The geomagnetic field was at quiet to unsettled levels on September 17 under the decreasing influence of effects related to CH1316. Solar wind speed at SOHO ranged between 542 and 802 km/sec, averaging 659 km/sec (-74 km/sec compared to the previous day). The high latitude magnetometer at Andenes recorded quiet to minor storm levels. The above 10 MeV proton flux was at background levels all day.
Solar flux density measured at 20h UT on 2.8 GHz was 147.2 - increasing 26.2 over the previous solar rotation. The 365 day average solar flux at 1 AU was 169.04 (183 days ago). The 81 day average solar flux at 1 AU was 148.7 (41 days ago, this is 50.4% of the total SC25 solar flux range from the max 81 day average peak on September 4, 2024 at 227.3 and an average 81 day flux of 69 at solar minimum). SC25 365d peak: 195.9 on October 13, 2024.
The Potsdam WDC planetary A index was 9 (STAR Ap - based on the average of three hour interval ap indices: 9.1). Three hour interval K indices: 23332122 (planetary), 22332222 (Boulder), 33333255 (Andenes).
The background x-ray flux is at the class B9 level (GOES 19).
At the time of counting spots (see image time), spots were observed in 9 active regions using 2K resolution (SN: 232) and in 9 active regions using 1K resolution (SN: 165) SDO/HMI images.
AR 14216 [N10W04] gained a magnetic delta
configuration before noon when negative polarity flux emerged within the
leading positive polarity spot section. This caused an increase in flare
activity. After noon the magnetic delta faded and the region continued to
decay slowly. C1 flares: C1.5 @ 09:06, C1.3 @ 11:39, C1.1 @ 12:08, C1.7 @
12:19, C1.3 @ 14:55, C1.2 @ 15:00, C1.3 @ 20:50 UT
AR 14217 [S15E31] decayed slowly and was mostly quiet. C1 flares:
C1.0 @ 01:59 UT
AR 14219 [N26W40] decayed slowly and quietly.
AR 14220 [S18W00] developed quickly becoming a compact spot group
with minor polarity intermixing. M flares are possible. C1 flares: C1.1 @
05:06, C1.0 @ 08:25, C1.3 @ 08:57, C1.8 @ 09:52, C1.6 @ 10:15, C1.5 @ 14:05,
C1.2 @ 15:23, C1.3 @ 19:53, C1.4 @ 22:38, C1.6 @ 22:47 UT
AR 14221 [N04E28] decayed slowly and quietly.
AR 14223 [S16E50] lost the tiny magnetic delta. Spots are still
located within a fairly small area and C flares are possible.
AR 14224 [S09E30] decayed slowly and quietly.
AR 14225 [N10E67] decayed slowly and quietly.
Spotted regions not observed (or interpreted
differently) by SWPC/USAF:
S11435 [N22W02] decayed slowly and quietly.
C2+ flares
| Magnitude | Peak time (UT) | Location | Source | Recorded by | Comment |
| C2.3 | 15:41 | 14216 | GOES18 | ||
| C2.8 | 00:04 | 14223 | GOES18 | flare started at 23:58 |
September 15-17: No obviously Earth directed CMEs.
[Coronal
hole history (since October 2002)]
[Compare today's report to the situation one solar rotation ago:
28 days ago
27 days ago
26 days ago]
A large recurrent trans equatorial negative polarity coronal hole (CH1316) rotated across the central meridian on September 10-13. A recurrent northern hemisphere negative polarity coronal hole (CH1317) will become Earth facing on September 19-20.
The geomagnetic field is expected to be at quiet to unsettled levels on September 18 due to effects related to CH1316. Quiet levels are likely on September 19-21. Effects related to CH1317 could cause quiet to minor storm levels on September 22-23.
| Coronal holes (1) | Coronal mass ejections (2) | M and X class flares (3) |
1) Effects from a coronal hole could
reach Earth within the next 5 days. When the high speed stream has arrived
the color changes to green.
2) Effects from a CME are likely to be observed at Earth within 96 hours.
3) There is a possibility of either M or X class flares within the next 48 hours.
Green: 0-30% probability, Yellow: 30-70% probability, Red: 70-100% probability.
(Click on image for 2K resolution).
4K resolution.
Compare to the previous day's image. 0.5K image
When available the active region map has a coronal hole polarity overlay where red (pink) is negative and blue is positive.
Data for all officially numbered solar regions according to the Solar Region Summary provided by NOAA/SWPC, all other regions are numbered sequentially as they emerge using the STAR spot number. Comments are my own, as is the STAR spot count (spots observed at or inside a few hours before midnight) and data for regions not numbered by SWPC or where SWPC has observed no spots. SWPC active region numbers in the table below and in the active region map above are the historic SWPC/USAF numbers. SWPC data considered to be not sufficiently precise (location, area, classification) are colored red.
| Active region | SWPC date numbered STAR detected |
Spot count | Location at midnight | Area | Classification | SDO / HMI 4K continuum image with magnetic polarity overlays |
Comment | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| SWPC/ USAF |
STAR | SWPC | STAR | Current | Previous | ||||||
| 2K | 1K | ||||||||||
| S11418 | 2025.09.08 | N21W47 |
 |
||||||||
| S11421 | 2025.09.09 | S08W56 | |||||||||
| S11422 | 2025.09.10 | N20W58 | |||||||||
| 14216 | 2025.09.10 2025.09.11 |
17 | 33 | 20 | N10W04 | 0200 | CAI | DSI |
 |
 |
beta-gamma was AR S11423 area: 0360 |
| S11424 | 2025.09.11 | N09W40 | |||||||||
| S11427 | 2025.09.12 | S17W37 | |||||||||
| S11428 | 2025.09.12 | N08W31 | |||||||||
| 14220 | 2025.09.12 2025.09.15 |
18 | 45 | 26 | S18W01 | 0080 | DAI | DKC |
 |
 |
beta-gamma was AR S11429 area: 0500 location: S18W00 |
| 14219 | 2025.09.13 2025.09.15 |
7 | 2 | N26W42 | 0015 | CRO |
 |
 |
was AR S11431 location: N26W40 |
||
| 14217 | 2025.09.13 2025.09.14 |
12 | 16 | 8 | S17E36 | 0200 | EAO | EAO |
 |
 |
was AR S11433 location: S15E31 area: 0320 |
| S11434 | 2025.09.13 | S23W55 | |||||||||
| S11435 | 2025.09.13 | 8 | 2 | N22W02 | 0015 | BXO |
 |
 |
|||
| 14221 | 2025.09.14 2025.09.15 |
6 | 8 | 6 | N04E29 | 0110 | DSO | DSO |
 |
 |
was AR S11436 area: 0190 |
| S11437 | 2025.09.14 | N23W02 | |||||||||
| S11438 | 2025.09.14 | S13E16 | |||||||||
| 14222 | 2025.09.16 | N13E08 | trailing spots of AR 14216 | ||||||||
| 14223 | 2025.09.15 2025.09.16 |
5 | 13 | 7 | S16E50 | 0040 | CAO | DRI |
 |
 |
beta-gamma was AR S11439 area: 0080 |
| 14224 | 2025.09.15 2025.09.16 |
7 | 3 | S09E30 | 0015 | BXO |
 |
 |
was AR S11440 | ||
| S11441 | 2025.09.16 | N12E54 |
 |
||||||||
| 14225 | 2025.09.16 2025.09.16 |
1 | 5 | 1 | N10E68 | 0120 | HSX | CSO |
 |
 |
was AR S11442 area: 0230 location: N10E67 |
| Total spot count: | 59 | 142 | 75 | ||||||||
| Sunspot number: | 119 | 232 | 165 | (total spot count + 10 * number of spotted regions) | |||||||
| Weighted SN: | 89 | 178 | 111 | (Sum of total spot count + classification weighting for each AR. Classification weighting: X=0, R=3, A/S=5, H/K=10) | |||||||
| Relative sunspot number (Wolf number): | 131 | 128 | 132 | ||||||||
| Month | Average solar flux | International sunspot number (WDC-SILSO) |
Smoothed sunspot number (4) | Average
ap (3) |
|
|---|---|---|---|---|---|
| Measured | 1 AU | ||||
| 2024.03 | 154.4 | 152.9 | 103.7 | 141.4 (+4.5) | 11.03 |
| 2024.04 | 161.3 | 162.6 | 137.0 | 144.5 (+3.1) | 9.69 |
| 2024.05 | 187.7 | 191.9 | 172.1 | 149.1 (+4.6) | 23.56 (SC25 peak) |
| 2024.06 | 184.3 | 190.2 | 164.1 | 152.8 (+3.7) | 10.24 |
| 2024.07 | 196.6 | 203.0 | 196.8 | 155.0 (+2.3) | 7.13 |
| 2024.08 | 246.1 (cycle peak) |
252.2 | 216.0 (SC25 peak) | 156.8 (+1.8) | 15.96 |
| 2024.09 | 195.7 | 197.8 | 141.1 | 159.4 (+1.6) | 15.36 |
| 2024.10 | 221.0 | 219.6 | 165.8 | 160.9 (+1.5) (SC25 solar max) | 18.99 |
| 2024.11 | 199.6 | 195.3 | 154.1 | 157.1 (-3.8) | 9.33 |
| 2024.12 | 197.3 | 191.2 | 154.6 | 151.3 (-5.8) | 8.42 |
| 2025.01 | 190.3 | 184.2 | 137.0 | 146.3 (-5.0) | 13.17 |
| 2025.02 | 184.2 | 179.7 | 155.7 | 139.9 (-6.4) | 14.42 |
| 2025.03 | 163.7 | 162.1 | 134.2 | (135.6 projected, -4.3) | 15.96 |
| 2025.04 | 162.5 | 163.6 | 140.6 | (132.5 projected, -3.1) | 17.62 |
| 2025.05 | 135.3 | 138.3 | 79.2 | (127.6 projected, -4.9) | 17.26 |
| 2025.06 | 131.3 | 135.4 | 116.3 | (122.8 projected, -4.8) | 21.43 |
| 2025.07 | 137.8 | 142.3 | 125.6 | (118.9 projected, -3.9) | 11.80 |
| 2025.08 | 154.9 | 158.7 | 133.6 | (114.1 projected, -4.8) | 9.97 |
| 2025.09 | 142.1 (1) | 61.4 (2A) / 108.4 (2B) / 136.2 (2C) | (109.7 projected, -4.4) | (14.3) | |
| 2025.10 | (105.8 projected, -3.9) | ||||
| 2025.11 | (103.6 projected, -2.2) | ||||
| 2025.12 | (101.9 projected, -1.7) | ||||
| 2026.01 | (97.8 projected, -4.1) | ||||
| 2026.02 | (93.7 projected, -4.1) | ||||
1) Running average based on the daily 20:00 UTC observed solar flux value
at 2800 MHz and any corrections applied to that measurement.
2A) Current impact on the monthly sunspot number based on the Boulder
(NOAA/SWPC) sunspot number (accumulated daily sunspots / month days).
2B) Boulder SN current month average to date.
2C) STAR SDO 1K Wolf number 27 day average.
3) Running average based on the quicklook and definitive Potsdam WDC ap
indices. Values in red
are based on the definitive international
GFZ Potsdam WDC ap
indices.
4) Source: WDC-SILSO, Royal Observatory
Of Belgium, Brussels
Sunspot activity has been on a decreasing trend since September
2024 (using the average solar flux over 3 solar rotations as a proxy).
Compared to the peak of SC25 solar flux has been at a low level in both May
and June 2025. Taking a closer look at the 81 day (3 solar rotations)
average solar flux proxy, its peak was in early September 2024 and had on
May 12 decreased to less than half of the solar cycle flux range.
Subtracting the 81 day average of approximately 69 at solar cycle minimum
from all measurements, the SC25 range is 0 - 158.3.
All 365 day smoothed sunspot numbers peaked on October 11-12, 2024,
very likely making this the peak of SC25 (365 day smoothed peaks: STAR 1K: 236.1, STAR 2K: 364.3, NOAA SN: 156.0, ISN: 163.9). The 365d
smoothed solar flux peaked on October 13, 2024, just 1.7% below the SC23
peak, and 62.6% above the SC24 peak.
--------------------------------------------------------------------------------------
This report has been prepared by Jan Alvestad. It is based on the analysis of data from whichever sources are available at the time the report is prepared. All time references are to Universal Time. Comments and suggestions are always welcome.
SDO images are courtesy of NASA/SDO and the AIA, EVE, and HMI science teams.
