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تسجيل مستخدم جديدALMA detection of dark chromospheric holes in the quiet Sun
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We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of a quiet-Sun region at a wavelength of 3 mm, obtained during the first solar ALMA cycle on April 27, 2017, and compare them with available chromospheric observations in the UV and visible as well as with photospheric magnetograms. ALMA images clearly reveal the presence of distinct particularly dark/cool areas in the millimeter maps having temperatures as low as 60% of the normal quiet Sun at 3 mm, which are not seen in the other data. We speculate that ALMA is sensing cool chromospheric gas, whose presence had earlier been inferred from infrared CO spectra.
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Aims. The main aim of the present analysis is to decipher (i) the small-scale bright features in solar images of the quiet Sun and active regions obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and (ii) the ALMA correspondence o
f various known chromospheric structures visible in the H-alpha images of the Sun. Methods. Small-scale ALMA bright features in the quiet Sun region were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz) and in an active region using interferometric ALMA measurements (3 mm, 100 GHz). With the single-dish observations, a full-disk solar image is produced, while interferometric measurements enable the high-resolution reconstruction of part of the solar disk, including the active region. The selected quiet Sun and active regions are compared with the H-alpha (core and wing sum), EUV, and soft X-ray images and with the magnetograms. Results. In the quiet Sun region, enhanced emission seen in the ALMA is almost always associated with a strong line-of-sight (LOS) magnetic field. Four coronal bright points were identified, while other small-scale ALMA bright features are most likely associated with magnetic network elements and plages. In the active region, in 14 small-scale ALMA bright features randomly selected and compared with other images, we found five good candidates for coronal bright points, two for plages, and five for fibrils. Two unclear cases remain: a fibril or a jet, and a coronal bright point or a plage. A comparison of the H-alpha core image and the 3 mm ALMA image of the analyzed active region showed that the sunspot appears dark in both images (with a local ALMA radiation enhancement in sunspot umbra), the four plage areas are bright in both images and dark small H-alpha filaments are clearly recognized as dark structures of the same shape also in ALMA.
ALMA observations of the Sun at mm-$lambda$ offer a unique opportunity to investigate the temperature structure of the solar chromosphere. In this article we expand our previous work on modeling the chromospheric temperature of the quiet Sun, by incl
uding measurements of the brightness temperature in the network and cell interiors, from high resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We also examine the absolute calibration of ALMA full-disk images. We suggest that the brightness temperature at the center of the solar disk in Band 6 is $sim440$ K above the value recommended by White et al. (2017) and we give improved results for the electron temperature variation of the average quiet Sun with optical depth, as well as the derived spectrum at the center of the disk. We found that the electron temperature in the network is considerably lower than predicted by model F of Fontenla et al. (1993) and that of the cell interior considerably higher than predicted by model A. Depending upon the network/cell segregation scheme, the electron temperature difference between network and cell at $tau=1$ (100 GHz) is from $sim$660 to $sim$1550 K, compared to $sim$3280 K predicted by the models; similarly, the $T_e$ ratio is from $sim$1.10, to 1.24, against $sim$1.55 of the model prediction. We also found that the network/cell $T_e(tau)$ curves diverge as $tau$ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior.
Using Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quiet Sun at 1.26 and 3 mm, we study spatially resolved oscillations and transient brightenings, i.e. small, weak events of energy release. Both phenomena may have a bearin
g on the heating of the chromosphere. At 1.26 mm, in addition to power spectra of the original data, we degraded the images to the spatial resolution of the 3 mm images and used fields of view of equal area for both data sets. The detection of transient brightenings was made after the oscillations were removed. At both frequencies we detected p-mode oscillations in the range 3.6-4.4 mHz. In the corrected data sets, the oscillations at 1.26 and 3 mm showed brightness temperature fluctuations of ~1.7-1.8% with respect to the average quiet Sun, corresponding to 137 and 107 K, respectively. They represented a fraction of 0.55-0.68 of the full power spectrum and their energy density at 1.26 mm was 0.03 erg cm$^{-3}$. We detected 77 transient brightenings at 1.26 mm and 115 at 3 mm. Although the majority of the 1.26 mm events occurred in cell interior, their occurrence rate per unit area was higher than that of the 3 mm events. The computed low-end energy of the 1.26 mm transient brightenings ($1.8 times 10^{23}$ erg) is among the smallest ever reported, irrespective of the wavelength of observation. However, their power per unit area is smaller than that of the 3 mm events, probably due to the detection of many weak 1.26 mm events. We also found that ALMA bright network structures corresponded to dark mottles/spicules seen in broadband H$alpha$ images from the GONG network.
Using ALMA observations, we performed the first systematic survey for transient brightenings (i.e. weak, small-scale episodes of energy release) in the quiet solar chromosphere at 3 mm. Our dataset included images of six 87 x 87 regions of the quiet
Sun obtained with angular resolution of a few arcsec at a cadence of 2 s. The transient brightenings were detected as weak enhancements above the average intensity after we removed the effect of the p-mode oscillations. A similar analysis, over the same regions, was performed for simultaneous 304 and 1600 AA data obtained with the Atmospheric Imaging Assembly. We detected 184 3 mm transient brightening events with brightness temperatures from 70 K to more than 500 K above backgrounds of $sim 7200-7450$ K. Their mean duration and maximum area were 51.1 s and 12.3 Mm$^2$, respectively, with a weak preference of appearing at network boundaries rather than in cell interiors. Both parameters exhibited power-law behavior with indices of 2.35 and 2.71, respectively. Only a small fraction of ALMA events had either 304 or 1600 AA counterparts but the properties of these events were not significantly different from those of the general population except that they lacked their low-end energy values. The total thermal energies of the ALMA transient brightenings were between $1.5 times 10^{24}$ and $9.9 times 10^{25}$ erg and their frequency distribution versus energy was a power law with an index of 1.67. We found that the power per unit area provided by the ALMA events could account for only 1% of the chromospheric radiative losses (10% of the coronal ones). Therefore, their energy budget falls short of meeting the requirements for the heating of the upper layers of the solar atmosphere and this conclusion does not change even if we use the least restrictive criteria possible for the detection of transient brightenings.
We need to establish a correspondence between the magnetic structures generated by models and usual stellar activity indexes to simulate radial velocity time series for stars less active than the Sun. This is necessary to compare the outputs of such
models with observed radial velocity jitters and is critical to better understand the impact of stellar activity on exoplanet detectability. We propose a coherent picture to describe the relationship between magnetic activity, including the quiet Sun regions, and the chromospheric emission using the Sun as a test-bench and a reference. We analyzed a time series of MDI magnetograms jointly with chromospheric emission time series obtained at Sacramento Peak and Kitt Peak observatories. This has allowed us to study the variability in the quiet Sun over the solar cycle, and then, based on available relations between magnetic fields in active structures and chromospheric emission, to propose an empirical reconstruction of the solar chromospheric emission based on all contributions. We show that the magnetic flux covering the solar surface, including in the quieted regions, varies in phase with the solar cycle, suggesting a long-term relationship between the global dynamo and the contribution of all components of solar activity. We have been able to propose a reconstruction of the solar S-index, including a relationship between the weak field component and its chomospheric emission, which is in good agreement with the literature. This allows us to explain that stars with a low average chromospheric emission level exhibit a low variability. We conclude that weak flux regions significantly contribute to the chromospheric emission; these regions should be critical in explaining the lower variability associated with the lower average activity level in other stars as compared to the Sun and estimated from their chromospheric emission.
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