Search

We have measured the average radial (cell center to network boundary) profile of the continuum intensity contrast associated with supergranular flows using data from the Precision Solar Photometric Telescope at the Mauna Loa Solar Observatory. After removing the contribution of the network flux elements by the application of masks based on Ca II K intensity and averaging over more than 105 supergranular cells, we find a [?]0.1% decrease in red and blue continuum intensity from the supergranular cell centers outward, corresponding to a [?]1.0 K decrease in brightness temperature across the cells. The radial intensity profile may be caused either by the thermal signal associated with the supergranular flows or a variation in the packing density of unresolved magnetic flux elements. These are not unambiguously distinguished by the observations, and we raise the possibility that the network magnetic fields play an active role in supergranular scale selection by enhancing the radiative cooling of the deep photosphere at the cell boundaries.

We have examined images from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa Solar Observatory (MLSO) in search of latitudinal variation in the solar photospheric intensity. Along with the expected brightening of the solar activity belts, we have found a weak enhancement of the mean continuum intensity at polar latitudes (continuum intensity enhancement ~0.1%-0.2%, corresponding to a brightness temperature enhancement of ~2.5 K). This appears to be thermal in origin and not due to a polar accumulation of weak magnetic elements, with both the continuum and Ca II K intensity distributions shifted toward higher values with little change in shape from their midlatitude distributions. Since the enhancement is of low spatial frequency and of very small amplitude, it is difficult to separate from systematic instrumental and processing errors. We provide a thorough discussion of these and conclude that the measurement captures real solar latitudinal intensity variations.

We have found evidence of an alignment of the solar supergranulation in the direction parallel to the Sun's rotation axis. Signatures of the alignment are apparent in both time-averaged images and in three-dimensional power spectra. The north-south organization is persistent in time, extending over many supergranular lifetimes. It occurs over a wide latitudinal extent, to +-60deg, and shows variation on a 10deg-30deg scale. These properties, as well as the rotation rate of the pattern, suggest a underlying larger scale dynamical cause. We examine a mechanism by which giant cell motions may contribute to such alignment.

It has recently been suggested that the solar supergranulation undergoes oscillations, with a spectrum of superposed traveling waves of unknown origin showing excess prograde power to yield superrotation. We show here that the observed supergranular spectrum does not necessarily imply a wave origin but is instead consistent with two components of nonoscillatory bulk motions having differing rotation rates and somewhat asymmetrically distributed in space. The two components are identified with solar mesogranulation and supergranulation, and the spatial asymmetry is shown to be caused by a weak north-south alignment of the supergranular flows. The source of both the supergranular alignment and its enhanced rotation is likely underlying giant cell motions. Because no single rotation rate characterizes all components of a solar image, the spectral properties, including the Fourier dispersion relation, are extremely sensitive to the rate at which the solar disk is tracked when making up the time series. A spuriously wavelike spectrum is obtained when the image tracking rate falls between the actual mesogranular and supergranular rotation rates.

Two Precision Solar Photometric Telescopes (PSPT) designed and built at the U.S. National Solar Observatory (NSO) are in operation in Rome and Hawaii. A third PSPT is now in operation the NSO at Sunspot, NM. The PSPT system records full disk solar images at three wavelengths: K line at 393.3 nm and two continua at 409 nm and 607 nm throughout the observing day. We currently study properties of limb darkening, sunspots, and network in these images with particular emphasis on data taken in July and September 1998. During this period, the number of observations per month was high enough to show directional properties of the radiation field surrounding sunspots. We show examples of our PSPT images and describe our study of bright rings around sunspots.

This paper presents a numerical study of the dynamics and stability of two-dimensional thermal plumes in a significantly stratified layer. Motivated by stellar envelope convection in which radiative cooling at the star's photosphere drives vigorous down flows, we examine cool plumes descending through an adiabatically stratified layer of increasing density with depth. Such flows are inaccessible by laboratory experiments, yet are important to the understanding of heat and momentum transport, magnetic field generation, and acoustic excitation in stars like the Sun. We find that the structure of thermal plumes in a stratified compressible medium is significantly different from that in an incompressible one, with pressure perturbations playing an important dynamical role. Additionally, we find that the plumes are subject to vigorous secondary instabilities even in a quiescent background medium. While the flows studied are not fully turbulent but transitional, the nature of the compressive instabilities and their influence on subsequent flow evolution suggests that advective detrainment of fluid from the plume region results. Simplified plume models assuming a hydrostatic pressure distribution and velocity-proportional entrainment may thus be inappropriate in this context.

We study a simplified model of solar acoustic oscillations and show how asymmetries in spectral lines depend both on the acoustic source depth, as previously recognized, and on the acoustic source type. We provide a unified description of modal line asymmetries and high-frequency pseudomode locations, suggesting an inversion on power spectra minima to determine source properties and a correction to Lorentzian line shapes based upon the relative locations of spectral peaks and valleys. We also consider nonadiabatic effects due to Newtonian cooling and demonstrate that these do not lead to notable differences between velocity and intensity power spectral line shapes. We argue more generally that it is unlikely that any nonadiabatic effect can be responsible for the observed differences. Finally, we discuss the importance of both multiplicative and additive background power to the spectra and show how additive noise can reduce the apparent line asymmetry of a mode. We note that information on solar convective motions can be potentially extracted from three components of the acoustic power spectra: the additive background yielding information on the spectrum of nonoscillatory motions at the height of observation, the multiplicative background reflecting the source spectrum, and the power minima providing the source depth and physical nature. For stochastically excited linear waves only the first of these contributes significantly to spectral differences between observed variables.