We study effects of the interplanetary magnetic field (IMF) orientation on the terrestrial tail bow shock location and shape by using global MHD magnetosphere model and empirical bow shock models. It is shown that the tail bow shock cross section is well approximated by an ellipse with the direction of the major axis roughly perpendicular to the IMF clock angle direction. With the increasing IMF clock angle, the eccentricity of the bow shock cross section increases for northward IMF but decreases for southward IMF.

No abstract

The frozen-in interplanetary magnetic field (IMF) in the solar wind is one of the most important parameters affecting the Earth’s space weather. In the early studies of the IMF’s influence on space weather, significant effects of the north–south component of the IMF B Z were emphasized, while the radial component of the IMF B X was largely ignored. However, the IMF near the Earth is not always dominated by the north–south component of the IMF B Z , and the radial component of the IMF B X also plays an important role. However, while the effects of the IMF B X (cone angle) on the magnetopause have been studied in recent years, there has been much less effort to quantify the B X (cone angle) effects on the bow shock. In this paper, using the bow shock crossing data from multiple satellites, we investigate the IMF cone angle effect on the dayside and nightside of the bow shock. Our results show that under the radial IMF condition, the dayside of the bow shock is located closer to the Earth than the average. At the same time, on the nightside, the bow shock is farther away from the Earth than the average. The mechanism explaining the bow shock location under the radial IMF is not completely understood. We believe that the magnetosonic Mach number and unusual conditions of the magnetosheath, especially for low dynamic pressure, play an important role. In the future, more work is needed to describe the reactions of the Earth’s magnetosphere to different IMF orientations, especially to the radial IMF.

Using the bow shock crossing events from four spacecraft: IMP 8, Geotail, Magion‐4, and Cluster 1, a new three‐dimensional asymmetric bow shock model is constructed. The model is parameterized by the solar wind dynamic pressure, the interplanetary magnetic field, magnetosonic Mach number, solar wind β, and the Earth's dipole tilt angle. It is shown that the shape and size of bow shock are both affected by the dipole tilt angle. The dipole tilt angle causes asymmetries in the meridional plane: (1) the bow shock subsolar standoff distance and the north‐south asymmetry increase with the dipole tilt angle; (2) as the dipole tilt angle increases, the shock flaring angle in the equatorial plane is slightly reduced, while in the meridional plane the flaring angle obviously decreases in Southern Hemisphere and keeps almost unchanged in the Northern Hemisphere. The flaring angle in the Northern Hemisphere is larger than in the Southern Hemisphere; (3) the effects of negative dipole tilt angle on shock flaring are just the opposite of those for positive tilt, and the effects of dipole tilt angle on the shape of the bow shock are north‐south symmetric. The model results are also validated by comparing with one previous empirical model and with observational crossings, and it is demonstrated that the new model is able to predict the observed crossings more accurately and can better describe the rotational asymmetry and north‐south asymmetry of the Earth's bow shock.

In this study we use the bow shock crossings contained in the Space Physics Data Facility database, collected by four spacecraft (IMP 8, Geotail, Magion‐4, and Cluster1) to analyze the effect of the interplanetary magnetic field (IMF) By component on the bow shock position and shape. Although the IMF Bz component is usually considered much more geoeffective than By, we find that the dayside bow shock is more responsive to the eastward component of the IMF than the north‐south one. We believe that the explanation lies in the changes that the Bz component induces on the magnetopause location and shape, which largely compensate the corresponding changes in the dayside bow shock location. In the tail, we find that the bow shock cross section is elongated roughly in the direction perpendicular to the IMF direction, which agrees with earlier modeling studies.