Four years of IMP 8 magnetic field measurements, 1978‐1982, when ISEE 3 took upstream interplanetary magnetic field (IMF) measurements, have been analyzed to produce full cross‐section magnetic maps of the magnetotail at about 33RE downwind from Earth. This paper describes how the field geometry in the cross‐sectional plane responds to different IMF orientations: dominant By, dominant +Bz, and dominant −Bz. The dominant By case exhibits marked departures from bilateral symmetry that have the sense of superimposing a fraction of the IMF on the symmetrical tail field. However, the “superimposed” perturbation field, measured as a fraction of the IMF, is highly nonuniform: It is maximum in the flanks of the plasma sheet and minimum in the lobes. There is also a rotation of the current sheet which varies with IMF strength and with distance. The two dominant Bz cases show no systematic departures from bilateral symmetry. However, the shapes and the relative sizes of the dipolar and flaring field regions of their cross sections are markedly different. The difference field, obtained by subtracting the positive Bz case from the negative Bz case, shows that the strongest perturbations run the north‐south extent of the flanks, instead of residing in the lobes, as might be expected from the dayside reconnection model.

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Abstract. As revealed in MHD simulation, the magnetospheric sash is a band of weak magnetic field that, for the usual case in which the IMF is approximately perpendicular to thi geomagnetic dipole, runs tailward along the highlatitude magnetopause flanks from one dayside cusp to the other, closing via the cross-tail neutral sheet. On the magnetopause flanks, it contains the magnetic separator line, at which all three topological types of field lines meet. Seen in a cross-sectional plane through the near-Earth tail, the magnetospheric sash takes the form of the cross-tail S, a weak-field feature comprised of the tail neutral sheet with diagonally symmetric extensions along the magnetopause flanks connecting it to the separator line. The cross-tail S is evident in the MHD results and in cross-sectional maps based on IMP 8 data. The magnetopause expression of the sash is latent in prior works that described the geometry of antiparallel fields across the magnetopause and the consequent cancellation of the fields within the magnetopause layer. The sash picture bears a strong resemblance to antiparallel merging geometry. A new global magnetospheric structureGlobal MHD simulation reveals a hitherto unrecognized global magnetospheric structure. It is perhaps best described as a low field strength feature that runs, ribbon like, from one dayside cusp tailward along one flank of the magnetopause, then through the near-Earth tail as the neutral sheet to the other magnetopause flank, along which it runs back to the other dayside cusp. In keeping with the tradition of using sartorial designations for magnetospheric plasma structures--e.g., hood, mantle, sheet, and belt--we propose calling this newly recognized ribbon-like feature the magnetospheric sash. Our purpose here is to use a particular MHD simulation to document the sash's geometry, to ground its existence in global magnetic topology, and to identify its presence in data and its adumbration in prior work. The ISM MHD codeBefore describing the sash, we give some details of the MHD code and the boundary and initial conditions of the simulation to be presented here.

A comprehensive examination of 2 yr of radiosonde data to determine the surface duct conditions over Istanbul (4°N, 29°E), Turkey, was made. The refractivity of the atmosphere is a function of air temperature and water vapor pressure. Any negative gradient in the modified refractivity results in the presence of a duct in the atmosphere. Therefore, the occurrence of ducts strongly depends upon both the synoptic and the local meteorological conditions that prevail over the region. The characteristics of surface ducts occurring over Istanbul were examined statistically. It was found that most of the ducts occur in May and July. The highest occurrence rate of surface ducts was observed in the summer season, and the lowest rate was observed in the winter season. The median duct thickness and duct strength are found to be the highest and the strongest in summer, whereas they are the lowest and the weakest in winter. When the data are separated into stable and unstable atmospheric subgroups, it is seen that surface duct characteristics show clear seasonal differences. Surface ducts in a stable atmosphere are found to be stronger than those in an unstable atmosphere. Also, daytime (1200 UTC) surface ducts occur more frequently than nighttime (0000 UTC) surface ducts in Istanbul. These statistics are discussed in association with local meteorological conditions and weather systems affecting the Istanbul region, and comments are made on the importance of their possible consequences in the region.

This paper gives maps of magnetosheath magnetic field lines in a roughly 12RE thick annulus circling the tail approximately 30RE downwind from Earth. There are maps for northward and southward IMF and for quasi‐equatorial IMF (the usual case). The maps are based on IMP 8 data taken during the 127 orbits (four years) when ISEE 3 measured the upstream IMF. In these four years, IMP 8's orbit swept the mapped annulus eight times. The maps project the field onto a plane perpendicular to the sun‐earth line, after correcting for solar wind aberration and tail flaring. Each shows draping around the tail: The field enters from one side, bifurcates at the tail boundary, “flows” around in both directions, rejoins at the opposite side, and exits. A new finding is that, contrary to expectations, the draping pattern is rotated relative to the plane formed by the IMF and the aberrated x‐axis. For example, it is rotated relative to the equatorial plane when the IMF lies in the equatorial plane. The degree of rotation varies from essentially zero for strongly northward and southward IMF to a peak ∼17° for moderately southward IMF. The sense of rotation is clockwise for toward IMF sectors and counterclockwise for away IMF sectors—the same as the dayside merging line. We also find that the tail is fatter and the magnetosheath field less ordered for strongly southward IMF than for strongly northward IMF. The new findings are understandable in terms of dayside magnetic merging.