Kristian Birkeland's pioneering investigations of geomagnetic disturbances

More than 100 years ago Kristian Birkeland (1967–1917) addressed questions that had vexed scientists for centuries. Why do auroras appear overhead while the Earth's magnetic field is disturbed? Are magnetic storms on Earth related to disturbances on the Sun? To answer these questions Birkeland devised terrella simulations, led coordinated campaigns in the Arctic wilderness, and then interpreted his results in the light of Maxwell's synthesis of laws governing electricity and magnetism. After analyzing thousands of magnetograms, he divided disturbances into 3 categories: 1. Polar elementary storms are auroral-latitude disturbances now called substorms. 2. Equatorial perturbations correspond to initial and main phases of magnetic storms. 3. Cyclo-median perturbations reflect enhanced solar-quiet currents on the dayside. He published the first two-cell pattern of electric currents in Earth's upper atmosphere, nearly 30 years before the ionosphere was identified as a separate entity. Birkeland's most enduring contribution toward understanding geomagnetic disturbances flowed from his recognition that field-aligned currents must connect the upper atmosphere with generators in distant space. The existence of field-aligned currents was vigorously debated among scientists for more than 50 years. Birkeland's conjecture profoundly affects present-day understanding of auroral phenomena and global electrodynamics. In 1896, four years after Lord Kelvin rejected suggestions that matter passes between the Sun and Earth, and two years before the electron was discovered, Birkeland proposed current carriers are "electric corpuscles from the Sun" and "the auroras are formed by corpuscular rays drawn in from space, and coming from the Sun". It can be reasonably argued that the year 1896 marks the founding of space plasma physics. Many of Birkeland's insights were rooted in observations made during his terrella experiments, the first attempts to simulate cosmic phenomena within a laboratory. Birkeland's ideas were often misinterpreted or dismissed, but were verified when technology advances allowed instrumented spacecraft to fly in space above the ionosphere.

Latitudinally propagating on–off switching aurorae and associated geomagnetic pulsations: A case study of an event of February 20, 1980

Poleward propagating on–off switching aurorae and equatorward propagating aurorae, otherwise similar, were observed simultaneously at Rabbit Lake and La Ronge, respectively, for about 40 min before dawn of Feb. 20, 1980. Rabbit Lake is a high auroral latitude site, at the northern end of the Saskatchewan chain of stations for the Pulsating Aurora Campaign, whereas La Ronge, due south of Rabbit, is almost at the southern edge of the auroral zone. The repetition periods of the on–off switching aurorae are about 6 to 13 s. The poleward propagating aurorae had well defined fronts of light which extended a few hundred kilometres or more in the east–west direction. The light fronts of the equatorward propagating aurorae, though comparable in extent, were less well defined: they were thicker and fuzzier. The poleward propagating aurorae moved with a speed ~10 km/s whereas the equatorward ones did so with a slightly greater velocity. Geomagnetic field fluctuations were concurrent with the aurorae at both sites. At Rabbit Lake, northward (southward) field changes were associated with upward (downward) changes whereas the trend is reversed at La Ronge, viz., northward (southward) changes with downward (upward) changes. These trends are consistent with a model of a periodic occurrence of two line currents, westward and eastward, the former moving poleward north of Rabbit Lake and the latter approaching La Ronge from the north.

Electron Precipitation at an Auroral Latitude – Saskatoon, L = 4.4. I. Photometer, Magnetometer, and Radiowave Data

A study of the quiet and disturbed lower ionosphere (60–100 km) near solar maximum years (1970–1971) has been carried out at Saskatoon (52 °N, 106 °W, L = 4.4) using a variety of techniques: a 5577 Å photometer, a magnetometer, a 'partial reflection radiowave system' (2.2 MHz), and an all-sky camera. Good correspondences between local and planetary magnetic disturbances and the green line intensity I(5577 Å) have been found. Radiowave data, ordered in terms of I(5577 Å), have been used in seasonal epoch analyses for the four seasons. It is shown that after the photometric maxima near geomagnetic midnight, ionospheric disturbances below 80 km continue to increase in magnitude towards sunrise. They are evident until at least noon the following day. For a given level of I(5577 Å), the ionospheric disturbance (< 90 km) is largest during the night hours in summer and winter; and after sunrise, largest in winter and fall months. There is good general correspondence between these results, and fluxes of precipitated electrons measured by satellite techniques.