SOT time-distance helioseismology in and around
active regions
T. Sekii, J. Zhao & A.G. Kosovichev
Dopplergrams produced by the MDI instrument, aboard the SOHO
spacecraft, have been used extensively for time-distance
helioseismology to probe sub-surface flow and soundspeed
anomaly. The Solar Optical Telescope aboard Solar-B also has
capability of producing high-resolution Dopplergrams, which can
be used for time-distance analysis. One problem with Doppler
measurement, however, is that it is not very reliable in active
regions, since the algorithm of converting filtergrams to
Dopplergrams is tuned for quiet regions, though how this should
affect time-distance measurement in active regions is not readily
known. Intensity data, though known to be noisier than Doppler
velocity data, are less likely to be affected by magnetic field.
We have analyzed MDI Doppler as well intensity data, in a quiet
region and in an active region, to make a comparison. The
travel-times from intensity data are systematically longer than
the ones from Doppler data. This is understood to have arisen
largely from different power distributions for two signals and
does not necessarily indicate any fundamental difference.
However, we have noted that the difference in travel times
increases in active region. Implication on inversions for
sub-surface velocity field, which will be carried out for SOT
data, is discussed.
Correspondence
Takashi Sekii (sekii@solar.mtk.nao.ac.jp), NAOJ
presentation
oral