Investigations of solar wind parameters based on multi-satellite observations from the first Lagrangian point of the Sun-Earth System

Space weather refers to the variations in the particle, radiation, and electromagnetic field environments due to the Sun, which impacts the interplanetary medium, magnetosphere, radiation belt, thermosphere and ionosphere. These variations occur during solar flares, and during the passage of coronal mass ejections (CMEs), corotating interaction regions (CIRs), etc. through the interplanetary medium. The disturbances on the Sun and in the interplanetary medium can affect radio communication and maritime applications, cause power grid failure, impact astronaut safety, and cause satellite degradation and damage. Modelling and predicting the impact of space weather and its early forecast can help mitigate such problems.

CMEs, large solar eruptions from the Sun, take about two to three days to reach the Earth, though all CMEs do not cause geomagnetic storms; it depends on the interplanetary magnetic field (IMF) and its orientation. CIRs can also cause moderate geomagnetic storms. A comparative study of these solar events for solar cycle 24 is carried out in this work. A two-point multi-satellite correlation length and decorrelation length between the satellites ACE and Wind at the Sun-Earth L1 position are measured for various solar parameters, e.g., IMF components, solar wind proton density, proton velocity, and alpha-proton density ratio.

The correlation length quantifies the distance between two data points from the satellites at which a physical quantity maintains its correlation. This definition mathematically characterises the statistical dependence of a physical quantity at two points separated by a distance R_E, where correlation persists. The correlation coefficient can be analysed to derive insights into solar wind scale lengths and to assist in forecasting space weather using satellite monitors. According to the study, CMEs have the largest correlation length and the ambient solar wind has the lowest correlation length, while CIRs exhibit a moderate correlation length during solar cycle 24. According to the analysis of the various phases of solar cycle 24, the correlation length is largest for solar maximum and lowest for solar minimum. Further, the correlation lengths in solar cycle 24 are higher during the descending phase of the solar activity than during the ascending phase.

Keywords: Solar wind, IMF, Space weather events, correlation/decorrelation length, geoeffectiveness.