Frequency Extensions of Auroral Kilometric Radiation: The Key to Unlocking Earth's Radio Response to Substorms

Abstract Auroral kilometric radiation (AKR) provides a sensitive probe of magnetospheric dynamics during substorms. We analyze an extended data set of AKR bursts observed by the WIND spacecraft to investigate how the frequency structure of AKR responds to substorm onset and evolution. A new adaptive method is introduced to identify low and high frequency extensions (LFEs and HFEs) of AKR by defining dynamic frequency thresholds for each substorm individually. Using substorm onset lists derived from both auroral imagery and ground‐based magnetometers, we examine how occurrence rates and integrated power contributions of LFEs and HFEs depend on substorm epoch and magnetic local time (MLT). LFEs and HFEs are most frequently observed near the average substorm onset location (≈23 hr MLT) and within minutes of substorm onset. LFEs show the clearest enhancement, contributing a greater fraction of the total AKR power post‐onset and in the pre‐midnight sector, suggesting that they represent a distinct substorm‐driven component of AKR. In contrast, HFEs show weaker and less systematic responses, with variability consistent with the main AKR band, suggesting that HFEs are an extension of the main band AKR rather than a separate population. Finally, substorm onset times derived from auroral imagery yield a sharper AKR response than those from ground magnetometer lists, underscoring the stronger physical link between aurora and AKR and highlighting limitations of ground magnetometer‐based onset identification. These results demonstrate that frequency extensions of AKR can be used to improve substorm identification.