The large-scale structure of the heliospheric current sheet during the Ulysses epoch

Abstract

Ulysses is traversing the Sun's polar regions for the first time a year or two before solar minimum. If the heliospheric magnetic field behaves as we expect, the heliospheric current sheet (HCS) during this stage of the sunspot cycle should be quite stable and lie nearly flat, close to the equator. The high latitude solar fields should be unipolar and nearing their maximum strength. The overlying polar coronal holes should be well developed, producing a nearly uniform high-speed solar wind. Because the Sun's polar fields liver longer than any other solar magnetic phenomenon, Ulysses will remain within a single coronal hole for an extended period and probe its structures in a unique way.

Of course everything will almost certainly not turn out to be as quiet and well-ordered as we expect. We know now that the photospheric field strength in the northern polar cap is less than the south. What will this mean for the solar wind speed and the magnitude of the interplanetary magnetic field? Solar and coronal observations suggest that the first magnetic signatures of the next solar cycle may already be emerging at high latitudes. Will Ulysses sense these fields “leaking” into the heliosphere or is the interplanetary magnetic structure completely dominated by the large-scale field as the models currently predict? How will the sources of variations in field, velocity, density, and composition fit into our conceptual picture?

Ulysses' rapid dash from south to north provides a unique opportunity to determine the latitudinal width of the equatorial region influenced by the HCS. This rapid change in latitude over a short interval during which the structure should be stable, should provide a definitive measurement of the latitudinal gradients of various quantities relative to the heliomagnetic equator.

In the next several years Ulysses will gradually decrease in latitude, eventually meeting the HCS as it gradually expands away from the equator during the rising part of the cycle. It will then race the HCS to the poles as maximum approaches. The situation in six years, at solar maximum, will be radically different. There will be no unipolar high latitude field, the HCS will extend to the poles, existing structures will be less stable, and there will likely even be multiple current sheets. Ulysses will have another opportunity to confirm or disprove our understanding of the high latitude heliosphere in a much different environment. The extended mission is essential to advance our understanding of the heliospheric field at the extremes of the solar cycle.