Aurora and the Earth’s Magnetotail Part 2: From Birkeland to Cluster

On 21 December 1833 "The Penny Magazine" had a drawing of the aurora on its front page. The corresponding article reads "The Aurora Borealis is a beautifully luminous meteor, appearing in the form of streams of light, rays, arches and crowns. A description of this splendid phenomenon, which enlivens the long darkness of the Arctic regions, has been given by Mr. A. De Chapell Brooke, in his 'Winter in Lapland' to which work we are indebted for the subject of our cut."

As per the end of the last post, we will start this story in 1963.

A Rocket in the Aurora

Magnetic field measuremens of
the US Navy rocket 1963-38C
showing the strong transverse
field fluctuations

After the start of the space age, basically starting with the launch of Sputnik on 4 October 1957, the investigation of the upper layers of our atmosphere also started to be done with so-called sounding rockets. The US Navy rocket 1963-38C was launched up into the ionosphere (100 - 600 km above Earth's surface) and it was equipped with magnetometers. When the rocket crossed the auroral regions strong transverse (i.e. perpendicular to the background field) magnetic field fluctuations were observed. This is a clear indication of the presence of magnetic field aligned currents (although it was first thought that they were hydromagnetic waves), the ones that Birkeland proposed to exist in the auroral regions. However, when the finding was published, there was no mention of Birkeland or of the currents. But as space missions continued, more and more evidence for these field aligned currents was gathered. Nowadays these are called Birkeland currents.

What is the source?

A simple model of the Earth's
magnetosphere, with the solar wind
(orange) interacting with the Earth's
magnetic field

To understand the generation of these Birkeland currents we have to understand how the solar wind interacts with the Earth's magnetosphere. The solar wind consists of a hot ionized gas (or plasma) which flows out of the Sun and embedded in the flow is the solar magnetic field. Due to the radial outflow of the plasma and the rotation of the Sun, the magnetic field gets rolled up around the Sun on its way outward, which creates the so-called Parker spiral. The solar wind interacts with the Earth's magnetic field in various ways, depending on the direction of the solar wind magnetic field. If the field is pointing up (i.e. northward) then the solar wind just compresses the Earth's magnetic field, and (keeping it simple) things remain quiet. However, if the field is pointing southward, it has the opposite direction as the Earth's magnetic field at the front, and then things happen, but let's first take a quick look behind the Earth.

At the front of the Earth the magnetic field is compressed by the solar wind. At the back of the Earth, however, the magnetic field gets pulled along with the solar wind and is stretched into a long tail. This stretched magnetic field, like a stretched rubber band, can store a lot of energy that can be released after a certain trigger is pulled.

The simplest version of magnetic
reconnection, field lines are pushed
together in the middle, and at the
central part (the X-point) new
connections are made and the tension
of the fieldmoves the field lines away from
the X-point

When the solar wind turns southward, the solar wind magnetic field connects itself to the Earth's magnetic field, opening up the magnetosphere at the front. This process is called magnetic reconnection. The continuous flow of the solar wind pulls along the magnetic field, and thus at the front the field is stripped off, and transported to the back of the Earth, where it is added to the magnetotail. One can, however, not keep on adding magnetic field to the magnetotail without end, and something drastic happens. In the centre of the tail, where the magnetic field is oppositely directed in the northern and southern part, the field gets squeezed together enough to start the same process as at the front, reconnection. The stretched magnetic field in the tail explosively reconnects and shoots the plasma towards the Earth and away from it, depending on which side of the reconnection point you are watching.

The "complete" solar wind - magnetosphere
interaction

This reconfiguration of the magnetotail, from very stretched and full of stored energy, to a less stretched lower energy shape, brings along the generation of currents, as with every temporal change of a magnetic field (e.g. a bicycle dynamo where a rotating magnet generates currents in a wire coil, which then let the light bulb shine). These (Birkeland) currents and highly accelerated particles flowing along the magnetic field towards the Earth create the aurora through their interaction with the Earth's atmosphere.

Exhibition models of the four
Cluster spacecraft

In 2000 the four-spacecraft Cluster II mission was launched. Four spacecraft that would be flying in a tetrahedron shape in regions of interest of the Earth's magnetosphere. All four spacecraft carry the same instruments and thus there are simultaneous measurements  of various quantities like magnetic field strength, plasma properties, etc. at four different points. Due to the specific configuration of the spacecraft this helps understanding the processed that are taking place in the magnetosphere, because we can now find out whether signals that are observed are caused by spatial of temporal structures.

Magnetic field measurements by
Cluster at three different locations
around a reconnection region and
an artist's impression of how the
spacecraft moved though this region.

This mission was used to make measurements in the magnetotail and in October 2001 an event was measured which showed clear signatures of this reconnection process [Runov et al., Current sheet structure near magnetic X-line observed by Cluster, 2003] . The spacecraft flew through the region where the field lines get together and reconnect and flow out again, like the moving image above. All the parameters that were measured were in agreement with the picture of magnetic reconnection. Cluster started at point A, where the plasma moved towards the tail region (away from the Earth) and then crossed a region where the field gets together (B) and then entered a region where the plasma moved towards the Earth (D). The direction of the magnetic field also agreed with the picture above and the extra magnetic fields that should be present because of the extra currents that flow in the reconnection region were also measured.

So now we have found the driver for the energetic processes which generate the aurora. In the next part we will take a look closer to Earth again, the region just above the aurora. We will try to follow the processes along the magnetotail with the THEMIS mission. And we will take a closer look at how the Earth's magnetosphere reacts at the dayside to changes in the solar wind magnetic field direction.