The Sun's atmosphere is dominated by a wonderful rhythm of ebb and flow of activity. Sunspots, the largest of which are visible even without a telescope, are areas of extremely strong magnetic fields on the surface of a star. A typical mature spot is white and daisy-shaped. It consists of a dark central core called the umbra, which is a loop of magnetic flux extending vertically from below, and a lighter ring of fibers around it, called the penumbra, in which the magnetic field extends outward horizontally.
Sunspots
At the beginning of the twentieth century. George Ellery Hale, using his new telescope to observe solar activity in real time, found that the spectrum of sunspots is similar to that of cool red M-type stars. Thus, he showed that the shadow appears dark because its temperature is only about 3000 K, much less than the ambient temperature of 5800 K.photosphere. The magnetic and gas pressure in the spot must balance the surrounding pressure. It must be cooled so that the internal pressure of the gas becomes significantly lower than the external one. In the "cool" areas are intensive processes. Sunspots are cooled by the suppression of convection, which transfers heat from below, by a strong field. For this reason, the lower limit of their size is 500 km. Smaller spots are quickly heated by ambient radiation and destroyed.
Despite the lack of convection, there is a lot of organized movement in the patches, mostly in partial shade where the horizontal lines of the field allow it. An example of such movement is the Evershed effect. This is a flow with a speed of 1 km/s in the outer half of the penumbra, which extends beyond its limits in the form of moving objects. The latter are elements of the magnetic field that flow outward over the region surrounding the spot. In the chromosphere above it, the reverse Evershed flow appears as spirals. The inner half of the penumbra is moving towards the shadow.
Sunspots also fluctuate. When a patch of the photosphere known as the "light bridge" crosses the shadow, there is a fast horizontal flow. Although the shadow field is too strong to allow movement, there are rapid oscillations with a period of 150 s in the chromosphere slightly higher up. Above the penumbra there are 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 star between 40°latitude, which indicates the global nature of this phenomenon. Despite the significant fluctuations in the cycle, it is overall impressively regular, as evidenced by the well-established order in the numerical and latitudinal positions of the sunspots.
At the beginning of the period, the number of groups and their sizes increase rapidly until after 2–3 years the maximum number is reached, and after another year - the maximum area. The average lifetime of a group is about one rotation of the Sun, but a small group can only last 1 day. The largest sunspot groups and largest eruptions usually occur 2 or 3 years after the sunspot limit has been reached.
May have up to 10 groups and 300 spots, and one group can have up to 200. The course of the cycle may be irregular. Even near the maximum, the number of sunspots can temporarily decrease significantly.
11 year cycle
The number of sunspots returns to a minimum about 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 sunspots begin to appear at higher latitudes, between 25° and 40°, with opposite polarity from the previous cycle.
At the same time, new spots can exist at high latitudes and old spots 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 11 year cyclethe configuration of the magnetic polarity of sunspot groups is the same in a given hemisphere and is in the opposite direction in the other hemisphere. It changes in the next period. Thus, new sunspots at high latitudes in the northern hemisphere can have a positive polarity and then a negative polarity, and the groups from the previous cycle at low latitude will have the opposite orientation.
Gradually, old spots disappear, and new ones appear in large numbers and sizes at lower latitudes. Their distribution is shaped like a butterfly.
Full cycle
Because 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 break, the magnetic flux is divided into sections with a positive and a negative sign. After many spots appear and disappear in the same zone, large unipolar regions with one sign or another are formed, which move towards the corresponding pole of the Sun. During each minimum at the poles, the flux of the next polarity in that hemisphere dominates, and this is the field as seen from Earth.
But if all magnetic fields are balanced, how do they divide into large unipolar regions that govern the polar field? This question has not been answered. Fields approaching the poles rotate more slowly than sunspots in the equatorial region. Eventually the weak fields reach the pole and reverse the dominant field. This reverses the polarity that the leading spots of the new groups should take, thus continuing the 22-year cycle.
Historical evidence
Although the cycle of solar activity has been fairly regular over several centuries, there have been significant variations in it. In 1955-1970, there were much more sunspots in the northern hemisphere, and in 1990 they dominated in the southern. The two cycles, peaking in 1946 and 1957, were the largest in history.
English astronomer W alter Maunder found evidence for a period of low solar magnetic activity, indicating that very few sunspots were observed between 1645 and 1715. Although this phenomenon was first discovered around 1600, few sightings 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 between these phenomena has not been proven.
There is some evidence for other similar periods at roughly 500 year intervals. When solar activity is high, strong magnetic fields generated by the solar wind block high-energy galactic cosmic rays approaching Earth, resulting in lessthe formation of carbon-14. Measuring 14С in tree rings confirms the low activity of the Sun. The 11-year cycle was not discovered until the 1840s, so observations prior to that 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 portion of the sun's magnetic flux. 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 rather than heat fluxes.
The plasma of ions and electrons that make up the solar atmosphere cannot cross horizontal field lines, despite the force of gravity. Prominences occur at the boundaries between opposite polarities, where the field lines change direction. Thus, they are reliable indicators of abrupt 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 the prominences a beautiful pink hue. Their density is much lower than that of the photosphere, since it is toofew collisions. They absorb radiation from below and emit it in all directions.
The light seen from Earth during an eclipse is devoid of ascending rays, so the prominences appear darker. But since the sky is even darker, they appear bright against its background. Their temperature is 5000-50000 K.
Types of prominences
There are two main types of prominences: quiet and transitional. The former are associated with large-scale magnetic fields that mark the boundaries of unipolar magnetic regions or groups of sunspots. Since such areas live for a long time, the same is true for quiet prominences. They can have various shapes - hedges, suspended clouds or funnels, but they are always two-dimensional. Stable filaments often become unstable and erupt, but can also simply disappear. Calm prominences live for several days, but new ones can form at the magnetic boundary.
Transient prominences are an integral part of solar activity. These include jets, which are a disorganized mass of material ejected by a flare, and clumps, which are collimated streams of small emissions. In both cases, some of the matter returns to the surface.
Loop-shaped prominences are the consequences of these phenomena. During the flare, the electron flow heats the surface up to millions of degrees, forming hot (more than 10 million K) coronal prominences. They strongly radiate, being cooled, and deprived of support, descend to the surface in the formelegant loops, following the magnetic lines of force.
Flashes
The most spectacular phenomenon associated with solar activity are flares, which are a sharp release of magnetic energy from the region of sunspots. Despite the high energy, most of them are almost invisible in the visible frequency range, since the energy emission occurs 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 brighter than in the neighboring chromosphere, and 3 times brighter than in the surrounding continuum. In Hα, a large flare 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 1033 erg, which is equal to the output of the entire star 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 particle impact on the chromosphere.
Types of outbreaks
The size range of flares is wide - from gigantic, bombarding the Earth with particles, 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, 10-5and 10-4 W/m2 respectively. So M3 on Earth corresponds to a 3× flux10-5 W/m2. This indicator is not linear as it only measures the peak and not the total radiation. The energy released in the 3-4 largest flares each year is equivalent to the sum of the energies of all the others.
Types of particles created by flashes change depending on the place of acceleration. There is not enough material between the Sun and Earth for ionizing collisions, so they retain their original state of ionization. Particles accelerated in the corona by shock waves show a typical coronal ionization of 2 million K. Particles accelerated in the flare body have significantly higher ionization and extremely high concentrations of He3, a rare isotope of helium only with one neutron.
Most major flares occur in a small number of hyperactive large sunspot groups. Groups are large clusters of one magnetic polarity surrounded by the opposite. Although the prediction of solar flare activity is possible due to the presence of such formations, researchers cannot predict when they will appear, and do not know what produces them.
Earth Impact
In addition to providing light and heat, the Sun impacts the Earth through ultraviolet radiation, a constant stream of solar wind and particles from large flares. Ultraviolet radiation creates the ozone layer, which in turn protects the planet.
Soft (long wavelength) X-rays from the solar corona create layers of the ionosphere that makepossible shortwave radio communication. On days of solar activity, the radiation from the corona (slowly varying) and flares (impulsive) increases, creating a better reflective layer, but the density of the ionosphere increases until radio waves are absorbed and shortwave communication is hindered.
Harder (shorter wavelength) X-ray pulses from flares ionize the lowest layer of the ionosphere (D-layer), creating radio emission.
The Earth's rotating magnetic field is strong enough to block the solar wind, forming a magnetosphere that particles and fields flow around. On the side opposite the luminary, the field lines form a structure called the geomagnetic plume or tail. When the solar wind increases, there is a sharp increase in the Earth's field. When the interplanetary field switches in the opposite direction to Earth's, or when large particle clouds hit it, the magnetic fields in the plume recombine and energy is released to create the auroras.
Magnetic storms and solar activity
Every time a large coronal hole orbits the Earth, the solar wind accelerates and a geomagnetic storm occurs. This creates a 27-day cycle, especially noticeable at the sunspot minimum, which makes it possible to predict solar activity. Large flares and other phenomena cause coronal mass ejections, clouds of energetic particles that form a ring 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 other long conductors.
Perhaps the most intriguing of all earthly phenomena is the possible impact of solar activity on the climate of our planet. The Mound minimum seems reasonable, but there are other clear effects. Most scientists believe that there is an important connection, masked by a number of other phenomena.
Because 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 mostly protons, because hydrogen is the dominant constituent element of the sun. Many particles moving at a speed of 1000 km/s second create a shock wave 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.
