Five-minute Power Maps from GONG and MDIFive-minute Power Maps from GONG and MDIIntroduction

The power of acoustic oscillations at the solar surface is closely related to the presence of magnetic activity, with power in the five-minute band generally being suppressed in active regions. In this poster, we examine the relationship between power in the five-minute band and magnetic index at a variety of spatial scales, using data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI).

Analysis•The data consist of Doppler velocity images of the Sun, acquired at 1-minute cadence with 2.5 arcsec pixels. •For the GONG network, images from different sites, where available, were first registered and merged as part of the standard analysis pipeline for local helioseismology. •The data were then remapped into heliographic latitude/longitude co-ordinates and subdivided into 189 overlapping 16°×16° patches, each of which was tracked for 1664 minutes at the Snodgrass surface rotation rate for its central latitude, as part of the processing for the ‘ring-diagram’ technique of local helioseismology. Missing frames were zero-filled.•Finally, for each pixel in each patch, we performed a Fourier analysis and computed the total power in the frequency range from 1.5 to 5.5 mHz.

We have analyzed 458 days of GONG data obtained over the period from August 2001 to January 2004. For comparison, we also look at two days of data from MDI that overlap the GONG observations.

Two days in detailWe consider two days for which we have both GONG and MDI data; the more active 2002-04-03 [020403] and the relatively low-activity 2003-11-03 [031103] when the very large active region AR10486 had just rotated out of the region used for the analysis.

•The top row shows the 5-minute power on 020403 (left two columns) and 031103 (right two columns), for GONG (left) and MDI (right). •Because the oscillations are mostly radial, power away from disk center is reduced by foreshortening.•The second row shows the residual power after correction for foreshortening. The correction term was derived from a linear fit with log (1-ρ2) as the independent variable and log (power) as the dependent variable, where ρ is the fractional distance from disk center. The fit excluded the pixels where the magnetic index was above the median value for the day.•On the bottom row are the corresponding portions of the MDI synoptic magnetograms (left) and continuum intensity maps (right) for each day.•In all cases, the locations of the active regions are clearly marked by reduced power. •The GONG maps are coarser in appearance, reflecting the poorer resolution of the GONG instruments due to atmospheric seeing. •The GONG maps sometimes show excess power associated with strong sunspots. This is believed to be an artifact due to the interaction of seeing with strong intensity gradients.•There are clear gradients across the disk in the MDI residual maps, indicating that the circularly symmetric geometric term is not a perfect fit to the data. This may be a result of distortions in the MDI instrument.

Quantitative Comparison: Two DaysFor the two days under consideration, we plot (left) the power in each pixel as a function of 1-ρ2, with the linear fit indicated, and (right) the residual power as a function of magnetic index. The GONG and MDI data are differently scaled. It is also interesting that while the slope of the log-log plot for the MDI data is very close to unity, that for the GONG data is about 1.2, implying that the dependence on 1-ρ2 is not strictly linear. This may be a consequence of the attenuation of power at finer spatial scales in the GONG data due to the poorer resolution, and effect which becomes more severe away from disk center.

The relationship between five-minute power and magnetic index is clearly present, but shows considerable scatter and non-linearity.

Quantitative Analysis: Many DaysWe next consider the overall data set, consisting of 458 days of GONG data. For this analysis, the total 5-minute power in each 16° square patch is considered, for a total of about 86,000 patches. Again, the left-hand plot shows the power as a function of (1-ρ2) and the right the residual after division by the geometric term. The slope of the log-log plot is very similar to that found on the pixel-by-pixel level.

GONG MDI

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AcknowledgmentsThis work utilizes data obtained by the Global Oscillation Network Group (GONG) program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísicade Canarias, and Cerro Tololo Interamerican Observatory. We also used data from the Solar Oscillations Investigation (SOI), obtained using the MDI instrument aboard SOHO. SOHO is a joint project of NASA and ESA. This work was supported in part by NASA grants S-92698-F and NAG5-11703 to the National Solar Observatory, and NAG5-10917, NAG5-11920 and NAG-121491, and by NSF grant ATM-0219581 to the University of Colorado.

SP13A-01

Comparison with Ring Diagram ResultsThe power in the 5-minute oscillations can also be estimated from fits for the parameters of the 3-dimensional power spectrum of the data cube, as in the ring diagram technique. Howe et al. (2003) have shown that such fits exhibit reduction of the power with increasing magnetic index, except for the highest-frequency peaks where the power is instead enhanced.

The plots show the variation of the total estimated power in the modes fitted in the ring diagram analysis, for 458 days of GONG data collected mostly during 2001-2002 (top) and for data from the 2002 MDI dynamics program (bottom). The MDI data were restricted to one year because otherwise the effects of instrument focus changes become too confusing.

R. Howe, R. W. Komm, F. Hill, National Solar Observatory

D. A. Haber and B. W. Hindman,University of Colorado

R. Howe, R. W. Komm, F. Hill, National Solar Observatory

D. A. Haber and B. W. Hindman,University of Colorado

Comparison of Different Spatial Scales•In the plots above, the GONG power maps for 020403 were averaged over successively finer subdivisions, from 16x16° to 1x1° squares. The left column shows the geometric fit, and the center and right columns the residuals after division by the geometric term, as a function of magnetic index and as a spatial map. The red symbols in the center column represent binned averages of the points.•The curve describing the sensitivity of total power to magnetic index appears to be similar across all spatial scales, saturating at about a 60% power deficit for magnetic indices above about 500 G. •As a comparison, we note that the global mode power shows a decrease of about 10% for a magnetic field strength of 30G. This lies somewhere in the scatter of the plots shown here. The very low-degree global modes seem to exhibit a somewhat greater sensitivity to the global magnetic field, changing by about 30% from maximum to minimum.

Summary•We confirm the suppression of power in the five-minute oscillations in the presence of magnetic activity.•The dependence of the 5-minute power on magnetic index appears to follow the same curve over a range of spatial scales.•The existence of the large databases of tracked and remapped data from GONG and MDI now makes it possible to study this phenomenonmore systematically than has been possible before.•The suppression of power levels off at higher magnetic field strengths.• This suggests that the use of the power maps as a proxy for magnetic field measurements is limited.•In further work, we intend to examine the behavior of the higher-frequency power in detail.

Electronic versions of this and many other GONG posters are available on the GONG CD and also at

http://gong.nso.edu/gallery/cd_data_2005/