Overview

Target Description

Low-energy (< 1 keV) ions and electrons play significant roles in the magnetospheric system. While this plasma often dominates the number density and mass density of the magnetosphere, it is amongst the least characterized and understood plasma populations, particularly in the very cold < 20 eV energy range, which has always been challenging to measure. Our knowledge of the composition and distribution function characteristics of this cold population is still limited, yet it is this population that plays a crucial controlling role in many magnetospheric processes - from the generation, amplification, damping and propagation of plasma waves that reside in the magnetosphere; to the wave-particle interaction processes that couple between other plasma populations: the ring current, the electron radiation belt, substorm-injected electrons, plasma-sheet ions, and plasma-sheet electrons. Mass-loading effects of this population can also alter dayside reconnection and thus influence solar-wind/magnetosphere coupling.

This FST targets two important plasma populations: the cold plasmasphere and its drainage plume; and the plasma cloak. Our knowledge of the refilling rates for the plasmasphere is still insufficient, while there are basic questions on the plasma cloak's source - where, when, and how much?

Both these populations may play an important role in the recirculation of the plasma into the magnetotail. There are outstanding questions on the recirculation pathways, and the resulting composition, spatial extend and density of these populations along the magnetopause boundary.

Further details on the composition and distribution function of the low-energy and very cold plasma would also allow new investigations on the role these populations play on other magnetospheric processes, including heating, loss and transport of this population itself.

This FST addresses the Decadal Survey Key Science Goal 2 (Determine the dynamics and coupling of Earth's magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs). This FST further addresses two Key Questions for Future Study in the 2013 Decadal Survey Consensus Report: "How are plasmas produced, lost, and energized in the magnetosphere?" and "How does the ionosphere-thermosphere system regulate the flow of solar energy throughout geospace?" and several LWS Strategic Science Areas including SSA-IV (Variability of the Geomagnetic Environment) and SSA-V (Dynamics of the Global Ionosphere and Plasmasphere).

Goals, Objectives, and Measures of Success

The main goal for this FST is to make significant progress towards understanding and predicting the complex feedback between ionospheric outflows and magnetospheric plasma on the coupling of the solar wind to the system. To address this goal, proposed investigations should include one or more of the following objectives:

• Provide a better understanding of plasma sources in the ionosphere: the refilling of the plasmasphere, and the origin of the warm plasma cloak; and to understand the factors controlling these sources;
• Investigate the evolution of the plasmasphere and plasma cloak and the impact that it has on the magnetospheric system;
• Determine the recirculation of the low-energy plasmas from the dipolar region into the magnetotail, and the impact this may have on the magnetospheric system;
• Determine the properties and controlling factors of the low-energy electron and ion populations on downstream magnetospheric processes.

Compared to the modeling of higher energy populations in the ring current and radiation belts the characterization, physics and dynamics of the cold plasma population requires further progress. Therefore, this FST specifically focuses on enhancing our understanding of the sources and distribution of this population in the magnetospheric system, and processes controlled by the cold plasma within the framework of LWS needs.

Measures of success for this FST include, but are not limited to studies that demonstrate:

• Understanding of cold plasma sources;
• Improved understanding of the impact of cold plasma on magnetospheric processes and the magnetospheric system; and
• Predictive capability of temporal, spatial, and spectral characteristics of the cold plasma population by using observations and existing models.

Magnetospheric research has seen a recent boon of observations (e.g., Van Allen Probes, THEMIS, Los Alamos Magnetospheric Plasma Analyzer (LANL-MPA), and MMS) and historic ones (e.g., FAST, Cluster, and Dynamics Explorer) that have greatly expanded the availability of high-quality data. First-principles and empirical models have also benefited from this unprecedented wealth of information against which their performance can be measured and improved.

Types of Investigations

In general, studies could include investigations that target the physical understanding of plasma processes within, and affected by, the cold population; coordinated data and modeling studies between ionospheric, thermospheric, and magnetospheric researchers to understand plasma sources and their global evolution; and development of empirical/ machine-learning models that can supplement the development of - and serve as comparison basis for - first-principle models for these populations.

These investigations include, but are not limited to the following types of studies:

• Efforts to improve models utilizing first-principle plasmasphere-refilling simulation capabilities, and efforts to test these simulations with magnetospheric spacecraft observations;
• Efforts to improve the characterization of cold plasma composition and distributions functions, through either data or modeling studies;
• Surveys of low-altitude and equatorial spacecraft measurements from multiple missions in coordination with global magnetospheric modeling; and
• Efforts to advance the self-consistent modeling of cold plasma processes and processes controlled by the cold plasma.

Investigations within this FST may include theoretical, numerical, and data analysis methods. Relevant observational sources for these studies include present-epoch spacecraft and ground-based observations, as well as historical records of proxy observations of the cold plasma population.