Latest Results from GONG: Helioseismic Studies of the Solar Cycle and Space Weather

1. Latest Results from GONG: Helioseismic Studies of the Solar Cycle and Space Weather Frank Hill Dec. 3, 2009 NAOC, Beijing

2. Outline • Brief overview of helioseismology • The GONG system • Latest results • Future Hα observations

3. Helioseismology • In 1960, it was discovered that the solar surface oscillates at a dominant period of five minutes. • In 1975, it was demonstrated that the oscillations are acoustic waves trapped inside the Sun. • Since the sun is filled with sound, the characteristics of the waves are determined by the internal solar structure and dynamics. • Thus we can infer the properties of the solar interior by measuring the wave frequencies, amplitudes, and life times.

4. Properties of the acoustic waves • Amplitudes: up to 20 cm/s • Periods: 3 to 10 min (5 min has most power) • Life times: hours to months • Sound is generated by surface granulation • Waves are trapped in the internal temperature gradient • Discrete allowed frequencies associated with vertical wavelengths that “fit” into the thermal cavities • 5,000,000 distinct modes • The Sun is a huge musical instrument

5. Two types of helioseismology • Global: • Waves are standing normal modes • Data decomposed into spherical harmonics • Inferred solar properties are averaged over entire sun • Need long (months to years) nearly continuous data sets without gaps (so GONG network, SOHO and SDO space missions) • Local: • Waves are travelling • Data decomposed into functions (e.g. sine waves) on localized areas • Inferred solar properties are typically averaged over small patches • Short (hours to days) data sets OK, but need to continually observe to identify artifacts and study temporal variations

6. X = X = X = Global helioseismology data reduction Three spherical harmonics Time signal for 3 modes Observed Doppler shift movie Σ

7. Some Pictures Of The Solar Acoustic Spectrum

8. Local Helioseismology • Probe a portion of the sun, not entire sun • 3 methods: • Ring diagrams • Time-Distance • Holography

9. Ring diagrams 3-d Fourier power spectrum of 16° patches Spectrum sliced at constant frequency

10. Time-distance Close analogy to terrestrial seismology Cross-correlation amplitude as a function of time and distance from a source Ray paths from a source

11. Acoustic holography The interference pattern from an object Waves emitted by subsurface sources are observed on the surface Waves are “time-reversed” to image the sources at a chosen depth

12. What is GONG? • Global Oscillation Network Group • GONG is a observing system for helioseismology and solar magnetic field studies • The instruments are geographically distributed to observe the Sun continually • Operating since 1995, camera change in 2001, polarization modulator change in 2006, Hα coming in 2010.

13. What does GONG observe? • Full-disk Doppler velocity, line-of-sight magnetic field, and intensity • Uses Ni I 676.8-nm spectral line • Solar image is 800x800 pixels (2.5” pixels) • One data set every 60 sec at each site • Semi-automated operation • Coming in mid 2010: H-α intensity images

14. GONG instrument locations

15. The GONG Instrument

16. Why a network? • Diurnal setting of sun produces a periodic gap once a day at a single site. • The solar acoustic spectrum is convolved with the temporal window spectrum, contaminating solar spectrum with many spurious peaks. • A well-designed network greatly decreases the amplitude of these artifacts. • Other observational strategies are space and Antarctica.

17. GONG temporal coverage 1995 2007 Overall average duty cycle: 0.849 Last year: 0.893 No day without data since July 2001

18. Latest results • Solar cycle and the extended minimum • Magnetic field changes • Space weather

19. The current minimum is unusual • Longer than average • Most spotless days (so far) since cycle 15 • Lowest global solar wind pressure of the space age • Solar magnetic field 36% weaker than last minimum • Lowest irradiance yet measured • Lowest sustained 10.7-cm radio flux since 1947 • Unusually high tilt of dipole field • No classical quiescent equatorial streamer belt • Very high cosmic ray flux

20. Spotless days of the last 150 years W. Livingston & M. Penn

21. Helioseismicview of the minimum • Frequency shifts • Travel time differences • Meridional flow • Tachocline oscillation • Convection zone dynamics • Zonal flows/torsional oscillations

22. Activity-related frequency shifts Jain & Tripathy

23. Temporal variation of frequencies and activity indicies The temporal evolution of mean frequency shifts (bottom panel) and activity indices represented by sunspot number (top panel) and 10.7 cm radio flux (middle panel). The quantities are calculated on a time scale of nine days and cover the period of May 7, 1995 to Dec 11, 2008. Both the magnitude and the fluctuations of the frequency shifts of this minimum are smaller than those of the last cycle.

24. Odd behavior of p-mode frequency shifts Temporal evolution of GONG intermediate-degree frequency shifts (red) calculated from 72 day time series during the (a) previous (cycle 22/23) and (b) current (cycle 23/24) minima of the solar cycle. The blue line represents the linearly scaled 10.7 cm radio flux (F10.7). The dash-dot and dash-dot-dot-dot lines in both panels of the figure display the minimum value in activity and frequency shifts between the cycle 22/23, respectively. Note that the frequency shifts are anti-correlated during the current minimum, unlike the previous one. This is also seen in MDI data.

25. Travel-time changes Relative to 2000 maximum Sound speed is roughly 20 m/s slower compared to 1996 minimum Independent of separation  near surface effect Implies either much cooler layers (T/T 0.5%) or lower B Consistent with reduced irradiance and very low activity S. Kholikov

26. Near-surface meridional flow

27. Deep meridional flow

28. Three meridional cells in latitude near the surface? R. Komm

29. Variations at the Tachocline • 1.3y period (Howe et al. 2000) • GONG (black) and MDI (red) agree. • Disappears after solar max • Not affected by reanalysis, but still unconfirmed. See Howe et al. (2000; Science 287, 2456)

30. Convection zone dynamics Courtesy R. Howe

31. A solar cycle of internal flows – the movie R. Howe

32. Torsional Oscillation at depth of 1 Mm • Cycle 24 migration started in 2003 • Activity turns on when flow reaches latitude of about 22° • Cycle 24 migration has taken 1.5 yrs longer to reach critical latitude • Poleward branch yet to appear Symmetric global inversion Courtesy R. Howe

33. Zonal Flow Patterns (Time-Radius) 15 30 0 45 60 MDI OLA MDI RLS GONG RLS Howe et al 2005 Cycle 24 flow is weaker during its rise phase below the surface

34. Surface TO Clear north/south asymmetry Courtesy R. Ulrich

35. GONG Magnetic Field Observations • GONG produces full-disk 800X800 magnetograms every minute 24-7. • 10-min averages are available on the Internet a few minutes after acquisition. • Synoptic maps and magnetic field extrapolations are generated every hour, also available on the Internet. • Movies of all synoptic maps & field extrapolations are available on the Internet.

36. Sample images from GONG’s web page • Each image is a 10-min average created at the instrument. • Bad images are rejected. • An approximate calibration is performed. • Images are registered and circularized to a common radius. • Data is delivered to the web a few minutes after acquisition. • All previous images are available in an FTP directory.

37. GONG Synoptic Maps

38. Field extrapolation products Line of sight coronal hole plot Synoptic coronal hole plot Synoptic field plot Line of sight synoptic field plot

39. Synoptic view Ecliptic-plane projections North polar view Line of sight view Synoptic view

40. Movies of hourly magnetic field products • All projections are also provided as movies • Example: Line-Of-Sight plane of sky projection

41. AFRL ADAPT product 10-min magnetic field average 10-min magnetic field standard deviation 10-min intensity average Weights for synoptic map

42. High-cadence magnetic field changes From December 12 2006: Mosaic plot of line-of-sight field changes over a four-hour period centered at 1829UT, the start time of a X6.5 flare in AR10930. Each plot corresponds to one pixel, and the mosaic covers most of AR10930. There is a systematic pattern to the changes, which should yield information about flare mechanisms. Courtesy G. Petrie, J. Harvey, J. Sudol.

43. Rings from magnetic field data – Quiet Sun Partial rings – apparent suppression in some directions

44. Rings from magnetic field data – Active Sun Rings suppressed in direction towards active region Information on active region dynamics contained in 3-d power spectrum

45. Spherical harmonic decomposition of magnetic field Also shows apparent suppression (retrograde here)

46. Helioseismology and space weather • Far-side imaging • Emerging active regions • Subsurface vorticity

47. Farside imaging and frontside activity

48. Far-side magnetic fields Irene Gonzalez-Hernandez & Charlie Lindsey have developed a calibration between the far-side phase shift and the magnetic field strength (above). It is now possible to create “magnetograms” of the far side (above images), but without polarity information.

49. New, improved far-side maps A comparison of the current far-side maps (on the right) and the new improved version on the left. The improved ones have been created from four maps over two days, which strengthens the persistent features and reduces the noise. Thus, some faint features that could not be reliably identified as active regions in the original maps have become candidate regions. These are identified with a red circle and a number that quantifies the probability that the feature is an active region.

50. Solar acoustic radius variability Temporal variation in disk-averaged far-side phase shift (Courtesy I. GonzálezHernández). Temporal variation in lag of low-degree autocorrelation function peak – could be applied to asteroseismic observations (courtesy S. Kholikov).