Mesoscale ionospheric anomalies not associated with space weather

Abstract

Using the Austrian network of 10 two‐frequency GPS geodetic reference stations, whose location is known with high precision, a study of mesoscale anomalies in the ionosphere over central Europe has been made (and is still in progress for the following years). From the large number of individual observations during the period from 2000 to 2004, maps of small‐scale ionospheric behavior (with a pixel size of 0.5° in latitude and longitude) have been constructed from which local anomalies (at an equivalent height of 250 km) in terms of departure from the model total electron content (TEC) can be recognized. Not all magnetic storms which were most common during the 2000–2002 period of solar maximum showed local effects in the regional ionosphere, but mesoscale ionospheric anomalies which are not (necessarily) associated with space weather were also observed up to 20% of the days of observation, with their highest frequency of occurrence during the winter months (November–February). These anomalies have a typical duration of several hours and seem to be associated with traveling ionospheric disturbances (TIDs) whose origin could lie in gravity waves initiated in the lower atmosphere. Thus TIDs not associated with space weather may play a role in the regional ionosphere but can be detected only by virtue of the dense spatial coverage obtained by a network of GPS stations for precise positioning. From 2000 to 2004, the solar activity and the background ionization decreased considerably. Since TIDs depend on both the properties of atmospheric gravity waves and on the level of ionization, we observed smaller absolute TID amplitudes in 2003–2004 compared with 2000–2001. There is indication that the relative amplitudes have increased slightly, which is consistent with the influence of ion drag on thermosphere dynamics.