Three-Dimensional Structure of Coronal Mass Ejections From LASCO Polarization Measurements K. P. Dere, D. Wang and R. Howard ApJL, 620; L119-L February Monday Seminar; Rapid; Taro Morimoto

Abstract Polarimetric measurements with LASCO The LASCO movie of the 2002 August 1 CME. Three-dimensional structure of a CME. Two particularly well observed events are discussed August 1 CME August 7 CME. A rising arcade of loops. A flux-rope type.

Introduction Another CME event on 2002 August 7. An understanding of the three-dimensional density and magnetic structure of CMEs in the solar corona is still lacking. It is very important from the “space weather” point of view. for understanding the physics of CMEs. The angle at which a CME emerges. Direction of the magnetic field. CME shape – spherical shell? expanding loop? CME evolution. The method described in the papers can provide the information on

Introduction Previous studies 1.Poland and Munro 1976 (ApJ, 209; ) White-light and He 304 images from Skylab. Ha images from SacPeak. Composite photographs of He 304 & white-light image UT 1441 UT 1511 UT1615 UT They measured the angle of the CME’s leading edge from the plane of the sky, and demonstrated that it was quite different from that of the accompanying prominence Aug 21 CME & prominence eruption.

Introduction 2. Crifo, Picat and Cailloux 1982 (SolPhys, 83, ) Bubble and loop type CME topologies. 1973, Aug 10 CME. White-light images from Skylab. They performed polarization measurements and found that the CME is more likely to be a bubble-shaped structure, than a loop.

Introduction 3. Moran and Davila 2004 (Science, 305, 66-70) x (West) z y (North) Sun Earth 03:56 UT Brightness Side View (z, y) Top View (x, z) Topological map (distance from the [x,y] plane LASCO movie of the 1998, Oct 31 event. [Ro]

Method Thomson Scattering of Radiation The observed radiation of a CME is assumed to be due to scattering from electrons in the CME, thus it becomes polarized. Its polarization fraction depends on the angle of the scattering. To measure I, Q, and U, the fraction of linear polarization, p, is given as. The polarization brightness is defined as In order to remove the contributions from F-corona and instrumental polarization, “background” polarization and brightness must be subtracted. f : contributions from CME alone b: background (pre CME)

Method The polarization for Thomson scattering as a function of angle (  ) from the plane of the sky. Calculation Observation Weighted average angle (distance) of the scattering plasma from the plane of the sky.

Data LASCO C2 data during a special month-long (2002 July – August) campaign “LASCO C2 Polarization Measurement”. Binning = 2 (512 x 512 sized images; Pixel size = 22.8”) 1hr cadence. (Exposure time = 100s for each image). Default wavelength = 550 nm. 41 CME observations during the period. Especially well observed CMEs on 2002, Aug 1, 2002, Aug 7.

Result 2002 August 1 CME. LASCO C2 movie of the 2002, Aug 1 CME. LASCO images of the event at 06:48 UT. When the CME plasma is out of the plane of the sky, the polarization (polarized brightness) become would be reduced.

Result deg rotation deg rotation. Loop arcade. Visualization of the 2002 August 1 CME event.

Result Can be interpreted as consisting of flux-rope or helical structure. The observation and visualization of the 2002, Aug 7 CME Aug, 7 CME

Summary The foreground/background ambiguity. The assumption that the line-of-sight integrated intensity can be placed at a single (x, y, z) point. This is convenient for visualization, but may cause misunderstandings of the topology of a CME. Three-dimensional visualization of CMEs has significant impact on understanding CME physics space weather forecast etc. The polarization technique, STEREO imaging, and spectroscopic techniques can complementary provide mutual measure to view a solar ejection structure.

LASCO data The absolute calibration of C2 = 3 %. The polarization characteristics of the C2 coronagraph is less known. Assumption that the polarizers are ideal is made. There are two flat mirrors on the path of the light.