I. INTRODUCTION Coronal loops are thin and bright structure of hot plasma emitting intense radiation in X-ray and EUV. (1) HOW CAN WE EXPLAIN THE CONSTRICTION OF PLASMA INTO A LOOP WITHOUT DISPERSION.1) By gas pressure2) By magnetic pressure However, there has been no reported observationl evidence for these possibilities.3) By magnetic tension If Magnetic fields were twisted, then the magnetic tension force will cause the constriction of plasma into the coronal loop. We estimate the magnetic twist of coronal loops that can explain the constriction of plasma into a loop without dispersion!!II. DATA TRACE EUV(17.1nm) Data SOHO/MDI 96minutes MagnetogramThe solid boxes represent the regions used for the linear force free extrapolation.ABSTRACT

EUV images taken by TRACE clearly display a number of thin coronal loops that represent one million degree plasma tracing magnetic field line in the corona. We estimate the magnetic twist of coronal loops that can explain the constriction of plasma into a loop without dispersion. We assume that the segment of a coronal loop taken by TRACE 171 image is a part of a straight, non-force-free twisted flux tube and that the variation of the axial field strength along the tube is determined by the large scale three-dimensional configuration of the coronal magnetic field calculated by linear force-free extrapolation of photospheric magnetic field observed by SOHO/MDI. We selected a number of conspicuous loops which are bright enough and well separated from other adjacent loops on TRACE EUV images so that we can fit a magnetic field line to each loop from one footpoint to the other footpoint. We have applied our method to several coronal loops and found that these loops have twist values from 1.5 to 2.5, which suggests that the winding number of EUV coronal loop may be around one.RyunYoung Kwon, Jongchul Chae

Astronomy Program, School of Earth and Environmental ScienceSeoul National University, Korea

IV. RESULTIII. MODEL Expression for the twist value .(2) DETERMINATION OF AXIAL FIELD STRENGTH . We decompose the coronal magnetic field B into a large-scale component and small-scale component We take the z axis perpendicular to the solar surface which the boundary conditions are specified on the z=0 plane. Large scale field is uniform at scales of the loop width and smaller but varies at larger scales. Small scale field varies ate scales of loop width and smaller but is zero at larger scale. Using the force free condition for the large scale field, We assume that the plasma constriction is wholly attributed to nonzero , we set , Hence the magnetohydrostatic equation describing the force balance across a loop at the small scale is given by Therefore this twisted flux tube model is characterized by three independent parameters: axial field strength B0, pressure excess p, and loop width a. We determine these parameters from observations!! ~ Then we can get solution at z > 0. Now, we can draw the magnetic field lines using solutions for different alpha. We choose the field line which has minimum distance between the loop and the curve. So we determined the axial field strength at each selected points.12456(1) DETERMINATION OF LOOP WIDTH a AND PRESSURE EXCESS p.where is FWHM defined by . Intensity profile across a loop assuming isothermal loop, and pressure excess p Intensity profile fitting The diamond symbols indicate the intensity profile across a loop observed by TRACE, the curve refer to the intensity profile model fits I (x) at the selected points(right).(3) DETERMINATION OF TWIST V. CONCLUSIONIf the twist on the axis exceeds 4.8 , flux tube is instable. (Mikic, Schnack, & van Hoven(1990) The on-axis magnetic twist of the loop is found to be from 1.5 to 2.5 which corresponds to a winding number about from 0.75 to 1.25. There is a tendency that the twist in the middle of the loop is larger than both footpoints.