geomagnetic alerts, solar flux peaks as sunspot counts rises

The Australian Space Weather Forecasting Centre (ASWFC) has issued a geomagnetic disturbance warning due to a coronal mass ejection (CME). The announcement expects four halo CMEs to arrive at Earth on 10-May, starting at 1000 UTC +/- 10 hours. G4 geomagnetic conditions are expected on 10-May, reducing to G3 with a chance of G4 on 11-May.

This week, sunspot groups decreased from nine on Monday to six on Wednesday. However, two of them are large, and both have a beta-gamma-delta magnetic configuration, indicating the possibility of strong solar flares. Several moderate flares (M-class) were observed daily, with large flares (X-Class) also being frequent. Although one sunspot is nearing the northwestern limb of the solar disk, solar activity is increasing. The other prominent sunspot, AR3664, has merged with AR3668 and is comparable in size to the infamous Carrington spot of 1859.

If AR3664 were to erupt in a CME similar to the Carrington Event and directed toward Earth, it could repeat the Carrington Event, causing potentially devastating consequences for power and communications grids. Shortwave fading (SWF) is a phenomenon where shortwave radio communication is disrupted due to solar flares and is named after John Howard Dellinger and Hans Mogel, abbreviated as the Dellinger or Mogel-Dellinger effect.

Solar flares with CMEs in the western half of the solar disk tend to increase geomagnetic activity and fluctuate shortwave propagation conditions around the weekend, with a slow return to average conditions in the following days. Large sunspots have caused increased attenuation on the lower shortwave bands, disrupting communications for tens of minutes to hours.

Predicted solar flux is 240 and 225 on May 10-11, 220 on May 12-13, then 215 on May 14, then 210 on May 15-16, and 200, 195, 190, 185, 180, 175, 170, 165, and 170 on May 17-25, then 175 on May 26-27, 170 on May 28, then 165 on May 29-31, then 175, 180, 185, 190, and 185 on June 1-5, 175 on June 6-9, 180 on June 10, 205 on June 11-12, then 200, 195, 190, and 185 on June 13-16.

Predicted planetary A index is 8, 5, 8, 12, and 10 on May 10-14, 5 on May 15-22, then 8, 12, 8, 5, 12, and 8 on May 23-28, then 5, 5, and 8 on May 29-31, and 12 on June 1-3, then 8, 10, 5, and 5 on June 4-7, then 8, 15, and 10 on June 8-10, and 5 on June 11-18.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere - May 9, 2024, from OK1HH, concludes that although the number of sunspot groups was smaller than in previous weeks, two of them were large and had a beta-gamma-delta magnetic configuration, indicating the possibility of producing strong solar flares. The author also discusses solar flares, CMEs, and the impact on radio communications and the aurora.

Sunspot numbers for May 2 through 8 2024 were 125, 121, 136, 152, 148, 144, and 142 with a mean of 138.3. 10.7 cm flux was 141.9, 156, 166.6, 176.9, 171.2, 203.6, and 227.1, with a mean of 177.6. Estimated planetary A indices were 44, 10, 6, 12, 15, 7, and 7, with a mean of 14.4. Middle latitude A index was 24, 16, 5, 12, 13, 6, and 10, with a mean of 12.3.

Archives of past propagation bulletins are at, and provides more information concerning shortwave radio propagation. For an explanation of numbers used in this bulletin, see

Contact information for sending tips, reports, observations, questions, and comments is provided. When reporting observations, the author encourages you to tell us which mode you were operating. More information can be found at the ARRL Technical Information Service web page,, and a QST article about Solar Indices at

Instructions for starting or ending email distribution of ARRL bulletins are located at, and more information on solar indices and propagation is at Let me know if you would like more information on the above announcement.

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