Solar Soft X-ray Irradiance (1991-2001) Derived from Yohkoh/SXT

Aki Takeda (Montana State Univ.), Nicole Albanese (Tufts Univ.) and Loren Acton (MSU)

0. Summary

Solar soft X-ray irradiance (3-45A) was calculated using the mission-long Yohkoh/SXT full-Sun images (1991-2001) obtained with two analysis filters (thin-Al and Al/Mg/Mn). We applied the most recent results of the re-calibated SXT effective area functions (see sub-section 1.2 and section 3) to this calculation. We assumed so-called, 'hybrid' elemental abundances by Schmeltz (2012). It turned out that the 'coronal' and 'photospheric' abuncances provide ~10% lower and ~25% higher values, respectively, than the hybrid abundance. SXT irradiance calculated in 1-10A shows best correlation with GOES/XRS, especially when the flux level is around 10^7 W/m^2 (see Fig. 6). This indidates that SXT's 1-10A irradiance is useable as the correction of GOES/XRS irradiance around the solar minimum (1995-1998).


Fig. 1 (top left): Yohkoh/SXT irradiance for 1991-2001 in 3-45A.
Fig. 2 (middle left): Filter-ratio temperatures derived from the thin-Al and Al/Mg/Mn filter pairs.
Fig. 3 (bottom left): Filter-ratio emission measures derived from the thin-Al and Al/Mg/Mn filter pairs.
Fig. 4 (top right): Yohkoh/SXT irradiance calculated in 1-8A.
Fig. 5 (middle right): GOES/XRS 1-8A flux for 1991-2001 from YLA seamless GOES data archive.
Fig. 6 (bottom right): Comparison of SXT irradiance with GOES/XRS.

1. Introduction

1.1 Yohkoh/SXT and the YLA

1.2 Most recent calibration on SXT effective area function

A significant decrease of the signal level of SXT's aspect sensor (visible telescope) was observed for the first 14 months after the launch of the satellite (see Fig. 8, left plot). It was long considered only affect visible light signals, and not affect the X-ray signals. However, the variation of the total coronal emission measures shows artificial dip that is not related to the solar activity. To correct this, Acton (2018) introduced the accumulation of some material(evaporated from the telescope body) on the outer surface of SXT (that affect aspect sensor and pre-filter). Acton(2018) assumed the material contains Cabon, and estimated the growth rate of the carbon layer that explains the decrease of the visible light signals. The transmission of the pre-filter was modified using the same accumulation rate, and propagated to the SXT effective area function.

1.3 Seamless GOES/XRS data archive

2. Data Processing

The Yohkoh Legacy data Archive (YLA) provides the mission-long SXT full-sun composite images as SXT level_2 data. Data were processed with the following steps.

3. Comparison of Old and New Calibration

Fig. x shows the SXT irradiance for 1992 derived with old and new SXT effective area functions. The difference is the largest for the period until the first pre-filter failure on 27-Oct-1992.

4. Assumption of Elemental Abundances

Our calculation assumes the elemental abundances by Schmelz et al. (2012), which is so-called 'hybrid' aboundance set. We also tried other abundance assumptions : so-called 'coronal' abundance by Feldman (1992) and 'photospheric' abundance by Caffau et al.(2011). It turned out that the coronal and phospheric abundance sets provide ~10% lower and ~25% higher values, respectively, than the hybrid abundance set.

5. Comparison with GOES/XRS

The derived temperatures and emission measures allow us to calculate the soft X-ray irradiance in other wavelength range. We calculated the irradiance in the 1-8A range (Fig. 4) to compare with the data from GOES/XRS low band flux (Fig. 5). It turned out that : These indidate that SXT's 1-10A irradiance is useable as the correction of GOES/XRS irradiance around the solar minimum (1995-1998).

References