This study presents results from magnetic field line conjunctions between the medium‐Earth orbiting Demonstration and Science Experiments (DSX) satellite and the low‐Earth orbiting (LEO) very low frequencies (VLF) Propagation Mapper (VPM) satellite. DSX transmitted at VLF toward VPM, which was equipped with a single‐axis dipole electric field antenna, when the two spacecraft passed near the same magnetic field line. VPM did not observe DSX signals in any of the 27 attempted conjunction experiments; the goal of this study, therefore, is to explain why DSX signals were not received. Explanations include (a) the predicted power at LEO from DSX transmissions was too low for VPM to observe; (b) VPM's trajectory missed the “spot” of highest intensity due to the focused ray paths reaching LEO; or (c) rays mirrored before reaching VPM. Different combinations of these explanations are found. We present ray‐tracing analysis for each conjunction event to predict the distribution of power and wave normal angles in the vicinity of VPM at LEO altitudes. We find that, for low‐frequency (below 4 kHz) transmissions, nearly all rays mirror before reaching LEO, resulting in low amplitudes at LEO. For mid‐ and high‐frequency transmissions (∼8 and 28 kHz respectively), the power at LEO is above the noise threshold of the VPM receiver (between 0.5 μV/m and 1 μV/m). We conclude that the antenna efficiency and plasmasphere model are critical in determining the predicted power at LEO, and are also the two most significant sources of uncertainty that could explain the apparent discrepancy between predicted amplitudes and VPM observations.

Very-low-frequency (VLF, 3-30 kHz) electromagnetic waves are prevalent in near-Earth space, and are produced by a variety of space-based and ground-based sources. Lightning and VLF transmitters launch powerful waves that propagate through the Earth's ionosphere and into the magnetosphere. Within the magnetosphere, naturally occurring and locally generated waves include chorus, hiss, and electromagnetic ion-cyclotron (EMIC) waves. Each of these whistler-mode waves propagating in the magnetospheric plasma can induce pitch-angle scattering and precipitation of trapped energetic particles (e.g. Inan & Carpenter, 1987;Imhof et al., 1983). For example, Abel and Thorne (1998) concluded that VLF waves radiated from both lightning and ground-based VLF transmitters play a significant role in maintaining the slot region of depleted fluxes between the inner and outer radiation belts. Each of these waves occurs in different regions of space, and with different amplitudes and frequencies, and therefore each affect somewhat different populations of energetic particles.The propagation of these VLF waves within the magnetosphere, as well as from the ground through the ionosphere, is complex and difficult to experimentally assess. Chorus and hiss waves, for example, are regularly measured by spacecraft such as the Van Allen Probes, but without knowledge of the source region of these waves, the propagation characteristics are difficult to characterize. Propagation characteristics such as the propagation direction, amplitude decay and/or growth, and reflections within the magnetosphere, are critical to understanding the quantitative effect these waves have on energetic particles.

The surfaces of many planetary bodies, including asteroids and small moons, are covered with dust to pebble-sized regolith held weakly to the surface by gravity and contact forces. Understanding the reaction of regolith to an external perturbation will allow for instruments, including sensors and anchoring mechanisms for use on such surfaces, to implement optimized design principles. We analyze the behavior of a flexible probe inserted into loose regolith simulant as a function of probe speed and ambient gravitational acceleration to explore the relevant dynamics. The EMPANADA experiment (Ejecta-Minimizing Protocols for Applications Needing Anchoring or Digging on Asteroids) flew on several parabolic flights. It employs a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible probe during its insertion into a system of bi-disperse, centimeter-sized model grains. We identify the force chain structure throughout the system during probe insertion at a variety of speeds and for four different levels of gravity: terrestrial, Martian, lunar, and microgravity. We identify discrete, stick-slip failure events that increase in frequency as a function of the gravitational acceleration. In microgravity environments, stick-slip behaviors are negligible, and we find that faster probe insertion can suppress stick-slip behaviors where they are present. We conclude that the mechanical response of regolith on rubble-pile asteroids is likely quite distinct from that found on larger planetary objects, and scaling terrestrial experiments to microgravity conditions may not capture the full physical dynamics.

No abstract