Solar flare X-ray imaging: coronal loop hydrodynamics and diagnostics of the rising phase.

Details Solar flare X-ray imaging: coronal loop hydrodynamics and diagnostics of the rising phase. Solar flare X-ray imaging: coronal loop hydrodynamics and diagnostics of the rising phase.

Jul 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...299..225r&link_type=abstract

Astronomy and Astrophysics, v.299, p.225

Astronomy and Astrophysics

Astrophysics

27

Sun: Corona, Sun: Flares, Sun: X-Rays

Scientific paper

We discuss the flare diagnostics possible with the images of flaring loops obtained with the Soft X-ray Telescope (SXT) on board Yohkoh. We use hydrodynamic models of individual flaring loops to synthesize in detail the distribution of surface brightness, and its evolution, in the pass-band of the SXT during the rising thermal phase. The point is to use our synthesized sequences as a guide to interpret data from Yohkoh SXT. On the basis of synthesized images (and the insight on hydrodynamics), we recognize four phases of the flare: the conduction phase (practically unobservable, whenever present), the chromospheric evaporation, the brightening at the loop apex and the peak phase. For each phase we discuss the possible diagnostics and, when possible, we discuss its connection with the heating mechanisms. A common feature of our results for observations through the Be filter is the initial propagation of brightness fronts upwards from the base to the top of the loop, in fact associated with chromospheric evaporation fronts: only the details of this propagation can depend on the flare heating mechanism and on its location. Bright evaporation fronts are preferentially produced by a localized thermal heating (especially at the loop footpoints); heating by electron beams instead produce a more gradual emission evolution. All our models predict a brightening of the top of the loop after the evaporation phase, independently of the site and mechanism of energy release. We find that the hardest SXT filter (the Be filter) should access a wider variety of emission structures and therefore provide a richer diagnostics, while the other filters should preferentially show bright loop feet, especially during hot (~2x10^7^K) flares in relatively long loops (e.g. 6x10^9^cm). We show that, for a correct analysis of the images, it is crucial to evaluate accurately loop length and the plasma temperature; for instance, some observable features can be properly interpreted only knowing the loop temperature.

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