Super-Hot Thermal Plasmas in Solar Flares Amir Caspi Research advisor: R.P. Lin

Why study solar flares? The most powerful explosions in the solar system - energies of up to 109-1010 H-bombs! Provide a “local” laboratory to explore the physics that govern other astrophysical phenomena (stellar flares, accretion disks, etc.) Allow us to explore plasma physics in regimes not (easily) re-creatable in the lab

“Typical” flare characteristics Durations of 100-1000 seconds Electrons and ions accelerated up to 100s of MeV and 10s of GeV (respectively) Plasma temperatures up to 10-50 MK Densities of ~1010 to ~1012 cm-3 Energy content up to ~1032-1033 ergs Generally, loop structure with thermal emission from the looptop, non-thermal emission from footpoints

Open questions Evolution of the thermal plasma Energetics What are the dominant heating mechanisms, especially for super-hot (T > 30 MK) plasmas? Where does heating occur? Is there a fundamental limit on the plasma temperature? What is the relationship between the thermal plasma and accelerated particles? Energetics How much energy contained in thermal electrons? Compared to the energy in accelerated electrons (and ions)?

Basic flare model

Basic flare model

X-ray emission mechanisms Electron bremsstrahlung (free-free continuum emission) Radiative recombination (free-bound continuum emission) Electron excitation & decay (bound-bound line emission)

Free-free (bremsstrahlung) Thermal: Maxwellian electron distribution yields Nonthermal: “injected” electron spectrum yields

Free-bound & bound-bound Free-bound continuum: free (thermal) electrons recombine and emit a photon of energy Bound-bound lines: bound electron excited (primarily through collisions with ambient free electrons) and de-excites via emission of a photon of energy Line profile (peak energy, FWHM, amplitude, shape) depends on T, v, n In X-rays, primary solar lines are from ions of O, Si, Ca, Fe, and Ni

X-ray Flare Classification Photometers on board the GOES satellites monitor solar soft X-rays GOES class is determined by peak flux in the 1-8Å channel Rough correlation between GOES class and temperature, energy

X-ray Flare Phases Impulsive (rise) phase - bursty HXR, fast but smoothly rising SXR Gradual (decay) phase - little to no HXR, gradual decline in SXR Pre-impulsive gradual rise observed in some flares

Early X-Ray Observations Balloon, rocket, satellite Broadband spectrometers Bragg crystal (narrowband) spectrometers Broadband imagers Instrumental limitations BBS: coarse energy resolution allowed interpretation of HXR spectra as thermal w/ T > 100 MK BCS: lines suggested T ~ 20 MK No “complete” picture of flare emission (Crannell et al. 1978)

X-Ray Observations: TNG Germanium detectors: much higher broadband spectral resolution Allow more accurate ID of thermal vs. non-thermal emission First results HXR emission likely non-thermal Emission from “super-hot” (T > 30 MK) thermal component RHESSI offers the first “complete” picture of flare emission: SXR/HXR continuum and line emission, plus imaging in arbitrary energy bands (Lin et al. 1981)

RHESSI Spectra and Imaging

Benefits of RHESSI Good spectral resolution - can distinguish between thermal/non-thermal emission Good temporal resolution - can observe evolution of spectra over short times Good angular resolution - can distinguish spatially-separate sources (and do spectroscopy) First broadband instrument with simultaneous spectral and imaging observations of continuum (thermal + nonthermal) and lines Now have multiple measurements of thermal emission

Fe & Fe/Ni line complexes Line(s) are visible in almost all RHESSI flare spectra Fluxes and equivalent width of lines are strongly temperature-dependent (Phillips 2004)

Fe & Fe/Ni line complexes Differing temperature profiles of line complexes suggests ratio is unique determination of isothermal temperature (Phillips 2004) Only weakly dependent on abundances

Fe & Fe/Ni line complexes Lines are cospatial with thermal continuum source No significant emission from footpoints Lines are a probe of the same thermal plasma that generates the continuum We can directly compare continuum temperature to line-ratio temperature

Analytical method Fit spectra with isothermal continuum, 3 Gaussians, and power law Calculate temperature from fit line ratio; may also calculate emission measure & equiv. widths from line fluxes Compare to continuum temperature

Two flares: 23/Jul/02 & 02/Nov/03

Flux ratio vs. Temperature

Flux ratio vs. Temperature

Flux ratio vs. Temperature

Fe Equivalent Width vs. Temperature Method of Phillips et al. (2005) Defined as integrated line flux divided by continuum flux (at peak energy) Compared to predictions, trend is opposite from ratio temperatures Not independent of abundances

Flux ratio vs. Temperature

23 July 2002: Pre-impulsive phase Fit equally well with or without thermal continuum! Iron lines indicate thermal plasma must be present, but much cooler than continuum fit implies

24 Aug 2002: Pre-impulsive phase

Flux ratio vs. Temperature

Flux ratio vs. Temperature

Flux Ratio vs. Temperature Possible explanations: Instrumental effects and coupled errors in multi-parameter fits Ionization non-equilibrium Incorrect assumptions about ionization fractions Multi-thermal temperature distribution … small contribution … unlikely … possible … needs further investigation

Emissivity vs. Temperature

Emissivity vs. Temperature

Emissivity vs. Temperature Possible explanations: Instrumental effects and coupled errors in multi-parameter fits Ionization non-equilibrium Multi-thermal temperature distribution Incorrect assumptions about ionization fractions Line excitation by non-thermal electrons Abundance variations during the flare … small contribution … unlikely … needs further investigation

Conclusions Fe & Fe/Ni line complexes provide a probe of the thermal plasma in addition to continuum emission Help constrain fits to thermal continuum Provide thermal information even when continuum is difficult to analyze Not all flares exhibit the same line/continuum relationship May suggest different temperature distributions Other differences (e.g. spectral hardness) may contribute Ratio & equivalent width results are not self-consistent Suggests theoretical predications may need corrections Assumptions about ionization fractions may be incorrect

Future Work Statistical survey of Fe & Fe/Ni emission in M/X flares Differential Emission Measure (DEM) analysis Determine effects of multi-temperature distribution on relationship between line ratio and isothermal approx. Use line emission to constrain DEM models Imaging Spectroscopy Obtain and analyze spectra for spatially-separated sources (e.g. footpoints and looptop) Isolate presumed thermal and non-thermal sources to determine individual thermal/non-thermal properties Place limits on the extent of non-thermal excitation of the lines

EXTRA SLIDES

Basic flare model (cartoon and data) (Aschwanden & Benz 1997)

(Krucker)

RHESSI Spectra and Imaging

Flux ratio vs. Temperature

Flux ratio vs. Temperature

Flux ratio vs. Temperature

Flux ratio vs. Temperature

Flux ratio vs. Temperature

Emissivity vs. Temperature

Emissivity vs. Temperature

Emissivity vs. Temperature

Emissivity vs. Temperature

Emissivity vs. Temperature

Emissivity vs. Temperature

Emissivity vs. Temperature

Flare location/size

Centroids of emission Higher energy emission from higher in the looptop Strongly implies multi-thermal distribution Centroid of Fe line complex emission consistent with high-EM, lower-T plasma lower in looptop