X-class Flares and Solar Particle Event of September 2017

The most significant activity of solar cycle 24 occurred in September 2017. The resulting radiation storm was detected on multiple ESA spacecraft. This article gives a brief overview of the activity.


The largest solar X-ray flare seen in 12 years, class X9.3, took place on at 12:24 UTC on 6th September 2017. This eruption was accompanied by a fast expulsion of coronal plasma known as a Coronal Mass Ejection (CME) and an associated interplanetary shock accelerating particles resulting in an enhancement of radiation seen near the Earth known as a Solar Particle Event (SPE).

This was followed by another huge X8.2 flare which took place at 16:06 UTC on Sunday 10th September 2017 with another very fast CME this time resulting in a stronger high-energy SPE. These events show that although the Sun is currently approaching the next minimum in its Solar Cycle, significant eruptions may still occur.

 

These events were observed by various space observatories. The video below shows this period of high activity with the succession of flares as observed by the AIA instrument on-board the NASA Solar Dynamics Observatory (SDO) at three overlaid wavelengths (171, 193 & 304 Angstrom) and the X-ray fluxes observed in the 1-8 Angstrom range by the NOAA GOES satellite.

 


View video on youtube (if embedded version is not working)

 

The associated high-energy proton increase from the 10 September event was promptly seen by the radiation monitoring instruments on a number of ESA spacecraft, including Integral, XMM-Newton, Proba-1, Proba-V, Giove-A, AlphaSat and Galileo. This type of SPE is known as a Ground Level Event (GLE) as the particle radiation was sufficiently energetic to reach the Earth’s atmosphere and induce secondary neutrons which were detected by neutron monitors at the Earth’s surface. Another interesting feature of the 10 September event was that the energetic protons arrived very quickly following the X-ray flare, implying a very good magnetic field connection to the site. Due to the solar rotation good connectivity is usually between 45 and 60 degrees in solar longitude West of the central meridian (that's the right portion of the Sun as we look at it). However, this flare occurred on the Sun's limb (~90 degrees West - see 0:19.5 in the video above) implying rapid angular expansion of the associated CME low in the solar corona. Operational aspects of such events on various ESA spacecraft are followed by the ESA SSA programme who have produced an article on this SPE.

 

Energetic protons as detected by the SREM (Standard Radiation Environment Monitor) instrument on Integral and Proba-1 spacecraft, respectively, are shown below, together with the X-ray emission graph from the event as seen by the US NOAA spacecraft. Comparison of SREM data to the GOES energetic proton measurements is also included with interpolation to the same energy ranges, showing a good agreement between the different instruments. These data were extracted using the TEC-EPS Open Data Interface (ODI) and are available to the public via that free tool. A set of prototype, ODI-based dynamic plots can be found here.

INTEGRAL, PROBA-1 and GOES 20 MeV proton flux time series
INTEGRAL, PROBA-1 and GOES 20 MeV proton flux time series
Mean Event proton spectra
Mean Event proton spectra

The CMEs (initially travelling at speeds between 1000 - 2000 km/s) as seen by the ESA/NASA SOHO mission (with EUV images from SDO) are shown in the video below. The SPE was also detected by SOHO as the substantial increase of transient tracks and spots seen on the images after 10 September result from the high-energy particles hitting the imaging instruments. Both this video and the one above were made using the JHelioviewer software; a user-friendly Java tool developed by the ESA JHelioviewer team which can be downloaded for free.



View video on youtube (if embedded version is not working)

 

Energetic proton and alpha particle (Helium ion) time series derived from the EPT (Energetic Particle Telescope) instrument on ESA’s Proba-V satellite are depicted for the parts of the Proba-V orbit most exposed to the interplanetary space, i.e. over the Earth's polar regions where the geomagnetic shielding is at its weakest. Although the majority of high-energy particles generated during SPEs are protons (left hand panel), heavier ions such as alpha particles are also present, and the EPT is equipped to observe these (right hand panel). The image shows preliminary results from EPT integrated over time periods of 5-minutes (thick lines) and then interpolated and averaged over longer time periods to fill gaps where Proba-V is shielded from the SPE by the Earth’s magnetosphere (thin coloured lines). These data are compared to data from the NOAA GOES spacecraft (black lines) which have been corrected and interpolated to match the nominal energy channels of EPT.

Fluxes recorded by EPT from 2017-09-10 compared to GOES (black lines). Left panel: protons; Right panel: helium (results are preliminary)
Fluxes recorded by EPT from 2017-09-10 compared to GOES (black lines). Left panel: protons; Right panel: helium (results are preliminary)

The final figure shows an early estimate of the cumulative radiation dose from the event as compared to past solar events as seen by the US NOAA GOES spacecraft in geostationary orbit. These data show that the event was a large but not exceptional one and that the dose was relatively low (~2 Gy at 4mm shielding), especially compared to orbits where doses are dominated by the radiation belts. Contacts with some operators indicate limited impact on operating spacecraft although astronauts on the ISS (international Space Station) were warned since exposure to the event at higher ISS latitudes, in low shielding situations could have been hazardous. Sources from DLR report that the event was also well recorded by the DOSimetry TELescopes (DOSTELs) mounted at a fixed position inside the Columbus module of the ISS.

Top 5 solar particle event dose depth curves with 10 Sep 2017 event
Top 5 solar particle event dose depth curves with 10 Sep 2017 event. Ranking based on 4 mm solid sphere dose.
2017/09/10 SEP Total Event dose compared to those since Nov '97
2017/09/10 SEP Total Event dose compared to those since Nov '97

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