On the source location of low-frequency heliospheric radio emissions

We used a combination of three different types of source localization to determine the two-dimensional source positions for low-frequency heliospheric radio emissions in ecliptic coordinates. These techniques include time of flight, rotating dipole direction finding, and a comparison of received power from two widely spaced receivers. Most of the source locations are within a few tens of degrees of the heliospheric nose, although some sources are located as far as 90degrees from the nose. The distribution is roughly linear and aligned generally parallel to the galactic plane. We conclude that the magnetic field in the local interstellar medium is the only obvious means by which to impose an asymmetry to the set of source locations about the nose; it is likely these measurements confirm that the previously proposed model for the galactic magnetic field with a primarily longitudinal orientation parallel to the galactic plane is present just upstream of the heliosphere. The apparent sizes of the radio sources range from very small (a few degrees at the most) to a few tens of degrees. In a couple of cases the source half-angle could approach 80 degrees. We suggest that the true source size is quite small and that the larger sizes found here are likely due to scattering in the intervening medium. The fact that the apparent source size varies considerably implies that scattering in the outer heliosphere is a function of time, position, or both.