A new mechanism for polar patch formation

Abstract

Polar patches are regions within the polar cap where the F-region electron concentration and airglow emission at 630 nm are enhanced above a background level. Previous observations have demonstrated that polar patches can be readily identified in Polar Anglo-American Conjugate Experiment (PACE) data. Here PACE data and those from complementary instruments are used to show that some polar patches form in the dayside cusp within a few minutes of the simultaneous occurrence of a flow channel event (short-lived plasma jets -2 km s 'l) and azimuthal flow changes in the ionospheric convection pattern. The latter are caused by variations of the y-component of the interplanetary magnetic field. The physical processes by which these phenomena cause plasma enhancements and depletions in the vicinity of the dayside cusp and cleft are discussed. Subsequently, these features are transported into the polar cap where they continue to evolve. The spatial scale of patches when formed is usually 200-1000 km in longitude and 20-3 ø wide in latitude. Their motion after formation and the velocity of the plasma within the patches are the same, indicating that they are drifting under the action of an electric field. Occasionally, patches are observed to occur simultaneously in geomagnetic conjugate regions. Since some of these observations are incompatible with the presently-accepted model for patch formation involving the expansion of the high latitude convection pattern entraining solar-produced plasma, further modeling of the effects of energetic particle precipitation in the cusp, the consequences of flow channel events on the plasma concentrations, and the time dependence of plasma convection as a result of interplanetary magnetic field By changes is strongly recommended. Such studies could be used to determine the relative importance of this new mechanism compared with the existing theory for patch formation as a function of universal time and season. 2. Patches drift antisunward across the polar cap at 300-1000 m s -l. 3. Patches are observed during periods when the interplanetary magnetic field (IMF) Bz is negative. Paper number 93JA01501, 0148-0227/94/93JA-01501 $05.00 4. Patches have been observed in summer and winter at sunspot maximum and minimum. 5. Patches are associated with intermediate-scale irregularities (1-10kin) as indicated by spread F on ionograms and considerable scintillation activity. 6. The electron temperature of a patch is low and unstructured, indicating that energetic particles do not precipitate into it when it is within the polar cap. 7. Regions of enhanced electron concentration are also seen at 800-kin altitude. 8. Patches may form simultaneously in geomagnetically conjugate regions. The HF backscatter signature of polar patches has been identified in the Polar Anglo-American Conjugate Experiment (PACE) data by comparison with a series of simultaneous riometer, photometer, and ionosonde observations from South Pole station [Rosenberg et al., 1993]. The characteristics of the polar patches as they pass...

Plasma structure within poleward-moving cusp/cleft auroral transients: EISCAT Svalbard radar observations and an explanation in terms of large local time extent of events

“…In particular, the concentration of sub-auroral plasma that is convected into the cusp region (Lockwood and Carlson, 1992) may vary and/or the fast ion¯ows that raise the ion temperature may also cause decreases in N e because the loss rates of the plasma are enhanced (Schunk et al, 1975). Rodger et al (1994) suggested these velocity-dependant loss rates may be a signi®cant factor in the cusp/cleft ionosphere. Balmforth et al (1998Balmforth et al ( , 1999 have modelled the e ect of a transient¯ow channel lasting 5 minutes and found results to be consistent with EISCAT observations of a co-located plasma depletion.…”

“…They interpreted the concentration enhancements as being due to changes in the pattern of¯ow modulating the entry into the polar cap of EUV-produced sub-auroral plasma. However, Rodger et al (1994) pointed out that plasma production by soft particle precipitation and plasma loss by enhanced electric ®elds and reaction rates may also be signi®cant factors in introducing structure into the plasma concentrations on the timescales for¯ux tubes to enter the polar cap.…”

Modelling the tongue-of-ionisation using CTIP with SuperDARN electric potential input: verification by radiotomography

“…At high F-region altitudes the lifetime of the plasma is sufficiently long for the ionisation to be drawn through the polar region and into the nightside sector. During its transit through the polar cap the TOI may be modulated into polar patches and various processes have been proposed for this (for example Anderson et al, 1988;Sojka et al, 1993;Rodger et al, 1994;Milan et al, 2002). A modelling study by Bowline et al (1996) indicated that the TOI is expected to be prominent at Ny-Ålesund in the European sector in the evening under conditions of IMF B y negative, and observations of the remnants of the TOI and polar patches on the nightside in this sector are reported by Middleton et al (2008) and Pryse et al (2006).…”

Interplanetary magnetic field control of polar patch velocity

“…However, their exact drift direction has to follow the polar ionospheric convection pattern [Rodger et al, 1994]. It is well known that the dawn-dusk component of the IMF (B y ) has a major influence on the orientation of the high-latitude convection pattern when the IMF is southward [Heelis, 1984;Heppner and Maynard, 1987;Greenwald et al, 1990].…”