Polar cap inflation and deflation

Abstract

Dayside and nightside merging rates usually differ; first one dominates and then the other to maintain long‐term flux balance. The polar cap, defined as the area in the ionosphere penetrated by open field lines, expands when closed field lines open during dayside merging and contracts when open field lines close during tail merging. The patterns of convection, electric field and current for polar cap inflation and deflation differ from those of steady state convection. This report models the flow and electrical parameters of polar cap inflation and deflation. We treat the idealized case of uniform conductivity, a planar ionosphere, and “pure Bz” merging (i.e., no By effect, although we note how the approach used here is readily modified to account for the By effect). For this idealized case, the potential associated with polar cap inflation and deflation is seen to be described by the same equations that govern the two‐dimensional hydrodynamic flow into and out of an expanding and contracting cylinder with a gap in its side. The gap corresponds to the breach in the polar cap boundary through which magnetic flux passes as a result of merging. The calculated potential pattern displays the usual two cell convection configuration, but the foci of the two cells are the edges of the flux gaps. Thus the cells are displaced sunward during dayside merging and tailward during tail merging.

Excitation of twin-vortex flow in the nightside high-latitude ionosphere during an isolated substorm

“…We now discuss the physical origins of the flow excitation, and, following the discussion in the Introduction and that of Siscoe and Huang (1985), , and Cowley et al ( , 1998, we presume that it was due principally to an interval of enhanced reconnection and net open flux closure in the tail. Here we will make estimates of the reconnection rates involved, and of the net flux closure, on this basis.…”

“…Now, however, the flow will be strongest on the nightside, with the foci of the twin-vortex flow located at either end of the nightside merging gap, and is associated with a contraction of the open polar cap and surrounding auroral zone. This two-component flow picture, while first being presented in the above form by , is related to earlier ideas on boundary motions and flows presented by Siscoe and Huang (1985) and Freeman and Southwood (1988). Further discussion may be found in the more recent papers by Cowley and Lockwood (1996) and Cowley (1998).…”

A simple model of the flux content of the distant magnetotail

“…[7] The dayside and nightside reconnection processes are time-dependent and independent of each other, and so the proportion of the terrestrial magnetic field that is interconnected with the IMF, that is ''open,'' is determined by the competition between the opening of flux at the subsolar magnetopause and the closure of flux in the magnetotail [e.g., Coroniti and Kennel, 1972;Siscoe and Huang, 1985;Cowley and Lockwood, 1992;Russell, 2000;Milan et al, 2003Milan et al, , 2004. The amount of open flux F PC which threads the ionospheric polar cap (or equally the whole surface of the ''connected'' magnetosphere) is governed by the following statement of Faraday's Law:…”

Forecasting ionospheric electric fields: An interplanetary coupling perspective

“…Consequently, the observed northern hemisphere convection pattern was stirred in part by merging of the IMF with closed field lines near the poleward edge of the southern hemisphere cusp thereby adding open flux to the northern polar cap. Subsequent motions of adiaroic polar cap boundaries were detected in the rocket electric field measurements, [Siscoe and Huang, 1985]. The observations indicate that IMF Bx significantly affected the location and tim ing of merging interactions.…”

“…The addition of new open flux to the polar caps expands the adiaroic portion of the openclosed boundaries, thereby communicating the flow to the whole pattern. As used by Siscoe and Huang [1985], the term "adiaroic" describes a boundary across which there is no flux transfer. After 5 to 10 min, the "new" open flux becomes "old."…”