Altitudinal Variation of <i>O</i>^<i>+</i> Scale Height at the Equatorial Topside Ionosphere

Abstract Altitude variation of the topside ionospheric electron density or its scale height has been widely investigated in the past. However, as the oxygen ion ( O + ) is an important indicator for separating the topside ionosphere and plasmasphere, the altitude variation of O + , which has not been well investigated, is crucial to understand the topside ionosphere. In this study, we provided analysis on how the O + scale height varies with altitude under different solar and geomagnetic activities, by using 12‐year measurements from the incoherent scatter radar (ISR) located at Jicamarca. Constant scale height Chapman (CSC) as well as Linearly Varying Chapman (LVC) functions are used to reconstruct the O + profile. The corresponding scale heights of O + based on both approaches have been compared. The O + profile derived from LVC function shows better agreement with the ISR measurements than that from CSC function. We found that the O + scale height increases with increasing solar/geomagnetic activity, and its height gradient varies significantly with local time, reaching a maximum of 0.05 at sunrise (around 06:00 local time) and a minimum of about −0.08 at noon (around 12:00 LT). We further investigated possible drivers causing the O + scale height variations at topside ionosphere, based on simulations from the SAMI2 physics‐based model. The model results show that the solar extreme ultra‐violet (EUV) radiation plays a key role in the positive gradient of O + scale height observed around sunrise, while the vertical plasma drift caused by E × B significantly contributes to the negative gradient observed around noon.