Wave localization in solar type III events

A suggested application of the theory of wave localization to type III solar radio events in the solar wind is discussed critically. A classical wave theory that enables one to relate wave localization to the observed spectrum of density fluctuations is summarized. Localization (in one dimension) is due to backscattering and depends on the density spectrum at a wavenumber equal to twice that of the scattered wave. The localization length is estimated for the Langmuir waves, for which the appropriate density fluctuations require ion sound waves, and for transverse waves, for which (at least for the fundamental) the spectrum of the appropriate density fluctuations has been measured in situ. In all cases the localization length is much shorter than the size of a type III event. For fundamental radiation the localization length can be even shorter than the observed sizes of clumps of Langmuir waves.It is concluded that although wave localization may be significant in type III events, most of its consequences have already been recognized in models that invoke multiple scattering. A notable exception is localization of fundamental transverse waves to the clumps of Langmuir waves, which provides a natural explanation for the observed brightness temperatures and for the initial predominance of fundamental over harmonic emission.     

Energetic solar particle events in a stream-structured solar wind

Theoretical considerations lead to a solar cosmic ray diffusion coefficient which varies with heliolongitude in a stream-structured solar wind. By solving numerically the time dependent convection-diffusion equation for the particle transport we investigate the effect of the azimuthal variation of the diffusion coefficient on intensity-time profiles as seen by a stationary observer. Depending on the position of the observer relative to the solar wind stream at the time of flare occurrence, completely different intensity-time profiles will be observed. When the spacecraft is at the time of the flare occurrence right at the leading edge of a solar wind stream, the large mean free path leads to rapid steepening of the initial phase of the intensity profile. The longitudinally decreasing mean free path 1 day in front of the leading edge will lead to intensity-time profiles similar to long-time injection events if the event occurs before the stationary observer enters the flux tubes with the decreasing diffusion coefficient.     

On the occurrence rate of high-speed solar wind events

We have investigated the rate of occurrence of solar wind phenomena observed between 1972–1984 using power-spectrum analysis. The data have been taken from the high-speed solar wind (HSSW) stream catalogue published by Mavromichalaki, Vassilaki, and Marmatsouri (1988). The power-spectrum analysis of HSSW events indicates that HSSW stream events have a periodicity of 9 days. This periodicity of HSSW events is of the 27-day period of coronal holes, which are major sources of solar wind events. In our opinion, the 9-day period may be the energy build-up time for coronal hole regions to produce the HSSW stream events.     

Velocities of type II solar radio events

Radial velocities for 144 simple but representative type II bursts were determined from measured frequency time histories. The velocity distribution is peaked in the region between 500 and 700 km s ^–1 (with the exact value dependent upon the coronal density model assumed) and skewed towards the larger velocities. In 85 % of the cases it was found that the velocities were constant with height. In the remaining 15 % the drift rate decreased drastically at low frequencies. This tended to occur for events having high initial velocities. The measured velocity is dependent upon the properties of the flare event but does not appear to be related to other characteristics of the radio burst. Comparisons show that the group of type II events studied had a velocity distribution which was comparable with that for coronal mass ejection events seen in association with type II bursts. The measured velocities were however statistically smaller than those of interplanetary type II bursts.     

Enigmatic solar wind disappearance events – Do we understand them?

At the Sun-Earth distance of one astronomical unit (1 AU), the solar wind is known to be strongly supersonic and super Alfvenic with Mach and Alfven numbers being on average 12 and 9 respectively. Also, solar wind densities (average ∼10cm^-3) and velocities (average ∼450kms^-1) at 1AU, are known to be inversely correlated with low velocities having higher than average densities andvice versa. However, on May 11 and 12 1999 the Earth was engulfed by an unusually low density (< 0.1cm^-3) and low velocity (< 350km s^-1) solar wind with an Alfven Mach number significantly less than 1. This was a unique low-velocity, low-density, sub-Alfvénic solar wind flow which spacecraft observations have shown lasted more than 24 hours. One consequence of this extremely tenuous solar wind was a spectacular expansion of the Earth’s magnetosphere and bow shock. The expanding bow shock was observed by several spacecraft and reached record upstream distances of nearly 60 Earth radii, the lunar orbit. The event was so dramatic that it has come to be known asthe solar wind disappearance event. Though extensive studies of this event were made by many authors in the past, it has only been recently shown that the unusual solar wind flows characterizing this event originated from a small coronal hole in the vicinity of a large active region on the Sun. These recent results have put to rest speculation that such events are associated with global phenomenon like the periodic solar polar field reversal that occurs at the maximum of each solar cycle. In this paper we revisit the 11 May 1999 event, look at other disappearance events that have ocurred in the past, examine the reasons why speculations about the association of such events with global phenomena like solar polar field reversals were made and also examine the role of transient coronal holes as a possible solar source for such events.     

Wave-trains in the solar wind

Applying an Alfvén-Wave-Extended-QRH-approximation and the method of characteristics, we solve the equations of motion for outwardly propagating Alfvén waves analytically for three different cases of an azimuthal dependence of the background solar wind, (a) for a pure fast-slow stream configuration, (b) for the situation where the high-speed stream originates from a diverging magnetic field region, and (c) for the case of (b) and an initially decreasing density configuration (‘coronal hole’). The reaction of these waves on the background state as well as mode-mode coupling effects are neglected. These three solar wind models are discussed shortly. For the superimposed Alfvén waves we find, on an average, that there is a strong azimuthal dependence of all relevant wave parameters which, correlated with the azimuthal distributions of the solar wind variables, leads to good agreements with observations. The signature of high-speed streams and these correlations could clearly indicate solar wind streams originating from ‘coronal holes’. Contrary to the purely radial dependent solar wind, where outwardly propagating Alfvén waves are exclusively refracted towards the radial direction, we now find a refraction nearly perpendicular to the direction of the interplanetary magnetic field in the compression region and closely towards the magnetic field direction down the trailing edge and in the low-speed regime.     

Fundamental and harmonic radiation in solar type III bursts

There are three kinds of observations that provide indirect evidence for the contentions that (a) some type III radiation is fundamental radiation; and (b) type III's are at times emitted simultaneously as fundamental and second-harmonic plasma radiation.Presented by D. Melrose     

Wave-trains in the solar wind

The main results of Whitham's averaged Lagrangian method for the treatment of linear wave-trains in a weakly inhomogeneous, moving medium are presented briefly. This method is then applied to an ideal, isotropic, one-fluid plasma which can be taken for the lowest order approximation for the interplanetary solar wind expansion.     

Wave-trains in the solar wind

The equations of motion of all relevant parameters of Alfvén waves propagating from the sun outwardly into the expanding interplanetary medium are discussed for the case of a quiet, ideal, isotropic, one-fluid solar wind plasma. It is found that the frequency of the wave reamains constant, while the wave vector and the amplitudes depend, in general, on the evolution of the background medium and on the angle between the wave vector and the interplanetary magnetic field. This latter dependence cancels approximately for the evolution of the amplitudes in the case of a pure, overall spiral magnetic field. It is shown that in this case the results of earlier discussions can be derived by less decisive restrictions.     

The brightness temperatures of solar type III bursts

There is a characteristic maximum brightness temperature T _B 10¹⁵K for type III solar radio bursts in the solar wind. The suggestion is explored that the maximum observed values of T _Bmay be attributed to saturation of the processes involved in the plasma emission. The processes leading to fundamental and second harmonic emission saturate when T _Bis approximately equal to the effective temperature T _Lof the Langmuir waves. The expected maximum value of T _Bis estimated for this saturation model in two ways: from the growth rate for the beam instability, and from the maximum amplitude of the observed Langmuir turbulence. The agreement with the observed values is satisfactory in view of the uncertainties in the estimates (a) of the intrinsic brightness temperature from the observed brightness temperature, (b) of the actual growth rate of the beam instability, which must be driven by local, transient features (that are unobservable using available instruments) in the electron distribution, and (c) in the k-space volume filled by the Langmuir waves, and this is consistent with the observational data on two well-studied events at the orbit of the Earth and with statistical data for events over a range of radial distances from the Sun.     

A model exciter for type III solar radiobursts

In an earlier paper (Harvey and Aubier, 1973) the large scale radial electron density gradient in the corona and solar wind was shown to cause the phase velocity of plasma waves to decrease as they propagate away from the Sun, thus leading to appreciable Landau damping of the plasma waves. It is proposed here that this same phase velocity decrease creates conditions which facilitate the stabilisation of a beam of exciter electrons of finite duration, provided that three conditions are fulfilled. Two of these conditions concern the velocity-time distribution of the exciter electrons at their point of ejection from the Sun, while the third is simply that, above a certain altitude, the coronal electron density decreases with altitude r faster than r ^?2. The plasma wave source is then associated with the leading edge of the electron stream. The spatial density of the power converted into plasma waves is calculated as a function of position and time, and is shown to be independent of the nature of the stabilisation mechanism. The maximum of this power density is found to move outwards from the Sun at a uniform speed when a simple electron injection model with a Maxwellian velocity distribution is introduced.     

A review of solar type III radio bursts

Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type III bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type III radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.     

Generation of solar type III bursts at the second harmonic

Using the data from our experiments on the IMP-6 (Explorer 43) satellite, we have examined over 200 type III bursts at kilometric wavelengths, including 16 bursts which were accompanied by >18 keV electron events with sharp onsets, in a search for the electrostatic waves which, according to theory, should be the primary source of type III bursts. No electrostatic waves of sufficient intensity to generate the type III bursts by any of the wave-wave scattering theories which produce the second harmonic of the plasma frequency, have been found.     

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.     

Solar wind density model from km-wave type III bursts

Solar radio bursts were observed with a 4-channel radiometer and polarization analyser at wavelenghts around 12 m. The time and frequency resolutions were 10 ms and 100 kHz respectively. Observations on the duration, time profile and frequency splitting are described.     

Fundamental and harmonic radiation in type III solar radio bursts

Type III solar radio bursts are investigated by modelling the propagation of the electron beam and the generation and subsequent propagation of waves to the observer. Predictions from this model are compared in detail with particle, Langmuir wave, and radio data from the ISEE-3 spacecraft and with other observations to clarify the roles of fundamental and harmonic emission in type III radio bursts. Langmuir waves are seen only after the arrival of the beam, in accord with the standard theory. These waves persist after a positive beam slope is last resolved, implying that sporadic positive slopes persist for some time, unresolved but in accord with the predictions of stochastic growth theory. Local electromagnetic emission sets in only after Langmuir waves are seen, in accord with the standard theory, which relies on nonlinear processes involving Langmuir waves. In the events investigated here, fundamental radiation appears to dominate early in the event, followed and/or accompanied by harmonic radiation after the peak, with a long-lived tail of multiply scattered fundamental or harmonic emission extending long afterwards. These results are largely independent of, but generally consistent with, the conclusions of earlier works.     

Progress and problems in the theory of type III solar radio emission

The experimental and theoretical status of type III solar radio emission is considered in detail. We emphasize very recent developments which are relevant to the underlying plasma physics. In particular we discuss the identity of the sub-megahertz emissions as fundamental, or second harmonic, the degree of correlation between emissivities, electron streams, and plasma (Langmuir) waves, paradoxes concerned with the time-ordering of these phenomena, and the role of background density irregularities and ion-acoustic turbulence in the solar wind. From the theoretical point of view we discuss the current picture of the underlying Langmuir turbulence, including such effects as the interaction between Langmuir waves and stream electrons, induced scatter off ions, and strong turbulence effects such as modulational instability and soliton collapse.     

Tangential discontinuities in the solar wind

This paper considers six discontinuity surfaces which were observed by magnetometers on 3 spacecraft in the solar wind. It is shown that the actual surface orientations, determined from the measured time delays and solar wind speed, are consistent with the theoretical orientations which were computed from the relation , where is the normal to the surface of a hydromagnetic tangential discontinuity across which the magnetic field direction changes from to . The plasma and magnetic field data for these discontinuities are consistent with the pressure balance condition, and the magnetic field vectors in the associated current sheets are parallel to the discontinuity surface, as required theoretically. The 6 discontinuity surfaces extended without much distortion over ∼ 0.002 AU. A seventh surface is discussed which satisfies the condition but which extended without much distortion over 0.01 AU. This latter is not a typical surface, however, and its curvature is larger than average. Most of the surfaces tended to lie along the spiral direction, but one was nearly perpendicular to the spiral direction.     

Cyclotron waves in the solar wind

Cyclotron waves in the solar wind near 1 AU with frequencies well below the electron cyclotron frequency and wavelengths much larger than the electron cyclotron radius but less than the proton cyclotron radius are considered. The cyclotron radii are defined from parallel thermal velocity of electron component and proton component with respect to the interplanetary magnetic field. No LH cyclotron waves are found to propagate for _p < 0, where _p 1 –T _p/T _p is the temperature anisotropy of the proton component with respect to the interplanetary magnetic field. The damping or growth of RH cyclotron waves is found to depend on the frequency range and the temperature anisotropy of the proton component. The RH cyclotron waves are damped in the frequency range _r | _p | _p for _p < 0, where _p is the proton cyclotron frequency. RH cyclotron instabilities occur in the frequency range | _p | _p > _r > | _p | _p /(1– _r ) for _p < 0. The marginal state is at _r =| _p | _p .Abstract presented at theInternational Symposium on Solar-Terrestrial, São Paulo, Brazil, 17–22 June, 1974     

Heat transport in the solar wind

Heat transport is considered both for quiet and disturbed solar winds. It is shown that heat may be transferred during solar flares by sharp fronted thermal wave pulses. Energy dissipation in the wave front arises from the firehose instability excitation. The effects of ionosonic turbulence on heat transport in a quiet solar wind are also investigated. A quasi-steady state, in which there is a balance between wave-particle interations and particle collisions is found. It is shown that the effect of wave-particle ‘collisions’ is to produce a significant decrease of the electron heat flow and electron temperature, and increase of the ion temperature relative to calculations which take into account particle particle collisions only.