Spheroidal core–mantle particle absorption, scattering, and polarisation in the long-wavelength limit

Context . The numerical calculation of optical properties (extinction, absorption, scattering, and polarisation efficiencies) is often time-consuming for non-spherical and inhomogeneous particles. Where possible, analytical methods are therefore to be preferred. Aims . We provide an analytical tool for deriving the optical properties of mantled spheroidal particles, of arbitrary axis ratios, in the long-wavelength limit ( a ≪ λ ), where the mantle form may be confocal, co-axial, or of a constant depth with respect to the particle core. Methods . We have developed an analytical approach to spheroidal core–mantle (CM) particle optical property calculations. Results . The analytical method compares well with DDSCAT numerical calculations and, under limited circumstances, with those made using the Bruggemann effective medium theory (EMT). Conclusions . The analytical method presented here provides a useful tool for exploring the optical and polarisation properties of CM spheroidal particles at long wavelengths ( λ ≳ 8 μ m) and is simpler and faster to implement than corresponding numerical methods. We caution against the use of EMT methods in approximating the optical properties of CM particles.