JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A8, PAGES 17,217-17,232, AUGUST 1, 2002

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Latitudinal dynamics of auroral roar emissions

S. G. Shepherd 1 and J. LaBelle
Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire
1Now at Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland.
C. W. Carlson
Space Sciences Laboratory, University of California, Berkeley

G. Rostoker
Department of Physics, University of Alberta, Edmonton, Alberta, Canada

Received February 25, 1999; revised April 30, 1999; accepted April 30, 1999

Abstract:

Auroral roar, a narrowband ( $\delta f/f < 0.1$) emission near 2 and 3 times the ionospheric electron gyrofrequency (2$f_{ce}$ and 3$f_{ce}$), is observed with a meridional chain of LF/MF/HF radio receivers located in northern Canada spanning 67$^\circ$ to 79$^\circ$ invariant latitude. Observations of these emissions are compared with the auroral electrojet location inferred from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer array. Variations in the intensity of the observed auroral roar emissions and in the invariant latitude of the most intense emissions are correlated with movements of the poleward boundary of the electrojet. For example, substorm onsets, which appear as rapid poleward expansions of this boundary, result in screening of the emissions from the underlying ground stations because of precipitation-induced ionization in the lower ionosphere. In four of the five study days the peak emission intensity is located 0$^\circ$-9$^\circ$ poleward of the poleward electrojet boundary inferred from the magnetometers. In one case the peak emission intensity is up to 10$^\circ$ equatorward of the poleward electrojet boundary. In all cases, there is a tendency for the latitude of the most intense auroral roar emissions to track the movements of the electrojet location inferred from the magnetometer data. For two examples, the footprint of the Fast Auroral Snapshot (FAST) satellite passes within 3$^\circ$ of one or more of the ground stations, and the satellite detects unstable electron populations in the polewardmost auroral arc, reinforcing the scenario that auroral roar emissions are generated by these electrons in the polewardmost arc and propagate into the polar cap where conditions are often favorable for their detection at ground level.