Observations of Strong Range Spread F and Nighttime Ionospheric Enhancement in the Mid-Latitude Region Under Geomagnetically Quiet Conditions

Spread F is one of most widely observed nighttime ionospheric irregularities by ionosondes in the equatorial, low- and mid-latitude regions. One type of spread F, known as strong range spread F (backscatter echoes beyond the critical frequency of the F2 layer, foF2), has been observed in the equatorial and low-latitude regions but not at middle latitudes. This study reported for the first-time observations of strong range spread F at Zhangye Station (ZHY, Geographical latitude 39.4°N, longitude 100.0°E, Dip Lat 29.6°N) on 29 November 2019 (under quiescent conditions). Observations show that strong range spread F occurred at first during midnight (∼16:30 UT), accompanied with mid-latitude nighttime ionospheric enhancements in foF2 and total electron content (TEC). Then, it becomes more pronounced when ionospheric collapse occurred. The measurements from satellite revealed that there are large opposite temperature and density gradients (temperature decreased sharply from ∼3100 to ∼2500 K, however, electron density increased sharply from ∼0.3 e5 to ∼1.2 e5 cm⁻³ around Zhangye Station) during this event. Results suggest that mid-latitude strong range spread F might be attributed to temperature gradient instabilities.