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.

     

The evolution of massive close binaries

The results of evolutionary computations for massive binary systems (initial masses of the primary 10M ) with mass ratios between 0.3 and 0.8 are summarized and compared with observations in order to verify how far one can go with the conservative assumption of mass exchange. It is found that conservative mass exchange leads to acceptable first-order models of W-R and massive X-ray binaries. However, the comparison between this theory and observation reveals that for the observed systems (W-R and X-ray binaries) a preference exists for low intial mass ratios; moreover, the X-ray luminosities of the theoretical models are systematically too low, though this may be due to the adopted wind model. In addition, the influences of several parameters (distance between the components, chemical composition, primary mass, mass ratio and atmosphere) are examined. These parameters influence the remnant mass and any further evolution only marginally. Attention is also given to the effect on the system parameters of a supernova explosion of the remnant of the mass-losing component. For a large range of systems a disruption probability smaller than 25% is found.     

Analysis of stellar occultation data: Effects of photon noise and initial conditions

A new inversion technique for obtaining temperature, pressure, and number density profiles of a planetary atmosphere from an occultation light curve is described. This technique employs an improved boundary condition to begin the numerical inversion and permits the computation of errors in the profiles caused by photon noise in the light curve. We present our assumptions about the atmosphere, optics, and noise and develop the equations for temperature, pressure, and number density and their associated errors. By inverting in equal increments of altitude, Δh, rather than in equal increments of time, Δt, the inversion need not be halted at the first negative point on the light curve as required by previous methods. The importance of the boundary condition is stressed, and a new initial condition is given. Numerical results are presented for the special case of inversion of a noisy isothermal light curve. From these results, simple relations are developed which can be used to predict the noise quality of an occultation. It is found that fractional errors in temperature profiles are comparable to those of pressure and number density profiles. An example of the inversion method is shown. Finally, we discuss the validity of our assumptions. In an appendix we demonstrate that minimum fractional errors in scale height determined from the inversion are comparable to those from an isothermal fit to a noisy isothermal light curve.     

A statistical study of crater-associated wind streaks in the North Equatorial Zone of Mars

A statistical study of all crater-related wind streaks visible on Mariner 9 A-camera frames between latitudes 0 and 30°N has been completed. Of the 2325 streaks identified 1914 (82%) are light tone streaks, 189 (8%) are dark tone, and the remaining 222 (10%) are of mixed tone. Nine parameters characterizing each streak and its associated crater were measured and intercorrelated. Because of the large number of light streaks in our sample fir findings for this type of streak are most significant statistically: light tone streaks occur preferentially in Pc terrain (heavily cratered plains); they are preferentially associated with fresh craters; the surface density of light streaks is not a strong function of elevation; a significant latitude effect does emerge—the density of light tone streaks reaches a maximum between 10 and 15°N, and drops off appreciably both toward the equator and toward higher latitudes; the mean angular width of light streaks is about 25°—long light streaks are significantly narrower than short ones; about 50% of streaks have streak length/crater diameter ratios of ?4; light streak directions conform closely to the wind regime expected at the season of global dust storms (southern summer). Generally speaking, the results for dark and mixed tone streaks in the northern equatorial zone are similar, with the following possible exceptions: dark streaks may show a slight preference to form at higher elecations; dark streaks may be slightly wider on average than light or mixed tone streaks; mixed tone streaks do not share the preference for sharp craters exhibited by light and dark streaks; in general, the directions of dark streaks do not conform to the general circulation pattern expected at the season of global dust storms as well as do those of the light streaks.     

On a mechanism for the formation of VLF electrostatic emissions in the high latitude F-region

The presence of highly anisotropic ion velocity distributions in the weakly-ionized plasma of strongly convecting areas of the high latitude F-region leads to the excitation of electrostatic microinstabilities (λ ～ 50 cm) at frequencies of the order of the lower hybrid frequency and smaller. We have estimated the threshold conditions for the excitation of the unstable waves under various physical circumstances. For some representative conditions we have also calculated the frequencies, growth rates, and wavelengths for the fastest growing modes using the linear approximation. We stress that the present theory breaks down in regions where the plasma cannot be treated as locally homogeneous. The altitude range over which the theory is applicable also varies with conditions. For highly disturbed conditions the upper altitude limit may be as high as 400 km.     

Interhemispheric flow of thermal plasma in a closed magnetic flux tube at mid-latitudes under sunspot minimum conditions

The coupled H⁺ and O⁺ time-dependent continuity and momentum equations are solved within a region of the L = 3 magnetic flux tube lying between (and including) the F2-layers of conjugate hemispheres. The method of solution is an extended and modified version of the Murphy et al. (1976) method. The model is used to study the coupling between the F2-layers of conjugate hemispheres during magnetically quiet periods.The results of the calculations strongly indicate that the protonosphere acts as a reservoir, with variable H⁺ content, which prevents direct coupling between the F2-layers of conjugate hemispheres. However there is generally a significant interhemispheric flow of plasma. This flow is caused by conditions in the summer and winter topside ionospheres and it maintains continuity in the plasma concentration within the protonosphere. There are times when the direction of flow is from the winter hemisphere to the summer hemisphere. It is suggested that maintenance of the winter F2-layer at night is not assisted directly by the F2-layer of the conjugate summer hemisphere.It is shown that during the first few days of protonosphere replenishment after a magnetic storm there is an upflow of H⁺ in the topside ionosphere at all times in the summer hemisphere. There is also an upflow of H⁺ during the daytime in both hemispheres. A comparison with the results obtained when the interhemispheric H⁺ flux is held permanently at zero shows that both F2-layers are little affected by the interhemispheric H⁺ flux. Nevertheless both F2-layers are affected by the H⁺ tube content of the protonosphere. When the H⁺ flux at 1000 km in one hemisphere is much greater than the H⁺ flux at 1000 km in the conjugate hemisphere, there is a corresponding signature in the interhemispheric H⁺ flux.The results suggest that there is insufficient time between magnetic storms for complete replenishment of the protonosphere to occur.     

On flares,substorms, and the theory of impulsive flux transfer events

Solar flares and magnetospheric substorms are discussed in the context of a general theory of impulsive flux transfer events (IFTE). IFTE theory, derived from laboratory observations in the Double Inverse Pinch Device (DIPD), provides a quantitative extension of neutral sheet theories to include nonsteady field line reconnection. Current flow along the reconnection line increases with magnetic flux storage. When flux build-up exceeds the level corresponding to a critical limit on the current, instabilities induce a sudden transition in the mode of conduction. The resulting IFTE, indifferent to the specific modes and instabilities involved, is the more energetic, the lower the initial resistivity. It is the more violent, the greater the resulting resistivity increase and the faster its growth. Violent events can develop very large voltage transients along the reconnection line. Persistent build-up promoting conditions produce relaxation oscillations in the quantity of flux and energy stored (build-up-IFTE cycles). It is difficult to avoid the conclusion: flares and substorms are examples of IFTE.     

Solar micro-bursts at 22.2 GHz and their relationship to events observed at lower frequencies

Observations of McMath region 10433 at 22 GHz using a telescope with a 4 beam during July 1974 revealed the existence events or microbursts with intensities below the sensitivity limit of normal solar patrol instruments. Many of these events were simply the high frequency counterpart of more intense bursts observed at lower frequencies. This note considers the small number of events which suggest that the gyro-synchrotron mechanism alone is incapable of explaining the observations and indicates that a thermal mechanism is needed to explain the high frequency event.On leave of absence from Department of Physics, University of Surrey, Guildford, U.K.