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Title: Dynamics of Coronal Hole Boundaries

Abstract

Remote and in situ observations strongly imply that the slow solar wind consists of plasma from the hot, closed-field corona that is released onto open magnetic field lines. The Separatrix Web theory for the slow wind proposes that photospheric motions at the scale of supergranules are responsible for generating dynamics at coronal-hole boundaries, which result in the closed plasma release. We use three-dimensional magnetohydrodynamic simulations to determine the effect of photospheric flows on the open and closed magnetic flux of a model corona with a dipole magnetic field and an isothermal solar wind. A rotational surface motion is used to approximate photospheric supergranular driving and is applied at the boundary between the coronal hole and helmet streamer. The resulting dynamics consist primarily of prolific and efficient interchange reconnection between open and closed flux. The magnetic flux near the coronal-hole boundary experiences multiple interchange events, with some flux interchanging over 50 times in one day. Additionally, we find that the interchange reconnection occurs all along the coronal-hole boundary and even produces a lasting change in magnetic-field connectivity in regions that were not driven by the applied motions. Our results show that these dynamics should be ubiquitous in the Sun and heliosphere.more » We discuss the implications of our simulations for understanding the observed properties of the slow solar wind, with particular focus on the global-scale consequences of interchange reconnection.« less

Authors:
;  [1]; ;  [2];  [3]
  1. Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109 (United States)
  2. Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
  3. Universities Space Research Association, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771 (United States)
Publication Date:
OSTI Identifier:
22661304
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 837; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; APPROXIMATIONS; DIPOLES; HELIOSPHERE; MAGNETIC FIELDS; MAGNETIC FLUX; MAGNETIC RECONNECTION; MAGNETOHYDRODYNAMICS; PLASMA; SOLAR WIND; SUN; SURFACES; THREE-DIMENSIONAL CALCULATIONS

Citation Formats

Higginson, A. K., Zurbuchen, T. H., Antiochos, S. K., DeVore, C. R., and Wyper, P. F. Dynamics of Coronal Hole Boundaries. United States: N. p., 2017. Web. doi:10.3847/1538-4357/837/2/113.
Higginson, A. K., Zurbuchen, T. H., Antiochos, S. K., DeVore, C. R., & Wyper, P. F. Dynamics of Coronal Hole Boundaries. United States. doi:10.3847/1538-4357/837/2/113.
Higginson, A. K., Zurbuchen, T. H., Antiochos, S. K., DeVore, C. R., and Wyper, P. F. Fri . "Dynamics of Coronal Hole Boundaries". United States. doi:10.3847/1538-4357/837/2/113.
@article{osti_22661304,
title = {Dynamics of Coronal Hole Boundaries},
author = {Higginson, A. K. and Zurbuchen, T. H. and Antiochos, S. K. and DeVore, C. R. and Wyper, P. F.},
abstractNote = {Remote and in situ observations strongly imply that the slow solar wind consists of plasma from the hot, closed-field corona that is released onto open magnetic field lines. The Separatrix Web theory for the slow wind proposes that photospheric motions at the scale of supergranules are responsible for generating dynamics at coronal-hole boundaries, which result in the closed plasma release. We use three-dimensional magnetohydrodynamic simulations to determine the effect of photospheric flows on the open and closed magnetic flux of a model corona with a dipole magnetic field and an isothermal solar wind. A rotational surface motion is used to approximate photospheric supergranular driving and is applied at the boundary between the coronal hole and helmet streamer. The resulting dynamics consist primarily of prolific and efficient interchange reconnection between open and closed flux. The magnetic flux near the coronal-hole boundary experiences multiple interchange events, with some flux interchanging over 50 times in one day. Additionally, we find that the interchange reconnection occurs all along the coronal-hole boundary and even produces a lasting change in magnetic-field connectivity in regions that were not driven by the applied motions. Our results show that these dynamics should be ubiquitous in the Sun and heliosphere. We discuss the implications of our simulations for understanding the observed properties of the slow solar wind, with particular focus on the global-scale consequences of interchange reconnection.},
doi = {10.3847/1538-4357/837/2/113},
journal = {Astrophysical Journal},
number = 2,
volume = 837,
place = {United States},
year = {Fri Mar 10 00:00:00 EST 2017},
month = {Fri Mar 10 00:00:00 EST 2017}
}
  • We investigate the effect of magnetic reconnection on the boundary between open and closed magnetic field in the solar corona. The magnetic topology for our numerical study consists of a global dipole that gives rise to polar coronal holes and an equatorial streamer belt, and a smaller active-region bipole embedded inside the closed-field streamer belt. The initially potential magnetic field is energized by a rotational motion at the photosphere that slowly twists the embedded-bipole flux. Due to the applied stress, the bipole field expands outward and reconnects with the surrounding closed flux, eventually tunneling through the streamer boundary and encounteringmore » the open flux of the coronal hole. The resulting interchange reconnection between closed and open field releases the magnetic twist and free energy trapped inside the bipole onto open field lines, where they freely escape into the heliosphere along with the entrained closed-field plasma. Thereafter, the bipole field relaxes and reconnects back down into the interior of the streamer belt. Our simulation shows that the detailed properties of magnetic reconnection can be crucial to the coronal magnetic topology, which implies that both potential-field source-surface and quasi-steady magnetohydrodynamic models may often be an inadequate description of the corona and solar wind. We discuss the implications of our results for understanding the dynamics of the boundary between open and closed field on the Sun and the origins of the slow wind.« less
  • Sequences of Skylab X-ray images near central meridian passages of a large solar coronal hole are used here to examine hole boundary changes with a time resolution of 90 min. It is found that X-ray bright points play a key role in both the expansion and contraction of the coronal hole. The size scale of the boundary changes is about 20,000 km, and the time scale is 3 hr or less. A scenario is suggested in which a large-scale closed magnetic field interacts with a bright point to effect a change in the hole boundary. 29 refs.
  • With the observations from the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we investigate the coronal hole boundaries (CHBs) of an equatorial extension of the polar coronal hole. At the CHBs, many extreme-ultraviolet jets, which appear to be the signatures of magnetic reconnection, are observed in the 193 A images, and some jets occur repetitively at the same sites. The evolution of the jets is associated with the emergence and cancellation of magnetic fields. We note that both the east and west CHBs shift westward, and the shift velocities are close tomore » the velocities of rigid rotation compared with those of the photospheric differential rotation. This indicates that magnetic reconnection at CHBs results in the evolution of CHBs and maintains the rigid rotation of coronal holes.« less
  • The interaction of open and closed field lines at coronal hole (CH) boundaries is widely accepted to be due to interchange magnetic reconnection. To date, it is unclear how the boundaries vary on short timescales and at what velocity this occurs. Here, we describe an automated boundary tracking method used to determine CH boundary displacements on short timescales. The boundary displacements were found to be isotropic and to have typical expansion/contraction speeds of {<=}2 km s{sup -1}, which indicate magnetic reconnection rates of {<=}3 x 10{sup -3}. The observed displacements were used in conjunction with the interchange reconnection model tomore » derive typical diffusion coefficients of {<=}3 x 10{sup 13} cm{sup 2} s{sup -1}. These results are consistent with an interchange reconnection process in the low corona driven by the random granular motions of open and closed fields in the photosphere.« less
  • Recurrent low energy (> or approx. =0.5 MeV) proton flux enhancements, reliable indicators of corotating plasma interaction regions in interplanetary space, have been observed on the Voyager 1 and 2 and Pioneer 11 spacecraft in the heliographic latitude range 2/sup 0/S to 23/sup 0/N and the heliocentric radial range 11 to 20 AU. After a period of rather high correlation between fluxes at different latitudes in early 1983, distinct differences develop. The evolution of the fluxes appears to be related to the temporal and latitudinal dynamics of solar coronal holes, suggesting that information about the latitudinal structure of solar windmore » stream sources propagates to these distances.« less