A burst of solar wind reached Earth this week and pushed the northern lights far enough south to become visible across nine northern U.S. states. The National Oceanic and Atmospheric Administration’s Space Weather Prediction Center forecast a planetary Kp index of 5 for Wednesday night, September 23, into early Thursday, a threshold the agency treats as the marker of an active geomagnetic storm.
The forecast put the aurora’s expected reach at northern Washington, northern Idaho, much of Montana, the Wyoming-Montana border, North Dakota, northern South Dakota, northern Minnesota, northern Wisconsin, and Michigan’s Upper Peninsula, with Alaska carrying better odds than any of the Lower 48 states. That is a wide band for a single Kp 5 event, and it reflects how sensitive the aurora oval is to even a modest jump in geomagnetic activity during the current stretch of the solar cycle. By Thursday, observers as far south as the Upper Peninsula were confirming the display NOAA had called two days out.
A coronal hole, not a flare, drove the display
The forecast, first reported by Forbes from NOAA’s outlook, traced the pulse to a large coronal hole rather than a coronal mass ejection, the explosive plasma eruptions that produce the biggest storms. According to EarthSky’s solar-activity log, “analysts detected no Earth-directed coronal mass ejections” during the period; instead, the coronal hole — a cooler, lower-density patch in the sun’s corona — opened a channel for high-speed solar wind roughly two days before it reached Earth on September 24. EarthSky described the arriving stream’s magnetic field as tilted southward, calling that geometry “favorable for energy coupling with Earth’s magnetosphere,” the condition that lets solar wind particles funnel into the upper atmosphere and excite the oxygen and nitrogen molecules that glow as aurora.
That geometry got an assist from the calendar. EarthSky and spaceweather.com both tied the week’s heightened aurora chances partly to the Russell-McPherron effect, a seasonal alignment near the equinoxes that tilts Earth’s magnetic field into a more receptive angle toward incoming solar wind. Spaceweather.com’s own tracking, current as of September 24, logged a 24-hour maximum Kp of 4.33 alongside a photograph submitted from Marquette, Michigan, deep in the Upper Peninsula NOAA had flagged as a viewing zone. The observer’s note accompanying the image read simply, “Even a tinge of the red Aurora came out to play.”
What a Kp of 5 actually forecasts
NOAA’s Space Weather Prediction Center, which issues the planetary K-index used across the aurora-forecasting world, describes the measure on spaceweather.gov as derived from “the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval,” averaged across 13 geomagnetic observatories positioned between 44 and 60 degrees of geomagnetic latitude. The scale runs from 0 to 9, and NOAA’s own documentation states plainly that a reading of “5 or more” marks a geomagnetic storm, the point at which the agency begins issuing formal alerts and watches to power-grid operators, satellite owners, and airlines whose high-latitude routes carry HF radio traffic vulnerable to disruption. The agency maintains a running record of those readings on its Kp-index product page.
On NOAA’s companion G-scale for geomagnetic storms, a Kp of 5 corresponds to G1, the mildest of five storm categories, one the agency associates with weak power-grid fluctuations and aurora that can become visible as far from the poles as the northern tier of the continental United States. That is a low bar as space weather events go, but the visibility footprint tracks closely with the nine states plus Alaska that Wednesday night’s forecast named.
Activity fades fast after the peak
Forbes reported that the viewing window narrows quickly once a coronal-hole stream passes its peak, with the visible footprint expected to shrink to roughly three states by Thursday night. That pattern is typical of these events: unlike the multi-day storms triggered by large coronal mass ejections, a coronal-hole stream tends to deliver one strong night and then taper. EarthSky’s log framed this particular stream as “minor” even as it produced a G1 storm and aurora reported as far away as Scotland and southern New Zealand on the opposite side of the planet during the same window.
Solar activity overall has also been trending down from a recent peak. EarthSky’s coverage places the current stretch well below the intensity of May 2024’s extreme geomagnetic storm, which pushed the aurora as far south as Florida and northern Mexico during the sun’s 11-year activity maximum. This week’s event was ordinary by comparison, a routine coronal hole rather than a flare-driven storm.
It still delivered a verified, NOAA-forecast Kp 5 aurora to nine named states within days of the autumn equinox, when the geometry of Earth’s magnetic field is most cooperative for exactly this kind of solar wind pulse. Whether the next coronal hole lines up with the same favorable tilt is the open question NOAA’s forecasters will be watching heading into October.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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