Seasonal and solar cycle dynamics of the AKR source region

J. L. Green, S. Boardsen, L. Garcia, S. F. Fung, and B. W. Reinisch


Several year's worth of observations from the plasma wave instruments on
both IMAGE and Polar spacecraft are used to study the seasonal and solar
cycle variations in the spectrum of auroral kilometric radiation (AKR). 
Only AKR observations when the spacecraft were in the northern
hemisphere emission cones were used. The results from the seasonal
analysis show significant changes in the AKR spectrum as a function of
dipole tilt. The average AKR spectral peak for positive dipole tilt is
~150 kHz but is ~260 kHz during times of negative dipole tilt.  In
addition, the average emission spectrum for positive tilt is up to two
orders of magnitude weaker over the 200 to 500 kHz frequency range when
compared with the average emission spectrum for negative tilt.  Assuming
the cyclotron maser mechanism for AKR these results imply that the AKR
source region (the auroral density cavity) moves to higher altitudes
during the summer and to lower altitudes during the winter.  Using data
from the DE-1 plasma wave instrument the magnetic local time of average
AKR source region is also investigated with dipole tilt. From these
observations it is found that for negative dipole tilt a broad AKR
source region exists, ranging from ~18 to ~24 MLT, with peak emission
coming from ~20 MLT. In comparison, under positive dipole tilt the
source region narrows (~20 to ~24 MLT) with peak emission at ~22 MLT. 
Taking into account the above seasonal effect a comparison of the
average spectra from IMAGE and Polar plasma wave data also demonstrates
a solar cycle effect.  The average AKR spectrum at solar maximum has the
same structure with dipole tilt as at solar minimum but is typically
lower (by as much as 1 to 2 orders of magnitude). The observations
presented support the concept that the expected increases in ionospheric
densities (with positive dipole tilt and solar UV increases during solar
maximum) play a significant role in magnetospheric-ionospheric coupling
by: 1) shortening the altitude range of the auroral plasma cavity , 2)
confining the cavity to a smaller range of MLT and closer to midnight,
and 3) decreasing the overall intensity of AKR by lessening the depth of
the auroral density cavity.

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Submitted to J. Geophys. Res., 2003JA10311, 2003.