The above results, when considered as relating to different aspects of the same physical process, suggest the probable existence of correlations between periods of significant Alfvénic turbulence, higher time-integrated AE and HILDCAA periods, and longer storm recovery phases. However, a statistical demonstration of the coupling between the solar wind Alfvénicity and the ring current dynamics, aimed at providing evidence in support of this possible scenario and distinguishing between a temporal coincidence and a causal linkage, is still missing in the literature and motivates the present paper, which deals with the correlation between the presence of Alfvénic streams and long-living periods of geomagnetic activity at low latitudes, after the main phases of storms induced by recurrent or non-recurrent solar transients. Specifically, by exploiting a 16 yr survey of interplanetary and geomagnetic data, an attempt is here sought to provide an empirical law that quantitatively links the duration of the recovery phase (as inferred from the SYM-H index-related magnetospheric output at low latitudes) with the Alfvénic content of the solar wind fluctuations. The goal is twofold. First, this would allow a robust establishment of the extent to which Alfvénic intervals contribute to slow recovery phases. Second, interesting predictions of the duration of the total geomagnetic storm (main plus recovery phase) would be available once the empirical law between the two periods was established. This is crucial since, although space weather forecasting often focuses on estimating the onset and intensity of geomagnetic storms (Joselyn 1995; Lundstedt et al. 2002; Abunina et al. 2013), the impacts on ground infrastructure are also related to the whole storm duration (closely linked to the storm intensity; e.g., Haines et al. 2019) through time-integrated effects (Balan et al. 2016; Lockwood et al. 2016; Mourenas et al. 2018). Indeed, the investigation of the different storm indices for low, intermediate, and high latitudes has shown that infrastructures on Earth, namely, the equipment of electric power transmission networks, are affected by the geomagnetically induced currents. In particular, transformers and some electrical substations have been observed to be very sensitive to prolonged periods of substorms, with delayed increases in anomalies (Švanda et al. 2020). Moreover, highly energetic charged particles in near-Earth space represent a risk for satellite operations. A strong relationship exists between relativistic electrons and HILDCAA events: the longer the HILDCAAs last, the higher the energy reached by relativistic electrons (Thorne et al. 2013). Hajra et al. (2015), taking into account solar cycle 23, found a flux enhancement of relativistic electrons in the outer radiation belt during HILDCAAs, corroborating the results of Meredith et al. (2002, 2003). Furthermore, there, large events are always followed by high flux peaks of 2 MeV electrons (Mourenas et al. 2019). It is worth pointing out that the investigation of the particle flux for electron energy larger than 2 MeV is extremely important for the space weather forecasting as a key indicator of serious hazard for damage of spacecraft in low, medium, and geosynchronous Earth orbits (Forsyth et al. 2020). Therefore, prolonged periods of strong Alfvénic turbulence in the solar wind could be precursors of many adverse effects in both ground installations and space satellites.
Citation: Lundstedt et al. 2002
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