The Evolution of Parallel Electron Temperature in Magnetospheric Reconnection Inflows.

Using data from NASA's Magnetospheric Multiscale mission captured in a reconnection inflow on the magnetospheric side of Earth's dayside magnetopause, we find a region where the heat flux density gradient term balances the parallel compression term in the electron parallel temperature equation. Combining these observations with analysis of the generalized fluid equations indicates that such a behavior represents a quasi‐isothermal region, where cold magnetosheath beams that have transported across the magnetopause introduce non‐zero gradients in parallel heat flux density. This region should prevail near dayside reconnection X‐lines in inflows on the magnetospheric side due to the formation of mixed electron distributions and increased parallel temperatures that arise from three‐dimensional boundary dynamics. Plain Language Summary: The equation that describes how electron temperature evolves with space and time includes several terms that account for the mixing of different plasmas across a boundary. NASA's Magnetospheric Multiscale mission can measure these terms and has found that under certain circumstances, the terms in the equations balance to result in a more constant electron temperature than expected. This phenomena should be common near similar interfaces and requires a three‐dimensional model of the boundary physics to explain the observed behavior. Key Points: The transport of cold magnetosheath electrons across the magnetopause impacts the dominant terms in the electron temperature equationParallel heat flux density gradient terms can balance those of parallel bulk velocity gradientsThree‐dimensional boundary effects are key to understanding electron properties in the magnetospheric inflows [ABSTRACT FROM AUTHOR]