Visible Spectro-Polarimeter Use Case

Transcription

1 Project Documentation RPT-0032 Revision A Visible Spectro-Polarimeter Use Case Alexandra Tritschler, David Elmore, Craig DeForest, Peter Nelson, Rebecca Centeno-Elliot Science Working Group May 2008 Approved for use: Project Scientist Date Project Manager Date Systems Engineer Date Original on file Advanced Technology Solar Telescope 950 N. Cherry Avenue Tucson, AZ Phone atst@nso.edu Fax

2 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. Abstract: Section 1: Observing Proposal 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 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 RPT-0032, Revision A Page 2 of 16

3 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 multiwavelength 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 and 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 RPT-0032, Revision A Page 3 of 16

4 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 / nm, FeI 900 nm) and one of the chromospheric CaII IR triplet lines. SNR 1000 (0.001 I c in 10 sec). Spectral resolution > 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 <10 min). Slit oriented perpendicular to horizon if inversions are intended to be applied. Complementary observations 1.1: High-spatial resolution fast cadence two-dimensional spectroscopic observations with the VTF in one photospheric line (FeI nm) and one chromospheric line (Na D nm). Spectral resolution Pixel size arcsec. FOV 60 x 60 arcsec centered on FOV covered by the ViSP in stripe mode. 40 wavelength steps in total, non-equidistant. Goal 2: Determine the overall 3d magnetic and kinematic structure of the sunspot. Core observations 2: Spectropolarimetric observations featuring good spatial resolution and high polarimetric accuracy in two photospheric lines (Fe / nm, Fe I 900 nm) and one of the chromospheric CaII IR triplet lines. Spectral resolution > Pixel size along slit and slit width arcsec (24 microns), 2-3 binning along slit. Step width 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 nm) and one chromospheric line (Na D nm). Spectral resolution Pixel size arcsec. FOV 60 x 60 arcsec centered on FOV covered by the ViSP. 40 wavelength steps in total, non-equidistant. RPT-0032, Revision A Page 4 of 16

5 Instruments: Section 2: Observation Details Instrument Status Wavelength Range ViSP(*) required nm Spectral Lines/Features Fe 630 lines Fe 900 nm Ca II IR line(s) Image Size and Scale ~0.03 nm 4K x 4K Context (**) required 500/656.3nm N/A 4k x 4k VTF(***) required nm Fe nm NaD 589.6nm 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. ~0.015 nm 4K x 4K Cameras 3 fps 1 fps 2 fps Sequences: Instrument ViSP(**) VTF Stepping and Slit Width spatial stepping (1ab) 2 x diff limit, 0.03 arcsec (2) coarse grid 24 micron slit spectral stepping non-equidistant Number of Steps (1ab) 2 arcsec/slit width (2) 80 arcsec/ arcsec slit 2 filters, 40 wavelength positions Sampling Definition diffraction limit (λ/d) Desired Cadence (1ab) <10 min (2) <1 hour Repeats and Duration (1ab) 12 / 2 h (2) 1 / 1 h critical (λ/2d) <10 sec(*) 1260 / 3 h (*) Assumed that the VTF can run at 10 fps (includes filter wheel turn ~2 x 2sec). (**) If a multi-slit approach is realized than timing will improve significantly allowing for full resolution. This would lead to more repetitions in the same time. This will also increase the data volume. RPT-0032, Revision A Page 5 of 16

6 Data Rates: (*) Instrument ViSP(**) Data Volume/Sequence 56GB (67 steps) 900GB(1081 steps) Data Rate 85MB/sec Repeats/ Duration 12 / 2 h 1 / 1h Calibration Data Volume ~100GB Total Data Volume 0.67TB 0.9TB VTF 2.68 GB(in 10sec) 268 MB/sec 1260 / 3 h 134GB(***) 3.38 TB (*) Assuming 4k x 4k detectors, 16 bit. Total data volume does not include calibration data volume. (**) Un-demodulated raw data : 8 spectra / slit position, 1 slit jaw camera with 1 image per slit position. The ViSP calibration data volume is an estimate including data for lamb scans, laser profile measurements polarization calibrations, flats and darks. (***) 100 Flat fields per wavelength position. Data Processing: Instrument Processing Types Reduction Purpose Output ViSP VTF off-mountain, NSO home base off-mountain, NSO home base demodulation, dark, gain, instrumental polarization(*) gain, dark, wavelength calibration, destretch instrument corrected Level 1 instrument and seeing corrected data Level 2 Processing -- Not (necessarily) in real-time or on-site. Reduction -- on demand. level 0 raw data. level 1 instrumental signatures removed. In case of spectro-polarimetry this includes calibration for instrumental polarization. level 2 seeing corrected (destretch procedure). level 3 physical parameter maps. (*) remove skew/curvature, align beams, merge beams. RPT-0032, Revision A Page 6 of 16

7 Calibrations: Instrument Dark/Bias Frame Flat Field Target/Grid Polarimetric Calibration Other ViSP accuracy: < frames? accuracy: <0.01 Yes accuracy: Lamb flats Laser profiles VTF accuracy: frames accuracy: frames/λ Yes No Lamb scans Laser profiles Telescope Setup: Telescope Tracking Fixed Position 1 Tracking Solar Rotation x Tracking Feature 4 Mosaicing Off-Limb Tracking 5 Carrington Rotation Rate Differential Rotation 2 Custom Rate 3 1 : Fixed position may be used for limb or fixed disk center observations. 2 : Will there be a single standard model rotation curve, or multiple curves for different features? 3 : User specified tracking curve (tracking motion only in heliographic longitude or also in latitude?). 4: Using AO-offload to coordinate telescope motion with observed feature drift. 5 : Same as fixed position? Image Rotation Fixed Heliographic Orientation 1 Fixed Geographic Orientation Parallactic Angle 2 x Stepped Parallactic Alignment 3 1 : Fixed orientation with respect to Sun e.g. solar north aligned with detector axis 2 : Fixed orientation with with respect to Earth's horizon i.e. slit aligned with direction of dispersion 3: Orientation is kept sufficiently close to the parallactic angle, but between adjustments image is rotated to keep a fixed orientation in heliographic coordinates. Scattered Light Control 1 Mirror Cleaning Telescope Scattered Light Level 2 1 : Primarily for coronal observations (or other observations such as sunspot umbrae). RPT-0032, Revision A Page 7 of 16

8 2 : Requirement on level of scattered light introduced by telescope (e.g. depending on cleanliness of other optical surfaces?). AO Telemetry 1 x Triggered 2 x Covariances 3 Engineering x Seeing Monitor 4 x Light-Level Monitor? 5 1 : Information from the AO system that can be recorded during data acquisition for use in dara analysis. 2 : Recording of AO data triggered synchronously with images or sequences acquired by an instrument. 3 : Basic information needed to compute long-exposure PSF. 4 : Real-time monitoring of r 0 or other atmospheric parameters as determined by the AO system. 5 : Measurement of (integrated) illumination. May be determined from AO or other system. (*) In fact it would be very useful to have a monitor that allows to follow the development of atmospheric conditions like r0, wind, light level, etc. Atmospheric Conditions: resolution wavelength nm Desired Resolution nm Minimum Resolution 2 derived median r0 clear Sky Condition Constraints 3 Sky Light Level 4 1 : What is the desired spatial resolution for these observations? 2: What is the minimum acceptable spatial resolution for these observations. 3: Are there any particular sky conditions that may (or may not) be needed for these observations? For example are cirrus and fog acceptable? 4 : Primarily for coronal observations. Target Selection: RPT-0032, Revision A Page 8 of 16

9 Required Solar Conditions 1 x Y N N/A Active Regions Flares Quiet Sun Filaments/Prominences Coronal Hole 1 : What particular solar features are needed (or must be present) for the observations? Auxiliary Target Selection Data 1 Y N N/A Full-Disk Magnetogram 2 x High-Resolution Magnetogram x Hα Full-Disk Image 2 x EUV Image 3 Hα High-Resolution Image X-Ray Image 3 Other x Grid Overlay 4 URL: x Time Series 5 1 : What other types of solar data should be used to select the target? Data should be displayed with a fixed orientation (e.g. solar north up) for all images. 2 : Will these be available from a local telescope? 3 : Can these be obtained from a single, fixed source? 4 : Will time series of any of these data be necessary to choose the target (e.g. to find the location of flares or emerging flux)? 5 : Data should be displayed with a grid overlay in a choice of coordinate systems. Will all auxiliary data have WCS meta-data? Target Definition Fixed Coordinates 1 x Feature Defined 2 Disk Center Limb (N/S/E/W) Interactive ATST Staff x Definition x Investigators 3 Coordinate System 4 x Heliographic RPT-0032, Revision A Page 9 of 16

10 Heliocentric Carrington Helioprojective 1 : Target defined by a fixed position in some coordinate system. 2 : Target coordinates derived from feature position, possibly based on external feature lists (e.g. pointing based on active region 3 : Final pointing can be only defined based on investigator input (as opposed to ATST staff choosing appropriate target for observing program). 4 : Coordinate system in which target pointing will be given (if applicable). Data Display: Data Display Purposes 1 Target Selection x Quality Assurance 2 x Activity Monitoring 3 Instrument Monitoring 4 Data Selection 5 x Examination of Processed Data Instrument Setup 6 1 : For what reasons will the investigator or ATST staff need to display or examine the data obtained as part of the execution of the observing program. 2 : To confirm acquired data is being correctly obtained and suffices for the observational program. 3 : To monitor changes of solar conditions. 4 : To monitor proper operation of instruments. 5 : To identify or flag data that does or does not meet certain quality standards 6 : To confirm instrument operation during instrument setup and calibration. Quick Look Display 1 Raw Data x Pre-processed data (e.g. flat field/dark) (*) x Instrument Specific Reduction (**) x Scan/Map 2 x Profiles/Spectrum 3 Fixed Interval Display (frame number, time step) Frame Selection 4 x Stokes Parameters RPT-0032, Revision A Page 10 of 16

11 Name: Additional Algorithms 5 Source 6 : 1 : The Quick-Look System (QLS) will allow for real-time examination of the data obtained by any instrument or camera. The system can extract data at any point in the processing stream and optionally apply certain algorithms or plugins to the data before display. None of the displayed data will be recorded, though it may be possible to make notes (i.e. entries into a log or the header database) via the QLS. The nature of the data displayed may be fixed (e.g. show every 10 th image) or may involve user interaction with the display itself (e.g. zoom and pan controls, blinking two images, cursor-defined profile display, or scrolling through a sequence of images) through a predefined set of implemented display tools. The user may choose to hold the currently displayed image for closer examination and then release and allow the display to be updated with the latest data. The system should always indicate the currency of the displayed data (e.g. real-time, one minute old). 2 : Display of multiple images from a single set of images obtained with a single instrument as part of an observing sequence. Because of data rates only a subset of the scans obtained during the observations might be shown. This may be a fixed or an interactive display. 3 : Interactive display of one or two axes through a 3-D data cube (one axis is typically wavelength). 4 : Choose a single image within a fixed interval based on some predefined criteria (e.g. contrast). 5 : Additional plugins may be applied to data prior to display, but these will require prior testing and approval. 6 : Who will provide the plug in? Investigator? Instrument Scientist? Other ATST staff? (*): A rough correction for instrumental polarization? (**) : Display of additional physical information derived from the spectra like maps of: total circular polarization, total polarization, total linear polarization, net circular polarization, Doppler shifts, magnetic field strength and inclination (weak field limit). Interactive Examination 1 x Standard Packages 2 IDL User-Provided Routines 3 Instrument Software Other Software Computer 4 Interactive Work flow Definition 5 1 : What types of tools does the investigator expect to need for immediate examination of the data? 2 : For example SolarSoft 3 : Standard instrument-specific data reduction pipelines. 4 : Allow access to data store from external computers (i.e. investigators laptop) while on-site. May be provided subject to bandwidth limitations or with read-only access. Only selected sequences or scans will be provided to the user with this mechanism (in FITS format and perhaps in processed form). An interface may be needed to allow user to select which data to be exported or made available. 5 : User-defined data analysis processing pipelines. RPT-0032, Revision A Page 11 of 16

12 Remote Data Display 1 Real-Time Images Target Selection Quality Assurance Post-Observation Examination Activity Monitoring Telescope and Environmental Data Engineering Data 1: What information and images may need to be displayed away from the telescope for remote investigators and off-site ATST staff? RPT-0032, Revision A Page 12 of 16

13 Section 3: Observation Definition Observation Definition for the Core Observation 1 and 2: Über-Instrument Configuration File: Set Tracking: Feature tracking. Depends on sunspot. Image Rotation: Stepped Parallactic Alignment or Parallactic depending on ORACLE response. Image Orientation: Slit oriented perpendicular to horizon, fixed geographic orientation. Coordinate System: Heliographic. AO Telemetry: triggered, covariances, seeing and light level monitor. Seeing Monitor: 5 sec integration (pass integration time to AO, include seeing information in FITS header). Pointing: real time refinement. Warnings: Lightlevel varies too much or drops: light level (1 min average) < 3 (DST light-level units) r 0 (5 min average) < 10 cm Modulators out of sync? Calibration: Dark Flat 0.001(telescope motion, AO unflat, in focus) Polarization calibration (GOS), 5 second accumulation (telescope motion, AO unflat, in focus, disc center, ND filter required) Standard calibration optical configurations (clear, dark, retarder only at 0, 45º, 90º, 135º, linear polarizer at 0º, 45º, 90º, 135º, linear polarizer at 0º plus retarder at 0º, 45º, 90º, and 135º, and linear polarizer at 45º plus retarder at 0º, 45º, 90º, and 135º. Lamp Flat Increase slit width for same flux as solar observations, 5 seconds Target standard Grid standard VTF : Wavelength Calibration: +/ 15 days? Telescope Calibration Use current valid telescope calibration, else perform telescope calibration sequence using telescope calibration unit. Laser profile measurement. Pixel Sampling: (1ab) 0.03 arcsec along slit. (2) arcsec along slit. Binning: (1ab) no binning along slit. (2) 2-3 binning along the slit. Area: (1ab) 2 x 120 arcsec. Step width matches slit width. (2) 80 x 120 arcsec. Step width of arcsec. ; Science Observations Pointing Sequence 1: Slew telescope to selected target. Lock AO on sunspot (sub-structure?). pointing relative, 0, 0 + [[-5,5],[-5,5]] arcsec Polarimetric Sequence: Integral number of full rotations of achromatic polarization modulator at RPT-0032, Revision A Page 13 of 16

14 frame rate determined from solar flux level. Applicable to ViSP and VTF. Start Observation 1: Position the modulator at the GOS. Loop 1: Move slit to start position. Start scanning sequence. Repeat: 12 times Loop 2: Obtain Modulation states at slit position S, then move to slit position S+1. Repeat: 67 times. End Observation 1: Pointing Sequence 2: Slew telescope to selected target. Lock AO on sunspot. pointing relative, 0, 0 + [[-5,5],[-5,5]] arcsec Polarimetric Sequence: Integral number of full rotations of achromatic polarization modulator at frame rate determined from solar flux level. Applicable to ViSP and VTF. Start Observation 2: Position the modulator at the GOS. Loop 1: Move slit to start position. Start scanning sequence. Repeat: 1 time Loop 2: Obtain Modulation states at slit position S, then move to slit position S+1. Repeat: 1081 times. End Observation 2: ; Dark Observations Procedure should be defined by instrument team. Pointing Sequence 3: not needed. Start Observation 3: Position the modulator at the GOS. Move dark slide into the light path. Loop 1: Move slit to start position. Repeat: N times. Loop 2: Obtain Modulation states at slit position S, then move to slit position L (L=S means that darks are taken only at slit position S). Repeat: 100 times? End Observation 3 ; Flat Field Observations Procedure should be defined by instrument team. Pointing Sequence 4: Slew telescope to disk center or user defined position. Command AO unflat / flat. pointing relative, 0, 0 + [[-5,5],[-5,5]] arcsec (does not really matter) RPT-0032, Revision A Page 14 of 16

15 Start Observation 4: Command telescope to move (random guider movement) Loop 1: Move slit to start position. Repeat: N times. Loop 2: Obtain Modulation states at slit position S, then move to slit position L (L=S means that flats are taken only at slit position S). Repeat: 100 times? End Observation 4 ;Polarization Calibration Measurements 1: This measurement determines the polarimeter response function only (X Mueller matrix) assuming that the telescope influence is either modeled or determined in an independent way. Procedure should be defined by instrument team. Suggestion: Pointing Sequence 5: Slew telescope to disk center or user defined position. Command AO unflat. pointing relative, 0, 0 + [[-5,5],[-5,5]] arcsec (does not really matter) Start Observation 5: Position the modulator at the GOS. Position calibration optics at the GOS in front of the modulator. Command telescope to move (random guider movement) Loop 1: Move slit to start position. Repeat: N times. Loop 2: Obtain modulation states for different input states defined by calibration optics in GOS Repeat: P times (P defined by different relative orientations of POL and RET in GOS). End Observation 5 ;Polarization Calibration Measurements 2: This measurement determines the combined XT(?) Mueller matrix as proposed by Socas-Navarro (2004). Procedure should be defined by instrument team. Suggestion: Pointing Sequence 6: Slew telescope to disk center or user defined position. Command AO unflat. pointing relative, 0, 0 + [[-5,5],[-5,5]] arcsec. Start Observation 6: Position the modulator at the GOS. Position calibration optics above primary mirror. RPT-0032, Revision A Page 15 of 16

16 Position blocking mask in collimated beam. Loop 1: Move slit to start position. Repeat: N times. Loop 2: Obtain modulation states for different input states defined by calibration unit above primary mirror (defined by telescope polarization calibration sequence) Repeat: P times (P defined by different relative orientations of POL and RET). Remove calibration optics. Leave blocking mask in the beam. Perform solar observations preferably on sunspot through mini-aperture telescope. Remove blocking mask. Repeat solar observations with full aperture telescope. End Observation 6 RPT-0032, Revision A Page 16 of 16