Superstrong photospheric magnetic fields in sunspot penumbrae (1909.13619v1)

Abstract: Recently, there have been some reports of unusually strong photospheric magnetic fields (which can reach values of over 7 kG) inferred from Hinode SOT/SP sunspot observations within penumbral regions. These superstrong penumbral fields are even larger than the strongest umbral fields on record and appear to be associated with supersonic downflows. The finding of such fields has been controversial since they seem to show up only when spatially coupled inversions are performed. Here, we investigate and discuss the reliability of those findings by studying in detail observed spectra associated with particularly strong magnetic fields at the inner edge of the penumbra of active region 10930. We apply classical diagnostic methods and various inversions with different model atmospheres to the observed Stokes profiles in two selected pixels with superstrong magnetic fields, and compare the results with a magnetohydrodynamic simulation of a sunspot whose penumbra contains localized regions with strong fields (nearly 5 kG at $\tau=1$) associated with supersonic downflows...

• The paper investigates superstrong magnetic fields (>7 kG) in sunspot penumbrae using classical diagnostics and various inversion techniques on Hinode observations and compares results with MHD simulations.
• Findings show variable outcomes across different inversion methods, with some suggesting strong fields tied to downflows, while MHD simulations indicate conditions favoring large field strengths around 5 kG in counter-Evershed flows.
• The potential existence of these fields highlights the dynamic nature of penumbrae, stressing the need for advanced inversion techniques and further research into effects like the Paschen-Back effect to refine understanding.

Superstrong Photospheric Magnetic Fields in Sunspot Penumbrae

The paper examines the presence of superstrong magnetic fields within sunspot penumbrae, specifically those exceeding 7 kG, a figure that surpasses even the most intense fields observed in umbral regions. These fields have garnered attention due to their controversial nature, largely because they have predominantly surfaced through spatially coupled inversion techniques.

Main Objectives and Methodology

The paper investigates observations from the penumbra of active region 10930 using classical diagnostic methods along with various inversion approaches to validate the occurrence of these strong magnetic fields. Detailed analyses were conducted using Hinode SOT/SP observations, and different inversion techniques were applied, such as:

• SPINOR 2D Inversions: Applied a height-dependent, single-component model, showing magnetic field strengths above 7 kG tied to supersonic downflows.
• Height-dependent 2-component Inversions: Suggests the coexistence of an umbral and a penumbral component, with the latter experiencing high velocities and strong fields.
• Height-independent Inversions: Provides moderate strength fields around 4 kG with notable velocity shifts.

The paper engaged in comparing these inversion results with a comprehensive magnetohydrodynamic (MHD) simulation to garner insights into the reliability of the estimated field strengths.

Key Findings

1. Variable Inversion Outcomes: Different inversion methodologies yielded disparate results, indicating the complexity and potential limitations of accurately identifying these field strengths. While the SPINOR 2D inversions predict notably high magnetic field strengths correlating with supersonic downflows, 2-component inversions imply a combination of weaker internal penumbral fields.
2. MHD Simulation Correlation: The presence of counter-Evershed flows in the sunspot penumbra, as demonstrated by the MHD simulations, suggests conditions that could foster large field strengths (roughly 5 kG). The simulations highlight how the intricate dynamics and gradients characterize the physical environment of these sunspots.
3. Bayesian Information Criterion (BIC) Results: The BIC analysis favored SPINOR 2D inversions despite the slight advantage. Given the anticipated gradation of parameters, height-dependent solutions are deemed more appropriate.

Implications

The potential existence of extreme magnetic fields in sunspot penumbrae suggests that these regions are far more dynamic than conventionally perceived. Such conditions might considerably affect solar atmospheric modeling and illustrate the need for refined inversion techniques that adequately capture the complexities of solar magnetic phenomena.

Future Directions

Further advancements in inversion methodologies, particularly those accounting for height variance and potential non-linear aspects of solar magnetic fields, are necessary. It is also recommended to explore the Paschen-Back effect's role in strong field scenarios to refine the understanding of polarization signals under such conditions.

This paper contributes significantly to solar physics by challenging existing paradigms of sunspot magnetic fields, urging the need for precision in inversion techniques coupled with robust simulations to uncover the behavior of these formidable solar magnetic structures.