A document by Jeffrey Michael Broll. Click on the document to view its contents.

The Parker Solar Probe (PSP) spacecraft completed its second Venus gravity assist maneuver (VGA2) on 26 December 2019. For a 20 minute interval surrounding closest approach, the PSP/FIELDS Radio Frequency Spectrometer (RFS) was set to ‘burst mode’, recording radio spectra from 1.3–19.2 MHz at sub-second cadence. We analyze this burst mode data, searching for signatures of radio frequency ‘sferic’ emission from lightning discharges. During the burst mode interval, only 4 spectra were observed with strong impulsive signals, and all 4 could be attributed to saturation of the RFS high gain stage by \emph{in situ} electrostatic plasma waves. These RFS measurements during VGA2 are consistent with previous non-detection of radio frequency lightning signals from Venus reported by Gurnett et al. (2001).

Our solar system is filled with meteoric particles, or cosmic dust, which is either interplanetary or interstellar in origin. Interstellar dust (ISD) enters the heliosphere due to the relative motion of the sun and the interstellar flow. Interplanetary dust (IPD) comes primarily from asteroid collisions or comet sublimation, and comprises the bulk of material entering Earth’s atmosphere. This study examines variations in ISD and the IPD flux at Earth using observations from three different satellite techniques. First are size-resolved in situ meteoroid detections by the Ulysses spacecraft, and second are in situ indirect dust observations by Wind. Third are measurements of meteoric smoke in the mesosphere by the Solar Occultation For Ice Experiment (SOFIE). Wind observations are sorted into the interstellar and interplanetary components. Wind ISD show the anticipated correlation to the 22-yr. solar magnetic cycle, and are consistent with model predictions of ISD. Because Wind does not discriminate particle size, the IPD measurements were interpreted using meteoric mass distributions from Ulysses observations and from different models. Wind observations during 2007-2020 indicate a total meteoric influx at Earth of 22 metric tons per day (t d-1), in reasonable agreement with long-term averages from SOFIE (25 t d-1) and Ulysses (32 t d-1). The SOFIE and Wind influx time series both show an unexpected correlation to the 22-yr. solar cycle. This relationship could be an artifact, or may indicate that IPD responds to changes in the solar magnetic field.

Dust impacts on spacecraft are commonly detected by antenna instruments as transient voltage perturbations. The signal waveform is generated by the interaction between the impact-generated plasma cloud and the elements of the antenna – spacecraft system. A general electrostatic model is presented that includes the two key elements of the interaction, namely the charge recollected from the impact plasma by the spacecraft and the fraction electrons and cations that escape to infinity. The clouds of escaping electrons and cations generate induced signals, and their vastly different escape speeds are responsible for the characteristic shape of the waveforms. The induced signals are modeled numerically for the geometry of the system and the location of the impact. The model employs a Maxwell capacitance matrix to keep track of the mutual interaction between the elements of the system. A new reduced-size model spacecraft is constructed for laboratory measurements using the dust accelerator facility. The model spacecraft is equipped with four antennas: two operating in a monopole mode, and one pair configured as a dipole. Submicron-sized iron dust particles accelerated to > 20 km/s are used for test measurements, where the waveforms of each antenna are recorded. The electrostatic model provides a remarkably good fit to the data using only a handful of physical fitting parameters, such as the escape speeds of electrons and cations. The presented general model provides the framework for analyzing antenna waveforms and is applicable for a range of space missions investigating the distribution of dust particles in relevant environments.

While not specifically designed as a planetary mission, NASA’s Parker Solar Probe (PSP) mission uses a series of Venus gravity assists (VGAs) in order to reduce its perihelion distance. These orbital maneuvers provide the opportunity for direct measurements of the Venus plasma environment at high cadence. We present first observations of kinetic scale turbulence in the Venus magnetosheath from the first two VGAs. In VGA1, PSP observed a quasi-parallel shock, $\beta\sim1$ magnetosheath plasma, and a kinetic range scaling of $k^{-2.9}$. VGA2 was characterised by a quasi-perpendicular shock with $\beta\sim 10$, and a steep $k^{-3.4}$ spectral scaling. Temperature anisotropy measurements from VGA2 suggest an active mirror mode instability. Significant coherent waves are present in both encounters at sub-ion and electron scales. Using conditioning techniques to exclude these electromagnetic wave events suggests the presence of developed sub-ion kinetic turbulence in both magnetosheath encounters.

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.

Electrostatic solitary waves (ESWs) are a type of nonlinear time-domain plasma structure (TDS) generally defined by bipolar electric fields and propagation parallel to the local magnetic field. Formation mechanisms for TDSs in the magnetosphere have been studied extensively and are associated with plasma boundary layers and the braking of bursty bulk flows (BBFs). However, the rapid timescales over which these TDSs occur (< 2 ms) make them infeasible to count by eye over large time periods. Furthermore, high-cadence data are not always available. The Solitary Wave Detector (SWD) on NASA’s Magnetospheric Multiscale (MMS) mission quantifies the occurrence and amplitude of TDS throughout the constellation’s orbit; analysis of burst (65 kS/s) parallel electric field data indicates that the SWD captures appx. 60% of all bipolar TDS encountered in the tail region, enabling large-scale examination of their occurrence. Maps of TDS occurrence rates during several years of the MMS mission were generated from SWD data, showing enhanced TDS density in the tail region between 6-9 Re; enhance occurrence in or near shocks; and an unexpected enhancement in the dawn side of the tail and in the radiation belt.

The solar wind is slowed, deflected, and heated as it encounters Venus's induced magnetosphere. The importance of kinetic plasma processes to these interactions has not been examined in detail, due to a lack of constraining observations. In this study, kinetic-scale electric field structures are identified in the Venusian magnetosheath, including plasma double layers. The double layers may be driven by currents or mixing of inhomogeneous plasmas near the edge of the magnetosheath. Estimated double layer spatial scales are consistent with those reported at Earth. Estimated potential drops are similar to electron temperature gradients across the bow shock. Many double layers are found in few high cadence data captures, suggesting that their amplitudes are high relative to other magnetosheath plasma waves. These are the first direct observations of plasma double layers beyond near-Earth space, supporting the idea that kinetic plasma processes are active in many space plasma environments.

Plasmaspheric hiss waves are a dominant source of scattering for keV – MeV radiation belt electrons within the plasmasphere. Previous simulation and modeling work concerning hiss waves has often incorporated them via particle-based parameterizations (e.g., L-shell). However, recent work has shown that not only is hiss wave power loosely dependent on L-shell, but that proximity to the plasmapause may yield more accurate wave power distributions as it pertains to the modeling and scattering of electrons. This work serves to expand upon those previous studies by creating a low frequency (20 – 150 Hz) hiss wave model and incorporating a previously crafted high frequency (> 150 Hz) hiss wave model based on plasmapause location, proximity to the plasmapause, and Kp activity level. Diffusion coefficients created using this method produced shorter lifetimes for electrons between ~300 keV – 4 MeV than their L-sorted counterparts. Furthermore, 3D-simulations using and comparing the different hiss wave models (plasmapause sorting vs. L-sorting) find that the plasmapause based approach yields more accurate results when compared to Van Allen Probe-A observations.

Antenna instrument carried by spacecraft is complementary to dedicated dust detectors by registering transient voltage perturbations caused by impact-generated plasma. The signal waveform contains information about the interaction between the impact-generated plasma cloud and the elements of spacecraft – antenna system. Variability of antenna signals from dust impacts has not yet been systematically characterized. A set of laboratory measurements are performed to characterize signal variations in response to spacecraft parameters (bias voltage and antenna configuration) and impactor parameters (impact speed and composition). These measurements demonstrate that dipole antenna configurations are sensitive to impact location because of how the asymmetric expansion of impact plasma cloud produces different signals among antennas. This result revises previous conclusions that dipole antenna configurations should be insensitive to impacts. When dust impacts occur at low speeds, antenna instruments typically register smaller amplitudes and less characteristic impact signal shapes. In this case, impact event identification becomes challenged by low signal-to-noise ratios and complex waveforms, indicating the compound nature of non-fully developed impact-generated plasmas. Laboratory studies of aluminum dust particle hypervelocity impacts were used to explore the dependence of impact waveform variability on dust composition. No significant variations were determined compared to common iron dust measurements, consistent with prior studies. Additionally, electrostatic model fitting is used to obtain impact plasma parameters from antenna-detected waveform signals. The recovered parameters are comparable to those from Fe dust. This suggests a similarity of fully developed impact plasma cloud behaviors upon hypervelocity impact.

In planetary bow shocks, binary particle collisions cannot mediate the conversion of upstream bulk flow energy into downstream thermal energy, and wave-particle interactions in part assume this role. Understanding the contribution of waves to shock heating requires knowledge of the modes that propagate in different classes of shocks; we describe the growth patterns of whistler waves within the Venusian bow shock. Waves with frequencies $f \lesssim 0.1 f_{ce}$, where $f_{ce}$ is the electron cyclotron frequency, preferentially grow at local minima in the background magnetic field $|B_0|$. Quasi-parallel propagating whistlers with frequencies between $0.1 f_{ce}$ and $0.3 f_{ce}$ are strongest at the downstream ramps of these $|B_0|$ minima. Immediately downstream of the shock, whistlers with frequencies $f < 0.1 f_{ce}$ propagate more than 80$^\circ{}$ oblique from $\vec{B}_0$ and have elliptically polarized $\vec{B}$ fields. A prediction from kinetic theory of the orientation of these waves’ $\vec{B}$ ellipses is confirmed to high accuracy.

Depressions in magnetic field strength, commonly referred to as magnetic holes, are observed ubiquitously in space plasmas. Sub-proton-scale magnetic holes with spatial scales smaller than or on the order of $\rho_p$, are likely supported by electron currents vortices, rotating perpendicular to the ambient magnetic field. While there are numerous accounts of sub-proton-scale magnetic holes within the Earth’s magnetosphere, there are no reported observations in other space plasma environments. We present the first evidence of sub-proton-scale magnetic holes in the Venusian magnetosheath. During Parker Solar Probe’s first Venus Gravity Assist, the spacecraft crossed the planet’s bow shock and subsequently observed the Venusian magnetosheath. The FIELDS instrument suite onboard the spacecraft achieved magnetic and electric field measurements of magnetic hole structures. The electric field associated with magnetic depressions are consistent with electron current vortices with amplitudes on the order of 1 $\mu$A/m$^2$.