SV 42The Magnetic Helicity of an Interplanetary Hot Flux Rope

S. Dasso , C. Mandrini , P. Demoulin 
Observations of helical magnetic structures in the solar photosphere, corona and solar wind have attracted considerable attention in the last years, with the consequent interest in magnetic helicity (one of the few global quantities that is preserved) studies both in the solar and interplanetary contexts. Interplanetary flux ropes, e.g., magnetic clouds, have been in general modelled using three different approaches in cylindrical geometry: the Gold Hoyle model (or constant twist angle model), the Lundquist model (or linear force-free field model), and the constant current model [Hidalgo et al., Solar Physics, 194, 165-174 (2000)]. At present, it is not at all evident which of these models give the best representation of these structures. However, even if these models are physically different, the growing interest in the relation between the helicity generated in the Sun and ejected to the interplanetary medium, has lead us to evaluate if the magnetic helicity of magnetic clouds can be model independent. We present in this work the results corresponding to the fitted set of free parameters for a hot tube observed by Wind on October 24-25, 1995, for each of the three models mentioned above. We derive analytically the expression for the magnetic helicity for the constant current model. We find that the magnetic helicity values determined when using the Gold Hoyle and Lundquist models are the same; while for the constant current model we obtain a value which is lower by a factor 3. From our point of view, our results mean that whatever be the approach used magnetic helicity is a well-determined quantity and, thus, it is worth using it to understand the link between solar and interplanetary phenomena. This work was partially supported by the Argentinean CONICET (grant PIP 02693/00) and UBA (grant UBACYT X059).