Transient brightenings of interconnecting loops

We discuss three different kinds of dynamic events related to interconnecting loops observed in soft X-rays aboard Skylab: (1) A newly born transequatorial loop that was either emerging from subphotospheric layers or gradually filled in with hot plasma. (2) Large-scale twists of interconnecting loops which never relax, and often only form, after the loop brightenings. (3) Three events where the loop that later interconnected two active regions had been visible long before one of the interconnecting regions was born. Several impacts this observation might have upon our understanding of the process of flux emergence are suggested.     

On the filamentary nature of solar magnetic fields

A method is presented for obtaining information about the unresolved filamentary structure of solar magnetic fields. A comparison is made of pairs of Mount Wilson magnetograph recordings made in the two spectral lines Fei 5250 Å and Fei 5233 Å obtained on 26 different days. Due to line weakenings and saturation in the magnetic filaments, the apparent field strengths measured in the 5250 Å line are too low, while the 5233 Å line is expected to give essentially correct results. From a comparison between the apparent field strengths and fluxes and their center to limb variations, we draw the following tentative conclusions: (a) More than 90 % of the total flux seen with a 17 by 17 arc sec magnetograph aperture is channeled through narrow filaments with very high field strengths in plages and at the boundaries of supergranular cells. (b) An upper limit for the interfilamentary field strength integrated over the same aperture seems to be about 3 G. (c) The field lines in a filament are confined in a very small region in the photosphere but spread out very rapidly higher up in the atmosphere. (d) All earlier Mount Wilson magnetograph data should be multiplied by a factor that is about 1.8 at the center of the disk and decreased toward the limb in order to give the correct value of the longitudinal magnetic field averaged over the scanning aperture.Guest Investigator at the Hale Observatories, on leave from Astronomical Observatory, Lund, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Observations of short period oscillations in two dimensions

Observations of the photospheric velocity field at the disk center with a cadence of five frames per second strongly support the idea that short period oscillations arise from a combination of image motion and horizontal gradients of the line of sight velocity field. Any genuine solar short period oscillations are effectively masked by these false short period oscillations.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation.Visiting Astronomer, Solar Division, KPNO.     

Velocity fields in the solar atmosphere

Observations of velocity fields in the solar atmosphere made with the Mount Wilson solar magnetograph are analyzed. These observations, which were made with very high velocity sensitivity, cover nearly 250 hours and were made with apertures of several sizes and at various parts of the solar disk, and in strong and weak magnetic fields. The amplitudes of the 300-sec oscillations are about 25% weaker in regions where the magnetic field is greater than 80 gauss than where the field is less than 10 gauss. No difference in the frequencies of the oscillations could be found between strong-field and field-free regions. It is suggested that the oscillations occur only where the field is absent and the lower amplitude in a strong field represents the fraction of the magnetograph aperture occupied by a magnetic field. The element sizes for the 300-sec oscillations are probably at least 5–10 arc seconds.Observations made simultaneously with two lines formed at different depths in the solar atmosphere showed small phase differences in the 5-min oscillations. The upper level showed shorter period oscillations when the lower level oscillations underwent phase changes.A short period oscillation is found superposed on the 300-sec oscillation. These SPOs come in bursts that last for a minute or two and have average amplitudes that fall in the range 0.05–0.10 km/sec peak to peak. All attempts to explain them as instrumental or seeing effects have failed. Their periods fall in the range 1–5 seconds. The horizontal scale of these oscillations is smaller than that of the 300-sec oscillations, and the SPOs are more nearly isotropic oscillations than are these around 300 seconds. They do not represent a high-frequency tail of the latter. These observations did not have a digitizing interval short enough to analyze the SPOs for power spectra, but it is clear from the tracings that they are not a nearly monochromatic oscillation as are the longer waves. The amplitudes of the SPOs in the solar atmosphere must be very large and they contribute greatly to the non-radiative energy flux. It is suggested that they represent a large microturbulence line-broadening effect.     

Bias in the estimate of seismic moment tensor by the linear inversion method

Summary. We investigate the effects of various sources of error on the estimation of the seismic moment tensor using a linear least squares inversion on surface wave complex spectra. A series of numerical experiments involving synthetic data subjected to controlled error contamination are used to demonstrate the effects. Random errors are seen to enter additively or multiplicitively into the complex spectra. We show that random additive errors due to background recording noise do not pose difficulties for recovering reliable estimates of the moment tensor. On the other hand, multiplicative errors from a variety of sources, such as focusing, multipathing, or epicentre mislocation, may lead to significant overestimation or underestimation of the tensor elements and in general cause the estimates to be less reliable.     

Large-scale photospheric magnetic field: The diffusion of active region fields

The large-scale photospheric magnetic field has been computed by allowing observed active region fields to diffuse and to be sheared by differential rotation in accordance with the Leighton (1969) magnetokinematic model of the solar cycle. The differential rotation of the computed field patterns as determined by autocorrelation curves is similar to that of the observed photospheric field, and poleward of 20° latitude both are significantly different from the differential rotation of the long-lived sunspots (Newton and Nunn, 1951) used as an input into the computations.Now at Department of Physics, Victoria University of Wellington, Wellington, New Zealand.     

Polar Plume Anatomy: Results of a Coordinated Observation

On 7 and 8 March 1996, the SOHO spacecraft and several other space- and ground-based observatories cooperated in the most comprehensive observation to date of solar polar plumes. Based on simultaneous data from five instruments, we describe the morphology of the plumes observed over the south pole of the Sun during the SOHO observing campaign. Individual plumes have been characterized from the photosphere to approximately 15 R⊙ yielding a coherent portrait of the features for more quantitative future studies. The observed plumes arise from small (∼ 2-5 arc sec diameter) quiescent, unipolar magnetic flux concentrations, on chromospheric network cell boundaries. They are denser and cooler than the surrounding coronal hole through which they extend, and are seen clearly in both Feix and Fexii emission lines, indicating an ionization temperature between 1.0–1.5 x 10⁶ K. The plumes initially expand rapidly with altitude, to a diameter of 20–30 Mm about 30 Mm off the surface. Above 1.2 R⊙ plumes are observed in white light (as ‘coronal rays’) and extend to above 12 R⊙. They grow superradially throughout their observed height, increasing their subtended solid angle (relative to disk center) by a factor of ∼10 between 1.05 R⊙ and 4–5 R⊙ and by a total factor of 20–40 between 1.05 R⊙ and 12 R⊙. On spatial scales larger than 10 arc sec, plume structure in the lower corona (R < 1.3 R⊙) is observed to be steady-state for periods of at least 24 hours; however, on spatial scales smaller than 10 arc sec, plume XUV intensities vary by 10–20% (after background subtraction) on a time scale of a few minutes. (Dr. Hassler is now employed by Southwest Research Institute, Boulder, CO)     

Some observations bearing on the problem of the short-period oscillations

Observations of solar velocity fields made simultaneously at Mount Wilson and at Kitt Peak with the same size aperture (5 arc-sec) and same position on the disk (± 1 arc-sec) are presented. The object is to clarify whether the short-period oscillations (SPO's) previously reported (Howard, 1967), could be caused by local seeing conditions. The time of onset and general character of the SPO's are found to be well correlated for the two sites, a condition that favors a solar origin. However, because correlation in complete detail did not prove possible, some doubt must remain regarding the source of the SPO's.Kitt Peak National Observatory Contribution No. 287.Operated by The Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Studies of solar magnetic fields

Magnetic flux data from the Mount Wilson magnetograph are examined over the interval 1967–1973. The total flux in the north is greater than that in the south by about 7% over this interval, reflecting a higher level of activity in the northern hemisphere. Close to 95% of the total flux is confined to latitudes equatorward of 40°, which means that close to 95% of the flux cancels with flux of opposite polarity before it can migrate poleward of 40°. It is pointed out that a consequence of this flux distribution is that ephemeral regions must make a negligible contribution to the long-term largescale magnetic flux distribution. A broad peak in the total flux may be seen centered about one year after activity maximum in the north below 40°. In the south there is a very sharp increase in flux about the same time. In the north, several poleward migrations of flux may be seen. Two of these may correspond with the two poleward prominence migrations seen by Waldmeier. In both the north and the south there is a poleward migration of negative flux about the time of activity maximum. Poleward flux drift rates are about 20 m s^?1.