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Introduction

Auroral roar is a relatively narrowband radio emission of auroral origin observed at frequencies of 2.6-3.0 MHz and 4.0-4.5 MHz Kellogg:78, Weatherwax:93. The emissions are left-hand polarized with respect to the magnetic field Shepherd:97a and are composed of fine structures as narrow as $\sim<$6 Hz LaBelle:95, Shepherd:98. Recently, using radio receivers at sites extending from 66.5$^\circ$ to 79.9$^\circ$ invariant latitude ($\Lambda$), Hughes:98 studied the variation of 2$f_{ce}$ and 3$f_{ce}$ auroral roar with latitude. They found that the auroral roar frequency increases with increasing latitude, confirming that auroral roar is associated with electron gyroharmonics and that the auroral roar at 2.6-3.0 MHz originates at $\sim$275 km altitude if it is generated at the second gyroharmonic. Surprisingly, the peak occurrence rate of 2$f_{ce}$ emissions was found to be at 75$^\circ$-76 $^\circ \, \Lambda$, near the poleward edge of the statistical auroral zone.

This study motivates a closer examination of auroral roar radio emissions measured with the chain of five radio receivers located at the magnetic observatories along an approximate magnetic meridian in northern Canada (see Table 1 and Figures 8 and 9). Magnetometers located at these sites and many others can be used to infer the locations of auroral currents and in particular the latitudes of the electrojet boundaries. By comparing these boundaries with the observed auroral roar emissions on selected days, the latitude dependence of these emissions can be understood in greater detail than was possible in the statistical approach of Hughes:98.

For this study, 5 days were selected from nearly 4 years of programmable stepped-frequency receiver (PSFR) data (1995-1998). Auroral roar is detected at all five sites in 3 out of the 5 days and at the four most poleward stations in the other 2 days. On all five selected days the intensity of the auroral emissions, as a function of latitude, may be compared with the locations of auroral currents inferred from the magnetometers. During two of the selected days the footprint of NASA's Fast Auroral Snapshot (FAST) satellite passes within several degrees of one or more ground stations at the same time auroral roar is recorded with PSFRs located at these stations. The FAST electron measurements provide a check of the auroral boundary inferred from the magnetometers and measure characteristics of the electron distribution above the probable source region of the emissions.


next up previous
Next: Instrumentation Up: Latitudinal dynamics of auroral Previous: Latitudinal dynamics of auroral


Simon Shepherd 2002-06-05