arXiv subjects

Tohru Hada

Publications and source records attributed to Tohru Hada.

5 recordsLinked to original sources

arXiv2024

Review of the August 1972 and March 1989 Space Weather Events: Can We Learn Anything New From Them?

Updated summaries of the August 1972 and March 1989 space weather events have been constructed. The features of these two events are compared to the Carrington 1859 event and a few other major space weather events. It is concluded that solar active regions release energy in a variety of forms (X-rays, EUV photons, visible light, coronal mass ejection (CME) plasmas and fields) and they in turn can produce other energetic effects (solar energetic particles (SEPs), magnetic storms) in a variety of ways. It is clear that there is no strong one-to-one relationship between these various energy sinks. The energy is often distributed differently from one space weather event to the next. Concerning SEPs accelerated at interplanetary CME (ICME) shocks, it is concluded that the Fermi mechanism associated with quasi-parallel shocks is relatively weak and that the gradient drift mechanism (electric fields) at quasi-perpendicular shocks will produce harder spectra and higher fluxes. If the 4 Augusut 1972 intrinsic magnetic cloud condition (southward interplanetary magnetic field instead of northward) and the interplanetary Sun to 1 au conditions were different, a 4 August 1972 magnetic storm and magnetospheric dawn-to-dusk electric fields substantially larger than the Carrington event would have occurred. Under these special interplanetary conditions, a Miyake et al. (2012)-like extreme SEP event may have been formed. The long duration complex 1989 storm was probably greater than the Carrington storm in the sense that the total ring current particle energy was larger.

physics.space-ph↗

arXiv2008

Parametric instabilities of large-amplitude parallel propagating Alfven waves: 2-D PIC simulation

We discuss the parametric instabilities of large-amplitude parallel propagating Alfven waves using the 2-D PIC simulation code. First, we confirmed the results in the past study [Sakai et al, 2005] that the electrons are heated due to the modified two stream instability and that the ions are heated by the parallel propagating ion acoustic waves. However, although the past study argued that such parallel propagating longitudinal waves are excited by transverse modulation of parent Alfven wave, we consider these waves are more likely to be generated by the usual, parallel decay instability. Further, we performed other simulation runs with different polarization of the parent Alfven waves or the different ion thermal velocity. Numerical results suggest that the electron heating by the modified two stream instability due to the large amplitude Alfven waves is unimportant with most parameter sets.

astro-ph↗

arXiv2006

Kinetically modified parametric instabilities of circularly-polarized Alfven waves: Ion kinetic effects

Parametric instabilities of parallel propagating, circularly polarized finite amplitude Alfven waves in a uniform background plasma is studied, within a framework of one-dimensional Vlasov description for ions and massless electron fluid, so that kinetic perturbations in the longitudinal direction (ion Landau damping) are included. The present formulation also includes the Hall effect. The obtained results agree well with relevant analysis in the past, suggesting that kinetic effects in the longitudinal direction play essential roles in the parametric instabilities of Alfven waves when the kinetic effects react "passively". Furthermore, existence of the kinetic parametric instabilities is confirmed for the regime with small wave number daughter waves. Growth rates of these instabilities are sensitive to ion temperature. The formulation and results demonstrated here can be applied to Alfven waves observed in the solar wind and in the earth's foreshock region.

physics.plasm-ph↗

arXiv2006

Remarks on nonlinear relation among phases and frequencies in modulational instabilities of parallel propagating Alfven waves

Nonlinear relations among frequencies and phases in modulational instability of circularly polarized Alfven waves are discussed, within the context of one dimensional, dissipation-less, unforced fluid system. We show that generation of phase coherence is a natural consequence of the modulational instability of Alfven waves. Furthermore, we quantitatively evaluate intensity of wave-wave interaction by using bi-coherence, and also by computing energy flow among wave modes, and demonstrate that the energy flow is directly related to the phase coherence generation.

physics.plasm-ph↗

arXiv2004

Disperson relation of finite amplitude Alfven wave in a relativistic electron- positron plasma

The linear dispersion relation of a finite amplitude, parallel, circularly polarized Alfvén wave in a relativistic electron-positron plasma is derived. In the nonrelativistic regime, the dispersion relation has two branches, one electromagnetic wave, with a low frequency cutoff at $\sqrt{1+2ω_p^2/Ω_p^2}$ (where $ω_p=(4πn e^2/m)^{1/2}$ is the electron/positron plasma frequency), and an Alfvén wave, with high frequency cutoff at the positron gyrofrequency $Ω_p$. There is only one forward propagating mode for a given frequency. However, due to relativistic effects, there is no low frequency cutoff for the electromagnetic branch, and there appears a critical wave number above which the Alfvén wave ceases to exist. This critical wave number is given by $ck_c/Ω_p=a/η$, where $a=ω_p^2/Ω_p^2$ and $η$ is the ratio between the Alfvén wave magnetic field amplitude and the background magnetic field. In this case, for each frequency in the Alfvén branch, two additional forward propagating modes exist with equal frequency. A simple numerical example is studied: by numerically solving the coupled system of fluid and Maxwell equations, normal incidence of a finite amplitude Alfvén wave on an interface between two electron-positron plasmas of different densities is considered.

physics.plasm-ph↗