A global study of ionosheric winter anomaly rate of occurrence using GPS-TEC from the last two solar cycles

AZPILICUETA FRANCISCO; NAVA BRUNO

Buenos Aires

Workshop; COLAGE XI - 2018; 2018

COLAGE

Since the first report of it existence by Berkner and colleagues in the 30s, the winter anomaly (or non-seasonal variation as named by Berkner) has been subject of continue study and modeling. As it is the case with other ionospheric phenomena (e.g.  the semi-annual anomaly) the aeronomic community has not reached to a complete formulation of the physics involved in this phenomenon. Even there isn't a clear definition of the anomaly in terms of the effects expected to observe in the presence of the winter anomaly. The criterion used for this work was that the winter anomaly is present when the maximum NmF2 (or equivalently the TEC - Total Electron Content) value during the local winter is higher than the corresponding value during local summer over a single station. This definition highlights the anomalous characteristic since local summer values would be expected higher than winters.For this contribution we have processed the complete GPS data files from set of IGS stations (International GNSS Service) with LPIM software, resulting on a database of  TEC(Total Electron Content) for almost the last two solar cycles. The chosen stations provide a good coverage in latitude and reasonable good in longitude. Comparing the results obtained for the winter/summer season, we were able to identify the presence of the winter anomaly and to produce a latitude-year scheme covering solar cycles 23 and 24.The main conclusions of the work can be summarized as follows: 1) the winter anomaly was found to occur in several northern hemisphere and southern equatorial stations, only one station at southern mid-latitude shows signs of winter anomaly; 2) the rate of occurrence during cycle 23 was significantly higher than during cycle 24, confirming the relation between the rate with the solar ionization level (cycle 23 level was approx. 40% higher than cycle 24); 3) for non equatorial stations the winter anomaly is absent for low solar activity years. These results are in agreement with several results already published and supports the idea the phenomenon is more likely a non-seasonal (annual anomaly) rather than a seasonal anomaly.