HF radar observations of ionospheric backscatter during geomagnetically quiet periods

Abstract

Abstract. The quiet-time coherent backscatter from the Fregion observed by the Tasman International Geospace Environment Radar (TIGER) Bruny Island HF radar is analysed statistically in order to determine typical trends and controlling factors in the ionospheric echo occurrence. A comparison of the F-region peak density values from the IRI-2007 model and ionosonde measurements in the vicinity of the radar's footprint shows a very good agreement, particularly at subauroral and auroral latitudes, and model densities within the radar's footprint are used in the following analyses. The occurrence of F-region backscatter is shown to exhibit distinct diurnal, seasonal and solar cycle variations and these are compared with model trends in the F-region peak electron density and Pedersen conductance of the underlying ionosphere. The solar cycle effects in occurrence are demonstrated to be strong and more complex than a simple proportionality on a year-to-year basis. The diurnal and seasonal effects are strongly coupled to each other, with diurnal trends exhibiting a systematic gradual variation from month to month that can be explained when both electron density and conductance trends are considered. During the night, the echo occurrence is suggested to be controlled directly by the density conditions, with a direct proportionality observed between the occurrence and peak electron density. During the day, the echo occurrence appears to be controlled by both conductance and propagation conditions. It is shown that the range of echo occurrence values is smaller for larger conductances and that the electron density determines what value the echo occurrence takes in that range. These results suggest that the irregularity production rates are significantly reduced by the highly conducting E layer during the day while F-region density effects dominate during the night.

Variations in the occurrence of SuperDARN F region echoes

“…The 300 km sunset line does not indicate a significant change in echo occurrence during nighttime of [2001][2002], from the fall equinox to winter and to spring equinox. This result is consistent with that of Kane et al (2012). For other two winter times of Fig.…”

“…The enhanced electron density is required to meet orthogonality condition. The density effect in occurrence of HF echoes was well demonstrated by Kane et al (2012) for the TIG radar. The authors also explained that enhanced echo detection in the post noon to midnight sectors is maintained because F region density decreases slowly while the E region density decays quickly (which removes the shortening effect on the polarization electric field on the gradient-drift instability in the F region, see e.g., Tsunoda, 1988).…”

“…Recently, Kane et al (2012) investigated the TIG (Bruny Island) SuperDARN radar data in the southern hemisphere and identified a pattern of enhanced F region echo occurrence on the month-MLAT plane. The pattern (that they termed "butterfly") is very similar to the pattern we report here for the SAS radar with an offset in appearance because the first month of the year is summer in the southern hemisphere while it is winter in the northern hemisphere.…”

“…For periods of solar maximum, Kane et al (2012) noticed that the terminator line is not really a special line on the occurrence plots as echoes are equally frequently detected well prior and well after the sunset. This is despite the fact that only quiet days were selected in their paper.…”

“…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.…”