Helioseismic Tracers of Magnetic Activity S. C. Tripathy

Size: px

Start display at page:

Download "Helioseismic Tracers of Magnetic Activity S. C. Tripathy"

Meryl Dennis
6 years ago
Views:

1 Helioseismic Tracers of Magnetic Activity S. C. Tripathy In collaboration with Kiran Jain and Frank Hill

2 Outlook Historical Background of Frequency Shifts Recent results from Medium-Degree Results from Local Analysis Summary 2

3 Historical Development Understanding the source of the varying solar oscillation frequencies began since mid-eighties. A decrease of 0.42 μhz (1.3 parts in 10 4 ) in mode frequencies between 1980 (maximum) and 1984 (minimum) was initially reported by Woodard & Noyes (1985) using data from the Active Cavity Radiometer for Irradiance Monitor (ACRIM). Using BBSO data, Woodard et al. (1991) showed that the frequencies are strongly correlated with magnetic field strength. 3

4 From Woodard et al. (1991), Ap.J. 373, L81 Tripathy et al. 2007, Solar Phys. 243, 105 4

5 Recent Results show complexity A significant decrease in correlation between frequencies and activity indices is seen at minimum and high-activity phases and a strong correlation during rising and declining phases (Jain, Tripathy & Hill, 2009, Ap.J., in Press) 5

6 What Causes the Frequency Shifts? The mechanism is believed to be located close to the solar surface since the shifts depend strongly on frequency. Rabello-Soares et al., 2008, J. Phys. Conf. Ser. 118, 1 6

7 What Causes the Frequency Shifts? The mechanism is believed to be located close to the solar surface since the shifts depend strongly on frequency. However, theoretical models using only direct magnetic effects (photosphere (1985, 1991), tachocline(1986), chromosphere (1989), sunspot anchoring zone (50 Mm (2005)) are not capable of producing the observed magnitude of frequency shifts. Indirect effect of temperature changes (1988) and an effect of a change in acoustic cavity (2005) have also been proposed. The exact mechanism is still an open question. 7

8 Frequency shifts from high-degree modes For high degree modes obtained from ring-diagram analysis, the frequency shifts associated with active regions have also been analyzed (Hindman et al. 2000, Rajguru et al. 2001, Howe et al. 2004, Rabello- Soares et al. 2008). Hindman et al. have argued that the active regions change the local frequencies of high-degree p and f modes. 8

9 Fractional Frequency Shifts and coeval magnetograms Hindman et al., 2000, Solar Phys., 192, 363 Active regions appear as locations of large positive frequency shifts. 9

10 Our study We have carried out a quantitative analysis using data from two Carrington rotation periods one each during a) descending phase (CR 2009) b) minimum phase (CR 2058) CR 2009: October 21 November 19, 2003 (includes high-activity period of Halloween events) CR 2058: June 20 July 18,

11 The Technique We have used the ring-diagram technique to compute the mode frequencies from GONG Dopplergrams. The frequencies are determined as a function of ring-day (1664 minutes). We analyze 189 individual regions on the solar disk (densepack) covering up to 52.5 in latitude and longitude, separated by 7.5 (over lapping tiles since each region is about 15 x 15 ). For each sampling day and tile, the frequency shift is measured relative to the spatial average over the dense-pack frequencies. The strength of the magnetic field (MAI) is obtained from the magnetograms mapped and tracked in the same way as Dopplergrams. MAI represents the average over all the pixels in a given tile. 11

12 Single mode statistics, n= 3, l= 248, ν = 3 mhz High activity period (CR 2009) MAI δν Low activity period (CR 2058) 12

13 Comparison of δν for different radial orders High activity period (10/26/2003, CR2009) 13

14 Comparison of δν for different radial orders Low activity period (07/11/2007, CR2058) rp =

15 Comparison of δν for different radial orders δν is larger for high-degree modes and modes of higher radial order. Since the shifts are larger, the correlation also increases with mode order and degree. The shift increases with field strength. 15

16 Binned by like magnetic field strength High activity period (10/26/2003) Low activity period (07/11/2007) 16

17 Binned by frequency shifts GONG MDI The shifts for a given radial order and ν = 3.5 mhz are calculated in comparison with a quiet region. The frequency bins are 80 μhz wide. The error bars correspond to the standard deviation of the mean. From Rabello-Soares et al., 2008, J. Phys. Conf. Ser., 118,1. 17

18 Single mode, Averaged over dense-pack tiles The spatial average by definition would have been zero if active regions are not present on the solar disk. Thus, the small shifts arise due to the active regions but we do not find any correlation between their temporal evolution. 18

19 Averaged over all the modes in a given tile rp = 0.91 rp = 0.93 rp = 0.88 The frequency shifts act as tracers of magnetic field. 19

20 Averaged over all the modes in a given tile rp = 0.74 rp = 0.77 rp = 0.85 This suggests that during low activity phase, the shifts do not act as a good tracer of the activity. 20

21 Summary We find that the correlation between frequency shifts and the surface magnetic activity measured locally are appreciably different during the two activity periods. This is consistent with those reported recently for global modes. The small frequency shifts do not have a counter-part in active regions. Thus, this analysis only partially supports the idea that the variations in the local mode frequencies are due to active regions. We comprehend that the weak component of the magnetic field ( e.g. ubiquitous horizontal field or turbulent field) plays an important role, particularly during the minimal-activity phase of the solar cycle. 21

22 Recent Results using GONG data The variation of the frequencies with the changing magnetic activity is now well established. Jain et al., 2009, Ap.J., in Press Pearson s linear correlation coefficient (rp) for different activity indices are > 94% implying linear relation on solar cycle timescales ( e.g. RI = 0.98; F10 = 0.99) 22

23 Binned Frequency Differences 23

24 Binned Frequency Differences 24

25 Binned, Averaged over dense-pack tiles 25

26 Binned, Averaged over dense-pack tiles 26

27 Binned by like magnetic field strength Low activity period (CR2058) 27

Post-hoc derivation of SOHO Michelson Doppler Imager flat fields

Astronomy & Astrophysics manuscript no. flatfield c ESO 8 October 28, 8 Post-hoc derivation of SOHO Michelson Doppler Imager flat fields Hugh E. Potts and Declan A. Diver Dept. Physics and Astronomy, University