The Gannon storm of May 10–11, 2024, triggered the strongest geomagnetic activity recorded since October 2003, driving auroras to unusually low latitudes and causing an unprecedented contraction of Earth’s plasmasphere, according to space-weather data published in Earth, Planets and Space.
Solar Triggers and Geomagnetic Intensity
Active region 13664 on the Sun released repeated powerful flares and Earth-directed coronal mass ejections beginning May 8, 2024, according to event records. These ejections crossed the 150-million-kilometre gap between the Sun and Earth, with the first interplanetary shock reaching the planet’s magnetic environment at about 17:05 UTC on May 10.
Solar-wind density and speed jumped while the embedded magnetic field repeatedly pointed south. This orientation coupled efficiently with Earth’s oppositely directed field through magnetic reconnection, transferring energy into the magnetosphere. The planetary Kp index reached 9, hitting G5 on the NOAA geomagnetic-storm scale, the highest category, and the first such event since the Halloween storms of October 2003. NASA data lists a peak Dst of -412 nanoteslas, while the higher-time-resolution SYM-H index fell to -518 nanoteslas, according to the later plasmasphere study.
Arase Satellite Observations of the Plasmasphere Collapse
While ground observers photographed red auroras over Mexico, blue and purple structures across Japan, and southern lights over Australia, Japan’s Arase satellite recorded a massive erosion of the plasmasphere. Launched by the Japan Aerospace Exploration Agency in 2016, Arase uses its Plasma Wave Experiment to measure plasma waves and calculate local electron density through upper-hybrid resonance frequencies.
A study led by Dr. Atsuki Shinbori of Nagoya University’s Institute for Space-Earth Environmental Research found that the plasmapause moved from an L-shell near 7 (about 44,600 kilometres from Earth’s centre) to near 1.5 (about 9,600 kilometres from the centre) within nine hours of the storm’s sudden commencement, according to Earth, Planets and Space. Dividing 1.5 by 7 yields a radial distance contraction to roughly one-fifth of its earlier position.
Did you know? The storm is frequently called the Mother’s Day storm or named after Jennifer Gannon, a space-weather physicist whose work focused on reducing the risks posed by geomagnetic disturbances.
Negative Ionospheric Storms and Delayed Recovery
The plasmasphere’s recovery was unusually slow, requiring more than four days to refill. Shinbori’s team measured refilling times of 4.70 days on inbound passes and 4.31 days on outbound passes, marking the longest recovery recorded since Arase began monitoring in 2017.
Ground-based Global Navigation Satellite System receivers mapped total electron content in the ionosphere and showed that electron content became depleted across much of the globe during the recovery phase. According to the study, intense energy input at high latitudes altered upper-atmosphere composition, creating a negative ionospheric storm that diminished the source reservoir of charged particles normally flowing upward to refill the plasmasphere.
Low-Latitude Auroral Displays
The compression of Earth’s magnetic field allowed charged particles to travel much further along magnetic field lines toward the equator. In Mexico, a Space Weather study of 45 locations documented red auroras in low-latitude states, including a display near Colima at 19.4 degrees geographic latitude (about 27.5 degrees magnetic latitude) that lasted more than 40 hours.
In Japan, citizen scientist photographs reconstructed a blue-dominant structure near 40 degrees magnetic latitude over northern and central Honshu. The Australian Bureau of Meteorology recorded G5 planetary conditions on May 11 and G4 conditions locally, with displays widely photographed across southern Australia and far north of Tasmania. NASA noted that modern cameras are more sensitive than human night vision, with credible reports placing auroras as low as 26 degrees magnetic latitude.
Frequently Asked Questions
What caused the May 2024 Gannon storm?
The storm was triggered by repeated solar flares and coronal mass ejections from active region 13664 beginning on May 8, 2024, sending dense plasma and southward-pointing magnetic fields toward Earth.
How much did Earth’s plasmasphere shrink during the storm?
According to research led by Dr. Atsuki Shinbori, the outer boundary of the plasmasphere contracted radially from an L-shell of 7 to 1.5 within nine hours, placing it at roughly one-fifth of its normal distance from Earth’s centre.
Why did the plasmasphere take so long to recover?
Intense high-latitude energy input created a negative ionospheric storm that depleted the supply of charged particles available to flow upward and refill depleted flux tubes, resulting in a recovery time exceeding four days.
Where were auroras visible during the Gannon storm?
Vivid auroras appeared at unusually low latitudes, including Mexico, Japan, and southern Australia, due to the intense compression of Earth’s magnetic field.
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