FG: The Impact of the Cold Plasma in Magnetospheric Physics
Co-chairs:
Gian Luca Delzanno (Los Alamos National Laboratory), Natalia Buzulukova (Goddard Space Flight Center), Barbara Giles (Goddard Space Flight Center), Roger Varney (SRI International), Joe Borovsky (Space Science Institute).
Research area:
The cold plasma impacts all the research areas of GEM but a significant focus will be on ‘Inner Magnetosphere (IMAG)’ because of the critical role of waves and wave-particle interactions in magnetospheric dynamics.
Description:
The cold-particle populations that exist in the magnetosphere (cf. Table 1) include (1) plasmaspheric ions (including the plume), (2) plasmaspheric electrons (including the plume), (3) cloak ions (including the oxygen torus), (4) cloak electrons, (5) outflowing cold electrons, (6) outflowing cold ions and (7) charge-exchange-byproduct cold protons (CHEX protons). Outflowing (from the ionosphere) cold electrons are anticipated for the maintenance of charge neutrality in the magnetosphere: two places where they should occur are (a) in the post-midnight to dawn region where the electron plasma sheet precipitates away to make diffuse aurora and (b) at the inner edge of the electron plasma sheet where (owing to gradient-curvature drift effects) the ion plasma sheet flows radially Earthward while the electron plasma sheet flows eastward. There are also plasmaspheric-refilling cold-ion and cold-electron outflows into open-drift-trajectory flux tubes on the dayside.
While the hot (ring current/plasma sheet) and energetic (radiation belts) populations have received a lot of attention because of their potential harm to space infrastructure, the cold-plasma populations are the least studied so that in some cases they have been referred to as the ‘hidden populations’. This is because (1) spacecraft are almost always charged to values that make measuring the cold-ion properties very difficult and (2) spacecraft surfaces exposed to sunlight and bombarded by energetic particles emit copious amounts of low-energy secondary and photoelectrons that blind the measurement of magnetospheric cold electrons.
Yet, the cold ~eV plasma plays multiple roles in magnetospheric dynamics (see Table 1). All of the magnetosphere’s cold ions flow to the dayside magnetopause, where the cold ions can reduce solar-wind/magnetosphere coupling by mass-loading dayside reconnection. The presence of cold cloak and CHEX ions when magnetospheric convection slows down can increase the early-time refilling rate of the plasmasphere. Cold ions and cold electrons can affect waves and wave-particle interactions by changing (1) the resonant conditions between particles and waves, (2) the wave growth rates, (3) the saturation level of the waves, and (4) wave-particle diffusion coefficients, all with strong implications on the dynamics of plasma sheet, ring current, and radiation belts. The low-energy oxygen of the cloak drastically changes ULF frequencies, which impacts the radial diffusion of energetic populations. Cold plasma has repeatedly been implicated for the spatial structuring of diffuse and pulsating aurora. See Table 1 for some connections between cold plasma populations and known impacts in magnetospheric physics.
We cannot understand the full complexity of the magnetospheric system until we can (1) reliably measure the full properties of the cold ions and electrons, (2) learn what controls these properties, and (3) learn all of the impacts of the cold populations. We also need to include all of the related couplings into global models. This FG is proposed to advance our understanding of the impact of the cold particle populations in magnetospheric physics, from both theoretical/modeling and observational perspectives. The cold plasma represents a clear outstanding issue that needs to be addressed to make progress towards a full understanding of magnetospheric dynamics.
