Seeding solar storms: suprathermal particles and the challenges researchers face
Dr Maher Dayeh is a senior research scientist at the Southwest Research Institute in San Antonio, Texas. His research includes heliospheric and magnetospheric physics, space weather, lightning and thunder microphysics, and the design and development of particle detectors for atmospheric and space applications. Dr Dayeh’s current work focuses on measuring the characteristics of suprathermal particles in the heliosphere, and understanding how they may affect and contribute to space weather prediction. This is important work as suprathermal particles play a key role in seeding solar storms and are particularly difficult to measure.
Sketch showing a solar flare and CME driving an interplanetary shock. The suprathermal particle population provides material that gets picked up by the shock and gets accelerated into very high energies [after Desai and Bergess 2008].
Suprathermal particles are charged ions and electrons that are found throughout interplanetary space, and move at speeds two to hundreds of times faster than the thermal plasma of the solar wind. Their origin and properties are unsolved questions, however, there are currently two prevailing theories that describe their origin and acceleration, i.e., where do these particles come from, and how do they energise? One theory suggests that they originate from solar winds or similar low-energy particles in space and become energised by localised compressions, while the other suggests that they are partially remnants of previous events such as solar flares, coronal mass ejections (CMEs), and co-rotating interaction regions (CIRs), which are often associated with shocks that accelerate particles in the interplanetary medium. Studying these suprathermal particles is critical to the understanding of space weather and its effects on the near-Earth environment.
Suprathermal particles are difficult to measure as they exist at a wide range of energies, from about 1–2 kilo-electron volts (keV) to hundreds of keV, and most instruments that have been used in solar particle research in the past were designed to measure either low or high energy particles, but not both at the same time. Such instruments are unable to capture the full energy range of suprathermal particles, making these particle measurements very sparse. It is thus important to develop novel instrumentation that can capture the full energy range of suprathermal particles.
Suprathermal particle population plays a key role in providing seed material to energetic solar storms that drive space weather. Yet they remain puzzling.
Dr Maher Dayeh has studied solar, heliospheric, and magnetospheric physics, along with space weather, and is currently studying suprathermal particles in different regions of the heliosphere (a bubble-like cocoon inflated by the continuous outflow of the solar wind from the Sun that extends far beyond our solar system) by studying data from a variety of instruments on board NASA, NOAA and ESA spacecraft. His work aims to identify the properties of suprathermal particles in different regions of the heliosphere, identify characteristics that can be used to distinguish between the different acceleration processes, and understand how suprathermal particles may affect space weather prediction.
Suprathermal Particles
When a solar event occurs, such as solar flares and coronal mass ejections, charged particles (e.g., ions and electrons) are released into the heliosphere. These particles can be found at a range of energies, from 1 keV up to millions of keV. Particles that hold a level of energy higher than that of their surrounding plasma or matter are known as suprathermal particles. Populations of these particles are diverse and dynamic; they often contain ions from a variety of sources, including solar and interplanetary shock-accelerated events, cometary and interstellar sources. Suprathermal particles also contain a variety of elements, including helium, iron and oxygen. This, combined with the massive range of energies they can be found at, sheds light on the complexity of understanding their properties. It has been found that, in the heliosphere, the composition and size of suprathermal particle populations are related to solar cycle variability. In a recent study, Dr Dayeh found that during “quiet times”, which are periods characterised by very low levels of solar activity and a quiet interplanetary space, particles measured near the Earth are dominated by matter that traces back to previous solar and interplanetary events.
Diagram showing the particle sources believed to contribute to the suprathermal seed population [adapted from Mason 2000].
These suprathermal particles are believed to serve as a seed population for the large energetic particle events that are accelerated by fast-moving interplanetary shocks. Understanding the properties of these particles is key to understanding particle acceleration and transport throughout the solar corona and the heliosphere.
Elusive particles and impacts
Suprathermal particle populations are difficult to study. As these particles exist within a wide range of energies, they cannot be fully characterised with dedicated instruments that are used to solely study plasma or energetic particles. Despite this, the limited research conducted has given crucial hints about the composition of suprathermal particle populations. They contain a wide variety of elements from a variety of sources, making them very diverse and dynamic.
Suprathermal particle populations are important as they act as seeds for large solar particle events, such as CME-driven shocks in the interplanetary space that drive space weather events. CME-driven shocks are responsible for accelerating particles to very high energies, forming a radiation hazard for astronauts and space instrumentation. These CMEs can also create intense geomagnetic storms resulting in unwanted space weather consequences on Earth, such as damage to the power grid and GPS communication systems interruptions.
Understanding suprathermal particles is critical for understanding solar energetic particle events and ultimately space weather at Earth.
Astronauts on future planetary missions are typically exposed to higher levels of radiation as they are not protected by the Earth’s atmosphere and magnetosphere, however, during solar events the levels of radiation increase dramatically and can lead to severe health effects. Space instrumentation is also sensitive to radiation as it can penetrate the spacecraft itself and either damage or destroy electronics, in addition to causing satellites to drift from their assigned orbits. For example, a storm in April 2010 is thought to have caused an important communications satellite, Galaxy-15, to founder and drift, taking almost a year to return to its station.
Oxygen fluence measured by several instruments on NASA’s Advanced Composition Explorer (ACE) spacecraft during a 3-year period. Black data points show the energy spectrum from the low-energy solar wind up to the very energetic particles. The suprathermal seed population bridges the energy gap between the solar wind tails and the low end of
the energetic particle spectrum. [Adapted from Mewaldt 2001].
Understanding the composition, sources, and properties of suprathermal particles is thus an important key to ultimately understanding the complexities of the Sun-Earth connection in space.
Particles near the Earth’s orbit
Dr Dayeh and his team are currently studying the properties of suprathermal particles in the heliosphere near the Earth’s orbit. Their work uses data from a variety of instruments on board several NASA and ESA spacecraft, which can provide a wider picture of the characteristics of these particles in different parts of the solar system, identify tracers that can be used to distinguish between different sources, and determine how understanding these particles might affect the prediction of space weather. This work will further our understanding of how these particles are accelerated in shocks, as well as aid our understanding of solar energetic particle events — the theoretical framework used to study suprathermal particles in space — and further our understanding of particle transport and acceleration in the interplanetary space.
