We have used Global Oscillation Network Group (GONG) magnetograms to characterize the changes in the photospheric longitudinal magnetic field during 15 X-class solar flares. An abrupt, significant, and persistent change in the magnetic field occurred in at least one location within the flaring active region during each event. We have identified a total of 42 sites where such field changes occurred. At 75% of these sites, the magnetic field change occurred in less than 10 minutes. The absolute values of the field changes ranged between 30 and almost 300 G, the median being 90 G. Decreases in the measured field component were twice as frequent as increases. The field changes ranged between 1.4 and 20 times the rms noise of the observations. In all but one equivocal case, the field changes occurred after the start of the flare. In all cases, the field changes were permanent. At least two-thirds of the field changes occurred in the penumbrae of sunspots. During three events for which simultaneous Transition Region and Coronal Explorer (TRACE ) images are available, we have found excellent spatial and temporal correlation between the change in the magnetic field and an increase in brightness of the footpoints of flare ribbons, but not vice versa. Among many possible explanations for the observations, we favor one in which the magnetic field changes result from the penumbral field relaxing upward by reconnecting magnetic fields above the surface. One of the basic assumptions of flare theories is that the photospheric magnetic field does not change significantly during flares. These results suggest that this assumption needs to be re-examined.

We characterize the changes in the longitudinal photospheric magnetic field during 38 Xclass and 39 M-class flares within 65 • of disk-center using 1-minute GONG magnetograms. In all 77 cases we identify at least one site in the flaring active region where clear, permanent, stepwise field changes occurred. The median duration of the field changes was about 15 minutes and was approximately equal for X-class and for M-class flares. The absolute values of the field changes ranged from the detection limit of ∼10 G to as high as ∼450 G in two exceptional cases. The median value was 69 G. Field changes were significantly stronger for X-class than for M-class flares and for limb flares than for disk-center flares. Longitudinal field changes less than 100 G tended to decrease longitudinal field strengths, both close to disk-center and close to the limb, while field changes greater than 100 G showed no such pattern. Likewise, longitudinal flux strengths tended to decrease during flares. Flux changes, particularly net flux changes near disk-center, correlated better than local field changes with GOES peak X-ray flux. The strongest longitudinal field and flux changes occurred in flares observed close to the limb. We estimate the change of Lorentz force associated with each flare and find that this is large enough in some cases to power seismic waves. We find that longitudinal field decreases would likely outnumber increases at all parts of the solar disk within 65 • of disk-center, as in our observations, if photospheric field tilts increase during flares as predicted by Hudson et al.Whereas SH05 limited their study to field changes at representative points in each active

The Navy Prototype Optical Interferometer (NPOI) has been used to measure the angular diameters of 41 late-type giant and supergiant stars previously observed with the Mark III optical interferometer. Sixteen of these stars have published angular diameters based on model atmospheres (infrared Ñux method, IRFM). Comparison of these angular diameters shows that there are no systematic o †sets between any pair of data sets. Furthermore, the reported uncertainties in the angular diameters measured using both interferometers are consistent with the distribution of the di †erences in the diameters. The distribution of diameter di †erences between the interferometric and model atmosphere angular diameters are consistent with uncertainties in the IRFM diameters of 1.4%. Although large di †erences in angular diameter measurements are seen for three stars, the data are insufficient to determine whether these di †erences are due to problems with the observations or are due to temporal changes in the stellar diameters themselves.

No abstract