5-May-2022: Scientists develop a new model for inferring density inhomogeneity in the solar corona

Scientists have developed a new theoretical model to quantify the inhomogeneity in density of solar corona generated by the turbulence in the electrically conducting, magnetized fluid present as plasma in it.

The solar corona is an extremely dynamic medium. It is well established from the last few decades that electrically conducting, magnetized fluid present as plasma which causes magnetohydrodynamic (MHD) wave-driven turbulence, plays an important role in the heating of the solar corona by millions of degrees.

The heating is a factor of a thousand times higher than the solar surface. The exact reason behind heating of the solar corona to such a high temperature is still an open question and is known as the ‘coronal heating problem’ in the solar physics community.

Turbulent motion in a fluid is characterized by chaotic changes in pressure and flow velocity. But, due to the extremely hot temperature in the solar corona, the material in the solar corona becomes like a soup of charged particles, which is called plasma. Propagation of the MHD waves in a plasma medium can cause turbulence. When the MHD-wave propagates through the medium, it carries energy, and the amount of energy depends on the density inhomogeneity of the medium.

Scientists from Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institute under the Department of Science & Technology, Government of India, have developed a new approach to estimate the amount of density inhomogeneity in the solar atmosphere, which can be quantified by the density filling factor (fraction of the volume occupied by the over dense regions with respect to the entire volume of the medium). This method involves numerical simulation. 

This model, Developed by Dr. Samrat Sen, a post-doctoral researcher at KU Leuven, Belgium, and Dr. Vaibhav Pant, published in The Astrophysical Journal, talks about two different methods to calculate the density filling factor. One of the methods involves calculation of the density filling factor from the area measurement obtained from the simulated data. Whereas, in the second method, forward modelling (use of specific model to produce an outcome according to the user interests) is used for the spectroscopic measurement to estimate the density filling factor.

The researchers have found that the theoretically estimated values are in good agreement with the observed values in the solar corona. According to the study, MHD wave-driven turbulence increases the filling factor of the over dense structures, and the medium becomes more density homogeneous.

11-Mar-2022: A simple image-processing technique to unravel the dynamics of Solar Corona can help detect Coronal Mass Ejections better

Indian researchers have developed a simple technique of separating the constant background of the Solar Colona and revealing the dynamic corona.

The simple approach of subtracting the constant background can improve efficiency of identification of Coronal Mass Ejections (CME) -- events in which a large cloud of energetic and highly magnetized plasma erupts from the solar corona into space, causing radio and magnetic disturbances on the earth. It can also give a clear picture of the characteristics of CMEs and make their study easier.

Coronal mass ejections (CMEs) are dynamic structures in the Solar Corona and are capable of driving the Space Weather in near-Earth space. It becomes imperative to separate such structures and visually or automatically identify the CMEs through the radial distances in the images taken using coronagraphs.

The density of the outermost layer of the atmosphere of the Sun – Corona – decreases with distance radially outwards. As the intensity of the corona observed in white light depends on the density of particles in the atmosphere, it decreases exponentially. If the contrast between the constant corona and transient CMEs is not high, detection of CMEs becomes a challenge.

A new method developed by Mr. Ritesh Patel, Dr. Vaibhav Pant, and Prof. Dipankar Banerjee from Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, along with Satabdwa Majumdar from the Indian Institute of Astrophysics (IIA), Bengaluru, autonomous institutes under DST, Government of India, called the Simple Radial Gradient Filter (SiRGraF), is capable of separating the background revealing the dynamic corona. This research has been accepted for publication in the Solar Physics journal.

This method, which subtracts the constant background, brings out the transient corona, followed by dividing the result by an azimuthally uniform background to reduce the radial decrease in intensity. A combination of these two steps allows us to identify the structures such as CMEs throughout the field of view of the coronagraph images.

An example of the application of SiRGraF on a white-light coronagraph image of STEREO/COR-1A is shown in Figure 1. The images on the left and right panels are before and after processing by the proposed algorithm, respectively. An interesting aspect of this algorithm is that when a bulk of such images are to be processed, it will take only a few seconds for completion while also maintaining the image quality for scientific analysis.

7-Mar-2022: Science behind jets of plasma occurring all over Sun’s chromosphere unravelled

Scientists have unravelled the science behind the jets of plasma - the fourth state of matter consisting of electrically charged particles that occur just about everywhere in the sun’s chromosphere, which is the atmospheric layer just above the Sun's visible surface.

These jets, or spicules, appear as thin grass-like plasma structures that constantly shoot up from the surface and are then brought down by gravity. The amount of energy and momentum that these spicules can carry is of fundamental interest in solar and plasma astrophysics. The processes by which plasma is supplied to the solar wind, and the solar atmosphere is heated to a million degrees Celsius, still remain a puzzle.

Led by astronomers at the Indian Institute of Astrophysics, an autonomous institute of the Department of Science & Technology (DST), Govt. of India a team of interdisciplinary researchers from India and UK have explained the origin of ‘spicules’ on the Sun, using laboratory experiments as an analogy. They found that the physics underlying paint jets when excited on a speaker is analogous to the solar plasma jets.

In trying to explore the underlying physics of spicule dynamics, the team turned to an an audio speaker. A bass speaker responds to excitation at low frequencies like the rumbling sounds heard in movies. When a liquid is placed above such a speaker and the music is turned on , the free surface of the liquid becomes unstable beyond a particular frequency and starts vibrating. A beautiful example of “Faraday excitation” observed in nature is when droplets of water splashes on the back of a partially submerged male alligator during mating display. However, a fluid like paint or shampoo will result in unbroken jets when excited on a speaker since its long polymer chains give it directionality. 

The authors of the article realized that the physics underlying these paint jets must be analogous to the solar plasma jets. They then asked what it would take to generate such jets in plasma? Sahel Dey, from the Indian Institute of Astrophysics (IIA), and the first author of the study explained: “The solar plasma can be imagined as threaded by magnetic field lines, much like the long chains in polymer solutions. This makes both the systems anisotropic, with properties varying with the direction in space.” Mathematically too, there exists an analogy in the treatment of stresses involved, though there are obvious differences as well.

“Spurred by the visual similarity between the solar spicules and the jets of paint on the speaker, we investigated the roles of magnetic field on the Sun using state-of-the-art numerical simulations of the solar plasma. In parallel, we explored the role of polymer chains by using slow motion videography on Faraday waves in polymeric solutions.” elaborated, Murthy O. V. S. N., co-author from the Azim Premji University where the laboratory experiments were conducted. They found that the jets are kept intact against instabilities by the magnetic field in the Sun, and by the polymer chains in the polymeric solution respectively. The research has been published on 3rd March 2022 in the journal ‘Nature Physics’.

The scientists elaborated that the plasma right below the visible solar surface (photosphere) is perpetually in a state of convection, much like boiling water in a vessel heated at the bottom. This is ultimately powered by the nuclear energy released in the hot-dense core. The convection serves almost periodic but strong kicks to the plasma in the solar chromosphere, the shallow semi-transparent layer right above the visible solar disk. The chromosphere is 500 times lighter than the plasma in the photosphere. Therefore, these strong kicks from the bottom, not unlike alligator bellowing, shoot the chromospheric plasma outward at ultrasonic speeds in the form of thin columns or spicules.

Spicules come in all sizes and speeds. The existing consensus in the solar community has been that the physics behind the short spicules is different from that of taller and faster spicules.

The study challenges this widespread belief to show that solar convection can by itself form all kinds of jets - short as well as tall. “The simulations were able to reproduce a forest of jets because they explored a more realistic range of parameters than earlier studies,” summarised Piyali Chatterjee, the corresponding author and lead investigator from IIA.  The team members used three different supercomputers, all from India, including a National Supercomputing Mission facility at JNCASR (Bengaluru) to run their massively parallel scientific code.

Professor Annapurni Subramaniam, Director of IIA said, “This novel coming together of solar astronomers and condensed matter experimentalists was able to reveal the underlying cause of the poorly understood solar spicules. The power of unifying physics that connects physically disparate phenomena will prove to be the driving force of much more interdisciplinary collaboration.”