fl097.phillips_k01 Posted: 31-Dec-92 Updated: 12-Dec-93, 2-Feb-94, 8-Oct-94, 29-Jun-95, 26-Apr-96 Events specified: N/A
K. J. H. Phillips -- Group Leader
Project involves work of C. D. Pike, T. Watanabe, J. Lang, A. Fludra and may involve others.
Channel 1 of the BCS covers a wavelength range that, for most flares, just covers the wavelength of the Fe K beta line at 1.756 A, shortward of the Fe XXVI Lyman alpha lines and Fe XXV satellites. The K beta line is due to 1s - 3p transitions in photospheric iron, and most likely is excited by fluorescence, with X-rays having energies above 7.1 keV emitted by hot flare-produced plasma shining down on the photosphere to K-shell ionize photospheric iron atoms with consequent photon emission as the atoms undergo rearrangement transitions. More than one transition is possible, but the most important lines are the almost coincident lines K beta 1 and K beta 3, at 1.756 A. The line was first identified in solar flare spectra by Tanaka in Hinotori X-ray data.
The chief objective of this proposed Bulletin Board topic is to confirm that the excitation mechanism of the Fe K beta is in fact fluorescence. Possible alternatives include ionization of the photospheric iron atoms by energetic particles. The two mechanisms can quite easily be distinguished from light-curves of the K beta line feature, as particle excitation would give an impulsive burst at the flare start, and fluorescence a curve roughly following soft X-rays with energies of about 7 keV, which will be much more gradual. However, very few flares have K beta line fluxes large enough to be able to trace out a light curve, and most likely distinguishing between the excitation mechanisms will be from calculations of the expected flux. Preliminary work suggests that the observed flux is within a factor of three of that expected from the fluorescence mechanism.
The possibility exists of directly measuring the photospheric-to-coronal iron abundance from observations of the K beta line and the Fe XXV line, seen in BCS Channel 2. A preliminary theoretical analysis suggests that the ratio depends on this abundance ratio and only weakly on temperature, which could be obtained from Fe XXV line ratios. Alternatively, the excitation of the K beta line could be obtained from directly measured continuum above 7 keV, and this is being investigated using SXS measurements.
Update 26-Apr-96
The paper on the coronal/photospheric abundance of Fe using Yohkoh BCS observations of the Fe II K-beta line and the Fe XXV resonance line was published in ApJ, 435, 888 (1994). Work has since progressed along similar lines using the K-alpha lines of Fe II which (like Fe K-beta) are formed by fluorescence during solar flares. These lines and the Fe XXV resonance line were observed by X-ray spectrometers on SMM and the P78-1 spacecraft. A preliminary analysis was done and this indicated a similar result to that obtained from the Yohkoh results, namely, the photospheric and coronal abundances of Fe are generally insignificantly different. However, a few flares do show the K-alpha lines to be relatively strong compared with the Fe XXV line, indicating a coronal Fe abundance somewhat less (up to a factor 2) than the photospheric. KJHP and Dr Andrzej Fludra plan to investigate these flares further, using both the K-alpha/Fe XXV line ratio technique as well as the line/continuum technique that Dr Fludra has used to investigate the Fe and Ca abundance in the past.
Update 29-Jun-95
Work on the Yohkoh BCS spectra containing the Fe K beta and Fe XXV lines was completed and a paper on this and the coronal/photospheric iron abundance written and published in ApJ, 435, 888 (1994 Nov. 10). Work is now proceeding on analysis of SMM, P78 and Hinotori data in which the Fe K alpha lines as well as the Fe XXV lines are visible. It is expected that these data will enable an even more definitive statement on the coronal/photospheric Fe abundance to be made.
Update 8-Oct-94
Further analysis of the Fe K beta line, at 1.757 A and seen in some Yohkoh BCS spectra of flares, has been made with a view to determining the ratio of the photospheric to coronal Fe/H abundance. This has been done by taking the observed line intensity ratio of the Fe K beta line with the Fe XXV resonance line (w), at 1.85 A and seen with channel 2 of the BCS (K beta is seen with channel 1). This intensity ratio has been plotted with the heliocentric angle of each flare. The plot shows a small scatter of points around a constant value for theta less than about 60 degrees, falling to zero at higher thetas, as would be expected if the K beta line is formed by fluorescence in the photosphere (cf. theory of Bai 1979). The line intensity ratio for theta less than about 60 degrees agrees to within a factor of two with that from theory in which the photospheric and coronal Fe/H abundances are set equal to one another, so our work is consistent with a coronal abundance that is not significantly time-varying and equal to the photospheric abundance.
The work has been written up and accepted for publication in Astrophysical Journal. It has been presented at Japanese conferences also.
Work is now proceeding on the similar Fe K alpha lines (1.936, 1.940 A) which are seen in SMM, P78 and Hinotori spectra and are formed in exactly the same way as the K beta line (i.e. through fluorescence of the solar photosphere by a hot flare plasma in the low corona. Preliminary work suggests the same result, using SMM and P78 data at least. This has been reported at the Kofu meeting (Sept. 1993) and Cospar (July 1994). Comparison is being made with Fe abundance determinations from Fe XXV line to continuum measurements in a collaboration with Dr A. Fludra.
Update 2-Feb-94
The work will be extended to look at K alpha data from previous missions, notably the SOLFLEX instrument on the US P78-1 satellite, the Bent Crystal Spectrometer on the NASA SMM satellite and the SOX1 instrument on the Japanese Hinotori satellite. The analysis will follow that of the K beta work.
Update 12-Dec-93
A paper "Iron K-beta line emission in solar flares observed by Yohkoh and the solar abundance of iron" by K J H Phillips, C D Pike, J Lang, T Watanabe and M Takahashi has been submitted to Astrophys. J. The abstract is:
"A weak line at 1.757A in solar flare X-ray spectra from the BCS is identified as the K beta or inner shell 1s - 3p transition in photospheric iron. The observed line intensity and light curve during several flares suggest that, as with the K alpha lines at 1.936 and 1.940A, excitation occurs by fluorescence, with X-ray emission from the hot flare plasma in the low corona exciting photosheric iron atoms. The intensity ratio of the K beta line to the Fe XXV resonance line, which is near in wavelength (1.875A) and also observed by the BCS, depends on the photospheric and coronal Fe/H abundance ratio as well as heliocentric distance and height of the X-ray source. Contrary to recent work, the BCS observations indicate that the photospheric and coronal Fe/H abundance ratios are approximately equal, with little variation from flare to flare."
For the 31 flares involved in the above work, it is proposed to combine some of the results of topic fl027 on "The abundances of sulphur, calcium and iron" in collaboration with A Fludra to investigate the abundances of calcium and sulphur relative to the photospheric and coronal Fe/H abundance ratios.