Drivers of chorus in the outer dayside magnetosphere

Abstract

Using ELF/VLF wave data recorded in 2007 from two high-latitude (Lambda = 69.8 degrees, 71.8 degrees) Antarctic ground stations, the dayside variation of chorus wave occurrence and amplitude are analyzed in conjunction with geomagnetic and solar wind driving parameters. Both stations observe chorus (defined here as discrete rising emission tones together with unstructured hiss) in a broad window of local time across the dayside when the stations are on closed magnetic field lines. Wave occurrence rates rise rapidly from similar to 0.06-0.12 at dawn to their maximum value of similar to 0.5-0.6 near local noon. The event amplitudes also peak near noon. Occurrence probabilities at the lower-latitude station are consistently higher with the average difference in the rate between the two stations being 0.15. In addition, similar to 80% of the time, event amplitudes are larger at the lower-latitude site. When the stations are in the dawn local time sector (5.5 < MLT < 10), the onset of waves is clearly linked to substorms, as seen by the AE index as well as by energetic electron injections observed at geosynchronous orbit. However, as the stations rotate to noon (MLT > 10), wave occurrence rates appear to be relatively independent of geomagnetic activity as measured by K-p and AE. Chorus near noon at times appears related to substorm activity, but intense waves can also be observed during extended quiet periods. Waves across the entire dayside are more likely during higher solar wind dynamic pressure as well as during significant changes in pressure. We attribute the high occurrence rate of outer dayside chorus to several effects resulting from solar wind compression of the dayside magnetosphere; the first is electron drift shell splitting, and the second is the creation of a region of high magnetic field homogeneity which is particularly favorable for wave growth.