UCL Department of Space and Climate Physics Mullard Space Science Laboratory
Multi-­‐wavelength observations of active region evolution
Lucie Green & Lidia van Driel-­‐Gesztelyi
Active Region -­‐ Definition
• Active regions are the totality of observable phenomena in a 3D volume represented by the extension of magnetic field from the photosphere to corona, revealed by emissions over a wide range of wavelengths from radio to X-­‐rays and 𝞬-­‐rays (only during flares) accompanying and following the emergence of mildly twisted magnetic flux (kG, ≥ 1020 Mx) through the photosphere into the chromosphere and corona. • The simplest ARs have bipolar magnetic field configurations, but ARs may be built-­‐up by several bipoles emerging in close succession. Active Region -­‐ Definition
strong active
magnetic field number
is manifested by the • In the photosphere the presence Aof NOAA
region
is
region
has spots
appearance of dark sunspots or assigned
pores and whilst
bright the
faculae representing concentrated and dispersed magnetic fields, respectively. • In the chromosphere arch filament systems connect opposite polarity magnetic concentrations, filaments form along the magnetic inversion line and the bright regions which appear above dispersed fields are called plages. • In the transition region and corona bright, hot, dense loops connect the opposite magnetic polarities. Active Regions & plasma beta
•
The lower corona is a unique band in the
Sun’s atmosphere where the magnetic
pressure dominates over the plasma
pressure (plasma β= 8πp/B2).
•
Plasma β <1 up to 1.2 to 1.5 solar radii
(Gary, 2001).
•
Emerging AR magnetic field carries
energy into the solar atmosphere.
•
Emerging AR field interacts with preexisting field driving changes in topology
and dynamic energy release events.
Gary, 2001
4
SDO HMI & AIA – AR 12119 18-­‐23 July 2014 – evolution of a small AR of 2.5 x 1021 Mx
HMI
1600 T≈105 + 5•103
304 T≈5•104
171 T≈6.3•105
193 T≈1.2•106 + 2•107
211 T≈2•106
Flux budget of the Sun
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Phase I: Active region formation
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Structure of the magnetic field
•
Portions of the toroidal flux layer become buoyant and rise up as omega loops •
•
Early emergence suggests that flux is shredded •
Scenario is consistent with the emergence of a fragmented omega loop Later emergence and formation of sunspots suggests that flux is in coherent bundle Zwann, 1985
Strous & Zwann, 1999
8
Active region formation: serpentine flux emergence
Cheung, LRSP (2014)
Reconnection at bald patches leads to plasma The effect of granular flows heating (Ellerman bombs in Hα) Pariat et al. (2004)
on serpentine field lines (Cheung et al., 2010)
Serpentine flux emergence – coronal reorganisation
Hinode/EIS
Valori et al. (2012)
Hinode/SOT/SP
Emerging flux in AR 11024: NLFF analysis using the magneto-­‐frictional method using Hinode/
SOT/SP vector data.
Serpentine magnetic field lines
Valori et al. (2012)
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Inherent twist?
•
Electric currents computed from the gradients of the magnetic field vectors grow with the
emerging flux (Leka et al., 1996)
•
Vector magnetogram data show twist when plotted over an emerging active region
!
!
Location of flux tube boundary
Model:
pitch angle:
13.9o
7o
•
untwisted flux tubes are
destroyed by vortices that
form in its wake in All
the major flux which has crossed the
convection zone convection zone and emerged into the
•
twist can conservecorona
the
must
integrity of the flux tube
(Emonet & Moreno-Insertis,
1998)
2.5o
0o
be twisted?
!
wake structures
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Polarity patterns in emerging ARs indicate that they are twisted ! (Leka et al., 1996; …)
magnetic tongues: trace of azimuthal B flux rope partly emerged
…and this is observable even in the LOS field! (López-­‐Fuentes et al., 2000; Luoni et al., 2011)
Luoni, Mandrini, Démoulin, van Driel-­‐Gesztelyi, Solar Phys. (2011)
First days of an AR emergence
trace of: axial B new bipole
70 Mm
time
=> apparent rotation of the AR
In MHD simulations of twisted flux emergence such tongues (or tails) are clearly present (Archontis and Hood, 2010; MacTaggart, 2011; Jouve, Brun, Aulanier, 2013; review by Hood, Archontis, McTaggart, Solar Phys., 2012). Analytical modelling – effect of twist level
!
!
!
!
!
•
•
AR 8757
!
!
!
Luoni et al. (2011)
The direction of the inversion line and the elongation of tongues depend on the level of twist (Nt, the number of turns) in the half-­‐torus model flux-­‐rope. Using coronal extrapolations, sigmoidal shape, helicity flux for 40 ARs the sign of helicity inferred from the tongue pattern was consistent with that deduced from these other known helicity sign proxies. – This simple method works!
Evolution owing to a global twist of the emerging flux tube: tongues/tails •
•
•
•
•
•
Follow-­‐up to work by Poisson et al. (2015) on 41 bipolar ARs.
Magnetic tongues/tails: analytic model for N=1 (end-­‐to-­‐end twist)
τ is the angle between the PIL and orthogonal to the main bipole axis
φ is the tilt angle
The AR rotates owing to the evolution of the tongues.
The total magnetic flux reaches maximum because an azimuthal component is added to the LOS axial flux. As the emergence continues the azimuthal flux contribution is decreasing and disappears, which may be interpreted as flux cancellation.
Poisson, Mandrini, Démoulin, López-­‐Fuentes, Solar Phys, 2015
Are all ARs twisted and if so, how much?
Poisson, et al., Solar Phys, 2015
• Selected sample of 41 ARs 01/2003-­‐01/2011 (isolated ARs, low background flux, emergence close to disc centre). • Overall twist is relatively low, but it is evident in practically all ARs. MDI
HMI
Inherent twist
There is a weak hemispheric preference for:
•
•
negative twist in northern hemisphere
positive twist in southern hemisphere
(Canfield & Pevtsov, 1999)
!
This isn’t dependent on solar cycle.
!
Q. Where do the flux tubes acquire their
twist? During the dynamo generation
process or through buffeting in the
convection zone?
Some (extreme?) cases show rotating
sunspots
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Formation of AR 12017
• Use Jhelioviewer or solar monitor to look at the formation of AR 12017 • Are there any tongues? • what is the trend in that hemisphere?
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Quick look at HMI line-­‐of-­‐sight magnetograms for AR 12017 on 23/24 March 2014
Set-up IDL to include aia & sdo paths, gunzip data if needed and extract files -> ‘tar xvf'
!
•
•
•
•
•
IDL> spawn, ‘ls *.fits’, hlist
IDL> print, hlist
IDL> ss=indgen(10)
IDL> ss=ss*12.
IDL> aia_prep, hlist(ss), -1, index, data, /uncomp_delete
!
(see sheet about missing values)
!
• IDL> index2map, index, data, hmaps
!
•
•
•
•
IDL> sub_map, hmaps, shmaps, xrange=[-900, -400], yrange=[100, 400]
IDL> rmaps = drot_map(shmaps, ref_map=shmaps(9))
IDL> plot_map, rmaps(0), dmin=-1000, dmax=1000
IDL> movie_map, rmaps, dmin=-1000, dmax=1000, xrange=[x1, x2], yrange=[y1,y2]
Phase II: Active region decay
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Démoulin et al., 2002
Active region evolution
•
•
Largest spots may last for months
•
Magnetic structure simplifies and
becomes more bipolar
•
It is dispersed by random walk process
and gathers along the supergranular
boundaries
•
The flux tube of the active region
apparently gets disconnected from its
toroidal roots.
•
Lifetime:
Sunspots break up as flux gets pealed off
the strong concentrations
tAR (days) = 15 Φ / 1021 Mx
!
where Φ is the total magnetic flux of the
active region. Approx. 5 months for a
major AR (Schrijver & Title, 1999)
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The best AR to study long-­‐term magneUc evoluUon: AR 7978
EFR -­‐ 1st rotation, Jul. 1996
2nd rotation
3rd rotation
4th rotation
5th rotation
Q. To what extent do the active region
rotation
fields remain connected to6th
the
subsurface fields which connect down to the
(van Driel-­‐Gesztelyi et atachocline?
l, 1999, 2003; 7th rotation
Démoulin et al, 2002, 2003, Mandrini et al., 2004, Li and Welsch, 2008, …)
During solar minimum between cycles 22 and 23, the 4th of a 5-­‐AR activity nest (50% of ARs emerge in a nest). SOHO/MDI magnetograms July-­‐December 1996
How many rotations can AR 12017 be tracked for?
Rotation 1
Rotation 2
Rotation 2
Rotation 3
MagneUc area and flux density evoluUon
van Driel-­‐Gesztelyi et al., ApJ (2003)
Area
Flux density
Mean magnetic flux density is total magnetic flux divided by the surface area of the AR.
MagneUc flux evoluUon of AR 7978 van Driel-­‐Gesztelyi et al., ApJ (2003)
Fixed area: 1.6 x 1011 km2
Not corrected for MDI’s underestimation of flux Φ x 1.45
Flux in AR 12017
Use poly_manual_HMI.pro on (last) HMI image(s)
!
IDL> .r wdefroi
IDL> poly_manual_HMI, rmaps, posflux, negflux, pospix, negpix, movie
!
rmaps is the input
!
posflux, negflux, pospix, negpix, movie are all outputs
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Milestones in the long-­‐term evoluUon
B decrease, shearing, polarity separaUon
• Southern hemisphere AR • The PIL (NL) angle is changing in accordance with shearing by differential rotation. • The weighted polarity separation hardly changes from the 3rd to the 4th rotation Li and Welsch (2008)
Active region evolution
The long-term evolution can
reflect the 3D structure of the
emerging flux rope.
(Lopez Fuentes et al, 2000).
Green et al., 2002
30
Removal of magnetic flux
•
Ohmic dissipation
•
The diffusion timescale is found by equating
the LHS of the induction equation with the 2nd
term on the RHS.
•
This is the timescale of Ohmic decay due to the
finite electrical resistance. For a sunspot with
L=106 m, tD ~ 30,000 years. Ohmic dissipation
becomes effective on sub-granular length
scales.
Flux retraction
Only works for very small elements where there is a
large curvature such that the tension force
overcomes buoyancy.
Cancellation
A combination of magnetic reconnection and flux
retraction due to the tension force.
!
LifeUme – funcUon of total flux
AR
Flux (Mx)
Lifetime
Emerge/Lifetime
Large w. spots
5x10
weeks-­‐months
15%-­‐3%
Small w. pores
1x10
Days-­‐week(s)
15%-­‐27%
Ephemeral
3x10
Hours-­‐≈1 day
~30%
Schrijver and Zwaan, 2000
•
•
The lifetime of an AR can depend, however, on its interaction with surrounding
magnetic fields.
The lifetime of two ARs with the same flux content can be very different
depending on the phase of the cycle: a large AR may be traceable for up to 10
months during solar minimum, when it can evolve undisturbed, while its identity
can be lost in less than 4 months during solar maximum (Schrijver and Harvey,
1994).
Timescales
• How long do ARs spend emerging?
• How long do ARs spend decaying?
-> Most of their life spent in the decay phase
Activity evolution
ARs are the principal source of a broad range of solar activity phenomena: ranging from small-­‐scale brightenings and jets to the largest flares and coronal mass ejections (CMEs). The level and type of activity is dependent on the evolutionary stage of an AR, being highest at the emergence stage and decreasing after that. Activity evolution in AR 7978
Yohkoh / SXT
SoHO / MDI
– Shearing coronal loops
– Converging motions at PIL
– Flux dispersal and B decrease
– Flux cancellation at PIL
!
– Energetic flare rate î – CME rate ~ const
Démoulin et al. (2002) & van Driel Gesztelyi et al. (2003) Martin et al. (1985), Schmieder et al. (2008), Park et al. (2010), Green et al. (2011)
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Flare and CME activity in AR 7978
15
18
11
12
Démoulin et al., A&A (2002)
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AR 8100
37
Q. How is the CME rate maintained
during the decay phase?
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56.1
38
Finding flare and CME activity
•
in IDL ‘pr_gev’ as a quick look, but only whilst there is an active region with a
NOAA label!
•
Soft X-ray light curves in the active region which can be mapped to the GOES
light curves
•
CME identification through coronagraph images and lower coronal signatures
such as filament eruptions, rising EUV/SXR structures, dimming regions and
EUV global waves
Formation of eruptive structures in decay phase
• Flux emergence • Flux fragmentation and dispersal • Elongation along the Y direction • Flux cancellation
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Model for flux rope formation
van Ballegooijen
& Martens, 1989
Filaments form during
this phase
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Formation: AR decay phase
1) Flux emergence
2) Increasing shear
3) Double-J formation
4) Continuous S
Feb 2007
17 hr
7 hr
9 hr
12 hr
12 hr
24 hr
ON FAR
SIDE
AR 10977
AR 8005
Evolutionary stages in isolated bipolar regions
17 hr
ON FAR
SIDE
42
Formation: decay phase
2) Increasing shear
3) Double-J formation
4) Continuous S
7 hr
AR 10977
9 hr
12 hr
12 hr
24 hr
AR 8005
17 hr
Feb 2007
Evolutionary stages in isolated bipolar regions
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Flux cancellation – formation of flux rope (sigmoid)
Green, Kliem, Wallace (2011)
• 34% of the peak –ve flux cancels 2.5 days prior to the eruption. • If this all was built up in the flux rope, then it would contain ~60% of the AR flux. (van Ballegooijen & Martens, 1989).
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Cancellation -­‐ flux rope formation
Green, Kliem, Wallace (2011)
• But: cancellation can only create a flux rope from sheared fields, in fact it is re-­‐distributing the shear, concentrating it along the PIL. !
• The above example shows that only 2/5 cancellations were forming a flux rope out of the sheared arcade. !
• Poloidal and axial flux in the flux ropes for most models amounts to about 60%-­‐70% of the cancelled flux and 30%-­‐50% of the total flux in the regions. (Savcheva et al., 2012) !
However, more cancellations – more massive fluxrope? 45
46
Pre-eruption structure
Lites, 2005
+
Flux rope versus arcade
-
Identifying flux ropes:
• Inverse crossing of vector field
Concave up field could be produced by field
lines at the bottom of a flux rope (Athay et al.,
1983; Lites, 2005; Canou et al., 2009)
Green &
Kliem, 2009
• (some?) sigmoids
!
Inverse crossing of the PIL by S shaped field
lines which survive an eruption (Fan & Gibson,
2006; Green & Kliem, 2009) so that S shaped
field lines in a sheared arcade can be
excluded (Antiochos et al., 1994)
• Plasmoids/hot flux ropes
!
Formed and heated during magnetic
reconnection. Temperature of ~10 MK (Shibata
et al., 1995; Reeves & Golub, 2011; Zhang et
al., 2012; Patsourakos et al., 2013)
Shibata et al.,
1995
!
Image from
Reeves &
Golub, 2011
47
The helicity budget in AR 7978
Démoulin et al., A&A, 2002 Mandrini et al., 2004
All values are in units of 1042 Mx2 !
Estimated helicity carried away per CME !"!" ≈ 2 x 1042 Mx2 • The total amount of helicity which should come from twisted flux emergence and and replenished via torsional waves from the sub-­‐surface reservoir after CMEs can be estimated to be between 55 – 241 x 1042 Mx2 in this simple bipolar region, that produced 30 CMEs. • During the same period, the differential rotation injected only 8. 3 x 1042 Mx2 , so it clearly 48
was a minor contributor to the magnetic-­‐helicity budget of AR 7978. Helicity evolution in two bipolar ARs
The helicity source in these small
bipolar active regions appears to
be driven by photospheric flows
49