Arecibo Multifrequency IPS Observations: Solar-Wind Density Turbulence Scale Sizes and Their Anisotropy

Abstract

We present an analysis of interplanetary scintillation (IPS) observations conducted with the Arecibo 305-m radio telescope during the minimum phase at the end of Solar Cycle 24 and the onset of Solar Cycle 25. These observations span a broad frequency range of ∼ 300 to 3100 MHz, encompassing the P-, L-, and S-bands, and cover heliocentric distances from ∼ 5 to 200 solar radii. Each L-band observation provided simultaneous measurements across a bandwidth of approximately 600 MHz. Furthermore, whenever feasible, the near-simultaneous measurements of a source acquired across all three frequency bands were useful to study the scintillation characteristics over a much wider frequency band along the same line of sight through the heliosphere. The dynamic spectrum of the scintillations obtained at the L-band shows a systematic decrease in the scintillation index from the lowest to the highest frequency, offering valuable insight into the influence of the solar wind density microstructures responsible for scintillation. Analyses of the scintillation index (\(m\)) for multiple sources at the L-band, along with near-simultaneous observations of selected sources covering the P-, L-, and S-bands, clearly demonstrate a wavelength dependence of \(m \propto \lambda ^{\omega }\), which inherently leads to a dependence of \(m\) on the Fresnel scale, when considering the effective distance to the scattering screen, \(z\). The index \(\omega \) ranges between ∼ 1 and 1.8. The average \(\omega \) value of a source, determined from observations made on multiple days (i.e., at a range of solar offsets to mitigate the influence of possible day-to-day variations in solar-wind turbulence) exhibits variability across sources. The results on the radial dependence of scintillation agree with earlier IPS measurements. The temporal power spectra obtained over the wide frequency range exhibit a power-level evolution in accordance with the wavelength dependence and a broadening with an increasing observation frequency. Furthermore, the increased temporal–frequency rounding of the “Fresnel knee” in the spectrum with the observing frequency suggests a novel phenomenon: an increase in anisotropy as the scale size of the density–turbulence structure decreases.