Research Achievements

October 8, 2020 

On the trail of causes of radiation events during space flight

Lessons from conjunction observations with ISS detectors and the Arase satellite.


Scientists have made significant progress in understanding the sources of radiation events that could impact human space-flight operations. Relativistic Electron Precipitation (REP) events are instances when high energy electrons move through areas of space at significant fractions of the speed of light. These REP events may pose challenges to human spaceflight, specifically during extravehicular activity (EVA).


These hazards motivate the question of whether REP events can be forecasted in order to avoid unnecessary human exposure to radiation. In order to predict REP events, their cause must first be determined.


A scientific team in Japan has made strides in answering that question. Their findings were published on August 14 in the Journal of Geophysical Research: Space Physics.


The team, consisting of Dr. Ryuho Kataoka at the National Institute of Polar Research, Prof. Yoshizumi Miyoshi and Asst. Prof. Masafumi Shoji at the Institute for Space-Earth Environmental Research of Nagoya University, and other scientists in Japan, pinpointed the cause of REP events and emphasized that REP events must be accounted for in human spaceflight missions.


It has been hypothesized that electromagnetic ion cyclotron (EMIC) waves play an important role in REP events at the ISS. It was still an open question, however, whether other mechanisms played a role in REP event generation. EMIC waves are electromagnetic waves that propagate through the plasma in Earth's magnetosphere, causing disturbances in the charged particles within the plasma.



Figure1  Left: the Arase satellite. Right: Three different detectors on the ISS were complementarily used. SEDA-AP (Space Environment Data Acquisition equipment-Attached Payload), CALET (Calorimetric Electron Telescope), and MAXI (Monitor of All-sky X-ray Image). Credit: Left: ©ERG science team.Right: ©JAXA/NASA



Using multiple sensors aboard the ISS, as well as data from the Arase satellite, the research group was able to show that at least three separate processes contributed to REP events. One is indeed EMIC waves. But the data also suggested two other sources: Whistler mode chorus waves and electrostatic whistler waves. Whistler mode waves can be excited by high energy electrons associated with auroral activities, such as the Northern Lights.



(Click on the image for a larger version.)


Figure2   Plasma waves observed by the Arase satellite (left) and count rate of MeV electrons observed with the CALET detector on the ISS (right). Credit: Ryuho Kataoka et al., 2020



With a better understanding of the physical causes of REP events, the team is working towards ways to predict future events. Kataoka says, "The next step is the space weather forecast of REP events at the ISS by modeling different kinds of plasma wave activities. The ultimate goal is to obtain a unified theory to understand the interaction between energetic particles and plasma waves, and their impact of radiation dose on the atmosphere, space craft, and human beings."


This study, "Plasma Waves Causing Relativistic Electron Precipitation Events at International Space Station: Lessons From Conjunction Observations With Arase Satellite," was published online in JGR Space Physics on August 14, 2020 at DOI: 10.1029/2020JA027875


About the ERG Science Center

The ERG (Exploration of energization and Radiation in Geospace) Science Center is operated by the Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), and the Institute for Space-Earth Environmental Research (ISEE) at Nagoya University. All data related to the ERG project are open to the public on the center's website:



This article was originally published on the National Institute of Polar Research website:

(The content has been edited by Nagoya University.)


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