THE EVOLUTION OF PLASMA PARAMETERS ON A CORONAL SOURCE SURFACE AT 2.3R_☉DURING SOLAR MINIMUM

Abstract

We analyze data from the Solar and Heliospheric Observatory to produce global maps of coronal outflow velocities and densities in the regions where the solar wind is undergoing acceleration. The maps use UV and white light coronal data obtained from the Ultraviolet Coronagraph Spectrometer and the Large Angle Spectroscopic Coronagraph, respectively, and a Doppler dimming analysis to determine the mean outflow velocities. The outflow velocities are defined on a sphere at 2.3 R from Sun-center and are organized by Carrington Rotations during the solar minimum period at the start of solar cycle 23. We use the outflow velocity and density maps to show that while the solar minimum corona is relatively stable during its early stages, the shrinkage of the north polar hole in the later stages leads to changes in both the global areal expansion of the coronal hole and the derived internal flux tube expansion factors of the solar wind. The polar hole areal expansion factor and the flux tube expansion factors (between the coronal base and 2.3 R ) start out as super-radial but then they become more nearly radial as the corona progresses away from solar minimum. The results also support the idea that the largest flux tube expansion factors are located near the coronal hole/streamer interface, at least during the deepest part of the solar minimum period.

Coronal Heating by Surface Alfvén Wave Damping: Implementation in a Global Magnetohydrodynamics Model of the Solar Wind

“…The edges of coronal hole boundaries must expand much more rapidly than radially (so-called superradially) in order to fill the space above the streamers. This superradial expansion of field lines at heights of 1-2 solar radii from the surface has been verified observationally with SOHO/UVCS observations (Dobrzycka et al 1999;Strachan et al 2012). Strachan et al (2012) confirm that, for solar minimum conditions, the largest flux tube expansion occurs near the interface of coronal holes and streamers (i.e., the open/closed magnetic field boundaries).…”

“…This superradial expansion of field lines at heights of 1-2 solar radii from the surface has been verified observationally with SOHO/UVCS observations (Dobrzycka et al 1999;Strachan et al 2012). Strachan et al (2012) confirm that, for solar minimum conditions, the largest flux tube expansion occurs near the interface of coronal holes and streamers (i.e., the open/closed magnetic field boundaries). Previous modeling efforts of one-and two-dimensional (1D/2D) open flux tubes with prescribed magnetic field geometry and solar wind parameters demonstrated that SAW damping could be a significant contribution to heating in these regions (Jatenco-Pereira & Opher 1989;Narain & Sharma 1998).…”

Observations of the Solar Corona from Space

“…The comparison of coronal and heliospheric abundance data support the interpretation that the main contribution to the slow wind comes from the boundary layers of coronal holes, where the outflowing plasma is dynamically fractionated because of the low proton fluxes associated with the large geometrical expansion factors of the flow tubes surrounding coronal streamers. Strachan et al (2012) analyzed the evolution of the outflow velocity and density of the corona, combining the UVCS and LASCO data, in order to measure the expansion of the solar wind from the coronal base to the source surface height, set at 2.3 R , during the transition from the solar minimum of cycle 22 to the rising phase of cycle 23, in the period from 1996 to 1998. When the activity rises and the polar holes shrink, Strachan et al (2012) observe changes not only in the global area of the coronal hole but also in the areal divergence of the flux tubes channeling the solar wind.…”

“…Strachan et al (2012) analyzed the evolution of the outflow velocity and density of the corona, combining the UVCS and LASCO data, in order to measure the expansion of the solar wind from the coronal base to the source surface height, set at 2.3 R , during the transition from the solar minimum of cycle 22 to the rising phase of cycle 23, in the period from 1996 to 1998. When the activity rises and the polar holes shrink, Strachan et al (2012) observe changes not only in the global area of the coronal hole but also in the areal divergence of the flux tubes channeling the solar wind. The very large expansion factors of the flux tubes observed near the streamer belt during solar minimum tend to vanish when the overall size of the polar holes decreases.…”

“…The very large expansion factors of the flux tubes observed near the streamer belt during solar minimum tend to vanish when the overall size of the polar holes decreases. According to Strachan et al (2012), larger coronal holes have the largest areal expansion and emanate the fastest solar wind. This result is consistent with the finding that there is a correlation of the area of coronal holes and the solar wind speed (Zhao et al 2017;Tokumaru et al 2017).…”