Search Publications
Comparison of HMI Dopplergrams with GONG and MDI data
Howe, R.; Komm, R.; Hill, F. +3 more
We compare sample Dopplergrams from the Helioseismic and Magnetic Imager, the Michelson Doppler Imager and the Global Oscillation Network Group. Each instrument has a distinct static velocity patterm across the disk; once this has been subtracted and the images interpolated to a common grid, the agreement is satisfactory.
Progress in sunspot helioseismology
Birch, A. C.
Local helioseismology is a set of methods that are used to study wave propagation and infer physical conditions in the solar interior. Sunspots are a particularly challenging target for local helioseismology. In this review, I will show that some new methods (magnetoconvection simulations and numerical wave propagation simulations) lead to shallow…
Temporal Variations of High-Degree Solar p-Modes from GONG and MDI
Howe, Rachel; Hill, Frank; Jain, Kiran +4 more
We study temporal variations in the amplitudes and widths of high-degree acoustic modes in the quiet and active Sun by applying ring-diagram technique to the GONG+ and MDI Dopplergrams during the declining phase of cycle 23. The increase in amplitudes and decrease in line-widths in the declining phase of the solar activity is in agreement with pre…
Angular-degree dependence of p-mode frequencies during solar cycle 23
Hill, F.; Salabert, D.; Garcia, R. A. +3 more
We analyze simultaneous helioseismic observations collected by the ground- and space-based instruments during solar cycle 23 by computing oscillation frequencies for low- and intermediate-degree p-modes on a time scale of 36 days. We find that the frequency shifts corresponding to different angular degree, ell, indicate different epochs for the on…
Localized averaging kernels for probing the deep meridional flow with data from GONG, MDI and HMI
Roth, M.; Doerr, H. -P.
The Fourier-Legendre decomposition (FLD) method to study wave absorption in sunspots is a helioseismic technique which is also suited for the measurement of the meridional flow. Because the FLD method is sensible to low-degree oscillation modes, it bears the potential to be used to probe the average meridional flow in much deeper layers as it is c…
The Need for Physics-based Inversions of Sunspot Structure and Flows
Birch, A. C.; Braun, D. C.; Crouch, A. D. +1 more
Current controversy exists in the interpretation and modeling of helioseismic signals in and around magnetic regions like sunspots. Unresolved issues include the dependence of the sign of both the inferred flows and wave speed on the type of filtering used, and the discrepancy between the relatively deep two-layer wave-speed models derived from st…
Characteristics of Solar Meridional Flows
Basu, Sarbani; Antia, H. M.
We have done a ring-diagram analysis of MDI full-disc data to determine the properties of solar meridional flow in the outer 2% of the Sun over the solar cycle 23. The meridional flows show a migrating pattern with higher-velocity flows migrating toward the equator as activity increases. Additionally, we find that the migrating pattern of the meri…
Variation of high-degree mode frequencies during the declining phase of solar cycle 23
Hill, F.; Jain, K.; Tripathy, S. C.
We investigate the spatial and temporal variation of the high-degree mode frequencies during the declining phase of the solar cycle 23 and the extended minimum between the cycle 23 and 24. We find that the frequency shifts of high-degree modes obtained through the ring-diagram analysis in different phases of the solar cycle are not equally correla…
Investigating the correlation between high-frequency global oscillations and solar flares
Rabello-Soares, M. C.; Bogart, R. S.; Chakraborty, S. +1 more
Global oscillations have been observed in the high-frequency spectrum in velocity and intensity measurements above the acoustic cutoff frequency of the solar atmosphere. In a recent work, strong correlation at high frequencies, determined between helioseismic spectra and soft x-ray flux has been cited as evidence that solar flares may be one of th…
A search for coherent structures in subsurface flows
Howe, R.; Komm, R.; Hill, F.
We search for coherent patterns in horizontal subsurface flows obtained from Global Oscillation Network Group (GONG) Dopplergram data using ring-diagram analysis. The existence of north-south aligned downflow patterns near the equator has been predicted by numerical models of the solar convection zone. We analyze time series of daily flow measurem…