Showing papers in "Solar Physics in 1968"

Solar Spicules (Invited Review Paper)

Abstract: 7. ConclusionThe author of a review article is undoubtedly the one who benefits most from it. Only by reviewing an entire subject does it become clear how much is known of it and in what areas more information is desired. In the past 10 years, spicules have probably been the best studied fine structures on the sun. A substantial amount of observational information on spicules is available as seen in Section 3. Some of the most important questions that remain to be answered in greater detail than available now are, in my opinion, the following:(1)Do spicules indeed diffuse after they reach their maximum growth and brightness? If so, is this an expansion of the spicule magnetic field, a change of the properties of the gases surrounding the spicule, or an actual diffusion of spicular matter across the spicular magnetic fields?(2)Is the group behaviour of spicules as that described by Lippincott (1957)? If true, it would imply that the spicule mechanism extends over a large area of the sun (100000 km), which conflicts with the spicule theories as given in Section 4.(3)Is the ‘tilt’ of spicule-emission lines real? If so, is it caused by spicule rotation and how does it differ between the various spicule-emission lines?(4)Is the spicule diameter different in the different spicule-emission lines?(5)Is it possible to measure the spicular magnetic field How large is it?(6)Do the physical conditions vary from spicule to spicule? How do they vary with height and time within one spicule? For this, one needs simultaneous spectra of spicules in many lines which have a different temperature behaviour.(7)Is it true that both the bright and dark elongated fine mottles seen in disk spectroheliograms are spicules? This requires renewed study of the solar disk.(8)After a definite identification of spicules with disk structure is made, what can one learn about spicule properties from the disk study? One can, for example, try to find a direct link with the solar granulation.(9)Are there spicules in active regions of the sun? How do they differ from spicules in quiescent regions?(10)Are chromospheric grains perhaps spicules which do not grow upwards because of a lack of magnetic fields? Are they perhaps related to granules?(11)What are the implications of a breakdown of the ‘statistically steady-state’ assumption in the spicule-intensity calculations? The answers to many of these questions are of great importance in the precise understanding of a spicule, and in the derivation of a magnetohydrodynamic model for it.

Propagation of hydromagnetic disturbances in the solar corona and Moreton's wave phenomenon

Abstract: Moreton's wave phenomenon, observed to propagate across the solar disk during certain solar flares, may be explained as the intersection line between an expanding coronal wavefront surface and the chromosphere: The propagation of a hydromagnetic disturbance in a spherically symmetric model corona with a radial magnetic field is studied in the WKB-approximation, and the propagation paths of wavepackets, the shape of the wavefronts expanding in time, and the variation of the direction of material oscillation along the paths are discussed for disturbances in hydromagnetic wave modes. It is shown that the wavefront expanding in fast-mode in the corona has a circular intersection line with the chromosphere, which sweeps on the solar surface with a velocity and a direction of material oscillation just favorable for explaining the observed features of the phenomenon.

The intensity, velocity and magnetic structure of a sunspot region

Abstract: The observational set-up for a detailed study of the velocity, intensity and magnetic-field fine structure in and around a sunspot is described. On highly resolved spectra we detected in the vicinity of a sunspot a large number of points with strong magnetic fields (magnetic knots). The magnetic field in these knots causes a striking decrease of the line depth (or a ‘line gap’ after Sheeley, 1967). The properties of the magnetic knots are: (1) magnetic fields up to 1400 gauss; (2) diameter ≈ 1100 km; (3) coincidence with dark intergranular spaces; (4) generally downward material motion; (5) lifetime>30min; (6) estimated total number around an unipolar spot ⩾ 2000; (7) combined magnetic flux comparable to the sunspot flux; (8) coincidence with Ca+ plages. For the smallest sunspots (pores) we obtained magnetic fields >1500 gauss. Hence a magnetic field of about 1400–1500 gauss appears to be a rather critical level for pore and spot formation. We found a large number of small areas producing line gaps without measurable magnetic field. These ‘non-magnetic gap-regions’ coincide with bright continuum structures. Some aspects arising from the occurrence of hundreds of magnetic knots in an active region are discussed in the last section.

Micro-scale structures in the interplanetary medium

Abstract: This report describes and analyses the following micro-scale (≲.01 AU) structures which were found in combined interplanetary magnetic field and plasma data obtained by the deep-space probe, Pioneer-6: (1) Several types of simultaneous discontinuities in the magnetic field and plasma parameters; (2) at least one clear example of a transitional region (D-sheet) associated with a plasma discontinuity; (3) other D-sheets which give evidence of magnetic-field annihilation; (4) inhomogeneous isothermal regions in which the square of the magnetic-field intensity is proportional to the density; (5) periodic variations in the magnetic-field intensity which are associated with discontinuities in the bulk speed. It is suggested that small velocity discontinuities play a fundamental role in reducing stresses in the interplanetary medium, and that large-velocity discontinuities may give rise to waves and turbulence.

The Bilderberg Model of the Photosphere and Low Chromosphere

Abstract: From 17 through 21 April 1967, an international study week was held in the ‘Bilderberg’ near Arnhem, Netherlands, with the aim of obtaining an internationally acceptable model of the solar photosphere and low chromosphere. It was found that such a model, based on observed intensities and center-to-limb observations of the solar continuous spectrum, could indeed be established. This model, henceforth called the Bilderberg Continuum Atmosphere (BCA), is shown in Table I, which gives the temperature, gas and electron pressures, and other data as functions of the continuous optical depth at 5000 A between τ5000 = 10−7 and 25. The model is characterized by a flat temperature minimum of 4600°K between τ5000 ≈ 10−2 to 10−4. The model is homogeneous, and in hydrostatic equilibrium. A hydrogen-helium ratio of 10 has been assumed.

Magnetic fields and flares in the region cmp, 20 september 1963.

Abstract: The position of bright knots of 30 flares at their very beginning relative to the high-resolution isogauss maps of the longitudinal component (H ∥) and maps of the transverse component (H ⊥) of magnetic field are considered for seven days during the passage of the active and large spot group in Sept 1963 (see Table I and maps on Figures 1–8) The flare bright knots occur simultaneously in regions of opposite magnetic polarity, and the majority of these knots are adjacent to neutral line H ∥ = 0, although not coinciding precisely with this line (Figure 9) Lenticular form of flare knots and the motions of bright material of flares is restrained by transversal field H ⊥ Also flares are closely associated (83%) with so-called ‘bifurcated regions’, where specific crossing of transverse components takes place (Figures 4–5) There is well-expressed (80%) coincidence of flare knots with the strongest (positive or negative) electric currents as determined from the relation j = c/4π rot H The relation of results obtained to some existing theories of flares is briefly discussed

The photospheric network

Abstract: Spectroheliograms, obtained in certain Fraunhofer lines with the 82-cm solar image at the Kitt Peak National Observatory, show a bright photospheric network having the following properties: (1) It resembles, but does not coincide with, the chromospheric network, the structure of the photospheric network being finer and more delicate than the relatively coarse structure of the chromospheric network. (2) It is exactly cospatial with the network of non-sunspot photospheric magnetic fields. (3) Its visibility in a given photospheric Fraunhofer line is primarily dependent on the states of ionization and excitation from which the line is formed and secondarily dependent on the Zeemansensitivity of the line-being most visible in low-excitation lines of neutral atoms and least visible in high-excitation lines of singly ionized atoms.

Spectral observations of spicules at two heights in the solar chromosphere.

Abstract: An observational program at the Sacramento Peak Observatory in 1965 provided high-dispersion spectra of the solar chromosphere in several spectral regions simultaneously. These regions included various combinations of the spectral lines Hα, Hβ and Hɛ, the D3-line of Hei, the infrared triplet of Oi, and the H- and K-lines and the infrared triplet of Caii. With the use of an image slicer the observations were made simultaneously at two heights in the solar chromosphere separated by several thousand kilometers. From these data we draw the following conclusions: (a) Emission of different lines arises in the same chromospheric features. The intensity ratio of lines of different elements varies significantly from spicule to spicule. For the H- and K-lines of ionized calcium, this ratio remains constant, independent of wavelength throughout the line, overall intensity, and height in the chromosphere. Two rare-earth lines in the wing of the H-line show no spicular structure at all. (b) The line-of-sight velocities of many features reverse as a function of time, although most spicules show velocities in only one direction. The simultaneous spectra at two heights show most spicules to have the same line-of-sight velocity at both. There may be an additional class of features, mostly rapidly moving, whose members have line-of-sight velocities that increase with height. These features comprise perhaps 10% of the total. Velocity changes occur simultaneously, to within 20 sec, at two heights separated by 1800 km, indicating velocities of propagation of hundreds of km/sec. The velocity field of individual features is often quite complicated; many spectral features are inclined to the direction of dispersion, implying that differential mass motions are present. (c) The existence of anomalously broad H and K profiles is real. Even with high dispersion and the best seeing, such profiles are not resolved into smaller features. The central reversal in K, H and Hα appears to remain unshifted when the wings are displaced in wavelength, indicating that the reversal is non-spicular.

The stability of force-free magnetic fields with cylindrical symmetry in the context of solar flares

Abstract: The problem of the accumulation and storage of the energy released in solar flares is discussed; it is proposed that convective energy of the photosphere is transformed into magnetic energy of the chromosphere and corona. The consequences of a large ratio of magnetic pressure to gas pressure are investigated. In this case the field must be approximately force-free. The only suitable force-free fields which allow an analytical treatment are those of cylindrical symmetry. The stability of these fields is studied with the energy principle. It is shown that they are always unstable due to kink type instabilities. The shape of the unstable perturbations is described in detail and an upper limit for their amplitude is estimated. The consequences for the proposed mechanism of energy storage are briefly discussed.

Coronal polar plumes

Abstract: The purpose of this investigation was to determine what connection exists between coronal plumes and polar surface features. To this end the properties of plumes were re-examined by making a detailed statistical analysis of photographs of three eclipses (1962, 1963, and 1965) of the last sunspot minimum. It is found that a ‘typical’ plume has a core density ≈ 108 cm−3, a half width ≈ 3.3 × 104km, and a density profile with distance r from its axis characterized by $$N \approx {\text{10}}^{\text{8}} \left( {1 - \frac{r}{{3.9 \times 10^4 }}} \right)^{1.6} $$ There is some (although only weak) regularity in the projected spacing of plumes with a mean separation of ≈ 7 × 104 km. The relation between plumes and various surface features is examined. Although little direct evidence can be assembled, we conclude that a direct connection exists between plumes and photospheric faculae, bright K3 faculae, and the small-scale magnetic structure present in the chromospheric network. It is hypothesized that plumes originate at the bright cores of the rosettes which lie along the chromospheric network. The distribution of magnetic field in the corona above a surface covered with idealized chromospheric network cells is calculated. The fact that the shape and size of the magnetic flux tubes originating from the rosette agrees with that of observed plumes supports the hypothesis.

Magnetic fields and the temperature structure of the chromosphere-corona interface

Abstract: The temperature structure of the transition region between the chromosphere and corona is discussed in the context of current ideas about magnetic fields in these layers. Magnetic channeling of the downward conductive heat flow from the corona into the regions of enhanced field at the supergranulation boundaries is proposed as a mechanism for explaining the measured intensities of solar ultraviolet emission lines which originate in layers with temperatures below 105 °K. It is shown that nearly all of the observed ultraviolet line emission originates in interspicule regions, and that this emission plays an important part in the energy balance of the cooler layers of the transition region. It is suggested that certain motions observed in the upper chromosphere may represent the earliest visual evidence for conversion of inflowing conduction energy into kinetic motions.

High-resolution photography of the solar chromosphere

Abstract: High-resolution observations of the low chromosphere beyond the limb at the centre of the Hα line reveal the existence of two types of fine structure whose presence has hitherto been unobserved: (1) small bright features which show a close correspondence in properties with the bright mottles on the disk and are unequivocally identifiable as the latter seen beyond the limb; and (2) a narrow dark band lying immediately above the photospheric limb whose physical significance is not yet clear.

The intensity, velocity, and magnetic structure of a sunspot region. ii. some properties of umbral dots.

Abstract: A time sequence of high-resolution sunspot photographs, exposed almost simultaneously in two continuum wavelengths (4680 Å and 6400 Å), was used to study some properties of umbral fine structures (‘umbral dots’). The lifetime of the umbral dots is found to be 1500 sec. Photometry of some bright dots leads to an observed intensity excess of 0.129 Iphot and 0.134 Iphot in the blue and red respectively. The observed mean diameter of the dots is found to be 420 km. These values still include the action of image blurring. From the color index the true intensity and diameter of the dots are estimated. It appears that the umbral dots are in reality of photospheric brightness having true diameters of 150–200 km. The spatial distribution of the dots in sunspot umbrae is discussed. Some peculiarities in recent sunspot magnetic-field observations may be explained by magnetic inhomogeneities associated with umbral dots.

Relativistic electrons from solar flares

Abstract: Observations of interplanetary relativistic electrons from several solar-flare events monitored through 1964 to mid-1967 are presented. These are the first direct spectral measurements and time histories, made outside the magnetosphere, of solar-flare electrons having relativistic velocities. The 3- to 12-MeV electrons detected have kinetic energies about two orders of magnitude higher than those solar electrons previously studied in space, and measurements of both the time histories and energy spectra for a number of events in the present solar cycle were carried out. These measurements of interplanetary electrons are also directly compared with solar X-ray data and with measurements of related interplanetary solar protons.

Solar flare injection and propagation of low-energy protons and electrons in the event of 7–9 July, 1966

Abstract: Solar flare injection and propagation of low energy protons and electrons in 7-9 July 1966 solar particle event

A Measurement of the Solar H and K Profiles

Abstract: A new series of photometric observations of the H and K lines were obtained at Sacramento Peak Observatory in 1964 and 1965. In both the observations and the data reduction special attention was given to obtaining a suitable average over the solar fine structure and to defining a proper reference continuum. The results are that the central intensities of H and K are the same and equal to 4.2% of the continuum. The limb-darkening curves at the line centres are also the same for both lines.

Some general properties of helmeted coronal structures.

Abstract: A simplified analysis of helmeted coronal structures is carried out and some of the gross properties of such structures discussed. It is found that the magnetically closed region can have but a limited extension into the corona. For temperatures in excess of 1.5 × 106 °K, the maximum height above the limb is about 1.6 R ⊙. The maximum possible extension of the helmet from the solar center is exactly one-half the distance to the critical point (where the flow velocity passes through the speed of sound). For this reason, a helmet streamer, at least out to a few solar radii, is essentially a magnetostatic structure - the flow adjacent to the helmet having little effect upon its properties. For given base dimensions, there is a maximum temperature for which a helmet streamer can exist - giving an indication of why such streamers do not appear over young active regions. If the temperature in the helmet and in the streaming region are approximately the same, the helmet height, helmet shape, external flow velocity, and rate of outward decline in the magnetic field are shown to be much more dependent upon the photospheric field distribution than upon the field strength. The density enhancement, however, is a strong function of the field strength. This enhancement is preserved out to the top of the helmet with both the density inside and outside decreasing approximately as predicted by hydrostatic equilibrium. The possible existence of both domed helmets and cusped helmets is demonstrated with the former existing at lower temperatures and the latter at higher temperatures. Cusped helmets occur, however, over a relatively narrow temperature range and are, hence, expected to be less common. The expansion velocity outside the helmet is higher than that predicted by radial flow but increases outward much more slowly. The magnetic field decreases outward proportionally to the square root of the density and inversely proportionally to the velocity - bearing, in general, no relation to a potential field since the rate of decline in field strength is determined by the temperature.

A large scale pattern in the solar magnetic field

Abstract: A clearly evident large-scale pattern in the interplanetary magnetic field during 1964 is used to search for a similar large-scale pattern in the solar magnetic field. It is found that such a pattern did exist in the photospheric field observations on both sides of the equator over a range of at least 40°N to 35°S. The pattern is basically similar at all these latitudes, and differs from that to be expected from solar differential rotation in three important respects. It is found that the solar magnetic pattern changed at all latitudes investigated within an interval of a few solar rotations.

Étude de la localisation des éruptions dans la structure magnétique évolutive des régions actives solaires

Abstract: A l'aide d'un materiel d'observation exceptionnellement complet reuni dans le cadre de la ‘Cooperative Study of Solar Active Regions’, nous avons tente de preciser les conditions spatiotemporelles de l'occurrence des eruptions dans la structure magnetique evolutive des Regions Actives.

Emission cores in H and K lines

Abstract: Enhancement of the violet K2 emission peak results when the atmospheric layers at heights where K3 forms are moving downward with velocities of 10–20 km/sec or when the K2 layers and those immediately below are moving upward with velocities of 3–7 km/sec. Evidence favoring the former alternative is cited.

On long-term forecasts of proton flares

Abstract: 174 proton flares which were observed during the period from 1956 to 1965, occurred in 81 different active regions. It is shown that these active regions formed in complexes of activity, which stayed on the solar surface for many months, and in some cases even for several years. Since the proton-flare regions develop very rapidly and reach the proton-flare active stage within a few days, these complexes of activity represent the areas on the sun, where proton-flare regions can form at any time. Reference is made to contributions by Bumba and Howard, who investigated the birth of active regions and detected some properties of complexes of activity; nevertheless, at the present time, we do not know any method to predict when a proton-flare region begins to develop in such a complex of activity. On the other hand, there is a chance of predicting the dangerous longitudes on the sun, as soon as such a complex of activity has been well recognized or, from the opposite point of view, to predict the safe proton-flare free periods on the sun. If, however, all the complexes on both the hemispheres are taken into account and every complex is considered ‘proton-dangerous’ from 2 days before to 7 days after the central meridian passage, one can prove that no proton-flare free periods existed for more than 3 years around the maximum of the last solar cycle. Applying this result to the present cycle, one can conclude that no safe forecasts of proton-flare free periods can be made from the beginning of 1968 to the end of 1970. During the remaining 7 or 8 years of the solar cycle, long-term forecasts of proton flares could be made provided that our knowledge of the formation and development of the complexes of activity is improved. It is of interest to notice some properties of the complexes formed in the last solar cycle. While the complexes on the Northern solar hemisphere remained at fairly constant heliographic longitudes for many years, the complexes formed on the Southern hemisphere seemed to travel in two rows around the sun, in the direction opposite to the solar rotation. Another interesting fact is a yearly periodicity in the formation of proton-flare regions in the complexes of activity, with a maximum in the summer period and a deep minimum in the winter season. Such a seasonal variation also appears, if one considers the flare activity, type-IV bursts, PCA's, great magnetic storms, and magnetic crochets. Therefore, one can reasonably believe that this yearly variation, even when similar to the seasonal variation at the earth, is of solar origin.

High-resolution measurements of photosphere and sunspot velocity and magnetic fields using a narrow-band birefringent filter

Abstract: Modifications to a Zeiss 1/4 A filter are described which allow high spatial resolution observations of the line-of-sight velocities and magnetic fields in the photosphere and in sunspots. First results show: (1) the granular velocity field to be very strong; differences in upward motions in the granules and downward motions in between are as much as 6 km/sec; (2) the Evershed effect in sunspots to originate primarily in the dark regions between bright penumbral filaments.

Influence of a solar active region on the interplanetary magnetic field

Abstract: Interplanetary magnetic field sectors evolution /1964-1965/, discussing solar active region development leading to magnetic loops in interplanetary medium

Magnetic Field Structure Associated with Coronal Streamers.

Abstract: Magnetic field structure associated with coronal streamers observed during solar eclipse of May 30, 1965

Solar abundances of lithium, beryllium and boron

Abstract: New solar abundances have been derived for Li, Be and B They are mainly based on high-resolution spectra obtained at the Jungfraujoch Scientific Station (Switzerland) For Li, the abundance results from a discussion of the photospheric and sunspot spectra Our results, log N Li = 042, log N Be = 117 and log N B < 280 (in the log N H = 1200 scale), are lower than the previously admitted abundances for these elements The far UV spectrum (λ < 3000 A) has also been considered in each case The meaning of our results is discussed from the point of view of the destruction of these elements during the evolution of the sun

Magneto--gravity waves and the heating of the solar corona.

Abstract: It is generally believed that the heating of the solar corona is caused by waves originating in the photosphere and propagating into the corona where their energy is dissipated. The medium through which these waves propagate is in general permeated by magnetic fields complicating the behaviour of this propagation considerably. We have therefore analysed the wave motions in a plasma permeated by constant magnetic and gravitational fields. In general, three waves modes were found, which we called the + mode, −mode, and the Alfven mode. Each mode was found to be strongly coupled to each of the three kinds of motion; acoustic, gravity, and hydromagnetic. However, the Alfven mode was found to be separable from the dispersion relation, and therefore independent of compressibility and gravity. The local dispersion relation is derived and expressed in nondimensional form independent of the constants that describe a particular atmosphere. From the dispersion relation one can show that rising waves propagate either with a constant or a growing wave amplitude depending on the magnitudes and directions of the gravitational field, magnetic field, and the wave vector. The variation of the density with height is taken into account by a generalized W.K.B. method. Equations are found which give the height at which wave reflection occurs, giving the upper bound for possible wave propagation.

A new method of magnetograph observation of the photospheric brightness, velocity, and magnetic fields

Abstract: : Several improvements have been made to the Mt. Wilson Observatory solar magnetograph, including changes to the guider, the Doppler compensator, and the data handling system. The improved magnetograph has been used for a new type of solar observation consisting of several hundred scans back and forth along a straight line of length 3/4 R sub o perpendicular to central meridian. The data reduction, which is done entirely with a computer, eliminates those effects which have their origin in the earth-sun geometry. The spatial and temporal properties of the 5-minute oscillations are discussed. (Author)

The proton flare of August 28, 1966

Abstract: The proton flare of August 28, 1966 began on Hα records at 15h21m35s UT. It presented an unusually complex development with flare emission occurring in two distinct plages. The brightest part of the flare attained maximum intensity, 152 % of the continuum, between 15h30m and 15h32m UT. Photometric measurements show that a long-enduring part of the flare continued to decline in intensity until at least 21h20m UT.

The Solar H and K Lines of Ionized Calcium

Abstract: Photographic observations of the mean H and K profiles are given for several positions across the solar disk (0.1 < μ <.6). The core profiles of H and K are nearly identical at the same position on the sun.

A concentric ellipse multiple-arch system in the solar corona

Abstract: 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.