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Ann. Geophys., 23, 509-521, 2005
www.ann-geophys.net/23/509/2005/
© European Geosciences Union 2005
On the motion of dayside auroras caused by a solar wind pressure pulse
A. Kozlovsky1,2, V. Safargaleev3, N. Østgaard4, T. Turunen1, A. Koustov5, J. Jussila2, and A. Roldugin3
1Sodankylä Geophysical Observatory, Sodankylä, FIN-99600, Finland
2Department of Physical Sciences, University of Oulu, Oulu, FIN-90014, Finland
3Polar Geophysical Institute, Apatity, 18 4200, Russia
4Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94 720–7450, USA
5Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
Abstract. Global ultraviolet auroral images from the IMAGE satellite were used to
investigate the dynamics of the dayside auroral oval responding to a sudden
impulse (SI) in the solar wind pressure. At the same time, the TV all-sky
camera and the EISCAT radar on Svalbard (in the pre-noon sector) allowed for
detailed investigation of the auroral forms and the ionospheric plasma flow.
After the SI, new discrete auroral forms appeared in the poleward part of the
auroral oval so that the middle of the dayside oval moved poleward from about
70° to about 73° of the AACGM latitude. This poleward shift
first occurred in the 15 MLT sector, then similar shifts were observed in the
MLT sectors located more westerly, and eventually the shift was seen in the 6
MLT sector. Thus, the auroral disturbance "propagated" westward (from 15 MLT
to 6 MLT) at an apparent speed of the order of 7km/s. This motion of the
middle of the auroral oval was caused by the redistribution of the luminosity
within the oval and was not associated with the corresponding motion of the
poleward boundary of the oval. The SI was followed by an increase in the
northward plasma convection velocity. Individual auroral forms showed
poleward progressions with velocities close to the velocity of the northward
plasma convection. The observations indicate firstly a pressure
disturbance propagation through the magnetosphere at a velocity of the order
of 200km/s which is essentially slower than the velocity of the fast
Alfvén (magnetosonic) wave, and secondly a potential (curl-free)
electric field generation behind the front of the propagating disturbance,
causing the motion of the auroras. We suggest a physical explanation for the
slow propagation of the disturbance through the magnetosphere and a model for
the electric field generation. Predictions of the model are supported by the
global convection maps produced by the SuperDARN HF radars. Finally, the
interchange instability and the eigenmode toroidal Alfvén oscillations are
discussed as possible generation mechanisms for the dayside auroral forms
launched by the SI.
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