We will present signal processing techniques based on shape analysis of the time-frequency signatures of chorus elements in the Van Allen Radiation Belts. Specifically, we will employ the Radon transform and the Distance Transform, which are well-known for isolating local shapes and patterns in the image processing literature, to achieve automated detection of the spectral morphology of chorus elements. In particular, we will introduce “shrink-wrapping” techniques that quantify the salient features defining the microstructure of chorus elements. We will present preliminary results based on case studies of the EMFISIS data from the Van Allen Radiation Belts mission.

During the Twin Rockets to Investigate Cusp Electrodynamics (TRICE-2) High-Flyer rocket’s passage through the cusp the high frequency (HF) radio wave receiver observed three intervals of banded Upper-Hybrid (UH) waves. The bands begin at the UH frequency ($\sim$1.2–1.3 MHz), descending to as low as 1.1 MHz, with amplitudes of hundreds of mV/m. The spacing of the bands are $\sim$4.5–6 kHz and the number of bands ranges from three to ten. Simultaneously, the very low frequency (VLF) radio wave receiver observed Lower-Hybrid (LH) waves with amplitudes ranging from 1–10 mV/m and frequencies of 4.5-6 kHz. Slight variations of the spacings of the bands in the UH waves were closely correlated with variations in the LH peak frequencies. Two possible wave-wave interactions are explored to explain this phenomenon: decay of an UH wave into a lower frequency UH wave and a LH wave, and coalescence of independent UH waves and LH waves that spawn UH waves. Using a dispersion relation calculator with electron and ion distribution functions based off those observed by the particle instruments suggests that UH waves, and to a lesser degree LH waves, can be excited by linear instabilities. Kinematic analysis of the waves dispersion relations and the wave matching conditions show that wave-wave interactions linking UH and LH modes are possible through either decay or coalescence. This analysis along with comparisons of the energy densities of the waves, and the ratio of their occupation numbers suggest that the decay process is more likely than coalescence.

We performed a comprehensive statistical study of electromagnetic ion cyclotron (EMIC) waves observed by the Van Allen Probes and Exploration of energization and Radiation in Geospace satellite (ERG/Arase). From 2017 to 2018, we identified and categorized EMIC wave events with respect to wavebands (H+ and He+ EMIC waves) and relative locations from the plasmasphere (inside and outside the plasmasphere). We found that H-band EMIC waves in the morning sector at L>8 are predominantly observed with a mixture of linear and right-handed polarity and higher wave normal angles during quiet geomagnetic conditions. Both H+ and He+ EMIC waves observed in the noon sector at L~4-6 have left-handed polarity and lower wave normal angles at |MLAT|< 20˚ during the recovery phase of a storm with moderate solar wind pressure. In the afternoon sector (12-18 MLT), He-band EMIC waves are dominantly observed with strongly enhanced wave power at L~6-8 during the storm main phase, while in the dusk sector (17-21 MLT) they have lower wave normal angles with linear polarity at L>8 during geomagnetic quiet conditions. Based on distinct characteristics at different EMIC wave occurrence regions, we suggest that EMIC waves in the magnetosphere can be generated by different free energy sources. Possible sources include the freshly injected particles from the plasma sheet, adiabatic heating by dayside magnetospheric compressions, suprathermal proton heating by magnetosonic waves, and off-equatorial sources.

The nonlinear growth theory of chorus emissions is used to develop a simple model of the subpacket formation. The model assumes that the resonant current, which is released from the source to the upstream region, radiates a new whistler mode wave with a slightly increased frequency, which triggers a new subpacket. Saturation of the growth in amplitude is controlled by the optimum amplitude. Numerical solution of advection equations for each subpacket, with the chorus equations acting as the boundary conditions, produces a chorus element with a subpacket structure. This element features an upstream shift of the source region with time and an irregular growth of frequency, showing small decreases between adjacent subpackets. The influence of input parameters on the number of subpackets, the shift of the source, the frequency sweep rate and the maximum amplitude is analyzed. The model well captures basic features of instantaneous frequency measurements provided by the Van Allen Probes spacecraft. The modeled wave field can be used in future particle acceleration studies.

The MMS spacecraft routinely observe electron “microinjections” at energies in the 10s-100s keV range across the nightside magnetosphere at distances <20 Re. Microinjections are typically observed in clusters where multiple dispersed injection signatures are recorded in succession over a short time interval (e.g., ~10 in one hour) and may be related to surface wave activity at the magnetopause. Recent work has shown that microinjections have distinctive features in the angular distributions, where field-aligned distributions are observed near dusk, while trapped distributions are observed near dawn. Due to their recent discovery, the origin and generation of microinjections has yet to be conclusively identified and detailed studies thus far have largely been done on a case-by-case basis. In an effort to elucidate more general properties and characteristics of microinjections, we describe an automated routine designed to identify microinjection signatures in the MMS/FEEPS measurements. The algorithm uses image processing techniques (the radon transform) and is based on a similar method developed to identify whistler-mode chorus elements in Van Allen Probes wave observations (Sen Gupta et al., [2017]). We present an initial set of results from a statistical database of microinjection events obtained from the automated algorithm to further our understanding of this intriguing phenomenon.

High-quality measurements of electromagnetic fields and electron velocity distributions by the Magnetospheric Multiscale (MMS) mission in Earth’s magnetosheath present a unique opportunity to characterize heliospheric plasma turbulence and to determine the mechanisms responsible for its dissipation. We apply the field-particle correlation technique to the MMS measurements to identify the dissipation mechanism and quantify the dissipation rate. It is found that 95% of the intervals have velocity-space signatures of electron Landau damping that are quantitatively consistent with linear kinetic theory for the collisionless damping of kinetic Alfvén waves. About 75% of the intervals have asymmetric signatures, implying that often the collisionless damping is stronger for waves propagating one direction along the magnetic field than the other. Nearly half of the samples have the same electron energization rates as order-of-magnitude estimates of the turbulent energy cascade rate, suggesting that electron Landau damping is frequently responsible for the dissipation of magnetosheath turbulent energy.

Auroral whistler mode radio emissions called saucers are of fundamental interest because they require an unusually stationary emission process in the dynamic auroral environment, and it is a mystery how that can happen in this or similar conditions elsewhere in geospace. The Cusp Alfven and Plasma Electrodynamics Rocket (CAPER-2), launched into the polar cusp and obtained the first rocket measurements of a large-scale, multiple-armed dayside saucers, similar to those recently observed by the the DEMETER satellite, with the addition of in situ particle measurements and simultaneous conjugate ground-based measurements. For 300 s prior to cusp entry, CAPER-2 detected ~15 truncated saucer arms lasting 5–50 s. Directional analysis using waveforms, combined with ground-based data, suggests that these originate within the cusp. Ray-tracing analysis indicates source altitudes ~2500 km. On-board particle instruments show dispersed electron bursts in the cusp, presumed Alfvenically accelerated, corresponding to approximately the same source heights as the saucers.

Autonomous large-scale detection of whistler-ode chorus elements in the Van Allen radiation belts has been an open computational challenge. This is primarily due to: (i) Variability of the spectral morphology of chorus elements, and (ii) Structured background interference from hiss-like chorus that can make elements difficult to detect using traditional signal processing and pattern recognition techniques. We will present computational techniques drawing on pixel connectivity, signal-to-noise (SNR) considerations as well as supervised and unsupervised pattern recognition techniques. Specifically, we will explore the efficacy of popular machine learning techniques trained on unfiltered spectral images versus those trained on culled features generated by unsupervised feature extraction techniques. Representative results will be presented based on magnetic field measurements taken by the EMFISIS instrument suite in the Van Allen probes mission.