Distribution and solar wind control of compressional solar wind-magnetic anomaly interactions observed at the Moon by ARTEMIS

Abstract

A statistical investigation of 5 years of observations from the two-probe Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission reveals that strong compressional interactions occur infrequently at high altitudes near the ecliptic but can form in a wide range of solar wind conditions and can occur up to two lunar radii downstream from the lunar limb. The compressional events, some of which may represent small-scale collisionless shocks ("limb shocks"), occur in both steady and variable interplanetary magnetic field (IMF) conditions, with those forming in steady IMF well organized by the location of lunar remanent crustal magnetization. The events observed by ARTEMIS have similarities to ion foreshock phenomena, and those observed in variable IMF conditions may result from either local lunar interactions or distant terrestrial foreshock interactions. Observed velocity deflections associated with compressional events are always outward from the lunar wake, regardless of location and solar wind conditions. However, events for which the observed velocity deflection is parallel to the upstream motional electric field form in distinctly different solar wind conditions and locations than events with antiparallel deflections. Consideration of the momentum transfer between incoming and reflected solar wind populations helps explain the observed characteristics of the different groups of events.

Figure 1.

Schematic illustration of the geometry of compressional solar wind-magnetic anomaly interactions simulated by Fatemi et al. [2014] and observed by Halekas et al. [2014]. For these events, the upstream motional electric field of the solar wind (E_c) pointed toward the lunar surface near the limb. Reflected protons would therefore be initially decelerated by the force associated with this electric field (F_ref), in turn resulting in a corresponding outward force (F_sw) on the solar wind, in order to conserve linear momentum. At later points in the reflected proton’s trajectories, after they have accelerated to velocities higher than the solar wind (if they have not impacted the Moon), the forces between the two ion populations would switch direction as shown (see section 5 for discussion and detailed force equations).

Figure 2.

Event with upstream motional electric field roughly parallel to the observed velocity deflection, with panels showing upstream ion and electron energy spectra (corrected for spacecraft potential) in units of eV/[cm² s sr eV]; near-Moon ion and electron energy spectra (corrected for spacecraft potential) in the same units; upstream (dashed) and near-Moon (solid) ion density, magnetic field, and velocity; and the velocity deflection calculated by differencing the near-Moon from the upstream velocity. The black lines on electron energy spectra show scalar electron temperature. All vector quantities utilize SSE coordinates. Text labels indicate the position of the near-Moon probe.

Figure 3.

Event with upstream motional electric field roughly antiparallel to the observed velocity deflection, with all panels and labels the same as Figure 2.

Figure 4.

Event observed during variable IMF conditions, with all panels and labels the same as Figures 2 and 3. In this case, we time-shifted the upstream observations by 40 s to account for propagation delays between the two probes.

Figure 5.

The top two panels show observed vector velocity deflections during all identified events, in Selenocentric Solar Ecliptic (SSE) coordinates, colored by Δv_y (red = positive, blue = negative). The bottom two panels show the degree of alignment of the observed velocity deflection with the upstream motional electric field E (green = parallel, purple = anti parallel).

Figure 6.

(top row) The vector velocity deflections for each identified event, with the vector length corresponding to the magnitude of the peak velocity deflection. (bottom row) The average degree of alignment of the observed velocity deflection with the upstream motional electric field E, with a point for each event located at the time of maximum velocity deflection. In Figure 6 (bottom row), the squares represent steady IMF events, the diamonds represent variable IMF events, and the plus signs indicate steepened ULF waves (corresponding to the categories of Table 1). The plus signs inside of squares or diamonds indicate ULF precursors for coherent events.

Figure 7.

Surface locations 30° in longitude sunward of the terminator and at the same SSE z coordinate as events observed by ARTEMIS, along with 20, 50, and 100 nT contours of surface magnetic field estimated from Lunar Prospector Electron Reflectometer measurements [Mitchell et al., 2008]. Event symbols and color code are the same as in Figure 6.

Figure 8.

Distribution of upstream solar wind parameters during compressional events observed by ARTEMIS (same symbols and color code as Figures 6 and 7) compared to the overall statistical distribution of solar wind parameters observed by ARTEMIS over its entire mission, for the same range of lunar phases (gray-scale 2-D frequency distributions). We define cone angle as the angle between the IMF and the SSE x axis and clock angle as the angle from the y axis in the y-z plane, with positive angles representing positive z. We define the Alfvén Mach number ${\mathit{M}}_{\mathit{A}}=V_{\mathrm{sw}}\mathrm{/}\left(\frac{B}{\sqrt{{\mathit{\mu }}_o{\mathit{m}}_{\mathit{p}}{n}_{\mathit{p}}}}\right)$, ${\mathit{\beta }}_{\mathit{p}}={\mathit{n}}_{\mathit{p}}\mathit{k}{\mathit{T}}_{\mathit{p}}\mathrm{/}\left(\frac{B^{\mathrm{2}}}{\mathrm{2}{\mathit{\mu }}_o}\right)$, ion inertial length $D_{\mathit{p}}=\mathit{c}/{\mathit{\omega }}_{pi}$, and convected ion gyroradius ${\mathit{r}}_{\mathit{c}}=\frac{\mathit{m}{v}_{\mathrm{sw}\mathrm{\perp }}}{qB}$.

Figure 9.

Peak transverse velocity deflection versus near-Moon to upstream ratios of plasma density, magnetic field magnitude, and ion and electron thermal pressure (same symbols and color code as Figures 6–8).

Figure 10.

Peak transverse velocity deflection versus the convected ion gyroradius r_c in the upstream solar wind (same symbols and color code as Figures 6–9).

Figure 11.

Reflected proton trajectories (gray lines) traced from the strong farside lunar magnetic anomalies under the assumption of uniform magnetic and electric fields equal to upstream values, along with observed velocity deflections (same color scale as Figures 5 and 6), for the events of Figure 2 (parallel case, top row) and Figure 3 (antiparallel case, bottom row). The orange arrows show direction of upstream motional electric field. The green and red arrows show the direction of the force on the reflected protons (green) and solar wind (red) at selected locations.