The Radio Sun

NoRH 17 GHz / 1.8 cm

VLA 1.4 GHz / 21 cm

NRH 0.327 GHz / 91 cm

Karl-Ludwig Klein

Observatoire de Paris - MeudonLudwig.klein@obspm.fr

The solar corona

© C. Viladrich, SAF

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Magnetograms - SoHO/MDI

A >1 MK plasma whose structure and dynamics are governed by magnetic fields

emerging fom the interior.

• Emission processes 1 : thermal plasma– free-free– gyroresonance (enhanced opt depth at = sce; s=2, 3, 4)

Radio observations of the solar atmosphere

• Radio waves from the solar atmosphere : – propagation at >pe ne decreases with increasing altitude

– sounding of different heights at different frequencies (0 RS-1 AU)

NoRH 17 GHz / 1.8 cm

VLA 1.4 GHz / 21 cm

NRH 0.327 GHz / 91 cm

Emission processes 2 : radio bursts– (gyro)synchrotron (cm-

m-) – collective emission at

pe or 2pe (pene;

bursts, dm-m-) identification of moving

exciters : electron beams, shock waves

Radio observations of the solar atmosphere

• Radio waves from the solar atmosphere : – propagation at >pe ne decreases with increasing altitude

– sounding of different heights at different frequencies (0 RS-1 AU)

ETH ZurichAIP Potsdam - OSRA Tremsdorf

Solar radio astronomy in Europe

• Accessible from ground: 1 mm–30 m (300 GHz-10 MHz)

Solar radio instrumentation

• 2 types of observations :Spectroscopy of the whole Sun (bursts)Aperture synthesis imaging

• Plasma diagnostics of the corona (ne, T, B) and the origin of the solar wind

• Large-scale coronal disturbances: mass ejections (CME), shocks

• The Sun as a particle accelerator :– Mildly relativistic electrons in flares (gyrosynchrotron) – « Quiet-time » non thermal e--populations– e- accelerated during CME and at coronal shocks– Energetic particle acceleration and propagation in the corona and

interplanetary space

Observations of the solar corona at radio

• Plasma diagnostics of the corona (ne, T, B) and the origin of the solar wind

• Large-scale coronal disturbances: mass ejections (CME), shocks

• The Sun as a particle accelerator :– Mildly relativistic electrons in flares (gyrosynchrotron) – « Quiet-time » non thermal e--populations– e- accelerated during CME and at coronal shocks– Energetic particle acceleration and propagation in the corona and

interplanetary space

• Outlook: solar radio telescopes for the future

Observations of the solar corona at radio

Radio emission of the quiet solar atmosphere

Plasma diagnostics (ne, T, B) of an

extended region from the

chromosphere to the corona

+-

h

A multi frequency view of the radio Sun

2004 Jun 25 2004 Jun 27 2004 Jun 28 2004 Jun 29

Nan

çay

Rad

iohe

liogr

aph

410

MH

z

Sib

eria

n S

olar

R

adio

Tel

esco

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7GH

z

Nob

eyam

a R

adio

helio

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h 17

GH

z

Different structures at : active regions (GHz), coronal holes

Brig

htne

ss te

mpe

ratu

re [K

]

106

105

Wavelength [m]

1 20.6

T b (c

oron

al ho

le)Low :

Tb = Te = 6.7105 K

High :

Tb << Te

ne = 2.3108 cm-3

T b (average corona)

Mercier & Chambe 2009 ApJ 700, L137

Coronal plasma parameters

Bremsstrahlung: brightness spectrum depends on ne & Te

Nançay Radioheliograph

Gyromagnetic radiation

• Electron cyclotron frequency

• Low speed electron (T=106 K) : cyclotron line (unobservable in corona , since pe>ce ) and low harmonics (=s0 , 0=ce , s=1,2,3)

• Synchrotron rad., relativistic e: 0=ce/ ; beaming high s, max. intensity at

23

2c ce =

Inte

nsit

y

Time Frequency

B

4

12.8 [MHz]

2 10 Tcee

eB B

m

π −= ≈

€

τ ,gr =ξs2s

2s+1s!

neLB

ν1,77 ×10−10Te( )

s−1×

1± cosϑ( )2

sinϑ( )2s−2

δ ν − sν ce( )

=sce (s=2 … 4 for Te2106 K)

-> 5 GHz (6 cm) if s=3, B=600 G

Resonant surf., depth ~100 km

=5

GHz, s=

3

=8,4

GHz,

s=3

chromosphère

600 G

1000 G

>3 ce

,max

€

Tb = Te τ( )exp −τ( )dτ0

τ 0

∫ , τ = τ ff + τ gr

Gyroresonance emission: a tool for coronal magnetic field measurements

• Gyroresonance emission

• τgr>1 : Tb on iso-B surface (=sce ; in general not plane)• Above sunspots (intense B)• Confirmed technique: cf. Alissandrakis, Kundu, Lantos 1980, A&A 82, 30

• Future: broadband spectrographic imaging

Lee et al. 1999, ApJ 510, 413Lee et al. 1998, ApJ 501, 853

Gyroresonance emission: a tool for coronal magnetic field measurements

Optical + VLA

• The corona emits bremsstrahlung at cm-to-m- (quiet corona), optically thin or thick.– Determination of coronal plasma parameters from bremsstrahlung

spectrum (ne, Te); comparison with othe diagnostics (EUV line spectroscopy); origin of solar wind; nature of coronal electron population (maxwellian ?)

– Determination of coronal magnetic fields: circular polarisation of optically thin bremsstrahlung, depolarisation (not shown here), gyroresonance emisssion.

• Perspective : Multi-frequency mapping of the Sun by the Frequency Agile Solar Radiotelescope (FASR).

• Not addressed here: recombination lines from the chromosphere. ALMA ?

• Further reading : Aschwanden, Physics of the Solar Corona; papers in Solar and Space Weather Radiophysics, see FASR web site http://www.ovsa.njit.edu/fasr

Thermal radio emission from the solar corona: summary

Bursts of gyrosynchrotron radiation from solar flares

Evidence of electron

acceleration to relativistic energies

in the corona

Observed microwave spectra

Whole Sun spectra of solar radio bursts: Nita et al 2004 ApJ 605, 528

Ow

ens Valley S

olar Array

• Observation of a microwave burst spectrum with dense frequency coverage (Owens Valley Solar Array)

• Continuous spectrum (practically)

>>0, >1 : gyrosynchrotron radiation

• (1, >>1 : synchrotron radiation)

• Corona: hundreds of keV, occasionally higher energies

Observations : Owens Valley,Nita, Gary, Lee 2004, ApJ 605, 528

Opt

. thi

ck Opt. thin

Gyrosynchrotron interpretation

• Solar radio burst : usually observed up to some tens of GHz.

• New: =212 GHz (SST1): synchrotron emission from relativistic e-:

=10

• Slope of the microwave spectrum

Trottet et al. 2002 A&A

Relativistic electrons at the Sun

(1) Univ. Mackenzie Sao Paulo

232 ce ≈

€

Sν ∝ν− δ −1( )

2 for N E( )dE ∝ E−δ dE

• Time profile (microwaves, HXR, gamma-rays): electron acceleration from 100 eV (quiet corona) to hundreds (sometimes tousands) of keV in a few seconds to a few tens of seconds

• Consistent with e-spectrum

inferred from gamma-ray bremsstrahlung (h> 300 keV; Trottet et al. 1998 AA 334, 1099 )

Relativistic electrons at the Sun

A gyrosynchrotron model source

Bastian, Benz, Gary 1998, ARAAOptically thick:

loop top

Optically thin: foot points

More detailed models: Preka-Papadema & Alissandrakis AA 139, 507; 1988 AA 191, 365: 1992 AA 257, 307 Klein & Trottet 1984, AA 141, 67

Microwave source morphologies

Loop (LF) + footpoints (HF): Nindos et al. 2000, ApJ 533, 1053

Compact loop : Kundu et al. 2001, ApJ 547, 1090

17 GHz

SXR+HXR

Multiple sources :• footpoints (cospatial 17 GHz,

HXR)• compact or extended loops Nishio et al. 1997, ApJ 489, 976Hanaoka 1996, 1997, Solar Phys.

• Microwaves from solar flares are gyrosynchrotron rad.

• Co-evolution with HXR, gamma-ray continuum; electron acceleration to MeV energies (from 100 eV in the corona) within a few seconds.

• Electron spectrum consistent with that inferred from the gamma-ray continuum (NOT HXR continuum: mildly relativistic electrons !)

• Further reading : Bastian, Benz, Gary 1998 ARAA 36, 131; Pick, Klein, Trottet 1990 ApJS 73, 165; Benka & Holman 1994 ApJ 435, 469)

Gyrosynchrotron radiation from solar flares : summary

Particle acceleration and magnetic reconnection

Hard X-ray

and

radio bursts, and a cartoon scenario

Particle acceleration in a simple flare

• A set of complementary observations of EM emissions from flare-accelerated electrons :

– Hard X-rays (h > 20 keV): energy spectra and imaging

– Radio emission : spectra and imaging from ground (400 GHz > > 20 MHz)

– Radio emission : spectra from space ( < 14 MHz)

Vilm

er et al. 2002 Solar P

hys 210, 261

5 min

Hard X-ray emission from electron beams

e beam

HXR

Image EUV TRACE / NASA

• Beam of suprathermal electrons travelling downward through the corona.

• Collisions with ambient protons : bremsstrahlung,

h < energy(e)• Particularly efficient when

ambient density high (chromosphere) : frequently observed ‘footpoint’ sources at h>20 keV.

RHESSI HXR + TRACE : Krucker et al. 2008 ApJ 678, L63

Hard X-ray emission from electron beams

• Beam of suprathermal electrons travelling downward through the corona.

• Collisions with ambient protons : bremsstrahlung,

h < energy(e)• Particularly efficient when

ambient density high (chromosphere) : frequently observed ‘footpoint’ sources at h>20 keV.

• Low energy e deposit their E in the corona.

Particle acceleration in a simple flare

• A set of complementary observations of EM emissions from flare-accelerated electrons :

– Hard X-rays (h > 20 keV): energy spectra and imaging

– Radio emission : spectra and imaging from ground (400 GHz > > 20 MHz)

– Radio emission : spectra from space ( < 14 MHz)

Vilm

er et al. 2002 Solar P

hys 210, 261

5 min

1) Electromagnetic waves

2) Langmuir waves = electron plasma oscillations (ES waves, cannot exist in vacuum):

… but can couple to EM waves and than escape from the source (cf. solar radio bursts)

2 2 2 23

2pe thkω ω υ= +

ω

ω/k=c

k

ωpe

EM w

ave

Langmuir wave

High-frequency waves in a plasma : isotropic case (B=0)

• Beam of suprathermal electrons travelling through the corona

• “Bump in tail” instability

f/υ// > 0 : growth of Langmuir waves,

pene

• Plateau (quasi-linear relaxation)

Maxwellian

Beam

υ//

f(υ//)

The Langmuir waves cannot escape from the corona, but …

Radio emission from electron beams

• Electron beam rising into the corona Langmuir waves at decreasing

• Coupling with ion sound waves (S<<L) or Langmuir waves EM waves at T = L + S L pe “fundamental”

T = L + L= 2L 2pe “harmonic”

• Short radio burst that drifts from high to low (“type III” burst)

e beam

Height (time)

Fre

quen

cy

high ne

high

low ne

low

Radio emission from electron beams

• Hard X-rays from the low atmosphere (chromosphere) - e precipitated downward to ne > 1012 cm-3, bremsstrahlung with ambient p, h<energy(e).

• Radio emission (type III) from outward propagating e beams, =2pene, start < 400 MHz : ne < 109 cm-3, energy ~10 keV.

Acceleration region in the corona, injects particles downward (chromosphere) & upward (high corona, IP space)

Particle acceleration in a simple flareV

ilmer et al. 2002 S

olar Phys 210, 261

5 min

Particle acceleration associated with magnetic reconnection ? A simple scenario.

Particle acceleration region in a reconnecting coronal current sheet.

Electric fields : - plasma inflow (-VB)

- turbulence- termination shock (outflow/ambient plasma)

Vilmer et al. 2002 Solar Phys

Mechanisms of charged particle acceleration

• Extended CS cannot exist in the solar corona : instabilities (e.g., tearing), fragmentation. Also : pb with high particle fluxes.

• Numerous regions with small-scale E fields, X points, O points and (contracting) magnetic islands.

• Multiple acceleration sites embedded in coronal plasma sheets.

Aschwanden 2002 SSR 101, 1

Non thermal electrons in the corona outside flares

• Hot plasma (17 & 5.7 GHz), non thermal electrons (164 MHz)• electron beams in IP space (1000-20) kHz (1 day overview) Quasi-continuous electron acceleration in an active region, origin of

non-maxwellian particle populations in IP space ?

17 GHz Nobeyama 5.7 GHz Irkutsk 0.164 GHz Nançay

(20-1000 kHz) WAVES/WIND

24 h

Wind/WAVES

Outlook: solar radio telescopes for the future

• The ideal solar imaging radio telescope : broadband cm-m-, 0.01-1 R above the photosphere, high cadence– The Frequency-Agile Solar Radio Telescope (FASR) 30 MHz-30 GHz– dm-: Chinese RH (underway) 400-1600 MHz– Nobeyama Radioheliograph 17 & 34 GHz (chromosphere/low corona -

flares and quiescent)– Siberian Solar Radio Telescope Irkutsk 5 GHz (low corona)– Nançay Radioheliograph 450-150 MHz (corona 0.5 R)

• General purpose synthesis arrays at m-– LOFAR, Europe : (200-30) MHz (NL; under construction/deployment)– MWA, Australia : (300-30) MHz (MIT-australian cooperation)– Solar use to be explored, under discussion

• Sub-mm-IR imaging at high cadence: SST; extend to FIR (space)

• Maintain whole Sun patrol instrumentation: flares mm-Dm-