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New Insights on Particle Acceleration from RHESSI Data: Unraveling Electron Dynamics and Constraints

This report discusses the key goals and constraints related to particle acceleration based on data from the RHESSI mission, emphasizing the acceleration and transport of electrons in solar flares and coronal mass ejections (CMEs). Topics include the production of high-energy electrons, bremsstrahlung mechanisms, and the role of magnetic topology. The findings aim to connect new acceleration theories to observational data, addressing existing challenges such as electron flux calculations and the impacts of magnetic configurations on flare dynamics.

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New Insights on Particle Acceleration from RHESSI Data: Unraveling Electron Dynamics and Constraints

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  1. Group V: Report Regular Members: K. Arzner, A. Benz, C. Dauphin, G. Emslie, M. Onofri, N. Vilmer, L. Vlahos Visitors: E. Kontar, G. Mann,R. Lin, V. Zharkova

  2. Main Goals • Constrains on particle acceleration from the RHESSI data (close collaboration with all WGs) and other available sources of information on high energy particles • Discuss new theories on particle acceleration • Connecting theories on particle acceleration with the global magnetic topologies hosting flares and CMEs

  3. Constraints on Acceleration/Transport(Electrons) • Must produce an electron flux of at least 1037 electrons per second • Must be able to accelerate electrons on time scales at most 10 milliseconds • Must sometimes produce electron energies greater than at least 10’s of GeV • Mechanism must be able to produce a flattening of the electron distribution at energies on the order of 500 keV • Higher nonthermal hard X-ray flux statistically associated with harder spectra

  4. The Electron “Problem” • Efficiency of bremsstrahlung production ~ 10-5 (ergs of X-rays per erg of electrons) Electron flux ~ 105 hard X-ray flux • Electron energy can be 1032 – 1033 ergs in large events • Total number of accelerated electrons up to 1040 (cf. number of electrons in loop ~1038). • replenishment and current closure necessary

  5. Revised Numbers

  6. X/ -ray spectrum Thermal components T= 2 10 7 K T= 4 10 7 K Electron bremsstrahlung Ultrarelativistic Electron Bremsstrahlung -ray lines (ions > 3 MeV/nuc) SMM/GRS Phebus/Granat Observations GAMMA1 GRO GONG Pion decay radiation (ions > 100 MeV/nuc) sometimes with neutrons RHESSI Energy range

  7. Electron-Dominated Events • First observed with SMM (Rieger et al, 1993) • Short duration (s to 10 s) high energy (> 10 MeV) bremsstrahlung emission • No detectable GRL flux • Photon spectrum > 1 MeV (X-1.5—2.0) • For 2 PHEBUS events • if Wi>1MeV/nuc  We>20 keV • No detectable GRL above continuum • Weak GRL flares? Vilmer et al (1999) BATSE PHEBUS

  8. non-thermal thermal RHESSI two component fits: T, EM γ, F35

  9. spectral index flux Grigis & B.

  10. Energy dependent photon spectral index Interval 3 (peak of the flare) Spectral index evolution:

  11. Mean Electron Spectrum: Temporal evolution 1 3 5 RHESSI Lightcurves 3-12keV; 12-25keV; 25-50keV; 50-300keV 2 4 Temporal evolution of the Regularized Mean Electron Spectrum (20s time intervals) 3 1 2 4 5

  12. GOES 1-8 A DERIVATIVE Non-thermal preflare coronal sources

  13. RHESSI SPECTRA 5-50 keV Thermal+broken powerlaw Preflare period: 01:02:00-01:11:00 Broken powerlaw extends down to 5 keV Thermal component never dominates EM and T are poorly determined Chisquare ~ 1 if EM=0 (NB similar source in July 23rd 2002 event) White = photons, Green = thermal model, Red = broken powerlaw, Purple = background

  14. Electron spectrum at 1AU Typical electron spectrum can be fitted with broken power law: Break around: 30-100 keV Steeper at higher energies Oakley, Krucker, & Lin 2004

  15. Ions • Tens of MeV ions and hundreds of MeV particles can be accelerated at the same time; • We also see cases where we see a stage when hundreds of MeV ions are primarily accelerated.

  16. g-ray line emission can be delayed from hard X-rays from <2 to 10’s of sec. 50- 180 keV 275- 325 keV 4 – 6.4 MeV |-----20 sec----| 50- 180 keV 275- 325 keV 4 – 6.4 MeV |------100 sec------|

  17. June 3, 1982 - Evidence for delayed high-energy emission

  18. Constraints for Theory Radio spectral features and flares • Connection between hard X-ray features and spikes in the range 300-3000 MHz, corresponding to densities of 109 -1011, has always been a promising diagnostic of energy release • But there are some aspects hard to understand: frequently the spikes occur in a narrow frequency range for 10s of seconds, implying a fixed density in the energy release site. Energy release widespread over a large volume would produce spikes over a wide frequency (i.e. density) range • Wide range of burst types in this frequency range is hard to understand: what controls frequency drift rates of different features?

  19. Radio Emission at Decimetric Wavelengths

  20. Constraints for Theory Magnetic configuration of flares in the low corona • See configurations of all types in radio images: single “loops”, double “loops”, complex configurations • Frequently see magnetic connections over very large spatial scales • Magnetic field strength: spectra typically imply 500-1000 G in the radio source • But radio spectra are frequently flat-topped: hard to model, range of fields in the source (need FASR) • See both prompt precipitation, implying either rapid scattering of electron pitch angles or loops with little height dependence for B, and trapping, where radio is strong but X-rays are weak, implying little pitch angle scattering.

  21. Radio Flare Loop

  22. Geometry The MHD incompressible equations are solved to study magnetic reconnection in a current layer in slab geometry: Periodic boundary conditions along y and z directions Dimensions of the domain: -lx < x < lx, 0 < y < 2ly, 0 < z < 2lz

  23. Description of the simulations: equations and geometry Incompressible, viscous, dimensionless MHD equations: B is the magnetic field, V the plasma velocity and P the kinetic pressure. and are the magnetic and kinetic Reynolds numbers.

  24. Time evolution of the electric field The surfaces are drawn for E=0.005 from t=200 to t=300

  25. Kinetic energy as a function of time <E> (erg) t=400 t=300 Magnetic energy: Total final energy of particles: t (s)

  26. Energy spectra: e (blue) and p (black)upper panel – neutral, middle – semi-neutral,lower – fully separated beams 1.8 for p 2.2 for e 1.8 for p 2.2 for e 1.7 for p 4-5 for e 4-5 for p 2.0 for e 1.5 for p 1.8 for e

  27. Proton beam compensated by proton-energised electrons precipitate about 10s Pure electron beams, compensated by return current, precipitate in 1s The suggested scheme of proton/electron acceleration and precipitation

  28. Electron Acceleration in Solar Flares basic question: particle acceleration in the solar corona energetic electrons  non-thermal radio and X-ray radiation electron acceleration mechanisms:  direct electric field acceleration (DC acceleration) (Holman, 1985; Benz, 1987; Litvinenko, 2000; Zaitsev et al., 2000)  stochastic acceleration via wave-particle interaction (Melrose, 1994; Miller et al., 1997) shock waves (Holman & Pesses,1983; Schlickeiser, 1984; Mann & Claßen, 1995; Mann et al., 2001)  outflow from the reconnection site (termination shock) (Forbes, 1986; Tsuneta & Naito, 1998; Aurass, Vrsnak & Mann, 2002) radio observations of termination shock signatures HXR looptop HXR footpoints

  29. Outflow Shock Signatures During the Impulsive Phase Solar Event of October 28, 2003: • • X17.2 flare • RHESSI & INTEGRAL data (Gros et al. 2004) • • termination shock radio signatures start • at the time of impulsive HXR rise • • signatures end when impulsive • HXR burst drops off The event was able to produce electrons up to 10 MeV.

  30. Discussion I basic coronal parameters at 150 MHz ( 160 Mm for 2 x Newkirk (1961)) (Dulk & McLean, 1978) (flare plasma) shock parameter total electron flux through the shock

  31. Summary •  The termination shock is able to efficiently generate energetic electrons • up to 10 MeV.  Electrons accelerated at the termination shock could be the source of • nonthermal hard X- and -ray radiation in chromospheric footpoints • as well as incoronal loop topsources. • The same mechanism also allows to produceenergetic protons (< 16 GeV).

  32. Summary • The constrains on the acceleration are becoming so many and the ability of a single acceleration to handle all this become impossible- No unique acceleration • Shocks, stochastic E-Fields and turbulent acceleration enters into the picture • Synchronized from photosheric motions complex magnetic topologies maybe be the answer

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