Heliospheric plasma sheet and coronal streamers

Helios 2 measurements of solar wind plasma and magnetic field are used to investigate the structure of the heliospheric plasma sheet between 0.3 and 1 AU. In agreement with previous observations at 1 AU, the plasma sheet thickness is much larger than that of the embedded current sheet. The plasma sheet appears surrounded by a density halo, a region of slightly raised density. High‐time resolution data show that decreases in relative helium abundance coincide with the plasma sheet boundaries, reinforcing the notion that the solar wind within the plasma sheet is of a different nature (with different solar origins) than that outside it. Although radio occultation measurements of the corona were not available at the time of the Helios data, a synthesis of recent results on coronal streamers shows that there is a remarkable similarity between their major features and those of plasma sheets, demonstrating that the coronal counterpart of the plasma sheet is the stalk of the coronal streamer. These measurements also suggest that the density halo seen in the Helios data is associated with the radial extension of the boundaries of the streamer observed in the extended corona before the streamer narrows to a stalk.