Abstract

Regarding geomagnetic measurements, the impact of the Moon is twenty times smaller than that of the Sun, so it is not easy to determine from daily magnetograms. The influence can be studied after a carefully selected time series of magnetograms obtained in specific conditions. Specifically, this is in minimum solar intensity with no charges or transitions of weather storm fronts, and no major earthquakes in the wider area. The beginning of the 25th Solar Cycle and 2019 Winter Solstice was an entirely appropriate timeframe for conducting a study of the lunar impact on geomagnetic noise. At the end of 2019, we were able to perform a time series of geomagnetically calm days at the PIA observatory (Piran) during the three-day period from the Third Quarter to the New Moon, which almost coincided with the Winter Solstice. The results of the processing of the geomagnetic measurements were compared with the independent sea level measurements from the tide gauge station in Koper. The spectral analysis of two consecutive lunar periods was used to evaluate the gravitational effect of the Moon. Furthermore, the results were compared with sea tides in Koper. We showed that the estimated influence of the Moon from geomagnetic measurements coincides with the modelled influence of the Moon from the results of sea tides. The results of the geomagnetic noise caused by the influence of the Moon can be used as a starting point for the investigation of other natural and anthropogenic influences on geomagnetic measurements at the geomagnetic observatory PIA (Piran).

Key words: geomagnetic noise, gravity of the Moon, 25th Solar cycle, Winter Solstice