THE EFFECT OF THE MIDLATITUDE IONOSPHERIC TROUGH ON WHISTLER-MODE DUCTING DURING MAGNETIC STORMS

Whistler-mode signals observed at Faraday, Antarctica (65 degrees S, 6 4 degrees W, Lambda=50.8 degrees) show anomalous changes in group dela y and Doppler shift with time during the main phase of intense geomagn etic activity. These changes are interpreted as the effect of refracti ng signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough . The refraction region is observed to propagate equatorwards at veloc ities in the range 20-85 ms(-1) during periods of high geomagnetic act ivity (K-p greater than or equal to 5), which is in good agreement wit h typical trough velocities. Model estimates of the time that the trou gh edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46 degrees S, 171 degrees E, Lambda = 52.5 degrees) that have propagated at an average L-shell. of 2.2 (La mbda=47.60) do not show such trough-related changes in group delay. Th ese observations are consistent with a lower occurrence of the trough at lower invariant latitudes.