Binzheng Zhang

Magnetosphere Sawtooth Oscillations Induced by Ionospheric Outflow

Numerical simulations show that a magnetospheric disturbance is caused by an influx of O+ ions from the ionosphere. 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.

Ionospheric control of magnetotail reconnection

How the ionosphere gains influence In Earths upper atmosphere, the reconnection of magnetic field lines converts latent magnetic energy into the thermal and kinetic energy of plasma flows. But reconnection appears to produce faster flows before midnight compared with after. To find out why, Lotko et al. simulated this energy exchange. Challenging common assumptions about our space weather environment, they conclude that the ionosphere plays an active role when coupled to the magnetosphere in driving the behavior of the magnetotail. Science, this issue p. 184 Asymmetric plasma flows on the night side of Earth are regulated by magnetosphere-ionosphere coupling. Observed distributions of high-speed plasma flows at distances of 10 to 30 Earth radii (RE) in Earth’s magnetotail neutral sheet are highly skewed toward the premidnight sector. The flows are a product of the magnetic reconnection process that converts magnetic energy stored in the magnetotail into plasma kinetic and thermal energy. We show, using global numerical simulations, that the electrodynamic interaction between Earth’s magnetosphere and ionosphere produces an asymmetry consistent with observed distributions in nightside reconnection and plasmasheet flows and in accompanying ionospheric convection. The primary causal agent is the meridional gradient in the ionospheric Hall conductance which, through the Cowling effect, regulates the distribution of electrical currents flowing within and between the ionosphere and magnetotail.

How does mass loading impact local versus global control on dayside reconnection

This paper investigates the effects of magnetospheric mass loading on the control of dayside magnetic reconnection using global magnetospheric simulations. The study iys motivated by a recent debate on whether the integrated dayside magnetic reconnection rate is solely controlled by local processes (local-control theory) or global merging processes (global-control theory). The local-control theory suggests that the integrated dayside reconnection rate is controlled by the local plasma parameters. The global-control theory argues that the integrated rate is determined by the net force acting on the flow in the magnetosheath rather than the local microphysics. Controlled numerical simulations using idealized ionospheric outflow specifications suggest a possible mixed-control theory, that is, (1) a small amount of mass loading at the dayside magnetopause only redistributes local reconnection rate without a significant change in the integrated reconnection rate and (2) a large amount of mass loading reduces both local reconnection rates and the integrated reconnection rate on the dayside. The transition between global-controland local-control-dominated regimes depends on (but not limited to) the source region, the amount, the location, and the spatial extension of the mass loading at the dayside magnetopause.

Influence of ion outflow in coupled geospace simulations: 1. Physics‐based ion outflow model development and sensitivity study

We describe a coupled geospace model that includes causally regulated ion outflow from a physics-based ionosphere/polar wind model. The model two-way couples the multifluid Lyon-Fedder-Mobarry magnetohydrodynamics (MHD) model to the ionosphere/polar wind model (IPWM). IPWM includes the H+ and O+ polar wind as well as a phenomenological treatment of energetic O+ accelerated by wave-particle interactions (WPI). Alfvénic Poynting flux from the MHD simulation causally regulates the ion acceleration. The wave-particle interactions (WPI) model has been tuned and validated with comparisons to particle-in-cell simulations and empirical relationships derived from Fast Auroral Snapshot satellite data. IPWM captures many aspects of the ion outflow that empirical relationships miss. First, the entire coupled model conserves mass between the ionospheric and magnetospheric portions, meaning the amount of outflow produced is limited by realistic photochemistry in the ionosphere. Second, under intense driving conditions, the outflow becomes flux limited by what the ionosphere is capable of providing. Furthermore, the outflows produced exhibit realistic temporal and spatial delays relative to the magnetospheric energy inputs. The coupled model provides a flexible way to explore the impacts of dynamic heavy ion outflow on the coupled geospace system. Some of the example simulations presented exhibit internally driven sawtooth oscillations associated with the outflow, and the properties of these oscillations are analyzed further in a companion paper.

Influence of ion outflow in coupled geospace simulations: 2. Sawtooth oscillations driven by physics-based ion outflow

We present the first simulations of magnetospheric sawtooth oscillations under steady solar wind conditions that are driven internally by heavy ion outflow from a physics-based model. The simulations presented use the multifluid Lyon-Fedder-Mobarry magnetohydrodynamics model two-way coupled to the ionosphere/polar wind model (IPWM). Depending on the type of wave-particle interactions utilized within IPWM, the coupled simulations exhibit either sawtooth oscillations or steady magnetospheric convection. Contrasting the simulations that do and do not develop sawtooth oscillations yields insights into the relationship between outflow and sawtooth oscillations. The total outflow rate is not an adequate predictor of the convection mode that will emerge. The simulations that develop sawtooth oscillations are characterized by intense outflow concentrated in the midnight auroral region. This outflow distribution mass loads the tail reconnection region without excessively mass loading the dayside reconnection region and leads to an imbalance between the dayside and nightside reconnection rates.

Effects of Electrojet Turbulence on a Magnetosphere-Ionosphere Simulation of a Geomagnetic Storm†

This material is based upon work supported by NASA grants NNX14AI13G, NNX13AF92G, and NNX16AB80G. The National Center for Atmospheric Research is sponsored by the National Science Foundation. This work used the XSEDE and TACC computational facilities, supported by National Science Foundation grant ACI-1053575. We would like to acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCARs Computational and Information Systems Laboratory, sponsored by the National Science Foundation. We thank the AMPERE team and the AMPERE Science Center for providing the Iridium derived data products. All model output, simulation codes, and analysis routines are being preserved on the NCAR High-Performance Storage System and will be made available upon written request to the lead author of this publication. (NNX14AI13G - NASA; NNX13AF92G - NASA; NNX16AB80G - NASA; National Science Foundation; ACI-1053575 - National Science Foundation)

Asymmetric Kelvin‐Helmholtz Instability at Jupiter's Magnetopause Boundary: Implications for Corotation‐Dominated Systems

The multifluid Lyon-Fedder-Mobarry (MFLFM) global magnetosphere model is used to study the interactions between solar wind and rapidly rotating, internally driven Jupiter magnetosphere. The MFLFM model is the first global simulation of Jupiter magnetosphere that captures the Kelvin-Helmholtz instability (KHI) in the critically important subsolar region. Observations indicate that Kelvin-Helmholtz vortices are found predominantly in the dusk sector. Our simulations explain that this distribution is driven by the growth of KHI modes in the prenoon and subsolar region (e.g., >10 local time) that are advected by magnetospheric flows to the dusk sector. The period of density fluctuations at the dusk terminator flank (18 magnetic local time, MLT) is roughly 1.4 h compared with 7.2 h at the dawn flank (6 MLT). Although the simulations are only performed using parameters of the Jupiter’s magnetosphere, the results may also have implications for solar wind-magnetosphere interactions at other corotation-dominated systems such as Saturn. For instance, the simulated average azimuthal speed of magnetosheath flows exhibit significant dawn-dusk asymmetry, consistent with recent observations at Saturn. The results are particularly relevant for the ongoing Juno mission and the analysis of dawnside magnetopause boundary crossings for other planetary missions.

Long‐Lasting Response of the Global Thermosphere and Ionosphere to the 21 August 2017 Solar Eclipse

Previous studies have been devoted to examining the ionosphere and thermosphere response during the solar eclipse, but the posteclipse response of the global ionosphere and thermosphere has not been well quantified. In this study, for the first time, we quantitatively investigate the posteclipse response of the global ionosphere and thermosphere system to the recent Great American Solar Eclipse using a high-resolution, global, coupled ionosphere-thermosphere-electrodynamics model. It was found that the posteclipse response of the ionosphere and thermosphere is significant and worldwide, which was not expected. Specifically, even 9 hr after the eclipse ended, the globally averaged ionospheric total electron content perturbations were about 0.2 total electron content unit, and the corresponding changes in neutral temperature and winds reached 2 K and 2 m/s. The changes in the global dynamic and energetic processes associated with the solar eclipse contributed to this long-lasting response of the ionosphere and thermosphere during the posteclipse period. Plain Language Summary The thermosphere is the layer of the Earth’s atmosphere above the mesosphere between about 60 and 1,000 km, and the ionosphere is the ionized part of the atmosphere. In this region, the neutral gas and the ionized plasma have significant impact on low Earth orbiting determination and satellite radio communications. This work is the first to use a state-of-the-art, first-principles model of the coupled thermosphere and ionosphere, with self-consistent electrodynamics, to systematically investigate the dynamics and electrodynamic behavior of the global ionosphere and thermosphere after an eclipse. Although the solar eclipse is a transient local event, its impact on the ionosphere and thermosphere can persist for a long time over the entire globe, rather than just being an impulse event with a localized response as was previously expected. This effort paves the way for improving the understanding of the upper atmospheric variability.

Suppression of the Polar Tongue of Ionization During the 21 August 2017 Solar Eclipse

It has long been recognized that during solar eclipses, the ionosphere-thermosphere system changes greatly within the eclipse shadow, due to the rapid reduction of solar irradiation. However, the concept that a solar eclipse impacts polar ionosphere behavior and dynamics as well as magnetosphere-ionosphere coupling has not been appreciated. In this study, we investigate the potential impact of the 21 August 2017 solar eclipse on the polar tongue of ionization (TOI) using a high-resolution, coupled ionosphere-thermosphere-electrodynamics model. The reduction of electron densities by the eclipse in the middle latitude TOI source region leads to a suppressed TOI in the polar region. The TOI suppression occurred when the solar eclipse moved into the afternoon sector. The Global Positioning System total electron content observations show similar tendency of polar region total electron content suppression. This study reveals that a solar eclipse occurring at middle latitudes may have significant influences on the polar ionosphere and magnetosphere-ionosphere coupling. Plain Language Summary The ionosphere is the ionized part of Earth’s upper atmosphere extending from about 60 to 1,000 km. During solar storm events, the dayside ionospheric plasma can be transported from middle latitude into the polar region by the electric field, leading to a “tongue-like” structure, which is called the “tongue of ionization (TOI).” Meanwhile, the solar eclipse can dramatically decrease the ionospheric plasma density within the Moon’s shadow by the reduction of solar irradiation. Since the TOI structure is closely related to the middle latitude plasma density, it is interesting to explore the solar eclipse influences at middle latitudes on the polar ionospheric behavior associated with the polar TOI structure. On the basis of high-resolution simulations of the 21 August 2017 solar eclipse, it was reported that a significantly suppressed TOI occurred during the solar eclipse. The results provide new insights into the broad impacts of middleand highlatitude eclipses on the behavior and dynamics of the high-latitude ionosphere and the geospace system.

Transition from global to local control of dayside reconnection from ionospheric-sourced mass loading

We have conducted a series of controlled numerical simulations to investigate the response of dayside reconnection to idealized, ionosphere-sourced mass loading processes to determine whether they affect the integrated dayside reconnection rate. Our simulation results show that the coupled solar wind-magnetosphere (SW-M) system may exhibit both local and global control behaviors depending on the amount of mass loading. With a small amount of mass loading, the changes in local reconnection rate affects magnetosheath properties only weakly and the geoeffective length in the upstream solar wind is essentially unchanged, resulting in the same integrated dayside reconnection rate. With a large amount of mass loading, however, the magnetosheath properties and the geoeffective length are significantly affected by slowing down the local reconnection rate, resulting in an increase of the magnetic pressure in the magnetosheath, with a significant reduction in the geoeffective length in the upstream solar wind and in the integrated dayside reconnection rate. In this controlled simulation setup, the behavior of dayside reconnection potential is determined by the role of the enhanced magnetic pressure in the magnetospheath due to magnetospheric mass loading. The reconnection potential starts to decrease significantly when the enhanced magnetic pressure alters the thickness of the magnetosheath.