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Acoustic Mode Frequencies of the Sun during the Minimum Phase between Solar Cycles 23 and 24  

sushanta tripathy   Submitted: 2012-10-04 17:29

We investigate the spatial and temporal variations of the high-degree mode frequencies calculated over localized regions of the Sun during the extended minimum phase between solar cycles 23 and 24. The frequency shifts measured relative to the spatial average over the solar disk indicate that the correlation between the frequency shift and magnetic field strength during the low-activity phase is weak. The disk-averaged frequency shifts compu ted relative to a minimal activity period also reveal a moderate correlation with different activity indices, with a maximum linear correlation of about 72%. From the investigation of the frequency shifts at different latitudinal bands, we do not find a consensus period for the onset of solar cycle 24. The frequency shifts corresponding to most of the latitudes in the northern hemisphere and 30 {dg} south of the equator indicate the minimum epoch to be February 2008, which is earlier than inferred from solar activity indices.

Authors: S.C. Tripathy, K. Jain, F. Hill
Projects: GONG

Publication Status: To be published in Solar Physics
Last Modified: 2012-10-05 09:24
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Comparison of High-degree Solar Acoustic Frequencies and Asymmetry Between Velocity and Intensity Data  

sushanta tripathy   Submitted: 2008-09-25 17:53

Using the local helioseismic technique of ring diagram we analyze the frequencies of high-degree f- and p-modes derived from both velocity and continuum intensity data observed by MDI. Fitting the spectra with asymmetric peak profiles, we find that the asymmetry associated with velocity line profiles is negative for all frequency ranges agreeing with previous observations while the asymmetry of the intensity profiles shows a complex and frequency dependent behavior. We also observe systematic frequency differences between intensity and velocity spectra at the high end of the frequency range, mostly above 4~mHz. We infer that this difference arises from the fitting of the intensity rather than the velocity spectra. We also show that the frequency differences between intensity and velocity do not vary significantly from the disk center to the limb when the spectra are fitted with the asymmetric profile and conclude that only a part of the background is correlated with the intensity oscillations.

Authors: S. C. Tripathy, H. M. Antia, Kiran Jain, F. Hill
Projects: SoHO-MDI

Publication Status: ApJ (in Press)
Last Modified: 2008-09-26 09:30
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Solar Oscillation Frequency Changes on Time Scales of Nine Days  

sushanta tripathy   Submitted: 2007-07-03 18:09

We establish that global solar p-mode frequencies can be measured with sufficient precision on time scales as short as nine days to detect activity-related shifts. Using ten years of GONG data, we report that mode-mass and error-weighted frequency shifts derived from nine days are significantly correlated with the strength of solar activity and are consistent with long-duration measurements from GONG and the SOHO/MDI instrument. The analysis of the year-wise distribution of the frequency shifts with change in activity indices shows that both the linear-regression slopes and the magnitude of the correlation varies from year to year and they are well correlated with each other. The study also indicates that the magnetic indices behave differently in the rising and falling phases of the activity cycle. For the short-duration nine-day observations, we report a higher sensitivity to activity.

Authors: S.C. Tripathy, F. Hill, K. Jain, J.W. Leibacher
Projects: SoHO-MDI, GONG

Publication Status: Accepted in Solar Physics
Last Modified: 2007-07-03 20:56
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Abstracts by Author
Acoustic Mode Frequencies of the Sun during the Minimum Phase between Solar Cycles 23 and 24
Comparison of High-degree Solar Acoustic Frequencies and Asymmetry Between Velocity and Intensity Data
Solar Oscillation Frequency Changes on Time Scales of Nine Days

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