Auroral Kilometric Radiation Source Region Variations with Season and Solar Cycle

Data from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and the Polar plasma wave instruments were used to study the seasonal and solar cycle variations of the northern hemisphere auroral kilometric radiation (AKR) source region. Our results show that there are significant seasonal changes in the AKR spectrum. We find that the spectral peak drifts from 260 kHz in the winter to 150 kHz in the summer and the overall intensity is weaker. Assuming that the AKR is generated by means of the cyclotron maser mechanism this implies that the AKR source region (the auroral density cavity) moves to higher altitudes during the summer months. Data from the DE-1 plasma wave instrument were used to study the seasonal variation of the AKR region in magnetic local time (MLT). During the winter months the MLT range of the AKR region extends from 18-24 MLT with a source centroid near 20 MLT. During the summer months the MLT range narrows to 20-24 MLT and the source centroid moves closer to 22 MLT. Comparing Polar and IMAGE data also allowed for an analysis of the solar cycle variations of the AKR. The average AKR spectrum is typically weaker by about an order of magnitude during solar maximum. These results illustrate that variations in ionospheric density with solar EUV flux both seasonally and with solar cycle have significant effects on the AKR source region due to strong magnetospheric-ionospheric coupling. The effect of this coupling is that increases in ionospheric density result in a narrowing of the altitude and magnetic latitude range of the AKR density cavity and a decrease in the intensity of AKR by lessening the density depth of the auroral density cavity. These results will need to be accounted for in any development of an AKR-related substorm index and for statistical studies of the AKR emission pattern.