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Ann. Geophys., 23, 3513-3532, 2005
www.ann-geophys.net/23/3513/2005/
© European Geosciences Union 2005
Motion of the dayside polar cap boundary during substorm cycles: II. Generation of poleward-moving events and polar cap patches by pulses in the magnetopause reconnection rate
M. Lockwood1,*, J. A. Davies1, J. Moen2,**, A. P. van Eyken3, K. Oksavik2,***, I. W. McCrea1, and M. Lester4
1Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
2Department of Physics, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway
3EISCAT Headquarters, Kiruna, Sweden
4Department of Physics and Astronomy, University of Leicester, Leicester, UK
*also at: Department of Physics and Astronomy, Southampton University, Southampton, Hampshire, UK
**also at: Arctic Geophysics, University Centre on Svalbard, N-9170 Longyearbyen, Norway
***also at: Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, USA, Norway
Abstract. Using data from the EISCAT (European Incoherent Scatter)
VHF and CUTLASS (Co-operative UK Twin-Located Auroral Sounding System) HF
radars, we study the formation of ionospheric polar cap patches and their
relationship to the magnetopause reconnection pulses identified in the
companion paper by Lockwood et al. (2005). It is shown that the poleward-moving,
high-concentration plasma patches observed in the ionosphere by EISCAT on
23 November 1999, as reported by Davies et al. (2002), were often associated with
corresponding reconnection rate pulses. However, not all such pulses
generated a patch and only within a limited MLT range (11:00-12:00 MLT) did a
patch result from a reconnection pulse. Three proposed mechanisms for the
production of patches, and of the concentration minima that separate them,
are analysed and evaluated: (1) concentration enhancement within the patches
by cusp/cleft precipitation; (2) plasma depletion in the minima between the
patches by fast plasma flows; and (3) intermittent injection of
photoionisation-enhanced plasma into the polar cap. We devise a test to
distinguish between the effects of these mechanisms. Some of the events
repeat too frequently to apply the test. Others have sufficiently long
repeat periods and mechanism (3) is shown to be the only explanation of
three of the longer-lived patches seen on this day. However, effect (2) also
appears to contribute to some events. We conclude that plasma concentration
gradients on the edges of the larger patches arise mainly from local time
variations in the subauroral plasma, via the mechanism proposed by Lockwood et al. (2000).
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