Magnetospheric Response to Solar Wind Forcing: ULF Wave – Particle Interaction Perspective

Solar wind forcing, e.g. interplanetary shock and/or solar wind dynamic pressure pulses impact on the Earth’s magnetosphere manifests many fundamental important space physics phenomena including producing electromagnetic waves, plasma heating and energetic particle acceleration. This paper summarizes our present understanding of the magnetospheric response to solar wind forcing in the aspects of radiation belt electrons, ring current ions and plasmaspheric plasma physics based on in situ spacecraft measurements, ground-based magnetometer data, MHD and kinetic simulations. Magnetosphere response to solar wind forcing, is not just a “one-kick” scenario. It is found that after the impact of solar wind forcing on the Earth’s magnetosphere, plasma heating and energetic particle acceleration started nearly immediately and could last for a few hours. Even a small dynamic pressure change of interplanetary shock or solar wind pressure pulse can play a non-negligible role in magnetospheric physics. The impact leads to generate series kind of waves including poloidal mode ultra-low frequency (ULF) waves. The fast acceleration of energetic electrons in the radiation belt and energetic ions in the ring current region response to the impact usually contains two contributing steps: (1) the initial adiabatic acceleration due to the magnetospheric compression; (2) followed by the wave-particle resonant acceleration dominated by global or localized poloidal ULF waves excited at various L-shells. Generalized theory of drift and drift-bounce resonance with growth or decay localized ULF waves has been developed to explain in situ spacecraft observations. The wave related observational features like distorted energy spectrum, boomerang and fishbone pitch angle distributions of radiation belt electrons, ring current ions and plasmaspheric plasma can be explained in the frame work of this generalized theory. It is worthy to point out here that poloidal ULF waves are much more efficient to accelerate and modulate electrons (fundamental mode) in the radiation belt and charged ions (second harmonic) in the ring current region. The results presented in this paper can be widely used in solar wind interacting with other planets such as Mercury, Jupiter, Saturn, Uranus and Neptune, and other astrophysical objects with magnetic fields.

The Role of Mesoscale Plasma Sheet Dynamics in Ring Current Formation

During geomagnetically active periods ions are transported from the magnetotail into the inner magnetosphere and accelerated to energies of tens to hundreds of keV. These energetic ions, of mixed composition with the most important species being H+ and O+, become the dominant source of plasma pressure in the inner magnetosphere. Ion transport and acceleration can occur at different spatial and temporal scales ranging from global quasi-steady convection to localized impulsive injection events and may depend on the ion gyroradius. In this study we ascertain the relative importance of mesoscale flow structures and the effects of ion non-adiabaticity on the produced ring current. For this we use: global magnetohydrodynamic (MHD) simulations to generate self-consistent electromagnetic fields under typical driving conditions which exhibit bursty bulk flows (BBFs); and injected test particles, initialized to match the plasma moments of the MHD simulation, and subsequently evolved according to the kinetic equations of motion. We show that the BBFs produced by our simulation reproduce thermodynamic and magnetic statistics from in situ measurements and are numerically robust. Mining the simulation data we create a data set, over a billion points, connecting particle transport to characteristics of the MHD flow. From this we show that mesoscale bubbles, localized depleted entropy regions, and particle gradient drifts are critical for ion transport. Finally we show, using identical particle ensembles with varying mass, that O+ non-adiabaticity creates qualitative differences in energization and spatial distribution while H+ non-adiabaticity has non-negligible implications for loss timescales.

Partitioning Hückel–London Currents into Cycle Contributions

Ring-current maps give a direct pictorial representation of molecular aromaticity. They can be computed at levels ranging from empirical to full ab initio and DFT. For benzenoid hydrocarbons, Hückel–London (HL) theory gives a remarkably good qualitative picture of overall current patterns, and a useful basis for their interpretation. This paper describes an implemention of Aihara’s algorithm for computing HL currents for a benzenoid (for example) by partitioning total current into its constituent cycle currents. The Aihara approach can be used as an alternative way of calculating Hückel–London current maps, but more significantly as a tool for analysing other empirical models of induced current based on conjugated circuits. We outline an application where examination of cycle contributions to HL total current led to a simple graph-theoretical approach for cycle currents, which gives a better approximation to the HL currents for Kekulean benzenoids than any of the existing conjugated-circuit models, and unlike these models it also gives predictions of the HL currents in non-Kekulean benzenoids that are of similar quality.

Offsets in the geomagnetic indices of the magnetospheric ring current

The paper considers changes in the daily average values of the Dst, SYM-H, ASY-H, and ASY-D indices and their dependence on the level of magnetic disturbance for the period 1981–2016. These indices are geomagnetic characteristics of the magnetospheric ring current. It has been established that the indices of the asymmetric component of the ring current ASY-H and ASY-D during relatively magnetically quiet periods are not equal to zero. The values of the offsets in the dependences of the ASY-H and ASY-D indices on the level of magnetic disturbance have been determined. The behavior of the index of the degree of symmetry of the ring current, the ratio SYM-H / ASY-H, is analyzed during the year at different levels of disturbance. This ratio has been found to grow in absolute value with increasing disturbance and to exceed 1 at large disturbances (at Dst <–50).

Simulation study of the energetic neutral atom (ENA) imaging monitoring of the geomagnetosphere on a lunar base

Since the moon’s revolution cycle is exactly the same as its rotation cycle, we can only see the moon always facing Earth in the same direction. Based on the clean particle radiation environment of the moon, a neutral atomic telemetry base station could be established on the lunar surface facing Earth to realize long-term continuous geomagnetic activity monitoring. Using the 20°×20° field of view, the 0.5°×0.5° angle resolution, and the ~0.17 cm²sr geometric factor, a two-dimensional ENA imager is being designed. The magnetospheric ring current simulation at a 4–20 keV energy channel for a medium geomagnetic storm (Kp=5) shows the following: 1) at ~60 Rᴇ (Rᴇ is the Earth radius), the imager can collect 10⁴ ENA events for 3 min to meet the statistical requirements for 2D coded imaging data inversion, so as to meet requirements for the analysis of the substorm ring current evolution process of magnetic storms above medium; 2) the ENA radiation loss puzzles in the magnetopause and magnetotail plasma sheet regions have been deduced and revealed using the 2-D ENA emission model. High spatial-temporal resolution ENA imaging monitoring of these two important regions will provide the measurement basis for the solar wind energy input process and generation mechanism; 3) the average sampling interval of ENA particle events is about 16 ms at the moon’s orbit; the spectral time difference for the set energy range is on the order of minutes, which can provide location information to track the trigger of geomagnetic storm particle events.