First of all, let me clear your confusion. I am not referring to the Corona virus! Actually, there is another important meaning of the term ‘corona’, which is closely related to our cosmos. This is also known as ‘Solar Corona’ or ‘Stellar Corona’. To put it simpler words, Corona is the outer atmosphere of the Sun which consist of hot ionized gas, also known as, plasma. The plasma expands to millions of kilometers above the solar surface and eventually end up being a nomad i.e., the solar wind. Not to mention, being in the (solar) wind, it can travel freely through our entire solar system.
The outermost region of the Sun’s atmosphere consisting of plasma is called Corona. More precisely, the corona is located above the chromosphere, the Sun’s lower atmosphere. The plasma is extremely hot but very tenuous at the same time. So, it is only normal that corona’s temperature becomes around 2 million kelvins. Even though the temperature of corona is way hotter than the Sun’s surface temperature which is only around 5,780 kelvins, its density is considerably low, even lower than Earth’s atmosphere. That’s why, corona can yield very little heat and consequently, shine only half brightly as the moon. Unfortunately, this dim shinning of corona gets overpowered by the brightness of solar surface as a result of which we cannot see corona in naked eye, without any instruments.
To be more scientific, the reason for the low density of corona in such high temperature is because the constituent gas molecules get so sparse in corona that the energy content per cm3 becomes much lower than that of the interior solar region.
Anyway, this high temperature really accelerates the speed of the solar particles in corona. Then, the corona gases get expanded which only ends at the heliopause. Consequently, those accelerated kinetic particles break through the shackles of the solar gravity and travel freely like a gypsy throughout the solar system
Now, it might seem confusing how come corona has greater temperature than its surface. In fact, NASA has researched for a while to determine the exact reason and it is still mysterious to all. However, the NASA scientists have come up with a hypothesis. A NASA mission called IRIS discovered packets of extremely hot elements that travel from the Sun into the corona. They named these hot elements as ‘heat bombs’. They opine that the heat bombs explode in the corona and release their heat energy there, consequently, leading to the sudden hot temperature
However, the corona is separated from the chromosphere by a relatively narrow zone known as the transition region. Transitional temperatures range from thousands of degrees in the chromosphere to more than a million degrees in the corona. The density of plasma quickly decreases as it moves higher from the chromosphere to the corona.
The solar surface is kind of totally covered in magnetic fields. Even because of magnetism, the size and shape of corona often varies. As presented in the following schematic, the magnetic fields of corona affect the charged particles and manipulate them into shaping miscellaneous forms including streamers, loops and plumes. These strange and creative formations are actually observed by NASA scientists using special telescopes.
The solar atmosphere, including the corona, is typically invisible to us. The Sun’s surface is too luminous to leave the stage to the dimmer corona to be seen by the spectators. During a complete solar eclipse, the fragile and dim corona appears for a very short moment when the Moon blocks out all the coruscating light from the photosphere. So, finally, with the moon’s interference, we can see the actual dim and beautiful corona on a full eclipse.
However, in this 21st century, we don’t need to rely on the eclipse anymore to witness corona. In fact, a special instrument has been discovered, called coronagraph, to observe the corona at usual times under non-eclipse conditions. Some coronagraphs are used with ground-based telescopes and some are even attached to satellites.
The sun is literally everyday above our head, blessing or cursing us with its scorching fire-like heat. But if we try to think deeper, we might find these apparently simple events quite amusing. To make these simple little incidents happen, there have been occurred a thousand of sequential, scientifical and mysterious events. Sometimes in life, let’s all just take a moment to think about the mystery of universe. It would certainly mesmerize us and make us realize how little our daily hassles are. However, don’t let the exploration on corona and universe distract you from being careful about the other corona, yes, the corona virus!
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