Showing papers in "Solar Physics in 1969"

A model of interplanetary and coronal magnetic fields.

Abstract: Green function solution to Maxwell equations for interplanetary and coronal magnetic fields above photosphere, considering field at source surface

Magnetic Fields and the Structure of the Solar Corona. I: Methods of Calculating Coronal Fields

Abstract: Several different mathematical methods are described which use the observed line-of-sight component of the photospheric magnetic field to determine the magnetic field of the solar corona in the current-free (or potential-field) approximation. Discussed are (1) a monopole method, (2) a Legendre polynomial expansion assuming knowledge of the radial photospheric magnetic field, (3) a Legendre polynomial expansion obtained from the line-of-sight photospheric field by a least-meansquare technique, (4) solar wind simulation by zero-potential surfaces in the corona, (5) corrections for the missing flux due to magnetograph saturation. We conclude (1) that the field obtained from the monopole method is not consistent with the given magnetic data because of non-local effects produced by monopoles on a curved surface, (2) that the field given by a Legendre polynomial (which is fitted to the measured line-of-sight magnetic field) is a rigorous and self-consistent solution with respect to the available data, (3) that it is necessary to correct for the saturation of the magnetograph (at about 80 G) because fields exceeding 80 G provide significant flux to the coronal field, and (4) that a zero-potential surface at 2.5 solar radii can simulate the effect of the solar wind on the coronal magnetic field.

Chromospheric inhomogeneities in sunspot umbrae.

Abstract: The properties of rapidly changing inhomogeneities visible in the H and K lines above sunspot umbrae are described. We find as properties for these ‘Umbral Flashes’: (a) A lifetime of 50 sec. The light curve is asymmetrical, the increase is faster than the decrease in brightness. (b) A diameter ranging from the resolution limit up to 2000 km. (c) A tendency to repeat every 145 sec. (d) A ‘proper motion’ of 40 km/sec generally directed towards the penumbra. (e) A Doppler shift of 6 km/sec. (f) A magnetic field of 2100 G. (g) A decrease in this field of 12 G/sec. This decrease is probably related to the flash motion. (h) At any instant an average of 3–5 flashes in a medium-sized umbra. A weak feature often persists in the umbra after the flash. This post-flash structure initially shows a blue shift, but 100–120 sec after the flash, it shows a rapid red shift just before the flash repeats.

The evolution of the photospheric network

Abstract: A time-lapse sequence of spectroheliograms in the bandhead of CN at λ3883 reveals the following behavior of the photospheric network with time: (1) There is a steady flow of bright ‘points’ (≃ 1000 km in diameter) laterally outward from sunspots at speeds on the order of 1 km·sec−1. After traveling about 10 000 km from a sunspot they either conglomerate to form fragments of the photospheric network or disappear. (2) Spatial changes in the network pattern seem to take place by means of the shifting of network fragments laterally on the solar surface. Although most small-scale details are recognizable after 5–10 minutes, within 30 minutes nearly all the details have changed completely. In contrast to this, the large-scale network pattern seems relatively unchanged after 2 1/2 hours. (3) Occasionally ‘new’ network, not resulting from the lateral motion of bright features from either previously existing network or sunspots, appears on the solar surface. This process consists of the formation in approximately 10 minutes of bright points and a darker-than-average feature between them. The dark feature disappears in another 5–10 minutes and the bright points separate at a relative speed of a few km·sec−1. If the event is of a sufficiently large magnitude, a sunspot will appear.

Directional discontinuities in the interplanetary magnetic field

Abstract: It is shown that the interplanetary magnetic field has different characteristics on different scales, and it is noted that a given physical theory may not be applicable or relevant on all scales. Four scales are defined in terms of time intervals on which the data may be viewed. Many discontinuities in the magnetic-field direction are seen on the mesoscale (≈ 4 days, ≈ 1 AU). The characteristics of such directional discontinuities which were observed by Pioneer 6 during the period December 16, 1965-January 4, 1966 are presented, with special emphasis on their distribution in time. Previously, it was suggested that such discontinuities are simply boundaries of spaghetti-like filaments extending from the sun to the earth. Here it is shown that on the mesoscale unique filaments with sharp boundaries containing well-ordered magnetic fields are not always seen although discontinuities are always present at ≈ 1 AU. Thus, the interplanetary medium appears to be discontinuous rather than filamentary. The filamentary model implies that discontinuities originate at the sun and are convected with the solar wind. The discontinuous model allows the additional possibility that the discontinuities form in the interplanetary medium far from the sun.

Power spectra of the interplanetary magnetic field.

Abstract: Power spectra studies of interplanetary magnetic field fluctuations based on data from Pioneer 6 space probe

Tangential discontinuities in the solar wind

Abstract: Solar wind discontinuity surfaces observed by spacecraft magnetometers determined from measured time delays and solar wind speed

Solar Activity and Recurrences in Magnetic-Field Distribution

Abstract: A study of the Mount Wilson magnetic-field synoptic chart material divided into latitude zones for the interval 1959–67, and a comparison of the data with sunspot groups have provided a better understanding of the structure of the background-field pattern and its relation to activity. The interaction of old and new fields within the pattern seems to result in long-lived sections of alternating polarity in both hemispheres. We postulate subsurface sources with rotation periods of about 27 days which produce active regions over a longitude zone of some tens of degrees. There is a tendency for the background-field features with strong fields to resist to some extent the shearing effects of differential rotation. A prediction is made concerning the nature of the interplanetary magnetic field above the ecliptic.

Sudden commencement associated discontinuities in the interplanetary magnetic field observed by IMP 3

Abstract: The magnetic field measurements made by the magnetic field experiment on the IMP 3 (Explorer 28) spacecraft have been examined at the time of geomagnetic s.s.c. events. Thirty-six such events occurred while IMP 3 was in the interplanetary medium during 1965, 1966 and 1967 and have been analysed. Of these events 8 must have been tangential discontinuities, 2 are either tangential discontinuities or rotational discontinuities and 26 are possible shock waves. (Two of these 26 events have been shown by other authors to be shocks.) These 26 possible shocks have similar magnetic signatures: an increase of 20 % or more in the magnetic field magnitude and a relatively small (always less than 90°) change in direction. The larger s.s.c. events were more likely to be caused by possible shocks while the smaller events were often associated with tangential discontinuities. The orientation of the discontinuity surfaces of the 26 possible shocks shows a preference to be aligned somewhere between a direction perpendicular to the sun-earth line and a direction tangent to the local spiral angle of the magnetic field. It was possible to associate solar flares with 14 of the 26 possible shock events. Of these 14, a reliable orientation was deduced for 8 events. By considering the orientation of these 8 events in relation to the position of the parent flares on the solar disk it is suggested that a typical shock front propagating out from the sun has a radius of curvature less than but of the order of 1 AU.

Magnetograph Measurements with Temperature-Sensitive Lines

Abstract: Certain discrepancies between theoretical and empirical calibrations of magnetograph response are resolved by recognizing the existence of line profile changes in magnetic regions Many of the photospheric lines commonly used for magnetic field measurements weaken greatly in magnetic regions outside of sunspots Unless due account is made of the line profile change, the magnetograph measurements underestimate magnetic flux and field strengths

Motions in Arch Filament Systems

Abstract: A new analysis of Hα filtergrams and Hα spectra of arch filament systems (AFS) shows that material flows downwards in both branches of the arch filaments (v ≈ 50 km/sec) while the top of the arches ascends (v ≲ 10 km/sec). It is suggested that AFS are produced by the magnetic field which expands, between growing spots, into higher levels carrying material with it that subsequently slides down along the magnetic field following gravity.

Current limitation and solar flares

Abstract: A flare model based on force-free currents in the solar atmosphere is considered. The energy of the flare is supposed to be stored as magnetic energy in the current system. If the current density exceeds a certain critical limit an over-voltage may arise in the circuit which will give rise to a rapid release of the stored energy. At the end of the paper some results yielded by the model are compared with observational evidence of flares.

Microwave and hard X-ray bursts from solar flares

Abstract: We have applied detailed theories of gyro-synchrotron emission and absorption in a magnetoactive plasma, X-ray production by the bremsstrahlung of non-thermal electrons on ambient hydrogen, and electron relaxation in a partially ionized and magnetized gas to the solar flare burst phenomenon. The hard X-ray and microwave bursts are shown to be consistent with a single source of non-thermal electrons, where both emissions arise from electrons with energies < mc 2. Further-more, the experimental X-ray and microwave data allow us to deduce the properties of the electron distribution, and the values of the ambient magnetic field, the hydrogen density, and the size of the emitting region. The proposed model, although derived mostly from observations of the 7 July 1966 flare, is shown to be representative of this type of event.

The Profiles of Fraunhofer Lines in the Presence of Zeeman Splitting. I: The Zeeman Triplet

Abstract: For the case of pure absorption lines (LTE) a method is described which enables the general computation of Zeeman-split line profiles. The magnetic field vector, the Doppler shift and the line absorption coefficient is permitted to vary arbitrarily with optical depth. Elliptical birefringence (e.g., Faraday rotation) of the solar atmosphere is taken into account. Some numerical examples are given and some interesting behaviors of the line profiles are discussed.

Oscillatory phenomena in quiescent prominences

Abstract: A model for horizontal oscillations of prominences is presented. It is found that the model of a freely oscillating prominence surrounded by coronal matter explains satisfactorily the observed periods and damping times, as well as the changes in the prominence shape.

Helium Abundance in the Solar Wind

Abstract: Explorer 34 observations of hydrogen and He ions in solar wind, noting number density variations and tentative association with geomagnetic storms

Measurements in the solar spectrum between 1400 and 1875 A with a rocket-borne spectrometer

Abstract: Rocket-borne spectrometer intensity measurements UV region of solar spectrum, noting agreement with IR observations of solar temperature minimum

Anisotropies of solar cosmic rays

Abstract: The behavior of the anisotropy during solar cosmic-ray events is discussed in terms of a simple model in which cosmic-ray particles propagate along the mean interplanetary magnetic field, undergoing pitch-angle scattering. It is shown that a generalized form of the telegraph equation should be used when the anisotropy is large (i.e. greater than 30%), but that the usual diffusion equation is adequate otherwise. The behavior of the anisotropy during the decay phase of solar cosmic-ray events is then considered, and several effects which can give rise to a small persisting anisotropy are described. Finally, observations of solar cosmic-ray anisotropies following flares are reviewed and it is concluded that the simplified mathematical treatment presented here adequately describes some of the general features of the behavior of the anisotropy, but does not provide a detailed quantitative description of the particle behavior, especially during the highly anisotropic phase of an event.

Corotating Structure in the Solar Wind

Abstract: The hydrodynamic equations which describe the radial solar wind expansion are linearized and specialized to treat corotating perturbations. Approximate solutions are found which are time stationary in the corotating reference frame. The solutions predict the behavior of corotating structures for a given boundary condition close to the sun. In particular, the structure resulting from the interaction of fast and slow streams is described. Comparison with sector structure data shows reasonable qualitative and quantitative agreement.

Brightness variations of the white light corona during the years 1964-67

Abstract: Observations of the white light corona were made on over 900 days during the years 1964–67 at heights between 1.125 and 2.0 R⊙ with the K-coronameter at Mount Haleakala and Mauna Loa, Hawaii. The brightness distribution of the minimum corona was elliptical with average equatorial intensities three times the polar. Coronal features of the new cycle at 1.125 R⊙ occurred predominantly in the sunspot zones at 25–30° latitude and in a high latitude zone which migrated toward the North pole before solar maximum. The brightness of the inner corona doubled over this period and a close association is found between the average corona and 10.7-cm solar radio flux. Electron densities in the equatorial regions were nearly twice those of Van de Hulst's model corona, in agreement with the results of recent eclipse observations.

Differential Rotation of the Solar Electron Corona

Abstract: Autocorrelation analyses of K-coronameter observations made at Haleakala and Mauna Loa, Hawaii, during 1964–1967 have established average yearly rotation rates of coronal features as a function of latitude and height above the limb. At low latitudes the corona was found to rotate at the same rate as sunspots but at higher latitudes was consistently faster than the underlying photosphere. There were differences as large as 3–4% in the rate at specific latitudes from year to year and between the two hemispheres. In 1967 a nearly constant rotation was found for heights ranging from 1.125 to 2.0 R 0. For 1966 there was a more complicated pattern of height dependence, with the rate generally decreasing with height at low latitudes and increasing at high latitudes.

Solar velocity fields: 5-min oscillations and supergranulation

Abstract: One dimensional magnetograph scans have been used to study the 5-min photospheric velocity oscillations and the supergranulation. The oscillations in wing brightness lead the oscillations in velocity by less than 90° in the photosphere, and about 90° in the chromosphere, suggesting that they are traveling waves at lower levels and standing waves at higher levels. Downward flows have been observed to be coincident with the chromospheric network confirming the hypothesis that material is flowing downward at supergranular boundaries.

Observational Evidence for Quantization in Photospheric Magnetic Flux

Abstract: Observations are presented which suggest that away from sunspots photospheric magnetic flux is quantized. Assuming the elemental area of a magnetic region to be 1 (arc-sec)2 the elemental field strength is 525 G.

A Rossby-wave dynamo for the Sun, II

Abstract: To make the analysis more tractable, we simplify the equations of Part I to apply to two superposed layers of fluid, with horizontal variations in the motion and magnetic field represented by a small number of Fourier harmonics. The resulting set of eighteen ordinary nonlinear differential equations in time for the Fourier amplitudes is integrated numerically. We analyze in detail the dynamo action from a typical Rossby wave motion and compare it with the solar cycle. The field reversal process is similar in some respects to that put forth by Babcock. Toroidal fields are dragged up by vertical motions in the Rossby waves to form large-scale vertical fields, whose polarities alternate with longitude roughly like bipolar magnetic regions. Vertical fields of preferentially one polarity are carried toward the pole by the meridional motion in the wave to form an axisymmetric poloidal field. This poloidal field is then stretched out by the differential rotation into a new toroidal field of the opposite sign from the original. The poloidal field changes sign when the toroidal and bipolar region like fields are maximum, and vice versa. For the case studied, the reversal period is too short (∼ 2 years) and the poloidal fields too large (∼ 40 G) for the sun. Improvements for the model are discussed.

A mechanism for the build-up of flare energy

Abstract: It is shown how the kinetic energy of the rotational motion of a sunspot can be transferred to electromagnetic energy in filamentary currents. The time needed for preconditioning the solar atmosphere for a flare varies within wide limits. For small flares it may be of the order of minutes; for large flares, of the order of hours or days.

Hydromagnetic shocks in the solar wind

Abstract: The Rankine-Hugoniot relations are applied to shock-like discontinuities measured by both magnetic field and plasma instruments on the satellite Explorer 34 between May 30, 1967 and Jan. 11, 1968. Shock normals were either determined from the magnetic field observations, or from the times of occurrence of the discontinuity at Explorers 33, 34 and 35. The Rankine-Hugoniot relations are obeyed to the accuracy of the observations, and the values of shock velocities, density ratios, and Mach numbers indicate that at 1 AU the typical interplanetary shock is not strong, although all the events studied caused geomagnetic impulses.

A study of the composition of the lower solar corona.

Abstract: Some trends in the composition of the lower solar corona due to mixing and diffusion have been examined. Mixing has been treated through plausible inference from the thermal gradient and through analogy with the neutral atmosphere of the earth. These indicate that diffusion may be important in the lower corona. Changes in composition due to pressure and thermal gradients have been calculated with multi-component diffusion equations. Results indicate that (i) large enhancements of heavy elements are possible and (ii) the composition of the corona can show large variations with values of the thermal gradient in the lower corona.

‘Bartels' active longitudes’, sector boundaries and flare activity

Abstract: The flare activity and especially the proton-flare activity is concentrated in the zones of ‘Bartels' active longitudes’ and in the neighbourhood closest to the sector boundaries of the interplanetary magnetic field. This concentration seems to be greater if the importance of the event increases.