Interplanetary shock waves: Recent developments

Direct and indirect observations of interplanetary shock waves have been extended to the study of (i) the shock structure itself; (ii) the disturbed solar wind in its wake; (iii) additional discontinuities such as reverse shocks and pistons; and (iv) the shock's kinematic behavior. The last item — the trajectory — has benefited by the procedure (suggested by Pintér) of matching type II radio drift-inferred velocities with indirectly-inferred initial velocities found from at least two successive measurements in space. The significance of making type II observations at hectometric and kilometric wavelengths (as made, for example, by Slysh and Malitson, Feinberg and Stone) cannot be over-emphasized due to this technique's ability to make unambiguous solar terrestrial relationships. More direct and physically-meaningful observations, however, are still dependent uponin situ plasma and magnetic field measurements. Additional emphasis is presently being placed on numerical modeling of shock-induced disturbances in the solar wind as generated by both flares and stream-stream interactions. The former mechanism is emphasized in this review with several recommendations for further research: (a) further numerical modeling for shocks, starting when they are born within relatively low-Alfvén speed coronal regions; (b) expanded synoptic studies by spacecraft at various heliocentric longitudes, radii, and (eventually) latitudes with coordinated diagnostics; and (c) extended patrol of natural probes, such as comets, augmented with theoretical studies of possible shock-induced mechanical and chemical effects.     

Auroral flares and solar flares

The morphology of development of auroral flares (magnetospheric substorms) for both electron and proton auroras is summarized, based on ground-based as well as rocket-borne and satellite-borne data with specific reference to the morphology of solar flares.The growth phase of an auroral flare is produced by the inflow of the solar wind energy into the magnetosphere by the reconnection mechanism between the solar wind field and the geomagnetic field, thus the neutral and plasma sheets in the magnetotail attaining their minimum thickness with a great stretch of the geomagnetic fluxes into the tail.The onset of the expansion phase of an auroral flare is represented by the break-up of electron and proton auroras, which is associated with strong auroral electrojets, a sudden increase in CNA, VLF hiss emissions and characteristic ULF emissions. The auroral break-up is triggered by the relaxation of stretched magnetic fluxes caused by cutting off of the tail fluxes at successively formed X-type neutral lines in the magnetotail.The resultant field-aligned currents flowing between the tailward magnetosphere and the polar ionosphere produce the field-aligned anomalous resistivity owing to the electrostatic ion-cyclotron waves; the electrical potential drop thus increased further accelerates precipitating charged particles with a result of the intensification of both the field-aligned currents and the auroral electrojet. It seems that the rapid building-up of this positive feedback system for precipitating charged particles is responsible for the break-up of an auroral flare.     

Interplanetary shock waves: Ulysses observations in and out of the ecliptic plane

Between its launch in October 1990 and the end of 1993, approximately 160 fast collisionless shock waves were observed in the solar wind by the Ulysses space probe. During the in-ecliptic part of the mission, to February 1992, the observed shock waves were first caused mainly by solar transient events following the solar maximum and the reorganisation of the large scale coronal fields. With the decay in solar activity, relatively stable Corotating Interaction Regions (CIRs) were observed betwen 3 and 5.4 AU, each associated with at least one forwardreverse shock pair. During the out-of-ecliptic phase of the orbit, from February 1992 onwards, CIRs and shock pairs associated with them continued to dominate the observations. From July 1992, Ulysses encountered the fast solar wind flow from the newly developed southern polar coronal hole, and from May 1993 remained in the unipolar magnetic region associated with this coronal hole. At latitudes beyond 30°, CIRs were associated almost exclusively with reverse shocks only. A comprehensive list of shock waves identified in the magnetic field and solar wind plasma data from Ulysses is given in Table 1. The principal characteristics were determined mainly from the magnetic field data. General considerations concerning the determination of shock characteristics are outlined in the Introduction.     

Interplanetary scintillation observations of the high-latitude solar wind

Until the ULYSSES spacecraft reached the polar regions of the solar wind, the only high-latitude measurements available were from indirect techniques. The most productive observations in regions of the solar wind between 5R and 200R have been the family of radio scattering techniques loosely referred to as Interplanetary Scintillation (IPS) (Coles, 1978). Useful observations can be obtained using a variety of radio sources, for example spacecraft beacons, planetary radar echoes and compact cosmic sources (quasars, active galactic nuclei, pulsars, galactic masers, etc.). However for measurement of the high-latitude solar wind cosmic sources provide the widest coverage and this review will be confined to such observations. IPS observations played a very important role in establishing that polar coronal holes (first observed in soft x-ray emission) were sources of fast solar wind streams which occasionally extend down to the equatorial region and are observed by spacecraft. Here I will review the IPS technique and show the variation of both the velocity and the turbulence level with latitude over the last solar cycle. I will also outline recent work and discuss comparisons that we hope to make between IPS and ULYSSES observations.     

Observational support of reconnection in solar flares

We present a detailed analysis of the magnetic topology of flaring active region. TheH kernels are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibrils or/and transverse magnetic field direction, is taken into account. We show that the kernels are magnetically connected by field lines passing close to the separator. We confirm, for other flares, previous studies which show that photospheric current concentrations are located at the borders of flare ribbons. Moreover we found two photospheric current concentrations of opposite sign, linked in the corona by field lines which follow separatrices. These give evidence that magnetic energy is released by reconnection processes in solar flares.     

Interaction of non-perpendicular/parallel solar wind shock waves with the earth's magnetosphere

The interaction of travelling interplanetary shock waves with the bow shock-magnetosphere system is considered. We consider the general case when the interplanetary magnetic field is oblique to the Sun-planetary axis, thus, the interplanetary shock is neither parallel nor perpendicular. We find that an ensemble of shocks are produced after the interaction for a representative range of shock Mach numbers. First, we find that the system S ₊ R _– CS _– S ₊ appears after the collision of travelling fast shock waves S ₊ (Mach number M = 2 to 7) with the bow shock. Here, S _– and R _– represent the slow shock wave and slow rarefaction wave, and C represents the contact surface. It is shown that in the presence of an interplanetary field that is inclined by 45° to the radial solar wind velocity vector, the waves R _– and S _– are weak waves and, to the first degree of approximation, the situation is similar to the previously studied normal perpendicular case. The configuration, R ₊ C _m S _– S ₊ or R ₊ C _m R _– S ₊ where C _m is the magnetopause, appears as the result of the fast shock wave's collision with the magnetopause. In this case the waves S _– and R _– are weak. The fast rarefaction wave reflected from the magnetosphere is developed similar to the case for the collision of a perpendicular shock. The shock wave intensity is varied for Mach numbers from 2 to 10. Thus, in the limits of the first approximation, the validity of the one-dimensional consideration of the nonstationary interaction of travelling interplanetary shock waves with the bow shock-magnetosphere system is proved. The appearance of the fast rarefaction wave, R ₄, decreasing the pressure on the magnetosphere of the Earth after the abrupt shock-like contraction, is proved. A possible geomagnetic effect during the global perturbation of the SSC or SI⁺ type is discussed.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Nuclear processes and accelerated particles in solar flares

Nuclear processes and particle acceleration in solar flares are reviewed. The theory of gamma-ray and neutron production is discussed and results of calculations are compared to gamma-ray, neutron, and charged-particle observations from solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies, electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site, are presented. The information on elemental and isotopic abundances derived from gamma-ray observations is compared to abundances obtained from escaping accelerated particles and other sources.NAS/NRC Resident Research Associate.     

Space-system timekeeping in the presence of solar flares

It has become increasingly clear that the enhanced space radiation environment associated with solar activity can play havoc with satellite systems. An important special case of this problem concerns precise timekeeping among orbiting communications satellites, where the loss of synchronization can mean the loss of communications. Here, we discuss one satellite system's solution to this problem, where radiation-sensitive spacecraft crystal oscillators are "slaved" to radiation-insensitive spacecraft atomic clocks.     

Observational studies of gamma-rays and neutrons from solar flares

Gamma-ray observations from HINOTORI satellite and possible neutron observations from the Tokyo neutron monitor are reviewed. Time histories of gamma-ray and X-ray emissions for both typical impulsive and gradual flares are discussed in connection with the particle acceleration time. The gamma-ray spectral hardening observed around 400 keV is explained from superimposition of two different electron bremsstrahlung spectra. Proton-energy spectra derived from the gamma-ray observations are compared with the solar energetic particle spectra in interplanetary space. The weak correlation between the gamma-ray fluence and the proton flux is discussed in connection with the particle trapping and escaping in the flare region. The limb darkening of the 2.22 MeV line resulting from neutron-proton capture is interpreted in terms of the attenuation by the Compton scattering in the photosphere. Possible solar neutron events recorded by the Tokyo neutron monitor are presented and the correlation between the gamma-ray fluence and the neutron fluence are described.     

Variations of the magnetic fields in large solar flares

We present preliminary results from high resolution observations obtained with the Michelson Doppler Imager (MDI) instrument on the SOHO of two large solar flares of 14 July 2000 and 24 November 2000. We show that rapid variations of the line-of-sight magnetic field occured on a time scale of a few minutes during the flare explosions. The reversibility/irreversibility of the magnetic field of both active regions is a very good tool for understanding how the magnetic energy is released in these flares. The observed sharp increase of the magnetic energy density at the time of maximum of the solar flare could involve an unknown component which deposited supplementary energy into the system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Particle acceleration in solar flares and some related phenomena

Observational features concerning solar energetic particles are compactly reviewed with some emphasis on the spectra and time histories. Velocity dependent characteristics in the energy spectra are pointed out, and compared to the results of the interplanetary shocks. A shock drift acceleration is introduced in order to interpret the observational features, especially a very fast acceleration to MeV energies within an order of second. There is a strong evidence of the shock drift acceleration in the interplanetary shocks. When some conditions are satisfied in the corona, only one or several encounters of particles with a near perpendicular shock accelerates protons to gamma-ray emitting energies (> 10 MeV). Pre-acceleration is inevitable for any kind of acceleration mechanisms in solar flares. To fulfill the requirements from the abundance ratios between various species of accelerated ions, pre-acceleration to the same velocities before the injection into a main acceleration process turns out to be absolutely necessary.     

Energy build-up and release mechanisms in solar and auroral flares

Flare phenomena in the solar atmosphere and in the terrestrial magnetosphere exhibit many similarities. The mechanical energy of enhanced photospheric motion is converted and stored in the form of magnetic potential energy in sunspot fields, which is analogous to the case of the growth phase of magnetospheric substorms. The energy release during the explosive phase is initiated by a sudden collapse in the magnetic field topology and the X-type magnetic neutral point is created in the corona. Subsequent electrical discharge takes place in the form of an intense electrojet current flowing in the base of the chromosphere at the altitude where the Cowling conductivity is a maximum. It is suggested that the acceleration of particles by field-aligned electric fields and the Ohmic heating in the chromosphere result in major features of solar flares.This article also appears inSolar Physics 40 (1975) 217–226. By way of exception this paper is reproduced here for the sake of completeness.     

The analysis of energy release in solar flares based on X-ray observations

Extended review of selected papers which deal with the problem of flare heating in solar coronal loops is presented. Discussed methods of the analysis of flare heating based on the X-ray observations have been worked out using the Palermo-Harvard hydrodynamic code. The case is presented when the assumption of the uniform heating across the loop is made. The existence of multiple elementary heating episodes is postulated as well. Next the possibility of the non-uniform heating across the loop is assumed and its manifestation in the X-ray observations is investigated. The application of proposed methods of the analysis to the observations of solar flares in X-rays is presented.     

Observation of flare-generated shock waves by Helios-2 near the sun

We present the observation of three flare generated shock waves which were measured by Helios-2 at a radial distance from the Sun of 0.476 AU, 0.444 AU, and 0.297 AU. These results show that two of the shocks fulfill the classification scheme of F-events stated in the past while the third shock profile is that of a R-event being interpreted as a driven wave. Because of the close distance to the Sun a flare-association was possible. Only in one case there is some evidence for a piston in the downstream region of the shock. One of the shocks is characterized by a distinct hole in the frequency distribution of directional discontinuities in the downstream region.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

可控肥皂膜气柱界面与激波相互作用的实验研究

基于肥皂膜技术提出了一种生成气柱界面的简单方法。形成的界面不需要支撑网格,因此初始条件可以很好地控制。在水平激波管中结合高速纹影开展了平面激波与二维或三维气柱界面作用的实验。纹影照片显示实验结果杂波更少,演变中的界面也比文献的结果更对称。此外,还特别研究了由极小曲面特征导致的三维效应,发现三维效应使界面的演变和发展变慢了。本研究将有助于更多了解三维性对Richtmyer-Meshkov不稳定性发展的影响。     

Jets and brightenings generated by energy deposition in the middle and upper solar chromosphere

Numerical simulations of energy depositions in the middle and upper solar chromosphere result in ejection of chromospheric material into the corona and heating of the chromospheric gas. These simulations may be capable of describing some of the features seen by the soft X-ray telescope on board theYohkoh satellite.