Ion Velocity and Temperature Variation Around Topside Nighttime Irregularities: Contrast Between Low- and Mid-Latitude Regions

Studies on low-/mid-latitude ionospheric irregularities have a long history, but ion temperature and drift velocity variations within the irregularities have been rarely investigated, especially at mid-latitudes. To fill this knowledge gap, we statistically investigate ion temperature and velocity of mid-latitude ionospheric irregularities and compare them to those of low-latitude ones. This study mainly relies on in situ plasma moment data from the newly launched Ionospheric Connection Explorer (ICON) satellite and is supplemented by Republic of China Satellite 1 (ROCSAT-1) data in 2000–2004. At low latitudes (|MLAT|<10°) plasma density irregularities encountered by ICON have the following properties: lower-density regions have cooler ions and more poleward/westward/outward velocity than the ambient plasma, which agrees with previous reports on equatorial plasma bubbles. At mid-latitudes (|MLAT| = 10°∼30°), the opposite trends are observed: lower-density regions have hotter ions and more equatorward/eastward/inward drift than the ambient, which is a new finding in this study. The mid-latitude observations can be put into the context of Medium-Scale Traveling Ionospheric Disturbance (MSTID) properties reported previously, which evidences that topside irregularities at nighttime mid-latitudes generally represent MSTIDs. ROCSAT-1 data support the contrast between low- and mid-latitude ionospheric irregularities.