Frank Hill

Variability in the power spectrum of solar five-minute oscillations

Two-dimensional power spectra of solar five-minute oscillations display prominent ridge structures in (k, ω) space, where k is the horizontal wavenumber and ω is the temporal frequency. The positions of these ridges in k and ω can be used to probe temperature and velocity structures in the subphotosphere. We have been carrying out a continuing program of observations of five-minute oscillations with the diode array instrument on the vacuum tower telescope at Sacramento Peak Observatory (SPO). We have sought to establish whether power spectra taken on separate days show shifts in ridge locations; these may arise from different velocity and temperature patterns having been brought into our sampling region by solar rotation. Power spectra have been obtained for six days of observations of Doppler velocities using the Mg i λ 5173 and Fe i ω 5434 spectral lines. Each data set covers 8 to 11 hr in time and samples a region 256″ × 1024″ in spatial extent, with a spatial resolution of 2″ and temporal sampling of 65 s. We have detected shifts in ridge locations between certain data sets which are statistically significant. The character of these displacements when analyzed in terms of eastward and westward propagating waves implies that changes have occurred in both temperature and horizontal velocity fields underlying our observing window. We estimate the magnitude of the velocity changes to be on the order of 100 m s-1; we may be detecting the effects of large-scale convection akin to giant cells.

Response of the Solar Five-Minute Oscillations to a Major Flare

Solar five-minute oscillations of intermediate-degree l were observed both before and after a very strong white-light flare. Intensity images of the full Sun taken on two sides of the Fe I λ 5576 spectral line were recorded on film, digitized with 8 spatial resolution, and then converted into Doppler velocities. The data were projected onto both equatorial and polar sectoral modes and Fourier transformed in time. Comparing the resulting power spectra, we find a substantial increase in power in the ps ridge of the equatorial modes on the day after the flare; such an increase may be a consequence of the solar flare. When data from all the ridges are considered, there is an average increase in power of only a few percent the day after the flare. This overall increase is probably not significant due to uncertainties from effects of the beating of unresolved modes.

The far-side solar magnetic index

Several magnetic indices are used to model the solar irradiance and ultimately to forecast it. However, the observation of such indices are generally limited to the Earth-facing hemisphere of the Sun. Seismic maps of the far side of the Sun have proven their capability to locate and track medium-large active regions at the non-visible hemisphere. We present here the possibility of using the average signal from these seismic far-side maps as a proxy to the non-visible solar activity which can complement the current front-side solar activity indices.

Kinetic helicity of subsurface flows and magnetic flux

We study the relation between the vorticty of solar subsurface flows and surface magnetic activity, analyzing more than five years of GONG+ data with ring-diagram analysis. We focus on the enstrophy, defined as the square of vorticity, and the kinetic helicity density, defined as the scalar product of velocity and vorticity, and derive them from the surface to a depth of about 16 Mm. We find that enstrophy and helicity density of subsurface flows are rather constant at low flux values (less than about 10 G), while at higher flux values there is a linear relation between flux and the logarithm of enstrophy or unsigned helicity. In addition, we analyze the temporal variation of thirteen emerging active regions. At the locations of these active regions, there is little enstrophy or helicity before the regions emerge, while after flux emergence the vorticity and helicity values are large. The crosscorrelation in time between flux and enstrophy shows that they are correlated and that shallow layers lag behind deeper layers. This signal might be a hint of the emergence of active regions.

Variation of high-degree mode frequencies during the declining phase of solar cycle 23

We investigate the spatial and temporal variation of the high-degree mode frequencies during the declining phase of the solar cycle 23 and the extended minimum between the cycle 23 and 24. We find that the frequency shifts of high-degree modes obtained through the ring-diagram analysis in different phases of the solar cycle are not equally correlated with the local magnetic activity index.

Meridional Flow Observations: Implications for the current Flux Transport Models

Meridional circulation has become a key element in the solar dynamo flux transport models. Available helioseismic observations from several instruments, Taiwan Oscillation Network (TON), Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI), have made possible a continuous monitoring of the solar meridional flow in the subphotospheric layers for the last solar cycle, including the recent extended minimum. Here we review some of the meridional circulation observations using local helioseismology techniques and relate them to magnetic flux transport models.

Solar oscillations in cycle 24 ascending

Solar oscillation frequencies are known to follow the trend of solar cycle and show a strong correlation with various activity indices. However, the extended minimum between cycles 23 and 24 has raised several questions on the correlation between frequencies and solar activity where frequencies with different mode sets sensed different minima. In this paper, we analyze intermediate-degree mode frequencies as the Sun emerges from the unusually long period of minimal magnetic activity to study their behaviour with activity indices and compare results with the corresponding phase of cycle 23. We show that a model based on the rising phase of cycle 23 is a good predictor for behaviour in the rising phase of cycle 24.

Using SONG to probe rapid variability and evolution of starspots

The Stellar Observations Network Group (SONG) is being developed as a network of 1-meter spectroscopic telescopes designed for and primarily dedicated to asteroseismology. It is patterned after the highly successful GONG project. The Danish prototype telescope will be installed in Tenerife in early 2011. Ultimately we hope to have as many as 8 identical nodes providing continuous high-resolution spectroscopic observations for targets anywhere in the sky. The primary scientific goals of SONG are asteroseismology and the search for Earth-mass exoplanets. The spectroscopic requirements for these programs push the limits of current technology, but the resulting spectrograph design will enable many secondary science programs with less stringent requirements. Doppler imaging of starspots can be accomplished using continuous observations over several stellar rotations using identical instrumentation at each node. It should be possible to observe the evolution of starspot morphology in real-time, for example. We discuss the design and status of the SONG project in general, and we describe how SONG could be used to probe short timescale changes in stellar surface structure.

Latitudinal distribution of travel times in the upper solar convection zone

We applied the time-distance technique to GONG+ data in 2001 (at solar maximum) and 2006 (at solar minimum) to study the influence of surface activity on the latitudinal distribution of travel times of acoustic waves in the upper solar convection zone. We find that surface activity is the dominant source of travel time differences over the solar cycle. Removal of the surface activity with a masking method reveals a residual travel-time shift of 0.5 sec, corresponding to a surface temperature change of 0.25° K over the solar cycle.

Effects of surface magnetic activity on meridional circulation measurements

Temporal variations of the subsurface meridional flow with the solar cycle have been reported by several authors. This work examines the possible contamination of the helioseismic measurements by the extra velocity fields associated with active regions as well as the uncertainties in the data obtained where strong magnetic fields are present. We compare meridional flows obtained by both ring-diagram and time-distance analysis before and after removing the areas of strong magnetic field. The preliminary results suggest that a careful examination of the contribution of magnetic regions to the longitude averaged meridional flow is required.