Seasonal Variation of F-region Echo Occurrence in the Midnight Sector

In this study we consider long-term data (1995-2001) in the midnight sector for the Saskatoon (Canada), Hankasalmi (Finland) and Halley (Antarctica) SuperDARN radars to reveal the seasonal effect upon echo occurrence. The selected radars monitor scatter at about the same magnetic latitudes but significantly different geographic latitudes. The seasonal variations vary between these radars and slightly change with the phase of the solar cycle. The Halley radar clearly shows maxima during equinoxes at solar activity minimum (geomagnetic latitudes ~75-80 deg). As the solar activity increases, progressively more echoes are observed during the austral winter, especially at geomagnetic latitudes of ~70 deg. The Finland radar, whose location allows it to monitor scatter ~5 deg geographically more equatorward than Halley, shows similar features except for more pronounced winter echo occurrence. This radar shows little seasonal effect at low latitude of ~65 deg. The Saskatoon radar can observe echoes at even lower geographic latitudes. It measures two equinox maxima at high latitudes and a summer maximum at magnetic latitudes of 65-70 deg during the solar minimum. This maximum changes to a more uniform distribution at the solar maximum. The reasons for the observed differences are discussed in terms of solar illumination conditions, enhanced electric fields controlled by the magnetosphere-solar wind interaction and propagation conditions for radio waves.