DOUBLE-GAUSSIAN MODELS OF BRIGHT POINTS OR WHY BRIGHT POINTS ARE USUALLY DARK

We have modeled the structure of small bright features, ''bright point s'' seen in an outstanding CH filter (G-band) image. In our model, bri ght points consist of a Gaussian bright core centered in a Gaussian da rk surround. The basis for this approach is the observation that nearl y all of the bright points in the image exist within intergranular lan es, vertices between granules, or local brightness depressions. Using reasonable estimates for the size and depth of vertices and lanes, the model predicts that bright points clearly detectable in images with 0 ''.2 resolution will seldom be detectable in images with resolutions b eyond 0''.4. This occurs because the transfer function of the telescop e and atmosphere averages the bright points with their comparably size d dark surroundings to near zero contrast when blurred beyond 0''.4. T hese results explain the great rarity of images that clearly show brig ht points. Furthermore, the image shows many bright points with core d iameters equal to that of the FWHM of a point-spread function of a per fect telescope. If the intensity profiles of these bright points were Gaussian on a hat background, then their intrinsic brightness would ha ve to be unrealistically high and they would not disappear on images b lurred beyond 0''.4, but would simply gradually expand in size and dro p in contrast as the blur increased. Because the bright points are sit es of magnetic fields, our model helps to explain lower resolution dis k center observations that show magnetic fields occur in regions that are dark relative to the mean continuum level. The modeling also sugge sts that bright points with diameters of 0''.1 or less would be undete ctable in the current generation of 0.5 m high-resolution solar telesc opes, under any seeing conditions.