The great solar storms of late 2003, seen from New Zealand

This address first carried notes on the late-2003 solar storms as they were happening. The page has been rewritten with the benefit of hindsight, and a comparison with the big storm of May 2024.

What happened

In late October and early November 2003, the Sun produced one of the most intense bursts of activity of the space age. The events have become known as the Halloween storms. Several large, complex sunspot groups rotated across the face of the Sun at the same time, well after the peak of that solar cycle, and they released a series of powerful flares and coronal mass ejections (CMEs).

Highlights included:

• A run of X-class flares. X-class is the top of the flare scale. Several X-class events erupted over about two weeks, including flares rated above X10.
• The very large flare of 4 November 2003. This flare was so intense it saturated the X-ray sensors on the monitoring satellites. It was initially rated X28, and later analyses suggested it may have been stronger still. It remains among the largest flares ever measured. Fortunately for aurora watchers and power grids alike, it erupted near the Sun's edge and its CME was not aimed squarely at Earth.
• Fast CMEs that did hit Earth. Two CMEs from late October crossed to Earth in well under a day each, unusually fast, and produced severe geomagnetic storms at the end of October.

Aurora in the southern hemisphere

The storms at the end of October pushed the auroral ovals a long way from the poles in both hemispheres. Observers across southern Australia and New Zealand reported aurora, with displays seen from the South Island and reports from further north than usual. In the northern hemisphere the aurora reached deep into the mid-latitudes.

The storms also had practical effects: satellites were put into safe modes, some spacecraft were damaged, aircraft on polar routes were diverted, and power systems at high latitudes had problems. They became a reference case for space weather planning around the world.

What made them exceptional

• Timing. They came during the declining phase of the solar cycle, a reminder that the biggest events do not always cluster at solar maximum.
• Several active regions at once, each capable of major flares.
• Speed. The fast CMEs gave very little warning between eruption and impact.
• Strength of the magnetic field. The CMEs carried strongly southward-pointing magnetic fields, which couple efficiently with Earth's field and drive strong storms.

If the space weather terms are unfamiliar, the NOAA Space Weather Prediction Center's aurora page explains them clearly, and NASA's Sun pages cover flares and CMEs.

Comparing with May 2024

On 10 and 11 May 2024, a cluster of CMEs from a large sunspot region produced the strongest geomagnetic storm in about two decades, reaching the top of the NOAA storm scale. For New Zealand it was arguably a better show than 2003. Skies were clear over much of the country, the storm peaked during our night, and phone cameras were everywhere. Aurora was seen and photographed far up the North Island, including around Auckland and beyond, and in the south the display filled much of the sky.

The difference is partly geometry and timing: how strong a storm is matters, but so does whether its peak happens during New Zealand's dark hours and whether the cloud plays along. The 2003 storms had a more extreme flare; May 2024 delivered the better aurora for this part of the world.

Lessons for watchers

Big storms can come at any point in the solar cycle, warnings may be short, and a camera will show far more than the eye. Our guide on how to see and photograph the aurora australis covers the practical side.