by neufer » Tue Nov 23, 2021 10:41 pm
https://en.wikipedia.org/wiki/Stellar_corona wrote:
<<Many coronal heating theories have been proposed, but two theories have remained as the most likely candidates: wave heating and magnetic reconnection (or nanoflares).
In 2012, high resolution soft X-ray imaging with the High Resolution Coronal Imager aboard a sounding rocket revealed tightly wound braids in the corona. It is hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the active solar corona to temperatures of up to 4 million kelvin. The main heat source in the quiescent corona (about 1.5 million kelvin) is assumed to originate from magnetic MHD waves. In plasma physics, an Alfvén wave, named after Hannes Alfvén, is a type of magnetohydrodynamic wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.
The convection zone of the sun, the region beneath the photosphere in which energy is transported primarily by convection, is sensitive to the motion of the core due to the rotation of the sun. Together with varying pressure gradients beneath the surface, electromagnetic fluctuations produced in the convection zone induce random motion on the photospheric surface and produce Alfvén waves. The waves then leave the surface, travel through the chromosphere and transition zone, and interact with the ionized plasma. The wave itself carries energy and some of the electrically charged plasma.
Recently, Alfvénic motions have been found in the lower solar atmosphere and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the Solar Dynamics Observatory. These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the Hinode spacecraft. After the seminal work of Jess et al. (2009), in 2017 Srivastava et al. detected the existence of high-frequency torsional Alfvén waves in the Sun's chromospheric fine-structured flux tubes. They discovered that these high-frequency waves carry substantial energy capable of heating the Sun's corona and also in originating the supersonic solar wind. In 2018, using spectral imaging observations, non-LTE (local thermodynamic equilibrium) inversions and magnetic field extrapolations of sunspot atmospheres, Grant et al. found evidence for elliptically-polarized Alfvén waves forming fast-mode shocks in the outer regions of the chromospheric umbral atmosphere. They provided quantification of the degree of physical heat provided by the dissipation of such Alfvén wave modes above active region spots. NASA's Parker Solar Probe is intended to approach the Sun to a distance of approximately 9.5 solar radii to investigate coronal heating and the origin of the solar wind.>>
[quote=https://en.wikipedia.org/wiki/Stellar_corona]
[float=right][img3=Alfvén(?) with tightly wound braids & MHD wave stocking]https://upload.wikimedia.org/wikipedia/en/7/78/L%C3%A5ngstrump_G%C3%A5r_Ombord.jpeg[/img3][/float][float=left][img3=Hinode image (Jan. 12, 2007) reveals the filamentary nature of the corona.]https://upload.wikimedia.org/wikipedia/commons/d/da/171879main_LimbFlareJan12_lg.jpg[/img3][img3=Magnetic MHD Alfvén S-waves, flow from the base of black hole jets.]https://upload.wikimedia.org/wikipedia/commons/f/fe/PIA19822-MagneticBlackHoleWaves-AlfvenS-waves-20150709.jpg[/img3][/float]
<<Many coronal heating theories have been proposed, but two theories have remained as the most likely candidates: wave heating and magnetic reconnection (or nanoflares).
[b][color=#FF0000]In 2012, high resolution soft X-ray imaging with the High Resolution Coronal Imager aboard a sounding rocket revealed tightly wound braids in the corona. It is hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the [u]active solar corona[/u] to temperatures of up to 4 million kelvin.[/color] [color=#0000FF]The main heat source in the [u]quiescent corona[/u] (about 1.5 million kelvin) is assumed to originate from magnetic MHD waves. In plasma physics, an Alfvén wave, named after Hannes Alfvén, is a type of magnetohydrodynamic wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.[/color][/b]
The convection zone of the sun, the region beneath the photosphere in which energy is transported primarily by convection, is sensitive to the motion of the core due to the rotation of the sun. Together with varying pressure gradients beneath the surface, electromagnetic fluctuations produced in the convection zone induce random motion on the photospheric surface and produce Alfvén waves. The waves then leave the surface, travel through the chromosphere and transition zone, and interact with the ionized plasma. The wave itself carries energy and some of the electrically charged plasma.
Recently, Alfvénic motions have been found in the lower solar atmosphere and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the Solar Dynamics Observatory. These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the Hinode spacecraft. After the seminal work of Jess et al. (2009), in 2017 Srivastava et al. detected the existence of high-frequency torsional Alfvén waves in the Sun's chromospheric fine-structured flux tubes. They discovered that these high-frequency waves carry substantial energy capable of heating the Sun's corona and also in originating the supersonic solar wind. In 2018, using spectral imaging observations, non-LTE (local thermodynamic equilibrium) inversions and magnetic field extrapolations of sunspot atmospheres, Grant et al. found evidence for elliptically-polarized Alfvén waves forming fast-mode shocks in the outer regions of the chromospheric umbral atmosphere. They provided quantification of the degree of physical heat provided by the dissipation of such Alfvén wave modes above active region spots. NASA's Parker Solar Probe is intended to approach the Sun to a distance of approximately 9.5 solar radii to investigate coronal heating and the origin of the solar wind.>>[/quote]