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Solar magnetic field

Weak magnetic field in the quiet area of the solar surface

The solar magnetic field is distributed between the sun and Interplanetary space The magnetic field can be divided into large-scale structure and small-scale structure. The former mainly refers to that the sun is universal magnetic field And the overall magnetic field. The latter is unipolar and mainly concentrated near the solar active region, and most of them are bipolar magnetic fields. sunlight Universal magnetic field It refers to the weak magnetic field in the quiet area of the solar surface. Most of the sun's material is High temperature plasma , the state of the sun motion And evolution are closely related to the magnetic field. Sunspot 、 solar flare 、 Prominence Such activity phenomenon is directly controlled by the magnetic field. Therefore, the sun magnetic field It is of great significance to make a study of.

Chinese name: Solar magnetic field
Foreign name: solar magnetic field
Basic magnetic field strength: 1 Gauss

Two origins: Fossil theory, dynamo theory
Observation principle: Zeeman effect
Distribution: Sun and interplanetary space

catalog

definition

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Distributed in the sun and Interplanetary space Magnetic field. It can be divided into large-scale structure and small-scale structure. The former mainly refers to the general magnetic field of the sun and the whole magnetic field , they are unipolar, and the latter is mainly concentrated in Solar active region Near, and most of them are bipolar magnetic fields.

The solar magnetic field extends almost vertically above the sunspot

Under the action of the solar wind, the solar magnetic field also permeates the whole planet Inter space, formation Interplanetary magnetic field 。 Its polarity is consistent with the overall magnetic field of the sun, and decreases with the distance from the sun. All kinds of solar activity phenomena are closely related to the magnetic field: before and after the flare, the magnetic field in the nearby active area has sharp changes (such as magnetic field annihilation); The magnetic field of sunspot is strongest and small sunspot About 0.1 tesla, and the big sunspot can reach 0.3~0.4 tesla or even higher. The magnetic field of the plaque is about 0.02 tesla. The formation and evolution of prominence are also governed by the magnetic field.^[1]

origin

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Fossil theory

The source of the sun's magnetic field is an unsolved problem. The existing theories can be divided into two categories: the first is that the existing magnetic It was formed billions of years ago sunlight The material left behind. Theoretical calculations show that the natural decay period of the solar universal magnetic field is up to 10 billion years, so it is possible for the magnetic field to remain for a long time.

Dynamo theory

（ Mechanism of solar average magneto current generator ）It is believed that the magnetic field of the sun is weak due to the movement of charged matter neutron The magnetic field is amplified. Since most of the sun's matter is plasma motion state , then we can use the dynamo effect to explain some problems about the origin of the solar magnetic field.

An important topic of the solar magnetic field theory is the formation mechanism of the solar cycle. The theory of differential rotation is now generally accepted. It thinks that the sun is poor rotation Make photosphere The lower horizontal magnetic force line tube is wound up, and at a certain time, it will float up to the solar surface to form a double pole sunspot 。 Due to a large number of Bipolar sunspot With the expansion and diffusion of the magnetic field, the original universal magnetic field is neutralized, and then a universal magnetic field with opposite polarity will appear. This can explain the 22 year magnetic cycle of the sun.

Research

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brief history

hale

In 1908, the United States astronomy Home Hale is waiting Mount Wilson Observatory (now called Haile Observatory ）, using Spectral line The Zeeman effect measurement of the magnetic field of sunspots was later carried out at Potsdam Observatory (1942), Crimea Astrophysics Observatory (1955) and other places.

In 1912, Haile and others began to measure the general magnetic field of the sun, but the results obtained were quite wrong

In 1953, H.D. Babcock developed Solar photoelectric magnetic imager In the following twenty years, various types of magnetic imagers have been successfully developed to observe the weak magnetic field on the solar surface, thus the local magnetic field on the solar surface, the global magnetic field of the sun and magnetic nodes have been found. While great progress has been made in the field measurement, theoretical research has also flourished, such as the structure of sunspot magnetic field, the origin of solar activity cycle, the mechanism of flare eruption, and the internal magnetic field Spectral line Important progress has been made in research on formative theory,

Observation means

measure celestial bodies The magnetic field is mainly used Spectral line Of Zeeman effect , that is, two properties of radiation in the magnetic field are used: ① Zeeman splitting or broadening of spectral lines; ② Zeeman Branch polarization 。 General use rendering Normal Zeeman effect Magnetic sensitive spectral lines; For example, FeI λ 6303. The splitting amount Δλ H of spectral line in magnetic field and magnetic field intensity H is proportional, and the corresponding formula is Δλ H=4.67 × 10 -5 gλ two H, Where g is the spectral line Lande splitting factor 。 For FeI λ 6303, g=2.5.

The sunspot is the region with the strongest magnetic field on the solar surface, and its intensity can reach three or four thousand gauss. At this time, Δλ H of FeI λ 6303 is about 10 -1 Aye. large Solar spectrograph This value can be measured accurately. The specific method is to place a quarter wave wafer (making circular polarized light become plane polarized light) and a polarizing film mesh (allowing light with different polarization directions to pass through in turn) in front of the slit of the spectrograph, so that the interlaced spectral lines can be obtained on the negative film. This makes it easy for us to directly measure Δλ H, and the magnetic field strength H can be calculated by substituting the above formula. But for the area outside the sunspot, the magnetic field is weak and Δλ H is small, so it is difficult to measure accurately. At this time, it is necessary to use a magnetic imager to modulate in a certain way to obtain two Zeeman branch lines alternately, so the radiation flow through the exit slit of the alignment line wing will constantly change. The H value can be determined according to the amplitude of this change.

The full heliospheric vector magnetic imager is the first space solar magnetic field measurement equipment in China. Its time resolution is relatively high, which can realize continuous observation of the full heliospheric vector magnetic field^[4] 。

Active area magnetic field

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Sunspot magnetic field

Generally speaking, there are two main sunspots in a sunspot group, and their magnetic polarity is opposite. If the leading sunspot is N-pole, the following sunspot is S-pole in the same hemisphere (for example Northern Hemisphere ）The distribution of magnetic polarity of each sunspot group is the same; And in the other half of the ball（ southern hemisphere ）On the contrary, at the end of one solar activity cycle (about 11 years) and the beginning of another cycle, the above magnetic polarity distribution will be reversed. Therefore, the polarity distribution of sunspot magnetic field experiences a cycle every 22 years, which is called a magnetic cycle. Strong magnetic field is the most basic feature of sunspots. The low temperature, material movement and structure model of sunspots are closely related to the magnetic field,

Flare and magnetic field

Flare is the most intense solar activity phenomenon. One big solar flare Burst can release 10 thirty ～10 thirty-three Erg's energy, which may come from the magnetic field. Once a magnetic field with an intensity of several hundred gauss in the active region is annihilated, all the magnetic energy contained in it will be released, enough to supply a large flare eruption. Before and after the flare eruption, the magnetic field in the nearby active region often changes violently. The magnetic field with complex structure has become simpler after the flare. This is the evidence of the magnetic field annihilation theory of the flare eruption.

Magnetic field of prominence

Prominence Its temperature is about 10000 degrees, but it can exist in the corona with a temperature of up to one or two million degrees for a long time, neither collapsing rapidly nor falling to the sun's surface, which is mainly due to the heat insulation and support of the magnetic lines of force. The magnetic field intensity of the quiet prominence is about 10 Gauss, and the magnetic field lines are basically parallel to the solar surface; The magnetic field of active prominence is stronger, up to 200 gauss, and the magnetic field structure is more complex.

Solar universal magnetic field

In addition to the solar active area, there is also a weak magnetic field in the quiet area of the solar surface. On the whole, the sun and earth Similarly, there is a general magnetic field. However, due to the interference of the magnetic field in the local active region, the general magnetic field of the sun is only significant in the polar region, not as complete as the earth's magnetic field. The magnetic field intensity in the solar polar region is only 1-2 gauss. The intensity of the sun's general magnetic field often changes, and even the polarity will suddenly change. This situation was observed twice from 1957 to 1958 and 1971 to 1972,

Global magnetic field of the sun

If the sun is regarded as a star and the non imaging solar beam shines into the magnetic imager, the overall magnetic field mixed from all parts of the solar surface can be measured. The intensity of this magnetic field shows regular changes, and the polarity changes from positive to negative, and from negative to positive. Roughly speaking, in every Solar rotation It is easy to explain this phenomenon by changing twice in a week (about 27 days): there is a large magnetic area with opposite polarity between east and west on the solar surface. As the sun rotates from east to west, scientists can observe the positive and negative overall magnetic fields alternately. In short, the sun has both a general magnetic field and a global magnetic field. The former is the opposite of North and South, while the latter is the confrontation between East and West,

structure

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fine

High resolution observations since the 21st century have shown that the solar magnetic field has a very complex fine structure. As far as the active region is concerned, the magnetic field intensity in the same sunspot range is often very different; And in a sunspot with a certain polarity (such as N pole) as a whole, there are often small magnetic nodes with another polarity (S pole). Therefore, strictly speaking, there is no monopole sunspot. In the transverse magnetic field map, not only the intensity is different everywhere, but also the azimuth is different. In the penumbra of sunspots, the magnetic fields of the brighter stripes and the darker regions between them are also significantly different. In the active region, the diameter of the magnetic node is about 1000 km, and the magnetic field intensity is 1000~2000 Gauss. The natural decay time of sunspot magnetic field is very long,

In the quiet area of the solar surface, it used to be thought that there was only a weak magnetic field with an intensity of about 1-10 gauss. However, new observations show that the strength of the magnetic field in the quiet area is also very uneven, and also contains many magnetic nodes. They occupy a small area on the solar surface, but they contain most of the Magnetic flux 。 Specifically, the range of magnetic nodes in the quiet area is less than 200km, and the magnetic flux they contain accounts for about 90% of the total quiet area. Due to the concentration of magnetic flux magnetic field intensity It can reach thousands of gauss, far exceeding the average magnetic field strength in a large range of quiet areas.

Sector of interplanetary magnetic field

When the magnetic field is "frozen", the particles of the solar wind run with the magnetic line of force, so the solar magnetic field will permeate the whole solar system Space. Because the magnetic line of force carried by the solar wind when the sun rotates is not a straight line, but a spiral line. In addition, there is a global magnetic field on the solar surface, and the polarity of adjacent magnetic regions is opposite. These factors work together to form the sector structure of the interplanetary magnetic field. It is closely related to the overall magnetic field of the sun, and their polarity is almost identical. Once the polarity of the overall solar magnetic field is converted, the polarity of the interplanetary magnetic field will be converted immediately,

As the solar magnetic field expands outward, its strength becomes weaker and weaker. In the outer space of the earth, the intensity of the magnetic field is less than one thousandth of Gauss. However, due to the extremely thin gas in interplanetary space, such a weak magnetic field can also Material movement Have a dominant role. Under the action of the solar wind, the geomagnetic field is compressed Magnetosphere Cannot extend outwards within the range of.

Solar internal magnetic field

Up to the 21st century Magnetic field measurement It is limited to the solar atmosphere. As for the internal magnetic field of the sun, it cannot be measured directly, but can only be estimated roughly by theoretical methods. Some people think that it may be much stronger than the atmospheric magnetic field,

Sleep state

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brief introduction

The sleep state of the solar magnetic field is the latest research of scientists, which reveals that the cycle of solar activity is prolonged, Sunspot 、 solar flare and solar storm The real reason for the gradual reduction of such phenomena. According to the US magazine Wired, the latest research by scientists shows that the flow of magnetic gas on the sun's surface will explain why the sun is in "sleep".

background

From 2008 to the first half of 2009, what puzzled scientists were sunspots, flares and solar storm The phenomenon is very rare, and it has been 11 years Solar cycle The final period was extended by 15 months. The latest observations from the Solar and Heliospheric Observatory (SOHO) show that it can better predict the intensity and duration of the solar cycle in the future.

Magnetic field weakening

Solar magnetic field

Better forecasting solar activity Sex is crucial, because the eruption of solar activity can release a large number of charged particles to the earth magnetization Material that can destroy the ground power supply network and damage communication satellites. A new research report published in Science on March 12 pointed out that David Hathaway of NASA's Marshall Space Flight Center and Lisa Rightmire of the University of Memphis analyzed the relationship between the sun and the solar wind layer detector tracking the solar equator to the poles over the past 13 years ionization Gas changes, researchers found a kind of airflow The ionized gas of the commercial flow continued to intensify its activity for many years before the start of the minimum solar activity in 2008. Research shows that 11 years ago, the solar activity gradually weakened, accompanied by meridional airflow The activity of. Neil Sheeley of the US Naval Research Laboratory said that it is likely that the current delayed activity cycle 2009-2010 was caused by the relatively weak magnetic field in 2007-2009 magnetic field intensity Is critical for the next solar cycle,

Material cycle

Halliway and Wright Mayer believe that the faster warp direction airflow This causes the solar pole to produce a weaker magnetic field, which extends the solar slow activity cycle.

Halliway said that the meridional direction airflow The influence of magnetic field changes can significantly inhibit the circulation of magnetic materials on the solar surface. The faster meridional airflow will result in the inability of the solar polar magnetic field to become strong,

magnetic reversal

According to the measurements of the observatory, the solar magnetic field region across the entire solar system is turning over in recent months

"Throughout the solar system, this change will have a chain reaction," said a solar physicist

With the internal generator restructuring itself, the solar magnetic field region will probably have polar opposition at the peak of the solar cycle every 11 years. The next reversal will mark the exact center of the 24th solar cycle, which is only the fourth time that it has been discovered since 1976, when it was first tracked.

"During the reversal of the magnetic field region, the magnetic field region at the two poles of the sun will disappear, the magnetic force will be reduced to zero, and it will be merged with the opposite polarity again", explained a solar physicist,

Scientists have now observed some signs of the beginning of the reversal: the sun's north pole has begun to change, and the south pole is speeding up to catch up, which means that at least until now, the sun has effectively owned two south poles. In less than four months, the solar magnetic field will be completely changed

What does the reversal of the magnetic field mean?

Because of the dominance of the solar magnetic field (we call it the solar circle, which can extend to Pluto), the reversal of the solar magnetic field will have a series of consequences for the entire solar system. In the inverted solar magnetic field area, what plays the central role is the "current sheet" - an irregular surface protruding from the solar equator, where the slow radial magnetic field area of the sun causes current.

The current itself is small, but the amount is large, and the entire solar circle is surrounded by it. During the reversal of the magnetic field, the current sheet becomes undulating. Scientists compare this undulation to the stitches on a baseball.

The role of the current sheet is to block cosmic rays and prevent them from penetrating the interior of the solar system. A corrugated sheet of current seems to be able to create a better shield to block these energy particles.

The era of reversal of the earth's magnetic field is coming

The sun is not the only exotic flower in the solar system where magnetic field reversal will occur. The earth will also, and it has happened many times in the past billion years

This is not surprising, scientists said, because both the sun and the earth's magnetic fields are considered to be generated by generators involving rotating and convection heat conducting liquids. However, their difference is that the frequency of the reversal of the earth's magnetic field is far less than that of the sun. On average, it only happens once every 200000 to 300000 years. Scientists predict that the earth's magnetic field may reverse one day in the next 1000 years,

Main impacts

The weather will no longer be so hot, and it is likely to reproduce the weather conditions of the cold period 300 years ago

When the sun has these two conditions, will the earth's temperature drop significantly. If the decline, we are worried about whether the global warming will slow down. More importantly, the consensus so far is that the global warming is not due to Greenhouse emissions The scientists caused by gas pay more attention to this problem,^[2]

Scientific proof

The magnetic field located below the sun's surface will further affect the formation of sunspot magnetic field, and the intensification of sunspot magnetic field activity indicates a new Solar cycle Start signal. The weak polar magnetic field will take more time to reach the intensity of sunspots, thus delaying the solar activity cycle. In addition, the weaker polar magnetic field will generate less activity in the subsequent solar cycle,

Halliway said: "Meridian direction airflow The polar magnetic field plays an important role in the next cycle, suggesting that future observations will help us predict future solar activity cycles. "

Hiller said that it is necessary to add that physicists' understanding of the solar cycle and the transformation of the magnetic field below the sun's surface is not complete.

Natchimuthuk Gopalswamy of NASA Goddard Space Flight Center stressed that the research that aroused scientists' interest will help distinguish different types of solar magnetism.

N According to Xinhua News Agency

NASA released a batch of high-quality solar images taken by the "Sun B" observation satellite on the 21st, and said that these "unprecedented" solar observation images show that the solar magnetic field is much more violent and dynamic than previously known.

Dick Fisher, director of the Heliophysics Division of the NASA Science Program Board, said that these high-quality solar observation pictures will, to a certain extent, "rewrite the textbook", usher in a "new era" of solar activity research, and further help humans understand how solar activity affects the Earth, space orbiting satellites, and the entire solar system.

The picture shows the solar image taken by the "Sun-B" observation satellite released by NASA on March 21, 2007, showing the outward diffusion of the magnetic field above the sunspot.

The sun has a magnetic field

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Solar magnetic field

On the whole, the sun, like the earth, has a universal magnetic field. However, due to the interference of the magnetic field in the local active region, the general magnetic field of the sun is only significant in the polar region, not as complete as the earth's magnetic field. The magnetic field intensity in the solar polar region is only 1-2 gauss. The intensity of the sun's general magnetic field often changes, and even the polarity will suddenly change. This situation was observed twice from 1957 to 1958 and 1971 to 1972.

Global magnetic field of the sun

If the sun is regarded as a star and the non imaging solar beam shines into the magnetic imager, the overall magnetic field mixed from all parts of the solar surface can be measured. The intensity of this magnetic field shows regular changes, and the polarity changes from positive to negative, and from negative to positive. Roughly speaking, it changes twice in each solar rotation cycle (about 27 days). It is easy to explain this phenomenon: there is a large magnetic area with opposite polarity on the solar surface. As the sun rotates from east to west, scientists can observe the positive and negative overall magnetic fields alternately. In short, the sun has both a general magnetic field and a global magnetic field. The former is the opposite of North and South, while the latter is the confrontation between East and West.

The source of the universal solar magnetic field is an unsolved problem. The existing theories can be divided into two categories:

The first is that the existing magnetism is left over from the material that formed the sun billions of years ago. Theoretical calculations show that the natural decay period of the solar universal magnetic field is up to 10 billion years, so it is possible for the magnetic field to remain for a long time.

The second is that since most of the sun's matter is plasma and is often in motion, we can use the dynamo effect to explain some problems about the origin of the solar magnetic field.

The vast majority of the sun's material is high-temperature plasma. The state, movement and evolution of the sun are closely related to the magnetic field. Sunspots, flares, prominences and other active phenomena are directly controlled by the magnetic field. Therefore, the study of solar magnetic field is of great significance.^[3]

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