ASTRONOMY: ADITYA MISSION X-ray spectrometers on-board Aditya-L1 for solar flare studies K. Sankarasubramanian1,2,3,*, Manju Sudhakar1, Anuj Nandi1, M. C. Ramadevi1, Abhijit Avinash Adoni1, Ankur Kushwaha1, Anil Agarwal1, Arjun Dey1, Bhuwan Joshi4, Brajpal Singh1, V. Girish1, Ishan Tomar1, Kamal Kumar Majhi1, Kumar1, Manjunath Olekar1, Monoj Bug1, Manohar Pala1, Mukund Kumar Thakur1, Rajeev R. Badagandi1, B. T. Ravishankar1, Sarthak Garg1, N. Sitaramamurthy1, N. Sridhara1, C. N. Umapathy1, Vinod Kumar Gupta1, Vivek Kumar Agrawal1 and B. Yougandar1 1ISITE Campus, ISRO Satellite Centre, Outer Ring Road, Marathahalli, Bengaluru 560 037, India 2Indian Institute of Astrophysics, 2nd Block, Koramangala, Bengaluru 560 034, India 3CESSI, Indian Institute of Science Education and Research, Kolkata 741 246, India 4Udaipur Solar Observatory, Physical Research Laboratory, Udaipur 313 004, India Aditya-L1 mission will carry two high-spectral resolu- of the solar atmosphere (corona, chromosphere and tion X-ray spectrometers to study solar flares. The photosphere). soft X-ray spectrometer will cover the energy range Though solar flares are observed in all energy bands, from 1 to 30 keV, while the hard X-ray spectrometer X-ray energies provide the best observations due to high will cover from 10 to 150 keV. These two instruments temperatures generated during the flares. Solar flares are together will provide opportunities to study the plas- ma parameters during solar flares as well as accelera- tion mechanisms of energetic particles during the flaring time. Keywords: Coronal heating, solar flares, X-ray spec- trometers. Introduction SOLAR flares were first observed by Carrington and Hodgson in 1859 as a sudden increase of brightness for a few minutes on the solar atmosphere. Flares have then been observed regularly in the hydrogen alpha line origi- nating in the chromosphere. The complex nature of struc- tures like different source sizes, associated plasma ejections and Morton waves have been reported. Radio observations during the flaring time indicated that the flares not only heat up the plasma, but also accelerate electrons which produce these radio emissions. In late 1950s when space observations were feasible, flares were well observed in X-ray energies. It has been realized from these observations that a sizeable portion of the flare en- ergy is emitted in hard X-rays. Gamma-ray emissions are also seen during solar flares, making it a phenomenon observed in all wavebands (Figure 1)1. It is now under- stood that although the flaring phenomenon is initiated in the corona, energy is deposited by the flare in all layers 1 *For correspondence. (e-mail: sankark@isac.gov.in) Figure 1. Typical flare brightening at different wavelengths . CURRENT SCIENCE, VOL. 113, NO. 4, 25 AUGUST 2017 625 ASTRONOMY: ADITYA MISSION classified as A, B, C, M and X by their brightening in the A typical X1 class flare and the spectral parameters of soft X-ray band, with the X-class flare having an energy the hard X-ray (HXR) emitting electrons8 have been stud- range 10–4 Wm–2 (X1) to 10–3 Wm–2 (X10) with M1 flux ied and plotted in Figure 2 as the flare evolves over time. smaller by an order to X1 flux, and so on. Two solar spectrometers covering soft (1 to 30 keV) and hard (10 to 150 keV) X-ray bands will be flown on Solar Low Energy X-ray Spectrometer Aditya-L1 mission to study solar flares and their dynamics. Solar Low Energy X-ray Spectrometer (SoLEXS) will cover the energy band from 1 to 30 keV with a spectral Overall science objectives resolution of <4% (i.e. <250 eV at 6 keV). This energy band will help in obtaining the thermal energy of the The two X-ray spectrometers covering the energy band flares. To cover the large class of flares, from A- to from 1 to 150 keV will allow us to carry out the follow- X-class, SoLEXS will carry two identical detectors with ing science objectives: (i) Study of heating mechanisms different apertures. The large area aperture will cater to during the flare2. (ii) Quantitative measure of flare ener- small flares (A- to C-class), while the small aperture will gies by measuring the temperature of the plasma (thermal observe intense flares (other classes). energy) and also studying the acceleration of particles SoLEXS is configured as two packages, viz. detector (non-thermal energy)2. (iii) Coronal abundance during package and electronics package (Figure 3). The detector flare evolution3. (iv) Precursor phase activities possibly package carries the two detectors and the associated high related to reconnection mechanisms4. (v) Time variation voltage power supply along with charge sensitive pream- of spectral parameters as well as quasi-periodic oscilla- plifiers. The electronics package carries the required tions, especially seen in hard X-ray energy5. (vi) Associa- processing and power electronics to cater to the instru- tion of flare and Coronal Mass Ejections (CMEs)6. (vii) ment. Prominence eruption and flare trigger7. SoLEXS will also carry an on-board calibration source for the gain calibration over time to obtain high spectral quality for its data. With its on-board processing, SoLEXS will provide spectra with 1 s cadence during a flare. A flare trigger using the count rate threshold is im- plemented to operate this instrument in quiet as well as flare mode. This flare trigger is also provided as a hard- ware line to the SUIT instrument on-board Aditya-L1. Currently, the engineering model of SoLEXS (both the packages) is being integrated to test for its performance9. High Energy L1 Orbiting X-ray Spectrometer High Energy L1 Orbiting X-ray Spectrometer (HEL1OS) is a high-energy X-ray spectrometer (10 to 150 keV) for studying the impulsive phase of solar flares. HEL1OS aims to take advantage of the location of the spacecraft at Sun–Earth L1 in order to obtain uninterrupted observations Figure 2. (Top) Light curve of X1 class flare of 19 January 2005 showing pulsations during the impulsive phase. (Bottom) Temporal variation of low-energy cut-off of the hard X-ray emitting electron dis- tribution and its spectral index. Figure 3. Solar low energy X-ray spectrometer engineering model. 626 CURRENT SCIENCE, VOL. 113, NO. 4, 25 AUGUST 2017 ASTRONOMY: ADITYA MISSION Figure 4. Engineering model of the HEL1OS payload. of the short-lived impulsive phase of solar flares. This dances, break energy shift for different classes of flares, energy band helps identify the non-thermal energy release separation of thermal and non-thermal energies, etc. during the flares. These two instruments will also carry out flare–CME stu- HEL1OS is being developed with two different types dies in combination with coronagraph and also promi- of detectors10: CZT and CdTe. The CZT detector is a nence–flare–CME relations with SUIT and coronagraph. state-of-the-art, near-room-temperature device. In order Aditya-L1 being at Sun–Earth L1 point provides us with to achieve a total geometric area of 32 sq. cm, two such a unique opportunity to study the flare phenomenon con- detectors (16 sq. cm per detector) are used to cover the tinuously in X-rays without any data gap, like in previous energy range 20 to 150 keV and operate in the tempera- missions. ture range 5C to 20C. The individual detectors are pixe- lated with 256 pixels, with pixel dimension 2.46 mm  2.46 mm. The CdTe detector, which has better resolution 1. Benz, A. O., Flare observations. Living Rev. Sol. Phys., 2017, at lower energies, is used for detailed spectroscopic stud- 14(2), 1–59. ies from 10 to 40 keV, and operates in the temperature 2. Hannah, I. G. et al., Microflares and the statistics of X-ray flares. Space Sci. Rev., 2011, 159, 263–300. range –35C to –25C. The overall geometric area of 3. Narendranath, S. et al., Elemental abundances in the solar corona 0.5 sq. cm will be achieved using two CdTe detectors, as measured by the X-ray solar monitor onboard Chandrayaan-1. each with an area of 0.25 sq. cm. The field-of-view of the Sol. Phys., 2014, 289, 1585–1595. instrument has been constrained to 6  6 using a 4. Joshi, B. et al., Pre-flare activity and magnetic reconnection dur- stainless steel mesh-type collimator. ing the evolutionary stages of energy release in a solar eruptive flare. ApJ, 2011, 743, 195–208. At present, the engineering model of HEL1OS is under 5. Rao, A. R. et al., RT-2 detection of quasi-periodic pulsations in development (Figure 4). All electronics cards (front-end the 2009 July 5 solar hard X-ray flare. ApJ, 2010, 714, 1142– electronics with detectors mounted, processing electron- 1148. ics and power conditioning electronics) are fabricated and 6. Yashiro, S. et al., Spatial relationship between solar flares and populated; electrical testing and performance verification coronal mass ejections. ApJ, 2008, 673, 1174–1180. 7. Chifor, C. et al., The early phases of a solar prominence eruption with detectors is underway. and associated flare: a multi-wavelength analysis. A&A, 2006, 458, 965–973. Summary 8. Warmuth, A. et al., Rapid changes of electron acceleration charac-teristics at the end of the impulsive phase of an X-class solar flare. ApJ, 2009, 699, 917. Solar flares in X-rays have been studied in detail using 9. SoLEXS Team, SoLEXS PDR document, ISRO-ISACADITYA- the RHESSI mission for the last several years, albeit with L1-RR-1343, 2016. low spectral resolution. The X-ray instruments on-board 10. HEL1OS Team, HEL1OS PDR document, ISRO-ISAC-ADITYA-L1-RR-1342, 2016. Aditya-L1 will provide a large energy coverage (1 to 150 keV) with a spectral resolution better than of RHESSI providing opportunities to study coronal abun- doi: 10.18520/cs/v113/i04/625-627 CURRENT SCIENCE, VOL. 113, NO. 4, 25 AUGUST 2017 627