The heliospheric current sheet

The heliospheric current sheet (HCS) is the boundary between open oppositel y directed magnetic field lines which commonly originate as the outward ext ension of the solar magnetic dipole. The dipole tilt, the rotation of the S un, and the outward propagation of the solar wind cause peaks and valleys i n the current sheet which spiral outward. The HCS extends throughout the he liosphere to the greatest distances reached by Pioneer and Voyager. It serv es as a magnetic equator, and solar wind parameters including speed, temper ature, density, and composition vary with distance from the HCS. Extrapolat ed back to the Sun, especially near solar minimum, the HCS corresponds to t he low-latitude streamer belt. Both features are closely related to a neutr al line obtained by extrapolating photospheric magnetic fields to a source surface at several solar radii. The current sheet and sector structure pers ist throughout the solar cycle including solar maximum. At 1 AU the width o f the HCS is approximately 10,000 km while a surrounding plasma sheet is th icker by a factor of similar to 30. The field inside the HCS does not simpl y decrease to a null and then reappear with the opposite sense. Instead, th e field rotates at nearly constant magnitude from one polarity to the other . In spite of theoretical expectations that fields on opposite sides of the HCS will merge or reconnect, there is little evidence that such is occurri ng. Many scientific questions remain unanswered. What are the global proper ties of the HCS near solar maximum, and how faithfully are they reproduced by source surface models? Are multiple HCS crossings caused by waves on the current sheet or by multiple current sheets? What is the effect of coronal mass ejections on the HCS and vice versa?