A new study has found that different phases of geomagnetic storms bring significant variation in rates of electromagnetic ion cyclotron (EMIC) waves-- intense wave mode observed in the Earth’s magnetosphere, that can scatter satellite damaging killer electrons, into ionosphere. Pre-onset phase of geomagnetic storms show highest onset of EMIC and this is followed by the main and recovery phases. Tracing the dynamics of these waves and how they interact with such energetic particles can help reduce damage to the high altitude satellites traversing the radiation belts.
Geomagnetic storms are global disturbances in the Earth's magnetic field caused by the interaction with magnetized plasma ejected from the Sun. A large amount of energy is transferred into the Earth’s magnetosphere during geomagnetic storms which then causes disturbances in the electric and magnetic fields in the Earth’s vicinity. Various oscillating electric and magnetic fields, or waves, can interact with high-energy particles in near-Earth space and alter the dynamics of the Earth’s magnetosphere. Hence it is important to know when and where these waves occur so as to plan any future space missions.
A study of 7 years of satellite magnetometer measurements of storm and non-storm periods by scientists of Indian Institute of Geomagnetism (IIG), an autonomous institution of Department of Science and Technology showed that the occurrence probability of EMIC waves are not well captured by a specific geomagnetic activity index alone, but is rather well manifested by considering individual storm phases-- pre-onset, main and recovery. The large database was used to understand the global distribution of EMIC wave occurrence and wave and plasma parameters, like wave amplitude, plasma density, etc. under different geomagnetic conditions.
EMIC waves occur 2.9 times more often during geomagnetic storms than during non-storm times. As the drivers of EMIC waves are different during non-storm, storm time and during individual storm phases, the scientists used geomagnetic activity indices, to illustrate that the occurrence rates during different storm phases even for a given geomagnetic index.
They found that the majority (72%) of storm time EMIC waves occur during the recovery phase due to long recovering time, while the highest occurrence rates are in the pre-onset phase, followed by main and recovery phases. Wave amplitudes are found to be evenly distributed across different Magnetic Local Time (MLT) sectors during all geomagnetic conditions.
EMIC waves are known to cause precipitation of relativistic (∼MeV) electrons or killer electrons from the radiation belts into the upper atmosphere. Such energetic particle precipitation can lead to enhancements in local ionization and can have possible impacts on the radio wave communication in the ionosphere. These high energetic particles are also a concern of radiation dosage to astronauts at the International Space Station, as well as to crew members of high altitude aircrafts. Understanding the spatial distribution of EMIC waves is hence vital from the point of view of its impact on atmosphere or ionosphere.
A schematic showing EMIC wave – relativistic electron interaction and precipitation to high-latitude ionosphere/ atmosphere (Adopted from Yuan et al., GRL, 2018)
The study of complex wave phenomena and its wave-particle interaction processes are vital in understanding the dynamics of geospace for planning of future space missions. As EMIC waves are known to scatter relativistic (~MeVs) of electrons from the Earth’s magnetosphere and radiation belts, they are now being planned as a tool for removing highly energetic radiation belt particles during satellite launches to avoid damage to its electronics. Understanding the near-Earth space plasma processes will help advance the knowledge of plasma waves and instabilities which can be applied in laboratory plasma studies leading to several other plasma applications.
An example of a Van Allen Probe satellite orbit showing dynamic spectra of magnetic field measurement used to identify EMIC waves. The three white lines are the Hydrogen, Helium, and Oxygen cyclotron frequencies observed by Van Allen Probes.