Magnetic twist of an extreme-ultraviolet coronal loop inferred from plasma constriction

Magnetic twist is a very likely way of constricting coronal plasma into loops. An attempt is made to determine the magnetic twist that can explain the constriction of coronal loops indirectly from observations. We decompose the coronal magnetic field into a large-scale magnetic field and a small-scale magnetic field. The large-scale field is assumed to be force-free, but the small-scale field is not. We derive a magnetohydrostatic equation describing the relationship between the small-scale field variation and the pressure variation across the loop. We adopt a simple flux rope model with the following properties. (1) There exists a close physical relationship between the twist-creating process and the plasma-injecting process that keeps the axial field constant as the loop forms. (2) The large-scale force-free field is so close to the current-free configuration that its associated current is negligible in the plasma constriction. (3) Pressure peaks on the loop axis and monotonically decreases with the distance from the axis. This flux tube model is characterized by three independent parameters: axial field strength, peak pressure excess, and loop width. The peak pressure excess and the loop width are determined from an analysis of the EUV data taken by TRACE, and an estimate of the axial field strength is found from the linear force-free extrapolation of the photospheric magnetic field observed by SOHO MDI. By applying this method to a specific coronal loop, we have found that the magnetic twist on the loop axis is about 1.5 pi, and the twist per unit length is more concentrated near its top than near its footpoints. This twist value has been obtained using the simplifying assumption of uniform axial field strength. In the case where the axial field is stronger inside the loop than in the surroundings, the twist will turn out to be greater than 1.5 pi, since the twist is required to constrict not only the plasma but also the magnetic flux. On the other hand, if the axial field is weaker than that in the surroundings, a smaller twist may be enough to constrict the plasma.