Seasonal variation of HF radar F region echo occurrence in the midnight sector

Abstract

Long‐term data (1996–2001) for a number of Super Dual Auroral Radar Network (SuperDARN) HF radars in both the Northern and Southern Hemispheres are used to study the midnight F region echo occurrence. We confirm the previously reported increase of echo occurrence toward the solar cycle maximum for all radars considered and a clear winter maximum for some of them. The echo occurrence rate experiences clear equinoctial maxima for many radar locations, especially at higher latitudes and in Antarctica. We attribute the solar cycle echo increase in the midnight sector to the more frequent occurrence of enhanced electric fields and strong plasma density gradients. The equinoctial maxima are believed to be controlled entirely by the electric field increase due both to the Russell‐McPherron effect and to differences in conjugate ionospheric conductances controlled by the tilt of the Earth's axis. For the low geographic latitude portion of the Saskatoon radar observations, the echo statistics differ from the other radars; there is a clear summer maximum in echo occurrence and no definite signature of equinoctial maxima. A summer maximum in low‐latitude echo occurrence also is observed by the Hankasalmi radar during the solar cycle minima. The effect is attributed to improved propagation conditions for HF radio waves during summer periods for the latitudes where, for other seasons, there is a deficiency in the electron density.

Variations in the occurrence of SuperDARN F region echoes

“…Our results imply that during the solar cycle maximum there are additional factors (besides transition from sunlit to dark ionosphere) controlling the onset of F region echoes. Koustov et al (2004) argued that increases in the occurrence of strong plasma gradients and electric fields are potentially two additional factors for nighttime observations. Unfortunately, there is no obvious way to check the validity of this hypothesis although there are several indirect arguments.…”

“…They also noticed a separate region of enhanced echo detection in the afternoon-dusk sector and related those to the sectors of favorable conditions for the gradient drift instability operation in the ionospheric trough which is highly expected at these latitudes and during these hours. Koustov et al (2004) considered SAS observations near midnight for years 1996-2001 (i.e., from the minimum to the maximum of solar cycle 23). The paper focused on two discrete bands of magnetic latitude, 70.5 • and 75.5 • .…”

“…It has been reported that solar illumination conditions (Bristow et al, 2011), interplanetary magnetic field (IMF) magnitude and orientation (Ballatore et al, 2000), occurrence of magnetic storms (Kumar et al, 2011), and substorms (Wild and Grocott, 2008) affect echo detection. The characteristics of the variations seem to be somewhat different in the northern and southern hemispheres; for example, the southern hemisphere radars show much stronger equinoctial maxima (Koustov et al, 2004). Changes in propagation conditions for radio waves to reach ionospheric irregularities and irregularity production factors have been discussed as reasons for variations in echo occurrence.…”

“…Only ionospheric echoes stronger than 3 dB were counted. Our approach is similar to that of Koustov et al (2004). For presentation, obtained occurrence rate values were further averaged over 1 h intervals of each month.…”

“…An abundance of such echoes is a critical factor for producing reliable convection maps. It has been established that the rate of echo detection with the SuperDARN radars varies with radar location, time of day, season and solar cycle (e.g., Milan et al, 1997;Ruohoniemi and Greenwald, 1997;Koustov et al, 2004;Kane et al, 2012). It has been reported that solar illumination conditions (Bristow et al, 2011), interplanetary magnetic field (IMF) magnitude and orientation (Ballatore et al, 2000), occurrence of magnetic storms (Kumar et al, 2011), and substorms (Wild and Grocott, 2008) affect echo detection.…”