1. Observatoire de Paris - Meudon
The Radio Sun
NoRH 17 GHz / 1.8 cm
VLA 1.4 GHz / 21 cm
NRH GHz / 91 cm
Karl-Ludwig Klein
Observatoire de Paris - Meudon

2. The solar corona
Magnetograms - SoHO/MDI
EUV images - SoHO/EIT
A >1 MK plasma whose structure and dynamics are governed by magnetic fields emerging fom the interior.
© C. Viladrich, SAF

3. 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 GHz / 91 cm
Emission processes 1 : thermal plasma
free-free
gyroresonance (enhanced opt depth at  = sce; s=2, 3, 4)

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)
Emission processes 2 : radio bursts
(gyro)synchrotron (cm-m-)
collective emission at pe or 2pe (pene; bursts, dm-m-)
 identification of moving exciters : electron beams, shock waves
ETH Zurich
AIP Potsdam - OSRA Tremsdorf

5. Solar radio astronomy in Europe

6. Solar radio instrumentation
Accessible from ground: 1 mm–30 m (300 GHz-10 MHz)
2 types of observations :
Spectroscopy of the whole Sun (bursts)
Aperture synthesis imaging

7. 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

8. 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

9. Radio emission of the quiet solar atmosphere
Plasma diagnostics (ne, T, B) of an extended region from the chromosphere to the corona

10. A multi frequency view of the radio Sun
2004 Jun 25
2004 Jun 27
2004 Jun 28
2004 Jun 29
Nançay Radioheliograph 410 MHz + - h
Siberian Solar Radio Telescope
5.7GHz
Nobeyama Radioheliograph 17 GHz
Different structures at  : active regions (GHz), coronal holes

11. Coronal plasma parameters
Bremsstrahlung: brightness spectrum depends on ne & Te
Nançay Radioheliograph
temperature [K]
Brightness
106
105
Wavelength [m] 1 2
0.6
Tb (average corona)
Tb (coronal hole)
Low  :
Tb = Te = 6.7105 K
High  :
Tb << Te
ne = 2.3108 cm-3
Mercier & Chambe 2009 ApJ 700, L137

12. Gyromagnetic radiationB
Electron cyclotron frequency
Low speed electron (T=106 K) : cyclotron line (unobservable in corona , since pe>ce ) and low harmonics (=s0 , 0=ce , s=1,2,3)
Synchrotron rad., relativistic e: 0=ce/ ; beaming  high s, max. intensity at
Frequency
Intensity
Time

13. Gyroresonance emission: a tool for coronal magnetic field measurements
=sce (s=2 … 4 for Te2106 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
Gyroresonance emission

14. Gyroresonance emission: a tool for coronal magnetic field measurements
Optical + VLA
Lee et al. 1998, ApJ 501, 853
Lee et al. 1999, ApJ 510, 413
gr>1 : Tb on iso-B surface (=sce ; in general not plane)
Above sunspots (intense B)
Confirmed technique: cf. Alissandrakis, Kundu, Lantos 1980, A&A 82, 30
Future: broadband spectrographic imaging

15. Thermal radio emission from the solar corona: summary
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

16. Bursts of gyrosynchrotron radiation from solar flares
Evidence of electron acceleration to relativistic energies in the corona

17. Observed microwave spectra
Owens Valley Solar Array
Whole Sun spectra of solar radio bursts: Nita et al 2004 ApJ 605, 528

18. Gyrosynchrotron interpretation
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. thick
Opt. thin

19. Relativistic electrons at the Sun
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 A&A
(1) Univ. Mackenzie Sao Paulo

20. Relativistic electrons at the Sun
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 AA 334, 1099 )

21. A gyrosynchrotron model source
Optically thick:
loop top
Bastian, Benz, Gary 1998, ARAA
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

22. 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, 976
Hanaoka 1996, 1997, Solar Phys.

23. Gyrosynchrotron radiation from solar flares : summary
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)

24. Particle acceleration and magnetic reconnection
Hard X-ray
and
radio bursts, and a cartoon scenario

25. Particle acceleration in a simple flare
5 min
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)
Vilmer et al Solar Phys 210, 261

26. 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.
e beam
HXR
Image EUV TRACE / NASA

27. 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.
RHESSI HXR + TRACE :
Krucker et al ApJ 678, L63

28. Particle acceleration in a simple flare
5 min
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)
Vilmer et al Solar Phys 210, 261

29. High-frequency waves in a plasma : isotropic case (B=0)
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) ω
ω/k=c k
ωpe
EM wave
Langmuir wave

30. Radio emission from electron beams
Beam of suprathermal electrons travelling through the corona
“Bump in tail” instability
f/// > 0 : growth of Langmuir waves, pene
Plateau (quasi-linear relaxation)
//
f(//)
Maxwellian
Beam
The Langmuir waves cannot escape from the corona, but …

31. Radio emission from electron beams
e beam
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= 2L  2pe “harmonic”
Short radio burst that drifts from high  to low  (“type III” burst)
low ne
high ne
Height (time)
Frequency
high 
low 

32. Particle acceleration in a simple flare
5 min
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, =2pene, start < 400 MHz : ne < 109 cm-3, energy ~10 keV.
 Acceleration region in the corona, injects particles downward (chromosphere) & upward (high corona, IP space)
Vilmer et al Solar Phys 210, 261

33. Particle acceleration associated with magnetic reconnection
Particle acceleration associated with magnetic reconnection ? A simple scenario.
Vilmer et al.
2002
Solar Phys
Particle acceleration region in a reconnecting coronal current sheet.
Electric fields : - plasma inflow (-VB)
- turbulence
- termination shock (outflow/ambient plasma)

34. Mechanisms of charged particle acceleration
Aschwanden 2002 SSR 101, 1
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.

35. Non thermal electrons in the corona outside flares
17 GHz Nobeyama
5.7 GHz Irkutsk
0.164 GHz Nançay
Wind/WAVES
( kHz) WAVES/WIND
24 h
Hot plasma (17 & 5.7 GHz), non thermal electrons (164 MHz)
electron beams in IP space ( ) kHz (1 day overview)
 Quasi-continuous electron acceleration in an active region, origin of non-maxwellian particle populations in IP space ?

36. Outlook: solar radio telescopes for the future
The ideal solar imaging radio telescope : broadband cm-m-,  R above the photosphere, high cadence
The Frequency-Agile Solar Radio Telescope (FASR) 30 MHz-30 GHz
dm-: Chinese RH (underway) MHz
Nobeyama Radioheliograph 17 & 34 GHz (chromosphere/low corona - flares and quiescent)
Siberian Solar Radio Telescope Irkutsk 5 GHz (low corona)
Nançay Radioheliograph 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-