LASCO C1 Doppler Shift and Width Analysis
LASCO C1 Doppler Shift and Width Analysis
Questions? call (202) 767 2263 ask for the LASCO C1 Calibration Person

Introduction

The LASCO C1 interferometer has the ability to generate a series of images which are separated in wavelength by as little as 0.03 A. A "scan" such as this allows an emission line to be characterized as a function of wavelength at each point in an image. For the most part, the line profile is well represented by a Gaussian profile, shifted from its nominal wavelength, and with a varying width and amplitude. The shift of the line from its nominal wavelength is mostly due to the doppler shift of the solar plasma generating the line as it moves towards or away from the instrument. The width of the emission line corresponds roughly to the temperature of the solar plasma. A series of scans may be made in rapid succession, which allows a movie of the velocity and temperature to be made, each scan constituting one frame of the movie. In the following article, the method and results of an analysis of this type will be discussed. The dataset used will be the FeXIV scan data specifically intended for this purpose which was taken using the LASCO C1 instrument March 28-30, 1998.

Analysis

During the above mentioned time period, a series of about 50 scans were taken of the west limb of the sun and about 35 scans were taken of the east limb of the sun. Each scan consisted of about 15 on-line images (192 x 288 binned 2x2) with command wavelengths ranging from 5299.7 A to 5303.9 A, spaced at 0.3 A intervals, along with two off line images at command wavelengths of 5309.2363 A and 5297.6001 A. The on-line data for each scan was reduced using the IDL fpc1_3img routine along with the two off-line open-door images and the closed door images from 05/Mar/1998 to yield a set of 15 signal images for each scan. In cases where the closed door images were more than 0.01 A away from the wavelenth of an on-line image, the closed door image was interpolated from the measured closed door scans.

Since there is evidence of a shift in the vacuum wavelength as a function of command wavelength, the wavelengths of each on-line image were adjusted by measuring the location of the Fraunhofer minimum at 5303.78 A (vacuum).

Due to solar rotation with a period of about 27.275 days, any rigidly co-rotating plasma will have a doppler shift of:

	dw = w R u f/c         Eq (1)
where w is the wavelength, R is the solar radius, u is the angular rate of rotation of the sun, c is the speed of light, and f is the horizontal (x) distance on the image from sun center in units of solar radii. Note that the component of velocity along a line of sight due to rotation is the same at all points along the line of sight, so that dw is a function only of x-position on the image. (The x-axis of the image is perpendicular to the axis of rotation of the sun.) Part of the data reduction process for the doppler images has been to eliminate shifts due to a rigid rotation of the solar corona.

Gaussian Profile Fits to Data

Over most of the images, the line profile is approximately Gaussian, except in low signal regions where the shape is rather noisy, and in certain regions where multiple overlapping lines give a double-line shape to the measured profile. For each scan, at each pixel, a Gaussian-plus-constant function has been fitted to the data, and the resultant shift and width of this fitted Gaussian is then taken to be the shift and width of the respective emission line. An example of the results of a Gaussian fit at pixel [83,193] for the scan taken on 29/Mar/1998 04:02-04:20 (Frame 16) is shown in Fig. 1.

Results

In Fig 2, three images are shown: The first is the integrated line intensity (zeroth moment, with an EIT image at 195 A included), the second is the wavelength shift (first moment) and the third is the line width (second moment). In all cases, the values of the moments are derived from the fitted Gaussian curves. All images are from frame 16 of the west limb doppler data. Note that the doppler shift image is given in units of velocity according to Equation 1 above. This particular frame is unique in that it contains the beginning of a large CME which is directed at an angle away from the instrument, resulting in the large red shift visible in the doppler image.

The above picture is taken from the MPEG MOVIE of the west limb data which was shown at the AGU meeting of May, 1998. (The web version has been reduced in size and compressed)


PR Nov 05, 1999