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Hydraulic effects in a radiative atmosphere with ionization
Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Inter University Centre for Astronomy and Astrophysics, India; Princeton University, USA.
Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. University of Colorado, USA.ORCID-id: 0000-0002-7304-021X
Rekke forfattare: 22016 (engelsk)Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 587, artikkel-id A90Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Context. In his 1978 paper, Eugene Parker postulated the need for hydraulic downward motion to explain magnetic flux concentrations at the solar surface. A similar process has also recently been seen in simplified (e.g., isothermal) models of flux concentrations from the negative effective magnetic pressure instability (NEMPI).

Aims. We study the effects of partial ionization near the radiative surface on the formation of these magnetic flux concentrations.

Methods. We first obtain one-dimensional (1D) equilibrium solutions using either a Kramers-like opacity or the H-opacity. The resulting atmospheres are then used as initial conditions in two-dimensional (2D) models where flows are driven by an imposed gradient force that resembles a localized negative pressure in the form of a blob. To isolate the effects of partial ionization and radiation, we ignore turbulence and convection.

Results. Because of partial ionization, an unstable stratification always forms near the surface. We show that the extrema in the specific entropy profiles correspond to the extrema in the degree of ionization. In the 2D models without partial ionization, strong flux concentrations form just above the height where the blob is placed. Interestingly, in models with partial ionization, such flux concentrations always form at the surface well above the blob. This is due to the corresponding negative gradient in specific entropy. Owing to the absence of turbulence, the downflows reach transonic speeds.

Conclusions. We demonstrate that, together with density stratification, the imposed source of negative pressure drives the formation of flux concentrations. We find that the inclusion of partial ionization affects the entropy profile dramatically, causing strong flux concentrations to form closer to the surface. We speculate that turbulence effects are needed to limit the strength of flux concentrations and homogenize the specific entropy to a stratification that is close to marginal.

sted, utgiver, år, opplag, sider
2016. Vol. 587, artikkel-id A90
Emneord [en]
radiative transfer, hydrodynamics, Sun: atmosphere, sunspots
HSV kategori
Identifikatorer
URN: urn:nbn:se:su:diva-129991DOI: 10.1051/0004-6361/201425396ISI: 000371589800101OAI: oai:DiVA.org:su-129991DiVA, id: diva2:927749
Tilgjengelig fra: 2016-05-13 Laget: 2016-05-09 Sist oppdatert: 2022-02-23bibliografisk kontrollert

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