Structure of the Transition Region and the Low Corona from TRACE and SDO Observations Near the Limb

We examined the structure near the solar limb in TRACE images of the continuum and in the 1600 and 171 angstrom bands as well as in SDO images in the continuum (from HMI) and all AIA bands. The images in different wavelength bands were carefully coaligned by using the position of Mercury for TRACE and Venus for SDO during their transit in front of the solar disk in 1999 and 2012, respectively. Chromospheric absorbing structures in the TRACE 171-angstrom band are best visible 7('') above the white-light limb, very close to the inner limb, defined as the inflection point of the rising part of the center-to-limb intensity variation. They are correlated with, but are not identical to, spicules in emission, seen in the 1600-angstrom band. Similar results were obtained from AIA and SOT images. Tall spicules in 304 angstrom are not associated with any absorption in the higher temperature bands. Performing azimuthal averaging of the intensity over 15 degrees sectors near the N, S, E and W limbs, we measured the height of the limb and of the peak intensity in all AIA bands. We found that the inner limb height in the transition region AIA bands increases with wavelength, consistent with a bound-free origin of the absorption from neutral H and He. From that we computed the column density and the density of neutral hydrogen as a function of height. We estimated a height of (2300 +/- 500)km for the base of the transition region. Finally, we measured the scale height of the AIA emission of the corona and associated it with the temperature; we deduced a value of (1.24 +/- 0.25)x10(6) K for the polar corona