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 2013 and 2018. 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 variations were correlated with magnetic field ones with ~ 90 deg. phase shift. These observations are mainly interpreted in terms of energetic proton structures drifting toward the probe locations, while being accompanied by standing waves.

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.

The Van Allen Probes Mission consists of two identical spacecraft flying in highly elliptical orbits, with perigee altitudes originally near 600 km. During the low altitude periods of the orbits, the spacecrafts are immersed in a region of high-density atomic Oxygen. Atomic Oxygen is known to change and degrade the properties of spacecraft surfaces, such as those of the Van Allen Probes Electric Field and Waves (EFW) instrument. The consistency of the sensor surfaces in EFW is important because the mechanisms used to ensure the collection of high quality electric field measurements requires that the photoemission properties of each sensor are uniform and stable. Oxidation or erosion of the sensor surfaces could limit the instrument’s ability to balance the currents produced by both the plasma electrons and the controlled bias current applied to the sensors, and thus to properly operate the device. We have modeled the atomic Oxygen exposure to the spacecraft to help determine the impact it has had on the sensors. We have calculated the fluence (time integrated flux) of atomic Oxygen particles that have collided with the spacecrafts over the entire course of the mission. We have also looked at the distribution of atomic Oxygen flux over time to further analyze malfunctions in the sensor readings at different points along the course of the mission. Additionally, we have investigated how different surfaces of the spacecraft are affected differently due to their orientation with respect to the spacecraft’s motion.

We present new and previously unreported in-situ observations of Hertz frequency multi-harmonic mode field line resonances detected by the Electric Field and Waves (EFW) instrument on-board the NASA Van Allen Probes during low-L perigee passes. Spectral analysis of the spin plane electric field data reveals the waves in numerous perigee passes, in sequential passes of Probes A and B, and with harmonic frequency structures from ~0.5 to 3.5 Hz which vary with L-shell, altitude, and from day to day. Comparing the observations to wave models using plasma mass density values along the field line given by empirical power laws and from the International Reference Ionosphere model (IRI), we conclude the waves are standing Alfvén field line resonances, and that only odd-mode harmonics are excited. The model eigenfrequencies are strongly controlled by the density close to the apex of the field line, suggesting a new diagnostic for equatorial ionospheric density dynamics.

We report observational evidence for a class of coherent magnetic dipolarization structures that are long lived and radially extensive. The reported dipolarization structures, a subset of general dipolarizations, typically remain coherent over 20-30 min in real time, 3-6 hours in MLT, and 10-20 Re in radial distance. Arrays of more than three spacecraft in non-collinear geometry are used to determine the propagation vector, including both the normal speed and direction, of such dipolarizations in the equatorial plane. The determined azimuthal propagation is ~3 deg/min, which corresponds to ~50 km/s at 6.6 Re. This speed is consistent with those obtained from two azimuthally separated spacecraft in previous works. Further analysis suggests that these azimuthally propagating dipolarizations (APDs) are often finger-like in shape, ranging from 5 to 20 Re in length and several Re in width. The reported APD may accompany the earthward flow channel and dipolarizing flux bundle (DFB).

A dipolarization of the background magnetic field was observed during a conjunction of the Magnetospheric Multiscale (MMS) spacecraft and Van Allen Probe B on 22 September 2018. The spacecraft were located in the inner magnetosphere at L~6-7 just before midnight magnetic local time (MLT). The separation between MMS and Probe B was ~1 Re. Gradual dipolarization or an increase of the northward component Bz of the background field occurred on a timescale of minutes. Since both MMS and Probe B measured similar gradual increases, the spatial scale was of the order of the separation between these two. On top of that, there were Bz increases, and a decrease in one case, on a timescale of seconds, accompanied by large electric fields with amplitudes > several tens of mV/m. Spatial scale lengths were of the order of the ion inertial length and the ion gyroradius. The inertial term in the momentum equation and the Hall term in the generalized Ohm’s law were sometimes non-negligible. These small-scale variations are discussed in terms of the ballooning/interchange instability (BICI) and kinetic Alfven waves. It is inferred that physics of multiple scales was involved in the dynamics of this dipolarization event.

The spatial scales of whistler-mode waves, determined by their generation process, propagation, and damping, are important for assessing the scaling and efficiency of wave-particle interactions affecting the dynamics of the radiation belts. We use multi-point wave measurements by two Van Allen Probes in 2013-2019 covering all MLTs at L=2-6 to investigate the spatial extent of active regions of chorus and hiss waves, their wave amplitude distribution in the source/generation region, and the scales of chorus wave packets, employing a time-domain correlation technique to the spacecraft approaches closer than 1000 km, which happened every 70 days in 2012-2018 and every 35 days in 2018-2019. The correlation of chorus wave power dynamics using is found to remain significant up to inter-spacecraft separations of 400 km to 750 km transverse to the background magnetic field direction, consistent with previous estimates of the chorus wave packet extent. Our results further suggest that the chorus source region can be slightly asymmetrical, more elongated in either the azimuthal or radial direction, which could also explain the aforementioned two different scales. An analysis of average chorus and hiss wave amplitudes at separate locations similarly shows the reveals different radial and azimuthal extents of the corresponding wave active regions, complementing previous results based on THEMIS spacecraft statistics mainly at larger L>6. Both the chorus source region scale and the chorus active region size appear smaller inside the outer radiation belt (at L< 6) than at higher L-shells.