Towards understanding how near-Earth space erupts

One of the greatest mysteries of near-Earth space physics is the eruption that occurs on the nightside of our magnetic domain, the magnetosphere. These eruptions have a silly name – substorms – because once upon a time many individual substorms were thought to comprise a larger disturbance.
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Towards understanding how near-Earth space erupts

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Substorms cause sudden brightenings of the aurora. The largest Geomagnetically Induced Currents (GICs), which may disrupt power grids, are invariably connected to large substorms. Substorm-related injections bring particles to the Earth’s radiation belts, where they are locally accelerated to high energies, causing satellite anomalies. Substorms also occur on other planets that have a magnetosphere, and the process is analogous to Solar eruptions.

During a substorm (see Fig. 1), a large cloud of plasma, called a plasmoid, is torn away from our magnetosphere and ejected into space. From the 1960s onwards, people have debated how the eruption occurs: i.e., what is “the process” that ejects the plasmoid? This debate was one of the biggest reasons I initiated and led the development of our near-Earth space simulator, Vlasiator. It took more than ten years of software development, and finally we are in a position to start discussing these plasmoids.

Figure 1:
A schematic figure of the classical substorm sequence unearthed from my PhD thesis dating almost exactly 20 years prior to writing this blog. a) Earth’s magnetic field is depicted from the side, with the Sun to the left, and the magnetospheric tail on the nightside on the right. b) The tail grows as a response to energy input from the dayside, and a process begins to sever a part of the tail. c) A large plasma cloud called a plasmoid is ejected, and d) the system returns to its ground state. The satellite in the tail observes variations of the magnetic field X and Z components in time (the panels in the right), and the ground magnetic field variations are constructed to an index called AE, showing the substorm sequence in time.

Early on, the debate polarised into two seemingly contradictory physical concepts based on magnetic reconnection and plasma instabilities. Magnetic reconnection (see schematics from Fig. 2) occurs between oppositely directed magnetic domains, leading to plasma mixing, energy transfer, and the change of magnetic topology. An instability, in turn, is a process through which small perturbations grow into oscillations, which may eventually disrupt the plasma system.

Figure 2: Simplified concepts of reconnection (a) and plasma stability (b, adapted from Wikipedia). a) Two separate plasma systems that are marked with blue and red magnetic fields come into contact, and their respective plasmas (coloured pearls) mix as the magnetic topology changes (purple field). b) Unstable plasmas have free energy, depicted on the right by a ball sitting on top of a hill. If something perturbs the seemingly stable system, the ball drops from the hill, disrupting the initial configuration.

Modelling both the reconnection and plasma instabilities requires very sophisticated modelling tools, which cover the domain where substorms occur – i.e., the entire solar wind – magnetosphere – ionosphere system. This is a highly ambitious challenge not only because of the sheer volume to be modelled. Additional complexity arises from the fact that only reconnection has been addressed by earlier-generation global modelling tools. However, these simulations, based on the fluid description of plasmas, cannot fully describe the instabilities.

Vlasiator is a newly completed simulation that not only describes the vast space where plasmoids occur, but also uses a beyond-fluid plasma description that reproduces both reconnection and instabilities in detail. This makes it possible – for the first time – to investigate both phenomena in the same simulation box.

We carried out a simulation run (see Fig. 3) that reproduced the large-scale plasmoid in a similar manner as they are seen in satellite observations. Our paper concerns the physics behind the launch of the plasmoid, and I can finally – and to my joy – say that in our opinion, both debating camps are both right and wrong at the same time.

Figure 3: Overall view of the Vlasiator first 6D global run. The solar wind comes to the simulation box from the right, interacting with a flow obstacle formed by our magnetosphere. The interaction drags the nightside of the magnetosphere into a long tail, where the plasmoids are ejected. The figure shows in the tail the current sheet surface, and the flapping waves which are  connected to the process of ejecting the plasmoid. (Credit: Markku Alho).

Our simulation produces a similar sequence supporting the reconnection-based theory – we see reconnection in the flank of the magnetospheric tail, then disruption of the current sheet that supports the tail, and the launch of the plasmoid. However, unlike what the reconnection-based theory supposes, the disruption is not caused by the fast plasma jets launched by reconnection (see Fig. 2, the black arrows).

On the other hand, similarly as the instability-based camp argues, we see a current disruption due to a kinetic instability in the centre of the tail, propagating tailward before the plasmoid is ejected. However, the current disruption originally requires reconnection-generated ions that perturb the system, leading to growing flapping waves (see Fig. 3) that steepen to disrupt the current. The current disruption does not cause any new reconnection but only helps to release the plasmoid from the already-existing reconnection sites.

Why was this not seen before? This is mainly because we only have good in situ coverage by satellites on one meridian plane of the vast magnetospheric tail. In our simulations, the instability and reconnection both develop at a larger volume, and there is a considerable amount of dawn-dusk directed information flow that is not currently measured at all.

However, now that we know what to look for, maybe we can go through past satellite events to see if some of the different missions might have been in the right place at the right time. Maybe we can also help the new mission designers to build a constellation that will finally put an end to the discussion and find out why our magnetosphere recurrently burps plasma clouds into space.

There’s room for improvement, too. The current simulation did not have an ionosphere connected to the solar wind – magnetosphere system. The lack of the ionosphere rules out detailed comparison to the full substorm process, because at the time of the plasmoid release, some of the tail current diverts through to the ionosphere, producing spectacular auroral displays. We cannot yet model that with this version of Vlasiator.

However, I can reveal that we already have a run coming out with the ionosphere plugged in. The results look amazing – while the same conclusions remain, the ionosphere plays a significant role in the process – but more on that in upcoming papers!

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