Large-diameter thermionic hollow cathodes allow for higher-current operation than smaller cathodes but present new design and operational challenges. Significantly higher heater powers are required for ignition, and heater-insulating ceramics typically used can chemically interact with refractory metal heaters at high temperatures. In order to meet the mission requirements for the next generation of Hall thrusters, hollow cathodes will have to produce 300 to 700 A of discharge current while operating for tens of thousands of hours. A large-diameter hollow cathode intended to combine both long-life and highcurrent operation has been constructed as part of the development of the RF-Controlled Hollow Cathode. This concept proposes the use of RF power to control the dense-plasma attachment area, reducing peak current density while maintaining high current output. The ignition behavior of this hollow cathode was found to depend heavily on gas species, with krypton allowing for plasma ignition at much lower voltages than argon for comparable flow rates. Steady-state operation was achieved for discharge currents from 20 to 225 A using krypton for which current-voltage traces are presented. While operating at 20 A, for both argon and krypton, cathode temperatures were found to decrease with increasing mass flow, contrasting with the behavior of prior large cathodes. Heater failures due to arcing were mitigated with an appropriate ignition procedure and electrical design. Chemical reactions due to high heater temperatures were identified as a source of frequent heater failures and using a graphite standoff is proposed.

Though comparable to Earth in its size, internal structure, and distance from the Sun, Venus lacks an intrinsic magnetic field, thus allowing the solar wind to closely interact with its atmosphere and generate interesting plasma phenomena (Futaana et al., 2017). For example, as the solar wind is diverted around Venus, the interplanetary magnetic field (IMF) drapes around the planet and generates currents in the ionosphere, resulting in an induced magnetosphere. The extent of this magnetosphere is much smaller (∼0.05 Venus radii at the subsolar point) than that of Earth's magnetosphere (∼10 Earth radii) (Russell et al., 2016), but still provides a dayside magnetic barrier that diverts the solar wind's flow, causes the IMF field lines to pile up, and forms an upstream bow shock (Luhmann, 1986;Zhang et al., 1991). Studying such phenomena at Venus provides insight into how other unmagnetized atmospheric bodies interact with magnetized plasma flows, either in our Solar System (e.g., Mars

Abstract. In this series of papers, we present statistical maps of mirror mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on Extreme Ultraviolet (EUV) solar flux levels (high and low). The detection of these structures is done through magnetic field-only criteria and ambiguous determinations are checked further. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution facilitating comparisons between planetary and cometary environments. Applied to the magnetometer data of the Venus Express (VEX) spacecraft from May 2006 to November 2014, we detect structures closely resembling MMs lasting in total more than 93,000s, corresponding to about 0.6 % of VEX's total time spent in the Venus's plasma environment. We calculate MM-like occurrences normalised to the spacecraft's residence time during the course of the mission. Detection probabilities are about 10 % at most for any given controlling parameter. In general, MM-like structures appear in two main regions, one behind the shock, the other close to the induced magnetospheric boundary, as expected from theory. For solar maximum, the active region behind the bow shock is further inside the magneosheath, near the solar minimum bow shock location. The ratios of the observations during solar minimum and maximum are slightly dependent on the depth Δ B / B of the structures, deeper structures are more prevalent at solar maximum. A dependence on solar EUV (F10.7) flux is also present, where at higher F10.7 flux the events occur at higher values than the daily average value of the flux. Combining the plasma data from the Ion Mass Analyser with the magnetometer data shows that the instability criterion for MMs is reduced in the two main regions where the structures are measured, whereas it is still enhanced in the region in-between these two regions, implicating that the generation of MMs is transferring energy from the particles to the field. This study is the second of two on the magnetosheaths of Mars and Venus, and a third paper comparing the results obtained at the two planets will follow.

Abstract. Out of the two Venus flybys that BepiColombo uses as a gravity assist manoeuvre to finally arrive at Mercury, the first took place on 15 October 2020. After passing the bow shock, the spacecraft travelled along the induced magnetotail, crossing it mainly in the YVSO direction. In this paper, the BepiColombo Mercury Planetary Orbiter Magnetometer (MPO-MAG) data are discussed, with support from three other plasma instruments: the Planetary Ion Camera (SERENA-PICAM) of the SERENA suite, the Mercury Electron Analyser (MEA), and the BepiColombo Radiation Monitor (BERM). Behind the bow shock crossing, the magnetic field showed a draping pattern consistent with field lines connected to the interplanetary magnetic field wrapping around the planet. This flyby showed a highly active magnetotail, with e.g. strong flapping motions at a period of ∼7 min. This activity was driven by solar wind conditions. Just before this flyby, Venus's induced magnetosphere was impacted by a stealth coronal mass ejection, of which the trailing side was still interacting with it during the flyby. This flyby is a unique opportunity to study the full length and structure of the induced magnetotail of Venus, indicating that the tail was most likely still present at about 48 Venus radii.

Abstract. In this series of papers, we present statistical maps of mirror mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on Extreme Ultraviolet (EUV) solar flux levels (high and low), and, specific to Mars, on Mars Year (MY) as well as atmospheric seasons (four solar longitudes Ls). We first use magnetic field-only criteria to detect these structures and present ways to mitigate ambiguities in the nature of the detected structures. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution facilitating comparisons between planetary and cometary environments. Applied to the magnetometer data of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft from November 2014 to February 2020 (MY32–MY35), we detect structures closely resembling MMs lasting in total more than 170,000 s, corresponding to about 0.1 % of MAVEN's total time spent in the Martian plasma environment. We calculate MM-like occurrences normalised to the spacecraft's residence time during the course of the mission. Detection probabilities are about 1 % at most for any given controlling parameter. In general, MM-like structures appear in two main regions, one behind the shock, the other close to the induced magnetospheric boundary, as expected from theory. Detection probabilities are higher on average in low solar EUV conditions, whereas high solar EUV conditions see an increase in detections within the magnetospheric tail. We tentatively link the former tendency to two combining effects: the favouring of ion cyclotron waves the closer to perihelion due to plasma beta effects, and, possibly, the nongyrotropy of pickup ion distributions. This study is the first of two on the magnetosheaths of Mars and Venus.