AKR Observations From All Local Times Indicate Substorm Activity

Overview & Synopsis

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 ∼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.Plain Language Summary Auroral Kilometric Radiation (AKR) is terrestrial radio emission that is produced along magnetic field lines above the auroral oval. It coincides with acceleration of electrons into the upper atmosphere and is colocated with bright, discrete optical and UV aurora. Substorms, a global process of energy transfer in the terrestrial magnetosphere, are related to the production of low frequency AKR. Due to the beaming pattern of AKR sources the spacecraft position must be considered when relating remote observations to magnetospheric processes. We address the question of how remote AKR observations from different positions can be associated with substorms. Through binary classification analyses of AKR observations and substorm event lists from 1995 to 2005, we show that observations from all local times can indicate substorm activity. Observations made on the flanks of the magnetosphere show positive correlation, with AKR occurring frequently with events due to morphological changes in the ionosphere during substorms. Observations near noon have a high probability of coinciding with extended auroral features during substorms and can also be a reliable indicator. While midnight observations have weaker correlation due to the spatial distribution of typical auroral activity, but low frequencies improve the correlation.