Detection of langmuir solitons: implications for type III burst emission mechanisms at 2ω PE

We present the experimental verification of existing theoretical models of emission mechanisms of solar type III bursts at the second harmonic of the plasma frequency, _pe . This study is based on the detection of Langmuir and envelope solitons by the Ulysses spacecraft inside three type III burst source regions. We show that the oscillating-two-stream instability, coherent radiation by Langmuir solitons and stochastic phase mixing of the Langmuir waves in the strong turbulence regime are the appropriate emission mechanisms at 2 _pe .     

A self-consistent model for the storm radio emission from the Sun

A self-consistent theoretical model for storm continuum and bursts is presented. We propose that the Langmuir waves are emitted spontaneously by an anisotropic loss-cone distribution of electrons trapped in the magnetic field above active regions. These high-frequency electrostatic waves are assumed to coalesce with lower-hybrid waves excited either by the trapped protons or by weak shocks, making the observed brightness temperature equal to the effective temperature of the Langmuir waves.It is shown that whenever the collisional damping ( _c ) is more than the negative damping (- _A ) due to the anisotropic distribution, there is a steady emission of Langmuir waves responsible for the storm continuum. The type I bursts are generated randomly whenever the collisional damping ( _c ) is balanced by the negative damping (- _A ) at the threshold density of the trapped particles, since it causes the effective temperature of Langmuir waves to rise steeply. The number density of the particles responsible for the storm radiation is estimated. The randomness of type I bursts, brightness temperature, bandwidth and transition from type I to type III storm are self-consistently explained.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

A relationship between the brightness temperatures for type III bursts

Type III bursts often have brightness temperatures at the fundamental greater than 10⁹K. If the fundamental emission is due to scattering of Langmuir waves into transverse waves by thermal ions, this implies that induced scattering dominates over spontaneous scattering, which in turn requires that the energy density in Langmuir waves be greater than some minimum value, e.g. W ^l > 3 × 10^-10 erg cm^-3 for bursts at f _p = 100 MHz. Such Langmuir waves become isotropic on a time-scale shorter than the rise-time of type III bursts, e.g. < l s at f _p = 100 MHz. Consequently, their coalescence, leading to emission at the second harmonic, proceeds. The above inequalities would imply a brightness temperature at the second harmonic in excess of 10⁹K at f = 200 MHz.The predicted values of the brightness temperatures _T1 ^t and _T2 ^t (at the fundamental and second harmonic respectively) can be expressed in terms of an optical depth . After is eliminated a functional relation between _T1 ^t , _T2 ^t and the plasma frequency, f _p , remains. The form of this relation is not dependent on a quantitative theory of how the Langmuir waves are generated by the stream of electrons. Consequently, comparison with observed quantities should provide further insight into the detailed properties of the emission processes.     

Some features of the solar microwave emission and their connection with geomagnetic activity

A statistical investigation of the slowly varying component of the 9.1-cm solar radio emission is based upon the Stanford radioheliograms covering the years 1962–66. On the average the peak value of the brightness temperature T _b is proportional to the area covered by the corresponding spot group. However, in individual cases the observed T _b is definitely lower or higher than is to be expected from the size of the spot group. We introduce the concept microwave importance I _m of the spot group, which is the T _b to be expected from the Zürich class and spot number; and the concept relative brightness B _r, which is the ratio of the observed T _b to I _m. This leads to the distinction of faint, normal and bright sources with B _r 0.8, 0.8 < B _r < 1.2 and B _r 1.2 respectively. B _r is correlated with the maximum magnetic-field strength H observed in the spot group and with the flux-density spectrum of the source. The yearly average of B _r and the average flux-density spectrum vary with the phase of the solar cycle.An analysis of the results is based upon the electron-density distribution in the condensation, which was visible at the solar limb during the eclipse of February 5, 1962, and on an adopted temperature distribution with a central value of 4 × 10⁶ K. The computed T _b, including gyro-resonance absorption, agrees with the value derived from the microwave importance of the spot group and B _r = 0.5, which shows that in the current gyro-resonance models the electron density is underestimated. The variation of T _b with the size of the spot group can be explained by varying the dimensions of the condensation in area and in height, if the central density and temperature remain constant. The statistical relationships between B _r and H and between B _r and the flux-density spectrum yield a model for the differences between faint and bright sources: B _r increases with the contribution from the gyro-resonance absorption and with the central electron density.For paper I see Solar Phys. 7, 448. For paper III see Solar Phys. 8, 450.     

Spectral peculiarities of strong solar bursts

We have investigated spectral features of strong radio burst emission for the 21st cycle of solar activity. The maximum daily radio fluxes in 8 frequency ranges are analyzed. For every year, the classification of these daily spectra is obtained by the cluster analysis method.We have shown that strong bursts are characterized by the stable shape of the mean radio emission spectra. For these bursts the total level of radio emission does not depend on the phase of the solar 11-yr cycle and varies with the quasi-period of 4 yr.The basic features of burst spectra can be explained by the gyrosynchrotron radiation of nonthermal electrons and plasma radiation at the second harmonic of plasma frequency. We supposed that in the generation region of centimetric emission, if the strength of the magnetic field B 100 G, the number of microbursts can amount to (6–7) × 10³. In the generation region of decimetric emission, the energy of Langmuir waves changes as W _l n _e ^0.4.     

Excitation of whistler waves driven by an electron temperature anisotropy

A 3-D particle simulation of excitation of whistler waves driven by an electron temperature anisotropy (T > T ) is presented. Results show that whistler waves can have appreciable growth driven by the anisotropy. The maximum intensity of the excited whistler waves increases as a quadratic function of the anisotropy. Due to the presence of a threshold, one needs a relatively large electron temperature anisotropy above threshold to generate large-amplitude whistler waves. The average amplitude of turbulence in the context of whistler waves is up to as large as about 1% of the ambient magnetic field when T /T . The total energy density of the whistler turbulence is adequate for production of relativistic electrons in solar flares through stochastic acceleration.     

Ion-Sound Model of Microwave Spikes with Fast Shocks in the Reconnection Region

A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+st. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10 UT and 28 November 1997 between 05:00–05:10 UT using the multichannel spectrograph in the range 2.6–3.8 GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in T _e SXT maps (up to 20 MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.     

Some features of the solar microwave emission and their connection with geomagnetic activity

The quiet component of the 9.1-cm solar radio emission is studied from the Stanford radioheliograms covering the period April–October 1964. The distribution of the brightness temperature in heliographic coordinates is not entirely uniform, but positive and negative departures from the average value appear at a number of stable locations. The most important negative departure crosses the central meridian 4 days before the maximum of the recurrent geomagnetic activity. Two out of three less important brightness depressions are connected with geomagnetic disturbances in the same manner. It is suggested that the brightness depressions are identical with M-regions.The result is confirmed by the construction of polytrope models for the solar wind, for various values of the parameters (the polytrope index) and T (the temperature in the inner corona). The velocities near the earth's orbit and in the inner corona are computed as functions of the model parameters, the density results from the observed proton flux at 1 AU. For quiet conditions the model with T = 1.26 × 10⁶ K and = 1.10 is appropriate. The corresponding density and temperature in the corona lead to a value of 4000 K for the contribution of the corona to the 9-cm brightness. For disturbed conditions the suitable model has the parameters T 2.0 × 10⁶ K, a 1.04. It being given that the proton flux at 1 AU is relatively constant, the equation of continuity leads to a low coronal density because of the high solar-wind velocity. The corresponding coronal contribution to the 9-cm brightness is of the order of 10 K. This confirms that the brightness temperature is considerably reduced in the regions where the enhanced solar wind originates. We suggest the name coronal depression for such regions.Papers II and III will appear in forthcoming issues of this journal.     

Implications of liouville's theorem on the apparent brightness temperatures of solar radio bursts

Liouville's theorem for radiation, of which the generalized étendue is a consequence, implies ² d² d² A = constant along the ray path, where is the refractive index and d² and d² A are the ranges, respectively, of solid angle and of area that define a ray (actually a bundle of rays). Implications of this concept on the propagation of radio waves from the actual to the apparent source in the solar corona (i.e., the scatter image of the true source) are discussed. The implications for sources of fundamental plasma radiation include: (1)The observed solid angle (defining the directivity) and apparent area A of the source are compatible with Liouville's theorem only if the apparent source (the scatter image of the true source) corresponds to the envelope of subsources with a small filling factor f. (2) The brightness temperature T _Bof the actual source is greater than that of the apparent source by f ^-1. (3) For sources of fundamental plasma radiation the factor f is very small ( 10^-2). (4) A long-standing discrepancy between the observed low value of T _B at meter/decameter wavelengths for the quiet Sun and the known coronal temperature may be resolved by noting that the implied coronal temperature is given by T _B f and that the factor f must be significantly less than unity.A brief discussion is included of the relation between Liouville's theorem, the generalized étendue, Milne's laws, occupation numbers, extension in phase, and suppression of emission by a medium with refractive index unequal to unity.     

Interaction of turbulent solar with with cometary plasma tails

The longitudinal electric field associated with the observed electrostatic turbulence in the solar wind is shown to modify the dispersive characteristics of the hydromagnetic waves propagating along the interface between the solar wind and the cometary plasma. Extremely weak turbulence has a tendency to stabilize these surface waves, whereas turbulence of moderate level can be stabilizing or destabilizing depending on the strength of the cometary magnetic fieldB _oc relative to the interplanetary magnetic fieldB _os . ForB _oc B _os , destabilization is not possible.     

Observations and interpretation of the slowly varying component of solar radio emission at decameter wavelengths

We have observed the slowly varying component of solar radio emission at a frequency of 34.5 MHz with half power beam widths of 26/40 in the east-west and north-south directions, respectively. It is found that the observed brightness temperatures vary within the limits of 0.3×10⁶K to 1.5×10⁶K, and the average half power widths of the brightness distribution on the Sun is about 3R . Thermal emission from coronal regions of various electron densities and temperatures with and without the magnetic field has been computed and compared with the observed results.     

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.     

Clumpy Langmuir waves in type III radio sources

A model is developed for the clumpy Langmuir waves observed in type III source regions. In this model the waves are generated by instability of a beam which propagates outward from the Sun in a state close to marginal stability. Ambient density perturbations cause fluctuations about the marginally stable state, leading to nonuniformities in both beam and waves and, hence, to spatially inhomogeneous growth. High damping rates and high wave levels are strongly anti-correlated, leading to suppression of the net damping. Below saturation stochastic growth causes the waves to follow a random walk in the logarithm of their energy density and the resulting probability of observing a field of magnitude E is approximately proportional to E ^-1. Comparison with observations shows that this model can account for the levels and clumpiness of the Langmuir waves, the small net dissipation required for the beams to propagate to 1 AU, the characteristic decay time of type III electromagnetic emission, and the negative mean growth rate observed in situ in type III sources. At 1 AU only the very highest fields approach the threshold for nonlinear wave collapse, but this threshold may be more commonly exceeded closer to the Sun.     

Non-thermal solar wind heating by supra-thermal ions

The effect of a new energy source due to energies transferred from supra-thermal secondary ions on the temperature profile of the solar wind has been considered. For this purpose a solution of a tri-fluid model of the solar wind including solar electrons, protons, and -particles, and starting with the boundary conditions of Hartle and Barnes at 0.5 AU is given. On the base of the assumption that suprathermal He⁺-ions which have four times the temperature of suprathermal protons are predominantly coupled to solar -particles by Alfvén waves, it is shown that the temperature T of solar -particles should be appreciably higher than those T _p of solar protons beyond the orbit of the Earth. For 1 AU a temperature excess T over T _p according to that which has been found in some solar wind ion spectrograms can only be explained for a small part of the orbit of the earth which is inside the cone of enhanced helium densities. Around 1 AU the temperatures T and T _p are found to decrease much slighter with solar distance than given in the two-fluid model of Hartle and Barnes. Beyond 1.7 and 2.2 AU the temperatures T and T _p even start increasing with solar distance and come up to about 10⁵ at about 10 AU. These predictions should lend some support to future temperature measurements with deep-space probes reaching Solar distances of some AU.Forschungsberichte des Astronomischen Institutes, Bonn, 72-10.     

The saturation of electron-cyclotron maser and the time profile of emitted spikes

In this paper the evolution of the hollow beam distribution of energetic electrons giving rise to ECM instability is investigated and the spatial dispersion term is included in the equation of wave energy. The instability causes the growth of wave energy, while the propagation of waves evacuates the electromagnetic energy from the source region. By analysing these two effects spike-like time profiles of waves are obtained. It is found that the saturation time t _s of ECM emission and the duration of spikes increase with the decrease of the frequency of solar radio spike emission. The approximate expressions of t _sand of the peak wave energy density are derived.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 leave from the Department of Astronomy, Nanjing University, Nanjing, People's Republic of China.     

On the nature of some active regions in the microwave range

The radio emission of a selected number of solar active regions has been investigated with high angular resolution at two frequencies: 10 and 17 GHz. By comparing the results of the two observations the following conclusions can be drawn:

1. The brightness temperature distribution of an active region is often composed of very bright cores of small dimension (angular extent θ?20″) imbedded in extended halos of lower brightness.
2. The radio emission of such structures as well as the degree of polarization can be explained with a thermal process. The halos can originate by pure thermal bremsstrahlung while in the case of the very bright cores found at 10 GHz (brightness temperature T _b?1–9 × 10⁶K) the emission at the harmonics of the gyrofrequency is needed.

     

The K-corona under hydrostatic density distribution: Relevance to solar wind

An analytical expression is obtained for the K-corona brightness as a function of the distance from the solar limb. The agreement of the theory with the numerous observational data indicates that the density distribution at the levels located at 1.2–2.5 R heliocentric distances may be treated as hydrostatic with T = const. The deviation of the observed K-corona brightness from the expression obtained may be indicative of supersonic fluxes of matter. These concepts may be used to verify the various theoretical models for solar wind source.     

Electromagnetic lower hybrid instability in the solar wind

Low frequency electromagnetic lower hybrid waves (so-called hybrid whistlers) propagating nearly transverse to the magnetic field can be driven unstable by a resonant interaction with halo electron distributions carrying solar wind heat flux. The electromagnetic lower hybrid instability is excited when the halo electron drift exceeds the parallel phase velocity of the wave. The growth rate attains a maxima at a certain value of the wavenumber. The maximum growth rate decrease by an increase in _e (the ratio of electron pressure to magnetic field pressure) and halo electron temperature anisotropy. At 0.3 AU the growth time of the electromagnetic lower hybrid instability is of the order of 25 ms or shorter, whereas the most unstable wavelengths associated with the instability fall typically in a range of 27 to 90 km. The instability would give rise to a local heating of solar wind ions and electrons in the perpendicular and parallel directions relative to the magnetic field, B₀. The observations of low frequency whistlers having high values ofB/E ratios (B andE being the magnitude of the wave magnetic and electric field, respectively) and propagating at large oblique angles to B₀ behind interplanetary shocks, can be satisfactorily explained in terms of electromagnetic lower hybrid instability. The instability is also relevant to the generation mechanism of correlated whistler and electron plasma oscillation bursts detected on ISEE-3.     

Fundamental emission for type III bursts in the interplanetary medium: The role of ion-sound turbulence

It is argued (a) that the onset times of type III radio emission and of the streaming electrons implies that type III bursts in the interplanetary medium are generated predominantly at the fundamental, (b) that in view of recent observations of ion-sound waves in the interplanetary medium the theory of the generation of the bursts should be revised to take account of these waves, and (c) the revised theory favours fundamental emission. A detailed discussion of the effect of ion-sound waves on type III bursts is given. The most important results are: (1) Ion-sound waves cause enhanced (over scattering off thermal ions) fundamental emission. (2) Second harmonic emission is also enhanced for T _e> 5 × 10⁵ K, e.g., low in the corona, but is suppressed for T _e< 5 × 10⁵ K, e.g., in the interplanetary medium. (3) The bump-in-the-tail instability for Langmuir waves can be suppressed by the presence of ion-sound waves; it may be replaced by an analogous instability in which fundamental transverse waves are generated directly, with no associated second harmonic, but there are unresolved problems with theory for this process. (4) Very low frequency ion-sound waves can scatter type III radiation. (5) Although the ion-sound waves which have been observed are at too high a frequency to be relevant for these processes, it seems likely that ion-sound waves of the required frequencies are present and do play important roles in the generation of type III bursts.