Transcript Document

OBSERVATION, DATA ASSIMILATION
AND SIMULATION OF GLOBAL SOLAR
MAGNETIC FIELDS
Mausumi Dikpati
High Altitude Observatory, NCAR
High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR)
The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research
under sponsorship of the National Science Foundation. An Equal Opportunity/Affirmative Action Employer.
Observations of the Sun’s global magnetic
field evolution with 11 year periodicity
HISTORICAL BACKGROUND
All global solar dynamo theory can be traced back to papers by Parker,
Babcock and Leighton
.
1. Generation of dynamo waves from differential rotation and helical
convection acting on seed magnetic fields (Parker 1955a)
2.
Fluxtubes rising due to magnetic buoyancy (Parker 1955b)
3.
Polar reversal due to poleward migration of poloidal flux (Babcock
1961; Leighton 1969)
FLUX TRANSPORT DYNAMO PROCESSES
+
Meridional
circulation
Wang & Sheeley, 1991
Choudhuri, Schüssler, & Dikpati, 1995
Durney, 1995
Dikpati & Charbonneau, 1999
Küker, Rüdiger & Schültz, 2001
And certainly many others
2D Babcock-Leighton flux-transport dynamos
Turbulent diffusivity
Longitude-averaged alpha-effect
𝛀
⟨ρv⟩
Results
Observed NSO map of
longitude-averaged
photospheric fields
Contours: toroidal
fields at CZ base
Gray-shades:
surface radial fields
3D Babcock-Leighton Flux-transport dynamo
3D kinematic dynamo equation
+S
Spot-maker recipe
1. Spot-producing toroidal
fields from tachocline
center
2. Threshold field strength
defined
3. Delay time (and randomness)
Miesch & Dikpati (2014)
Deposit spot pairs
on the surface in
response to
dynamo-generated
toroidal field at
tachocline
Spot-maker recipe
Field of the bipolar spots at the surface :
Gaussian or polynomial profiles
with tilt given by Joy’s Law
(Stenflo & Kosovichev 2012)
3D structure of a bipolar spot is computed by using
potential field extrapolation below the surface
Assumption: Spots quickly
decouple from deep roots
(Schussler & Rempel 2005)
Results from 3D Babcock-Leighton Flux-transport Dynamo
At low latitudes, small-scale
features appear due to eruption of
tilted bipolar spots, but their
dispersal by diffusion, meridional
circulation and differential rotation
produces mean poloidal fields
Miesch & Dikpati (2014)
• Trailing flux drifts towards
the poles in a series of
streams and cause polar
reversal
• Toroidal field butterfly
diagram shows equatorward
migration, cycle period is
governed by meridional
circulation
see also Yeates & Munoz-Jaramillo (2013)
Toroidal, poloidal and nonaxisymmetric magnetic energies
Normalization
-3
Toroidal magnetic energy:
4 ´10 7 ergcm
Nonaxisymmetric mag. energy:1.5 ´105 ergcm -3
Poloidal magnetic energy: 6 ´10 3 ergcm -3
Unresolved issues with global magnetic fields
 Why did we not observe a dipolar corona in the last solar
minimum?
 What produced an unusually large phase difference (~3 years)
between North and South hemispheres’ solar cycles?
 What are the statistics and origins of buoyant and coalesced
spots, as revealed by SDO/HMI observations? Do GONG
magnetograms analysis agree with SDO/HMI?
 What if the 2nd meridional circulation cell in depth is confirmed?
 Is solar dynamo on the verge of a paradigm shift?
Solar minimum corona during past three cycles
1986
1996
2008
 Solar minimum corona is normally a dipolar corona, but it was not at the
end of cycle 23
 Is it due to large phase difference between North and South cycles
during the declining phase of cycle 23 and rising phase of cycle 24?
 Global coronal structure is governed by the evolution of dynamogenerated magnetic fields; therefore it is necessary to understand the
phase shift between North and South dynamos
Phase shift between North and South hemispheres
Sun et al. (2014)
•
An unusually large
phase-shift of ~3 years
between North and
South in cycle 24 was
observed
• Polar fields took much
longer than normal to
reverse, but the North
reversed about a year
before the South.
Causes of phase shift between solar cycles in
North and South hemispheres
Phase shift can be caused by North-South asymmetries in dynamo
ingredients, such as in
(i) Babcock-Leighton poloidal source (Belucz et al. 2013) ,
(ii) Meridional circulation profile and speed (Belucz & Dikpati 2013)
(iii) Inflow cells associated with active regions (Shetye et al. 2015, see also
Cameron & Schussler 2012).
But, what physics initiates the North-South asymmetry
in the dynamo ingredients?
Two types of spot-emergence
SDO/HMI reveals that spots manifest at the surface by their buoyant eruption
and also by coalescence at the surface from salt-pepper type small bipoles
Difference in properties between coalesced and buoyant spots
Buoyant
Coalesced
Total unsigned flux is more for buoyant
than coalesced spot
Buoyant
Coalesced
Buoyant spot (NOAA11948)
The angle of the field to radial varies smoothly
rotated faster than coalesced
with time for the buoyant spot, but strongly
fluctuates for the coalesced spot
spot (NOAA 11588)
(Sainz-Dalda, Dikpati, Rajaguru & Judge 2015, in preparation)
Statistics and origin of buoyant and coalesced spots
SDO/HMI images indicate there were many more coalesced than buoyant
spots during early phase of cycle 24
The number of buoyant versus coalesced spots may vary with cycle phase,
and from cycle to cycle.
GONG magnetograms should be analysed to confirm these two types of spotemergences in cycle 24 as well as cycle 23
Possible origins:
•Perhaps all spots originate from tachocline toroidal field, but coalesced spots may have
fragmented into small, salt-pepper type bipoles before reaching the surface, which
coalesced later to produce spots.
•Coalesced spots may also be coming from local dynamo action at the surface. In that
case Joy’s law tilt can be weak and random, leading to weaker polar fields.
•Could a change in the proportions of buoyant and coalesced spots cause the low polar
field strength in cycle 24?
Meridional Circulation is the Biggest Challenge for
Flux-transport Dynamos
Is meridional circulation single or
multiple celled in depth?
Single cell reproduces solar cycle
features well. But two cells in depth
generally do not.
Three cells in depth have not been
observed, but have been used in
simulation of flux-transport dynamos
(Hazra, Karak & Choudhuri 2014) to
reproduce solar cycle features
However many cells there are in depth, the flux-transport dynamo models
need equatorward flow near the bottom, where toroidal fields can be held
and amplified, to get the correct sense of migration and butterfly diagram
Spatial profile of the Sun’s meridional circulation
is not settled
Observations:
• Doppler measurments from MWO data indicate poleward surface flow
up to ~60 degree latitude, after which it reverses (Ulrich 2010)
• Time distance heiloseismology using SDO/HMI data indicates two cells
stacked in depth (Zhao et al. 2013)
• Perturbation technique using SoHO/MDI data indicates four cells in
latitude (Schad et al. 2013)
• Ring diagram analysis from GONG data indicate a long primary cell
going all the way down to the bottom of convection zone (Kholikov et
al. 2014)
Spatial profile of the Sun’s meridional circulation
is not settled (contd.)
Models:
• Mean-field models produce
one long primary cell with
poleward surface flow,
often associated with a
weak reverse flow for a
solar-like differential
rotation (Ruediger 1989;
Rempel 2005; Dikpati 2014)
• Convective simulations
indicate a more complex
pattern consisting of
multiple cells in latitude and
depth, which are nonsolarlike (Gilman 1983;
Guerrero et al. 2013;
Featherstone and Miesch
2015)
Antisolar DR;
solar-like mc
Solar-like DR;
non-solarlike mc
Can we infer solar meridional circulation by applying
data assimilation in solar dynamo models?
Reconstruction using 1 observation
True state and “synthetic”
and 16 ensemble members
observation
• Reconstruction is reasonably good, except for two windows in time
• For an initial guess far-off from truth, reconstructed state
asymptotically converges toward the truth
Dikpati, Anderson & Mitra, GRL, 2014
Prospects of combining data assimilation technique
Reconstruction using 1 observation
True state and “synthetic”
and 16 ensemble members
observation
• Reconstruction is reasonably good, except for two windows in time
• For an initial guess far-off from truth, reconstructed state
asymptotically converges toward the truth
Dikpati, Anderson & Mitra, GRL, 2014
Potential of EnKF data assimilation for reconstructing
spatio-temporal pattern of meridional circulation
What happens
if the initial
guess about
the spatial
pattern of
meridional
circulation is
wrong?
Obviously
reconstructed
flow-speed is not
so good
Correct
spatial
profile
Optimum reconstruction
using 160 observations
and 192 ensemble
members
Incorrect
spatial
profile
Dikpati, Anderson & Mitra, GRL, 2014;
(see also Alex Fournier’s AGU Poster of last fall)
Flux-transport dynamos will not work for the Sun if
meridional circulation consists of multiple cells in depth
(Belucz, Dikpati & Forgacs-Dajka 2015, submitted)
See also
Jouve &
Brun (2007)
for another
four-celled
case
What if the 2nd cell in depth is confirmed?
• Surface transport mechanism for generation and evolution of polar fields
would remain unaffected
• Polar fields would not be able to get to the bottom to provide the seed for next
dynamo cycle.
• Tachocline toroidal fields would migrate poleward
Then a paradigm shift in solar dynamo models is necessary:
 Stronger equatorward propagation needs to be created by some other
mechanism, by helical flow in the tachocline combined with radial shear.
 Tachocline shear needs to suppress small-scale dynamo action in the matter
described by Cattaneo & Tobias (2014)
However it might be premature to embrace paradigm shift, because:
 There is skepticism about 2nd cell in depth
 It is theoretically difficult to produce multiple cells in depth
 Another independent analysis using different instrument’s data did not confirm
2nd cell in depth yet
Future measurements of greatest importance
• Meridional circulation profile, particularly below the outermost 30 megameters,
and near the tachocline
• Departure of thermodynamic structure, particularly density, from spherical
symmetry as in “standard” solar model
Hypothesis: Every meridional circulation pattern should have a non-spherically
symmetric density pattern associated with it
Questions:
1. Does thermodynamic structure found “match” with meridional circulation
observed?
2. Could evidence of 2nd circulation cell in depth actually be evidence of
thermodynamic structure?
Or, Are all these helioseismic inferences independent of each other?
Longitude dependent motion and fields in the tachocline:
1. Can large-scale nonaxisymmetric flows and magnetic fields be observed, as
found in calculations of global magnetorotational instability of tachocline?
2. Can also the related longitude-dependent thermodynamic structure in the
tachocline be observed?
Unresolved issues with polar field measurements
•WSO polar field is the longitude-averaged
mean surface line-of-sight field within the polar
cap from 65-degree latitude to pole
•Hinode observations indicate polar regions
have many “patches” of magnetic flux > 10^18
Mx, which vary strongly with cycle phase, in
addition to many more small patches of weaker
fields that do not vary much with cycle phase.
So the solar cycle signal is carried only by the
big patches.
Tsuneta et al (2008)
1. What is the polar field measurement that dynamo simulations should be
compared with?
2. Is there an observed process in polar region regarding polar field strength
and evolution that dynamo models are not accounting for?
3. Is it possible to have an instrument flown in an orbit around the Sun
that will observe the poles on a regular basis for at least a decade?
Summary of requirements of new explorations of
available data and requirements for new data
•Large North-South asymmetry in the current cycle is now an important topic of
study; extend the study to longitude-dependent features
•GONG magnetograms need to be analysed for distinguishing buoyant-eruption
versus coalescence in cycle 24 first, for calibration with SDO/HMI and then cycle
23
•Does a 2nd cell in depth exist? If so, is it a permanent or intermittent feature?
•Is the meridional circulation at the bottom of the convection zone poleward or
equatorward?
•If the meridional circulation is poleward at the bottom, then another paradigm
shift in solar dynamo is necessary. What form would this paradigm shift take?
•Search for evidence of nonspherically symmetric thermodynamic structure in the
convection zone and tachocline that can be compared with theoretically obtained
thermodynamic patterns
•Measurements of global longitude-dependent flows at the base of the convection
zone, which most likely govern the global longitude-dependent magnetic features
we see at the surface
Thank you