The directivity of solar type III bursts at hectometer and kilometer wavelengths: Wind-Ulysses observations

We investigate the directivity of solar type III radio bursts at hectometer and kilometer wavelengths using radio data recorded simultaneously by the Wind and Ulysses spacecraft which are widely separated in heliocentric longitude and latitude. From the positions of the associated flares and the solar wind speed and plasma density measured aboard the spacecraft, we estimate the location of the type III radio sources along the approximately spiral magnetic field lines connected to the flare sites. Using the ratios of the radio flux densities measured from 1995 to 2005 at the same frequencies (940-80 kHz) on these spacecraft, we have for the first time derive the average directivity diagram of type III bursts in two dimensions, longitude and latitude relative to the emission source. This diagram is found to exhibit no significant variation with the solar activity. The diagram in longitude is found to shift east of the local magnetic field direction at the source. The angular width and eastward shift of the diagram are observed to increase with decreasing frequencies; around 800 kHz, its full width at maximum/10 level is about 190 degrees and its eastward shift 23 degrees. The diagram in latitude, obtained uniquely thanks to the out-of-ecliptic orbit of Ulysses, shows no significant variation with latitude.