A new way to probe deeper into the Sun’s secrets has been found by studying the magnetic field at different layers of the solar atmosphere using data from the Kodaikanal Tower Tunnel Telescope.  The solar atmosphere is composed of various layers interconnected through magnetic fields. The magnetic field acts as a conduit to transfer energy and mass from the inner layers to the outer layers, commonly known as the “coronal heating problem,” and is also the prime driver of the solar wind. To understand the physical mechanisms behind these processes, measurements of magnetic fields at different heights of the solar atmosphere are important. The strength of the magnetic field can be inferred by precise measurements of the spectral line intensities across the Sun in full polarization. Simultaneous multiline spectropolarimetry is an observational technique that captures this magnetic field at different layers of the solar atmosphere. Recent studies have demonstrated the technique's capability to detail the magnetic structure of sunspots, umbral flashes, and chromospheric variations during solar flares.

A study led by astronomers at the Indian Institute of Astrophysics, an autonomous institute of the Department of Science and Technology (DST), examined an active region (sunspot) with complex features, including multiple umbrae and a penumbra, through simultaneous observations in the Hydrogen-alpha and Calcium II 8662 Ã… lines from the Kodaikanal Tower Tunnel telescope.

The Kodaikanal Solar Observatory (KoSO), operated by the Indian Institute of Astrophysics, is known for the discovery of the Evershed Effect in 1909. The study used data from multiple spectral lines acquired simultaneously, especially the Hydrogen-alpha line, at 6562.8 Angstroms (â„«), to infer the magnetic field's stratification at various heights of the solar atmosphere, taken from the Tunnel Telescope at the Kodaikanal Solar Observatory, which is operated by IIA.

The primary mirror (M1) of the 3-mirror setup at the Tunnel Telescope tracks the Sun, the secondary mirror (M2) redirects sunlight downwards, and the tertiary mirror (M3) makes the beam horizontal. This kind of setup, where the primary mirror is rotated to track a moving object in the sky, in this case, the Sun, is called a Coelostat. An achromatic doublet (38 cm aperture, f/96) focuses the Sun’s image at a distance of 36 m with an image scale of 5.5 arcsec per mm. The chromospheric magnetic field in the spectral lines is typically inferred using the Calcium II 8542 Å and Helium I 10830 Å line. However, these diagnostic probes have certain limitations which limit their applicability across diverse solar features. “The Hα line, however, turned out to be a crucial probe to infer the chromospheric magnetic field because it is less sensitive to local temperature fluctuations. This allows us to probe the chromospheric magnetic field in events with sudden temperature fluctuations, such as flaring active regions”, said Harsh Mathur, the lead author and a Ph.D. student at IIA.

The study, accepted for publication in The Astrophysical Journal, provides a comprehensive analysis of the line-of-sight (LOS) magnetic field through these simultaneous observations. The results indicate that the Hα line core consistently infers weaker magnetic field strengths than the Ca II 8662 Å line inversions, suggesting that the Hα line samples higher atmospheric layers than the Ca II IR triplet. The study found the values of magnetic fields to be 2000 G at the photosphere and 500 G at the chromosphere. “In regions exhibiting localized heating or temperature enhancements, the full Hydrogen-alpha line became sensitive to the chromospheric magnetic field. The study highlights the Hydrogen-alpha line's effectiveness as a chromospheric diagnostic tool, particularly when the other spectral lines, such as Calcium II 8542 Å, probe deeper layers of the solar atmosphere.

This multi-line approach is crucial for understanding the intricate magnetic field stratification in the chromosphere,” said Dr. K. Nagaraju, principal investigator of this study. “The study underscores the necessity for further spectropolarimetric observations of the Hα line using advanced telescopes with superior spatial and spectral resolution. Telescopes like the Daniel K. Inouye Solar Telescope (DKIST), the forthcoming European Solar Telescope (EST), and the National Large Solar Telescope (NLST) hold great potential in providing deep insights into the chromospheric magnetic field's stratification,” said Dr. Jayant Joshi, a co-author of the study.

The National Large Solar Telescope is a proposed ground-based 2-m class optical and near-infrared (IR) observational facility. It is proposed to be built in Merak village in Ladakh in India. The Bengaluru, India team includes Mr. Harsh Mathur, a Ph.D. student at IIA, Dr. K. Nagaraju from IIA, and Dr. Jayant Joshi from IIA. The USA team comprises Dr. Rahul Yadav from the Laboratory for Atmospheric and Space Physics, Boulder. This research marks a significant step towards a more detailed and nuanced understanding of the Sun's magnetic field, paving the way for future studies and observations to elucidate the complexities of solar magnetic phenomena further.