The Day the Solar Wind Disappeared at Mars

In December 2022, an extremely low-density solar wind stream encountered first the Earth and then Mars, shortly after the radial alignment of the two planets (i.e., Mars opposition). As the low-density stream passed Mars, the properties of the Mars-solar wind interaction changed dramatically in response to the low solar wind Alfvenic Mach number and dynamic pressure. The Martian magnetosphere and its boundaries expanded by thousands of kilometers, extending outside of the nominal average bow shock location. The low upstream Mach number resulted in a low ratio of plasma to magnetic field pressure in the magnetosheath, allowing the formation of a very high but stable ion temperature anisotropy, together with a very low level of electromagnetic instabilities. Meanwhile, the decrease in solar wind dynamic pressure caused the Martian upper ionosphere at the terminator to transition from a magnetized state to an unmagnetized state. This event provides an opportunity to study a unique end-member state of the Mars-solar wind interaction. The solar wind consists of a supersonic flow of hot ionized gas (plasma) from the Sun that flows outward through the solar system. The solar wind interacts with planetary atmospheres and magnetic fields, which form shielded regions known as magnetospheres. The characteristics of the solar wind, including its density and speed, vary over a wide range. The shape, extent, and physical characteristics of planetary magnetospheres in turn respond to these changes in the solar wind. In this work, we study the response of the Martian magnetosphere to an extremely low-density solar wind stream. During the passage of this stream, the Martian magnetosphere expanded by thousands of kilometers, and its physical characteristics changed dramatically. This event reveals a unique outlier state for the Mars-solar wind interaction. An extremely low-density solar wind stream encountered the Earth and Mars just after Mars opposition in December 2022The Martian magnetosheath developed very unusual properties, with a low level of plasma instabilities and a high ion anisotropyThe Martian magnetosphere and its boundaries expanded by thousands of km, extending outside of the location of the nominal bow shock