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Line Features in RHESSI Spectra. Kenneth J. H. Phillips Brian R. Dennis GSFC RHESSI Workshop Taos, NM 10 – 11 September 2003. Line Features in RHESSI Spectra. Two features from highly ionized Fe and Ni. ~6.7 keV Fe XXV lines and satellites Fe XVIII – Fe XXIV lines

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Line features in rhessi spectra
Line Features in RHESSI Spectra

Kenneth J. H. Phillips

Brian R. Dennis


RHESSI Workshop

Taos, NM

10 – 11 September 2003

Line features in rhessi spectra1
Line Features in RHESSI Spectra

Two features from highly ionized Fe and Ni.

  • ~6.7 keV

    • Fe XXV lines and satellites

    • Fe XVIII – Fe XXIV lines

    • Fe XXVI lines at higher T (>80MK)

  • ~8 keV

    • Fe XXV & Fe XXVI lines, more highly excited

    • Ni XXVII, Ni XXVIII lines

The fe line and fe ni line features
The Fe-line and Fe/Ni-line Features

  • 6.7-keV feature resolved with crystal spectrometers in flares many times.

  • 8-keV feature hardly ever seen from flares with crystal spectrometers.

  • RHESSI is the first instrument (apart from NEAR-PIN) to see this part of the spectrum of flares.

  • RHESSI resolution (~0.8 keV FWHM for detector 4) allows for some diagnostic work.

Chianti spectrum t 20mk

Chianti Spectrum, T=20MK




Fe XXV +satellites

Ni XXVII + sats

Fe edge

Chianti spectrum t 50mk
Chianti Spectrum, T=50MK


Fe XXV, XXVI high-n lines

Ca XIX, XX lines

Fe XXV + sats

Ni XXVII + sats

Temperature t dependences
Temperature (T) Dependences

  • Line features are made up of many different lines.

  • Each line has different T dependence of intensity - G(T) functions.

  • Thus, line feature intensity varies with T relative to continuum.

  • Line-to-continuum ratio is best measured by “equivalent width”, i.e. width in keV of the line feature having intensity equal to continuum.

Equivalent width of fe line feature
Equivalent Width of Fe-line Feature

ChiantiCoronal Fe abundance

Temperature t dependences1
Temperature (T) Dependences

  • For multithermal plasma, must use differential emission measure, DEM(T).

  • Ken Phillips has used

    • DEM = K T-α

    • DEM = K exp(-T/T0)

  • Equivalent width vs. α and T0

Fe line equivalent width vs


Fe-line Equivalent Width vs α

Equivalent Width in keV/A

DEM = A T-α cm-3 K-1

Integral from 10 to 100 MK

Fe line equiv width vs t 0
Fe-line Equiv. Width vs. T0

Equivalent Width in keV/B

DEM = B exp (-T/T0) cm-3 K-1

Integral from 10 – 100 MK

T0 (MK)

Intensity ratio of the 2 features
Intensity Ratio of the 2 Features

  • The intensity ratio of the Fe-line to Fe/Ni-line features depends on T,but only weakly for T>40MK.

  • For T<30MK, the Fe/Ni-line feature is weak.

  • Thus, intensity ratio is useful asT-diagnostic for flares with T > 30 MK.

Fe line centroid energy
Fe-line Centroid Energy

  • As T increases, Fe XXIV satellites in the Fe-line feature decrease relative toFe XXV resonance line.

  • Thus, centroid energy of the Fe-line feature increases with T.

  • RHESSI’s small gain change with count rate make the modest energy change difficult to measure at present.

Fe line feature centroid energy vs t
Fe-line Feature Centroid Energy vs. T

Stars – SMM/BCS

Diamonds – Yohkoh/BCS

Curve - Chianti

Synthesizing x ray spectra
Synthesizing X-ray Spectra

  • Chianti used for most plots here.

  • Comparisons between Chianti and SMM/BCS spectra show significant differences. Incorrect Fe XXIII line intensities in Chianti

  • Comparisons with APEC spectra also show differences. More lines included in APEC than in Chianti but may not be significant for RHESSI’s application.

Other atomic codes
Other Atomic Codes

  • Currently, SPEX uses

    • Mewe et al. (1985) data,

    • Arnaud & Rothenflug (1985) ion fractions,

    • cosmic element abundances.

  • Mazzotta et al. (1998) ion fractions are better (rates based on better cross section data now available, some experimentally verified).

  • With APEC and Chianti, it’s possible to choose ion fraction calculation and abundances.

Element abundances in flares
Element Abundances in Flares

  • RHESSI line features depend only on Fe/H and Ni/H abundances.

  • Coronal Fe, Ni abundances are higher than photospheric by a factor of

    • 4 (Feldman et al.)

    • 1.5-2.0 (Fludra & Schmelz’s “hybrid” model)

    • 1 (Meyer).

  • Abundances in flares may vary

    • from flare to flare (Feldman et al.)

    • during flares (Sylwester et al.).

Possible rhessi projects
Possible RHESSI Projects

  • Determine flare Fe abundances

    • continuum + line fits to RHESSI spectra

    • continuum slope gives T

    • line feature equivalent width gives Fe abundance.

  • Compare measured Fe abundance with nonthermal parameters, flare size, duration, etc.

  • Images in Fe line show location of high temperature plasma.


  • Diagnostic potential of Fe and Fe/Ni line complexes in RHESSI flare spectra:

    • Equivalent width  Fe abundance with T from continuum

    • Fe to Fe/Ni ratio  T (~30 – 40 MK)

    • Fe feature centroid energy  T (problems)

  • Old version of Mewe code used in SPEX is inaccurate.

  • Chianti problems at the 20 – 30% level.

  • APEC similar to Chianti for RHESSI’s purposes.

  • Plan to switch from Mewe to Chianti in SPEX.