2 SMM FCS solar flare spectra

The SMM spacecraft operated fully for nine months in 1980, from the time of launch (February) to the time when an attitude control unit on the spacecraft failed (November), and from April 1984 when Space Shuttle astronauts repaired the attitude control unit to December 1989 when the spacecraft re-entered the Earth's atmosphere. The FCS instrument was unable to operate while the attitude control unit was unavailable (November 1980 to April 1984). The full spectral range of the FCS was 1.5-20 Å (in principle slightly longer wavelengths were accessible, but the sensitivity was too small to make spectra in this range useful). The instrument consisted of seven Bragg-diffracting crystals mounted on a single rotatable shaft, with solar X-rays incident on them via a fine collimator (FWHM response 14$^{\prime\prime}$). X-ray photons were detected by proportional counters (one per crystal). The whole instrument could be moved in a scanning motion with the crystals fixed in wavelength at so-called "home position" spectral lines so that images of flares and active regions could be formed. Alternatively, with the collimator pointed at, e.g., the brightest point of a flare, the crystal shaft could be rotated through all or part of its range to obtain spectra in all seven channels (these are numbered 1 to 7 in decreasing order of wavelength). A summary of the spectroscopic results is given by [25, Phillips (1999)]. During the first part of the mission, in 1980, when the Sun was near the peak of its activity cycle, the FCS was used largely as an imaging instrument, and relatively few flare spectra were obtained; only on one occasion were the FCS crystals driven over their entire range to get full spectral coverage. From 1984 to 1989, when the Sun was at first near minimum activity but later at a high state of activity, many spectra of active regions and flares were obtained, but by this time one then later other detectors had failed so that the spectral coverage was never complete.

Despite these operating problems, sets of spectra taken on two occasions are very suitable for analysis and comparison with MEKAL theoretical spectra. Those obtained during the decay of an M3 flare on 1980 August 25 form one of these sets. The spectral coverage is complete in this one case, but there is significant line emission only in the range 5-20 Å, covered by channels 1 to 4. The scan was started at 13:10 U.T. and lasted approximately 17.5 minutes, during which time the flare X-ray emission appreciably decreased. A notable feature of the spectra obtained is the large number of Fe ion lines in the 10.5-17 Å range, mostly due to n=3-2 transitions in Fe XVII-Fe XIX. The analysis and a full line list with identifications are given by [26, Phillips et al. (1982)].

On the second occasion, several spectral scans were made during an M4.5 flare on 1985 July 2 between 21:19 U.T. and 21:41 U.T., which included the peak at about 21:25 U.T. As a consequence, much higher-excitation lines are apparent in these spectra, notably in the 7.3-10 Å range which includes intense $n \geq 4 - 2$ lines in various Fe ions (mostly from Fe XIX to Fe XXIV). The analysis, in which several lines were identified for the first time, is described by [12, Fawcett et al. (1987)]. Some of the Fe XXI and Fe XXII lines are density-dependent, and values obtained in an analysis of the spectra were discussed by [28, Phillips et al. (1996b)]. The sequence of spectral scans was briefly as follows. The home position lines were first scanned, and having checked that the flux in them was large, the on-board software controlling the FCS commanded a spectral scan over the short-wavelength range. The home position line flux was again checked, then a second scan was made covering longer wavelengths, then a third and finally a fourth scan were made to cover the entire range of the FCS crystals. Four of the seven channels were operating at the time of this flare. The scans from channel 3, covering the wavelength range 7.33-10.09 Å (with two small data drop-outs), are the ones discussed here and which include the high-excitation Fe ion lines.

  

Figure 1: a) Spectral resolution $\Delta\lambda$ (FWHM, in Å) and b) resolving power $\lambda/\Delta \lambda$ of channels 1 to 4 of the FCS instrument, determined by the crystal rocking curve. The thermal Doppler line profile width (FWHM) of four representative lines in this range are individually plotted: the wavelengths are O VIII 18.97 Å, Fe XVII 15.01 Å, Mg XI 9.17 Å, Si XIII 6.65 Å

As mentioned, the FCS had unprecedentedly good spectral resolution for the soft X-ray region, and even now there has not been a space-based instrument with better resolution except for limited spectral ranges. (Some spectrometers on plasma devices such as the PLT tokamak do now have better resolution than the FCS.) The instrumental resolution of the FCS was determined largely by the width of the crystal rocking curve, a function of wavelength over the range of each crystal. As this is an important characteristic that determines the precision of line wavelengths, we plot both the FWHM (Å) of the rocking curve $\Delta\lambda$ (Fig. 1a) and the resolving power $\lambda/\Delta \lambda$ (Fig. 1b) as a function of wavelength. As can be seen, the resolving power of channels 1 and 2 is between 1000 and 2500, but is much greater for channels 3 and 4 covering shorter wavelengths. For channels 1 and 2, the instrumental width (FWHM) determined by the crystal rocking curve is much larger than the width (FWHM) of the thermal Doppler profile of spectral lines in this range. Examples of the latter, given by $1.665 \times (\lambda/c)\sqrt(T_{\rm ion}/m_{\rm ion})$ ($\lambda$ in Å, $T_{\rm ion}$ the ion temperature in K of peak line emissitivity, $m_{\rm ion}$ the ion mass), are plotted in Fig. 1a. As can be seen, the thermal Doppler width is larger than the rocking curve width for channels 3 and 4.

Plasma turbulence is a significant line broadening mechanism for flare spectra at the time of the flare impulsive (onset) phase, but is likely to be much smaller or nearly zero for later stages, as with the flare spectra we analyzed. In general we found that a combination of thermal Doppler and the calculated FCS instrumental broadening adequately represented the observed line profiles.

Both the relative and absolute wavelength scale of FCS should be very precise. This is because of the high-precision Baldwin drive-encoder unit that was used for the FCS crystal drive. The output of this unit is crystal drive address rather than wavelength directly. An algorithm exists in the standard analysis software package (now incorporated in the SolarSoftWare or SSW system: see [13, Freeland & Handy 1998)] for the conversion. The usual Bragg condition $N\lambda = 2d \,\,{\rm sin}\,\, \theta$ (N is the spectral order, generally 1 except for a few lines that are intense enough to be significant in second order) must be corrected for crystal refractive index and non-flatness. These were precisely measured before launch. The accuracy of relative wavelengths is estimated by [26, Phillips et al. (1982)] to be about 2 mÅ for $\lambda < 15$ Å (i.e. over most of the range considered here). Absolute wavelengths were obtained using the home position lines in each of the seven channels as reference. For channel 1, this was the O VIII Ly-$\alpha$ line, but for the remaining channels the reference was the resonance line ($1{\rm s}^2 \, ^1{\rm S}_0 - 1{\rm s}2{\rm p} \,\, ^1{\rm P}_1$) of various He-like ions. The theoretical wavelengths of these lines are known to better than 1 mÅ so the absolute wavelengths should be accurate to 2 or 3 mÅ over much of the FCS wavelength range used for this analysis. It should be noted, however, that many of the line wavelengths given by Phillips et al. above about 15 Å are up to 5 mÅ larger measured against the wavelength scale in the more recent SSW software. A few wavelengths at shorter wavelengths have also been slightly adjusted (generally by not more than 1 mÅ).

The conversion of photon count rates in the FCS spectra to absolute intensity units (photons cm^-2 s^-1 Å^-1) is possible through pre-launch data referring to parameters such as measured crystal sizes, reflectivities etc. Again, standard software does this conversion (the procedure for the latter was described by [26, Phillips et al. 1982]). Since a large number of parameters are involved and the accuracy was impossible to check after SMM was launched, the absolute intensities are probably not more accurate than 25%.

With flare spectra from the FCS, there is a large background emission which is instrumental in origin and is much larger than the theoretical free-bound and free-free continuum emission. It is due to fluorescence by solar hard X-rays (with photon energies of 9 keV or more) of the crystal material; the fluoresced radiation is emitted in all directions from each crystal, and so forms a continuous background when detected by the proportional counters. During the time it takes for the crystals to be scanned over their particular wavelength ranges (about 17.5 minutes in the case of the August 25 flare), the flare intensity will have changed. This has two consequences important for this work. First, the radiation fluorescing the crystals varies with time, so that the background is strongly wavelength-dependent. Secondly, the line intensities will also be a function of time and therefore wavelength, so that the emission measure $EM = N_{\rm e}^2 V$ as a function of temperature T ($N_{\rm e}$ is the electron density, V the volume) will apparently vary, reflecting the state of the emitting plasma at the time of the scan.

Channels 1 and 2 of the FCS overlap in the region 13.10-14.94 Å, a fact which is useful in comparing the line intensities and wavelength scales. As the wavelength range 13.10-14.94 Å was observed in channel 1 soon after the start of the spectral scans during the 1980 August 25 flare but much later in channel 2, we expect higher-ionization stages to be prominent in the channel 1 scan but less obvious in the channel 2 scan. This is well illustrated for the region 13.45-13.70 Å which contains the well-known triplet of Ne IX (due to $1{\rm s}^2 \, ^1{\rm S}_0 - 1{\rm s}2{\rm p} \,\, ^1{\rm P}_1$, $1{\rm s}2{\rm p}\,\,^3{\rm P}_{1,2}$, and $1{\rm s}2{\rm s}\,\,^3{\rm S}_1$ transitions) as well as several Fe XIX lines of the $2{\rm p}^4 - 2{\rm p}^3 3{\rm d}$ array. Because Fe XIX is a hotter ion than Ne IX, the Fe XIX lines are more intense relative to the Ne IX lines in the channel 1 scan than in the channel 2 scan. There are other more minor differences of a similar nature. The wavelength scales of the two channels are very slightly different over this range: channel 1 wavelengths are approximately 1-2 mÅ longer than those of channel 2.

For the original analysis of the 1980 August 25 flare, [26, Phillips et al. (1982)] listed 205 recognizable line features, of which 165 were assigned identifications. The analysis of the 1985 July 2 flare by [12, Fawcett et al. (1987)] in the 7.85-10.0 Å range resulted in a list of 47 recognizable line features, of which 38 were not in the list of Phillips et al. Identifications of these lines (which are mostly due to L complex lines with $n \geq 4 - 2$transitions in various Fe ions) were assigned by comparison with data calculated from a Hartree-Fock code [10, (Cowan 1981)].

  

Table 1: Wavelength ranges of spectral intervals for spectra analyzed

¹ See text and Figs. 3-6. Scan s22 (range 13.115-14.908 Å) was not used in the final analysis.

  

Table 2: Fit parameters of the 1980 August 25 flare spectra observed by SMM/FCS (see text for explanation of symbols)

  

Table 3: Fit parameters of SMM FCS 1985 July 2 flare spectra (see text for explanation of symbols)