Solar X-ray Spectrometer (Soxs) Mission on Board GSAT2 Indian Spacecraft: The Low-Energy Payload

The first space-borne solar astronomy experiment of India, namely Solar X-ray Spectrometer (SOXS), was successfully launched on 08 May 2003 on board geostationary satellite GSAT-2 of India. The SOXS is composed of two independent payloads, viz. SOXS Low-Energy Detector (SLD) Payload and SOXS High-Energy Detector (SHD) Payload. The SOXS aims to study the full-disk integrated X-ray emission in the energy range from 4 keV to 10 MeV. In this paper we present the first report on the SLD instrumentation and its in-orbit performance. The SLD payload was designed and developed at the Physical Research Laboratory in collaboration with various centers of Indian Space Research Organisation (ISRO). The basic scientific aim of the SLD payload is to study solar flares in the energy range from 4 to 60 keV with high spectral and temporal resolution. To meet these requirements, the SLD payload employs state-of-the-art solid state detectors, the first time for a solar astronomy experiment, viz. Si PIN (4 –25 keV), and cadmium–zinc–telluride (4 –60 keV). With their superb high-energy resolution characteristics, SLD can observe iron and iron–nickel complex lines that are visible only during solar flares. In view of its 3.4 FOV, the detector package is mounted on a Sun Aspect System, for the first time, to get uninterrupted observations in a geostationary orbit. The SLD payload configuration, its in-flight operation, and the response of the detectors are presented. We also present the first observations of solar flares made by the SLD payload and briefly describe their temporal and spectral mode results.     

Solar x-ray spectrometer (SOXS) mission – Low energy payload – First results

We present the first results from the ‘Low Energy Detector’ pay-load of ‘Solar X-ray Spectrometer (SOXS)’ mission, which was launched onboard GSAT-2 Indian spacecraft on 08 May 2003 by GSLV-D2 rocket to study the solar flares. The SOXS Low Energy Detector (SLD) payload was designed, developed and fabricated by Physical Research Laboratory (PRL) in collaboration with Space Application Centre (SAC), Ahmedabad and ISRO Satellite Centre (ISAC), Bangalore of the Indian Space Research Organization (ISRO). The SLD payload employs the state-of-the-art solid state detectors viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices that operate at near room temperature (-20°C). The dynamic energy range of Si PIN and CZT detectors are 4–25 keV and 4–56 keV respectively. The Si PIN provides sub-keV energy resolution while CZT reveals ∼1.7keV energy resolution throughout the dynamic range. The high sensitivity and sub-keV energy resolution of Si PIN detector allows the measuring of the intensity, peak energy and equivalent width of the Fe-line complex at approximately 6.7 keV as a function of time in all 8 M-class flares studied in this investigation. The peak energy (E _p) of Fe-line feature varies between 6.4 and 6.8 keV with increase in temperature from 9 to 34 MK. We found that the equivalent width (ω) of Fe-line feature increases exponentially with temperature up to 20 MK but later it increases very slowly up to 28 MK and then it remains uniform around 1.55 keV up to 34 MK. We compare our measurements ofw with calculations made earlier by various investigators and propose that these measurements may improve theoretical models. We interpret the variation of both E_pand ω with temperature as the changes in the ionization and recombination conditions in the plasma during the flare interval and as a consequence the contribution from different ionic emission lines also varies.     

Evidence of decay of flux ratio of Fe to Fe-Ni line features with electron temperature in solar flares

We report observational evidence of the decay of the flux ratio of Fe to Fe-Ni line features as a function of plasma electron temperature in solar flares in comparison to that theoretically predicted by Phillips (2004). We present the study of spectral analysis of 14 flares observed by the Solar X-ray Spectrometer (SOXS) — Low Energy Detector (SLD) payload. The SLD payload employs the state-of-the-art solid state detectors, viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices. The sub-keV energy resolution of Si PIN detector allows us to study the Fe-line and Fe-Ni line features appearing at 6.7 and 8 keV, respectively, in greater detail. In order to best-fit the whole spectrum at one time in the desired energy range between 4 and 25 keV we considered Gaussian-line, the multi-thermal power-law and broken power-law functions. We found that the flux ratio of Fe to Fe-Ni line features decays with flare electron temperature by the asymptotic form of polynomial of inverse third order. The relative flux ratio is ∼30 at temperature 12 MK which drops to half, ∼15 at 20 MK, and at further higher temperatures it decreases smoothly reaching to ∼8 at ∼50 MK. The flux ratio, however, at a given flare plasma temperature, and its decrease with temperature is significantly lower than that predicted theoretically. We propose that the difference may be due to the consideration of higher densities of Fe and Fe-Ni lines in the theoretical model of Phillips (2004). We suggest revising the Fe and Fe-Ni line densities in the corona. The decay of flux ratio explains the variation of equivalent width and peak energy of these line features with temperature.     

The Fe-Line Feature in the X-Ray Spectrum of Solar Flares: First Results from the SOXS Mission

We present the first results from the low-energy detector payload of the solar X-ray spectrometer (SOXS) mission, which was launched onboard the GSAT-2 Indian spacecraft on May 08, 2003 by the GSLV-D2 rocket to study solar flares. The SOXS low-energy detector (SLD) payload was designed, developed, and fabricated by the Physical Research Laboratory (PRL) in collaboration with the Space Application Centre (SAC), Ahmedabad and the Indian Space Research Organization (ISRO) Satellite Centre (ISAC), Bangalore. The SLD payload employs state-of-the-art, solid-state detectors, viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices that operate at near room temperature (−20 ^°C). The energy ranges of the Si PIN and CZT detectors are 4 – 25 and 4 – 56 keV, respectively. The Si PIN provides sub-keV energy resolution, while the CZT provides ~1.7 keV energy resolution throughout the energy range. The high sensitivity and sub-keV energy resolution of the Si PIN detector allows measuring the intensity, peak energy, and the equivalent width of the Fe-line complex at approximately 6.7 keV, as a function of time in all ten M-class flares studied in this investigation. The peak energy (E _p) of the Fe-line feature varies between 6.4 and 6.7 keV with increase in temperature from 9 to 58 MK. We found that the equivalent width (w) of the Fe-line feature increases exponentially with temperature up to 30 MK and then increases very slowly up to 40 MK. It remains between 3.5 and 4 keV in the temperature range of 30 – 45 MK. We compare our measurements of w with calculations made earlier by various investigators and propose that these measurements may improve theoretical models. We interpret the variation of both E _p and w with temperature as being to the changes in the ionization and recombination conditions in the plasma during the flare, and as a consequence, the contribution from different ionic emission lines also varies.     

Data acquisition, control, communication and computation system of solar X-ray spectrometer (SOXS) mission

The Solar X-ray Spectrometer (SOXS) mission onboard GSAT-2 Indian Spacecraft was launched on 08 May 2003 using GSLV-D2 rocket by Indian Space Research Organization (ISRO). SOXS aims to study solar flares, which are the most violent and energetic phenomena in the solar system, in the energy range of 4–56 keV with high spectral and temporal resolution. SOXS employs state-of-the-art semiconductor devices, viz., Si-Pin and CZT detectors to achieve sub-keV energy resolution requirements. In this paper, we present an overview of data acquisition, control, communication and computation of low energy payload of the SOXS mission.     

Very High-Resolution Solar X-Ray Imaging Using Diffractive Optics

This paper describes the development of X-ray diffractive optics for imaging solar flares with better than 0.1 arcsec angular resolution. X-ray images with this resolution of the ???10?MK plasma in solar active regions and solar flares would allow the cross-sectional area of magnetic loops to be resolved and the coronal flare energy release region itself to be probed. The objective of this work is to obtain X-ray images in the iron-line complex at 6.7?keV observed during solar flares with an angular resolution as fine as 0.1 arcsec ?C over an order of magnitude finer than is now possible. This line emission is from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma at temperatures in excess of ???10 MK. It provides information on the flare morphology, the iron abundance, and the distribution of the hot plasma. Studying how this plasma is heated to such high temperatures in such short times during solar flares is of critical importance in understanding these powerful transient events, one of the major objectives of solar physics. We describe the design, fabrication, and testing of phase zone plate X-ray lenses with focal lengths of ???100 m at these energies that would be capable of achieving these objectives. We show how such lenses could be included on a two-spacecraft formation-flying mission with the lenses on the spacecraft closest to the Sun and an X-ray imaging array on the second spacecraft in the focal plane ???100 m away. High-resolution X-ray images could be obtained when the two spacecraft are aligned with the region of interest on the Sun. Requirements and constraints for the control of the two spacecraft are discussed together with the overall feasibility of such a formation-flying mission.     

Balloon-Borne Hard X-Ray Spectrometer Using CdTe Detectors

Spectroscopic observation of solar flares in the hard X-ray energy range, particularly the 20 ∼ 100 keV region, is an invaluable tool for investigating the flare mechanism. This paper describes the design and performance of a balloon-borne hard X-ray spectrometer using CdTe detectors developed for solar flare observation. The instrument is a small balloon payload (gondola weight 70 kg) with sixteen 10×10×0.5 mm CdTe detectors, designed for a 1-day flight at 41 km altitude. It observes in an energy range of 20−120 keV and has an energy resolution of 3 keV at 60 keV. The second flight on 24 May 2002 succeeded in observing a class M1.1 flare.     

Hard X-ray bursts from flares behind the solar limb

The determination of the location of the region of origin of hard X-rays is important in evaluating the importance of 10–100 keV electrons in solar flares and in understanding flare particle acceleration. At present only limb-occulted events are available to give some information on the height of X-ray emission. In fifteen months of OSO-7 operation, nine major soft X-ray events had no reported correlated Hα flare. We examine the hard X-ray spectra of eight of these events with good candidate X-ray flare producing active regions making limb transit at the time of the soft X-ray bursts. All eight bursts had significant X-ray emission in the 30–44 keV range, but only one had flux at the 3σ level above 44 keV. The data are consistent with most X-ray emission occurring in the lower chromosphere, but some electron trapping at high altitudes is necessary to explain the small nonthermal fluxes observed.     

Gamma-ray radiation of solar flares in October–November 2003 according to the data obtained with the AVS-F instrument onboard the CORONAS-F satellite

Thirty active regions were observed on the Sun during the period from October 19 to November 20, 2003. Hard X-ray and gamma-ray radiation was detected from four active regions (10484, 10486, 10488, and 10490): 14 solar flares stronger than M5.0 according to the GOES classification were recorded during this period by detectors onboard the Geostationary Operational Environmental Satellite (GOES), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and other satellites. Five of these flares (and also the M2.7 flare of October 27, 2003) were also observed by the AVS-F apparatus onboard the CORONAS-F satellite. This paper discusses the time profiles and energy spectra of the solar flares of October 26, 2003 (M7.6), and October 29, 2003 (X10), and of the initial phase of the flare of November 4, 2003 (X18), obtained by the AVS-F instrument during the passage of the satellite over the regions near the geomagnetic equator. The spectra of the M7.6 flare of October 26, 2003, and of the initial phase of the X18 flare of November 4, 2003, in the energy band from 0.1 to 17 MeV contain no lines, whereas the spectrum of the flare of October 29, 2003, exhibits nuclear lines and the 2.2-MeV line during the entire flare gamma-ray emission registration. We also report the time profiles of the flare of October 29, 2003, in the energy bands corresponding to the continuum in the energy band 0.3–0.6 MeV, the nuclear lines of ⁵⁶Fe, ²⁴Mg, ²⁰Ne, ²⁸Si, ¹²C, and ¹⁶O, and the 2.2-MeV neutron-capture line. The analysis of these temporal profile periodograms shows the presence of a thin structure with characteristic scales from 34 to 158 s at the 99% confidence level. The AVS-F apparatus analyzes temporal profiles of low-energy gamma-ray emission with a temporal resolution of 1 ms within the first 4.096 seconds of solar flares. The analysis of the data reveals no regularities in the time series on time scales ranging from 2 to 100 ms at a confidence level of 99% for these three solar flares.     

Study of microflares through soxs mission

We present a study of 10 microflares observed in 4–30 keV by SOXS mission simultaneously with Hα observations made at NAOJ, Japan during the interval between February and August 2004. The X-ray and Hα light curves showed that the lifetime of microflares varies between 4 and 25 min. We found that the X-ray emission in all microflares under study in the dynamic energy range of 4–30 keV can be fitted by thermal plus non-thermal components. The thermal spectrum appeared to start from almost 4 keV, low level discriminator (LLD) of both Si and CZT detectors, however it ends below 8 keV. We also observed the Fe line complex features at 6.7 keV in some microflares and attempted to fit this line by isothermal temperature assumption. The temperature of isothermal plasma of microflares varies in the range between 8.6 and 10.1 MK while emission measure between 0.5 and 2x10⁴9 cm^-3. Non-thermal (NT) emission appeared in the energy range 7–15 keV with exponent -6.8 ≤γ≤-4.8. Our study of microflares that had occurred on 25 February 2004 showed that sometimes a given active region produces recurrent microflare activity of a similar nature. We concluded from X-ray and simultaneous Hα observations that the microflares are perhaps the result of the interaction of low lying loops. It appears that the electrons that accelerated during reconnection heat the ambient coronal plasma as well as interact with material while moving down along the loops and thereby produce Hα bright kernels.     

Non-thermal processes in large solar flares

We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:

1. The ～10–10² keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
2. The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
3. The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
4. Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～10³¹ ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
5. High energy protons are accelerated later than the 10–10² keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
6. The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×10³⁰ ergs in >20 keV electrons is required for detectable white-light emission.

The highly efficient electron energization required in these flares suggests that the flare mechanism consists of rapid dissipation of chromospheric and coronal field-aligned or sheet currents, due to the onset of current-driven Buneman anomalous resistivity. Large proton flares then result when the energy input from accelerated electrons is sufficient to form a shock wave.     

X-ray emission characteristics of flares associated with CMEs

We present the study of 20 solar flares observed by “Solar X-ray Spectrometer (SOXS)” mission during November 2003 to December 2006 and found associated with coronal mass ejections (CMEs) seen by LASCO/SOHO mission. In this investigation, X-ray emission characteristics of solar flares and their relationship with the dynamics of CMEs have been presented. We found that the fast moving CMEs, i.e., positive acceleration are better associated with short rise time (< 150 s) flares. However, the velocity of CMEs increases as a function of duration of the flares in both 4.1–10 and 10–20 keV bands. This indicates that the possibility of association of CMEs with larger speeds exists with long duration flare events. We observed that CMEs decelerate with increasing rise time, decay time and duration of the associated X-ray flares. A total 10 out of 20 CMEs under current investigation showed positive acceleration, and 5 of them whose speed did not exceed 589 km/s were associated with short rise time (< 150 s) and short duration (< 1300 s) flares. The other 5 CMEs were associated with long duration or large rise time flare events. The unusual feature of all these positive accelerating CMEs was their low linear speed ranging between 176 and 775 km/s. We do not find any significant correlation between X-ray peak intensity of the flares with linear speed as well as acceleration of the associated CMEs. Based on the onset time of flares and associated CMEs within the observing cadence of CMEs by LASCO, we found that in 16 cases CME preceded the flare by 23 to 1786 s, while in 4 cases flare occurred before the CME by 47 to 685 s. We argue that both events are closely associated with each other and are integral parts of one energy release system.     

Energy-Dependent Timing of Thermal Emission in Solar Flares

We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the “Solar X-ray Spectrometer” (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(ε) from the flare by integrating a series of isothermal plasma flux. We find that the multi-temperature integrated flux F(ε) is a power-law function of ε with a spectral index (γ)≈−4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E=4 – 15 keV is dominated by temperatures of T=12 – 50 MK, while the multi-thermal power-law DEM index (δ) varies in the range of −4.4 and −5.7. The temporal evolution of the X-ray flux F(ε,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling time varies between 296 and 4640 s and the electron density (n _e) varies in the range of n _e=(1.77 – 29.3)×10¹⁰ cm⁻³. Employing temporal evolution technique in the current study as an alternative method for separating thermal from nonthermal components in the energy spectra, we measure the break-energy point, ranging between 14 and 21±1.0 keV.     

Estimates of the neutron emission during large solar flares in the rising and maximum period of solar cycle 24

We searched for solar neutrons using the data collected by six detectors from the International Network of Solar Neutron Telescopes and one Neutron Monitor between January 2010 and December 2014. We considered the peak time of the X-ray intensity of thirty five ≥ X1.0 class flares detected by GOES satellite as the most probable production time of solar neutrons. We prepared a light-curve of the solar neutron telescopes and the neutron monitor for each flare, spanning ± 3 h from the peak time of GOES. Based on these light curves, we performed a statistical analysis for each flare. Setting a significance level at greater than 3σ, we report that no statistically significant signals due to solar neutrons were found. Therefore, upper limits are determined by the background level and solar angle of these thirty five solar flares. Our calculation assumed a power-law neutron energy spectrum and an impulsive emission profile at the Sun. The estimated upper limits of the neutron emission are consistent within the order of magnitude of the successful detections of solar neutrons made in solar cycle 23.     

Electron acceleration in solar flares

We present observations of an intense solar flare hard X-ray burst on 1980 June 27, made with a balloon-borne array of liquid nitrogen-cooled germanium detectors which provided unprecedented spectral resolution (≲1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 10⁸–10⁹K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting ∼3–15 s, whch have a hard spectrum and a break energy of 30–65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 keV to ≳100 keV through the event. The double power-law shape indicates that acceleration by DC electric fields parallel to the magnetic field, similar to that occurring in the Earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission. The total potential drop required for flares is typically ∼10² kV compared to ∼10 kV for auroral substorms.

     

Latest results on Jovian disk X-rays from XMM-Newton

We present the results of a spectral study of the soft X-ray emission (0.2–2.5 keV) from low-latitude (‘disk’) regions of Jupiter. The data were obtained during two observing campaigns with XMM-Newton in April and November 2003. While the level of the emission remained approximately the same between April and the first half of the November observation, the second part of the latter shows an enhancement by about 40% in the 0.2–2.5 keV flux. A very similar, and apparently correlated increase, in time and scale, was observed in the solar X-ray and EUV flux.The months of October and November 2003 saw a period of particularly intense solar activity, which appears reflected in the behavior of the soft X-rays from Jupiter's disk. The X-ray spectra, from the XMM-Newton EPIC CCD cameras, are all well fitted by a coronal model with temperatures in the range 0.4–0.5 keV, with additional line emission from Mg XI (1.35 keV) and Si XIII (1.86 keV): these are characteristic lines of solar X-ray spectra at maximum activity and during flares.The XMM-Newton observations lend further support to the theory that Jupiter's disk X-ray emission is controlled by the Sun, and may be produced in large part by scattering, elastic and fluorescent, of solar X-rays in the upper atmosphere of the planet.     

Silicon Abundance from RESIK Solar Flare Observations

The RESIK instrument on the CORONAS-F spacecraft obtained solar flare and active-region X-ray spectra in four channels covering the wavelength range 3.8?–?6.1 Å in its operational period between 2001 and 2003. Several highly ionized silicon lines were observed within the range of the long-wavelength channel (5.00?–?6.05 Å). The fluxes of the Si?xiv Ly-β line (5.217 Å) and the Si?xiii 1s ²?–?1s3p line (5.688 Å) during 21 flares with optimized pulse-height analyzer settings on RESIK have been analyzed to obtain the silicon abundance relative to hydrogen in flare plasmas. As in previous work, the emitting plasma for each spectrum is assumed to be characterized by a single temperature and emission measure given by the ratio of emission in the two channels of GOES. The silicon abundance is determined to be A(Si)=7.93±.21 (Si?xiv) and 7.89±.13 (Si?xiii) on a logarithmic scale with H=12. These values, which vary by only very small amounts from flare to flare and times within flares, are 2.6±1.3 and 2.4±0.7 times the photospheric abundance, and are about a factor of three higher than RESIK measurements during a period of very low activity. There is a suggestion that the Si/S abundance ratio increases from active regions to flares.     

Flare Characteristics from X-ray Light Curves

A new methodology is given to determine basic parameters of flares from their X-ray light curves. Algorithms are developed from the analysis of small X-ray flares occurring during the deep solar minimum of 2009, between Solar Cycles 23 and 24, observed by the Polish Solar Photometer in X-rays (SphinX) on the Complex Orbital Observations Near-Earth of Activity of the Sun-Photon (CORONAS-Photon) spacecraft. One is a semi-automatic flare detection procedure that gives start, peak, and end times for single (“elementary”) flare events under the assumption that the light curve is a simple convolution of a Gaussian and exponential decay functions. More complex flares with multiple peaks can generally be described by a sum of such elementary flares. Flare time profiles in the two energy ranges of SphinX (1.16?–?1.51 keV, 1.51?–?15 keV) are used to derive temperature and emission measure as a function of time during each flare. The result is a comprehensive catalogue – the SphinX Flare Catalogue – which contains 1600 flares or flare-like events and is made available for general use. The methods described here can be applied to observations made by Geosynchronous Operational Environmental Satellites (GOES), the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and other broad-band spectrometers.     

Onboard performance of the RT-2 detectors

The RT-2 Experiment onboard the CORONAS-PHOTON satellite is designed to study the spectral, temporal, and spatial details of solar hard X-ray flares in the 15–150 keV range. Above this energy (and upto 1000 keV), it also acts as an omni-directional gamma-ray detector with a capability to study gamma-ray bursts (GRB), bright solar flares, and X-ray pulsars. With an ensemble of hard X-ray detectors with different fields of view and coding devices, it also has the capability to investigate the spectrum of Cosmic Diffuse X-ray Background. The performance of the detectors from 2009 February to November is described in this paper. Results obtained on a few GRBs and solar flares are also briefly discussed.