Charlie Lindsey (NSO), Doug Braun (Solar Physics Research Corporation), Stuart Jefferies (NSO), Martin Woodard (Bartol Research Institute), Yuhong Fan (NSO), Yeming Gu (NSO), and Seth Redfield (Summer Research Assistant, 1995) are using soundwaves to explore the subsurface structure of solar magnetic fields. They believe that they are seeing outflow patterns below the surface of active regions. This program grew out of the discovery by Braun et al. (1988) that sunspots absorb acoustic waves. The Lindsey-Braun team subsequently developed a technique called Doppler acoustic seismology and have applied it to the Bartol-NASA-NSO South-Pole observations of 1991.
Soundwaves traveling under the solar surface have periods of several minutes and surface wavelengths of several thousand kilometers. Because these waves penetrate to various depths beneath the solar surface, and they are advected by flows that they pass through, their surface speeds carry useful information about subsurface velocity patterns. Helioseismic observations made continuously over a period of up to 50 hours yield images of a vector field called the horizontal Doppler acoustic signature, which is known to be sensitive to horizontal flows. The figure shows horizontal Doppler acoustic signature derived from surface waves observed over a 50-hour period on 5-6 January 1991 from the South Pole. Frames a and b show K-line images of the Sun before and during the acoustic observations respectively. The large active region (NOAA 6442) just below and left of the center of Frame b is newly emerged. Frames c and e show the Doppler acoustic signatures for northward flows for relatively long waves (c), which penetrate up to 20,000 km beneath the photosphere and for shorter waves (e), which penetrate only to about 10,000 km depths. Under this interpretation, the lighter shades indicate positive northward flows, where north is in the direction of the top of the frame. The darker shades indicate southward flow.
Frames d and e show upflow velocities estimated by taking the divergence of the horizontal Doppler signatures. These two are strongest at great depths for the two mature active regions but stronger at intermediate depths for the newly emerging one. The outflow speeds that Lindsey and Braun propose give rise to the Doppler signatures are roughly zero at the photosphere (except in narrow annuli around sunspots, called moats), and increase roughly linearly with depth to speeds of order 200 m/s at 10,000km below the photosphere. The flows appear around three major active regions that are visible at the solar surface during 1-6 January 1991, and extend several tens of thousands of kilometers laterally, into the quiet Sun. The newly emerged active region shows a considerably shallower flow than those of the mature active regions.
The Doppler acoustic signatures seen by Lindsey et al. appear to be somewhat in conflict with recent results in time-distance helioseismology, also based on the Bartol-NASA-NSO observations (Duvall et al. 1996). That work considered time-delay asymmetries that suggest rapid downflows and inflows, with speeds up to 2 km/s. This discrepancy suggests that effects besides flows may contribute both to the time-distance measurements and the Doppler acoustic signatures. The Doppler acoustic signatures measured by Lindsey et al. are calibrated by introducing artificial flows into selected regions of the South-Pole observations. This exercise assures that extended inflows of the order of 100 m/s would provide a strong Doppler acoustic signature. While this makes it difficult to interpret the time-distance measurements in terms of rapid, extended downflows, it does not necessarily assure that the much more moderate signatures that Lindsey et al. see are themselves due to flows. Lindsey, Braun, and Woodard are independently examining a range of schemes other than acoustic Doppler shifts due to submerged flows that could possible give rise to the signatures they see.
If the Doppler acoustic signatures are indeed caused by flows, or deeply submerged structure of any kind, Lindsey et al. think that it will be possible to use acoustic waves to examine solar interior magnetic structure just as our eyes use light to examine perturbations in a vacuum (or air) that scatter electromagnetic waves. This is made possible by a computational scheme ("helioseismic holography") introduced in earlier publications by Lindsey and Braun (1990) and Braun, Lindsey, Fan and Jefferies (1992). A number of modeling schemes have been tried (Kosovichev 1996) or proposed (D'Silva 1996) that do not rely on holography. Lindsey and his colleagues suspect that computational holography will be the key to diagnostics and modeling of flow structure on a small scale that lies deep under the solar photosphere.