Inertial Alfvén Waves and the Discrete Aurora

Abstract:

Substorm onset viewed from the ground is characterized by the appearance of highly structured, rapidly moving auroral forms. Within a few minutes the arcs begin to fade and become more diffuse. Electron inertia in Alfvén waves becomes important when the wavelength perpendicular to the magnetic field becomes smaller than the electron inertial length. Under these conditions the waves carry a significant electric field parallel to the magnetic field. Simulations have shown that this parallel electric field can accelerate electrons parallel to the magnetic field in a way that demands the fine-scale structure so characteristic of the aurora. Simulations also show that a substantial fraction of the electrons are accelerated upward to precipitate in the conjugate hemisphere and a substantial fraction of electrons are also trapped in the geomagnetic field. The more diffuse auroral forms during the declining phase of the substorm can be attributed to electrons accelerated in the conjugate hemisphere and to subsequent precipitation of quasi-trapped electrons. A major gap in our understanding is the origin of the inertial Alfvén waves. The association between the aurora and Alfvén waves has been well documented by observations made from the Polar Satellite that showed Alfvén waves in transit between the magnetotail and the auroral ionosphere. Hybrid-code simulations have shown the generation of earthward-propagating Alfvén waves as a consequence of substorm-associated depolarization of the tail field. Needed are in situ measurements of perpendicular wavelengths. There is also a need for simulations of the generation region that are capable of resolving spatial scales that will map to electron inertial length scales. My bet at this time is that these waves are generated by partially magnetized ions in colliding plasma populations.