When observed in theHα spectral line, the chromosphere appears deep red.
Achromosphere ("sphere of color", from theAncient Greek words χρῶμα (khrôma) 'color' and σφαῖρα (sphaîra) 'sphere') is the second layer of astar's atmosphere, located above thephotosphere and below thesolar transition region andcorona. The term usually refers to theSun's chromosphere, but not exclusively, since it also refers to the corresponding layer of astellar atmosphere. The name was suggested by the English astronomerNorman Lockyer after conducting systematic solar observations in order to distinguish the layer from the white-light emittingphotosphere.[1][2]
In theSun's atmosphere, the chromosphere is roughly 3,000 to 5,000 kilometers (1,900 to 3,100 miles) in height, or slightly more than 1% of the Sun's radius at maximum thickness. It possesses a homogeneous layer at the boundary with the photosphere. Narrow jets ofplasma, calledspicules, rise from this homogeneous region and through the chromosphere, extending up to 10,000 km (6,200 mi) into the corona above.
The chromosphere has a characteristic red color due toelectromagnetic emissions in theHαspectral line. Information about the chromosphere is primarily obtained by analysis of its emitted electromagnetic radiation.[3] The chromosphere is also visible in the light emitted by ionized calcium, Ca II, in the violet part of the solar spectrum at a wavelength of 393.4 nanometers (theCalcium K-line).[4]
Chromospheres have also been observed onstars other than the Sun.[5] On large stars, chromospheres sometimes make up a significant proportion of the entire star. For example, the chromosphere ofsupergiant starAntares has been found to be about 2.5 times larger in thickness than the star's radius.[6]
The density of the Sun's chromosphere decreases exponentially with distance from the center of the Sun by a factor of roughly 10 million, from about2×10−4 kg/m3 at the chromosphere's inner boundary to under1.6×10−11 kg/m3 at the outer boundary.[7] The temperature initially decreases from the inner boundary at about6000 K[8] to a minimum of approximately3800 K,[9] but then increasing to upwards of35,000 K[8] at the outer boundary with thetransition layer of thecorona (seeStellar corona § Coronal heating problem).
The density of the chromosphere is 10−4 times that of the underlyingphotosphere and 10−8 times that of theEarth's atmosphere at sea level. This makes the chromosphere normally invisible and it can be seen only during atotal eclipse, where its reddish colour is revealed. The colour hues are anywhere between pink and red.[10] Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere.
The chromosphere'sspectrum is dominated byemission lines when observed at the solar limb.[11][12] In particular, one of its strongest lines is theHα at awavelength of656.3 nm; this line is emitted by ahydrogen atom whenever itselectron makes a transition from then=3 to then=2energy level. A wavelength of656.3 nm is in the red part of the spectrum, which causes the chromosphere to have a characteristic reddish colour.
High-resolution observations of the solar chromosphere show hair-like spicules, here shown in a false colored image made in borderline ultraviolet radiation of calcium K-line.
Many different phenomena can be observed in chromospheres.
The most commonly identified feature in the solar chromosphere are spicules. Spicules rise to the top of the chromosphere and then sink back down again over the course of about 10 minutes.[14]
Since the first observations with the instrument SUMER on boardSOHO, periodic oscillations in the solar chromosphere have been found with a frequency from3 mHz to10 mHz, corresponding to a characteristic periodic time of three minutes.[15] Oscillations of the radial component of the plasma velocity are typical of the high chromosphere. The photospheric granulation pattern usually has no oscillations above20 mHz; however, higher frequency waves (100 mHz, or a10 s period) were detected in the solar atmosphere (at temperatures typical of the transition region and corona) byTRACE.[16]
Plasma loops can be seen at the border of the solar disk in the chromosphere. They are different fromsolar prominences because they are concentric arches with maximum temperature of the order0.1 MK (too low to be considered coronal features). These cool-temperature loops show an intense variability: they appear and disappear in some UV lines in a time less than an hour, or they rapidly expand in 10–20 minutes. Foukal[17] studied these cool loops in detail from the observations taken with the EUV spectrometer onSkylab in 1976. When the plasma temperature of these loops becomes coronal (above1 MK), these features appear more stable and evolve over longer times.
Images taken in typical chromospheric lines show the presence of brighter cells, usually referred to as thenetwork, while the surrounding darker regions are namedinternetwork. They look similar togranules commonly observed on the photosphere due to the heatconvection.
^"II. Spectroscopic observation of the sun, No. II., was resumed and concluded".Proceedings of the Royal Society of London.17:131–132. 1869-12-31.doi:10.1098/rspl.1868.0019.ISSN0370-1662.
^Wilkinson, John (2012).New eyes on the sun: a guide to satellite images and amateur observation. Berlin: Springer.ISBN978-3-642-22839-1.OCLC773089685.
