Substructures in Earth's bow shock
Using data from ESA's Cluster mission, scientists have revealed the mechanisms at play when our planet's bow shock – its 'shield' against cosmic radiation and an efficient particle accelerator – transfers energy throughout space.
The researchers found direct evidence of small-scale structures within Earth's magnetic field that are key in helping waves of plasma to 'break'. These waves behave somewhat like a wave in the sea: as a wave approaches the beach, it seems to grow in size as the depth decreases, until it breaks. This is because the crest of the wave moves faster than the trough, causing it to fold over and break. This kind of 'breaking' also occurs – albeit in a more complex way – for waves of plasma, and redistributes energy in the process.
This graph portrays data on Earth's magnetic field strength obtained by two of Cluster's four spacecraft – one plotted in red (Cluster 3) and one in black (Cluster 4). The data were gathered on 24 January 2015 within a particularly thin and variable part of the shock known as the shock ramp. The two spacecraft were sitting just 7 kilometres apart, taking in-situ measurements at a never-before-achieved close separation.
Differences in the two sets of measurements provide evidence that small-scale magnetic field structures exist within the broader extent of the bow shock, within a particularly thin and variable part of the shock known as the shock ramp. These small structures are key in facilitating the breaking of plasma waves within the shock, and thus the transfer of energy, in this portion of the Earth's magnetic environment.