The Center-to-Limb Dependence of Flare Hα Redshift Velocities

Details The Center-to-Limb Dependence of Flare Hα Redshift Velocities The Center-to-Limb Dependence of Flare Hα Redshift Velocities

May 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...180.4103b&link_type=abstract

American Astronomical Society, 180th AAS Meeting, #41.03; Bulletin of the American Astronomical Society, Vol. 24, p.794

Astronomy and Astrophysics

Astronomy

Scientific paper

It is well known that impulsive phase flare spectra show redshifts in Hα and blueshifts in soft X-ray lines. Various authors have argued that this is a consequence of chromospheric evaporation and momentum balance. During the Solar Maximum Mission it was found that a center-to-limb dependence exists in X-ray blueshifts. This is understood to imply that the motions of the X-ray emitting material have a significant radial preference. If this is true, and the motions are due to chromospheric evaporation (so momentum balance should obtain in the observed Hα and X-ray plasmas), we should expect to see a center-to-limb dependence in Hα redshifts, as well. In this paper we present the results of a statistical study of the Hα spectra of ~ 35 flares observed with the CCD Imaging Spectrograph at Mees Solar Observatory. For purposes of comparison of Hα to the X-ray data, we have spatially averaged the spectrum of each flare, subtracted off the preflare spectrum, and determined the line-of-sight velocity of the usually redshifted component as a function of time using a two-Gaussian fit. We force one component to appear in the preflare line center, and determine the maximum observed velocity of the second (shifted) component for each flare. No center-to-limb dependence is obvious when we plot these velocities as a function of the usual direction cosine mu = cos theta . But when we compare this velocity to a modeled one based on the chromospheric evaporation model, a relationship emerges between the observed and modeled velocity. Finally, the relationship is improved still further when we take disk position into account, using a linear mu relation. We conclude that the center-to-limb dependence of Hα Doppler shifts in these flares supports the chromospheric evaporation model. As well, we infer that there is considerable statistical departure from strictly radial motion due to non-radial magnetic field geometry in the chromosphere at the sites of these flares.

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