We recently reported the finding of elementary rising‐tone emissions embedded within each harmonic of magnetosonic waves, by investigating wave electric field waveforms measured by Van Allen Probes. The present study further uncovers a new set of fine structures of magnetosonic waves, namely, each elementary rising‐tone may consist of a series of mini harmonics spaced around the O + gyrofrequency. The measured ion distributions suggest that the proton ring distribution provides free energy to excite the waves, whilst the O + ions suppress the wave growth around multiples of O + gyrofrequency, resulting in the formation of mini harmonics. Further investigation suggests that the warm plasma dispersion relation, that is, the ion Bernstein mode instabilities, may contribute to the formation of mini harmonics. The mini harmonic structure implies a new mechanism of energy redistribution among ion species in space plasmas, potentially providing a new acceleration mechanism for O + ions in the magnetosphere.

Rapid relativistic electron enhancements (REE) in the outer radiation belt have long been an intriguing phenomenon for space weather. In this study, we investigate rapid REE from October 2012 to December 2017 using multi-spacecraft observations. A total of 27 rapid REE events are identified from the Van Allen Probes (RBSP) measurements with a 5 times increase of MeV electrons at the center of the outer radiation belt (L = 4.5-5.5) in a half RBSP orbit (similar to 4.5 hr). All REE events are found to be in association with pulse-like injections of MeV electrons in the outer radiation belt. Electron fluxes in each injection at L similar to 6.6 and the overall electron enhancements at L = 4.5-5.5 are quantified. The 500 keV and 0.8-1 MeV electron fluxes are correlated in injections and in overall enhancements. Substorm strength is more intense before/during the REE than intervals after the REE. The statistical study suggests that substorm-associated MeV electron injections are highly correlated with rapid REE in the outer radiation belt. Rapid relativistic electron enhancements (REE) in the outer radiation belt are one of the most important phenomena in space weather. The fast increases of MeV energy electrons can have significant impacts on satellite operations and safety. In this study, we identify 27 events of relativistic electron enhancements in which MeV electron increases five times in similar to 4.5 hr at a radial distance of 4.5-5.5 Earth radius. All REE events are found in association with multiple pulse-like injections of MeV electrons. The electron fluxes during each injection at geosynchronous altitude and the overall enhancements in the center of the radiation belt are quantified. The increases in sub-relativistic and MeV electron fluxes are highly correlated. Overall, this research provides insights into the correlation between substorm-associated MeV electron injections and the rapid intense REE in the outer radiation belt, contributing to a better understanding of the dynamics and processes involved in the radiation belt particle energization. All 27 events of rapid enhancement of relativistic electrons are found to be associated with injections of MeV electrons The 500 keV and 0.8-1 MeV electron fluxes are highly correlated during injections at L similar to 6.6 and in overall enhancements at L = 4.5-5.5 Substorm strength is more intense during rapid relativistic electron enhancements

We use the full NASA Van Allen Probes mission (2012-2019) to extract the electron plasma density from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) and Electric Field and Waves (EFW) instruments and discuss the evolution of the plasmasphere. We generate new statistics including mean and standard deviations of the plasma density with respect to L-shell, magnetic local time (MLT), and various geomagnetic indices. These statistics are generated to be applied in radiation belt physics and space weather codes (with fits provided). The mean plasmasphere is circular around Earth with respect to MLT for Kp < 1. The mean 100 cm-3 level line is above L = 5 and mean 10 cm-3 level expands above the Van Allen Probes apogee for Kp < 1. The outer electron belt lies within the plasmasphere for 60% of all times. As activity increases (Kp > 2), a gradual MLT asymmetry forms with higher mean density in the afternoon sector due to plumes expanding outward. Conversely, the mean density decreases on the dawn and night sectors. The mean density is between similar to 500 and similar to 50 cm-3 between L similar to 4 and L similar to 6 during quiet and moderately active times (Kp < 3), representing similar to 80% of all times. Statistics in regions of high density below L = 2 are underdefined for intense activity. The highest standard deviation of density represents a factor 2.5 to 3 times the mean above L = 5 and for active times. We find the percent difference between the EFW and EMFISIS densities is bounded by +/- 20% for quiet and moderate activity (Kp < 5) and goes up to +/- 100% for extreme activity. The Earth's plasmasphere, discovered in the 1950s, is a region of cold plasma made of ions and electrons of a few electronvolts in energy, originating from upwelling ionized gas from the ionosphere and forming a rotating torus around the Earth. The radial profile of the electron cold plasma density within the plasmasphere decays from 10,000 electrons per cubic centimeter at similar to 1,000 km altitude to 10s electrons per cubic centimeter at its outer edge, sometimes exceeding similar to 36,000 km in altitude at the equator. The state of the plasmasphere is highly dependent on geomagnetic conditions, with geomagnetic storms and substorms eroding parts of this plasma. Here, we analyze 7 years of NASA Van Allen Probes measurements of the electron plasma density and generate statistics with respect to L-shell, magnetic local time, and geomagnetic indices. In this way, we show statistical variations of the plasmasphere, a strong magnetic local time dependence, and erosion with increasing geomagnetic activity. New mean electron densities and their standard deviation are generated and fitted with model functions that can be incorporated into space weather codes. This is important because the electron density is a key parameter influencing the strength of wave-particle interactions that accelerate and scatter energetic particles in the inner magnetosphere.

The cusp ion outflow on Earth has been extensively studied for decades. However, the energy flux associated with the ion outflow, which is of equivalent importance to the number flux or mass flux, has been rarely studied. Here, we present the first systematic study on the cusp ion energy flux and the energy budget along the cusp magnetic flux tube using quasi-conjunction observations from the Polar and Fast satellites. Significant ion energy fluxes (several 10s mW/m2 mapped to 100 km altitude) away from the Earth are frequently observed in the mid-altitude cusp (3-6 Re). Observations at low altitudes (<2 Re) show that the upward ion energy flux is associated with ion outflows originating from the ionosphere. In addition, we show that the ion outflows experience intense energization well above the ionosphere. The only possible energy source for this energization is the earthward Poynting flux in mid-altitude. The electrons are accelerated downward and are another energy sink (not source). The altitude profile of the energy fluxes suggests a transition region between 2 and 4 Re where both ion heating and electron acceleration primarily occur and where significant Poynting flux is dissipated. Analysis of the E/B ratio shows that the Poynting flux is carried by both the Alfven waves and quasi-static structures. The above results place important constraints on possible local ion mechanisms.

The present study uncovers the fine structures of magnetosonic waves by investigating the EFW waveforms measured by Van Allen Probes. We show that each harmonic of the magnetosonic wave may consist of a series of elementary rising-tone emissions, implying a nonlinear mechanism for the wave generation. By investigating an elementary rising-tone magnetosonic wave that spans a wide frequency range, we show that the frequency sweep rate is likely proportional to the wave frequency. Furthermore, we reveal that each elementary rising-tone magnetosonic waves consist of multiple mini-harmonics spaced at O+ gyrofrequency. We reveal that O+ ions can suppress the generation of magnetosonic waves at multiples of O+ gyrofrequency, resulting in the mini-harmonic structure. The commonly observed mini-harmonics indicate an energy transfer between different ion species.

Waves which couple to energetic electrons are particularly important in space weather, as they drive rapid changes in the topology and intensity of Earth’s outer radiation belt during geomagnetic storms. This includes Ultra Low Frequency (ULF) waves that interact with electrons via radial diffusion which can lead to electron dropouts and rapid acceleration and inward transport of electrons during. In radiation belt simulations, the strength of this interaction is specified by ULF wave radial diffusion coefficients. In this paper we detail the development of new models of electric and magnetic radial diffusion coefficients derived from in-situ observations of the azimuthal electric field and compressional magnetic field. The new models use L* as it accounts for adiabatic changes due to the dynamic magnetic field coupled with an optimized set of four components of solar wind and geomagnetic activity, Bz, V, Pdyn and Sym-H, as independent variables (inputs). These independent variables are known drivers of ULF waves and offer the ability to calculate diffusion coefficients at a higher cadence then existing models based on Kp. We investigate the performance of the new models by characterizing the model residuals as a function of each independent variable and by comparing to existing radial diffusion models during a quiet geomagnetic period and through a geomagnetic storm. We find that the models developed here perform well under varying levels of activity and have a larger slope or steeper gradient as a function of L* as compared to existing models (higher radial diffusion at higher L* values).

Dipolarization events with inductive, radial electric fields are examined, using Van Allen Probes data between 2012 and 2019. Two cases are studied, followed by statistical analyses. These events were observed between evening and premidnight magnetic local times (MLTs) under moderate geomagnetic activities. Radial electric field variations, azimuthal magnetic field variations, and energetic protons were often observed when horizontal magnetic fields started to decrease in the dip region. Magnetic field lines were stretched with their motion similar to the gradient B/curvature drift velocities of energetic protons. Signs of electric fields changed when horizontal magnetic fields started to increase in the dipolarization front (DF). Electric field and magnetic field variations were correlated and in quadrature regarding the phase relation. Among other explanations, we interpret these observations in terms of energetic proton structures drifting toward the probe locations, while being accompanied by standing waves.

The Van Allen Probes mission operations materialized through a distributed model in which operational responsibility was divided between the Mission Operations Center (MOC) and separate instrument specific SOCs. The sole MOC handled all aspects of telemetering and receiving tasks as well as certain scientifically relevant ancillary tasks. Each instrument science team developed individual instrument specific SOCs proficient in unique capabilities in support of science data acquisition, data processing, instrument performance, and tools for the instrument team scientists. In parallel activities, project scientists took on the task of providing a significant modeling tool base usable by the instrument science teams and the larger scientific community. With a mission as complex as Van Allen Probes, scientific inquiry occurred due to constant and significant collaboration between the SOCs and in concert with the project science team. Planned cross-instrument coordinated observations resulted in critical discoveries during the seven-year mission. Instrument cross-calibration activities elucidated a more seamless set of data products. Specific topics include post-launch changes and enhancements to the SOCs, discussion of coordination activities between the SOCs, SOC specific analysis software, modeling software provided by the Van Allen Probes project, and a section on lessons learned. One of the most significant lessons learned was the importance of the original decision to implement individual team SOCs providing timely and well-documented instrument data for the NASA Van Allen Probes Mission scientists and the larger magnetospheric and radiation belt scientific community.

Auroral beads are spatially wavy forms routinely seen before the onset of auroral substorms and are closely related to the onset‐related instabilities. To date, the acceleration mechanism of electrons that create auroral beads is not fully determined. Here, we present a fortuitous event when the Van Allen Probe A (RBSP‐A) was in magnetic conjunction with auroral beads. RBSP‐A observed Alfvén waves, locally generated kinetic Alfvén waves (KAWs) and Alfvénic accelerated electrons at several 100 eV. The Alfvén waves and KAWs carried sufficient Poynting flux to power visible aurora and may control the beads' motion. These observations and previous simulations support that the Alfvénic acceleration is the acceleration mechanism of the auroral beads. Specifically, KAWs are generated around the equator and accelerate local cold electrons to several 100 eV. The waves are suggested to propagate to both hemispheres and accelerate electrons to several keV, which directly account for the auroral beads.

Isolated proton auroras (IPAs) appearing at subauroral latitudes are generated by energetic protons precipitating from the magnetosphere through interaction with electromagnetic ion cyclotron (EMIC) waves. Thus, an IPA is the ionospheric projection of the spatial and temporal variation of wave‐particle interaction regions in the magnetosphere. In this study, we conducted unique multi‐event analysis of simultaneous observations of IPAs and their source regions on 22 April, 7 September, and 22 March 2018, using all‐sky imagers at subauroral latitudes and the Van Allen Probes. When the satellite footprint passed over the IPAs associated with ground Pc1 geomagnetic pulsations, locally generated He + ‐band EMIC waves with the same frequencies as the ground Pc1 pulsations were observed in all events. The IPAs and EMIC waves had comparable narrow widths in the latitudinal direction. The EMIC waves appeared during the rapid enhancement of the ring current proton flux at energy range of ∼10–50 keV, while they disappeared at the rapid decrease of the electron density. From these results, we conclude that the boundaries of the localized IPAs and EMIC waves were determined by the overlap region of energetic proton enhancement and the plasmasphere. This overlap of ring‐current protons and plasmasphere is a favorable condition for the pitch‐angle scattering of protons by the EMIC waves. Characteristic magnetic and electric field variations with the IPAs were not observed by the satellite, indicating that the IPAs were not accompanied by field‐aligned currents comparable to that of oval auroral arcs.

The Van Allen Probes Electric Fields and Waves (EFW) instrument provided measurements of electric fields and spacecraft floating potentials over a wide dynamic range from DC to 6.5 kHz near the equatorial plane of the inner magnetosphere between 600 km altitude and 5.8 Re geocentric distance from October 2012 to November 2019. The two identical instruments provided data to investigate the quasi-static and low frequency fields that drive large-scale convection, waves induced by interplanetary shock impacts that result in rapid relativistic particle energization, ultra-low frequency (ULF) MHD waves which can drive radial diffusion, and higher frequency wave fields and time domain structures that provide particle pitch angle scattering and energization. In addition, measurements of the spacecraft potential provided a density estimate in cold plasmas ( <20 eV ) from 10 to 3000 cm^-3 . The EFW instrument provided analog electric field signals to EMFISIS for wave analysis, and it received 3d analog signals from the EMFISIS search coil sensors for inclusion in high time resolution waveform data. The electric fields and potentials were measured by current-biased spherical sensors deployed at the end of four 50 m booms in the spacecraft spin plane (spin period ∼11 sec ) and a pair of stacer booms with a total tip-tip separation of 15 m along the spin axis. Survey waveform measurements at 16 and/or 32 S/sec (with a nominal uncertainty of 0.3 mV/m over the prime mission) were available continuously while burst waveform captures at up to 16,384 S/sec provided high frequency waveforms. This post-mission paper provides the reader with information useful for accessing, understanding and using EFW data. Selected science results are discussed and used to highlight instrument capabilities. Science quantities, data quality and error sources, and analysis routines are documented.

The Van Allen Probes spacecraft flew through the inner radiation belt and the plasmasphere, probing the extremely dynamic coupling between the thermosphere, the ionosphere and the subauroral magnetosphere. Examining the electrodynamics of this region using Van Allen Probes data required that the ∼1 mV/m electric field (E‐field) of interest be measured in a region where the E‐field due to spacecraft motion was ∼100 mV/m. This means that the E‐field, the magnetic field, and the spacecraft velocity had to be measured to better than 1%. The instruments on board the Van Allen Probes have achieved this accuracy, delivering reliable near equatorial E‐field measurements even below three Earth radii. The objective of this commentary is to summarize the methodology developed over the years to maximize the accuracy and scientific return of these double probe E‐field measurements below L = 3.

This paper presents the highlights of joint observations of the inner magnetosphere by the Arase spacecraft, the Van Allen Probes spacecraft, and ground-based experiments integrated into spacecraft programs. The concurrent operation of the two missions in 2017-2019 facilitated the separation of the spatial and temporal structures of dynamic phenomena occurring in the inner magnetosphere. Because the orbital inclination angle of Arase is larger than that of Van Allen Probes, Arase collected observations at higher L -shells up to L∼10 . After March 2017, similar variations in plasma and waves were detected by Van Allen Probes and Arase. We describe plasma wave observations at longitudinally separated locations in space and geomagnetically-conjugate locations in space and on the ground. The results of instrument intercalibrations between the two missions are also presented. Arase continued its normal operation after the scientific operation of Van Allen Probes completed in October 2019. The combined Van Allen Probes (2012-2019) and Arase (2017-present) observations will cover a full solar cycle. This will be the first comprehensive long-term observation of the inner magnetosphere and radiation belts.

Spherical double probe electric field sensors become electrically coupled to magnetospheric plasma during operation, leading to an instrument response that varies with the local plasma environment. Here, a method is developed for determining this variable coupling impedance for each measurement direction by using periods of favorable boom, wave, and magnetic field geometry. Comparing electric field complex amplitudes between 30 Hz and 10 kHz observed along each boom direction to those predicted from simultaneous magnetic field measurements and cold plasma theory allows for the amplitude and phase response of the instrument to be quantified over the full range of plasma densities encountered on‐orbit. A sheath model is developed to describe how the sheath resistance, sheath capacitance, and relative effective length vary as a function of plasma density. An additional empirical correction is also included to describe the phase response along the spin‐axis. The modeled sheath correction is subsequently tested for case studies of burst‐mode data and statistical analyses of survey‐mode data. It is demonstrated that the levels of agreement between observations and theoretical predictions based on Faraday's Law are substantially greater for the sheath corrected data than for uncorrected observations. Comparisons between observations with oppositely directed Poynting vector directions reveals that the sheath correction reconciles a bifurcated distribution in the uncorrected data to a single peak centered on agreement with Faraday's Law. A full sheath corrected EMFISIS L4 survey mode data set has been produced for final archive. Full details of the sheath correction are also provided for manual data correction.

We deduce the cold electron plasma density from NASA Van Allen Probes measurements throughout 2012-2019. We then extract two of the plasmasphere boundaries. We first use the gradient method for locating the plasmapause at L pp and, then, the $100\mathrm{~cm}^{-3}$ density threshold for the plasmasphere outer edge located at L 100 . The sharp gradient of the plasmapause is only defined in 53% of cases, while L 100 is defined for $\sim 85\%$ of cases. Differences and similarities between L pp and L 100 are discussed. L 100 is demonstrated to bound the plasmasphere at large L-shell in the dusk where L pp gradients often lack. We generate new empirical density models of the plasmasphere boundaries binned by L-shell, magnetic local time (MLT), and the maximum of the Kp index over 24 hours (Kp*).

Although electromagnetic ion cyclotron (EMIC) waves are commonly observed in the magnetosphere and are believed to energize background cold ions, it is not clear whether EMIC waves play a significant role in determining spacecraft potential change. In this paper, we present two strong He‐band EMIC wave events observed by the Van Allen Probe‐B spacecraft inside the plasmasphere. One event occurred on 11 March 2016 when the spacecraft was on the dayside, and the other occurred on 9 October 2016 when the spacecraft was in the postmidnight sector. When a strong He‐band EMIC wave activity was detected, low‐energy ion flux enhancements occurred nearly simultaneously with the EMIC wave power enhancements. Both events presented in this study are clearly unique in that He‐band wave power and enhanced proton flux are extremely high. During the wave activity interval, we found that the spacecraft charged more positively without a significant change in the ambient electron density. We discuss whether low‐energy ions energized by EMIC waves can contribute to the spacecraft potential change.

This summary article gathers a series of new results related to the characterization of the electromagnetic properties of lightning strokes both on Earth and in space through lightning-generated whistlers.

Relativistic electrons in the Earth's radiation belts are highly dynamic on a variety of timescales during the geomagnetic storm. Using Van Allen Probe spacecraft data, we investigate rapid enhancements of relativistic electrons in the outer radiation belt during a corotating interaction region (CIR) driven storm. Successive dipolarizations associated with 100keV‐MeV electron injections are identified. The evolution of energetic electrons is analyzed in the space of adiabatic invariants ( μ , K and L *). Within less than a few hours, the phase space density (PSD) of the relativistic electrons promptly increases corresponding to injections of MeV electrons. The PSD of MeV electrons cumulatively increases by a factor of 4–10 at L * = 4.5–5.8 which is likely due to successive groups of dipolarizations and injections. Both near‐equatorial (small K ) and off‐equatorial (large K ) energetic electrons increase significantly. The increases in the near‐equatorial electrons are still dominant, suggesting the operation of betatron acceleration. The event study shows that successive dipolarizations associated with the CIR‐driven storm may rapidly affect relativistic electrons of the outer radiation belt over a wide range in the phase space.

Lower-band whistler-mode chorus waves are important to the dynamics of Earth’s radiation belts, playing a key role in accelerating seed population electrons (100’s of keV) to relativistic ($>$ 1 MeV) energies, and in scattering electrons such that they precipitate into the atmosphere. When constructing and using statistical models of lower-band whistler-mode chorus wave power, it is commonly assumed that wave power is spatially distributed with respect to magnetic L-shell. At the same time, these waves are known to drop in power at the plasmapause, a cold plasma boundary which is dynamic in time and space relative to L-shell. This study organizes wave power and propagation direction data with respect to distance from the plasmapause location to evaluate what role the location of the plasmapause may play in defining the spatial distribution of lower band whistler-mode chorus wave power. It is found that characteristics of the statistical spatial distribution of equatorial lower band whistler mode chorus are determined by L-shell, and are largely independent of plasmapause location. The primary physical importance of the plasmapause is to act as an Earthward boundary to lower band whistler mode chorus wave activity. This behavior is consistent with an equatorial lower band whistler mode chorus wave power spatial distribution that follows the L-shell organization of the particles driving wave growth.