Where is the base of the Transition Region? Evidence from TRACE, SDO, IRIS and ALMA observations

Classic solar atmospheric models put the Chromosphere-Corona Transition Region (CCTR) at similar to 2 Mm above the tau(5000) = 1 level, whereas radiative MHD (rMHD) models place the CCTR in a wider range of heights. However, observational verification is scarce. In this work we review and discuss recent results from various instruments and spectral domains. In SDO and TRACE images spicules appear in emission in the 1600, 1700 and 304 angstrom bands and in absorption in the EUV bands; the latter is due to photo-ionization of H I and He I, which increases with wavelength. At the shortest available AIA wavelength and taking into account that the photospheric limb is similar to 0.34 Mm above the tau(5000) = 1 level, we found that CCTR emission starts at similar to 3.7 Mm; extrapolating to lambda = 0, where there is no chromospheric absorption, we deduced a height of 3.0 +/- 0.5 Mm, which is above the value of 2.14 Mm of the Avrett and Loeser model. Another indicator of the extent of the chromosphere is the height of the network structures. Height differences produce a limbward shift of features with respect to the position of their counterparts in magnetograms. Using this approach, we measured heights of 0.14 +/- 0.04 Mm (at 1700 angstrom), 0.31 +/- 0.09 Mm (at 1600 angstrom) and 3.31 +/- 0.18 Mm (at 304 angstrom) for the center of the solar disk. A previously reported possible solar cycle variation is not confirmed. A third indicator is the position of the limb in the UV, where IRIS observations of the Mg II triplet lines show that they extend up to similar to 2.1 Mm above the 2832 angstrom limb, while AIA/SDO images give a limb height of 1.4 +/- 0.2 Mm (1600 angstrom) and 5.7 +/- 0.2 Mm (304 angstrom). Finally, ALMA mm-lambda full-disk images provide useful diagnostics, though not very accurate, due to their relatively low resolution; values of 2.4 +/- 0.7 Mm at 1.26 mm and 4.2 +/- 2.5 Mm at 3 mm were obtained. Putting everything together, we conclude that the average chromosphere extends higher than homogeneous models predict, but within the range of rMHD models.. (C) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.