| 1. |
- Eklund, Henrik, et al.
(författare)
-
Using the slope of the brightness temperature continuum as a diagnostic tool for solar ALMA observations
- 2023
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 669
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The intensity of radiation from the solar atmosphere at millimetre wavelengths is closely related to the plasma temperature, and the formation height of the radiation is wavelength dependent. It follows from this that the slope of the intensity continuum, or the brightness temperature continuum, samples the local gradient of the gas temperature of the sampled layers in the solar atmosphere.Aims. We aim to show the added information and diagnostics potential of the solar atmosphere that comes with measuring the slope of the brightness temperature continuum.Methods. We used solar observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA) and estimated and predicted the slope using a numerical three-dimensional radiation-magnetohydrodynamic simulation. The slope was estimated by the differences between observables at wavelengths corresponding to different sub-bands at opposite sides of the ALMA receiver band 3 (2.8–3.2 mm) and band 6 (1.20–1.31 mm).Results. The sign of the brightness temperature slope indicates temperature changes with increasing height at the sampled layers. A positive sign implies an increase in temperature, while a negative sign implies a temperature decrease. The differences in brightness temperature between the sub-bands across the field of view of the simulation typically span from −0.4 kK to 0.75 kK for band 3 and −0.2 kK to 0.3 kK at band 6. The network patches are dominated by large positive slopes, while the quiet-Sun region shows a mixture of positive and negative slopes. As the slope of the continuum is coupled to the small-scale dynamics, a negative slope is seen typically under quiet-Sun conditions as a result of propagating shock waves and the corresponding post-shock regions. The temporal evolution of the slopes can therefore be used to identify shocks. The observability of the slope of the brightness temperatures is estimated at bands 3 and 6 for different angular resolutions corresponding to ALMA observations. The simulations also show that the intensity of the radiation at bands 3 and 6 can simultaneously originate from several major components at different heights, which is strongly dependent on the small-scale dynamics and is seen in both quiet-Sun and network patches. Our in-depth analysis of selected shock waves that propagating upward in the atmosphere shows that the delay of shock signatures between two wavelengths (e.g., bands 6 and 3) does not necessarily reflect the propagation speed of the shock front, but might be cause by the rate of change in opacity of higher layers at these wavelengths.Conclusions. The slope of the brightness temperature continuum sampled at different ALMA receiver sub-bands serves as an indicator of the slope of the local plasma temperature at the sampled heights in the atmosphere. This offers new diagnostic possibilities for measuring the underlying physical properties of small-scale dynamic features and thus contributes to the understanding of these features and the related transport of energy and heat in the chromosphere.
|
|
| 2. |
- Gómez, Juan Camilo Guevara, et al.
(författare)
-
The Sun at millimeter wavelengths : IV. Magnetohydrodynamic waves in small-scale bright features
- 2023
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 671
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. We used solar observations of a plage-enhanced network with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 3 and Band 6, together with synthetic continuum maps from numerical simulations with Bifrost in the same bands, to carry out a detailed study of bright small-scale magnetic features.Methods. We made use of an algorithm to automatically identify and trace bright features within the field of view (FoV) of the ALMA observations and the simulation. In particular, the algorithm recovers information of the time evolution of the shape, motion of the centre of gravity, temperature, and size for each feature. These quantities are used to determine the oscillatory properties of each feature utilising wavelets analysis.Results. We found 193 and 293 features in the Bands 3 and 6 observations, respectively. In the degraded simulation, the total number of features were 24 for Band 3 and 204 for Band 6. In the original simulation, the total number of features were 36 for Band 3 and 392 for Band 6. Based on the simulation, we confirm the magnetic nature of the features. We have obtained average oscillation periods of 30–99 s for the temperature, 37–92 s for size, and 37–78 s for horizontal velocity. There are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1–5.0 km s−1. We find a predominant anti-phase behaviour between temperature and size oscillations suggesting that the variations of the bright features are caused by compressible fast-sausage magnetohydrodynamics (MHD) modes. For the first time to our knowledge, we estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by ALMA as 453–1838 W m−2 for Band 3 and 3640–5485 W m−2 for Band 6.Conclusions. We have identified MHD waves, both transverse (kink) and compressible sausage modes, in small-scale (magnetic) structures, independently, in both ALMA Band 3 and Band 6 observations, along with their corresponding synthetic images from simulations. The decrease of wave energy-flux with height (from Band 6 to Band 3) could possibly suggest energy dissipation at chromospheric heights, namely, wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere.
|
|
