An M6.7 solar flare erupted on Oct. 8 and pushed X-ray output past the M5 threshold that sets off NOAA’s R2 (Moderate) radio blackout rating. The Space Weather Prediction Center’s alert went out at 15:47 UTC, one minute after the threshold was crossed, and the follow-up summary at 16:04 UTC said to expect a limited blackout of high-frequency radio communication lasting tens of minutes.
That wording is a forecast of impact, written minutes after the event began, and it describes the sunlit side of Earth at the moment of the flare.
The M6.7 flare in the SWPC summary: 15:29 to 15:58 UTC
The summary, issued as serial 327, logs the event with a start at 15:29 UTC, a peak at 15:48 UTC and an end at 15:58 UTC, so the flare took about half an hour from onset to finish. It gives an X-ray class of M6.7, an optical class of 2b and a location of N12W20 on the solar disk, which puts the source north of the sun’s equator and west of the central meridian.
An M6.7 flare is six and seven-tenths times the M1 baseline. NASA explains on its solar storms and flares page that each flare class is a 10-fold increase in energy over the one below it, with A the weakest and X the strongest, and that the numbers 1 through 9 divide the lower classes. An M6.7 therefore reaches about two-thirds of the X1 level, the threshold for the R3 rung.
The light reaches Earth in about eight minutes, NASA notes, so by the time the flare is seen most of its effects have already arrived.
R2 on the NOAA scale: M5 and tens of minutes
NOAA’s space weather scales define R2 by a physical measure of M5, or 5 times 10 to the minus 5 watts per square meter. Its listed effects are a limited blackout of HF radio communication on the sunlit side, with loss of radio contact for tens of minutes, and degradation of low-frequency navigation signals for tens of minutes. The scale puts R2 at about 350 events per solar cycle, or roughly 300 days.
The rungs on either side give context. R1 starts at M1 and means weak or minor HF degradation with occasional loss of contact, at about 2,000 events per cycle. R3 starts at X1 and means a wide-area HF blackout with about an hour of lost contact on the sunlit side.
The alert itself, serial 542 in the same feed, states that the impact is centered on the sub-solar point, where the sun is directly overhead, and that the extent of the HF blackout depends on current X-ray flux intensity. It points forecasters and radio operators to the D-RAP product for real-time area and duration.
D-RAP and what an HF blackout looks like on a map
D-RAP, short for D-Region Absorption Predictions, is the SWPC model that turns X-ray and particle measurements into a map. The D-RAP page describes a global display of the highest frequency affected by at least 1 decibel of absorption, driven by solar X-ray flux, energetic particles or both, with an estimated recovery clock.
The physics is in the D region of the ionosphere, roughly 50 to 90 kilometers up. HF signals between 3 and 30 megahertz normally bounce off the higher F2 layer, but they lose strength in the D region on the way up and back down, and the page says that at low latitudes the D region is driven mainly by photons, producing prompt changes. The page lists emergency management, aviation and maritime operators among the users, and says the data update at one-minute cadence from GOES X-ray measurements.
No SWPC product read for this report records a measured outage, so the extent of the actual loss on Oct. 8 is not documented in the alert feed. The impact language is the center’s expectation for a flare of this size.
Sunspot activity and the geomagnetic outlook
An independent tally on SpaceWeather.com lists the strongest flare of the day as an M6 at 1548 UT, which matches the SWPC peak time of 15:48 UTC. It describes sunspot 4549 as growing, with a beta-gamma-delta magnetic field that can produce X-class flares, and says NOAA put the chance of M-class flares at 40 percent and X-class at 5 percent for the next 24 hours.
The same page says CMEs launched from that sunspot are modeled to arrive on Oct. 9 and Oct. 11. The center had already published a geomagnetic storm watch for Oct. 9 predicting G1 to G2 conditions from a CME linked to an M1.4 flare on Oct. 6. Those arrivals are a separate hazard from the radio blackout, which the alert ties to the flare’s X-ray flux.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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