The “little fountain” on the sun may be the culprit in the corona’s high temperatures.

When we bake a fire, the farther away from the source of the fire, the lower the temperature. If the core of the sun is seen as a source of fire, does this rule be equally satisfied? Since the sun’s energy comes from the nuclear fusion reaction in its inner core region, the further away from the sun’s core, the lower the temperature, according to the second law of thermodynamics. In fact, from the sun’s core to the sun’s surface (the sphere of light), the temperature dropped from about 15 million degrees Celsius to about 5,500 degrees Celsius, which does meet this law.

However, as the temperature moves out of the sphere of light, the temperature is abnormally high, and the coronal layer (the outermost atmosphere of the sun) is even as high as a million degrees Celsius.

How is the coronal high temperature generated and maintained? This is the question of coronal heating, one of the long-standing challenges in the fields of solar and space physics. In 2012, the issue of coronal heating was selected by Science magazine as one of the eight unsolved mysteries of contemporary astronomy.

Recently, a team led by Tian Qi, a professor at Peking University’s School of Earth and Space Sciences, and his international partners published a study in the journal Science, providing a new window into the mystery of the coronal heat.

“Little Fountain” offers clues to crack the puzzle

First, the corona was discovered during a total solar eclipse. The coronaist brightness is about one millionth the surface of the sun, and when the moon completely blocks the sun’s disk, weak coronal radiation is visible.

In the middle of the last century, high ions were found in the corona, and the temperature of the corona was inferred to be as high as one million degrees, more than two more than the temperature on the sun’s surface.

When people take pictures of the sun’s color sphere, it is common to see that the sun’s edges have a lot of burr-like jets, needles between the photosphere and the corona. These needles, which are usually only about 200 km wide (about 700,000 km radius of the sun), are intermittently ejected from the sun’s surface into the corona. At any one time, there are about a million needles on the surface of the sun, Tian told Science and Technology Daily.

“The needle moves outwards like a fountain, so its trajectory is slender. The needle appears to be dark in the H-alpha image because the hydrogen atom H-alpha spectrum radiation emitted by the background material below is absorbed by the needle as it transmits outwards. “Tian said the needle was formed when the magnetic relink accelerates the throwing of material located in the lower atmosphere (color sphere). “The substances thrown include substances with a temperature of about 10,000 degrees Celsius, such as neutral atoms, electrons and ions. “

In 2014, Tian and others published a paper in the journal Science, based on observations from the Interface Layer Imaging Spectrometer Satellite (IRIS), that a large proportion of the needles were heated to at least 100,000 degrees Celsius. In addition, some observations have shown that some needles may be heated to a magnitude of one million degrees Celsius. “These studies show that needles play a very important role in the material and energy supply of the corona, and understanding its generation and transmission processes is key to solving the problem of coronal heating. Tian said.

However, there is no understanding of the mechanism by which needles are produced. Tian told reporters that many scholars have proposed a variety of theoretical models of needle-like material production, the core physical processes of these models include slow shock waves, alfin wave, the interaction between neutral gas and ionizing gas, the distortion of the flaky magnetic field structure, the vortex motion, the magnetic reconnection between the magnetic field structure in the opposite direction, and so on.

However, these claims are hardly universally shared by the heliophysical community. This is mainly due to the lack of direct observational evidence to substantiate. Limited by the resolution and sensitivity of telescopes in the past, it is extremely difficult to observe the production of needles.

Large-calibre solar telescopes are a great success.

Tian and his postdoctoral fellow, Tanmoy Samanta, and others, worked with the Great Bear Lake Observatory in the United States to observe the mechanism and heating of needles in the sun’s tranquil ity , except for the sun spots and their surrounding speckle.

Using the hydrogen atom H-alpha spectrum, the team conducted long-term (about 3.5 seconds) and high spatial resolution (about 45 kilometers) of imaging observations of the needle. By measuring the polarization profile of the 1.56 micron spectrum of the iron atom, the team obtained high-quality data on the evolution of the magnetic field in the depths of the light sphere, and the spatial resolution of the magnetic map reached about 150 kilometers.

After analyzing the data in detail, they found that the interaction between the different polar magnetic field structures was closely related to the production of needles. These needles are usually produced near the strong magnetic field area (called a network tissue) at the boundary of a convective unit on the sun. Needles are usually produced when small-scale weak magnetic field structures of opposite polarity appear near network tissue. Some of the opposite polar magnetic field structures gradually smaller and eventually disappear as they approach the magnetic field of the network tissue, observing the accompanying needle activity in the process.

“These observations provide strong support for the view of magnetic reconnected drive needles. Tian said magnetic recoupling is a physical process in which the magnetic field topology in the plasma changes, causing the energy of the magnetic field to release heating and accelerating matter.

The solar sun is a common process of small-scale magnetic currents (i.e. magnetic field structures floating from the sun’s interior into the sun’s atmosphere). “Magnetic reconnection can occur when these newly emerging small-scale magnetic field structures are close to the network tissue of a strong magnetic field, and the magnetic field polarity on the two contact surfaces is opposite. Tian said.

Magnetic relink accelerates the throwing of material located in the lower atmosphere to form a needle. This is in contrast to the two most popular needle-producing mechanisms (the interaction between magnetic fluid shock waves, neutrality and ionizing components) today. This image is also different from the image described in the numerical models of several existing magnetically reconnected drive needles.

The Atmospheric Imaging Telescope (AIA), which is on board the U.S. Solar Dynamics Observatory (SDO), also observed the observation area of the Gowdy Sun Telescope. The data show that the upper end of the needle has an enhanced 171-inches of radiation (mainly from Fe8-plus ions, produced in an environment of about 1 million degrees Celsius), indicating that the needle is heated to a million-degree magnitude during propagation.

In the past, a few observations of the sun’s edge and the sun’s active region (the area around the sun) have shown that the jet stream of the sun’s lower atmosphere causes the solar corona to heat up. Observations of the most common quiet areas on the sun surface show that it is a very common phenomenon for needles to be heated to coronal temperatures.

Study ideas for re-sorting the causes of coronal high temperature

“The heating mechanism of needles during outward transmission is still unclear and requires further study in the future. The possible mechanisms include the dissipation of plasma waves, the dissipation of current, the effects of turbulence, and so on.

Experts say the results re-examine the idea of coronal heating. “In the past, people used to look only for heating clues in coronal observations, and theoretical studies have mostly explored physical processes in the corona. This result shows that coronal heating is closely related to magnetic activity in the sun’s lower atmosphere, and to demystify coronal heating, attention must be paid to the process of energy and matter transmitting from the lower atmosphere, i.e. the coupling between the atmospheres of the sun.” Tian told Science and Technology Daily.

The results of this study will promote the study of coronal heating and magnetic recombination related theoretical and numerical simulations. The sun’s lower atmosphere is partially ionized and has a large number of neutral gases, and the characteristics of magnetic reconnection in this environment are different from those in a completely ionized environment, which still needs further study.

The corona’s high temperatures are a direct cause of the formation of the solar wind, which is flooded with regions among the planets and is arguably the basic medium in the solar system, Tian said. “If the corona is not so hot, then the sun will not emit solar wind, interplanetary space is basically a vacuum. So it is important to understand the cause of the coronal high temperature formation. “

In addition, understanding the high temperature of the corona has implications for our understanding of other similar phenomena in the universe. “Many stars, like the sun, have coronas that are much hotr than their surfaces. There may also be high-temperature coronal layers around the black hole accretion disk. Our observations also provide a reference for understanding their causes. Tian explained.

Tian admitted that the breakthrough in this study was due to the collaborative observation of different levels (different temperatures) in the sun’s atmosphere by the ground and space telescopes. “In the next three years, satellites such as our Advanced Space-based Solar Observatory (ASO-S), Solar Orbiter in Europe, and Aditya-L1 in India will be launched, and these large devices will be launched in multiple electromagnetic bands to make high-resolution and highly sensitive observations of the sun’s atmosphere. This will help us to better understand the relationship between coronal heating and the magnetic activity of the lower atmosphere. “

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