Exploring Ionospheric Variability Through Doppler Residuals: A Study Utilizing the HamSCI Grape V1 Receiver

This study leverages the capabilities of the Grape V1 low-IF receiver to analyze both long and short-term patterns of high frequency (HF; 3-30 MHz) skywave signals. The HF spectrum, often used for global long-range communications, also spans the frequencies used for remote sensing of the near-Earth plasma environment. The Grape receiver (callsign K2MFF) used in this study is located at the New Jersey Institute of Technology (NJIT) in Newark, NJ. At a rate of 1 Hz, it samples its link to the WWV broadcasting station transmitting at 10 MHz from Fort Collins, CO. The Doppler shift in this radio link, caused by its interactions with the ionosphere, is measured to study fluctuations in the ionosphere's electron density. This methodology provides insight into the effects of geomagnetic activity on the terrestrial ionosphere, caused by complex processes in the coupled Sun-Earth plasma environment. Our results show that the signal received during the daytime is less prone to Doppler shift than when received during the nighttime. This night-day contrast is consistent across most 24-hour cycles, barring dates of antenna maintenance or severe geomagnetic storms. We also found a strong correlation between daytime measurements and Cauchy statistics, and between nighttime measurements and a mixture of exponential power / lognormal statistics, wherein day and night at the geographic midpoint between WWV and NJIT are considered. The identification of these differing statistical regimes per time of day has led us to characterize long-term trends in the dataset by the medians of day and night Doppler measurements, independently. Additionally, the receiver's sensitivity and versatility was affirmed through case-studies of atypical Doppler traces captured in the data stream, by identifying characteristic markers of solar flares and solar eclipses.