An X8.2 class solar flare flashes at the edge of the Sun on 10 September 2017, part of a burst of solar activity lasting for two weeks which led to a wide range of space weather effects at Earth. Note that this image is rotated in order to highlight the west limb of the Sun, so north is to the left. Credit: NASA/GSFC/SDO

Never take things for granted. That was the message that the Sun sent to the space weather community in early September 2017. After months, indeed years, of low solar activity, our star exhibited a sudden burst of activity leading to a wide range of space weather effects at Earth, including intense X-ray fluxes, a significant geomagnetic storm, a long-lasting radiation storm in space, and even a small increase in radiation levels at the surface of the Earth.

It is probably one of the most comprehensively observed space weather events, and a great opportunity to learn lessons both for our scientific understanding, and for how the space weather operations community responds to such events.

The burst of solar activity was largely associated with an active region (AR2673) that traversed the visible face of the Sun between 29 August and 10 September. When it first appeared at the east side of the Sun it was a simple seemingly benign feature. It remained that way until 3 September, when it developed rapidly into a magnetically complex region, suggesting a growing potential for major activity.

Figure 1. Solar activity on 1-12 September 2017 as shown by the uncorrected X-ray radiance measured from NOAA’s GOES-13 satellite. The X-ray background rises markedly as the active regions on the Sun become more complex. Superposed on this background are a series of X-ray bursts from solar flares, many rising above the M-class threshold of 10-5 W m-2 and a few well above the higher X-class flare threshold. Following the X8 flare on 10 Sep, the X-ray irradiance falls markedly as the active region rotates past the west limb of the Sun. Credit: Data courtesy of NOAA; plot by author

This was not long in coming as the region produced a series of large solar flares on 4 and 5 September (see Figure 1).

The largest of these flares, a M5.5 flare late on 4 September, was associated with launch of a coronal mass ejection (CME) that arrived at Earth late on 6 September producing a marked and temporary compression of Earth’s magnetic field (a so-called sudden impulse event) that could be seen by ground-based magnetometers. However there was no geomagnetic storm because the magnetic field in the CME was predominantly northward.

It was 6 September that saw the most spectacular activity on the Sun – in particular two large X-class flares in quick succession: an X2.2 peaking at 9:10 a.m. and X9.3 at 12:02 p.m. UTC.

Figure 2. The X9 flare on 6 September is shown in the lower right of this image, taken at 9.4 nm wavelength by the AIA instrument on NASA’s Solar Dynamics Observatory. The high intensity of the flare has generated a number of artifacts as shown by the solid and dashed lines aligned with the center of the flare. Credit: NASA

The latter was the largest solar flare seen from Earth since 2006 and is shown in Figure 2.

It was associated with another major CME launch – one that in this case produced a significant geomagnetic storm starting late on 7 September and continuing during 8 September.

AR2673 continued to be very active as it rotated towards the west limb of the Sun, producing a series of M-class flares and one just reaching into X class.

Figure 3. The CME launched on 10 September appears in the right hand (west) side of this coronagraph image taken by the LASCO instrument on the ESA/NASA SOHO mission. Credit: NASA

Then just as it was disappearing over the limb, it produced another monster event – an X8.2 flare at 4:06 p.m. on 10 September and large CME heading well away from Earth, as can be seen in Figure 3.

Throughout this time our planet also experienced a series of radiation storms, starting promptly after the first CME launch and flare on 4 September.

It was enhanced after the second CME launch and X9.2 flare on the 6th before falling to a lull on 9th. There was a huge enhancement after the CME launch and flare on 10th, and then a gradual decay, falling below storm levels late on 14th.

The last enhancement was rich in very high energy particles (>100 MeV); it included particles with sufficient energy (>1 GeV) to generate increased natural radiation levels at the surface of the Earth, giving the first “ground level enhancement” since 2012. It was preceded by a marked reduction in the cosmic ray fluxes reaching Earth – a so-called Forbush decrease caused by the passage of the CMEs that arrived at Earth on 6 and 7 September.

At the time of writing it seems that the geomagnetic and radiation impacts on technological systems were limited, reflecting that the intensity of these storms was much below that of historically extreme space weather events such as those in March 1989 and October 2003.

Intense X-ray fluxes had a serious impact on some emergency communications systems  – and did so at the worst possible moment

However, the intense X-ray fluxes did have a serious impact on some emergency communications systems – and did so at the worst possible moment, namely the passage of Hurricane Irma through the Caribbean.

High frequency (HF) radio communications were widely used for the vital coordination of emergency response work across the Caribbean.

This radio technology provides over-the-horizon communications by reflecting 5 to 20 MHz radio waves off the plasma (ionized gas) that is naturally produced high in the upper atmosphere by solar extreme ultra-violet radiation. But the intense X-rays from solar flares penetrate more deeply into the atmosphere producing a layer of ionization that absorbs these radio waves. This happened on both 6 and 10 September leading to many hours during which HF radio waves were blocked by this absorption.

Adverse space weather can magnify the problems associated with other natural hazards

This radio blackout was a severe hindrance to relief efforts following Hurricane Irma. It was a clear example of how adverse space weather can magnify the problems associated with other natural hazards occurring at the same time. The global reach of space weather means that there is a high likelihood of such coincidences in time.

However, we should recognize that this event could have been much worse. We were lucky in that the growth of magnetic complexity on the Sun’s surface began only as AR2673 passed the central meridian. Thus our planet only received two glancing blows from the CMEs launched by this burst of activity and, in particular, the large fast CME launched on 10 September missed us completely, though it did impact Mars the following day, leading to a global aurora at that planet [LASP, 2017].

If the activity had started seven days earlier, that CME might have come directly at Earth, leading to an extreme geomagnetic storm. That we did not need – right in the middle of a period of major natural hazards, not just the Caribbean hurricanes but also severe floods in south Asia, and rising political tensions (adverse space weather can be a confusing factor in monitoring security of our world – as discussed by Knipp et al. [2016]).

The good news is that we can learn lessons from this space weather event. It was a substantial event that exhibited the full range of space weather effects, and at a time when we have a wealth of space weather measurements, as well as growing range of space weather services and increasing space weather awareness in the wider community. Most obviously, it has given us a new dataset that we can use to challenge and advance our scientific understanding. But it was also a good test of the systems now in place to deliver observational data to forecast centers, as well as of the accuracy of those forecasts.

Figure 4. The CME launched from the solar farside on 17 September appears upper part of this coronagraph image taken by the LASCO instrument on the ESA/NASA SOHO mission. It shows how farside CMEs are easily seen from near Earth, once they have grown significantly larger than the Sun. Credit: NASA

The high solar activity continued as AR2673 transited the farside of the Sun. For example, a large CME was launched from the farside on 17 September (since CMEs quickly become much larger than the Sun, our instruments can easily see past the Sun to detect farside CMEs – see Figure 4).

It appeared to be associated with a flare in AR2673, which was by then observable by cameras on NASA’s STEREO A satellite, located to the east of the Sun.

Thus there was much anticipation when AR2673 reappeared at the east limb on 24 September, but it proved to be that this region (now renumbered as AR2682) had reverted to a benign state.

But it did take a day or two to confirm that as it is difficult to assess the magnetic structure of active regions near the limb of the Sun, given that we only have magnetographs near Earth (such as the HMI instrument on NASA’s Solar Dynamics Observatory). It is, perhaps, an example of how space weather forecasting can be improved by observations from a spacecraft off the east limb of the Sun [Kraft et al., 2017].

We must always be ready for a major space weather event

In summary, the space weather events of early September 2017 have been a timely reminder that we must always be ready for a major space weather event, even as we approach solar minimum.

The historical record shows this clearly, but it is helpful for the Sun to remind us with an interesting but in the end limited event. It is a lesson that the space weather community needs to share effectively – with the operators of vulnerable systems, with policy-makers and with the general public. We know that a big space weather event will come – we don’t know when, but we know we need to be ready as we can be.

—Michael Hapgood, Editor of Space Weather and RAL Space, STFC Rutherford Appleton Laboratory, UK; email:

AGU’s Space Weather journal is organizing a special collection to highlight the “Space Weather Events of 4–10 September 2017.” Find out more about the call for papers and how to submit an article.


Hapgood, M. (2017), A wake-up call from the Sun, Eos, 98, Published on 12 October 2017.

Text © 2017. The authors. CC BY-NC-ND 3.0
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