Tenuous solar winds: Insights on solar wind–magnetosphere interactions

During solar cycle 23 quasi-dropouts of the solar wind (density < 1 cm - 3 ) were observed. These tenuous winds allow us to probe properties of the magnetosphere and its coupling to the solar wind which would otherwise be obscured by the effect of high density. We focus on five areas which provided new insights into the response of geospace to solar wind variations: (i) the magnetospheric magnetic configuration; (ii) the polar rain; (iii) dayside flux erosion; (iv) magnetosheath waves; and (v) ring current constants. We find: (i) the geostationary field had dipolar strength and was inclined by ⩽ 5 ∘ to the dipolar direction; (ii) the solar wind strahl, and consequently the polar rain, were intensified; (iii) the depression of the geostationary field ( Δ B GS ) due to dayside flux erosion could be measured and was related to IMF B z by Δ B GS = - 2.8 + 2.3 B z (nT); (iv) right-hand electromagnetic ion cyclotron waves were excited alone in the magnetosheath and were generated directly from the temperature anisotropy of the solar wind; (v) ring and magnetopause currents decreased to asymptotic values of 5 and 3 nT, respectively, which are substantially smaller than quiet-time values obtained from statistics.