Explosions at the edge of the magnetosphere
Today I will talk about a new result from THEMIS concerning explosions that naturally occur on the edge of the Earth’s magnetosphere. The blow-back from these explosions strikes the magnetosphere and the effects can be seen all the way down to the ground. I will begin with some background material to set the scene. I will then talk about the THEMIS observations in space, the THEMIS observations on the ground, and end with some comments about the importance of these observations.
The Earth’s magnetosphere is permanently bathed in the solar wind, a never-ending flow of plasma from the Sun’s atmosphere. Just like a boat moving on water, the magnetosphere also has a bow wave – usually called the bow shock. This is where the solar wind is slowed and deflected around the Earth.
Now so far as water is concerned, it is pretty featureless and easily flows around obstacles. But the solar wind is fundamentally different. This is because it contains a magnetic field. The magnetic field gives it structure, much like the grain in a piece of wood.
And just like wood grain, there are all sorts of features in the solar wind’s magnetic field – for example ‘knots’ and ’whorls’ - but what interests us today are the joins between different ‘pieces’ of solar wind. This is illustrated in our first figure. The figure shows the Earth, surrounded by the magnetosphere. The Sun is on the right hand side, and solar wind flows in from the right. The bow shock sits in front of the magnetosphere.
Now in one part of the solar wind, the magnetic field points away from the Earth, and in another part, it points towards the Earth. These two pieces are joined together, like two pieces of wood glued together, at a discontinuity. This discontinuity comes from the Sun and is carried along in the solar wind – I’ve shown a snapshot here, but it is moving towards the Earth with the solar wind.
Inside the discontinuity there is a concentration of electrical current. Now, if this current hits the bow shock in just the right way – or wrong way, depending on your point of view, it can create an explosion at the bow shock. The solar wind, traveling at nearly a million miles an hour, is stopped dead in its tracks and is heated ten-fold (as high as 10 million degrees).
These events were seen for the first time in the 1980s and were originally called ‘funnies’ because they were so strange. Now they have a more technical name – Hot Flow Anomaly, or for short an HFA.
Some progress has been made using older spacecraft data, and computer simulations, so we are not completely in the dark. Here, I am showing an example of a computer simulation which shows the temperature of the solar wind. The line corresponds to the discontinuity at the heart of the HFA, and the explosion – marked by the hot plasma – is localized where the bow shock and discontinuity meet.
However, they are still quite mysterious. Why do they get so hot? How do they stop the solar wind? And what is their overall impact on the magnetosphere?
THEMIS is the perfect tool for these problems. With the five spacecraft in a special configuration, on the 4th of July we flew through the heart of a HFA as it was being born. This gave us a snapshot of its interior, in unprecedented detail. In particular, we made the first simultaneous observations with five spacecraft, and this allowed us to examine its size and shape. The energy released in the explosion was about 10 gigajoules – equivalent to a July 4 fireworks display of about 100 million fireworks set off simultaneously! The data have given us new insight into the fundamental physical mechanisms that caused the heating.
Each time an HFA occurs, the blow-back from the explosion strikes the magnetosphere. Its effects are felt all the way to the ground. We used the THEMIS ground based observatories to look at what happened on the ground. We think the HFA generated a pressure pulse that struck the magnetosphere. This pressure pulse passed over the magnetosphere and caused a huge ripple in the magnetic field. We used the ground observatories to track the ripple across the magnetosphere. It traveled from the east to the west side of northern America in about three minutes – at a speed of 60 000 miles an hour. The image shows how each of the THEMIS ground stations saw the pulse as it propagated from left to right across north America.
To conclude, on July 4 2007, the THEMIS spacecraft flew through the heart of a Hot Flow Anomaly – an explosion at the edge of the magnetosphere. These explosions are naturally caused by electrical currents in the solar wind. The blow back from the explosion struck the magnetosphere, creating a pressure pulse that was picked up by the THEMIS ground based observatories. The unique combination of the THEMIS satellites and the ground based observatories allowed us to follow the event from the cradle to the grave, and has given us new insights into the basic physics that drive these and other magnetospheric events.
We are investigating fundamental processes, whose roots lie in some extremely complicated physics! Yet at the same time, these processes are important from a practical point of view, because they are the building blocks of space weather. In this way, THEMIS lies at the intersection of basic and applied science.
Visuals:
1. Cartoon showing what a HFA is
2. Simulation
3. Impact on the ground