Dynamics of the Storm Time Magnetopause and Magnetosheath Boundary Layers: An MMS-THEMIS Conjunction

This letter uses simultaneous observations from Magnetosphere Multiscale (MMS) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) to address the dynamics of the magnetopause and magnetosheath boundary layers during the main phase of a storm during which the interplanetary magnetic field (IMF) reverses from south to north. Near the dawn terminator, MMS observes two boundary layers comprising open and closed field lines and containing energetic electrons and ring current oxygen. Some closed field line regions exhibit sunward convection, presenting an avenue to replenish dayside magnetic flux lost during the storm. Meanwhile, THEMIS observes two boundary layers in the pre-noon sector which strongly resemble those observed at the flank by MMS. Observations from the three THEMIS spacecraft indicate the boundary layers are still evolving several hours after the IMF has turned northward. These observations advance our knowledge of the dynamic magnetopause and magnetosheath boundary layers under the combined effects of an ongoing storm and changing IMF. Earth's magnetic field, the magnetosphere, acts as a shield protecting the near Earth environment from the solar wind. During active times, this shield can be damaged and reduced in size, but during quieter times, the shield can expand and repair itself. This letter uses observations from multiple spacecraft to better understand how the edges of our magnetosphere change during the transition from active to more quiet times. We observe the development of boundary layers at the dayside and flanks of the magnetosphere. Convection in and around these layers can contribute to the growth of the magnetosphere after it has been eroded during active times. These observations also place limits on the size of the boundary layers and how quickly they may develop. Direct evidence of sunward transport of closed magnetic flux is observed by Magnetosphere Multiscale (MMS) at the dawn flank terminator MMS and Time History of Events and Macroscale Interactions during Substorms (THEMIS) observe nearly identical boundary layers at the dawn flank and pre-noon sectors Boundary layers and sunward convecting flux tubes are populated with energetic electrons and ring current oxygen ions