Space physics can be characterised as the physics of the Earth's environment in space and the interplanetary medium. Loosely speaking, space physics comprises everything that can be reached with spacecraft. Astrophysical objects like stars and galaxies, on the other hand, can only be studied using remote sensing instruments (telescopes). Further neighbouring science disciplines are aeronomy (physics and chemistry of the Earth's upper atmosphere) and planetology.
The medium between the planets, the so-called solar wind, is a magnetised plasma (ionised gas) of solar origin, and can be understood as the expanding solar atmosphere. Planets with a significant internal magnetic field like the Earth are magnetised obstacles in the solar wind. Interactions of magnetised planets with the solar wind give rise to the formation of planetary cavities called magnetospheres with characteristic boundary layers like the bow shock and the magnetopause. Magnetospheric plasmas are not only of solar origin but stem to a large extent also from the planetary atmosphere and in some cases even from satellites within the planetary magnetosphere (e.g., the inner Galilean moon Io in the Jovian magnetospere).
The global structure of a magnetosphere depends very much on the efficiency of solar wind-magnetosphere coupling. This in turn is governed by the actual state of the magnetosphere itself, and by solar wind parameters like the direction of the interplanetary magnetic field (IMF) which are subject to strong variations. Solar wind-magnetosphere coupling is thus an intermittent rather than a continuous process. The Earth's magnetosphere, e.g., strongly responds to periods of southward directed IMF. Periods of strong magnetospheric activity are called magnetic storms. Of particular interest are magnetic substorms which can be understood as global reconfigurations of the magnetosphere on the timescale of 30 minutes. Substorms are associated with, e.g., auroral emissions in the polar zones, coherent magnetic field disturbances called pulsations, failures of Earth-orbiting communication satellites, and failures of power stations on the ground. The state of the magnetosphere as controlled by the solar wind is a prominent aspect of space weather.