type-III Solar Radio Bursts Research Articles

The growth of the longitudinal beam–plasma instability in the presence of an inhomogeneous background

We study the longitudinal stability of beam–plasma systems in the presence of a density inhomogeneity in the background plasma. Previous works have focused on the non-relativistic regime where hydrodynamical models are used to evolve pre-existing Langmuir waves within inhomogeneous background plasmas. Here, for the first time we study the problem with kinetic equations in a fully relativistic way. We do not assume the existence of Langmuir waves, and we focus on the rate and the mechanism by which waves are excited in such systems from an initial perturbation. We derive the structure of the unstable modes and compute an analytical approximation for their growth rates. Our computation is limited to dilute and cold beams, and shows an excellent agreement with particle-in-cell simulations performed using the SHARP code. We show that, due to such an inhomogeneity, the virulent beam–plasma instabilities in the intergalactic medium are not suppressed but their counterparts in the solar wind can be suppressed as evidenced by propagating type-III solar radio bursts.

PLASMA EMISSION BY COUNTER-STREAMING ELECTRON BEAMS

ABSTRACT The radiation emission mechanism responsible for both type-II and type-III solar radio bursts is commonly accepted as plasma emission. Recently Ganse et al. suggested that type-II radio bursts may be enhanced when the electron foreshock geometry of a coronal mass ejection contains a double hump structure. They reasoned that the counter-streaming electron beams that exist between the double shocks may enhance the nonlinear coalescence interaction, thereby giving rise to more efficient generation of radiation. Ganse et al. employed a particle-in-cell simulation to study such a scenario. The present paper revisits the same problem with EM weak turbulence theory, and show that the fundamental (F) emission is not greatly affected by the presence of counter-streaming beams, but the harmonic (H) emission becomes somewhat more effective when the two beams are present. The present finding is thus complementary to the work by Ganse et al.

Whistler wave generation by non-gyrotropic, relativistic, electron beams

Particle-in-cell code, EPOCH, is used for studying features of the wave component evident to propagate backwards from the front of the non-gyrotropic, relativistic beam of electrons injected in the Maxwellian, magnetised background plasma with decreasing density profile. According to recent findings presented in Tsiklauri [Phys. Plasmas 18, 052903 (2011)], Schmitz and Tsiklauri [Phys. Plasmas 20, 062903 (2013)], and Pechhacker and Tsiklauri [Phys. Plasmas 19, 112903 (2012)], in a 1.5-dimensional magnetised plasma system, the non-gyrotropic beam generates freely escaping electromagnetic radiation with properties similar to the Type-III solar radio bursts. In this study, the backwards propagating wave component evident in the perpendicular components of the electromagnetic field in such a system is presented for the first time. Background magnetic field strength in the system is varied in order to prove that the backwards propagating wave's frequency, prescribed by the whistler wave dispersion relation, is proportional to the specified magnetic field. Moreover, the identified whistlers are shown to be generated by the normal Doppler-shifted relativistic resonance. Large fraction of the energy of the perpendicular electromagnetic field components is found to be carried away by the whistler waves, while a small but sufficient fraction is going into L- and R-electromagnetic modes.

Suprathermal particle events observed by WIND spacecraft in interplanetary space during 1995–1999 and their classification

Fifty-five suprathermal particle events were selected from WIND observations between 1995 and 1999. Based on systematic analysis on the observational characteristics of these events a two-parameter (the rising time and the flux ratio of electrons to protons in each event) classification method was proposed to classify these events. The three classified classes are (1) impulsive electron events with the flux ratio of electrons to protons being bigger than 1 and rising time being shorter than 200 min, (2) impulsive proton events with the flux ratio being smaller than 1 and rising time being shorter than 200 min, and (3) gradual proton events with the flux ratio being smaller than 1 and the rising time being longer than 200 min. In the past, “impulsive solar electron events” were under intense research. However, because the selection standards of their velocity dispersions or pitch-angle distributions were inadequate, statistical surveys of selected events were different from each other and even some conclusions were not consistent with the theory, for example, the relation of type-III solar radio bursts to the “impulsive solar electron events”. The first class of impulsive electron events are associated with type-III radio bursts and with clear velocity dispersions; therefore they ought to originate from the Sun. The second class of the events, which have short continuance time and usually are not associated with type-III radio bursts and without velocity dispersion, are still far away from interplanetary shocks and most of them do not one-to-one correspond to corrotating interacting regions (CIRs); such events are possible results of local interplanetary magnetic field reconnection or electromagnetic disturbances. Finally, about 2/3 gradual proton events of the third class occur with interplanetary shocks, the delay times of which are almost equal to the rising time. Some of these events can be understood as particle accelerations by shocks.

Radiation Pattern of Plasma Emission in Beam–Plasma Interaction

The physical process of type-III solar radio bursts by an electron beam in a plasma involves electromagnetic radiation at the plasma frequency and its second harmonic. Recently, multiple-harmonic radiation of plasma frequency has drawn renewed interests in electromagnetic radiation in theoretical and numerical studies. The radiation pattern of electromagnetic waves in nonlinear beam-plasma interaction at the plasma frequency and its multiple harmonics is presented, by using a 2-D electromagnetic particle-in-cell simulation.

Numerical Simulations of Type-III Solar Radio Bursts

The first numerical simulations are presented for type-III solar radio bursts in the inhomogeneous solar corona and interplanetary space, that include microscale quasilinear and nonlinear processes, intermediate-scale driven ambient density fluctuations, and large scale evolution of electron beams, Langmuir and ion sound waves, and fundamental and harmonic electromagnetic emission. Bidirectional coronal emission is asymmetric between the upward and downward directions, and harmonic emission dominates fundamental emission. In interplanetary space, fundamental and/or harmonic emission can be important. Langmuir and ion sound waves are bursty and the statistics of Langmuir wave energy agree well with the predictions of stochastic growth theory.

Physics of Plasma Radiation

We present a theory of the fundamental plasma emission of type-III solar radio bursts. Starting from the generalized Zakharov equations, considering the pump wave as a pair of oppositely propagating Langmuir waves with different amplitudes, and the excitation of electromagnetic and induced Langmuir waves, we obtain a general dispersion relation for the coupled waves. We numerically solve the general dispersion relation using the pump wave amplitude and plasma parameters observed in the interplanetary medium.

A theory of the fundamental plasma emission of type-III solar radio bursts

Results from plasma wave experiments in spacecraft give support to nonlinear interactions involving Langmuir waves, electromagnetic waves and ion-acoustic waves in association with type III solar radio bursts. In this paper we present a theory of the fundamental plasma emission of type-III solar radio bursts. Starting from the generalized Zakharov equations, considering the pump wave as a pair of oppositely propagating Langmuir waves with different amplitudes, and the excitation of electromagnetic and induced Langmuir waves, we obtain a general dispersion relation for the coupled waves. We numerically solve the general dispersion relation using the pump wave amplitude and plasma parameters as observed in the interplanetary medium. We compare our results with previous models. We find that the stability properties depend on the pump wave numbers and on the ratio of wave amplitude between the forward and backward pump wave. The inclusion of a second pump wave allows the simultaneous generation of up and down converted electromagnetic waves. The presence of a second pump with different amplitude from the first one brings a region of convective instability not present when amplitudes are the same.

AKR signal increases caused by triggering

This paper presents a study of the amplitude increases which accompany the triggering of auroral kilometric radiation (AKR) by Type‐III solar radio bursts. IMP‐8 data were used to determine the signal increases observed during one‐hour periods before and after Type‐III bursts at 100 kHz, 178 kHz and 500 kHz and these were compared with similar observations when the Type‐III bursts were absent. The results indicate that between 8 to 16% of the Type‐III bursts caused statistically significant intensity increases and that infrequent large signal increases of sometimes 20 dB or more tended to characterize the triggered AKR, rather than a large proportion of small increases.

On the spectra of type-III solar radio bursts observed at low frequencies

The spectra of strong bursts observed at low frequencies by OGO-5 during 1968–1970 are presented. They usually exhibit an intense main peak between 100 kHz and 1 MHz, and sometimes a less intense secondary peak between 1 and 3.5 MHz. Main peaks of 10−12 Wm−2 Hz−1 or more were obtained in very strong events, but because of antenna calibration problems those could be one or two orders of magnitude too high. Recently published work supports the finding that type III bursts at low frequencies can be at least four orders of magnitude more intense than at ground-based frequencies of observation. It is found that the energy received at the Earth increases with decreasing frequency approximately as f−n, where 3 ≲ n ≲ 4.

Harmonic emission from adiabatically collapsing Langmuir solitons

Numerical studies of radiation at 2ωp from a Langmuir envelope collapsing adiabatically in three dimensions show that the emissivity is higher than expected. A volume emissivity obtained from an approximate density of collapsing packets leads to favorable comparisons with measurements of type-III solar radio bursts.

Type-III radio bursts and their interpretation

The observations of type-III solar radio bursts are briefly reviewed to set requirements on a model for their interpretation. The most important of these requirements is that the source must be an electron stream which is in a state of continuous quasilinear relaxation and which initially must have a nearly monotonically decreasing velocity distribution. The problem of constructing a model is broken into three parts: (1) The plasma wave source which depends on the interaction of the electron stream with electron plasma waves. (2) The radiation source which depends on the interaction of plasma waves and transverse electromagnetic waves or in a magnetized plasma the ordinary and extraordinary modes of magnetoionic theory. (3) The propagation of radiation between the source and the observer which depends on the transmission of radiation through a scattering refracting absorbing magnetized plasma. Progress on a model for the plasma wave source is reviewed and it is concluded that no existing models are adequate. The equations which would lead to an adequate model are written down, but not solved. These include, in addition to collisional damping, Landau damping both by the exciting stream and the background plasma, and spontaneous and induced processes for a three-dimensional distribution of plasma waves. Possible limitations to a quasilinear approach such as pile-up of plasma waves and nonlinear effects are considered. Processes which affect the gross structure of the source such as electron trajectories in coronal streamers and electron scattering by inhomogeneities are reviewed. Progress on the radiation source is considered both in the absence and presence of a magnetic field. At high frequencies (e.g., 80 MHz) observations of radiation near the fundamental and second harmonic of the plasma frequency allow a unique determination of source size and the energy density in plasma waves within the uncertainties of geometry by source ray tracing. This determination is extremely critical because the fundamental must be amplified and thus production of the fundamental is effectively a much more highly nonlinear process than production of the second harmonic. At low frequencies (e.g., 500 kHz) the second harmonic is shown to be dominant because amplification of the fundamental becomes an inefficient process. Calculations of scattering of radiation in a random medium are reviewed. It is concluded that these are adequate at high and low frequencies, but have not been carried out properly at intermediate frequencies where amplification of the fundamental may still be present. It is shown in particular that when scattering is taken into account at high frequencies all observations can be explained by isotropic emission near the second harmonic. At low frequencies the nature of the scatterers is determined by source occultations unlike the case at high frequencies where these are free parameters. This fact allows the possibility of determining true source sizes at low frequencies by subtracting out the contribution due to scattering. A mechanism for producing the possibly observed linear or highly elliptical polarization of type-III bursts, which must be imposed far from the source due to Faraday rotation, is reviewed. Finally, the questions of what remains to be done and what we can hope to obtain upon completion of this work are briefly considered.