Hironori Shimazu

A self-similar solution of expanding cylindrical flux ropes for any polytropic index value

We found a new class of solutions for MHD equations that satisfies the condition that cylindrical flux ropes can expand self-similarly even when the polytropic index γ is larger than 1. We achieved this by including the effects of elongation along the symmetry axis as well as radial expansion and assuming that the radial expansion rate is the same as the elongation rate. In previous studies (Osherovich et al., 1993a, 1995), a class of self-similar solutions was described for which cylindrical flux ropes expand only in the medium where γ is less than 1. We compare the models including elongation and excluding elongation observationally by using the WIND key parameters. The difference in the fitting results of the magnetic field between these two models is slight. However the fitting of the velocity is improved when elongation is included and when new geometric parameters that are necessary to represent the elongation are introduced. The values of these parameters are almost the same scale as the radius of flux ropes, which is consistent with the assumption of the isotropic expansion. This new exact solution to a time-dependent two-dimensional MHD problem can also be used to test numerical codes.

Three-dimensional hybrid simulation of magnetized plasma flow around an obstacle

The interaction between the magnetized plasma flow and an obstacle was investigated in the computer simulations described here by using a three-dimensional hybrid code (kinetic ions and massless fluid electrons). The results, which are relevant to the interaction between the solar wind and an unmagnetized planet (Venus or Mars), show that fundamental structures (bow shock and magnetotail) are formed. When a reflecting boundary is used at the obstacle, the magnetic field configuration was clearly asymmetrical in the direction of the convection electric field. This asymmetry is a result of differences in ion acceleration due to the convection electric field. Asymmetry is also evident when the size of the obstacle is close to the Larmor radius of protons. The shock of a smaller obstacle is weaker than that of a larger obstacle, but the shock size is almost independent of the obstacle size.

New method for detecting interplanetary flux ropes

We identified candidate flux ropes by using interplanetary magnetic field data obtained by the Wind spacecraft and selecting the periods when the third derivative of each interplanetary magnetic field component with respect to time was continuously smaller than a certain value. If the data gathered during these periods fit a cylindrical force-free field model, the candidates were classified as flux ropes. There were too many flux ropes that had a large impact parameter value to be explained by the finite lengths of the flux ropes (or possibly the curvatures of the flux rope axes). We thus excluded flux ropes having a large impact parameter value from our analysis of fitted parameters. We took into account short-duration flux ropes (2–10 hours), and we found that the number of small flux ropes is much larger than that of large ones. We found that this method works well in identifying flux ropes provided that valid criteria have been established.

Prediction of the Dst index from solar wind parameters by a neural network method

Using the Elman-type neural network technique, operational models are constructed that predict the Dst index two hours in advance. The input data consist of real-time solar wind velocity, density, and magnetic field data obtained by the Advanced Composition Explorer (ACE) spacecraft since May 1998 (http://www2.crl.go.jp/uk/uk223/service/nnw/index.html). During the period from February to October 1998, eleven storms occurred with minimum Dst values below -80 nT. For ten of these storms the differences between the predicted minimum Dst and the minimum Dst calculated from ground-based magnetometer data were less than 23%. For the remaining one storm (beginning on 19 October 1998) the difference was 48%. The discrepancy is likely to stem from a imperfect correlation between the solar wind parameters near ACE and those near the earth. While the IMF Bz remains to be the most important parameter, other parameters do have their effects. For instance, Dst appears to be enhanced when the azimuthal direction of IMF is toward the sun. A trapezoid-shaped increase in the solar wind density enhances the main phase Dst by almost 10% compared with the case of no density increase. Velocity effects appear to be stronger than the density effects. Our operational models have, in principle, no limitations in applicability with respect to storm intensity.

Operational models for forecasting Dst

Abstract We have constructed operational models for forecasting the geomagnetic storm index (Dst) two hours in advance from six parameters: the velocity and density of the solar wind, the magnitude of the interplanetary magnetic field (IMF), and the x, y, and z components of the IMF. Our models use an Elman-type neural network, and we forecast space weather by using real-time solar-wind data from the Advanced Composition Explorer spacecraft.The models have worked well since April of 1998 and the Dst values forecast using them have been made available to the public at http://www.crl.go.jp/uk/uk223/service/nnw/index.html. From February to October 1998 there were 11 storms with minimum Dst values below −80 nT, and for ten the difference between the forecast minimum Dst and the Dst calculated from data measured by ground stations was less than 23%.For the storm starting on 19 October, however, the difference was 40% because of the weak correlation between the ACE environment and the earths environment during this event.The Dst depends on the orientation of the IMF relative to the solar magnetospheric x-y plane and seems to be relatively large when the y component of the IMF is positive and perhaps also when the x component is positive.

Effect of Finite Larmor Radius on Cosmic-ray Penetration into an Interplanetary Magnetic Flux Rope

We discuss a mechanism for cosmic-ray penetration into an interplanetary magnetic flux rope, particularly the effect of the finite Larmor radius and magnetic field irregularities. First, we derive analytical solutions for cosmic-ray behavior inside a magnetic flux rope, on the basis of the Newton-Lorentz equation of a particle, to investigate how cosmic rays penetrate magnetic flux ropes under an assumption of there being no scattering by small-scale magnetic field irregularities. The results show that the behavior of a particle is determined by only one parameter f 0, that is, the ratio of the Larmor radius at the flux rope axis to the flux rope radius. The analytical solutions show that cosmic rays cannot penetrate into the inner region of a flux rope by only gyration and gradient-curvature drift in the case of small f 0. Next, we perform a numerical simulation of a cosmic-ray penetration into an interplanetary magnetic flux rope by adding small-scale magnetic field irregularities. The results show that cosmic rays can penetrate into a magnetic flux rope even in the case of small f 0 because of the effect of small-scale magnetic field irregularities. This simulation also shows that a cosmic-ray density distribution is greatly different from that deduced from a guiding center approximation because of the effect of the finite Larmor radius and magnetic field irregularities for the case of a moderate to large Larmor radius compared to the flux rope radius.

Macroparticle simulation of collisionless parallel shocks generated by solar wind and planetary plasma interactions

An implicit-particle simulation of the collisionless parallel shock created at the interface between an injected beam and a stationary plasma is performed in one-dimensional geometry. The solar wind plasma, which consists of ions and electrons, is injected into a stationary dense plasma that corresponds to the planetary ionosphere. Electromagnetic waves with right-hand circular polarization that propagate upstream (R− waves) are generated at the interface of the two plasmas, which decelerate the solar wind to form a shock. The shock transition region is not monotonic but consists of two distinct regions, a pedestal and a shock ramp. The transition region, which contains the ionopause, is a few thousand electron skin depths long. The parallel shock varies in time and periodically collapses and re-forms. The right-hand circularly polarized electromagnetic waves that propagate downstream (R+ waves) are excited at the shock ramp. Nonlinear wave-particle interaction between the solar wind and the R+ waves causes wave condensation and density modulation. These R+ waves may be sweeping away the downstream plasma to suppress its thermal diffusion across the shock. The electrons at the shock ramp exhibit a flat-topped velocity distribution along the magnetic field owing to the ion acoustic-like electrostatic waves.

A bandwidth challenge at Super Computing (SC) Conference: Large-scale data transfer using 10Gbps network

Results of bandwidth challenge contest at Super Computing (SC) 2007 by NICT are discussed. The target of this contest is to make full use of 10 Gbps network for any scientific applications. We, NICT team, attempted to transfer large-scale numerical data at NICT (in Japan) to SC 2007 venue in Reno, USA. We optimized our data transfer system for the real-time computer simulation data, and finally obtained through-put from Japan to USA as high as 4 Gbps.

Effects of charge exchange and photoionization on the interaction between the solar wind and unmagnetized planets

The effects of photoionization and charge exchange were compared by using three-dimensional hybrid code simulations of the interaction between the solar wind and Venus. The direction of the shock altitude asymmetry in the direction of the convection electric field when photoionization was included was contrary to observations near Venus. However, when charge exchange was included, the direction of the asymmetry did agree with the observations. An analysis of the simulation results showed that the velocities of pickup ions and electrons differed near locations where charge exchange occurred. This velocity difference caused an electric current, and it generated a strong magnetic barrier on the side of the planet to which the convection electric field was pointing. This magnetic barrier acted as an obstacle to the solar wind, and the shock was inflated on this side.

Development of the global simulation model of the heliosphere

The heliospheric structure ranging from the solar surface to the earth’s orbit is self-consistently reproduced from a time-stationary three-dimensional (3D) magnetohydrodynamic (MHD) simulation. The simulation model incorporates gravity, Coriolis, and centrifugal forces into the momentum equation, and coronal heating and field-aligned thermal conduction into the energy equation. The heating term in the present model has its peak at 2.8 solar radius (Rs) and exponentially falls to zero at greater distance from the solar surface. The absolute value of heating depends on the topology of the solar magnetic field so as to be in inverse proportion with the magnetic expansion factor. The results of the simulation simultaneously reproduce the plasma-exit structure on the solar surface, the high-temperature region in the corona, the open- and closed-magnetic-field structures in the corona, the fast and slow streams of the solar wind, and the sector structure in the heliosphere.