Sky & Telescope May 1985 [antikvár]

Birkeland and the Electromagnetic CosmologyAnthony L. Peratt, Los Alamos National LaboratoryThe history of science is every where replete with controversies, per sonal prejudices, and fierce animosities be tween camps holding conflicting theoretical views. In astrophysics, the iconoclastic efforts of Copernicus, Galileo, Subrahman yan Chandrasekhar, Fred Hoyle, and Hal ton Arp immediately spring to mind.Lesser known among the disputes in as trophysics is the half-century-long contro versy between the advocates of Kristian Birkeland. who suggested that electron streams following the Earth's magnetic-field lines were responsible for auroras, and Sydney Chapman, who maintained that the Earth was surrounded by "vacuum." It was not until 1974, three-quarters of a century after the theories were proposed, that space-probe measurements decided the controversy in Birkeland's favor.Today, plasma physicists believe that many of the phenomena in the cosmos can be explained in terms of a spaghetti of "Birkeland currents" (electrical currents flowing along magnetic lines of force; see Table I), and that "pinched" Birkeland currents may be the mechanism responsible for initiating the gravitational collapse of matter in the plasma state. However, it is not generally known that Birkeland, whom many regard as the founder of experimental astrophysics, had himself developed an extensive cosmological theory based upon his experiments.Kristian Olaf Bernhard Birkeland (1867-1917) received his education in Bonn, Geneva, and Leipzig, studying under such notables as Henri Poincari and Heinrich Hertz, and was appointed a professor at the University of Oslo in 1898, when he was 31. His remarkable achievements in technology and applied physics made him rich and famous (see Table II) and financed his auroral investigations.The main point of his auroral theory was that electrically charged particles ejected from sunspots are captured by the Earth's magnetic field and directed along the field's lines into the polar regions. Because the particles are electrically charged, they can be deflected by the Earth's magnetic field in such a way that they arrive on the night side of the planet. As the incoming particles reach the upper atmosphere they are slowed down by the increasing density of atoms and molecules, and in the process the atmospheric constituents become excited and ionized. Birkeland performed extensive laboratory experiments to illustrate his points.It was amazing that he could actually demonstrate the new theory. His basic idea was to study the motions of electrons in a magnetic dipole field where the air density is low. The experiment, labeled a "terrella," was a model of the conditions in Earth's upper atmosphere. In many ways it reminds us today of how a televi-Birkeland, the founder of experimental astrophysics, is shown here with his assistent, K. Devik, and his "terrella," a magnetized metellic globe representing the Earth.sion picture is produced by energetic electrons striking a phosphor screen {S&T: December, 1982, page 534). At the turn of the century, Birkeland's work was considered to be state-of-the-art physics.The terrella model was really rather simple. He placed a sphere containing an electromagnet inside a large vacuum chamber, which represented the space around the Earth and its magnetic field. He then shot clouds of electrons toward this simulated Earth to produce a light phenomenon that looked like the aurora. (We nowTABLE IPhenomena in which Birkeland currents are thought to play a role:In situ observationsAuroral rays, arcs, and draperies Auroral eiectrojet Magnetospheric inverted V events "Flux ropes" in ionosphere of Venus Observations not accessible to in situ measurementsSolar prominences, spicules, coronal streamers, and polar plumes Cometary tails {in situ. 1985) Interstellar medium (e.g., Veil nebula) Interstellar clouds (e.g Lagoon nebula. Orion nebula) Plasma within the galactic center Lobes of double radio galaxies and galactic "jets"TABLE 11 Birkeland's other contributions to science and technology:Derived the general expression for the Poynting vectorGave the first general solution to Maxwell's equationsPioneered the field of charged-particle beamsUtilized the concept of "longitudinal-mass"Constructed the first foil diodes Pioneered the field of visible-light photography of electrical dischargesAdvocated charged-particle propulsion engines for space travelCreated Norsk Hydro's nitrogen-fertilizer industry (the Birkeland-Eyde method for production of potassium nitrate)Invented an electromagnetic rail gun capable of firing a 10-kg projectile Established Birkeland's Firearms Anticipated cosmic rays (discovered in 1911) with his calculations involving energies of several billion electron voltsHeld patents on the electromagnetic cannon, electric blankets, solid margarine, and hearing aids