Pulsating radio emissions from the solar corona

Three solar outbursts which show pulsating radio emissions at metric waves (239 MHz) are examined. The behaviour of the single frequency, high-time resolution records and the spectral diagrams seem to indicate that such phenomena are peculiar phases of type IV radiation, perhaps connected with absorptions in the solar corona. The spectral analysis of the low-frequency modulation of the emissions show a very definite spectral line with a period ranging from 1^s.7 to 3^s.1.     

Thermal cyclotron radio emission of neutral current sheets in the solar corona

Cyclotron radiation (its frequency spectrum and polarization) of thermal electrons in a neutral current sheet is considered. It is shown that cyclotron radiation is able to escape only from a thin edge of the sheet where the magnetic field is practically homogeneous. Due to this, the frequency spectrum has the form of comparatively narrow lines with integer ratio of frequencies. This fact enables one to recognize neutral current sheets in the solar corona by their radio emission.     

On a cold emission in the solar corona

The faint emission of hydrogen, helium and metals in the corona which appeared near an active prominence is studied. The calculations showed that the temperature of the emission region is in the limits from 10 000 K to 30 000 K and the electron density is between 10⁹-10¹⁰ cm^-3, respectively.     

Structure in the solar corona from radio scintillation measurements

Since the 1950s, a wide variety of radio observations based on scattering by electron density fluctuations in the solar wind has provided much of our information on density fluctuations and solar wind speed near the source region of the solar wind. This paper reviews recent progress in the understanding of the nature of these density fluctuations and their relationship to features on the Sun. The results include the first measurements of fine-scale structure within coronal streamers and evidence for structure in solar wind speed in the inner corona.     

Hard X-ray emission from the solar corona

This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration.     

Multiple radio echoes in the solar corona

Echo-type solar radio bursts are associated with preceding short-lived bursts in double events. The peculiar and rather rare decameter echoes are observed with a UTR-2 antenna. The initial narrow-band burst is followed, some 5 to 10 s later, by an echo-like burst at the same frequency. The observational data obtained for decameter echo evens are, on the whole, consistent with the model of a pulsed source emitting radio signals at the plasma-frequency harmonic, which is placed in a non-uniform corona and rotates together with the Sun.Intensity-time profiles of 25 MHz echo bursts are of an unusual shape, featuring an extended leading edge and an abrupt decay at the trailing edge and also showing some fine structures in the form of an additional, weakly pronounced maximum or a step at the final stage of the burst. Time parameters characterizing the profiles are evaluated. The step is delayed with respect to the main pulse at about two times longer than the principal echo maximum. At the same time, the time delays depend essentially on the heliolongitude of the active region and achieve their maximum values at the meridian. The step height does not exceed 0.5 of the echo maximum. At this level, the echo-decay time almost coincides with the initial burst duration but is about 1.7 times less than the echo-rise time. The feature at the echo tail can be interpreted as a result of a repeated reflection of the burst from the source region. The causes and conditions for the formation of multiple echoes are discussed. The extended leading edge of the echo permits us to assume a quasi-radial fibrous structure of the corona, capable of back-scattering the incident radiation.     

Scattering of radiowaves from cosmic sources in the solar corona

The scattering of radiowaves in the outer solar corona is discussed. Results are given of the decametric wave observations. In the theoretical analysis both regular refraction and the gradients in the electron density fluctuations are considered. The theory is in satisfactory agreement with the experimental data. From their comparison the ratio n=(l _r /l _t ) is deduced of the correlation scales in the radial (l _r )and transverse directions. This value is not equal to the ratio of the observed distribution dimensions. The frequency dependence of the angular spectrum rms width is not λ² at longer wavelengths. At small separations from the Sun, however, the rms angular size cannot serve as the only characteristic of the spectrum, the latter being non-Gaussian. Referred to in the paper as the Ukr. IRE.     

Cyclotron wave instability in the corona and origin of solar radio emission with fine structure

The longitudinal waves (Bernstein modes and plasma waves near the hybrid frequency) in a mixture of equilibrium coronal plasma and a small group of energetic electrons are investigated. The energetic electrons have a nonequilibrium momentum distribution inherent in trapped particles. The frequency dependence of the cyclotron instability increments is studied. Attention is paid to a significant role of the relativistic effects for the cyclotron instability of longitudinal waves. For sufficiently large velocity of nonequilibrium electrons the increments are shown to increase when the hybrid frequency coincides with one of the gyrofrequency harmonics (double plasma resonance). The results obtained are used in Parts II and III to explain tadpoles and zebra-pattern in solar radio bursts.     

An interferometric investigation of emission lines from the solar corona

The profiles of the Fe xiv, 5303, and Fe x, 6374, emission lines of the solar corona have been observed at different positions using a photoelectric scanning Fabry-Perot interferometer. These profiles were obtained during the eclipse of 7th March 1970, in Mexico and at the Pic-du-Midi coronagraph in October, 1970. The half-widths of these profiles were determined for both the coronal lines and temperatures were derived from these widths. No systematic temperature variation was discovered, however there was some suggestion of the existence of a fluctuation with time in the width of the emission lines.     

Whistlers in the solar corona and their relevance to fine structures of type IV radio emission

This short report concerns a general consideration of whistler manifestations in fine structures, including possible trajectories of obliquely propagating whistlers, the role of quasilinear diffusion and an interpretation of new observations. A whistler ray tracing and kinetic whistler growth rates under arbitrary angles to the magnetic field in the solar corona were calculated. Different regimes of a whistler instability yield divers elements of fine structures: different stripes in emission and absorption or millisecond pulsations, moreover, zebra-stripes can convert into fiber bursts and inversely. A new explanation of low-frequency absorption in fibers is proposed: it is connected with an attenuation of plasma-wave instability due to the fast electron diffusion by whistlers. Rope-like chains of fiber bursts are explained by a periodic whistler instability in a magnetic reconnection region.     

On association of radio emission with solar surges

About 45% non-flare surges are found to be associated with radio bursts inferred from spectral data. Associated surges are mostly accompanied by type I (24%) and type III (29%) bursts.     

The solar corona above active regions: A comparison of extreme ultraviolet line emission with radio emission

The observation of extreme ultraviolet (EUV) emission lines of Fe ix through Fe xvi made by Orbiting Solar Observatory-1 are discussed and applied to a study of the solar corona above active regions. Ultraviolet and radio emission are determined and compared for several levels of activity classified according to the type of sunspot group associated with the active region. Both radio emission and line radiation from Fe xvi, the highest stage of ionization of Fe observed, are observed to increase rapidly with the onset of activity and are most intense over an E-spot group early in the lifetime of the active region. As activity diminishes, radiation from Fe xv and Fe xvi becomes relatively more prominent. The observations imply that the coronal temperature reaches a maximum during the period of highest activity, as indicated by sunspot-group complexity and the occurrence of chromospheric flares. A maximum coronal electron temperature of 4.0 × 10⁶ °K is estimated when taking into account the mechanism of dielectronic recombination. Concurrently, the average coronal electron density increases by a factor of 10–12. Both electron temperature and density decrease as activity subsides. The coronal temperature above the remaining Ca ii plage is estimated to be 2.5–3.0 × 10⁶ °K after flare activity has ceased and sunspots have disappeared.     

The gyro-synchrotron radiation from moving type IV sources in the solar corona

Calculations of the gyro-synchrotron emission are made for conditions which might be expected in moving type IV sources in the solar corona. Two simple models for an evolving source are treated: a uniform cube and an inhomogeneous sphere. The results suggest that most moving sources have the following features: (1) A rather strong magnetic field, 10 G, is carried out within the source. This is required to achieve the high degree of circular polarization often observed. (2) Synchrotron self-absorption causes the source to be optically thick at frequencies less than about 100 MHz, thus restricting the bandwidth of the radiation. The self-absorption decreases as the source moves outward and expands. The turnover frequency, which separates the optically thick and thin spectral regimes, moves rapidly to lower frequencies, accompanied by a change from low to high circular polarization. In the case of an inhomogeneous source, the source appears to be larger at the lower frequencies. (3) Razin-Tsytovich suppression cannot be an important factor in determining the characteristics of most sources.Exchange visitor from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado, U.S.A.     

A U-type solar radio burst originating in the outer corona

The observation of a U-type solar radio burst with a reversing frequency of approximately 0.7 MHz suggests the presence of a magnetic bottle extending out to about 35 R . A possible model of this loop structure is developed from the data. The occurrence of low-frequency U-bursts seems to be extremely rare although magnetic bottles may develop frequently during solar maximum.     

Evidence for arc sec radio burst sources in the upper corona

Some interpretations of solar S burst spectra are presented. It is shown that the spectra provide evidence for small (～ 500 km) radio sources in the corona which radiate at the fundamental plasma frequency. The possibility of S burst fringes corresponding to coronal MHD waves of wavelength λ ～- 10³ km is discussed.     

Interferometer observation of pulsating sources associated with a type IV solar radio burst

An extremely complex outburst, part of which showed unsually rapid intensity fluctuations of a few second interval, was observed on 1970 November 5 with the 160 MHz interferometer of the Nobeyama Solar Radio Station. The pulsating source, which was stable in position and strongly circularly polarized ( 60 %), had an extension as large as 17 (7.5 × 10⁵ km) in the east-west direction. The structure of the source remained unchanged while the source darkened and brightened repeatedly. The change of the source brightness occurred in a time shorter than a second.Two alternative mechanisms responsible for the pulsating phenomenon are suggested; (1) gyroresonance absorption of continuum radiation by a fast particle beam injected in a quasi-periodic manner into a large region of weak magnetic field, or (2) magnetohydrodynamic oscillation of the continuum source itself, which is intrinsically much smaller than observed. It is observed as a large source as a consequence of scattering of the emitted radiation in a region situated above the source.     

On a possible mechanism of zebra-pattern generation in solar radio emission

A mechanism is proposed for the generation of zebra-patterns in solar radio bursts due to the excitation of nonlinear ion-sound waves in a nonisothermal plasma and their scattering on fast particles. The appearance of the ion sound at the fundamental frequency can take place in the interaction of two opposing Alfvén or whistler waves. The presence of quasi-equidistant stripes in electro-magnetic radiation is ultimately determined by weak ion-sound dispersion resulting in the formation of higher harmonics.     

A model for drift pair and hook burst emission from the solar corona

Combination scattering of the Cerenkov plasma waves generated by a fast electron beam on the electron density fluctuations in a magnetoactive plasma is assumed to be the cause of the emission of the drift pair (or the hook burst) from the solar corona. The features of the combination emission are studied numerically with parameters appropriate to the solar corona condition. It is found that the major properties of the drift pair and the hook burst can be accounted for.     

On altitude structure of centimeter-wave radio emission of solar active regions

A method is presented for the direct measurement of the heights of the radio emission of solar active regions when they are located at the limb in order to reconstruct the vertical structure of the magnetic field in solar active regions. The method involves an analysis of radio source positions in the scans based on high frequency resolution one-dimensional centimeter-wave measurements performed on the RATAN-600 radio telescope. Radio sources are difficult to identify at many frequencies when observed at the limb at zero position angle because of abrupt signal variations at the solar limb. To eliminate edge effects on the scan, special observing periods are used (near vernal and autumnal equinoxes), when the source at the limb is located far from the scan edge because of the large position angle of the Sun. As a result of these observations, the spectra of relative heights are constructed for a number of sources for the period from 2007 through 2012. Source heights are shown to generally increase with wavelength. The height difference between the 5 and 2 cm emission is equal to 5.2 ± 2.0 Mm, and the corresponding height difference between the 8 and 2 cm emission is equal to 9.6 ± 3.0 Mm. It is shown that such characteristics can be obtained for a field generated by a dipole submerged under the photosphere at a depth of 17 Mm irrespective of the possible reduction of relative altitudes to absolute altitudes.     

Low frequency turbulence in the solar corona and fundamental radiation of type III solar radio burst

On the basis of the previous numerical simulations, a new mechanism for the emission of the fundamental radio waves of solar radio type III bursts is presented. This hypothesis is to attribute the fundamental radio emission to the coalescence of the plasma waves with the low frequency turbulence, whistler or ion acoustic waves, pre-existing on the way of the electron beam which excite the plasma waves.It is estimated that ion acoustic waves could be occasionally unstable in the solar corona due to that drifting bi-Maxwellian distribution of electrons as observed in the solar wind, which is probably caused by collision-less heat conduction.It is also suggested that the reduced damping of the ion acoustic waves in such a distorted electron distribution in the corona may decrease the threshold electric current to cause the anomalous resistivity to be the onset of the solar flares.