Kanako Seki

Outer radiation belt boundary location relative to the magnetopause: Implications for magnetopause shadowing

[1]xa0Relativistic electron fluxes of the outer radiation belt often decrease rapidly in response to solar wind disturbances. The importance of the magnetopause shadowing (MPS) effect on such electron losses has yet to be quantified. If the MPS is essential for outer radiation belt electron losses, a close relationship between the outer edge of the outer belt and the magnetopause standoff distance is expected. Using GOES and THEMIS data, we examined earthward movement of the outer edge of the outer belt during electron loss events at geosynchronous orbit and its correlation with the magnetopause standoff distance. In events with significant earthward movement, we found a good correlation. There were no clear correlations in events without significant earthward movement, however. Comparing the observational results with a test particle simulation, the observed dependence between the outer edge and the magnetopause standoff distance is consistent with the MPS effect.

A split in the outer radiation belt by magnetopause shadowing: Test particle simulations

[1]xa0We developed a three-dimensional relativistic test particle code and used it to calculate the trajectories of relativistic electrons in the outer radiation belt. By applying time-varying magnetic field data calculated from the Tsyganenko model and using observed solar wind data and the Dst index, we examined the drift loss of relativistic electrons by magnetopause shadowing (MPS). Since other loss processes such as wave-particle interactions are not included in this simulation, the pure MPS effect can be discussed. A split was found in the outer radiation belt after the enhancement of the solar wind dynamic pressure. Isolated electrons outside of the split have a narrow pitch angle distribution around 90° and are confined to a narrow range of the L shell. We found that the existence of the isolated electrons depends on the large geomagnetic tilt angle. These findings indicate that the split can be seen during summer and winter after MPS occurs. We suggest that this split in the outer radiation belt during summer and winter is evidence that MPS actually causes the loss of the outer radiation belt.

Coordinated EISCAT Svalbard radar and Reimei satellite observations of ion upflows and suprathermal ions

[1]xa0The relationship between bulk ion upflows and suprathermal ions was investigated using data simultaneously obtained from the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and the Reimei satellite. Simultaneous observations were conducted in November 2005 and August 2006, and 14 conjunction data sets have been obtained at approximately 630 km in the dayside ionosphere. Suprathermal ions with energies of a few eV were present in the dayside cusp region, and the ion velocity distribution changed from an isotropic Maxwellian near the cusp region to tail heating at energies above a few eV in the cusp region. The velocity distribution of the suprathermal ions has a peak perpendicular or oblique to the geomagnetic field, and the temperature of the suprathermal ions was 0.9–1.4 eV. An increase in the phase space density (PSD) of the suprathermal ions, measured with the Reimei, was correlated with bulk ion upflow observed at the same altitude using EISCAT, and with the energy flux of precipitating electrons with energies of 50–500 eV. The PSD also has a good correlation with the electron temperature, which was increased by precipitation, but not with the ion temperature (0.1–0.3 eV) at the same altitude measured with EISCAT. These results suggest that plasma waves such as broadband extremely low frequency (BBELF) wavefields associated with precipitation are connected to the bulk ion upflows in the cusp and effectively cause the heating of suprathermal ions. The heating of suprathermal ions disagrees with anisotropic heating due to O+−O resonant charge exchange.

Formation of a broad plasma turbulent layer by forward and inverse energy cascades of the Kelvin–Helmholtz instability

[1]xa0The 2-D MHD simulations of the Kelvin–Helmholtz instability (KHI) with transverse magnetic field and highly asymmetric density configurations in a large simulation domain show that rapid formation of a broad plasma turbulent layer can be achieved by forward and inverse energy cascades of the KHI. The forward cascade is triggered by growth of the secondary Rayleigh-Taylor instability excited during the nonlinear evolution of the KHI. The inverse cascade is accomplished by nonlinear mode couplings between the fastest growing mode of the KHI and other KH unstable modes. As a result of the energy transport by the inverse cascade, the growth rate of the largest vortex allowed in the system reaches 3.7 times as large as the linear growth rate. A PIC simulation under the similar initial configuration is also conducted and shows the quick growth of the largest vortex and the resultant spread of the turbulent layer in which plasmas are effectively mixed. The proposed mechanism enables rapid formation of a large scale mixing layer which resembles observational characteristics of the low-latitude boundary layer of the magnetosphere.

Small-scale auroral current sheet structuring

[1] We simulate the 3-D evolution of a thin current sheet as it impinges on the ionosphere from a magnetospheric source in a manner analogous to that which may occur during the onset of an auroral substorm. We consider two scenarios: one in which electron inertia alone acts to allow motion between the plasma and the geomagnetic field, and a second where a resistive layer at the interface between the ionosphere and magnetosphere is included. These two scenarios in our fluid model are intended to represent what have become known as Alfvenic and Quasi-static or Inverted-V aurora, respectively. In the absence of resistivity the evolution is shown to be driven by a combination of Kelvin-Helmholtz and tearing instabilities leading to vortices similar to folds and the eventual break-up of the planar arc into distorted fine-scale sheets and filamentary currents. The later stage of this evolution is driven by an instability on the steep transverse current gradients created by the former instabilities. With a resistive layer present the K-H instability dominates leading to the formation of auroral curls. We show how these evolutionary processes can be ordered based on the ratio of the transverse electric and magnetic fields (ΔE X /ΔB Y ) across the current sheet relative to the Alfven speed, and demonstrate how the evolution is dependent on wave reflection from the topside ionosphere.

Cross-scale coupling in the auroral acceleration region

[1]xa0High resolution imaging within regions of auroral luminosity reveal complex, highly structured dynamic and often vortical forms which evolve on time scales of the order of several seconds and less. These features are inherently multi-scale in nature with different sizes moving and evolving at different rates. Recent analyses have shown how the scale dependency of these motions can provide new insights into the nature of energy transport across scales occurring in current sheets through the auroral acceleration region. However the processes driving this transport and thus facilitating particle acceleration and the formation of bright and dynamic aurora remain unknown. This is a basic issue not only for advancing understanding of auroral arc formation but moreover for understanding dissipation and particle acceleration in current sheets generally. In this Frontier article we show how dedicated space-borne auroral imagery combined with magnetically conjugate field and particle measurements can be used to advance understanding of this universal physical process. By coupling these measurements with numerical simulations we show how flow shear, magnetic reconnection and tearing may launch a cascade toward smaller scales and conspire to form, shape and structure auroral forms. The simulations show that these processes evolve toward a robust scaling of structured magnetic fields (Bx) with wavenumber (ky) perpendicular to the geomagnetic field where Bx2(ky)/Δky ∼ ky−7/3 as observed.

Development of a magnetohydrodynamic simulation code satisfying the solenoidal magnetic field condition

Abstract We have developed a new magnetohydrodynamic (MHD) simulation code that automatically satisfies the solenoidal magnetic field ( B ) property ∇ ⋅ B = 0 . We use the vector potential ( A ) instead of the magnetic field itself in the magnetohydrodynamic equation. To solve the advection term, we adopt a Rational-CIP algorithm in the simulation code. The non-advection terms are solved by the 4th order Runge–Kutta method for time and 4th order central difference for space in a regular grid system. Code assessments are carried out to evaluate the properties of the developed code. A remarkable feature of the new code is the description of Alfven wave propagation with less numerical dispersion. After the code assessments, we apply the code to a global simulation of the planet Mercurys magnetosphere. Fundamental structures of the magnetosphere are successfully reproduced.

Self-consistent kinetic numerical simulation model for ring current particles in the Earth's inner magnetosphere

[1]xa0A new self-consistent and kinetic model for ring current particles in the inner magnetosphere is presented. A closed set of nonlinear time evolution equations is derived that incorporates kinetic particle dynamics and self-consistent development of the electromagnetic field. The particle transport is described by a five-dimensional collisionless drift kinetic equation, in which particle trajectories are approximated by their guiding centers under the influence of a time-dependent electromagnetic field. The time evolution of the electromagnetic field follows the Maxwell equations with the feedback from particles through electric currents. A numerical simulation code solving the system of equations in a global inner magnetosphere in three spatial dimensions (or five dimensions in phase space) is developed. It is demonstrated that the propagation of magnetohydrodynamic waves can successfully be described by the present model. It is also found that the self-consistent coupling could affect the transport of energetic particles especially at low energies as well as the intensity and spatial distribution of field-aligned currents. These preliminary results suggest the importance of the self-consistent coupling in the global development of geomagnetic storms.

Implementation of the CIP algorithm to magnetohydrodynamic simulations

Abstract An implementation of the Constrained Interpolation Profile (CIP) algorithm to magnetohydrodynamic (MHD) simulations is presented. First we transform the original momentum and magnetic induction equations to unfamiliar forms by introducing Elsasser variables [W.M. Elsasser, The hydromagnetic equations, Phys. Rev. (1950)]. In this formulation, while the compressional and pressure gradient terms remain as non-advective terms, the advective and magnetic stress terms are expressed in the form of an advection equation, which enables us to use the CIP algorithm. We have examined some 1D test problems using the code based on this formula. Linear Alfven wave propagation tests reveal that the developed code is capable of solving any Alfven wave propagation with only small numerical diffusion and phase errors up to k ▵ h = 2.5 (where ▵ h is the grid spacing). A shock tube test shows good agreement with a previous result with less numerical oscillation at the shock front and the contact discontinuity which are captured within a few grid points. Extension of the 1D code to the multi-dimensional case is straightforward. We have calculated the 3D nonlinear evolution of the Kelvin–Helmholtz instability (KHI) and compared the result with our previous study. We find that our new MHD code is capable of following the 3D turbulence excited by the KHI while retaining the solenoidal property of the magnetic field.

The ERG Science Center

The Exploration of energization and Radiation in Geospace (ERG) Science Center serves as a hub of the ERG project, providing data files in a common format and developing the space physics environment data analysis software and plug-ins for data analysis. The Science Center also develops observation plans for the ERG (Arase) satellite according to the science strategy of the project. Conjugate observations with other satellites and ground-based observations are also planned. These tasks contribute to the ERG project by achieving quick analysis and well-organized conjugate ERG satellite and ground-based observations.