Diffuse, monoenergetic, and broadband aurora: The global precipitation budget

摘要

We have developed an auroral precipitation model which separately categorizes the discrete aurora and both the electron and ion diffuse aurora. The discrete aurora includes acceleration by two distinct physical mechanisms, namely, quasi-static electric fields, producing monoenergetic peaks, and dispersive Alfven waves, producing broadband electron acceleration. The new model is not merely finer in magnetic latitude (MLAT) and magnetic local time (MLT) resolution than previous models but is parameterized by solar wind driving instead of Kp and is based on functional fits to the solar wind coupling function which best predicts auroral power. Each of the four auroral types in each MLAT and MLT bin is separately fitted, a departure from the traditional compilation of a handful of discrete models, each assigned to represent a Kp (or other activity index) range. The variation of any of these four types of aurora at any local time can be predicted on the basis of the specific solar wind history of an epoch. This approach permits perhaps the first comprehensive comparison of the hemispheric contribution of each type of aurora. It turns out that the diffuse aurora is surprisingly dominant, constituting 84% of the energy flux into the ionosphere during conditions of low solar wind driving (63% electrons, 21% ions). The diffuse aurora is far from quiescent, tripling in power dissipation from our low to high solar wind—driving conditions. Even under the latter condition, the diffuse aurora contains 71% of the hemispheric energy flux (57% electrons, 14% ions). The monoenergetic aurora contributes more energy flux (10% quiet, 15% active) than does broadband acceleration signatures (6% quiet, 13% active). However, the broadband aurora rises fastest with activity, increasing by a factor of 8.0 from low to high driving. Moreover, this most dynamic auroral type contributes very high number fluxes, even exceeding monoenergetic aurora under active conditions (28% of hemispheric precipitation versus 21%). Thus, dynamic ionospheric heating and ion outflow is likely heavily affected by the wave aurora. Although energy flux peaks on the nightside, number flux peaks on the dayside. The cusp, as previously reported, is much better defined by ions than electrons. Hence, the ion number flux peak is confined, corresponding to the cusp, while the region with high electron number flux is broad (a cleft, corresponding to the boundary layers, including the closed low-latitude boundary layer).