Flares, CMEs and solar cosmic rays Karl-Ludwig Klein Observatoire de Paris F Meudon SoHO/EIT

Outline High energy particles from the solar corona Magnetic reconnection and particle acceleration: a simple solar flare and a cartoon scenario Particle acceleration for pedestrians Bibliography : J. Miller et al Journal Geophys. Res. 102, A00976 M. Aschwanden 2002 Soa Sci Rev 101, 1 L. Vlahos 2007, in The High Energy Solar Corona : Waves, Eruptions, Particles, Springer LNP 725

Introduction : High-energy particles from the solar corona

High-energy particles in the Universe Cas A (X rays; Chandra; NASA) Earth (gamma rays; Compton Gamma Ray Observatory; NASA) Many astrophysical objects can only be seen because of the radiation emitted by high- energy particles (electrons, ions): SNR, pulsars, AGN … How are these particles accelerated to the high energies required ?

High-energy particles in the Universe Cosmic rays have different origins: –Galactic + occasional solar (up to several GeV) –Galactic (up to eV; « knee ») –Probably extragalactic (> knee) © University of Utah

High-energy particles in the Universe - clues from the Sun The Sun is also a particle accelerator, where we can –observe the radiation of the particles, –map the environment, –see time evolution, –measure the escaping particles in space. Solar energetic particles (SEP) have an impact on technology and human beings (aircraft & beyond). Energies: – eV thermal corona –Up to 100s of MeV (electrons), up to several GeV (protons; neutron monitors, muon telescopes) in flares/CMEs.

At MeV to 10s of MeV (GOES spacecraft): sudden rise (depends on longitude) + slow decay, hours - days Neutron monitors (>500 MeV): shorter duration A solar energetic particle (SEP) event

SEP (GOES) CME height-time (SoHO) Flares (soft X-rays, GOES) 4 hrs GLE (Bütikofer et al. 2008) Proton flux spectrum

The acceleration of charged particles in the solar corona How can charged particles be accelerated to high speeds ? An electric field is required : –An electric field that is perpendicular to the magnetic field does not accelerate particles. –In the corona a magnetic field-aligned electric field is easily short-circuited by free electrons. –The electric fields require peculiar (transient) conditions in the corona : current sheets, magnetic reconnection, shock waves.

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

EUV: dark photosphere (T=5800 K) Coronal structures revealed by the plasma they confine: open and closed (loops) magnetic structures TRACE (NASA) : Fe line (T > 10 6 K) Magnetic field structures in the corona

A plasma tube pervaded by magnetic field is less dense / lighter than its environment: buoyancy. If a minor outward deformation of the tube occurs, gravitation pulls the material downward and reduces the amount of matter: the tube becomes lighter. The deformation of the flux tube grows, its summit rises toward the photosphere, emerges through it into the outer atmosphere. Appearance of bipolar regions in the photosphere, loops above. Buoyancy force (Archimedes) on a flux tube in the solar interior: g Emergence of a magnetic flux tube Why does magnetic flux emerge from the solar interior (Parker instability) ? The emergence of magnetic flux from the interior into the atmosphere is the basic driver of solar activity.

Coronal mas ejections (CMEs) Ejections of large- scale coronal magnetic field structures. Magnetically driven from the corona to IP space. STEREO/Secchi

CMEs and shock waves Distribution of measured CME speeds (plane of the sky) + semi-empirical profile of the Alfvén speed in the corona => Fast CME (  3000 km/s) likely drive (fast MS) shocks (M A  3), which can accelerate particles detectable in space. Mann et al A&A 400, 329 Warmuth & Mann 2005 A&A 435, Gopalswamy 2006, Springer LNP Ontiveros & Vourlidas 2009 ApJ 693, 267

A simple solar flare and a cartoon scenario Hard X-ray and radio bursts, magnetic reconnection

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) Vilmer et al Solar Phys 210, 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 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 energy 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) Vilmer et al Solar Phys 210, min

Beam of suprathermal electrons travelling through the corona “Bump in tail” instability  f/   // >0 : growth of Langmuir waves,  pe  n e Plateau (quasi-linear relaxation) Maxwellian Beam  // f(  // ) 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 = 2 L  2 pe “harmonic ” Short radio burst that drifts from high to low (“type III” burst) e beam Height (time) Frequency high n e high low n e low Radio emission from electron beams

Hard X-rays from the low atmosphere (chromosphere) - e precipitated from corona to n e > cm -3, bremsstrahlung with ambient p, h <energy(e). Radio emission (type III) from outward propagating e beams, =2 pe  n e, start < 400 MHz : n e < 10 9 cm -3, energy ~10 keV.  Acceleration region in the corona, injects particles downward (chromosphere) & upward (high corona, IP space) Particle acceleration in a simple flare Vilmer et al Solar Phys 210, min

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

Plasma inflow into the CS: convective electric field Stationarity:  Equilibrium diffusion (B annihiliation) - inflow B=0 A simple reconnecting current sheet    j  Diffusion region

A simple reconnecting current sheet  j Plasma inflow into the CS: convective electric field Reconnection in DR:   B=0  Diffusion region jet

Hard X-ray sources : simultaneous double footpoints on top of H  ribbons (=heated chromosphere) Rapidly varying source positions (fragmented acceleration region) Rapid energy transport to the low solar atmosphere (H , WL, … FIR) Krucker et al ApJ 678, L63 Supporting evidence : HXR source morphology

Supporting evidence : energy transport from the corona to the chromosphere Time profiles : thermal response of the chromosphere (H  ), HXR &  waves from NT e. Fast energy transport: NT particles (also : WL : Fletcher et al ApJ 656, 1187) H ,  line polarisation (THEMIS obs. : Hénoux & Karlicky 2003 ; Karlicky & Hénoux 2002 ; Xu et al ApJ 631, 618) 1 min Trottet et al A&A 356, 1067

Temperature distribution in above-looptop hard X-ray sources ( Sui & Holman 2003 ApJ 596, L251 ) RHESSI: higher T close to the presumed reconnection region 6-8 keV Supporting evidence : temperature distribution in coronal hard X-ray sources

An unsolved problem : huge fluxes of accelerated particles HXR emission : requires >10 35 e/s at E>20 keV. How can such an electron beam propagate through the corona ? Problems: charge separation & electric current neutralisation. Fast protons (1836 times more energy  inconsistent) Ambient e : Neutralisation of the beam current ? >10 35 e/s m 2

An unsolved problem : huge fluxes of accelerated particles Fast protons (1836 times more energy  inconsistent) Ambient e : Neutralisation of the beam current ? >10 35 e/s m 2

An unsolved problem : huge fluxes of accelerated particles Fast protons (1836 times more energy  inconsistent) Ambient e :  amb  1000 km/s>>c s Instability when too fast w/r to ions (> ion sound speed) Remedy: filamentation ? Neutralisation of the beam current ? >10 35 e/s m 2 A further argument for filamentation : typical dimension of CS = ion cyclotron radius; corona: << spatial resolution

Particle acceleration for pedestrians Qualitative discussion of particle acceleration scenarios in the solar corona: DC E field, turbulence, shocks

Mechanisms of charged particle acceleration: 1. « Direct » electric field A simple scenario of charged particle motions in a current sheet (CS). Plasma motion towards the CS, induces an E. Stationary configuration: Charged particles (here : electron) E  B - drift into the CS. Those crossing B=0 are accelerated by the E field (“meander orbit”).      

Evolution of a reconnecting current sheet Diffusion Convection Smaller scale, enhanced diffusion, magnetic field reconnection Tearing mode instability, magnetic island formation magnetic X-line magnetic O-line

magnetic X-line magnetic O-line Mutual attraction of parallel currents Evolution of a reconnecting current sheet Coalescence instability

Mechanisms of charged particle acceleration: 1. « Direct » electric field Trapping in coalescing magnetic islands: prolonged acceleration, high energies. Particles wander from island to island to gain more energy (Drake et al Nature 443, 553). Aschwanden 2002 SSR 101, 1Kliem 1994 ApJS 90, 719

Evolution of a reconnecting current sheet Numerical simulation of island formation starting with two parallel current sheets ( Drake et al Nature 443, 553 ): sequence tearing-coalescence; fragmentation of the current sheet  fragmented energy release, multiple acceleration regions.

Doppler effect : resonance EM wave - gyrating particle Pure gyration (harmonic s): Resonance condition : B   P 0   P 0 cos   Mechanisms of charged particle acceleration: 2. Wave turbulence

Alfvén waves  coll  ci  pi  ce  pe Sound waves Ion- sound waves Whistler Magnetosonic waves Wave number k Electromagnetic waves Langmuir waves Frequency  Doppler resonance : particle gyration resonant with Doppler shifted wave vector - maximum energy exchange between wave & particle Which wave modes can resonate with a given particle ? Example: Alfvén waves + electrons After Aschwanden 2002, Fig. 4.4 Wave modes in a plasma  c <  p, k  B 0 :

Mechanisms of charged particle acceleration: 2. Wave turbulence Following the same lines of reasoning: Alfvén waves + electrons: Alfvén waves + ions: Whistlers + electrons: Many - but not all - resonant acceleration processes have a threshold energy. Particles can be accelerated out of the thermal background by –Waves near the cyclotron resonance (require a cascade of waves from long wavelengths at the beginning to successively shorter wavelengths/higher frequencies) –A pre-acceleation process (e.g. s=0 - transit time damping)

Reames 1999 SSR 90, 413 Z/A (plasma of 3.2  10 6 K) Abundance (w/r to corona) Mechanisms of charged particle acceleration: 2. Wave turbulence - ion abundances Abundances of ions measured by s/c during SEP events. Characteristic enhance-ments of abundances with respect to coronal values: –strong Fe-Ar, –moderate Ne, Si…, –no enh t C, N, O Ordering by the average charge state of the ions in the coronal plasma. Explained by waves cascading from large scales (low frequencies) to small scales.

Mechanisms of charged particle acceleration: 3. Shock waves Large-scale shock waves generated by fast moving large-scale structures (CMEs). Small-scale shocks in outflow jets from reconnecting current sheets. Fast mode shocks: magnetic field compression behind the shock. Mann et al A&A 454, 969 Slow shocks Fast shock Slow shocks Diffusion region Flare loops Ejected plasma H  ribbons

Mechanisms of charged particle acceleration: 3. Shock waves In the frame of the shock: incoming plasma (upstream) flows across upstream magnetic field Electric field Compression of the magnetic field at the shock (fast mode): particle drifts in the shock front –grad B –curvature Shock drift acceleration : downstreamupstream  E  B  B  erB erB

Mechanisms of charged particle acceleration: 3. Shock waves In the frame of the shock: incoming plasma (upstream) flows across upstream magnetic field Electric field Compression of the magnetic field at the shock (fast mode) : particle drifts in the shock front –grad B drift dominates over curvature drift when B u, B d nearly parallel: quasi-perpen- dicular shock. Shock drift acceleration : downstreamupstream  E  B  B  erB erB

Mechanisms of charged particle acceleration: 3. Shock waves At quasi-parallel shocks: –First accelerated particles create MHD turbulence up- stream. Subsequent particles are elastically scattered (trapped) by these waves. –Waves are overtaken by the shock (faster than MHD waves), particles are reflected at the shock or by its down- stream turbulence. –Multiple shock crossings of a charged particle: acceleration (Fermi 1; analogy: tennis ball between approaching rackets). Lee 2005 ApJS 158, 38 Diffusive shock acceleration

Mechanisms of charged particle acceleration: which mechanism prevails ? Long-debated problem : attempts to decide which mechanism will produce the highest energies and the most numerous HE particles. In fragmented energy release regions, several mechanisms can/will act together: direct E, stochastic, shock acceleration all possibly active in reconnecting CS. Open question: roles of flares and large-scale (CME) shocks in accelerating escaping particles.

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Questions What tells us that the corona is a plasma structured by magnetic fields ? Where does the magnetic field of the corona come from ? What are “energetic particles”, “solar energetic particle events”, “ground level enhancements” ? How are solar cosmic rays distinguished from CR of other origins ? With which kind(s) of solar phenomena are solar energetic particle events related ? Cite some observed signatures of energetic particles in the solar atmosphere and in space. Describe in a few words what “magnetic reconnection” signifies, and why it can give rise to energetic particles.