CONDITIONS FROM THE SUN · 2 min read

Solar wind: speed, density and arriving energy

What the km/s reading measures, and why faster wind is only part of the picture.

A stream of charged particles

Solar wind flows continuously from the Sun. It consists mainly of protons and electrons and carries a magnetic field. Its interaction with Earth’s magnetic environment supplies energy that can drive northern lights activity. This is different from the weather wind measured at your destination.

Speed is measured in kilometres per second

Slow solar wind is commonly around 400 km/s. Fast streams from coronal holes can reach roughly 500–800 km/s. These are useful reference ranges, not viewing thresholds: a high speed does not automatically mean a bright display.

Density and magnetic direction also matter

Density describes how many particles occupy a given volume, usually reported as particles per cubic centimetre. Changes in density, speed and the carried magnetic field all affect the interaction with Earth. Southward Bz can make energy transfer more effective; look at these conditions together.

The measurement is taken upstream

Spacecraft near the L1 point measure the solar wind before it reaches Earth. The travel time varies with speed and conditions, so the observation timestamp is not a local arrival time. Follow several readings and check for stale or missing data before drawing a conclusion.

What to look for

Read solar-wind speed together with Bz and the recent trend. A fast stream with a favourable magnetic field is more useful context than speed alone.