Two Correlations with Enhancement Near the Proton Gyroradius Scale in Solar Wind Turbulence: Parker Solar Probe (PSP) and Wind Observations

Based on the Parker Solar Probe mission, this paper presents the observations of two correlations in solar wind turbulence near the Sun for the first time, demonstrating the clear existence of the following two correlations. One is positive correlation between the proton temperature and turbulent magnetic energy density. The other is negative correlation between the spectral index and magnetic helicity. It is found that the former correlation has a maximum correlation coefficient (CC) at the wavenumber k ρ p ≃ 0.5 (ρ p being the proton thermal gyroradius), and the latter correlation has a maximum absolute value of CC at k ρ p ≃ 1.8. In addition, investigations based on 11 yr of Wind observations reveal that the dimensionless wavenumbers (k ρ p ) corresponding to the maximum (absolute) values of CC remain nearly the same for different data sets. These results tend to suggest that the two correlations enhanced near the proton gyroradius scale would be a common feature of solar wind turbulence.

Unveiling the three-dimensional magnetic texture of skyrmion tubes

Magnetic skyrmions are stable topological solitons with complex non-coplanar spin structures. Their nanoscopic size and the low electric currents required to control their motion has opened a new field of research, skyrmionics, that aims for the usage of skyrmions as information carriers. Further advances in skyrmionics call for a thorough understanding of their three-dimensional (3D) spin texture, skyrmion–skyrmion interactions and the coupling to surfaces and interfaces, which crucially affect skyrmion stability and mobility. Here, we quantitatively reconstruct the 3D magnetic texture of Bloch skyrmions with sub-10-nanometre resolution using holographic vector-field electron tomography. The reconstructed textures reveal local deviations from a homogeneous Bloch character within the skyrmion tubes, details of the collapse of the skyrmion texture at surfaces and a correlated modulation of the skyrmion tubes in FeGe along their tube axes. Additionally, we confirm the fundamental principles of skyrmion formation through an evaluation of the 3D magnetic energy density across these magnetic solitons.

Relativistic Modifications in Galactic Rotation Curves under a Toroidal Galactic Field

Rotational dynamics of galaxies exhibits an increase beyond the Keplerian velocity which corresponds to a missing mass up to six times the dynamic mass in the observable universe. In this paper we show that the observed increase in galactic rotation velocities is a general relativistic effect resulting from the combined effect of toroidal magnetic energy density in galaxies and spacetime dragging due to the rotating compact mass in galactic center. The effect of magnetic energy density on spacetime vorticity is derived from Maxwell equations in axially symmetric spacetime where the dragging effects are shown to extend farther in the galactic disc via the toroidal field, modifying the rotational speed of the galactic matter. This is shown to lead to the diverse rotation curves of spiral galaxies, along with the Tully-Fisher relation for total galactic mass and maximum rotational velocity.

Can the Local Bubble explain the radio background?

ABSTRACT The ARCADE 2 balloon bolometer along with a number of other instruments have detected what appears to be a radio synchrotron background at frequencies below about 3 GHz. Neither extragalactic radio sources nor diffuse Galactic emission can currently account for this finding. We use the locally measured cosmic ray electron population, demodulated for effects of the Solar wind, and other observational constraints combined with a turbulent magnetic field model to predict the radio synchrotron emission for the Local Bubble. We find that the spectral index of the modelled radio emission is roughly consistent with the radio background. Our model can approximately reproduce the observed antenna temperatures for a mean magnetic field strength B between 3 and 5 nT. We argue that this would not violate observational constraints from pulsar measurements. However, the curvature in the predicted spectrum would mean that other, so far unknown sources would have to contribute below 100 MHz. Also, the magnetic energy density would then dominate over thermal and cosmic ray electron energy density, likely causing an inverse magnetic cascade with large variations of the radio emission in different sky directions as well as high polarization. We argue that this disagrees with several observations and thus that the magnetic field is probably much lower, quite possibly limited by equipartition with the energy density in relativistic or thermal particles (B = 0.2−0.6 nT). In the latter case, we predict a contribution of the Local Bubble to the unexplained radio background at most at the per cent level.

Self-inhibiting thermal conduction in a high-, whistler-unstable plasma

A heat flux in a high-$\unicode[STIX]{x1D6FD}$ plasma with low collisionality triggers the whistler instability. Quasilinear theory predicts saturation of the instability in a marginal state characterized by a heat flux that is fully controlled by electron scattering off magnetic perturbations. This marginal heat flux does not depend on the temperature gradient and scales as $1/\unicode[STIX]{x1D6FD}$. We confirm this theoretical prediction by performing numerical particle-in-cell simulations of the instability. We further calculate the saturation level of magnetic perturbations and the electron scattering rate as functions of $\unicode[STIX]{x1D6FD}$ and the temperature gradient to identify the saturation mechanism as quasilinear. Suppression of the heat flux is caused by oblique whistlers with magnetic-energy density distributed over a wide range of propagation angles. This result can be applied to high-$\unicode[STIX]{x1D6FD}$ astrophysical plasmas, such as the intracluster medium, where thermal conduction at sharp temperature gradients along magnetic-field lines can be significantly suppressed. We provide a convenient expression for the amount of suppression of the heat flux relative to the classical Spitzer value as a function of the temperature gradient and $\unicode[STIX]{x1D6FD}$. For a turbulent plasma, the additional independent suppression by the mirror instability is capable of producing large total suppression factors (several tens in galaxy clusters) in regions with strong temperature gradients.

Mean Energy Density of Photogenerated Magnetic Fields Throughout the EoR

There seems to be magnetic fields at all scales and epochs in our Universe, but their origin at large scales remains an important open question of cosmology. In this work we focus on the generation of magnetic fields in the intergalactic medium due to the photoionizations by the first galaxies, all along the Epoch of Reionization. Based on previous studies which considered only isolated sources, we develop an analytical model to estimate the mean magnetic energy density accumulated in the Universe by this process. In our model, without considering any amplification process, the Universe is globally magnetized by this mechanism to the order of, at least, several 10−18 G during the Epoch of Reionization (i.e. a few 10−20 G comoving).

Spectropolarimetric diagnostics of unresolved magnetic fields in the quiet solar photosphere

A few years before the Hinode space telescope was launched, an investigation based on the Hanle effect in atomic and molecular lines indicated that the bulk of the quiet solar photosphere is significantly magnetized, due to the ubiquitous presence of an unresolved magnetic field with an average strength 〈B〉, ≈ 130 G. It was pointed out also that this “hidden” field must be much stronger in the intergranular regions of solar surface convection than in the granular regions, and it was suggested that this unresolved magnetic field could perhaps provide the clue for understanding how the outer solar atmosphere is energized. In fact, the ensuing magnetic energy density is so significant that the energy flux estimated using the typical value of 1 km/s for the convective velocity (thinking in rising magnetic loops) or the Alfvén speed (thinking in Alfvén waves generated by magnetic reconnection) turns out to be substantially larger than that required to balance the chromospheric energy losses. Here we present a brief review of the research that led to such conclusions, with emphasis on a new three-dimensional radiative transfer investigation aimed at determining the magnetization of the quiet Sun photosphere from the Hanle effect in the Sr I 4607 Å line and the Zeeman effect in Fe I lines.