The role of spicules in heating the solar atmosphere Implications of EUV observations

Measurements of the spatial and temporal variations of EUV emission from spectral lines formed at temperatures between 2 x 10/sup 4/ and 2 x 10/sup 6/ K are used to demonstrate that EUV emitting inhomogeneities, which we associate with spicules, disappear at heights approx.15,000 km above the limb, the same height at which H..cap alpha.. spicules disappear. The mean scale height of EUV spicules observed in several quiet regions and a polar coronal hole is 2000 km +- 300 km, in good agreement with the scale height of H..cap alpha.. spicules. An upper limit of 2--3 x 10/sup -7/ g cm/sup -2/ was determined for the mass of cool (T17,000 km. The EUV observations suggest that release of gravitational potential energy of downward flowing cool material from spicules is not the primary source of global heating for the upper chromosphere, although it does make a nonnegligible contribution. If spicular material plays an important role in the global chromospheric-coronal energy balance, it most likely does so by being heated to coronal temperatures at low heights (h< or approx. =15,000 km). At such heights the thermal energy of heated spicular material is much larger than its gravitational potential energy. The absence of significant temporal intensity variations of the highest temperature line studied (Mg x lambda625) limits the range of conditions under which spicular material can be heated to coronal temperatures.