Download
1 / 23
230 likes | 233 Vues
This study analyzes the lifetimes of high solar p-modes using absorption in sunspots and the time-distance technique. The results show variations in lifetimes during different solar cycles and the influence of magnetic fields on acoustic waves.
E N D
Measurements of lifetimes of high-lsolar p-modes in sunspots Olga Burtseva, Frank Hill and Shukur Kholikov(NSO/GONG) LoHCo Meeting – Tucson – January 30, 2008
Outline • Introduction • Lifetime measurements technique • Analysed data • Lifetimes at solar min and max • Lifetimes in sunspots • Conclusions and future work
Introduction • P-mode lifetimes can be determined from the line profile widths in power spectra of global modes ( < 200). For high- interval this method doesn’t work. • One of the tools to determine the lifetimes of high- p-modes is to use absorption of p-modes in sunspots (Braun, Duvall & LaBonte 1987, 1988; Bogdan et al. 1993; Chen, Chou and the TON team 1996). • Time-distance technique is an alternative way to estimate the lifetimes of high- solar acoustic waves.
Lifetime measurements technique • The amplitude of the cross-correlation function decreases exponentially with the number of skips • This phenomenon is interpreted as the dissipation of solar p-mode power (D.-Y.Chou et al., 2001) • Lifetimes computed with this method are lifetimes of wave packets (not individual modes)
Lifetime measurements technique • The amplitude (A) of the wave packet decreases due to dissipation • Dispersion causes decrease of A and increase of the width (W) of the wave packet • Dispersion doesn’t change the energy of the wave packet (A2W=const) with number of skips (n). Thus: • Anand Wnare amplitude and width of the cross-correlation function of the wave packet, enis one skip travel time, Тis lifetime
Lifetimes at solar min and maxAnalysed Data • MDI full disk velocity data:two 512-minute time series at minimum (May 24 and September 18, 1996) and two 512-minute time series at maximum (September 9 and 11, 2000). • - on the selected days in 1996, there is no active regions present in the solar disk. • - on the two days in 2000, there are active regions on the solar disk. • MDI full disk magnetograms to estimate strength of magnetic field.
Data analysis • Remapping, tracking, SH decomposition, Gaussian filtering • Phase velocity filtering to isolate wave packets with central0 and ℓ0 • Reconstruction of the images and computing of the cross-correlation functions of the wave packets • Region of target points is at the disk center
Lifetimes at solar minimum and maximum • The results show that the lifetime at maximum is systematically shorter than that at minimum. • It is known that magnetic regions absorb or scatter the acoustic waves. • We suppose that exclusion of the strong magnetic field areas from cross-correlation analysis will allow us to conclude if the lifetime change caused by effect of active regions or it changes with solar cycle because of global variations of the properties of the solar plasma with solar cycle.
Masking of active regions • B of the quiet regions on the solar disk is upto 30 G, same in both 1996 and 2000. • B in active regions is upto 1000 G.
Conclusions • The results without masking of active regions show that the lifetime at solar maximum is systematically shorter than that at solar minimum. • After exclusion of active regions lifetime at maximum is still systematically shorter than that at minimum, but by less quantity, as it increases. • Next … • Separate ingoing and outgoing waves, and compare lifetimes. • Compute lifetime in sunspots.
Lifetimes in sunspotsAnalysed Data • GONG and MDI full disk velocity data.
Conclusions • In the quiet regions: lifetimes of ingoing and outgoing waves are close in most of the cases. • In the sunspots: lifetimes of outgoing waves are shorter in comparison with lifetimes of ingoing waves. • Lifetimes in the sunspots are shorter then the lifetimes in the quiet regions.
