Antarctic Observations of Dayside Auroral Hiss

Ground-level auroral hiss, particularly at LF (>30 kHz), is strongly correlated with onsets of polar substorms and occurs predominantly in the pre-midnight sector. Although satellites record auroral hiss in the cusp, there have been only a few reports of dayside auroral hiss at ground level. We performed a statistical study of LF hiss recorded at South Pole Station in 2004, 2005, and 2007. As expected, the vast majority of the events occur in the premidnight sector. However, there is a secondary peak in the occurrence rate in the pre-noon sector (69 examples of prenoon LF hiss), at 1000-1500 UT, which is about 0630-1130 MLT. Individual events appear similar to nightside hiss, but statistically they do not extend to as high frequencies: the median upper frequency bound was about 200 kHz, but in some cases the upper bound extended to 500 kHz. The median duration was <1-2 minutes, with three events lasting more than 30 minutes. These features are indistinguishable from nightside hiss. Using ACE satellite data, the IMF By component could be unambiguously determined for 50 events: 11 corresponded to positive By, 35 to negative By and 4 to By ~0. So, there is a propensity for By negative. We found no evidence of solar wind signatures or impulses on the magnetopause at times of these events. We examined all sky camera data for 34 events: 21 of these events had auroral arcs overhead, sometimes isolated and often active; 13 events had cloudy skies or no activity or an arc on the horizon. One event, on July 19, 2005, was associated with a faint red arc. We examined magnetometer, riometer, VLF and photometer signatures for 43 events: 24 of these events were associated with magnetometer signatures; 16 with riometer signatures; 37 with VLF signatures; and 21 of 28 events with photometer signatures (15 events had no photometer data). This study shows that the dayside LF hiss often extends to VLF frequencies, and further study will determine the relative intensity.