PLASMA IN THE UNIVERSE
SunIn the Sun we find a whole range of examples of fibrous, helicoidal structures with Birkeland currents flowing along the lines of force of the local magnetic fields: protuberances (1011 A), spicules, coronary currents, eruptions and some others. The Sun is the source of the magnetic field with the shape of an Archimedean spiral (form due to the Sun’s rotation), in which it is found immersed the totality of the solar system. The zone influenced by the Sun’s magnetic field is known as heliosphere. The probe Voyager 1 reached in 2005 the boundary of the heliosphere (heliopause), which is estimated to be between 110÷160 Astronomic Units (1 AU=150 million of kilometers) far away. Hopefully the radioisotope thermal electric generators will last until 2020, which could be enough time to send valuable data about the solar wind in the heliopause. In the heliopause the Voyager detected a drop in the velocity of the solar wind from 1.6 million of km/hr down to 250 thousand of km/hr, the place where the solar wind clashes against the stellar wind. The Voyagers’ cosmic ray detector, magnetometer, plasma wave detector and low-energy charged particle detector are operational and still sending data back to Earth, as of today, June 2005. From the Sun it is expelled an uninterrupted current of neutral and electrically charged particles, which we call solar wind.
Planetary magnetospheresThe original bipolar field of the planets is deformed by the interaction with the solar wind, and that becomes the characteristic shape of the magnetospheres. Just in the planetary vicinity it is usually found the corotating plasmosphere; towards the Sun it is the shock wave, within which the solar wind parameters change abruptly. In the direction from the Sun is elongated a plasma tail. The plasma system circumscribes the boundary of the magnetospheric layer. In the polar zones it is formed a characteristic surface of electric discharges, due to the action of trapped charged particles – the polar lights. The current flow in the surface along the force lines of the planetary field and it is about the so-called Birkeland current. Earth’s magnetosphere: In the corotating plasmosphere the temperature of the particles is 1 eV, in the plasma tail is 1 up to 10 keV, with a particle concentration of 0.5 cm−3. The plasma tail elongates until a hundredfold of Earth’s radius and has a thickness of 20 Earth radii. The boundary of the magnetospheric layer separates the Earth’s magnetic field from its surroundings and has a particle concentration of 1 cm−3.
Jupiter’s magnetosphere is similar to the magnetospheres of the rest of the planets. Moreover, it has the so-called plasma torus. The volcanic activity of its moon Io throws away plasmas rich in sulfur, which along its entire trajectory creates an ample plasma torus. Along the lines of force of planet Jupiter’s magnetic field (perpendicular to the torus) there is a flow of a Birkeland current, which closes through the moon Io and partially heats it. The size of the Birkeland current is estimated to be of some millions of Amperes. The Birkeland current contributes, together with the tidal forces, to the heating of the moon and the maintenance of its volcanic activity. ![]()
Saturn’s magnetosphere has also a plasma torus, similar to the one in Jupiter. Saturn’s plasma torus is the biggest plasma structure in the solar system [after the Sun itself and the spirally shaped plasma within the heliosphere, N. of the T.]. It reaches from 15 times the radius of Saturn up to 25 times the radius of the planet. Inside the torus there are approximately 3000 particles per cm3.
Magnetosphere of the comets. The comets also have their own magnetosphere. For example, in the well-known comet Halley, on its last passage near the Earth, it was measured a magnetic field on its tail of 70 nT (700 microgauss), the particle concentration is of 1 000 per cm3 and its temperature is 1,5 eV (1 eV ~ 10 000 K). In the comet Hyakutake of the year 1996 it was found on its tail an entwined plasma fiber and the satellite ROSAT detected X-ray radiation coming out of its nucleus.
Planetary atmospheresIn the atmosphere of the planets the plasma is found mostly within an ample ionized zone – the ionosphere. In the Earth’s atmosphere the best-known layer, front the plasma point of view, is layer F (140 until 1 000 km.), within which it is reached concentrations of ionized particles of up to 106 in a single cm3. In the ionosphere of Venus it was detected a conductive fiber with a Birkeland current with a length of up to 20 km. Other interesting phenomena are the electrostatic discharges in the atmospheres – the lightnings or thunderbolts. The typical energy of an earthly lightning is of 6×108 J, the thunderbolts in Venus have an energy of about 2×1010 J and in Jupiter is of 3×1012 J. In the polar zones occurs a conductive discharge in the form of a surface – the polar lighjts. In the Earth there are observed with frequency filaments in the longitudinal direction with a length of about 100 m. The polar shine has been observed in Jupiter and Saturn as well.
NebulaeIn many nebulae we observe helicoidal fibrous structures. Here we do not have a direct observation, which would confirm that they are indeed filaments with a Birkeland current, but there are indirect clues: the observation of the polarized, synchrotronic shine, which emerges only in the zones with magnetic fields, and the detection of high-energy particles manifestations, which can be accelerated precisely through a pinch structure. By plasma it can be considered even an ample range of neutral hydrogen atoms (region H I). Although the degree of ionization on these nebulae is of only 10−4, given their great size, even this concentration is enough for a marked collective behavior (the nebula reacts to local fields, electric and magnetic). . Extensive fibrous structures are observed, particularly within the remnants of the explosion of a supernova. From the shine reaching us from the Crab nebula is conjectured to have the presence of a magnetic field of about 16 nT.
GalaxiesThere are monitored filaments in the center of our Galaxy with a length of about 60 pc, which remind us of a twisted rope and therefore they have helicoidal structures. Probably it is also about plasma structures sustained by a magnetic field. The size of this field and the current are founded on assumptions over scarcely precise dimensional extrapolations. Also in radio galaxies, active galactic nuclei and in the jets expelled by quasars are observed ample fibrous structures. The same jets of the quasars are highly collimated hot plasmas.
Translation: Arturo Ortiz Tapia, 2005 |