Acceleration of Pick‐Up Ions in the Martian Magnetosheath: A Tianwen‐1 Case Study

This study delves into a Pick‐Up Ion (PUI) event captured by the Mars Ion and Neutral Particle Analyzer aboard Tianwen‐1, revealing a faster acceleration than expected within the Martian magnetosheath. This event suggests the presence of a convection electric field considerably stronger than that typically observed in the solar wind. Through Magnetohydrodynamic simulation, we identified two regions of intensified convection electric fields within the inner magnetosheath, prominently manifested at mid to high solar zenith angles (SZA = 40°–70°) in the X‐Z plane of the Mars Solar Electric field coordinates. Tianwen‐1 observed that the peak of the electric field strength, up to five times of that in the solar wind, is located at the upper edge or within the Magnetic Pileup Boundary. Further study by test particle simulations showed acceleration by the electric field effectively doubles the energy gain of PUIs, in comparison to scenarios absent of this extra acceleration. It is revealed that the presence of such electric field regions within the Martian magnetosheath is a prevailing feature, shaped by the solar wind's interaction with Mars and modulated by the planet's rotating crustal magnetic fields. Plain Language Summary The interaction between planets and their upstream solar wind holds great significance for the long‐time climate evolution of planets. Mars lacks a global dipole magnetic field, which makes it easier for these planetary ions to escape through acceleration by the solar wind convection electric field. The observations of Tianwen‐1 and Mars Atmosphere and Volatile Evolution satellites provide a great opportunity to study the acceleration of planetary ions by this electric field. Through a case study involving observation and simulation, we confirm that there are two regions in the Martian magnetosheath, where the convection electric field is stronger than in the solar wind. This electric field may rapidly energize the ions and possibly result in intensified ion escape. Key Points In the Martian magnetosheath, joint effects of the plasma flow around Mars and the interplanetary magnetic field pileup result in enhanced convection electric field The strong E field envelops a large part of the Magnetic Pileup Boundary (MPB), with the maximum at the med‐high solar zenith angles, at the upper edge or within the MPB The crustal magnetic fields modulate the distribution of the strong E field, leading to hemispheric asymmetry