Calculating CME Mass in SECCHI COR1 and COR2

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Before level 0.5 images can be converted from brightness to mass, they must be combined into calibrated total brightness images.  

secchi_prep, files, header, images, /polariz_on, /smask_on

To isolate the CME from the total background, we subtract a pre-event image. The pre-event image should also be a calibrated total brightness image. For the COR2 data, if you are using both the on-board double images and the total brightness images created from the polarization sequence, we recommend that you choose a pre-event image for each type of image.

Now the images can be converted from brightness to mass. The program scc_calc_cme_mass.pro is a modification of the LASCO program calc_cme_mass.pro.  Both these programs call eltheory.pro, which has the encoded the equation from Billings (1966).  

The input to scc_calc_cme_mass.pro is a single image array of Total Brightness units and header. To calculate the mass per pixel for the entire image use the keyword /ALL. The keyword POS=[angle in deg] allows the user to specify the angle of CME propagation relative to the plane of the sky. It is 20 deg by default.

diff_image = images[*,*,0] - pre_event_image
mass_image = scc_calc_cme_mass(diff_image ,header[0], /ALL, POS=0)

The units in mass_image are now grams per pixel. The values should be greater than 1e10 grams.  (There can be negative values in the image. Negative values are interpreted to be where mass has been evacuated from the pre-event image.) We recommend that you write the mass image to a file since the mass processing does take a second.

sccwritefits, 'mass'+header[0].filename, mass_image, header[0]

To find the total mass of the CME you need to sum the mass over the area of the CME. Without the /ALL keyword and with a 3rd argument (box), scc_calc_cme_mass returns a total mass for a region specified by box:

total_mass = scc_calc_cme_mass(diff_image ,header[0], box, POS=0)

A similar program, scc_cme_massimg2total.pro , also sums mass over an area and allows interactive selection of this area. The input to the program is a mass image filename. To point-and-click the area of the CME use the /SECTOR keyword alone:

total_mass = scc_cme_massimg2total(mass_filename, /SECTOR)

The text output of scc_cme_massimg2total.pro is:

Date-Obs Cen-PA Width Height ROI-Type Pts PixRad Mass Mass/Pt Potential Cm_r Cm_A
2008-01-02T09:55:00 98.43 38.91 1.02 DEFROI 24307 132.19 7.659e+14 3.15094e+10 2.258e+27 3.55 -60.04
 

To write the text output to a file use the keyword SAVE = logfilename :

total_mass = scc_cme_massimg2total(mass_filename, /SECTOR, SAVE=logfilename )

To sum over the area non-interactively, you can use the RADII = [R1, R2] and ANGLES = [T1, T2] keywords together or separately to specify an annular sector.

total_mass = scc_cme_massimg2total(mass_filename, /SECTOR, SAVE = logfilename,
RADII = [1.5, 3.0], ANGLE =[75, 120])

The text output of scc_cme_massimg2total.pro, IF keywords SECTOR, RADII or ANGLES is set, is:

Date-Obs Cen-PA  Width Height ROI-Type  (R1 R2 T1 T2 Pts) PixRad Mass Potential Cm_r Cm_A
2008-01-02T09:55:00 98.43 38.91 1.02 SECTOR 1.52 2.54 78.98 117.89 24307 132.19 7.659e+14 2.258e+27 3.55 -60.04
 

Enjoy finding your own CME masses.

 


Billings, D. E. 1966, a Guide to the Solar Corona (New York: Academic)