Adrian Doicu's research while affiliated with German Aerospace Center (DLR) and other places

... The interference of this kind manifests itself in the mutual shadowing and may considerably influence the opposition effects . While the description of the light scattering by densely packed media in terms of the ladder and cyclic diagrams has been considerably improved in recent years [see Doicu and Mishchenko, 2019b;Doicu and Mishchenko, 2019c) and references therein], the contribution of the diagrams corresponding to the interference of waves of different scattering orders has not been taken into account yet. ...

... The interference of this kind manifests itself in the mutual shadowing and may considerably influence the opposition effects . While the description of the light scattering by densely packed media in terms of the ladder and cyclic diagrams has been considerably improved in recent years [see Doicu and Mishchenko, 2019b;Doicu and Mishchenko, 2019c) and references therein], the contribution of the diagrams corresponding to the interference of waves of different scattering orders has not been taken into account yet. ...

... and that, when considering only one type of particle, the above reduces to an equation which is found in much of the literature [38,18,56,41,15,28]. 12) and the angles θ p and φ p can be complex numbers, meaning that we may have |k p | = 1, but we do have thatk p ·k p = 1 for the real inner product. ...

... In this paper we consider five models of a discrete random densely-packed medium (from A to E below), which differ by both the scattering characteristics of a volume element and the behavior of the extinction in a medium. The medium is considered as a semi-infinite layer, and the incident radiation is assumed to propagate perpendicularly to its boundary, since the theory for a more general case of the obliquely incident radiation is still at an early stage of development [see, e.g., Doicu and Mishchenko, 2019a)]. We compare the results of these models to each other and to the simulation results reported by Penttilä et al. (2021). ...

... This is a self-consistent system Green's function equation analogous to the Dyson equation of quantum field theory [74][75][76][77]. After slightly rearranging Eq. (13) and converting back to standard representation [78,79], we obtain ...

... However, the implementation of radiative transfer theory often involves some approximations even in one dimension (after already assuming a "plane-parallel atmosphere"; see, e.g., [10]) and requires more sophisticated assumptions and techniques in three dimensions when horizontal homogeneity cannot be assumed (see, e.g., [11,12]). In fact, radiative transfer theory itself is based on an approximation; as Mishchenko [13], Doicu and Mishchenko [14], and Doicu et al. [15] show, the radiative transfer equation can be derived from Twersky's approximation, according to which photons (scattered waves) do not loop through the same scattering center more than once during their trajectories (see, e.g., Twersky [16,17], Ishimaru [18], and Haspel and Sela [19]), whereas a full first-principles calculation would account for all scattered wave paths in the medium. Thus, though it has been shown that radiative transfer theory should be a good approximation in the case of a large and sparsely spaced collection of spherical scatterers (see, e.g., [7,15]), with modern computational capabilities, a first-principles-based calculation, if feasible, may actually present some advantages over radiative transfer theory in such a case. ...

... As there are many particles in a particulate medium, the scattered photon from a particle may interact with those from other particles. This means incident beams can be scattered more than one time and they can be scattered again by the nearby particles (Mishchenko et al., 1999;Dai and Haussener, 2018;Doicu and Mishchenko, 2018;Mishchenko and Dlugach, 2018). ...

... Similarly, the Q matrix can also be transformed into a block diagonal matrix. According to the IIM T-matrix theory, the Q matrix is calculated using the U matrix with the following equation: Because the matrix element of g(r n , r n ) is independent of the azimuthal mode m [20,22,31], we can rearrange the layout of the matrix element of g(r n , r n ) into an N-block diagonal matrix according to the arrangement rule of the U matrix [20,22], as shown in Figure 5. ...

... The passive atmospheric composition sensors (ACS) detect and record the radiance reflected by the Earth atmosphere in the ultraviolet (UV), visible (VIS), and thermal infrared (IR) regions. The information about the atmosphere is then retrieved from the spectral data by using the so called atmospheric processors, i.e. codes which are specifically designed to invert ACS measurements [1]. Extracting the information about geophysical parameters (level-2 data) from spectral radiance distributions (level-1 data) turns out to be a major computational challenge and requires high performance computing (HPC) [2]. ...

... In practice, we usually apply a scheme in which the regularization strength is gradually decreased during the iterative process, i.e. q ≥ 0.8. Doicu et al. [2002Doicu et al. [ , 2003 analyzed the numerical performances of this algorithm for nonlinear ill-posed problems without/with bound-constraint by means of simulated infrared spectra. ...