Aditya-L1 is a spacecraft mission of ISRO to study the Sun. It has been designed and will be built in collaboration between the Indian Space Research Organization (ISRO) and various Indian research institutes. It is planned to launch in end of 2020, but due to CONVID 19 and other ambitious projects such as Gaganyaan it is unclear when ISRO will launch Adithya.
The Aditya-1 mission was conceived as a 400kg class satellite carrying one payload, the Visible Emission Line Coronagraph (VELC) and was planned to launch in a 800 km low earth orbit. A Satellite placed in the halo orbit around the Lagrangian point 1 (L1) of the Sun-Earth system has the major advantage of continuously viewing the Sun without any occultation/ eclipses. Therefore, the Aditya-1 mission has now been revised to “Aditya-L1 mission” and will be inserted in a halo orbit around the L1, which is 1.5 million km from the Earth.
A Lagrangian point is a region of space that lies partway between the Earth and the Sun where gravitational forces of both bodies are balanced. ESA explains that a spacecraft in orbit around these points will simply hover without the influence of any gravitational force. This makes it possible to place objects in a 3D orbit around these points which is known as a halo orbit.
The first point of these points is L1 that lies 1.5 million km away from the Earth towards the Sun. It is the future destination of Aditya L1 and the current abode to the Solar and Heliospheric Observatory (SOHO), an ESA/NASA collaboration. The main advantage of placing the Aditya spacecraft in a halo orbit around L1 is continuous observations of the Sun without any obstructions or eclipses.
Aditya-L1 is a 1500 kg-class satellite carrying seven payloads. The payloads cover the Sun’s photosphere (ultraviolet (UV) and soft and hard X-rays), chromosphere (UV) and corona (visible and NIR). In addition, particle payloads will study the particle flux emanating from the Sun and reaching the L1 orbit, while the magnetometer payload will measure the variation in magnetic field strength at the halo orbit around L1. These payloads will make in situ measurements and therefore have to be placed outside the interference from the Earth’s magnetic field for useful measurements’
Indian Space Research Organisation (ISRO) made an announcement of opportunity (AO) and invited payload proposals from various Indian research institutions and universities. In response, several proposals were received. Seven payloads were selected based on their scientific merit and technical feasibility. The main payload continues to be the coronagraph of Aditya-1 with enhanced capabilities of spectroscopy and spectro-polarimetry.
The payloads on-board Aditya-L1 is
Visible Emission Line Coronagraph (VELC)
Visible emission line coronagraph (VELC) is the prime payload on board Aditya-L1.VELC is designed to be an internally occulted reflective coronagraph to meet the observational requirements of wide wavelength band and closer to the solar limb (1.05 Ro) as against the externally occulted and refractive coronagraphs. VELC has the capability of simultaneous spectroscopic observations of solar corona in emission lines 7892 Å (FeXI), 10747 Å (FeXII), and 5303 Å (FeXIV), spectropolarimetric observations in emission line 10747 Å (FeXII) and imaging of solar corona in continuum at 5000 Å (pass band 10 Å). Field of view (FOV) for spectroscopy and spectropolarimetry is 1.05 Ro to 1.5Ro (0.28 deg to 0.4 deg) and for continuum imaging is 1.05Ro to 3Ro (0.28 deg to 0.8 deg). The uniqueness of VELC stems from the fact that it is capable of simultaneous observations in multiple wavelength bands closer to the limb (1.05 Ro) with high pixel resolution and cadence as compared to the other space solar missions and ground-based solar coronagraphs.
VELC Primary mirror (M1) is an off-axis parabolic mirror of clear aperture 192 mm with an off-axis distance of 152 mm is at a distance of 1570 mm from entrance aperture (EA). The diameter of EA is 147 mm to cover the desired FOV. Secondary mirror (M2), positioned at prime focus, acts as an internal occulter, which ejects the disc light (up to 1.05 Ro) through a central elliptical hole and reflects the coronal light over the FOV 1.05 Ro to 3Ro. M1, M2, and the collimator together reimages the entrance pupil. Lyotstop located after the collimator cuts off the diffracted light from EA. M4 directs the coronal light to different spectroscopic and imaging channels.
Proposed science goals of VELC on board Aditya-L1 are:
The major contributors to the instrument back ground are (i) scattered disk light into the FOV due to the microroughness and particulate contamination over M1 surface and (ii) diffraction of solar disk light at EA. A detailed study will help in deciding the maximum allowed limit on the microroughness and surface cleanliness level of M1.
Solar Ultraviolet Imaging Telescope (SUIT)
SUIT will provide near simultaneous observations of lower and middle layers of the solar atmosphere, namely the Photosphere and Chromosphere. These observations will help to improve our understanding of coupling and dynamics of various layers of the solar atmosphere, mechanisms responsible for stability, dynamics and eruption of solar prominences and Coronal Mass ejections, and possible causes of solar irradiance variability in the Near and Middle UV regions, which is of central interest for assessing the Sun’s influence on climate.
SUIT has two main sub-units: the optical bench and the electronics box. The optical bench has a two mirror offaxis telescope designed and optimized to take high-resolution images in the 200-400 nm region with a passively cooled CCD detector. The optical bench consists of the mirrors, focal plane assembly, filter wheel, shutter & focusing mechanisms, baffles, aperture filter, enclosure covers and structural support elements. The optical bench will be mounted on the top deck of the spacecraft along with some of the other instruments.
There are total 11 science filters (8 Narrow-band and 3 Broadband) that will be mounted on two filter wheels each with 8 filter slots (a total of 16 slots). The 5 other slots will have 1 clear glass filter, 3 neutral density filters and 1 closed position for taking dark frames. The filter wheels will be driven by two independent drives that will bring a predefined combination of neutral density filter and science filter into the beam path. The exposure control is done using a diaphragm shutter that is located in front of the first filter wheel. Depending on the combination of the science filters chosen, the exposure time can vary between few tens to a few hundred milliseconds.
The SUIT instrument will take images of the Sun 24x7 throughout its operational life, except for the in-orbit calibration (initial and periodic) phases of the instrument and the periodic orbit correction maneuvers for the spacecraft. The entrance aperture is proposed to have a multi-operation door mechanism that can be opened and closed during the calibration and orbit maneuvers.
The electronics box consists of all the processing and control electronics for the detector and the mechanisms of SUIT. This box will be mounted inside the S/C bus below the top deck. The electronics have been separated from the optical bench to minimize the contamination of optics due to molecular out gassing. The front-end electronics located in the vicinity of the CCD will be interfaced with the readout electronics through interface cables for data and power. After the exposure, the shutter will remain closed while the detector is read and the filter wheels are moved into the position for the next exposure.
The aim is to provide a high-degree of autonomy to the system to operate 24x7 with minimum interventions from the ground operations team. Nevertheless, there are provisions in the design to override sequences on the onboard computers by ground commands. This will provide flexibility for operating the instrument in different operations modes as per the requirement of the science team.
The optical design of SUIT was done based on the instrument functional performance requirements and the design constraints. The two-mirror off-axis configuration was selected to minimize the scattering effects. It also prevents any direct straylight from the telescope entrance from reaching the focal plane. The use of aspheric surfaces for the mirrors reduces the number of components to correct for abberation all over the field of view and only a single element field corrector lens is used just before the image plane. The final design configuration of SUIT has a primary mirror with a clear aperture (CA) of 141mm, which is sufficient to give diffraction limited images of 1 arcsec diameter at 280nm wavelength.
The SUIT image plane uses a 4096×4096 CCD sensor with 12 micron square pixels and offers a pixel sampling of 0.7 arcseconds; providing a minimum angular resolution of ~1.4 arcseconds.
The focal length of the system is 3500mm with a field of view of approximately 0.8º (up to ~1.6 Solar Radius); covering the entire solar disk and leaving sufficient margin for potential misalignments between the optical axes of SUIT and VELC. The field correcting lens produces uniform image quality throughout the field of view with an acceptable dispersion due to wideband filters. It also allows compensating for image focus shift due to any possible change in the thermal configuration of the instrument.
All the opto-mechanical and electronic systems of SUIT, including mirrors, filters, baffles, mechanisms and focal plane assembly, will be mounted on a light-weighted optical bench made of Titanium alloy. The optical bench would be covered with an enclosure that will provide protection from external environment, straylight and contamination. The optical bench will be mounted on the spacecraft top deck with six mounting legs. The control systems for the instrument and the detector readout electronics will be housed separately in an electronics box mounted inside the spacecraft below the top deck. The data, control and power cables from the mechanisms and the focal plane assembly will be relayed to the electronics box.
Aditya Solar wind Particle Experiment (ASPEX)
The primary focus of the ASPEX payload on-board the ADITYA-L1 satellite is to understand the solar and interplanetary processes (like shock effects, wave-particle interactions etc.) in the acceleration and energization of solar wind particles. In order to achieve this, it is necessary that ASPEX measures low as well as high energy particles that are associated with both the slow and fast components of solar wind, suprathermal population, shocks associated with CME and CIR and SEPs. Among these, it is expected that the slow and fast components of the solar wind and some part of the suprathermal population can be measured in a predominantly radial direction. In addition, a part of the suprathermal population, CME and CIR-accelerated particles and SEPs are expected to arrive at the detectors along the Parker spiral.
It is known that He++/H+ ratio is an important proxy that can be used to determine the arrival of a CME driven shock front at 1 AU . ASPEX will use the Heþþ/Hþ ratio as a compositional “flag” to differentiate (and identify) the arrivals of CME, CIR, SEP-related particles from those of the quiet solar wind origin. Therefore, it is necessary that the measurements are planned suitably so that all the science objectives are fulfilled. The major science objectives of the ASPEX payload are as follow:
Addressing the above mentioned issues require systematic observations of particle fluxes at selected energy ranges as well as measurements of the He++/H+ number density ratio at the L1 Lagrangian point. Keeping this in mind, the ASPEX payload has been configured as two independent subsystems. The SWIS subsystem consisting of two independent units will have the capability of measuring solar wind particles in the energy range of 100 eV to 20 keV in the plane of the ecliptic and normal to the plane of the ecliptic, using an electrostatic analyzer (ESA) coupled to a micro channel plate (MCP) detector. While one of the SWIS units, referred to as Top Hat 1 (THA-1) will receive and differentiate particles (H+ and He++ ions) in the ecliptic plane (species differentiation mode), the second SWIS unit, referred to as Top Hat 2 (THA-2) will measure the total flux irrespective of types of species across the ecliptic plane (species integrated mode). The STEPS subsystem will measure the particle flux in the 20 keV/n to 20 MeV/n energy range in the Sunward, anti-Sunward, Parker, ecliptic North and ecliptic South directions, using custom designed silicon detectors. Three STEPS units (Sunward, Parker and anti-Sunward) are designed to operate in the species differentiation mode while the remaining three STEPS units (between Sunward and Parker, ecliptic North and ecliptic South) will operate in the species-integrated mode.
Plasma Analyzer Package for Aditya (PAPA)
Main objective of PAPA is to understand the composition of solar wind and its energy distribution like, Continuous Measurement of the solar wind and interplanetary electron distribution functions in the energy range 0.01-3keV to extract the interplanetary magnetic field structure and topology. Study of the composition of solar wind and there by understanding about the origin of solar wind and particle acceleration mechanism.
Solar Wind Electron Energy Probe (SWEEP) to measure the solar wind and interplanetary electron distribution functions in the energy range 0.01-3keV.
Solar Wind Ion Composition Analyzer (SWICAR) to measure the kinetic temperatures and mean speeds of all major ion species in the mass range 1-30 amu.
X-ray spectrometers on-board Aditya-L1
Two solar spectrometers covering soft (1 to 30 keV) and hard (10 to 150 keV) X-ray bands will be flown on Aditya-L1 mission to study solar flares and their dynamics. The two X-ray spectrometers covering the energy band from 1 to 150 keV will allow us to carry out the following science objectives:
Solar Low Energy X-ray Spectrometer (SoLEXS)
Solar Low Energy X-ray Spectrometer (SoLEXS) will cover the energy band from 1 to 30 keV with a spectral resolution of <4% (i.e. <250 eV at 6 keV). This energy band will help in obtaining the thermal energy of the flares. To cover the large class of flares, from A- to X-class, SoLEXS will carry two identical detectors with different apertures. The large area aperture will cater to small flares (A- to C-class), while the small aperture will observe intense flares (other classes).
SoLEXS is configured as two packages, viz. detector package and electronics package . The detector package carries the two detectors and the associated high voltage power supply along with charge sensitive preamplifiers. The electronics package carries the required processing and power electronics to cater to the instrument. 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 implemented to operate this instrument in quiet as well as flare mode. This flare trigger is also provided as a hardware line to the SUIT instrument on-board Aditya-L1.
High Energy L1 Orbiting X-ray Spectrometer (HEL1OS)
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 of the short-lived impulsive phase of solar flares. This energy band helps identify the non-thermal energy release during the flares.
HEL1OS is being developed with two different types of detectors: CZT and CdTe. The CZT detector is a state-of-the-art, near-room-temperature device. In order to achieve a total geometric area of 32 sq. cm, two such detectors (16 sq. cm per detector) are used to cover the energy range 20 to 150 keV and operate in the temperature range 5C to 20C. The individual detectors are pixilated with 256 pixels, with pixel dimension 2.46 mm * 2.46 mm. The CdTe detector, which has better resolution at lower energies, is used for detailed spectroscopic studies from 10 to 40 keV, and operates in the temperature range –35C to –25C. The overall geometric area of 0.5 sq. cm will be achieved using two CdTe detectors, each with an area of 0.25 sq. cm. The field-of-view of the instrument has been constrained to 6o* 6o using a stainless steel mesh-type collimator.
Fluxgate Digital Magnetometer
Aditya-L1 spacecraft includes a Fluxgate Digital Magnetometer (FGM) to measure the local magnetic field which is necessary to supplement the outcome of other scientific experiments onboard. The insitu vector magnetic field data at L1 is essential for better understanding of the data provided by the particle and plasma analysis experiments, onboard Aditya-L1 mission. Also, the dynamics of Coronal Mass Ejections (CMEs) can be better understood with the help of insitu magnetic field data at the L1 point region. This data will also serve as crucial input for the short lead-time space weather forecasting models.
The proposed FGM is a dual range magnetic sensor on a 6 m long boom mounted on the Sun viewing panel deck and configured to deploy along the negative roll direction of the spacecraft. Two sets of sensors (tri-axial each) are proposed to be mounted, one at the tip of boom (6 m from the spacecraft) and other, midway (3 m from the spacecraft). The main science objective of this experiment is to measure the magnitude and nature of the interplanetary magnetic field (IMF) locally and to study the disturbed magnetic conditions and extreme solar events by detecting the CME from Sun as a transient event. The proposed secondary science objectives are to study the impact of interplanetary structures and shock solar wind interaction on geo-space environment and to detect low frequency plasma waves emanating from the solar corona at L1 point. This will provide a better understanding on how the Sun affects interplanetary space.
Since the quiet mode ambient Inter planetary Magnetic Field at L1 point is of the order of few nT and the magnetic field generated by the spacecraft field is of the order of few thousand nT, the 6 m boom will reduce the effect of spacecraft generated field by a factor of 200.
The Aditya-L1 FGM sensors are ring core based with ferromagnetic material as the core material provides the best noise performance and stability with temperature and time.
The primary goal of FGM aboard Aditya-L1 mission is to measure the magnitude and nature of IMF and its temporal evolution. It is also aimed at studying the disturbed magnetic conditions arising from extreme solar events such as Coronal Mass Ejection (CME)
Besides, Aditya-L1 FGM experiment will also enable the study of the consequences of IP shocks and IP counterparts of CMEs viz. magnetic clouds, etc., in the geospace environment. The IMF measurements are expected to provide important insights into the physical processes occurring in the Sun such as solar plasma wave generation and will also provide complementary measurements to that made by other plasma experiments onboard viz. ASPEX and PAPA.
One of the outstanding questions related to solar activity cycle presently is whether it is heading towards a minimum like Maunder Minimum in the past. This is based on the findings that the current solar cycle 24 is the weakest launched in the ascending phase of the solar cycle 25, it is expected to shed light on the behavior of the next activity cycle also.
Apart from providing vital information on the long term variation of the solar activity cycle mentioned above, the Aditya-L1 FGM instrument will play a key role in identification of transient events occurring on the Sun viz. CMEs, in particular their IP counterparts. Attempts to relate the three part structure of CMEs viz. leading edge, cavity and prominence with their ICME counterparts like sheath, magnetic cloud and cold plasma are crucial to understand the propagation of CMEs in the IP medium and in forecasting the arrival time of CMEs a t the Earth. Combining data obtained from the plasma experiments on Aditya-L1 spacecraft e.g. PAPA and ASPEX, various signatures of the ICMEs can be traced to solar eruptions.
The Aditya-L1 mission is expected to provide a multi-pronged holistic approach to the understanding of some of the outstanding problems of solar physics.
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5/20/2020 07:38:01 am
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