Context. Resistive Ohmic dissipation has been suggested as a mechanism for heating the solar chromosphere, but few studies have established this association. Aims. We aim to determine how Ohmic dissipation by electric currents can heat the solar chromosphere. Methods. We combine high-resolution spectroscopic Ca II data from the Dunn Solar Telescope and vector magnetic field observations from the Helioseismic and Magnetic Imager (HMI) to investigate thermal enhancements in a sunspot light bridge. The photospheric magnetic field from HMI was extrapolated to the corona using a non-force-free field technique that provided the three-dimensional distribution of electric currents, while an inversion of the chromospheric Ca II line with a local thermodynamic equilibrium and a nonlocal thermodynamic equilibrium spectral archive delivered the temperature stratifications from the photosphere to the chromosphere. Results. We find that the light bridge is a site of strong electric currents, of about 0.3 A m −2 at the bottom boundary, which extend to about 0.7 Mm while decreasing monotonically with height. These currents produce a chromospheric temperature excess of about 600−800 K relative to the umbra. Only the light bridge, where relatively weak and highly inclined magnetic fields emerge over a duration of 13 h, shows a spatial coincidence of thermal enhancements and electric currents. The temperature enhancements and the Cowling heating are primarily confined to a height range of 0.4−0.7 Mm above the light bridge. The corresponding increase in internal energy of 200 J m −3 can be supplied by the heating in about 10 min. Conclusions. Our results provide direct evidence for currents heating the lower solar chromosphere through Ohmic dissipation.
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The magnetic topology of the inverse Evershed flow
Context. The inverse Evershed flow (IEF) is a mass motion towards sunspots at chromospheric heights. Aims. We combined high-resolution observations of NOAA 12418 from the Dunn Solar Telescope and vector magnetic field measurements from the Helioseismic and Magnetic Imager (HMI) to determine the driver of the IEF. Methods. We derived chromospheric line-of-sight (LOS) velocities from spectra of H α and Ca II IR. The HMI data were used in a non-force-free magnetic field extrapolation to track closed field lines near the sunspot in the active region. We determined their length and height, located their inner and outer foot points, and derived flow velocities along them. Results. The magnetic field lines related to the IEF reach on average a height of 3 megameter (Mm) over a length of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9) sunspot radii. The average field strength difference Δ B between inner and outer foot points is +400 G. The temperature difference Δ T is anti-correlated with Δ B with an average value of −100 K. The pressure difference Δ p is dominated by Δ B and is primarily positive with a driving force towards the inner foot points of 1.7 kPa on average. The velocities predicted from Δ p reproduce the LOS velocities of 2–10 km s −1 with a square-root dependence. Conclusions. We find that the IEF is driven along magnetic field lines connecting network elements with the outer penumbra by a gas pressure difference that results from a difference in field strength as predicted by the classical siphon flow scenario.
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- Award ID(s):
- 2050340
- NSF-PAR ID:
- 10404163
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 662
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A25
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202142585
