Ulysses returns to the Sun’s polar cap

The Ulysses spacecraft continues to go where no other spacecraft has gone before, namely, over the Sun’s poles to study the Sun and its influence on the space environment. On Friday, 17 November, Ulysses once again crossed into the Sun’s south polar cap as it tracks the sunspot cycle and the cycle of solar activity. Previous spacecraft have remained near the Sun’s equator where the Earth and other planets are located, however, Ulysses’ orbit is perpendicular or highly inclined to all other spacecraft orbits providing a unique perspective from which to study the Sun and its effect on surrounding space.

Ulysses has twice before orbited the Sun’s polar caps, first during a minimum in the number of sunspots and then when the sunspot number was at its maximum. This third orbit is also occurring during a sunspot minimum but under very different circumstances after a reversal in the magnetic poles of the Sun.

Understanding solar activity is important not only because the Sun is an average star that is available for close scrutiny but because it has important consequences for Earth and its inhabitants as we continue to move into a new era of space-based technology and are able to send people into space and its hazards. Solar activity and the sunspots are driven by the solar magnetic field that changes dramatically over a 22-year cycle. During the first Ulysses orbit, the Sun’s magnetic poles were positive with outward fields in the north and negative or inward fields in the south. During Ulysses second orbit at sunspot maximum, the Sun’s polar fields disappeared and then reappeared with the opposite sense, negative or inward in the north and positive or outward in the south. In addition, the strength of the polar fields is now about one-half of what they were during the first orbit. Consequently, Ulysses is currently poised to investigate this changed magnetic field and its effect on the solar wind, galactic cosmic rays and the other constituents in space.


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