Nongyrotropic electron velocity distribution functions near the lunar surface

We have analyzed nongyrotropic electron velocity distribution functions (VDFs) obtained near the lunar surface. Electron VDFs, measured at ∼10–100 km altitude by Kaguya in both the solar wind and the Earth's magnetosphere, exhibit nongyrotropic empty regions associated with the ‘gyroloss’ effect; i.e., electron absorption by the lunar surface combined with electron gyromotion. Particle-trace calculations allow us to derive theoretical forbidden regions in the electron VDFs, thereby taking into account the modifications due to nonuniform magnetic fields caused by diamagnetic-current systems, lunar-surface charging, and electric fields perpendicular to the magnetic field. Comparison between the observed empty regions with the theoretically derived forbidden regions suggests that various components modify the characteristics of the nongyrotropic electron VDFs depending on the ambient-plasma conditions. On the lunar nightside in the magnetotail lobes, negative surface potentials slightly reduce the size of the forbidden regions, but there are no distinct effects of either the diamagnetic current or perpendicular electric fields. On the dayside in the solar wind, the observations suggest the presence of either the diamagnetic-current or solar wind convection electric field effects, or both. In the terrestrial plasma sheet, all three mechanisms can substantially modify the characteristics of the forbidden regions. The observations imply the presence of a local electric field of at least 5 mV/m although the mechanism responsible for production of such a strong electric field is unknown. Analysis of nongyrotropic VDFs associated with the gyroloss effect near solid surfaces can promote a better understanding of the near-surface plasma environment and of plasma–solid-surface interactions

Interaction between terrestrial plasma sheet electrons and the lunar surface: SELENE (Kaguya) observations

Analysis of the data obtained by SELENE (Kaguya) revealed a partial loss in the electron velocity distribution function due to the "gyro-loss effect", namely gyrating electrons being absorbed by the lunar surface. The Moon enters the Earth's magnetosphere for a few days around full moon, where plasma conditions are significantly different from those in the solar wind. When the magnetic field is locally parallel to the lunar surface, relatively high-energy electrons in the terrestrial plasma sheet with Larmor radii greater than SELENE's orbital height strike the lunar surface and are absorbed before they can be detected. This phenomenon can be observed as an empty region in the electron distribution function, which is initially isotropic in the plasma sheet, resulting in a non-gyrotropic distribution. We observed the expected characteristic electron distributions, as well as an empty region that was consistent with the presence of a relatively strong electric field (∼10 mV/m) around the Moon when it is in the plasma sheet

Three case reports on the cometary plasma tail in the historical documents

Cometary tails visually manifest the solar wind and became an initial hint for its discovery. While the solar wind is being directly monitored with satellites, its time series before the space age has been controversially reconstructed with multiple proxies. Recently, archival reports of cometary plasma tails have been subjected to consideration to indirectly measure the solar wind but brought conclusion that no plasma tails had been reported prior to 1769 probably due to their brightness. However, historical records have occasionally reported comets with two tails even before 1769. These cases have been tentatively associated with visual reports of cometary plasma and dust tails. Therefore, we examined three such cases (C/1577 V1, 1P/837, and 1P/760), and compared the descriptions in historical records with calculated direction of their plasma tails. Our comparisons show that the records and calculations agree in these cases and plasma tails were visually recorded corresponding to these three great comets. These cases certify the capability of plasma tail observations with the unaided eye even before 1769, qualitatively imply their extreme brightness, proximities with the Sun and the Earth, relative enhancements of UV radiations, and interaction of cometary neutral atmosphere with solar wind plasma and magnetic field, while the lack of their detailed length or kink hinders us from their quantitative measuring. Further investigations will likely lead to the re-discovery of even more visual evidence of cometary plasma tails and, hence, improve our understanding on past space climate