Local Helioseismology & Holography


A cut-away of the interior of the Sun, showing the wave fronts of
acoustic radiation used to image active regions on the far surface.
Click on the picture for a high-resolution version, or click here
for a QuickTime animation.

Local Helioseismology, a term first appearing in print in 1993, is a relatively new field in solar physics which uses ambient sound waves (called ``p modes'') propagating throughout the interior of the Sun to probe local inhomogeneities in the solar interior or on its surface. This field complements the traditional approach of what we may now term ``global'' helioseismology, whose principal goal has been the measurement, identification, and modeling of the resonant oscillation modes of the Sun (especially their frequencies) to deduce the global properties of the solar interior. Since the discovery in 1987 that sunspots are strong absorbers of incident p modes, Doug Braun and his collaborators have been designing and applying new tools in local helioseismology to explore the acoustic properties of solar magnetic regions and other small-scale perturbations. Although sunspots and other magnetic regions are in many ways the most obvious local structures on the Sun, there are other types of perturbations which local helioseismology is ideally suited to explore, such as the solar meridional flow.

An ongoing collaboration between Braun and Charles Lindsey has lead to the successful development and application of Helioseismic Holography, first discussed over a decade ago. Helioseismic Holography is a general formalism for producing diffraction-limited seismic images of the solar interior. Diagnostics of acoustic power holography include the detection of sources and sinks of acoustic waves on the Sun, such as the absorption inside sunspots and the high-frequency emission (acoustic glories) surrounding complex active regions. This figure shows helioseismic images of the active region complex NOAA 8179 obtained for the 24 hour period 1998 March 16. The figure shown is reduced in resolution for bandwidth. Click the image for a high-resolution version. Frame a shows a concurrent SOHO SOI-MDI magnetogram. Frames b, c, and d show 5 mHz helioseismic images of the regions with depths at 0, 11.2 and 19.5 Mm below the solar surface, respectively. Sunspots and their immediate surrounding show up dark in these seismic images, indicating that up to half of all incident acoustic radiation is absorbed. The acoustic glory is quite conspicuous in frame b as a bright halo of excess 5 mHz emission surrounding the entire active region complex. (figure from Braun and Lindsey 1999) Phase-sensitive procedures are also being applied to solar data extend the utility of holography to probe the thermal, magnetic and Doppler properties of active regions and their surroundings.

Helioseismic holography has also been used to image active regions on the far surface of the Sun (the side facing away from the Earth). The figure appearing at the top of this page shows acoustic wave fronts traveling between a point source on the far (left) side and the near (right) side where they are observed. These observed waves are propagated backwards in time (computationally) through a model of the Sun to reconstruct the wave amplitudes on the far-side, thereby constructing an image of the acoustic perturbations (mainly surface magnetic fields) at the far surface. With this procedure, we now have the ability to predict the appearance of solar active regions up to a week or more before they rotate around to the front side. This has important implications for space weather forecasting. Large active regions are often the sites of solar flares and coronal mass ejections which can disrupt communications and electrical power transmission on Earth, and endanger the health of astronauts working in space. Thus an early-warning system to detect the presence of these regions well in advance of their appearance on the near side of the Sun has significant practical utility.

For a list of relevant publications see Doug Braun's homepage

Also see the following pages for more information:

Far-side Imaging of the Sun

Holography of active region NOAA 9636: A comparison of SOI-MDI and GONG+ observations.

Acknowledgements

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.