We present statistical analysis of 16,903 current sheets (CS) observed over 641 days aboard Ulysses spacecraft at 5 AU. We show that the magnetic field rotates across CSs through some shear angle, while only weakly varies in magnitude. The CSs are typically asymmetric with statistically different, though only by a few percent, magnetic field magnitudes at the CS boundaries. The dataset is classified into about 90.6\% non-bifurcated and 9.4\% bifurcated CSs. Most of the CSs are proton kinetic-scale structures with the half-thickness of non-bifurcated and bifurcated CSs within respectively 200–2,000 km and 500–5,000 km or 0.5–$5\lambda_{p}$ and 0.7–$15\lambda_{p}$ in units of local proton inertial length. The amplitude of the current density, mostly parallel to magnetic field, is typically within 0.05–0.5 nA/m$^{2}$ or 0.04–$0.4J_{A}$ in units of local Alfv\’{e}n current density. The CSs demonstrate approximate scale-invariance with the shear angle and current density amplitude scaling with the half-thickness, $\Delta \theta\approx 16.6^{\circ}\;(\lambda/\lambda_{p})^{0.34}$ and $J_0/J_{A}\approx 0.14\;(\lambda/\lambda_{p})^{-0.66}$. The matching of the magnetic field rotation and compressibility observed within the CSs against those in ambient solar wind indicate that the CSs are produced by turbulence, inheriting thereby its scale-invariance and compressibility. The estimated asymmetry in plasma beta between the CS boundaries is shown to be insufficient to suppress magnetic reconnection through the diamagnetic drift of X-line, but magnetic reconnection is probably suppressed by other processes. The presented results will be of value for future comparative analysis of CSs observed at different distances from the Sun.

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.

The “zebra stripes” are peaks and valleys commonly present in the spectrograms of energetic particles trapped in the Earth’s inner belt. Several theories have been proposed over the years to explain their generation, structure and evolution. Yet, the plausibility of various theories has not been tested due to a historical lack of ground-truth, including in-situ electric field measurements. In this work, we leverage the new visibility offered by the database of NASA Van Allen Probes electric drift measurements to reveal the conditions associated with the generation of zebra stripe patterns. Energetic electron flux measurements by the Radiation Belt Storm Probes Ion Composition Experiment between January 1, 2013 and December 31, 2015 are systematically analyzed to determine 370 start times associated with the generation of zebra stripes. Statistical analyses of these events reveal that the zebra stripes are usually created during substorm onset, a time at which prompt penetration electric fields are present in the plasmasphere. All the pieces of experimental evidence collected are consistent with a scenario in which the prompt penetration electric field associated with substorm onset leads to a sudden perturbation of the trapped particle drift motion. Subsequent inner belt drift echoes constitute the zebra stripes. This study exemplifies how the analysis of trapped particle dynamics in the inner belt and slot region provides complimentary information on the dynamics of plasmaspheric electric fields. It is the first time that the signature of prompt penetration electric fields is detected in near‐equatorial electric field measurements below L=3.

No existing instrument is capable of consistently measuring all three components of the DC and low frequency electric field (E-field) throughout the heliosphere with sufficient accuracy to determine the smallest, and most geophysically relevant component: the E-field component parallel to the background magnetic field. E-field measurements in the heliosphere are usually made on spinning spacecraft equipped with two disparate types of double probe antennas: (1) long wire booms in the spin plane, and (2) ~10 times shorter rigid booms along the spin axis. On such systems, the potential difference (signal + noise) is divided by the boom length to produce a resultant E-field component. Because the spacecraft-associated errors are larger nearer the spacecraft, the spin plane components of the E-field are well measured while the spin axis component are poorly measured. As a result, uncertainty in the parallel E-field is usually greater than its measured value. Grotifer leverages more than fifty years of expertise in delivering highly accurate spin plane E-field measurements, while overcoming inaccuracies generated by spin axis E-field measurements. Its design consists of mounting detectors on two rotating plates, oriented at 90° with respect to each other, on a non-rotating central body. Each rotating plate has two component measurements of the E-field such that the Twin Orthogonal Rotating Platforms (TORPs) provide four instantaneous measurements of the E-field, and the three E-field components are well-measured by the rotating detectors. Grotifer (Giant rotifer) is a reference to the rotifer, also known as the “wheel animalcule”, which has twin crowns of antenna-like cilia that appear to rotate in all directions. Grotifer marks a profound change in E-field instrument design that represents the best path forward to close the observational gap that currently hampers resolution of significant science questions at the forefront of space plasma physics research. Here, we present the Grotifer design concept implemented as a 27-U CubeSat, discuss the important features in the design and operation of Grotifer, and demonstrate the feasibility of implementing Grotifer using existing sub-systems and technologies.

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$^2$.