The Sun is a magnetically variable star which produces profound consequences and impacts on planets with intrinsic magnetic field (like Earth). Its magnetic activity and the associated changes evolve on timescales that range from minutes up to billions of years, involving nonstationary and nonequilibrium active processes, broadly regarded as solar activity. Whereas this concept is quite common nowdays, it is neither straightforwardly interpreted nor unambigously defined. For instance, magnetic variability on the solar surface, eruptive phenomena, coronal
activity, interplanetary transient phenomena, solar energetic particles as well as geomagnetic storms and substorms, tail current distruption, wave-particle interactions can be related to the concept of solar activity. The short-term (from minutes up to months) variability of the solar
activity is generally associated with eruptive phenomena (i.e., flares, coronal mass ejections) which extend from the solar surface to the inner heliosphere, carried out from the Sun by the solar wind. They affect Earth’s magnetospheric dynamics and produce several Space Weather
effects (high-latitude ionospheric auroral activity, large electrical ground currents, GPS and high-frequency communication failures), with a considerable amount of solar wind energy entering Earth's magnetosphere and both regular and irregular variations/fluctuations observed on a very wide interval of timescales, ranging from a few tens of minutes up to two hundreds minutes.
In this work, we present a detailed study of the timescale coupling between solar wind changes and the magnetospheric response in the course of two geomagnetic storms occurred in 2013 and 2015. To investigate the range of the coupled timescales we use the Empirical
Mode Decomposition (EMD), which is particularly suitable for the analysis of nonlinear and nonstationary time series, and the Delayed Mutual Information (DMI), which is capable of providing a measure of the total linear and nonlinear correlation. Our findings support the common idea that the Earth’s magnetosphere response consists of both directly driven and internal processes, where the internal processes instead of being driven have to be considered more reasonably to be triggered by external solar wind changes. Furthermore, there is a clear separation of timescales between the internal processes and the directly driven one, being the characteristic separation timescale of the order of 100-200 min.