The geostationary field during dayside erosion events 1996–2001: A joint Wind, ACE, and GOES study

Abstract

We carry out a statistical investigation on dayside magnetosphere erosion during the period 1996–2001, using Wind, ACE, and GOES observations supplemented by ground‐based magnetograms. The inner magnetospheric signature of erosion that we focus on is the depression of the geostationary magnetic field strength (ΔBtot) within 2 hours of local magnetic noon. We relate this decrease to the southward component of the interplanetary magnetic field (IMF) in the range −16 ≤ Bz ≲ 0 nT and the solar wind dynamic pressure in the range ∼0–6 nPa. Thus both relevant interplanetary parameters extend to well beyond average values at 1 AU. The geostationary field strength (Btot) is derived from measurements by the GOES 8, 9, and 10 spacecraft. Our data set consists of 288 measurements. We adopt two approaches. In one approach, we first correct for the effect of dynamic pressure on the geostationary field strength by studying days with various dynamic pressure values but low IMF clock angles, θ (<45°). With the latter condition we mean to exclude dayside reconnection to a reasonable degree. We then use this pressure correction to infer the erosion contribution to Btot (ΔBtot). We find ΔBtot[nT] = −3.58 + 1.67Bz with a correlation coefficient of 0.59. This analysis is most reliable in giving the change of the geostationary field to a decrease in IMF Bz, for which we obtain ΔBtot/Bz = −1.67 nT per 1 nT decrease in Bz. In the second approach, we do a bivariate fit of Btot to the (Bz, Pdyn) data pairs and obtain Btot[nT] = 97.64 + 20.22 + 1.56Bz. Major sources of error are discussed, and the results are compared to previous work.