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

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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 Ð 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 Ð 0.3 (or 0.4) nm, which we refer to as XS and 0.1 Ð 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 Ð 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 concurrently with other techniques. Published GOES-8 0.1 Ð 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 Ð 0.4 nm pass band, and a long X-ray (XL) channel, with a nominal 0.1 Ð 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 Ð 0.5 nm, 0.1 Ð 0.8 nm, 0.1 Ð 2.0 nm, 0.8 Ð 1.6 nm, and 4.4 Ð 6.0 nm) of soft X-ray emission were monitored with ion-chamber photometers. A GeigerÐMüller counter was flown in 1959 to monitor 0.05 Ð 0.3 nm fluxes (Gregory and Kreplin, 1967). Subsequent missions, starting with SOLRAD 10, used an ion chamber for the 0.05 Ð 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 freeÐfree and freeÐbound 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 Ð 0.4 nm and 0.1 Ð 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 applicable 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 transfer function (see next section). However, the reductions in GOES-8 fluxes were made in data processing after first computing fluxes (both XS and XL) using the pre-launch transfer functions, which are still the ones listed in GOES publications (Garcia, 1994; White, Thomas, and Schwartz, 2005).

3. Analysis

3.1. Deriving Solar X-Ray Fluxes from Ion-Chamber Signals

The primary signal provided by broad band soft X-ray sensors, a current [I ] produced as a result of ionization of the gas in an enclosed chamber, cannot be directly interpreted as a level of X-ray ﬂux from the Sun. Rather, that current is the result of a convolution of the incoming solar spectrum with the transmission of the “window” of the chamber through which the radiation passes, the absorbing and ionizing properties of the contained gas, and geometrical effects of the detector system:

∞ I = eωA G(λ)(λ) dλ, (1) 0 Author's personal copy

W.M. Neupert

Table 1 Modiﬁcations made to GOES sensors and to ion-chamber current-to-soft X-ray ﬂux conversions during the operational interval from SMS-1 to GOES-8.

Spacecraft Short-wavelength Sensor (XS) Long-wavelength Sensor (XL)

SMS-1 Transfer function derived from laboratory Transfer function derived from laboratory calibration results and observed flare spectra calibration results and observed flare spectra used to infer solar X-ray fluxes used to infer solar X-ray fluxes SMS-2 Transfer function based on theoretical calcu- Transfer function based on theoretical calculation and measured window weights lation and measured window weights GOES-1 Additional beryllium window added to sup- Additional beryllium window added to sup- press solar UV. Fluxes for this and subse- press solar UV. Fluxes for this and subsequent GOES based on theoretical calcula- quent GOES based on theoretical calculations and measured window weights tions and measured window weights GOES-2 Observed fluxes increased by 17.2% to Observed fluxes increased by 8.9% to match match SMS-2a SMS-2a GOES-3 Transfer function based on a flat spectrum Transfer function based on a flat spectrum adopted for this and subsequent GOES. adopted for this and subsequent GOES Spectral band reduced from 0.05 Ð 0.4 nm to 0.05 Ð 0.3 nm in deriving transfer function GOES-4 GOES-5 GOES-6 GOES-6 XL fluxes reduced by 17% to match GOES-5 fluxes GOES-7 GOES-8 Use of efficiency curve published by Hanser Use of efficiency curve published by Hanser and Sellersb and Sellersb Original spinning spacecraft design replaced Original spinning spacecraft design replaced with a three-axis stabilized spacecraft with a three-axis stabilized spacecraft GOES-8 XS fluxes reduced 15% to match GOES-8 XL fluxes reduced 30% to match GOES-7 fluxesc GOES-7 fluxesc a Panametrics (1987). b Hanser and Sellers (1996). c Hill et al. (2005). where G(λ) is the sensitivity of the sensor system as a function of wavelength, and (λ) is the wavelength-dependent radiant flux due to the flare. A is the window area of the sensor, ω is the number of ion pairs produced in the gas per unit absorbed energy, and e is the electronic charge (Garcia, 1994; Sylwester, Garcia, and Sylwester, 1995). Note that G(λ) includes all geometric (e.g. collimator) factors. In practice, a spectrally weighted sensitivity, called the transfer function [G¯ ] is used:

∞ G(λ)(λ) dλ ¯ = 0 G λ . (2) j (λ)dλ λi

The ﬂare ﬂux within the spectral interval λi to λj is then estimated as ¯ F(λi ,λj ) = I/eωAG. (3)

The incoming spectral distribution [(λ)] is aprioriunknown and one must be assumed to obtain an estimate of F(λi ,λj ). In the absence of accurate characterization of the solar ﬂare Author's personal copy

Intercalibration of Solar Soft X-Ray Broad Band Measurements

Table 2 Computed ion-chamber sensitivity values [G¯ ]a as in Table 3 for SOLRAD 11.

Short X-ray sensor (XS)

XL Levelb and spectrum 0.05 Ð 0.3 nm 0.05 Ð 0.4 nm 0.05 Ð 0.3 nm 0.05 Ð 0.4 nm − − − − 10 MK “Gray Body” spectrum 9.38 × 10 10 5.56 × 10 10 9.38 × 10 10 5.56 × 10 10 Mewe elemental abundances Meyer elemental abundances − − − − X 6.0 Aschwanden and Alexander (2001)c 1.055 × 10 9 5.61 × 10 10 1.074 × 10 9 5.82 × 10 10 − − − − M1.2 Dere and Cook (1979)9.77 × 10 10 4.57 × 10 10 9.99 × 10 10 4.75 × 10 10 − − − − C8.0 Dere and Cook (1979)8.01 × 10 10 3.10 × 10 10 9.02 × 10 10 3.54 × 10 10 − − − − C2.0 Dere and Cook (1979)7.74 × 10 10 2.14 × 10 10 7.93 × 10 10 2.22 × 10 10

Long X-ray sensor (XL)

0.1 Ð 0.8 nm 0.1 Ð 0.8 nm − − 2 MK “Gray Body” spectrum 1.518 × 10 10 1.518 × 10 10 Mewe elemental abundances Meyer elemental abundances − − X 6.0 Aschwanden and Alexander (2001)7.65 × 10 10 7.51 × 10 10 − − M 1.2 Dere and Cook (1979)6.84 × 10 10 6.64 × 10 10 − − C 8.0 Dere and Cook (1979)5.24 × 10 10 5.22 × 10 10 − − C 2.0 Dere and Cook (1979)4.13 × 10 10 3.85 × 10 10

− a Units are coulombs erg 1. b XL Levels referenced to GOES-2. c Spectra derived using the CHIANTI spectral modeling program (Dere et al., 1997) and flare models by As- chwanden and Alexander (2001) and Dere and Cook (1979) at three times during the decay of an M1.3 flare. spectrum, NRL adopted several “gray-body” spectral distributions, i.e. the spectral distributions but not radiation levels characteristic of black bodies having temperatures ranging from 10 MK for XS measurements to 2 MK for XL. These were maintained throughout the years of NRL flare-flux reports. The recalculated G¯ factors needed to convert SOL- RAD fluxes from “gray-body” spectra to typical flare spectra, derived from differential emission measures (DEM) published by Aschwanden and Alexander (2001) at the peak of an X5 flare and by Dere and Cook (1979) from the beginning of the decay phase an M1.3 flare, are given in Table 2. These models were selected as they incorporated extreme- ultraviolet observations and therefore represented a greater range of flare plasma temperatures than do solely soft X-ray based analyses. Elemental abundances published by Mewe (1972) were commonly in use during the 1970s but more recent abundance determinations (Meyer, 1985) are most often now used. Both have been considered in Table 2 to demonstrate the rather small effect that differences in adopted elemental abundances may have on final sensitivity values. The CHIANTI version 5 spectral modeling program (Dere et al., 1997; Landi et al., 2006) with 0.001 nm spectral resolution was used for these calculations. For its ion-chamber calibrations up to and including GOES-2, NOAA/SWPC initially adopted spectral distributions derived from soft X-ray proportional-counter flare observations (Culhane et al., 1969; Pounds, 1970; Donnelly, Grubb, and Cowley, 1977)butthen switched to a “flat” spectrum beginning with GOES-3, that is, using:

∞ G(λ)(λ) dλ G¯ = 0 . (4) (λj − λi ) Author's personal copy

W.M. Neupert

Table 3 Computed ion-chamber sensitivity values [G¯ ]a for GOES-8.

Short X-ray sensor (XS)

XL Levelb 0.05 Ð 0.3 nm 0.05 Ð 0.4 nm 0.05 Ð 0.3 nm 0.05 Ð 0.4 nm − − − − “Flat” spectrum 1.60 × 10 5 1.14 × 10 5 1.60 × 10 5 1.14 × 10 5 Mewe elemental abundances Meyer elemental abundances − − − − X 6.0 Aschwanden and Alexander (2001)c 2.23 × 10 5 1.19 × 10 5 2.08 × 10 5 1.20 × 10 5 − − − − M 1.2 Dere and Cook (1979)2.47 × 10 5 1.16 × 10 5 2.44 × 10 5 1.16 × 10 5 − − − − C 8.0 Dere and Cook (1979)2.94 × 10 5 1.14 × 10 5 2.91 × 10 5 1.13 × 10 5 − − − − C 2.0 Dere and Cook (1979)4.11 × 10 5 1.14 × 10 5 4.00 × 10 5 1.12 × 10 5

Long X-ray sensor (XL)

0.1 Ð 0.8 nm 0.1 Ð 0.8 nm − − “Flat” spectrum 4.09 × 10 6 4.09 × 10 6 Mewe elemental abundances Meyer elemental abundances − − X 6.0 Aschwanden and Alexander (2001)3.73 × 10 6 3.69 × 10 6 − − M 1.2 Dere and Cook (1979)3.87 × 10 6 3.86 × 10 6 − − C 8.0 Dere and Cook (1979)3.66 × 10 6 3.65 × 10 6 − − C 2.0 Dere and Cook (1979)3.51 × 10 6 3.52 × 10 6

− a Units are amp W 1 m2. b XL levels referenced to GOES-2. c Spectra derived using the CHIANTI spectral modeling program (Dere et al., 1997) and ﬂare models by Aschwanden and Alexander (2001) and Dere and Cook (1979) at three times during the decay of an M 1.3 ﬂare.

Also, for GOES-3 and later missions, the estimated flat-spectrum XS flux was based on a spectral range of 0.05 Ð 0.3 nm as contrasted with 0.05 Ð 0.4 nm used for earlier missions. The reported XS fluxes, beginning with GOES-3, were reduced by a factor of 2.5/3.5(Gar- cia, 1994) although the Solar Geophysical Data published by the National Geophysical Data Center (NGDC) continues to label XS fluxes as 0.05 Ð 0.4 nm. Transfer functions for a flat- spectrum and for flare spectra based on sensor parameters published by Hanser and Sellers (1996) for GOES-8 are provided in Table 3. For comparison, GOES-2 fluxes were originally determined using G¯ values of 1.17 × 10−5 and 3.98 × 10−6 for XS and XL, respectively (Garcia, 1994). The impact of differing elemental abundances is benign but the change from an assumed 0.05 Ð 0.3 nm to 0.05 Ð 0.4 nm spectral range for XS fluxes is significant. As pointed out by Wende (1972), an XS flux based on an 0.05 Ð 0.4 nm spectral range results in a far more linear relation between ion-chamber current and estimated X-ray flux over a wide range of flare intensities than does the use of an 0.05 Ð 0.3 nm range. Wende’s work was at least in part the basis for the original spectral range selections used through GOES-2. ¯ Using G values calculated for alternative spectral ranges [λk,l] and assumed spectral distributions [b], GOES XS or XL fluxes can be converted from published values for a flat spectral assumption [F(λi,j ,flat)] to the corresponding flux:

¯ ¯ F(λk,l,b) = F(λi,j ,ﬂat)G(λi,j ,ﬂat)/G(λk,l,b). (5) Author's personal copy

Intercalibration of Solar Soft X-Ray Broad Band Measurements

Table 4 Comparison of computed and observed SOLRAD 11 to GOES-2 ﬂare-ﬂux ratios.

XL ﬂare levelc Computed ratiosa Observed ratiosb XS XL XS XL

X6.0 0.60 5.04 0.90 9.0 M1.2 0.49 4.50 0.66 3.5 C8.0 0.33 3.45 0.63 3.15 C2.0 0.23 2.72 0.51 2.3 a Computed using gray-body spectra for SOLRAD 11 and, for GOES-2, spectra derived by the CHIANTI spectral modeling program (Dere et al., 1997) from Aschwanden and Alexander (2001) and Dere and Cook (1979) ﬂare models using Mewe (1972) abundances. b Ratios from empirical algorithms given by Kahler and Kreplin (1991). c XL ﬂare level referenced to GOES-2.

3.2. Comparisons of Soft X-Ray Fluxes Using Overlapping Spacecraft Observations

3.2.1. Direct Comparison of SOLRAD 11 with SMS-2 and GOES-2

Kahler and Kreplin (1991) compared SOLRAD 11 with SMS-2 (the Synchronous Meteoro- logical Satellites, were the forerunners of GOES) and GOES-2 observations of many flares and provided algorithms for relating SOLRAD fluxes to GOES. Measurements of concurrent SOLRAD 11 and GOES-2 fluxes reported in Solar Geophysical Data were repeated using a consistent method of background subtraction with results similar to those of Kahler and Kreplin (1991) except at very low flux levels. XL values were read at flare maximum; XS values were also read at XL flare maximum. Table 4 provides a comparison of computed SOLRAD 11 to GOES-2 fluxes ratios, using Table 2, with observed flux ratios (from Kahler and Kreplin, 1991) for four different flare levels. Note that the ratios are flare-size (actually XL/XS) dependent as SOL- RAD and GOES sensor spectral sensitivities differed somewhat due to differing ion- chamber window thicknesses and gas fillings (Kahler and Kreplin, 1991). Comparison of computed and observed ratios implies that the SOLRAD XS channel is high compared to GOES-2 and its fluxes should be multiplied by 0.5 to 0.66 depending on flare level, to achieve consistency. SOLRAD 11 XL fluxes may be low by 25% relative to GOES-2 for M-level (1.0 Ð 10.0 × 10−5 Wm−2) flares with a poorer match for X-level (≥ 1.0 × 10−4 Wm−2) and a better match for weaker C-level (0.1 Ð 1.0 × 10−5 Wm−2) events.

3.2.2. Comparison of GOES Observations from GOES-2 through GOES-12

Direct comparison of GOES-2 and GOES-12 observations is, of course, not possible, but can be estimated using successive GOES data sets. Continuity of GOES observations and calibrations was achieved by overlapping operation of successive GOES satellites sufﬁ- ciently so that solar-ﬂare events could be concurrently observed and compared. Such comparisons were done at the beginning of each new mission (Panametrics, 1987) and have repeated and extended here for GOES-2 and GOES-5 (1983), GOES-5 and GOES-6 (1986), GOES-6 and GOES-7 (1988 Ð 1994), GOES-7 and GOES-8 (1995 Ð 1996), and GOES-8 and GOES-12 (2003). Three-second archival GOES data were used when available (at http://www.ngdc.noaa.gov); otherwise, one-minute data were used (comparisons of the two Author's personal copy

W.M. Neupert

Figure 1 Ratios of GOES-7 (launched in 1988) to GOES-6 (in operation six years as of 1988) XS and XL peak flare fluxes for observations from 1988 to 1994 plotted against time and against GOES flux levels. Changes in the observed ratios with flare intensity correlate with transitions from one GOES electronics gain range to the next as soft X-ray fluxes increased (Amplifier ranges labeled R1, R2, and R3 were used during the observations.) Differences from one range to the next reflect uncertainties in electronics calibrations.

Table 5 GOES flare-flux ratios for flares observed concurrently by two GOES satellites.

GOES-5/ GOES-6/ GOES-7/ GOES-8/ GOES-8/ GOES-12/ GOES-2 GOES-5 GOES-6 GOES-7 GOES-2 GOES-8 (Product of Means)

XS/Range 3 0.825 0.88 1.05 Ð 1.04 XS/Range 2 0.825 0.87 1.05 0.96 1.04 XS/Range 1 0.81 0.90 1.15 0.99 1.06 XS/Range 0 Ð 0.94 1.2 0.98 1.08 Mean of all ranges 0.82 0.90 1.10 0.98 0.80 1.07 Mean and variance 0.80 ± 0.07 0.90 ± 0.03 1.07 ± 0.095 0.98 ± 0.04 0.75 ± 0.10 1.06 ± 0.02 of all data Panametrics mean 0.78 0.732

XL/Range 3 0.92 0.97 1.0 0.89 0.98 XL/Range 2 0.93 0.95 0.93 Ð 0.99 XL/Range 1 0.98 0.95 0.89 0.96 1.00 XL/Range 0 Ð 0.96 Ð 0.97 Ð Mean of all ranges 0.94 0.96 0.94 0.94 0.80 0.99 Mean and variance 0.95 ± 0.04 0.96 ± 0.01 0.96 ± 0.04 0.94 ± 0.03 0.82 ± 0.05 1.00 ± 0.02 of all data Panametrics mean 1.005 1.16 1.025 Author's personal copy

Intercalibration of Solar Soft X-Ray Broad Band Measurements

Table 6 Computed and observed GOES-8 to GOES-2 ﬂare-ﬂux ratios.

Flare levelc Computed ratiosa Observed ratiosb XS XL XS XL

X6.0 0.75 0.90 M1.2 0.73 0.94 C8.0 0.71 0.89 0.75 ± 0.10 0.82 ± 0.05 C2.0 0.70 0.86 a Computed using a flat spectrum for GOES-8 and, for GOES-2, spectra derived by the CHIANTI spectral modeling program (Dere et al., 1997) using flare models from Aschwanden and Alexander (2001)andDere and Cook (1979). b Product of flare-flux ratios for flares observed concurrently by two successive GOES (Table 5). c Spectra derived using the CHIANTI spectral modeling program (Dere et al., 1997) and flare models by Aschwanden and Alexander (2001) and Dere and Cook (1979) at three times during the decay of an M1.3 flare. XL flare level referenced to GOES-2.

types when both were available show that using one-minute averages did not impact results). A quiescent level, within 30 minutes of the beginning of the XL flare, was subtracted for each detector and for each event. Figure 1 presents such a comparison for joint GOES-6 and GOES-7 flare observations. Note the improvement in ratio consistency when data are seg- regated by amplifier range used. Flux ratios for each of four gain levels for each sensor are listed in Table 5. Median values of observations are reported as it was felt that outliers due to instrumental effects (e.g., mis-pointing, error in subtracting particle background) might be unduly affect averages. These median values and their averages are given without error estimate. Means and variances of ratios were calculated for each sensor, summing over all amplifier ranges (even though the ratios differ from range to range). These results are shown in Table 5. Corresponding values reported by Panametrics are also included. The product of ratios relating GOES-8 to GOES-2 provides an estimate of flare fluxes that GOES-8 might have reported for events observed by GOES-2. In computing the compounded product using Panametrics results we used our own values for GOES later than GOES-5. The GOES-5/GOES-2 XL mean of 0.95 is well within the uncertainty of electronic calibration and cannot be attributed to a long-term drift in sensitivity of GOES-2 which had been operating for six years when GOES-5 was launched. It can be seen from Figure 1 and Table 5 that an accurate match of flare fluxes between two consecutive GOES spacecrafts was rarely achieved. Table 6 provides a comparison of modeled and observed flux ratios for GOES-8 relative to GOES-2 (GOES-8 data rather than GOES-12 were used for this comparison as GOES-8 observations are frequently referenced in analyses of Yohkoh observations; e.g., Aschwanden and Alexander, 2001). Given the large variance of the observed GOES-8 to GOES-2 XS ratio (Table 5), the observed GOES-8 XS fluxes are consistent with no significant cumulative error between GOES-2 and GOES-8 after spectral range and shape compensation. However, with a lesser variance of 6% in the XL channel, the GOES-8/GOES-2 XL observed-to-computed ratio of 0.92 (Table 6) suggests the possibility of a discrepancy there. These estimates can be refined using a graphic means of comparing SOLRAD 11, GOES-2, and GOES-8 data sets. Author's personal copy

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Figure 2 Reported maximum XL flare flux vs. the ratio of XL to XS at XL maximum using originally reported flux values for SOLRAD and GOES. The differences between detectors are primarily the result of differing spectral assumptions adopted in the original data reductions. Squares: SOLRAD 11 (“gray-body” spectra); Diamonds: GOES-2 (observed flare spectrum Culhane et al., 1969; Pounds, 1970; Crosses: GOES-8 (flat spectrum).

3.3. Data Set Comparisons Using Plots of the Maximum XL Flux vs. the Ratio of XL to XS Flux at XL Maximum

3.3.1. Comparisons between GOES Data Sets

Originally illustrated by Donnelly, Grubb, and Cowley (1977) and applied extensively by Thomson, Rodger, and Cliverd (2005), the locus of points of the XL (0.1 Ð 0.8 nm) flux of a flare plotted against XL/XS traces an open loop-like feature as the flare progresses. This trace generally has a minimum XL/XS value prior to XL maximum. XL/XS then increases slowly as XL approaches a maximum value. (The inverse ratio [XS/XL] has been related to an “isothermal” or “color” flare temperature (Garcia, 1994; White, Thomas, and Schwartz, 2005).) Such an open loop then describes the initial increase of isothermal flare temperature as the XL flux increases, with maximum temperature being reached shortly before the flare’s maximum XL value. Using the plotting convention employed by Thomson, Rodger, and Cliverd (2005), the ratio XL/XS at the peak of XL obtained for many flares by a sin- gle instrument (i.e., a GOES or SOLRAD two-sensor complement) results in a distribution such that XL/XS at XL maximum decreases as maximum XL increases. The plots produced by two soft X-ray ion-chamber pairs (i.e. XL and XS fluxes) using the same flare spectral distributions and spectral ranges should coincide, even if observing a different group of flares. Any difference implies a difference in performance or calibration of either one or the other (possibly both) detectors of the pair of detectors. Figure 2 presents the SOLRAD 11, GOES-2, and GOES-8 observations at the XL maxima of observed flares, as published. SOLRAD 11 data are offset from the GOES-2 distribution, reflecting principally the dis- parate spectral assumptions that were adopted. GOES-8 points overlie GOES-2 but that is coincidental as different assumptions were made in the original data reduction. Figure 3 presents GOES-2 and GOES-8 data after GOES-8 fluxes have been converted to the same spectral model and spectral ranges employed for GOES-2, using the C8.0 flare model of Dere and Cook (1979) and the computed transfer functions in Table 3. Note the reduced horizontal scale used in Figure 3 and subsequently. An offset between GOES-8 and GOES- 2 remains. If, instead, the experimental GOES-8 to GOES-2 ratios (Table 5)areusedto match the two data sets then better agreement (Figure 4) is obtained but still with a remaining offset. This remaining offset can be removed (Figure 5) by decreasing the GOES-8 XS fluxby9%,0.8oftheXSGOES-8/GOES-2varianceinTable5, and increasing the GOES-8 Author's personal copy

Intercalibration of Solar Soft X-Ray Broad Band Measurements

Figure 3 Reported maximum XL flare flux vs. the ratio of XL to XS at XL maximum for GOES-2 and GOES-8 with GOES-8 observations converted to the same flare spectrum and spectral ranges that had been assumed originally for GOES-2, using the computed Gbar values of Table 3. Squares: SOLRAD 11 (“gray-body” spectra); Diamonds: GOES-2 (observed flare spectrum Culhane et al., 1969; Pounds, 1970; Crosses: GOES-8 (flat spectrum). Dark lines are linear best fits applied to each data set. Note change in horizontal scale from Figure 2.

Figure 4 Reported maximum XL flare fluxes vs. the ratio of XL to XS at XL maximum for GOES-2 (diamonds) and GOES-8 (crosses) but with conversion factors for GOES-8 derived from experimental GOES-8/GOES-2 ratios (Table 5) rather than computed Gbar values. Dark lines are linear best fits applied to each data set.

XL flux by 5%, 0.8 of the variance of the experimental ratio. (Other solutions are possible, of course, but only at the expense of increasing the required adjustment to either XS or XL.) Alternatively, the offset shown in Figure 3, which used computed ratios (Table 4)rather than experimental ratios, can be removed by decreasing GOES-8 XS by 20%, 1.5 times the variance, and an increasing GOES-8 XL by 9%, again, 1.5 times its variance. In the absence of further methods of evaluating the errors implicit in each of these two approaches the means of the above results can be adopted, that is, GOES-8 XS fluxes (already converted to the C8 flare spectrum) reduced by 14% (i.e., multiplied by 0.86, with a probable error of 0.06) and XL increased by 7% (i.e., multiplied by 1.07, with a probable error of 0.02). GOES-2 was here used as the reference not only because of its overlap with SOLRAD, but also because the original GOES-2 fluxes were derived using realistic flare spectra (Donnelly, Grubb, and Cowley, 1977) in the conversion from ion-chamber current to flux. A summary of conversion factors that could be applied to the GOES-8 published fluxes for three different reference flux systems is provided in Table 7,usingtheG¯ values of Table 3. It is possible that GOES-8 fluxes, before adjustments were made prior to their dissemination (Hill et al., 2005), are the most reliable and hence factors by which they are Author's personal copy

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Figure 5 Reported maximum XL ﬂare ﬂuxes vs. the ratio of XL to XS at XL maximum for GOES-2 (diamonds) and GOES-8 (crosses) but with GOES-8 XL conversion increased by 5% (0.8 of the GOES-8/GOES-2 XL variance shown in Table 5) and GOES-8 XS conversion decreased by 9% (0.8 of the XS variance) from the conversion factors already appliedtoGOES-8inFigure4. The two lines shown in Figure 4 are now superimposed.

Table 7 Multipliers to convert published GOES-8 flare fluxes to several assumed spectral distributions and spectral range and for three possible flux reference systems.

Flat spectral assumption Modeled C 8.0 ﬂare spectruma

XS 0.05 Ð 0.3 nm 0.05 Ð 0.4 nm 0.05 Ð 0.3 nm 0.05 Ð 0.4 nm Conversion based on optimized GOES-8-to-GOES-2 match 0.86 1.21 0.47 1.22 (from Figure 5) Conversion using published GOES-8 XS as reference ﬂuxes 1.00 1.40 0.55 1.41 Conversion to original GOES-8 ﬂuxes (i.e., prior to GOES-7 matchb) 1.18 1.65 0.65 1.66

XL 0.1 Ð 0.8 nm 0.1 Ð 0.8 nm Conversion based on optimized GOES-8-to-GOES-2 match (from Figure 5) 1.07 1.19 Conversion using published GOES-8 XL as reference ﬂuxes 1.00 1.12 Conversion to original GOES-8 ﬂuxes (i.e., prior to GOES-7 matchb) 1.43 1.60 a Dere and Cook (1979). bHill et al. (2005).

to be inferred from published GOES-8 ﬂuxes are included in Table 7. The transition from a spinning spacecraft (up to GOES-7) to a three-axis stabilized spacecraft (for GOES-8 and later) may have improved the accuracy of determining the solar ion-chamber current (Hill et al., 2005). Adjustments for GOES-10 and GOES-12 would be very nearly the same as those provided for GOES-8 as these later sensors gave very consistent results. Author's personal copy

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Figure 6 Reported maximum XL flare fluxes vs. the ratio of XL to XS at XL maximum for published SOLRAD 11 (squares) and published SOLRAD 9 (stars) observations. A linear fit to the SOLRAD 11 data is superimposed. Note the displacement of SOLRAD 9 relative to SOLRAD 11.

Figure 7 Maximum SOLRAD 9 and SOLRAD 11 XL ﬂare ﬂuxes vs. the ratio of XL to XS at XL maximum after applying published adjustments to SOLRAD 9 observations: Kahler and Kreplin, 1991 (triangles); Kreplin and Horan, 1992 (stars). The Kreplin and Horan adjustments to SOLRAD 9 appear to provide the best match to SOLRAD 11.

3.3.2. Comparison of SOLRAD 9 with SOLRAD 11

The same technique of plotting XL vs. XL/XS can be applied to two SOLRAD sensor sets using data reported in Solar Geophysical Data. Figure 6 compares SOLRAD 9 observations (the ﬁrst SOLRAD to have carried onboard data recording) with SOLRAD 11, which overlapped in time with SMS and GOES. A clear discrepancy between their respective plots is found. Two sets of correction factors for SOLRAD 9 were reported: Kahler and Kreplin (1991) reported correction factors of 1.2 and 2.1 to be applied to SOLRAD 9 XS and XL, respectively; Kreplin and Horan (1992) gave a factor of 2.2 for XL with no mention of XS changes. Figure 7 demonstrates that the Kreplin and Horan (1992)factorforXL,with no change in XS, provides a slightly better ﬁt to SOLRAD 11 than the earlier Kahler and Kreplin (1991) factors. Applying the algorithms of Kahler and Kreplin (1991)toSOLRAD 11 then provides continuity with GOES. Author's personal copy

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3.3.3. Comparison of GOES with Other Concurrent Observations

Several comparisons of GOES with other solar soft X-ray flare observations, using images, spectra, and other broad band techniques, have recently been made (Aschwanden and Alexander, 2001;Sylwesteret al., 2005; Rodgers et al., 2006;Chiforet al., 2007;Alhaet al., 2008; Väänänen, Alha, and Huovelin, 2009). These results are discussed in order of decreasing solar flare-flux level: Aschwanden and Alexander (2001)usedYohkoh soft X-ray and TRACE extreme- ultraviolet images of an X5 flare (the “Bastille Day Flare” of 14 July 2000) to derive a DEM and X-ray spectrum. Although their computed spectrum accurately reproduced observed Yohkoh fluxes, their computed 0.1 Ð 0.8 nm flux was a factor of 2.1 above the published GOES-8 XL value and the 0.05 Ð 0.4 nm flux was 1.6 times greater than GOES-8 XS. Sylwester et al. (2005) found the continuum in the 3.4 Ð 6.1 Å (2.0 Ð 3.7 keV) spectral range observed by the RESIK Bent Crystal Spectrometer onboard the Russian CORONAS- F solar mission to be a factor of two above the GOES continuum, derived using GOES published fluxes, during the declining phase of an M2.1 flare on 26 April 2003. Chifor et al. (2007) similarly compared a C5.8 flare, with GOES; RESIK again being higher by about 1.5. Their computed GOES continua were based on algorithms for GOES-8 by White, Thomas, and Schwartz (2005) that were derived for unmodified (i.e., unreduced) GOES-8 flux values. Had unmodified GOES-8 fluxes been used by Chifor et al. (2007), GOES and RESIK continua would have been in agreement. Rodgers et al. (2006) compared GOES-8 XL fluxes for 29 events with fluxes from modeled spectra using broad-band measurements made by TIMEDÐSEE. In contrast to the As- chwanden and Alexander (2001)andSylwesteret al. (2005) results for large events, their modeled 0.1 Ð 0.8 nm fluxes for four events above a level of M4 (4.0×10−5 Wm−2) matched published GOES-8 levels. For weaker flares their model results were 1.5 to 2.0 times reported GOES levels. Alha et al. (2008) using X-ray spectra obtained by the X-ray Solar Monitor (XSM) on the Finnish SMART-1 mission to the Moon applied the SPEC modeling program with a broken power-law spectrum and several Gaussian lines to compute the solar-flare flux at a C4 level, finding good agreement with the reported GOES XL flux. Väänänen, Alha, and Huovelin (2009), assuming Mewe spectral models, analyzed ten events and found a GOES to XSM ratio of 0.94 ± 0.09 (i.e. a ratio of XSM to GOES of 1.06) for weak flares (all but two at levels below C1 recorded by XSM). The authors note that their spectra were extrapolated downward in energy from 2.0 keV to 1.5 keV (i.e. upward in wavelength from 6.1 to 8.0 Å). This spectral interval contains prominent emission lines of Si XIII, which, if neglected, could result in an underestimate of fluxes attributed to XSM. The ratio of the mean of these non-GOES measurements to published GOES-8 XL values, for flares above GOES class M2, is 1.7 with a nominal variance of 0.6. The comparison for flares below M1 yields a mean of 1.5 with a variance of 0.5. Using the ratios of Table 7 andEquation(5), we conclude that: GOES-8 XL (flat spectrum) published fluxes are 0.59 of the mean of non-GOES concurrent measurements; GOES-8 XL published fluxes after adjustment to a C8 flare spectrum are 0.59 × 1.12 = 0.66 of other measurements; GOES-8 XL fluxes after both spectral adjustment and restoration of the 30% reduction during data processing are 0.66/0.7 = 0.94 of the mean of other measurements. GOES-2 XL fluxes are 0.59 × 1.19 = 0.70 of the mean of measurements concurrent with GOES-8. Author's personal copy

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Only one comparison of a modeled flux with a GOES-8 XS flux is available. Aschwanden and Alexander (2001) found a ratio of 1.6. In this instance the comparison was between a computed 0.05 Ð 0.4 nm flux and a reported GOES 0.05 Ð 0.3 nm value. Adjusting the reported GOES value to the spectrum and wavelength interval used by Aschwanden and Alexander (2001), the ratio would be reduced to 1.13 and, had the GOES-8 XS data not been reduced by 15% in processing, to 0.96. Thus, the GOES-8 XS flux after both spectral adjustment and restoration of the 15% reduction would be 1.04 of the modeled result. These very limited comparisons suggest that the GOES observations beginning with GOES-8 (which were made from a three-axis stabilized platform), after compensation for a realistic flare spectrum and not altered to be consistent with the preceding GOES-7, are a good match to the averages of non-GOES data and could be used as estimates of the integrated soft X-ray fluxes in the 0.1 Ð 0.8 nm and 0.05 Ð 0.4 nm spectral bands. This conclusion applies to M-class flares and greater but may not be applicable for very small events.

3.3.4. Uncertainties in the Analysis

Uncertainties in the SOLRAD and GOES fluxes may be introduced, particularly for small flares, by the manner of subtracting the underlying (and possibly varying) solar background X-ray level on which the flare flux is superimposed (Rodgers et al., 2006). Background GOES fluxes were subtracted for events analyzed for this article but several earlier analyses (Kahler and Kreplin, 1991; Thomson, Rodger, and Cliverd, 2005; Väänänen, Alha, and Huovelin, 2009) made no mention of how or whether GOES background levels were taken into account. Not subtracting a non-flare background signal from low-level GOES flare fluxes could depress the non-GOES to GOES ratios reported for weak events. Each of the non-GOES flux comparisons has its observational uncertainties, usually estimated at 10% (Rodgers et al., 2006) to 20% (Chifor et al., 2007). Spectra calculated using CHIANTI are only slightly affected by changing abundances of neon and possibly magnesium, which may vary from flare to flare (Schmelz, 1993). The impact of these (as well as slightly different abundances for other elements) on final flux values is small compared to the still remaining uncertainties in calibrations in the SOLRAD and GOES measurements.

4. Discussion and Conclusions

Two analyses of the GOES data base have been presented. The first (Table 7) provides a means of converting GOES-8 fluxes between differing sets of spectral assumptions that can be applied to these observations. It was found that reported GOES-8 XL fluxes are approximately 20% below early measurements (GOES-2) based on realistic flare spectra. Sixty three percent of this difference can be attributed to the change in spectral assumptions and the remainder is evidently due to uncertainties in calibrations and matching of flux levels recorded during overlapping observations. To maintain a consistent flare scale for space-weather purposes GOES-8 through GOES-12 XL fluxes could be increased by 1.07 to be consistent with GOES-2 when the latter are expressed in terms of flat-spectrum fluxes. A second comparison, between GOES-8 data and other flare observations made concurrently, provides cross-calibration between GOES and other measurements. Flux determinations relative to GOES by other methods differ among themselves by a factor of as much as 1.5 (and up to a factor of two for the weakest flares). Spectrally adjusted GOES-8 XL fluxes are estimated to be 0.66 of the mean of these other observations for large flares. If the pre- publication reductions to GOES-8 fluxes noted above were removed, the spectrally adjusted Author's personal copy

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GOES-8 values would be only a few percent below the non-GOES mean (for XL) or a few percent above (for XS). This good agreement may be fortuitous and more comparisons with GOES are to be encouraged. Users of the GOES databases should be aware of the following. i) Published GOES fluxes since GOES-2 are based on a flat spectral distribution and are not directly comparable with integrated flux values based on realistic flare spectra. ii) The short-wavelength GOES spectral band currently represents the integrated flat spectral flux between 0.05 and 0.3 nm, not 0.05 Ð 0.4 nm. iii) Algorithms provided by White, Thomas, and Schwartz (2005) for calculation of color temperature and flare emission measure for GOES-8 and later are based on pre-launch G¯ values. However, XL and XS fluxes, after being calculated with these G¯ values, were subsequently reduced (each by different amounts) before publication.

Acknowledgements The author thanks Thomas Bogdan and Ernest Hildner, Director and former Director of the NOAA/Space Weather Prediction Center, respectively, for their hospitality and support. The CHIANTI atomic database and code is an international project involving the Naval Research Laboratory, George Mason University (USA), the Rutherford Appleton Laboratory, the Mullard Space Sciences Laboratory and the Uni- versity of Cambridge (UK), and the University of Florence (Italy). The author thanks an anonymous reviewer for suggestions that improved the clarity and usefulness of the article.

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