The pole-equator variation of solar chromospheric height

We measured the height of the solar chromosphere as a function of position angle on seven circumsolar sets of high spatial resolution H alpha filtergrams obtained during 1994-1995. Typically 40 CCD frames were needed to cover the solar limb. We also measured the frequency of visible macrospicules along the limb as a function of position angle. Two different height measures based on the gradient of the limb profile are defied in the Paper. They show variation with a pole-equator difference of about 2'', the poles being higher. In centerline Her the derived height is typically 4300-4400 lan at the equator and just below 6000 lan at the solar poles with a true local scatter of 500 lan (rms). A slight increase in height is also seen above active regions. We see a similar but smaller height variation in the wing of Ha. We also measured the height difference between the chromosphere in centerline and the wing of Ha. The off-band limb is always lower than the centerline limb by an average value of 500 km (0 ''.7). Because of the self reversal of the chromospheric Ha line, the off-band chromosphere is twice as bright as the centerline. As a result, the gradient of the off-band limb profile is considerably steeper just above the photosphere. Thus, the popular misconception that spicules rise above the chromosphere is incorrect, except insofar as the macrospicules are concerned. We find the latter much more frequent above at the pole than at the equator, confirming the result of Bohlin et al. The number of macrospicules near the pole reaches 20 per 150'', or about three per arcmin(2) on the disk, while we only count a few (sometimes none) near the equator. While we defer a serious study of the macrospicules to a later work, we can already state that these features are asymmetric in time, i.e., they are seen to go up but not come down. Comparison with Yohkoh soft X-ray data shows a high correlation between the enhanced chromospheric height, the macrospicule frequency, and the location of the polar coronal holes. This agrees with earlier measurements obtained in lower resolution and suggests a magnetic cause of the height difference, namely that the presence of a vertical magnetic field permits the jets to ascend higher.