MAX's two energy ranges happen to be extraordinarily well suited for this study.  The lower range includes the positron-annihilation line, whose Doppler profile is very sensitive to the temperature and density of the plasma in which the positrons (produced by nuclear and pi-meson decays) annihilate.  The first high-resolution measurement of a solar flare annihilation line was recently made by RHESSI (Share et al. 2003) and the line turned out to be surprisingly broad (8.1+/-1.1 keV FWHM).  There are two possible scenarios for a line this broad: thermalization in a very hot plasma (about 500,000K) or annihilation "in flight" in a cooler plasma in a very narrow energy band of approximately 6000K +/- 500K.  In the quiet solar atmosphere, there is not enough solar material in either state to stop and annihilate positrons, so we are being told something very specific about the flaring atmosphere; but more data are needed to know which atmosphere and annihilation mode is responsible.  The broadened line is confirmed in other RHESSI flares not yet published. 
 

Fig 1 : Spectrum of the solar 511 keV annihilation line derived by subtracting the instrumental and background components from the total spectrum observed during the flare. The uncertainties are too large for the scatter in the data; therefore, we use the difference in chi-squared to compare the fits of different line shapes. The solid curve is the best-fitting Gaussian. The dashed curve showing the calculated line shape formed in a quiet atmosphere at 6000 K fits the data equally well. There is only a 1% probability that the line shape at 5000 K (dotted curve) fits as well (difference in chi-squared = 6.7). Figure from Share et al. 2003

A bright de-excitation line of iron is included in MAX's high energy channel, being in fact the same 847 keV transition MAX is designed to study in radioactive decay in supernovae.  When the nuclear excited state is formed by collision of accelerated protons and alpha particles with a stable 56Fe nucleus, however, it decays in picoseconds, so that the Doppler shift of the line records the recoil of the nucleus from the original interaction.  Thus the Doppler profile of the line contains information on the energy spectrum, the angular distribution, and the composition (protons versus alpha particles) of the accelerated flare ions.  De-excitation line profiles at high resolution have been observed by RHESSI (Smith et al.  2003) and used to put some constraint on the angular distribution of accelerated particles, but more photons are needed to get full value out of the line; RHESSI has only 26 cm2 of effective area at 847 keV. 

Finally, there is an exciting possibility of detecting 56Co radioactivity in the solar atmosphere in the aftermath of a major flare (Ramaty and Mandzhavidze 2000).  If this line is seen, its decay rate can be used to investigate atmospheric mixing in active regions, a process difficult to study in any other way.  The line is expected to be faint, but extremely well-matched to MAX's capabilities, since there is no flare continuum underneath it to lower the sensitivity (the  prompt de-excitation lines are extremely bright but carry their own flare continuum background underneath them, so that effective area becomes more important than background rejection). 
 

Fig. 2: Model 20Ne line shape for a power-law index of -3.75, a viewing angle of 30°, and a forward-isotropic distribution. Dashed curve: Line shape from interacting protons. Dash-dotted curve: Line shape from interacting a-particles. Solid curve: Total line shape for an a /proton ratio of 0.5. Vertical solid line: Rest energy of the de-excitation line, 1634 keV. [Note: the Fe line is much narrower (a few keV) but could still be resolved by a carefully made GeD]. Simulations by R. Murphy presented in Smith et al. 2003
 

Most of the largest X-class flares, which produce copious gamma-ray lines, occur from large, complicated active regions which often produce several such flares as they cross the Solar surface.  MAX could thus be pointed at an active region which had proven itself capable, yielding an excellent chance of a repeat performance within a few days.  In the meantime, smaller flares from the same region would allow MAX to test the competing hypotheses that ions are accelerated to high energies only in the largest flares, or that they are always present at some level when electrons are accelerated.  The next flare location could probably be predicted to within one arcminute, and MAX would have to track the region across the Solar surface, with pointing updates several times a day (either automated or commanded). 
 

Ramaty, R., & Mandzhavidze, N. 2000, in IAU Symp. 195, Highly
Energetic Physical Processes and Mechanisms for Emission from
Astrophysical Plasmas, ed. P. C. H. Martens, S. Tsuruta, & M. A. Weber (San Francisco: ASP), 123
Share, G. H. et al. 2003, ApJL 595, L85
Smith, D. M. et al. 2003, ApJL 595, L81 

mise à jour : mars 2004
questions et commentaires :Peter von Ballmoos