A theorist’s assessment/wish list of future observational needs

1. A theorist’s assessment/wish listof future observational needs Gordon Emslie

2. RHESSI Proposal Excerpts “N(E,r,t), together with information on ambient density, magnetic field strength and topology, will allow the electron loss processes to be directly evaluated. This will decide whether the X-ray emission is thermal or nonthermal, since the energy loss characteristics of the emitting electrons are so different in the two cases.” “HESSI's high sensitivity imaging spectroscopy extending down to ~3 keV will allow systematic survey of the microflare non-thermal contribution to coronal heating.”

3. RHESSI Proposal Excerpts “HESSI will provide the first determination of line shapes, which are direct probes of the angular distribution of the interacting accelerated ions. The derived angular distributions will discriminate between various transport and acceleration mechanisms.” “HESSI will provide the first high-resolution spectroscopy and imaging of the 20Ne line. The ratio of the flux in this line to other lines will probe the ion energy spectrum (and total ion energy content) down to ~1 MeV.” “HESSI can detect a narrow line at 339 keV due to the bombardment of 56Fe by energetic alpha-particles leading to 59Ni in an excited state. This will distinguish between enhanced alpha-abundance in accelerated particles or in the ambient medium.”

4. RHESSI Proposal Excerpts • “The shape of the 0.511 MeV positron annihilation line gives information about the ambient medium since the positrons slow down before annihilating. HESSI's energy resolution at 0.511 MeV is sufficiently good to measure temperatures down to 104 K, and it can easily distinguish among annihilation sites located below the transition region, in the corona, or in the hot (~107 K) flare plasma.” • “Time variation of the 2.223-MeV line can also be used to determine the unknown 3He abundance in the photosphere. The solar 3He abundance is essential for studies of galactic abundance evolution and may have cosmological implications.”

5. So how have we done?

6. Problems “Solved” True shape of hard X-ray spectra; domain of thermal dominance Microflares CANNOT be source of quiescent coronal heating Stronger connection between flares and CMEs (no “solar flare myth”) Solar oblateness determined to < 1 mas; dominated by EUV magnetic elements

7. The thermal/non-thermal transition in the electron distribution function • Existence (but not ubiquity?) of HXR structure at very small spatial scales • Directivity/location of primary hard X-ray source (albedo studies) • Location/physical properties (n, T, B) of acceleration region • Widths/shifts of gamma-ray lines • Generation of electron flux maps – but not yet at stage to study variation of F(E,r,t) Issues “Advanced”

8. Issues “Advanced” (EM,T) analysis of soft X-ray-emitting plasma Partition of flare energy (SEP shock-acceleration efficiency) Fourier-based Imaging Algorithms Soft-hard-soft spectral evolution (“pivot point”?) Skype teleconferencing skills

9. Pervasive evidence for “shockingly” bright nonthermal sources in the corona • Hard X-ray/gamma-ray emission locations different • Gamma-ray redshifts inconsistent with vertical guiding field • Different electron spectra deduced from HXR vs. radio observations Issues “Uncovered”

10. Issues on which major progress is expected Relationship between EUV and HXR sources (SDO) Ultra-energetic ion diagnostics (Fermi) Flare radiant energy content (EVE)

11. To what extent can limitations in existing data point the way to new observations?

12. Pattern of residuals • Shows inadequacy of fit model • Suggests how to improve it!

13. New Observations Suggested HXR/-ray imaging (occulting?) spectropolarimetry (x~2”, 20-300 keV, / ~ 0.1-0.2, t ~ (1-10)s, 10%P; 100:1 dynamic range) Large-area HXR instrument to provide very short timescale measurements (e.g., acceleration, time-of-flight) Focusing-optics HXR telescope Stereo (Quad?) HXR imaging for directivity/3-D imaging studies

14. New Observations Needed Gamma-ray imaging spectroscopy (x~5”, 0.5-2.3 MeV, / ~ 1%) Observations of weak proton-capture gamma-ray lines, produced by low-energy (~0.5 MeV) ions High-resolution (~1 eV) imaging spectroscopy of lines (principal, satellite, fluorescence,…) and continua in the (1-10) keV range

15. New Observations Needed Comparison of in situ and remote-sensing signatures of accelerated particles Frequency-agile radio interferometry (FASR) Coronal magnetic field diagnostics AIA-type instrument with an agile “flare mode” (narrower field of imaging and more rapid time cadence)

16. The RHESSI Book • A special edition of Space Science Reviews • First conceived at Santa Cruz 2007 • Nine Articles/”Chapters” • Introduction (Dennis et al.) • Multi-Wavelength Observations (Fletcher et al.) • Electrons (Holman et al.) • Ions (Vilmer et al.) • Radio (White et al.) • Statistical Properties (Hannah et al.) • Hard X-Ray/Electron Math (Kontar et al.) • Theory (Zharkova et al.) • Conclusions (Lin)