ELFIN_radbelts_cutaway_webWhile terrestrial weather has long been a field of study, space weather is very much a fledgling discipline. The damage or loss of satellites such as CRRES as a result of particles released during solar activity serves as a poignant reminder of the implications of space weather.

The sun continuously emits a stream of ions and electrons toward Earth. These charged particles interact with Earth’s magnetic field, altering the trajectories of the particles. The resulting trajectories resemble a spiral or helical rotation around Earth’s magnetic field lines. The pitch angle of a particle is defined to be the angle between the particle’s velocity vector and the direction of the Earth’s magnetic field.

Particles with some velocity component parallel to the direction of the magnetic field (i.e. those with a pitch angle less than 90°) will be drawn closer to Earth. As these particles move closer, they encounter stronger magnetic field strengths, which further alter their motion by changing their pitch angle; for some particles, this change in pitch angle is great enough to cause them to reverse direction and travel back along magnetic field lines toward the opposite pole. The point at which this reflection occurs is called a mirror point, and particles that continuously oscillate between mirror points are said to be trapped. Trapped electrons, often referred to as “killer electrons,” can possess nearly a thousand times more energy than a conventional dental X-ray and present a significant threat to astronauts and spacecraft hardware.

Other particles, however, do not become or remain trapped. Particles whose mirror point lies at altitudes in or below the Earth’s atmosphere can collide with atmospheric particles and eventually be lost to the atmosphere, generating the auroras. Particles lost to the atmosphere in this way have characteristic pitch angles, and the range of pitch angles at which a particle is lost is referred to as the loss cone.

The mechanisms by which trapped particles can precipitate into the loss cone are still subjects under investigation. Computer modeling suggests that equatorial Electromagnetic Ion Cyclotron (EMIC) waves may primarily be responsible for electron losses, but the level of contribution from other effects has not yet been determined observationally. Energetic particle measurements from the ELFIN mission will address this contentious issue by determining whether electron losses bear the characteristic signatures of EMIC wave scattering.