Soft X-ray irradiance study using Hinode/XRT synoptic composite images

Soft X-ray irradiance study
using Hinode/XRT synoptic composite images

Aki Takeda at HJC on 20 October, 2021


Sample pages from the XRT Synoptic Composite Image Archive (SCIA) `(http://solar.physics.montana.edu/HINODE/XRT/SCIA/)`

Hinode 14 / IRIS 11 Joint Science Meeting 2021

1. Introduction

Hinode's X-Ray Telescope :
- launched in 2006, September.
- grazing incidence telescope with apeture size ≈ 35cm
- 9 X-ray analysis filters
- observing wavelength range : 6 - 200 Å
- CCD : back-illuminated 2048 × 2048 pix
- field of view : 2040 × 2040 arcsec.

Full-Sun observations by Hinode/XRT --- Synoptic program ---
- 2007.2 --- 2008.2 : 4 times/day with 2048 × 2048 pix
- 2008.2 --- present : 2 times/day with 1024 × 1024 pix
- 2018.2 --- present : additional synoptic images as HOP 349.
- synoptic composite images : short/medium/long exposure images taken closely in time are processed into a single composite image to increase the dynamic range. These products are accessible from the XRT Synoptic Composite Image Archive (SCIA). For detail, see Takeda et al. (2016) Sol.Phys. 291, 317

XRT synoptic composite image statistics

Number of synoptic compsite images obtained with selected filters
(as of 30 September, 2021). Colored cells indicate the data used in this study.

year	Al_mesh	Ti_poly	Al_poly	thin_Be	Alpl/Tipl	med_Al	thick_Be
2006	0	2	366	138	0	3	0
2007	1150	1129	1614	368	285	24	0
2008	426	919	360	38	1	1	0
2009	383	727	379	0	0	117	0
2010	843	849	6	1	6	2	0
2011	714	669	41	371	7	6	0
2012	759	764	87	451	6	2	0
2013	618	629	88	444	0	0	0
2014	690	692	54	641	53	52	50
2015	604	450	337	606	109	112	79
2016	645	0	667	658	142	171	97
2017	664	0	830	652	153	153	114
2018	1214	0	1214	1239	144	144	127
2019	1639	0	1644	1632	134	135	135
2020	1593	0	1592	1602	126	126	135
2021	716	0	744	721	65	63	63
Total	12658	6830	10023	9562	1231	1111	792

2. Method of Analysis

Step 1 : Derive filter-ratio Te and EM (isothermal corona assumed).
- collect soft X-ray full-Sun signals (< 1.1R_⦿) from 2 different analysis filters.
  - 2008 - 2015.5 : Al_mesh and Ti_poly
  - 2015.6 - 2021.6 : Al_mesh and Al_poly
- apply Filter Ratio Method to derive Te and EM.
  - traditional coronal elemental abundance (Feldman 1992) assumed.
Step 2 : Calculate isothermal coronal spectrum using Te and EM.
- CHIANTI atomic DB ver.9.0.1 (isothermal.pro)
- traditional coronal elemental abundance (sun_coronal_1992_feldman_ext.abund)
- CHIANTI's default ionization equilbrium data (chianti.ioneq)
Step 3 : Integrate coronal spectrum over a certain wavelength range.
- 5 -- 60 Å as the XRT irradiance.
- 1 -- 8 Å for comparison with GOES/XRS
- etc.

3. Calibration challenges and difficulties

Irradiance study assumes uniform quality of data throughout the mission, but the instruments are aging:

Visible stray light contamination --- caused by a crack developed on the pre-filter.

event	date	open area fraction	note
1st	9-May-2012	~0.1%	Ti_poly and C_poly filters started to be affected.
2nd	14-Jun-2015	~3.3%	Ti_poly filter single use disabled.
3rd	27-May-2017	~7.5%	Al_mesh filter started to be affected.
4th	29-May-2018	~9.9%

Open/Ti_poly image in 2014 (after the 1st pre-filter failure).

Before
stray light subtraction. Stray light image
obtained in eclipse seasons. Stray light subtracted.
How to correct
1. Subtraction of visible light component from the X-day images.
2. Modification of XRT's effective area function due to the open area of the pre-filter.
(How to best correct the XRT effective area functions is still under discussion among the XRT team.)

Growth of contaminant layers on CCD and X-ray analysis filters.
--- caused by the evaporation of some material used in the telescope.

Previous calibration by Narukage et al.(2011) needs to be modified.

contamination thickness on CCD (plotted by K. Yoshimura)	thickness of contamination on X-ray filters from Narukage et al. (2011)

The CCD contaminant layer thickness is now measured allowing the existence of the residual layer (since 2017).
The additional growth of the FILTER contaminant layer after 2007 is likely.
(Installation of the FILTER conaminant correction after 2007 is under discussion among the XRT team.)

4. Results

4.1 Mission-long XRT irradiance (5-60A), filter-ratio Te and EM

4.2 Comparison with GOES/XRS

Comparison of GOES/XRS 1-8Å and Hinode/XRT 1-8Å irradiance.
* GOES/XRS data source:
- 2008 --- 2010 : Seamless GOES/XRS Archive (1-min averaged) in Yohkoh Legact data Archive (YLA@MSU).
- 2011 --- 2020 : 1-min averaged data from GOES15 reprocessed science-quality data (NCEI@NOAA).
* XRT data : integrate the isothermal spectrum calculated from the Te and EM over the range, 1 to 8Å
- XRT irradiance (1-8Å) is roughly 2 orders of magnitude lower!
- XRT irradiance do not contain flare data.
- difference in time resolution (XRT: a few image/day, GOES: 1-min.)
- GOES/XRS response function (log energy channel)

Comparison of GOES/XRS 1-8A and Hinode/XRT 0.3-10A irradiance.

Scatter plot of GOES/XRS 1-8A and Hinode/XRT 0.3-10A irradiance.

4.3 Comparison with Yohkoh/SXT

Yohkoh/Soft X-ray Telescope :
- Grazing incidence telescope operated in 1991-2001.
- Observing wavenength range: 3-30Å (cf. XRT: 5-60Å)
- Irradiance study performed by Takeda, Acton & Albanese (2019) ApJ 887:225.
Comparing the solar cycle evolution of Te and EM.
- years 2008 though 2014 (rising phase of solar cycle 24)
  
  [Note: dotted lines in each plot is the equi-irradiance curve in the unit of W/m² in log scale.]
- years 2015 though 2021 (decending phase of solar cycle 24)
- all years 2008 though 2021
- comparison with SXT (AlMg and thin_Al filter-ratio Te and EM)
  - XRT plot includes 2 solar minimum periods and 1 max period.
  - SXT plot includes 1 solar minimum period and 2 max periods.
  - SXT poorly detects coronal signals in solar minimum.
  - XRT filters sample the cooler portion of the coronal plasma than SXT filters.
  - XRT samples more dominant portion of coronal plasma because of its higher EM.
  - Enormously high frequency on XRT's lowest Te and EM histgram bin(s) might be showing that XRT observes the very bottom of the solar minumum condition.

4.4 Use of spatial information

Applying the Filter Ratio Method to images, instead of full-Sun integrated signals.

2D-Te ⟶ EM weighted average Te v.s. full-Sun integrated Te

2D-EM ⟶ total EM v.s. full-Sun integrated EM
3-temperature analysis:
XRT 3-temperature movie

cool component (red) : below 1.5MK
medium component (green) : 1.5 to 2.5MK
hot component (yellow) : above 2.5MK
Solar cycle variation of cool/med/hot components

5. Summary

We investigated the long-term variation of the filter-ratio Te, EM and soft X-ray irradiance using a series of XRT full-Sun images.
- caveat: details of calibration not finalized yet.
Comparison with GOES/XRS 1-8Å channel
- XRT's 0.3-10Å irradiance gives proximate irradiance level.
- GOES15 and XRT both have detection limit at around 10^-9 W/m².
Comparison of Te and EM with Yohkoh/SXT
- XRT sees more majority component of coronal plasma.
- mimimum condition at around Te~1.15MK and VEM~10⁴⁹ (cm^-3).
Use of spatial information.
- use of spatial information in irradiance study has high potential for investigating the termal evolution of the corona, but obtaining the uniform quality data for a long period of time is a key.

Before stray light subtraction.	Stray light image obtained in eclipse seasons.	Stray light subtracted.

2D-Te ⟶ EM weighted average Te	v.s.	full-Sun integrated Te
2D-EM ⟶ total EM	v.s.	full-Sun integrated EM