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The flares of August 1972

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We present the analysis of observations of the August flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms and spectra. In each flare the observations fit a model of particle acceleration in the chromosphere with emission produced by impact and by heating by the energetic electrons and protons. The region showed inverted polarity and high gradients from birth, and flares appear due to strong magnetic shears and gradients across the neutral line produced by sunspot motions. Post flare loops show a strong change from sheared, force-free fields parallel to potential-field-like loops, perpendicular to the neutral line above the surface.

We detected fast (5 s duration) small (1′') flashes in λ3835 at the footpoints of flux loops in the August 2 impulsive flare at 18∶38 UT, which may be explained by dumping of > 50 keV electrons accelerated in individual flux loops. The flashes show excellent time and intensity agreement with > 45 keV X-rays. In the less impulsive 20∶00 UT flare a less impulsive wave of emission in λ3835 moved with the separating footpoints. The thick target model of X-ray production gives a consistent model for X-ray, λ3835 and microwave emission in the 18:38 UT event.

Spectra of the August 7 flare show emission 12 Å FWHM in flare kernels, but only 1 to 2 Å wide in the rest of the flare. The kernels thus produce most of the Hα emission. The total emission in Hα in the August 4 and August 7 flares was about 2 × 1030 erg. We belive this dependable value more accurate than previous larger estimates for great flares. The time dependence of total Hα emission agrees with radio and X-ray data much better than area measurements which depend on the weaker halo.

Absorption line spectra show a large (6 km/s-1) photospheric velocity discontinuity across the neutral line, corresponding to sheared flow across that line.

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This work has been supported by NASA under NGR 05 002 034, NSF Atmospheric Sciences program under GA 24015, and AFCRL under FI9628-73-C-0085.

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Zirin, H., Tanaka, K. The flares of August 1972. Sol Phys 32, 173–207 (1973).

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