Ulysses

Launched in October1990, Ulysses will explore the Sun's polar regions. It has an orbit which swings far from the ecliptic plane, giving it a unique vantage point from which to study the sun. It measures radio frequency emissions produced by energetic electrons ejected by the sun. These electrons follow the magnetic field lines from the sun, and their interactions with the plasma of the solar wind generate radio emissions called Type III radio bursts.

Ulysses used a gravity-assist maneuver with Jupiter to rotate its orbital plane far out of the ecliptic, becoming the first spacecraft to go to high ecliptic latitudes (max 80¡) and over the poles of the sun. To monitor radio emissions, it has a 72.5 meter long dipole antenna and a sensitive receiver with 76 discrete frequency channels between 1 and 940 kHz. The radio bursts further from the sun have lower characteristic frequencies, so the multiband detector can map the bursts at different distances to picture the sun's magnetic field.

The electrons from the sun travel at speeds in the range 0.1 to 0.3c so that they sweep out to the radius of the earth's orbit in about 30 minutes. This allows mapping of a magnetic field line in a short time to get a near-instantaneous plot of the sun's magnetic field. The field lines were found to be a spiral form, as had been predicted as a result of the outward solar wind and the rotation of the sun. The form is called an archimedean spiral.

Sun's Magnetic Field from Ulysses

The electrons from the sun travel at speeds in the range 0.1 to 0.3c so that they sweep out to the radius of the earth's orbit in about 30 minutes. This allows mapping of a magnetic field line in a short time to get a near-instantaneous plot of the sun's magnetic field. The field lines were found to be a spiral form, as had been predicted as a result of the outward solar wind and the rotation of the sun. The form is called an archimedean spiral.