Polar routes and cosmic dose: why latitude matters

Two airliners cruise at FL370 on the same day. One is on a great-circle from New York to London that crosses 58° N latitude at its northernmost point. The other is on a transpolar great-circle from New York to Hong Kong that crosses 80° N. According to the FAA's CARI-7A model, the dose rate inside the second aircraft is roughly twice that inside the first, even though the cabin altitude, aircraft type, and date are identical ^[1]. The reason is geomagnetic shielding — and understanding why is the single most useful piece of cosmic-radiation knowledge a frequent flier can have.

The Earth's geomagnetic field is a particle shield

Galactic cosmic rays are charged particles — primarily protons (about 87% of the flux by particle count above 1 GeV/nucleon), helium nuclei (12%), and heavier nuclei (1%) ^[2]. Charged particles moving through a magnetic field are deflected. Earth's roughly-dipole magnetic field bends low-energy cosmic rays away from the planet entirely; only particles with enough rigidity (momentum per unit charge) can punch through and produce the atmospheric cascade that delivers dose to flight altitudes.

The minimum rigidity a particle needs to reach a given latitude is called the vertical cutoff rigidity, R_c. At the geomagnetic equator, R_c is around 14–17 GV (gigavolts) — only the most energetic cosmic rays get through. At 60° geomagnetic latitude, R_c drops to about 1–2 GV. At the geomagnetic poles, R_c approaches zero, meaning essentially the entire GCR spectrum reaches the top of the atmosphere unimpeded. Because the GCR spectrum is steeply falling — there are many more low-energy particles than high-energy ones — admitting low-rigidity particles dramatically increases the flux at the top of the atmosphere, and therefore the dose at cruise altitude.

What that means in numbers

Dose rate at FL370 in mid-2026, for an ICRP-103 reference person, computed with CARI-7A:

Geomagnetic latitude band	Approx vertical cutoff rigidity (GV)	Effective dose rate (µSv/hr)
0–20° (equatorial)	14–17	2.2–2.8
30–45° (most US domestic, southern Europe)	6–11	3.8–4.6
50–60° (typical transatlantic mid-band)	1.5–4	5.0–5.8
60–70° (polar transatlantic, Iceland, northern Canada)	< 1.5	5.6–6.4
70°+ (transpolar)	~ 0	6.0–7.0

The ratio between equatorial and polar dose rate at the same altitude is roughly 2.5×–3×. That's the headline number. ICRP Publication 132 (2016) summarises the same physics for the aviation context and gives broadly consistent figures ^[3].

Why polar routes still dominate dose for transatlantic fliers

The mid-latitude dose rate is already two-to-three times higher than equatorial. But the polar premium on top of mid-latitude is itself only about 10–25%. So why do polar segments dominate annual dose for many frequent fliers?

The arithmetic is route geometry, not rate. North-Atlantic Track System (NATS) routings between northeast US / eastern Canada and northern Europe typically peak at 55–65° N. London-Tokyo, Toronto-Hong Kong, San Francisco-Singapore via polar great-circles spend hours above 70° N. A transpolar New-York-to-Hong-Kong segment is roughly 15 hours block time, of which 6–8 hours can be above 70° geomagnetic. At an average 6.5 µSv/hr that's about 40–50 µSv of dose accumulated just in the polar segment, on top of the climb / descent / mid-latitude dose. By contrast, a transatlantic crossing might deliver 50–80 µSv total per one-way segment ^[1].

In our sample report (Subject A), four polar transatlantic round-trips a year produced 38% of the subject's annual dose from 4.2% of segments. That ratio is not unusual.

Which routes are "polar" for dosimetry purposes

The aviation industry distinguishes polar (ICAO Polar Operations: above 78° N or below 60° S) from cross-polar (routes between Asia / north America that fly over the pole). For dose purposes, the relevant threshold is lower. Anything that spends meaningful time above 60° geomagnetic latitude is in the regime where geomagnetic shielding has dropped enough to matter. That includes:

• All NATS transatlantic crossings (great-circle routes peak around 55–62° N).
• All Europe-to-east-Asia routes that fly over Russia or the polar regions.
• Toronto / Chicago / NYC-to-Asia transpolar routes.
• Most domestic flying within the upper US, Canada and Scandinavia.

Routes that stay mostly equatorial — Brazil to South Africa, Singapore to Sydney, intra-Africa — are in the lower dose regime.

Why mid-latitude routings exist (and why you cannot always choose them)

The great-circle is the shortest path between two airports. ATC and operational constraints can deviate aircraft from the great-circle for fuel economy (riding favourable jet-stream winds), weather avoidance, ETOPS (extended twin-engine operations) restrictions, and political airspace. None of those is driven by dose minimisation. For carriers operating under EURATOM Basic Safety Standards (the EU regulatory framework), aircrew dose is monitored, but route selection is overwhelmingly driven by economics, not dose ^[3].

As a passenger, you generally cannot select a non-great-circle routing. If you have a choice between two carriers and your annual exposure matters to you, the rough heuristic is: the more northerly the planned routing (which you can usually infer from the airway charts or from a tool like FlightRadar24 historical playback), the higher the per-segment dose. The dose-rate premium for a polar transpolar over a mid-latitude transatlantic is real but modest in percentage terms; the cumulative effect over a year of heavy flying can be substantial.

Solar particle events compound polar risk

The geomagnetic shielding that suppresses GCR at low latitudes also suppresses the high-energy solar protons released by major flares and coronal mass ejections. A solar particle event (SPE) on a polar transatlantic crossing can deliver dose at rates many times the GCR background; the same event at the equator might deliver almost nothing. NOAA SWPC classifies SPEs on the S-scale (S1 minor through S5 extreme); for the largest events, transpolar airlines historically reroute. See our SPE guide for the historical record and how the response works.

What polar-route awareness means for you

For the occasional flier — one or two transatlantic round-trips a year — the polar premium is real but small in absolute terms. Even ten polar transatlantic round-trips a year, at the high end of the dose-rate band, is on the order of 4–6 mSv/yr. That is below the FAA aircrew action level of 6 mSv/yr ^[4], and far below the ICRP-103 occupational limit of 20 mSv/yr ^[5].

For very frequent flyers and aircrew, polar segments are the single largest reducible exposure. Carriers operating in EURATOM jurisdictions are required to optimise crew rosters to keep individual aircrew dose below regulatory limits, and route selection is part of that optimisation. For an individual passenger doing 100+ flights a year, choosing carriers and routings that minimise polar time can reduce annual dose by 20–40%.

The CARI-7 model and the FlightRadiation report quantify exactly how much polar exposure your specific pattern produces and what fraction it represents of your annual total. The number matters; doing something about it is your call.

The northern Hemisphere asymmetry

Almost all high-latitude civil flying happens in the northern hemisphere. The high-traffic NATS, Pacific cross-polar, and Asia-Europe routes all transit northern high latitudes. The southern equivalents — Australia-South Africa, southern South America-Antarctica adjacency routes — see vastly less traffic. As a result, the global aircrew dose burden is concentrated in northern-hemisphere routes, and the dosimetric findings in the literature are dominated by northern-hemisphere data. The physics is symmetric north-south but the traffic patterns are not.

For practical purposes this means almost every dosimetric study, every polar attribution analysis, and every CARI-7A validation run uses northern-hemisphere routes as its baseline. Southern-hemisphere fliers can use the same numbers with confidence because the underlying physics is symmetric; the absolute dose at the same geomagnetic latitude on either hemisphere is essentially identical.

Latitude geographic vs geomagnetic

Throughout this guide we have referred to geomagnetic rather than geographic latitude. The distinction matters because Earth's magnetic dipole is tilted roughly 11° from the rotation axis and the magnetic poles drift over geological time. The geomagnetic north pole in 2026 sits in the Canadian Arctic near 86° N geographic / 175° W longitude. A great-circle from New York to Hong Kong over the geographic pole passes much closer to the geomagnetic pole on the Pacific side than on the Atlantic side, and the dose-rate profile reflects this rather than a clean geographic-latitude function.

The International Geomagnetic Reference Field (IGRF), maintained by the IAGA Working Group V-MOD, is the authoritative model for converting between geographic and geomagnetic coordinates and is updated every five years to track secular variation in the Earth's field ^[2]. CARI-7 uses the appropriate IGRF coefficients internally and we use them for the polar-attribution metric in our reports.

When polar routings change

Operational polar routings opened up at scale in the late 1990s as ETOPS rules and onboard navigation improvements allowed long-range twins to fly polar great-circles. Pre-1998, the great-circle from New York to Hong Kong was technically the polar route, but most carriers flew either south-pacific or via Anchorage refuelling stops, both of which kept time at high latitudes shorter. Modern operations spend more time at high geomagnetic latitudes than legacy operations did, and this is one reason aircrew dose figures from the 1990s under-represent current crew exposure.

Solar-cycle phase additionally modulates the polar dose-rate premium. During solar minimum, when GCR flux is elevated and SPE activity is low, the polar premium on GCR alone is larger in absolute terms (because the underlying baseline is higher). During solar maximum, GCR is suppressed but SPE risk is concentrated at polar latitudes, so the polar premium is more episodic but bounded by event timing.

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