Ionospheric longitudinal variations at midlatitudes: Incoherent scatter radar observation at Millstone Hill

Incoherent scatter radar （ISR） extra-wide coverage experiments during the period of 1978-2011 at Millstone Hill are used to investigate longitudinal differences in electron density. This work is motivated by a recent finding of the US east-west coast difference in TEC suggesting a combined effect of changing geomagnetic declination and zonal winds. The current study pro- vides strong supporting evidence of the longitudinal change and the plausible mechanism by examining the climatology of electron density Ne on both east and west sides of the radar with a longitude separation of up to 40% for different heights within 300-450 kin. Main findings include： 1） The east-west difference can be up to 60% and varies over the course of the day, being positive （East side Ne 〉 West side Ne） in the late evening, and negative （West side Ne 〉 East side Ne） in the pre-noon. 2） The east-west difference exists throughout the year. The positive （relative） difference is most pronounced in winter; the negative （relative） difference is most pronounced in early spring and later summer. 3） The east-west difference tends to enhance toward decreasing solar activity, however, with some seasonal dependence; the enhancements in the positive and negative differences do not take place simultaneously. 4） Both times of largest positive and largest negative east-west differences in Ne are earlier in summer and later in winter. The two times differ by 12-13 h, which remains constant throughout the year. 5） Variations at different heights from 300-450 km are similar. Zonal wind climatology above Millstone Hill is found to be perfectly consistent with what is expected based on the electron density difference between the east and west sides of the site. The magnetic declination-zonal wind mechanism is true for other longitude sectors as well, and may be used to understand longitudinal variations elsewhere. It may also be used to derive thermospheric zonal winds.

Variability of the Critical Frequency foF2 during Minimum and Maximum Phases of Solar Cycles 20 and 21: A Comparative Study between American and African Equatorial Regions

The present work is a comparative study between the foF2 variabilities for two equatorial regions (Ouagadougou: lat. 12°21'N;long. 1°30'E, dip. 1.43° in Africa and Huancayo: Lat. 12°S;Long. 75°12'W in America) during solar cycles 20 and 21 minima and maxima phases under geomagnetic extreme conditions (quiet and disturb). Profiles from these two stations are very similar for all the seasons over the solar cycles. However, measured data from Huancayo station are higher than those from Ouagadougou station during winter with a reverse phenomenon for summer. The investigations suggest that the gap between foF2 values and the reverse phenomenon observed for the two stations may be explained by their hemispheric location (Huancayo in south hemisphere and Ouagadougou in North one). Longitudinal irregularities in ionosphere may also contribute to that little difference observed during the time interval of our investigation.

The nighttime ionospheric response and occurrence of equatorial plasma irregularities during geomagnetic storms:a case study

Recent studies revealed that the long-lasting daytime ionospheric enhancements of Total Electron Content(TEC)were sometimes observed in the Asian sector during the recovery phase of geomagnetic storms e.g.,Lei(J Geophys Res Space Phys 123:3217-3232,2018),Li(J Geophys Res Space Phys 125:e2020JA028238,2020).However,they focused only on the dayside ionosphere,and no dedicated studies have been performed to investigate the nighttime ionospheric behavior during such kinds of storm recovery phases.In this study,we focused on two geomagnetic storms that happened on 7-8 September 2017 and 25-26 August 2018,which showed the prominent daytime TEC enhancements in the Asian sector during their recovery phases,to explore the nighttime large-scale ionospheric responses as well as the small-scale Equatorial Plasma Irregularities(EPIs).It is found that during the September 2017 storm recovery phase,the nighttime ionosphere in the American sector is largely depressed,which is similar to the daytime ionospheric response in the same longitude sector;while in the Asian sector,only a small TEC increase is observed at nighttime,which is much weaker than the prominent daytime TEC enhancement in this longitude sector.During the recovery phase of the August 2018 storm,a slight TEC increase is observed on the night side at all longitudes,which is also weaker than the prominent daytime TEC enhancement.For the small-scale EPIs,they are enhanced and extended to higher latitudes during the main phase of both storms.However,during the recovery phases of the first storm,the EPIs are largely enhanced and suppressed in the Asian and American sectors,respectively,while no prominent nighttime EPIs are observed during the second storm recovery phase.The clear north-south asymmetry of equatorial ionization anomaly crests during the second storm should be responsible for the suppression of EPIs during this storm.In addition,our results also suggest that the dusk side ionospheric response could be affected by the daytime ionospheric plasma density/TEC variations during the recovery phase of geomagnetic storms,which further modulates the vertical plasma drift and plasma gradient.As a result,the growth rate of post-sunset EPIs will be enhanced or inhibited.