URAP Science

Solar/Interplanetary radio emissions

Radio emissions occurring in the solar corona and in the interplanetary medium are usually produced by streams of electrons traveling at speeds many times faster than the solar wind velocity. The radio bursts observed by Ulysses fall into two categories -- type II and type III bursts. The former are produced by electrons accelerated at shocks traveling through the solar wind. They typically drift from 1 MHz to 10 kHz in about a day, which corresponds to the motion of the shock waves outward from the Sun. Consequently, the type II burst permits remote tracking of the motion of the shock in the solar wind. The type III bursts result when electrons are accelerated in the low corona, in association with solar flares or other solar activity. These electrons may then stream out from Sun with velocities approaching one-half the speed of light. They produce radio bursts whose drift from high to low frequencies occurs much faster than the type II bursts.

Example of type II burst observed by URAP
Example of type III bursts observed by URAP
 Explanation of URAP  dynamic spectrum

Waves in the Solar Wind

Fluctuations in the solar wind plasma and magnetic field are the natural consequences of unstable particle distributions. One example is the streaming electrons that escape from the corona at the time of a solar flare. These energetic electrons excite oscillations of the solar wind plasma at a frequency known as the electron plasma frequency (f_pe). URAP observes these waves when the electrons stream past the spacecraft. These waves play an important role in the production of type III radio bursts. Indeed, the study of this complicated relationship between the electrons, plasma waves, and radio waves continues today.

Example of waves at f_pe and associated type III burst

A variety of other plasma wave species also exist in the solar wind.

Jupiter and Other Planets

Each of the planets with an internal magnetic field produces a magnetosphere which surrounds the planet and deflects the solar wind around it. Typically, these magnetospheres are sources of a variety of radio emissions, produced by the energetic electrons that are trapped in the magnetic fields. Ulysses has observed radio emissions from Jupiter, Saturn, and the Earth; of these three planets, Jupiter has by far the most varied population of radio emissions. Analysis of the Jovian radio bursts by Ulysses was greatly enhanced by the close flyb-by of the planet. Ulysses passed within 6 Jovian radii of Jupiter (about 420,000 km) on February 8, 1992 (see image below). Inside the Jovian magnetosphere, URAP also observed many intense waves at frequencies below that of the freely-propagating radio waves; these are described in the tutorial (to be available soon).

Jovian radio emissions on the day of the Ulysses fly-by

Jupiter Distant Encounter

Ulysses's orbit brought it into the vicinity of Jupiter during 2003 and 2004. During this encounter it made a unique path that reached high Jovian latitudes. Ulysses investigators made detailed observations in order to take advantage of this unique opportunity. Details of the encounter are given here.

Other applications of URAP data

The waves observed by URAP are primarily studied to better understand the physical mechanisms that create them, but the observations also permit the determination of important parameters describing the plasma and magnetic field in which the spacecraft travels. Examples include:

  • measuring the electron density, temperature, and bulk velocity by fitting the thermal noise spectrum of the electrons (see example of spectrum below)
  • measuring the electron density in the solar wind (and electron density and magnetic field strength in the Jovian magnetosphere) using the resonance sounder
  • identifying rapid fluctuations in the solar wind, which produce waves but which occur too quickly to be detected by the Ulysses particle instruments

Sample thermal noise spectrum