Tanmoy Samanta
Astrophysicist,
Indian Institute of Astrophysics
Indian Institute of Astrophysics
• Chromospheric, coronal heating and solar wind • Spicules, jets, loops, small-scale energetic events • MHD waves and magneto-seismology • Solar magnetism, variability, eruptive phenomena and space weather • Solar UV and Extreme-UV imaging, spectroscopy and spectropolarimetry • Ground and space-based imaging and spectroscopic instrumentations.
The atmosphere of the Sun which is extended over several million kilometers from its surface is called the solar corona. Corona is made of with hot gas called plasma which is confined in magnetic fields. Even if the temperature in the core of the Sun does reach 14 million degrees, it drops to a mere 5700 degrees at the surface. The temperature should be even lower farther away from the Sun, but the temperature of the corona is measured at more than a million degrees. What causes this rapid increase in temperature is still one of the big mysteries. Popular theories which attempt to explain coronal heating can be broadly grouped into two categories. One possibility is that a large number of magnetic reconnection. The other theory argues that the heating is dominated by the damping of magneto-hydrodynamic (MHD) waves. It is now evident the solar atmosphere is highly structured and it is likely that various heating mechanisms operate in different atmospheric structures. Observational tests of a specific heating mechanism may be difficult because several mechanisms might operate at the same time. Theoretical estimates often predict very small spatial scales where the ultimate dissipation occurs, sometimes of the order of a few hundred meters, that even with current high spatial resolution satellite techniques cannot be resolved. Heating the solar plasma is a fundamental problem in solar physics, astrophysics and space weather forecast, which may also have industrial applications in laboratory devices. Although the details of the answer are not completely known, it does seem that the solution is near.
Spicules are dynamic, jet-like plasma structures observed in the solar chromosphere that rapidly shoot upward and often fade without returning. Despite numerous high-resolution observations from instruments like Hinode and IRIS, and advances in MHD simulations, the precise mechanism responsible for the origin and evolution of spicules remains elusive. One of the fundamental challenges is to identify what triggers the onset of Type-II spicules, which are faster, shorter-lived, and more energetic compared to their Type-I counterparts. Do they arise primarily from magnetic reconnection at small spatial scales, from magneto-acoustic shock wave interactions, or from Alfvénic turbulence in the lower solar atmosphere?
An equally pressing question is: What role do spicules play in transporting mass and energy into the corona? Spicules have been proposed as a conduit for both thermal and wave energy transfer, potentially accounting for a significant portion of the energy required for coronal heating. However, the energy flux estimates vary widely, and the dominant dissipation mechanisms (e.g., resonant absorption, phase mixing, or turbulent cascade) remain topics of ongoing investigation.
Furthermore, how do spicules evolve as they ascend into the transition region and corona? Do they carry sufficient Poynting flux to offset radiative losses? Are there diagnostic signatures (in EUV or spectropolarimetric lines) that can trace their upward energy transport unambiguously?
These questions make the generation of spicules and their coronal connection a deeply compelling problem. A better understanding would not only shed light on the dynamics of the solar atmosphere but also inform models of stellar activity across a wide range of stars.
Magnetohydrodynamic (MHD) waves permeate the solar atmosphere and are crucial for diagnosing plasma conditions and energy transport. The field of solar magneto-seismology (SMS) leverages observations of these waves to infer local plasma parameters, such as magnetic field strength, density scale heights, and temperature distributions. However, the accuracy and reliability of these diagnostics face significant challenges.
A fundamental problem in SMS is disentangling the different wave modes—kink, sausage, torsional Alfvén, and slow modes—especially in structures with fine sub-resolution structuring. What are the most robust observational signatures that distinguish these modes, particularly under realistic conditions of line-of-sight superposition and temporal resolution limits?
Furthermore, how can we accurately model wave propagation in inhomogeneous, stratified, and dynamic media such as coronal loops, spicules, or prominence threads? The presence of density gradients, transverse structuring, and magnetic twist can lead to mode coupling, phase mixing, and resonant absorption—all of which complicate wave analysis but are also potential mechanisms for wave energy dissipation.
A central question remains: To what extent do MHD waves contribute to coronal heating, and how efficiently is wave energy dissipated in different regions of the atmosphere? Can current or next-generation instruments (e.g., DKIST, Solar Orbiter) resolve the spatial and temporal scales required to observe wave damping directly?
The ultimate goal is to build a framework where observed wave properties can be inverted reliably to provide real-time diagnostics of magnetic field and plasma conditions. Doing so requires not just improved observations, but also advancements in forward modeling and statistical inversion techniques. The challenge lies in bridging the gap between idealized theory and the complex reality of the Sun’s dynamic, multi-scale atmosphere.
Solar flares are among the most violent phenomena in the solar system, involving sudden and massive energy release via magnetic reconnection in the corona. In the aftermath of these events, particularly above post-flare arcades, supra-arcade regions exhibit complex plasma dynamics and a host of poorly understood features. Among these, Supra-Arcade Downflows (SADs)—dark, tadpole-shaped voids descending into the flare arcade—pose intriguing questions.
What is the true nature of SADs? Are they retracting flux tubes, voids formed by instabilities, or the by-products of turbulent reconnection outflows? Current models differ in attributing their formation to magnetic tension release, thermal instabilities, or Rayleigh-Taylor-like effects. SADs appear to leave behind bright trails, suggesting interaction with surrounding hot plasma, but their density, temperature, and magnetic structure remain poorly constrained.
Another fascinating, yet underexplored, aspect is the possibility of vortex shedding in these regions. In classical fluid dynamics, vortex shedding occurs when flow past a blunt object becomes unstable, forming oscillatory patterns. Theoretical MHD models predict similar behavior in the turbulent current sheets of flares. But: Can vortex shedding be directly observed in the solar corona? What observational signatures (e.g., Doppler shifts, transverse displacements) would conclusively confirm their presence? And how does this turbulence impact reconnection efficiency and flare energetics?
The interplay of SADs and vortex dynamics raises further questions: Are they manifestations of the same instability spectrum or independent phenomena? Can advanced numerical simulations replicate both features simultaneously under realistic solar conditions?
Investigating these questions will help decode the physics of energy transport, plasma turbulence, and instability formation in high-temperature, low-beta astrophysical environments—key to understanding solar and stellar flares.
Coronal Mass Ejections (CMEs) are large-scale expulsions of plasma and magnetic field from the solar corona, capable of triggering severe space weather effects such as geomagnetic storms, communication disruptions, and radiation hazards to spacecraft and astronauts. Despite extensive study, predicting CME initiation, evolution, and impact remains a formidable challenge.
One key unresolved question is: What precise physical conditions lead to CME eruption? While many models point to flux rope destabilization, torus instability, or breakout reconnection as triggers, the pre-eruptive magnetic configuration and energy build-up mechanisms are still not well understood. Can we identify reliable pre-eruption signatures using multi-wavelength data and vector magnetograms?
After eruption, the CME propagates through the heliosphere, often interacting with the ambient solar wind and other CMEs. This leads to another problem: How does the internal magnetic structure of a CME evolve as it travels toward Earth? The geoeffectiveness of a CME is strongly tied to the orientation and strength of its magnetic field upon arrival. However, this internal structure is rarely observed directly and is challenging to infer from remote sensing alone.
Current models struggle with predicting arrival time and magnetic orientation at Earth with sufficient accuracy. What role can data assimilation, heliospheric imagers, and in-situ measurements (e.g., from Parker Solar Probe or Solar Orbiter) play in improving forecast capabilities?
Finally, how do CMEs couple with the magnetosphere-ionosphere system to produce specific space weather impacts? Answering these questions demands a cross-disciplinary approach that combines solar physics, heliophysics, magnetospheric science, and machine learning.
Improving our understanding of CME initiation and propagation is not only a scientific imperative but also a societal one, as human technology becomes increasingly vulnerable to solar activity.
