Kinetic instabilities in the lunar wake: ARTEMIS observations

Abstract

The Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission is a new two‐probe lunar mission derived from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. On 13 February 2010, one of the two probes, ARTEMIS P1 (formerly THEMIS‐B), made the first lunar wake flyby of the mission. We present detailed analysis of the electrostatic waves observed on the outbound side of the flyby that were associated with electron beams. Halekas et al. (2011) derived a net potential across the lunar wake from observations and suggested that the net potential generated the observed electron beams and the electron beams in turn excited the observed electrostatic waves due to kinetic instabilities. The wavelengths and velocities of the electrostatic waves are estimated, using high‐resolution electric field instrument data with cross‐spectrum analysis and cross‐correlation analysis. In general, the estimated wavelengths vary from a few hundred meters to a couple of thousand meters. The estimated phase velocities are on the order of 1000 km s−1. In addition, we perform 1‐D Vlasov simulations to help identify the mode of the observed electrostatic waves. We conclude that the observed electrostatic waves are likely on the electron beam mode branch.

High and low frequency instabilities driven by a single electron beam in two-electron temperature space plasmas

“…However, for normalised beam speeds which are greater than 2.15, we identify the instability as the electron beamresonant instability, typified by x r / kv dbz . 24 The transition from the electron-acoustic to the electron beam-resonant instabilities is clearly seen in the real frequency curve corresponding to v dbz ¼ 2.15 C h which exhibits the characteristics of both instabilities.…”

Kinetic electron and ion instability of the lunar wake simulated at physical mass ratio

“…2 That the regions with signs of ion-beam disruption lie close to the center of the wake is suggestive, however, since that is where the seeds of the ion-disrupting holes originate in the simulations. It is also where the high-resolution data showed possible signs of slow-moving electron holes, 27 and appears to be the source of an outward-propagating electron beam; 3 electron holes generally increase the parallel electron temperature, and deep ones can appear as beams.…”

“…That could make them easier to detect and characterise, but may also make them more difficult to disentangle from other signal variations. One of the high-resolution observations reported by Tao et al 27 revealed broad-band electrostatic fluctuations at around the ion plasma frequency, for which electron holes were proposed as one possible cause (cf. Ref.…”

“…Such a hole may thus for instance extend 100 m along the magnetic field, be moving with a parallel speed of 10 6 m=s, and have a potential amplitude of 0.5 V. That would give an electric field signal of $ 10 mV=m with duration $0.1 ms, which-though weaker and faster than those of electron holes observed in the Earth's plasma sheet 29,30 -may still be detectable with ARTEMIS. High-frequency electrostatic fluctuations have been observed in the lunar wake, 27 and could in principle be poorly resolved electron holes.…”

Statistical characterization of the foremoon particle and wave morphology: ARTEMIS observations

“…In contrast, higher‐frequency, ∼100 Hz whistlers are weakened when the field lines are connected to strong crustal fields, suggesting electron anisotropy caused by surface absorption as a plausible free energy source [ Harada et al , ]. Broadband electrostatic emission is frequently detected on the field lines connected to the Moon or to the lunar wake, as well as inside the wake [ Bale et al , ; Farrell et al , ; Hashimoto et al , ; Nishino et al , ; Tao et al , ; Harada et al , ].…”