Outer Heliosphere: Investigation on the signal fluctuations of the data collected by the Voyager probes

The following thesis analyses deep-space data recorded by the Voyager 1 and Voyager 2 probes, covering the period between January 1, 2016, and December 31, 2020. These data were collected at distances ranging from 133.57 to 151.41 astronomical units (AU) for Voyager 1 and from 109.98 to 125.83 AU for Voyager 2. The data were provided by NASA's @Omniweb database and include information on the magnetic field in its three components and magnitude, as well as data related to three proton parameters, as velocity, density, and temperature, when available. Based on the available data, the investigation aims to contribute to predicting the future trends, with the goal of identifying the positions of the probes within the Interstellar Medium. Two methods of analysis are employed: the Fluctuation Theory, which examines the oscillations of variables over time, and a simple Fourier interpolation, which allows for the Fourier Transform of the signal into a function of frequency and thus, the power of the signal. The thesis begins by examining the current state of the probes, including their positions and the paths they have reached. The data are contextualized through estimates of different regions of the Interstellar Medium found in the literature, based on energy conservation law calculations. The core of the paper is the implementation of the two analysis methods. In the chapter applying Fluctuation Theory, trends in both the mean and the anomaly (deviation) of individual measurements from the data mean over hourly intervals of varying durations are studied to assess the probability distribution and density of probability distribution, both analytically and asymmetric Laplace probability distribution. It is noted that the curves narrow and lengthen as the duration of the averaging interval changes. Additionally, the analysis of different components under examination results in varying numbers of peaks in the probability density function (PDF). Subsequently, calculations of variables, fluctuations, interpolation of the PDF at the identified values, and the scale factor between probability fluctuation functions are performed. This aims to estimate, through statistical indices, the evolution of data to identify the point at which the appropriate magnetic field value can indicate the conclusion of the Outer Heliosphere region, thereby officially marking the probes' exit from the solar system. In the chapter dedicated to Fourier analysis, the hypotheses from the previous study are confirmed. There are sharp jumps in magnetic field values near critical positions (AU), and after estimating the transform, which allows for the evaluation of data as a function of frequency (highlighting periodic trends), information about the signal's decreasing power spectrum is obtained. In conclusion, from the data collected by the two probes, two critical periods are observed in which distinct variations in magnetic field magnitude and proton-related parameters occur. Both probes experience a decrease in magnetic field values, but Voyager 1, the more distant of the two, experiences a second, more significant drop, which could signify the definitive exit from the solar system.