Correlation and spectral analysis of daily solar radio flux

Correlation and spectral analysis of solar radio flux density and sunspot number near the maximum of the sunspot cycle has indicated the existence of

1. long period amplitude modulation of the slowly varying component (SVC) of radio emission
2. coronal storage over a period of the order of three solar rotations
3. fast decay (one solar rotation period or less) of gyromagnetic emissions from radio sources
4. shift in location of chromospheric sources compared to those of either the upper corona or the photosphere.

     

Motions and magnetic fields in filaments

Based on the developed method of jointly using data on the magnetic fields and brightness of filaments and coronal holes (CHs) at various heights in the solar atmosphere as well as on the velocities in the photosphere, we have obtained the following results:

The upward motion of matter is typical of filament channels in the form of bright stripes that often surround the filaments when observed in the HeI 1083 nm line.

The filament channels observed simultaneously in Hα and HeI 1083 nm differ in size, emission characteristics, and other parameters. We conclude that by simultaneously investigating the filament channels in two spectral ranges, we can make progress in understanding the physics of their formation and evolution.

Most of the filaments observed in the HeI 1083 nm line consist of dark knots with different velocity distributions in them. A possible interpretation of these knots is offered.

The height of the small-scale magnetic field distribution near the individual dark knots of filaments in the solar atmosphere varies between 3000 and 20000 km.

The zero surface separating the large-scale magnetic field structures in the corona and calculated in the potential approximation changes the inclination to the solar surface with height and is displaced in one or two days.

The observed formation of a filament in a CH was accompanied by a significant magnetic field variation in the CH region at heights from 0 to 30000 km up to the change of the predominant field sign over the entire CH area. We assume that this occurs at the stage of CH disappearance.

     

The results of statistical analysis of the coronal profiles above the solar active regions

In order to establish some regularities or variations in the distribution of widths and intensities of the coronal line profiles λ 5303 and λ 6374 depending upon the solar activity, a statistical analysis was made for more than 3000 profiles (the data covering the period 1966–1972). The following results obtained:

1. The distribution of coronal line profile widths changes depending upon the solar activity phase.
2. The character of the relation between the intensities and widths varies with variation of the solar activity phase.

     

A concentric ellipse multiple-arch system in the solar corona

A typical concentric ellipse multiple-arch system was observed in the solar corona during the February 4, 1962 eclipse in New Guinea. The following results have been obtained from analysis of a white-light photograph taken by N. Owaki (see Owaki and Saito, 1967a).

1. The arches are composed of four equidistant components, elliptical in shape, and almost concentric with a prominence at the common center of the ellipses.
2. The prominence and arch system appears to be the lower region of a helmet-shaped streamer.
3. The widths of the arches are observed to increase with height.
4. Analysis was made in the light of three models for the coronal structures that could lead to the observed arches: (a) rod-like concentrations of electrons; (b) tunnel-shaped elliptical shells of electrons; and (c) dome-like ellipsoidal shells of electrons. Electron densities are derived for the models, and the dome-like model is excluded as a possibility for arch systems exhibiting a coronal cavity.
5. The scale height in the arch-streamer region is found to be almost the same as that of the K-corona, suggesting equal temperatures, density distributions, etc. in each region.
6. There is a dark space (a coronal cavity) between the innermost arch and the prominence. The brightness of this cavity is 1/5 that of the adjacent arch. It is 3% brighter than the background corona of the arch-streamer system.
7. A comparison is made between the deficiency of electrons in the coronal cavity and the excess of electrons in the prominence. It is found that the ratio of the excess to the deficiency lies between 0.9 and 40.
8. A comparison between the electron efflux from the ‘leaky magnetic bottle’ possibly formed by rod-shaped coronal arches and the electron influx into those arches from the chromosphere leads us to the conclusion that the rod model is probably valid and that spicules appear to be an adequate supply for the electrons observed in the arches. The tunnel model may be valid, but in that case spicules are probably not the sources of the electrons observed in coronal arches.

     

A survey of coronal holes and their solar wind associations throughout sunspot cycle 20

To gain insight into the relationships between solar activity, the occurrence and variability of coronal holes, and the association of such holes with solar wind features such as high-velocity streams, a study of the period 1963–1974 was made. This period corresponds approximately with sunspot cycle 20. The primary data used for this work consisted of X-ray and XUV solar images obtained from rockets. The investigation revealed that:

1. The polar coronal holes prominent at solar minimum, decreased in area as solar activity increased and were small or absent at maximum phase. This evolution exhibited the same phase difference between the two hemispheres that was observed in other indicators of activity.
2. During maximum, coronal holes occurred poleward of the sunspot belts and in the equatorial region between them. The observed equatorial holes were small and persisted for one or two solar rotations only; some high latitude holes had lifetimes exceeding two solar rotations.
3. During 1963–74 whenever XUV or X-ray images were available, nearly all recurrent solar wind streams of speed ?500 km s^?1 were found associated with coronal holes at less than 40° latitude; however some coronal holes appeared to have no associated wind streams at the Earth.

     

Alfvén waves in the solar atmosphere

We examine the propagation of Alfvén waves in the solar atmosphere. The principal theoretical virtues of this work are: (i) The full wave equation is solved without recourse to the small-wavelength eikonal approximation (ii) The background solar atmosphere is realistic, consisting of an HSRA/VAL representation of the photosphere and chromosphere, a 200 km thick transition region, a model for the upper transition region below a coronal hole (provided by R. Munro), and the Munro-Jackson model of a polar coronal hole. The principal results are:

1. If the wave source is taken to be near the top of the convection zone, where n _H = 5.2 × 10¹⁶ cm^?3, and if B _⊙ = 10.5 G, then the wave Poynting flux exhibits a series of strong resonant peaks at periods downwards from 1.6 hr. The resonant frequencies are in the ratios of the zeroes of J ₀, but depend on B _⊙, and on the density and scale height at the wave source. The longest period peaks may be the most important, because they are nearest to the supergranular periods and to the observed periods near 1 AU, and because they are the broadest in frequency.
2. The Poynting flux in the resonant peaks can be large enough, i.e. P _⊙ ≈ 10⁴–10⁵ erg cm^?2s^?1, to strongly affect the solar wind.
3. ¦δv¦ and ¦δB¦ also display resonant peaks.
4. In the chromosphere and low corona, ¦δv ≈ 7–25 kms^?1 and ¦δB¦ ≈0.3–1.0 G if P _⊙≈10⁴-10⁵ erg cm^?2s^?1.
5. The dependences of ¦δv¦ and ¦δB¦ on height are reduced by finite wavelength effects, except near the wave source where they are enhanced.
6. Near the base, ¦δB¦ ≈ 350–1200 G if P _⊙ ～- 10⁴–10⁵. This means that nonlinear effects may be important, and that some density and vertical velocity fluctuations may be associated with the Alfvén waves.
7. Below the low corona most wave energy is kinetic, except near the base where it becomes mostly magnetic at the resonances.
8. ?₀ < δv ² > v _A or < δB ² > v _A/4π are not good estimators of the energy flux.
9. The Alfvén wave pressure tensor will be important in the transition region only if the magnetic field diverges rapidly. But the Alfvén wave pressure can be important in the coronal hole.

     

Solar flare physics enlivened by TRACE and RHESSI

The Transition Region and Coronal Explorer (TRACE) gave us the highest EUV spatial resolution and the Ramaty High Energy Solar Spectrometric Imager (RHESSI) gave us the highest hard X-ray and gammaray spectral resolution to study solar flares. We review a number of recent highlights obtained from both missions that either enhance or challenge our physical understanding of solar flares, such as:

1. Multi-thermal Diagnostic of 6.7 and 8.0 keV Fe and Ni lines
2. Multi-thermal Conduction Cooling Delays
3. Chromospheric Altitude of Hard X-Ray Emission
4. Evidence for Dipolar Reconnection Current Sheets
5. Footpoint Motion and Reconnection Rate
6. Evidence for Tripolar Magnetic Reconnection
7. Displaced Electron and Ion Acceleration Sources.

     

Coronal holes,solar wind streams,and geomagnetic activity during the new sunspot cycle

We have extended our previous study of coronal holes, solar wind streams, and geomagnetic disturbances from the declining phase (1973–1975) of sunspot cycle 20 through sunspot minimum (1976) into the rising phase (1977) of cycle 21. Using daily He I 10830 Å spectroheliograms and photospheric magnetograms, we found the following results:

1. As the magnetic field patterns changed, the solar atmosphere evolved from a structure having a few, large, long-lived, low-latitude coronal holes to one having numerous small, short-lived, high-latitude holes (in addition to the polar holes which persisted throughout this 5-year interval).
2. The high-latitude holes recurred with a synodic rotation period of 28–29 days instead of the 27-day period already known to be characteristic of low-latitude holes.
3. During 1976–1977 many coronal holes were intrinsically ‘weak’ in the sense that their average intensities did not differ greatly from the intensity of their surroundings. Such low-contrast holes were rare during 1973–1975.

An updated Bartels display of the occurrence of holes, wind speed, and geomagnetic activity summarizes the evolution of their characteristics and interrelations as the sunspot cycle has progressed. Long-lived, low-latitude holes have become rare but remain terrestrially effective. The more common high-latitude holes are effective only when the Earth lies at a relatively high heliographic latitude in the same solar hemisphere.     

Rotational Tomography For 3d Reconstruction Of The White-Light And Euv Corona In The Post-Soho Era

Improving our understanding of the mechanisms that energize the solar wind and heat structures in the solar corona requires the development of empirical methods that can determine the three-dimensional (3D) temperature and density distributions with as much spatial and temporal resolution as possible. This paper reviews the solar rotational tomography (SRT) methods that will be used for 3D reconstruction of the solar corona from data obtained by the next generation of space-based missions such as the Solar and Terrestrial Relations Observatory (STEREO), Solar-B and the Solar Dynamics Observatory (SDO). In the next decade, SRT will undergo rapid advancement on several frontiers of 3D image reconstruction:

1. Electron density reconstruction from white-light coronagraph images.
2. Differential emission measure (DEM) reconstruction from EUV images.
3. Dual-spacecraft (STEREO) observing geometry.
4. Fusion of data from multiple spacecraft with differing instrumentation.
5. Time-dependent estimation methods.

Although the principles described apply to many different wavelength regimes, this paper concentrates on white-light and EUV data. Previous work on all of these subjects is reviewed, and major technical issues and future directions are discussed.     

Radio spectrum of two active regions

The results of the total solar eclipse of November 12, 1966, observed at 8 different wave-lengths between 3 and 21 cm, are studied and the spectrum of two active regions present on the disk is deduced. It is shown that the observed increase of the flux of the most intense source in the range 3–10 cm is due to geometrical effects. Neglecting the influence of the magnetic field, the following quantities are deduced.

1. the mean and central temperature of the coronal condensation.
2. the ∫ _corona N ²dh (N = electron density).

Both these quantities are in good agreement with optical observations.     

Structure and evolutionary history of the solar system,I

1. Introduction and Survey. The method for studying the structure and evolution of the solar system is discussed. It is pointed out that theories that account for the origin of planets alone are basically insufficient. Instead one ought to aim for a general theory for the formation of secondary bodies around a central body, applicable both to planet and satellite formation. A satisfactory theory should not start from assumed properties of the primitive Sun, which is a very speculative subject, but should be based on an analysis of present conditions and a successive reconstruction of the past states.
2. Orbits of Planets and Satellites. As a foundation for the subsequent analysis, the relevant properties of planets and satellites are presented.
3. The Small Bodies. The motion of small bodies is influenced by non-gravitational forces. Collisions (viscosity) are of special importance for the evolution of the orbits. It is pointed out that the focusing property of a gravitational field (which has usually been neglected) leads to the formation of jet streams. The importance of this concept for the understanding of the comet-meteoroid relations and the structure of the asteroidal belt is shown.
4. Resonance Structure. A survey is given of the resonances in the solar system and their possible explanation. It is concluded that in many cases the resonances must already be produced at the times when the bodies formed. It is shown that resonance effects put narrow limits on the post-accretional changes of orbits.
5. Spin and Tides. Tidal effects on planetary spins and satellite orbits are discussed. It is very doubtful if any satellite except the Moon and possibly Triton has had its orbit changed appreciably by tidal effects. The isochronism of planetary and asteroidal spins is discussed, as well as its bearing on the accretional process.
6. Post-accretional Changes in the Solar System. The stability of the solar system and upper limits for changes in orbital and spin data are examined. It is concluded that much of the present dynamic structure has direct relevance to the primordial processes.

     

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.     

The flare of September 7, 1973: A typical example of a newly recognized class of solar transients

X-ray, extreme-ultraviolet and optical observations of a solar flare are discussed. It is shown that the flare exemplifies a class of transient events characterized by long duration and long decay time and by the development of high systems of loops, generally brighter at the top. In contrast with compact short lifetime events, the distinctive properties of this class of transients are:

1. The disruption of the magnetic configuration at the flare onset, as indicated by prominence eruption or activation and by associated white-light coronal transients;
2. a continuous energy deposition, presumably at the top of loops, during a large fraction of the flare development and well after the intensity peak;
3. a continuous supply of additional material to the top of loops, with subsequent downflows and out-of-hydrostatic equilibrium conditions.

     

A statistical study of the spiral spots on the solar disc

After adding the data observed in the years from 1979 to 1982 to those obtained earlier (Ding et al., 1981), we re-examine the previous results and conclude:

1. The longitudinal distribution of spiral spots on the solar disc is generally the same as that of sunspot groups with areas of S _p ≥ 400, but their active longitudes seem to be more concentrated.
2. The distribution of spiral patterns in the southern and northern hemispheres shows that the differential rotation may be a fundamental solar dynamo for the formation of the spiral spots.
3. The statistical directions of the emerging twisted magnetic vectors in the active regions in the southern and northern hemispheres are synchronously inverse with a period of about two years. This period seems to be detected in other solar observations.

     

Magnetohydrodynamic tube waves and high speed solar wind streams

It has been widely conjectured that magnetohydrodynamic (MHD) waves may provide the extra momentum or energy required to explain the high speed solar wind streams that originate in coronal holes. Although the magnetic structuring inherent in this problem has been incorporated into models of the bulk flow, this is not generally true of the associated treatments of wave propagation. In particular, as pointed out by Davila (1985), we might generally expect the magnetic geometry to substantially modify those waves whose wavelength is comparable to the hole width. Using both a geometrical optics and an eigenmode approach, Davila addressed the question of wave propagation in a simple uniform width flux slab model of a coronal hole and concluded

1. the hole may act as a ‘leaky wave guide’, i.e., waves travelling along it may leak into the surrounding corona, but
2. the group velocity of waves with periods in a physically relevant range (around 100 s) is downward, indicating that such waves cannot carry energy into the solar wind and therefore cannot be driving it.

We agree with (i) but argue that (ii) results from a mistaken interpretation of a dispersion relation, and is incorrect. Furthermore, we apply the cylindrical tube leaky wave approach of Cally (1986) to a simple coronal hole model, and find two wavetypes with substantial upward energy fluxes. However, of these, we argue that the so-called ‘trig modes’ (geometry modified fast waves) leak so profusely that they are unable to transport energy over the distance required; the non-axisymmetric ‘thin tube’ modes, though, do not suffer from this disability.     

Effect of perturbed potentials on the stability of libration points in the restricted problem

The location and the stability in the linear sense of the libration points in the restricted problem have been studied when there are perturbations in the potentials between the bodies. It is seen that if the perturbing functions satisfy certain conditions, there are five libration points, two triangular and three collinear. It is further observed that the collinear points are unstable and for the triangular points, the range of stability increases or decreases depending upon whetherP> or <0 wherep depends upon the perturbing functions. The theory is verified in the following four cases:

1. There are no perturbations in the potentials (classical problem).
2. Only the bigger primary is an oblate spheroid whose axis of symmetry is perpendicular to the plane of relative motion (circular) of the primaries.
3. Both the primaries are oblate spheroids whose axes of symmetry are perpendicular to the plane of relative motion (circular) of the primaries.
4. The primaries are spherical in shape and the bigger is a source of radiation.

     

Global solar activity on long time scales

We report the results of the application of our approach to study the behavior of solar activity in the past, where:

When reconstructing the variations of solar activity, geomagnetic parameters, and the interplanetary magnetic field in the past we select a sequence of increasing time scales, which can be naturally represented by the potentials of available observational data. We select a total of four time scales: 150–200 years, 400 years, 1000 years, and 10000 years.

When constructing the series of each successive (in terms of length) time scale we use the data of the previous time scale as reference data.

We abandon, where possible, the series of traditional statistical parameters in favor of the series of physical parameters.

When deriving the relations between any parameters of solar activity, geomagnetic disturbance, and the interplanetary magnetic field, we take into account the differential nature of relations on different time scales. To this end, we use the earlier proposed MSR and DPS methods.

To verify the resulting reconstructions, we use the “principle of witnesses”, which uses independent (in some cases, indirect) information as initial data.

     

Magnetic field configurations associated with fast solar wind

In this paper, we consider the implications of the observed inverse correlation between solar wind speed at Earth and the expansion rate of the Sun-Earth flux tube as it passes through the corona. We find that the coronal expansion rate depends critically on the large-scale photospheric field distribution around the footpoint of the flux tube, with the smallest expansions occurring in tubes that are rooted near a local minimum in the field. This suggests that the fastest wind streams originate from regions where large coronal holes are about to break apart and from the facing edges of adjacent like-polarity holes, whose field lines converge as they transit the corona. These ideas lead to the following predictions:

1. Weak holes and fragmentary holes can be sources of very fast wind.
2. Fast wind with steep latitudinal gradients may be generated where the field lines from the polar hole and a lower-latitude hole of like polarity converge to form a mid-latitude ‘apex’.
3. The fastest polar wind should occur shortly after sunspot maximum, when trailing-polarity flux converges onto the poles and begins to establish the new polar fields.

     

Non-thermal processes in large solar flares

We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:

1. The ～10–10² keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
2. The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
3. The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
4. Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～10³¹ ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
5. High energy protons are accelerated later than the 10–10² keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
6. The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×10³⁰ ergs in >20 keV electrons is required for detectable white-light emission.

The highly efficient electron energization required in these flares suggests that the flare mechanism consists of rapid dissipation of chromospheric and coronal field-aligned or sheet currents, due to the onset of current-driven Buneman anomalous resistivity. Large proton flares then result when the energy input from accelerated electrons is sufficient to form a shock wave.     

Ephemeral active regions in 1970 and 1973

A study of ephemeral active regions (ER) identified on good quality full-disk magnetograms reveals:

1. On the average 373 and 179 ER were present on the Sun in 1970 and 1973 respectively. The number varies with the solar cycle.
2. The median lifetime of ER depends on observation quality and selection rules but is estimated as about 12 hr for our data.
3. The latitude distribution is very broad but not uniform. The distribution peaks near the equator and shows variations similar to distributions of large active regions.
4. The longitude distribution is essentially homogeneous.
5. The spatial orientation of ER is almost random. In 1973 there is a hint of an excess of new cycle orientations at high latitudes.

A comparison of parameters of ER and regular active regions suggests that ER are the small-scale end of a broad spectrum of active regions. The role of ER in the light of present theories of solar activity is investigated but is not yet clear. Heating of the chromosphere and corona may be significantly affected by ER.