Harang Discontinuity Observed by Multi-Instrument Satellites in the Topside Ionosphere During Substorms

Implied by the terminologies “Harang Reversal” and “Harang Discontinuity”, there are two significant features of the Harang region: (i) the reversal of auroral electrojets along with the underlying plasma convection flow and electric (E) fields, and (ii) the discontinuity between the electrojets/convection flows/E-fields. Even the earliest studies reported the discontinuity observed in the meridional E-field. Conversely, some of the previous studies state that convection flow and E-field reversals do not involve any physical discontinuity. We investigate these two features (i–ii) observed in five topside-ionosphere Harang scenarios. Each scenario occurred during a sequence of events that led to the onset of the substorm expansion phase, when the Harang region was newly formed. Our results show (1) the newly formed Harang region between the dusk and dawn convection cells, where one convection cell wraps around the other, (2) the zonal drift- and E-field reversals, (3) the discontinuity between the dusk and dawn convection flows and also between the reversing E-field components, and (4) the Earthward electromagnetic energy deposition locally minimizing or diminishing within the discontinuity and peaking within the reversing zonal drift and E-fields. Thus, the observed convection flow and E-field reversals involved the development of discontinuity.