About the Sun

• The Sun can be split into two regions:
• The interior is a sphere with radius R = 7x108m. There are three main parts to the Sun’s interior:

1. 1. 1. The core is at the center. It the hottest region, where the nuclear fusion reactions that power the Sun occur
      2. The radiative (or radiation) zone – Its name is derived from the way energy is carried outward through this layer, carried by photons as thermal radiation.
      3. The convective (or convection) zone – It is also named after the dominant mode of energy flow in this layer; heat moves upward via roiling convection.

• The atmosphere lies on top and has the following layers (from innermost to outermost):

1. The photosphere is about 300 km thick. Most of the Sun’s visible light that we see originates from this region. It is the boundary between the Sun’s interior and the solar atmosphere.
2. The chromosphere is about 2000 km thick. We only see this layer and the other outer layers during an eclipse. It is the lower region of the solar atmosphere.
3. The corona extends outwards for more than a solar radius. It is the upper region of the solar atmosphere. The upper corona gradually turns into the solar wind, a flow of plasma that moves outward through our solar system into interstellar space. The solar wind is, in a sense, just an extension of the Sun’s atmosphere that engulfs all of the planets. 

The puzzle of solar atmosphere

• The temperature at the core of the Sun is nearly 15 million degrees Celsius, while that at its surface layer, photosphere, is merely 5,700 degrees C. 
• The natural thing to expect is that still further outwards, in its atmosphere, known as the corona, the temperatures would be comparable to that at the surface (photosphere). 
• However, the temperature of the corona is much higher. 
• It starts increasing outside the photosphere, reaching a value of about one million degrees or more in the corona.

Eruptions of the Sun

• Another puzzle is a set of questions pertaining to the mechanisms of eruptions of the Sun, such as solar flares and coronal mass ejections. 
• These are driven by magnetic reconnections happening in the Sun’s corona. 
• Solar flares are a sudden explosion of energy caused by tangling, crossing or reorganizing of magnetic field lines near sunspots (darker and cooler spots).
• Coronal Mass Ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun’s corona.
• Magnetic reconnection is a process where oppositely polarity magnetic field lines connect and some of the magnetic energy is converted to heat energy and also kinetic energy which leads to the generation of heating, solar flares, solar jets, etc.

Why in News?

A significant advancement has been made by an international team of solar physicists who has measured the global magnetic field of the Sun’s corona, or outer atmosphere, for the very first time.

News in Detail

• Since popular attempts to explain these solar puzzles invoke the magnetic field of the corona, the present work will help understand and verify these theories better.
• The team used the improved measurements of the Coronal Multi-channel Polarimeter (CoMP) and advanced data analysis to measure the coronal magnetic field. 
• CoMP is an instrument operated by the High Altitude Observatory of the U.S. at Mauna Loa Solar Observatory, Hawaii.
• It is very important to measure the corneal magnetic fields regularly since the solar corona is highly dynamic and varies within seconds to a minute time scale. 
• While photospheric magnetic fields are measured regularly from space, the measurement of global coronal magnetic fields was missing in the past since the coronal magnetic fields are very weak.

References:

1. https://www.thehindu.com/sci-tech/science/global-magnetic-field-of-suns-atmosphere-measured-for-the-first-time/article32295461.ece
2. https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect8/lecture8.html
3. https://socratic.org/questions/what-is-the-middle-layer-of-the-sun-s-atmosphere
4. https://scied.ucar.edu/sun-regions
5. https://www.nasa.gov/content/goddard/the-difference-between-flares-and-cmes