, in 1993-95 for 3 <; L <; 6. During sunspot minimum (1996), there were significant electron events only briefly around the spring and autumn equinoxes. The SAMPEX electron data for 2003 and throughout 2004 and 2005 show the shifted position of the outer Van Allen zone and the filling of the slot region (L<; 3). A persistent new belt of electrons was produced in the wake of the Halloween storms and this was clearly seen for L <; 2 for several years. We note that SAMPEX data demonstrate that in 2008 and 2009, the radiation belts virtually disappeared due to very weak solar wind driving conditions associated with the recent profound solar activity minimum period. Building on this historical record, we describe the new, exciting results from the Relativistic Electron-Proton Telescope (REPT) instrument that were launched successfully onboard the Radiation Belt Storm Probes mission on 30 August 2012. Key areas of scientific progress using REPT will be addressed. Excellent new data from the twin REPT instruments are available from the initial turn-on (Launch+3 days) of the instruments to the present. Inner and outer zone electron spectra have been compared with model expectations.