Lab study creates artificial magnetosphere to explore spontaneous excitation of chorus emissions

A dipole magnetic field, created by a ring current, is the most fundamental type of magnetic field found both in laboratories and in space. Planetary magnetospheres, like that of Jupiter, effectively confine plasma.

The RT-1 project aims to learn from nature and create high-performance magnetosphere-like plasma to realize advanced fusion energy. At the same time, the artificial magnetosphere offers a means of experimentally understanding the mechanisms of natural phenomena in a simplified and controlled environment.

The Ring Trap-1 (RT-1) is an experimental device located at the University of Tokyo. Using high-temperature superconducting technology, a dipolar field coil is magnetically levitated, allowing plasma experiments to be conducted in an environment close to that of the planetary magnetosphere.

Whistling chorus emission, observed in the space surrounding Earth, known as “geospace”, is an important phenomenon linked to aurora and space weather. Choral emission has been actively studied primarily through spacecraft observations, theoretical studies, and numerical simulations.

While spacecraft are powerful tools for studying the real space environment, the planetary magnetosphere is a huge and complex system, difficult to understand in its entirety. Moreover, it is not easy for human beings to manipulate the space environment.

In contrast, laboratory environments allow us to create a simplified research object extracted from the complex properties of nature in a controlled environment. Therefore, experimental studies should play a complementary role in the observation and theory of choral broadcast comprehension. However, it is not simple to create a magnetospheric environment in the laboratory. Until now, no laboratory experiments on chorus emissions in a magnetospheric dipole magnetic field have been conducted.

A research team from the National Institute of Fusion Sciences in Toki, Japan, and the Graduate School of Frontier Sciences at the University of Tokyo in Kashiwa, Japan, successfully conducted laboratory studies on the choir broadcast in whistler mode using the RT-1 device. This “artificial magnetosphere” has a magnetically levitating superconducting coil to create a planetary magnetosphere-type dipole magnetic field in the laboratory.

Using high-temperature superconducting technology, a 110 kg coil is magnetically levitated in a vacuum chamber and the generated magnetic field confines the plasma. This unique configuration allows operation without any mechanical support structure for the coil, allowing plasma to be generated in an environment similar to that of a planetary magnetosphere, even within a ground-based installation.

In this study, the research team filled the RT-1 vacuum vessel with hydrogen gas and injected microwaves to create a high-performance hydrogen plasma, primarily by heating electrons.

During the experiments, plasmas were generated in different states and research on wave generation was carried out. Therefore, spontaneous production of the whistling wave chorus emission was observed when the plasma contained a considerable amount of electrons at high temperatures.

Measurements were also taken of the strength and frequency of the plasma chorus emission, focusing on its density and the state of electrons at high temperatures.

The results, published in Natural communications, revealed that the generation of chorus emission is caused by an increase in the number of high-temperature electrons, responsible for plasma pressure. Additionally, increasing the overall plasma density had the effect of suppressing the generation of chorus emission.

Through this study, it was clarified that chorus emission is a universal phenomenon occurring in a plasma with electrons at high temperature in a simple dipole magnetic field. The properties revealed by the experiment, including appearance conditions and wave propagation, could improve our understanding of chorus emission and associated phenomena observed in geospace.

Electromagnetic waves from a choral emission have the potential to further accelerate hot electrons to higher energy states, leading to the formation of aurora and satellite breakdowns. These electromagnetic waves, along with energetic particles, play a crucial role in space weather phenomena.

In geospatial, when explosive events (flares) occur on the surface of the Sun, they cause magnetic storms, causing large fluctuations in the electromagnetic field and generating large quantities of energetic particles. Not only does this cause satellite failures and impact the ozone layer, but it is also known to disrupt power and communications networks on the ground.

With the expansion of human activity today, understanding space weather phenomena is becoming increasingly important. However, many mechanisms and phenomena in this area remain unresolved. The results of this study should contribute to a better understanding of the mechanisms behind various space weather phenomena.

In the field of fusion plasma, which aims to ultimately solve energy problems, particle loss and structure formation due to interaction with waves is one of the central research questions. A precise understanding of the complex interactions between spontaneously excited waves and plasma is essential to achieve fusion.

Wave phenomena with frequency variations have been widely observed in high-temperature fusion plasmas, indicating the existence of a common physical mechanism with chorus emission.

The results of this study represent a step forward in understanding common physical phenomena observed in fusion and space plasmas. It is anticipated that future research will advance further through increased cooperation between these two fields.>

Whistler waves are one of the fundamental waves that propagate in plasma. In chorus emissions observed around geospace and Jupiter, fluctuation events with frequency variations similar to birdsong occur repeatedly. They are thought to be closely linked to auroras and space weather phenomena, such as the production and transport of high-energy electrons.