P.I.: T.M. Brown (HAO/NCAR)
Co-Is: T.J. Bogdan (HAO/NCAR), A. Skumanich (HAO/NCAR)
This study aims to combine seismic and polarimetric observations to deduce the time-dependent structure of solar active regions, both above and below the photospheric level. New analysis methods (including time-distance analysis, p-mode absorption and scattering measurements, and characterization of acoustic sources) will be applied to MDI helioseismology data to infer subsurface flows and magnetic field structures in the general neighborhood of selected magnetic active regions. These regions will also be observed at high spatial resolution with the Advanced Stokes Polarimeter, yielding both vector magnetic field maps and moderate-resolution Doppler time series that may be used for further seismic analysis. These various observations will be combined in an attempt to clarify the relations between flows, electric currents, and field patterns in and around centers of magnetic activity.
Scientific Objectives
The aim of the proposed study is to use a combination of seismic and
other kinds of information to improve our understanding of the structure
and evolution of solar active regions, and of the mechanisms by which
they interact with acoustic waves.
Several newly-developed techniques of observation and of data analysis
encourage us to believe that a study can now be conducted that
will yield unprecedented insights into the processes responsible for
the surface manifestations of solar activity.
These new techniques suggest several specific questions that are ripe
for inquiry, among which are the following:
(1) What is the 3-dimensional morphology of an active region, and how
does it evolve in time?
(2) What is the connection between the magnetic field concentrations and
the thermal and velocity fields that make up an active region?
(3) What is the importance of magnetized regions as sources and sinks of
acoustic radiation?
(4) What physical processes govern the evolution of magnetic structures?
Are these processes the same for structures of all sizes?
Our strategy for answering the questions listed above involves a combination of data from several sources, advanced analysis techniques, and rigorous interpretation and modeling. MDI data will serve a crucial function in this combined analysis, since it can be used to infer both the large-scale environment of the active regions under study, and (in the high-resolution mode) the smaller-scale structure of the regions and their interactions with p-modes. Data of other sorts will also be needed, however, to dissect the internal structure of the magnetized regions, and to provide boundary matching information for the seismic analysis.
As one observational component, we shall obtain new observations of one or more active regions, using the Advanced Stokes Polarimeter (ASP) (Elmore et al. 1992). The ASP gives estimates of the vector magnetic field, the magnetic filling factor, and atmospheric current structures, as well as the thermal and velocity fields, all with spatial resolution of 1-2 Mm on the Sun. For regions that are the size of a typical active region and its immediate surroundings, such parameter maps may be obtained roughly once every 10 to 15 minutes, at each of two different heights in the atmosphere. The ASP observations will provide the best available estimates of the magnetic, thermal, and flow configuration of the active region above &theta& = 1 in the continuum. The second major observational component will be MDI image data, spanning the time period during which the ASP data are available, and extending if possible for some time interval before and after. The MDI data will be used to estimate the subsurface flows, magnetic fields, and thermodynamic structure of the active region and its surroundings with as much precision and resolution as is feasible. To infer ther larger-scale background of the active region, we shall appeal to GONG data. Because of its continuous nature, we can be confident that GONG images and frequencies will be available for whatever time is necessary to supplement the MDI/ASP observations. Finally, we will use high-resolution (typically 1 Mm) filtergrams obtained simultaneously with the ASP magnetic data (but at a higher temporal cadence) to augment the MDI seismic data, and to extend it to higher wavenumber. Although confined to relatively short time intervals and to a restricted portion of the Sun, these data should permit the study of smaller structures, closer to the surface, than is possible with the MDI data alone.
In somewhat more detail, the research to be conducted will be as follows: We will plan and execute a set of ASP observations in coordination with planned MDI high-resolution activity-oriented campaigns. The ideal times for such observations would be the spring or fall of 1996, when long observing days are possible at NSO/Sunspot. The ASP observations will provide both rasters of magnetic, flow, and thermal properties at fairly slow time resolution, and filter-based Dopplergrams at a time cadence fast enough to sample the high-frequency acoustic waves. These data will be reduced using extant analysis techniques.
Once suitable ASP and MDI observations have been obtained, GONG data will be requested for an interval of at least two GONG months (i.e., 72 days) centered on the time of the ASP observations. This data set will allow observation of the area occupied by the active region for three full disk passages, including the ones before and after the ASP observations. It will also provide long sequences of at least one ``control'' region, i.e., a large area of quiet Sun, to be used for comparison with the behavior of the active regions. The MDI and GONG images will be remapped as necessary for the analysis methods described below, and the data from different GONG sites will be combined by a merging method. We will develop mappings that allow superposition of the MDI, GONG, and ASP data sets with errors that are small compared to the MDI pixel size.
Analysis of the three seismic data sets (MDI, GONG, and ASP/filtergrams) will at first concentrate on approaches with which we have the most experience: p-mode absorption (Braun et al. 1988, Bogdan et al. 1993) and local p-mode and high-frequency power analyses (Brown et al. 1992). Lower priorities (but ones that we expect to achieve on a one- to two-year time scale) are to implement and apply methods involving phase shifts of waves propagating through the active region (Braun et al. 1993) and time-distance analysis of waves suffering bounces at different distances from the active region center (Duvall et al. 1993). In addition we expect to make some progress on (but not necessarily bring to completion) a full 3-dimensional time-distance inversion technique and a local depth inversion method based on high-frequency wave interference.
Simultaneously with these observational activities, we will work on improving the theoretical understanding of the processes whereby sound waves scatter from and are absorbed by magnetic flux concentrations. These studies will build on the work described above, and will be pursued mostly in the context of wave interactions with thin flux tubes or with collections of such tubes. Much of this work will be done in collaboration with other visitors to HAO, who bring with them the necessary analytic or computational skills.
Finally, we will attempt to combine information from the various sources so as to gain a clearer picture of the processes controlling the structure and evolution of the active regions studied. It is hard to predict what might be learned from this process without a better knowledge of the sensitivity and limitations of the seismic data. Nevertheless, we can list a few questions that are of great interest, and that might be approachable in the ways described.
Required Data and Resources
For this project, it will be necessary to organize a set of observations that are coordinated between the MDI and NSO/Sunspot (the site of the ASP). For this purpose we suggest planning two ASP observing runs, each one coincident with 5 daily 8-hr high-resolution MDI campaigns. It would be possible to use the partial high-resolution images obtained as part of the dynamics program in place of full-sized campaign images, provided that suitable activity centers are visible within the partial images. The observables returned from the MDI would be the usual ones required for activity-related seismology campaigns: velocity and continuum intensity, with line intensity and magnetograms taken occasionally. Of these 10 campaigns, at most 4 would be selected for initial study. We request computing resources from the SOI SSC sufficient to perform the time-distance and ingoing/outgoing wave analysis for these 4 data sets. Analysis of the ASP and GONG data will be performed using HAO computing resources.
References
Bogdan, T.J., Brown, T.M., Lites, B.W., & Thomas, J.H., Astrophys. J. 406, 723, 1993.
Braun, D.C., Duvall, T.L., Jr. & LaBonte, B.J., Ap. J. 335, 1015, 1988.
Braun, D.C., LaBonte, B.J., Duvall, T.L., Jr., Jefferies, S.M., Pomerantz, M.A. & Harvey, J.W., in GONG 1992: Seismic Investigation of the Sun and Stars, ed: T.M. Brown, (San Francisco, Astronomical Society of the Pacific), p. 77, 1993.
Brown, T.M., Bogdan, T.J., Lites, B.W. & Thomas, J.N., Ap. J. Letters 394, L65, 1992.
Duvall, T.L., Jr., Jefferies, S.M., Harvey, J.W. & Pomerantz, M.A., Nature 362, 430, 1993.
Elmore, D.F., Lites, B.W., Tomczyk, S., Skumanich, A.P., Dunn, R.B., Schuenke, J.A., Streander, K.V., Leach, T.W., Chambellan, C.W., Hull, H.K. & Lacey, L.B., Proceedings of the SPIE 1746, 22, 1992.