Helicity Observations by Huairou Vector Magnetograph Mei Zhang National Astronomical Observatory, Chinese Academy of Sciences Plan of the Talk: 1.Huairou vector magnetic field measurements 2.Some scientific results obtained by using Huairou vector magnetograms

In connection to this meeting: Why should you care? -The magnetic field we observed on the surface comes from the interior. -So, could or will local helioseismology detect the same characters in the interior? Why do I care?  If the local helioseismology can tell me what characters are also observed in the interior, they are probably more fundamental characters.

Basics of field measurement A.We use Zeeman effect to measure the magnetic field on the solar photosphere. B.We measure polarized light to derive vector magnetic field: Circular polarization (Stokes V) for longitude field (B z ) and linear polarization (Stokes Q and U) for transverse field (B x and B y ). C.There are two types of vector magnetographs: 1.Spectrograph: measure the full spectrum of a line; use inversion codes to derive vector B (advantage: accurate) 2.Filter-magnetograph: measure at a fixed wavelength; use calibration methods based on linear assumption (advantage: better temporal-resolution, large sample)

Huairou filter-type magnetograph 35cm in diameter FeI Å line Stokes V: Å from line center Stokes Q and U: line center Lande factor g=1.5 Seeing: arcsec Routine observation since 1988 No optical system change (Only a better CCD in 2002) Typical noise: σ z ~5G, σ x,y ~40-60G

Physical quantities that can be derived Shear angle Current (J z ) Twist (  z ) Twist (  best ) Current helicity (h c ) Current helicity imbalance (  h ) All these quantities indicate the non-potentiality (helicity) of the measured field.

Scientific questions that could be targeted Measure non-potentiality: -Shear angle -Current (J z ) -Twist (  z ) -Twist (  best ) -Current helicity (h c ) -Current helicity imbalance (  h ) Vector magnetic field on the photosphere Extrapolation of coronal magnetic field For example ( in this talk ): Variation associated with flares and CMEs Flux emergences and indication of dynamo

Variation associated with flares and CMEs Flares and CMEs are two major forms of solar activities in the corona. The drive is presumably from solar interior by flux emergence or from the shearing on the photosphere. So, searching for indication of energy release and initiation trigger.

Variation associated with flares and CMEs But the shear angle could also increase after a flare, in a flux emergence region. (Zhang H, 1995, AA, 304, 541) A dramatic change of shear angle at the key site of eruption (that is, at the site of flux cancellation) Energy buildup and release process (Deng et al., 2001, Solar Physics, 204, 13) Total J z Total h c Total energy

Variation associated with flares and CMEs A case where a new flux system of negative helicity emerges into an active region (NOAA 8210) with positive overall helicity (  best =0.010 arcsec -1 ) suggests that the emergence of opposite helicity might be the trigger of the CME. (Wang JX et al., 2004, ApJ, 615, 1021) EFR

Flux emergence process Magnetic fields, in the form of flux tubes, emerge from the solar interior to form sunspots and active regions. So, what does this process bring up into the photosphere, in addition to the magnetic flux? What are the properties of the flux emergence?

Flux emergence process During the emergence, total current increases linearly, while the twist (  z ) remains constant. (Wang TJ & Abramenko, 2000, A&A, 357, 1056)

Flux emergence process A negative correlation between the signs of the tilt angle and the current helicity suggests the helicity conservation in a flux tube with zero total magnetic helicity. (Tian et al., 2001, A&A, 374, 294) 203 simple bipolar active regions

Flux emergence process A positive correlation between the signs of the tilt angle and the twist indicates where a kink instability presents. (Tian et al., 2005, Solar Physics, 229, 63) 104 complicated δ active regions

Helicity produced by dynamo So, observing the helicity pattern produced by solar dynamo would hence help us understand the dynamo process in the interior. Solar dynamo produces magnetic field in the convection zone. The field emerges to the surface and presents as solar cycle, with toroidal field to form sunspots and poloidal field to form large-scale dipolar field.

Helicity produced by dynamo (Bao & Zhang H., 1998, ApJ, 496, L43) Current helicity imbalance Twist (  z ) (Pevtsov et al., 1995, ApJ, 440, L109) Statistically, In the northern hemisphere:  z,  best, h c,  h <0 In the southern hemisphere:  z,  best, h c,  h >0

Helicity produced by dynamo Active regions with current helicity signs opposite to most others in the same hemispheres occur normally at some heliographic longitudes and persist over long periods. (Zhang H. & Bao. SD, 1999, ApJ, 519, 876) northern hemispheresouthern hemisphere Current helicity of active regions

Helicity produced by dynamo (Zhang M., 2006, ApJ Letters, 646, L85) Weak fields: Following established hemispheric rule Strong fields: Helicity sign opposite to that of weak fields A large sample of Huairou magnetograms (January August 2004) weighted average, over a whole year weights: -1: northern +1: southern

Implications (1): Helicity conservation in solar dynamo? Theories have predicted that solar dynamo would produce opposite helicity signs in the mean field and in the fluctuation (e.g. Blackman & Field 2000). Weak and strong fields have opposite helicity signs!

(for 100G<|B z |<500G) Weaker in field strength, More dominate by numbers (for |B z | > 1000G) Stronger in field strength, less dominate by numbers Implications (2): Solving a puzzle? Current helicity (h c =B z (  B) z =  B z 2 ) when calculated from the whole field usually shows a weaker hemispherical law than  best does? (In Pevtsov et al and ours)  best: each point equally weighted; h c: Weighted by B z 2

(1976 May – 1977 June) Pevtsov & Latushko (2000) studied the current helicity of the large- scale photospheric magnetic field, using derived vector magnetogram from MDI data. They find: h c follow the usual hemispheric rule in high latitudes but not within  40 o of solar latitude --- another puzzle. Implications (3): Fields outside active regions also twisted?

Concluding Remarks (1) Vector magnetic field measurements provide us a chance to study the magnetic non-potentiality (helicity) in solar active regions. The distribution and evolution of non-potentiality within active regions allow us to discuss what are the favorable conditions for coronal activities such as flares and CMEs. The hemispheric rule and helicity observations provide information of flux emergence process as well as indication of dynamo process in the solar interior.

Concluding Remarks (2) Further studies are certainly necessary and are advocating measurements with better spatial and temporal resolutions. Full-disk vector magnetic field measurements are also in order, for understanding large-scale structures and connections between active regions. In this connection, Solar-B and the upcoming SDO missions may provide us new insights from where we are now in both theory and observation.

A new solar full disk vector magnetograph at Huairou Stokes V Stokes QStokes U 10cm in diameter FeI Å line A test-bed for HMI/SDO?

Thank you ！

Filter-type magnetographs Solution: Nonlinear calibrations, for both longitudinal and transverse fields? Problems: 1.Linear calibration 2.Faraday rotation Advantage: High-temporal resolution

Faraday rotation Vector B + Trace images HR: Huairou MTK: Mitaka SPM: HSP/Mees (Zhang H. et al., 2003, Sol. Phys., 213, 87) Comparison seems indicating that the Faraday effect is not as serious as we have thought. --- Why?