Heliospheric 2-3 kHz radio emissions are produced when an interplanetary shock interacts with the heliopause, which is the boundary between the solar wind and the interstellar plasma. The sequence of events is illustrated in Figures 1 and 2 below. About once per solar cycle, usually shortly after solar maximum, a period of exceptional solar activity occurs, with numerous coronal mass ejections over a period of a month or more. As time advances, the shock waves from these disturbances merge into an exceptionaly strong quasi-spherical interplanetary shock wave and associated plasma disturbance (called a global merged interaction region) that can propagate to great distances from the Sun. Figure 1 shows such a shock wave. Interplanetary shock waves of this type have been detected by various spacecraft, such as Pioneers 10 and 11 (now no longer operating) and Voyagers 1 and 2, indicated by P10, P11, V1, and V2 in Figure 1. As this shock wave propagates outward from the Sun it eventually crosses the termination shock, where the solar wind becomes subsonic, and continues on until it reaches the heliopause. When the shock crosses the heliopause into the cooler, more dense interstellar medium it generates radio emissions at the electron plasma frequency, f p , and its harmonic, 2f p . The radio emission is then detected by Voyager, as illustrated in Figure 3, which shows a typical profile of the electron plasma frequency as a function of radial distance from the Sun. The color frequency-time spectrogram shown above, which covers a time period of 22 years (from 1982 through 2003), has three such events, a moderately strong event in 1983-84, a very strong event in 1992-94, and a new weak 2002-03 event. Typically it takes a year or more for the shock to travel from the Sun to the heliopause, but less than a day for the radio emission to propagate from the source region to Voyager. Don Gurnett

Principal Investigator of the Voyager PWS

