14:45 〜 15:00
[PEM15-11] Ionosphere-thermosphere coupling effects due to geomagnetic storms
キーワード：ionosphere, thermosphere, geomagnetic storm, coupling, solar wind high-speed stream, EISCAT
Geomagnetic storms can be caused by different solar drivers including interplanetary coronal mass ejections (ICMEs) as well as solar wind high-speed streams (HSSs) and associated stream interaction regions (SIRs). Significant changes to the ionosphere-thermosphere (I-T) system can be caused even by moderate storms with the Dst index ranging between -100 and -50 nT. In this presentation, we will review some of the effects caused by a long-lasting moderate geomagnetic storm driven by an HSS/SIR and observed by the meridian scanning EISCAT Tromsø and Svalbard incoherent (IS) radars, global total electron content (TEC) measurements by GNSS satellites, and supplementary instruments including SuperDARN and AMPERE. The presentation underlines the need to carry out multi-instrument studies to understand I-T coupling.
Storm effects include significant decreases in F-region electron density (Ne) and TEC over a large latitude range in the afternoon and evening sector, and narrower high-latitude decreases in the morning to pre-noon sector. Increases are also found, and they are associated with particle precipitation and polar cap patches. About half of the decreases at high latitudes are associated with enhanced ion temperatures as observed by the EISCAT radars, indicating local ion-neutral frictional heating source by strong electric fields. Ion-neutral frictional heating speeds up the loss of atomic O+ and the production of molecular NO+ ions, which then subsequently recombine with electrons. The overall effect is to decrease electron density. An additional interesting point is that high-latitude F-region decreases seem to favour large-scale downward current regions (R2/R1/R0).
We also observe decreases in Ne and TEC at mid-latitudes (sometimes called the storm negative phase), even though no local ion-neutral frictional heating (aka Joule heating) takes place. A plausible explanation is that the Joule heating in the high-latitude E region lifts the neutral atmosphere up and increases the content of molecules (N2 and O2) in the F region, which is one factor to speed up the loss of O+. The molecular-rich air can be transported to lower latitudes by winds, where subsequently the recombination rate is increased. Indeed, the TIMED/GUVI instrument shows indication of such composition changes.
A strong positive phase of the storm with increased TEC is observed in the beginning of the main phase at mid-latitudes. The measurements by the Millstone Hill IS radar are used to study the changes in the mid-latitude ionosphere. Uplift of the ionosphere and arrival of traveling ionospheric disturbances (TIDs) takes place coincident with the start of the positive phase. The possible mechanisms behind the observations are discussed.
Storm effects include significant decreases in F-region electron density (Ne) and TEC over a large latitude range in the afternoon and evening sector, and narrower high-latitude decreases in the morning to pre-noon sector. Increases are also found, and they are associated with particle precipitation and polar cap patches. About half of the decreases at high latitudes are associated with enhanced ion temperatures as observed by the EISCAT radars, indicating local ion-neutral frictional heating source by strong electric fields. Ion-neutral frictional heating speeds up the loss of atomic O+ and the production of molecular NO+ ions, which then subsequently recombine with electrons. The overall effect is to decrease electron density. An additional interesting point is that high-latitude F-region decreases seem to favour large-scale downward current regions (R2/R1/R0).
We also observe decreases in Ne and TEC at mid-latitudes (sometimes called the storm negative phase), even though no local ion-neutral frictional heating (aka Joule heating) takes place. A plausible explanation is that the Joule heating in the high-latitude E region lifts the neutral atmosphere up and increases the content of molecules (N2 and O2) in the F region, which is one factor to speed up the loss of O+. The molecular-rich air can be transported to lower latitudes by winds, where subsequently the recombination rate is increased. Indeed, the TIMED/GUVI instrument shows indication of such composition changes.
A strong positive phase of the storm with increased TEC is observed in the beginning of the main phase at mid-latitudes. The measurements by the Millstone Hill IS radar are used to study the changes in the mid-latitude ionosphere. Uplift of the ionosphere and arrival of traveling ionospheric disturbances (TIDs) takes place coincident with the start of the positive phase. The possible mechanisms behind the observations are discussed.