The study of magnetic fields in different layers of the solar system using data from the Kodaikanal Tower Tunnel Telescope has revealed a new way to probe deeper into the mysteries of the Sun. The solar system is made up of different layers interconnected through magnetic fields. The magnetic field acts as a conduit for transferring energy and mass from the inner layers to the outer layers, commonly known as the “coronal heating problem”, and is also a major factor in the solar wind. Measuring magnetic fields at different heights of the solar system is important to understand the physical mechanism behind these processes.
In this, the strength of the magnetic field can be estimated by accurately measuring the intensity of spectral lines around the Sun in a state of full polarization. Multilinear spectropolarimetry, which measures the polarization of light waves at different distances, is an observational technique that captures the magnetic field in different layers of the Solar System. Recent studies have demonstrated the technical capabilities of detailing the magnetic structure of sunspots, umbral flashes and chromospheric variations during solar flares.
A study conducted by astronomers from Indian Institute of Astrophysics, an autonomous institute of the Department of Science and Technology (DST), using simultaneous observations in the hydrogen-alpha and calcium II-8662 A-lines from the Kodaikanal Tower Tunnel Telescope, examined an active region (sunspot) with complex features including multiple deep shadows and a penumbra.
The Kodaikanal Solar Observatory (COSO), operated by the Indian Institute of Astrophysics, is known for the discovery of the Evershed effect in 1909. This study used data from multiple spectral lines, particularly the hydrogen-alpha line, 6562.8 Angstrom (A), obtained simultaneously from the tunnel telescope of the Kodaikanal Solar Observatory, operated by IIA, to detect different layers of magnetic fields at different altitudes of the solar system.
In a three-mirror tunnel telescope, the primary mirror (M1) tracks the Sun. The secondary mirror (M2) redirects the sunlight downwards and the tertiary mirror (M3) brings the rays parallel to the horizontal. Such facilities where the primary mirror rotates tracking an object moving in the sky, here the Sun, are called coelostats. 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 spectral lines is typically estimated using the Calcium II 8542 Å and Helium I 10830 Å line. However, these diagnostic probes have certain limitations, which limits their utility across different solar systems. “The Hα line, however, may be an important probe in terms of results in chromospheric magnetic field, as 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 in a flaring active region,” said Harsh Mathur, a PhD student at IIA and lead author of the report.