Statistical correlations between geomagnetic activity and high-latitude TIDs investigated with the Tromso Dynasode

Travelling Ionospheric Disturbances (TID) and the underlying Atmospheric Gravity Waves (AGW) have long been associated with cases of extreme geomagnetic activity. This is especially true for geomagnetic storms that have been linked to large scale TIDs (LSTID). However, in the case of mid-scale TIDs (MSTID), a clear correlation has yet to be shown with geomagnetic activity, except for isolated cases. Such a correlation has been long suspected and assumed, since the mechanism by which these waves are generated must be linked to particle precipitation and localized heating, which occurs to some extent at all levels of geomagnetic activity due to the solar wind - magnetosphere - ionosphere coupling.

Our study used 7 years of measurements from the Tromso dynasonde, specifically the electron density and ionospheric tilts height profiles. These are generally available at a 2-minute cadence and thus can be used to resolve the bulk of the gravity wave spectrum. The excellent height resolution of the data, as well as the directionality of the two ionospheric tilts allow us to quantify the TID activity using an estimator based on the power spectral density integral. This proxy for overall TID activity accounts for all waves from all sources: auroral, orographic, from tropospheric weather, etc. We then demonstrate the relative importance of auroral waves by showing that TIDs with a north-south propagating direction correlate very well with geomagnetic indices indicative of geomagnetic disturbances at high-latitude (the AE, Kp and Polar Cap Indices), while less so for the SYM-H index, which is more indicative of lower latitudes. Long-term correlations are discussed, with an emphasis on changes to the statistical distribution describing the correlation. Finally, a high-precision analysis was performed to pin-point the time-delay between geomagnetic and TID activity at Tromso. The results show a dominant population of TIDs characterized by a delay of 2-6 hours, with a secondary population with delays larger than 10 hours.