TRIPPEL-SP Solar Spectropolarimeter

• TRIPPEL-SP spectropolarimeter is a solar instrument integrating advanced polarimetric modulators and high-speed CMOS detectors to capture the full Stokes vector.
• It employs a lens Littrow configuration with a narrow slit and echelle grating (R≈200,000) to achieve high spectral resolution for detailed solar feature analysis.
• Its rapid cadence and robust calibration methods enable precise diagnostics of dynamic solar events, including flares and subtle quiet Sun variations.

The TRIPPEL-SP spectropolarimeter is a high-resolution, rapid-polarimetry solar instrument, conceived as a polarimetric enhancement to the TRI-Port Polarimetric Echelle-Littrow (TRIPPEL) spectrograph at the Swedish 1-m Solar Telescope (SST). It is optimized for detailed spectropolarimetric diagnostics of the solar atmosphere, enabling precise measurements of the full Stokes vector across selected chromospheric and photospheric spectral lines. The scientific utility of TRIPPEL-SP is demonstrated in both quiet Sun studies—such as probing potential latitude dependence of the solar spectrum—and in high-cadence analyses of dynamic events including solar flares, where resolving rapid evolution in thermodynamic and magnetic parameters is essential for understanding solar activity.

1. Technical Architecture and Instrumental Design

The TRIPPEL-SP builds upon the core design of the TRIPPEL spectrograph, which employs a lens Littrow configuration with a ruled echelle grating to achieve moderate-to-high spectral resolution (R ≈ 200,000; Δv ≈ 1.1–1.4 km s⁻¹). The main spectrograph features a narrow 25 μm slit, mounted on a chromium-coated glass plate, offering fine spatial resolution (~0.11″) with the capability for positional rotation to mitigate instrumental artifacts such as spectral smile and keystone distortions.

The unique polarimetric capability of TRIPPEL-SP derives from the integration of modulators—specifically ferroelectric liquid crystals, adapted from the Fast Solar Polarimeter—and three high-speed, high-QE CMOS detectors at the A, B, and C exit ports. Each port is paired with narrow-band filters to isolate discrete orders, enabling simultaneous multi-wavelength observation. A wideband “slit-jaw” imaging channel operates in parallel to capture context images for post-facto image restoration. During typical flare observations, frame rates are adjusted (e.g., to 100 Hz) to maximize photon-limited SNR in spectral regions like Ca II 8542 Å, where detector sensitivity can otherwise be a bottleneck (Baso et al., 18 Oct 2025).

2. Observational Modalities and Data Acquisition

TRIPPEL-SP is engineered for both static and time-resolved spectropolarimetric measurements across the solar disk. Observational protocols involve targeting multiple spatial positions with a precisely maintained heliocentric angle, as in latitude-differentiation studies (μ = cos θ = √2/2 ≈ 0.71) (Kiselman et al., 2011), as well as employing rapid multi-scan “sit-and-stare” or raster modes to track temporal evolution during active events (Baso et al., 18 Oct 2025).

Data acquisition is coordinated via auxiliary systems:

• Correlation-tracking ensures the slit is scanned or held over regions of interest to average across solar granulation or to resolve compact dynamic features,
• Flat-fielding employs a method of random signal injection into the SST’s adaptive mirror to mitigate fixed-pattern noise and spatial inhomogeneities,
• Simultaneous dark exposures are acquired for baseline subtraction,
• Calibration observations incorporate disk-center reference spectra and standard stars for cross-calibrating Stokes vectors.

3. Calibration, Data Reduction, and Spectral Analysis

Data reduction pipelines for TRIPPEL-SP address numerous instrumental and atmospheric effects:

• Geometric distortions (e.g., field curvature), wavelength calibration (via FTS atlas cross-referencing), and spectral stray light (typically 5–6% of continuum) are systematically corrected.
• Telluric contamination is minimized by rejecting affected intervals or applying airmass-dependent corrections.
• Spatial averaging of raw spectrograms along the slit collapses the data into high-SNR, one-dimensional spectra amenable to precise line profile and equivalent width measurements.

Spectropolarimetric analyses rely on full-Stokes retrieval, facilitated by the rapid modulation and synchronized readout. Key analytic steps include integration over carefully defined wavelength intervals for equivalent widths and Doppler shifts, as well as direct fitting or NLTE inversion of line profiles to retrieve physical stratifications in temperature, velocity, and magnetic field (Baso et al., 18 Oct 2025).

4. Quantitative Spectropolarimetric Diagnostics

TRIPPEL-SP is optimized for quantitative retrievals of both thermodynamic and magnetic parameters, exemplified by:

• Equivalent width differencing at varying solar latitudes to test latitude dependence of the solar spectrum. The relative difference is computed as

$$\frac{\Delta W}{\langle W\rangle} = \frac{W_{\text{low}} - W_{\text{high}}}{\langle W\rangle}$$

with an effective difference for blended spectra given by

$$\frac{\Delta W^{(\text{eff})}}{\langle W\rangle} = \frac{1}{2} \frac{\Delta W}{\langle W\rangle}$$

such differences, when translated to abundance variations via LTE curves of growth, are confirmed to be ≤0.005 dex even for refractory elements (Kiselman et al., 2011).

• Non-LTE (NLTE) inversion of chromospheric lines (notably Ca II 8542 Å), using codes such as STockholm Inversion Code (STiC), yielding stratified atmospheric parameters as functions of optical depth (e.g., log τ = 0, −2, −4). This approach supports robust detection of localized heating, plasma flows, and vector magnetism during active events (Baso et al., 18 Oct 2025).
• Magnetic field extrapolation from vector magnetogram boundaries using non-force-free field (NFFF) models, enabling estimation of free magnetic energy,

$$E_F = \int_V \frac{B^2_{\text{NFFF}}}{8\pi} dV - \int_V \frac{B^2_{\text{P}}}{8\pi} dV$$

and quantifying the residual between extrapolated and observed transverse field components by

$$E_n = \frac{\sum_{i=1}^M |\mathbf{B}_{t,i} - \mathbf{b}_{t,i}|\,|\mathbf{B}_{t,i}|}{\sum_{i=1}^M |\mathbf{B}_{t,i}|^2}$$

(Baso et al., 18 Oct 2025). This enables direct connection of atmospheric diagnostics to the underlying magnetic topology.

5. Scientific Outcomes and Impact on Solar Physics

TRIPPEL-SP has been pivotal in resolving fundamental questions on solar spectral invariance and solar activity:

• In the context of solar twin abundance comparisons, it was established that latitude-dependent variations in spectral line strength across the solar disk (at constant heliocentric angle) are ≤1.5% for all studied lines, corresponding to negligible abundance differences (≤0.005 dex). This definitively rules out the possibility that observed Sun–twin abundance anomalies are due to geometric (aspect-angle) effects (Kiselman et al., 2011).
• In studies of eruptive events, TRIPPEL-SP was crucial for temporally resolving the evolution of thermodynamic and magnetic quantities throughout a C5.1-class solar flare. Pre-flare diagnostics unraveled localized heating (~2000 K temperature increase) and downflows (10–20 km/s) at bald-patch (BP) magnetic topologies, linked to reconnection-driven destabilization of a filament. Dynamic tracking of flare rise, eruption, and decay showed loss of ~30% free magnetic energy and characteristic chromospheric heating (up to ~8500 K) along flare ribbons (Baso et al., 18 Oct 2025).
• The instrument’s spatial–spectral fidelity enables detection of subtle variations due to weak magnetic fields, solar activity, or solar cycle phase—providing a baseline for long-term solar atmospheric monitoring as well as magnetic field diagnostics.

6. Operational Limitations and Methodological Considerations

Several operational constraints and methodological limitations are intrinsic to TRIPPEL-SP observations:

• Pointing precision is critical: positional drifts (~1′) can artefactually modulate equivalent widths by up to 5% for sensitive lines, though differential measurement strategies mitigate much of this scatter (Kiselman et al., 2011).
• Atmospheric seeing and variable airmass impact both spatial resolution and telluric contamination, particularly in weak or blended lines.
• Instrumental effects—stray light, spectral curvature, alignment drifts—are only partially correctable, potentially contributing to residual systematic errors, especially in differential abundance studies.
• Observations to date have targeted predominantly quiet-Sun and moderately active regions. While some strong chromospheric lines are activity-sensitive, most weak photospheric lines used for abundance determinations are minimally affected, except for cases involving hyperfine structure (e.g., Mn I lines).

7. Broader Relevance and Future Prospects

TRIPPEL-SP sets a benchmark for precision solar spectropolarimetry at moderate-aperture ground-based telescopes. Its design and methodology highlight:

• The tactical value of multi-port, high-speed, full-Stokes-capable solar spectrographs with broad multiplexing flexibility.
• The necessity of combining spectroscopic/polarimetric data with advanced inversion and field-extrapolation tools to fully exploit the diagnostic potential in studies of stellar atmospheres.
• The promise for similar designs in dedicated space or synoptic instruments, where high-fidelity, rapid spectropolarimetric diagnostics are required for both basic research and real-time forecasting in heliophysics.

The instrument’s demonstrated measurement accuracy and sophistication in handling instrumental artefacts, atmospheric variability, and error analysis provide a template for future high-resolution spectropolarimetric investigations targeting nuanced solar and stellar atmospheric phenomena.