Sciences and Exploration Directorate

Brief Bio


Atul Mohan did his PhD under Dr. Divya Oberoi from the National Centre for Radio Astrophysics - Tata Institute of Fundamental Research (NCRA-TIFR). His thesis titled "Metrewave High Dynamic Range Snapshot Spectroscopic Imaging Studies of Weak Coronal Events". Thesis explored weak radio bursts in the solar corona using snapshot spectroscopic radio imaging facilitated by modern interferometric arrays, multi-waveband observations and modelling. He also worked on building tools and techniques for automated calibration and imaging of large interferometric data from the Murchison Widefield Array (MWA). Tools and techniques to extract science relevant information from large volume of image datacubes were also developed. He did his first postdoc at Rosseland Centre for Solar Physics (RoCS), University of Oslo under Dr. Sven Wedemeyer. He worked with millimeter image datasets from ALMA during this period. HE explored the stellar and solar datasets from ALMA to study the chromospheric heating and activity across the main sequence. Searching across ALMA observation meta-database and cross matching with other observation archives/stellar database they presented a catalogue of Millimeter detected sample of main sequence stars along with the respective spectral energy distribution from radio - Xray band and other physical and spectroscopic properties enlisted across various databases. Much of the science during this period was done analysing this database. He also guided a Master student who looked into various astronomical survey datasets across in sub-THz band and revealed the spectral structure as a function of spectral types and stellar physical properties. Analysing these datasets and exploring them across various physical parameter axes for the stars, helped understand stellar activity evolution across the main sequence. Currently Atul is working as a NASA-PHaSER fellow in the STEREO working group under Dr. Natchimuthuk Gopalswamy.

Research Interests


Weak ubiquitous energy release events in Solar corona

Heliophysics: Solar Atmosphere

My PhD thesis was focused on understanding the ubiquitous and frequent weak energy release events across the solar corona. These events collectively contribute to the steady heating and particle acceleration in solar atmosphere. With modern radio imaging facilities like MWA (Murchison Widefield Array), LoFAR etc., high spectro-temporal resolution (<0.5s , ~10 kHz) imaging studies of solar corona at high imaging dynamic range and fidelity is now possible. Exploiting this new and exciting window of exploration, during my PhD, I had explored weak activity across solar corona specifically focussing on active regions during periods when no or very weak flares (energy <~ microflare (GOES class B or less) were reported. Spectroscopic radio imaging offers a unique means to explore the cross coronal propagation of accelerated particle beams in the corona and with sub-second cadence imaging capability we can also explore their temporal variability at a cadence finer than possible ever before. Combining with lesding space based instrumentation by various NASA/ESA mission and models we can now explore the full thermal/ non-thermal and accelerated particle dynamics across solar corona at fine spatio-temporal scales.


Solar chromspheric heating: chromosphere - corona link

Heliophysics: Solar Atmosphere

With the advent of ALMA (Atacama LArge Millimeter/sub-Millimeter Array) high resolution (~1 -10") mm imaging of solar chromospheres at ~s cadence have become a reality. Millimeter spectroscopic imaging helps probe the multi-thermal chromospheric layers. Thus accompanied by high cadence imaging we can now probe the dynamics of hot plasma and shocks across various chromospheric structures. Combined with simultaneous data from radio band and spectroscopy using TRACE and IRIS spectral lines that probe transition region and lower corona, we can now explore the energy transfer - particularly thermal flux since mm-continuum is sensitive to electron thermal emission - across chromosphere to corona.


Robust automation strategies in big data analysis: from calibration to identifying science relevant information

Heliophysics: Analysis

During my PhD I developed algorithms and techniques to calibrate and image raw radio interferometric data from modern telescopes. An automated imaging pipeline was developed and new techniques were implemented to identify and extract science relevant information from large multi-dimensional image data cubes. I'm trained in analysing radio intereferometric data specifically, but would like to extend the algorithms tools and techniques to built to analyse large data sets to newer database. I had developed Python based routines to query metadata of various database and cross match information to identify science relevant datasets and accumulate relevant data from various sources. I also like to explore data analysis tools to automatise infromation extraction on large datasets using Python and CASA (image analysis package written in python which now can be called as a module) based toolkits. A major hurdle here is often the robustness in the identified features and patterns which requires training over multiple sets and coming up with optimal parameters. Some of these tools have already been used in my works like Gaussian mixture decomposition, automatic source identification and model fitting etc. Such tools applied across mutliwaveband datasets will help identify complex patterns and multi-scale dynamics across solar atmosphere.


Stellar activity evolution across main-sequence: exo-space weather

Astrophysics: Stars

Stellar activity is a major determinant of exo-planet habitability. Millimeter to radio band observables offer a unique means to perform tomographic exploration of stellar active atmospheres from chromosphere to corona. Millimeter band specifically explore the thermal emission from chromospheric layers providing us a direct way to infer thermal stratification in stellar atmospheres where by quantifying the chromopsheric heating gradients across main sequence. Due to this unique property, with modern sensitive mm-telescopes we can now explore the origin and characteristics of chromospheric heating across the main sequence as a function of stellar physical type and age.This is a nascent field since in the pre ALMA/NOEMA era there existed no facility with required sensitivity to perform such studies for even the near neighborhood stars. Meanwhile modern instruments in the radio band have already started providing radio dynamic spectra for nearby stars which has lead to discoveries of star-planet interaction and super flares from strongly magnetised active regions in nearby stars. Radio spectrum from a few to 10 GHz band can also provide information on stellar wind bremsstrahlung emission. Combined with thermal and non-thermal spectral line diagnostics and continuum X-ray data we can better constrain stellar atmospheric models and physics of flaring phenomena across stellar type. Eventually these efforts will help define useful multi-waveband metrics to identify habitable exo-space weather conditions.

Current Projects


NASA-PHaSER researcher

Space Weather

I'm currently working with the STEREO working group under Dr. Nat Gopalswamy. I'm exploring the nature of radio bursts associated with CMEs and their links to SEPs and heliospheric shocks.

Positions/Employment


Postdoctoral Researcher

Rosseland Centre for Solar Physics, University of Oslo (RoCS, UiO) - Oslo, Norway

February 2020 - February 2023

I was the lead postdoctoral researcher working for the project "Exploring Millimetre Indicators of Solar/Stellar Activity" (PI: Dr. Sven Wedemeyer, RoCS). I explored the radio - Xray spectral energy distribution (SED) of the sun-as-a-star and other cool main-sequence stars using primarily archival data. Tools and techniques were built to cross match various archival multi waveband datasets and retrieve the stellar SEDs, which was then modelled using existing atmospheric emission models. Our analysis lead to the characterisation of solar/stellar atmospheric activity across the main sequence. I derived robust metrics of atmospheric heating and activity for sun-like stars based on mm observables facilitated my new generation telescopes. We demonstrated that mm-based activity indicators directly probe the chromospheric heating gradients in stellar atmospheres and provide a robust quantitative characterisation of stellar activity than traditional ones based on IR - Xray band spectral line and continuum observables.

Teaching Experience


07/2022 - 06/2023: Master thesis "Searching for stellar emission in the sub-THz bands."

Student: Mats Kirkaune, RoCS, University of Oslo

11/2022: Lecture on “Stellar activity and impact on exoplanets"

Platform: Norwegian research school for Dynamics and Evolution of Earth and Planets

Education


2014-2019: PhD, National Centre for Radio Astrophysics - Tata Institute of Fundamental Research (NCRA-TIFR), Pune, Maharashtra, India.


2009-2014: BS-MS, (Physics Major, Mathematics Minor), Indian Institute of Science Education and

Research, Trivandrum, Kerala, India.

Professional Societies


IAU

Junior member

2023 - Present

Professional Service


2021 - 2023: Reviewed 40 observation proposals for ALMA and 2 for uGMRT.

2022: Reviewed manuscript for AAS.

Awards


12/2018: Max Hammond Student Travel Grant Award, awarded to the student with a high ranking scientific

abstract among the AGU applicants in Space Physics and Aeronomy section.

11/2010: CNR Rao Foundation prize, awarded to the outstanding student in the 1st semester of BS-MS

Programme.

Other Professional Information


I have successfully acquired observing time across various competitive multi-waveband instruments. These have been targeted at studying stellar flares and activity.


Accepted proposals as PI:

AstroSat Xray - NUV A11_067 Accepted: 2021

uGMRT Meterwave 41 051 Accepted: 2021

Nordic Optical Telescope Optical 64-405 Accepted: 2021

MeerKAT Meterwave MKT-22-093.S Accepted: 2022

NOEMA Millimeter W22BF Accepted: 2022

Nordic Optical Telescope Optical 66-408 Accepted: 2022

Special Experience


I developed first successful software pipeline to automatically calibrate and image large interferometric dataset from the Murchison Widefield Array leading to the first sub-second spectroscopic images in metrewaveband to reveal sub-second scale dynamics in the quiet sun disk.

Publications


Refereed

Oberoi, D., S. K. Bisoi, K. S. Raja, et al. D. Kansabanik, A. Mohan, S. Mondal, and R. Sharma. 2023. Preparing for solar and heliospheric science with the SKAO: An Indian perspective Journal of Astrophysics and Astronomy 44 (1): 40 [10.1007/s12036-023-09917-z]

Mohan, A., S. Wedemeyer, P. H. Hauschildt, S. Pandit, and M. Saberi. 2022. EMISSA (Exploring millimetre indicators of solar-stellar activity) Astronomy &amp; Astrophysics 664 L9 [10.1051/0004-6361/202244385]

Henriques, V. M., S. Jafarzadeh, J. C. Guevara Gómez, et al. H. Eklund, S. Wedemeyer, M. Szydlarski, S. V. Haugan, and A. Mohan. 2022. The Solar ALMA Science Archive (SALSA) Astronomy &amp; Astrophysics 659 A31 [10.1051/0004-6361/202142291]

Mohan, A., S. Wedemeyer, S. Pandit, M. Saberi, and P. H. Hauschildt. 2021. EMISSA (Exploring Millimeter Indicators of Solar-Stellar Activity) Astronomy &amp; Astrophysics 655 A113 [10.1051/0004-6361/202142095]

Mohan, A. 2021. Characterising coronal turbulence using snapshot imaging of radio bursts in 80–200 MHz Astronomy &amp; Astrophysics 655 A77 [10.1051/0004-6361/202142029]

Mohan, A. 2021. Discovery of Correlated Evolution in Solar Noise Storm Source Parameters: Insights on Magnetic Field Dynamics during a Microflare The Astrophysical Journal Letters 909 (1): L1 [10.3847/2041-8213/abe70a]

Mondal, S., D. Oberoi, and A. Mohan. 2020. First Radio Evidence for Impulsive Heating Contribution to the Quiet Solar Corona The Astrophysical Journal 895 (2): L39 [10.3847/2041-8213/ab8817]

Mohan, A., P. I. McCauley, D. Oberoi, and A. Mastrano. 2019. A Weak Coronal Heating Event Associated with Periodic Particle Acceleration Episodes The Astrophysical Journal 883 (1): 45 [10.3847/1538-4357/ab3a94]

Mohan, A., S. Mondal, D. Oberoi, and C. J. Lonsdale. 2019. Evidence for Super-Alfvénic Oscillations in Solar Type III Radio Burst Sources The Astrophysical Journal 875 (2): 98 [10.3847/1538-4357/ab0ae5]

Mondal, S., A. Mohan, D. Oberoi, et al. J. S. Morgan, L. Benkevitch, C. J. Lonsdale, M. Crowley, and I. H. Cairns. 2019. Unsupervised Generation of High Dynamic Range Solar Images: A Novel Algorithm for Self-calibration of Interferometry Data The Astrophysical Journal 875 (2): 97 [10.3847/1538-4357/ab0a01]

Mohan, A., and D. Oberoi. 2017. 4D Data Cubes from Radio-Interferometric Spectroscopic Snapshot Imaging Solar Physics 292 (11): 168 [10.1007/s11207-017-1193-1]

Talks, Presentations and Posters


Invited

SOLAR/STELLAR ATMOSPHERIC TOMOGRAPHY WITH MM-RADIO SNAPSHOT SPECTROSCOPIC IMAGING

October 2022

The talk was given in Planetary, Solar, and Heliospheric Radio Emissions Conference, Dublin. It was a brief overview of the various research avenues and novel insights gained using the modern radio - mm imaging instruments that offer high spectro-temporal resolution. Link: https://pre2022.dias.ie/programme/

Other