Engineering topics
Publications and source records attributed to Solodyna, C. V..
Skylab S-054 data have been used to examine the flux from X-ray bright points with approximately 90 s time resolution. There is evidence of a steady heating input, similar to one reported for active region loops. Also observed are impulsive brightenings of bright points and rapid decays which are consistent with a sudden turn-off of the steady heating.
The relationship between coronal hole evolution and solar active regions during the Skylab period is examined. A tendency is found for holes to grow or remain stable when the activity nearby, seen as calcium plages and bright regions in X-rays, is predominantly large, long-lived regions. It is also found that there is a significantly higher number of small, short-lived active regions, as indicated by X-ray bright points, in the vicinity of decaying holes than there is near other holes. This is interpreted to mean that holes disappear at least in part because they become filled with many small scale, magnetically closed, X-ray emitting features. This interpretation, together with the observation that the number of X-ray bright points was much larger near solar minimum than it was during the Skylab period, provides a possible explanation for the disappearance of the large, near-equatorial coronal holes at the time of solar minimum.
The positions of X-ray coronal transients outside of active regions observed during Skylab were superposed on H-alpha synoptic charts and coronal hole boundaries for seven solar rotations. A detailed spatial association between the transients and neutral lines was confirmed. It was found that most of the transients were related to large-scale changes in coronal hole area and tended to occur on the borders of evolving equatorial holes.
A 90 s time resolution study of the soft X-ray emission from three active region loops shows the emission to be constant to about two percent over the half hour period of observation. Soft X-ray observations in two wavebands are used to deduce the temperature and density of these loops. The data unambiguously demonstrate that energy is supplied to each loop during the observations. If heating is due to discrete events, the time interval between events is shown to be less than 10 min, which is short relative to the radiative cooling time of the loops.
The evolution of coronal hole boundary positions on a time scale of approximately 1 day is studied on the basis of an examination of all coronal holes observed by Skylab from May to November 1973. It is found that a substantial fraction (an average of 38%) of all coronal hole boundaries shifted by at least 1 deg heliocentric in the course of a day. Most (70%) of these changes were on a relatively small scale (less than 3 times the supergranulation cell size), but a significant fraction occurred as discrete events on a much larger scale. The large-scale shifts in the boundary locations involved changes in X-ray emission from these areas of the sun. There were generally more changes in the boundaries of the most rapidly evolving holes, but no simple relationship between the amount of change and the rate of hole growth or decay.
Sudden shifts in coronal-hole boundaries observed by the S-054 X-ray telescope on Skylab between May and November, 1973, within 1 day of CMP of the holes, at latitudes not exceeding 40 deg, are compared with the long-term evolution of coronal-hole area. It is found that large-scale shifts in boundary locations can account for most if not all of the evolution of coronal holes. The temporal and spatial scales of these large-scale changes imply that they are the results of a physical process occurring in the corona. It is concluded that coronal holes evolve by magnetic-field lines' opening when the holes are growing, and by fields' closing as the holes shrink.
Soft X-ray images and magnetograms of several active regions and coronal holes are examined which support the interpretation that some of the dark X-ray gaps seen between interconnecting loops and inner cores of active regions are foot points of open field lines inside the active regions. Characteristics of the investigated dark gaps are summarized. All the active regions with dark X-ray gaps at the proper place and with the correct polarity predicted by global potential extrapolation of photospheric magnetic fields are shown to be old active regions, indicating that field opening is accomplished only in a late phase of active-region development. It is noted that some of the observed dark gaps probably have nothing in common with open fields, but are either due to the decreased temperature in low-lying portions of interconnecting loops or are the roots of higher and less dense or cooler loops.
Using soft X-ray data from the S-054 X-ray spectrographic telescope aboard Skylab, we observed temporal changes in the emission structure of the X-ray corona associated with the birth of a small coronal hole. Designated as CH6, this coronal hole was born near the equator in a time interval less than 9-1/2 hr. By constructing a light curve for a point near the center of CH6, we observed a sudden 40% decrease in X-ray emission associated with the birth of this coronal hole. On a time scale of hours, the growth of CH6 in area proceeded faster than the average rate predicted by the diffusion of solar fields. The short term decay of CH6 followed the diffusive rate to within experimental uncertainty. On a time scale of one rotation, the subsequent development of CH6 was not consistent with steady growth at the average rate predicted by diffusion.
The paper examines plasma and magnetic-field changes occurring across 1359 directional discontinuities taken from interplanetary data spanning almost four solar rotations. The plasma field characteristics of these events exhibit a distinct variation with large-scale solar-wind velocity. At low velocities, tangential discontinuities appear to predominate. At higher velocities, a substantial and increasing fraction of directional discontinuities exhibits the plasma field properties expected of outwardly propagating rotational discontinuities. The results of Sari (1972, 1975) and of the present study suggest that in the calculation of propagation diffusion coefficients for low-energy cosmic rays, the effects of directional discontinuities should be subtracted from the magnetic fluctuation spectrum during relatively quiet wind conditions. It is not clear that such subtraction is necessary during more disturbed periods.
The paper examines the properties of microscale magnetic field fluctuations for diverse classes of large-scale azimuthal velocity structures in the solar wind, with particular reference to the orientation of the wave normal vector for MHD waves in the interplanetary medium. It is shown that the direction of minimum variance does not exhibit any characteristic refraction pattern with respect to large-scale velocity structures in the solar wind. It is suggested that the direction of minimum variance is not indicative of the wave normal vector directions. The determination of the wave normal vector directions for the predominantly transverse MHD waves in the interplanetary medium is presently beyond the capability of single spacecraft observations.
The properties of directional discontinuities occurring in selected periods of interplanetary magnetic field data are investigated. Microscale fluctuations in these selected periods have previously been identified as being predominantly Alfvenic on time scales ranging from 5 min to a few hours. We argue that the majority of directional discontinuities in such periods are rotational and outwardly propagating.
Investigation of the properties of low-frequency transverse waves in an expanding plasma. The wave vector, the background magnetic field, and the streaming velocity of the plasma are all assumed to lie along the radial direction. Expressions are presented for the radial dependence of the amplitude and phase of left and right circularly polarized waves, correct to first order in the wave frequency divided by the proton cyclotron frequency. Differences in the phase velocities of these two circular polarizations result in a Faraday rotation that can be substantial for typical interplanetary conditions near 1 AU. The implications of these results for a realistic solar wind model with the interplanetary magnetic field along the spiral direction are also considered.