Friday, May 31, 2013

Aurora and the Earth’s Magnetotail Part 1: From History to Birkeland

On 1 August 2010 there was a huge explosion on the Sun, sending out a large cloud of plasma and magnetic fields (a so called Coronal Mass Ejection or CME) which hit the Earth on 3 August, creating majestic aurorae (great pictures of aurorae by Emil Kepko) even at low latitudes. What happened here? Before we answer that, we will first take a little trip through history and look how the study of aurorae and modern space physics developed. 


Anders Celcius
In  1716 there was large scale auroral activity over Europe, which fascinated the natural scientists of that time, like Anders Celcius. Naturally, due to their geographical location the Scandinavians have had a leading role in the study of the northern lights. Together with Olof Hiorter Celcius decided to look at how a compass needle behaves, and therefore they wrote down the compass needle reading every hour for a whole year. What they found was that every time there was auroral activity the needle of the compass would change direction and wiggle. The stronger the aurora, the more the compass needle moved, and thus is was decided that aurora is “magnetic in nature,”  however a model for how this works exactly could not be given by these two researchers. Alexander von Humboldt would later call this phenomenon a “magnetic storm” (a term still used in modern space physics). These people were on the right track.

A solar pillar
However, there were also other proposed mechanisms that were, as we now know, incorrect. The director of the observatory of Vienna’s university, Father Maximilian Hell studied the northern lights between 1767 and 1770, for which he moved to Vardø in the Barents Sea. Although Hell did bring a compass with him, he did not observe any motion of the needle when there was activity and in the end he concluded that the aurora was created by the light of the sun and moon, reflected and refracted by frozen water vapour in the atmosphere. Although this frozen vapour does create interesting effects like “solar pillars,” it is not an explanation for aurorae. 

Start of space physics 


Let’s make a jump in time and move towards the twentieth century, with a short stop in 1859. Richard Carrington observed the Sun and made drawings of the sunspots. On 1 September 1859 he noticed a large group of sunspots with bright spots turning into a connecting bright ribbon. This bright ribbon, or solar flare, would create the largest auroral activity in modern times on 2 September of that year: the so-called "Carringtong Event." The northern lights were even seen in the Caribic, telegraph lines stopped working and fire broke out in the stations. Now a solar connection was found: something happened on the Sun and a day later the Earth reacted. This event started the study of the Sun-Earth connection (see “The Sun Kings” by Stuart Clark for the full story) and with that modern space physics.

The idea that there could be a connection between the Earth and the Sun was strange, the distance between the two being so vast. But somehow, when something happens on the Sun the Earth’s magnetic field responds to it. William Herschel e.g. showed that the magnetic disturbances at Earth walked in lockstep with the number of sunspots. 

Kristian Birkeland 

The scientific report of Birkeland's
auroral investigation
Moving back to Scandinavia, to Norway to be specific, we find professor Kristian Birkeland, who was famous for inventing a method to produce saltpeter and for building a hydroelectric plant. This work he did only to finance his studies of the aurora. His was the first extensive study of the aurora, using magnetic field measurements at various locations during "the Norwegian Aurora Polaris Expedition 1902 - 1903." He sent his students and employees to the far north to record magnetic measurements during times of aurora, however he also went himself. Being well aware of Maxwell’s theory of electromagnetism, he knew that if there are deviations in the Earth’s magnetic field, they have to be related to electrical currents. But how exactly does the Sun generate currents in the Earth’s magnetic field?

Birkeland in his laboratory
with the Terrella in action
In order to study the Earth’s magnetic field (and its possible connection to the Sun) Birkeland build a so-called “Terrella” (small Earth) in the laboratory. This terrella consisted of a metal sphere with an electromagnetic inside. This was placed inside a vacuum chamber, the metal sphere was acting as an anode and electrons were emitted from a cathode inside the chamber. When Birkeland turned on the machine, he observed that there were bright rings around the magnetic poles, similar to aurorae. This was an indication for the electic nature of the northern lights.

Based on his experiments with the terrella Birkeland concluded that the aurorae are somehow formed by “solar rays,” particle emissions from the Sun and that: “From a physical point of view it is most probable that solar rays are neither exclusively negative nor positive rays, but of both kinds.

In other words (as we would say now): the solar wind consists of both negative electrons and positive ions. A bold statement from Birkeland without actual observations of the solar wind.

The solar rays enter into the Earth’s magnetic field; generate the aurora through interaction with the atmosphere; and the magnetic field disturbances through electric currents. How this all worked in detail he could not say, but it seemed to him the most logical explanation.

Unfortunately, the most influential magnetospheric scientist of that time, Sidney Chapman, was not in favour of Birkeland’s hypothesis on the generation of the aurora, and through his influence blocked these ideas from the scientific world. 

Birkeland died in 1917, his ideas about currents in the auroral regions could not be tested by satellite observations until 1963. To read all about Birkeland’s interesting scientific life take a look in “Northern Lights” by Lucy Jago.

In part 2 "From Birkeland to THEMIS"  we will move to 1963 and the space age: did Birkeland get it right?

This story is also available as a lecture:
"Aurora and the Earth's magnetotail: From Birkeland to THEMIS"

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