Global Driving of Auroral Precipitation: 1. Balance of Sources

The accurate determination of auroral precipitation in global models has remained a daunting and rather inexplicable obstacle. Understanding the calculation and balance of multiple sources that constitute the aurora, and their eventual conversion into ionospheric electrical conductance, is critical for improved prediction of space weather events. In this study, we present a semi-physical global modeling approach that characterizes contributions by four types of precipitation-monoenergetic, broadband, electron, and ion diffuse-to ionospheric electrodynamics. The model uses a combination of adiabatic kinetic theory and loss parameters derived from historical energy flux patterns to estimate auroral precipitation from magnetohydrodynamic (MHD) quantities. It then converts them into ionospheric conductance that is used to compute the ionospheric feedback to the magnetosphere. The model has been employed to simulate the 5-7 April 2010 Galaxy15 space weather event. Comparison of auroral fluxes show good agreement with observational data sets like NOAA-DMSP and OVATION Prime. The study shows a dominant contribution by electron diffuse precipitation, accounting for ∼74% of the auroral energy flux. However, contributions by monoenergetic and broadband sources dominate during times of active upstream solar conditions, providing for up to 61% of the total hemispheric power. The study also finds a greater role played by broadband precipitation in ionospheric electrodynamics which accounts for ∼31% of the Pedersen conductance.

Global GNSS-RO Electron Density in the Lower Ionosphere

Lack of instrument sensitivity to low electron density (Ne) concentration makes it difficult to measure sharp Ne vertical gradients (four orders of magnitude over 30 km) in the D/E-region. A robust algorithm is developed to retrieve global D/E-region Ne from the high-rate GNSS radio occultation (RO) data, to improve spatiotemporal coverage using recent SmallSat/CubeSat constellations. The new algorithm removes F-region contributions in the RO excess phase profile by fitting a linear function to the data below the D-region. The new GNSS-RO observations reveal many interesting features in the diurnal, seasonal, solar-cycle, and magnetic-field-dependent variations in the Ne morphology. While the D/E-region Ne is a function of solar zenith angle (χ), it exhibits strong latitudinal variations for the same χ with a distribution asymmetric about noon. In addition, large longitudinal variations are observed along the same magnetic field pitch angle. The summer midlatitude Ne and sporadic E (Es) show a distribution similar to each other. The distribution of auroral electron precipitation correlates better with the pitch angle from the magnetosphere than from one at 100 km. Finally, a new TEC retrieval technique is developed for the high-rate RO data with a top reaching at least 120 km. For better characterization of the E- to F-transition in Ne and more accurate TEC retrievals, it is recommended to have all GNSS-RO acquisition routinely up to 220 km.

Ballooning-Interchange Instability in the Near-Earth Plasma Sheet and Auroral Beads: Global Magnetospheric Modeling at the Limit of the MHD Approximation

Explosive magnetotail activity has long been understood in the context of its auroral manifestations. While global models have been used to interpret and understand many magnetospheric processes, the temporal and spatial scales of some auroral forms have been inaccessible to global modeling creating a gulf between observational and theoretical studies of these phenomena. We present here an important step toward bridging this gulf using a newly developed global magnetosphere-ionosphere model with resolution capturing ≲ 30 km azimuthal scales in the auroral zone. In a global magnetohydrodynamic (MHD) simulation of the growth phase of a synthetic substorm, we find the self-consistent formation and destabilization of localized magnetic field minima in the near-Earth magnetotail. We demonstrate that this destabilization is due to ballooning-interchange instability which drives earthward entropy bubbles with embedded magnetic fronts. Finally, we show that these bubbles create localized field-aligned current structures that manifest in the ionosphere with properties matching observed auroral beads.

Contribution of Bursty Bulk Flows to the Global Dipolarization of the Magnetotail During an Isolated Substorm

This paper addresses the question of the contribution of azimuthally localized flow channels and magnetic field dipolarizations embedded in them in the global dipolarization of the inner magnetosphere during substorms. We employ the high-resolution Lyon-Fedder-Mobarry global magnetosphere magnetohydrodynamic model and simulate an isolated substorm event, which was observed by the geostationary satellites and by the Magnetospheric Multiscale spacecraft. The results of our simulations reveal that plasma sheet flow channels (bursty bulk flows, BBFs) and elementary dipolarizations (dipolarization fronts, DFs) occur in the growth phase of the substorm but are rare and do not penetrate to the geosynchronous orbit. The substorm onset is characterized by an abrupt increase in the occurrence and intensity of BBFs/DFs, which penetrate well earthward of the geosynchronous orbit during the expansion phase. These azimuthally localized structures are solely responsible for the global (in terms of the magnetic local time) dipolarization of the inner magnetosphere toward the end of the substorm expansion. Comparison with the geostationary satellites and Magnetospheric Multiscale data shows that the properties of the BBFs/DFs in the simulation are similar to those observed, which gives credence to the above results. Additionally, the simulation reveals many previously observed signatures of BBFs and DFs, including overshoots and oscillations around their equilibrium position, strong rebounds and vortical tailward flows, and the corresponding plasma sheet expansion and thinning.

Global MHD modeling of resonant ULF waves: Simulations with and without a plasmasphere

We investigate the plasmaspheric influence on the resonant mode coupling of magnetospheric ultralow frequency (ULF) waves using the Lyon-Fedder-Mobarry (LFM) global magnetohydrodynamic (MHD) model. We present results from two different versions of the model, both driven by the same solar wind conditions: one version that contains a plasmasphere (the LFM coupled to the Rice Convection Model, where the Gallagher plasmasphere model is also included) and another that does not (the stand-alone LFM). We find that the inclusion of a cold, dense plasmasphere has a significant impact on the nature of the simulated ULF waves. For example, the inclusion of a plasmasphere leads to a deeper (more earthward) penetration of the compressional (azimuthal) electric field fluctuations, due to a shift in the location of the wave turning points. Consequently, the locations where the compressional electric field oscillations resonantly couple their energy into local toroidal mode field line resonances also shift earthward. We also find, in both simulations, that higher-frequency compressional (azimuthal) electric field oscillations penetrate deeper than lower frequency oscillations. In addition, the compressional wave mode structure in the simulations is consistent with a radial standing wave oscillation pattern, characteristic of a resonant waveguide. The incorporation of a plasmasphere into the LFM global MHD model represents an advance in the state of the art in regard to ULF wave modeling with such simulations. We offer a brief discussion of the implications for radiation belt modeling techniques that use the electric and magnetic field outputs from global MHD simulations to drive particle dynamics.

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.