SEARCH

SEARCH BY CITATION

Extremely low frequency (10–1500 Hz) magnetospheric chorus has been analyzed to investigate a possible dependence on substorms. Care was taken to separate spatial effects from temporal effects by analyzing an entire year of data acquired by the Ogo 5 search coil magnetometer. A major finding of the study of spatial dependences is that chorus occurs principally in two magnetic latitude regions. Equatorial chorus is detected near the equator, and high-latitude chorus is found at magnetic latitudes above 15°. When chorus in these two regions is analyzed separately, substorm dependences become apparent. Comparisons with AE indicate that equatorial chorus occurs primarily during substorms. High-latitude chorus is not strongly dependent on AE and often occurs during intervals of prolonged quiet with AE < 100 γ for the previous 12 hours or more. The dependence of equatorial chorus on local time, magnetic latitude, and L is consistent with generation by a cyclotron resonance between the whistler mode chorus and 10- to 100-keV trapped substorm electrons. Equatorial chorus has an abrupt onset in the postmidnight sector and a second enhancement from dawn to noon, a pattern which is similar to that of energetic electron precipitation. The occurrence frequency of equatorial chorus peaks at the equator, |magnetic latitude| < 5°, a region where cyclotron resonance is most efficient. The L value of maximum chorus occurrence increases from 5–8 postmidnight to 7–11 postdawn, a dependence which is consistent with generation by electrons which have undergone drift shell splitting. Delay times between substorms and the onset of equatorial chorus are consistent with a gradient drift of ∼25-keV electrons. Equatorial postmidnight chorus and postdawn chorus have similar occurrence rates and wave intensities. The maximum chorus occurrence rates are 54% postmidnight and 56% postdawn. Time-averaged equatorial chorus intensities ≥ 10−3 γ² are detected up to 17% of the time for 6 ≤ L ≤ 7 postmidnight and up to 14% of the time for 7 ≤ L ≤ 10 from dawn to noon. Such wave intensities are sufficient to cause near-strong pitch angle diffusion of electrons for L > 6 and strong diffusion for L > 8. Instantaneous diffusion rates may be considerably higher owing to the discrete burstlike nature of the chorus. The spatial and temporal dependences of high-latitude chorus are considerably different from those of equatorial chorus. High-latitude chorus occurs in local day and evening and at large L. The emission is detected primarily on the dayside, at 0800 ≤ LT ≤ 1600, and often within 1–2 RE of the magnetopause. The occurrence of high-latitude chorus during quiet intervals is consistent with local generation within ‘minimum B pockets.’