4 H $\vec{\alpha}$ equatorial coronal hole enhancement

In spite of the lack of an ECH enhancement in any of the UV lines observed, the two detections of H$\alpha$ increased emission in ECHs coincident with radio bright spots were unequivocal. We examined the H$\alpha$ images in an attempt to gain insight into the cause of the radio enhancements. The morphology of the enhanced H$\alpha$ emission is apparent in the full resolution H$\alpha$ image, which shows that the H$\alpha$ enhancement results from three phenomena: the presence of bright fibrils and bright points, and the absence of dark fibrils. At full resolution, the H$\alpha$ enhancements peak at a contrast of 1.8 above the QS level, rather than the peak value of 1.3 measured in the images which were smoothed to match the radio resolution. The contrast of the original image is greater, since the bright H$\alpha$ fibrils have widths of 2 $^{\prime\prime}$ to 3 $^{\prime\prime}$, much narrower than the smoothing function width. The minimum brightness level in the enhancement lies at the average level outside the enhancement, due to the lack of dark fibrils. Since the fibrils are not fully resolved at the BBSO resolution, the true peak contrast may be greater than 1.8, which suggests that the true peak radio enhancement may also be greater than 1.8. The source of the bright H$\alpha$ fibrils in enhanced regions must be an increase in either intrinsic fibril brightness or filling factor. An increase in filling factor of 1.3 could explain the observation, and would occur if fibrils were either thicker or more densely packed. This seems plausible, since the well defined radio enhancements and H$\alpha$ bright spots occur in regions of strong magnetic fields, distributed among many flux tubes, which are likely the bases of the fibrils observed in H$\alpha$.

In order to test whether the increase in radio and H$\alpha$ emission in the enhancements might be due to a filling factor increase, we computed the histograms of the radio and H$\alpha$ emission in the area within both the CH and the SUMER field of view. If the enhancements are caused by a fibril filling factor increase, the integrated enhanced emission in both images should be equal. The background intensity in the images is normalized to unity, and both histograms are plotted in Fig. 4.

Figure 4: Histograms of the 17 GHz radio (solid) and H$\alpha$ (dotted) emission in the area intersected by the CH boundary and SUMER field of view.

The H$\alpha$ distribution extends to higher intensity, possibly due to insufficient smoothing of the H$\alpha$image. If the radio resolution element is larger than 19 $^{\prime\prime}$, a wider smoothing function should be used. Since the background and enhancement distributions overlap, we follow the method developed by Skumanich et al. (1975), to determine the proportion of integrated bright point emission. The distribution from zero to the intensity at the peak is reflected about the peak to create the background distribution, which is subtracted from the total to obtain the bright pixel distribution. Integrating the resulting distributions yields integrated enhanced emission fractions of 3.3% for H$\alpha$ and 2.8% for 17 GHz radio. Given the uncertainty in the technique used to obtain the bright pixel distributions, the two emission fractions might be equal. This result is consistent with a fibril filling factor increase causing both the radio and H$\alpha$ enhancements.

To date, there are no 17 GHz radio images of ECHs at resolution less than $\approx$18 $^{\prime\prime}$. However, higher resolution images of other types of solar regions have been obtained at several other frequencies. Images of a macrospicule on the limb at 4.8, 8.5 and 15 GHz have been made using the Very Large Array (VLA) at 4 $^{\prime\prime}$ resolution (Habbal & Gonzalez 1991), but no spicules are visible, since they would be unresolved. Microwave images of the QS on the disk have been obtained at 4.9 GHz with 6 $^{\prime\prime}$ resolution (Gary & Zirin 1988), 8.5 GHz at 4 $^{\prime\prime}$  resolution (Gary et al. 1990) and 19.9 and 22.5 GHz at 3 $^{\prime\prime}$ and 5 $^{\prime\prime}$ resolution, respectively (Bastian et al. 1996). In all of these observations, radio fine structure at the resolution limit was present. The 4.9 and 8.4 GHz images showed strong correlation with H$\alpha$ images. Bright H$\alpha$fibrils and points corresponded to enhanced radio emission and magnetic flux. However, none of the microwave observations had sufficient resolution to resolve fibrils. The close correlation detected between 4.8, and 8.5 GHz emission and H$\alpha$ emission is consistent with ECH 17 GHz radio enhancements being caused by bright fibrils.