The magnetism of the very quiet Sun

Solar-B 5, Tokyo, November 2003

J. Sánchez Almeida

Instituto de Astrofísica de Canarias,Spain


What is quiet Sun? Why is it important?

Main observational properties

Surface coverage Degree of tangling Magnetic field strengths Magnetic flux and energy Variations with the solar cycle

Influence on the corona

Origin of the QS magnetism (hints from theory)

Conclusions

Connection with Solar-B


Network

Inter-Network


1”x1”

Do

mín

gue

z C

erd

eña

et a

l. (0

3))

Inter-Network Quiet Sunangular resolution mag. 0.5”sensitivity 20 GVTT (obs. Teide), speckle reconstructedUnsigned flux density 20 G


Most of the (unsigned) magnetic flux and energy existing on the solar surface at any given time is in the quiet Sun.

It may play a significant role in all the physical processes pertaining to the global solar magnetic properties (dynamo, coronal heating, sources of the solar wind,…).

This role has been neglected so far.

Easy target for the new spectro-polarimeters.


Between 90% at solar maximum and 99% at solar minimum (e.g., Harvery 1994)

Surface Coverage


Degree of tangling

The magnetic structures of the quiet Sun are not spatially resolved. Actually, the magnetic field vector varies within scales smaller than the smallest that we can resolve

350 km 0.5”

observer

100 km

line of sight

Resolution Element

magnetic field vector


IR spectral lines Visible spectral lines

SA et al. 2003b

Solar-B 5, Tokyo, November 2003Stokes Profiles


Stokes V profiles observed in the Quiet Sun (SA & Lites, 2000)

Usual hypothesis


polarization signals in complex (tangled) magnetic fields cancel out

1B

2B

Q2 = -Q1 Q1+Q2 = Qobs = 02B

1B

V2 = -V1 V1+V2 = Vobs = 0


SA et al. (2003)

The symbols correspond to observations of IN Quiet Sun Magnetic fields


In short, due to the complex topology of the QS magnetic fields,

The measure of the magnetic field properties is a non-trivial issue. It involves a big deal of modeling and assumptions on the underlying atmosphere: Inversion Codes.

All measurements are bound to underestimate the magnetic flux content of the QS fields .


Magnetic Field Strengths

Sunspots:

Plage & Network regions:

Inter-Network quiet Sun:

G 3000 --G 2000 B

G 1500 B

G] 1500 G, 0[B

Solar-B 5, Tokyo, November 2003SA et al. 2003b


SA

et a

l. 20

03b;

Soc

as N

avar

ro &

SA

200

3

PDF: probability of finding a given field strength (per unit field strength).

from Hanle effect

from Zeeman effect


dBBB

B max

min

)PDF(factor fill

dBBBB

B max

min

)PDF(density flux (unsigned)

dBBBB

B max

min

)PDF(8

1density energy magnetic 2


Most of the quiet Sun (IN) surface is covered byweak magnetic fields.

more than 95% of the surface has G 500 B

Magnetic flux density in the form of weak and strong magnetic field strengths.

Flux (B < 500 G) = 55 G = 75 % of the flux

Flux (B < 100 G ; Hanle signals) = 50 G = 60 % of the flux

Flux (B > 500 G) = 20 G = 25 % of the flux

75 G < B < 120 G


Network

AR + Network

Quiet Sun (visible + IR)

AR

+N

dat

a fr

om S

chrij

ver

& H

arve

y, 9

4, S

Ph,

150

, 1

Quiet Sun (Hanle)


Energy density in the form of weak and strong magnetic field strengths.

Energy (B < 500 G) = 24 % of the mag. energy

Energy (B < 100 G ; Hanle signals) = 19 % of the mag. energy

Energy (B > 500 G) = 76 % of the mag. energy


Variation along the solar cycle

Unknown, but it is one of the clear observational targets. To be achieved thanks to the new synoptic magnetograms (e.g., SOLIS www.nso.noao.edu/solis/ ).

Claims on the the variation:

No flux density variation along the cycle within 40% (SA 2003c). Refers to the tail of kG of the PDF

Variation of some 100% (Faurobert et al. 2001). Refers to the weakest fields, deduced from Hanle signals.

No variation of hanle signals (Trujillo-Bueno & Shukina 2003).

If existing, the variations are negligible as compared to the variation observed in active regions.


Options for the origin of the quiet Sun magnetic structures

Debris from active regions produced by the global solar dynamo

Turbulent local dynamo driven by granulation. (Petrovay & Szakaly 1993, Cattaneo 1999,…)

Turbulent global dynamo (Stein & Nordlund 2001, Schussler et al. 2003, etc.)


Debris from Active Regions: Unlikely

ARs emerge (and so decay) at a rate of 6 x 1021 Mx day-1

IN magnetic flux > 1.2 x 1024 Mx

then IN cannot decay in less than 200 days,

(200 days = 1.2 x 1024 Mx / 6 x 1021 Mx day-1)

otherwise they would be gone before fresh AR flux replenishes it.

200 days is too long since the IN fields vary in timescales of min … Lifetimes found in the literature are hours.


BzTemperature

1”

Turbulent local dynamo

Cattaneo & Emonet, 2001


SA et al. (2003)


Turbulent global dynamo

Unclear how to distinguish this mechanism from the local dynamo.

complex topology no variation along the cycle tight coupling with granular motions

Stein & Nordlund 2001



Traditionally, the influence of the IN fields on the coronal magnetic field is neglected.

Argument: the magnetic field is so complex that most of the field lines close in very low loops and never reach the corona.

However:

The base of the corona is very low: 2500 km (VALC)

Cancellation is often non-local (e.g., Schrijver & Title, 2002) so a fraction actually makes it to the corona.

The IN flux is so large that a small fraction makes a significant absolute contribution

Solar-B 5, Tokyo, November 2003Hoffman et al. 2003

force free extrapolations

)0( BB

loop height < 500 km

1000 km < height < 2000 km


magnetic energy equals thermal energy density

Hoffman et al. 2003

prominences


The topology of the network fields reaching the corona is significantly modified (in a non-trivial way) by the presence of IN fields.

Schrijver & Title (2003)

Solar-B 5, Tokyo, November 20030.3” Solar B resolution

nm 630.2 I Fe @

104 V/I G 20 -3c

IN fields would be routinely detected with normal mapping of the spectropolarimeter of SOT.

(however, only the tail of kG IN fields)


Studying the IN magnetism is appealing, since it allows to address basic problems of solar physics (solar dynamo, coronal structure and heating, sources of the solar wind, magnetic decay, …)

Because of the complications of the magnetic field, non-trivial magnetic field measurements are needed. Stokes profiles are needed for this task (provided complex magnetic fields are allowed for by the inversion techniques).


The quiet Sun is a component of the solar magnetism whose role has been neglected so far, but whose true role is not understood yet.

Quantitatively important in terms of the global magnetic properties (e.g., carries more unsigned flux than all active regions at the solar max.) It occupies most of the solar surface.

Complex magnetic topology. The diagnostic of their properties based on the observed polarization is a non-trivial issue. Stokes profiles + inversion techniques needed!

It is not possible to describe the magnetic field of a IN pixel with a single magnetic field vector.


(?) fairly easy to detect with high angular resolution (easy to achieve by the new generation of ground based 1m-class + Adaptive Optics or space-borne magnetometers, line SOLAR-B.)

(??) No strong variation along the cycle. Refers to the tail of kG fields detected in visible magnetograms.

(?) Magnetic Flux and magnetic energy dominated by the tail of kG fields.

Distribution of magnetic field strengths: from 0 to 1.5 kG.


(??) Does it reach coronal heights? Probably it does in the quiet Corona

(??) Relationship with the origin and acceleration of the solar wind?

(??) Coronal heating due to nano-flares?

(??) Role within the global solar dynamo responsible for the 22 years solar cycle: passive, leading role …

(??) Responsible for the chromospheric basal flux.


Documents

The magnetism of the very quiet Sun