High-Density Magnetospheric He+ at the Dayside Magnetopause and Its Effect on Magnetic Reconnection.

Observations from the Magnetospheric Multiscale (MMS) mission are used to quantify the maximum effect of magnetospheric H⁺ and He⁺ on dayside magnetopause reconnection. A data base of current-sheet crossings from the first 2 years of the MMS mission is used to identify magnetopause crossings with the highest He⁺ concentrations. While all of these magnetopause crossings exhibit evidence of plasmaspheric plume material, only half of the crossings are directly associated with plasmaspheric plumes. The He⁺ density varies dramatically within the magnetosphere adjacent to the magnetopause, with density variations of an order of magnitude on timescales as short as 10 s, the time resolution of the composition instrument on MMS. Plasma wave observations are used to determine the total electron density, and composition measurements are used to determine the mass density in the magnetosheath and magnetosphere. These mass densities are then used with the magnetic field observations to determine the theoretical reduction in the reconnection rate at the magnetopause. The presence of high-density plasmaspheric plume material at the magnetopause causes transient reductions in the reconnection rate of up to -40%. Plain Language Summary As the solar wind propagates from the Sun to the Earth, it encounters two boundaries that limit its access to the near-Earth environment. The first is a bow shock that heats, slows, and deflects the solar wind. The second is the Earth's magnetopause, where the shocked solar wind is deflected around the Earth's magnetic field region called the magnetosphere. Magnetic reconnection at the magnetopause creates an interconnection between the magnetic field of the shocked solar wind and the Earth's magnetic field. The rate at which these magnetic fields interconnect, the reconnection rate, depends on the amount of plasma (ions and electrons) on either side of the magnetopause. This research uses observations from the Magnetospheric Multiscale mission to look at this rate for times when there is substantial plasma on the magnetospheric side of the magnetopause. These instances are rare; however, when they do occur, they reduce the reconnection rate by a substantial amount (up to 40%). [ABSTRACT FROM AUTHOR]