Jan. 7, 2022
3:30 p.m. - 5 p.m.
NASA’s THEMIS-ARTEMIS mission entered lunar orbit in mid-2011 and since then, has returned a treasure trove of data for studies of the lunar plasma environment. To date, the ARTEMIS mission has resulted in over 200 total publications, nearly 100 of which are directly related to the lunar plasma environment and/or the subsequent effects of the Moon on its surroundings. Now past its 10-year mark in lunar orbit, I look back and highlight results from the ARTEMIS mission, focusing on work related to the Moon’s interaction with ambient plasma. In particular, I will discuss ARTEMIS studies of: (1) the lunar wake as an example for fundamental space-plasma processes throughout the solar system; (2) the lunar neutral and ionized exosphere, which both responds to and perturbs the near-lunar space environment; and, (3) electrostatic lunar surface charging, driven by a combination of solar wind and magnetospheric plasmas, as well as interplanetary dust. ARTEMIS has made broad and fundamental contributions to our understanding of each of these areas, transforming our view of the Moon from a simple passive absorber to a host of rich and complex plasma processes that actively perturb the lunar environment. Finally, I will conclude by discussing on-going and future avenues of research with ARTEMIS, including upcoming synergies with the NASA Heliophysics Division’s HERMES payload to be hosted on the Lunar Gateway.
Jan. 14, 2022
noon - 1 p.m.
A major discovery of Parker Solar Probe (PSP) was the presence of large numbers of localized increases in the radial solar wind speed and associated sharp deflections of the magnetic field - switchbacks (SB). A possible generation mechanism of SBs is through magnetic reconnection between open and closed magnetic flux near the solar surface termed interchange reconnection that leads to the ejection of flux ropes (FR) into the solar wind. The role of FRs merging in controlling the structure of SB in the solar wind is explored through direct observations, analytic analysis, and numerical simulations. Observations also suggest that SBs undergo merging, which is shown to be energetically favorable to reduce the strength of the wrapping magnetic field and that this drives the observed elongation of SBs. A further consequence is a resulting dominance of the axial magnetic field within SBs that leads to the characteristic sharp rotation of the magnetic field into the axial direction at the SB boundary that is revealed in observations. Observations by PSP reveal the existence of intensive plasma wave bursts with frequencies below 0.1 fce (from tens of Hz to 150 Hz in the spacecraft frame) collocated with the switchbacks boundaries. Sunward propagation with depletion of magnetic field magnitude leads to a significant Doppler frequency downshift of whistler waves from 200-300 Hz (0.2-0.5 fce) down to 20-80 Hz in the spacecraft frame. Their peak amplitudes can be as large as 2 to 4 nT. Such values represent approximately 10-20% of the background magnetic field. We have evaluated the properties of these waves collocated with dips of magnetic field related to switchback boundaries, the mechanisms of wave generation: the generation of these waves is supported by the modified electron distribution with increased transverse temperature anisotropy inside the magnetic holes; and the effects on solar wind suprathermal particles from interaction with these waves: sunward propagating whistler waves efficiently interact with the high energy solar wind electrons (in the energy range up to 1 keV) scattering the strahl population of suprathermal electrons into a halo population due to the most efficient cyclotron resonance interaction.
Jan. 21, 2022
noon - 1 p.m.
During its first eight orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale Heliospheric Current Sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the common detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 16-107 solar radii. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. Ion and electron signatures of the reconnection separatrix layers are also observed adjacent to some exhausts. The common detection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. Thus, the PSP findings suggest that large-scale dynamics either locally in the solar wind or within the coronal source of the HCS (e.g., at the tip of helmet streamers) plays a critical role in triggering reconnection onset