Aurora Chaser

Early warning from 1.5 million km away

A small fleet of spacecraft sits between the Earth and the Sun, sampling the solar wind before it reaches us. They give forecasters, and aurora hunters, up to an hour’s notice of what’s coming.

What is L1?

L1, the first Lagrange point, is a spot about 1.5 million km from Earth in the direction of the Sun, roughly 1% of the way there. A spacecraft that close to the Sun would normally orbit it faster than Earth does, but at L1 Earth’s gravity pulls back just enough that it keeps pace with us. It stays between the Earth and the Sun all year round, with a constant view of the Sun and of the solar wind heading our way.

The balance at L1 is unstable, like a ball on top of a hill, so spacecraft don’t sit exactly on the point. They fly wide loops around it called halo orbits, taking about six months per loop and firing their thrusters every so often to stay on track. Staying off the direct Sun–Earth line also matters for communications: a spacecraft directly in front of the Sun would be drowned out by the Sun’s own radio noise.

The L1 point between the Sun and Earth The solar wind flows from the Sun past a spacecraft looping around the L1 point, 1.5 million km from Earth, before reaching Earth. A to-scale line below shows that L1 is only 1% of the 150 million km from the Sun to Earth. Not to scale Solar wind (300–800 km/s) L1 Spacecraft loop around it Earth 1.5 million km To scale: L1 is only 1% of the way to the Sun (150 million km) Sun L1 Earth
Spacecraft at L1 meet the solar wind before Earth does. They are close to us compared with the Sun, but still about four times further away than the Moon.

How much warning?

The warning time is simply how long the solar wind takes to cover the last 1.5 million km. Typical solar wind at 400 km/s takes about an hour. A fast stream at 800 km/s takes about half an hour, and the fastest coronal mass ejections (CMEs), at 2,000 km/s or more, cut it to a quarter of an hour or less. So the bigger the storm, the less notice you get.

450 km/s

Warning before it reaches Earth: about 56 minutes

Fast solar wind, such as a stream from a coronal hole.

The spacecraft

Several spacecraft share the space around L1. Some are operational weather stations that feed forecasters around the clock; others are science missions that also send useful data.

  • SOLAR-1

    NOAA · launched September 2025 · at L1 since January 2026

    Primary real-time monitor

    Launched as SWFO-L1, it’s the first US satellite built purely for operational space weather. Since 30 June 2026 it has been NOAA’s main source of real-time solar wind data. Its CCOR-2 coronagraph gets images of CMEs to forecasters within 30 minutes, where older research instruments can take up to eight hours.

  • ACE

    NASA · launched 1997

    Backup monitor

    The Advanced Composition Explorer, a research mission that has streamed real-time solar wind data for decades. It is now the backup to SOLAR-1.

  • IMAP

    NASA · launched September 2025 · at L1 since January 2026

    Next backup

    Launched on the same rocket as SOLAR-1. Its main job is mapping the edge of the Sun’s influence in space, but its I-ALiRT link streams solar wind and particle data in near real time (public since February 2026). It will take over as SOLAR-1’s backup once that link runs around the clock.

  • SOHO

    ESA and NASA · launched 1995

    CME images

    The veteran. For three decades its LASCO coronagraphs have been forecasters’ main view of CMEs leaving the Sun, although their images can take hours to come through. It passed 30 years in space in December 2025.

  • DSCOVR

    NOAA, NASA and US Air Force · launched 2015

    Replaced in 2026

    NOAA’s main real-time solar wind monitor from 2016 until June 2026, when the Space Weather Prediction Center switched to SOLAR-1. Also known for its EPIC camera’s full-disc pictures of Earth.

  • Aditya-L1

    ISRO (India) · launched 2023 · at L1 since January 2024

    Science mission

    India’s first solar observatory, carrying a coronagraph, solar imagers and instruments that sample the solar wind. It studies the Sun rather than providing an operational warning service.

Days ahead and minutes ahead

The L1 fleet gives two very different kinds of warning:

Days ahead: a CME leaves the Sun
Coronagraphs on SOLAR-1 and SOHO block out the Sun’s bright disc to see CMEs leaving it. Forecasters estimate each CME’s speed and direction and predict when it will arrive, usually one to four days later. This is when the Bureau of Meteorology may issue an Aurora Watch. Arrival times can be hours out, and nobody can yet tell which way the CME’s magnetic field will point.
Minutes ahead: it sweeps past L1
Magnetometers and plasma sensors measure its real speed, density and magnetic field. If the field turns strongly southward, a storm is likely to follow within the hour, and once it’s under way the Bureau issues an Aurora Alert.

The OVATION aurora forecast

The US Space Weather Prediction Center turns the L1 measurements into a short-range aurora forecast using the OVATION model (Oval Variation, Assessment, Tracking, Intensity, and Online Nowcasting), developed at the Johns Hopkins University Applied Physics Laboratory. OVATION takes the solar wind speed and magnetic field measured at L1 and works out where the aurora will be and how bright, for both the northern and southern hemispheres.

Because its input is solar wind that hasn’t reached Earth yet, OVATION looks 30 to 90 minutes ahead: exactly the L1-to-Earth travel time. If the L1 data drop out, it falls back on the current Kp index and loses that lead time.

Check the aurora 30-minute forecast and look at the southern map: the further the oval reaches towards Australia, the better your chances.

OVATION or this site’s map? The viewing map is a rule of thumb for where the aurora could be seen at a given Kp, which is useful for planning where to go. OVATION is a live forecast for the next hour or so, which is useful for deciding when to look.

Reading the solar wind yourself

The raw L1 data are public, on the Space Weather Prediction Center’s solar wind display and the Bureau of Meteorology’s solar wind speed page. Here’s what aurora watchers look for:

  • Bz: the north–south part of the magnetic field carried by the solar wind, in nanotesla (nT). Negative means southward, which is what lets energy into Earth’s magnetosphere. A few minutes of southward Bz does little. As a rule of thumb, look for Bz staying around −10 nT or lower for an hour or more.
  • Speed: quiet solar wind blows at about 300–450 km/s. Faster wind delivers more energy for the same Bz.
  • Density: a sudden jump in density, speed and field strength together usually marks the shock at the front of an arriving CME.
  • Bt: the total strength of the magnetic field. A high Bt gives Bz room to swing far south.

The catch: L1 is a single point in space. The solar wind can change on its way to Earth, and Bz can flip north as quickly as it turned south. Treat the data as a short-range forecast, not a promise.

What’s next

Everything at L1 looks straight at the Sun, so CMEs heading towards us are seen head-on, which makes their speed hard to judge. The European Space Agency’s Vigil mission, planned for launch in 2031, will watch from a different spot, the L5 point 60° behind Earth in its orbit. From there it will see Earth-bound CMEs side-on and spot active regions on the Sun before they rotate into view from Earth, giving up to four to five days’ notice of some space weather.

Sources