The dayside magnetopause location during radial interplanetary magnetic field periods: Cluster observation and model comparison

Abstract:

The magnetopause locations under radial interplanetary field conditions are investigated. Among 262 (256) earthward (sunward) RIMF events from years of 2001 to 2009, Cluster satellites have crossed the magnetopause 22(12) times, with 10 (7) events occurring at mid-latitude. The observed magnetopause positions are compared with two empirical magnetopause models (Shue98 and Boardsen00 model). The observation–model differences exhibit local time asymmetry. For earthward RIMF cases, the Shue98 model underestimates the magnetopause positions in the post noon sector, while it overestimates the magnetopause positions in the dawn and dusk sectors. The Boardsen00 model generally underestimates the magnetopause after 6 magnetic local time, with larger deviations in the post noon sector as compared to those in the pre-noon. For sunward RIMF cases, the selected events are mainly clustered around the dawn and dusk sectors. The comparison with the Shue98 model indicates contractions in the dawn and expansions in the dusk sector, while the comparison with Boardsen00 indicates general expansions, with larger expansions in the later local time sectors. The local time variations in the differences between observations and Shue98 and the Boardsen00 models indicate that the real magnetopause could be asymmetrically shaped during radial IMF periods, which should be considered by magnetopause models. The observation–model differences in the magnetopause positions during RIMF periods correlate well with the solar wind dynamic pressure, with larger MP for larger Pd. The southern magnetopause expands further outward relative to the model prediction when the dipole tilt angle is more negative (local summer in the Southern Hemisphere). For earthward RIMF cases, the generally good correlations between MP and the IMF cone angle are consistent with the previous hypothesis that, with more radial IMF, the subsolar magnetopause will expand further outward.