Proceedings of IAU Symposium 233,
St. Petersburg 2004, submitted

First Determinations of Differential Emission Measure Distribution from RESIK X-ray Spectra

A. Kepa, J. Sylwester, B. Sylwester and M. Siarkowski

Space Research Centre, Polish Academy of Sciences,
Kopernika 11, 51-622 Wroclaw, Poland
e-mail: ak@cbk.pan.wroc.pl

Abstract

RESIK Bragg spectrometer aboard CORONAS-F solar observatory collected spectra of hundreds of solar flares in the unique spectral range between 3.3 Å and 6.1 Å. The instrument's absolute and relative calibrations are now finished which allows for the first time to interpret the observations in terms of the differential emission measure (DEM) distribution. With known DEM shape it is possible to derive and study in detail flaring plasma composition and energy balance. Here we report results of using an improved Maximum Likelihood iterative algorithm (Withbroe-Sylwester) for determination of the DEM distributions for 7 January 2003 flare at 23:30 UT. The study is based on the absolute flux values observed in a number of strong H- and He-like transitions of K XVIII, Ar XVIII, Ar XVII, S XVI, S XV, Si XIV and Si XIII ions. In this respect, we use line and continuum X-ray emissivities as provided by CHIANTI package from SolarSoft.

1 Introduction

We present analysis of differential emission measure distribution (DEM) for solar flares. The DEM describes the distribution of emitting plasma with temperature. With known DEM shape it is possible to derive and study in detail flaring plasma composition and energy balance. For calculations of the DEM shape we use Withbroe-Sylwester multiplicative algorithm (Sylwester, Schrijver and Mewe, 1980).

2 Method

The Withbroe-Sylwester multiplicative algorithm is the iterative procedure in which the next approximation of the DEM distribution

(T) is calculated from preceding one

(T)_. using the following expression:

where:

c_i is the correction factor taken as c_i = F_oi/F_ci, F_oi and F_ci are the measured and predicted fluxes in particular line/band i (i = 0, 1, 2 ... k) and w_i(T) is the weight function defined by the following relationship:

where:

a - is the optimizing parameter, f_i(T) is the emission function in line/band i and F_ci is the flux calculated (in j^th iteration) as:

In the set of selected bands used for the DEM analysis the fluxes consist of line and continuum emissions. The emission functions for spectral bands selected have been calculated using CHIANTI (Dere et al., 2001) package ver. 4.2.

3 Results

>From a large RESIK database covering hundreds of flares, we choose well observed (class M4.9) flare on 7 January 2003 ( ~ 23:30 UT). Derived variations of DEM for this event are consistent with similar determinations of DEM shape from the FCS SMM observations.

Figure 1: L: The theoretical emission functions (in photons cm^-2 s^-1, lines plus continuum included) used in the calculations; R: Example DEM curves for 5 time intervals (a, b, c, d - rise phase, e - late decay) for the 7 January 2003 flare.

Particularly interesting findings obtained are: a) development of high temperature plasma component (T > 20 MK) during flare rise phase, b) presence of "flat DEM phase" near flare maximum, c) development of two-temperature DEM pattern during decay phase with a slow cooling of the hotter plasma component. RESIK (PI - J. Sylwester) is a common project between NRL (USA), MSSL and RAL (UK), IZMIRAN (Russia) and SRC (Poland). The authors acknowledge support from grant 2.P03D.002.22 of the Polish Committee for Scientific Research.

References

Sylwester J., Schrijver J. & Mewe R. 1980 Solar Physics 67, 285-309.

Dere, K. P., Landi, E., Young, P. R. & Del Zanna, G. 2001 ApJS 134, 331-354.

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Proceedings of IAU Symposium 233, St. Petersburg 2004, submitted