gh EXTERNAL MAGNETIC FIELD CAUSES THE PUZZLING HEATING OF THE SOLAR CORONA

The Russian astrophysicists have theoretically modelled coronal loop oscillations and have shown that the plasma present in coronal loops is quite "normal". So, the puzzle of the Sun's atmosphere heating remains unresolved. Coronal loops, immense magnetic arches more hot and dense than the coronal gas, are formed by magnetic fields. They are anchored in the Sun's visible surface rising up to 100 - 200 thousand kilometres. In the active regions of the Sun these loops, which are 1000 - 3000 kilometres in diameter, form bunches and tangles.

In 1998 the American Transition Region and Coronal Explorer (TRACE) spacecraft with the imaging telescope onboard has been launched. Images of hot coronal loops were obtained and researchers observed oscillations of coronal loops near the epicentre of a solar flare. But these oscillations were damped quickly within a few oscillation periods in the way the process occurred not in the rare coronal plasma but in the rather viscous environment resembling honey.

Calculations made by the American scientists showed that the viscosity of the coronal plasma was billion times higher than supposed. And if the plasma is so viscous, its conductivity must be billion times lower than in the normal one and electric currents related with magnetic fields would effectively damp in this kind of plasma. And such a mechanism can cause heating of the solar corona.

After studying the images obtained by TRACE spacecraft the Russian scientists have made a physical model which describes loop oscillations in the plasma of normal density but they introduced in their model an external magnetic field. They saw that a loop oscillating with observed amplitude transforms all energy of the primary boost into the wave energy dissipation in the environment during only two or three periods of oscillation. What can be the cause of such a magnetic field? Nobody can measure magnetic fields in the coronal plasma. "We have made an assumption. Gas density and temperature in a loop are higher than in its environment, so a force exists that protects the loop from simultaneous disintegration. Only pressure of external magnetic field can play the role of such a force", says Alexander Solovjev who leads the Russian research team. According to Solovjev, the puzzle of the Sun's corona heating remains unresolved prompting new theories and hypotheses.

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