Motion and origin of noon high‐latitude poleward moving auroral arcs on closed magnetic field lines

Abstract

We present observations of the near‐noon high‐latitude poleward moving auroral arcs, which show essential differences from well‐known poleward moving auroral forms associated with reconnection and flux transfer events. We suggest that the observed arcs are a specific class of dayside auroras referred to in the paper as poleward moving auroral arcs. The arcs were investigated together with the ionospheric plasma flows derived from the European Incoherent Scatter VHF measurements over Svalbard. The auroral arc motion was monitored at 1000–1300 magnetic local time by the all‐sky camera on 9 December 1998. It has been found that the noon auroral arcs move poleward at the velocity of the order of 150–350 m s−1, and this velocity does not show any dependence on the velocity of the ionospheric plasma convection along the same direction. The arcs appear at 10–20 min after strong changes (up to 3 km s−1) in the east‐west plasma flow, which resulted from variations in the interplanetary magnetic field. Ionospheric plasma drift and ground magnetometer data show the location of the auroras equatorward of the convection reversal. Spectrums of the northward electric field variations measured by the radar demonstrate clear peaks corresponding to the magnetic field line eigenfrequency oscillations with amplitude on the order of 10 mV m−1. The period of the oscillations increases from 7 min at 74.3° magnetic latitude (MLAT) to 14 min at 76.4° MLAT, and corresponding spectral characteristics were observed in the magnetic field measured on the ground. The observed features allow us to suggest that the noon auroral arcs arise on closed magnetic field lines as a result of interference between Alfvén field line eigenmode toroidal oscillations on different L shells. The field line resonance oscillations at the near‐cusp L shells are excited by the Alfvén impulse associated with the convection disturbance following after variations in interplanetary parameters.