Understanding how the Sun’s activity regulates the response of the Earth’s and other planets’ atmospheres and/or magnetospheres has attracted great interest from the scientific community over the last few years. Planetary Space Weather is the general term referring to the science devoted to the study of the interaction between the Sun and the environment of a Solar System body (e.g., Lilensten et al., 2014). Although the solar activity is naturally the main driver for any planetary (or satellite) disturbance, it is worth reminding that in the outer Solar System, other interplanetary drivers (e.g., galactic cosmic rays and interstellar pick-up ions, which can alter the composition and properties of the solar wind) may play some role in the variability of planetary environments. It follows that both solar and non-solar impinging plasma/radiation are different actors at different locations in the Solar System and should be thus both considered in Planetary Space Weather. It turns out that Planetary Space Weather is a cross-discipline that spans planetology, heliophysics, and aeronomy, at the interface between fundamental science and applications. The fleet of solar and planetary space missions significantly advances our understanding of space weather phenomena in the Solar System and greatly supports theoretical models. This kind of missions enables a large variety of plasma and magnetic field observations, both with remote-sensing and in-situ instruments and dedicated to different bodies in the heliosphere. With the recent launches in the last 4 years of Parker Solar Probe (Fox et al., 2016), Solar Orbiter (Müller et al., 2020), and BepiColombo (Benkhoff et al., 2010) for the study of the Sun and Mercury, respectively, the insertion of the Juno mission into orbit around Jupiter in 2016 (Bolton et al., 2017) and the upcoming launch of JUpiter ICy moons Explorer (JUICE, Grasset et al., 2013) scheduled in 2023 for the exploration of the largest Solar System planet and three of its Galilean moons (Ganymede, Callisto, and Europa), we have entered what is universally recognized as the golden age of the heliophysics and planetology. This unprecedented observational capability will be allowing scientists to investigate in great detail the relationship between solar and terrestrial/planetary environments, with particular reference to the magnetic connectivity between these environments and the solar corona, the outer layer of the solar atmosphere, where the solar wind originates permeating the whole heliosphere.

This Research Topic collects six original research papers (both observational and modeling), one perspective article, and one review, all of them addressing space weather science and space weather awareness related to the chain of phenomena at the base of the onset and propagation of disturbances (both transient, such as Coronal Mass Ejections—CMEs—or shocks, or advected by the solar wind as turbulence and flux ropes) from the Sun into the inner heliosphere and their interaction with terrestrial/planetary environments, in particular highlighting how space weather phenomena are characterized by different intensities and morphologies across the entire Solar System.

Although our physics-based understanding of solar-wind/magnetosphere coupling is still far from being satisfactory (Borovsky), the papers collected in this Research Topic will help advance our knowledge in tracing propagation of solar events through the Solar System, detecting and predicting planetary events driven by the Sun’s activity, and finally forecasting the effectiveness of interplanetary disturbances on planetary magnetospheres and atmospheres. Specifically, based on a statistical study on 79 CMEs, Kilpua et al. clearly show that the fluctuation properties in CME sheath regions are very different from those generally observed within planetary magnetosheaths and that CME-driven shocks do not reset the solar wind turbulence at a similar extent than planetary bow shocks, thus advancing our understanding of the role of shocked solar-wind plasma in modulating geomagnetic activity and magnetospheric evolution. Interesting in the context of magnetic connectivity studies, Borovsky assesses the possibility of using the electron strahl as an indicator of the magnetic connection between the solar corona and the near-Earth environment. This study complements and is somehow supported by observational evidence that the magnetic-flux-tube structure of the solar wind can be mapped by measuring density and temperature of the core-electron population (Borovsky et al.). Magnetic connections from the Sun to the lunar nightside are explored by Borovsky and Delzanno as being very important in space weather science for identifying the pathways for impulsive Solar-Energetic-Electron (SEE) events, which may pose hazards for space objects orbiting around the Moon in the form of damaging radiation. Riley and Issan provide the interplanetary scientific community with a refined technique dedicated to Planetary Space Weather for magnetically connecting different regions of the inner heliosphere to their source regions at the Sun. This represents a breakthrough in understanding how coronal structures propagate and evolve from the Sun into the heliosphere, shaping the structure of the solar wind through dynamic stream-stream interactions. More accurate estimates of magnetic connectivities require the use of Carrington synoptic maps built from white-light observations (Poirier et al.) against which the different coronal and heliospheric models should be compared to rank their performance, e.g., in inferring the shape of the magnetic sector structure. Finally, as if to conceptually link in a rather comprehensive view all the results collected in this Research Topic, Beedle et al. briefly review the main solar-wind drivers governing the magnetosphere-ionosphere system, emphasizing the crucial roles played by magnetic topology and connectivity. The authors thus provide a concise though exhaustive description of the magnetically connected space weather system, intended for a growing cross-disciplinary audience interested in applied aspects of modern space weather research and forecasting.