2020 ARTEMIS SCIENCE NUGGETS
The Acceleration of Lunar Ions by Magnetic Forces in the Terrestrial Magnetotail Lobes
by Xin Cao
University of Iowa
Introduction
The Moon has a tenuous exosphere, which is mainly composed of neutral particles released from the lunar surface [Stern, 1999]. There are multiple processes through which neutral particles in the exosphere and on the lunar surface are transformed to heavy ions with the same mass, including photoionization, photon and electron-stimulated desorption, charge exchange, impact ionization [McGrath et al., 1986; Huebner & Mukherjee, 2015; Sarantos et al., 2012], and micrometeoroid bombardment [Stern,1999; Hartle and Killen, 2006; Halekas et al., 2011; Horanyi et al., 2015]. Studying these lunar ions can help us understand the fundamental physical processes that occur at the Moon and the linkages between the lunar surface, the exosphere, and the ambient plasma environment.
In a dense flowing magnetized plasma such as the solar wind, newborn ionized particles are picked up and accelerated by a –v × B motional electric field during the so-called pickup process [Hilchenbach et al., 1992; Mall et al., 1998; Yokota et al., 2009; Halekas et al., 2012, 2013, 2015; Hartle et al., 2011; Sarantos et al., 2012]. In the terrestrial magnetotail lobes, Zhou et al. [2013] showed that the average mass of lunar pickup ions was ~28 amu, based on the plasma quasi-neutrality requirement and the fact that ARTEMIS instruments underestimate the ion density. In contrast to the ambient solar wind, in the terrestrial magnetotail lobes the density of lunar ions is commonly comparable to or even larger than the density of the ambient lobe plasma [Halekas et al., 2018]. This condition invalidates the test-particle approximation commonly used to describe pure –v × B pickup, since the ambient plasma flow cannot provide the momentum required to pick up the lunar ions. The lobes in the magnetotail therefore provide a unique environment to investigate the dynamics of the lunar ions, since the local plasma environment can be appreciably perturbed by the presence of the lunar ions. In this scenario, the magnetic perturbations produced by the interaction with the ambient flow may provide the dominant forces that accelerate newborn ions [Vasyliunas, 2016]. The acceleration of ions is dominantly controlled by the J × B force, which can be divided into magnetic tension and magnetic pressure.
|
| Figure 1. The overall magnetic field structure caused by mass loading near the Moon. The black curves represent the disturbed field structure. In each left plot, the solid and dashed curves represent the upstream and downstream structure, respectively. The red curve represents the trajectory of ARTEMIS. The blue‐shaded regions are the locations where the accelerated ions were observed, which does not necessarily represent the entire regions of mass loading. |
Results
This study utilized ion data from ESA and magnetic field data from FGM to investigate the ion velocity and magnetic field perturbations along the dayside trajectory. Figure 2a (the upper subfigure of Figure 2) shows ARTEMIS P2 original measurements of ion velocity and magnetic field difference from ARTEMIS P1 made during 10:45-11:35 UTC on 11 November 2011, when P1 was far from the Moon (~[2, -7.5, 0.8] RU in SSE coordinates) and P2 was above the dayside of the Moon. In Figure 2b, the first panel shows the magnetic field measured by P2 during the event of 11 Nov 2011. The magnetic field was Bx dominated, which indicates the satellite was in the north tail lobe. The seventh panel shows the predicted velocity of the lunar ions in the Y SSE direction due to acceleration by magnetic pressure, and the eighth panel shows the predicted velocity due to acceleration by magnetic tension. Both of the two variables derived from the discussion in section 2 (the details of the algorithm are described in the supporting information). From the observation, the initial time when ARTEMIS detected the long-period beam was about 11:02:30 UTC, which was marked by the left vertical dashed line in the figure. Therefore, this is the earliest time for the generation of lunar ions with energy above the spacecraft potential in this beam, and no such lunar ions were accelerated before this time point. The ninth panel compares the predicted velocity components due to acceleration by the magnetic tension and pressure with the measured ion velocity during the time of the long-duration beam, in SSE coordinates. The orange and red curves are the Vy estimated by the two types of magnetic forces and that measured by ARTEMIS respectively. The predicted velocity component driven by the magnetic tension (orange color) and that driven by the magnetic pressure (red color) generally match the Vy of lunar ions very well during the whole period of the beam. The tenth panel shows that the overall velocity of lunar ions by the combination of the motion driven by magnetic tension and pressure, which is consistent with the corrected Vy measured by ARTEMIS, by observing whether the magnitude or the shape of the curve.
|
| Figure 2. The upper subfigure (a) reveals the ion velocity (red arrows) and magnetic field perturbation (black arrows) measured by ARTEMIS P2 during 10:45-11:35 UTC on 11 November 2011. The arrows are normalized to the unit magnitude and therefore they only indicate the direction. The coordinates are converted from Solar Selenocentric Ecliptic (SSE) into a coordinate system such that the X-Y plane is aligned with the upstream B-U plane (see more details in the text). The left represents the X-Z plane and the right represents the X-Y (B-U) plane. The lower subfigure (b) reveals multi-panels for the ARTEMIS data and estimation of the lunar ion acceleration by the magnetic tension and pressure in the event of November 11, 2011. |
In addition, Figure 3a provides an overview of the data from ARTEMIS P1 original measurements of ion velocity and magnetic field difference from ARTEMIS P2 made during 04:45-05:45 UTC on 29 October 2012, when P2 was far upstream from the Moon (~[7, -7, -1] RU in SSE coordinates) and P1 was above the dayside of the Moon, which is analogous with the first event in Figure 2a. The Moon was at ~[-60, 8, 2] RE in GSE coordinates.
|
| Figure 3. The upper subfigure (a) reveals the ion velocity ARTEMIST P1 measured (red arrows) and the magnetic field perturbation (black arrows) during 04:45-05:45 UTC on 29 October 2012. The arrows are normalized to the unit magnitude and therefore they only indicate the direction. The coordinates is converted from Solar Selenocentric Ecliptic (SSE) coordinate system such that the X-Y plane where the average lunar ion velocity and lobe’s magnetic field lie on is approximately consistent with the convective B-U plane (see more details in the text). The left represents the X-Z plane and the right represents the X-Y (B-U) plane. The lower subfigure (b) reveals multi-panels for the ARTEMIS data and estimation of the lunar ion acceleration by the magnetic tension and pressure in the event of October 29, 2012. |
In Figure 3b, the variables in each panel correspond with those in Figure 2b. The first panel shows the magnetic field measured by P1 during the event of 29 Oct 2012. The magnetic field was Bx dominated, which indicates the satellite was in the north tail lobe as well. The seventh panel shows the predicted velocity of the lunar ions in the Y SSE direction due to acceleration by magnetic pressure, and the eighth panel shows the predicted velocity due to acceleration by magnetic tension. Both of the two variables are derived from the method we have discussed in section 2. From the observation, the initial time when ARTEMIS detected the long-period beam was about 05:14 UTC, which was marked by the right vertical dashed line in the figure. Similarly as the method of the first event, the predicted velocity component in the Y direction due to acceleration by the magnetic tension increases from 0 initially to about 30 km/s at the end of the beam. In contrast, the local magnetic pressure accelerated the lunar ions in the Y direction in the beginning but then decelerated them in a certain of locations during the end of the beam. Given the magnitude of the predicted velocities due to acceleration by magnetic forces, the local magnetic field likely played an important role in transferring momentum to the lunar plasma. The ninth panel compares the predicted velocity components due to acceleration by the magnetic tension and pressure with the measured ion velocity during the time of this beam, in SSE coordinates. The tenth panel shows that the overall velocity of lunar ions by the combination of the motion driven by magnetic tension and pressure is consistent with the corrected Vy measured by ARTEMIS, by observing whether the magnitude or the shape of the curve, except a slight overestimation during about 05:00-05:03. (See the detailed discussion in the paper) The last two panels show the magnetic field perturbation ARTEMIS measured during this event in the converted coordinate system and in the SSE coordinate system.
Conclusion
In summary, This study have presented two ARTEMIS observations for studying the acceleration of lunar ions in the magnetotail lobes. This study investigated the magnetic field perturbation along the trajectory of the satellites and showed that the magnetic structure is consistent with a combined mass loading disturbance and plasma wake structure in the lobes. This study made the first-order estimate of the acceleration driven by the magnetic field perturbation, by calculating the magnetic tension and pressure along the trajectory of the satellites above the dayside of the Moon. The results suggest that magnetic field forces could play a crucial role in accelerating the lunar ions in the magnetotail lobe. This work can help us to more deeply understand the interaction of lunar ions with the ambient environment in the magnetotail lobes. Such a physical mechanism is also expected to play an important role in the ion acceleration at other Moons with a similar ionosphere and analogous ambient environment.
References
Cao, X., Halekas, J., Poppe, A., Chu, F., Glassmeier, K. H. (2020). The Acceleration of Lunar Ions by Magnetic Forces in the Terrestrial Magnetotail Lobes. Journal of Geophysical Research: Space Physics, 125(6), e2020JA027829.(Please see the full list of references in the references section of the paper above)
Biographical Note
Xin Cao is a postdoctoral researcher at University of Iowa. His research primarily focuses on the moon-magnetosphere interaction, the magnetotail dynamics of the Earth and the magnetospheric systems of the Ice Giants (Uranus and Neptune systems).
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu