In Situ Observation of a Magnetopause Indentation that Is Correspondent to Throat Aurora and Is Caused by Magnetopause Reconnection

Throat auroras have been suggested to correspond to magnetopause indentations, but how such indentations can be generated is an open question. Using coordinated magnetopause in situ and two‐dimensional auroral imaging observations, we showed that a transient magnetopause crossing observed by Time History of Events and Macroscale Interactions during Substorms near the dayside magnetopause corresponds to a throat aurora observed by Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager. The magnetopause crossing was confirmed to present a localized magnetopause indentation rather than arise from motion of the whole magnetopause. Using the minimum variance analysis and Walén test methods, we captured clear reconnection signatures at either boundary of the indentation. Also, an upstream solar wind discontinuity and a magnetosheath variation were identified to correlate well with the magnetopause indentation occurrence. These observations suggest that the magnetopause indentation was generated by reconnection and lend solid support to an assumption that throat aurora is produced by a particular reconnection that can develop inward the magnetosphere at a much‐localized region. Plain Language Summary The equatorward boundary of the discrete auroral oval near magnetic local noon is generally regarded as the open‐close field line boundary, which corresponds to the roughly smooth magnetopause boundary. Throat auroras are some north‐south aligned discrete auroral arcs attached at the equatorward edge of discrete auroral oval near local noon. Therefore, the throat auroras have been suggested to correspond to magnetopause indentations. However, no in situ observations show the existence of such indentations and how the indentations can be generated is also an open question. Taking advantage of coordinated Time History of Events and Macroscale Interactions during Substorms and Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observations, we provided multipoint in situ observations, confirming that a transient magnetopause crossing correspondent to a typical throat aurora event was a magnetopause indentation. Also, with these observations, we captured clear reconnection signatures at either boundary of the indentation. Combined with the two‐dimensional morphology of the throat aurora, such magnetopause reconnection signatures may imply that during the generation of the throat aurora there exist