NEW LIGHT ON THE HEART OF DARKNESS OF THE SOLAR CHROMOSPHERE

Solar carbon monoxide spectra indicate the existence of a cool (less t han 4000 kelvin) component to the solar chromosphere coexisting with t he hot, bright gas at 6000 to 7000 kelvin. However, both the existence and the location of the cool component have been controversial. New h igh-resolution spectra show that carbon monoxide goes into emission ju st beyond the limb, allowing it to be probed without photospheric cont amination. The cool component has temperatures as low as 3000 to 3500 kelvin and appears to cover 50 to 85 percent of the quiet solar surfac e. There is a steep temperature rise to normal chromospheric temperatu res at a height of 900 to 11 00 kilometers. Large horizontal velocitie s are seen, suggesting that the cool component is maintained by the su personic adiabatic expansion of upwelling gas in overshooting granules .