A new method for measuring the sun's magnetic field. Astrophysicists have found the strongest magnetic field on the sun in the entire history of measurements. A device that measures the magnetic fields of the sun.
Magnetic field of the sun
Magnetic fields are present, apparently, on all stars. The magnetic field was first discovered on the closest star to us - the Sun - in 1908 by the American astronomer J. Hale, who measured the Zeeman splitting of spectral lines in sunspots.
According to modern measurements, the maximum magnetic field strength of sunspots = 4000 Oe. The field in sunspots is a manifestation of the general azimuthal magnetic field of the Sun, the field lines of which have different directions in the Northern and Southern hemispheres of the Sun
Unlike nearby outer space, direct measurement of magnetic fields on the Sun with magnetometers is impossible not only because of the technical difficulties of sending a space probe to the Sun, but also because high temperature its substance, which no device can withstand). Therefore, both on the Sun, and even more so on other more distant objects, magnetic fields can be measured only indirectly - by analyzing electromagnetic radiation.
On the Sun, the magnetic field is captured by hot matter or “frozen” into it. As it moves, solar matter carries with it as much of the magnetic field as it can. Since the rotation speed at the equator is faster than the rotation speed at the poles, the magnetic field lines are stretched, but the field lines do not break off during such winding; they are more like extremely elastic rubber. Like rubber, the more they stretch, the more energy they store.
The magnetic field of the spots suppresses convection in the upper layers of the convective zone, the energy transfer here sharply decreases, therefore the gas temperature in the spot area decreases by 1,500-2,000 K. In the close vicinity of the spot, where the field strength is relatively low, the magnetic field, on the contrary, enhances convective energy transfer. This is how bright formations arise - torches.
Estimates show that buoyancy is effective down to depths of about 15,000 km, while the thickness of the convective zone is about seven times greater. It follows that the magnetic fields of sunspots are formed in the upper part of the convective zone of the Sun.
In this regard, the following question arises: how is the inhomogeneous rotation of the Sun maintained? After all, the strengthening of magnetic fields and the formation of magnetic tubes occurs due to the inhibition of the rotational motion of the equatorial regions, and if this energy were not continuously supplied, then after several revolutions the Sun would begin to rotate as absolutely solid, i.e. the angular velocity of rotation at the poles and at the equator would be the same.
The sun as a variable star
Variable stars are those luminaries whose luminosity changes over time as a result of physical processes occurring in its region.
It turns out that our Sun is such a star.
Information collected by the solar wind particle sensor Swoops probe Ulysses, allowed us to conclude that the solar wind has been continuously “weakening” since the mid-1990s. Moreover, this process apparently began much earlier. Currently, the speed of the solar wind has reached its absolute minimum for at least half a century - since direct research began using spacecraft. The decrease in solar wind speed over a decade is relatively small - about 3%, but it is a consequence of a decrease in the temperature and pressure of solar wind particles by 13% and 20%, respectively. It is impossible to say yet how long the process will take and how far it has gone. The cooling of the solar wind is also accompanied by a decrease in the strength of the Sun's magnetic field by a third over the same period.
Thus, the radiation situation in the Solar System and in near-Earth space has worsened - the flux density of especially dangerous high-energy protons coming from deep space has increased by about 20%.
.The anomalous decrease in solar wind activity complements the picture of difficult-to-explain anomalies in the behavior of the star itself. The unique activity of the star at the end of the last cycle was replaced by an abnormally long absence of spots - an indicator of activity - on the star.
A decrease in the number of sunspots, generally speaking, is characteristic of solar activity minima, but this time the process took too long. For almost a year now, there have been practically no spots on the Sun at all.
It is obvious that the scale of processes currently occurring on the Sun goes beyond the hypothesis of their 11-year cyclicity.
Sunspots provide us with the most visual examples of non-stationary processes on the Sun. First of all, this is their rapid development. Sometimes two or three days are enough for a large spot or a large group of spots to develop in a “clean” area of the photosphere. As a rule, however, their development is slower and in large groups reaches a maximum after 2-3 weeks. Small spots and groups appear and disappear within a week, while large ones last for several months. One spot is known to have existed for 1.5 years. When a spot appears, when its penumbra is still small, the same photospheric granulation is visible in it (Gansky, Thiessen), which with further development takes on a fibrous appearance; fibers are much more durable than granules. When a round spot of regular shape approaches the solar edge, it is observed by us in projection and its diameter in the direction of the radius of the solar disk is greatly reduced (proportionally; see Fig. 8). In this case, the so-called Wilson effect is often observed, which consists in the fact that the penumbra of the spot on the side of the edge of the disk is clearly visible, but on the side facing the center of the disk, it is greatly reduced. This phenomenon allows for the geometric likening of a sunspot to a giant depression with conically tapering walls. But not all spots reveal this.
Typically, a group of sunspots is stretched along heliographic longitude (in exceptional cases - up to 20° or more). In this case, the group often contains two of the largest sunspots with separate penumbraes, which have slightly different movements on the surface of the Sun. The eastern spot is called the leading one, the western one is called the next one. Often this tendency to form in pairs is also observed in individual spots that do not form groups with a large number of small satellite spots.
Rice. 38. Vortex structure of spots in the bipolar group. The directions of the vortices are opposite. (Spectrogram in Na rays)
Observations of radial velocities along different spectral lines in different places spots and from different angles to it show the presence of strong (up to 3 km/s) movements in the penumbra of the spot - spreading of matter in its deep parts and flow of matter inside at high altitudes. The latter is confirmed by the vortex structure visible above the spots on spectroheliograms in the rays. The directions of these vortices are opposite in the southern and northern hemispheres of the Sun and indicate in single spots the inflow of matter in accordance with how the Coriolis force should deflect it.
Typically, systematic movements are no longer observed at the outer edge of the penumbra.
As mentioned above, sunspots have strong magnetic fields. Intensities of 1000-2000 Oe are common, and in one group at the end of February 1942, a intensity of 5100 Oe was measured. Detailed studies of the distribution of the direction and strength of the magnetic field inside the spot showed that in the center of the spot, magnetic field lines run along the axis of the spot (up or down), and as they move towards the periphery of the spot, they increasingly deviate from the normal to the surface, almost up to 90° at the edge of the penumbra. In this case, the magnetic field strength decreases from a maximum to almost zero.
Rice. 39. Changes in the average latitude and magnetic polarity of sunspots in successive cycles of solar activity
The larger the spot, the stronger its magnetic field, as a rule, but when a large spot, having reached its maximum size, begins to decrease, the strength of its magnetic field remains unchanged, and the total magnetic flux decreases in proportion to the area of the spot. This can be interpreted as if the spot only contributes to the removal of the magnetic field that has existed for a long time under the surface. This is also confirmed by the fact that often the magnetic field does not disappear after the disappearance of the spot, but continues to exist there and intensifies again when the spot reappears in the same area. The presence of permanent flare fields here suggests that stable active regions exist in these places.
In groups with two large spots, the leading and following spots have opposite magnetic polarity (Figs. 38 and 39), which justifies the name of such groups - bipolar, as opposed to unipolar groups that include single spots. There are complex groups in which spots of one and the other polarity are randomly mixed. In each cycle of solar activity, the polarities of the leading and next sunspots in the northern and southern hemispheres are opposite to each other.
So, if in the northern hemisphere of the Sun the polarity of the leading spot is northern (N), and the next one is southern (S), then at the same time in the southern hemisphere the polarity of the leading spot is S, and the next one is N. For those rare spots that are intersected by the equator , the polarity of the northern and southern halves is opposite. But with the end of the solar activity cycle, when its minimum passes, in each hemisphere the distribution of magnetic polarity at the spots of the bipolar group changes to that which was in the previous cycle on the opposite hemisphere. This important fact was established by Hale and his colleagues in 1913.
Although the local magnetic fields of the Sun can be very strong, its overall magnetic field is very weak and only barely stands out against the background of local fields only in years of sunspot minimums. Moreover, it is changeable. In the years 1953-1957, its intensity corresponded to a dipole with an induction of 1 G, the sign was opposite to the sign of the Earth's magnetic field, and the axis of the dipole coincided with the axis of rotation. In 1957, the sign of the field changed to the opposite in the southern polar regions of the Sun, and at the end of 1958 - in the northern ones. Last modified field sign was observed in 1970-1971.
The change in the magnetic polarity of sunspots with the end of the solar activity cycle is not the only sign of the end of the cycle. Sunspots rarely form far from the equator. Their preferred zone lies within heliographic latitudes from 1-2° to 30° in both hemispheres. At the equator itself, spots are rare, as well as at latitudes above 30°. But this picture has the peculiarity of its change over time: the first spots of the new cycle (after the imaginary) appear far from the equator (for example, spot c was recorded on March 15, 1914, from May 1943 and from October 1954), in while the last spots of the outgoing cycle are still observed near the equator. During the heyday of the cycle, near its maximum, spots can be found at all heliographic latitudes between - 45° and +45° (a group of spots is known even with a latitude of +50°, observed in June 1957 during the maximum solar activity), but mainly between 5 and 20°. Thus, the average heliographic latitude of the spots steadily decreases as the 11-year cycle of solar activity develops, and new spots appear closer and closer to the equator (Fig. 39). This pattern was first established in 1858 by Carrington and is sometimes called Spörer's law (although the latter established it 10 years later).
Thus, if by period we mean the period of time during which all properties change and return to their original state, then the true period of solar activity is not 11 years, but 22 years. Interestingly, some alternation in the height of the maximum through the cycle also confirms the 22-year periodicity. An 80-year cycle of solar activity is also planned. For some internal reasons, solar activity varies widely with characteristic time about a century.
So, between 1645 and 1715. there were almost no spots on the Sun, and the group appeared only once. This is the so-called Maunder minimum. Another minimum, the Spörer minimum, occurred between 1410 and 1510. On the contrary, the medieval maximum was between 1120 and 1280. was very energetic, similar to what we are experiencing now. The described variations were accompanied by fluctuations in the average annual temperature in England within 1 °C.
Magnetic field by modern ideas is formed inside the Sun in its convective zone, located directly below the solar surface (photosphere). The role of the magnetic field in the dynamics of processes occurring on the Sun is enormous. Apparently, it is the key to all active phenomena occurring in the solar atmosphere, including solar flares. We can say that if the Sun did not have a magnetic field, it would be an extremely boring star.
Many objects observed on the Sun also owe their origin to the magnetic field. For example, sunspots are places where giant magnetic loops emerging from the interior of the Sun penetrate the solar surface. It is for this reason that groups of sunspots, as a rule, consist of two regions of different magnetic polarity - northern and southern. These two regions correspond to the opposite bases of the floating flux tube. The solar activity cycle is also the result of cyclic changes in the magnetic field that occur in the solar interior. The prominences, which seem to float in the void above the surface of the Sun, are in fact supported by the magnetic field lines with which they are penetrated. Finally, many objects observed in the corona, in particular streamers and loops, simply repeat in their shape the topology of the magnetic fields surrounding them.
Magnetic field measurements
A magnetic field affects the movement of charged particles entering it. For this reason, the electrons that make up any atom, rotating around the nucleus in one direction, when placed in a magnetic field, will increase their energy, while electrons rotating in the other direction will decrease their energy. This effect (Zeeman effect) leads to the splitting of the emission lines of an atom into several components. Measuring this splitting makes it possible to determine the magnitude and direction of the magnetic field on objects distant from us that are inaccessible to direct study, such as the Sun. Modern methods measurements make it possible to determine the field on the solar surface with high accuracy, but are often powerless when measuring the three-dimensional field in the solar corona. In this case, special mathematical methods are used to reconstruct the complete three-dimensional picture of the field from surface measurements.
Space weather prediction
Understanding the nature of the solar magnetic field and its behavior will enable more reliable predictions of space weather. There are currently some indirect signs that indicate that a flare may occur in the active region. However, longer-term predictions, such as, for example, predicting the duration of the future solar cycle, are still extremely inaccurate and are not based on strict physical models, but on the search for various kinds of empirical dependencies. However, we hope that in the near future we will be able to understand the Sun well enough to model its future activity and predict space weather in the same way we now predict weather on Earth.
The Voyager 1 and Voyager 2 spacecraft are the most distant and fastest objects created by man. For several years now they have been flying through border solar system and will soon be completely abandoned. But even before they finally go to the stars, they transmit data that changes the understanding of our big house. It turns out that the solar system is surrounded by magnetic foam, whose giant bubbles can not only greatly influence our protection from galactic cosmic rays, but also distort our knowledge of the Universe...Voyagers have been in flight for more than 33 years old . After many discoveries made within the solar system, now. The devices have already crossed the heliospheric shock wave, ahead of them is the heliopause region (the boundary along which the pressure of the solar wind and the interstellar medium is balanced).
Voyager 1 has taken a slight lead and is now more than 17 billion km from the earth ( 116 astronomical units , those. 116 distances from the Earth to the Sun), its speed is greater 60000 km/h , and the signal from it goes to Earth about 14 hours . But even from such a distance, the devices transmit data leading to new discoveries.
One of the discoveries concerns solar magnetic field . The dimensions of the solar magnetic field are incomparably larger than the earth’s and go far beyond the limits of planetary orbits. Previously, it was believed that at the border of the Solar system everything is arranged in the usual way: the magnitude of the magnetic field decreases, the lines of force bend and return back to the Sun. Now it turns out that this is not so.
The magnetic fields at the edge of the solar system are very weak, so it took more than 4 years for enough data transmitted by both Voyagers to accumulate. When the data was finally available and the corresponding models were built, the scientists were extremely surprised. It turned out that the magnetic boundary of the solar system is a “foam” of gigantic proportions . Each “bubble” in this foam has a diameter of about 1 a.u. (1 astronomical unit = 150 million km). Voyager spends 3-4 months crossing such a “bubble”.
The reason for the appearance of magnetic “bubbles” is the rotation of the Sun around its axis, as a result of which, at the limit of their propagation, magnetic lines “get entangled” and their reconnection. The effect of magnetic reconnection was familiar to astrophysicists before - namely this effect is considered a source of energy from solar flares (see description of the effect in the comments), but they never expected to encounter it at the edge of the solar system.
The Sun's magnetic field lines are oriented in different directions in its different hemispheres. Because The axis of the magnetic field is inclined relative to the axis of rotation of the Sun, its magnetic field meanders in the form of a complex spiral, divided into sectors with different polarities. Beyond the shock wave boundary, with a decrease in solar wind speed ( which inflates the Sun's magnetic field into a huge bubble of the heliosphere) the distances between its heteropolar “folds” decrease sharply. As a result, the lines of magnetic fields are broken, reconnected, and new sections of the field—magnetic bubbles—bud off from the spiral.
As a result of reconnection, magnetic fields are formed that are separated from the “mother” magnetic field of the Sun. They “loop” into bubbles, partially connected to each other.
If this is indeed the case (and the data on the magnetic field transmitted from the Voyagers most logically line up precisely in this picture), then the boundary of the magnetic field of the Sun and, accordingly, of our Solar system, resembles not a simple “ coastline”, and the line of the surf separating our “island” from the ocean of interstellar matter.
And this new knowledge about the border of our solar system is not so far from ours Everyday life, as one might think. The fact is that The magnetic field of the Sun protects the Earth from various “messengers” of other stars and galaxies, just as the magnetic field of the Earth protects us from the Sun. And among these “messengers” there may be very dangerous for life on Earth - for example, high energy particles, accelerated by supernova explosions and/or passing near black holes...
Scientists are now faced with the task of determining - is such a boundary of the solar magnetic field more reliable protection than the usual one? ? Are charged particles, flying towards us from beyond the solar system, slowed down or, on the contrary, accelerated even more strongly in magnetic bubbles? Or is the magnetic foam so weak and so many “holes” in it that it has virtually no effect on cosmic rays?..
And a more abstract, but no less interesting question - if the boundary of the solar system is so “complicated”, then to what extent are we, being inside, are able to see what is there outside of our Solar System (naturally, we are talking, first of all, about the electromagnetic picture of the world)?..
A video created by NASA will help you visualize the story of the discovery of magnetic foam:
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Measurements of the Earth's magnetic field induction at distances of the order of 100 km (medium scale) are very important for understanding such geophysical phenomena as the behavior of the upper mantle, the evolution of ocean currents and the impact of the Sun's magnetic field on the planet's ionosphere. However, such research is expensive, since it involves launching special spacecraft into the upper layers of the atmosphere (at an altitude of about 100 km). A team of scientists from Germany and the USA has proposed a cheaper, ground-based method for measuring the geomagnetic field at a given scale, which is highly accurate and insensitive to magnetic interference from the environment.
Studying the structure and strength of the Earth’s magnetic field allows us to “look” into the depths of our planet: measuring the induction of the geomagnetic field and its variations at different scales provides information about the sources of this field at the corresponding depths. Thus, “mapping” the earth’s magnetism within a few meters can reveal underground ferromagnetic objects, such as unexploded shells and mines or mothballed containers with toxic waste. Measuring the magnetic field and its fluctuations over distances of several kilometers can be used to detect mineral deposits. On a global scale, the study of the Earth's magnetic "shell" provides data for the geodynamo model, a theory that describes the origin and subsequent evolution of the Earth's magnetic field.
The study of the distribution of geomagnetic field induction on a medium scale, that is, within 10–100 kilometers, is also of significant scientific interest. In particular, this makes it possible to assess the influence of the Sun’s magnetic field on the ionosphere, provides information about the behavior of the Earth’s upper mantle and the circulation of oceanic masses - one of the main factors regulating the climate on the planet (after all sea water is an electrolyte, and its movement actually represents an ionic current). To avoid unwanted environmental influences, geomagnetic field measurements at this scale must be carried out at heights corresponding to this spatial resolution. In other words, to “map” geomagnetism at distances of about 100 km, you need to go up the same amount.
For such measurements, satellites with a magnetometer are launched, which requires serious material and financial investments. Scientists from the USA and Germany have proposed ground method measurements of the Earth's magnetic field on a scale of about 100 km, which is highly sensitive and has a relatively low cost. They described their method in a recent publication Magnetometry with mesospheric sodium in the journal Proceedings of the National Academy of Sciences. The idea of the authors of the article is based on technology used in some observatories to create artificial laser guide stars.
What are artificial guide stars?
It is known that astronomical observations using an optical telescope located on the surface of the Earth are often difficult due to atmospheric turbulence. Random movements of air masses blur images of stars and significantly reduce resolution large telescopes with lenses over 1 m. Therefore, so-called adaptive optics are usually used. A special mirror is installed in the telescope, which can be deformed and adapt to changing external conditions. To take into account distortions, the telescope must be calibrated by pointing it at some bright star (called a reference star).
However, a natural guide star is not always detected in the telescope’s field of view, so they came up with the idea of creating guide stars with a laser. The laser irradiates a layer of sodium atoms about 10 km thick, located at an altitude of about 90 km above the Earth's surface (this sodium layer was formed as a result of the combustion of meteors). If the wavelength of the laser light is 589 nm, then in the small area where the laser beam hits, the sodium atoms go into an excited state: the outer electrons move to a higher energy level, live there for some time, and then return back, emitting yellow light. Next, this light from a small area of the sky irradiated by the laser is recorded by a telescope. As a result, an artificial reference star is born (Fig. 1), which is then used to correct the image in the telescope.
An important fact should be noted here. Since electrons have spin, they do rotational movement around the nucleus, and also due to some similarity of alkali metal atoms with the hydrogen atom (the total spin of all electrons in these atoms is 1/2), the above-mentioned higher energy level of the sodium atom is split into two levels closely spaced in energy, each of which can become temporary "home" for the excited electron. The emerging two levels of the sodium atom are called sodium doublet. It is identified on the discrete (line) spectrum of sodium as two closely spaced thin yellow lines, designated D 1 and D 2. This means that the excited sodium atom actually emits yellow light of two very close wavelengths.
Operating principle of a ground-based geomagnetic field detector
In 1961, it was discovered that under the influence of circularly polarized laser pulses, under a certain condition, spin polarization is observed in alkali metal vapors located in an external magnetic field - the spins of the atoms of these elements acquire a specific direction. This condition is the coincidence of the frequency of the laser pulses (not to be confused with the frequency of the light emitted by the laser) and the frequency with which the magnetic moment of the atoms precesses in an external magnetic field. The phenomenon of rotation of a particle's magnetic moment vector around the direction of a magnetic field line is known in physics as Larmor precession, and the frequency at which it rotates is called the Larmor frequency. For an atom, it is determined by its mass, the structure of energy levels and the induction of an external magnetic field.
Spin polarization will cause one of the lines of the sodium doublet, D 2, to become brighter and the other line (D 1) to dim when compared with the line spectrum of sodium obtained in the case of continuous irradiation, or when the frequency of the laser pulses does not coincide with Larmor frequency. Observation of the effect described above will mean that the Larmor frequency for sodium atoms has been found, and from it it is now easy to calculate the desired magnetic field induction. This is exactly what the principle of operation of a ground-based geomagnetic field detector on a scale of 100 km looks like in theory.
In practice, as conceived by the authors, the following should happen: the laser shoots into the sky a series of periodic pulses (with circular polarization), the direction of movement of which should be approximately perpendicular to the geomagnetic field lines (Fig. 2). The wavelength of the laser radiation is 589 nm, and the frequency of their pulses is experimentally selected to be equal to the Larmor frequency for sodium atoms located in the place where the laser pulses were sent. You can understand whether the frequencies coincide with the help of a telescope, which in this case will register in the spectrum of sodium atoms an increase in the brightness of the D 2 line and, accordingly, a weakening of the D 1 line. When this condition is met, the desired value of the magnetic field induction is found from the value of the Larmor frequency.
Let us pay attention to the non-randomness of the choice of the sodium layer as a kind of remote magnetometer. The altitude of its location (90 km) perfectly matches the conditions for measuring the Earth's magnetic field and its fluctuations on a given average scale.
Any device or instrument that measures any physical quantity, inevitably does this with a certain error, or, as experts say, “makes noise.” In the geomagnetic field detector proposed by the authors of the article, one of the sources of noise is laser radiation, which in reality is not monochromatic, but has, although very small, but nevertheless non-zero blurring in frequency or wavelength, associated with the quantum nature of the process of generating coherent radiation itself . The size of this blur, called the laser width, determines, among other things, the sensitivity of the detector. The smaller the radiation width, the more sensitive the measurements will be.
In addition, the accuracy of the device is also affected by the area of the telescope lens (the larger the better), laser intensity and duty cycle, characterizing the frequency of emission of laser pulses and equal to the ratio of the pulse duration to their repetition period. As follows from the definition, the fill factor is a dimensionless quantity that varies in the range from 0 to 1 or from 0 to 100%. If the duty cycle is 100%, then continuous, constant, non-pulsed radiation is observed. A decrease in the duty cycle value means that the time interval between pulses within their repetition period is continuously increasing.
As calculations have shown, for measuring the geomagnetic field it is best to monitor the change in the brightness of the D 1 line of the sodium doublet. In this case, if we set the laser beam width to 400 MHz, the optimal sensitivity is achieved at a fill factor of 20% and a laser intensity of about 30 W/m 2 . For these values it will be less than 0.5 nT (nanotesla, 10 –9 T). This is quite enough to monitor the circulation of oceanic masses and the influence of the solar magnetic field, which creates an induction of the order of 1–10 nT. As a comparison, let us recall that the average value of the Earth’s magnetic field induction is approximately 50 μT (microtesla), that is, almost 3–4 orders of magnitude greater.
The authors of the article believe that the proposed technology for measuring the geomagnetic field can, in principle, be installed in any observatory, regardless of whether it contains devices or objects that create magnetic interference. Moreover, scientists believe that based on their method, it is possible to implement a mobile platform that could monitor the Earth’s magnetic field on a scale of 100 km.