Temporal properties of magnetospheric line radiation

Magnetospheric line radiation (MLR) events are relatively narrowband VLF si gnals that sometimes drift in frequency and have been observed in both grou nd-based and satellite data sets. We present the results of a survey undert aken on the basis of measurements made of MLR events observed at Halley, An tarctica (75 degrees 30'S, 26 degrees 54'W, L approximate to 4.3), in June, July, September, and December 1995, specifically looking at the temporal p roperties of Halley MLR events. We find that (1) single MLR lines described in previous papers tend to be comprised of up to 3 lines with widths of 5- 10 Hz. (2) The multiple lines show highly variable spacings (e.g., < 10 to similar to 100 Hz). (3) The rate at which MLR lines appears from the noise is similar to 0.1 to 0.2 dB s(-1), considerably smaller than previously rep orted for other VLF emissions; MLR exhibits slow growth, relative frequency stability, and long lifetimes in comparison with other coherent whistler m ode emissions and thus may be generated by a separate mechanism. (4) The di urnal occurrence of MLR observed at Halley is twin-peaked, indicating an as sociation with a combination of chorus and midlatitude hiss. (5) MLR occurr ence rates can vary greatly within a given month, but the proportion of MLR present as part of the overall Halley wave activity is roughly constant (1 0-13%) throughout the year. (6) The occurrence of MLR activity at Halley is weakly linked to geomagnetic activity but only 24-48 hours after very larg e storms (Kp > 6), which affects only 8% of the total MLR events in this st udy. For smaller storms, there is little effect, although MLR events tend n ot to occur when the geomagnetic activity has been quiet in the previous 48 hours. (7) There is no dependence of MLR occurrence rates upon the instant aneous levels of geomagnetic activity. (8) The average duration of a typica l MLR event at Halley is similar to 30 min, quite similar to previous repor ts.