ANALYSIS AND CHARACTERISTICS OF THE UNPREDICTED G2-MODERATE GEOMAGNETIC STORM ON APRIL 20, 2018 IN SOLAR CYCLE 24 MINIMUM

This paper presents some results from the investigation of the unpredicted by the world prediction centers of G2 moderate geomagnetic storm, observed on 20 April 2018 in the minimum of the 24th solar cycle. This storm was generated by a trans-equatorial coronal hole CH861. Some data from the instruments of robotic spacecrafts ACE (Advanced Composition Explorer) and DSCOVR (Deep Space Climate Observatory) are used for tracking and researching of the solar wind variations. Calculations of the solar wind parameters from measurments by ACE and DSCOVR space probes in the Lagrange equilibrium point Ll are made: the kinetic (dynamic) energy density Ek, thermal energy density Et and magnetic energy density Em during the investigated period April 18-24, 2018 (Fig. 3). A stepwise change of the solar wind parameters is observed with a time delay of 24 hours before the geomagnetic storm. In other words, a preliminary shock wave in the solar wind variations is observed. This can be used as a precursor of geomagnetic storms in forecasting space weather. In our previous investigation [9] we established that there is a preliminary increase of solar wind parameter about 24 hours before the strong geomagnetic storms. In that case, the reaction in the kinetic and thermal energy densities has been essential. But the reaction in the magnetic energy density was insignificant. In the present case of G2 moderate geomagnetic storm, there is also a preliminary 24 hours change in the solar wind parameters. However there is a specific feature this variation is more important in the magnetic, than in the dynamical parameters, i.e. the situation is opposite to the case of the G4 - Severe geomagnetic storm (level G4, K = 8) in September 2017. The results obtained will help to improve the daily space weather forecasts prepared by Space Weather and Space Climate Forecast Center in Institute for Space Research and Technology of Bulgarian Academy of Sciences.