A cluster reveals the inner workings of the Earth's cosmic particle accelerator

The new study used observations from two of the four spacecraft of the Cluster mission, which flew in tight formation under the shock of the Earth's bow before, sitting just 7 kilometers apart.

The data was collected on January 24, 2015 at a distance of 90,000 km from the Earth, about a quarter of the way from the moon, and reveals arc shock properties that were previously unknown due to the absence similar in situ measurements. .

When a supersonic flow meets an obstacle, a shock is formed. This is often seen in the universe around stars, supernova remnants, comets and planets – including ours. Shocks are known to be very efficient particle accelerators and potentially responsible for creating some of the most energetic particles in the universe.

Shock around the Earth, known as shock to the bow, is our first line of defense against particles from the cosmos and our nearest testbed to study the dynamics of plasma shocks . This is explained by the high and supersonic velocities of solar wind particles, which create a phenomenon that appears somewhat at the shock wave formed when an airplane crosses the speed threshold. sound.

The new study, published today in Progress of science, reveals the mechanisms involved when this shock transfers energy from one type to another.

"The shock of the Earth's arc is a laboratory of natural shock and ideal," says lead author Andrew Dimmock of the Swedish Institute of Space Physics in Uppsala, Sweden.

"Through missions like Cluster, we are able to place several spacecraft in and around it, spanning ranges from hundreds to just a few kilometers.

"This means that we can distinguish how the shock changes in space and time, which is crucial for characterizing a shock of this type."

There are several types of shocks, defined by the way in which they transfer kinetic energy into other types of energy. In the Earth's atmosphere, kinetic energy is transformed into heat during the collision of particles – but the vast distances at play during the shock to the arc of our planet mean that particle collisions do not occur. can not play such a role in the transfer of energy because they are too far apart from each other.

This type of shock is therefore called shock without collision. Such shocks can exist across a wide range of scales, ranging from millimeters to the size of a cluster of galaxies, and transfer energy via processes involving plasma waves and fields. electric and magnetic.

"In addition to being collision-free, the Earth's bow shock can also be non-stationary," adds Michael Balikhin, co-author of the University of Sheffield, UK.

"In a certain way, it behaves like a wave in the sea: when a wave approaches the beach, it seems to get bigger as the depth decreases, until it reaches the sea. it breaks – this is because the crest of the wave moves faster than the hollow, causing it to fold back and break.

"This type of" break "also occurs for plasma waves, although physics is a bit more complicated."

To study in detail the physical scales of initiation of this surge (something unknown to date), the researchers called for a special campaign in which two of the four Cluster probes were moved to an unprecedented separation of less than 7 km resolution data from the inside of the shock itself.

In analyzing the data, the team found that the magnetic field measurements obtained by the two Cluster satellites were very different. These direct evidence of the existence of small-scale magnetic field structures in the widest range of arc shock indicates that they are essential to facilitate the disruption of plasma waves, and thus the energy transfer, in this part of the magnetosphere.

With sizes of a few kilometers, similar to the scales at which the electrons rotate around the magnetic field lines, these structures are located in a particularly thin and variable part of the shock, where the properties of the constituent plasma and surrounding fields may change more drastic .

"This part of the bow thruster is known as the damping ramp and can reach a thickness of a few kilometers.This discovery was also based on Cluster data a few years ago. ", said co-author Philippe Escoubet, also at ESA. Project scientist for the Cluster mission.

Launched in 2000, the four Cluster satellites fly in formation around the Earth, making it the first space mission capable of studying in three dimensions the physical processes taking place in and in the immediate vicinity of the Earth's magnetic environment.

"This type of study really shows the importance of Cluster as a mission," adds Escoubet. "By performing incredible spacecraft separations – seven kilometers, as used in this study, and even smaller, up to three kilometers away -, Cluster allows us to probe the planet." Magnetic environment of our planet at the smallest scale ever achieved.

"This allows us to better understand the bow shock of the Earth and its action as a giant particle accelerator, key element of our knowledge of the high energy universe."

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More information:
Andrew P. Dimmock et al. Direct proof of non-stationary collisions without collisions in the plasmas of space, Progress of science (2019). DOI: 10.1126 / sciadv.aau9926