Amateur Astronomers Association of Princeton

by S. Prasad Ganti

My tribute to Dr. Eugene Parker, who passed away a few days back. He was a pioneer in the studies of the Sun. His ideas were rejected initially and later gained strength on the finding of evidence. A NASA spacecraft launched in 2018 to study the Sun was named in his honor. I wrote a blog in September 2018 about the launch.  

The Sun was an enigma for thousands of years. Considered as a center of power, it has a representation in the form of godly status in almost every mythology the world over. Although the sustainer of life in the form of supplying heat and light to us, we knew very little about the inner workings. It was not until the early decades of the last century that nuclear forces were discovered and the nuclear fusion reactions occurring in the Sun were explained. The nuclear fusion reactions are burning Hydrogen to generate Helium and giving out the light and heat which is sustaining life on Earth. Once the Sun runs out of Hydrogen, it will die in about 5 billion years. 

The second part of the story was about how the Sun is layered. It is not just one giant ball having uniform characteristics. Like Earth has different layers from the center to the Surface, and the different characteristic features on the Surface, the Sun also consists of different layers. It certainly is a very violent place to get even close to. 

The core of the Sun is the hottest at about 15 million degrees. This temperature is enough to sustain the nuclear fusion reaction. The temperature gradually reduces till the surface which is known as the photosphere. This is the place from which we receive our light and heat. The temperature there is about 5800 degrees. The outer layer of the Sun is the Corona, which is visible during a total solar eclipse. It is very hot, running into ten to twenty million degrees. It is strange that Corona, which is the outer layer, is hotter, compared to the inner photosphere. 

In addition to the light and heat, the Sun also gives out charged particles, which are a mix of electrons and protons which shoot out at high speeds. Dr. Parker’s crucial insight was that this flow of particles would follow the same dynamics as the water and gasses from a comet, which do not really form a tail, but move in a direction away from the Sun.  The calculations showed that the flow started slow near the sun and accelerated to supersonic speeds as it moved away. This is also known as the solar wind. 

We don’t feel the solar wind on the Earth because the magnetic field of the Earth deflects the wind away. The winds can be observed as northern and southern lights in the sky closer to the poles.  Without this protection, there would be no life on Earth. Sometimes the winds are very intense and have the potential to destroy electronics in satellites in space and on Earth. Earth’s magnetism plays a crucial role other than helping us navigate using compasses on the ground. 

Just using conventional physics of Maxwell’s equations (formulated by James Clerk Maxwell), Dr. Parker was able to postulate the existence of solar winds. But back in the 1950s, the scientific community was very skeptical. His paper was published after Dr. Chandrasekhar Subrahmanyan, a colleague and an Editor from University of Chicago, intervened on his behalf and overrode the objections, although he himself was skeptical  too. Earlier, Dr. Subrahmanyan helped Dr. Parker to find a job. Four years later, Dr. Parker was vindicated when Mariner 2, a NASA spacecraft en route to Venus, observed energetic particles streaming through interplanetary space. Now solar wind is a scientific fact. 

In 1972, Dr. Parker proposed that a multitude of tiny solar flares were heating the corona and causing it to be hotter than the inner layers. To confirm this and study the Sun in detail, a NASA space probe named in Dr. Parker’s honor was launched in 2018. That study being done in the Sun’s neighborhood at 4 million miles away, is still work in progress. 

My respects to Dr. Parker who has enhanced our understanding of the Sun and the hope is that his namesake which is still out there closer to the Sun will add more so.