# Presentation: 394447

## 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
- 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

- Fe XXV

- Ni XXVII + sats

- Ca XIX

- Fe XXV

- Fe edge

## Chianti Spectrum, T=50MK

- Fe XXV + sats

- Fe XXVI

- Ni XXVII + sats

- Fe XXV, XXVI high-n lines

- Ca XIX, XX lines

## ~6.7-keV Fe-line FeatureComposition

- ~6.7-keV Fe-line FeatureComposition

## ~8-keV Fe-Ni-line FeatureComposition

- ~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

- DEM = A T-α cm-3 K-1
- Integral from 10 to 100 MK

- Equivalent Width in keV/A

## Fe-line Equiv. Width vs. T0

- Fe-line Equiv. Width vs. T0

- DEM = B exp (-T/T0) cm-3 K-1
- Integral from 10 – 100 MK

- Equivalent Width in keV/B

- 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

- Chianti

## 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

- 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.

## Conclusions

- 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.