Guides
Solar wind speed and Bz: the hour before the aurora
Kp tells you what the last three hours were like. The solar wind tells you what is about to arrive.
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The solar wind is the stream of charged particles and magnetic field that flows constantly from the Sun, and two of its numbers decide most aurora nights: its speed and the north-south direction of its magnetic field, called Bz. SWPC explains that energy pours into Earth's magnetosphere when the wind is fast and its field points south, opposite to Earth's own; sustained southward Bz over several hours is what builds a geomagnetic storm. Spacecraft near the L1 point, about 1.6 million km toward the Sun, measure the wind before it hits. At 400 km/s, the speed SWPC gives for the ordinary slow wind, that is about 67 minutes of warning; at 800 km/s, the top of SWPC's range for coronal hole streams, about 33 minutes. The clouds of coronal mass ejections travel faster still, and the fastest reach Earth in 15 to 18 hours after leaving the Sun. Kp and the OVATION map react only once the energy has arrived.
How much warning does the solar wind give?
Warning from the L1 monitors
59 min
| At the slow wind speed, 400 km/s | 67 min |
| At a fast stream, 800 km/s | 33 min |
| Sun to Earth at this speed | 3.8 days |
L1 distance: about 1.6 million km (SWPC). Real CMEs slow down or speed up on the way.
What the solar wind is, and the two numbers that matter
The Sun loses a continuous stream of protons and electrons, a plasma that carries the solar magnetic field outward with it. SWPC's solar wind page describes two broad kinds. The ordinary slow wind, around 400 km/s, fills the plane where the planets orbit. Coronal holes, regions where the Sun's field opens into space, release fast wind at 500 to 800 km/s. Because the Sun rotates in about 27 days, fast and slow wind form a spiral, and where a fast stream catches up with slow wind ahead of it, it builds a compressed region of dense plasma and strong field.
For the aurora, the wind matters through its coupling with Earth's magnetic field. SWPC's aurora tutorial makes a subtle point: the electrons that light the aurora do not come straight from the solar wind. They come from inside the magnetosphere, mostly from its tail on the night side, and the solar wind supplies the energy that accelerates them. That transfer works best when two conditions meet. The wind must be fast, which raises the energy available. And the magnetic field it carries must point south, opposite to Earth's field where the two meet on the dayside, so that the field lines can connect. The geomagnetic storms page names the southward field, sustained for several hours or more, as the most important ingredient of a storm.
The north-south component of that field is called Bz. On SWPC's displays, negative values mean southward. A Bz that swings between north and south every few minutes does little; a Bz that holds south for hours, especially when the total field strength is large, loads the magnetosphere until it releases the energy in substorms, and those substorms are the bright, moving displays that people drive out to see. The real-time solar wind display shades every interval with a southward Bz and offers an option to shade only values below minus 5 nanotesla, a sign of how much attention forecasters pay to this one number.
The monitors at L1 and the warning they give
About 1.6 million km from Earth toward the Sun, the gravity of the two bodies balances in a way that lets a spacecraft keep station between them. That point, called L1, is where the solar wind is measured before it arrives. According to SWPC's real-time solar wind page, NASA's ACE has been sending data since 1998 and NOAA's DSCOVR since 2016; in 2026 they are joined by NOAA's SOLAR-1 and the real-time link of NASA's IMAP mission, which SWPC says will replace the older measurements.
The warning time is simply the distance divided by the speed. The table shows it for the speeds SWPC uses to describe the wind, and for two faster speeds within the range it gives for coronal mass ejections. The last column is the trip from the Sun at the same speed, which no real wind keeps constant over the whole distance.
| Solar wind speed | From L1 to Earth | From the Sun to Earth at that speed |
|---|---|---|
| 400 km/s | 67 min | 4.3 days |
| 500 km/s | 53 min | 3.5 days |
| 800 km/s | 33 min | 2.2 days |
| 1,500 km/s | 18 min | 1.2 days |
| 3,000 km/s | 9 min | 0.6 days |
In practice the lead time is shorter than the table suggests, because the aurora is not lit the minute the wind arrives. The magnetosphere takes time to load, and a strong substorm can follow a southward turning by tens of minutes or more. SWPC's OVATION model, which runs on the L1 data, forecasts the oval about 30 to 90 minutes ahead, which is the realistic horizon of a short-term aurora forecast. The mini-simulator above gives the warning for any speed.
Coronal mass ejections and fast streams
The two families of disturbances look different in the data. A coronal mass ejection is a cloud of plasma and magnetic field thrown off the Sun, often after a flare in a complex sunspot region. SWPC describes speeds from below 250 km/s to nearly 3,000 km/s, and says the fastest Earth-directed CMEs arrive in as little as 15 to 18 hours while slower ones take several days. Forecasters see them leave in coronagraph images, mainly from the NASA SOHO mission, and estimate their speed and direction; the arrival time can still be off by hours. When a fast CME reaches L1, a shock shows up as a sudden jump in density, speed and field strength, and SWPC says this often gives 15 to 60 minutes of warning before the shock reaches Earth. What follows depends on the field inside the cloud: if it turns strongly south and stays there, the storm can climb to G4 or G5; if it stays mostly north, even a fast CME can bring less aurora than hoped, although SWPC notes that most CMEs reaching Earth turn favorable at some point.
A high-speed stream from a coronal hole builds more gradually. The compressed region at its front brings a rise in density and field, then the speed climbs and stays high for days. SWPC notes that storms from these streams are less intense than the biggest CME storms but can deposit more energy over a longer interval. They usually peak at G1 or G2, which is the level that matters most for the northern tier of the United States, and because coronal holes can last several solar rotations, a stream that brought aurora once often returns about 27 days later. SWPC's real-time display can overlay the data from 27 days earlier for exactly this reason.
Using the solar wind on an aurora night
The solar wind is the most immediate signal available, and also the noisiest. A good routine is to start with the forecast: the three-night forecast and the Kp page say whether a storm is expected and when. Once it is dark and the forecast is close to your town's threshold, the wind data tell you whether the night is shaping up: a speed well above 400 km/s and a Bz that has held south for an hour or more are good signs; a Bz stuck north is a reason for patience. Then the OVATION map shows whether the oval is actually growing toward you.
None of these numbers replaces a look at the sky. A substorm can light the north within minutes of a southward turning, and the best displays of a night often come in short bursts. The storm scale guide explains what each G level brings, and the solar cycle guide explains why CMEs and fast streams keep coming in the years after the solar maximum.