Arecibo Multi-frequency IPS Observations: Solar Wind Density Turbulence Scale Sizes and their Anisotropy
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 covered heliocentric distances from ~5 to 200 solar radii. The dynamic spectrum of the scintillations obtained at 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 L-band, along with near-simultaneous observations of selected sources covering the P-, L-, and S-bands, clearly demonstrate a wavelength dependence of $m \propto λ^ω$, which inherently leads to a dependence of $m$ on the Fresnel scale, when considering the effective distance to the scattering screen, $z$. The index $ω$ ranges between $\sim$1 and 1.8. The average $ω$ value of a source, determined from observations made on different days, 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 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.