In the atmosphere of the Sun, the wonderful rhythm of the ebbs and flows of activity dominates. Sunspots, the largest of which are visible even without a telescope, are regions of an extremely strong magnetic field on the surface of the sun. A typical mature spot is white and has the shape of a daisy. It consists of a dark central core called a shadow, which is a loop of magnetic flux extending vertically from below, and a lighter ring of fibers around it, called penumbra, in which the magnetic field extends outward horizontally.
Sun spots
At the beginning of the twentieth century. George Ellery Hale, observing real-time solar activity with his new telescope, found that the spectrum of spots is similar to the spectrum of cold red M-type stars. Thus, he showed that the shadow appears dark because its temperature is only about 3000 K, much less than 5800 K of the surrounding photosphere. The magnetic and gas pressure in the spot should balance the surrounding. It must be cooled so that the internal gas pressure becomes much lower than the external. In the "cool" areas are intensive processes. Sun spots are cooled due to the suppression of convection by a strong field, which transfers heat from below. For this reason, the lower limit of their size is 500 km. Smaller spots are quickly heated by the surrounding radiation and destroyed.
Despite the absence of convection, a lot of organized movement occurs in the spots, mainly in partial shade, where the horizontal lines of the field allow this. An example of such a movement is the Evershed effect. This is a stream with a speed of 1 km / s in the outer half of the partial shade, which extends beyond it in the form of moving objects. The latter are magnetic field elements that flow outward in the area surrounding the spot. In the chromosphere above it, the reverse flow of Evershed appears in the form of spirals. The inner half of the partial shade moves toward the shadow.
Fluctuations also occur in sunspots. When a portion of the photosphere known as the “light bridge” crosses the shadow, a fast horizontal stream is observed. Although the shadow field is too strong to allow movement, a little higher in the chromosphere, rapid oscillations occur with a period of 150 s. Over penumbra observed so-called. traveling waves propagating radially outward with a 300 s period.
Number of sunspots
Solar activity systematically passes over the entire surface of the luminary between 40 ° latitude, which indicates the global nature of this phenomenon. Despite significant fluctuations in the cycle, on the whole it is impressively regular, which is confirmed by a well established order in the numerical and latitudinal positions of the spots.
At the beginning of the period, the number of groups and their sizes increase rapidly until after 2-3 years their maximum number is reached, and after another year - the maximum area. The average life time of a group is about one rotation of the Sun, but a small group can last only 1 day. The largest sunspot groups and the largest eruptions usually occur 2 or 3 years after reaching the sunspot limit.
Up to 10 groups and 300 spots can appear, and one group can count up to 200. The cycle may be irregular. Even near the maximum, the number of spots can temporarily decrease significantly.
11 year cycle
The number of spots returns to a minimum of approximately every 11 years. At this time, there are several small similar formations on the Sun, usually at low latitudes, and for months they may be absent altogether. New spots begin to appear at higher latitudes, between 25 ° and 40 °, with the polarity opposite to the previous cycle.
At the same time, new spots may exist at high latitudes and old ones at low latitudes. The first spots of the new cycle are small and live only a few days. Since the rotation period is 27 days (longer at higher latitudes), they usually do not return, and newer ones are closer to the equator.
For an 11-year cycle, the configuration of the magnetic polarity of the groups of spots is the same in this hemisphere and in the other hemisphere faces in the opposite direction. It changes in the next period. Thus, new spots at high latitudes in the northern hemisphere can have a positive polarity and a negative one following it, and groups from the previous cycle at a low latitude will have the opposite orientation.
Gradually, the old spots disappear, and new ones appear in large quantities and sizes at lower latitudes. Their distribution is in the shape of a butterfly.
Full cycle
Since the configuration of the magnetic polarity of sunspot groups changes every 11 years, it returns to the same value every 22 years, and this period is considered the period of a complete magnetic cycle. At the beginning of each period, the total field of the Sun, determined by the dominant field at the pole, has the same polarity as the spots of the previous one. As the active regions rupture, the magnetic flux is divided into sections with a positive and negative sign. After many spots appeared and disappeared in the same zone, large unipolar regions with one or another sign are formed, which move to the corresponding pole of the Sun. During each minimum at the poles, the flux of the next polarity in this hemisphere prevails, and this field is visible from the Earth.
But if all magnetic fields are balanced, how are they divided into large unipolar regions that control the polar field? No answer was found to this question. Fields approaching the poles rotate more slowly than sunspots in the equatorial region. In the end, weak fields reach the pole and reverse the dominant field. This changes the polarity that the leading spots of the new groups must take, thereby continuing the 22-year cycle.
Historical evidence
Although the solar activity cycle has been fairly regular for several centuries, significant variations have been observed. In the years 1955-1970, much more spots were in the northern hemisphere, and in 1990 they dominated in the southern. The two cycles peaked in 1946 and 1957 were the largest in history.
English astronomer Walter Munder found evidence of a period of low solar magnetic activity, indicating that between 1645 and 1715 there were very few spots. Although this phenomenon was first discovered in about 1600, few cases of their observation were recorded during this period. This period is called the Mound minimum.
Experienced observers reported the appearance of a new group of spots as a great event, noting that they had not seen them for many years. After 1715, this phenomenon returned. It coincided with the coldest period in Europe from 1500 to 1850. However, the connection of these phenomena has not been proved.
There is some evidence of other similar periods at intervals of approximately 500 years. When solar activity is high, strong magnetic fields generated by the solar wind block high-energy galactic cosmic rays approaching the Earth, leading to less carbon-14 formation. Measurement of 14 C in tree rings confirms the low activity of the Sun. The 11-year cycle was not detected until the 1840s; therefore, observations up to this time were irregular.
Ephemeral areas
In addition to sunspots, there are many tiny dipoles called ephemeral active regions that exist on average less than a day and are found throughout the sun. Their number reaches 600 per day. Although the ephemeral regions are small, they can make up a significant part of the magnetic flux of the star. But since they are neutral and rather small, they probably do not play a role in the evolution of the cycle and the global field model.
Prominences
This is one of the most beautiful phenomena that can be observed during solar activity. They are similar to clouds in the Earth’s atmosphere, but are supported by magnetic fields and not by heat fluxes.
The plasma of ions and electrons that make up the solar atmosphere cannot cross the horizontal lines of the field, despite the force of gravity. Prominences arise at the boundaries between opposite polarities, where the field lines change direction. Thus, they are reliable indicators of sharp field transitions.
As in the chromosphere, prominences are transparent in white light and, with the exception of total eclipses, should be observed in Hα (656.28 nm). During an eclipse, the red Hα line gives prominences a beautiful pink tint. Their density is much lower than that of the photosphere, since there are too few collisions to generate radiation. They absorb radiation from below and emit it in all directions.
The light visible from the Earth during an eclipse is devoid of ascending rays, so the prominences look darker. But since the sky is even darker, it seems bright against its background. Their temperature is 5000-50000 K.
Types of prominences
There are two main types of prominences: calm and transitional. The former are associated with large-scale magnetic fields, denoting the boundaries of unipolar magnetic regions or groups of sunspots. Since such sites live long, the same is true for calm prominences. They can have a different shape - hedges, suspended clouds or funnels, but are always two-dimensional. Stable fibers often become unstable and erupt, but can also simply disappear. Calm prominences live for several days, but new ones can form on the magnetic boundary.
Transitional prominences are an integral part of solar activity. These include jets, which are a disorganized mass of material ejected by a flash, and clots are collimated flows of small emissions. In both cases, part of the substance returns to the surface.
Loop-like prominences are consequences of these phenomena. During the flash, the electron flux heats the surface to millions of degrees, forming hot (more than 10 million K) coronary prominences. They radiate strongly, cooling, and devoid of support, descend to the surface in the form of elegant loops, following the magnetic lines of force.
Outbreaks
The most spectacular phenomenon associated with solar activity are flares, which represent a sharp release of magnetic energy from the sunspot region. Despite the high energy, most of them are almost invisible in the visible frequency range, since energy is emitted in a transparent atmosphere, and only the photosphere, which reaches relatively low energy levels, can be observed in visible light.
Flares are best seen in the Hα line, where the brightness can be 10 times higher than in the neighboring chromosphere, and 3 times higher than in the surrounding continuum. In Hα, a large flash will cover several thousand solar disks, but only a few small bright spots appear in visible light. The energy released in this case can reach 10 33 erg, which is equal to the output of the entire luminary in 0.25 s. Most of this energy is initially released in the form of high-energy electrons and protons, and visible radiation is a secondary effect caused by the action of particles on the chromosphere.
Types of flashes
The range of flare sizes is wide - from giant particles bombarding the Earth to barely noticeable. They are usually classified by their associated X-ray fluxes with wavelengths from 1 to 8 angstroms: Cn, Mn or Xn for more than 10 -6 , 10 -5 and 10 -4 W / m 2, respectively. Thus, M3 on Earth corresponds to a flux of 3 × 10 -5 W / m 2 . This indicator is not linear, since it measures only the peak, and not the total radiation. The energy released in the 3-4 largest flares every year is equivalent to the sum of the energies of all the others.
The types of particles created by flares vary depending on the place of acceleration. Between the Sun and the Earth there is not enough material for ionizing collisions, so they retain their original state of ionization. Particles accelerated in the corona by shock waves exhibit a typical coronal ionization of 2 million K. Particles accelerated in the flare body have significantly higher ionization and extremely high concentrations of He 3 , a rare helium isotope with only one neutron.
Most large outbreaks occur in a small number of overactive large groups of sunspots. Groups are large clusters of the same magnetic polarity, surrounded by the opposite. Although the prediction of solar activity in the form of outbreaks is possible due to the presence of such formations, researchers can not predict when they will appear, and do not know what produces them.
Impact on the earth
In addition to providing light and heat, the Sun acts on the Earth through ultraviolet radiation, a constant stream of solar wind and particles from large outbreaks. Ultraviolet radiation creates the ozone layer, which, in turn, protects the planet.
Soft (long-wave) X-rays from the solar corona create layers of the ionosphere that make possible short-wave radio communication. On days of solar activity, the radiation of the corona (slowly changing) and flashes (impulsive) increases, creating a better reflective layer, but the density of the ionosphere increases until the radio waves are absorbed and the short-wave communication is not difficult.
More stringent (short-wave) X-ray pulses from flares ionize the lowest layer of the ionosphere (D-layer), creating radio emission.
Earth's rotating magnetic field is strong enough to block the solar wind, forming the magnetosphere that particles and fields flow around. On the side opposite the luminary, the field lines form a structure called a geomagnetic plume or tail. When the solar wind intensifies, there is a sharp increase in the Earth's field. When the interplanetary field switches in the opposite direction to the earth, or when large clouds of particles fall into it, the magnetic fields in the plume connect again and the energy that creates auroras is released.
Magnetic Storms and Solar Activity
Every time a large coronal hole turns to the Earth, the solar wind accelerates and a geomagnetic storm occurs . This creates a 27-day cycle, especially noticeable at a minimum of sunspots, which allows us to predict solar activity. Large outbreaks and other phenomena cause emissions of coronal mass, clouds of energy particles that form an annular current around the magnetosphere, causing sharp fluctuations in the Earth’s field, called geomagnetic storms. These phenomena disrupt radio communications and create power surges on long-distance lines and in other long conductors.
Perhaps the most intriguing of all earthly phenomena is the possible effect of solar activity on the climate of our planet. Mound's minimum seems reasonable, but there are other obvious effects. Most scientists believe that there is an important relationship masked by a number of other phenomena.
Since charged particles follow magnetic fields, corpuscular radiation is not observed in all large flares, but only in those located in the western hemisphere of the Sun. Lines of force from its western side reach the Earth, directing particles there. The latter are mainly protons, because hydrogen is the dominant constituent element of the star. Many particles moving at a speed of 1000 km / s second create a shock front. The flow of low-energy particles in large flares is so intense that it threatens the lives of astronauts outside the Earth's magnetic field.