THE SUN, OUR STAR

The Sun seen with X rays. Its surface appears rough due to the convective motions that cause the appearance on the surface of bubbles of incandescent gas. In the outer zone, the corona, a large protuberance can be seen.  (JPEG, 183 K)  (NASA-Skylab)

Detail of a protuberance. These structures are formed by huge concentrations of gas of the solar corona, that have a lower temperature compared to the surrounding environment. They can be millions of km long. The picture was taken using a coronograph, an instrument that conceals the disc of the Sun to highlight the external details.  (JPEG, 232 K)  (NASA-Skylab)

Sequence of the total  eclipse of the Sun of July 11th 1991, photographed in Mexico.  (JPEG, 56 K)  (Photo: R. Crippa)

Total eclipse of the Sun on July 11th 1991. When the Sun, the Moon and the Earth are aligned in precise mutual positions, the disc of the Moon eventually conceals the Sun completely, for a few minutes, leaving a darkened zone on the Earth. In such occasions it is possible to observe more clearly the corona of the Sun, which represents the atmosphere of our star, and the phenomenon of the protuberances.  (JPEG, 96 K)  (Photo: R. Crippa)

In this final phase of the eclipse of the Sun on March 18th 1988 it is possible to see the disc of the Sun still partially covered, while its surface clearly shows the solar spots, observed for the first time by Galileo. (JPEG, 281 K)  (Photo: V. Maffei, A. Mormando, A. Possenti)

One of the largest groups of solar spots in modern history: that of April-May 1947. The spots are zones of particular activity of the photosphere, connected to the magnetic field, that can last up to several weeks and tend to form following a cyclic period of eleven years that seems to influence the terrestrial climate. By studying their regular movement, Galilee understood that the Sun rotates on its own axis.  (JPEG, 329 K)

Launched by a Space Shuttle in 1990, Ulysses already explored once the poles of the Sun in 1994 and 1995. The probe is now traveling back to Jupiter, and will begin another exploration of the solar poles  in 2000-2001. 

Ulysses: discovering the poles of the Sun.  The systematic study of the Sun and of its properties had already begun in the sixties. Many interplanetary probes had reached the planets nearer to the star in the seventies. No satellite though had ever explored the poles of the Sun. In 1994, after an interplanetary voyage lasted 4 years, the space probe Ulysses of Esa flew over the South Pole of the Sun (300 million km far), and the following year it flew over the North Pole. The unique discovery was that... the Sun does not have magnetic poles! Ulysses is now traveling towards Jupiter. The gravitational attraction force of the gigantic planet will give the probe the push necessary to travel back to the poles of the Sun, just like a boomerang. (2000-2001).  (JPEG, 257 K)

 

The trajectory followed by Ulysses to reach the Sun exploited Jupiter's gravitational push.  (JPEG, 93 K)  (NASA)

 

The scientific goals of Ulysses. The probe studies: the solar wind and the magnetic field; the radio emissions and the plasma emissions, the x rays and the energetic particles; the interstellar gas and the cosmic dust, the gamma rays of cosmic origin.  (JPEG, 244 K)  (ESA)

 

Artistic picture of the probe in the vicinity of the solar poles. The joint ESA-NASA mission explores the heliosphere, the region dominated by the flux of the solar wind. (JPEG, 182 K)  (ESA)

 

Drawing of the large scale structure of the heliosphere. You can see the main boundaries that are supposable. The orbits of the external planets give an idea of the size of the system.  (JPEG, 329 K)  (ESA)

 

Computerized model of the main components of the heliosphere: the corona, the solar wind and the magnetic field.  (JPEG, 477 K)  (ESA)

THE SOHO MISSION OF ESA

Launched on December 2nd 1995 by an Atlas II American rocket, SOHO (Solar and Heliospheric Observatory) studies the solar wind and the Sun from within, up to the outer regions.  

One of the most surprising discoveries in the last years that regard the Sun is that our star "sings". The notes, whose frequencies are too low to be heard by the human ear, cannot propagate in interplanetary space. The solar physics specialists, however, succeed to reveal a rhythmic movement produced at the surface of the Sun, generated by the reverberation of the sound waves within. The consequent oscillations have precise frequencies, just like musical notes, and their analysis casts new light on the structure of the Sun. In December 1995 ESA launched the SOHO satellite to carry out a "hi-fi" recording of the singing of the Sun. The scheduled duration of the mission is 2 years.

Artistic vision of the satellite in its orbit. SOHO is in a region of balance between the solar attraction force and that of the Earth, at a distance of 1.5 million km from the Earth in the direction of the Sun.  (JPEG, 99 K)  (ESA)

 

SOHO during the assembly and tests at Marconi of Portsmouth (England).  (JPEG, 648 K)  (ESA)

 

UVCS, Ultraviolet Coronographic Spectrometer  The ultraviolet coronographic Spectrometer (UVCS) observes the radiation diffused by the solar corona in the Extreme Ultraviolet (EUV). It can determine the main parameters (temperature, density and mass flux speed) of the plasma that constitutes the solar corona. It works on the same principle of  solar eclipses, that allow to observe the corona, less luminous than the solar disc. In the case of UVCS the disc is hidden by a screen of adequate dimensions; the instrument, constantly pointing the centre of the Sun, can rotate on its own axis, thus allowing the complete mapping of the corona.  (JPEG, 113 K)

 

A view of the flight model of UVCS spectrograph, with the fissures for the three channels screened by the "baffles", (necessary to protect it from diffused light), and relative movements. The instrument was realized with the collaboration of the Smithsonian Astrophysical Observatory (Cambridge, USA), ETH (Zurich, Switzerland), Goddard Space Flight Center (USA) and the universities of Florence, Padua, Turin and Catania, coordinated by ASI.  (JPEG, 426 K)  (University of Padua, CISAS)

 

Ultraviolet image of the solar corona and disc, obtained with UVCS and EIT, respectively. The picture shows "polar plumes" that can be observed in the North solar pole, at a distance between 1.5 and 2.5 solar radii. I (JPEG, 89 K)  (University of Padua, CISAS)

For the optical, electronic and mechanical tests on the UVCS spectrometer, carried out at the Department of Electronics and Computer Science of the University of Padua, the Laboratory Evaluation Unit (LEU) was used.
 
 
 

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