arXiv subjects

Morifumi Mizuno

Publications and source records attributed to Morifumi Mizuno.

5 recordsLinked to original sources

arXiv2026

Continuation of Force-Free Electrodynamics upon the loss of magnetic dominance

Force-Free Electrodynamics (FFE) describes the evolution of the electromagnetic field in magnetically dominated plasmas, but ceases to be hyperbolic once the magnetic dominance condition $F^{ab}F_{ab}>0$ is lost. We demonstrate that, after the loss of magnetic dominance, FFE may be replaced by a theory of null fields, $F^{ab}F_{ab}=0=F^{ab}\tilde{F}_{ab}$, characterized by the condition that its principal null direction is tangent to a geodesic congruence. In flat spacetime, this theory may be equivalently stated as the condition that the field satisfies $\vec{B}^2-\vec{E}^2=0=\vec{E}\cdot\vec{B}$ and the integral curves of the drift velocity $\vec{E}\times\vec{B}/\vec{B}^{2}$ are straight lines. We develop the general structure and the properties of this theory and test it against 1D PIC simulations using the collision of planar symmetric Alfvén waves. We find that the force-free combined with the null continuation shows remarkable macroscopic agreement with PIC simulations, including the birth and evolution of the null region ($F^{ab}F_{ab}=0$) and the formation of a current sheet. As an independent test, we apply the null continuation to Adhikari's type-changing solution, an exact FFE solution exhibiting finite-time loss of magnetic dominance, and also find macroscopic agreement with 1D PIC simulations.

astro-ph.HE↗

arXiv2026

Schwinger effect with backreaction in 1+1D massive QED with a strong external field

In the presence of a strong electric field, the vacuum is unstable to the production of pairs of charged particles -- the Schwinger effect. The created pairs extract energy from the electric field, resulting in nontrivial backreaction. In this paper, we study 1+1D massive QED subject to strong external electric fields in a self-consistent and fully quantum manner. We use the bosonized version of the theory, which attains a cosine interaction term in the presence of nonzero fermion mass $m$. However, the assumption of strong electric field justifies a perturbative treatment of the cosine interaction, i.e., an expansion in $m$. We calculate the vacuum expectation value of the electric field to first order in $m$ and show that -- surprisingly -- it satisfies a classical nonlinear partial differential equation (related to the sine-Gordon equation). We show that the electric field exhibits dissipation-free oscillations (analogous to ordinary plasma oscillations) and calculate the plasma frequency analytically. We also compare to the semiclassical approximation commonly used to study backreaction, showing that it fails to capture the $O(m)$ shift in the plasma frequency.

hep-th↗

arXiv2025

Plasma flow in force-free magnetospheres: two-fluid model near pulsars and black holes

Force-free electrodynamics describes the electromagnetic field of the magnetically dominated plasma found near pulsars and active black holes, but gives no information about the underlying particles that ultimately produce the observable emission. Working in the two-fluid approximation, we show how particles can be "painted on" to a force-free solution as a function of boundary conditions that encode the particle output of "gap regions" where the force-free approximation does not hold. These boundary conditions also determine the leading parallel electric field in the entire magnetosphere. Our treatment holds in a general (possibly curved) spacetime and is phrased in language intrinsic to the 1+1 dimensional "field sheet spacetimes" experienced by particles stuck to magnetic field lines. Besides the new results, this provides an elegant formulation of some standard equations; for example, we show that the zero-gyroradius guiding center approximation is just the Lorentz force law on the field sheet. We derive a general perturbative method and apply it to pulsar and black hole magnetospheres with radial magnetic fields to produce fully analytic models that capture key features of the full problem. When applied to more realistic magnetic field configurations together with simulation-informed boundary conditions for the gap regions, this approach has the potential to provide global magnetosphere models without the need for global particle-in-cell simulations.

astro-ph.HE↗

arXiv2024

New Consistency Relations between Averages and Variances of Weakly Lensed Signals of Gravitational Waves

The lensing of gravitational waves (GWs) occurs when GWs experience local gravitational potential. In the weak lensing regime, it has been reported that a simple consistency relation holds between the variances of the magnification and phase modulation. In this paper, we present two additional consistency relations between the averages and variances of the weakly lensed GW signals in wave optics. We demonstrate that these consistency relations are derived as the weak lensing limit of the full-order relations for the averages of the amplification factor and its absolute square. These full-order relations appear to originate from energy conservation and the Shapiro time delay, and they are demonstrated to hold irrespective of the matter distribution.

gr-qc↗

arXiv2023

Weak lensing of gravitational waves in wave optics: Beyond the Born approximation

The Universe's matter inhomogeneity gravitationally affects the propagation of gravitational waves (GWs), causing the lensing effect. Particularly, the weak lensing of GWs has been studied within the range of the Born approximation to constrain the small-scale power spectrum. In this work, the validity of the Born approximation is investigated by accounting for the higher-order terms in the gravitational potential $Φ$. To do so, we formulate the post-Born approximation and derive the magnification $K$ and the phase modulation $S$ up to third order in $Φ$. We find that the average of $S$ and $K$ is non-zero and that the average of $S$ depends on the size of the point mass. Due to this size dependency, the signal is enhanced, and the number of GW events required for detecting the average of $S$ decreases. We find that this number can become comparable to or even smaller than the number required for detecting the variance of $S$ in certain scenarios. In addition, it is verified that, for lensing by dark low-mass halos, the post-Born corrections are a few orders of magnitude smaller than the Born approximation at $f\geq0.01$~Hz. However, in the presence of the point mass, there is a condition under which the Born approximation fails. We derive the correction terms to the Born approximation and identify the condition under which the Born approximation no longer holds. For the magnification, the Born approximation is valid as long as the wavelength of GWs is larger than the Schwarzschild radius of lenses, while for the phase modulation, this condition is modified due to the physical size of the point mass.

astro-ph.CO↗