Observatoire de Paris - Meudon 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 - Meudon Ludwig.klein@obspm.fr

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

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 1 : thermal plasma free-free gyroresonance (enhanced opt depth at  = sce; 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) 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

Solar radio astronomy in Europe

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

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

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

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

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

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

Gyromagnetic radiation B 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

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

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

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 http://www.ovsa.njit.edu/fasr

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

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

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

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. 2002 A&A (1) Univ. Mackenzie Sao Paulo

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. 1998 AA 334, 1099 )

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

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.

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)

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

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. 2002 Solar Phys 210, 261

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

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. 2008 ApJ 678, L63

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. 2002 Solar Phys 210, 261

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

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 …

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 

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. 2002 Solar Phys 210, 261

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)

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.

Non thermal electrons in the corona outside flares 17 GHz Nobeyama 5.7 GHz Irkutsk 0.164 GHz Nançay Wind/WAVES (20-1000 kHz) WAVES/WIND 24 h 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 ?

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-