The evolution and the secondary maximum of the green line intensity

A new relation has been given in order to calculate the intensity of the green line of the solar corona at 5303 Å as a function of the number of proton events N _P and the R (R) index of solar activity. This relation is available for the 19th and 20th solar cycles. Moreover there is given a theoretical justification of this relationship taking into account as a new parameter the evolution of the coronal magnetic field during the solar cycle.     

The interaction effect of fast and slow solar wind streams in interplanetary space on wind characteristics at the Earth's orbit

By analyzing observational data, it has been possible to determine quantitative relationships that represent the role of the interaction of fast and slow solar wind (SW) streams in the formation of characteristic SW properties at the Earth's orbit.It is shown that maximum values of magnetic field B _M and density n _M peaks in the neighbourhood of the sector boundary (SB) at the base of the high-speed stream front are associated with solar wind characteristics such as the SW minimum velocity near the SB, V _m, the maximum velocity in the central part of the fast stream, V _M, and the slope of the magnetic field neutral line to the solar equatorial plane at R = 2.5 R (R is the solar radius).It is concluded that enhancements of absolute values of the z-component of the magnetic field, ¦B _z¦, recorded at the Earth's orbit, are largely attributable, at sufficiently large values of , to the interaction of different-velocity SW streams.     

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.     

Generation of rotational discontinuities by magnetic reconnection associated with microflares

Magnetic reconnection can take place between two plasma regions with antiparallel magnetic field components. In a time-dependent reconnection event, the plasma outflow region consists of a leading bulge region and a trailing reconnection layer. Magnetohydrodynamic (MHD) discontinuities, including rotational discontinuities, can be formed in both the bulge region and the trailing layer. In this paper, we suggest that the rotational discontinuities observed in the solar wind may be generated by magnetic reconnection associated with microflares in coronal holes. The structure of the reconnection layer is studied by solving the one-dimensional Riemann problem for the evolution of an initial current sheet after the onset of magnetic reconnection as well as carrying out two-dimensional MHD simulations. As the emerging magnetic flux reconnects with ambient open magnetic fields in the coronal hole, rotational discontinuities are generated in the region with open field lines. It is also found that in the solar corona with a low plasma beta ( 0.01), the magnetic energy is converted through magnetic reconnection mostly into the plasma bulk-flow energy. Since more microflares will generate more rotational discontinuities and also supply more energy to the solar wind, it is expected that the number of rotational discontinuities observed in the solar wind would be an increasing function of solar wind speed. The observation rate of rotational discontinuities generated by microflares is estimated to be dN _RD/dt - f/63 000 s (f > 1) at 1 AU. The present mechanism favors the generation of rotational discontinuities with a large shock normal angle.     

North-south asymmetry in the heliospheric current sheet and the IMF sector structure

It is shown that in a heliomagnetic field the presence of a magnetic quadrupole in addition to a magnetic dipole introduces a north-south asymmetry in the heliospheric current sheet (HCS) about the heliographic equator. The dominant polarity of the interplanetary magnetic field (IMF) for the above type of current sheet reverses sign at a transition latitude _T, which lies in a heliohemisphere opposite to the one in which the HCS has more heliolatitudinal extension. The position of _T in the heliosphere and the north-south asymmetry introduced in the HCS change with the relative phase of the dipole and quadrupole components present in the solar magnetic field. The effect of the above type of asymmetric HCS in the IMF mean sector width is evaluated and the results are in agreement with the observations during the minima of solar cycle 21.     

Magnetic diffusion and flare energy buildup

Photospheric motion shears or twists solar magnetic fields to increase magnetic energy in the corona, because this process may change a current-free state of a coronal field to force-free states which carry electric current. This paper analyzes both linear and nonlinear two-dimensional force-free magnetic field models and derives relations of magnetic energy buildup with photospheric velocity field. When realistic data of solar magnetic field (B ₀ 10³ G) and photospheric velocity field (v _max 1 km s^–1) are used, it is found that 3–4 hours are needed to create an amount of free magnetic energy which is of the order of the current-free field energy. Furthermore, the paper studies situations in which finite magnetic diffusivities in photospheric plasma are introduced. The shearing motion increases coronal magnetic energy, while the photospheric diffusion reduces the energy. The variation of magnetic energy in the coronal region, then, depends on which process dominates.     

Evolution and nature of north-south asymmetry in the heliospheric current sheet

The nature and evolution of north-south asymmetry in the heliospheric current sheet (HCS) has been investigated using solar and interplanetary magnetic field (IMF) observations for the past few solar cycles. The mean heliographic latitude of the HCS (averaged over the solar longitude) a ₀ is found to be non-zero during many solar rotations indicating that the large-scale solar magnetic field is more ordered in a system where the origin is shifted away from the centre of the Sun. We have shown that the asymmetry in HCS manifests in different forms depending on the transition heliographic latitude of the reversal of dominant polarity of the IMF ( _T) and the difference in the maximum latitudinal extension of the HCS in the two solar hemispheres (). The classification of the observed asymmetry during 1971–1985 and its effect on IMF observations near Earth has been studied. We have also inferred the sign of _T during 1947–1971 using inferred IMF polarity data. The observed sign reversals of _T suggest the importance of periodicities less than the solar cycle period to be associated with the evolution of asymmetry in HCS. Asymmetry in sunspot activity about the solar equator does not seem to relate consistently well with the asymmetry in HCS about the heliographic equator.     

Mechanical wave-energy fluxin magnetoatmospheres: Discrete and continuous spectra

This paper deals with a rather general class of magnetoatmospheres — media for which the restoring forces of buoyancy, compressibility and magnetic tension/pressure are important in sustaining wave motion. The magnetic field has the general form (B ₀(z), 0,0) and there is also an aligned shear flow (U ₀(z), 0, 0) present. After discussion of the equilibrium and stability of such systems, and certain mathematical properties of a particular system (an isothermal atmosphere with uniform magnetic field, of interest in solar physics), theory is developed which enables expressions to be written down for the mechanical wave energy flux associated with wave motion due to a transient source. These analytic expressions are very general and contain contributions from the continuous and discrete frequency spectra, corresponding respectively to freely propagating and trapped (or surface) waves. These fluxes are evaluated for various ranges of magnetic field, horizontal wavenumber, characteristic source times and frequency, for a simple constant-parameter atmosphere. The source is taken to be a transient fluctuation of the lower boundary, (modelling convective overshoot) which is taken to be located at the level ₅₀₀₀=0.08 in the solar atmosphere. The relative distribution of wave energy flux in the various modes is discussed in the context of solar physics parameters. The possible significance of leaky modes arising from supergranular or other flow, for the local flux balance in the solar chromosphere is outlined.     

The large-scale solar magnetic field

The large-scale photospheric magnetic field, measured by the Mt. Wilson magnetograph, has been analyzed in terms of surface harmonics (P _n ^m )()cosm and P _n ^m ()sinm) for the years 1959 through 1972. Our results are as follows. The single harmonic which most often characterized the general solar magnetic field throughout the period of observation corresponds to a dipole lying in the plane of the equator (2 sectors, n = m = 1). This 2-sector harmonic was particularly dominant during the active years of solar cycles 19 and 20. The north-south dipole harmonic (n = 1, m = 0) was prominent only during quiet years and was relatively insignificant during the active years. (The derived north-south dipole includes magnetic fields from the entire solar surface and does not necessarily correlate with either the dipole-like appearance of the polar regions of the Sun or with the weak polar magnetic fields.) The 4-sector structure (n = m = 2) was prominent, and often dominant, at various times throughout the cycle. A 6-sector structure (n = m = 3) occasionally became dominant for very brief periods during the active years. Contributions to the general solar magnetic field from harmonics of principal index 4 n 9 were generally relatively small throughout this entire solar cycle with one outstanding exception. For a period of several months prior to the large August 1972 flares, the global photospheric field was dominated by an n = 5 harmonic; this harmonic returned to a low value shortly after the August 1972 flare events. Rapid changes in the global harmonics, in particular, relative and absolute changes in the contributions of harmonics of different principal index n to the global field, imply that the global solar field is not very deep or that very strong fluid flows connect the photosphere with deeper layers.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

On the relationship between polar faculae and large-scale magnetic field

We investigate a latitude–time distribution of polar faculae observed at Ussuriysk Observatory in years 1966–1986. The distribution is compared with the longitude-averaged (zonal) magnetic field of the Sun calculated from the data obtained at Mount Wilson Observatory in the years 1966–1976, and at Kitt Peak National Observatory during the period from 1976 to 1985. We found that slow, poleward-directed migration of the polar faculae zones occurring during the course of the solar cycle is not a continuous process, but it contains several episodes of appearance and fast poleward drift of new zones of polar faculae. At the rising phase of the solar cycle, new zones of polar faculae appear at latitudes as low as 40°, but the ones observed during the declining phase of the solar cycle originate at higher latitudes of 50–55°. Such episodes of appearance and fast migration of the polar faculae zones are associated with the poleward-directed streams of magnetic field originated at low latitudes. Moreover, we found some evidence for existence of an additional component of the polar faculae activity that reveals an equatorward migration during the course of the solar cycle. We also investigated a relationship between the number of polar faculae, n, and absolute magnetic flux _z of the zonal mode of the solar magnetic field. We found that within the polar zones of the Sun, substantial correlation between temporal variations of n and _z takes place both on the time scale of the solar cycle and on a shorter time scale of 2–4 years. The relationship between the number of polar faculae and magnetic flux may be approximated by a linear dependence n=0.12_z (where _z is expressed in 10²¹ Mx), except for time interval 1977 through 1980 for which the factor of proportionality is found to have a systematically larger value of 0.20.     

Effects of diverging coronal fields on the solar wind expansion

The expansion of the solar wind in divergent flux tubes is calculated by taking into account a magnetic acceleration of the particles, analogous to the magnetic mirror effect.The resulting force term included in the magnetohydrodynamical equations describes a conversion of thermal into kinetic energy. This causes an additional acceleration of the solar wind plasma which has never been taken into account before. The force is directed opposite to the magnetic field gradient. Consequently, in this case the solar wind velocity increases faster to its asymptotic value than it does for corresponding nonmagnetic solutions. Therefore inside and close to the solar corona markedly higher velocities are found. Compared to strictly hydrodynamical models, the critical point is shifted towards the Sun, and the radial decrease of the ratio of thermal to kinetic energy is faster.The necessary prerequisites for these calculations are (a) that the gyroperoid _g of the plasma particles is much shorter than the Coulomb collision time _c , and (b) that the collision time _c is shorter than the characteristic time _d in which an appreciable amount of thermal anisotropy is built up. Thus it is (a) insured that the particles have established magnetic moments and follow the guiding center approximation, and (b) an almost isotropic velocity distribution function is maintained which, in this first approximation of a purely radial expansion, justifies the use of isotropic pressures and temperatures.Both (a) and (b) are shown to be fulfilled in a region around the Sun out to about 20R , and thermal anisotropies developing outside of this region could explain the observed magnetically aligned anisotropies at 1 AU.     

The structure of the turbulent shock wave propagating in the solar atmosphere across the magnetic field

A consistent account of plasma turbulence in magnetohydrodynamics equations describing transport processes across the magnetic field is presented. The structure of the perpendicular shock wave generated in the solar atmosphere, as a result of either local disturbance of the magnetic field or dense plasma cloud motion with a frozen-in magnetic field, has been investigated. The region of parameters in the solar atmosphere at which the electron-ion relative drift velocity u exceeds the electron thermal velocity V _eand generation of radio emission becomes possible, has been determined. The plasma turbulence inside the front has been shown, under conditions of solar corona, not to cause the oscillation structure of shock front to break down. Under chromospheric conditions, the shock profile is aperiodical. Then, the condition u > V_ecan be satisfied and shock waves having an Alfvén Mach number M which exceeds the critical value M _c 3.3 for aperiodical shock waves can exist (Eselevich et al., 1971a). Arguments are given in favour of the fact that perpendicular shock waves are generated in the Sun's atmosphere when dense plasma clouds, with a frozen-in magnetic field, are expanded.     

Prompt proton and electron acceleration to relativistic energies in solar flares

As a possible mechanism for particle acceleration in the impulsive phase of solar flares, a new particle acceleration mechanism in shock waves is proposed; a collisionless fast magnetosonic shock wave can promptly accelerate protons and electrons to relativistic energies, which was found by theory and relativistic particle simulation. The simultaneous acceleration of protons and electrons takes place in a rather strong magnetic field such that _{ce pe} . For a weak magnetic field ( _{ce pe} ), strong acceleration occurs to protons only. Resonant protons gain relativistic energies within the order of the ion cyclotron period (much less than 1 s for solar plasma parameters). The electron acceleration time is shorter than the ion-cyclotron period.     

Effects of the Sector Structure of the Interplanetary Magnetic Field on Galactic Cosmic Ray Anisotropy

We study the effects of the sector structure of the interplanetary magnetic field (IMF) on the Galactic cosmic ray (GCR) anisotropy at solar minimum by using Global Network neutron monitor data. The hourly neutron monitor data for 1976 were averaged for the positive (+) and negative (–) IMF sectors (+ and – correspond to the antisolar and solar directions of magnetic field lines, respectively) and then processed by the global survey method. We found that the magnitude of the GCR anisotropy vector is larger in the positive IMF sector and that the phase shifts toward early hours. The derived GCR components A _r, A , and A for the different + and – sectors are then used to calculate the angle ( 46°) between the IMF lines and the Sun–Earth line, the solar wind velocity U ( 420 km/s), the ratio of the perpendicular (K ) and parallel (K _||) diffusion coefficients K /K _|| = ( 0.33), and other parameters that characterize the GCR modulation in interplanetary space.     

Solar flare activity and the structure of the coronal neutral line

In this study we analyse the positions of major flares from 1978 and 1979, with respect to the magnetic structure of the solar corona, as described by a potential field model. We find that major flares exhibit no strong association with the neutral line at the chromospheric level. However, when we calculate the neutral line's position at higher and higher altitudes in the corona, we find that major flares show an increasing tendency to be found close to these high-altitude coronal neutral lines. The correlation between flares and higher-altitude coronal neutral lines reaches a maximum at an altitude of 0.35R , and thereafter decreases as the neutral line is moved out to the source surface at an altitude of 1.50R . This indicates that major flares are strongly associated with coronal structure at the 0.35R level ( 250 000 km) - an altitude surprisingly high in the corona. This reinforces the idea that flares are associated with large-scale coronal magnetic fields and also indicates that the region of coronal magnetic topology important to solar flare processes may be larger than previously thought.     

The determination of coronal potential magnetic fields using line-of-sight boundary conditions

Previous efforts to construct solar coronal fields using surface magnetograph data have generally employed a least squares minimization technique in order to determine the spherical harmonic expansion coefficients of the magnetic scalar potential. Provided there is no source surface high up in the corona, we show that knowledge of the line-of-sight component of the surface magnetic field, B _i = B _r sin + B cos , is sufficient to uniquely determine the potential coronal magnetic field by an explicit construction of the magnetic scalar potential for an arbitrary B _l (, ).The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Solar coronal streamers

A unique combination of photographic and K-coronameter data were used to study the structure and evolution of two known coronal streamers. In addition, two other K-coronameter enhancements were studied as representing ideal second examples of the known streamers. As a general rule the observations indicate that these features were direct coronal manifestations of photospheric bipolar magnetic regions (BMR) and were of two basic types:active region, by which is meant a coronal streamer which develops radially over a low-latitude active region; andhelmet which denotes a streamer whose structure and development appear to be a consequence of a long-lived complex of activity, composed of both trailing magnetic fields and a parent center of disk activity.The similarity of growth rates during the first solar rotation of life led to derivation of a total streamer density of 4–5 × 10⁸ cm^–3 atr = 1.125R . This density may represent a characteristic maximum density at the base of streamers. The intensity gradient of the inner (r1.5R ) corona was used to establish a qualitative evolutionary model of streamers which synthesizes the observations. Briefly, streamers initially develop over active regions; the streamer growth rate may be as rapid as the disk activity, or at worst lags flare activity by solar rotation. The streamer can be the cause of interplanetary and geomagnetic effects at 1 AU within a solar rotation after birth. Thereafter the streamer follows an evolution dictated by the underlying solar magnetic fields. In any case the lowest level of the coronal enhancement has a lifetime not exceeding that of the solar disk activity.     

High resolution observations of small-scale magnetic elements and interpretation

This contribution deals with the properties of small-scale magnetic elements in plages. Spectro-polarimetric observations, obtained with the highest possible spatial resolution with the German solar telescopes at the Observatorio del Teide on Tenerife, were analysed. We conclude from the spread of line parameters measured in the Stokes I and V profiles of Fe I and Fe II lines that a wide range of magnetic properties is realised in the solar atmosphere. The flow velocities in small-scale magnetic flux tubes, deduced from the zero-crossing of the V profiles at high spatial resolution, show a fluctuation of v _Doppler = 580 m s^-1. This is substantially smaller than the turbulent broadening velocities of v _Doppler = 2 – 3 km s^–1 commonly derived by fitting V profiles from flux tube models to low spatial resolution data, e.g. from a Fourier Transform Spectrometer. Attempts to explain the high resolution I and V profiles by models of hydrostatic flux tubes are discussed. It appears impossible to accomplish agreement between the modeled and observed radiation of lines with strong and weak magnetic sensitivity at the same time. We suggest a scenario in which small-scale magnetic elements possess substructure and are dynamic, with gas flows and magnetic field strengths varying in space and time.     

A model of the magnetized solar wind

A steady state, inviscid, single fluid model of the solar win d in the equatorial plane is developed using magneto-hydrodynamics and including the heat equation wit h thermal conduction but no non-thermal heating (i.e. a conduction model). The effects of solar rotation and magnetic field are included enabling both radial and azimuthal components of the velocity and magnetic fields to be found in a conduction model for the first time.The magnetic field cuts off the thermal conduction far from the sun and leads to an increased temperature at 1 AU and relatively small changes to the radial velocity and density. Models have been found which fit the experimental electron densities in 2 R < r < 16 R . These models predict at 1 AU a radial velocity of 300–380 km·sec^-1 and a density of 8 protons·cm^-3. The latter velocity corresponds to a density profile obtained by Blackwell and Petford (1966) during the last sunspot minimum, and is about 100 km·sec^-1 above that found in previous conduction models which fit the coronal electron densities. The radial velocities are now consistent with the mean quiet solar wind, as are the densities when the experimental values are averaged over a magnetic sector. However, the azimuthal velocity at 1 AU is only 1–2 km·sec^-1 which is low compared to the experimental values, as found by previous authors.     

Anomalous magnetic diffusion in coronal current layers

The current-driven kinetic Alfvén instability is proposed as an anomalous transport mechanism for regions of concentrated, field-aligned currents in the solar corona. Anomalous magnetic diffusivity ( _{e f f} 10⁹cm²s^–1), produced by kinetic Alfvén turbulence in the vicinity of the saturation level, provides fast magnetic energy release with a local inflow Alfvén Mach numberM _in 0.1.