M. Gerassimenko

Short-term temporal variations of X-ray bright points

Skylab S-054 data have been used to examine the flux from X-ray bright points with ∼ 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 interpretation of simultaneous soft X-ray spectroscopic and imaging observations of an active region

Simultaneous soft X-ray spectroscopic and broad band imaging observations of an active region have been analyzed together to determine the parameters which describe the coronal plasma. From the spectroscopic data, models of temperature-emission measure-elemental abundance have been constructed which provide acceptable statistical fits. By folding these possible models through the imaging analysis, models which are not self-consistent can be rejected. In this way, only the oxygen, neon and iron abundances of Pottasch (1967), combined with either an isothermal or exponential temperature-emission measure model are consistent with both sets of data. Contour maps of electron temperature and density for the active region have been constructed from the imaging data. The implications of the analysis to the determination of coronal abundances and to future satellite experiments are discussed.

Coronal hole evolution by sudden large scale changes

We have compared 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 ≤ 40 °, with the long-term evolution of coronal hole area. We find 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. We conclude that coronal holes evolve by magnetic field lines opening when the holes are growing, and by fields closing as the holes shrink.

The relationship between solar activity and coronal hole evolution

We examine the relationship between coronal hole evolution and solar active regions during the Skylab period. We find a tendency 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. This is consistent with results of previous studies, using somewhat different methods. We also find 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. We interpret this 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 previously reported 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.

X-ray imaging studies of Alcator-A radiation and internal disruptions

Soft-X-ray data collected with an imaging system on the Alcator-A tokamak are used to study characteristics of quasi-equilibrium plasma radiation, and a method of using soft-X-ray data to determine the electron-temperature-profile change due to internal disruptions is described. It is found that, for a series of discharges characterized by increasing plasma density, X-radiation from the plasma core becomes more like hydrogen bremsstrahlung. For the highest-density discharges studied, local-X-ray enhancement factors are approximately 1, within the central half of the plasma column. Finally, the change in the electron temperature profile due to internal disruptions which occurred during a high-density discharge is determined.

Observational evidence of continual heating in X-ray emitting coronal loops

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 quantitative interpretation of solar X-ray images

We examine the interpretation of plasma parameters derived by quantitative analysis of solar X-ray photographs obtained through broad band filters. We find some of the recent criticisms of the filter ratio method to be unfounded. Using active region and quiet Sun emission measures derived mostly from spectroscopic observations we find that effective emission measures and temperatures derived from S-054 data by filter ratio analysis are within better than 20% of the total emission measure and average temperature, respectively. The uncertainties associated with filtered flux determination are found to produce an error of about 10% in the derived effective temperature. We thus conclusively demonstrate that parameters derived from S-054 data by filter ratio analysis are representative of the observed active region and quiet Sun material.

Do changes in coronal emission structure imply magnetic reconnection

We examine three major possible interpretations of observed reconfigurations of coronal X-ray and XUV emitting structures on a scale comparable to the size of the structures themselves. One possibility is that little change in the large-scale magnetic field configuration is associated with the change in emission. The other two possibilities are processes by which the magnetic field structure can change.We demonstrate that large changes in visibility in X-rays or XUV lines can be associated with relatively minor changes in the coronal magnetic field by showing the behavior of magnetic interconnections between individual active regions in a complex of activity observed by the S-054 X-ray spectrographic telescope on Skylab. While the large-scale interconnections are continuously present for at least several days, individual loops in these structures are visible for only relatively short times (≲1 day).The two theoretical possibilities which we discuss are ‘frozen-in’ motion of the fields, and field line reconnection. We emphasize that reconnection occurs in regions much smaller than telescopic resolution. Because there are no measurements of the magnetic field in the corona in projection against the disk, existing observations are generally not sufficient to show in detail howmuch reconnection has occurred.

Comparison of the 9.1cm and soft X-ray emission from an active region

Thermal bremsstrahlung from the X-ray observed plasma accounts for most of the observed 9.1 cm emission from McMath 12336, an old, spotless active region, on June 2, 1973. This implies that only a small fraction of the emission measure within the active region is in the range around 106 K and below.

Instrument for synchronizing plasma diagnostic measurements with tokamak internal disruptions

We describe an instrument which detects internal disruptions in real time, providing one pulse at each disruption and another pulse at a selectable phase of the internal disruption cycle (as determined by the interval between the two preceding disruptions). By appropriate gating, this instrument allows diagnostics requiring integration times long compared to the internal disruption period to be used in the study of plasma parameter changes produced by internal disruptions. It also permits selection of the phase of the internal disruption cycle at which diagnostics with short sampling times are used. Several features of the instrument are discussed in light of its performance in firing the Thomson scattering laser on the Alcator‐C tokamak.