On the dynamic cusp aurora and IMF By

Abstract

Using a new form of presentation of optical data from meridian scanning photometers, we demonstrate two important aspects of cusp dynamics concerning transients called midday auroral breakups or poleward moving auroral forms (PMAFs). Many previous studies have associated PMAFs with pulsed (in time) reconnection at the magnetopause, where these transients are thought to represent the auroral footprint of reconnected magnetic flux tubes (flux transfer events) and their motion is thought to reflect the motion of the corresponding flux tubes. Some studies have indicated that there is a prenoon‐postnoon asymmetry in the occurrence and motion pattern of PMAFs related to the east‐west component of the interplanetary magnetic field (IMF) By. In two case studies we point out the strong modulation in intensity that exists between the PMAF and the surrounding cusp emission, evident in both the red and green lines. We then show cases where the local occurrence of PMAFs is dependent on IMF By in the sense that PMAF activity at ∼1100 magnetic local time ceases when IMF By changes from positive to negative values. This is explained by reconfigurations of the magnetopause reconnection geometry and ionospheric convection pattern causing PMAFs to disappear from the field of view of the optical instruments, an interpretation which is supported by local ground magnetic field observations.