Size and shape of the distant magnetotail

We employ a global magnetohydrodynamic model to study the effects of the interplanetary magnetic field (IMF) strength and direction upon the cross section of the magnetotail at lunar distances. The anisotropic pressure of draped magnetosheath magnetic field lines and the inclusion of a reconnection‐generated standing slow mode wave fan bounded by a rotational discontinuity within the definition of the magnetotail result in cross sections elongated in the direction parallel to the component of the IMF in the plane perpendicular to the Sun‐Earth line. Tilted cross‐tail plasma sheets separate the northern and southern lobes within these cross sections. Greater fast‐mode speeds perpendicular than parallel to the draped magnetosheath magnetic field lines result in greater distances to the bow shock in the direction perpendicular than parallel to the component of the IMF in the plane transverse to the Sun‐Earth line. The magnetotail cross section responds rapidly to variations in the IMF orientation. The rotational discontinuity associated with newly reconnected magnetic field lines requires no more than the magnetosheath convection time to appear at any distance downstream, and further adjustments of the cross section in response to the anisotropic pressures of the draped magnetic field lines require no more than 10–20 min. Consequently, for typical ecliptic IMF orientations and strengths, the magnetotail cross section is oblate, while the bow shock is prolate. Key Points An ecliptic IMF causes prolate bow shock but oblate magnetotail cross sections The oblate lunar magnetotail cross sections include broad slow‐mode fans Lunar magnetotail and bow shock cross sections respond rapidly to IMF variations