Takeru K. Suzuki

Making the Corona and the Fast Solar Wind: A Self-consistent Simulation for the Low-Frequency Alfvén Waves from the Photosphere to 0.3 AU

By performing a one-dimensional magnetohydrodynamic simulation with radiative cooling and thermal conduction, we show that the coronal heating and the fast solar wind acceleration in the coronal holes are natural consequences of the footpoint fluctuations of the magnetic fields at the photosphere. We initially set up a static open flux tube with a temperature of 104 K rooted at the photosphere. We impose transverse photospheric motions corresponding to the granulations with a velocity dv⊥ = 0.7 km s-1 and a period between 20 s and 30 minutes, which generate outgoing Alfven waves. We self-consistently treat these waves and the plasma heating. After attenuation in the chromosphere by 85% of the initial energy flux, the outgoing Alfven waves enter the corona and contribute to the heating and acceleration of the plasma mainly by the nonlinear generation of the compressive waves and shocks. Our result clearly shows that the initially cool and static atmosphere is naturally heated up to 106 K and accelerated to 800 km s-1.

Forecasting Solar Wind Speeds

By explicitly taking into account the effects of Alfven waves, I derive from a simple energetics argument a fundamental relation that predicts solar wind (SW) speeds in the vicinity of Earth from physical properties on the Sun. Kojima et al. recently found from observations that the ratio of surface magnetic field strength to the expansion factor of open magnetic flux tubes is a good indicator of the SW speed. I show by using the derived relation that this nice correlation is evidence of Alfven wave acceleration of the SW in expanding flux tubes. The observations further require that the fluctuation amplitudes of magnetic field lines at the surface be almost universal in different coronal holes, which needs to be tested with future observations.

Coronal heating and acceleration of the high/low‐speed solar wind by fast/slow MHD shock trains

We investigate coronal heating and acceleration of the high- and low-speed solar wind in the open field region by dissipation of fast and slow magnetohydrodynamical (MHD) waves through MHD shocks. Linearly polarized Alfven (fast MHD) waves and acoustic (slow MHD) waves travelling upwardly along with a magnetic field line eventually form fast switch-on shock trains and hydrodynamical shock trains (N waves) respectively to heat and accelerate the plasma. We determine the one-dimensional structure of the corona from the bottom of the transition region (TR) out to 1 au under the steady-state condition by solving evolutionary equations for the shock amplitudes simultaneously with the momentum and proton/electron energy equations. Our model reproduces the overall trend of the high-speed wind from the polar holes and the low-speed wind from the mid- to low-latitude streamer except the observed hot corona in the streamer. The heating from the slow waves is effective in the low corona to increase the density there, and plays an important role in the formation of the dense low-speed wind. On the other hand, the fast waves can carry a sizable energy to the upper level to heat the outer corona and accelerate the high-speed wind effectively. We also study dependence on field strength, B 0 , at the bottom of the TR and non-radial expansion of a flow tube, f max , to find that large B 0 /f max ? 2 but small B 0 ≃ 2 G are favourable for the high-speed wind and that small B 0 /f max ≃ 1 is required for the low-speed wind.

Alfvén Wave-driven Proto-Neutron Star Winds and r-Process Nucleosynthesis

We propose magnetic proto-neutron star (PNS) winds driven by Alfven waves, as well as the neutrino heating, as an appropriate site for r-process nucleosynthesis. Alfven waves excited by surface motions of a PNS propagate outwardly, and they heat and accelerate the wind by dissipation. Compared with the wind purely driven by neutrino heating, a larger entropy per baryon and shorter dynamical timescale are achieved, which favors the r-process. We study reasonable cases in which the wave amplitude is 10% of the Alfven speed at the surface to find that a PNS with surface field strength 5 × 1014 G gives suitable wind properties for the r-process, provided that the dissipation length of the wave is at most ~10 times the PNS radius. We also compare properties of transcritical and subcritical winds in light of the r-process. We finally discuss possibilities of detections of γ-rays from radioactive nuclei and absorption lines due to Ba in supernova remnants that possess magnetars.

Tsunamis in Galaxy Clusters: Heating of Cool Cores by Acoustic Waves

Using an analytical model and numerical simulations, we show that acoustic waves generated by turbulent motion in intracluster medium effectively heat the central region of a so-called cooling flow cluster. We assume that the turbulence is generated by substructure motion in a cluster or cluster mergers. Our analytical model can reproduce observed density and temperature profiles of a few clusters. We also show that waves can transfer more energy from the outer region of a cluster than thermal conduction alone. Numerical simulations generally support the results of the analytical study.

On the Heating of Cluster Cooling Flows by Sound Waves

We investigate heating of the cool core of a galaxy cluster through the dissipation of sound waves excited by the activity of the central active galactic nucleus. Using a weak-shock theory, we show that this heating mechanism alone cannot reproduce the observed temperature and density profiles of a cluster, because the dissipation length of the waves is much smaller than the size of the core, and thus the wave energy is not distributed to the whole core. However, we find that if it is combined with thermal conduction from the hot outer layer of the cluster, wave heating can reproduce the observational results.

Grand Challenges in Protoplanetary Disc Modelling

The Protoplanetary Discussions conference—held in Edinburgh, UK, from 2016 March 7th–11th—included several open sessions led by participants. This paper reports on the discussions collectively concerned with the multi-physics modelling of protoplanetary discs, including the self-consistent calculation of gas and dust dynamics, radiative transfer, and chemistry. After a short introduction to each of these disciplines in isolation, we identify a series of burning questions and grand challenges associated with their continuing development and integration. We then discuss potential pathways towards solving these challenges, grouped by strategical, technical, and collaborative developments. This paper is not intended to be a review, but rather to motivate and direct future research and collaboration across typically distinct fields based on community-driven input, to encourage further progress in our understanding of circumstellar and protoplanetary discs.

Collisionless damping of fast magnetohydrodynamic waves in magnetorotational winds

We propose collisionless damping of fast MHD waves as an important mechanism for the heating and acceleration of winds from rotating stars. Stellar rotation causes magnetic field lines anchored at the surface to form a spiral pattern, and magnetorotational winds can be driven. If the structure is magnetically dominated, fast MHD waves generated at the surface can propagate almost radially outward and cross the field lines. The propagating waves undergo collisionless damping owing to interactions with particles surfing on magnetic mirrors that are formed by the waves themselves. The damping is especially effective where the angle between the wave propagation and the field lines becomes moderately large (~20°-80°). The angle tends naturally to increase into this range because the field in magnetorotational winds develops an increasingly large azimuthal component. The dissipation of the wave energy produces heating and acceleration of the outflow. We show using specified wind structures that this damping process can be important in both solar-type stars and massive stars that have moderately large rotation rates. This mechanism can play a role in the coronae of young solar-type stars that are rapidly rotating and show X-ray luminosities much larger than that of the Sun. The mechanism could also be important for producing the extended X-ray-emitting regions inferred to exist in massive stars of spectral type middle B and later.

Cosmic ray production of ⁶Li by virialisation shocks in the early Milky Way

The energy dissipated by virialisation shocks during hierarchical structure formation of the Galaxy can exceed that injected by concomitant supernova (SN) explosions. Cosmic rays (CRs) accelerated by such shocks may therefore dominate over SNe in the production of 6Li through α + α fusion without co-producing Be and B. This process can give a more natural account of the observed 6Li abundance in metal-poor stars compared to standard SN CR scenarios. Future searches for correlations between the 6Li abundance and the kinematic properties of halo stars may constitute an important probe of how the Galaxy and its halo formed. Furthermore, 6Li may offer interesting clues to some fundamental but currently unresolved issues in cosmology and structure formation on sub-galactic scales.

R-process Nucleosynthesis in Alfven Wave-Driven Proto-Neutron Star Winds

We propose magnetic proto-neutron star (PNS) winds driven by Alfvén waves as well as the neutrino heating as an appropriate site for the r-process nucleosynthesis. Alfvén waves excited by surface motions of a PNS propagate outwardly, and they heat and accelerate the wind by dissipation. In the Alfvén wave-driven wind, larger entropy per baryon and shorter dynamical time scale are achieved, which favors the r-process. A PNS with surface B0 > ∼ 5× 1014G, gives suitable wind properties for the r-process in a typical case. We also perform nuclear reation calcuations and confirm this result; the 3rd peak elements are sufficiently synthesized in the Alfvén wavedriven wind in such a condition.