Ground-based Polarimetric Observations Research Articles

Polarization of extragalactic radio sources: CMB foregrounds and telescope calibration issues

Radio source observations play important roles in polarimetric cosmological studies. On the one hand, they constitute the main foregrounds for cosmic microwave background (CMB) radiation on scales smaller than 30 arcmin up to 100 GHz, on the other they can be used as targets for validation of products of polarimetric experiments dedicated to cosmology. Furthermore, extragalactic high-redshift sources have been used for cosmic polarization rotation (CPR) investigation. In this paper, we will discuss the support to cosmological studies from ground-based polarimetric observations in the radio and millimetric wavelength bands. Most of the limits to accuracy improvements arise from systematic effects and low calibration quality. We will discuss some details of interferometric calibration procedures and show some of the perspectives that the Atacama large millimeter array (ALMA) could offer for CPR studies.

Quantitative estimation of Tropical Rainfall Mapping Mission precipitation radar signals from ground‐based polarimetric radar observations

The Tropical Rainfall Mapping Mission (TRMM) is the first mission dedicated to measuring rainfall from space using radar. The precipitation radar (PR) is one of several instruments aboard the TRMM satellite that is operating in a nearly circular orbit with nominal altitude of 350 km, inclination of 35°, and period of 91.5 min. The PR is a single‐frequency Ku‐band instrument that is designed to yield information about the vertical storm structure so as to gain insight into the intensity and distribution of rainfall. Attenuation effects on PR measurements, however, can be significant and as high as 10–15 dB. This can seriously impair the accuracy of rain rate retrieval algorithms derived from PR signal returns. Quantitative estimation of PR attenuation is made along the PR beam via ground‐based polarimetric observations to validate attenuation correction procedures used by the PR. The reflectivity (Zh) at horizontal polarization and specific differential phase (Kdp) are found along the beam from S‐band ground radar measurements, and theoretical modeling is used to determine the expected specific attenuation (k) along the space‐Earth path at Ku‐band frequency from these measurements. A theoretical k‐Kdp relationship is determined for rain when Kdp ≥ 0.5°/km, and a power law relationship, k = a Zhb, is determined for light rain and other types of hydrometers encountered along the path. After alignment and resolution volume matching is made between ground and PR measurements, the two‐way path‐integrated attenuation (PIA) is calculated along the PR propagation path by integrating the specific attenuation along the path. The PR reflectivity derived after removing the PIA is also compared against ground radar observations.

Mars aerosol optical thickness retrieved from measurements of the polarization inversion angle and the shape of dust particles

The estimates of the optical thickness of the “clear” atmosphere of Mars obtained from ground-based polarimetric observations are characterized by very low values. The sensitivity of polarization to the size and shape of scatterers raises the question of the effect of the irregular shape of solid aerosol particles on the optical thickness retrieved from the spectral dependence of the polarization inversion angle for Mars (the phase angle at which the polarization of the scattered radiation changes sign). The Stokes scattering matrix for irregular dust particles is calculated using the T-matrix method. The model of randomly oriented oblate polydisperse spheroids used in this study indicates a weak dependence of the polarization inversion angle on the optical thickness of aerosols τ d: an increase in τ d from 0.05 to 0.15 results in an inversion angle increase of less than 1°, whereas for spherical particles this change is about 5°. It is reasonable to expect that particles with more pronounced irregular shapes than spheroids may lead to more substantial depolarization and hence an even weaker sensitivity of the inversion angle to variations in the aerosol optical thickness. Thus, very low estimates of the optical thickness of the “clear” atmosphere of Mars retrieved from ground-based polarimetric observations using a spherical-shape approach for aerosol particles cannot be considered reliable. Ignoring the dust particle shape introduces similar uncertainties in the models of the Martian atmosphere as those caused by traditionally discussed errors in such parameters as refractive index or size distribution.