Aditya-1 Solar Coronagraph Mission
Aditya-1 is a solar coronagraph mission of ISRO (Indian Space Research Organisation) approved by the Space Commission of the Government of India. The project is a national effort involving the collaboration of the following institutions: ISRO, IIA (Indian Institute of Astrophysics), Udaipur Solar Observatory, ARIES (Aryabhatta Research Institute of Observational Sciences), TIFR (Tata Institute of Fundamental Research), and some Indian universities. In Sanskrit, Aditya is the name of the sun. 1) 2)
The major scientific objectives of Aditya-1 are to achieve a fundamental understanding of the physical processes that heat the solar corona (from the base to the extended region), accelerate the solar wind, and produce CMEs (Coronal Mass Ejections). To achieve these objectives, the Aditya coronagraph requires the capabilities for the following observations:
- High frequency intensity oscillations (~ 1 Hz)
- Dynamics of coronal loops with high cadence
- Magnetic field topology
- CMEs close to the solar disk.
The Aditya-1 observations will include coronal waves, cooling of coronal loops, temperature structure of coronal loops, formation and development of coronal loops, monitoring topology of coronal magnetic field, loop dynamics, dynamical scale heights of coronal loops, flows in the coronal loops and CME studies.
The goals are to detect the existence of waves in the solar corona and the nature of waves, to investigate the role of waves in heating the solar coronal plasma, to understand the formation of coronal loops, to understand the magnetic nature of coronal loops, to understand the cooling of post flare loops and to investigate the pre-eruption dynamics of CMEs in detail.
The Aditya-1 science instrument will be a 20 cm internally occult coronagraph using an off-axis parabolic mirror capable of taking images of the solar corona simultaneously in the visible emission lines at 5303 Å (Fe xiv) and at 6374 Å (Fe x). These emission lines are the brightest in the visible coronal spectrum and the FOV (Field-of-View) will be from 1.05 Rsun to 3.0 Rsun in the solar corona. The fast cadence of the imaging instrument will allow to study the high frequency waves and its association with the coronal heating. Linear polarization measurements will also be made to map the magnetic topology of the solar corona.
Background: The project considers the Aditya-1 mission more or less as a successor of the STEREO mission of NASA (launch Oct. 26, 2006), in particular for the time frame 2012-2013, the next solar maximum period, when the STEREO mission may not be functional anymore. Furthermore, as far as CME studies are concerned, most of the future missions may carry a coronagraph, but the FOV of these different missions will be very different from each other and none of the planned and future missions can go close to the solar disk (the best is 1.3 Rsun by the STEREO mission).
The LASCO (Large Angle Spectrometer Coronagraph) C1 coronagraph on SOHO (launch Dec. 2, 1995), had the capability to observe close to the sun (up to 1.1 Rsun), but functioned only for a short while; even when it did, its ability to address the CME initiation issues was hampered by its low cadence (tens of minutes) and the relatively poor signal to noise ratio of its green line images due to the use of Fabry-Perot etalon as well as the limited aperture size (of 5 cm) to determine the velocity structure of the solar corona. The C2 and C3 coronagraphs of LASCO only address CME propagation issues.
The COR1 coronagraphs of the SECCHI (Sun-Earth Connection Coronal and Heliospheric Investigation) instrument suite aboard the STEREO spacecrafts are used to obtain relatively low cadence data. While high cadence on-disk EUV data from the TRACE (Transition Region and Coronal Explorer) mission of NASA (launch April 2, 1998) and the SDO (Solar Dynamics Observatory) mission of NASA (launch Feb. 11, 2010) have the potential for addressing some aspects of CME initiation problems, the limited field of view of this instrument is a major limiting factor. Also note that SDO does not carry any coronagraph.
Figure 1: Conceptual view of the Aditya-1 spacecraft with the solar panels stowed (image credit: ISRO)
The Aditya spacecraft is 3-axis stabilized using the IMS (Indian MiniSatellite) bus of SARAL (Satellite with Argos and AltiKa) mission heritage (launch in 2011). The spacecraft is developed as a standard satellite bus by ISAC (ISRO Satellite Center), which can be used for missions for remote sensing, communications and space science applications.
The structure of Aditya-1 consists of aluminum skin honeycomb sandwich structure, a cuboid of size 89 cm x 89 cm x 61.5 cm. The payload will be accommodated on the top deck with localized corner brackets. The total mass of the spacecraft, including the payload, is expected to be around 300 kg.
AOCS (Attitude and Orbit Control System): The AOCS configuration consists of various types of sensors for measurement of attitude errors, control electronics and different types of actuators such as reaction wheels, magnetic torquers and reaction control thrusters to impart thrust/torque to the spacecraft in the desired direction. The sensor system consists of a star sensor, sun sensors (4π FOV), magnetometer and precision sun sensor and the IRU (Inertial Referencing Unit). The IRU consists of three miniature gyroscopes, dynamically tuned and cluster mounted, and suspended with vibration isolators. Considering the stability requirement of < 5 x 10-5 º/s, the option of using a FOG (Fiber Optic Gyro) solution is being studied. Three types of actuators are being used, reaction wheels, magnetic torquers and RCS (Reaction Control Subsystem) thrusters to control the orbit and the attitude of the spacecraft.
EPS (Electric Power Subsystem): The spacecraft features one solar wing with 2 panels, each of size 120 cm x 81 cm. The solar panels are stowed on the VD02 side in the launch configuration; they are programmed to auto deploy immediately after injection into the orbit. An average power of 400 W is available using multi-junction solar cells. A Li-ion battery provides power during the eclipse phase of the orbit. The EPS is configured as a single regulated, battery tied bus with one battery. The power electronics package performs the functions of solar array string switching, battery charge/discharge control and power conditioning and distribution. The power distribution scheme is based on the specific voltage and current requirements for all subsystems.
Figure 2: Block diagram of the Aditya-1 minisatellite (image credit: ISRO)
The BMU (Bus Management Unit) with the OBC takes care of all data handling functions using the MIL-STD-1553B standard interface with the star sensor and the payload. The payload data handling subsystem employs serial interfaces to the baseband data handling formatter. The formatter receives the payload data packets, annotates these with the housekeeping data, and deposits them onto the SSR. The BMU is built around state-of-the-art technologies using FPGAs and ASICs. It interfaces with TTC (RF), sensors, power, thrusters, MTCs (Magnetic Torque Controllers), gyros, wheel drive electronics and the payload.
Figure 3: Block diagram of the BMU (image credit: ISRO)
The SPS (Satellite Positioning System) provides the state vector (position, velocity and time) of the spacecraft in real-time. A miniaturized GPS receiver is being used. The state vector of the satellite is sent to BMU through a serial interface. The system works in cold start mode, that is, it searches the sky, receives the GPS signals, and computes position, velocity and time without any prior information of the spacecraft' state vector.
RF communications: The TT&C transponder consists of a S-band receiver, a transmitter and the antenna subsystem. The receiver is of a non-coherent type using the FM/PSK/PCM modulation scheme. The receiver provides frequency demodulation of the onboard received signal to extract the two switching PSK subcarrier signals. This output will be PSK demodulated to obtain data and clock. The FM/PSK demodulation is performed in the digital domain. The TT&C antenna offers a near omni-directional coverage to provide a reliable link during both the initial launch phase and on-orbit phase.
The DHS (Data Handling Subsystem) has two components: the BDH (Baseband Data Handling) subsystem and the RF transmitter. The payload data that is received by the BDH is formatted, randomized and fed to the transmitter. The transmitter accepts the digital data stream from the BDH, modulates the data in QPSX (Quadrature Phase Shift Keying) mode and transmits the data to a ground station using a conventional X-band (8.125 GHz) antenna. The DHS provides the following operational capabilities:
- Input data rates: ~42 / 4.8 /2.4/ 56/0.224 Mbit/s
- Data interface: Serial bit stream
- Channel coding: RS (Reed Solomon) coding (255, 233), I = 5
- Transmission data rate: 32 Mbit/s
- Modulation: QPSK
- SSR (Solid State Recorder) capacity: 32 Gbit on on-board storage. The SSR has the provision to support simultaneously data recording and playback. This feature is required to get uninterrupted observational data from the payload.
The X-band data transmission uses an 8 W SSPA (Solid State Power Amplifier). The data transmission is through a shaped beam antenna.
Orbit: Sin-synchronous dawn/dusk orbit, mean altitude of ~ 800 km, inclination = 98.55º, period = 100 minutes, LTDN (Local Time on Descending Node) = 18:00 hours.
The only scientific instrument on the Aditya-1 spacecraft is the solar coronagraph. The instrument optical axis is required to be pointed towards the center of the sun continuously. The solar coronagraph is being designed and developed at IIA (Indian Institute of Astrophysics) in Bangalore. ISRO is providing the detectors. 3) 4)
Observation technique considerations: The solar corona is ~ a million times fainter than the solar disk in the visible wavelengths. To observe them, a special telescope is needed, namely a coronagraph. A coronagraph produces an artificial eclipse by means of occulting the disk radiation either inside or outside the telescope; hence, the observation techniques are classified into two types: “internally occulted” and “externally occulted”.
- Internally occulted coronagraphs help in observing the corona close to the solar limb, but it has stringent requirements for controlling the scattered light within the instrument.
- In the case of externally occulted coronagraphs, the above requirements are not crucial, which eases the restriction on the quality of optical components used. However, there are two basic limitations for the externally occulted coronagraph: vignetting of inner coronal light, and boom length and its pointing accuracy.
To achieve vignetting free inner coronal images (< 1.5 Rsun), the boom length of the external occulter should be larger than 100 m. In general, an internally occulted coronagraph is used to observe the inner corona (< 2 Rsun) and an externally occulted coronagraph is used to observe the outer corona (> 2 Rsun).
To achieve the science goals in terms of the telescope and the back-end instrument, the whole system will need to possess the following capabilities:
• Good spatial resolution (~2 arcsec) to resolve the coronal loops and other structures
• Large enough FOV for CME studies (1.05 to 3 Rsun, where Rsun represents the radius of the visible solar disk)
• Low instrumental scatter (< 10-6 of the solar disk intensity near the limb and < 10-7 of the solar disk intensity for heights > 100 arcsec above the limb)
• Large enough telescope aperture (~ 20 cm) to provide enough photons and facilitate short exposures (or equivalently, high cadence observations)
• Simultaneous observations in two emission lines for coronal loop diagnostics
• Polarization measurements for understanding of the coronal magnetic field topology.
Considering the scientific goals and the required SNR levels, a 20 cm off-axis internally occulted coronagraph and the associated back-end instruments are selected. An off-axis design is chosen to reduce the scattered light resulting from the spiders that hold the secondary mirror. This is an important aspect that is considered for the accurate coronal intensity and polarization measurements. The selection of internal occultation is driven by the science requirement to study inner corona.
The 20 cm coronagraph uses an off-axis parabolic mirror which simultaneously images the visible emission lines at 5303 Å [Fe xiv] and 6374 Å [Fe x] using narrowband imaging in the emission lines, and imagery in the 5800Å range using the continuum/broadband imaging technique for CME studies. The FOV in the solar corona is from 1.05 to 3.0 Rsun for broadband imaging and 1.05 to 1.5 Rsun for narrowband imaging. A dichroic beam splitter, polarization optics, and narrowband filters are accommodated in the optical path.
The CCD detector array has a format of 2 k x 2 k with a pixel size of < 12 µm. The fast cadence of the imaging instrument will allow to study the high frequency waves and its association with the coronal heating. Linear polarization measurements will also be made to map the magnetic topology of the solar corona. The large FOV capability of this instrument allows the study of CMEs. The primary optics has a micro-roughness of better than 0.2 nm.
Figure 4: Optical layout of the coronagraph (image credit: ISRO)
Legend to Figure 4: EA (Entrance Aperture), M1 (Primary Mirror), M2 (Secondary Mirror), M3 (Mirror to reject the solar disk light), CL (Collimating Lens), LS (Lyot Stop), M4 (Imaging mirror), Pol. (Polarization Filter Wheel), DBS (Dichroic Beam Splitter), CCD (Charge-Coupled Device), and FM (steering mirror).
The instrument operates in four observing modes.
Table 1: Observational modes for the payload
Figure 5: Schematic view of the payload to satellite interface (image credit: ISRO)
The solar coronagraph has a mass of ~ 130 kg with an unregulated power requirement of 120 W. The shape of the solar coronagraph payload is a rectangular box of size: 1.7 m x 0.70 m x 0.40 m (red box in Figure 1). The mechanical structure of the payload consists of an optical bench unit which supports the optics and the detector assembly, covered by a secondary structure. The optical bench unit, i.e. the bottom plate of the corona box, is interfaced with the top deck of the satellite. The satellite features a four point mounting arrangement on its top deck.
1) V. Koteswara Rao, “Aditya-1, Indian Minisatellite Space Coronagraph,” Proceedings of the 61st IAC (International Astronautical Congress), Prague, Czech Republic, Sept. 27-Oct. 1, 2010, IAC-10.B4.2.2
2) Divya Gandhi, “ISRO planning to launch satellite to study the sun,” The Hindu, January 13, 2008, URL: http://www.hinduonnet.com/2008/01/13/stories/2008011354801000.htm
4) “IIA Developing Payload For ISRO's Solar Mission Aditya,” Space Daily, July 2, 2009, URL: http://www.spacedaily.com/reports/IIA_Developing_Payload_For_ISRO_Solar_Mission_Aditya_999.html
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.
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