RoCS publications September - October 2021

Four publications have been accepted for publication from RoCS in October. Researcher Henrik Eklund and Postdoctoral Fellows Ana Belén Griñón Marin and Atul Mohan present their latest findings.

Tree employees: one woman and two men

From left to right: Ana Belén Griñón Marin, Atul Mohan and Henrik Eklund. Photo: UiO


Title of the publication

Journal: The Astrophysical Journal

1st Author: Ana Belén Griñón Marín

Position: Postdoctoral Fellow

Short summary by the author:

 
A spectral line inversion code, Very Fast Inversion of the Stokes Vector (VFISV), has been used since May 2010 to infer the solar atmospheric parameters from the spectropolarimetric observations taken by the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). The magnetic filling factor, the fraction of the surface with a resolution element occupied by magnetic field, is set to have a constant value of one in the current version of VFISV. This report describes an improved inversion strategy for the spectropolarimetric data observed with HMI for magnetic field strengths of intermediate values in areas spatially not fully resolved. The VFISV inversion code has been modified to enable inversion of the Stokes profiles with two different components: one magnetic and one non-magnetic. In this scheme, both components share the atmospheric components except for the magnetic field strength, inclination, and azimuth. In order to determine whether the new strategy is useful, we evaluate the inferred parameters inverted with one magnetic component (the original version of the HMI inversion) and with two components (the improved version) using a Bayesian analysis. In pixels with intermediate magnetic field strengths (e.g. plages), the new version provides statistically significant values of filling fraction and magnetic field vector. Not only does the fitting of the Stokes profile improve, but the inference of the magnetic parameters and line-of-sight velocity are obtained uniquely. The new strategy is also proven to be effective for mitigating the anomalous hemispheric bias in the east-west magnetic field component in moderate field regions.

Title of the publication

The Sun at millimeter wavelengths III. Impact of the spatial resolution on solar ALMA observations

Journal: Astronomy and astrophysics

1st Author:  Henrik Eklund

Position: Researcher

Co-authors from RoCS:

  • Sven Wedemeyer
  • Mikołaj Szydlarski
  • Shahin Jafarzadeh

Short summary by the author:

A few years ago, observations of the Sun started with the Atacama Large Millimeter/sub-millimeter Array (ALMA), which provides a valuable diagnostics tool because of the high angular, spectral and temporal resolution compared to other radio observatories, but also as these measured intensities at millimeter wavelengths are closely related to the local gas temperature. 

Here we use a statistical approach to perform estimations on the degradation of the observable signatures of small-scale dynamics that comes with the angular resolution of the observations. 

A three-dimensional radiation-magnetohydrodynamic model of the solar atmosphere from the Bifrost code was used to calculate the radiative transfer at the wavelengths corresponding to the receiver bands of ALMA. The resulting synthetic observables were then degraded to angular resolutions corresponding to ALMA observations at different receiver bands and interferometric array configurations.

Brightening events were then systematically detected in the degraded data and their signatures were compared to the signatures at the original high resolution to derive correction factors that in principle can be applied in the analysis of observational data. The statistical estimations were performed for areas with different characteristic magnetic field topology.

Title of the publication

Characterising coronal turbulence using snapshot imaging of radio bursts in 80 – 200 MHz

Journal: Astronomy and astrophysics

1st Author: Atul Mohan (Single Author)

Position: Postdoctoral Fellow

Co-authors from RoCS: None

Short summary by the author:

Solar bursts offer a unique means to study the properties of turbulence in solar atmosphere (corona). The high energy particles produced by the burst event generate bright radio emission patches along their path as they travel across the corona. These bright emission that travel to the observer suffers a lot from the scattering in the ambient corona, causing the radiation to diffuse or spread out like a droplet of ink in water. Tracking the way this diffusing happens, one can understand the properties of turbulence in the corona at various regions along which the burst particles travelled.

However, observational studies remained difficult due to the lack of high speed imaging techniques that can produce radio burst images at intervals less than a second.

In this study, using data from a new generation telescope facility, namely Murchison Widefield Array (MWA), radio burst imaging was performed at 0.5s cadence. Analysing these images, the nature of coronal turbulence was studied for the first time over a very wide range of heights across the corona.

Earlier studies had probed turbulence characteristics at heights of  700,000 km above the solar photosphere. With this study, we now have the information down to ~ 300,000 km above photosphere. The strength of turbulence is found to decrease as we move closer to the Sun. The size of the radio burst emission regions broadened due to diffusion and the extent of broadening was found to decrease as we go closer to the Sun. This was found to be due to the fact that the radio waves were affected by turbulence only within a small region close to the region where the radiation originated, and the width of this region reduces systematically as we go closer to the Sun.

Title of the publication

EMISSA - Exploring Millimeter Indicators of Solar-Stellar Activity I. The initial millimeter -- centimeter main-sequence star sample

Journal: Astronomy & Astrophysics

1st Author:  Atul Mohan

Position: Postdoctoral Fellow

Co-authors from RoCS:

  • Sven Wedemeyer
  • Sneha Pandit
  • Maryam Saberi

Short summary by the author:

Stellar atmospheres have different stratified layers, like an onion, at different heights from its surface. These layers vary in the nature of energetic phenomena and temperature. Modern interferometric arrays sensitive to radiation at millimeter to centimeter (mm - cm) wavelength ranges can explore the energetic processes and temperature of these multiple layers. In this study we first gathered the different mm - cm observations of stars nearby and studied the properties of their emission at mm - cm bands. The sample we studied had 12 stars of which majority were sun-like stars with a surface temperature ranging from 3000 - 7000 degree Celsius. Our analysis suggest that these stars, unlike the more hotter ones, show evidences for the existence of an atmospheric structure similar to our Sun, whose surface temperature is around 6000 degrees. The outer atmospheres of these stars showed a tendency to get progressively hotter with height. This is in contrast to the usual expectation that the temperature should decrease as we move away from a hot object. This is a well known phenomena in the sun, which is generally referred to as coronal and chromospheric heating. We studied how the rate of heating varied as a function of stellar surface temperatures. We infer that stars with cooler surface temperatures show a higher rate of heating across the outer atmospheric layers. However, we need dedicated mm - cm sky surveys to identify more stars to have better statistics and to robustly model this trend.

By Eyrun Thune
Published Nov. 3, 2021 3:07 PM - Last modified Nov. 3, 2021 4:06 PM