![]() The V/ I profiles of the two Mn i lines at 279.91 nm and at 280.19 nm have only two lobes, which provide information on the longitudinal field component in the lower chromosphere (Supplementary Material 2D). The Stokes V/ I profiles of the Mg ii h & k resonance lines have two external lobes and two inner lobes, which encode information on the longitudinal component of the magnetic field in the middle chromosphere and at the top of the upper chromosphere, respectively (Supplementary Materials 2A to 2C). The small gap seen in (C) and (D) results from the lack of data in a few deteriorated pixels.ĬLASP2 detected clear circular polarization signals not only within the bright region of the plage but also at the enhanced network elements located at d and e ( Fig. The vertical axes indicate the distance in arc seconds along the spatial direction of the CLASP2 slit, measured from its center. The I and V/ I spectra are the result of temporally averaging the individual Stokes parameters during 150.4 s. ( D) Fractional circular polarization V(λ)/ I(λ) observed by CLASP2 around 280 nm. The labels at the top of (C) indicate the location of Mg ii k at 279.64 nm, Mg ii h at 280.35 nm, and the Mn i lines at 279.91 and 280.19 nm. ( C) Variation along the slit of the intensity profile I(λ) observed in the spectral region of the Mg ii h & k lines. ( B) Longitudinal component of the photospheric magnetic field inferred from the Stokes profiles observed by Hinode/SOT-SP in Fe i visible lines. The red line indicates the radially oriented slit of the CLASP2 spectrograph, which covers 196 arc sec. ( A) Broadband Lyman-α image obtained by the CLASP2 slit-jaw system. The present investigation is based on a unique dataset acquired by the Chromospheric LAyer Spectropolarimeter (CLASP2), a suborbital space experiment that on 11 April 2019 allowed us to measure the first ever spectrally resolved Stokes profiles across the Mg ii h & k lines in active and quiet regions of the solar disk. The theoretical investigations reported in the just quoted review paper led us to a series of suborbital space experiments called CLASP, which required the development of novel instrumentation (Supplementary Material 1A). ![]() To this end, we need to measure and model the polarization of ultraviolet spectral lines originating in such atmospheric regions ( 10). It is impossible to fully understand the solar chromosphere without mapping its magnetic structure, especially in the relatively hot layers of the upper chromosphere and TR where β < 1 ( 1– 9). This, together with the fact that the non-thermal energy needed to heat the corona must propagate through the chromosphere, explains why it is indeed a crucial interface region to solve many of the key problems in solar and stellar physics. Above the β = 1 surface, the magnetic field essentially dominates the structuring and dynamics of the plasma. As a result, the β = 1 corrugated surface, where the ratio of gas to magnetic pressure is unity, lies inside the chromosphere. Moreover, from the visible photospheric surface to the chromosphere-corona transition region (TR), the plasma density decreases exponentially by several orders of magnitude, more rapidly than the magnetic field strength. ![]() Although the temperature of the chromospheric plasma does not exceed 10 4 K, the fact that its density is much larger than that of the extended and rarified corona implies that much more mechanical energy is required to sustain the chromosphere than the million-degree corona. The chromosphere is a very important region of the solar atmosphere, with an extension of several thousand kilometers, located between the relatively cool surface layers of the photosphere and the overlying hot corona ( 1– 3).
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