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    Received 6 August 2008; received in revised form 21 October 2008; accepted 22 October 2008

    the composition, density, and temperature in Earths andMars ionosphere and upper atmosphere. Furthermore,the solar driven atmospheric processes are wavelength

    Accurate measurements of the solar ultraviolet spectralirradiance, along with an understanding of its variabilityon all time scales, are required for atmospheric studiesand application for space weather operations. While therehave been several recent broadband measurements of thesolar XUV irradiance by the XUV Photometer System

    * Corresponding author.E-mail address: (T.N. Woods).

    Available online at

    Advances in Space Research 41. Introduction

    The solar soft X-ray (XUV, dened here as 130 nm)radiation is highly variable on all time scales with varia-tions for both short-term (minutes, ares), mid-term(months, solar rotation), and long-term (years, solar cycle)ranging from factors of two to a hundred (wavelengthdependent). These solar XUV variations directly aect

    dependent and thus dependent on the intrinsic solar vari-ability at the appropriate wavelengths. The large aresare of major concern for space weather applications caus-ing detrimental eects on communication and navigationsystems due to ionospheric changes (e.g., Lanzerotti,2001; Kintner et al., 2007) and on satellite tracking asrelated to satellite drag changes due to solar forcing ofthe neutral density (e.g., Sutton et al., 2006).Abstract

    The solar soft X-ray (XUV; 130 nm) radiation is highly variable on all time scales and strongly aects the ionosphere and upperatmosphere of Earth, Mars, as well as the atmospheres and surfaces of other planets and moons in the solar system; consequently,the solar XUV irradiance is important for atmospheric studies and for space weather applications. While there have been several recentmeasurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during ares,has been hampered by the lack of high spectral resolution measurements in this wavelength range. The conversion of the XUV photom-eter signal into irradiance requires the use of a solar spectral model, but there has not been direct validation of these spectral models forthe XUV range. For example, the irradiance algorithm for the XUV Photometer System (XPS) measurements uses multiple CHIANTIspectral models, but validation has been limited to other solar broadband measurements or with comparisons of the atmosphericresponse to solar variations. A new rocket observation of the solar XUV irradiance with 0.1 nm resolution above 6 nm was obtainedon 14 April 2008, and these new results provide a rst direct validation of the spectral models used in the XPS data processing. Therocket observation indicates very large dierences for the spectral model for many individual emission features, but the dierencesare signicantly smaller at lower resolution, as expected since the spectral models are scaled to match the broadband measurements.While this rocket measurement can help improve a spectral model for quiet Sun conditions, many additional measurements over a widerange of solar activity are needed to fully address the spectral model variations. Such measurements are planned with a similar instru-ment included on NASAs Solar Dynamics Observatory (SDO), whose launch is expected in 2009. 2008 COSPAR. Published by Elsevier Ltd. All rights reserved.

    Keywords: Solar ultraviolet irradiance; Flares; Space weatherComparison of solar soft X-rphotometers to a high spectra

    Thomas N. Woods *,

    Laboratory for Atmospheric and Space Physics (LASP), Univer0273-1177/$34.00 2008 COSPAR. Published by Elsevier Ltd. All rights resedoi:10.1016/j.asr.2008.10.027irradiance from broadbandresolution rocket observation

    hillip C. Chamberlin

    of Colorado, 1234 Innovation Drive, Boulder, CO 80303, USA

    3 (2009) 349354rved.

  • es i(XPS), detailed understanding of the solar XUV irradiance,especially its variability during ares, has been hamperedby the lack of high spectral resolution from these measure-ments. The are variations are better understood in theextreme ultraviolet (EUV; 27120 nm) and far ultraviolet(FUV; 120200 nm) wavelengths where there are higherresolution spectral measurements of the solar variability(e.g., Woods et al., 2005a). However, the results with thebroadband measurements at shorter than 27 nm initiallyyielded conicting results with atmospheric responses toares (e.g., Strickland et al., 2007). The source for these ini-tial dierences is the choice of the solar spectral model usedin converting the XUV broadband photometer data intoirradiance units. A new algorithm has been developed forprocessing the XPS data with dynamic solar spectra thatinclude a are component, and the new XPS Level 4 prod-ucts are signicantly improved for are events (Woodset al., 2008).

    The focus for this paper is the direct validation of theCHIANTI spectral models used in processing the XPSdata. Prior validation has been limited to other solarbroadband measurements or with comparisons of theatmospheric response to solar variations (Woods et al.,2008). This new validation is based on the results from arecent rocket observation of the solar XUV irradianceabove 6 nm with 0.1 nm resolution on 14 April 2008.

    2. Improved irradiance algorithm for XPS

    XPS is a set of lter photometers that measure the solarirradiance from 0.1 to 27 nm with an additional channel atthe important H I Lyman-a line at 121.6 nm. The XPS isone of four dierent solar irradiance instruments onNASAs Solar Radiation and Climate Experiment (SOR-CE) satellite (Rottman, 2005; Woods et al., 2005b) and isalso part of the Solar EUV Experiment (SEE) on NASAsThermosphere, Ionosphere, Mesosphere, Energetics, andDynamics (TIMED) satellite (Woods et al., 2005a). In eachXPS there are a total of twelve silicon photodiodes, eightwith metal lms directly deposited on them, one with121 nm interference lters in front, and three with barephotodiodes. The lter material (either metal coating orinterference) establishes the wavelength sensitivity overbroadbands of about 710 nm and also blocks the longwavelength solar radiation that would overwhelm the rela-tively weak signal at these short X-ray wavelengths.

    The primary data products from XPS are the solar XUVirradiance in its broadbands and also at higher spectral res-olution based on scaling model spectra to the XPS signallevels. The time cadences for the XPS results include thedaily averaged irradiances and higher cadence of a fewminutes. Because of the satellite orbit and instrument con-guration, the TIMED XPS has about 3% duty cycle forsolar observations, and the SORCE XPS has about 70%duty cycle for solar observations. The TIMED daily mea-

    350 T.N. Woods, P.C. Chamberlin / Advancsurements began in January 2002, and the SORCE dailymeasurements began in March 2003, and data from bothcan be found at the LASP LISIRD database (

    The original XPS Level 4 algorithm scaled a daily refer-ence spectrum to match the signals (currents) of the XPSphotometers and is described in detail by Woods et al.(2005b). The scale factors for the reference spectrum aredetermined in three bands at 04, 414, and 1427 nm. Thisapproach of having a single reference spectrum for eachday to generate a XUV spectrum in 1 nm intervals for eachXPS measurement works quite well for non-are measure-ments but severely over estimates the are irradiance by afactor of 2 or more as determined from measured atmo-spheric response to the ares (e.g., Strickland et al., 2007;Tsurutani et al., 2005). The reason for this algorithmbreakdown for ares is due to the are spectrum changingsignicantly, namely to increase more at the shorter wave-lengths. Consequently, this algorithm is expected to overestimate the are irradiance with a static reference spec-trum because the photometers are most responsive at theshorter wavelengths. That is, the photometer signal goesup dramatically, by a factor of about 50 for very largeares, with the real spectral increase expected to be mostlynear 2 nm (Rodgers et al., 2006), but the irradiance calcu-lation uses the lower photometer responsivity at longerwavelengths, which in turn yields high estimates for theare irradiance.

    An improved algorithm, as described in detail by Woodset al. (2008), is to use a combination of reference spectrathat are representative of both are and non-are activity.The CHIANTI version 5.2 spectral model (Dere et al.,1997; Landi et al., 2006) is used to generate reference spec-tra representative for non-are and are measurements.The CHIANTI spectral model includes options with stan-dard dierential emission measures (DEMs) and also iso-thermal spectra appropriate for the Sun.

    Two non-are spectra are estimated on a daily basisusing the minimum current from the XPS Ti-coated pho-tometer. A combination of the standard DEM options inCHIANTI are used for the non-are components, beinga solar cycle minimum spectrum and an active region spec-trum. The active region reference spectrum is simply theactive region DEM in the CHIANTI spectral model;whereas, the solar cycle minimum reference spectrum is acombination of the quiet Sun DEM and coronal holeDEM. Although neither TIMED nor SORCE hadobserved solar cycle minimum conditions at the time whenthe algorithm was developed, the lowest XPS measure-ments t well with the average of the CHIANTI quietSun DEM spectrum and coronal hole DEM spectrum;therefore, this average is chosen to be the solar cycle min-imum reference spectrum, which is the focus here for thecomparison to the rocket observation in April 2008.

    The are spectrum is estimated for every observationusing the dierence from the


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