the radio sun norh 17 ghz / 1.8 cm vla 1.4 ghz / 21 cm nrh 0.327 ghz / 91 cm karl-ludwig klein...
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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
pe5.
7GH
z
Nob
eyam
a R
adio
helio
grap
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-
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