The very slow solar wind in the Inner Heliosphere

Abstract:

Measurements near 1AU of the bulk and compositional properties of the interplanetary plasma point to the existence of two solar winds that can be classified by their speeds, V, the fast (V>400 km s^-1) and slow winds (V<400 km s^-1). The slow solar wind is seldom observed slower than 300 km s^-1 at 1 AU. We show that, closer to the Sun, there is a big amount of solar wind slower than 300 km s^-1, hereafter very slow solar wind (VSSW). It is mostly detected inside 0.7 AU by the HELIOS spacecraft during solar maximum (1979-1980). The closer to the Sun the slower it can be observed, reaching velocities of 200 kms^-1 near 0.3 AU. This very slow wind usually contains the very dense heliospheric plasma sheet as well as the heliospheric current sheet. The very low speeds disappear by 1AU likely due to the interaction with the faster plasma. Solar Probe Plus will measure in-situ how low in the inner Heliosphere this interaction starts and whether even lower velocities are observed inside 0.3 AU.

The VSSW has higher density and lower temperature than regular slow solar wind, qualitatively extending the known scaling laws for the solar wind over 300 km s^-1(Lopez & Freeman, 1986) (Hundhausen, Bame, Asbridge, & Sydoriak, 1970). Like the rest of the slow solar wind, the helium abundance of the VSSW increase with solar activity, approaching to the fast wind composition at solar maximum. Combining a Potential Field Source Surface (PFSS) to a ballistic backmapping, we relate the ins-situ measurements to the solar surface. We compute the proton density flux just above the photosphere and find much higher fluxes in the VSSW than in the faster winds at solar maximum. Based on this, we propose a likely mechanism for the solar cycle variability of the helium abundance of the VSSW and slow solar wind, which will be tested by combining Solar Orbiter and Solar Probe Plus measurements of the VSSW with high resolution and high cadence Carrington maps.

This work was funded by the EU FP7 HELCATS project.