Directivity Patterns of Complex Solar Type III Radio Bursts: Stereoscopic Observations

Abstract:

Complex solar type III-like radio bursts are a group of type III bursts that occur in association with slowly drifting type II radio bursts excited by coronal mass ejection (CME) driven shock waves. We present
simultaneous observations of these radio bursts from the STEREO A, B and WIND spacecraft at low frequencies, located at different vantage points in the ecliptic plane. Using these stereoscopic observations, we
determine the directivity of these complex radio bursts. We estimate the angles between the directions of the magnetic field at the sources and the lines connecting the source to the spacecraft (viewing angles) by assuming that the sources are located on the Parker spiral magnetic field lines emerging from the associated active regions into the spherically symmetric solar atmosphere. We estimate the normalized peak intensities of these bursts (directivity factors) at each spacecraft using their time profiles at each spacecraft. These observations indicate that the complex type III bursts can be divided into two groups: (1) bursts emitting into a very narrow cone centered around the tangent to the magnetic field, and (2) bursts emitting into a wider cone. We show that the bursts , which are emitted along the tangent to the spiral magnetic field lines at the source are very intense, and their intensities steadily fall as the viewing angles increase to higher values. We have developed a ray tracing code and computed the distributions of the trajectories of rays emitted at the fundamental and second harmonic of the electron plasma frequency. The comparison of the observed emission patterns with the computed distributions of the ray
trajectories indicate that the intense bursts visible in a narrow range of angles around the magnetic field directions probably are emitted in the fundamental mode, whereas the relatively
weaker bursts visible to a wide range of angles are probably emitted in the harmonic mode.