Diffuse auroral precipitation provides a major source of energy input to the Earth's upper atmosphere. The precipitated particles lead to changes in the rate of ionization, which can modulate large natural current systems and induce electrical currents along transmission lines and pipelines. The additional ionization can also result in chemical changes to the neutral atmosphere, which has been linked with ozone-depletion. The precipitated particles also lead to changes in the electrical conductivity of the ionosphere, which maps along magnetic-field lines and further affect the transport of plasma in the magnetosphere.
The diffuse aurora is primarily caused by rapid pitch-angle scattering of plasma sheet electrons. Because the magnetosphere is essentially collisionless, the only viable mechanism that can cause pitch-angle scattering is wave-particle interactions. Previous studies have shown that scattering by both electrostatic electron cyclotron harmonic (ECH) waves and whistler-mode chorus could be important, but there is still no general consensus on the dominant process. This project will examine the roles of ECH and chorus waves by looking at the rate of wave-induced particle scattering by both classes of wave. Pitch-angle diffusion rates will be calculated with existing codes, based on a comprehensive analysis of the spectral properties of chorus and ECH waves observed by the CRRES satellite in the outer magnetosphere, under different levels of magnetic activity. Computed rates of electron scattering will be used to determine the equilibrium pitch-angle distribution and the rate of precipitation loss to the atmosphere compared to the limit imposed by strong diffusion. The scattering results will be used, together with statistical data on the global distribution of trapped electrons in the near-Earth plasma sheet, to model the global distribution of diffuse auroral emissivity. This study will provide a quantitative understanding of how microphysical processes regulate the transfer of energy in geospace. The proposed research is central to the primary science objectives of the new GEM Focus Group on Diffuse Auroral Precipitation.
Much of the research will be undertaken by a graduate student and a young research scientist. The scientific results obtained will be made available to other members of the magnetospheric community interested in modeling the global distribution of the ring current electron population and diffuse auroral precipitation under different geomagnetic conditions. The results of the study can be used to model the global distribution of ionospheric conductivity, which regulates the rate of magnetospheric convection, and is critical for the development of a Geospace General Circulation Model. Quantification of electron scattering rates in the near-Earth plasmasheet is also important for understanding the evolution of electron flux and pitch angle distributions during injection events, which provide the source of free energy for the excitation of waves affecting energetic radiation belt dynamics. The important physical processes studied at Earth, can be applied to other magnetized planets such as Jupiter and Saturn, where similar scattering can occur.
The diffuse auroral is a weak emission from the upper atmosphere caused by energetic electrons precipitating into the atmosphere. Although weaker than discrete auroral arcs the diffuse aurora occurs overa broad latitudinal region and the deposition of incoming electrons dominates the ionizing energy input coming into the upper atmosphere. Consequently, the diffuse aurora provides a major souce of ionization and conductivity in the upper atmosphere. This in turn affects the coupling between the upper atmosphere and ionosphere and can strongly affect the dynamics of the magnetosphere. The origin of the diffuse aurora had remained a mystery for several decades. Our research has clearly shown that the scattering of electrons into the atmosphere is cause by resonant interactions with an electromagnetic wave called chorus. The chorus waves are excited as electrons are driven into the inner magnetosphere during geomagnetic activity. The excited waves then cause scattering of the injected low energy population. But the same waves can also interact with much higher energy "killer" electrons, which are know to cause radiation damage to orbiting spacecraft. By monitoring the distribution of the diffuse aurora, we can also map out the distribution of the scattering chorus waves and thus obtain a global picture of how these waves influence the Earth's radiation belts..
