Langmuir probe-based lunar near-surface plasma measurements associated with the unique hop test of Chandrayaan 3 lander: observations and model simulations

ABSTRACT The study of lunar near-surface electron density provides crucial insights into the Moon’s space environment and its interaction with solar wind. This work discusses the changes in the lunar near-surface plasma environment when the Moon is in the geomagnetic tail just prior to, during, immediately after, and 24 h after the hop (in terminator and outside the geotail) of Chandrayaan 3 lander. The results obtained from the Langmuir probe measurements along with model simulations, highlight the variations in electron density (N$_e$) from $\approx$478 cc−1 before hop, to $\approx$26 cc−1 during and immediately after hop as well as $\approx$1100 cc−1 at the terminator. Substantial reduction in ambient ionization during hop is modelled in terms of the lander firing-induced hot exhaust and consequent movement of neutral species away from the lander vicinity and ensuing reduced refilling. In the terminator, the (N$_e$) increases to $\approx$1100 cc−1 due to the combined effects of increased neutral densities and consequent increased production. The mean electron temperatures (T$_e$) obtained during the sweep mode operation just prior to hop and in terminator are $\approx$2600 K and $\approx$3300 K, respectively. The Lunar Ionospheric Model is used to bring out the role of photoionization of molecular species along with solar wind proton/magnetospheric proton charge exchange processes and solar wind pickup/geomagnetic field-related removal of ions in modulating the lunar ionosphere during the varying phases. Thus, the hop experiment enabled the study of the response of the lunar ionosphere to impulsive changes in the ambient atmosphere due to man-made (lander operation) and natural causes.