RESIK
High-Sensitivity
Soft X-Ray Spectrometer
for
the Study of Solar Flare plasma
J. Sylwester1, I. Gaicki1, Z. Kordylewski1, M. Kowaliński1, S. Nowak1, M. Siarkowski1, W Trzebiński1,
R.D.
Bentley2,
M.W. Whyndham2,
P.R. Guttridge2,
J.L. Culhane2,
J.
Lang3,
K.J.H. Phillips3,
C.M. Brown4, G.A. Doschek4,
V.N.Oraevsky5,
S.I. Boldyrev5,
I. M. Kopaev5, A.I.
Stepanov5, V.Yu.
Klepikov5
1Space Research Centre, Polish Academy of Sciences, Wrocław, Poland
We present details concerning the rationale, objectives and construction of the RESIK Bragg crystal spectrometer which is to be launched aboard the CORONAS-F solar observatory next year.
1. INTRODUCTION
Flares are the impulsive release of energy that occur in solar active regions. They play an important role in the overall energy balance of the Sun, particularly during times of maximum solar activity. In order to understand what mechanism produce flares, it is important to measure the physical parameters of the flare plasma (temperature, emission measure and composition) as the flare evolves.
RESIK (REntgenovsky Spektrometr s Izognutymi Kristalami) is a Bragg crystal spectrometer designed to observe active region and solar flare plasmas and has been selected for the payload of the CORONASF satellite offered to the participants on the no-launch-payment and no-exchange-of-funds basis. This satellite is one of a series of observatories devoted to solar physics that will be operational during the current 23rd solar cycle. CORONAS-F follows CORONAS-I satellite launched in 1994.
The primary objectives of the experiments aboard the CORONASF satellite are to study global solar oscillations, solar constant variation, physics of flares and active regions.
CORONASF will carry instruments that make high resolution measurements of the γray flare spectra, in situ charged particles composition, solar neutron, UV and radio fluxes. The satellite is to be put in a circular orbit around the Earth with an altitude of 500 km and its orbital plane inclined at 82º to the equator this results in periods of several days of uninterrupted observations of the Sun (the longest satellite night will last for 35 minutes). The operational lifetime of the satellite is expected to be not less than 2 years.
RESIK is a Bragg crystal spectrometer designed to observe active region and solar flare plasmas. Its channels span ten different spectral bands in 1.1 Å - 6.1 Å range. Two pairs of crystals co-operate with two position-sensitive double proportional counters (spare Yohkoh BCS units).
The RESIK instrument is to be constructed by a consortium of five groups, namely, IZMIRAN in Russia, the Mullard Space Science Laboratory (MSSL) and the Rutherford Appleton Laboratory (RAL) in the UK, Naval Research Laboratory (NRL) in USA and the Space Research Centre (SRC) of Polish Academy of Sciences. All the parties involved share the same rights to access, use and interpret the data from the instrument. The data will be jointly analysed and general public access will be allowed immediately after reformatting. The arrangements for data distribution will be defined at a later date.
In the reminder of this paper the CORONAS-F satellite description is given, the RESIK performance, rationale and scientific objectives are pointed. Division of the tasks and responsibilities of the collaborating groups is presented.
The primary objective of the experiments aboard the CORONAS-F satellite is to study global solar oscillations, solar constant variation, physics of flares and active regions.
Besides RESIK, CORONASF will carry a number of instruments with complementary science objectives among them are DIOGENESS, a flat crystal spectrometer, and RESK, a spectro-polarimeter. DIOGENESS will measure spectra with a flat crystal spectrometer in the wavelength ranges 2.83 - 3.36 Å, 4.98 - 5.30 Å and 6.15 - 6.74 Å, overlapping two of the RESIK spectral bands. The short waveband of DIOGENESS covers important diagnostic lines of K XVIII ion λ = 3.531 Å, 3.549 Å and Ar XVIII λ = 3.731 Å, 3.736 Å, both of high first ionization potential (FIP) elements (see Table 1). DIOGENESS will also have the capability to measure the distribution of the soft X-ray brightness of the source in 2-4-8 keV bands with an angular resolution of ~ 5″ in the plane of the dispersion. This will help to locate the source offset relative to the satellite axis and will provide broad-band whole-Sun photometry in the ranges 2-4-8 keV with 10 ms time resolution during flares. The spectra from DIOGENESS will also be helpful in verifying in orbit the performance of the X-ray reflecting properties of the bent RESIK crystals.
Lebedev Physical Institute RES-K spectro-polarimeter will measure high resolution spectra in the vicinity of selected strong X-ray lines including Fe XXV group. The TEREK complex, similar to that flown on the CORONAS-I mission (http://X-ras.lpi.msk.su/) will produce images of the Sun in soft X-rays and XUV. There will also be instruments aboard the CORONAS-F satellite which will allow high resolution measurements of the γ-ray flare spectra, in situ charged particles composition, neutrons, and solar UV and radio fluxes (www.izmiran.rssi.ru).
The total mass of the scientific payload will be approximately 350 kg. The satellite will be re-pointed whenever its main axis drifts more than 10′ away from the centre of solar disc. The guaranteed drift rate will be less than 3.6 arcsec/s (probably much less as was the case for CORONAS-I) on each of the three gyro-stabilized axes. The operational lifetime of the satellite is expected to be not less than 2 years.
Passage through the radiation belts will require the X-ray detectors to be turned off for a maximum of 30% of the orbital period.
It
is known (Feldman, 1992) that the composition of solar plasma is different for
various structures (solar wind, SEP) and changes in time. So called first
ionization potential (FIP) effect
on plasma composition has been early noticed (Veck & Parkinson, 1981).
From 1984 it is known using the X-ray spectroscopy methods (Sylwester, Lemen
and Mewe, 1984) that the composition of the plasma may differ between flares.
UV, EUV and g-ray
observations further indicate for varying abundance of elements in different
structures in the corona.
RESIK
is the first X-ray spectrometer which design is optimized in order to perform
systematic study of solar plasma composition for a number of species (Table 1)
including both low and high FIP
elements. Reliable measurements of the continuum level will allow the absolute
(relative to hydrogen) abundance to be estimated for the first time for such a
large set of elements in a systematic way. In a process of abundance
determinations corresponding distributions of differential emission measure (DEM)
will have to be calculated. This is very important for understanding of
processes of coronal heating. Physical conditions in the so called
“super-hot” plasma component (T>30 MK) will be investigated in
detail in this respect.
Table 1.
Atomic numbers, abundance and FIP’s for elements, which will be investigated
by RESIK.
Z |
Element
|
Abundance * |
First IP [eV] |
13 |
Al |
11 |
5.986 |
14 |
Si |
126 |
8.151 |
15 |
P |
0.3 |
10.486 |
16 |
S |
19 |
10.360 |
17 |
Cl |
0.4 |
12.967 |
18 |
Ar |
3.8 |
15.759 |
19 |
K |
0.1 |
4.341 |
20 |
Ca |
8.5 |
6.113 |
21 |
Sc |
0.002 |
6.540 |
22 |
Ti |
0.14 |
6.820 |
23 |
V |
0.02 |
6.740 |
24 |
Cr |
0.7 |
6.766 |
25 |
Mn |
0.3 |
7.435 |
26 |
Fe |
126 |
7.870 |
27 |
Co |
0.13 |
7.860 |
28 |
Ni |
6.9 |
7.635 |
29 |
Cu |
0.03 |
7.726 |
30 |
Zn |
0.02 |
9.394 |
*
Based
on Feldman, Physica Scripta, 46, 202, 1992, compilation for the corona. In
this scale abundance of hydrogen is assumed to be 106.
The basic concept of RESIK is similar to that used in design of spectrometers flown on several previous missions: a diffracting curved crystal is used to select a limited wavelength range that covers spectral emission band of interest for given scientific objectives.
The RESIK instrument consists of the two separate modules:
two spectrometer sections
main electronics box
The main electronics box (including the microprocessor) is located inside the pressurised section of CORONAS-F. The spectrometers and front-end electronics are mounted in a box open to space.
Table 2. RESIK crystals
The RESIK instrument employs four bent crystals to diffract incident Xrays. The use of bent crystals allows the entire wavelength range to be integrated simultaneously, whereas a flat crystal spectrometer (like DIOGENESS) must scan to cover its wavelength range. The wavelength channels of RESIK spread over ten different spectral bands (Table 2) covering almost entirely 1.1 Å – 6.1 Å range. The crystals are bent to a convex cylindrical profiles since this geometry allows to reduce the overall size of the crystaldetector assembly. As a consequence of the wavelength coverage selected the widths of most of the lines to be measured are defined (in the first order of reflection) by the instrumental effects (i.e. the line widths observed will be larger than actual physical widths). The crystals selected for the spectrometers were chosen because they are not subject to fluorescence. Previous spectrometers have had problems with background flux caused by photons emitted from diffracting crystals when they were illuminated with energetic photons (E > 11.2 keV in the case of mostly used before germanium crystals). The elimination of this fluorescence contamination is of particular importance during flare impulsive phase. The absence of fluorescence contamination is essential when abundance measurements involving the ratio of the line to continuum fluxes are to be made.
In each of the two RESIK spectrometer units a double positionsensitive proportional counter is used in order to record the crystal-diffracted photons. The detectors used are spares from Solar-A (Yohkoh) BCS (Culhane, 1991). Spare analogue electronics processing cards are also being used. There are no moving parts (except for the stepper motors used with the Fe55 calibration sources) and no collimator is used – so the X-ray radiation of the whole Sun will contribute to the spectra.
Figure
1a. Example of the spectrum in the 3.3 Å ÷ 3.9 Å range
as expected to be observed by RESIK during the flare decay phase. Most of the
spectral features are marked at the appropriate spectral positions. Dashed
horizontal line represents here (and on the following figures) the detector
background. Dotted vertical lines are placed at the positions of the
ionization limit for appropriate He- and H-like ions (marked).
The RESIK instrument is to be constructed by a consortium of five groups (see the author list). Following division of tasks has been assumed:
IZMIRAN – satellite, telemetry, launch, data reformatting, overall operation
MSSL – detectors and front end electronics
NRL – crystals and alignment
RAL – high voltage supply and end-to-end tests
SRC – (acts as PI), mechanical construction, overall electronics, onboard computer, software, interface with telemetry, instrument control.
All the parties in the Consortium share the same rights to access, use and interpret the data from RESIK. The data will be jointly analysed and general public access will be allowed immediately after their reformatting. The arrangement for data distribution will be defined at a later date.
In Figure 1(a – c) we present example spectra as expected to be observed in RESIK channels during the flare decay phase. In the calculations of spectra the emission measure 1049 cm‑3 and the temperature 15 MK have been assumed. Provisional values of instrument effective areas have been used in order to determine expected detector count rates. The bound-bound line intensity (for stronger lines only) and continuum intensities were determined using Mewe et al., (1985, 1986) approximations respectively. Most of the spectral features expected to be present are marked at the appropriate spectral positions. However, due to the reduction of the scale they could not be read easily in
Fig
1b. Same as in
Fig. 1a, except for the wavelength coverage including the He-like Ar XVII “triplet”.
the Figures. The line features expected to be observed belong to elements with atomic number Z from 13 to 30 (Table 1) and represent resonant transitions in He- and H-like ions. Since the background level in the spectrometer is anticipated to be very low (see dashed line in the Figures) the level of the continuum is expected to be realistically determined for flares and hotter active regions. Investigation of synthetic spectra to be recorded by RESIK in various conditions of solar activity allows us to consider the following scientific objectives:
Determination
of relative and absolute coronal abundance (and/or the upper limits) for
elements with atomic numbers Z = 13 ÷ 30. Expected time resolution
is ~ 10 ÷ 20 s.
Studies of
coronal composition variability in time and between different coronal
structures, investigations of possible solar cycle effects.
Studies of ionization state of high temperature solar plasma.
Verification of atomic theory for ionization balance.
Search for
possible local non-Maxwellian velocity distribution of electrons.
Investigation of the spectrum close to ionization limits of individual ions and the relative intensities of higher members of H- and He-like resonance line series.
Fig
1c. Same as in Fig. 1a, except for the wavelength coverage including S XV
“triplet” and higher members of Al XII and Al XIII line series.
The RESIK Team would like to thank numerous people involved in making the hardware and keeping the project alive for many years now. In particular E. Stańczyk and J. Bąkała for performance of the mechanical structure and M. Oczyński for electronic assembly. Anna Kępa is acknowledged for her help in preparing the accompanying poster presentation.
From the Polish side, the project is supported by KBN grant 2 PO3C 005 08.
Culhane,
J.L. et. al. 1991, Solar Physics, 136, 89
Feldman,
U. 1992, Physica Scripta, 46, 202
Mewe,
R., Gronenschild, E.H.B.M. van den Oord, G.H.J. 1985, A&AS, 62, 197
Mewe,
R., Lemen, J.R. van den Oord, G.H.J. 1986, A&AS, 65, 511
Veck,
N.J., Parkinson, J.H. 1981, MNRAS, 190, 287
Sylwester,
J. Lemen, J.R., Mewe, R. 1984, Nature, 310, 665