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.     

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.     

On the observation of scattered radio emission from sources in the solar corona

The observations of a brief flaring region between two plages on the eastern limb of the Sun and the subsequent coronal transient are reported for June 16, 1972 by Koomen et al. (1974). Both of these events have unambiguous and closely timed associations with the solar noise bursts observed at 2800 and 2700 MHz and are also accompanied with good X-ray and SID effects but faint subflare (Solar-Geophysical Data). The two frequencies are those monitored at widely separated stations operated by the Astrophysics Branch of the National Research Council at Lake Traverse, Ontario and at Penticton, B.C.     

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.     

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.     

Photometric intensity and polarization measurements of the solar corona

During the 7 March, 1970, total eclipse of the Sun, the NASA Langley Research Center's Satellite Photometric Observatory (SPO) was used to measure the solar corona from Miahuatlan, Mexico. The SPO is equipped with a 24-inch Cassegrainian telescope, a four-channel photoelectric photometer, a Wollaston prism, and a rotating half-wave plate. Simultaneous measurements were made of the two orthogonal components of coronal light in the B and R bands of the UBVRI system. A 1-minute arc aperture was scanned from the lunar disk center out to five solar radii in a series of spirals of gradually increasing radius. For the first time, simultaneous multicolor intensity, degree, and angle of polarization profiles are computed from photoelectric measurements. Comparison of the variations of the measurements for each spiral scan yield a detailed picture of the intensity and polarization features in the K corona.     

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.     

Quasi-harmonic faraday-rotation fluctuations of radio waves when sounding the outer solar corona

A statistical analysis of the Faraday-rotation fluctuations (FRFs) of linearly polarized radio signals from the Helios 1 and Helios 2 spacecraft shows that the FRF time power spectra can be of three types. Spectra of the first type are well fitted by a single power law in the range of fluctuation frequencies 1–10 mHz. Spectra of the second type are a superposition of a power law and two quasi-harmonic components with fluctuation frequencies of about v₁=4 mHz (fundamental frequency) and v₂=8 mHz (second harmonic). Spectra of the third type exhibit only one of the two quasi-harmonic components against the background of a power law. The spectral density of the quasi-harmonic components can be represented by a resonance curve with a fairly broad [Δυ ≈ (0.5–1.3)υ_1,2] distribution relative to the v=v_{1, 2} peak. The intensity of the quasi-harmonic FRF has a radial dependence that roughly matches the radial dependence for the background FRF, while their period at the fundamental frequency is approximately equal to the period of the wellknown 5-min oscillations observed in the lower solar atmosphere. The fluctuations with 5-min periods in FRF records can be explained by the presence in the outer corona of isolated trains of Alfvén waves generated at the base of the chromosphere-corona transition layer and by acoustic waves coming from deeper layers.     

A type II solar radio burst observed in the corona and in interplanetary space

Solar Physics - Possible mechanisms of the third harmonic generation in solar radio bursts are investigated. It is shown that the most essential is the coupling of plasma waves in a source with...     

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.     

Electrons in the solar corona

During a balloon flight in France on September 13, 1971, at altitude 32 000 m, the solar corona was cinematographed from 2 to 5R during 5 hr, with an externally occulted coronagraph.Motions in coronal features, when they occur, exhibit deformations of structures with velocities not exceeding a few 10 km s^–1; several streamers were often involved simultaneously; these variations are compatible with magnetic changes or sudden reorganizations of lines of forces.Intensity and polarization measurements give the electron density with height in the quiet corona above the equator. Electron density gradient for one of the streamers gives a temperature of 1.6 × 10⁶ K and comparisons with the on-board Apollo 16 coronal observation of 31 July, 1971 are compatible with the extension of this temperature up to 25 R _bd.Three-dimensional structures and localizations of the streamers are deduced from combined photometry, polarimetry and ground-based K coronametry. Three of the four coronal streamers analysed have their axis bent with height towards the direction of the solar rotation, as if the upper corona has a rotation slightly faster than the chromosphere.     

Electrons in the solar corona

We discuss a model for the formation of the chromospheric Ca ii K line which does not make the usual assumption of complete redistribution. Using a physically reasonable scattering model, we find significant departures due to the frequency dependence of the line source function, particularly in the relative intensity and centre-to-limb behaviour of the K₁ parts of the line and in the asymmetry produced by differential velocity fields. We conclude that the frequency dependence of the K line source function must be considered in quantitative models for the formation of the K line.     

Electrons in the solar corona

The polarimetric survey of electrons in the K-corona initiated at Pic-du-Midi and Meudon Observatories in 1964 now covers a full solar cycle of activity. The measurements are photometrically calibrated in an absolute scale.In June 1967 a persistent coronal feature was fan-shaped as a lame coronale above quiescent prominences. We deduce an electron density of N ₀ = 1.5 × 10⁸ at 60 000 km above the photosphere, a total number of 14 × 10³⁹ electrons, a hydrostatic temperature of 1.7 × 10⁶ K, and a total thermal energy 3N _eKT = 1.0 × 10³¹ ergs. When a center of activity appeared, a major localized condensation developed to replace the old elongated feature, with N ₀ = 4.5 × 10⁸, a total of 4.5 × 10³⁹ electrons and the same temperature of 1.7 × 10⁶ K.Also, a fan-shaped feature of exceptional intensity was analysed on 8 September 1966, with N ₀ = 6 × 10⁸ and a total of 24 × 10³⁹ electrons.Fan-shaped features are frequent above quiescent prominences. They degenerate above a height of 2R into thinner isolated columns or blades with temperatures also around 1.7 × 10⁶ K.     

Variation in Ovii X-ray line-emission from the solar corona

The variation in intensity of the solar X-ray resonance (1s ^{2 1} S ₀ - 1s2p ¹ P ₁), intercombination (1s ^{2 1} S ₀ - 1s2p ³ P ₁), and forbidden (1s^{2 1} S ₀ - 1s2s ³ P ₁) lines of helium-like Ovii with 2800 MHz solar radio flux is presented for three solar rotations. A high correlation (r 0.80) exists between the intensities of all three X-ray lines and the 2800 MHz solar flux. The ratio of the forbidden to the intercombination line intensities is found to be essentially independent of long term solar activity. This ratio is used to determine upper limits on the coronal electron density and to make inferences concerning the change in density with solar activity.     

The digital system ARTEMIS for real-time processing of radio transient emissions in the solar corona

We present the real-time digital data processing system named ARTEMIS that was developed and constructed by the Space Research Department (DESPA) of Paris-Meudon Observatory to digitize, calibrate, format, date, process, compress, and archive in real time signals from multichannel receivers. This system is controlled by a multiprocessor computer based on Motorola MC 68010/68020 processors; it permits the automatic, routine recording of 128 parallel channels at a rate up to 300 samples per second and per channel with a 12-bit accuracy (4096 levels of intensity); it is used to process and record the 120 channels of a multichannel solar radiospectrograph in the frequency range 110–469 MHz; the remaining 8 channels are used for a scanning spectrograph in the frequency range 30–80 MHz and a two-dimensional multicorrelator interferometer at 75.5 MHz. The large quantity of raw data is reduced in real-time from about 1.3 Gbytes to about 40 Mbytes per day by the use of an original algorithm for real-time data compression. It is expected that this new facility will allow us to build a very large data base of digitized and accurately calibrated solar events, in order to achieve statistical measurements over long periods of time.     

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.     

Gyro-resonance absorption of plasma waves in the corona and the fine structure of solar radio bursts

We consider the important part played by plasma wave absorption at the higher harmonics of the electron gyrofrequency (under conditions of the so-called double plasma resonance) in the solar corona. An explanation is proposed for the fine structure of Type III bursts on the basis of this effect.     

Using scintillation measurements to achieve high spatial resolution in photometric solar observations

The RISE/PSPT (Radiative Inputs from the Sun to the Earth/Precision Solar Photometric Telescopes) experiment will attain high differential photometric precision in full-disk solar images with 1 arc sec pixels. To achieve this spatial resolution it will be necessary to use frame selection techniques to minimize the effects of atmospheric seeing. We report here on experiments to use a simple scintillation monitor as a trigger or veto for imaging observations.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

Re-interpretation of OSO-III scintillation counter measurements of hard solar X-ray spectra

Laboratory tests show that pulse pile-up requires a significant correction to the response of scintillation counters to solar X-ray bursts with steep energy spectra. The interpretation of the OSO-III and other satellite data in view of these corrections indicates that steep solar X-ray spectra with varying emission measure produce an effect similar to that produced by relatively hard spectra with a constant emission measure.     

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.