The solar wind is the continuous stream of charged particles — mostly protons and electrons — flowing outward from the Sun at 300–800 km/s. It shapes Earth's magnetosphere, powers the aurora, and its gusts and shocks are the driving force behind geomagnetic storms.
Understanding Solar Wind
Why "southward Bz" is the phrase that matters
The solar wind's energy only enters geospace efficiently through magnetic reconnection, and reconnection needs opposing fields. Earth's field points northward at the sunward boundary; when the interplanetary field carried by the wind tilts southward — negative Bz, in the jargon — the fields cancel and splice at the dayside, opening the magnetosphere and driving circulation that dumps energy into the tail, the radiation belts and the polar atmosphere. A dense, fast wind with northward field largely slides past; a slower flow with hours of strong southward field can brew a serious storm. This is why forecasters obsess over one component of one vector measured at L1: Bz is the gatekeeper, and its value in a CME's core is barely predictable before the ejecta actually washes over the monitors — the central reason storm forecasts firm up only an hour out.
A satellite's-eye view of the wind
Spacecraft experience the solar wind less as weather than as slow corrosion punctuated by assault. The steady wind sputters surfaces, charges insulators and seeds single-event upsets through its energetic minority. Fast-stream intervals recur with the Sun's ~27-day rotation, delivering predictable minor disturbances as each coronal hole swings past — the metronome of quiet-time space weather. The assaults come when storms driven by wind structures inflate the thermosphere (drag jumps, orbits sag, prediction accuracy collapses for days), pump the radiation belts, and set spacecraft potentials swinging. Operators read the upstream data accordingly: wind speed, density and Bz from L1 are the raw feed behind every alert their consoles raise, and behind the space-weather indices this site's tools track.