It rains everywhere in space. While it rains on Earth in the form of water, it rains liquid methane and nitrogen on Mars. Some planets even have rain made up of everything from lava to molten metal, but rain is not limited just to moons and planets. It rains on the Sun too and there are different types of it. This new type of rain could explain why Sun's atmosphere is hotter than the surface!
We all know the importance of rain. Water vapor condensed to form liquid droplets, which, when heavy enough, fall due to gravity and conducts life cycles of the entire food chain. It is a significant component of the water cycle and the primary source of fresh water on the Earth. While rain is common on Earth, it is also common on the Sun.
The rain on Sun is called Coronal rain. It is related to coronal heating and the magnetic field of the Sun. It also plays a fundamental role in the mass cycle between the hot, tenuous corona and the cold, dense chromosphere. It was overlooked for some time but was finally detected in April of 2019. Astronomers noticed “plasma rain” pouring down over the surface of the Sun, which explained why the outer atmosphere of the Sun is much hotter than the surface of the star.
What Is Corona?
A corona is the aura of plasma surrounding the Sun and other stars. The Sun’s corona, however, extends millions of kilometers into outer space and is easily seen when you watch a total solar eclipse. It can also be observed with a coronagraph. A coronagraph is an attachment of a telescope designed to block out the direct light from a star to make the nearby object more visible, which would have been hidden under the star’s light.
The scientists at NASA observed coronal rain in a smaller, previously overlooked magnetic loop on the Sun. The rain consists of large droplets of hot plasma that fall from the Sun’s corona, down toward the star’s surface, much like water rain on the Earth. The rain was observed by the high-resolution telescopes that were mounted on NASA’s Solar Dynamics Observatory.
Dr. Ll Leping
Coronal rain occurs in post-flare loops and the non-flaring active region coronal loops. These are classified into two categories: flare-driven and quiescent coronal rain—both of these form along with the structures that are magnetically closed. Recently, Dr Ll Leping from the National Astronomical Observatories from the Chinese Academy of Science (NAOC) and his research team found a new type of coronal rain forming along with open magnetic structures. Ll said, “Coronal condensation and rain are a crucial part of the mass cycle between the corona and chromosphere; in some cases, condensation and subsequent rain originate in the magnetic dips formed during magnetic reconnection. This provides a new and alternative formation mechanism for coronal rain.”
The researchers said that a new formation mechanism for coronal rain on the open magnetic structures is due to interchanged magnetic reconnection between open and closed magnetic systems. In this type of formation, the higher-lying open structures reconnect with the lower closed loops. As a result, a magnetic dip is formed. This dip is surrounded by plasma, converges into the dip, and enhances the density of the plasma in the dip. The cooling and condensation of the plasma is triggered by the thermal instability caused due to the density of the plasma. It is the cool condensation that falls on the solar surface as coronal rain. Since there was no flare detected during the reconnection and condensation process, the new type of rain belongs to the category of quiescent coronal rain.
Limb Of A Planet
Dr Ll says, “The quiescent coronal rain forming along the open structures is quite different from the flare-driven loops and the quiescent coronal rain in non-flaring active region loops that occur in the closed loops.” These reconnections and condensations that play an essential role in this rain process were investigated before took place above the limb.
The limb of a planet or a star or satellite is the object’s edge as seen against the dark sky background. Thus, collar ‘limb darkening’ refers to the fact that the Sun is way less bright towards its visible edge than towards its center. Its understanding allowed the early solar astronomers to construct models using this color difference. Prof. Hardi Peter, the co-author of the study, said that “whether the condensation facilitated by reconnection can still be observed on the disk, and how it performs, are open questions”. The researchers also found reconnection condensation events from September of 2010-11. They observed that above the eastern limb of the Solar Terrestrial Relations Observatory occurred on the disk of the Solar Dynamics Observatory.
The Hot Sun
Ll said, “If only the on-disc observations were available, the relation between the condensations and reconnections, shown clearly by the off-limb observations, could not have been identified. Thus, we suggest that some on-disc condensation events seen in the transition region and chromospheric lines may be facilitated by reconnection.”
While we have not been able to find out much about the Sun, this piece of information could prove fruitful for future studies. We have not been able to get close to the Sun due to its intense heat, and all we can find, is from observation only. To pinpoint it accuracy will always be a matter of debate.