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Session 86 - Coronal Activity.
Oral session, Thursday, June 13
Filaments with Ha fibril structure resembling right handed screws (dextral) predominate in the northern hemisphere while the left-handed ones (sinistral) predominate in the south. The "dextral" filaments modeled as flux ropes have left handed field lines and negative helicity while the "sinistral" filaments have positive helicity. The interplanetary magnetic clouds are caused by erupting filaments and they have the same helical field structure and hemispherical preference for handedeness as the filaments. Magnetic helicity conservation in expanding plasmas implies that the magnetic energy of the system might decrease with expansion. We argue that some of this lost magnetic energy can go into internal heating. Various expanding plasmas such as "Interplanetary Magnetic Clouds" and the "Solar Wind" do appear to be heated much more than what can be explained by simple adiabatic expansion. A simple flux rope model for magnetic clouds incorporating conservation of magnetic helicity and flux provides "scaling laws" (dependence of magnetic, geometric and thermodynamic quantities on distance from the Sun) in agreement with the observations. High temperatures in magnetic clouds are adequately explained by the lost magnetic energy even if one were to account for mechanical losses. The solar wind also shows a hemispherical preference for helicity in the deviation of the interplanetary spiral field's winding angle from the Parker solution. The helicity spectra of the solar wind also show presence of helicity at all the length scales. This suggests that the solar wind itself might consist of numerous flux ropes distributed over a large range of sizes and strengths, of which the magnetic clouds are the largest kind. Averaging over large number of such flux ropes would give us information about the "free streaming solar wind". Scaling laws obtained from a simple model of the solar wind on these lines, are in reasonable agreement with the observations.
Program listing for Thursday