Space weather, week 39 of 2026.
·Week 39 of 2026, September 21, 2026 to September 27, 2026, 1 of 7 days recorded. No day reached storm level; the most disturbed day was September 21, 2026, with the Kp index (0 to 9, storm at 5 and above) peaking at 1.7, quiet. The Sun let off 2 small (C-class) flares over the week. The daily sunspot count ran from 85 to 85, averaging 85, a moderately active Sun.
One section per complete UTC day, written by the daily data run from NOAA's feeds: the planetary Kp index every three hours, the GOES X-ray flare list, NOAA's daily sunspot number and 10.7 cm flux, and hourly solar wind at L1. Nothing here is edited by hand. The record index explains each figure, and the same numbers are in the open data file.
Monday, September 21, 2026
Earth's magnetic field was quiet: the Kp index, a 0 to 9 scale where 5 and above counts as a geomagnetic storm, peaked at 1.7 in the 00:00 to 03:00 UTC window and averaged 0.8 for the day. Aurora stayed up near the poles. The Sun let off 2 small (C-class) flares; the biggest was C1.2 at 06:29 UTC, too small to notice on Earth. NOAA's sunspot count was 85, a moderately active Sun (0 is no spots at all; 150 to 200 is typical at the peak of a cycle); 3 new sunspot groups appeared. The Sun's radio output, a second gauge of its activity that does not depend on counting spots, read 104, moderate (about 70 for a quiet Sun, 200 or more for a very active one). The solar wind, the stream of particles flowing past Earth, averaged 341 km per second, a slow day, and peaked at 595. Its magnetic field stayed mostly northward (Bz no lower than -4 nT), the direction that keeps storms away.
Kp through the day, one value per three hours from midnight UTC: 1.7, 1, 0.3, 0.7, 0.7, 0.3, 0, 1.3.
What do these numbers mean?
Four things are tracked each day. The Kp index says how disturbed Earth's magnetic field was, on a 0 to 9 scale measured every three hours; 5 and above is a geomagnetic storm, and the higher it goes the farther south the aurora reaches. Flares are sudden flashes on the Sun, sorted by size: C-class is small and harmless, M-class is medium and can briefly fade shortwave radio, X-class is large. The sunspot count is NOAA's daily tally of dark spots, weighted so that groups count more than lone spots; it runs from 0 on a blank Sun to 150 or more near the peak of a solar cycle. The radio output is the Sun's brightness at a 10.7 cm radio wavelength, a second activity gauge that does not depend on counting spots; 70 is quiet, 200 very active. The solar wind is the stream of particles flowing past Earth, measured about 1.5 million km upstream; its speed and the north-south direction of its magnetic field (Bz) decide whether a storm follows, because a southward field lets the wind's energy into Earth's field.