NAOJ Future Planning Symposium 2021
Nov. 9-10, 2021

Solar/Stellar magnetic fieldand dynamo

Hideyuki Hotta (Chiba University)
Materials provided by: Ryohko Ishikawa, Noriyuki
Narukage (NAOJ), Kazumasa Iwai (Nagoya-U),
Shinsuke Imada (U-Tokyo)
1

Contents
1. Current solar missions/projects in Japan
2. Solar magnetic field and dynamo
a. Theoretical view
b. Current observational achievementc. Future perspective
3. Connection to stellar magnetism (if time allows)
4. Summary2

Major problems in solar physics
Convection andmagnetic field
Solar windCoronal Mass
Ejection (CME)

Hinode/SOT

2. Flare

3. Coronal heating
Corona

1. Dynamo

Chromosphere

Chromosphericheating
Photosphere# Scale is not precise3Japanese projects in solar physics
Provided by JSPC(太陽研究者連絡会)

Chromosphere/coronalheating and solar wind

Solar flare and CME

Solar-C_EUVST

space

PhoENiX

space

Hinodespace○

SUNRISEballoon○

CLASP

sounding rocket

○

FOXSI

sounding rocket

ALMA

ground-based

Nobeyama Radioground-based

Mitaka Flareground-based○○

Hida DST

ground-based○○

Hida SMART

ground-based

Nagoya IPS

ground-based

Yamagawa Radioground-based○

ngGONG

ground-based

○

NIRTF

ground-based

SciCRT

ground-based○

AMATERAS

ground-based○

Simulations○

Solar cycle anddynamo○○○Orange: Future plan

○
○

○
○

○

○

○

○

○

○

○

○

○

○

○

○

4

Hinode/SOT (Solar Optical Telescope)ü Measurement of 𝑩 in the photosphereUbiquitous
granular-scale loops (hG) kG patches viewed from pole

𝑩 shear (flare trigger)

Intensity

Vertical 𝐵
Tsuneta+08, Shiota+12

Formation of superequipartition (kG) field
Horizontal 𝐵

Ishikawa+2008, Lites+2008

Nagata+08

Bamba+13

5

Hinode/SOT (Solar Optical Telescope)ü Dynamic activities in the chromosphereUbiquitous jets

© Okamoto

MHD waves
Shibata+07
De Pontieu+07

Okamoto+07

6CLASP rocket experiment
Chromospheric LAyer Spectro-Polarimeter

BL in the photosphereü Measurement of 𝑩
in the chromosphere

Ishikawa+21

Magnetic fields (BL) at threeheighs in the chromosphereFirst mapping ofmagnetic fields from thephotosphere to thevery top of thechromosphere7

Hinode/EIS (EUV Imaging Spectrometer)a glimpse
intoTurbulentmotion at the foot point of AR
Haraatet high
al. 2008
ospherexample, during
igh Resolution
et al. 2014)

ion of 0.3-0.4"
detect heating Figure 4.4: (a) Left) Plasma properties in solar wind source regions and their relationship with magnetic fields (Brooks
et al. 2015). Left) full-Sun AIA 193Å composite intensity image (blue) with solar windsourcewind
regionsacceleration(red and green),solar
ower resolution which is identified by abundance. Center) Overlay of magnetic field linesFast
from the potential field extrapolation& Savin of2013
on the Fe XIII 202.044 Å Doppler velocity map from Hinode/EIS.Hahn
Right) Percentage
total mass flux ashermore, Hi-C calculationa function of magnetic field line-starting latitude (red). The vertical blue lines on the left and right side mark thefield lines that latitude of 11° and 40°. (b) Signatures of Alfvén waves in plumes and inter-plumes. Left) Plume and inter-plumeobserved by multiple telescopes (Teriaca et al. 2003). Center) Slit positionsof Hinode/EIS
for thereconnection
observation of theFastflows fromoducing high- right panel overlaid on the SoHO/EIT 195 Å image. Right) off-limb observationof theet
non-thermalImadaal. 2013velocity (V ) fromthe strongest observed lines (symbols). The dashed line gives the predicted trend, proportional to the electron density
3).
n as n , for undamped waves. The observed behavior of V may be considered as a damping signature of Alfvén
AIA 193 A

Solar-Y (arcsec)

(a)

600

500

400

18:20:34
800
900
700
Solar-X (arcsec)

ster-scan observations on (left) 2007 January 18 and (right)-1/4 2007 January 24. Top to bottom: Detected photons, Doppler velocity,
io temperature deduced from Fe xv 284/Fe xiv 274. Solar north is up and west is right. Note the coordinate system is not thewaves (Hahn and Savin 2013).

60Im

350 km/sec

(b)

EIS

40

20

0

1000

8
16
24
0
Elapsed Time from 18:15:22 (min)Figure 2. (a) AIA 193 Å channel image taken at 18:20:34 UT on 2012 January 27 with the position of the CCD pixels (slit) used for preparing (b) shown sup
as a white bar. The dashed box indicates the EIS filed of view. (b) A time–distance diagram generated from the intensity distribution along the slit sh
The vertical axis shows distance along the slit, and the horizontal axis indicates time in minutes from 2012 January 27, 18:15:22 UT. The dashed line rep
transverse velocity of 350 km s−1 . The arrow shows the observing time of the distorted line profile in Figure 3(d) by EIS.
(An animation of this figure is available in the online journal.)

nt

Blue-wing 191.8 A

nt

Center 192.0 A

Red-wing 192.3 A

(b)

(a)

Solar Y (arcsec)

PLATE 1position along slit (arcsec)

The Astrophysical Journal Letters, 776:L11 (5pp), 2013 October 10(c)Diffuse
Structure

550
500

ay of magnetic
the first observing task (I-4-1) for the solar wind study is to diagnose the plasma properties
rvations of the Therefore,
in these regions and clarify the source regions, their contributions, and the relationships withvious missions, magnetic structures. One difficulty in analyzing the source regions of the solar wind is that a longer
cadence (~hours) is needed to observe faint structures (such as coronal holes and AR
ee section 3.3), temporal>400km/s
Figure2.1:
coronalstructuresofand
activities
in of magnitude
boundaries).
TheSolarthroughput
improvement
at least
one order
by Solar-C_EUVST
of slowsolar
windboth theOrigin
solar
soft-Xray.
will allow us to approach the timescales of their evolution (a few to several minutes). Also,8rolling
et al. 20088
ere. TheHarralatest24/47
450-400 -200

0

(d)

200

6000

Intensity

4000
2000

B

C

192.0

800

R

192.2

850

900

750

Solar X (arcsec)
Velocity [km/s]-400 -200

FeXXIV

191.8

750

900

850

0

(e)

B

C

191.8

192.0

R

192.2

800

-200

200

FeXXIV

0

200

400

FeXXIV

B

C

191.8

192.0

Wavelength [Angstrom]

900

850

(f)

1x104 2x104 3x104

800

1x105 2x105 3x105

750

R

192.2

Launch 2027
JAXA Epsilon M-class mission
EUV High-throughput Spectroscopic Telescope

Science objectives:
1. Chromospheric and coronal heating,Solar wind
2. Mechanism for solar flares, i.e., energy
release and eruption.

Solar-C Mission proposal — Supplemental material

Hinode/SOT
resolution~225 kmFigure 3.3: 46"×46" subfields of an Hi-C image both at native resolution (top) and convolved to EIS resolution (bottom). A
PSF value of 0.2" to 0.3" is quoted by the Hi-C team. Here a value of 0.25" (180 km) was used.

HiC (rocket) 225 km resolution

Key features:
A) Wide T-coverage (104-107 K)
Observe the whole regimes of the solar
atmosphere as a single coupled system
B) High resolution(spatial ~ 0.4”, temporal ~ 1 sec)
Capture the dynamic evolutions ofelementary structuresC) Spectroscopy
Determine the physical states of thetargets (𝑣, 𝜌, 𝑇, composition, ionization)

EIS 2250 km resolution
from Brooks et al. (2012) concludes that theseloops contain at most few strands with sizes of
few to several tenths of an arcsec. Moreover,very recent observations of the corona at 0.25
resolution obtained by the Hi-C NASA sounding
rocket show that a resolution better than 0.35
is required to study these coronal structures (J.
Cirtain, private communication). Figure 3.3 uses
Hi-C images to show the effect of increasing
spatial resolution from what currently available(EIS) to what achieved by Hi-C and foreseen for
EUVST. It can clearly be seen how most of theobtaining spectra over 0.3 seems capable ofmeasuring plasma parameters within the overallloop envelope. These observations will provide
the density, temperature and plasma motion profiles of the loops with far better accuracy thanpossible with current instruments. Understanding the sub-structure is crucial for determining
energy flow in coronal loops and ultimately forunderstanding the physics of coronal heating.

9

A possible scenario is that these loops are
NAOJ/JAXA
Solar-C
WGoftenheatedby small-scale energy
releases,Hinode/XRT (X-Ray Telescope)
Solar Observations in X-rays can evaluate
Ø Physical quantities of hot thermal plasma (~ 1 MK to > 10 MK) from soft X-ray
observations, e.g., Hinode/XRTØ Physical quantities of non-thermal (accelerated) electrons hard X-ray observations e.g.,
RHESSI
caused by magnetic reconnection.

Alfvenic waves(e.g., Cirtain et al. 2007)

Outflows from active regions(e.g., Sakao et al. 2007)

Turbulence(e.g., McKenzie et al. 2013)

Nano-flare heating(e.g., Terzo et al. 2011)10

PhoENiX
Physics Of Energetic and Non-thermal plasmas in the X-region
PhoENiX in 2030s –Imaging spectroscopy in soft and hard X-rays
Ø High dynamic range to evaluate entire flaring region
Ø High spatial, temporal, and energy resolutions
For the understandings of energetics in solar flares
including particle acceleration.
PhoENiX WG

Demonstrations with Sounding Rocket FOXSI series
0 5

(

5

2

2 2

-

2

- )5
5

2

0
-

0 5

2

-

100

3.90•104

photons

photons

pixels200

3.85•104
3.80•104

3.75•104
0
0

200

400

600
pixels

800

1000

0

5 0

-

105
104

3.95•104300

FOXSI-3 WG

-

20

40

60
time [sec]

80

100

103
102
101
100
10-1
0

1000

2000
3000
energy [eV]

4000

5000

11

Nagoya-U IPS (InterPlanetary Scintillation)

solar wind

Solar wind observation for
more than 30 years
Global structure of thesolar wind and its solar
cycle variation
Fujiki+2015, Tokumaru+2018

IPS

IPS

Detection of the CMEs andtheir real-time analysis
system are also used forspace weather forecasting.
Iwai+2019, 2021

IPS

IPS: Radio scattering by thesolar wind

New IPS observation
system (MP2023 submitted)
Ø x10 IPS observations.
Ø Resolve the global spatial
scale of the solar wind12

Japanese projects
for solar cycle, dynamo and interior
Chromosphere/coronalheating and solar wind

Solar-C_EUVST

space

PhoENiX

space

HinodeSUNRISE
CLASP
FOXSI
ALMA

ground-based
ground-based

Mitaka Flareground-based

Hida DST

ground-based

Hida SMART

ground-based

Nagoya IPS

ground-based

Yamagawa Radioground-based

ngGONG

ground-based

NIRTF

ground-based

SciCRT

ground-based

Simulations

Solar cycle anddynamo

○
○

○
space
There isalmost no observational○
balloonproject for the understanding the○sounding rocket
solar cycle and solar interior.
sounding rocket→We need it.

Nobeyama RadioAMATERAS○

Solar flare and CME○○

Observe long term evolutionof solarstatus○(flare,○ solar wind, etc)○○○

ngGONG
includes science○○○
goal for the solar interiorbut○○main motivationof Japan'scontribution is space ○○weather
○

○

○
○

○

○

As for the theory for the solar○ interior,ground-based
Japan has been doing world○leading○○large-scale project.

○13

Solar cycle
Average Daily Sunspot Area (% of Visible Hemisphere)

Sunspot area

0.5
0.4
0.3
0.2
0.1
0.0

1880

1900

1920

1940

1960

time [year]

1980

2000

2020

Hathaway/Upton web

Number of sunspots varies with the 11-year cycle.
This is confirmed with several hundred years ofobservations.

Galileo’s sunspot drawing
The Galileo Project

Physical mechanism to maintain the cycle has notbeen understood yet.
One of the most important problems in the solar
physics.

14

Features in solar cycle: Butterfly diagram

The solar cycle is not a simple oscillation in the 11-year cycle.
There are several interesting features:ü The sunspot latitude (active latitude) migrates from mid-latitudes
to low latitudes during each cycle.
ü The solar global field has a dipolar field. The polarity of the dipole
reverses in every cycle.
15

Features in solar cycle: Hale’s law
SOHO/MDI, 2001 (cycle 23)

-

+

+
-

+

SDO/HMI, 2012 (cycle 24)

+

+

+
-

-

+
-

+

-

More than 90% of sunspot pairs obey coherent polarity rule
ü In a cycle, proceeding spots in a hemisphere always has one polarity.
ü The polarity in the other hemisphere is opposite.
ü The relation is reversed in every 11-year cycle.
16Features in solar cycle: Hale’s law
2008-

1996-2008

E

-

+

+

-

W

E

+

-

-

+

W

Hale’s law indicates large-scale magnetic fields in the solar interior.
17

Magnetic field evolution (1/2)
We need to explain the reasons why:
ü the sunspot number varies in 11 years.
ü the large-scale magnetic field is constructed in the solar interior
Faraday‘s law inhigh conductivity limit

Magnetic induction equation

We want to know “large” scale magnetic field and we divide the field andflow to large
and small scale ′ as 𝑩 = 𝑩 + 𝑩′ and 𝒗 = 𝒗 + 𝒗′. Thenthe induction equation for large-scale fields becomes18

Magnetic field evolution (2/2)

Effect by large-scale flow Effect by small-scale flow
Convection
zone

Radiation zone

We need to understand at least:ü Large-scale flow 𝒗
ü Correlation between flow andmagnetic field 𝒗! ×𝑩′
in the convection zone.
In the outer 30% of the solar interior,the convection transports theenergy generated by nuclear fusionin the center of the sun.
This is turbulent.
19

Global helioseismology:“Fourier”
transformin time andspace

Frequency mHz

Solar oscillation
observed with SOHO/MDI

Relative differencein sound speed 𝛿𝑐 ! /𝑐 !

Technique to evaluate the internal flow:
Helioseismology (1/2)
Model − Seismology

𝑟/𝑅⊙
Christensen-Dalsgaard+2009

Inversion

Spherical harmonic degree ℓ

Acoustic wave in the solar convection zone constructs eigenmode.
Eigenmode must reflect solar internal structure (temperature, rotation, andflow).
Inversion of eigenmode observation leads to understandings of the solar
interior.
20

Technique to evaluate the internal flow:
Helioseismology (2/2)
Local helioseismology:

G-band intensity

In travel time method, travel time in two
points is evaluated.
Ø Mean travel time → Temperature or 𝑩
Ø Travel time difference → Flow
Travel time difference

We evaluate the travel timein multiple layers and carryout inversion to obtainphysical quantities.
If we want to evaluate thequantities in the deep layer,
we need to observe twopoints apart long enoughfor a long duration.

Inverted flow structure

Sekii+2007

21

Large-scale flow: Differential rotation
stretching

Result of global helioseismology

The details of the differential rotation havebeen revealed by helioseismology.
Error in pole and radiation zone is large, but
the distribution in most of the convection
zone has reached a consensus.
(see solar orbiter mission)
Data provided by R. Howe

22

Large-scale flow: Meridional flow
flux transport

Different observations show different
results(Zhao+2013, Rajaguru+2015, Jackiewics+2015,
Rajaguru+2015, Chen+2017, Gizon+2020)

The flow at the base of CZ is
expected to be several m/s and it isdifficult to detect it in 200 km/s soundspeed.

Zhao+2013
SDO

Gizon+2020
SoHO, GONG

Currently we cannot distinguish evenpoleward and equatorward flow at thebase of the convection zone.
23Current status of meridional flow
Prepared meridional flow by mean-field simulation

Stejko+2021

Gizon+2020’s dataü Local helioseismology is applied to thesimulation dataü Travel time difference is evaluated forsingle and multiple cell flow.
ü It is concluded that Gizon+2020 does
not have the precision to determine theflow direction at the base of CZ.
→Longer observation + α is needed.
24

Importance of meridional flow
1. Meridional flow determines cycle length (11-year) in flux transport
dynamo.
2. Information on turbulence (convection) in the solar convection zone.
Cycle length: 𝑇 = 56.8𝑣"#".%&𝑠"#".'( 𝜂)".** , 𝑣" : meridional flow velocity, 𝑠" :𝛼-effect, 𝜂) : turbulent diffusivity (Dikpati & Charbonneau, 1999)
Ex. of flux transport dynamo

Angular momentum transport by the
turbulence and the meridional flow
should be balanced.
The distribution of the meridional flow
directly reflects the turbulent status.
Hotta & Yokoyama, 201025

Small-scale flow: Turbulence
Mean field theory

It is difficult (almost impossible) to observe small-scale flow and 𝑩
simultaneously in the deep convection zone. We use theories toevaluate it. Mean-field theory suggests the electromotive force iswritten as (see Krause & Rädler, 1980 textbook)𝛼-effect𝛽-effect

Helical flow leads to construction oflarge-scale 𝑩. Cause of helical flow
is rotation (Coriolis force). Relation
between rotation and convection
flow determines the helicity.
→ We need to at least determine
convection velocity
26

Importance of turbulent flow
In 𝛼Ω dynamo, the cycle period (11 years) isdetermined by the value of 𝛽 (turbulent diffusivity).
The turbulence directly determines the cycle period.
𝑇~+ -." #'/*
~𝐿* /𝛽 (Parker, 1955)*, -,Charbonneau, 2005

In addition, turbulence is also important for the construction of the largescale flow (differential rotation and meridional flow) due to angularmomentum transport.

27

Small-scale flow: Turbulence
Mixing length theory (MLT) and simulation
L = 3.84 ⇥ 1033 erg s 1

Evaluate typicaltemperature
perturbation

Radiation energy flux

Comparison with numerical simulation

Convection zone
calculation domain

Radiation energy flux
(well known value)

Miesch+2008

0.71𝑅⊙

0.98𝑅⊙

MLT and simulations are consistent28

Current problems: Convective conundrum

Proxauf+2021, PhD thesis

Travel time measurement by helioseismology possibly is able to evaluate
the convective amplitude. Hanasoge+2012 suggest that the numericalsimulations overestimate it order of magnitude larger.
The observation is still controversial, but since then there have been a
lot of doubts on the convective amplitude in theory and numericalsimulations→Convective conundrum29

Convective conundrum: Supergranulation
There are typical two convection
scalesSupergranulation
several 10 Mm
Granulation
several Mm

Supergranulation

Granulation

SOHO/MDI

Hinode/SOT

Spherical harmonic expansionof Doppler velocity
Hathaway+2015, modified

Numerical simulation

Numerical simulations cannot reproduce
supergranulation peak. A larger
calculation shows the peak in the largest
scale determined by calculation box size.
Lord+201430

Convective conundrum: Differential rotation
We cannot reproduce the solar differential rotation with solar parametersΩ = 3Ω⊙Ω = 2Ω⊙Ω = Ω⊙In the numerical simulation, the convection automatically transports
angular momentum and construct the differential rotation. Recent highresolution simulation falls into “anti-solar” differential rotation.
Convective amplitude in the simulations is too large.
(see O’Mara+2016)31

Summary of convective conundrum
The solar convective conundrum is summarized as:ü Huge discrepancy between observation and simulations
Ø Observations is controversial
Ø Simulations are consistent with each other (consistently wrong?)ü Supergranulation cannot be reproduced with simulations
ü Solar-like differential rotation is not reproduced.
ü Recently solved with super high resolution simulation in Fugaku.
Our understanding of the thermal convection with a high Reynolds
number is highly premature.

Hotta & Kusano, 2021, Nature Astronomy

32Summary of solar observation

Effect by large-scale flow Effect by small-scale flow
ü Large scale flow 𝒗 part:Ø Differential rotation has reached a consensus
Ø Meridional flow in the deep layer is highly controversial
ü Small scale flow 𝒗′ part:Ø Observation is controversial
Ø Theory and simulation may be wrong.
We have finally reached a status “I know that I know nothing (無知の知)”.
We have defined the problem. The next 10-20 years must be exciting forthe solar interior study, but no Japanese mission challenges the
problem…
33

Future perspective of solar interior research
Solar-C Plan-A was planed 10 years ago.
Solar-C planned to be out-of-ecliptic orbit to observe thesolar polar region. This type of mission is needed tounderstand the solar interior.
Solar-C Plan-A
Plan-B → (part of Plan-B) → Solar-C EUVST
Solar-C Plan-A WG

Perspective to contribute the solar interior research fromJapan
1. Observation from multiple points is necessary
Ø Acoustic wave ray pass the deep layer.
Ø Long stable observation is required.
Ø Out-of-ecliptic orbit or L5 point is preferable
2. “Reliable” numerical simulation is needed.
Ø Strong point of JapanØ Useful for inversion
Ø Definition of reliable: solar cycle, large-scale flow, travel
time are consistent with observations.
34

Future perspective of solar interior research
We should observe:ü Convection (turbulence)Ø Angular momentum transport: Differential rotation andmeridional flowØ 𝑩 generation: Though the turbulent 𝛼- and 𝛽-effectØ The cycle period is determined by turbulence itself in the 𝛼Ω
dynamo.
Ø Convective amplitude in scales is important and then we canuse theoretical approaches.
ü Meridional flowØ The cycle period is determined by the meridional flow in the flux
transport dynamo (Dikpati & Charbonneau, 1999)
Ø Meridional flow directly reflects the turbulent angular momentum
transport → Important information on turbulence.
Ø At least direction at the base of the convection zone should bedetermined.
35

Stellar magnetic field cycle
G2

K1

K2

G2

K2

K5

G7

K3

K5

G8

K4

K1

K2

1970 1975 1980 1985 1990

1970 1975 1980 1985 1990

1970 1975 1980 1985 1990

The decade scale
stellar activity cycle isalso confirmed with thechromosphericactivity.
(Baliunas+1995)
36

Rotation, convection, magnetic field in stars

X ray luminosity

Increase withstrong rotational influence

Saturate at a certain
rotational influence
Gray circle:
partly convective star
Dark red and orange circle:
fully convective star
Wright+2016, Nature

Fast rotation
Rossby number =Slow rotation
01232415 2467 893:7;15<792415 2467 893:7

37

Differential rotation (DR) in stars
Evaluation of DR with light curve

Evaluation of DR withasteroseismology

Saar+2011

Benomar+2018, Science

We begin to evaluate the differential rotation ΔΩ for stars with different
Rossby number (Rotation/Convection).
This must be explained to understand the stellar magnetic activity.

38

Rotation evolution depend on magnetic field
Stars lose mass and angular momentum
during their life and becomes slow down.
𝑇#$$ = 5900 − 6200 K

Age-rotation relation is used for evaluate thestellar age (gyrochronology).
Recent observation indicates that some older
stars have rapid rotation rate than expected(van Saders+2016, Metcalf+2017).

𝑇#$$ = 5600 − 5900 K

The configuration of the magnetic field inthese stars have changed significantly andstop losing further angular momentum.
𝑇#$$ = 5100 − 5400 K
van Saders+2016, Nature

The dynamo is also the key to understand it.
39Summaryü The current and planed solar project in JapanØ There are a lot of projects to investigate:p Phospheric dynamics
p Chromospheric/Coronal heatings
p Magnetic reconnection
p Solar wind/Coronal Mass Ejection
Ø The central issue is the magnetic fieldü Solar interior researchØ Unfortunately, Japan has no mission for this topic except for thenumerical simulation
Ø Differential rotation has reached a consensus
Ø Meridional flow and convective amplitude have big uncertainty.
Ø Japan should join observational project with multiple point
helioseismology with taking advantage of the leading position of thenumerical simulation project.
ü Connection stellar researchØ Stars have cyclic activity variation
Ø Convection-rotation relation (Rossby number) is important to understandthe stellar activity and the differential rotation
Ø Rotation evolution is significantly constrained by the magnetic field(dynamo)Ø Stelar activity is also important for habitability in exoplanet.

40