  The explosion of space weather research since the early 1990s has been partly
fueled by the unprecedented, uniform, and extended observations of solar
disturbances from space and ground based instruments. Coronal mass ejections
(CMEs) from closed magnetic field regions and high speed streams (HSS) from
open field regions on the Sun account for most of the disturbances relevant to
space weather. The main consequences of CMEs and HSS are their ability to cause
geomagnetic storms and accelerate particles. Particles accelerated by CME
driven shocks can pose danger to humans and their technological structures in
space. Geomagnetic storms produced by CMEs and HSS related stream interaction
regions also result in particle energization inside the magnetosphere that can
have severe impact on satellites operating in the magnetosphere. Solar flares
are another aspect of solar magnetic energy release, mostly characterized by
the sudden enhancement in electromagnetic emission at various wavelengths from
radio waves to gamma rays. Flares are responsible for the sudden ionospheric
disturbances and prompt perturbation of Earths magnetic field known as magnetic
crochet. Nonthermal electrons accelerated during flares can emit intense
microwave radiation that can drown spacecraft and radar signals. This review
article summarizes major milestones in understanding the connection between
solar variability and space weather.
