Dean-Yi Chou

Solar Cycle Variations of Subsurface Meridional Flows in the Sun

We study the subsurface meridional flow in the Sun as a function of latitude and depth in the period from 1994 to 2000, covering the solar minimum in 1996 and the maximum in 2000, with the technique of time-distance helioseismology. It is found that the velocity of meridional flow increased when solar activity decreased from 1994 to 1997. As solar activity increased from 1997 to 2000, a new component of meridional flow, centered at about 20° latitude, was created in each hemisphere. It moves away from its center. The new flow changes the observed meridional flow from poleward at solar minimum to equatorward at solar maximum at low latitudes. The velocity of the new component increases with depth at least down to a depth of 70,000 km.

Ambient acoustic imaging in helioseismology

The increasing availability of high spatial resolution data of velocity and intensity variations on the Sun has stimulated the development of helioseismological techniques that probe the solar interior in localized regions. The techniques developed so far have yielded information on physical quantities (such as the flow velocity and magnetic field) below the surface, but are still far from providing a detailed picture of local subsurface inhomogeneities. Here we report the development and application of a new method for constructing three-dimensional solar images, utilizing acoustic noise (or stochastic P-mode oscillations) in the Sun. We treat a region of the solar surface as a phased array of acoustic sensors, which acts as a computational ‘lens’; acoustic waves ‘scattered’ by local inhomogeneities, such as sunspots, are collected and summed in phase, based on the knowledge of how (on average) they travel within the Sun. In this way, we are able to construct a three-dimensional image of a region of the solar interior.

Taiwan Oscillation Network

The Taiwan Oscillation Network (TON) is a ground-based network to measure solar intensity oscillations to study the internal structure of the Sun. K-line full-disk images of 1000 pixels diameter are taken at a rate of one image per minute. Such data would provide information onp-modes withl as high as 1000. The TON will consist of six identical telescope systems at proper longitudes around the world. Three telescope systems have been installed at Teide Observatory (Tenerife), Huairou Solar Observing Station (near Beijing), and Big Bear Solar Observatory (California). The telescopes at these three sites have been taking data simultaneously since October of 1994. Anl – v diagram derived from 512 images is included to show the quality of the data.

Dynamics of anchored flux tubes in the convection zone. I - Details of the model

A model for the emergence of buoyant segments of magnetic flux tubes whose ends are still anchored in the stable layers below the convection zone is developed. This model is based on the thin flux tube approximation of Spruit. Several specific examples of buoyant flux tube evolution computed with the model are described. In particular, attention is given to an example of a flux tube which results in upflows as it emerges, in contrast to the downflows generally believed to accompany the Parker instability. 49 refs.

The vertical structure of arch filament systems in solar emerging flux regions

The paper studies the vertical velocity of arch filaments in emerging flux regions by measuring their proper motion near the limb in H-alpha filtergrams and their radial velocity near the disk center in H-alpha spectrograms. Both show that arch filaments rise at a velocity between 10 and 15 km/s. The possible mechanisms producing this high velocity are discussed. None of them is satifactory, but it appears that the unloading of the arches by outflow must play a role. From the strength of H-alpha emission and D3 absorption, the temperature of arch filaments is estimated at 6000-15,000 K and the density at 10 to the 11th-12th/cu cm.

EVOLUTION OF SOLAR SUBSURFACE MERIDIONAL FLOWS IN THE DECLINING PHASE OF CYCLE 23

We study the evolution of meridional flows in the solar convection zone extending to a depth of 0.793 R☉ in the period 2000-2003 with helioseismic data taken with the Taiwan Oscillation Network (TON) using the technique of time-distance helioseismology. The meridional flows of each hemisphere formed a single-cell pattern in the convection zone at the solar minimum. An additional divergent flow was created at active latitudes in both hemispheres as the activity developed. The amplitude of this divergent flow correlates with the sunspot number: it increased from solar minimum to maximum (from 1996 to 2000), and then decreased from 2000 to 2003 with the sunspot number. The amplitude of the divergent flow increases with depth from 0.987 R☉ to a depth of about 0.9 R☉, and then decreases with depth at least down to 0.793 R☉.

Probing the subsurface structure of active regions with the phase information in acoustic imaging

We present the phase information of solar p-mode waves constructed with an acoustic imaging technique in the solar interior. There exists a phase shift between the time series constructed with ingoing waves and outgoing waves. We find that this phase shift is different in an active region and the quiet Sun. The p-mode travel time is shorter in the magnetic regions than in the quiet Sun. We construct a three-dimensional phase shift map of the solar interior. As with the acoustic absorption images, the phase shift features of the active region in maps at the surface correlate with magnetic fields. The vertical extension of phase shift features in the active region is smaller in the phase maps constructed with shorter wavelengths. This indicates the vertical spatial resolution of these three-dimensional phase maps is sensitive to the range of modes used in constructing the signal. The actual depths of the phase shift features in the active region may be smaller than those shown in the three-dimensional phase maps.

Acoustic Imaging in Helioseismology

The time-variant acoustic signal at a point in the solar interior can be constructed from observations at the surface, based on the knowledge of how acoustic waves travel in the Sun: the time-distance relation of the p-modes. The basic principle and properties of this imaging technique are discussed in detail. The helioseismic data used in this study were taken with the Taiwan Oscillation Network (TON). The time series of observed acoustic signals on the solar surface is treated as a phased array. The time-distance relation provides the phase information among the phased array elements. The signal at any location at any time can be reconstructed by summing the observed signal at array elements in phase and with a proper normalization. The time series of the constructed acoustic signal contains information on frequency, phase, and intensity. We use the constructed intensity to obtain three-dimensional acoustic absorption images. The features in the absorption images correlate with the magnetic field in the active region. The vertical extension of absorption features in the active region is smaller in images constructed with shorter wavelengths. This indicates that the vertical resolution of the three-dimensional images depends on the range of modes used in constructing the signal. The actual depths of the absorption features in the active region may be smaller than those shown in the three-dimensional images.

The separation velocity of emerging magnetic flux

We measure the separation velocity of opposite poles from 24 new bipoles on the Sun. We find that the measured velocities range from about 0.2 to 1 km s−1. The fluxes of the bipoles range over more than two orders of magnitude, and the mean field strength and the sizes range over one order of magnitude. The measured separation velocity is not correlated with the flux and the mean field strength of the bipole. The separation velocity predicted by the present theory of magnetic buoyancy is between 7.4Ba−1/4 cot θ and 13 cot θ km s−1, where θ is the elevation angle of the flux tube at the photosphere (see Figure 9), B is the mean field strength, and a is the radius of the observed bipole. The rising velocity of the top of flux tubes predicted by the theory of magnetic buoyancy is between 3.7Ba−1/4 and 6.5 km s−1. The predicted separation velocity is about one order of magnitude higher than those measured, or else the flux tubes are almost vertical at the photosphere. There is no correlation between the measured separation velocity and the theoretical value, 7.4Ba−1/4. The predicted rising velocity is also higher than the vertical velocity near the line of inversion in emerging flux regions observed by other authors.

IN SEARCH OF THE SOLAR CYCLE VARIATIONS OF p-MODE FREQUENCIES GENERATED BY PERTURBATIONS IN THE SOLAR INTERIOR

Observations show that the solar p-mode frequencies change with the solar cycle. The horizontal-phase-velocity dependence of the relative frequency change, scaled by mode mass, provides depth information on the perturbation in the solar interior. We find that the smoothed scaled relative frequency change varies along the solar cycle for horizontal phase velocities higher than a critical value, which corresponds to a depth near the base of the convection zone. This phenomenon suggests that the physical conditions in a region near the base of the convection zone change with the solar cycle.Observations show that the solar p-mode frequencies change with the solar cycle. The horizontal phase velocity dependence of the relative frequency change, scaled by mode mass, provides depth information on the perturbation in the solar interior. Our model study suggests that variations of smoothed, scaled relative frequency change versus horizontal phase velocity might be able to detect the weak signals generated by the perturbation near the base of the convection zone. We find that the smoothed scaled relative frequency change, derived from SOHO MDI data, might show the evidence of solar cycle variations for horizontal phase velocities higher than a critical value, which corresponds to a depth near the base of the convection zone. The magnitude of temporal variations approximately correlates with solar activity. If these temporal variations are real signals, it suggests that the wave speed in a region near the base of the convection zone changes with the solar cycle. This change might be related to solar cycle variations of magnetic fields near the base of the convection zone.