Given the space-based communication networks in daily use today, it is essential to constantly watch the emission of high-energy particles and radiation from the Sun. In particular, it is necessary to find out the intensity and timings of the emissions. Moreover, the basic physics too poses a challenge: the physical causes of such emissions. Hence, an ideal site for establishing a solar telescope is needed.
A solar telescope is best located in the middle of a large body of water, which reduces air turbulence due to ground heating during the day. The ground temperature at sites surrounded by water would be lower compared to the air temperature. This would considerably improve the visibility of the Sun’s fine features.
A solar observatory that exploits this feature is on a tiny island in the Fatehsagar Lake in Udaipur, Rajasthan. The lake area is a great tourist attraction. The Observatory was founded under the Vedshala Trust, Ahmedabad in 1975 by Arvind Bhatnagar (1936-2006), a well-known astronomer. The Department of Space of the Central Government took over the Observatory in 1981. Today, it is managed by the Physical Research Laboratory, Ahmedabad, founded by Dr Vikram Sarabhai in 1947, which is considered the cradle of space science in India.
Udaipur enjoys more than 270 days of clear skies every year. It is a city of lakes, ideal for minimizing the turbulence in the atmosphere that would affect the image in a telescope. It is possible to easily observe the details of the various solar flares.
Right from its inception, the Udaipur telescope has monitored solar activity on a daily basis. The observatory has been studying the Sun’s chromosphere for the last three decades. Currently, the telescope for this task is a 15-cm refractor, placed on a 10-m tall pillar. A protective dome safeguards it during the monsoon. Early detectors were photographic cameras. The data are recorded by CCD cameras for the last ten years. Microsoft Excel programme can enable a user to access the data stored in compact disks.
The observatory is today on the world map as one of the six sites participating in the international Global Oscillations Network Group (GONG). Six identical telescopes are deployed in as many sites around the world – in Canary Island (Spain), Chile, Learmouth (Australia), Hawaii and Big Bear (USA) for near continuous 24-hour monitoring of solar oscillations. The telescope monitors the Sun automatically and takes digital velocity images every minute. The precise velocity of the solar oscillation and magnetic field measurements are recorded on magnetic tape cassettes and shipped to the main station at Tucson, Arizona (USA). The data from all the six stations are then analyzed to unravel some fundamental problems of the solar interior and general astrophysics. The data are made available to the international scientific community.
GONG’s original goal was to determine the physical properties of the solar interior. The goal has been achieved. In fact, the precision in determining the internal structure of the Sun using this technique prompted physicists to look for other solutions for explaining the observed deficit of solar neutrinos. The latest explanation is that a fraction of the neutrinos change their properties while coming from the Sun to Earth, and thus escape detection.
Today, GONG is playing a new role: it provides a new tool for forecasting the space weather, which can disrupt satellites as well as manned missions in space. Observatories participating in acquiring the GONG, image the active regions on the far side of the Sun. The technique deployed for this purpose is called time-distance seismology. The beauty of this technique is that one can know about the active region, even before it appears on the front side of the Sun. Space weather can be predicted to some extent. It is an encouraging beginning. The work is now being pursued by satellites.
Daily observations of the Sun have triggered the curiosity of scientists engaged in basic research. One topic relates to the magnetic fields in the chromosphere, in sharp contrast to the conventional studies of the fields in the photosphere. Thus, the focus today is on the configurations of the solar magnetic fields that are about to lead to a flare. The “curled-up” nature of the magnetic fields was quantified recently and this has led to the need for magnetic field measurements in the chromosphere.
Another area of research concerns the relationship between physical properties of the coronal mass ejections from the Sun and their solar origins on the one hand and the severity of geomagnetic storms, on the other. Scientists in Udaipur have found that the initial ejection speed of a coronal mass could provide a clue to the severity of the storm. The scientists have also studied the magnetic power of the source needed for a mass ejection. The scientists have found that coronal mass ejection depends on one-fifth of the power of the total magnetic energy of the source active in the region. A solar flare can also produce changes in the oscillation patterns.
Contemporary solar research focuses on solar magnetism and its role in powering solar eruptions. Free energy available for eruptions should therefore be measured. And this needs vector magnetograph, which monitors a large number of active regions. It is also necessary to explore the triggering mechanisms for eruptions. This needs line-of-sight views of the magnetic fields with high spatial resolution.
Scientists can remotely measure the Sun’s magnetic field by observing the differences in the frequency of the light influenced by the magnetic field. The Udaipur Observatory has set up a solar vector magnetograph, which is a state-of-the-art device to study solar magnetic fields.
A magnetograph uses the basic principle that light has not only intensity or brightness but can also ‘vibrate’ in certain specific directions. The light from the photosphere (including Sunspots) is recorded at 6302 Angstroms. This spectral line is magnetically sensitive: in the presence of a magnetic field, the light is split into its components. The random motions of atoms in the solar atmosphere caused by high temperature broaden the spectral lines and prevent us from seeing the splitting caused by magnetic fields.
A solar vector magnetograph measures the polarization of light at different wavelength positions within a solar spectral line. The device can use a commercially available telescope to collect photons from the Sun. A light filter allows only the light of a particular wavelength band to pass through. This prevents the heating of the optics which would otherwise been very large, if all the Sunlight were to be allowed into the telescope. The telescope is kept rock-steady without any jitter during observations. The received solar light is modulated at a very fast rate to counter the effects of atmospheric distortion. The light obtained is processed to isolate the spectral lines, for which a stable device, called Fabry-Perot etalon, is installed. It measures wavelengths of light very precisely.
Every module in the magnetograph is computer controlled. A distinctive feature of the work is the participation of the students of the Mohan Lal Sukhadia University in developing the computer programme.
The Udaipur solar observatory is also setting up an adaptive optics system to improve the ground-based telescope’s imaging capability. The adaptive optics system will form part of a new telescope, called the Multiple Application Solar Telescope (MAST) being planned for the observatory. It will be 50-cm telescope in place of the currently operating 15-cm one, which takes synoptic images of the solar activity. The telescope will be of a steerable type, constantly tracking the Sun. This calls for a short focus telescope.
The telescope has a coude spectrograph for analyzing the light gathered by the telescope, while the adaptive optics system corrects the distorted wavefront. The corrected wavefront will be fed to a spectrograph or a narrow band filter. Thus spectrograms of smaller fields of view but with high spatial resolution can be obtained, which could lead to discoveries of new features.
The most significant scientific questions on the Sun include:
- The structure and dynamics of its interior and the structure of the heliosphere;
- Developing a predictive capability and quantifying the impact of the dynamical processes on the near-Earth space.
The work done at the Udaipur solar observatory would be useful in understanding some of the key links in this field. Given the large number of clear days and the latest instruments to process the light from the Sun, the Udaipur observatory is emerging as one of the best in the world.