  (abridged) The Sun's magnetic activity has steadily declined during its
main-sequence life. While the solar photospheric luminosity was about 30% lower
4.6 Gyr ago when the Sun arrived on the main sequence compared to present-day
levels, its faster rotation generated enhanced magnetic activity; magnetic
heating processes in the chromosphere, the transition region, and the corona
induced ultraviolet, extreme-ultraviolet, and X-ray emission about 10, 100, and
1000 times, respectively, the present-day levels, as inferred from young
solar-analog stars. Also, the production rate of accelerated, high-energy
particles was orders of magnitude higher than in present-day solar flares, and
a much stronger wind escaped from the Sun, permeating the entire solar system.
The consequences of the enhanced radiation and particle fluxes from the young
Sun were potentially severe for the evolution of solar-system planets and
moons. Interactions of high-energy radiation and the solar wind with upper
planetary atmospheres may have led to the escape of important amounts of
atmospheric constituents. The present dry atmosphere of Venus and the thin
atmosphere of Mars may be a product of early irradiation and heating by solar
high-energy radiation. High levels of magnetic activity are also inferred for
the pre-main sequence Sun. At those stages, interactions of high-energy
radiation and particles with the circumsolar disk in which planets eventually
formed were important. Traces left in meteorites by energetic particles and
anomalous isotopic abundance ratios in meteoritic inclusions may provide
evidence for a highly active pre-main sequence Sun. The present article reviews
these various issues related to the magnetic activity of the young Sun and the
consequent interactions with its environment.
