Donald F. Neidig

Multi-thermal observations of newly formed loops in a dynamic flare

The dynamic flare of 6 November, 1980 (max ≈ 15:26 UT) developed a rich system of growing loops which could be followed in Hα for 1.5 hr. Throughout the flare, these loops, near the limb, were seen in emission against the disk. Theoretical computations of deviations from LTE populations for a hydrogen atom reveal that this requires electron densities in the loops close to, or in excess of 1012 cm -3. From measured widths of higher Balmer lines the density at the tops of the loops was found to be 4 x 1012 cm -3 if no non-thermal motions were present, or 5 × 1011 cm -3 for a turbulent velocity of ~ 12 km s -1.It is now general knowledge that flare loops are initially observed in X-rays and become visible in Hα only after cooling. For such a high density, a loop would cool through radiation from 107 to 104 K within a few minutes so that the dense Hα loops should have heights very close to the heights of the X-ray loops. This, however, contradicts the observations obtained by the HXIS and FCS instruments on board SMM which show the X-ray loops at much higher altitudes than the loops in Hα. Therefore, we suggest that the density must have been significantly lower when the loops were formed and that the flare loops were apparently both shrinking and increasing in density while cooling.

A dynamic flare with anomalously dense flare loops

Abstract The dynamic flare of 6 November 1980 (max. ∼ 15:26 UT) developed a rich system of growing loops which could be followed in Hα for 1.5 hours. Throughout the flare, these loops, near the limb, were seen in emission against the disk. Theoretical computations of b-values for a hydrogen atom reveal that this requires electron densities in the loops to be close to 10 12 cm −3 . From measured widths of higher Balmer lines the density at the loops of the loops was found to be 4 × 10 12 cm −3 if no non-thermal motions were present. It is now general knowledge that flare loops are initially observed in X-rays and become visible in Hα only after cooling. For such a high density a loop would cool through radiation from 10 7 K to 10 4 K within a few minutes so that the dense Hα loops should have heights very close to the heights of the X-ray loops. This, however, contradicts the observations obtained by the HXIS and FCS instruments on board SMM which show the X-ray loops at much higher altitudes than the loops in Hα. Therefore, the density must have been significantly smaller when the loops were formed and the flare loops were apparently both shrinking and becoming denser while cooling.

Flare activity, sunspot motions, and the evolution of vector magnetic fields in Hale region 17244

Abstract We study the magnetic and dynamical circumstances leading to the 1B/M4 flare of November 5, 1980, and find a strong association between the buildup of magnetic shear and the onset of flare activity within the active region. The development of shear, as observed directly in vector magnetograms, is consistent in detail with the dynamical history of the active region and identifies the precise location of the optical and hard x-ray kernels of the flare emission.