On Flux Spectra of Solar Intranetwork Magnetic Elements

Jingxiu Wang, Guiping Zhou, Hui Li et al.

Solar Intranetwork (inner-network, inter-network) fields (IN) were first described by Smithson (1975), Livingston & Harvay (1975) as `discrete elements' of mixed polarities `interior to network'.A few key papers (Keller et al. 1994; Wang et al.1995; Lin 1995) in the middle of 1990s largely renewed the interests in IN field studies.A new era of relevant studies is opening by Hinode observations

I. Introduction

Are IN fields intrinsically weak or strong?What is the contribution of this weak field component to the Sun's magnetism and atmosphere heating?

How do they correlate with convection and plasma flow

What is their role in heating of solar atmosphere?

How do they Influence the opening coronal structure?

What is their origin? How do they act in solar

cycle?

There is a magnetic dichotomy on the Sun (Wang et al. 1995; Schrijver & Zwaan, 2000))

Some of the best ground-based observations

Bergers et al. 2004 A&A 428, 613 (SST)

Wang et al. 1995 Solar Phys. 160, 227 (Big Bear)

Very high resolution

Roughly the same size of FOV, 31X31

Lites, 2002, ApJ 573,431(ASP Sac Peak)

High sensitivity

Un

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Perspectives of current workIntrinsic fields strengthApparent flux densityFlux distributionsLocations & area occupiedDifference between infrared and visible

measurementsInternal StructuresPolarity distributionHorizontal IN

Perspectives of current work Evolution Velocity patterns Lifetime Rate of flux emergence and disappearance Non-potentiality & topology Atmospheric response Relative contributions to the Sun’s

magnetism Classification Origin Solar cyclic changes

Flux distributions Wang et al.(1985) – smallest observable flux 1016 Mx Shi et al. (1990) – 21016-1018 Mx; Wang et al. (1995) – magnetic dichotomy; IN with peak distribution

at 61016 Mx & NT at 21018 Mx; Power law after the peak distribution

Lin (1999) – majority of magnetic features < 51016 Mx Meunier et al.(1998) – lognormal distribution similar to sunspot of B

ogden et al. (1988) Parnell (2002) – Weibull PDF Khomenko et al.(2003) – TIP FeI 15648 noise (2-3) 10-4 smallest

21015 Mx Sanchez Almeida et al.(2003) – only 10% flux could be detected by

current magnetograms, IN far more flux in ARs Hartj & kneer (2002) – Gregory Coude Tel. Stokes V 6302 (1-5)1016

Mx appears different from Gaussian distribution (power law?) Lites & Socas-Navarro (2004) – no significant increase in unsigned

flux when resolution was improves to 0.6 Dominguez Cerdena et al.(2006) – PDF combined Hanle & Zeeman

measurements: significant fraction of unsigned flux (75-90%) contributed by fields < 500G

Rate of IN flux emergence

Zirin (1985) – earlier suggestion: 2 order of magnitude faster than AR, one order faster than Ephemeral Region (E.R.)

Wang et al. (1995) – Fully confirm Zirin’s estimation; Now, intranetwork E.R. were seen within each network (also Hinode) and at granulations

The IN elements seen to contribute the most of the Sun’s magnetic flux

II. Tentative work with Hinodeon flux spectra of IN elements

Understanding the calibrations Flux distribution and intrinsic field strength Appearance and disappearance Lifetime Understanding horizontal magnetic fields Overall relationship of IN fields, intensity,

and convection

Enhanced netwok (S04W02)20:51, 22:24, on 11 Dec. 2006

(0.16”, T=2min, △ 701x908 pixel2; TF Fe I 6302)

Quiet Sun (N03W05)17:35, 20:48 on 24 June 2007

(0.16”, T=1min, 914x995 pixel2; TF Na I 5896)△

Calibration: 9000*V/I= 1 MDI Gauss in enhanced

SOHO/MDI: Fe I Pixel area: 0.370 arcsec2

Hinode/SOT: Fe I 6302 APixel area: 0.026 arcsec2

Calibration: 8000*V/I= 1 MDI Gauss in quiet sun

SOHO/MDI: Fe IPixel area: 0.370 arcsec2

Hinode/SOT: Na-DPixel area: 0.026 arcsec2

Calibration: 1.2x105*V/I= 1 SP G in NET elements

SOT/SP: Fe I 6302 AIntegration: 20:00-21:03

SOT/FGIV: Fe I 6302 APixel area: 0.026 arcsec2

Calibration: 1.1x105*V/I= 1 SP G in IN elementsSOT/SP: Fe I 6302 AIntegration: 20:00-21:03

SOT/FGIV: Fe I 6302 APixel area: 0.026 arcsec2

Assuming a filling factor about 0.08 then the SP and FG provide the same apprant flux density

Flux distributions seen in FGIV data (More than 10,000 IN elements were identified and flux measured)

0

1

00 exp),,,(F

Here, is scale parameter; is shape parameter; 0 is location parameter. For the intra-netwrok elements, is close to 2.3×1016 Mx, can be taken as 1.13, 0 is 0.7×1016 Mx. The PDF is positively skewed with right tails. But what physics determines the scale and location parameters?

Flux distribution function fitted by Weibull function

Flux appeared in clustersof mixedpolarity;Many of them may, in fact,consist ofephemeral regions (ERs) in further high resolution

Flux emergence in the form of tiny ephemeral regions (ERs)

An opposite polarity pair approach and cancel first, then grow and separate

Disappearance of IN elements

cancellation

fading In-situ

fragmentation

coalescence

Overall Statistics of IN flux

Fractions of IN flux are 0.10-0.19 and 0.17-0.34 of Sun’s magnetic flux at any given time for enhanced and quiet network areas (allowing a half of NT flux were missing in our measurements).

Take IN life-time = 4 minutes, the total flux contributed by IN elements is (2.5-3.7)×1025 Mx per day; while assuming NT life-time = 20 hours, the total flux by NT is (2.6-8.0) ×1023 Mx per day.

Life-stories of IN elements

disappearance

fade In-situ 38%

Cancel. 45%

Coal. 13%

Frag. 4%

Appearance

In clusters 22%

ERs 10% (?)

In-situ 59%

Coal. 3%

Frag. 6%

Lifetime distrition for in-elements

Exponential fit (binsize=1min): y=340.3*(e(-x)/3.64)+1.57

Mean life-time = 5.8 m.

E-fold life time = 3.6 m.

More than 10000 IN-elements and 2000 NT-elements are identified. Their magnetic flux and size distributions are derived.

There are plenty of IN ephemeral regions, some of which has total unsigned flux of 1016 Mx and separation < 1 arcsec.

The IN and NT elements have flux distribution function with peak at 1x1016 & 1x1017 Mx, respectively.

IN elements have a life-time of 3-5 minutes, thusly IN fields do contribute the most magnetic flux to the solar surface each day.

The overall relationship among line-of-sight and horizontal magnetic fields, Doppler shifts, and intensity on the quiet Sun and IN elements is complicated. New evaluation needs to be made.

III. Concluding remarks Some fundamental results from ground-based

observations are confirmed Many things are still not known, say the horiz

ontal fields on the quiet Sun New quantitative measurements are timely an

d extremely important; All previous results need to be set on more solid ground

A new era of IN field studies is opening now by Hinode SOT SP/FG observations

More difficult tasks are getting idea on the magnetic coupling in the quiet solar atmosphere