Plasma Astrophysics

Plasma Astrophysics

More than 99% of dilute, high-temperature gasses in space are in an ionized state called a plasma. Understanding of plasma physics is, therefore, critical in various aspects of astrophysical phenomena. We consider the solar system as “a plasma physics laboratory in space,” and conduct research on, e.g., shock waves producing high-energy particles, magnetic reconnection leading to explosive phenomena such as solar flares and aurora breakups. We apply the understanding of these elementary plasma physics obtained with the solar system plasma research to more general astrophysical phenomena. We also conduct fundamental physics research such as nonlinear physics and non-equilibrium physics which often become important in space and astrophysical plasmas.

Origin of Cosmic Rays

The cosmic rays (CRs) are extremely high-energy charged particles in space. Although the number density of CRs is small compared to the thermal gas, the high individual particle energies of CRs make its energy density comparable to an averaged thermal energy of the interstellar medium as well as the magnetic field energy in the Galaxy. The origin of CRs is one of the most important issues in astrophysics. The most widely-accepted theory presumes that the CRs are produced at a shock wave driven by a supersonic flow interacting with an obstacle. Indeed, observations of high-energy radiation from shock waves in supernova remnants support this standard paradigm.

On the other hand, shock waves have been routinely observed in the solar system. For instance, the supersonic solar wind blowing out from the sun produces a bow shock when it interacts with a planet. Another example is an interplanetary shock produced when a high-speed material is emitted from the sun, often associated with a solar flare. Indeed, energetic particle flux enhancements have been observed around the shock waves. Since these shock waves in the solar system can be observed directly (or in-situ) by satellites in space, despite the small scale size of the problem, one may learn a lot more than remote-sensing astrophysical observations. By taking advantage of this, we conduct research for a unified understanding of particle acceleration in shock waves. The application to astrophysical phenomena includes shock waves in supernova remnants, as well as in relativistic outflows emanating from neutron stars and/or black holes.

Figure caption: Image taken by the Chandra X-ray observatory. Radiation from the outer shell is believed to be synchrotron emission from ultra-relativistic electrons accelerated by the shock wave.

Magnetic Explosion

A solar flare is the most energetic explosion in the solar system occurring on the solar surface, leading to abrupt brightening across the electromagnetic spectrum from radio to gamma-rays. The solar flare releases the magnetic energy stored in the solar corona via a mechanism called magnetic reconnection, and the free energy is rapidly converted into the plasma thermal and kinetic energies. A large amount of coronal material is often emitted to the interplanetary space by a solar flare, which, if it interacts with the earth, causes beautiful auroral activities at high-latitude regions near the geomagnetic poles. Magnetic reconnection is also believed to play a key role in the magnetospheric substorm or the aurora breakup.

Since the 1960s, understanding of magnetic reconnection has been advanced quite rapidly, in which the Japanese Yokoh and Geotail satellites played major roles for magnetic reconnection studies in the solar corona and the earth’s magnetosphere, respectively. The importance of magnetic reconnection is now well recognized in astrophysics as well, including plasma environments around neutron stars and black holes. Magnetic reconnection not only heats the plasma but also produces high-energy particles well beyond the thermal energy. This aspect is also important in application to astrophysics where non-thermal particles are ubiquitous.

How is the rapid energy release realized, and how are the high-energy particles produced in magnetic reconnection? These questions are still open and have been actively investigated. We try to understand the physics of magnetic reconnection with various approaches, including magnetohydrodynamics simulations, kinetic simulations, and observational data analysis. In particular, we use the data obtained by NASA’s MMS (Magnetospheric Multiscale) spacecraft launched in 2015, whose primary goal is to understand the physics of magnetic reconnection.

Figure caption: Solar flare on December 13, 2006 as observed by the Hinode satellite. Explosive energy release via magnetic reconnection heats and accelerates particles and observed as the enhanced emission.

Nonlinear Waves and Turbulence

Violent phenomena occurring in space often generate large-amplitude coherent waves, which exhibit complex behaviors because of strong nonlinearity. Such nonlinear waves may lead to plasma heating, bulk acceleration, as well as the generation of energetic particles. Understanding of nonlinear waves is thus important in astrophysics.

For instance, the solar corona having a temperature of million kelvins is high above the solar surface, whose temperature is only several thousand kelvins. Such a peculiar structure in view of thermodynamics is realized because of plasma heating provided by dissipation of nonlinear magnetohydrodynamics waves in the solar corona. Similarly, nonlinear waves play a key role in the acceleration of the supersonic solar wind. We study these problems using numerical simulations, as well as observational data analysis with the data obtained by the Hinode and other satellites.

Coherent nonlinear waves called whistlers are important for the generation of relativistic electrons trapped in the dipole geomagnetic field, also known as the radiation belts. In 2016, the Japanese Arase (formally known as ERG) satellite was launched to study the nonlinear wave-particle interactions. The data obtained by the Arase and other satellites have been investigated combined with numerical modeling.

Turbulence is another important subject in astrophysics. Extremely small viscosity or resistivity in space plasmas leads to a turbulent state, which has actually been observed in the interplanetary space and the earth’s magnetosphere. Turbulence plays the essential role in the transport of mass, momentum, and energy. For instance, turbulence is important in terms of the mass and momentum transport from the solar wind into the earth’s magnetosphere. Turbulent angular momentum transport is critical in accretion disks in protoplanetary nebulae or around black holes. Turbulence and associated transport phenomena have been studied both from theoretical and observational viewpoints.

Figure caption: Image of the Arase observation of the earth’s magnetosphere, which conducts in-situ measurements of the radiation belt electrons and nonlinear plasma waves.

Origin of Magnetic Field

Solar magnetic field is playing a role as an energy source in various active phenomena in the interplanetary space. The loop-like structures in the X-ray solar corona correspond to the magnetic field with sunspots at their footpoints.

Such magnetic field is considered to be maintained by the dynamo process. The location of the dynamo is the interior convection zone. It holds a non-uniformly rotating turbulent plasma. The dynamo is an energy conversion process from such flows to the magnetic fields.

Due to the opacity of the surface, no direct observations for dynamo processes are available. So numerical simulations are major tools for the research. However, such computations are resource demanding because of a requirement of both resolving small and short scales in turbulence and covering large and long ones in stellar magnetic cycles. For this reason, the mean-field approximations or local simulations for fundamental turbulent studies have been carried out along with such huge simulations.

Dynamo is a ubiquitous process that can be seen among various planetary and astronomical objects. Our Earth is a good example of them in which dynamo is working in the liquid iron in the outer core of the planet. In accretion disks of young stellar objects, X-ray binaries, and active galactic nuclei, the magnetorotational instability (MRI) plays a very important role for its gas accretion process.