Dipole model of the Earth's magnetic field

About: Dipole model of the Earth's magnetic field is a research topic. Over the lifetime, 2756 publications have been published within this topic receiving 83021 citations.

Magnetic braking of the present Sun

Abstract: Magnetic braking is essential for angular momentum transport in late-type stars which have convective envelopes. The mechanism may be entirely responsible for the slow rotation of the present Sun, on which a braking model is normally calibrated. Recent or current satellite missions such as Helios and Ulysses have jointly revealed a more complete picture of the solar corona, and more specifically of the solar wind. The wealth of these data at or near the solar minimum is valuable for constraining the dipolar solar braking model. In this paper, we use recently available observations (at or near the solar minimum) to constrain a solar magnetic braking model based on a dipolar field structure. It is found that the Ulysses data indicate a spherical Alfven surface at high latitudes. We infer from a thermal wind model that it is located at 16 Ro˙, which is larger than the 12 Rodot; deduced from the Helios data for the equatorial region near the solar minimum. It is also found that the braking model with a transition from a dipole to a split monopole field is generally consistent with Ulysses observations, provided that a linear relation between dead zone extent and dipole field strength is satisfied. Thus either the dipole field retains a sizeable dead zone but is much stronger than the standard value, ∼ 1 G, or the field has standard strength but an exceedingly small dead zone (<2 R x o˙). The magnetic braking rate as constrained by Ulysses data is found to be 2.1× 1030 dyn cm, which is about a quarter of the value deduced earlier from the Weber & Davis model and does not differ significantly from that deduced by Pizzo et al.

Interplanetary magnetic field control of high‐latitude activity on July 29, 1977

Abstract: Multisatellite particle and magnetic field data for the substorms of July 29, 1977, show auroral-like activity above 80 deg invariant latitude during the recovery period. The movement of auroral zone activity to high latitudes followed the substorm sequence, at which time the inferred interplanetary magnetic field (IMF) was strongly northward. Electron energy spectra indicative of a field-aligned potential drop, and the absence of supporting precipitating ions, are found at latitudes greater than 80 deg. The north-south symmetry of these observations suggests that the events are on closed field lines. It is noted the very strong northward IMF connected to the sunward tilted geomagnetic dipole field plays a role in the driving of strong Birkeland and ionospheric current systems in the northern polar regions, while eliminating them from the southern polar regions.

A comparison of model predictions for plasma convection in the northern and southern polar regions

Abstract: We have presented model calculations to show how the plasma flow distributions in the northern and southern polar regions differ when viewed from a geographic inertial frame. This reference frame was selected because it is the natural frame for geophysical plasma flow measurements, there being well-known velocity corrections for either satellite or ground-based observations. Although the magnetic invariant latitude, magnetic local time reference frame is better suited to studying magnetospheric processes, a transformation from the geographic inertial frame to this magnetic frame requires both a spatial and velocity transformation, and since the latter correction has generally been neglected, we prefer to present our results in the geographic inertial frame. However, we also present some of our results in the magnetic frame, taking account of the complete transformation. Our convection model includes the offset between the geographic and geomagnetic poles, the tendency of plasma to corotate about the geographic poles, and a dawn/dusk magnetospheric electric field mapped to a circle about a center offset by 5° in the antisunward direction from the magnetic pole. We considered both uniform and asymmetric magnetospheric electric field configurations. Our asymmetric electric field distribution contained an enhanced field in the dawnside northern hemisphere in conjunction with an enhanced field in the duskside southern hemisphere. From our study we have found the following: (1) In the geographic inertial frame the plasma flow patterns in both hemispheres exhibit significant variations with universal time because of the relative motion of the geomagnetic and geographic poles. (2) This universal time variation is greater in the southern polar region than in the northern polar region because of the greater displacement between the geomagnetic and geographic poles. (3) For the case of a uniform magnetospheric electric field the universal time dependence of the plasma flow distributions in the two hemispheres is similar, but there is a phase shift of about half a day between them. (4) For the case of an asymmetric magnetospheric electric field this half-day phase shift is still noticeable, but there are significant differences between northern and southern hemisphere convection patterns. (5) The transformation of plasma convection patterns from the geographic inertial to the geomagnetic quasi-inertial frame results in the same convection pattern for both hemispheres for the case of a uniform magnetospheric electric field, but results in different convection patterns for the two hemispheres for the more common case of an asymmetric electric field configuration. (6) Because the magnetospheric electric field distributions in the northern and southern polar regions are generally asymmetric, erroneous conclusions can be drawn about plasma convection patterns if data taken along satellite tracks from the northern and southern polar regions are overlaid. This is true whether the overlaying is done in the geomagnetic quasi-inertial frame or the geographic inertial frame.