Yet the corona is hundreds of times hotter than the Sun’s surface.Ī NASA mission called IRIS may have provided one possible answer. The corona is in the outer layer of the Sun’s atmosphere-far from its surface. This is the opposite of what seems to happen on the Sun.Īstronomers have been trying to solve this mystery for a long time. But when you walk away from the fire, you feel cooler. Imagine that you’re sitting next to a campfire. The corona’s high temperatures are a bit of a mystery. Image of corona from NASA's Solar Dynamics Observatory showing features created by magnetic fields. This low density makes the corona much less bright than the surface of the Sun. Why? The corona is about 10 million times less dense than the Sun’s surface. The corona reaches extremely high temperatures. Find tips on how to safely view an eclipse here. It is also confirmed that an atmosphere of homogeneous vertical magnetic fields does not produce the high temperatures observed in the corona above unipolar regions such as plage.Remember to never look directly at the Sun, even during an eclipse. This is probably because an increased mean separation distance between magnetic poles allows a more complex hierarchy of current sheets to evolve. The simulations indicate that the coronal heating increases with the typical separation distance between magnetic poles in the photosphere, at least when this separation distance is shorter than 6-7 Mm (which is approximately the numerical upper limit for typical separation distances in the models evolved in this thesis). It is plausible that the magnetic field structure in the QS photosphere is in the form of a “salt-pepper” pattern with poles of upwardand downward-oriented fields. The results confirms that both the tiny current sheets related to nanoflares and the hierarchy of largescale current sheets are plausible mechanisms for coronal heating. Five theoretical models with different magnetic field configurations are evolved over time intervals of 30-80 min of solar time, and the resulting coronal temperatures and amounts of Joule heating (ohmic heating) in each model are analyzed, compared to each other and compared to the corresponding results of a standard model evolved by Hansteen et al. To analyze the problem, the numerical code Bifrost is applied to solve the MHD equations on three-dimensional cutouts of the quiet-Sun (QS) atmosphere. ![]() Both mechanisms are actively referred to in the later chapters of this thesis. Two plausible heating mechanisms are discussed, both related to the generation of current sheets: 1) the stressing of a magnetic field which collapses into tiny current sheets (width of order 10 m) which eventually burst out as a nanoflare, a mechanism introduced by Parker (1988), and 2) a hierarchy of current sheets, analyzed by Galsgaard & Nordlund (1996), which also includes large-scale current sheets (width of several megameters) not related to nanoflares. Finally, the corona is brought into discussion, which leads us to the coronal heating problem. The question is: how does the coronal heating depend on the photospheric magnetic fields? That is the problem which this thesis focuses on.īefore investigating the problem, an introduction to the Sun is given, reviewing everything from the basics of a general star to the structure of the entire Sun, going through each layer, with focus on the atmosphere. As of today, most solar physicists agree that the mechanism that heats the corona is connected to the dynamics of the magnetic fields in the photosphere. This fact has puzzled solar physicists for more than six decades. AbstractThe solar corona has a temperature of order 1 MK, which is almost 200 times the temperature of the underlying surface.
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