Understanding the northern lights

Aurora Borealis

The Physics of Aurora Borealis

The solar wind consists of highly ionized electrons and protons emitted from our sun.[footnoteRef:2] When these charged subatomic particles interact with the Earth's magnetic field, it creates a spectacular light display called the Aurora Borealis in the northern hemisphere and Aurora Australis in the southern hemisphere. These displays of light are best understood using particle physics. The force on a charged particle (F) is equal to the charge (q) times velocity (v) times magnetic field strength (B), according to Lorentz force law, as long as the particle is moving parallel to the magnetic field. If the perpendicular and parallel components of the velocity vector are considered separately, the sine of the angle between the magnetic field strength and parallel component equal zero, therefore, the force acting on a charged particle is equal to q*vperp*B. The path of the particle, if it were visible to the naked eye, would be similar to a slinky toy pulled apart. In other words, the particle will move in the direction of the magnetic field but in a circular pattern due to vperp.[footnoteRef:3] The radius (r) of this circular pattern is equal to (m*vperp)/(q*B), where m is mass. [2: Walter Lewin. "Lecture 19: How do magicians levitate women?" Massachusetts Institute of Technology, Spring 2002, http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-19-how-do-magicians-levitate-women/, time 20:30. ] [3: Lewin, Lecture 19, time 22:15.]

When the solar winds reach the Earth's atmosphere the ionized particles become aligned with the magnetic lines of force surrounding the planet.[footnoteRef:4] The magnetic lines of force enter and exit the magnetic poles, which would bring the highly charged electrons and protons closest to the atmosphere near the poles. When these charge particles come into contract with the molecules forming the atmosphere, mainly oxygen and nitrogen, they become electronically excited. A return to the ground state results in the release of energy in the form of photons. The resulting spectacular light shows are mainly the product of oxygen returning to ground state, which appears green or red at lower or higher altitudes, respectively. By comparison, nitrogen returning to ground state will appear pink and red at lower altitudes. [4: R. Nave. "Aurora" Department of Physics and Astronomy, Georgia State University, n.d., http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/aurora.html. ]

The scientific advances required to reach this understanding of the Aurorae occurred at the beginning of the 20th century.[footnoteRef:5] Within the fields of astronomy and physics, significant progress had been made in developing spectrophotometers and spectroscopes capable of studying the emission spectra of excited gases. In 1869, Anders Jonas Angstrom, used a spectroscope to examine the aurora and discovered it was monochromatic.[footnoteRef:6] Over the subsequent decades the aurora emission were documented with greater accuracy, but the source of the green/yellow "aurora line" (green 5577 A)[footnoteRef:7] remained controversial. [5: Helge Kragh. "The Spectrum of the Aurora Borealis: From Enigma to Laboratory Science." Historical Studies in the Natural Sciences 39, no. 4 (2009): 381.] [6: Ibid, 382.] [7: Ibid, 384.]

A Norwegian physicist, Kristian Birkland, theorized that charged particles emitted from the sun were interacting with the Earth's magnetic field. He was able to provide empirical support for his thesis by directing a cathode ray (electron beam) towards a magnetized sphere in his laboratory.[footnoteRef:8] In 1924, Norwegian physicist Lars Vergard was able to create a thin layer of nitrogen dust using cyrogenics and bombard it with a high-energy cathode ray.[footnoteRef:9] This experiment produced several emission spectra similar to those produced by the aurora, but nothing happened when the same dust was bombarded with protons. This led to the hypothesis that nitrogen was responsible for the green aurora line. [8: Ibid, 386.] [9: Ibid, 393-396.]