arXiv subjects

Michele Berretti

Publications and source records attributed to Michele Berretti.

4 recordsLinked to original sources

arXiv2026

Standing oscillations in a resonant sunspot atmosphere captured by integral field spectroscopy

The solar atmosphere is replete with magnetohydrodynamic wave activity, with magnetic structures such as sunspots channelling wave energy flux efficiently into the outer atmosphere. Steep density and temperature gradients between the photosphere and chromosphere provide ideal conditions for magnetoacoustic resonance cavities, amplifying $\sim 5$ mHz oscillatory power in sunspot atmospheres. However, diagnosis of such cavities has largely been limited to lines such as Ca II H/K and He I 10830 Å, with no evidence yet from layers probed by the Na I D$_1$/D$_2$ doublet. Here we use the newly commissioned integral field unit FRANCIS to examine oscillations spanning the formation heights of the Na I D$_1$/D$_2$ lines and determine whether propagating and/or standing modes are present within a sunspot umbra. The RH1.5D code estimated formation heights for three windows: the Na I D$_1$ wing (core $-300$ mÅ; $\approx 355$ km), the Na I D$_1$ core ($\approx 750$ km), and the Na I D$_2$ core ($\approx 850$ km). Wavelet cross-correlation of line-core and bisector Doppler velocities yielded phase spectra versus height, classifying the dominant $\sim 5.5$ mHz oscillations as propagating or standing-like. At the umbra-penumbra boundary we find propagating modes with energy fluxes of $\sim 1.3 \times 10^{4}$ W m$^{-2}$ in the upper photosphere, falling to $\sim 3.1 \times 10^{3}$ W m$^{-2}$ in the lower chromosphere, implying a damping length $L_d \approx 363$ km, comparable to the local density scale height. In contrast, near-zero phase differences dominate regions of enhanced chromospheric power at the umbral centre, evidencing standing-wave behaviour and resonance-cavity dynamics. These results demonstrate the suitability of solar integral field units for mapping sunspot wave properties, with the Na I D$_1$/D$_2$ lines offering a novel diagnostic of resonance cavities and energy flux.

astro-ph.SR↗

arXiv2026

Multi-line Wave Signatures in a Sunspot from Near-UV Sunrise III/SUSI Observations

Magnetohydrodynamic waves redistribute energy in magnetic structures of the lower solar atmosphere, yet constraints on how wave power and dominant frequencies are organised above sunspots remain limited because most studies use only a few well-separated diagnostics. Here we present multiline wave signatures in a sunspot from near-UV spectroscopy with the Sunrise-III UV Spectropolarimeter and Imager (SUSI). We analyse a two-hour time series of repeated raster scans of a sunspot near disc centre in the 327-329 nm spectral window (> 100 lines). From these, we select 44 lines that radiative-transfer calculations suggest sample effective formation heights within the umbral core from deep photosphere toward the low chromosphere. For each line, we extract line-core intensity and line-of-sight velocity time series using a dedicated multi-line fitting routine and compute Morlet-wavelet power spectra. The refined global wavelet spectra show that most lines (in both intensity and velocity) are genuinely multi-frequency, with a dominant peak and substantial statistically significant power up to 12 mHz. Unsupervised clustering of the normalised spectra groups lines into families with similar spectral shapes and reveals a progression of dominant frequencies from ~2 to ~10 mHz across the ensemble, for both intensity and velocity (not necessarily in the same lines). This behaviour is not reproduced by a simple formation-height ranking, suggesting that uncertainties in the formation-height estimates and line-dependent diagnostic response together shape the ordering. These Sunrise-III/SUSI observations open a new regime for near-UV multi-line wave studies and provide the first systematic characterisation of frequency-structured sunspot wave behaviour in this spectral region.

astro-ph.SR↗

arXiv2026

The Influence of Opacity on Inferred MHD Wave Signatures in the Lower Solar Atmosphere

Magnetohydrodynamic wave activity in small-scale magnetic structures, such as solar pores, provides key insights into energy transport in the lower solar atmosphere. This study presents high-resolution observations of ten solar pores contained within a 43 x 43 Mm$^2$ field of view, obtained with the CRISP instrument at the Swedish 1-m Solar Telescope. We investigate the temporal behaviour of the line-of-sight velocity (vlos) and magnetic field (blos) oscillations within the pore structures. Using SIR inversions, we analyse the oscillatory signals at multiple optical depths (log $τ$ levels) to assess how variations in geometric height (z) and temperature relate to the observed blos fluctuations. Our results reveal that higher-frequency oscillations (> 6 mHz) exhibit strong coherences with in-phase fluctuations between blos and z across atmospheric layers, consistent with upward-propagating magneto-acoustic waves. In contrast, coherent lower-frequency oscillations display significant phase differences, which may arise from opacity effects contaminating the inversion response. These findings highlight the importance of accounting for opacity effects when interpreting magnetic oscillations, with direct implications for forthcoming high-precision magnetic diagnostics from facilities such as DKIST.

astro-ph.SR↗

arXiv2026

Adaptive multi-line fitting for stable line-core intensity and Doppler velocity

Next-generation solar spectrographs increasingly record dense wavelength windows in which tens to hundreds of spectral lines are sampled at each spatial location and time step. This expands the scope for multi-line, multi-height diagnostics of magnetohydrodynamic motions, but also raises a practical challenge: deriving stable line-core intensity and line-of-sight velocity time series when profiles evolve rapidly, become asymmetric, blend, or develop multi-lobed cores. Common fast estimators can perform well for simple, isolated absorption lines, yet can intermittently misidentify the core in crowded or morphologically complex cases. Even infrequent mis-tracking can leave step-like artefacts that redistribute power and bias spectral, phase, and coherence measures used in wave and dynamics analyses. We introduce LineFit, a fully reproducible adaptive multi-line fitting approach tailored to dense-window spectroscopy. LineFit models each line locally with bounded non-linear least-squares fits to a Voigt-family profile, including an asymmetric-Voigt option to accommodate unequal wing broadening, and incorporates close-pair ownership control together with conservative, per-line window adaptation and split-core-aware handling. Using a synthetic time series with unambiguous ground truth, we benchmark LineFit against four widely used fast baselines and assess both instantaneous centre errors and downstream time-series diagnostics. Several fast methods remain competitive for many lines, whereas LineFit is most robust in key stress cases involving intermittently split-core profiles and correspondingly yields power spectra that agree most closely with the truth. We also demonstrate a proof-of-principle that benchmarks hybrid acceleration of the LineFit software via supervised emulation, offering at least three orders-of-magnitude improvement in processing time.

astro-ph.SR↗