Icarus 182 (2006) 464–473 www.elsevier.com/locate/icarus The magnetic field draping direction at Mars from April 1999 through August 2004 David A. Brain ∗ , David L. Mitchell, Jasper S. Halekas UC Berkeley Space Sciences Laboratory, 7 Gauss Way, Berkeley, CA 94720, USA Received 12 April 2005; revised 19 September 2005 Available online 2 February 2006 Abstract Using more than five years of data from the magnetometer and electron reflectometer (MAG/ER) on Mars Global Surveyor (MGS), we derive the draping direction of the magnetic field above a given latitude band in the northern hemisphere. The draping direction varies on timescales associated with the orbital period of Mars and with the solar rotation period. We find that there is a strongly preferred draping direction when Mars is in one solar wind sector, but the opposite direction is not preferred as strongly for the other solar wind sector. This asymmetry occurs at or below the magnetic pileup boundary (MPB), is observed preferentially on field lines that connect to the collisional ionosphere, and is independent of planetary longitude. The observations could be explained by a hemispherical asymmetry in the access of field lines to the low-altitude ionosphere, or possibly from global modification of the low-altitude solar wind interaction by crustal magnetic fields. We show that the draping direction affects both the penetration of sheath plasma to 400 km altitudes on the martian dayside and the radial component of the magnetic field on the planetary night side.  2005 Elsevier Inc. All rights reserved. Keywords: Mars; Magnetic fields; Ionospheres; Solar wind 1. Introduction The interaction of solar wind charged particles with a planet can be classified according to the type of obstacle that planet presents to the solar wind. Mars is unique in the solar system in this regard because its solar wind obstacle is a combination of its atmosphere and strong crustal magnetic fields (see Nagy et al., 2004, for a recent review). Like Venus, the atmospheric in- teraction appears to dominate the global solar wind interaction (Cloutier et al., 1999). Crustal fields significantly modify this interaction locally, and perhaps globally (Acuña et al., 2001; Mitchell et al., 2001; Brain et al., 2003; Crider, 2004; Brain et al., 2005). These fields rotate with Mars so that it continuously presents a different obstacle to the solar wind. The solar wind carries the embedded interplanetary mag- netic field (IMF), which propagates through the bow shock and drapes around the planetary obstacle. It has long been * Corresponding author. Fax: +1 (510) 643 8302. E-mail address: brain@ssl.berkeley.edu (D.A. Brain). known that the orientation of the IMF will affect the global so- lar wind interaction with a planet (Dungey, 1961). At planets with large global magnetic fields, for example, IMF orientation determines whether magnetic reconnection is likely near the subsolar magnetopause (Paschmann et al., 1979). IMF direction also determines the direction of the solar wind convection elec- tric field,  E SW = − v SW ×  B , which in turn affects the motion of charged particles in the planetary interaction region. This ef- fect is especially important for atmospheric obstacles, where a significant plasma population is in motion with respect to the solar wind. At Mars, the IMF direction should influence the solar wind interaction in several ways. One main way is that planetary pickup ions are accelerated in the direction of  E SW , so that ions are preferentially lost over one hemisphere (Cloutier et al., 1969). Another is that the direction of gyromotion of so- lar wind charged particles may affect the size, shape, and na- ture of plasma boundaries in the interaction region (Brecht and Ferrante, 1991). A third is that the IMF direction should af- fect magnetic field topology (the connectivity of magnetic field lines) near crustal sources, as magnetic reconnection of crustal 0019-1035/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.09.023