Slowly varying component spectrum of the solar radio emission at millimetre wavelengths

This paper deals with the observed data on the solar S-component sources at millimetre wavelengths. The observations were made in 1968 and 1969 using the 22-m radio telescope of the Crimean Astrophysical Observatory at six wavelengths: 2, 4, 6, 8, 13 and 17 mm. The enhanced intensity of the solar active region in comparison with the quiet Sun level varies proportionally to ^–2 if the wavelength is within the range of 2 ÷ 6 mm. In the wavelength band of 6 ÷ 17 mm almost flat spectra of the solar S-component sources is observed. Assuming the bremsstrahlung mechanism of the radio emission for the quiet Sun and the solar active regions an attempt has been made to treat the above presented data. It appears that the most probable explanation of the 2 ÷ 6 mm spectrum is that the S-component sources are opaque. In the 6 ÷ 17 mm wavelength band there are two possibilities: the active region may be either transparent or opaque. But in the last case the source brightness temperature must be proportional to ². Some differences in the spectra of the sources, identified with flocculi and with bipolar sunspot groups, were mentioned. The cold regions (as compared with the quiet Sun) were observed up to = 2 mm and identified with the filaments. However, its visibility falls when the wavelength decreases.     

A multi-layered backpropagation artificial neural network algorithm for UV spectral classification

In this paper we present an application of an artificial neural network model based on a multi-layered backpropagation algorithm for spectral classification of UV data from the International Ultraviolet Explorer (IUE) low dispersion spectra reference atlas. The model used is similar to that of von Hippel et al. (1994), and is found to reduce the classification error as compared to the recently reported results on the same data set (Gulati et al. 1994b). The improved version of the network is much simpler in structure and the training time is reduced by a factor of almost 20. Such networks will prove very useful in efficient classification of large databases Subject headings: neural networks, stellar spectra, classification     

Nonlinear treatment for solar radio spikes

We emphasize that a nonlinear treatment is required to realize the diagnostic potentiality of solar spiky emission. The observational constraints including the latest data on the harmonic structure, degree, and sense of polarization are discussed. A set of coupled equations for energy density of high-frequency normal modes of a magneto-active plasma involving the most important nonlinear effects within the three-wave approximation is deduced. The equations include both previously known and new effects. The qualitative evaluations of the equations obtained have provided a few new findings: (i) quasi-linear relaxation of fast electrons on quasi-potential waves (_) occurs in a characteristic time scale of the order of 10 ms if the frequency,f, is about 1 GHz; (ii) the stimulated scattering of the transverse waves on the background plasma particles is shown to be important if the brightness temperature of the spiky emission exceeds 10¹⁵–10¹⁶ K; (iii) the Raman scattering of the transverse waves on background plasma density inhomogeneities may suppress the electron cyclotron maser instability if n _e ² /n _e ² 3 × (10^–4–10^–5).     

Wave propagation in the warm plasma and the spectrum of the solar radio bursts

The frequency bands of noise storms and type I-bursts as well as of type IVdm-bursts are shown to be in accordance with the consequences of a recently proposed mechanism (Mollwo, 1970). An explanation results of the observed spectral minimum near 600 MHz and interpretations are supposed of some features of type III- and of type IVmA-bursts. The magnetic field strength over active regions in two corona levels is deduced, too. The discussion leads to a conception of the corona parameters in the level of type IV-bursts which suggests an origin of these bursts by absolute instability of space charge waves.     

Colour display for Culgoora solar radio spectrograph

A real-time colour display of the Culgoora acousto-optical radio spectrograph is described and illustrated.     

Antenna performance analysis for decameter solar radio observations

Decameter wavelength radio emission is finely structured in solar bursts. For their research it is very important to use a sufficient sensitivity of antenna systems. In this paper we study an influence of the radiotelescope‐antenna effective area on the results of decameter solar radio observations. For this purpose we compared the solar bursts received by the array of 720 ground‐based dipoles and the single dipole of the radiotelescope UTR‐2. It is shown that a larger effective area of the ground‐based antenna allows us to measure a weaker solar emission and to distinguish a fine structure of strong solar events. This feature has been also verified by simultaneous ground‐ and space‐based observations in the overlapping frequency range (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A theory for the solar type-I radio continuum

The type-I radio continuum may arise from the combination of two electrostatic waves, both directed nearly normal to the magnetic field. One wave, near the upper-hybrid frequency, is generated by gyroresonance with superthermal electrons and comes into equilibrium with these electrons. The other wave, at the lower-hybrid frequency, is generated by the loss-cone instability of trapped superthermal protons in those wave directions for which the lower-hybrid frequency is an exact multiple of the proton gyrofrequency. The brightness temperature of the continuum indicates both the energy of the superthermal electrons and the existance of at least a small number of superthermal protons.     

A torsional wave model for solar radio pulsations

One of the widely accepted models for solar radio pulsations invokes radial oscillations of a magnetic flux tube. Due to acoustic, radiative damping, this theory does not easily explain the long length of the pulse trains, the large modulation depths or the great stability of the pulse repetition rate often observed. Torsional waves efficiently modulate synchrotron emission, and since they do not undergo radiative damping, can produce stable pulse repetition rates and long pulse trains.     

Theory of solar radio pulsation

We show that the observed modulation of some coronal microwave, X-ray and Type III emission into pulses of 10 sec intervals is a consequence of the stimulation of electron cyclotron waves propagated in the whistler mode in dipole-like bipolar regions of dimension 0.2 R . Assuming that a power law spectrum of 10 keV electrons with a slope similar to solar flare protons can be trapped in a bipolar region, we show that whistlers can be generated by pitch angle instability. The resultant 10 sec bounce motion of whistler wave trains leads to enhanced, modulated emission in microwave and X-ray frequencies by pitch angle scattering of MeV electrons, and to modulated Type III emission by scattering with coherent plasma waves. A direct prediction of the theory is the existence of sympathetic pulsations at two sources a fraction of a solar radius apart. A second test of the theory is that modulated Type III emission should show strong polarization.This work was conducted under U.S. Air Force Space and Missile Systems Organization (SAMSO) Contract No. F04701-69-C-0066.     

A solar spectrum for PFS data analysis

A measured calibrated solar radiance in the range 1.2-, with the spectral sampling of does not exist. When studying the measured Planetary Fourier Spectrometer (PFS) spectra of the Earth's or Mars's atmosphere we discover that the most used solar spectrum contains several important errors. Here we present a “calibrated” solar radiance in the wavelength range 1.2-, with the spectral resolution of PFS , which we are going to use for studying Martian spectra. This spectrum has been assembled using measurements from Kitt Peak and from ATMOS Spacelab experiment (uncalibrated high resolution) and theoretical results, together with low resolution calibrated continuum. This is the best we can have in this moment to be used with PFS, while waiting to have good solar calibrated radiances. Examples of solar lines at Mars are given.     

Towards a theory for type III solar radio bursts

The mechanisms for the transformation of plasma waves into radiation near the fundamental and second harmonic of the plasma frequency are reviewed and equations are given for both the emission and absorption coefficients for these mechanisms. Near the fundamental the process is the scattering of plasma waves on the polarization clouds of ions and the absorption coefficient can be negative, i.e. the radiation can be amplified. Near the second harmonic the process is the combination of two excited plasma waves for which the absorption coefficient can only be positive. These results are applied to construct models of the radiation source for type III solar radio bursts both at high frequencies where the fundamental is dominant and at low frequencies where the second harmonic is dominant using two model plasma wave spectra, one being one-dimensional, the other isotropic. At high frequencies second harmonic radiation is used to determine the source area for a given energy density in plasma waves W _p . The source size and W _p are detrmined uniquely for a given plasma wave spectrum by tracing rays in a model source taking into account amplification of the fundamental. The results for a strong source at the 80 MHz plasma level with a ratio of emissivities of the fundamental to second harmonic P(ω _p )/P(2ω _p ) ≈ 10 are that the source with a one-dimensional plasma wave spectrum is about 14000 km in diameter and W _p = 10^?6.52 erg cm^?3, and the source with an isotropic distribution of plasma waves is about 200 km in diameter and W _p = 10^?6.3 erg cm^?3. It is shown that at low frequencies, where amplification of the fundamental is no longer possible, second harmonic radiation must be dominant and thus very little information about the source can obtained from the radiation.     

Towards a theory for type III solar radio bursts

The possibilities for type III burst excitors are reviewed and it is concluded that particle streams are the most likely excitor. Possible methods of resolving the apparent discrepancy between the number of particle events observed in interplanetary space in the vicinity of the earth and the number of type III bursts are indicated. Observations relevant to the excitor are reviewed and translated into requirements for a theory of the exciting stream. Possibilities for an electron stream excitor are considered and it is concluded that, while such an excitor cannot be eliminated at the present time, there are definitely theoretical difficulties with it which can be overcome only by seemingly ad hoc and improbable assumptions. Possibilities for a proton stream excitor are examined and it is found that all theoretical difficulties can be overcome in a natural manner. The number of 50 MeV protons required to explain a strong type III burst is estimated conservatively as 3 × 10²⁵ which, after diffusion in interplanetary space, would be undetectable by the instruments flown thus far. This number is consistent with some theoretical ideas about the flare mechanism and also with present observational data.This paper concerns major type III bursts that have a measurable effect at low frequencies ( 10 MHz). The author is aware of the existence of different kinds of fast drift bursts which are fainter and mostly limited to the m-wave region (de Groot, 1970). These may be due to different kinds of excitors.Postdoctoral Fellow on the U.S.-U.S.S.R. Cultural Exchange Program.     

A new spectrum analyzer for radio astronomical studies with the radio telescope RATAN-600

The spectral measuring facility with a new Fourier analyzer for use with the radio telescope RATAN-600 is described and its experimental data are reported.     

A plasma-emission mechanism for type I solar radio emission

A theory for type I emission is developed based on fundamental plasma emission due to coalescence of Langmuir waves with low-frequency waves. The Langmuir waves are attributed to energetic electrons trapped in a magnetic loop over an active region. It is argued that the low-frequency waves should be generated in connection with the heating of the region. The continuum can be explained in terms of Langmuir waves generated by a gap distribution formed through collisional losses over a timescale of several tens of minutes. Bursts are attributed to local enhancements in the Langmuir turbulence associated with a loss-cone instability. No triggering mechanism for the bursts is identified. It is predicted that if the continuum is due to a large source then its brightness temperature should rise over several tens of minutes to a value which is roughly independent of frequency and of position across the source and which should not exceed 3 × 10⁹ K. For bursts, it is predicted that a fainter second harmonic component should accompany bright bursts.     

Towards a theory for type III solar radio bursts

The procedure developed in Smith (1974) to model the radiation source for type III bursts is modified to include scattering of radiation in the source itself. Since the inhomogeneities in the source must have the same statistical properties as the inhomogeneities used in tracing radiation from the source to the observer, these two parts of the type III problem are no longer uncoupled. Thus we use inhomogeneities consistent with the scattering inhomogeneities of Steinberg et al. (1971) and Riddle (1974) and apply the procedure to an archetype ‘fundamental-harmonic’ pair observed at Culgoora on 28 September, 1973 at 0319 UT. We find that it is impossible to model this burst with a source which is homogeneous in the sense that every part of the source has the same energy density in plasma waves. The density inhomogeneities in the source severely hamper amplification of the supposed fundamental. Possible ways out of this dilemma are discussed, including second harmonic pairs and a source with an inhomogeneous distribution of plasma waves. It is concluded that none of the possibilities are completely satisfactory to explain present observations and suggested that critical observations are missing.     

A classification scheme for solar flare models

We present a classification scheme for solar flare models that utilize magnetic free energycurrents. The classification scheme is geometry independent and delineates models into two categories: those models utilizing currents flowing parallel to B and those utilizing currents flowing perpendicular to B. This delineation of drivers allows us to specify what kinds of plasma-magnetic field configurations should be expected for a given current driver. Further, the delineation of drivers will allow us to identify both the strengths and the weaknesses of the various models.Based on invited talks given at the SERF Workshop (Aug. 13, 1979) and the IAU Meeting of Commission 10 (Aug. 15, 1979) held in Montreal.     

A model of the spectrum of the slowly varying component of solar radio emission from active regions

Statistical analyses of solar radio observations have shown that proton events are likely to occur when the flux at 3 cm, S₃, of the SVC is greater than 25 sfu and when S₃/S₈ is grater than 1. A theoretical explanation of this fact is attempted in this paper. I calculated the spectrum of the SVC using the gyro-radiation emission and found that the main reason for S₃ >; 25sfu is a ten-fold increase in the conductive energy flux in the active region over the quiet region, raising the height of the gyro-resonance layer for the 3 cm emission, and that the main reason for S₃/S₈ ? 1 is an increase in the coronal magnetic field gradient in the active region, causing a decrease in the optical thickness for the gyro-resonance absorption at 3 cm. It is precisely these active regions that are most favourable for the production of proton events.