An approximate flow equation for geomagnetic flux tubes and its application to polar substorms

Abstract

A recent model of polar substorms suggests they are the result of an impulsive recombination of magnetic field lines across the neutral sheet in the tail of the magnetosphere. The flow of flux tubes within the magnetosphere is shown to be dominated by the discharging action of the ionosphere on flux tubes. This fact enables an approximate flow equation for flow within the magnetosphere to be developed by integrating along flux tubes. This equation is applied to the above model of polar substorms and is shown to give reasonable agreement with the following experimental observations: (i) the velocity and flow pattern of auroral patches; (ii) the height and shape of the auroral breakup bulge; and (iii) the bay current system, including the westward electrojet under the assumption that the westward electrojet is current‐induced by the southward component of flow of flux tube feet in the high conductivity strips under auroral arcs. These strips are expected to exhibit a Cowling conductivity. The westward traveling surge is probably caused by an increasing width of the recombination slot in the tail of the magnetosphere. The current flow down field lines and through the ionosphere results in a density (and hence pressure) decrease in the neutral sheet at the west edge of the recombination slot, and thus causes a corresponding westward propagation of the west edge of the recombination slot. Application of the flow equations to the viscous bounday layer problem indicates an exponential falloff in velocity from the magnetosphere boundary with a 1/e distance of 5 × 104 m.

Substorm currents associated with magnetotail magnetic dipolarization: Geotail observations

“…The intense westward auroral electrojet in the nightside polar region observed in association with substorm initiation is the ionospheric portion of the substorm current wedge that is formed at the time of auroral breakups [e.g., Atkinson, 1967;McPherron et al, 1973]. The longitudinal distribution of the field-aligned portion of the current wedge can be estimated from magnetic field perturbations at midlatitude ground observatories; that is, the D component of the magnetic field is directed eastward in the evening-midnight sector and westward in the midnight-morning sector, indicating the existence of upward current and downward current, respectively.…”

Auroral electrojet oval

“…The blocked part of the cross-tail current must find a new circuit, and the obvious one would be the polar ionosphere circuit. In this way, a substorm current wedge is formed (Atkinson, 1967;McPherron et al, 1973). Along with tailward expansion of the blocked part of the cross-tail currents, the ionospheric partner of the current wedge as well as the westward electrojet will move poleward.…”

Morphology of spatial patterns in Pi 1-2 magnetic field pulsation activity - a review.

“…ATKINSON, 1967;CORONITI and KENNEL, 1973;ROSTOKER and BOSTROM, 1976;KAN and LEE, 1980) indicate that enhanced ionospheric conductivity will partially short-out the E field and thus inhibit convection, i.e. there is a trade-off between ionospheric dissipation of energy and the energy being expended in convection.…”