The Puzzle of Solar Corona

Sun is our energizing star and we have been studying it for a long time. Yet, it has many associated puzzles that are unexplained. The outermost atmosphere of the sun expands into the interplanetary space. The corona has been most easily seen during a total solar eclipse, it is when the moon is directly between the Earth and Sun which blocks the bright surface of the Sun.

The Puzzle

The corona is the outer atmosphere of the Sun. It extends many thousands of kilometers (miles) above the visible "surface" of the Sun, gradually transforming into the solar wind that flows outward through our solar system.

The material in the corona is extremely hot but very tenuous plasma. The temperature in the corona is more than a million degrees, surprisingly much hotter than the temperature at the Sun's surface which is around 5,500° C (9,940° F or 5,780 kelvins). The pressure and density in the corona is much, much lower than in Earth's atmosphere.

The corona is above the Sun's lower atmosphere, which is called the chromosphere. A relatively narrow area called the transition region separates the corona from the chromosphere. Temperatures rise sharply in the transition region, from thousands of degrees in the chromosphere to more than a million degrees in the corona. The density of plasma falls rapidly through the transition region moving upward from the chromosphere to the corona.

We normally cannot see the solar atmosphere, including the corona. The surface of the Sun is far too bright to allow a glimpse of the much fainter corona. During a total solar eclipse the wispy corona briefly comes into view as the Moon blocks out the solar surface. A special instrument called a coronagraph allows astronomers to view the corona at other times. Some coronagraphs are used with ground-based telescopes; others are carried on satellites.

Coronal Holes and the Solar Wind

The red-orange images superposed on the central disk are corresponding X-ray images taken of the Sun on the same days as the corona images. The dark regions in the X-ray images correspond to coronal holes. The coronal holes also are in evidence in the corona as regions with little bright structure, such as near the north solar pole (top).

The solar wind, the coronal holes are regions where the magnetic field lines of the Sun are open, allowing coronal gas to flow outward into space and producing the solar wind. In contrast, the capped looking streamers in the corona (helmet streamers) correspond to regions in which the magnetic field lines are closed in a loop, trapping the coronal gas and leading to enhanced X-ray emission because of the increased gas densities.

The preceding image (in false color) was taken with the Large Angle and Spectrometric Coronagraph on board the Solar and Heliospheric Observatory (SOHO). It shows rather graphically the streamers in the solar corona.

The Energy of the Corona

It is clear that the corona is very hot because of the electromagnetic radiation that it emits. We observe emission lines from the corona corresponding to very highly ionized atoms (for example, iron atoms in the +16 charge state). Such highly ionized atoms can only be produced at temperatures in the million degree range. The extremely high temperature of the corona is thought to be associated with effects of the solar magnetic field, which can store and transport energy from lower regions of the Sun to the corona.

Why is the Corona so Hot?

The corona’s high temperatures are a bit of a mystery. Imagine that you’re sitting next to a campfire. It’s nice and warm. But when you walk away from the fire, you feel cooler. This is the opposite of what seems to happen on the Sun. Astronomers have been trying to solve this mystery for a long time. The corona is in the outer layer of the Sun’s atmosphere—far from its surface. Yet the corona is hundreds of times hotter than the Sun’s surface.

A NASA mission called IRIS may have provided one possible answer. The mission discovered packets of very hot material called "heat bombs" that travel from the Sun into the corona. In the corona, the heat bombs explode and release their energy as heat. But astronomers think that this is only one of many ways in which the corona is heated.

India’s first solar mission, Aditya-L1 satellite will aim to measure the solar coronal magnetic fields regularly. This will help understand the spectacular solar eruptions and predictions of space weather and many more things.

Connecting the Article

Question for Prelims

The extremely high temperature of the corona is thought to be associated with the effects of

(a) Solar magnetic field
(b) Solar wind
(c) Nuclear fission
(d) None of the above

Question for Mains

Discuss the significance of India’s first solar mission, Aditya-L1 and its mission objectives.