Investigation of the Occurrence Characteristics and Possible Origins of Daytime Spread F Irregularities in Low Latitude Region

A study of occurrence characteristics and morphology of daytime spread F (DSF) irregularities was conducted using ionosonde from 2013 to 2020 over low-latitude Puer station in the China sector. In this study, a day with two or more consecutive F-region diffuse echo ionograms within one hour from local sunrise to sunset at an altitude of 250 km is considered a DSF day. The results show that the DSF irregularities observed in different solar activity years have a similar seasonal distribution, manifesting as frequent occurrences in June solstice with a maximum occurrence near sunrise. Furthermore, the morphology of most DSFs on the ionograms is predominantly frequency spread F, with a few observed near sunrise as MSF and RSF. In contrast to in high solar activity years (HSAY, 2013-2015), an interesting phenomenon of DSF irregularities in low solar activity years (LSAY, 2016-2020) is presented. That is, in the late afternoon hours (15-18 LT), DSFs have an unexpectedly high occurrence rate during the December solstice. By analyzing the virtual height variations for iso-frequency plots of the DSF event occurring in the afternoon, these DSF irregularities were found to be likely associated with daytime medium-scale traveling ionospheric disturbances (MSTIDs). Compared with the observed characteristics of the nighttime spread F, the fossils of nighttime plasma bubbles irregularities might play a key role in the development of DSF occurring in the early morning. At the same time, the influence of daytime MSTIDs on DSF irregularities occurring in the late afternoon hours cannot be ignored. Equatorial Spread F have been widely known as a large-scale plasma density depletions (bubbles) over the magnetic equator. The bubbles usually decay rapidly near sunrise due to strong photoionization. Due to the rarity of daytime spread F (DSF), and the difficulty in determining when and where these daytime F-region irregularities are initially generated, their occurrence characteristics and generation mechanisms remain unresolved well. In this study, we made the first long-term continuous observations of the DSF echoes at the low-latitude Puer ionosonde station from 2013 to 2020. The observations show that DSFs occur frequently in June solstice with a maximum occurrence near sunrise, and mostly appear on the ionogram in frequency spread F morphology. An interesting phenomenon of DSF irregularities in low solar activity years (LSAY) is that an unexpectedly high occurrence rate in the late afternoon hours (15-18 LT). The research results indicate that these DSFs could be associated with daytime medium-scale traveling ionospheric disturbances (MSTIDs). Our results indicate that the DSF irregularities in the low-latitude F region were not totally the continuations of nighttime plasma bubbles. The daytime MSTID is also responsible for the effects of DSFs that appear in the late afternoon hours during LSAY. DSFs show similar seasonal patterns in different solar activities: a high occurrence during the June solstice, peaking around sunriseThe morphology of most DSFs on the ionograms is mainly frequency spread F, with a few observed near sunrise as mix spread F (MSF) and range spread F (RSF) typesThe surprisingly high DSF occurrences at 15-18 LT during low solar activity years may be associated with daytime medium-scale traveling ionospheric disturbances