This page shows holographic analyses of the active region illustrated below, for both GONG+ and SOI-MDI observations spanning the a 24 hour interval starting 2001 September 29. The SOI-MDI observations were taken in the full-disk mode. The GONG+ images were obtained at the Big Bear, Learmonth, El Teide, and Cerro Tololo sites and have approximately 97% temporal coverage. The "continuum" image and magnetograms below are obtained from SOI-MDI. The images are Postel's projections.
The GONG+ and MDI full-disk images were interpolated to form Postel's
projections spanning
approximately 60 degrees (730 Mm), and tracked at the Carrington
rotation rate.
The figure below (click for higher resolution version)
shows a comparison of the power
spectra, integrated over temporal frequencies,
between 2 and 6 mHz.
Of primary importance for local helioseismology is
the sensitive of the instruments to p-modes of high wavenumber
(or spherical harmonic degree l). The fall-off of the power with l
observed in the GONG+ images, relative to that of MDI, clearly reflects
the effects of atmospheric seeing and attenuation.
We use p-modes of longer wavelengths (or smaller values of l) to form holographic images at increasing depth. The figure below illustrates how the optimal value of l (y-axis) decreases with depth (x-axis). The scale on the right side shows the horizontal wavelength of the mode, which, due to diffraction effects, indicates approximately the best horizontal resolution possible. The three different colors show the wavenumber for three different temporal frequencies. Note that higher spatial resolution may be achieved by considering p-modes with high temporal frequencies. The effects of atmospheric seeing and attenuation may be expected to affect holographic images constructed near the solar surface (i.e. at the left side of the plot).
Our comparison between MDI and GONG+ shown here uses "phase-sensitive" (or phase-correlation) seismic holography, which in this case employs the temporal correlation between the egression and ingression signals with the focal plane set at varying depths below the solar surface. The analysis is actually done in the frequency domain, so that the phase of this correlation is equal to the time-lag of the peak of the correlation divided by the temporal frequency. This time-lag defines the total ``travel-time perturbation'', which should be equal to zero for the quiet Sun. Negative values of this perturbation indicate that acoustic radiation takes less time passing through the active region than compared with the quiet Sun. Click here for an animated gif version of the full comparison. Some sample frames are shown below.
We have recently extended our applications of helioseismic holography to include Doppler diagnostics of active regions and quiet Sun. A comparison of Doppler-sensitive seismic images, derived from MDI and GONG+, is shown below (click on image for high-resolution version).
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A comparison of seismic images of the solar far side from the GONG+ network and MDI is available here.
This research uses solar observations obtained primarily from the Solar Oscillations Investigations - Michelson Doppler Imager on SOHO and the Global Oscillations Network Group (GONG), and is funded by the National Science Foundation and the National Aeronautics and Space Administration.
