Earth's ambipolar electrostatic field and its role in ion escape to space

Cold plasma of ionospheric origin has recently been found to be a much larger contributor to the magnetosphere of Earth than expected1-3. Numerous competing mechanisms have been postulated to drive ion escape to space, including heating and acceleration by wave-particle interactions4 and a global electrostatic field between the ionosphere and space (called the ambipolar or polarization field)5,6. Observations of heated O+ ions in the magnetosphere are consistent with resonant wave-particle interactions7. By contrast, observations of cold supersonic H+ flowing out of the polar ionosphere8,9 (called the polar wind) suggest the presence of an electrostatic field. Here we report the existence of a +0.55 ± 0.09 V electric potential drop between 250 km and 768 km from a planetary electrostatic field (E∥⊕ = 1.09 ± 0.17 μV m-1) generated exclusively by the outward pressure of ionospheric electrons. We experimentally demonstrate that the ambipolar field of Earth controls the structure of the polar ionosphere, boosting the scale height by 271%. We infer that this increases the supply of cold O+ ions to the magnetosphere by more than 3,800%, in which other mechanisms such as wave-particle interactions can heat and further accelerate them to escape velocity. The electrostatic field of Earth is strong enough by itself to drive the polar wind9,10 and is probably the origin of the cold H+ ion population1 that dominates much of the magnetosphere2,3.

The variable source of the plasma sheet during a geomagnetic storm

Both solar wind and ionospheric sources contribute to the magnetotail plasma sheet, but how their contribution changes during a geomagnetic storm is an open question. The source is critical because the plasma sheet properties control the enhancement and decay rate of the ring current, the main cause of the geomagnetic field perturbations that define a geomagnetic storm. Here we use the solar wind composition to track the source and show that the plasma sheet source changes from predominantly solar wind to predominantly ionospheric as a storm develops. Additionally, we find that the ionospheric plasma during the storm main phase is initially dominated by singly ionized hydrogen (H+), likely from the polar wind, a low energy outflow from the polar cap, and then transitions to the accelerated outflow from the dayside and nightside auroral regions, identified by singly ionized oxygen (O+). These results reveal how the access to the magnetotail of the different sources can change quickly, impacting the storm development.

On the relationship between energy input to the ionosphere and the ion outflow flux under different solar zenith angles

The ionosphere is one of the important sources for magnetospheric plasma, particularly for heavy ions with low charge states. We investigate the effect of solar illumination on the number flux of ion outflow using data obtained by the Fast Auroral SnapshoT (FAST) satellite at 3000-4150 km altitude from 7 January 1998 to 5 February 1999. We derive empirical formulas between energy inputs and outflowing ion number fluxes for various solar zenith angle ranges. We found that the outflowing ion number flux under sunlit conditions increases more steeply with increasing electron density in the loss cone or with increasing precipitating electron density (> 50 eV), compared to the ion flux under dark conditions. Under ionospheric dark conditions, weak electron precipitation can drive ion outflow with small averaged fluxes (~ 10⁷ cm^-2 s^-1). The slopes of relations between the Poynting fluxes and outflowing ion number fluxes show no clear dependence on the solar zenith angle. Intense ion outflow events (> 10⁸ cm^-2 s^-1) occur mostly under sunlit conditions (solar zenith angle < 90°). Thus, it is presumably difficult to drive intense ion outflows under dark conditions, because of a lack of the solar illumination (low ionospheric density and/or small scale height owing to low plasma temperature).

A Survey of Dense Low Energy Ions in Earth's Outer Magnetosphere: Relation to Solar Wind Dynamic Pressure, IMF, and Magnetospheric Activity

The properties of cold, dense, low energy ( < 150 eV) ions within Earth's magnetosphere between 6 and 14 R E distance are examined using data sampled by Time History of Events and Macroscale Interactions during Substorms spacecraft during a new low-energy plasma mode that operated from June 2016 to July 2017. These ions are a persistent feature of the magnetosphere during enhanced solar wind dynamic pressure and/or magnetospheric activity. These ions have densities ranging from 0.5 to tens of c m - 3 , with a mean of ∼ 1 c m - 3 and temperatures of a few to tens of eV, with a mean of ∼ 13 eV. These yield cold to hot ion density and temperature ratios that are 4.4 and 4 × 1 0 - 3 , respectively. Comparisons reveal that the cold ion densities are positively correlated with solar wind dynamic pressure. These ions are organizable, according to their pitch-angle distribution, as being transverse/convection dominated (interpreted as plume plasma) or magnetic field-aligned (FAL) (uni- or bi-directional characteristic of ion outflow or cloak plasma). Transverse ions preferentially occur in the prenoon to dusk sectors during sustained active magnetospheric conditions driven by enhanced solar wind dynamic pressure under southward B z and westward B y IMF orientations. Transverse ion velocities (reaching several tens of km/s) have a westward directed tendency with a slight radially outward preference. In contrast FAL ions preferentially occur from morning to noon during northward IMF orientations, enhanced solar wind dynamic pressure, and quiet magnetospheric conditions within several hours after moderate to strong activity. The FAL ions also have bulk velocities ≲ 30 km/s, with an eastward and radially outward tendency.

3DI: A novel ion composition and three-dimensional velocity analyzer for the topside ionosphere

A new ion composition and three-dimensional velocity analyzer, 3-Dimensional ion velocity and mass Imager (3DI), measures 3D velocity distribution functions (VDFs) for each major ion species in Earth's topside ionosphere. The 3DI instrument is composed of a miniaturized electrostatic analyzer (ESA) and a deflector, backed by a static, magnet-based, mass spectrometer. We have developed a micro-pixel read-out anode technique that significantly saves power in the particle detection system, and integrated it into an imaging microchannel plate (MCP). We tested the ESA and deflector, magnet-based mass spectrometer, and anode in the laboratory to demonstrate the 3DI prototype's performance. We have applied numerical calculations to evaluate and discuss 3DI's performance and dynamic range. Due to complexities associated with imaging 3D distribution functions during fast spacecraft motion, we also discuss the operation strategy for 3DI to capture and resolve the VDF within the field of view. Once applied to flight investigations, the 3DI observations will be extremely useful in identifying ionosphere composition, mass-dependent ion transport such as upflows, and mass-dependent ion heating. Furthermore, the precise measurement of non-thermal plasma VDFs provides information to improve ionospheric environment modeling and ground-based radar observations.

Including Kinetic Ion Effects in the Coupled Global Ionospheric Outflow Solution

We present a new expansion of the Polar Wind Outflow Model (PWOM) to include kinetic ions using the Particle-in-Cell (PIC) approach with Monte Carlo collisions. This implementation uses the original hydrodynamic solution at low altitudes for efficiency, and couples to the kinetic solution at higher altitudes to account for kinetic effects important for ionospheric outflow. The modeling approach also includes wave-particle interactions, suprathermal electrons, and an hybrid parallel computing approach combining shared and distributed memory paralellization. The resulting model is thus a comprehensive, global, model of ionospheric outflow that can be run efficiently on large supercomputing clusters. We demonstrate the model's capability to study a range of problems starting with the comparison of kinetic and hydrodynamic solutions along a single field line in the sunlit polar cap, and progressing to the altitude evolution of the ion conic distribution in the cusp region. The interplay between convection and the cusp on the global outflow solution is also examined. Finally, we demonstrate the impact of these new model features on the magnetosphere by presenting the first 2-way coupled ionospheric outflow-magnetosphere calculation including kinetic ion effects.

The Environment of the Young Earth in the Perspective of An Young Sun

Our Sun, a magnetically mild star, exhibits space weather in the form of magnetically driven solar explosive events (SEE) including solar flares, coronal mass ejections and energetic particle events. We use Kepler data and reconstruction of X-ray and UV emission from young solar-like stars to recover the frequency and energy fluxes from extreme events from active stars including the young Sun. Extreme SEEs from a magnetically active young Sun could significantly perturb the young Earth's magnetosphere, cause strong geomagnetic storms, initiate escape and introduce chemical changes in its lower atmosphere. I present our recent simulations results based on multi-dimensional multi-fluid hydrodynamic and magnetohydrodynamic models of interactions of extreme CME and SEP events with magnetospheres and lower atmospheres of early Earth and exoplanets around active stars. We also discuss the implications of the impact of these effects on evolving habitability conditions of the early Earth and prebiotic chemistry introduced by space weather events at the early phase of evolution of our Sun.

Magnetosphere sawtooth oscillations induced by ionospheric outflow

The sawtooth mode of convection of Earth's magnetosphere is a 2- to 4-hour planetary-scale oscillation powered by the solar wind-magnetosphere-ionosphere (SW-M-I) interaction. Using global simulations of geospace, we have shown that ionospheric O(+) outflows can generate sawtooth oscillations. As the outflowing ions fill the inner magnetosphere, their pressure distends the nightside magnetic field. When the outflow fluence exceeds a threshold, magnetic field tension cannot confine the accumulating fluid; an O(+)-rich plasmoid is ejected, and the field dipolarizes. Below the threshold, the magnetosphere undergoes quasi-steady convection. Repetition and the sawtooth period are controlled by the strength of the SW-M-I interaction, which regulates the outflow fluence.