Showing papers in "Solar Physics in 1974"

THEORETICAL MODEL OF FLARES AND PROMINENCES I: Evaporating Flare Model

Abstract: A theoretical model of flare which explains observed quantities in Ha, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10 a2 erg, the electron density of l013'5 cm -a in Hc~ flare, the temperature of the X-ray flare of 107.3K and the time to attain maximum Ha brightness (600 s) are derived consistent with observations. It is shown that the top height of the Ha flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences. The observed small thickness of the Ha flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.

The photospheric barium spectrum: solar abundance and collision broadening of Ba II lines by hydrogen

Abstract: A study of the solar Ba ii spectrum leads to a solar abundance of barium of loge ba = = 2.11±0.12, on the scale loge h = 12. The observed asymmetry of the resonance line λ4554 is consistent with an isotopic abundance ratio equal to the terrestrial one. The meteoritic Ba/Si abundance ratio found in carbonaceous chondrites appears to exceed the solar ratio by 0.1 to 0.2 dex (Section 5). The broadening by collisions with hydrogen atoms is determined from the solar spectrum (Section 4). Damping half-widths, γ h, of the three stronger Ba ii lines turn out to be larger by a factor of about 3.0 than predicted from pure van der Waals interaction of dipoles. Departures from LTE appear to be present in the cores of the resonance lines and of the lines arising from the metastable 5D levels (Section 6). The equivalent widths, however, remain practically unaffected. Equivalent widths of neutral barium lines are predicted and some new identifications of photospheric Ba i lines are suggested (Section 7).

A model of the solar convection zone

Abstract: A model of the convection zone is presented which matches an empirical model atmosphere (HSRA) and an interior model. A mixing length formalism containing four adjustable parameters is used. Thermodynamical considerations provide limits on two of these parameters. The average temperature-pressure relation depends on two or three combinations of the four parameters. Observational information on the structure of the outermost layers of the convection zone, and the value of the solar radius limit the range of possible parameter combinations. It is shown that in spite of the remaining freedom of choice of the parameters, the mean temperature-pressure relation is fixed well by these data. The reality of a small density inversion in the HSRA model is investigated. The discrepancy between the present model and a solar model by Mullan (1971) is discussed briefly.

Behavior of the flare-produced coronal MHD wavefront and the occurrence of type II radio bursts

Abstract: The propagation of the weak MHD fast-mode shock emitted into the corona by flares at their explosive phase is computer-simulated. It is shown as the result that the shock wave is refracted towards the low Alfven velocity regions pre-existing in the corona, and the strength of the shock, which is otherwise weak, is drastically enhanced on encountering low- V A regions due to the focussing effect by refraction and also due to the lowered propagation velocity of the shock in such regions. It is expected that electron acceleration takes place in such a drastic strengthening of the shock, leading to the local excitation of plasma waves and eventually to the occurrence of radio bursts at such locations. Such locations of shock strength enhancement, when computed by using HAO realistic models of coronal density and magnetic field of the day of certain type II burst events, actually coincide roughly with the observed positions of type II bursts. Peculiar configurations of type II burst sources as well as their occurrence even beyond the horizon of the responsible flare are explained consistently by the large scale refraction and the local enhancement of the shock due to the global and local distribution of Alfven velocity in the corona. A unified interpretation is given for the occurrence of type II bursts and Moreton's wave phenomena, and also the relation of our MHD fast-mode disturbance with other flare-associated dynamical phenomena is discussed.

Observations of photospheric faculae at the center of the solar disk

Abstract: Observations, in the near UV with very high spatial resolution, obtained with the Sacramento Peak Vacuum Tower Telescope, show that photospheric faculae are visible near the center of the disk. Faculae consist of bright point-like structures with typical dimensions of 100–200 km, and appear to be concentrated in the intergranular space. Preliminary results on measurements of size, brightness, life time, and on interaction of facular structures with granulation are presented, which support the view that faculae are identical with the recently discovered ‘solar filigree.’ Phenomenological arguments are discussed concerning the suggestion that facular points may represent magnetic flux concentrations in the photosphere.

The formation of solar quiescent prominences by condensation

Abstract: We model the formation of solar quiescent prominences by solving numerically the non-linear, time-dependent, magnetohydrodynamic equations governing the condensation of the corona. A two-dimensional geometry is used. Gravitational and magnetic fields are included, but thermal conduction is neglected. The coronal fluid is assumed to cool by radiation and to be heated by the dissipation of mechanical energy carried by shock waves. A small, isobaric perturbation of the initial thermal and mechanical equilibrium is introduced and the fluid is allowed to relax. Because the corona with the given energy sources is thermally unstable, cooling and condensation result. When magnetic and gravitational fields are absent, condensation occurs isotropically with a strongly time-dependent growth rate, and achieves a density 18 times the initial density in 3.5 × 104 s. The rapidity of condensation is limited by hydrodynamical considerations, in contrast to the treatment of Raju (1968). When both magnetic and gravitational fields are included, the rate of condensation is inhibited and denser material falls. We conclude that: (1) condensation of coronal material due to thermal instability is possible if thermal conduction is inhibited; (2) hydrodynamical processes determine, in large part, the rate of condensation; (3) condensation can occur on a time scale compatible with the observed times of formation of quiescent prominences.

OSO-7 observations of solar x-rays in the energy range 10–100 keV

Abstract: The solar X-ray experiment on the satellite OSO-7 has provided extensive observations of hard and soft X-ray bursts. We give a general description of the hard X-ray data here, in parallel with the description of the soft X-ray data already published (Datlowe et al., 1974). The data for this study consist of 123 hard X-ray bursts which occurred between 10 October 1971 and 6 June 1972. We examine the behavior of a typical event in terms of its spectral and flux variations. For the whole data sample, we find that 2/3 of the soft X-ray bursts have detectable hard X-ray emission. We present the distributions of frequency of occurrence of peak flux, spectral index, collisional energy loss, burst duration and the duration at half maximum of the flux profile. No correlation was found between the flux and the spectral slope of an individual data sample, nor was there a correlation between the peak flux and the full width at half maximum of a burst. We have utilized Hα flare identifications to study longitude variations. No statistically significant variation was found in the relative frequency of hard or soft X-ray bursts with longitude. However, the spectral slope does exhibit a center-to-limb variation, with limb spectra tending to be softer. We have compared the growth of energy in the hot flare plasma (soft X-ray source) with the collisional energy deposition (hard X-ray source) for the entire sample. This analysis shows that collision loss within the hot plasma is not the principal source of its heating.

Physical properties of solar chromospheric plages. II - Chromospheric plage models

Abstract: We propose chromospheric models of plages to explain profiles of the Ca ii H, K, λl8498, λ8542, and λ8662 lines described in Paper I. These models are consistent with boundary conditions imposed by the photosphere and the Lyman continuum. We find that increasing emission in these lines is consistent with a picture of increasing temperature gradient in the low chromosphere and the resulting increase in pressure and electron density at similar line optical depths. With this picture we suggest how to empirically determine the distribution of chromospheric parameters across the solar disk directly from Ca ii filtergrams. We also propose that the high density aspects of solar activity are produced by steep temperature gradients in the low chromosphere and thus by the enhanced heating mechanisms that steepen these gradients.

On the observation of scattered radio emission from sources in the solar corona

Abstract: The effects of scattering and refraction on radio waves in the solar corona are considered for several different coronal models. By considering a source near the plasma level in a spherically symmetric corona and in a streamer enhancement superimposed on a spherically symmetric corona we obtain results relating to bursts of types I, II and III.

A model combining the polar and the sector structured solar magnetic fields

Abstract: A phenomenological model of the interplay between the polar magnetic fields of the Sun and the solar sector structure is discussed. Current sheets separate regions of opposite polarity and mark the sector boundaries in the corona. The sheets are visible as helmet streamers. The solar sector boundary is tilted with respect to central meridian, and boundaries with opposite polarity change are oppositely tilted. The tilt of a given type of boundary [(+, −) or (−, +)] changes systematically during the sunspot cycle as the polarity of the polar fields reverses. Similar reversals of the position of the streamers at the limbs takes place. If we consider (a) a sunspot cycle where the northern polar field is inward (−) during the early part of the cycle and (b) a (+, −) sector boundary at central meridian then the model predicts the following pattern; a streamer at high northern latitudes should be observed over the west limb together with a corresponding southern streamer over the east limb. The current sheet runs now NW-SE. At sunspot maximum the boundary is more in the N-S direction; later when the polar fields have completed their reversal the boundary runs NE-SW and the northern streamer should be observed over the east limb and the southern streamer over the west limb. Observational evidence in support of the model is presented, especially the findings of Hansen, Sawyer and Hansen and Koomen and Howard that the K-corona is highly structured and related to the solar sector structure.

Coronal Heating by Alfvén Waves

Abstract: If Alfven waves are responsible for the heating of the solar corona, what are the various dissipation processes, under what conditions are they important, and what observational consequences may be expected? For wave periods longer than roughly one minute, the corona appears to the waves as turbulent and dissipation is efficient, but little more can be said. For shorter wave periods, a bend or twist in the magnetic field can convert Alfven waves into fast-mode waves, which then dissipate. But, if the waves travel nearly along the field, the main dissipation occurs via nonlinear interactions among Alfven waves, near the top of magnetic loops. Once the wave intensity is sufficient so that wave dissipation exceeds radiative energy losses, the temperature rise (ΔT) is limited by electron heat conduction. The basic result is that ΔT is independent of the wave intensity and it is also independent of the electron density. Therefore, regions high and low in the corona are heated similarly. Since ΔT is also proportional to the magnetic pressure, the heating by Alfven waves naturally outlines regions (loops) of enhanced magnetic fields. A numerical approximation, for wave period τ≲100 s, is $$\frac{{\Delta T}}{T} \approx 10^{ - 2.5} B^2 \left( {\frac{{2 \times 10^6 }}{T}} \right)^{5/2} \frac{\tau }{{30s}}$$ .

Propagation of flare protons in the solar atmosphere

Abstract: The velocity dispersion for a large number of solar proton events is analyzed in the energy regime of 10–60 MeV It is found for all events that the time from the flare to particle maximum t m is well represented by a sum of two components The first component which is energy independent describes the propagation in the solar atmosphere, the second component describes the propagation in the interplanetary medium giving a velocity dispersion v × t m = const The additional study of time intensity profiles, onset times, and multispaceprobe observations reveals that the propagation in the solar atmosphere consists of three processes: (1) A rapid transport process in the initial (≲ 1 h) phase after the event fills up a “fast propagation region (FPR)”, which may extend up to ≈ 60° from the flare site and which is tentatively identified with a large unipolar magnetic cell as seen on Hα synoptic charts, (2) a large-scale drift process which is energy independent with drift velocities v D in the range 1° ⩽ v D ⩽ 4°h-1, and simultaneously (3) a diffusion process which yields the general broadening of the intensity time profiles for eastern hemisphere events, which is, however, of less importance than previously assumed

Computation and Observation of Zeeman Multiplet Polarization in Fraunhofer Lines. II: Computation of Stokes Parameter Profiles

Abstract: A self-contained summary of the generalized Unno theory of LTE line formation in solar magnetic fields and its application to the numerical computation of Stokes parameter profiles is given. Within this context, computational details of general interest are described and numerical results for sunspot fields are given. Finally, a new method of computing the height of line formation is presented.

Observations of coronal disturbances from 1 to 9 solar radii. I - First event of 1973 January 11

Abstract: H alpha, white-light and radio observations of a coronal disturbance on Jan. 11, 1973, commencing at about 0036 hr UT show that a piston-driven shock wave propagated outward through the corona to heights of at least 9 solar radii. Probably most of the expelled coronal gas originated in a coronal enhancement in the lower corona. An estimate of the kinetic energy and the mass of the expelled gas is obtained which is compatible with observations of piston-driven shock waves near the earth. Shock-wave parameters are evaluated, and a model of the disturbance is outlined.

The nature of the sunspot phenomenon. I - Solutions of the heat transport equation

Abstract: It is pointed out that sunspots represent a disruption in the uniform flow of heat through the convective zone. The basic sunspot structure is, therefore, determined by the energy transport equation. The solutions of this equation for the case of stochastic heat transport are examined. It is concluded that a sunspot is basically a region of enhanced, rather than inhibited, energy transport and emissivity. The heat flow equations are discussed and attention is given to the shallow depth of the sunspot phenomenon. The sunspot is seen as a heat engine of high efficiency which converts most of the heat flux into hydromagnetic waves.

Studies of solar magnetic fields. II - The magnetic fluxes

Abstract: Magnetic flux data from the Mount Wilson magnetograph are examined over the interval 1967-1973. The total flux in the north is greater than that in the south by about 7% over this interval, reflecting a higher level of activity in the northern hemisphere. Close to 95% of the total flux is confined to latitudes equatorward of 40 degrees, which means that close to 95% of the flux cancels with flux of opposite polarity before it can migrate poleward of 40 degrees. It is pointed out that a consequence of this flux distribution is that ephemeral regions must make a negligible contribution to the long-term large-scale magnetic flux distribution. A broad peak in the total flux may be seen centered about one year after activity maximum in the north below 40 degrees. In the south there is a very sharp increase in flux about the same time. In the north, several poleward migrations of flux may be seen.

Type II radio bursts and particle acceleration

Abstract: 328 particle events recorded during 30 months from January 1, 1966 to June 30, 1968 (taken from the new Catalog of Solar Particle Events, 1955–1969) are compared with the occurrence of 166 type II radio bursts during the same period. The results of this comparison give a convincing evidence that proton acceleration to higher energies in flares (the ‘second acceleration step’) is closely connected with the type II burst occurrence. The shock wave appears to originate near the time when the impulsive burst occurs, and the second acceleration step follows immediately the first one; in some cases the second step sets in while the first step is still in progress.

Neutron propagation and 2.2 MeV gamma-ray line production in the solar atmosphere

Abstract: We have calculated the 2.2 MeV gamma-ray line intensity from the Sun using a Monte Carlo method for neutron propagation in the solar atmosphere. We provide detailed results on the total gamma-ray yield per neutron and on the time profile of the 2.2 MeV line from an instantaneous and monoenergetic neutron source. The parameters which have the most significant effects on the line intensity are the energies of the neutrons, the position of the neutron source on the Sun, and the abundance of He3 in the photosphere. For an isotropic neutron source which is not too close to the limb of the Sun, the gamma-ray yield is between about 0.02 to 0.2 photons per neutron, provided that the neutron energies are in the range 1 to 100 MeV and the ratio He3/H is less than about 5 × 10−5.

Equilibrium and stability of force-free magnetic field

Abstract: Force-free magnetic fields (f.f.f) are considered as the first approximation of magnetic hydrodynamic equations in the case when the energy of the field exceeds the thermal energy of the medium. Such a relation of energies takes place in the upper atmosphere of the Sun in active regions.

The profile and polarization of the coronal Lα line

Abstract: We calculate the profile and polarization of the Lα line in the solar corona. Coronal temperature variation, solar wind and other non-thermal motions have been taken into account. Because of the relatively low atomic weight of hydrogen the profile of the Lα line is a sensitive indicator of the coronal temperature. The line polarization contains relatively little information except for strong magnetic fields (> 70 G).

The structure of the middle corona from observations at 80 and 160 MHz

Abstract: Maps of the brightness distribution of the ‘quiet Sun’ at 80 and 160 MHz reveal the presence of features both brighter and darker than average. The ‘dark’ regions are well correlated with dark regions on UV maps; we deduce that they result from ‘coronal holes’. The ‘bright’ regions are associated with quiescent filaments and not plages or bright regions on microwave or UV maps; we deduce that they result from ‘coronal helmets’. When coronal holes appear near the centre of the disk we can estimate the density and kinetic temperature in the holes from the radio observations. For a hole observed on 1972 July 20–21, we find T ≈ 0.8 × 106 inside the hole and T ≈ 1.0 × 106 in average regions outside the hole. Inside the hole the density is estimated to be about one-quarter of that in Newkirk's model of the spherically symmetric corona. Variations in brightness at a fixed height above the limb are generally well correlated with scans at a similar height made with a K-coronameter. Occasional differences may result from streamers protruding beyond the limb from the back of the Sun. These can be seen by the K-coronameter but, because of refraction of the radio rays, not by the radio-heliograph.

Resonant scattering of particles and second phase acceleration in the solar corona

Abstract: Effective acceleration of particles by hydromagnetic turbulence requires that the particles be scattered at a rate ν comparable with the frequency ω of the turbulence. The only effective scattering process is due to resonant wave-particle interactions. The resonant waves are HM waves for ions with β≫βA(βc = particle speed, βAc = Alfven speed) and for electrons with γβ ⩾ 43β0(β0 ≈ 43βA), and are whistlers for electrons with β0 ≪ γβ≲ 43β0. The resonant waves can be generated by an anisotropic distribution of particles provided that the anisotropy factor A exceeds a threshold anisotropy A0 ≈ βA/β for HM waves and A0 ≈ β02/β2γ for whistlers. Turbulence with relative magnetic amplitude ɛ causes acceleration at a rate {ie0353-02} provided the following conditions are satisfied: (a) β ≫ βA for ions, β ≫ β0 for electrons; (b) ɛ ≫ A0; (c) n1/ne≲ω/gWi or n1/ne ≫(ω/Ωi) (γβ/43β0)2 for scattering by HM waves or whistlers respectively (n1 = number density of accelerated particles, Ωi = ion gyrofrequency).

Computation and Observation of Zeeman Multiplet Polarization in Fraunhofer Lines. III: Magnetic Field Structure of Spot Mt. Wilson 18488

Abstract: A brief introduction to the concept of Stokes parameters and their application to solar magnetic field measurements is followed by the description of a new photographic polarimeter which is capable of measuring profiles of all Stokes parameters simultaneously. Its performance characteristics and the method of measuring and correcting instrumental polarization are discussed.

Some properties of velocity fields in the solar photosphere. V. Spatio- temporal analysis of high resolution spectra

Abstract: Series of high resolution spectrograms taken simultaneously in three spectral regions were recorded with a lambda meter and subjected to a statistical analysis of the fluctuations of Doppler shift and brightness.(1)The 5-min oscillations are confirmed to be ‘evanescent’ waves. Their horizontal wavelengths range from 9″ to 15″ and probably still larger values.(2)Horizontal sound waves (Lamb waves) are only of minor importance in the photospheric velocity field.(3)Vertically propagating sound waves are observed and are used to establish a geometrical height scale. The horizontal wave numbers of these sound waves cover a wide range including the scales of both the 5 min oscillations and the granular convection.(4)The inversion of the granular brightness pattern at higher photospheric levels is confirmed for wavenumbers smaller than 8 Mm−1. At k ≈ 10 Mm−1, however, the correlation of brightness and blue shift observed in the photosphere holds also in lines originating as high as Na i(DI). This statistical result is related to structures directly visible in spectrograms.

Analysis of the August 7, 1972 White Light Flare: Its Spectrum and Vertical Structure

Abstract: Spectral data on a white light wave occurring during the explosive phase of the August 7 flare were obtained simultaneously with three telescopes at the Sacramento Peak Observatory. Spectrograms in the region λλ3530 to 5895 and sequences of filtergrams (∼ 200 A halfwidth) at 4950 A and 5900 A constitute the most complete record of white light flare emission obtained to date. Analysis of the iron line spectrum and of the CN and CH molecular lines shows that the maximum depth of the emission in the flare wave is about 200 km above the photosphere of the Harvard-Smithsonian Reference Atmosphere. Analysis of the Balmer lines gives an electron density of 3 × 1013 cm−3 where the continuum emission is present. From the Balmer line analysis, it is concluded that, in agreement with Canfield (1974) and Shmeleva and Syrovatskii (1973), the flare occurs in a thin layer and that the heating and ionization of the flaring layers are due to the injection of 100 keV electrons. There is no need to postulate filamentary structure in the flaring layer in order to explain the observations. Analysis of the continuum emission in the wave indicates that it is produced by free-bound transitions of hydrogen at a temperature of ∼ 8500 K. In the impulsive phase of the flare emission arose from short-lived bluish knots which could not be studied in detail. In the following phase, the one to which the conclusions in this paper refer, the continuum emission coincided with the Hα ribbon expansion (the explosive phase). We identify it with the ‘yellowish-white’ flares reported by Trouvelot (1891) and others.

The quiet corona: Temperature and temperature gradient

Abstract: A study of the lower corona thermal properties was made using the best examples of solar wind heavy ion spectra obtained with Vela 5 and 6 plasma analyzers at times of quiet solar wind (low speed, low temperature). The multiple Si and Fe ion species peaks in the spectra were fit with solutions of the ionization equilibrium equations to determine ‘freezing in’ temperatures for the various species over a range of heliocentric distances r. Assuming a power law electron temperature model, T = =t ⊙ (R ⊙/r) α , spherical symmetry, and mass conservation, the following results for the quiet corona were obtained: (1) The average freezing in temperatures ranged near 1.5 × 106 K at r ∼- 2.4 to 3.9 R ⊙. (2) Values for T ⊙ ranged between 1.7 × 106 K and 2.5 × 106 K with an average of 1.84 ± 0.13 × 106 K. (3) The temperature gradient parameter α lay between 0.20 and 0.41 with an average value of 0.29 ± ±0.06. This is consistent with the predicted value α = 2/7 derived from conduction dominated spherically symmetric models of the corona. (4) The 0 and N lines which freeze in at a distance of r ∼-1.5 R ⊙ indicate temperatures of ∼ 2.1 × 106 K. Temperatures higher at 1.5 R ⊙ than at 3 R ⊙, in agreement with extrapolations of the power law model, suggest that coronal heating in regions of open field geometry is not important beyond r ∼- 1.5 R ⊙.

The large-scale solar magnetic field

Abstract: The large-scale photospheric magnetic field, measured by the Mt. Wilson magnetograph, has been analyzed in terms of surface harmonics (P n m )(θ)cosmφ and P n m (θ)sinmφ) 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.

Observation of sectored structure in the outer solar corona - Correlation with interplanetary magnetic field

Abstract: Review of the daily images of the white light corona between 3 and 10 solar radii recorded by a coronagraph aboard the OSO-7 unmanned satellite since October 3, 1971. The observed sectored structure in the outer solar corona is discussed and correlated with the interplanetary magnetic field. The correlations support the observation of Hansen et al. (1973).

Rigid and differential rotation of the solar corona

Abstract: The rotation of the solar corona has been studied using recurrence properties of the green coronal line (5303 A) for the interval 1947–1970. Short-lived coronal activity is found to show the same differential rotation as short-lived photospheric magnetic field features. Long-lived recurrences show rigid rotation in the latitude interval ±57°.5. It is proposed that at least part of the variability of rotational properties of the solar atmosphere may be understood as a consequence of coexistence of differential and rigid solar rotation.