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- Knipp, D.J., et al.
(författare)
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Ionospheric Convection Response to Slow, Strong Variations in a Northward Interplanetary Magnetic Field: A Case Study for January 14, 1988
- 1993
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 98, s. 19273-19292
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Tidskriftsartikel (refereegranskat)abstract
- We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF B(y) and B(z) components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF B(x) component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio \textbackslashB(y)/B(z)\textbackslash exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of B(y) exceeded that of B(z), with potential drops of the order of 100 kV, even while B(z) remained northward. The later transition to southward B(z) produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through \textbackslashB(y)/B(z)\textbackslash = 1 than at the transition through B(z) = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.
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| 2. |
- Sitar, R J, et al.
(författare)
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Multi-instrument analysis of the ionospheric signatures of a hot flow anomaly occurring on July 24, 1996
- 1998
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. ; 103, s. 23357-23372
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Tidskriftsartikel (refereegranskat)abstract
- We present the analysis of a coordinated set of observations from the POLAR ultraviolet imager (UVI), ground magnetometers, incoherent scatter radar, solar wind monitors, and the DMSP satellite, focused on a traveling convection vortex (TCV) event on July 24, 1996. Starting at approximately 1036 UT, ground magnetometers in Greenland and eastern Canada observe pulsations consistent with the passing overhead of a series of TCV field-aligned current pairs. Azimuthal scans by the Sondrestrom incoherent scatter radar located near Kangerlussuaq (formerly Sondrestrom), Greenland, at this time show strong modulation in the strength and direction of ionospheric plasma flow. The magnetometer pulsations grow in magnitude over the next hour, peaking in intensity at 1137 UT. Images from the UVI instrument show a localized intensification of auroral emissions over central and western Greenland at 1139 UT. Subsequent images show the intensification grow in strength and propagate westward (tailward) until approximately 1158 UT, at which time the intensification fades, These observations are consistent with the westward passage of four pairs of TCVs over central Greenland. The intensification of auroral emissions at 1139 UT is associated with the leading vortex of the fourth TCV pair, thought to be the result of an upward field-aligned current. The modulated flow observed by the radar is the result of the strong electric fields associated with the field-aligned current systems responsible for the impulsive TCV as they pass through the field of view of the radar. Measurements taken in the solar wind by the Wind spacecraft suggest that a pressure change triggers the onset of TCV activity. A subsequent sudden change in the orientation of the interplanetary magnetic field produces a hot flow anomaly which forms at the bow shock. We believe that the interaction of the hot flow anomaly with the magnetopause intensified the fourth TCV pair and. produced the associated auroral brightening. DMSP particle data indicate that the TCVs occur on field lines which map to the boundary plasma sheet-low latitude boundary layer interface. The ground observations associated with the hot flow anomaly are the first of their kind and provide a mechanism to tie an interplanetary magnetic field orientation change into the existing theory that TCVs result from a deformation of the magnetopause.
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