AKR Observations From All Local Times Indicate Substorm Activity

Abstract

Auroral Kilometric Radiation (AKR) is known to present emission at lower frequencies near substorm onset, corresponding to activation of higher altitude sources along auroral magnetic field lines and acting as a proxy of the vertical distribution of acceleration processes during these events. While remote observations are more difficult to relate to ionospheric activity than those made in situ due to the illumination of the instrument by multiple AKR sources, there is a question of how the association of a particular observation with substorm activity can change with observer local time (LT) and more generally whether AKR observations can track geomagnetic activity in the same way as other proxies. This study addresses these questions by computing binary contingency tables and classification statistics including the Matthews Correlation Coefficient (MCC), using automatically selected AKR observations from Wind/WAVES and substorm event lists. AKR observations made on the dusk and dawn flanks present the greatest balance between true positive and negative associations with substorm activity, where active acceleration regions due to the enhanced substorm current wedge are distinct from typical nightside activity. Observations from the dayside and L1 can also be a reliable indicator, with AKR rarely observed from this region outside of substorm activity. Our results show that observations of AKR below 70 kHz (corresponding to active source regions above $\sim$12,000 km at an L-shell of 7) can act as a good discriminator of substorm activity, particularly for observations from LTs near midnight.