2. RHESSI OBSERVATIONS: A new flare pattern and a new model for the old pattern H. S. Hudson (SSL Berkeley)

3. Bozeman, April 10 2007 The new pattern The usual scheme for flare development features footpoint hard X-ray sources and the Neupert effect (the “thick target model”) The new pattern, mainly from RHESSI via the work of Säm Krucker, features: - Hard X-rays and  -rays from high in the corona - Minimal footpoint emission - Strong association with the biggest events (ie, CMEs and SEPs)

4. Bozeman, April 10 2007 A new explanation of the old pattern Modern data show that the thick-target model has insuperable problems Our proposal (Fletcher & Hudson 2007) replaces the particle beam with wave energy transport and places the electron acceleration in the chromosphere

5. Bozeman, April 10 2007 RHESSI vital statistics FOV whole Sun, 3 keV - 15 MeV Angular scales in 9 discrete collimators, 2.15 arc sec through 2.9 arc min (FWHM) Spectral resolution 1 keV for hard X-rays, somewhat larger for  -rays Launch February 2002 into middle-inclination low Earth orbit via a Pegasus rocket Many “firsts” from these observations, which continue RHESSI is the only high-energy solar observatory existing or planned

6. Bozeman, April 10 2007 Reuven Ramaty, 1967-2001

7. Bozeman, April 10 2007 The RHESSI “iris diaphragm” innovation and its front/rear segmentation Lin et al. 2002

8. Bozeman, April 10 2007 The RHESSI scheme RHESSI was built at SSL (Berkeley), NASA/GSFC and the Paul-Scherrer Institut (Switzerland). The RHESSI team is led by Bob Lin (Berkeley), Brian Dennis (NASA), and Arnold Benz (Zurich)

9. Bozeman, April 10 2007 Thanks to Gordon Hurford

10. Bozeman, April 10 2007 New improved science nuggets

11. Bozeman, April 10 2007 Topic 1: the new pattern for solar hard X-rays There’s some history to this, some brief comments about March 30, 1969 Then on to Säm’s new wonderful RHESSI material

12. Bozeman, April 10 2007 Frost & Dennis 1971 Enome & Tanaka 1971 (3.5 GHz) March 30, 1969: X-rays and Microwaves No H  flare, ~W105

13. Bozeman, April 10 2007 Palmer & Smerd, 1972 March 30, 1969: meter waves (Culgoora)

14. Bozeman, April 10 2007 RHESSI Coronal X-ray imaging Two-ribbon flare with both hard X-ray footpoints (blue) and thermal loop (red)

15. Bozeman, April 10 2007 Exponential decay reveals coronal source at 250 keV!

16. Bozeman, April 10 2007 Imaging spectroscopy Coronal source has HARDER/FLATTER spectrum than footpoints! spatially integrated  ~ 1.5 +- 0.3  ~ 2.9 +- 0.1

17. Bozeman, April 10 2007 Another high coronal event: 2002 Oct 27 MARS: GRS RHESSI 15-25 keV  -ray flare seen by GRS (MARS) GOES soft X-ray (thermal): tiny flare (B2) HXR emission up to 80 keV simultaneous onset exponential decay (  ~135 s) Earth

18. Bozeman, April 10 2007 Very large (>200 arcsec), expanding and rising, CME velocity ~2000 km/s October 27, 2002 ~400 km/s ~800 km/s size motion 300”

19. Bozeman, April 10 2007 Coronal source summary RHESSI sees many coronal sources, but no Masuda flares The large-scale sources have spectra that are extremely hard - recall  Kramers ~ 1/  E, hence relativistic electrons - stable trapping - large non-thermal pressure Sources can move or be stable

20. Bozeman, April 10 2007 Topic 2: the new explanation for the impulsive phase Wave transport of energy, not electron beams Translation of magnetospheric ideas Thanks to many magnetospheric physicists in Berkeley and elsewhere for explanations

21. Bozeman, April 10 2007 Death to the thick target!

22. Bozeman, April 10 2007 Basic ideas The electron beams of the thick-target model have become untenable Alfvén speeds in the flaring corona has been systematically underestimated Magnetic reconnection in a sheared field ought to have Alfvén-mode exhaust flows There are ways to get parallel electric fields, hence fast electrons, from dispersive Alfvén waves

23. Bozeman, April 10 2007 Collisional thick-target model (e.g. Kane & Anderson ‘69, Brown ’71, Hudson’72) Hard X-ray emission is primarily electron-proton bremsstrahlung from energetic electron beam in a cold, collisional chromosphere Beam also heats chromosphere producing white light and UV Coronal accelerator Coronal electron transport (generally no treatment of plasma collective effects) Bremsstrahlung HXRs and collisional heating HXRs, UV, WL chromosphere

24. Bozeman, April 10 2007 ‘Volumetric’ acceleration: Wave-particle turbulence (e.g. Larosa et al, Miller et al) Stochastic current sheets (e.g. Turkmani et al) Betatron acceleration (e.g. Brown & Hoyng, Karlicky et al) Diffusive shock or shock drift acceleration (e.g. Tsuneta & Naito, Mann et al) Reconnecting X-line or current-sheet acceleration Multiple X-lines/islands (e.g. Kliem, Drake) Single macroscopic current sheet (e.g. Litvinenko & Somov, Somov & Kosugi) Acceleration in the corona requires a high fraction of a large volume of electrons to be accelerated to high energies

25. Bozeman, April 10 2007 Orange=25-50keV Blue=WL RHESSI HXR and TRACE White Light White light footpoint area ~ 10 17 cm 2 From area and WL power, calculate beam number and energy flux. RHESSI 25-50keV 1 px = 0.5” ~ 300km

26. Bozeman, April 10 2007 Is there any evidence for a beam? By studying the photospheric albedo contribution to the HXR spectrum it is possible to discover the ratio of downward-going and upward-going electrons in the chromosphere. Ratio of downward/upward-going electrons is < 1 This is not consistent with a beam from the corona. Aug 20 2002Jan 17 2005 Kontar & Brown 2007

27. Bozeman, April 10 2007 Challenges for the impulsive phase “beam” (1)HXRs imply electron flux can be in excess of 2  10 36 e/s  typical flare ‘volume’ is emptied of electrons in ~10s but HXRs continue for several minutes. (2) Small WL & HXR footpoints imply electron beam density  0.1 of background plasma density  unstable beam propagation in corona (3) If corona is dense enough for stable propagation  no HXR footpoints are formed below ~ 50keV. (4) HXR albedo observations are not consistent with a downward-directed beam entering the chromosphere from the corona.

28. Bozeman, April 10 2007 The propagation time for these waves to the chromosphere is shorter than their damping time in the corona. Coronal B in core of active region ~ 500-1000G from microwave measurements. Pre-flare coronal n ~ 10 9 cm -3  Alfven speed ~ 35,000 km/s fast-mode -like The relaxation of a twisted, 3D field is an almighty magnetic ‘convulsion’, with a fast / Alfvénic character. Alfven-like Melrose 1992

29. Bozeman, April 10 2007 Coronal propagation B = 500 G B = 1000 G Use expression for damping time by phase-mixing (e.g. Roberts 2000) –  A <  damp for long wavelengths Cascade to short parallel wavelengths will not happen but cascade to short perpendicular wavelengths proceeds (Kinney & McWilliams 1998).

30. Bozeman, April 10 2007 Implications Wave Poynting flux dissipated in chromosphere produces local heating (white-light and UV?) and may cause local electron acceleration. Wave propagating to chromosphere and below will re-orient magnetic field – changes in line-of-sight B. In corona, parallel field will accelerate electrons to ~ 2v A, or ~ 20-40 keV. (NB this is the self-consistent electron current in 2-fluid MHD) Wave partially reflects in non-uniform chromosphere, draws electrons back into corona for re-acceleration. Process intimately connected to reconnection, so signatures related to magnetic topology changes are preserved.

31. Bozeman, April 10 2007 End Thanks for discussion and input: Lyndsay Fletcher, Säm Krucker