Magnetic cloud models with bent and oblate crosssection boundary 

Pascal Demoulin Submitted: 20090909 07:06
Magnetic clouds (MCs) are formed by magnetic flux ropes that are ejected
from the Sun as coronal mass ejections.
These structures generally have low plasma beta and travel through
the interplanetary medium interacting with the surrounding solar wind (SW).
Thus, the dynamical evolution of the internal magnetic structure of a MC
is a consequence of both the conditions of its environment
and of its own dynamical laws, which are mainly dominated by magnetic forces.
With insitu observations the magnetic field is only measured along the trajectory of the spacecraft across the MC. Therefore, a magnetic model is needed to reconstruct the magnetic configuration of the encountered MC.
The main aim of the present work is to extend the widely used cylindrical model
to arbitrary crosssection shapes.
The flux rope boundary is parametrized to account for a broad range of shapes.
Then, the internal structure of the flux rope is computed by expressing the magnetic field
as a series of modes of a linear forcefree field.
We analyze the magnetic field profile along straight cuts through the flux rope, in order to simulate the spacecraft crossing through a MC. We find that the magnetic field orientation is only weakly affected by the shape of the MC boundary. Therefore, the MC axis can approximately be found by the typical methods previously used (e.g., minimum variance).
The boundary shape affects mostly the magnetic field strength. The measure of how much the field strength peaks along the crossing provides an estimation for the aspect ratio
of the fluxrope crosssection. The asymmetry of the field strength between the front and
the back of the MC, after correcting the time evolution (i.e., its aging during the observation of the MC), provides an estimation of the crosssection global bending.
A flat or/and bent crosssection requires a large anisotropy of the total pressure imposed at the MC boundary by the surrounding medium.
The new theoretical model developed here relaxes the cylindrical symmetry hypothesis.
It is designed to estimate the crosssection shape of the flux rope using the insitu data of one spacecraft. This allows a more accurate determination of the global quantities, such as magnetic fluxes and helicity. These quantities are especially important for both linking an observed MC to its solar source and for understanding the corresponding evolution.
Authors: Demoulin P., Dasso S.
Projects: None

Publication Status: in press, A&A
Last Modified: 20090909 09:37


