Magnetic cloud fit by uniform-twist toroidal flux ropes

0484734 - ASÚ 2018 RIV FR eng J - Journal Article
Vandas, Marek - Romashets, E.
Magnetic cloud fit by uniform-twist toroidal flux ropes.
Astronomy & Astrophysics. Roč. 608, December (2017), A118/1-A118/12. ISSN 0004-6361. E-ISSN 1432-0746
R&D Projects: GA ČR(CZ) GA17-06065S
Institutional support: RVO:67985815
Keywords : magnetic fields * coronal mass ejections * solar wind
OECD category: Astronomy (including astrophysics,space science)
Impact factor: 5.565, year: 2017

Magnetic clouds are bent, and for some observations, a toroidal rather than a cylindrical flux rope is needed for a local approximation of the cloud fields. We therefore try to derive an axially symmetric uniform-twist force-free field in a toroid, either exactly, or approximately, and to compare it with observations. Equations following from the conditions of solenoidality and force-freeness in toroidally curved cylindrical coordinates were solved analytically. The magnetic field and velocity observations of a magnetic cloud were compared with solutions obtained using a nonlinear least-squares method. Three solutions of (nearly) uniform-twist magnetic fields in a toroid were obtained. All are exactly solenoidal, and in the limit of high aspect ratios, they tend to the Gold-Hoyle field. The first solution has an exactly uniform twist, the other two solutions have a nearly uniform twist and approximate force-free fields. The analysis of a magnetic cloud observation showed that these fields may fit the observed field equally well as the already known approximately linear force-free (Miller-Turner) field, but it also revealed that the geometric parameters of the toroid might not be reliably determined from fits, when (nearly) uniform-twist model fields are used. Sets of parameters largely differing in the size of the toroid and its aspect ratio yield fits of a comparable quality.

Permanent Link: http://hdl.handle.net/11104/0279879