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Above the neutral atmosphere, space is filled with charged particles, which are tied to the Earth’s magnetic field. The particles come from two sources, the solar wind and the Earth’s upper atmosphere. Most of the solar wind particles are deflected by the Earth´s magnetic field, but some can penetrate into near-Earth space. The ionized layer of the upper atmosphere is continuously ejecting particles into space, which have low energies and are difficult to measure. We investigate the relative importance of the two charged particle sources for the dynamics of plasma processes in near-Earth space. In particular, we consider the effects of these sources in magnetic reconnection.
Magnetic reconnection allows initially separated plasma regions to become magnetically connected and mix, and converts magnetic energy to kinetic energy of charged particles. Magnetic reconnection is the main driver of geomagnetic activity in the near-Earth space, and is responsible for the release of energy that drives a variety of space weather effects. We highlight the fact that plasma from the ionized upper atmosphere contributes a significant part of the density in the key regions where magnetic reconnection is at work, and that this contribution is larger when the geomagnetic activity is high.
Main regions of the Earth’s magnetosphere. Ionospheric ions (light blue) escape and fill the outer magnetosphere until they exit the Earth space environment. Credit: Toledo-Redondo et al. (2021)
Thanks to MMS mission, combined with high-performance numerical modelling, we now understand much better how ionospheric ions modify the reconnection process at a microphysical level. Ionospheric ions circulating in the magnetosphere are accelerated at reconnection sites and constitute a significant sink of energy for the reconnection process. In addition, depending on the ion mass, initial energy, and where the ions are entrained in a reconnection site, different energization mechanisms, some of them more efficient than others, come into play.
We still understand relatively little about how these recent discoveries of the magnetic reconnection microphysics shape the magnetosphere system as a whole. The impact of cold ions is still an open field of research, as cold ions introduce a new length-scale and many plasma processes depend on the coupling between different scales.
There is yet another ionospheric population, which is even less understood: cold electrons. They also outflow from the ionosphere, and these are even harder to characterize than cold ions. Electrons play crucial roles on magnetic reconnection and wave generation in the magnetosphere. So far, because of the immense difficulty of observing these low-energy electrons, the effects of cold electrons remain largely unexplored.
References:
Toledo-Redondo, S., André, M., Aunai, N., Chappell, C. R., Dargent, J., Fuselier, S. A., et al. (2021). Impacts of ionospheric ions on magnetic reconnection and Earth’s magnetosphere dynamics. Reviews of Geophysics, 59, e2020RG000707. https://doi.org/10.1029/2020RG000707
Open Access: Toledo, S., M. André, N. Aunai, C.R. Chappell, J. Dargent, S.A. Fuselier, A. Glocer, D.B. Graham, S. Haaland, M. Hesse, L.M. Kistler, B. Lavraud, W. Li, T. E. Moore, P. Tenfjord, and S.K. Vines (2021), Hidden atmospheric particles sculpt near-Earth space environment, Eos, 102, https://doi.org/10.1029/2021EO163314
