IMP 8 magnetic observations of the high‐latitude tail boundary: Locations and force balance

Abstract

IMP 8 (IMP J) magnetic field observations are used to determine the properties of the high‐latitude tail boundary, which separates the tail lobe and the magnetosheath. Boundary crossings are marked by transitions from quiet lobe to disturbed magnetosheath fields. From five years (1978–1982) of IMP 8 observations, a data set of 244 boundary crossings has been identified in the XGSM range of −15 to −40 RE and |Z′GSM| >15 RE, where Z′GSM is the distance from the neutral sheet. The results suggest the following: (1) the tail cross section is slightly flattened in the north–south direction, (2) the high‐latitude magnetotail field and boundary continue to flare antisunward out to 40 RE downstream of the Earth, and (3) interplanetary magnetic field (IMF) BZ and the solar wind dynamic pressure (but not, evidently, the solar wind magnetic pressure) control the boundary location. The dependence on IMF BZ is consistent with the buildup of magnetic fluxes in the magnetotail during periods of southward IMF BZ. Examination of the magnetic fields on each side of the boundary indicates the following: (1) The solar wind dynamic pressure determines the lobe‐side field strength. (2) The flaring angle is estimated on the basis of the classical model of the Chapman‐Ferraro problem. The result is consistent with the X dependence of the tail size and with the orientation of the lobe‐side field (all have flaring angles of 7°–8°). (3) The IMF strength controls the sheath‐side field strength, with a lesser contribution from the solar wind dynamic pressure. (4) The current flowing at the boundary primarily depends on the solar wind dynamic pressure. In summary, the present results indicate that the solar wind dynamic pressure is the most important factor determining the nature of the near‐Earth magnetotail magnetopause. We also present evidence that the IMF BZ plays an important role in these processes.