Multiple, discrete arcs on sunward convecting field lines in the 14‐15 MLT region

Ionospheric plasma flow measurements and simultaneous observations of thin (∼0.2° invariant latitude (ILAT)), multiple, longitudinally extended auroral arcs of transient nature within 74°‐76° ILAT and 1030‐1130 UT (∼14‐15 MLT) on January 12, 1989, are reported. The auroral structures appeared within the luminous belt of strong 630.0‐nm emissions located predominantly on sunward convecting field lines equatorward of the convection reversal boundary as identified by the European Incoherent Scatter UHF radar. The events occurred during a period of several hours quasi‐steady solar wind speed (∼ 700 km s −1 ) and a radially orientated interplanetary magnetic field (IMF) with a weak northward tilt (IMF B z >0). These typical dayside auroral features are related to previous studies of auroral activity related to the upward region 1 current in the postnoon sector. The discrete auroral events presented here may result from magnetosheath plasma injections into the low‐latitude boundary layer (LLBL) and an associated dynamo mechanism. An alternative explanation invokes kinetic Alfvén waves, triggered either by Kelvin‐Helmholtz instability at the inner (or outer) edge of the LLBL or by pressure pulse induced magnetopause surface waves.