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

Frank HillDec. 3, 2009

NAOC, Beijing

Outline

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

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.

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

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

Global helioseismology data reduction

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Observed Doppler shift movie

Three spherical harmonics Time signal for 3 modes

Some Pictures Of The Solar Acoustic Spectrum

Local Helioseismology

• Probe a portion of the sun, not entire sun• 3 methods:– Ring diagrams– Time-Distance– Holography

Ring diagrams

3-d Fourier power spectrum of 16° patches

Spectrum sliced at constant frequency

Time-distanceClose analogy to terrestrial seismology

Ray paths from a source Cross-correlation amplitude as a function of time and distance from a source

Acoustic holographyThe 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

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.

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

GONG instrument locations

The GONG Instrument

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.

GONG temporal coverageGONG monthly duty cycle

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GONG Month

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Overall average duty cycle: 0.849

Last year: 0.893

No day without data since July 2001

1995 2007

Latest results

• Solar cycle and the extended minimum• Magnetic field changes• Space weather

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

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

Helioseismic view of the minimum

• Frequency shifts• Travel time differences• Meridional flow• Tachocline oscillation• Convection zone dynamics• Zonal flows/torsional oscillations

Activity-related frequency shifts

Jain & Tripathy

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.

Temporal variation of frequencies and activity indicies

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.

Travel-time changesRelative 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

Near-surface meridional flow

Deep meridional flow

Three meridional cells in latitude near the surface?

R. Komm

Variations at the Tachocline

See Howe et al. (2000; Science 287, 2456)

1.3y period (Howe et al. 2000)

GONG (black) and MDI (red) agree.

Disappears after solar max

Not affected by reanalysis, but still unconfirmed.

Convection zone dynamics

Courtesy R. Howe

A solar cycle of internal flows – the movie

R. Howe

Torsional Oscillation at depth of 1 Mm

Courtesy R. HoweSymmetric global inversion

1. Cycle 24 migration started in 2003

2. Activity turns on when flow reaches latitude of about 22°

3. Cycle 24 migration has taken 1.5 yrs longer to reach critical latitude

4. Poleward branch yet to appear

Zonal Flow Patterns (Time-Radius)

MDI OLA

MDI RLS

GONG RLS

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Howe et al 2005 Cycle 24 flow is weaker during its rise phase below the surface

Surface TO

Courtesy R. Ulrich

Clear north/south asymmetry

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.

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.

GONG

Synoptic Maps

Synoptic coronal hole plot Line of sight coronal hole plot

Synoptic field plot

Field extrapolation products

Line of sight synoptic field plot

Ecliptic-plane projections

Synoptic view

Synoptic view

Line of sight view

North polar view

Movies of hourly magnetic field products

• All projections are also provided as movies

• Example: Line-Of-Sight plane of sky projection

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AFRL ADAPT product

10-min magnetic field average

10-min magnetic field standard deviation

10-min intensity average

Weights for synoptic map

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.

Partial rings – apparent suppression in some directions

Rings from magnetic field data – Quiet Sun

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

Spherical harmonic decomposition of magnetic field

Also shows apparent suppression (retrograde here)

Helioseismology and space weather

• Far-side imaging• Emerging active regions• Subsurface vorticity

Farside imaging and frontside activity

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.

New, improved far-side mapsA 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.

Solar acoustic radius variability

Temporal variation in disk-averaged far-side phase shift (Courtesy I. González Hernández).

Temporal variation in lag of low-degree autocorrelation function peak – could be applied to asteroseismic observations (courtesy S. Kholikov).

Vertical flows and emerging active regions

Solid line: vertical velocity averaged over all 801 regions and all ring days

Filled squares: regions with emerging flux – the 25% with highest increase in flux

Filled circles: regions with decaying flux – the 25% with greatest decrease in flux

Open squares: the rest (50% of regions)

Emerging flux: strong upflows in deeper layers, weaker downflows near the surface.

Decaying flux: stronger downflows

801 active regions, vertical flow and flux values for complete disk passage. Courtesy R. Komm

Emerging active regions

Underneath a Sunspot

Above: Sound speed: red is relatively high, blue is low. The variations are caused by either temperature or magnetic field.

Below: vorticity (twisting motions). Pattern shows two horizontal “tornadoes” with opposite sense of rotation. This pattern is under every active region that produces large numbers of X-class flares.

Subsurface vorticity and flare activity

Komm & Hill

Temporal behavior of kinetic helicity before a flare

Helicity increases before flares

Henthorn & Reinard

A superposed epoch analysis for active regions associated with X-class flares (red), M-class flares (blue), and C-class flares (cyan). Shown in green is an average value for active regions that donot flare.

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Statistics for flare forecasting based on NHGV and surface magnetic field strength

Time period Accuracy Hit rate False alarm rate Heidke Score

1 day before flare 96.7% 32.7% 62.4% 0.33

2 days before flare 97.2% 27.3% 71.0% 0.27

3 days before flare 97.7% 25.0% 71.7% 0.25

1-3 days before flare 94.4% 44.3% 65.8% 0.36

0-3 days before flare 91.2% 49.3% 65.1% 0.36

All days 86.9% 58.1% 65.0% 0.38

For M- and X-class flares

Heidke scores for surface magnetic field alone are 0.07-0.15

Hα in GONG

• Add filter, beam-splitter, 2048x2048 camera, DAS• Entrance window bandpass is adequate• Plenty of room on optical table• Acquire 1 image per minute at each site,

staggered by 20 sec between sites• Funded by US Air Force Weather Agency• Prototype running in Tucson• Deployment in spring 2010

GONG/AFWA Hα Concept

Existing GONG Calibration assembly

Hα camera

Hα filter

Focusing lens

Reimaging optics

Beam splitter

Existing instrument cover mounting rail

Prototype at Tucson

Sample GONG Hα image

Power spectral analysis of filament dynamics

Spectra of a synthetic rotating Gaussian

Spectra of area A on the image (an erupting filament)

The details of the spectral structures are related to the rates of rotation, translation, expansion, etc. of the features on the disk.

Can derive the relationships and then use the spectra for statistical studies of filament dynamics.

Spectra of a growing and translating Gaussian

Conclusion• GONG probes the solar interior using helioseismology• GONG also provides nearly continual surface

magnetic/Doppler fields at 1-min cadence• Will soon provide Hα intensity images• Results show deep connections between internal

dynamics and surface activity• Ultimate goal is to understand the solar cycle• Helioseismology can be used for space weather forecasts• GONG welcomes collaboration with Chinese scientists

interested in these areas