Solar particle events: when SPEs raise in-flight dose

The baseline dose at cruise altitude is set by galactic cosmic rays: a near-isotropic, steady background that varies slowly over the eleven-year solar cycle and somewhat with latitude and altitude (see our GCR vs solar guide). On top of that baseline sits a much smaller, much rarer, and much more dramatic contribution: solar particle events. SPEs are short bursts of high-energy protons (and a tail of heavier ions) accelerated from the solar atmosphere by flares and coronal mass ejections. The largest SPEs deliver dose at rates many times the GCR background. Understanding when they matter, when they don't, and what is being done about them is core to interpreting any in-flight dose number.

The physics of a solar particle event

When a major solar flare erupts, magnetic reconnection in the corona accelerates protons and ions to relativistic energies. A coronal mass ejection (CME) can additionally accelerate particles in the interplanetary shock front it drives ahead of itself. The accelerated particles propagate along the interplanetary magnetic field and, if the field geometry connects Earth's vicinity to the acceleration site, particles arrive at the magnetosphere within minutes to hours ^[1].

Most of those particles are deflected by the geomagnetic field exactly as GCR are; only particles above the local vertical cutoff rigidity penetrate to flight altitude. Because the SPE spectrum at the lower end (10–100 MeV) is much softer than the GCR spectrum, geomagnetic shielding at mid- and low-latitudes is even more effective against SPE protons. The dose enhancement from an SPE is therefore strongly concentrated at high geomagnetic latitudes. A polar-cruising aircraft can see a large dose-rate increase from a major event; the same event at the equator may be invisible.

The NOAA S-scale

NOAA Space Weather Prediction Center categorises SPEs on a five-step Solar Radiation Storm scale (S-scale), defined by the 10 MeV proton flux measured by GOES geostationary satellites ^[2]:

S-level	Descriptor	10 MeV proton flux (pfu)	Frequency per solar cycle (approx)
S1	Minor	10	50 events
S2	Moderate	100	25 events
S3	Strong	1,000	10 events
S4	Severe	10,000	3 events
S5	Extreme	100,000	<1 events

(pfu = particle flux units = particles cm^-2 sr^-1 s^-1, integrated above 10 MeV.) The frequencies above are NOAA's headline averages over a full eleven-year cycle; the distribution within a cycle is far from uniform, with most events clustered near solar maximum.

What in-flight dose actually looks like during an SPE

For events at S1–S2, the in-flight dose enhancement is typically a few microsieverts per hour at high latitudes, integrating to perhaps 10–50 µSv extra dose on a long-haul polar leg. This is comparable to, or smaller than, the per-segment GCR dose itself; it is real but not exceptional.

For S3 events, particularly with hard spectra, polar in-flight dose rates can reach several tens of microsieverts per hour for periods of several hours: total event-attributable dose of order 100–500 µSv on an exposed segment.

For the rare S4–S5 events, in-flight dose at high latitudes can briefly rival the dose from a chest CT scan ^[3]. The reference event most often cited in the dosimetry literature is the February 1956 ground-level event (GLE 5), which would have delivered roughly 10 mSv to a hypothetical polar-cruising airliner had one existed at the time. The more recent October 1989 GLE 42 and the 20 January 2005 GLE 69 were also exceptional. Even at these extremes, the per-event dose is bounded; SPEs do not accumulate into chronic exposure the way GCR does.

How airlines and aircrew respond

ICRP Publication 132 recommends that operators have procedures for SPE response, and major operators do ^[4]. The typical playbook:

• NOAA SWPC issues alerts in real time when proton flux thresholds are crossed. Operators with sophisticated dispatch can monitor these.
• For large events on polar routes, aircraft may be re-routed to lower latitudes or descended to lower altitudes (dose rate drops sharply with altitude; see altitude guide).
• Post-event, the dose accumulated during the event window is logged against the crew member's annual record.
• For S5 events specifically, full operational re-routing of all polar flights has occurred historically (some Asian carriers did this during the October 2003 Halloween Storms sequence).

For a passenger, none of this is your decision to make. The aircrew, dispatch, and the operator handle it. The point of mentioning it is that the system already accounts for SPE risk; you do not need to actively avoid polar flying out of SPE fear.

SPE contribution to lifetime dose for typical fliers

Quantitatively: integrated over a full eleven-year solar cycle, total SPE-attributable dose to a typical aircrew member with a polar-heavy schedule is on the order of 1–3 mSv, comparable to one extra year of normal flying, spread across a decade ^[3]. For non-occupational fliers with much less polar exposure, the SPE contribution to lifetime dose is small (often less than 0.5 mSv lifetime even for very frequent flyers).

This is the right way to think about SPE risk: it is the source of variance on top of a much larger predictable mean. The mean dose from GCR over a flying career is what dominates lifetime exposure; SPEs add a long-tailed perturbation. The perturbation can be dramatic in a single moment but is small over a lifetime.

What CARI-7 does and does not include

CARI-7 / CARI-7A models the galactic cosmic-ray background as a function of solar-cycle phase via the heliocentric potential. It does not model individual SPEs ^[5]. If you fly during a known SPE, the CARI dose for that segment is the GCR-only number and will under-report your actual dose.

For users who supply specific flight dates that overlap a NOAA-logged S2-or-larger SPE, FlightRadiation flags the segment on the report with an estimated SPE-additional-dose range based on the event's published proton flux profile. This is an estimate, not a measurement; we make the underlying uncertainty explicit.

For SPE-specific modelling at higher fidelity, the NAIRAS (Nowcast of Atmospheric Ionizing Radiation for Aviation Safety) project at NASA Langley provides real-time and historical event maps; we recommend it to readers who want the highest-resolution SPE dose reconstruction ^[6].

The bottom line

Solar particle events are real, are well-monitored, are concentrated at high latitudes, and contribute a modest fraction of lifetime dose for typical fliers. They are not a reason to avoid polar flying. They are a reason that any specific per-segment dose number is uncertain at the moment of the flight; you don't know whether you flew through an event until the dosimetry record is reconciled afterwards. For most readers, the practical takeaway is: trust the GCR baseline (CARI-7), understand that SPEs add a small lifetime perturbation, and recognise that the in-flight dose system already incorporates SPE response at the operator and regulator level.

The Forbush decrease and the recovery period

Large solar storms, particularly the coronal mass ejections that drive S-scale SPEs, also affect galactic cosmic ray flux in a separate way. As a CME-driven shock front sweeps past Earth, it temporarily depresses the GCR flux in the inner heliosphere, a phenomenon called a Forbush decrease. The decrease typically drops GCR flux 5–15% over a few days, then recovers gradually over the following week or two. The cosmic-ray monitor at Oulu in Finland and other neutron monitors provide real-time data on Forbush decreases ^[4].

For in-flight dose, the Forbush decrease and the SPE arrival are competing effects on the same flight: the SPE adds dose, the Forbush depression of GCR subtracts dose. For most large events the SPE addition dominates, sometimes by orders of magnitude, but for smaller events the two can be of similar magnitude. The net dose on a polar flight during a complex event sequence is harder to estimate than either contribution in isolation.

How SPE risk responds to the solar cycle

SPEs cluster around solar maximum but are not exclusive to it. The 11-year cycle has a roughly four-year rising phase, a two-year maximum, and a five-year declining phase. Counting all events S1 and above, the maximum years can have 10–15 events; the deep minimum years often have zero. The largest events (S4–S5) are rare in any phase but are not strictly limited to maximum: the August 1972 event (often described as one of the most dose-impactful events of the modern instrumented era) occurred in a declining phase.

For long-range planning, this means lifetime SPE exposure depends on which solar cycles your career spans. An aircrew member whose career happens to coincide with two solar-maximum periods will accumulate more SPE-attributable dose than one whose career happens to span two minima, even with identical route mix and flight hours.

Operator versus regulator dose tracking

Under the EURATOM Basic Safety Standards Directive (2013/59/Euratom) EU operators are required to assess crew exposure and to take it into account when organising work schedules. SPE-attributable dose is part of that assessment. The methodology recommended by EURADOS is to use the GCR-derived dose (from a code like CARI-7 or EPCARD) plus an SPE additional-dose estimate for any flight that occurred during a logged S2 or larger event. The SPE additional dose is typically estimated using the proton-flux integral measured by GOES-class satellites and a transport calculation specific to the event ^[2].

In US operations, AC 120-61B is silent on SPE-specific dose tracking; carriers that maintain dosimetry programs use methodologies broadly similar to the EURADOS recommendation. Most US carriers' dose records are dominated by the GCR baseline regardless.

Last reviewed 30 June 2026 · See our methodology and sources.