National Solar Observatory



ATST Science Use Case

Title: The magnetic and kinematic structure of Sunspot Penumbrae.

Prepared by: Alexandra Tritschler, David Elmore, Craig DeForest, Peter Nelson, Rebecca Centeno-Elliot.

Co-Team: Kevin Reardon, Gianna Cauzzi, Friedrich Wöger, Thomas Rimmele.

Section 1: Observing Proposal

Abstract:

We propose to perform a multi-wavelength study of the CLV of sunspot penumbrae with the ViSP in order to validate the concept of the uncombed penumbra in either one of the two scenarios: the embedded flux tube model or the gappy penumbra. We further aim to understand how the chromospheric loops and the inverse Evershed flow fit into either of the suggested models for the origin of the penumbral fine structure.

Scientific Justification:

The physics of sunspots and its fine structure is one of the most fascinating and demanding research fields in solar physics. The varying magnetic field strength and inclination combined with plasma flows make Sunspots a very challenging regime for modelers studying magneto-convection and observers that attempt to interpret the wealth of information delivered by spectro-polarimetric data. It was not until recently that our understanding of particularly the umbral fine structure greatly advanced by the combined improvements achieved in realistic three-dimensional MHD simulations of the strong-field regime and instrumentation and processing techniques.

Unfortunately the situation for the penumbral fine structure is by far more complicated by the presence of oblique fields with changing strength and inclination and noticeable flows. In an attempt to explain the observations particularly the asymmetries of the Stokes parameters, two model ideas evolved that are currently vividly disputed in the literature: the concept of the uncombed penumbra mimicked by a (thin) horizontal flux tube carrying the Evershed flow embedded in a static more vertical background magnetic field and the gappy penumbra characterized by field-free plumes (convective rolls) connected to the underlying convection zone that permeate quasi-intermittently the magnetic field from below. Although these two scenarios differ intrinsically, contempo observables (limited by spatial resolution and sensitivity of instrumentation) do not allow for an unambiguous interpretation and hence, recent attempts to differ between the scenarios have failed so far.

While most of the attention has been drawn to the photosphere the upper layers above sunspots in particular the chromosphere are in many aspects undiscovered land. What do we know about the

chromospheric fine-structure above sunspots and its dynamics?

When higher layers of the penumbral atmosphere are probed by using stronger lines (Hα, or like the Ca II line at 854.21 nm) the visible picture changes radically: the atmosphere is dominated by loop-like structures, dark fibrils that are more or less aligned radially, that begin within the umbra/penumbra and

extend far beyond the visible penumbra, and that fill almost the whole FOV. Inherent to these structures is a material inflow directed towards the umbra which was named the inverse Evershed flow. It is believed to be a material flow along the fibrils, which are almost certainly the result of the magnetic field organization in form of arched loops on a much larger scale (vertically and horizontally when compared with the photosphere). The inflow pattern is usually explained in light of a siphon flow along magnetic flux tubes, where the gas pressure difference between the two foot points of the loop structure acts as the driving force of the motion. The flow direction is from lower to higher magnetic field strength. When observed at higher spatial resolution the flow pattern itself seems to be very complex with large variations about the mean, including velocities in the opposite direction from the general chromospheric inflow pattern.

One of the most obvious questions that have not been addressed very clearly in the literature can be formulated as follows: how do the flows in the chromospheric loops connect to the observed flows in the photosphere or how do the chromospheric flows fit into the picture of the uncombed penumbra or gappy penumbra? Do we deal with two separated flow systems driven by different physical mechanisms? Might the chromospheric Evershed flow even be independent from the existence of a visible penumbra?

It seems obvious that the situation can be resolved only by high-spatial and spectral-resolution observations that allow to distinguish between the models in a direct way: via physical parameters that can be extracted directly from the line profiles of the Stokes parameters like e.g. Doppler induced wavelength shifts and the net-circular polarization (NCP). Information about the magnetic field strength and inclination must be derived from simultaneously inverting multiple lines covering a broad wavelength region which helps to minimize ambiguities in the results.

The Visible Spectro-Polarimeter (ViSP) is specifically designed to perform simultaneous multi-wavelength observations to fulfill the demanding requirements of solar spectropolarimetry with high sensitivity and accuracy.

Main Questions:

What is the origin of the penumbral fine structure in the photosphere and how is the penumbra heated?

How does the photospheric and chromospheric fine structure evolve in time? How do the chromospheric flows fit into the picture of the uncombed penumbra or gappy penumbra? Where do the foot points of the chromospheric loops end? Is the inverse Evershed flow driven by a siphon mechanism? What is the 3d magnetic topology of a sunspot?

Observation Description:

We propose to perform a spectro-polarimetric multi-wavelength study of sunspot penumbrae using the diagnostic lines Fe I 630.15 and 630.25 nm, Ca II IR triplet, and Fe I 900 nm with the ViSP. We envisage to scan the solar surface in two different modes: (1) fast cadence time sequences of stripes where the slit is preferably arranged along the filaments and (2) repetitions of full maps of a sunspot. The stripe mode could be performed in either of the following ways: (1a) covering the center- and limb-side penumbra at the same time including the umbra and areas outside the visible boundary of the sunspot (up to 10 arcsec) or (1b) centering the scan area first on either side of the penumbra covering part of the umbra, the full penumbra and parts of the outside area. Preferred targets should cover different mature sunspots with well developed penumbrae at different viewing angles for at least desirable 2 hours each. The ideal observing situation, however, would be that we can observe a sunspot during its disk passage even covering the late evolutionary stage when the sunspot lost all visible signature of the penumbra and only an umbral fragment is left. For program (1) the instrument should be operated in full resolution mode (non-binned along the slit) and the slit width should match twice the diffraction limit of the smallest wavelength observed in the sequence. When full FOV scanning is desired the configuration must be changed to allow for an acceptable scanning time. A slit-jaw device should provide complementary context information for each individual slit position.

Goal 1: Determine the origin of the penumbral fine structure in the photosphere and the chromosphere in terms of plasma flows and magnetic field geometry and its evolution in time. Clarify what the role of the chromospheric loops play in the context of the uncombed penumbra and what the nature of the inverse Evershed flow is by determination of physical parameters at the foot points of the loops.

Core observations 1: Spectropolarimetric observations featuring high spatial resolution and high polarimetric accuracy in two photospheric lines (FeI 630.15/630.25 nm, FeI 900 nm) and one of the chromospheric CaII IR triplet lines. SNR 1000 (0.001 Ic in 10 sec). Spectral resolution > 150000. Pixel size along slit and slit width 0.03 arcsec (24 microns). Step width adjusted to match slit width. Number of steps 67 corresponding to 2 arcsec. Cadence 10 min (desirable 150000. Pixel size along slit and slit width 0.074 arcsec (24 microns), 2-3 binning along slit. Step width 0.074 arcsec. Number of steps 1081 corresponding to 80 arcsec. Cadence 1.5 (desirable < 1 h). Slit oriented perpendicular to horizon.

Complementary observations 2.1: High-spatial resolution fast cadence two-dimensional spectroscopic observations with the VTF in one photospheric line (FeI 557.5 nm) and one chromospheric line (Na D 589.6 nm). Spectral resolution 100000. Pixel size 0.015 arcsec. FOV 60 x 60 arcsec centered on FOV covered by the ViSP. 40 wavelength steps in total, non-equidistant.

Section 2: Observation Details

Instruments:

|Instrument |Status |Wavelength Range |Spectral Lines/Features |Image Size and Scale |Cameras |

|ViSP(*) |required |630-900 nm |Fe 630 lines |~0.03 arcsec/pixel |3 cameras |

| | | |Fe 900 nm |@630 nm |@100 fps |

| | | |Ca II IR line(s) |4K x 4K | |

|Context (**) |required |500/656.3nm |N/A |4k x 4k |1 camera |

| | | | | |@12.5 fps |

|VTF(***) |required |550 – 590 nm |Fe 557.6 nm |~0.015 arcsec/pixel |2 cameras |

| | | |NaD 589.6nm |@557.6 nm |@12.5 fps |

| | | | |4K x 4K | |

|AO |required | |N/A | | |

(*) Windowing of cameras might be required.

(**) The context imager runs synchronized with the modulation.

(***) The VTF might have to run synchronized with the modulation.

Sequences:

|Instrument |Stepping and Slit Width |Number of Steps |Sampling |Desired |Repeats and |

| | | |Definition |Cadence |Duration |

|ViSP(**) |spatial stepping |(1ab) 2 arcsec/slit |diffraction limit (λ/D) |(1ab) ................
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