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

Abstract

We examined the structure near the solar limb in TRACE images of the continuum and in the 1600 and 171 Å 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-Å 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-Å band. Similar results were obtained from AIA and SOT images. Tall spicules in 304 Å are not associated with any absorption in the higher temperature bands. Performing azimuthal averaging of the intensity over 15^∘ 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 $(2300\pm 500)~\mbox{km}$ 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 $(1.24\pm 0.25)\times 10^{6}~\mbox{K}$ for the polar corona

Appendix: Relation Between the Column Density, \(N_{c}(h)\), and the Density, \(N(z)\)

Doing a simple geometric transformation we can write Equation 4 in the form

(15)

where $\ell$, as before, is along the line of sight, $r$ is the distance from the center of the Sun and $R_{{\odot }}$ the solar radius. The column density is then, from Equation 2,

(16)

The integral can be computed using the expression $3.461/6$ in the table of Gradshteyn et al. (2007), p. 364:

$$ \int _{0}^{\infty } \exp \bigl(-a\sqrt{x^{2}+b^{2}} \,\bigr)\,\mathrm{d}x=b K_{1} (ab), $$

(17)

where $K_{1}$ is the modified Bessel function of the first order. For , $b=r$, this gives

(18)

Since $r=R_{{\odot }}+h$, where $h$ is the distance from the limb, the argument of $K_{1}$ in Equation 18 is large; we can thus retain the first order term in the expansion of $K_{\nu }(z)$ for large $z$ (expression $8.451/6$ from p. 920 of Gradshteyn et al., 2007),

$$ K_{\nu }(z)\simeq \sqrt{\frac{\pi }{2z}} {\mathrm{e}}^{-z}, $$

(19)

and, substituting in Equation 18, we obtain, finally,

(20)