The cores of the stronger lines (Balmer lines, ionized calcium and magnesium) form mostly in the chromosphere, whereas weaker lines (e.g., G-band) originate from the low chromosphere/photosphere. This model is derived by semi-empirically inverting the observed extreme ultraviolet (EUV) line intensities. The VAL model (Vernazza, Avrett, & Loeser 1981) is the most widely used and complete model of the origin of the absorption lines. The corona, only viewable from earth when the photosphere is blocked out by a total eclipse or coronagraph, extends out though the solar system past Earth and on to the gas giants.īy careful comparison of atomic data with modelling of the solar atmosphere it is possible to estimate the height of formation of the Fraunhofer lines (Figure 1.4). This layer acts as an interface between the cool chromosphere and the hot tenuous corona. At a height of about 2000 km above the photosphere the electron density decreases sharply, and temperature increases dramatically from 25,000 K to a coronal temperature of 10 6K in a transition region only a few hundred kilometers thick. Moving up through the chromosphere the electron density drops off, while the temperature drops to a minimum of 4200 K then gradually increases again. A photon with a large range of energies can hence remove the electron by absorption.Ībove the photosphere the conditions change rapidly (Figure 1.4). The optical opacity of the photosphere and upper solar interior is mainly due to the negative hydrogen ion, where the second electron is only loosely attached.
Hence for the Sun, where the majority of photons are emitted in the visible wavelength, the photosphere is often defined as the layer where light emitted at 5000 Å (green) has an optical depth of 2/3,
As optical depth is a function of wavelength, it is reasonable to refer to the visible photosphere, the infrared-red photosphere, et cetera. The common definition of the photosphere is the layer from which the bulk of the visible photons are emitted, The photosphere has a small τ, so most photons can travel directly to earth without being absorbed. Photons originating in the core pass through the radiative and convective zones, with the respective transport mechanism dominating in each zone, before reaching the photosphere (Figure 1.3). This applies to the deep lying layers of the Sun. For a large τ, photons are absorbed and re-emitted numerous times along the line of sight, and so the plasma is described as optically thick.
Which of these best describes the photosphere free#
The optical depth is essentially the number of mean free paths along the line of sight. Where a λ is the extinction coefficient at a wavelength,λ, and z is a depth coordinate such that z outside the Sun.
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