Super-Hot Thermal Plasmas in Solar Flares Amir Caspi Research
advisor: R.P. Lin
Slide 2
2 Why study solar flares? The most powerful explosions in the
solar system - energies of up to 10 9 -10 10 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
Slide 3
3 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 ~10
10 to ~10 12 cm -3 Energy content up to ~10 32 -10 33 ergs
Generally, loop structure with thermal emission from the looptop,
non-thermal emission from footpoints
Slide 4
4 Open questions Evolution of the thermal plasma 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)?
Slide 5
5 Basic flare model
Slide 6
6
Slide 7
7 X-ray emission mechanisms Electron bremsstrahlung (free-free
continuum emission) Radiative recombination (free-bound continuum
emission) Electron excitation & decay (bound- bound line
emission)
Slide 8
8 Free-free (bremsstrahlung) Thermal: Maxwellian electron
distribution yields Nonthermal: injected electron spectrum
yields
Slide 9
9 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
Slide 10
10 X-ray Flare Classification Photometers on board the GOES
satellites monitor solar soft X-rays GOES class is determined by
peak flux in the 1-8 channel Rough correlation between GOES class
and temperature, energy
Slide 11
11 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
Slide 12
12 Early X-Ray Observations (Crannell et al. 1978) 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
Slide 13
13 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)
Slide 14
14
Slide 15
15 RHESSI Spectra and Imaging
Slide 16
16 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
Slide 17
17 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)
Slide 18
18 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
Slide 19
19 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
Slide 20
20 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
Slide 21
21 Two flares: 23/Jul/02 & 02/Nov/03
Slide 22
22 Flux ratio vs. Temperature
Slide 23
23 Flux ratio vs. Temperature
Slide 24
24 Flux ratio vs. Temperature
Slide 25
25 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
Slide 26
26 Flux ratio vs. Temperature
Slide 27
27 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
Slide 28
28 24 Aug 2002: Pre-impulsive phase
Slide 29
29 Flux ratio vs. Temperature
Slide 30
30 Flux ratio vs. Temperature
Slide 31
31 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
needs further investigation unlikely possible
Slide 32
32 Emissivity vs. Temperature
Slide 33
33 Emissivity vs. Temperature
Slide 34
34 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 needs further investigation unlikely
Slide 35
35 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
Slide 36
36 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
Slide 37
EXTRA SLIDES
Slide 38
38 Basic flare model (cartoon and data) (Aschwanden & Benz
1997)
Slide 39
39 (Krucker)
Slide 40
40 RHESSI Spectra and Imaging
Slide 41
41 Flux ratio vs. Temperature
Slide 42
42 Flux ratio vs. Temperature
Slide 43
43 Flux ratio vs. Temperature
Slide 44
44 Flux ratio vs. Temperature
Slide 45
45 Flux ratio vs. Temperature
Slide 46
46 Emissivity vs. Temperature
Slide 47
47 Emissivity vs. Temperature
Slide 48
48 Emissivity vs. Temperature
Slide 49
49 Emissivity vs. Temperature
Slide 50
50 Emissivity vs. Temperature
Slide 51
51 Emissivity vs. Temperature
Slide 52
52 Emissivity vs. Temperature
Slide 53
53 Flare location/size
Slide 54
54 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