A study of Traveling Ionospheric Disturbances and Atmospheric Gravity Waves using EISCAT Svalbard Radar IPY-data
Abstract. We present a statistical study of Traveling Ionospheric Disturbances (TIDs) as observed by the EISCAT Svalbard Radar (ESR) during the continuous IPY-run (March 2007–February 2008) with field-aligned measurements. We have developed a semi-automatic routine for searching and extracting Atmospheric Gravity Wave (AGW) activity. The collected data shows that AGW-TID signatures are common in the high-latitude ionosphere especially in the field-aligned ion velocity data (244 cases of AGW-TID signatures in daily records), but they can be observed also in electron density (26 cases), electron temperature (12 cases) and ion temperature (26 cases). During the IPY campaign (in solar minimum conditions) AGW-TID events appear more frequently during summer months than during the winter months. It remains still as a topic for future studies whether the observed seasonal variation is natural or caused by seasonal variation in the performance of the observational method that we use (AGW-TID signature may be more pronounced in a dense ionosphere). In our AGW-TID dataset the distribution of the oscillation periods has two peaks, one around 0.5–0.7 h and the other around 1.1–1.3 h. The diurnal occurrence rate has a deep minimum in the region of magnetic midnight, which might be partly explained by irregular auroral activity obscuring the TID signatures from our detection routines. As both the period and horizontal phase speed estimates (as derived from the classical AGW dispersion relation) show values typical both for large scale TIDs and mesoscale TIDs it is difficult to distinguish whether the generator for high-latitude AGW-TIDs resides typically in the troposphere or in the near-Earth space. The results of our statistical analysis give anyway some valuable reference information for the future efforts to learn more about the dominating TID source mechanisms in polar cap conditions, and to improve AGW simulations.