Beaming Angular Sizes of Individual AKR Bursts determined from Cluster WBD Observations

We have used the Wideband Data (WBD) instrument on four Cluster spacecraft to determine the beam angular size and sky distribution of more than 700,000 individual AKR bursts. We analyzed 15 observing epochs between July 2002 and May 2003 during which the Cluster constellation subtended at least 10 degrees angular separation as viewed from the AKR source region. After correcting for distance scaling, we compared flux densities at each of the four Cluster spacecraft in matching 300 ms by 1 KHz data windows. For each pair of spacecraft, we plotted the fractional number of bursts received at only one spacecraft compared with the total number. We fit a beaming model consisting of a randomly oriented uniformly filled beam. For most epochs the data are consistent with angular widths 39° +/- 7° (0.94 sr), 42° +/- 7° (0.98 sr), and 24° +/- 4° (0.73 sr) at emission frequencies of 125, 250, and 500 KHz respectively. These values are considerably smaller than previously reported values, which were determined by averaging over many individual AKR bursts. More complex beaming geometries are also possible, e.g., a hollow cone of emission. In the latter case, the derived beam sizes correspond to the cone thickness. These observations are the first direct measurement of the angular size of the radiation pattern of individual AKR bursts.