A document by Krishnaprasad Chirakkil. 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).

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.

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our Solar system: it is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H-dominated thermosphere for astrophysicists, or the Sun-like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth’s atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so-called Xenon paradox.

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.

The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high-spectral resolution designed to resolve D and H Lyman-α emissions. The high-spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman-α emissions. This work presents improved characterizations of the high-spectral resolution mode using in-orbit observations that allow for more robust detections of the faint D Lyman-α emission line. Additionally, the instrument was re-calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high-spectral resolution instrument. Comparisons to Lyman-α observations made with the low-resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint-emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data-reduction pipeline with favorable implications for the science utility of D and H Lyman-α emissions.

The canonical picture of the magnetotail of unmagnetized planets consists of draped interplanetary magnetic fields (IMF) forming opposite-directed lobes, separated by the current sheet. \citet{DiBraccio2018twisted} showed that Mars’ magnetotail has a twist departing from this picture. Magnetohydrodynamic (MHD) results suggest that open field lines connected to the planet that populate portions of the tail cause the apparent twist. To validate this interpretation, we compare the tail topology determined from MHD simulations to that inferred from data collected by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, in particular how each topology responds to the upstream IMF orientation. The occurrence rates for open topology from both data and MHD varies with IMF polarities in a similar fashion as the tail twisting. This suggests that Mars’ crustal fields have a global effect on the magnetosphere configuration, supporting the picture of a “hybrid” magnetotail that is partly induced/draped and partly intrinsic/planetary in origin.

Though ion escape to space is an important mechanism for atmospheric loss on Mars, the processes that accelerate ions to escape velocity have not been fully identified and quantified. The lowest altitude where suprathermal planetary ions appear is an important source region for ion escape, where electromagnetic forces and waves begin to energize ions to escape velocity. We have conducted a statistical study of O2+ distribution functions measured by Mars Atmosphere and Volatile EvolutioN SupraThermal And Thermal Ion Composition (MAVEN-STATIC) in order to identify the region where suprathermal tails appear. At all solar zenith angles, suprathermal ions appear just above the exobase region, where the mean free path between collisions exceeds the neutral gas scale height. O2+ temperature profiles are also presented. We also investigate the effects of crustal magnetism, finding that crustal fields protect planetary plasma on the dayside and enhance energization on the nightside.

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$^2$.