G-band intensity increase since May 2022
by Keiji Yoshimura, 30-June-2022 based on [xrt_co:47173]
3-Sep-2022, assumed thickness plot added. (A.Takeda)
3-Nov-2023, Long term thickness plot added. (A.Takeda)
Short summary
Small gradual increase of G-band intensity has been observed since the
late May 2022, which started before the intensity jump-up occurred
on 8-June. Because of this unexpected behaviour of G-band
intensity, we are currently unable to determine the CCD contamination
thickness with the existing method.
Background
The g-band intensity observed by the XRT is enhanced by the contaminant
layer on the CCD. The enhancement depends on the thickness of the layer.
See the
thickness v.s. gband_enhancement plot
The detail description can be found at:
https://solar.physics.montana.edu/HINODE/XRT/xrt_contam_db.html
These days the contaminant layer is getting over ~700A, the first peak of
the enhancement. (See
recent plot).
So the g-band intensity is expected to decrease with time (from CCD HTR off
through next HTR on).
G-band intensity increase
The g-band intensity looks to be increasing continuously since the end of
May before and after the big jump on June 8th, though the increase is very
small.
[click image to enlarge]
Comparison of several time periods
The plot below shows the observed g-band intensity change with time for
three time periods:
- 2022/04/13 - 2022/05/01 (Black pluses)
- 2022/05/04 - 2022/05/22 (Blue crosses)
- 2022/05/25 - 2022/06/12 (Red diamonds, before the "jump")
During the several time periods before 2022/04/13, the g-band intensity
changes are similar to the black plot.
You can see intensity increase in the red plot, while decrease in the others.
You may also notice a deviation started at the middle of the blue plot,
which may be due to the eclipse season (started early May).
Because the increasing rate of the contaminant layer tends to be small
during the eclipse season. This may cause "smaller decrease rate" of g-band
intensity", but never cause "increase".
[click image to enlarge]
What causes this?
I have no clear idea...
- Straylight may increase slowly?
- Unknown behavior of the G-band optics or straylight?
- Anything wrong in model or assumption?
The thickness of the contaminant layer on the CCD is calculated from the
g-band enhancement. The derived values are put into the database under
the SSW tree for the effective area calculation.
Since I have no idea what causes the g-band intensity increase as of now,
I cannot derive the thickness.
But, if we don't update the DB, the SSW software uses quite wrong values
for the data after 2022/05/25.
So I will put "likely" thickness into the DB to avoid the situation,
until we get better knowledge about this anomaly.
[Updated on 3-Sep-2022]
Assumed cotamination thickness in the SSW contamination thickness
DB in $SSW/hinode/xrt/idl/response/contam.
[click image to enlarge]
[Updated on 3-Nov-2023]
Red line shows the assumed thickness curve (with an emprical model) started since 2022/05/25.
[click image to enlarge]
