Peculiarities of X-ray emission of the solar flare on 29 October 2002

1. Introduction

[2] The experiment IRIS conducted since August 2001 on board the orbital station CORONAS-F continues the series of studies of the X-ray emission of solar flares carried out by the Physical and Technical Institute at high-apogee satellites of the PROGNOZ series and orbital station CORONAS-I.

[3] The experimental data on spectral and time characteristics of the X-ray emission of solar flares contain "direct'' information on the processes of explosion-type energy release during the flare and on physical parameters of the plasma in the flare regions and solar atmosphere. Being combined with observations of the Sun in the UV, optical, and radio ranges, they provide a basis for creation of physical models of solar flares. Especially important information is obtained from the measurements of X-ray emission characteristics with high sensitivity. The IRIS spectrometer created on the basis of the detectors with large area of entrance windows makes it possible to study fluxes of soft X-ray emission of a low intensity but during weak flares and during preflare periods of development of solar events.

[4] The device operates permanently in the monitoring regime. The time resolution in this regime is 2.5 s, the energy ranges are 2.9-22 keV and 15-250 keV. In the case of quick increase of the X-ray emission, a "Splash'' regime is switched on. In this regime the information is translated into the special telemetric system SCSI (System of Collection of Scientific Information) with the time resolution of 1.0 and 0.01 s. The measured flux of the X-ray emission is translated into SCSI within the energetic ranges: 2.9-14 and 15-160 keV, each range being subdivided into 32 energetic channels. Characteristics of the spectrometric channel of the IRIS device are shown in Table 1.

[5] Because of its physical parameters, the scientific equipment of IRIS makes it possible to study: (i) spectral-time characteristics of the soft X-ray emission not only from strong, but from weak solar flares as well, and also from the quiet Sun when the intensity is not more than sim 10^-5 erg cm^-2 s^-1 (~ 10 nW m^-2 ); and (ii) evolution of the energetic spectra of both soft and hard X-ray emission using the 32-channel spectra with the time resolution of 1 s. High temporal resolution (10 ms) of IRIS spectrometer makes it possible to single out separate pulses in hard X-ray emission of solar flares with the duration of several tens of milliseconds. Such pulses with duration more than 45 ms previously were registered on board the satellite "SMM'' [Kiplinger et al., 1983]. However, the millisecond structure was observed only within 10% of the registered flares [Dennis, 1985]. According to the observations performed on board the Burst and Transient Source Experiment (CGRO), peak in time distribution of 5000 spikes corresponds to the interval 300-1000 ms [Aschwanden, 2002; Aschwanden et al., 1996]. It means that for the majority of flares the duration of single pulses comprises hundreds of milliseconds. Pulses with duration of the order of 20-30 ms probably are rarer observed. Nevertheless, they are important for the understanding of physical processes related to plasma of solar flares.

Figure 1

[6] Using the flare registered by the IRIS device on 29 October 2002, we analyze what properties of time structures and dynamics of energetic spectra of solar flares can be studied on the basis of the fluxes of soft and hard solar emission measured by the device in the Splash regime. Figure 1a shows the picture of solar activity (X-ray emission flux with the time resolution of 1 min in units W m^-2 during 29 October 2002) in the soft X-ray range of wavelengths of 1-8 and 0.4-5Å (that corresponds to the energy ranges 1.6-12.4 and 3.1-24.8 keV, respectively) according to the data of the geostationary satellite GOES 10 available at http://spidr.ngdc.noaa.gov (Space Physics Interactive Data Resource Service). In Figure 1a the arrow marks the solar flare which was simultaneously with GOES 10 measured by the IRIS spectrometer in the Splash regime. This flare of the C1.8 class is shown in more detail in Figure 1b. The arrows in Figure 1b show the time interval of measurements of the X-ray solar emission by the IRIS device in the soft X-ray range simultaneously with the GOES 10 satellite. One can clearly see that the time of measurements of the flare by the IRIS device in the Splash regime in this energetic range covers only the main maximum of the event. Since the time profile of the main maximum of this flare is rather completely measured by the spectrometer, we can consider in more detail the time and spectral characteristics of this event.