[Paper Review] Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds

D. Cabrera Solana, L. R. Bellot Rubio+2 more|ArXiv.org|Sep 11, 2007

Solar and Space Plasma Dynamics23 references3 citations

TL;DR

The paper proposes that Evershed clouds (ECs) result from transient increases in the visibility of penumbral flux tubes due to localized pressure and density perturbations, not changes in magnetic field configuration. Uncombed Stokes inversions of high-resolution spectropolarimetric data reveal that ECs are not magnetic anomalies but enhanced tube visibility caused by downward shifts in the optical depth scale from reduced upper-tube pressure, explaining their Doppler velocity enhancements and motion along penumbral filaments.

ABSTRACT

Context: Evershed clouds (ECs) represent the most conspicuous variation of the Evershed flow in sunspot penumbrae. Aims: We determine the physical properties of ECs from high spatial and temporal resolution spectropolarimetric measurements. Methods: The Stokes profiles of four visible and three infrared spectral lines are subject to inversions based on simple one-component models as well as more sophisticated realizations of penumbral flux tubes embedded in a static ambient field (uncombed models). Results: According to the one-component inversions, the EC phenomenon can be understood as a perturbation of the magnetic and dynamic configuration of the penumbral filaments along which these structures move. The uncombed inversions, on the other hand, suggest that ECs are the result of enhancements in the visibility of penumbral flux tubes. We conjecture that the enhancements are caused by a perturbation of the thermodynamic properties of the tubes, rather than by changes in the vector magnetic field. The feasibility of this mechanism is investigated performing numerical experiments of thick penumbral tubes in mechanical equilibrium with a background field. Conclusions: While the one-component inversions confirm many of the properties indicated by a simple line parameter analysis (Paper I of this series), we tend to give more credit to the results of the uncombed inversions because they take into account, at least in an approximate manner, the fine structure of the penumbra.

Motivation & Objective

• To determine the physical properties of Evershed clouds (ECs) using high spatial and temporal resolution spectropolarimetric data.
• To resolve the long-standing ambiguity about whether ECs represent true magnetic perturbations or visibility effects in penumbral flux tubes.
• To investigate whether ECs are caused by changes in magnetic field geometry or by thermodynamic variations in the flux tubes.
• To test the feasibility of a pressure/density perturbation mechanism via numerical experiments on thick flux tubes in mechanical equilibrium.
• To compare one-component and uncombed inversion models to assess the reliability of physical interpretations in complex penumbral structures.

Proposed method

• Stokes profiles from four visible (630 nm) and three infrared (1565 nm) spectral lines were inverted using one-component atmospheric models assuming a single, homogeneous atmosphere per resolution element.
• Uncombed models were applied to account for the fine structure of the penumbra, explicitly modeling coexisting flux tubes and background atmosphere with distinct physical parameters.
• The inversion code used height-independent magnetic fields, velocity gradients, and separate stray light contributions for visible and infrared ranges, with the emergent intensity modeled as a linear combination of tube and background contributions.
• A physical model was constructed to simulate thick penumbral flux tubes in mechanical equilibrium, varying gas pressure and density to assess their impact on optical depth and line formation.
• The effect of reduced upper-tube pressure on the optical depth scale was simulated to determine its influence on the filling factor and polarization ratios observed in ECs.
• The results were compared between one-component and uncombed inversions to evaluate the robustness of the inferred physical properties.

Experimental results

Research questions

• RQ1 Are Evershed clouds caused by real perturbations in the vector magnetic field of penumbral filaments, or are they artifacts of visibility changes?
• RQ2 How do the physical properties of flux tubes hosting ECs differ from those of the surrounding penumbral structure?
• RQ3 Can variations in gas pressure and density within flux tubes explain the observed Doppler velocity enhancements and filling factor increases in ECs?
• RQ4 What is the role of the thermodynamic state of flux tubes in the formation and propagation of ECs, as opposed to magnetic field geometry?
• RQ5 Is the observed increase in filling factor in ECs due to changes in tube position, radius, or optical depth structure?

Key findings

• One-component inversions suggest ECs are associated with more inclined magnetic fields and stronger Evershed flows, implying a magnetic perturbation, but these results are considered less reliable due to model simplification.
• Uncombed inversions show that the magnetic field and temperature of flux tubes in ECs are nearly identical to those in surrounding intra-spines, indicating no significant magnetic change.
• The primary difference in uncombed inversions is a higher filling factor for the tube component in ECs, indicating increased visibility of the flux tubes.
• The enhanced filling factor is not due to changes in tube position or radius, but rather to a downward shift in the optical depth scale caused by reduced gas pressure and density in the upper half of the tube.
• Numerical experiments confirm that a pressure decrease in the upper tube leads to a density reduction that shifts the optical depth scale deeper, increasing the tube's contribution to the emergent Stokes profiles.
• The mechanism explains the observed linear-to-circular polarization ratio increases interpreted as higher filling factors, supporting a thermodynamic origin for ECs rather than a magnetic one.