Abstract
Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they emerge all over the Sun, these small-scale ...fields bring a substantial amount of flux, and therefore energy, to the solar surface. Because of this, IN fields are crucial for understanding the quiet Sun (QS) magnetism. However, they are weak and produce very small polarization signals, which is the reason why their properties and impact on the energetics and dynamics of the solar atmosphere are poorly known. Here we use coordinated, high-resolution, multiwavelength observations obtained with the Swedish 1 m Solar Telescope and the Interface Region Imaging Spectrograph (IRIS) to follow the evolution of IN magnetic loops as they emerge into the photosphere and reach the chromosphere and transition region. We studied in this paper three flux emergence events having total unsigned magnetic fluxes of 1.9 × 10
18
, 2.5 × 10
18
, and 5.3 × 10
18
Mx. The footpoints of the emerging IN bipoles are clearly seen to appear in the photosphere and to rise up through the solar atmosphere, as observed in Fe
i
6173 Å and Mg
i
b
2
5173 Å magnetograms, respectively. For the first time, our polarimetric measurements taken in the chromospheric Ca
ii
8542 Å line provide direct observational evidence that IN fields are capable of reaching the chromosphere. Moreover, using IRIS data, we study the effects of these weak fields on the heating of the chromosphere and transition region.
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.
The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important ...role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ∼3 minutes in the photosphere and ∼12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg ii h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.
Context. Imaging spectrographs are popular instruments used to obtain solar data. They record quasi-monochromatic images at selected wavelength positions. By scanning the spectral range of the line, ...it is possible to obtain bidimensional maps of the field-of-view with a moderate spectral resolution. Aims. In this work, we evaluate the quality of spectropolarimetric inversions obtained from various wavelength samplings during umbral flashes. Methods. We computed numerical simulations of nonlinear wave propagation in a sunspot and constructed synthetic Stokes profiles in the Ca II 8542 Å line during an umbral flash using the NLTE code NICOLE. The spectral resolution of the Stokes profiles was downgraded to various cases with differences in the wavelength coverage. A large set of wavelength samplings was analyzed and the performance of the inversions was evaluated by comparing the inferred chromospheric temperature, velocity, and magnetic field with the actual values at the chromosphere of the numerical simulation. Results. The errors in the inverted results depend to a large extent on the location of the wavelength points across the profile of the line. The inferred magnetic field improves with the increase of the spectral resolution. In the case of velocity and temperature, low spectral resolution data produce a match of the inverted atmospheres with the actual values comparable to wavelength samplings with finer resolution, while providing a higher temporal cadence in the data acquisition. Conclusions. We validated the NLTE inversions of spectropolarimetric data from the Ca II 8542 Å during umbral flashes, during which the atmosphere undergoes sudden dramatic changes due to the propagation of a shock wave. Our results favor the use of fine spectral resolution for analyses that focus on the inference of the magnetic field, whereas the estimation of temperature and velocity fluctuations can be performed with lower spectral resolution.
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PROPERTIES OF SUPERSONIC EVERSHED DOWNFLOWS Pozuelo, S. Esteban; Rubio, L. R. Bellot; Rodríguez, J. de la Cruz
The Astrophysical journal,
12/2016, Volume:
832, Issue:
2
Journal Article
Peer reviewed
Open access
ABSTRACT We study supersonic Evershed downflows in a sunspot penumbra by means of high spatial resolution spectropolarimetric data acquired in the Fe i 617.3 nm line with the CRISP instrument at the ...Swedish 1 m Solar Telescope. Physical observables, such as Dopplergrams calculated from line bisectors and Stokes V zero-crossing wavelengths, and Stokes V maps in the far red-wing, are used to find regions where supersonic Evershed downflows may exist. We retrieve the line-of-sight velocity and the magnetic field vector in these regions using two-component inversions of the observed Stokes profiles with the help of the SIR code. We follow these regions during their lifetime to study their temporal behavior. Finally, we carry out a statistical analysis of the detected supersonic downflows to characterize their physical properties. Supersonic downflows are contained in compact patches moving outward, which are located in the mid- and outer penumbra. They are observed as bright, roundish structures at the outer end of penumbral filaments that resemble penumbral grains. The patches may undergo fragmentations and mergings during their lifetime; some of them are recurrent. Supersonic downflows are associated with strong and rather vertical magnetic fields with a reversed polarity compared to that of the sunspot. Our results suggest that downflows returning back to the solar surface with supersonic velocities are abruptly stopped in dense deep layers and produce a shock. Consequently, this shock enhances the temperature and is detected as a bright grain in the continuum filtergrams, which could explain the existence of outward-moving grains in the mid- and outer penumbra.
Context.
Details of the magnetic field in the quiet-Sun chromosphere are key to our understanding of essential aspects of the solar atmosphere. However, the strength and orientation of this magnetic ...field have not been thoroughly studied at high spatial resolution.
Aims.
We aim to determine the longitudinal magnetic field component (
B
∥
) of quiet-Sun regions depending on their size.
Methods.
We estimated
B
∥
by applying the weak-field approximation to high-spatial-resolution Ca
II
854.2 nm data taken with the Swedish 1 m Solar Telescope. Specifically, we analyzed the estimates inferred for different spectral ranges using the data at the original cadence and temporally integrated signals.
Results.
The longitudinal magnetic field in each considered plasma structure correlates with its size. Using a spectral range restricted to the line core leads to chromospheric longitudinal fields varying from ∼50 G at the edges to 150–500 G at the center of the structure. These values increase as the spectral range widens due to the photospheric contribution. However, the difference between this contribution and the chromospheric one is not uniform for all structures. Small and medium-sized concentrations show a steeper height gradient in
B
∥
compared to their chromospheric values, so estimates for wider ranges are less trustworthy. Signal addition does not alleviate this situation as the height gradients in
B
∥
are consistent with time. Finally, despite the amplified noise levels that deconvolving processes may cause, data restored with the destretching technique show similar results, though are affected by smearing.
Conclusions.
We obtained
B
∥
estimates similar to those previously found, except for large concentrations and wide spectral ranges. In addition, we report a correlation between the height variation of
B
∥
compared to the chromospheric estimates and the concentration size. This correlation affects the difference between the photospheric and chromospheric magnetic flux values and the reliability of the estimates for wider spectral ranges.
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Abstract
The computational cost of fast non-LTE synthesis is one of the challenges that limits the development of 2D and 3D inversion codes. It also makes the interpretation of observations of lines ...formed in the chromosphere and transition region a slow and computationally costly process, which limits the inference of the physical properties on rather small fields of view. Having access to a fast way of computing the deviation from the LTE regime through the departure coefficients could largely alleviate this problem. We propose to build and train a graph network that quickly predicts the atomic level populations without solving the non-LTE problem. We find an optimal architecture for the graph network for predicting the departure coefficients of the levels of an atom from the physical conditions of a model atmosphere. A suitable data set with a representative sample of potential model atmospheres is used for training. This data set has been computed using existing non-LTE synthesis codes. The graph network has been integrated into existing synthesis and inversion codes for the particular case of Ca
ii
. We demonstrate orders-of-magnitude gain in computing speed. We analyze the generalization capabilities of the graph network and demonstrate that it produces good predicted departure coefficients for unseen models. We implement this approach in
Hazel2
and show how the inversions nicely compare with those obtained with standard non-LTE inversion codes. Our approximate method opens up the possibility of extracting physical information from the chromosphere on large fields of view with time evolution.
ABSTRACT We study the temporal evolution of downflows observed at the lateral edges of penumbral filaments in a sunspot located very close to the disk center. Our analysis is based on a sequence of ...nearly diffraction-limited scans of the Fe i 617.3 nm line taken with the CRisp Imaging Spectro-Polarimeter instrument at the Swedish 1 m Solar Telescope. We compute Dopplergrams from the observed intensity profiles using line bisectors and filter the resulting velocity maps for subsonic oscillations. Lateral downflows appear everywhere in the center-side penumbra as small, weak patches of redshifts next to or along the edges of blueshifted flow channels. These patches have an intermittent life and undergo mergings and fragmentations quite frequently. The lateral downflows move together with the hosting filaments and react to their shape variations, very much resembling the evolution of granular convection in the quiet Sun. There is a good relation between brightness and velocity in the center-side penumbra, with downflows being darker than upflows on average, which is again reminiscent of quiet Sun convection. These results point to the existence of overturning convection in sunspot penumbrae, with elongated cells forming filaments where the flow is upward but very inclined, and weak lateral downward flows. In general, the circular polarization profiles emerging from the lateral downflows do not show sign reversals, although sometimes we detect three-lobed profiles that are suggestive of opposite magnetic polarities in the pixel.
Abstract
We present data of 10 penumbral microjets (PMJs) observed in a H
α
, Ca
ii
8542 Å, and Fe
i
6302 Å line pair with the Swedish 1 m Solar Telescope (SST) with CRISP and Ca
ii
K with ...SST/CHROMIS in active region NOAA 12599 on 2016 October 12 at
μ
= 0.68. All four Stokes parameters of the Ca
ii
8542 Å and Fe
i
6302 Å lines were observed and a series of test pixels were inverted using the Stockholm inversion code. Our analysis revealed for the first time that PMJs are visible in H
α
, where they appear as dark features with average line-of-sight (LOS) upflows of 1.1 ± 0.6 km s
−1
, matching the LOS velocities from the inversions. Based on the H
α
observations we extend the previous average length and lifetime of PMJs to 2815 ± 530 km and 163 ± 25 s, respectively. The plane-of-sky (POS) velocities of our PMJs of up to 17 km s
−1
tend to give increased velocities with distance traveled. Furthermore, two of our PMJs with significant Stokes
V
signal indicate that the PMJs possess an increased LOS magnetic field of up to 100 G compared to the local pre-/post- PMJ magnetic field, which propagates as quickly as the PMJs’ POS velocities. Finally, we present evidence that PMJs display an on average 1 minute gradual precursory brightening that only manifests itself in the cores of the Ca
ii
lines. We conclude that PMJs are not ordinary jets but likely are manifestations of heat fronts that propagate at the local Alfvén velocity.
Investigating light bridges (LBs) helps us comprehend key aspects of sunspots. However, few studies have analyzed the properties of LBs in terms of the geometric height, which is a more realistic ...perspective given the corrugation of the solar atmosphere. We aim to shed light on LBs by studying the variation in their physical properties with geometric height. We used the SICON code to infer the physical quantities in terms of the optical depth and the Wilson depression values of three LBs hosted by a sunspot observed with Hinode/SP in the Fe i 630 nm pair lines. We also used SIR inversions to cross-check the height variation of the field inclination in the LBs. In both output sets, we performed linear interpolation to convert the physical parameters from optical depth into a geometric height scale in each pixel. Depending on their general appearance, we classified each LB as filamentary, grainy, or umbral. They appear as ridges that reach different maximum heights, with the umbral LB being the deepest. While the filamentary LB hosts a plasma inflow from the penumbra, the results for the grainy LB are compatible with an injection of hot plasma through convective cells of reduced field strength. Only a few positions reveal hints suggesting a cusp-like magnetic canopy. Moreover, strong gradients in the magnetic field strength and inclination usually exhibit enhanced electric currents, with the filamentary LB having remarkably strong currents that appear to be related to chromospheric events. The height stratification in filamentary and grainy LBs differ, indicating diverse mechanisms at work. Our results are in general incompatible with a magnetic canopy scenario, and further analysis is needed to confirm whether it exists along the entire LB or only at specific locations. Furthermore, this work assesses the usefulness of the SICON code when determining the height stratification of solar structures.
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