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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

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Line Features in RHESSI Spectra

Kenneth J. H. Phillips

Brian R. Dennis


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

    • 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

  • 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




Fe XXV +satellites

Ni XXVII + sats

Fe edge

Chianti Spectrum, T=50MK


Fe XXV, XXVI high-n lines

Ca XIX, XX lines

Fe XXV + sats

Ni XXVII + sats

~6.7-keV Fe-line FeatureComposition

~8-keV Fe-Ni-line FeatureComposition

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

ChiantiCoronal Fe abundance

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 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. 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

  • 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.

Intensity ratio of the 2 features


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

Stars – SMM/BCS

Diamonds – Yohkoh/BCS

Curve - Chianti

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

  • 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

  • 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

  • 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.

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