The slowly varying component of the solar radio radiation

 The slowly varying component of the solar radio radiation

The longer wavelengths from Sun that we receive on Earth originate in the corona where the characteristic temperature is high. The temperature relevant to the measured intensity of meter wavelengths is - 10° K. The intensity of this radiation undergoes slow variation, corresponding to the variations in the coronal structure. The measured radiation actually correspotids to a temperature of 10° K and a density nearly equel to three times the normal coronal density.

 The slowly varying component of the solar radio radiation thus fits with the concept that-it is thermal radiation trom regions of coronal condensations. The mechanism which gives risc to this radiation is free-free transition of clectrons in the hot coronal gas. The burst component of solar radio emission is sporadic, intense and nonthermal in character. This component is associated with the activity of the Sun. Different types of bursts have been recognized and the physics behind them is quite complex. As the name implies, bursts are manifested as the enhancement of. the radiosemission by onders of magnitude within periods as short as a few seconds.

 If the intensitaesof tursts are interpretedas of thermal origin, the corresponding equivalent blackbody temperatures would be of the order of 1012 K, which is absurd. The bursts are of different nature and they have been broadly classified into types 1, II, III. IV and V. Of these, all but the Type I bursts are correlated with flares.

 Type I bursts are not associated with flares or centres of activity. Type I bursts originate in noise storms that are produced high up in the corona usually above larger spots, their polarization showing no correlation with that o the spots. These bursts usually have lite-times of 0.1 to 0.2 seconds and during this period the wavelength of radiation remains fairly constant. The radiation.

 The theory of Gold and Hoyle thus predicts the release of the right amount of energy within the right amount of time as are observed in flares, through the meehanism of the magnetic field annihilation. The theory therefore implies the operation of a mechanism by which the twisted magnetic field which develops on the selar surface may become untwisted through flare discharge: Thistrunstormation irom twisted to untwisted field may have important bearing on the equilibriam configuration of the Sun. erg may be associated with such a twisted Soon after the production of radio frequency waves in the laboratory by Heinrich Hertz in 1888.

 Some scientists started thinking that the Sun might be a source of radie waves. Valuable work was one in this line by Wilsing. Scheiner, Nordmann, Jansky and others during the first few decades of this century and the end of the last century. But the actual detection of Sun.

The Second World War Independently By Stanley Hey of Britain

The second world war independently by Stanley Hey of Britain

  The shorter radio waves of wavelength - I cm come from the lower chromosphere which corresponds to a radiation temperature of 10 K. Meter and decameter wavelengths which are generated at the lower chromosphere are absorbed by ionized gas in the upper chromosphere and the corona, and therefore, cannot reach the Earth.

 The radio source was missed primarily due to poor technological advancement in those years. The discovery was finally made during the second world war independently by Stanley Hey of Britain, and by G.C. Southworth and G. Reber of the U.S.A. The subsequent decades were marked by intensive as well as extensive studies of the solar radio emission by scientists all over the world.

 The study of the radio-spectrum of the Sun has revealed that the radio emission from the Sun can be classified into two well-defined components: (a) a slowly varying component with sufficiently lone characteristic times, and (b) the sporadie burst component having much shorter characteristic times. The first component is of thermal origin and is recognized as the characteristic of the quiet Sun, while the second component is nonthermal which is characteristic of the disturbed Sun.

 As a hot body the Sun must emit radiations in rauio wavelengths, but since the temperature of the Sun is not uniquely defined (being different at different levels of the atmosphere) the nature of the solar thermal radio emission is altitude-dependent.

 The highly ionized corona is completely opaque to longer radio waves just as the case with the Earth's ionosphere. Thus, observing the Sun at different radio wavelengths means observing at different levels of its ätmosphere. This also enables us to distinguish between different levels in chromosphere and corona from which various disturbances give rise to radio emission.

Severny's theory has not emerged as fuli proof in magnetic field

Severny's theory has not emerged as fuli proof in magnetic field

 

 high gradient of fields. In fact, substantial modifications in the magnetic field structure are actually observed in the flare region after the flare has died out. Moreover, large magnetic field gradient is capable of accelerating particles to high energies which can subsequently emit radiation in wavelengths ranging from X-rays to long radio waves, as are actually observed in flares. But there are other observations of magnetic fields which Severny's cannot explain, and so Severny's theory has not emerged as fuli proof.

 A plausible theory of the origin of flares has also been proposed by Gold and Hoyle. Their theory relies for the energy source on coalescing and cancellation of two bundles of twisted magnetic lines of force. They imagine a filament consisting of a bundle of lines of force which emerges from a point in the photosphere and re-enters ät another point thus forming an arch.

 This bunch of lines of force may be twisted due to highly  In order that this energy may be released abruptly, a pair of such fil ments with initial magnetic fields in opposite directions but the same sense of twisting is hypothesized. Two such filaments placed close to each other will attract each other, interpenetrate, and a part of the field will be annihilated in a time-scale of the order of 10 seconds.convective motion in the subphotospheric layer.

 The energy density of this convective mass motion is 10 erg em only a few per cent of which is associated with flarë phenomena. The twisting of the field can greatly increase the intensit, of the initial moderate field. Calculation shows that an amount of magnetic energy equal to - 100 filament. This is the right amount of energy that is involved with flare phenomena.

The Earth's Ionosphere And Terrestrial Radio Communications

The Earth's ionosphere and terrestrial radio Communications

  The Earth's ionosphere and terrestrial radio communications are disrupted by flare X-rays. The solar ultraviolet radiation is greatly enhanced during solar flares. Sometimes, a single flare may outshine the entire Sun in ultraviolet radiation. Ultraviolet lines are generally enhanced but the L line remains remarkably unchanged during flares. It is interpreted that this line probably is radiated from the entire chromosphere. The Sun is the nearest source of cosmic rays. 

During large flares some of the solar (protons, eiectrons and heavier nuciei) are accelerated to very high energies-particles with energies even higher than 100 eV have been observed The highest flux of particles occurs, of course, in the MeV range. These high energy rays whose composition has been found not to differ significantly from the normal composition of the Sun itself. The most abundant particles are protons. Next comes the Alpha particles, and so on. The fraction of the flare energy contained in the flare cosmic ra s is, of course, quite small, usually less than one per cent. The particles are believed to be accelerated in regions of high magnetic field gradient but no entirely convincing explanation of the origin of solar cosmic rays has yet been established.

 Several theories have been proposed by various authors to explain the origin of solar flares. Flares have been speculated as manifestations of clectrical discharge. It has also been conjectured that flare energy might be produced by nuclear reactions in the surface layer of the Sun. Since both these theories have later been shown to be highly improbable, we shall particles are the solar cosmic not discuss these any further. Although none of the flare theories so far proposed appear completely satisfactory, we shafl briefly discuss two of the theories which can explain manyat the observed charactenstics of flares. Both these theories are based on the high magnetic Kelds in sunspot regions.

 Observationally, it is found that flares occur mostly in regions of complex spot groups, where magnetic fields are high. Magnetic fields of a few hundred gauss can supply energy of the order of a few times 10 erg cm, as flares are observed to possess (using E = B-18 TC). But the theory must also explain the extreme suddenness of the release of this energy. Severny proposed that flares originate as a result of magrnetic discharge near an X-type neutral point which may arise by interaction of fields of two neighbouring bimmagnetic spots in a complex spot group.

 The magnet field gradient is very high near neutral points. The intermetion of spot groups may initiate a kind of perturbation, which may trigger the magnetic discharge. thereby suddenly dissipating large amount of magnetic energy and neutralizing.

Particles leave the influence of solar gravity

 Particles leave the influence of solar gravity

 A vast range in size and brightness is observe  among flares and their importance is assigned accordingly. The importance is labelled 1, 2, 3 and 3 according as the areas covered by the flare lie in the ranges 100-250, 250-600, 600-1200 and 1200 respectively, which are measured inunts of a ailidntf part of the sotar disk. Besides the above classes, there are oflares iubelled with mporance and the microflares. Aithough the flare statistics partly depend on the observer, rough analysis indicate that about 80 per cent of flares are of importance I and about 3 per cent are of importance 3, while the remaining flares are of importance 2, the flares of importance 3" are of extremely rare occurrence. 

Flares, are events of frequent occurrence in the Sun and if we include subflares, there may be one flare every hour but flares of importance 3" hardly occur even once in a month. Flares are correlated with sunspot activity and the number of flares is observed to bear an approximate mathematical relation with the number of sunspots. It is found that the number.

Particles leave the influence of solar gravity and after a certain period depending on their speeds, impinge on the Earth's magnetosphere giving rise to geomagnetic storms and aurorae. This phenomenon, commonly known as solar wind, will be discussed in greater details in the Jast section of this Chapter. As we have already mentioned, solar flares are always accompanied by enhanced X-Tay and ultraviolet radiation.

 The first X-ray measurements during a flare were performed by H. Friedman in 1956 by sending rockets to heights in the range of 60 km. These data were extended by several other flights in subsequent years.

 Such measurements, in general, show two components of X-ray radiation: (a) a less energetic slow component lasting for a longer time; usually of order of an hour, and (b) a sudden short-lived high energetic component lasting for a few minutes. The first component originates from natural transitions in the coronal gas heated by flares to a temperature - 5 x 10 K. The second component is associated with bursts of nonthermal emission.

stares are the most energetic phenomena of highly transient nature

stares are the most energetic phenomena of highly transient nature

Stares are the most energetic phenomena of highly transient nature occurring in the Sun. While observing the Sun on spectroheliograms or filtergrams, sometimes the chromospheric emission lines, particularly the H and K Ca II lines originating in the sunspot and surrounding clage regions are observed to brighten up intensely within a few seconds or minutes.

 Such corcurrences are known as flares. The intensity of emission lines increases as much as four to five timies of those originating in plages within a few minutes, and sometimes within only 10 res. (for microflares). The strengths of lines then decay raher slowly within a time interval of 10 to 100 minutes.

 A very large flare can also be observed in the white light of the Sun when it appears as an intense bright spot against the continuous background of the spectrum of the surrounding solar disc. Flares are believed to originate in chromosphere but rise far up into the lower corona.

 They have a tendency to occur near the complex groups of spots and usually the occurrence is repeated many times in the same place. Occasionally, they are seen to occur when a spot group is rapidly developing. Some of these have also been observed to occur from the umbrae of large full-grown spots with complicated magnetic field structures.

 These observations clearly associate ilares with sunspot activity. Flares are classified according to their brightness and the areas covered by them. The area seems to be more important criterion of classification because usually, the larger the flare the brighter it is and with longer lifetime.

During his study of flare spectra with high dispersion instruments

 During his study of flare spectra with high dispersion instruments

Among other lines seen in emission are those of ionized metallic lines. the D, line of He I at  A and some H lines of the Paschen series. D, line in small flares is seen a absorption. Lines of Fe II and those of low excitation Fe I are strongly brightened in flares. All these observations are consistent with highe: excitation in flares. Electron densities in the range 10 to 105 cm and electron temperatures in the range   17,000 K have been suggested by various authors to be the most favourable state under which the observed spectra are produced.

 During his study of flare spectra with high dispersion instruments, Severny observed some fine structure granules which develop during the initial stage of the flares. These granules are short-lived with diameters - 400 km. Gas moves within granules with speeds as high  km snd it greatly influences the wings of lines of flare spectrum.

 Severny called these phenomena as "moastaches Moustaches are believed to be ditferent, fromU flares While flares are manifestations of release of large energies within a relatively short-time, moustaches are believed to be associated with small-explosion like events. Flares eject material with speeds ranging from a few hundred to as high as  km s. The streaming material perturbs prominences and filaments.

 The latter are sometimes destroyed or disrupted by flares, but strangely enough, reappear again after hours or days. Spectrographic studies indicate that large gaseous blobs which are manifestation of flare surges, rise high up into the solar atmosphere.

 Undergoing steady deceleration due to solar gravity and ome down into the Sun again with approximately the same speed. Besides this Sun-bound mass motion, flares also eject streams of corpuscular radiation in a wide range of energies. These high-speed.

Indicate that the rate of flare production associated with any particular

indicate that the rate of flare production associated with any particular

Flares per day of importance higher than or equal to I is - R/25, where R is the supe number given by (5.2). Thus, if R = 100 on any day, four flares of importance 21 can be expected. Since subflares are several times more numerous, if these are taken into acov a flare every hour will be more natural.

 Since flares originate almost always within abow 100,000 km of a spot group, the flare distribution on the solar disk bears close correspondenc with the distribution of spot groups. Further, quantitatively, observations indicate that the rate of flare production associated with any particular spot group depends on the area A of the spot group and on the time rate of change of the area dAldt.

 A typical flare has a surface area 5 x 10 cm (assuming radius = 4 x 10° cm and circular shape) and a height of -2 x 10° cm so that the typical volume is - 1029 cm. The total energy outpu' for such a flare is - 2 x 104 erg, so that the average energy density in a flare is - 2 x 10 erg cm. Most of this energy Is emitted in corpuscular radiation. Although flares radiate in most of the frequencies of the electromagnetic spectrum, from X-rays and rrays to long-wavelength radio waves, and also relativistic particles (cosmic rays), most of the energy is emitted in X-rays and ultraviolet radiation. The energy emitted in optical range is not too large, may be about 10 per cent and that in radio waves and cosmic rays is insignificant.

 Like size and intensity, the spectrum of flares also differ greatly from one another. In large flares the continuum is strongly brightened, but in normal flares of moderate importance the brightening of the continuum is not very marked, except over a region surrounding H H itself is observed in strong emission with large symmetrical emission wings.

 The wings, however, fade away quickly after the onset of the flare, while the flare may still linger. The central intensity of H becomes three to four times that of the continuum and the width may be as large as 15 Á.