Small-scale Flux Emergence, Coronal Hole Heating, and Flux-tube Expansion: A Hybrid Solar Wind Model. (arXiv:2104.04016v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Wang_Y/0/1/0/all/0/1">Y.-M. Wang</a>
Extreme-ultraviolet images from the Solar Dynamics Observatory often show
looplike fine structure to be present where no minority-polarity flux is
visible in magnetograms, suggesting that the rate of ephemeral region (ER)
emergence inside “unipolar” regions has been underestimated. Assuming that this
rate is the same inside coronal holes as in the quiet Sun, we show that
interchange reconnection between ERs and open field lines gives rise to a solar
wind energy flux that exceeds 10$^5$ erg cm$^{-2}$ s$^{-1}$ and that scales as
the field strength at the coronal base, consistent with observations. In
addition to providing Ohmic heating in the low corona, these reconnection
events may be a source of Alfv{‘e}n waves with periods ranging from the
granular timescale of $sim$10 minutes to the supergranular/plume timescale of
many hours, with some of the longer-period waves being reflected and dissipated
in the outer corona. The asymptotic wind speed depends on the radial
distribution of the heating, which is largely controlled by the rate of
flux-tube expansion. Along the rapidly diverging flux tubes associated with
slow wind, heating is concentrated well inside the sonic point (1) because the
outward conductive heat-flux density and thus the outer coronal temperatures
are reduced, and (2) because the net wave energy flux is dissipated at a rate
proportional to the local Alfv{‘e}n speed. In this “hybrid” solar wind model,
reconnection heats the lower corona and drives the mass flux, whereas waves
impart energy and momentum to the outflow at greater distances.
Extreme-ultraviolet images from the Solar Dynamics Observatory often show
looplike fine structure to be present where no minority-polarity flux is
visible in magnetograms, suggesting that the rate of ephemeral region (ER)
emergence inside “unipolar” regions has been underestimated. Assuming that this
rate is the same inside coronal holes as in the quiet Sun, we show that
interchange reconnection between ERs and open field lines gives rise to a solar
wind energy flux that exceeds 10$^5$ erg cm$^{-2}$ s$^{-1}$ and that scales as
the field strength at the coronal base, consistent with observations. In
addition to providing Ohmic heating in the low corona, these reconnection
events may be a source of Alfv{‘e}n waves with periods ranging from the
granular timescale of $sim$10 minutes to the supergranular/plume timescale of
many hours, with some of the longer-period waves being reflected and dissipated
in the outer corona. The asymptotic wind speed depends on the radial
distribution of the heating, which is largely controlled by the rate of
flux-tube expansion. Along the rapidly diverging flux tubes associated with
slow wind, heating is concentrated well inside the sonic point (1) because the
outward conductive heat-flux density and thus the outer coronal temperatures
are reduced, and (2) because the net wave energy flux is dissipated at a rate
proportional to the local Alfv{‘e}n speed. In this “hybrid” solar wind model,
reconnection heats the lower corona and drives the mass flux, whereas waves
impart energy and momentum to the outflow at greater distances.
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