Unusual observation of chorus at L=2.6

On the 4
^th
August 2010, Very Low Frequency (VLF) chorus was observed on Marion Island (L = 2.6). This is a very unusual occurrence as chorus is thought to be generated outside the plasmapause, which would only extend to such low L during periods of severe geomagnetic activity. A similar event was observed at Palmer Station, Antarctica (L = 2.44) during the Halloween storms of 2003. The 2003 event had a minimum Dst of around -350nT, while the 2010 event occurred after a significantly smaller storm (Dst ~ -70nT). A further difference is that the 2003 event occurred during solar maximum, while the 2010 event occurred during a period where the Sun had been unusually inactive. The spatial extent of the 2010 event is discussed, and data from a broadband VLF ground receiver is presented. Empirical plasmapause location models reveal that Marion Island was just within the plasmasphere at the time of the event. Preliminary analysis of low resolution VLF data from another high latitude station shows that a similar emission was observed at that site, but at a slightly later universal time, which indicates that the generation region was moving. This movement is likely linked to the eastward drift of electrons, and also possibly to the plasmapause relaxing to its quiet time position.