Solar Semidiurnal Variations in the Thermosphere and Ionosphere Forced From Above and From Below

Abstract

The nature and origins of solar-synchronous semidiurnal variations (SW2) in the thermosphere and ionosphere are explored through analysis of Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations that include and exclude forcing by tides at its 97-km lower boundary derived from Ionospheric Connection (ICON) explorer data. SW2 consists of two components, one excited in-situ (${\text{SW}2}_i$) and one due to vertical propagation from below the thermosphere (${\text{SW}2}_b$). ${\text{SW}2}_i$ in turn consists of a part forced by heating due to absorption of solar radiation (${\text{SW}2}_{is}$) and a part of “geomagnetic” origin (${\text{SW}2}_{ig}$) arising from magnetospheric coupling at high latitudes. At 300 km all three components achieve wind and temperature amplitudes of order 25 ${\text{ms}}^{-1}$ and 15–20 K within specific geographic domains and levels of solar activity. ${\text{SW}2}_{is}$ exhibits upward propagation from the lower thermosphere much like the $1^{st}$ symmetric component of ${\text{SW}2}_b$, refraction toward the winter hemisphere by mean winds, modulation by the $\sim$ 27-day solar rotation rate, and is estimated to exceed ${\text{SW}2}_b$ in general importance for F10.7 $>$ 133 sfu. At solar flux levels 75 sfu F10.7 125 sfu, electron density perturbations are of order 10%–35% for ${\text{SW}2}_b$ and 15%–25% for ${\text{SW}2}_{is}$. Vertical E $\times$ B drifts are of order 5–8 ${\text{ms}}^{-1}$ for ${\text{SW}2}_b$, 3–5 ${\text{ms}}^{-1}$ for ${\text{SW}2}_{is}$, and 5–8 ${\text{ms}}^{-1}$ for ${\text{SW}2}_{ig}$, the latter extending to the equator at elevated magnetic activity, and suggesting the presence of penetrating electric fields and/or disturbance dynamo action.