Nonlinear parametric instability and millisecond solar radio spikes

A model of nonlinear parametric instability (NLPI) modulation is suggested to explain the millisecond spikes in solar decimetric radio emission. The radio emission energy will periodically transfer to two other waves, and the intensity of the radio emission will be modified by NLPI processes in the corona, when the matching conditions of three-wave coupling are satisfied. This model can simultaneously explain the duration of spikes, why the millisecond spikes have not been observed in other solar radio emission, and the relation between the duration of the spikes and the intensity and the frequency of the radio emission as well.     

A time profile of millisecond pulsations of solar microwave radio bursts

An hypothesis on the interference origin of millisecond pulsations of solar-burst microwave radio emissions based on the fact that the signal scintillation appears as a result of radio-wave propagation through an inhomogeneous turbulent corona is considered. It is shown that the time profile of pulsations depends on the phase difference of interfering waves and can either look like pulses of “emission” and “absorption” or it can have a sawtooth form with slow buildup and fast drop. The observed properties of pulsations were compared with predictions of this model; this comparison showed that the formation of pulsations and their observed properties are satisfactorily explained by multipath propagation, which takes place at traversal of the coronal plasma by radio waves.     

Harmonic emission and polarization of millisecond radio spikes

The spectral distribution of millisecond radio spikes observed by the Zürich spectrometers in the 200–1100 MHz range has been studied. In one event out of a total of 36 we have found clearly developed harmonic structure. The ratio between the two bands of emission was 1:1.39 ± 0.01. We have also determined the sense of circular polarization of the spike events and compared it to the magnetic polarity of the leading spot of the flaring active region. According to the Leading Spot Rule the majority of the events (10 out of 13) were emitted in the ordinary mode.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

On the microwave millisecond spike emission and its associated phenomena during the impulsive phase of large solar flare

We propose a model of two acceleration regions, which can explain, on the basis of microwave maser caused by a “hollow-beam” distribution of electrons, the presence of millisecond spikes in the event of 1981 May 16 and their absence in the event of 1981 October 12, and the enhanced continuous emission in the latter. We have also uncovered relations among the features, e.g. the Type III_G, Type IV_DCIM and hard x-ray bursts, that accompany the microwave millisecond spikes during the impulsive phase of a large flare.     

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).     

Location and parameters of a microwave millisecond spike event

A typical microwave millisecond spike event on November 2, 1997 was observed by the radio spectrograph of National Astronomical Observatories (NAOs) at 2.6–3.8 GHz with high time and frequency resolution. This event was also recorded by Nobeyama Radio Polarimeters (NoRP) at 1–35 GHz and Radio Heliograph (NoRH) at 17 GHz. The source at 17 GHz is located in one foot-point of a small bright coronal loop of YOHKOH SXT and SOHO EIT images with strong photospheric magnetic field in SOHO MDI magnetograph. It is assumed that the electron cyclotron maser instability and gyro-resonance absorption dominate, respectively, the rising and decay phase of the spike event. For different harmonic number of gyro-frequency or magnetic field strength, a fitting program with free plasma parameters is used to minimize the difference between the observational and theoretical values of the exponential growth and decay rates for a given spike. The plasma parameters at third harmonic number are more comparable to their typical values in solar corona. Hence, it is able to provide a diagnosis for the source parameters (magnetic field, density, and temperature), the properties of radiations (wave vector and propagation angle), and the properties of non-thermal electrons (density, pitch angle, and energy). The results are also comparable with the diagnosis of the gyro-synchrotron radiation model, the frequency drift rates and a dipole magnetic field model, as well as the YOHKOH SXT and SOHO MDI data. This study is supported by the NFSC project nos. 10333030 and 10273025, and “973” program with no. G2000078403.     

The solar neutral iron spectrum

Mean profiles of eighteen neutral iron lines of varying strengths were measured at selected positions from the center of the solar disk to the limb. These profiles were obtained by rapid photoelectric scanning of the spectrum with a double-pass spectrometer. The Fe i lines selected are representative of most of the stronger low-lying transitions in the neutral iron atom. In addition to the iron lines, this observational program includes center-to-limb measurements of three Ti ii lines and of the Ca i resonance line 4226.7. The line profiles are presented here in graphical form after correction for instrumental effects and normalization to the local disk center continuum.Both Kitt Peak National Observatory and the National Center for Atmospheric Research are sponsored by the National Science Foundation.     

On the harmonic structure of solar radio spikes

This paper analyzes the frequency structure of the bands of electron-cyclotron maser instability. The calculations show that each term of the series describing the growth rate provides a double-peak structure, if we accept a nonthermal electron distribution with a two-side loss-cone. The ratio of central frequencies is found to be non-integer in general case. We conclude that the harmonic structure of solar radio spikes observed in a number of events can be imitated by electron-cyclotron maser emission of mildly relativistic electrons with a power-law momentum distribution.     

The secondary spectral component of solar microwave bursts

Microwave observations in the range 1 to 18 GHz with high spectral resolution (40 frequencies) have shown that many events display a complex microwave spectrum. From a set of 14 events with two or more spectral components, we find that two different classes of complex events can be distinguished. The first group (4 events) is characterized by a different temporal evolution of the spectral components, resulting in a change of the spectral shape. These events probably can be explained by gyrosynchrotron emission from two or more individual sources. The second class (10 events) has a constant spectral shape, so that the two spectral components vary together in intensity. For all ten events in this second class, the ratio of primary to secondary peak frequencies is remarkably similar, exhibiting an average value of 3.4, and both components show a common circular polarization. These properties suggest either a common source for the different spectral components or several sources which are closely coupled. An additional example of this class of burst was observed interferometrically to provide spatial resolution. This event suggests that the primary and secondary components have a similar location, but that the surface area of the secondary component is larger.Swiss National Science Foundation Fellow from the University of Bern.     

The microwave spectrum of Mars: An analysis

A weighted least squares fit to the best available data on the Martian microwave spectrum indicates that the brightness temperature decreases from long to short wavelengths, rather than increasing as expected from the solution of the one-dimensional equation of heat conduction. Reasonable assumptions on the ratio of electrical to thermal skin depths, on internal heat sources, on ferromagnetic materials, on radiative conduction, on compaction with depth, and on surface rpughness all fail in reproducing the deduced spectrum. A thin near-surface layer of a material with high dielectric constant and high millimeter wave absorption is needed. Since Mars exhibits marked surface overturn, a condensible material, namely liquid water, seems indicated. A layer of liquid water some tens of microns thick, on the average, localized in the top few millimeters of a Martian epilith with refractive index ? 1.6 fits the microwave spectrum, and the infrared and radar data as well. The origin of such a layer of liquid water and its possible exobiological significance are discussed. The distribution of water should be nonuniform over the disk and may help explain discordant microwave observations and the anomalous variation of infrared brigthness temperature with latitude. Further millimeter wave radio and radar studies of Mars are needed.     

Emission of solar radio spikes by beam-driven cyclotron resonance

The generation of bright solar radio spikes by the beam-driven cyclotron resonance maser mechanism (the resonant interaction of an electron beam with a circularly polarized wave in a background plasma under the action of a guide magnetic field) is studied. Nonlinear effects such as radiation damping and gyrophase bunching on electron energy and momentum are responsible for the enhanced direct energy conversion between the beam and the coherent wave. Factors such as beam energy spread and pitch angle distribution are analyzed. The intense maser radiation is carried at the source by the circularly polarized wave propagating along the magnetic field. Due to the magnetic field curvature, the outgoing maser radiation converts into extraordinary and ordinary modes. The extraordinary mode suffers from plasma absorption at the second harmonic layer, whereas the ordinary mode is likely to get through.     

Solar microwave bursts recorded at 2.84 GHz with millisecond time resolution

A millisecond recorder for solar observation at 2.84 GHz was put to work in 1981. From April 1981 to September 1982 it recorded 250spike pulse events, which are here listed together with information on the associated solar flares, hard X-ray bursts and radio bursts. In defining these spike events, particular attention has been paid to ensure the reliability of the recorded data. Statistical analysis of the data has given some interesting results. The spike pulses have shorter duration and higher flux density than were previously known, and the basic units of the spike pulse events are single spike pulses crowded together forming separate clusters. Many spike pulses observed are not yet resolved at 1 ms. Fast spike pulse events are closely correlated with complex magnetic field regions, and are often accompanied by hard X-ray bursts and fast drifting radio bursts. Some such events showed no correspondence with the radio bursts at the same frequency, 2.84 GHz, but corresponded to those occurring at dm and short cm wavelengths. Some theoretical investigations of the spike phonomena have been carried out, the details of which will be discussed elsewhere.     

Effects of the distribution function of non-thermal electrons and of cyclotron resonant absorption on millisecond radio spikes

A theoretical analysis of electron-cyclotron maser instabilities indicates that the distribution function of non-thermal electrons influences millisecond radio spikes in solar flares, and that a hollow beam distribution is more likely than a loss-cone distribution. The restrictions of classical theories of cyclotron resonant absorption are discussed and a formula is derived for the absorption coefficient near the resonant frequency. Finally, the computations show that for typical coronal parameters, the growth rates of the fundamental of fast extraordinary modes are much faster than those of their second harmonics; and because the directional angle of the fundamental is smaller, its resonant absorption may be neglected. Moreover, the band-width of the fundamental is consistent with observation of radio spikes; therefore, we claim that the millisecond radio spikes in the decimetric range are composed mainly of fundamentals of the fast extraordinary modes. The second harmonics of fast extraordinary modes may be generated for directions near to the vertical to the magnetic field, but it is impossible to observe both fundamental and second harmonics in the same direction.     

The spectrum of Nixix in the solar corona

The wavelengths and intensities of the stronger transitions in the spectrum of Ni xix are reduced from measurements of the X-ray spectra of three coronal active regions. The new measured wavelengths are consistent with prediction by isoelectronic extrapolation from the wavelengths of well established transitions but are about 0.01 Å longer than previously accepted laboratory measurements. This difference appears to be crucial to the correct assignment of features in the coronal spectrum to Ni xix. The relative intensities of our new assignments to Ni xix are in broad agreement with the Loulergue-Nussbaumer calculations.     

Millisecond spikes in a short decimetric solar radio burst

The characteristics of the millisecond spikes with short duration and weak flux density which were observed with high time resolution (1 ms) at 1420, 2000 and 2840 MHz during the great type IV solar radio burst of 30 July 1990 are introduced in detail in this article. The time profiles of the spikes are statistically analyzed and the parameters of the spike source are also estimated.     

Five minute microwave solar oscillations

Oscillations with a period of 5.6 min were observed on 10 July, 1978 while tracking at 22 GHz the active region McMath 15403. The oscillations were strong, clearly defined, had no damping, and lasted for about two hours. The rarity of the phenomenon is indicated by the fact that it occurred only once in more than 250 hr of solar observations. The possibility that these oscillations are due to a standing Alfvén wave driven by the photospheric velocity field is discussed.On sabbatical leave from Technion, Haifa, Israel.Formerly: Centro de Radio-Astronomia e Astrofísica Mackenzie, now with Brazilian National Research Agency CNPq, National Observatory.     

The interpretation of absorption-line shifts in the solar spectrum

The shifts of Fraunhofer lines of different chemical elements in a homogeneous medium with a plane monochromatic progressive adiabatic sound waves are derived. The calculations indicate that lines of neutral elements (6 ₀ 14) with lower excitation potentials ₀ i= 0–2 eV are red shifted, those with excitation potential ₀ i= 4–12 eV are blue shifted, and with ₀ i= 3 eV are both blue and red shifted. The lines of ions are shifted toward the blue. The shifts of Fraunhofer lines are found to decrease from the centre of the solar disk to the limb. These results agree qualitatively and quantitatively with observations.     

High-resolution microwave spectra of solar bursts

Microwave observations with exceptionally high spectral resolution are described for a set of 49 solar flares observed between May and October 1981. Total power data were obtained at 40 frequencies between 1 and 18 GHz by the Owens Valley frequency-agile interferometer with 10 s time resolution. Statistical analysis of this sample of microwave bursts established the following significant characteristics of their microwave spectra: (i) Most ( 80%) of the microwave events displayed complex spectra consisting of more than one component during some or all of their lifetime. Single spectral component bursts are rare. It is shown that the presence of more than one component can lead to significant errors when data with low spectral resolution are used to determine the low-side spectral index. (ii) The high-resolution data show that many bursts have a low-side spectral index that is larger than the maximum value of about 3 that might be expected from theory. Possible explanations include the effect of the underlying active region on the perceived burst spectrum and/or the necessity for more accurate calculations for bursts with low effective temperatures, (iii) the peak frequencies of the bursts are remarkably constant during their lifetimes. This is contrary to expectations based on simple models in which the source size and ambient field remain constant during the evolution of a burst.Swiss National Science Foundation Fellow from the University of Bern.