An investigation of the fine ray structure of the coronal streamer belt using LASCO data

It is shown that within R>3–4 Rfrom the solar center the coronal streamer belt consists in a sequence of radial brightness rays. A minimum angular size of the individual ray d2.0°–2.5°, which is about the same in the directions normal to and along the streamer-belt, is independent of the distance from the Sun at R=4–6 R. The lifetime of the rays can exceed 10 days. From time to time, inhomogeneities of material inside the rays begin to move in the antisunward direction. Plots of increase in their velocity with the distance from the Sun are similar to those obtained by Sheeley et al. (1997) for inhomogeneities that are carried by a quasi-stationary solar wind in streamers. It is concluded that the phenomena discussed in this paper and by Sheeley et al. (1997) share a common origin. It is suggested that a different origin of solar wind flows in streamers and in coronal holes may be associated with a different character of flows in microtubes of the magnetic field comprising a total solar wind flow. These tubes are observed as brightness rays in streamer belts and plumes in coronal holes.     

The anomalous distribution in heliocentric longitude of solar injected cosmic radiation

Concurrent observations of the solar flare of March 12, 1969 by two spacecrafts separated in solar longitude by 38° show that the accessibility at 1 AU to cosmic ray particles is not a simple function of the relative solar longitude. The cosmic ray flux, degree of anisotropy, and rise time all indicate that the favored path for cosmic ray propagation in this event was some 40° to the east of the nominal Archimedes spiral line of force from the flare location. This is interpreted as evidence for either (a) extreme stochastical wandering of the lines of force of the interplanetary magnetic field, or (b) the redistribution of the cosmic rays in coronal magnetic fields prior to escape onto the nominal Archimedes spiral lines of force.Now at CSIRO, G.P.O. Box 124, Port Melbourne, Victoria 3207, Australia.Now at Physical Research Laboratory, Ahmedabad, India.     

Solar-cycle evolution of the coronal general magnetic field of 1959–1974 and the synchronous variation of high-speed solar wind streams and galactic cosmic rays

The simultaneous enhancement or subsidence of both the high-speed solar wind streams and the galactic cosmic rays in the minimum or the maximum phase of the solar cycle are interpreted in a unified manner by the concept of geometrical evolution of the general magnetic field of the corona-heliomagnetosphere system. The coronal general magnetic field evolves from an open dipole-like configuration in the minimum phase to a closed configuration with many loop-like formations in the maximum phase of the solar cycle. This concept, developed in a theoretical solar-cycle model driven by the dynamo action of the global convection, is examined and found to be valid by studying the evolution of the coronal general magnetic field calculated from the observed surface general magnetic field of 1959–1974. It is also found that the energy density of the poloidal component of the general surface field, from which the coronal field originates, attained a maximum in the maximum phase and showed a evolution with virtually no phase delay with respect to that of the toroidal component of the field, to which the sunspot activity is related. The subsidence of the high-speed solar wind in the maximum phase is understood as a braking of the solar wind streams by the tightly closed and strong coronal field lines in the lower corona in the maximum phase. The field lines of the heliomagnetosphere, which originate from the coronal field lines drawn by the solar wind, are inferred to be also more tightly closed at the heliopause in the maximum phase than in the minimum phase. The decrease of the galactic cosmic rays in the maximum phase (known as the Forbush's negative correlation between the galactic cosmic ray intensity and the solar activity or the Forbush solar-cycle modulation of the galactic cosmic rays) is interpreted as a braking of the cosmic rays by the closed magnetic field lines at the heliopause. The observed phase lag (approximately one year) of the galactic cosmic ray modulation with respect to the evolution of the solar cycle, and the observed absence of the gradient of the total cosmic ray intensity between 1 AU and 8 AU, are discussed to support this view of the cosmic ray modulation at the remote heliopause, and reject other hypotheses to explain the phenomenon in terms of the magnetic irregularities of various kinds carried by the solar wind: The short-term Forbush decrease at a time of a flare shows that the magnetic irregularities can react on the cosmic rays relatively near the Sun if they even played a dominant role in the long-term modulation. The concept of the general magnetic field of the corona and the surface is also used to understand the basic nature of the surface field itself, by comparing the geometry of the calculated coronal field lines with the eclipse photographs of the corona, and by discussing, in the context of the coronal general magnetic field associated with the solar cycle, the process of the emergence of the coronal field lines from the interior and the formation of the transequatorial arches and loops connecting the two hemispheres in the corona.     

Strong cosmic-ray scattering in an anisotropic random magnetic field

We calculate the kinetic coefficients and the transport mean free paths of high-energy particles parallel to the regular magnetic field in the approximation of a large-scale anisotropic random magnetic field by using a nonlinear collision integral, i.e., by taking into account the processes of strong random scattering. We consider the diffusion of solar and Galactic cosmic rays by two-dimensional turbulence. Strong random scattering by two-dimensional turbulence is shown to reduce the parallel transport mean free path several fold. The momentum dependence of the parallel mean free path does not change, Λ_∥ ∝ p^2?v. In the case of strong random scattering by turbulence formed by several modes, the parallel transport mean free path is Λ_∥ ∝ p. We show that two-dimensional turbulence can make a major contribution to the parallel transport mean free paths of cosmic rays in the heliosphere and the interstellar medium.     

Ultra-high energy cosmic rays in the galactic corona

On the basis of recent new information on regular and chaotic magnetic fields in coronae of spiral galaxies, we discuss propagation of ultra-high energy cosmic rays of energies exceeding 10¹⁷ eV in the galactic corona. It is shown that the expected regular magnetic field is able to confine to the corona protons of energies up to 3×10¹⁹ eV. Chaotic magnetic fields of the corona play an important role in dynamics of cosmic-ray protons of energy up to 7×10¹⁸ eV.     

Interaction of relativistic dust grains with the solar radiation field

The effects of the solar radiation field on the propagation of relativistic dust grains are evaluated. It is concluded that relativistic iron grains with energies 10¹⁹ eV will melt in the solar radiation field before they reach the Earth's orbit around the Sun. However iron grains with lower energies will reach the Earth's orbit but grains travelling from the direction of the Sun will melt. This directional anisotropy or fingerprint may be used to search for relativistic dust grains in the primary cosmic rays. The fact that no significant solar system anisotropy has been detected places constraints on the hypothesis that the initiating particles of the extensive air showers are relativistic iron grains.     

In situ acceleration and gradients of charged particles in the outer solar system observed by the voyager spacecraft

The probable connection between cosmic rays and the electromagnetic state of the interplanetary medium was recognized by Hannes Alfvén as early as 1949 (Alfvén, 1949, 1950); he pointed out that the properties of cosmic rays necessitate a mechanism, external to Earth but within the solar system, capable of accelerating particles to extremely high energies. In advocating the view of local origin for part of the cosmic-ray spectrum, Alfvén and his colleagues developed a very general type of acceleration mechanism called magnetic pumping. The unique data set of the two Voyagers extends over an entire decade (1977–1987) and is most suitable to explore the problem of acceleration of charged particles in the heliosphere. The energy coverage of the Low Energy Charged Particle (LECP) experiment covers the range 30 keV to several hundred MeV for ions and 22 keV to several MeV for electrons. Selected observations of interplanetary acceleration events from 1 to 25 AU are presented and reviewed. These show frequent acceleration of ions to several tens of MeV in association with shocks; highest energies (220 MeV oxygen) were measured in the near-perpendicular ( _Bn 87.5°) shock of January 5, 1978 at 1.9 AU, where electron acceleration was also observed. Examples of ion acceleration in association with corotating interaction regions are presented and discussed. It is shown that shock structures have profound effects on high-energy (70 MeV) cosmic rays, especially during solar minimum, when a negative latitudinal gradient was observed after early 1985 at all energies from 70 MeV down to 30 keV. By early 1987, most shock acceleration activity in the outer heliosphere (25 to 30 AU) had ceased both in the ecliptic (Voyager-2) and at higher (30°) ecliptic latitudes (Voyager-1). The totality of observations demonstrate that local acceleration to a few hundred MeV, and as high as a few GeV is continually present throughout the heliosphere. It should be noted that in 1954 when Alfvén suggested local acceleration and containment of cosmic rays within the solar system, no one treated his suggestion seriously, at any energy. The observations reviewed in this paper illustrate once more Alfvén's remarkable prescience and demonstrate how unwise it is to dismiss his ideas.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

High coronal structure of high velocity solar wind stream sources

When solar wind plasma in the trailing (eastern) edge of a high-speed stream is mapped back to its estimated source in the high corona using the constant radial velocity (EQRH) approximation, a large range of velocities appears to come from a restricted range in longitude, often only a few degrees. This actually constitutes a sharp eastern coronal boundary for the solar wind stream source, and demands that the boundary have a three-dimensional structure. Using interplanetary data, we infer a systematic variation in source altitude (identified approximately with the Alfvén point), with faster solar wind attaining its interplanetary characteristics at lower altitudes. This also affects the accuracy of the source longitude estimates, so that we infer a width in the high corona of 4–6° for the source of the trailing edges of streams which appear to originate from a single longitude. We demonstrate that the possible systematic interplanetary effects (in at least some cases) are not large ( 2° in heliocentric longitude). The relatively sharp boundaries imply that high-speed streams are well-defined structures all the way down to their low coronal sources, and that the magnetic field structure controls the propagation of the plasma through the corona out to the vicinity of the Alfvén point ( 20 R ).     

SCR Steady State Distribution Function and Scattering Properties of the Interplanetary Medium

The propagation of energetic particles in the interplanetary space is considered on the basis of kinetic equation describing the scattering of charged particles by magnetic irregularities and the particle focusing by regular magnetic field. Our analysis confirms that angular distribution of solar cosmic rays contains a valuable information about properties of the particle scattering in the interplanetary magnetic field. Steady state solutions of the kinetic equation are applied to the analysis of solar proton events.     

Evidence for a two-component injection of cosmic rays from the solar flare of 1969, March 30

The solar flare of 1969 March 30, occurring 20° behind the west limb, produced very extensive 80 MHz radio emission at the Sun, and gave rise to the deployment of cosmic radiation over 360°long, in interplanetary space. The wide spread of this event may reflect a similar spread of coronal magnetic fields from the flare site. We interpret the solar proton data recorded by spacecraft at two separate points both at 1 AU, in terms of a two-component injection of particles at the Sun consisting of: (i) a soft component which arrived promptly; (ii) a harder component which arrived later. The radio spectral and positional data provide evidence of shock waves which propagated far and wide from the flare; we attribute the precursor injection of the soft ( 10 MeV) proton component to one of these shock waves.Radiophysics Publication RPP 1590, May, 1972.Now at University of California, LASL, Los Alamos, N.M., U.S.A.     

Measurements of magnetic fields in solar corona as based on the radio observations of the inversion of polarization of local sources at microwaves

The possibility of measuring magnetic fields of solar active regions at coronal heights up to 10¹⁰ cm by observing the inversion of circular polarization of local sources at microwaves is demonstrated. The observations by the radiotelescope RATAN-600 were accomplished with the angular resolution 17–34 in the wavelength range 2–4 cm. It is found that the inversion of polarization occured within a core of local source situated above the largest sunspot of Mc Math 14822. The inversion was followed during the period of June 30–July 3, 1977. The measured coronal magnetic field of 16 G is found to be at the height 12 × 10⁹ cm. This measured field proves to agree with a simulated potential structure of Mc Math 14822 coronal magnetic field. Our analysis of the inversion has been based on the theory of interaction (coupling) of the ordinary and extraordinary wave modes in the region of quasi-transverse propagation.     

Short and long-term variations in the cosmic ray intensity and their connection with the 5303 Å coronal line intensity in solar cycle No. 20

The cosmic ray modulation in the period 1965–70 is investigated by the comparison of the intensity data of groundbased stations with different response to primaries. The socalled step-like modulation, already observed by other authors, is found to be produced by the overlapping between the quasi-stationary solar cycle modulation and the Forbush decrease events. Moreover a good correlation between the cosmic-ray variance (Forbush decrease index) and the 5303 coronal intensity at middle heliolatitudes (17.5°–42.5°) is found, while the quasi-stationary solar cycle modulation is well correlated with the 5303 intensity near the solar equator (0°–17.5°). The different time behaviour of the solar activity at different heliolatitudes causes the step-like modulation.     

A study of the modulating effect of solar flares on the cosmic ray intensity using time series analysis

The cosmic ray 11-year variation for solar cycle 20 is attributed to the modulating effect of solar flare-induced shocks propagating through the interplanetary medium to the boundary of the heliosphere. The relative influence of these disturbances upon the cosmic ray intensity as a function of their travel time from the Sun is determined by a deconvolution of a linear system with the number of solar flares (importance 1) and the observed cosmic ray intensity as the input and output respectively of this system. The impulse response function so determined indicates that the solar flare - induced disturbances significantly modulate cosmic rays out to a distance of 70–90 AU where the modulating effect of the disturbances abruptly ends. This is interpreted as the boundary of the heliosphere.     

The release of energetic particles from the Sun

A model is developed to account for the release of solar cosmic rays from the Sun. The solar atmosphere out to 3–5 solar radii above the photosphere is permeated with magnetic field lines which trap low rigidity ( 50 MV) flare particles. Plasma heated by the flare process disturbs the trapping field, and not until the disturbance reaches 3–5 solar radii can the low rigidity flare particles have access to interplanetary space. If the plasma is not heated sufficiently to overcome the coronal field, flare particles are trapped, efficiently. Subsequent leakage of these particles into interplanetary space forms corotating streams. Reference is made to satellite observations of solar electromagnetic radiation and charged particles.     

The acceleration and propagation of solar cosmic rays as deduced from the relative abundance of protons to helium nuclei

Except for protons, the chemical composition of solar cosmic rays is very similar to the abundance of the elements at the photosphere of the Sun. If we consider the relative abundance ratio of protons to -particles (P/) at constant rigidity, this ratio is highly variable from one solar cosmic ray event to another. This ratio observed at the Earth, however, decreases monotonically with time from the onset of solar flares and, furthermore, is dependent on the heliocentric distance of the parent flares from the central meridian of the solar disk. P/'s which have been measured before the onset of SC geomagnetic storms change from 1.5 to 50 or more, being a function of the westward position of the source from the east limb of the Sun. These variations with respect to time and heliocentric distance suggest that the propagation of solar cosmic rays is strongly modulated in the interplanetary space. The major part of the -particles seem to propagate as if they are trapped within the magnetic clouds which produce SC geomagnetic and cosmic ray storms at the earth.The chemical composition and rigidity spectra of solar cosmic rays suggest that solar cosmic rays are mainly accelerated by the Fermi mechanism in solar flares. The observed variation of P/'s is produced mainly through the difference between the propagation characteristics of protons and -particles.NAS-NRC Associate with NASA.     

Large-scale structure of the interplanetary medium

We propose that the coronal source longitude and latitude of solar wind plasma can be estimated within 10°. Previous writers have argued that the solar wind in the ecliptic should originate near the equator and that a quasi-radial hypervelocity (QRH) approximation (constant radial flow) is valid beyond the magnetohydrodynamic critical points. We demonstrate that an extension of the QRH approximation (as if the solar wind flowed radially with constant velocity from the center of the Sun) yields a proper estimate of the high coronal source location at the release zone where the solar wind makes its transition to radial interplanetary flow. This extrapolated QRH (or EQRH) approximation succeeds because the two main corrections to this source estimate, coronal corotation and interplanetary acceleration, tend to cancel (the former correcting the source location eastward, the latter westward). Although this ideal spiral approximation was first suggested by Snyder and Neugebauer (1966), only recently has it been demonstrated that it relates a wide range of interplanetary plasma, magnetic field and energetic particle data to observed coronal magnetic structure. We estimate quantitatively the error in the EQRH approximation by comparison with steady-state streamlines predicted by azimuthally independent and dependent theoretical solutions to the steady-state plasma equations. We find the error in both cases 10° in longitude and therefore suggest that the EQRH approximation offers the means to relate observed solar initial conditions in the release zone directly to interplanetary measurements. If, in addition, the EQRH approximation also leads to agreement with low coronal structure, then there should be a straightforward correspondence to otherwise unobservable high coronal structure.     

Curvature and surface distribution of the polar rays in the solar corona on 12 November 1966

Following a technique developed by Saito, the positions and orientations of the polar rays visible in a photograph of the total solar eclipse of 12 November 1966 were examined to infer the length of an equivalent bar magnet representing the Sun's external magnetic field and the true distribution of polar rays from the two-dimensional projection seen in the photograph. In spite of the strong asymmetry in solar activity between the northern and southern solar hemispheres at the time of the eclipse, it was found that the magnetic field as delineated by the polar rays could be well represented by an extended dipole whose poles were each 0.43 R from the center of the Sun and on the axis of rotation. The polar rays are distributed mainly in a circular zone 9° away from the pole and about 8° wide, with this zone bounded by empty regions at the pole and 15° to 20° away from it, beyond which there are again some rays, but not so many. These results are in good accord with expectation from Saito's analysis.Operated by the Association of Universities for Research in Astronomy Inc. under contract with the National Science Foundation.     

Energy changes of cosmic rays in the interplanetary region

A clarification and discussion of the energy changes experienced by cosmic rays in the interplanetary region is presented. It is shown that the mean time rate of change of momentum of cosmic rays reckoned for a fixed volume in a reference frame fixed in the solar system is 〈p〉 =p V·G/3 (p=momentum,V is the solar wind velocity andG=cosmic-ray density gradient). This result is obtained in three ways:

1. by a rearrangement and reinterpretation of the cosmic-ray continuity equation;
2. by using a scattering analysis based on that of Gleeson and Axford (1967);
3. by using a special scattering model in which cosmic-rays are trapped in ‘magnetic boxes’ moving with the solar wind.

The third method also gives the rate of change of momentum of particles within a moving ‘magnetic box’ as 〈p〉_ad = ?p ?·V/3, which is the adiabatic deceleration rate of Parker (1965). We conclude that ‘turnaround’ energy change effects previously considered separately are already included in the equation of transport for cosmic rays.     

Long-Term Radio Scintillation Variations in the Circumsolar Plasma

Long-term scintillation measurements of the solar wind formation zone at solar elongations ranging from 1°–8° (Sun impact parameters: 4–30 R ) were recorded using the water maser source IRC-20431 at the wavelength =1.35 cm during its annual solar occultations in December 1981–1998. Dramatic changes in the spatial dependence of the scintillation index were recorded over the course of the 11-year solar cycle. Markedly diminished scattering, attributed to a pronounced heliolatitude effect, was observed at the closest solar approach distances in the years around solar activity minimum. From parallel investigations of the solar magnetic field structure it was determined that the field strength at the source of the solar wind streamlines is the governing factor for the solar wind acceleration process. Particularly apparent in the scintillation data during solar activity minimum is the increasing role of the polar coronal holes with their associated open magnetic field structure. The dependence of the solar scattering intensity on heliolatitude fades in the years of high solar activity as the level of scintillations increases at polar latitudes.     

Complete determination of the magnetic field vector and of the electron density in 14 prominences from linear polarizaton measurements in the HeI D3 and Hα lines

The present paper is devoted to the interpretation of linear polarization data obtained in 14 quiescent prominences with the Pic-du-Midi coronagraph-polarimeter by J. L. Leroy, in the two lines Hei D₃ andH quasi-simultaneously. The linear polarization of the lines is due to scattering of the anisotropic photospheric radiation, modified by the Hanle effect due to the local magnetic field. The interpretation of the polarization data in the two lines is able to provide the 3 components of the magnetic field vector, and one extra parameter, namely the electron density, because the linear polarization of H is also sensitive to the depolarizing effect of collisions with the electrons and protons of the medium. Moreover, by using two lines with different optical thicknesses, namely Hei D₃, which is optically thin, and H, which is optically thick ( = 1), it is possible to solve the fundamental ambiguity, each line providing two field vector solutions that are symmetrical in direction with respect to the line of sight in the case of the optically thin line, and which have a different symmetry in the case of the optically thick line.It is then possible to determine without ambiguity the polarity of the prominence magnetic field with respect to that of the photospheric field: 12 prominences are found to be Inverse polarity prominences, whereas 2 prominences are found to be Normal polarity prominences. It must be noticed that in 12 of the 14 cases, the line-of-sight component of the magnetic field vector has a Normal polarity (to the extent that the notion of polarity of a vector component is meaningful; no polarity can be derived in the 2 remaining cases); this may explain the controversy between the results obtained with methods based on the Hanle effect with results obtained through the Zeeman effect. A dip of the magnetic field lines across the prominence has been assumed, to which the optically thick H line is sensitive, and the optically thin Hei D₃ line is insensitive.For the Inverse prominences, the average field strength is 7.5±1.2 G, the average angle,, between the field vector and the prominence long axis is 36° ± 15°, the average angle, , between the outgoing field lines and the solar surface at the prominence boundary is 29° ± 20°, and the average electron density is 2.1 × 10¹⁰ ± 0.7 × 10¹⁰ cm^–3. For the Normal prominences, the average field strength is 13.2±2.0 G, the average angle,, between the field vector and the prominence long axis is 53° ± 15°, the average angle, , between the outgoing field lines and the solar surface at the prominence boundary is 0° ± 20° (horizontal field), and the average electron density is 8.7 × 10⁹ ± 3.0 × 10⁹ cm^–3.