Solar Wind and Interplanetary Magnetic Field Influence on Ultralow Frequency Waves and Reflected Ions Near the Moon

Abstract

With no global magnetic field or atmosphere, the Moon was traditionally seen as a perfect absorber of the incoming solar wind. Recently, it has become apparent that magnetic fields with sources in the lunar crust act to reflect a significant percentage of incoming solar wind particles, which can then interact with the surrounding plasma environment and drive plasma waves. Using data collected by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft, we look for simultaneous observations of reflected ions and 0.01 Hz waves to study the characteristics and conditions under which wave-particle resonant interactions occur. Analyzing the solar wind and interplanetary magnetic field during these observations reveals particular solar wind and interplanetary magnetic field conditions that favor the generation of these waves. We use an ion tracing program to produce reflected ion distributions for various ambient conditions. These distributions show that the conditions that lead to more ions crossing the equatorial region where ARTEMIS orbits are also those favored for wave observations. Low-frequency waves, such as those generated by cyclotron resonance with ions, can be heavily Doppler shifted, making it difficult to determine their intrinsic properties. Reflected ion distributions for the same ambient conditions as the observed waves suggest that most of the waves are intrinsically right-hand polarized.

Figure 1.

ARTEMIS observations taken on 31 January 2014 from 17:45 to 18:45: (a) magnetic field magnitude and components in Selenocentric Solar Ecliptic (SSE) coordinates near the Moon (B1) and in the undisturbed upstream solar wind (B2), (b) transverse polarization in the spacecraft frame (positive indicates right handed) as a function of frequency from wavelet calculations with the local proton cyclotron frequency overlaid (black), (c) FFT power spectrum of magnetic field measurements near the Moon and (d) in the undisturbed upstream solar wind, (e) difference spectrum narrowed to frequency range of interest, (f) ion differential energy flux (eV/(eV cm² s sr)) as a function of energy, (g) ion differential energy flux as a function of phi excluding the solar wind, and (h) ion differential energy flux as a function of energy over a phi range that excludes the solar wind.

Figure 2.

Parameter distributions for (a) solar wind speed, (b) magnetic field magnitude, (c) IMF cone angle, θ_IMF = arccos B_x∕B, and (d) IMF clock angle, ϕ_IMF = arctan B_y∕B_z. Black distributions are of accumulated ARTEMIS data, red distributions are of wave and ion events, blue distributions are of wave-only events, and green distributions are of ion-only events. Distributions are normalized by number of minutes of event/overall data. (e) A 2-D distribution showing θ_IMF and ϕ_IMF of the wave and ion events.

Figure 3.

(a) $\widehat{k}\cdot V_{\text{ion }}$ distributions of ions crossing the equatorial region generated from ion tracing simulations where |B| = 4 nT, θ_IMF = 130°, ϕ_IMF = 100°, and |V_sw| varies. (b) $\widehat{k}\cdot V_{\text{ion }}$ distributions of ions crossing the equatorial region generated from ion tracing simulations where |V_sw| = 350 km/s, θ_IMF=130°, ϕ_IMF = 100°, and |B| varies.

Figure 4.

Histograms of (a) average weighted ion counts per event, (b) full width half max of Gaussian fitted to distribution, and (c) center of Gaussian fitted to distribution for 500 cumulative $\widehat{k}\cdot V_{\text{ion }}$ distributions generated from random solar wind and IMF parameters. Red diamonds indicate value from $\widehat{k}\cdot V_{\text{ion }}$ distribution of wave and ion events. Blue diamonds indicate value from cumulative $\widehat{k}\cdot V_{\text{ion }}$ distribution of wave-only events.

Figure 5.

Example of ion distribution observable by ARTEMIS during an event. (a) Spatial distribution, where red(blue) indicates areas with ions that have velocities that can generate intrinsic right(left)-hand polarized waves. Purple indicates areas that have ions with velocities that can generate either right- or left-hand polarized waves. The spacecraft orbit is in green. The black lines mark the boundaries of the region calculated from the possible wave propagation directions. The solar wind direction is indicated by the teal arrow in the lower right corner of the plot. The orange arrow indicates the wave propagation direction. (b) $\widehat{k}\cdot V_{\text{ion }}$ distribution of the ions in the observable region. Red(blue) indicates ion velocities that can generate right(left)-hand polarized waves. Black indicates ion velocites that will not interact resonantly with waves.

Figure 6.

Combined $\widehat{k}\cdot V_{\text{ion }}$ distribution for (a) wave and ion events and for (b) wave-only events for all ions in the equatorial region (gray) scaled by a factor of 3.5 to more easily compare with the other distributions, all ions in the region observable by ARTEMIS during events (black), and ions with velocities that can generate right-/left-hand polarized waves during events (red/blue).

Figure 7.

Distribution of lunar locations around Earth at the times of observed (a) wave and ion events and (b) wave-only events in GSE coordinates. Earth is indicated by black circle.

Figure 8.

Simulated spatial distributions of ions passing through the equatorial region. |V_sw| = 350 km/s and |B| = 4 nT for all simulations. (left column) Simulations with an IMF direction of θ_IMF=40° and ϕ_IMF = 250°. (right column) Simulations with an IMF direction of θ_IMF = 130° and ϕ_IMF = 100°. From top to bottom, rows are of simulations when the lunar phase is third quarter moon, new moon, and first quarter moon.