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

Abstract

Abstract. The present work has investigated the midlatitudinal magnetopause locations under radial interplanetary field (RIMF) conditions. 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 midlatitudes. The observed midlatitudinal magnetopause positions are compared with two empirical magnetopause models (Shue et al., 1998;Boardsen et al., 2000) (hereafter referred to as the Shue98 model and the Boardsen00 model). The observation-model differences exhibit local time asymmetry. For earthward RIMF cases, the Shue98 model underestimates the magnetopause positions in the postnoon sector, while it overestimates the magnetopause positions in the dawn and dusk sectors. The Boardsen00 model generally underestimates the magnetopause after 6 MLT (magnetic local time), with larger deviations in the postnoon sector as compared to those in the prenoon. 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 the 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 ( R MP ) during RIMF periods correlate well with the solar wind dynamic pressure, with larger R MP for larger P d . 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 R MP and the IMF cone angle are consistent with the previous hypothesis (Dušík et al., 2010) that, with more radial IMF, the subsolar magnetopause will expand further outward, owever, this is not the case for the comparison with Boardsen00 during sunward IMF periods, as it shows less dependence on the IMF cone angle.

Evolution of the magnetic field structure outside the magnetopause under radial IMF conditions

“…The previous results show that the magnetopause moves about 2 R E sunward in comparison with the model predictions, having a bullet‐like or globally expanded structure [ Slavin et al ., ; Dušík et al ., ; Jelínek et al ., ; Suvorova et al ., , Suvorova and Dmitriev , , , Park et al ., ]. The magnetopause also shows a local time asymmetry in its size and shape [ Huang et al ., ]. It was reported that empirical models underestimate the magnetopause position in the postnoon sector (12–17 magnetic local time (MLT)) and overestimate the magnetopause positions in the dawn (5–8 MLT) and dusk (18–21 MLT) sectors.…”

Global expansion of the dayside magnetopause for long‐duration radial IMF events: Statistical study on GOES observations

“…Previous studies found that P SW under radial IMF intervals is smaller than that under nonradial IMF intervals [i.e., Merka et al ., ; Suvorova et al ., ; Pi et al ., ; Huang et al ., ]. Since the reduction of P SW also controls the expansion of the magnetopause for radial IMF, it is unclear whether the expanded magnetopause during radial IMF conditions is solely related to the IMF orientation [ Suvorova et al ., ].…”

Effect of interplanetary magnetic field <i>B</i>_<i>x</i> on the polar electrojets as observed by CHAMP and Swarm satellites

“…As utilized in previous work (e.g. Huang T et al, 2015), the specific criterion used to select dominant IMF B x events involves the IMF cone angle, calculated as the angle between the IMF vector and the Sun−Earth direction. It is calculated as…”