arXiv subjects

Masaru Nakanotani

Publications and source records attributed to Masaru Nakanotani.

5 recordsLinked to original sources

arXiv2026

Turbulence and its Potential Impact on Solar Chromospheric and Coronal Heating

Low-frequency turbulence in the solar chromosphere remains poorly understood. We address 1) the sources of low-frequency turbulence that potentially heat the chromosphere, and 2) how turbulence is transported and dissipated throughout the chromosphere and lower corona. We use particle-in-cell simulations to investigate mixed polarity magnetic fields corresponding to emergent magnetic carpet field in coronal holes or quiet Sun regions for strong (imbalanced) and weak (balanced) guide magnetic fields. The initial mixed polarity magnetic field transitions rapidly to a turbulent state dominated by advected small-scale nonlinear structures, with a minority slab turbulence population and the emergent field is largely annihilated. Turbulence is anisotropic for imbalanced magnetic field and more isotropic for balanced cases. We develop a transport model for turbulence advected and dissipated throughout the chromosphere by randomly distributed energy-containing scale dynamical flows described by log-normal statistics. We compute the expectations for the total energy per unit volume (h) J m^{-3}, the Elsasser specific energy (h) m^2 s^{-2}, the heating rate <\cdot{H}>(h) J m^{-3} s^{-1}, and the correlation length <λ>(h) km as functions of height h above the photosphere. Turbulent energy is injected into the low corona by a random "patchwork" of sites across the transition region surface. The expected energy injection rates <\cdot{S}> J m^{-2} s^{-1} for the chromosphere and at the base of the corona exceed the estimated energy requirements needed to heat both the chromosphere and corona. Similarly, we show that spicules can be heated gradually with increasing height by entrained magnetic carpet and photospheric turbulence.

astro-ph.SR↗

arXiv2026

Two-Dimensional Kelvin-Helmholtz Instability with Anisotropic Pressure

The Kelvin-Helmholtz (KH) instability occurs in multiple heliospheric (solar-wind stream interfaces, planetary magnetospheres, cometary tails, heliopause flanks) and interstellar (protoplanetary disks, relativistic jets, neutron star accretion disks) environments. While the KH instability has been well-studied in the magnetohydrodynamic (MHD) limit, only limited studies were performed in the collisionless regime, which is conducive to development of anisotropic pressures. Collisionless plasmas are often described using the Chew Goldberger and Low (CGL) equations which feature an anisotropic pressure tensor. This paper presents a comprehensive analysis of the CGL version of the KH instability using linearised and numerical techniques. We find that the largest growth rates and the greatest incidence of magnetic effects occur in the MHD limit. In the large relaxation time CGL limit, part of the energy goes into the formation of pressure anisotropies, resulting in smaller amounts of energy being available for bending the field lines. Consequently, when we cross-compare CGL and MHD simulations that are otherwise identical, the current densities are largest in the MHD limit, and the largest magnetic islands also form in that limit. Early and late time formation of pressure anisotropies have also been studied. We also find that the strongest trend for forming intermittencies in the flow also occurs in the MHD limit. The paper also discusses possible consequences of our results for turbulence and reconnection in the heliosheath (the layer between the solar wind termination shock and the heliopause).

astro-ph.SR↗

arXiv2024

Characterization of Turbulent Fluctuations in the Sub-Alfvenic Solar Wind

Parker Solar Probe (PSP) observed sub-Alfvenic solar wind intervals during encounters 8 - 14, and low-frequency magnetohydrodynamic turbulence in these regions may differ from that in super-Alfvenic wind. We apply a new mode-decomposition analysis (Zank et al 2023) to the sub-Alfvénic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfvén waves, including weakly/non-propagating Alfvén vortices, forward and backward fast and slow magnetosonic modes. Density fluctuations are primarily and almost equally entropy and backward propagating slow magnetosonic modes. The mode-decomposition provides phase information (frequency and wavenumber k) for each mode. Entropy-density fluctuations have a wavenumber anisotropy k_{||} >> k_{perp} whereas slow mode density fluctuations have k_{perp} > k_{||}. Magnetic field fluctuations are primarily magnetic island modes (delta B^i) with an O(1) smaller contribution from uni-directionally propagating Alfven waves (delta B^{A+}) giving a variance anisotropy of <{δB^i}^2> / = 4.1. Incompressible magnetic fluctuations dominate compressible contributions from fast and slow magnetosonic modes. The magnetic island spectrum is Kolmogorov-like k_{perp}^{-1.6} in perpendicular wavenumber and the uni-directional Alfven wave spectra are k_{||}^{-1.6} and k_{perp}^{-1.5}. Fast magnetosonic modes propagate at essentially the Alfvén speed with anti-correlated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due to beta_p<<1. Transverse velocity fluctuations are the dominant velocity component in fast magnetosonic modes and longitudinal fluctuations dominate in slow modes. Mode-decomposition is an effective tool in identifying the basic building blocks of MHD turbulence and provides detailed phase information about each of the modes.

astro-ph.SR↗

arXiv2023

Phase Coherence of Solar Wind Turbulence from the Sun to Earth

The transport of energetic particles in response to solar wind turbulence is important for space weather. To understand charged particle transport, it is usually assumed that the phase of the turbulence is randomly distributed (the random phase approximation) in quasi-linear theory and simulations. In this paper, we calculate the coherence index, $C_ϕ$, of solar wind turbulence observed by the Helios 2 and Parker Solar Probe spacecraft using the surrogate data technique to check if the assumption is valid. Here, values of $C_ϕ=0$ and $1$ indicate that the phase coherence is random and correlated, respectively. We estimate that the coherence index at the resonant scale of energetic ions ($10$ MeV protons) is $0.1$ at $0.87$ and $0.65$ au, $0.18$ at $0.29$ au, and $0.3$ ($0.35$) at $0.09$ au for super (sub)-Alfvénic intervals, respectively. Since the random phase approximation corresponds to $C_ϕ=0$, this may indicate that the random phase approximation is not valid for the transport of energetic particles in the inner heliosphere, especially very close to the Sun ($\sim0.09$ au).

physics.space-ph↗

arXiv2022

Observation of Magnetic Switchback in the Solar Corona

Switchbacks are sudden, large radial deflections of the solar wind magnetic field, widely revealed in interplanetary space by the Parker Solar Probe. The switchbacks' formation mechanism and sources are still unresolved, although candidate mechanisms include Alfvénic turbulence, shear-driven Kelvin-Helmholtz instabilities, interchange reconnection, and geometrical effects related to the Parker spiral. This Letter presents observations from the Metis coronagraph onboard Solar Orbiter of a single large propagating S-shaped vortex, interpreted as first evidence of a switchback in the solar corona. It originated above an active region with the related loop system bounded by open-field regions to the East and West. Observations, modeling, and theory provide strong arguments in favor of the interchange reconnection origin of switchbacks. Metis measurements suggest that the initiation of the switchback may also be an indicator of the origin of slow solar wind.

astro-ph.SR↗