Source of field‐aligned irregularities in the subauroral F region as observed by the SuperDARN radars

Abstract

Abstract. The HF radars of the Super Dual Auroral Radar Network (SuperDARN)observe the E x B convective drift, of ionospheric plasma when suitable small-scale field-Migned irregularities (FAI) from which to scatter are present. In order to estimate the distribution of FAIs in the subauroral F region, we have investigated the scattering occurrence percentage using data from six Northern Hemisphere radars during 39 months between July 1995 and September 1998. We have ident, ified a morphological tbature known as the dusk scatter event (DUSE; first reported by Ruohoniemi et al. [1988]) in detail and clarified its relation to the boundaries of the plasma density within the subm•roral F region. In all mont, hs, the DUSE appears within a fbw hours local time on the eveningside of sunset, where the magnetic latitude is slightly lower than the equatorward edge of the m•roral oval, which corresponds to the density-depleted structure known as the midlatitude trough. There exists a clear UT effect in the characteristics of DUSE, such that DUSE is more pronounced tbr UTs corresponding to dusk on the prime magnetic meridian (0 ø geomagnetic longitude). Furthermore, an enhancement of scattering occurrence around sunrise within t, he trough, which is termed dawn scatter event, (DASE), was newly identified. The region where the DUSE or DASE occurs has a close relationship with the duskside and dawnside ends of the midlatitude trough in the longitudinal direction (which are termed the sunward edges of the trough in this paper), where sunward directed steep plasma density gradients exist. The Kp dependence of the scattering occurrence was also examined. In disturbed conditions, the DUSE appee[rs at, earlier local times compared with that in quiet conditions, which is consistent with the Kp dependence of the sunward edge of the trough. Three models based on the gradient drift, instability were discussed. It, was tbund that the model which is based on the sunward plasma densit, y gradient at the sunward edges of the midlatitude trough and ambient electric field is most favorable tbr the growth of FAIs in this region and can consistently explain the statistical tbat, ures of DUSE and DASE. These facts indicate that we can est, imat, e t, he longitudinal extent of the midlatitude trough fi'om the appearance of DUSE and DASE observed by SuperDARN. In this study we exten(t the work of Ruoh, on'ie'mi et al.

On the factors controlling occurrence of F-region coherent echoes

“…Milan et al (1997) clearly demonstrated that Hankasalmi daytime echoes are typically cusp/cleft echoes obtained through this mode, and other reports (e.g. Ruohoniemi and Greenwald, 1997;Hosokawa et al, 2001) support this conclusion.…”

“…3, indeed echoes started to be observed more frequently right after the local sunset for the F-region, so the horizontal plasma gradient associated with the trough might have contributed well to the preferential irregularity formation during these hours. In addition, Hosokawa et al (2001) did find a second morning peak in echo occurrence for the Canadian radars, but not for the Hankasalmi radar. In our event, quite a few echoes were observed during morning hours on 10 and 11 February and perhaps on 9 February (EISCAT measurements are not available here, but Sodankylä ionosonde data support this conclusion) prior to local sunrise.…”

“…Over the last decade, statistics for various parameters of Fregion HF coherent echoes have been reported, including power, spectral width and velocity (Milan et al, 1997;Milan and Lester, 1998;Huber, 1999;Fukumoto et al, 1999Fukumoto et al, , 2000Hosokawa et al, 2001Hosokawa et al, , 2002. However, not much attention has been paid to a fundamental question on the major factor(s) controlling the appearance of such echoes.…”

“…One should mention that Hosokawa et al (2001) found quite a few afternoon/evening echoes being produced at the poleward edge of the mid-latitude ionospheric trough. As we mentioned in Sect.…”

Variations in the occurrence of SuperDARN F region echoes

“…Huber (1999) explored Saskatoon radar data for 1997 and reported that wintertime F region echoes are mostly detected in two time sectors: near noon and later in the afternoon close to the dusk sector. Hosokawa et al (2001) merged SAS data with data from other Canadian radars for periods of solar minimum and showed that winter and summer time echoes are more frequent on the dayside and nightside, respectively. 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.…”