The effect of an interaction of magnetic flux and supergranulation on the decay of magnetic plages

This paper studies how the properties of large-scale convection affect the decay of plages. The plage decay, caused by the random-walk dispersion of flux tubes, is suggested to be severely affected by differences between the mean size of cellular openings within and around plages. The smaller cell size within a plage largely explains the smaller diffusion coefficient within plages as compared to that of the surrounding regions. Moreover, the exchange of flux tubes between the inner regions of the plage and the surrounding network is suggested to be modified by this difference in cell size, and the concept of a partially transmitting plage periphery is introduced: this periphery preferentially turns back flux parcels that are moving out of the plage and preferentially lets through flux parcels that are moving into the plage, thus confining the flux tubes to within the plage. This semi-permeability of the plage periphery, together with the dependence of the diffusion coefficient on the flux-tube density, can explain the observed slow decay of plages (predicting a typical life time of about a month for a medium-sized plage), the existence of a well-defined plage periphery, and the observed characteristic mean magnetic flux density of about 100 G. One effect of the slowed decay of the plage by the semi-permeability of the plage periphery is the increase of the fraction of the magnetic flux that can cancel with flux of the opposite polarity along the neutral line to as much as 80%, as compared to at most 50% in the case of non-uniform diffusion. This may explain why only a small fraction of the magnetic flux is observed to escape from the plage into the surrounding network.     

Vertical structure of plages

A new spectroscopic method of deriving the three-dimensional information (escalation) worked out at the Pulkovo Observatory was applied to observations of solar faculae. The size of the investigated area was 70 × 18. Two series of spectra were obtained simultaneously in four spectral regions (H, D_1,2, (Nai), H, and H, K (Caii)) with a quadruple-camera spectrograph attached to the second horizontal solar telescope at Pulkovo on 13 August, 1968.It was found from the analysis of the maps of brightness distribution in the cores of the lines and continuum that the facular emission lies in ropes which incline to the solar surface.     

K-coronal enhancements and chromospheric plages

A systematic investigation was made of the K-corona immediately overlying the positions of the brightest and most isolated chromospheric plages during the years 1964–1967. In all cases, the corona was found to be enhanced with peak brightness proportional to the plage area. In the absence of plages, the K-coronal brightness remained at a quiet level which was essentially the same thoughout this part of the solar cycle.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Turbulence heating of plages

We consider the heating of the solar plages to be due to the Alfven waves having ponderomotive force. Assuming a certain field configuration, we derived the formula for the turbulence heating of plages by Alfven waves. Our calculated plage temperature distribution is in agreement with the observed distribution.     

Physical properties of solar chromospheric plages

We propose chromospheric models of plages to explain profiles of the Ca ii H, K, λl8498, λ8542, and λ8662 lines described in Paper I. These models are consistent with boundary conditions imposed by the photosphere and the Lyman continuum. We find that increasing emission in these lines is consistent with a picture of increasing temperature gradient in the low chromosphere and the resulting increase in pressure and electron density at similar line optical depths. With this picture we suggest how to empirically determine the distribution of chromospheric parameters across the solar disk directly from Ca ii filtergrams. We also propose that the high density aspects of solar activity are produced by steep temperature gradients in the low chromosphere and thus by the enhanced heating mechanisms that steepen these gradients.     

Physical properties of solar chromospheric plages

Double pass photoelectric observations are presented of five Caii lines (H, K, 8498 Å, 8542 Å, and 8662 Å) in a number of solar plages of different degrees of activity, quiet regions, and a sunspot. The data are compared with previous work. All five lines show increasing emission together in plages and the least opaque of the infrared triplet lines appears to exhibit core emission prior to the more opaque members of the multiplet. The question of source function equality is considered and the differences and similarities among plage profiles and between plage and quiet profiles are shown qualitatively and quantitatively.Staff Member, Laboratory Astrophysics Division, National Bureau of Standards.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy. Inc., under contract with the National Science Foundation.     

A model of the supergranulation network and of active-region plages

Analysis of magnetograph recordings made simultaneously in different spectral lines have shown that the quiet-region network and active-region plages with average field strengths less than about 100 G are made up by the same type of elementary structures, each having the same physical properties. Magnetograph data are used together with continuum, line profile, and EUV data to derive a model of these subarcsec, spatially unresolved elementary structures. The field strength at the center of each basic element is about 2 kG. The temperature enhancement starts at a height of about 180 km (above the level ₀ = 1 in HSRA), and increases rapidly with height. The brightness structures are coarser than the magnetic-field structures.The magnetic field cannot be contained by either gas pressure or dynamic pressure. The magnetic pressure must be balanced by the constricting force of strong electric currents along the magnetic filaments (pinch effect). A mechanism is proposed for the amplification of the field, involving vortex motions around the downdrafts in the network and plages. Efficient heating by hydromagnetic waves builds up an excess gas pressure inside the twisted fluxropes. The excess pressure is released by the ejection of spicules, which have to move out along the helically shaped field lines and thereby will acquire a spinning motion.The continuum emission in the fluxropes, which are located in the intergranular lanes, washes out the contrast between cell interiors and cell boundaries and creates an abnormal granulation pattern. When more and more magnetic flux is brought into a given area, the interaction between the fluxropes and the granulation starts to change the physical structure of the fluxropes. This begins at an average field B _obs 100 G, with a gradual transition to pores and sunspots as b _obs is increased.     

Power spectra of velocity fluctuations in plages

Solar Physics - Spatial power spectral densities of velocities in plages were compared with velocity power spectra of the quiet photosphere. The comparison suggests that photospheric oscillations...     

Physical properties of solar chromospheric plages

We compute a new grid of plage models to determine the difference in temperature versus mass column density structure T(m) between plage regions and the quiet solar chromosphere, and to test whether the solar chromosphere is geometrically thinner in plages. We compare partial redistribution calculations of Mg ii h and k and Ca ii K to NRL Skylab observations of Mg ii h and k in six active regions and Ca ii K intensities obtained from spectroheliograms taken at approximately the same time as the Mg ii observations. We find that the plage observations are better matched by models with linear (in log m) temperature distributions and larger values of m ₀ (the mass column density at the 8000 K layer in the chromosphere), than by models with larger low chromosphere temperature gradients but values of m ₀ similar to the quiet Sun. Our derived temperature structures are in agreement with the grid originally proposed by Shine and Linsky, but our analysis is in contrast to the study by Kelch which implies that stellar chromospheric geometrical thickness is not affected by chromospheric activity. We conclude that either the stellar Mg ii observations upon which the Kelch study was based are of poorer quality than had been assumed, or that the spatial averaging of inhomogeneous structures, which is inherent in the stellar data, does not lead to a best fit one-component model similar in detail to that of a stellar or a solar plage.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Staff member, Quantum Physics Division, National Bureau of Standards.     

The growth and decay of sunspot groups

Digitized Mount Wilson sunspot data from 1917 to 1985 are analyzed to examine the growth and decay rates of sunspot group umbral areas. These rates are distributed roughly symmetrically about a median rate of decay of a few hemisphere day^-1. Percentage area change rates average 502% day^-1 for growing groups and -45% day^-1 for decaying groups. These values are significantly higher than the comparable rates for plage magnetic fields because spot groups have shorter lifetimes than do plages. The distribution of percentage decay rates also differs from that of plage magnetic fields. Small spot groups grow at faster rates on average than they decay, and large spot groups decay on average at faster rates than they grow. Near solar minimum there is a marked decrease in daily percentage spot area growth rates. This decrease is not related to group area, nor is it due to latitude effects. Sunspot groups with rotation rates close to the average (for each latitude) have markedly slower average rates of daily group growth and decay than do those groups with rotation rates faster or slower than the average. Similarly, sunspot groups with latitude drift rates near zero have markedly slower average rates of daily group growth and decay than do groups with significant latitude drifts in either direction. Both of these findings are similar to results for plage magnetic fields. These various correlations are discussed in the light of our views of the connection of the magnetic fields of spot groups to subsurface magnetic flux tubes. It is suggested that a factor in the rates of growth or decay of spot groups and plages may be the inclination angle to the vertical of the magnetic fields of the spots or plages. Larger inclination angles may result in faster growth and decay rates.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

Some characteristics of the development and decay of active region magnetic flux

Mount Wilson synoptic data of both plages and sunspots are examined in an effort to determine in some detail the manner of the appearance and disappearance of the magnetic flux of active regions at the solar surface. Separating regions into leading and following portions by magnetic polarity in the case of the plages and by position in the case of sunspots (for which there is no magnetic information available in this data set), various characteristics of these features are studied, namely their rotation, their relative longitudinal motions, and the east-west inclinations of their magnetic fields. The evidence, taken together, suggests that the magnetic flux loops which comprise a region rise to the surface at the time of its formation, and (at least some of them) sink back below the surface at the time of the decay of the region. It is likely that not all the magnetic flux that arises sinks again below the surface.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

Solar cycle variations in the growth and decay of sunspot groups

We analysed the combined Greenwich (1874–1976) and Solar Optical Observatories Network (1977–2011) data on sunspot groups. The daily rate of change of the area of a spot group is computed using the differences between the epochs of the spot group observation on any two consecutive days during its life-time and between the corrected whole spot areas of the spot group at these epochs. Positive/negative value of the daily rate of change of the area of a spot group represents the growth/decay rate of the spot group. We found that the total amounts of growth and decay of spot groups whose life times ≥2 days in a given time interval (say one-year) well correlate to the amount of activity in the same interval. We have also found that there exists a reasonably good correlation and an approximate linear relationship between the logarithmic values of the decay rate and area of the spot group at the first day of the corresponding consecutive days, largely suggesting that a large/small area (magnetic flux) decreases in a faster/slower rate. There exists a long-term variation (about 90-year) in the slope of the linear relationship. The solar cycle variation in the decay of spot groups may have a strong relationship with the corresponding variations in solar energetic phenomena such as solar flare activity. The decay of spot groups may also substantially contribute to the coherence relationship between the total solar irradiance and the solar activity variations.     

Analysis of the magnetic structure of hale region 16398 and turbulent heating of plages

We use the Zirin-Frazier-McIntosh rules to interpret the plages and flares of Region 16398 (1979 November) as shown on H_α filtergrams taken at the Purple Mountain Observatory. The coupled equations of wave and flow fields are used to explain the vertical heating of plages. We think the observed long thin filament is the result of turbulent heating along a horizontal magnetic tube.     

The rotation of calcium plages in the years 1967–1970

An analysis of Ca II spectroheliograms obtained at Catania Observatory in the period 1967–1970 shows that plages rotate - in their first four days of lifetime - with a latitudinal differential rotation profile steeper than for older objects.A closer inspection reveals that plages slow down with age in some latitude strips, and accelerate in some others. That makes the profiles of old and young objects highly different from each other. In particular, the empirical laws of solar rotation usually adopted, appear to be inadequate to describe so complex a phenomenon as the rotation of these magnetic features. A close correspondence has been found between the latitudes at which plages accelerate (slow down) and the latitudes of westward (eastward) streams (Howard and LaBonte, 1980).Some implications of these results, in the light of the theory of a direct coupling of magnetic features to deep layers, are discussed.In the years concerned, Ca-plages of whatever age show differential rotation profiles steeper than in other phases of the solar cycle.     

Instrumental factors affecting the fringe contrast in optical interferometers

Interference fringes are the prime observable quantity in astronomical optical and radio interferometers. To maximize the signal-to-noise ratio it is important to keep instrumental causes of fringe contrast decrease to a minimum. I discuss these and derive the tolerances to be, placed on them.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Some particularities affecting the movements of Jupiter's red spot

The fluctuations in longitude of Jupiter's Red Spot are discussed. The long term fluctuations show behaviour similar to the fluctuations of zonal circulations on the Earth from 1830–1950. The three-monthly fluctuations have a temporal connection with the inferior conjunction of Mercury 1963–1971. Solar activity may be the key to both phenomena.     

The influence of sunspot canopies on magnetic inclination measurements in solar plages

Sunspots are known to have large, low-lying magnetic canopies, i.e. horizontal magnetic fields overlying a field-free medium, that cover substantial fractions of active region plage. In this paper we consider the influence of such canopies on the inclination of plage magnetic fields. We find that for observations in spectral lines like 5250.2Å the neglect of a sunspot canopy when determining magnetic inclination angles of plage fields can introduce errors exceeding 5–10°. This is particularly true if the observations do not have high spatial resolution. Thus this effect may explain some of the measurements of substantially inclined fields in solar plages. Furthermore we find that the Fe I 15648 Å line is far superior in giving correct flux-tube inclinations in the presence of a sunspot magnetic canopy. Finally, the inversion of full Stokes profiles is shown to produce more reliable results than results obtained by considering only ratios of individual Stokes profile parameters.     

How growth and decay of sunspot groups depend on axial tilt angles

Digitized Mount Wilson sunspot data covering the interval from 1917 to 1985 are analyzed to examine the average growth and decay rates of sunspot groups as a function of the tilt angles of the magnetic axes of the groups. It is found that in absolute terms, both growth and decay rates of groups peak at the average tilt angle of the groups (about +5°). In percentage terms these rates are a minimum near these tilt angles because average group areas are largest at the average tilt angle. The clear peaks at the average tilt angle (rather than at 0°) may be related to the structure or geometry of the subsurface flux loops that form the regions. One suggestion to explain this effect is that this is the angle that represents no twist of these subsurface flux loops. This implies, however, that these loops do not get twisted, on average, during their ascent to the surface by Coriolis forces, as has been suggested in the past. The average percentage growth rates for groups with negative tilt angles show high average values and large dispersions for certain tilt angle intervals, suggesting slower growth rates, for some unknown reason, for many small spot groups in certain tilt angle ranges.