Estimating energy spectra of electron precipitation above auroral arcs from ground‐based observations with radar and optics

Abstract

[1] In 2008, coordinated radar-optical auroral observations were organized in Northern Scandinavia using the European Incoherent Scatter Radar (EISCAT) and the Auroral Large Imaging System (ALIS). A bright auroral arc was imaged on 5 March 2008 from four ground-based stations, remaining stable between 18:41 and 18:44 UT and coinciding with increased electron densities. This work presents a unified inversion framework deriving the electron energy spectrum from either optical (N + 2 1NG(0, 1) emission at 4278 Å) or radar (electron density) observations. An updated forward model of the ionosphere based on a 1-D kinetic Monte Carlo model is described, characterizing the linear system to invert. The 3-D blue volume emission rate is first estimated with an iterative reconstruction technique. Also presented is a novel way to calculate an accurate initial guess for auroral tomography taking into account the horizontal/vertical nonuniformity of the emission region. This technique supersedes the often-used Chapman profile as initial guess when high spatial resolution is needed. The second step, performed for the first time with ALIS optical observations, uses the forward model to retrieve from the emission rates the 2-D latitude/longitude map of the electron energy spectrum. The same model and inversion methods are finally applied to the EISCAT electron density profiles to derive the temporal energy spectrum of precipitating electrons along the magnetic zenith. Energy spectra from radar and optics are in good agreement. Results suggest that the arc is generated by a complex energy spectrum reminiscent of dispersive Alfvén waves with two main peaks (2.5, 6 keV) and a typical latitudinal width of 7.5 km.Citation: Simon Wedlund, C., H. Lamy, B. Gustavsson, T. Sergienko, and U. Brändström (2013), Estimating energy spectra of electron precipitation above auroral arcs from ground-based observations with radar and optics,

The dependence of the strength and thickness of field‐aligned currents on solar wind and ionospheric parameters

“…Using low‐altitude and solar wind observations, we inferred reasonably well the sheared boundary layer thickness (Δ m ), which was found to be around 3000 km (consistent with in situ observations) and which appears to have some dependence on V sw . Our results indicate that these methods could serve as the basis for development of general tools for inferring boundary layer structures [ Simon Wedlund et al , ]. While there can be significant advantages to using this simplified analytic model for data analysis and modeling, it is also important to recognize the model limitations discussed in section 4.4 that may in some circumstances require a more comprehensive numerical treatment [e.g., Echim et al , ].…”

Temporal and spatial evolution of auroral electron energy spectra in a region surrounding the magnetic zenith

“…Tanaka et al [] studied the feasibility of combining multi‐instrument data, primarily images of the aurora and ISR measurements of electron density, to produce estimates of the spatial and energy distribution of the electron precipitation. Simon Wedlund et al [] applied such methods to study the energy spectra of electron precipitation above an auroral arc, but the spatial and temporal resolutions of the data and models, 2.5 km and between 5 and 20 s respectively, are unsuitable for studying the highly structured and dynamic small‐scale aurora.…”

Reconstruction of Fine-Scale Auroral Dynamics

“…The close-spaced optical instruments used in this study yield persistent observations of precipitation process outcomes [33], [34] complementing on-orbit and rocket-borne in situ measurements with a broader spatiotemporal context, along with improved B ⊥ resolution over widely spaced ground-based imagers. Observation of a typical rapidly moving (several km/s) auroral feature implicitly requires a frame rate on the order of 100 Hz for a narrow 9 • FOV and megapixel-class imager.…”