Spectral Background-subtracted Activity Maps

Initial ideas for extracting and visualizing temporal variations in time series were presented by Verma et al. (2012). The concept of a Background-Subtracted Activity Map (BaSAM) was later systematically introduced by Denker & Verma (2019) to study changes in the solar cycle, as seen in time series of full-disk UV images and photospheric magnetograms. Flare transients can be captured by BaSAMs (Pietrow et al. 2023), which, in combination with the Color Collapsed Plotting (Druett et al. 2022) technique, reveal the temporal evolution of spectral features. Kamlah et al. (2023) extended the scope of BaSAMs to examine high-spatial-resolution data of pores and light-bridges and to imaging spectroscopy of the chromospheric Hα line. These results motivated this work, which promotes BaSAMs as a tool for the analysis of multidimensional spectroscopic data (space, time, wavelength, and polarization state).

High-resolution spectroscopic data were obtained with the CHROMospheric Imaging Spectrometer (CHROMIS) at the 1 m Swedish Solar Telescope (SST, Scharmer et al. 2003) on La Palma, Canary Islands, Spain. The target was a small sunspot in the trailing part of active region NOAA 12723 from 08:18 to 09:14 UT on 2018 September 30, which was also observed by Kuckein et al. (2021) and Vissers et al. (2022). The CHROMIS data are publicly available at the SST Data Archive ³ after image restoration with multi-object multi-frame blind deconvolution (MOMFBD, van Noort et al. 2005) and processing with the CRISPRED (de la Cruz Rodríguez et al. 2015) data reduction pipeline. The dataset consists of 205 spectral scans of the Ca ii K  λ3933 Å and Hβ  λ4861 Å lines with 26 and 23 wavelength points, respectively. Since image rotation is present, the common field-of-view (FOV) is significantly reduced to 289 × 588.

The background-subtracted variation of a spectral quantity I_λ (t), that is, filtergrams or slit-reconstructed spectral maps, within a time series is given by

$$\begin{eqnarray}&&\begin{array}{l}\left\langle \left|{I}_{\lambda }-\left\langle {I}_{\lambda }\right\rangle \right|\right\rangle =\displaystyle \frac{1}{N}\displaystyle \sum _{i=1}^{N}\left|{I}_{\lambda }({t}_{i})-\left\langle {I}_{\lambda }\right\rangle \right|\\ \mathrm{with}\quad \left\langle {I}_{\lambda }\right\rangle =\displaystyle \frac{1}{N}\displaystyle \sum _{i=1}^{N}{I}_{\lambda }({t}_{i}),\end{array}\end{eqnarray} \tag{ 1 }$$

where the subscript λ refers to the wavelength position in a spectral line scan, and t_i denotes the temporal position within the time series of spectral scans, and N is the total number of scans (see, Denker & Verma 2019). Figure 1 aggregates approximately 3.67 × 10⁸ spectra in 26 Ca ii K  and 23 Hβ  background maps $\left\langle {I}_{\lambda }\right\rangle$ and BaSAMs $\left\langle \left|{I}_{\lambda }-\left\langle {I}_{\lambda }\right\rangle \right|\right\rangle$, where the angle brackets 〈...〉 are shorthand for time averaging.

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Calculating spectral BaSAMs first produces an averaged spectral scan $\left\langle {I}_{\lambda }\right\rangle$ from a series of two-dimensional narrow-band filtergrams. Persistent spectral features were associated with sunspots, pores, and an arch filament system (AFS), which is best seen in Hβ  line-core filtergrams. Bright footpoints mark the ends of horizontal dark fibrils connecting the sunspot on the left to a small pore embedded in bright plages on the right. Such regions associated with small-scale magnetic flux elements are best seen in the far wings of the Ca ii K  filtergrams, while Ca ii K  line-core filtergrams indicate chromospheric heating near the sunspot and pores associated with emerging flux.

The Ca ii K  line-wing BaSAMs $\left\langle \left|{I}_{\lambda }-\left\langle {I}_{\lambda }\right\rangle \right|\right\rangle$ clearly identify activity associated with the footpoints of the AFS, with the signal being stronger in the blue wing. In contrast, the right footpoint is more prominent in the Ca ii K  line-core BaSAMs. The central filament shows strong absorption but no activity features in the BaSAMs. However, strong variations delineate this filament, suggesting filament oscillations or flows aligned with the filament axis.

Minor activity also occurs near small-scale active-region filaments in the upper-right part of the FOV as a small kernel in the Ca ii K  line-core BaSAMs and as filament-like features in the Hβ  blue-wing and line-core BaSAMs. Overall, the morphology of the Ca ii K  and Hβ  BaSAMs falls into three categories related to the outer and inner line wings and the line cores, reflecting the transition from the photosphere to the chromosphere.

The potential of spectral BaSAMs is summarized in the two plots in Figure 1, where Ca ii K  and Hβ spectral lines for five locations are compared with the absolute disk-center intensity atlas spectrum (Neckel 1999) obtained with the McMath–Pierce Fourier Transform Spectrometer (FTS). Persistent intensity features in averaged spectral scans and regions with strong temporal variations, identified with spectral BaSAMs, served as objective criteria for selecting these locations. The averaged spectral profiles already show various spectral signatures, including blue- and red-shifts, line asymmetries, enhanced line wings, emission reversals, and (pseudo-)continuum and line-core intensities. These features, together with the averaged intensity and BaSAM values, can serve as a starting point for investigating the temporal evolution of spectra, for example, using machine learning techniques (e.g., Verma et al. 2021).

The CHROMIS data were obtained in a 2018 observing campaign by Joao da Silva Santos and Gregal Vissers. EU Horizon 2020 grant agreement 824135 (SOLARNET—Integrating High-Resolution Solar Physics).

Facilities: SST—Swedish Solar Telescope - , CHROMIS— Chromospheric Imaging Spectrometer - .

Software: BaSAMs (Denker & Verma 2019)—MOMFBD (van Noort et al. 2005)—CRISPRED (de la Cruz Rodríguez et al. 2015).