DEVELOPING A SOLAR RADIOMETRIC CALIBRATION SYSTEM USING SPECTRAL SYNTHESIS. Peter Fox (HAO/NCAR)

We present quantitative information on how we estimate the solar spectral irradiance (radiometric spectrum) and its day-to-day variations for bands in the ultraviolet, infrared and visible, and particular spectrum lines. We discuss the behavior of solar spectral variability in detail for different levels of solar magnetic activity by analyzing solar images observed between low and high activity periods for solar cycle 23 covering the years 1998-2004. Using three methods, we represent the solar surface with a set of structures that contribute to radiative variability - including sunspots, plages, enhanced and quiet network, and the quiet Sun. The synthesis calculations properly weight the computed specific intensities giving the Sun's irradiance at any particular wavelength and time. We discuss the validation of the computed spectra with observations and application to the radiometric calibration and variability problem. NCAR is sponsored by the National Science Foundation.

Synthetic radiance (specific intensity) spectra versus wavelength for each of the 7 solar structure types and at 10 disk positions. See examples below.

Theoretical calculations

Radiances

Solar spectral radiances in Wm-2 versus wavelength in nm (250-1500)

Quiet Sun at disk center (top) and near limb (lower)

Solar spectral radiances in Wm-2 versus wavelength in nm (250-1500)

Plage at disk center (top) and near limb (lower)

Solar spectral radiances in Wm-2 versus wavelength in nm (250-1500)

Sunspot at disk center (top) and near limb (lower)

Analyze full disk intensity images and decompose and assign a structure identification and disk position. See Decomposition.

Alternately, utilize full-disk measures of solar activity to represent fractional area coverage of mean disk radiances for each structure

Generate synthetic irradiance spectra using input from radiance spectra, and a distribution of solar activity. Results are given in the right panels of this poster.

Empirical solar atmosphere models: temperature versus height for each structure

Synthetic pseudo bolometric image from 2003/11/21 2001UT.

Peak in spot spectrum (color temperature) occurs at a redder wavelength than other solar features and varies with disk position.

Spot spectrum goes to a relative minimum in the near IRPeak in quiet Sun and

plage spectrum occur at similar wavelengths but center-to-limb spectral intensity changes are apparent

Observations

Solar Activity Decomposition

Example observational data set from SORCE: SIM (UV-IR) + SOLSTICE (UV) spectrum from 2003/11/22 - courtesy SORCE team.

Solar surface structures:A - the cell center faint component of the quiet sun,C - the average median intensity quiet sun component,E - the bright network component of the quiet sun,F - the enhanced network component for the active sun,H - the faint plage component for the active sun,P - the bright plage component for the active sun, andS - sunspots.

Area coverage of mean disk activity

PSPT Ca II K image (left) and mask representing solar activity (right) from 1998/06/12 2200UT (top) and 1998/06/20 2201UT (bottom)

PSPT Ca II K image (left) and mask representing solar activity (right) from 1998/06/10 1722UT (top) and 2200UT (bottom)

Series of PSPT Ca II K images (top) and masks representing solar activity (bottom) from 2003/11/01 (1711UT), 02 (0201UT), 04 (1722UT), and 21 (2001UT) left to right, representing ~ 7 hrs, 27 hrs, and ~ 20 days.

Model 1998/06/10 1998/06/12 1998/06/20

A 0.002 0.0025 0.0026

C 0.8723 0.8609 0.8550

E 0.066 0.069 0.057

F 0.029 0.035 0.035

H 0.015 0.017 0.029

P 0.015 0.015 0.021

S 0.0007 0.0006 0.0005

Relative intensity histograms of the Ca II K images from 1998/06/10 and 1998/06/12 which are used to develop the masks shown in panels to the left.

Irradiance spectra 1998/06/10 - UV

Summary:o very good agreement in visible through near IR on hours to years timescaleso some work to do in parts of UV/EUV with inaccurate absolute irradiances but variability levels are consistent with observationso consistency of image decomposition for short term variability needs attention, e.g. seeing effects

Days to month variability - UV

This material is based upon work partially supported by the National Science Foundation under Grant No. 0209480 (NCAR proposal number 1997-014) and the NASA SORCE project under a grant to CU/LASP (G. Rottman, PI). SOLSPEC data are courtesy of G. Thuillier. The spectral synthesis project at HAO was a joint effort between Juan Fontenla, Dick White and Peter Fox.

Synthesis - solid SOLSPEC - dash/dot SORCE/SIM - dash

Days to month variability - vis/nearIR

Synthetic Spectral Irradiance 2003/11/02

Irradiance spectra 2003/05/01 - Visible

Synthesis - solid SOLSPEC - dash

Irradiance spectra 2003/05/01 - nearIR

Variability in the UV spectrum (250-400nm), top: 2 day between 1998/06/10 and 06/12 middle: 10 days between 06/10 and 06/20, and bottom: years (1998 to 2003)

Solar spectra (250-1500nm), top: synthesis for 2003/11/21, next panel is variability spectrum between 2003/11/01 and 02, next is 11/04, and lower is 11/21. The spectra are obtained from decompositions using the images in the lower center panel of this poster.