Full non-LTE spectral line formation II. Two-distribution radiation transfer with coherent scattering in the atom's frame

In the present article, we discuss a numerical method of solution for the so-called "full non-LTE" radiation transfer problem, basic formalism of which was revisited by Paletou & Peymirat (2021; see also Oxenius 1986). More specifically, usual numerical iterative methods for non-LTE radiation transfer are coupled with the above-mentioned formalism. New numerical additions are explained in detail. We benchmark the whole process with the standard non-LTE transfer problem for a two-level atom with Hummer's (1962, 1969) $R_{\rm I-A}$ partial frequency redistribution function. We finally display new quantities such as the spatial distribution of the velocity distribution function of excited atoms, that can only be accessed to by adopting this more general frame for non-LTE radiation transfer.Comment: 7 pages, 5 figures, accepted A&

A numerical method to compute Euler potentials for non dipolar magnetic fields

International audienceThe magnetospheric magnetic field may be conveniently described by two scalar functions (a, ß), known as the Euler potentials. They are not uniquely defined, and they may be difficult to derive for configuration more complex than a simple dipole. We propose here a simple numerical method to compute one possible pair (a, ß). In magnetospheric regions of closed field lines, a can be chosen as a function of the tube volume of unit magnetic flux. The method can be applied to a wide class of magnetic fields which describe the magnetospheric domain of closed field lines and the conjugated ionosphere. Here, it is used with the T87 Tsyganenko model. The results coincide with the dipolar potentials at close distances from the Earth. At larger distances, they display an increasing distortion with the radial distance (or the invariant latitude in the ionosphere) and the magnetic activity. In the magnetosphere, the contours of a and ß are stretched towards the nightside. In the ionosphere, they also extend towards the nightside and present major distortions in a narrow ring at the polar cap boundary, which maps distant boundary layers in the magnetosphere

Relationships between field-aligned currents and convection observed by EISCAT and implications concerning the mechanism that produces region-2 currents: Statistical study

Fluid theories explain the origin of region-2 field-aligned currents as the closure of the ring current, driven itself by the azimuthal pressure gradients generated in the magnetospheric ring plasma by the sunward convection. Although the structure of pressure gradients appears experimentally complex, observations confirm that a close connection exists between the region-2 field-aligned currents and the ring current. The fluid linear theory of the adiabatic transport by convection of the ring plasma gives a first estimate of this process, and leads ultimately to phase quadrature (in terms of magnetic local time) between the region-2 field-aligned currents and the convection potential. When significant non-adiabatic processes are taken into account, such as precipitations at auroral latitudes, the theoretical phase difference rotates toward opposition. We determine experimentally the phase relationship between the region-2 field-aligned currents and the convection potential from recent statistics, depending on the magnetic activity index Kp, and performed from the EISCAT data base. For geometrical reasons of sufficient probing of region 2, it is only computed in the case of a moderate magnetic activity corresponding to 2 ≤ Kp<4. Region-2 field-aligned currents are found to be in phase opposition with the convection electrostatic potential at auroral latitudes. This confirms the importance of non adiabatic processes, especially ion losses, in the generation of region-2 field-aligned currents, as theoretically suggested

Full non–LTE spectral line formation I. Setting the stage

International audienceRadiative transfer out of local thermodynamic equilibrium (NLTE) has been increasingly adressed, mostly numerically, for about six decades now. However, the standard NLTE problem most often refers to the only deviation of the distribution of photons from their equilibrium, that is to say a Planckian distribution. Hereafter we revisit after Oxenius (1986, Kinetic theory of particles and Photons – Theoretical Foundations of non–LTE Plasma Spectroscopy, Springer) the so-called full NLTE problem, which considers coupling and therefore solving self–consistently for deviations from equilibrium distributions of photons as well as for massive particles constituting the atmospheric plasma

Large-scale distributions of ionospheric horizontal and field-aligned currents inferred from EISCAT

International audienceStatistical models for large-scale convection and for ionospheric conductances were previously derived from observations of the incoherent-scatter radar EISCAT. We complete this large-scale description with statistical models of the horizontal and field-aligned currents achieved from the same data base and for the same ranges of the magnetic activity index Kp. Except for the high-latitude dayside currents generally located poleward of the radar field of view, a large part of the whole current system can be probed with EISCAT. Globally consistent with previously published models, our results also exhibit some differences, such as the asymmetry in the local-time extension of the current sheets, concentrated to a few hours around 18 MLT in the evening sector, while widely spread from premidnight to prenoon magnetic local times on the morningside. This statistical description of the current system above EISCAT allowed us to examine several aspects of the large-scale auroral electrodynamics, namely the relationships between convection, conductances, and currents, in particular in the vicinity of the Harang discontinuity, and the features of the global current circuit

Polar cap convection patterns inferred from EISCAT observations

International audienceFrom data of the European incoherent scatter radar EISCAT, and mainly from its tristatic capabilities, statistical models of steady convection in the auroral ionosphere were achieved for various levels of magnetic activity. We propose here to consistently extend these models to the polar cap, by avoiding the use of a pre-defined convection pattern. Basically, we solve the second-order differential equation governing the polar cap convection potential with the boundary conditions provided by these models. The results display the classical twin-vortex convection pattern, with the cell centres around 17 MLT for the evening cell and largely shifted towards midnight (3–3.5 MLT) for the morning cell, both slightly moving equatorward with activity. For moderate magnetic activities, the convection flow appears approximately oriented along the meridian from 10:00 MLT to 22:00 MLT, while in more active situations, it enters the polar cap at prenoon times following the antisunward direction, and then turns to exit around 21:00 MLT. Finally, from these polar cap patterns combined with the auroral statistical models, we build analytical models of the auroral and polar convection expected in steady magnetic conditions