Realistic Worst Case for a Severe Space Weather Event Driven by a Fast Solar Wind Stream.

Satellite charging is one of the most important risks for satellites on orbit. Satellite charging can lead to an electrostatic discharge resulting in component damage, phantom commands, and loss of service and in exceptional cases total satellite loss. Here we construct a realistic worst case for a fast solar wind stream event lasting 5 days or more and use a physical model to calculate the maximum electron flux greater than 2 MeV for geostationary orbit. We find that the flux tends toward a value of 106 cm-2·s-1·sr-1 after 5 days and remains high for another 5 days. The resulting flux is comparable to a 1 in 150-year event found from an independent statistical analysis of electron data. Approximately 2.5 mm of Al shielding would be required to reduce the internal charging current to below the National Aeronautics and Space Administration-recommended guidelines, much more than is currently used. Thus, we would expect many satellites to report electrostatic discharge anomalies during such an event with a strong likelihood of service outage and total satellite loss. We conclude that satellites at geostationary orbit are more likely to be at risk from fast solar wind stream event than a Carrington-type storm.

Global model of low-frequency chorus (fLHR<f<0.1fce) from multiple satellite observations.

Whistler mode chorus is an important magnetospheric emission, playing a dual role in the acceleration and loss of relativistic electrons in the Earth's outer radiation belt. Chorus is typically generated in the equatorial region in the frequency range 0.1-0.8 fce, where fce is the local electron gyrofrequency. However, as the waves propagate to higher latitudes, significant wave power can occur at frequencies below 0.1fce. Since this wave power is largely omitted in current radiation belt models, we construct a global model of low-frequency chorus, fLHR

Scattering by chorus waves as the dominant cause of diffuse auroral precipitation.

Earth's diffuse aurora occurs over a broad latitude range and is primarily caused by the precipitation of low-energy (0.1-30-keV) electrons originating in the central plasma sheet, which is the source region for hot electrons in the nightside outer magnetosphere. Although generally not visible, the diffuse auroral precipitation provides the main source of energy for the high-latitude nightside upper atmosphere, leading to enhanced ionization and chemical changes. Previous theoretical studies have indicated that two distinct classes of magnetospheric plasma wave, electrostatic electron cyclotron harmonic waves and whistler-mode chorus waves, could be responsible for the electron scattering that leads to diffuse auroral precipitation, but it has hitherto not been possible to determine which is the more important. Here we report an analysis of satellite wave data and Fokker-Planck diffusion calculations which reveals that scattering by chorus is the dominant cause of the most intense diffuse auroral precipitation. This resolves a long-standing controversy. Furthermore, scattering by chorus can remove most electrons as they drift around Earth's magnetosphere, leading to the development of observed pancake distributions, and can account for the global morphology of the diffuse aurora.

The unexpected origin of plasmaspheric hiss from discrete chorus emissions.

Plasmaspheric hiss is a type of electromagnetic wave found ubiquitously in the dense plasma region that encircles the Earth, known as the plasmasphere. This important wave is known to remove the high-energy electrons that are trapped along the Earth's magnetic field lines, and therefore helps to reduce the radiation hazards to satellites and humans in space. Numerous theories to explain the origin of hiss have been proposed over the past four decades, but none have been able to account fully for its observed properties. Here we show that a different wave type called chorus, previously thought to be unrelated to hiss, can propagate into the plasmasphere from tens of thousands of kilometres away, and evolve into hiss. Our new model naturally accounts for the observed frequency band of hiss, its incoherent nature, its day-night asymmetry in intensity, its association with solar activity and its spatial distribution. The connection between chorus and hiss is very interesting because chorus is instrumental in the formation of high-energy electrons outside the plasmasphere, whereas hiss depletes these electrons at lower equatorial altitudes.

Wave acceleration of electrons in the Van Allen radiation belts.

The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earth's magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.