High-latitude dayside energetic precipitation and IMF Bz rotations

N. Ostgaard, D.L. Detrick, T.J. Rosenberg, R.R. Vondrak, H.U. Frey, S.B. Mende,
S.E. Haland, and J. Stadsnes


X-ray images from space and optical and riometer data from the ground are used
to examine two discrete high-latitude dayside arcs. The dayside auroral oval is
usually associated with soft precipitation of  1 keV electrons but the observed
X-ray features and riometer data indicate more energetic precipitation as the
X-rays are produced by electrons with energies >2.5 keV and cosmic radio
absorption is associated with  10 keV electron precipitation. Both the X-ray
images from space as well as the ground based optical and riometer data show two
energetic precipitation events appearing at high latitudes and subsequently
moving equatorward at  0.5 km/s. The average energy of the electrons is
estimated to be 4-8 keV, and the energy deposition is 6-10 mW/m2. Owing to the
high latitudes it is unlikely that these electrons were energized on the
nightside and adiabatically drifted to the dayside. Instead, we think that the
high-latitude events are controlled by the solar wind. The precipitation appears
at  77-79  magnetic latitude and coincides with a northward turning of the
interplanetary field. Owing to the uncertainties in determining the exact
location of open-closed boundary we interpret the events to be caused by either
KH instabilities at the inner edge of the LLBL or by lobe reconnection. In the
latter case the arcs are produced by electrons accelerated by parallel electric
fields resulting from converging horizontal electric fields associated with
convection enhancements due to lobe reconnection at the front surface of the
magnetotail. These flux tubes map to the poleward edge of the cusp. The
equatorward movement is attributed to erosion of field lines as the
interplanetary field turns southward with associated dayside reconnection on
flux tubes with footpoints at lower latitudes. 

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Journal of Geophysical Research., 108(A4), 8013, doi:10.1029/2002JA009350, 2003.