The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the nonlocal character of the observed radiation and plasma conditions in this ...layer. Inversion methods allow us to derive a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced approaches involve a depth-stratified model atmosphere described by temperature, line-of-sight velocity, turbulent velocity, the three components of the magntic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. In order to apply these techniques to spectral lines that sample the chromosphere, nonlocal thermodynamical equilibrium effects must be included in the calculations. We developed a new inversion code STiC (STockholm inversion Code) to study spectral lines that sample the upper chromosphere. The code is based on the RH forward synthesis code, which we modified to make the inversions faster and more stable. For the first time, STiC facilitates the processing of lines from multiple atoms in non-LTE, also including partial redistribution effects (PRD) in angle and frequency of scattered photons. Furthermore, we include a regularization strategy that allows for model atmospheres with a complex depth stratification, without introducing artifacts in the reconstructed physical parameters, which are usually manifested in the form of oscillatory behavior. This approach takes steps toward a node-less inversion, in which the value of the physical parameters at each grid point can be considered a free parameter. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere, and the optimizations that we applied to the code. We carry out some numerical experiments to show the performance of the code and the regularization techniques that we implemented. We made STiC publicly available to the community.
Understanding the complex dynamics and structure of the upper solar atmosphere strongly benefits from the use of a combination of several diagnostics. Frequently, such diverse diagnostics can only be ...obtained from telescopes and/or instrumentation operating at widely different spatial resolution. To optimize the utilization of such data, we propose a new method for the global inversion of data acquired at different spatial resolution. The method has its roots in the Levenberg-Marquardt algorithm but involves the use of linear operators to transform and degrade the synthetic spectra of a highly resolved guess model to account for the effects of spatial resolution, data sampling, alignment, and image rotation of each of the datasets. We have carried out a list of numerical experiments to show that our method allows for the extraction of spatial information from two simulated datasets that have gone through two different telescope apertures and that are sampled in different spatial grids. Our results show that each dataset contributes in the inversion by constraining information at the spatial scales that are present in each of the datasets, and no negative effects are derived from the combination of multiple resolution data. This method is especially relevant for chromospheric studies that attempt to combine datasets acquired with different telescopes and/or datasets acquired at different wavelengths. The techniques described in the present study will also help to address the ever increasing resolution gap between space-borne missions and forthcoming ground-based facilities.
We perform non-LTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes ...high-resolution Ca ii λ8542 observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier "sunspot plumes," which were invariably found to display strong downflows in sunspots. Finally, we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers.
Observationally Based Models of Penumbral Microjets Esteban Pozuelo, S.; de la Cruz Rodríguez, J.; Drews, A. ...
Astrophysical journal/The Astrophysical journal,
01/2019, Letnik:
870, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We study the polarization signals and physical parameters of penumbral microjets (PMJs) by using high spatial resolution data taken in the Fe i 630 nm pair, Ca ii 854.2 nm, and Ca ii K lines with the ...CRISP and CHROMIS instruments at the Swedish 1 m Solar Telescope. We infer their physical parameters, such as physical observables in the photosphere and chromospheric velocity diagnostics, by different methods, including inversions of the observed Stokes profiles with the STiC code. PMJs harbor overall brighter Ca ii K line profiles and conspicuous polarization signals in Ca ii 854.2 nm, specifically in circular polarization that often shows multiple lobes mainly due to the shape of Stokes I. They usually overlap photospheric regions with a sheared magnetic field configuration, suggesting that magnetic reconnections could play an important role in the origin of PMJs. The discrepancy between their low LOS velocities and the high apparent speeds reported on earlier, as well as the existence of different vertical velocity gradients in the chromosphere, indicate that PMJs might not be entirely related to mass motions. Instead, PMJs could be due to perturbation fronts induced by magnetic reconnections occurring in the deep photosphere that propagate through the chromosphere. This reconnection may be associated with current heating that produces temperature enhancements from the temperature minimum region. Furthermore, enhanced collisions with electrons could also increase the coupling to the local conditions at higher layers during the PMJ phase, giving a possible explanation for the enhanced emission in the overall Ca ii K profiles emerging from these transients.
Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the ...remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry-Pérot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si iv 1403 UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca ii K 3934 line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0 2 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere.
Context. High-resolution observations of the solar chromosphere at millimeter wavelengths are now possible with the Atacama Large Millimeter Array (ALMA), bringing with them the promise of tackling ...many open problems in solar physics. Observations from other ground and space-based telescopes will greatly benefit from coordinated endeavors with ALMA, yet the diagnostic potential of combined optical, ultraviolet and mm observations has remained mostly unassessed. Aims. In this paper we investigate whether mm-wavelengths could aid current inversion schemes to retrieve a more accurate representation of the temperature structure of the solar atmosphere. Methods. We performed several non-LTE inversion experiments of the emergent spectra from a snapshot of 3D radiation-MHD simulation. We included common line diagnostics such as Ca II H, K, 8542 Å and Mg II h and k, taking into account partial frequency redistribution effects, along with the continuum around 1.2 mm and 3 mm. Results. We find that including the mm-continuum in inversions allows a more accurate inference of temperature as function of optical depth. The addition of ALMA bands to other diagnostics should improve the accuracy of the inferred chromospheric temperatures between log τ ∼ −6, −4.5 where the Ca II and Mg II lines are weakly coupled to the local conditions. However, we find that simultaneous multiatom, non-LTE inversions of optical and UV lines present equally strong constraints in the lower chromosphere and thus are not greatly improved by the 1.2 mm band. Nonetheless, the 3 mm band is still needed to better constrain the mid-upper chromosphere.
Magnetic fields on the surface of the Sun and stars in general imprint or modify the polarization state of the electromagnetic radiation that is leaving from the star. The inference of solar/stellar ...magnetic fields is performed by detecting, studying and modeling polarized light from the target star. In this review we present an overview of techniques that are used to study the atmosphere of the Sun, and particularly those that allow to infer magnetic fields. We have combined a small selection of theory on polarized radiative transfer, inversion techniques and we discuss a number of results from chromospheric inversions.
Context.
Our knowledge of the heating mechanisms that are at work in the chromosphere of plage regions remains highly unconstrained from observational studies. While many heating candidates have been ...proposed in theoretical studies, the exact contribution from each of them is still unknown. The problem is rather difficult because there is no direct way of estimating the heating terms from chromospheric observations.
Aims.
The purpose of our study is to estimate the chromospheric heating terms from a multi-line high-spatial-resolution plage dataset, characterize their spatio-temporal distribution and set constraints on the heating processes that are at work in the chromosphere.
Methods.
We used nonlocal thermodynamical equilibrium inversions in order to infer a model of the photosphere and chromosphere of a plage dataset acquired with the Swedish 1-m Solar Telescope (SST). We used this model atmosphere to calculate the chromospheric radiative losses from the main chromospheric cooler from H
I
, Ca
II
, and Mg
II
atoms. In this study, we approximate the chromospheric heating terms by the net radiative losses predicted by the inverted model. In order to make the analysis of time-series over a large field of view computationally tractable, we made use of a neural network which is trained from the inverted models of two non-consecutive time-steps. We have divided the chromosphere in three regions (lower, middle, and upper) and analyzed how the distribution of the radiative losses is correlated with the physical parameters of the model.
Results.
In the lower chromosphere, the contribution from the Ca
II
lines is dominant and predominantly located in the surroundings of the photospheric footpoints. In the upper chromosphere, the H
I
contribution is dominant. Radiative losses in the upper chromosphere form a relatively homogeneous patch that covers the entire plage region. The Mg
II
also peaks in the upper chromosphere. Our time analysis shows that in all pixels, the net radiative losses can be split in a periodic component with an average amplitude of
amp̅
Q
= 7.6 kW m
−2
and a static (or very slowly evolving) component with a mean value of −26.1 kW m
−2
. The period of the modulation present in the net radiative losses matches that of the line-of-sight velocity of the model.
Conclusions.
Our interpretation is that in the lower chromosphere, the radiative losses are tracing the sharp lower edge of the hot magnetic canopy that is formed above the photosphere, where the electric current is expected to be large. Therefore, Ohmic current dissipation could explain the observed distribution. In the upper chromosphere, both the magnetic field and the distribution of net radiative losses are room-filling and relatively smooth, whereas the amplitude of the periodic component is largest. Our results suggest that acoustic wave heating may be responsible for one-third of the energy deposition in the upper chromosphere, whereas other heating mechanisms must be responsible for the rest: turbulent Alfvén wave dissipation or ambipolar diffusion could be among them. Given the smooth nature of the magnetic field in the upper chromosphere, we are inclined to rule out Ohmic dissipation of current sheets in the upper chromosphere.
With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This ...paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights from the photosphere to the chromosphere. The code features a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is written entirely in Fortran 90/2003 and complies strictly with the ANSI standards to ensure maximum compatibility and portability. It is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.
Context. Determination of solar magnetic fields with a spatial resolution set by the diffraction limit of a telescope is difficult because the time required to measure the Stokes vector with ...sufficient signal-to-noise ratio is long compared to the solar evolution timescale. This difficulty becomes greater with increasing telescope size as the photon flux per diffraction-limited resolution element remains constant but the evolution timescale decreases linearly with the diffraction-limited resolution. Aims. We aim to improve magnetic field reconstruction at the diffraction limit without averaging the observations in time or space, and without applying noise filtering. Methods. The magnetic field vector tends to evolve more slowly than the temperature, velocity, or microturbulence. We exploit this by adding temporal regularisation terms for the magnetic field to the linear least-squares fitting used in the weak-field approximation, as well as to the Levenberg-Marquardt algorithm used in inversions. The other model parameters are allowed to change in time without constraints. We infer the chromospheric magnetic field from Ca II 854.2 nm observations using the weak field approximation and the photospheric magnetic field from Fe I 617.3 nm observations, both with and without temporal regularisation. Results. Temporal regularisation reduces the noise in the reconstructed maps of the magnetic field and provides a better coherency in time in both the weak-field approximation and Milne-Eddington inversions. Conclusions. Temporal regularisation markedly improves magnetic field determination from spatially and temporally resolved observations.
PDF
