Intercalibration of Solar Soft X-Ray Broad Band Measurements from SOLRAD 9 Through GOES-12

Intercalibration of Solar Soft X-Ray Broad Band Measurements from SOLRAD 9 through GOES-12 Werner M. Neupert Solar Physics A Journal for Solar and Solar-Stellar Research and the Study of Solar Terrestrial Physics ISSN 0038-0938 Sol Phys DOI 10.1007/s11207-011-9825-3 1 23 Your article is protected by copyright and all rights are held exclusively by Springer Science+Business Media B.V.. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your work, please use the accepted author’s version for posting to your own website or your institution’s repository. You may further deposit the accepted author’s version on a funder’s repository at a funder’s request, provided it is not made publicly available until 12 months after publication. 1 23 Author's personal copy Solar Phys DOI 10.1007/s11207-011-9825-3 Intercalibration of Solar Soft X-Ray Broad Band Measurements from SOLRAD 9 through GOES-12 Werner M. Neupert Received: 1 March 2011 / Accepted: 12 July 2011 © Springer Science+Business Media B.V. 2011 Abstract The two-band soft X-ray observations of solar flares made by the Naval Research Laboratory’s (NRL) SOLar RADiation (SOLRAD) satellites and by the Geostationary Or- biting Environmental Satellites (GOES) operated by the National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction Center have produced a nearly contin- uous record of solar flare observations over a period of more than forty years (1969 – 2011). However, early GOES observations (i.e., GOES-2) and later (GOES-8 and subsequent missions) are not directly comparable due to changes in the conversion of measured currents to integrated fluxes in the two spectral bands that were adopted: 0.05 – 0.3 (or 0.4) nm, which we refer to as XS and 0.1 – 0.8 nm (XL). Furthermore, additional flux adjustments, using overlapping data sets, were imposed to provide consistency of flare-flux levels from mission to mission. This article evaluates the results of these changes and compares experimental GOES-8/GOES-2 results with changes predicted from modeled flare spectra. The factors by which recent GOES observations can be matched to GOES-2 are then optimized by adapt- ing a technique first used to extrapolate GOES X-ray fluxes above saturation using ionospheric VLF radio phase enhancements. A nearly 20% increase in published GOES-8 XL data would be required to match to GOES-2 XL fluxes, which were based on observed flare spectra. On the other hand, a factor of 1.07 would match GOES-8 and later flat-spectrum 0.1 – 0.8 nm fluxes to GOES-2 XL if the latter data were converted to a flat-spectrum basis. Finally, GOES-8 observations are compared to solar soft X-ray estimates made concur- rently with other techniques. Published GOES-8 0.1 – 0.8 nm fluxes are found to be 0.59 of the mean of these other determinations. Rescaling GOES to a realistic flare spectrum and removing a 30% downward adjustment applied to the GOES-8 measurements during initial data processing would place GOES-8 and later GOES XL fluxes at 0.94 of this XL mean. GOES-2 on the same scale would lie at about 0.70 of this mean. Significant uncertainties in the absolute levels of broad band soft X-ray fluxes still remain, however. Keywords GOES · Soft X-ray · Solar flares · SOLRAD W.M. Neupert () Boulder, CO, USA e-mail: [email protected] Author's personal copy W.M. Neupert 1. Introduction Broad-band soft X-ray observations of solar flares made over more than five solar sunspot cycles have long contributed to studies of ionospheric perturbations during such events, and, more recently, provided early warning of potential impacts of heliospheric disturbances on Earth-orbiting space missions (Hill et al., 2005). Initially such observations, using sounding rockets, were used to identify the spectral range of flare emissions responsible for D-region effects (Friedman, 1960; Kreplin, Chubb, and Friedman, 1962). They have subsequently been used to infer the properties of the regions on the Sun responsible for the enhanced radiation levels (e.g., Garcia, 1994; Feldman et al., 1996). More recently, GOES fluxes have been used in evaluating the impact of flare radiation on the Earth’s ionosphere (Meier et al., 2002 and references therein). Because of the reliance on the Naval Research Laboratory’s (NRL) SOLar RADiation (SOLRAD) satellites and, since 1974, on the National Oceanic and Atmospheric Adminis- tration’s (NOAA) Geostationary Operational Environmental Satellites (GOES) to charac- terize soft X-ray flare levels, it is worthwhile to examine the consistency of these many data sets. The soft X-ray sensors were always used in pairs: a short-wavelength channel, here- after called XS, with a nominal 0.05 – 0.4 nm pass band, and a long X-ray (XL) channel, with a nominal 0.1 – 0.8 nm pass band. I first compute factors required to adjust reported fluxes for the differing assumptions of incident soft X-ray spectral distribution and (in one instance) spectral range adopted by SOLRAD and GOES (Kahler and Kreplin, 1991; Don- nelly, Grubb, and Cowley, 1977). These factors are then compared to observed ratios of X-ray flare fluxes recorded simultaneously by two spacecraft, such as two different GOES. GOES-2 is used as a reference as it overlapped both SOLRAD 11 and later GOES missions. A relationship between the peak intensity of flares in the XL pass band and the ratio of XL and XS fluxes (Thomson, Rodger, and Cliverd, 2005) is then applied to refine the consistency of observations from SOLRAD 9 to GOES-8. Finally, recent GOES measurements are cross-calibrated against concurrent determinations of soft X-ray flare fluxes made with other techniques (Aschwanden and Alexander, 2001;Sylwesteret al., 2005; Rodgers et al., 2006; Väänänen, Alha, and Huovelin, 2009). Al- though general agreement is found, further measurements and intercomparisons are needed before absolute levels of the solar flare flux can be reported with confidence. 2. Broad Band Observations by SOLRAD and GOES 2.1. SOLRAD Observations The first soft X-ray observations to be reported routinely were made by the Naval Re- search Laboratory under the direction of Herbert Friedman and Robert Kreplin beginning with sounding rockets from 1949 through 1959 (Kreplin, 1961) followed by satellite observations, beginning in May 1961 (Kreplin, Chubb, and Friedman, 1962). Spectral bands (0.1 – 0.5 nm, 0.1 – 0.8 nm, 0.1 – 2.0 nm, 0.8 – 1.6 nm, and 4.4 – 6.0 nm) of soft X-ray emission were monitored with ion-chamber photometers. A Geiger–Müller counter was flown in 1959 to monitor 0.05 – 0.3 nm fluxes (Gregory and Kreplin, 1967). Subsequent missions, starting with SOLRAD 10, used an ion chamber for the 0.05 – 0.3 nm range (Dere, Horan, and Kreplin, 1974). Horan (1971) made use of the original currents from the NRL XS and XL sensors combined with expressions for free–free and free–bound emission by Culhane (1969) to infer flare isothermal temperatures and emission measures from the SOLRAD observations. Author's personal copy Intercalibration of Solar Soft X-Ray Broad Band Measurements A comprehensive calibration of the SOLRAD sensors was undertaken by Meekins et al., 1974). These laboratory calibrations were first applied to sensors on SOLRAD 11. In-flight comparisons of SOLRAD 11 with SOLRAD 10 indicated that these two sets of data were consistent (Kahler and Kreplin, 1991). Fluxes initially reported by SOLRAD 9 evidently were lower, as Kahler and Kreplin (1991) reported that multipliers of 1.2 for XS data and 2.1 for XL should be applied to SOLRAD 9 fluxes to be consistent with SOLRAD 11. Kreplin and Horan (1992) later recommended a factor of 2.2 for XL (with no mention of change for XS). SOLRAD observations used in the present analysis were published in graphical form in Solar Geophysical Data and extracted using the Un-Scan-It digitizing software package. 2.2. GOES Observations Observations in two spectral bands, 0.05 – 0.4 nm and 0.1 – 0.8 nm, were begun by NOAA’s Space Environment Laboratory (now Space Weather Prediction Center: SWPC) in May 1974 (Donnelly, Grubb, and Cowley, 1977) and continue to the present. Data are archived at the National Geophysical Data Center (http://www.ngdc.noaa.gov) and these are referred to in this article as “published” or “reported” fluxes. The primary mission of the GOES program is to support space-weather monitoring and forecasting at the SWPC (Hill et al., 2005). Detailed calibration of early sensors was carried out by Unzicker and Don- nelly (1974). The design of the sensors themselves has changed little since the inception of the program. A beryllium window was added over the GOES-1 sensors (and successor instruments) to provide ultraviolet shielding (Donnelly, Grubb, and Cowley, 1977). Pre- launch calibration from GOES-8 onward included accurate measurements of ion-chamber windows, chamber pressure, and sensitivity to X-rays from a radioactive 55Fe source. Beginning with GOES-3 (Garcia, 1994), a change in the nominal wavelength applica- ble to reported XS fluxes was made as were spectral assumptions used in data processing. At several points in the program modifications were made in the post-launch data processing to match newly launched sensor fluxes with then-operating sensors (Panamet- rics, 1987; Hill et al., 2005). These changes are summarized in Table 1. The reduction of 17% made to GOES-6 XL to match GOES-5 was made by increasing the XL trans- fer function (see next section).

Load more