Galactic cosmic rays vs solar radiation: the difference

Popular usage treats "cosmic radiation" as one phenomenon; the dosimetry literature treats it as two. The distinction matters because the two components respond to the eleven-year solar cycle in opposite directions, arrive with different spectra, and get shielded very differently by the geomagnetic field. Anyone reading a per-flight dose number is reading a galactic cosmic-ray number by default; whether solar-particle contribution mattered on that specific flight is a separate question.

Galactic cosmic rays: the steady background

GCR are charged particles (overwhelmingly protons at roughly 87% above 1 GeV/nucleon, helium nuclei at 12%, and a small contribution of heavier nuclei up through iron) that originate outside the solar system. The principal accelerator is believed to be supernova remnants and their shock waves, though high-energy GCR may also originate from active galactic nuclei and other astrophysical sources ^[1]. The accelerated particles propagate through the interstellar medium for millions of years before reaching the heliosphere, by which point the directional information about their source is scrambled. From Earth's vantage, GCR is highly isotropic, arriving roughly equally from all directions in space.

The GCR spectrum is steeply falling: there are vastly more low-energy particles than high-energy ones. The spectrum is conventionally described as a power law, dN/dE ∝ E^-2.7, modified at low energies by solar modulation and at very high energies by source physics. For aviation dose purposes, the dose-relevant energy range is roughly 100 MeV to several GeV per nucleon: particles energetic enough to penetrate the geomagnetic field at flight latitudes and to drive secondary cascades at cruise altitudes.

Solar energetic particles: episodic, event-driven

SEPs are accelerated locally, at solar flares (impulsive events) and at the shock fronts driven by coronal mass ejections (gradual events) ^[2]. They are released into the interplanetary medium when those eruptions occur, and propagate along the Parker-spiral magnetic field that connects the Sun to interplanetary space. The arrival of SEPs at Earth depends on whether the field line connecting the Sun's source region to Earth's location at the time of the event allows particle transit.

The SEP spectrum is much softer than the GCR spectrum, falling off rapidly above tens of MeV, so SEP dose is concentrated at lower energies. This means geomagnetic shielding is much more effective at suppressing SEP dose at mid- and low-latitudes than at suppressing GCR dose. SEP events are polar-only for in-flight dose.

SEPs are rare. A typical eleven-year solar cycle includes roughly 50 events classified S1 or above on the NOAA Solar Radiation Storm scale, with the largest events (S4–S5) once-per-cycle or rarer. See our SPE guide for the historical record.

The solar cycle modulates them in opposite directions

The Sun's eleven-year activity cycle changes the interplanetary magnetic field. At solar maximum, the heliospheric field is strong and tangled, and it deflects GCR away from the inner heliosphere. GCR flux at Earth is therefore suppressed at solar maximum. At solar minimum, the heliospheric field is weak, GCR enters more freely, and GCR flux at Earth is elevated.

Solar energetic particle events, by contrast, are more frequent at solar maximum (because flares and CMEs are themselves more frequent) and rare during solar minimum. So the two radiation sources are anti-correlated across the cycle. GCR-driven in-flight dose is roughly 20–30% higher at solar minimum than at solar maximum, all else equal ^[3]. SEP-driven contribution is concentrated in the few years around solar maximum.

How the heliocentric potential captures this

For practical dose calculation, the FAA CARI-7 series uses a single parameter, the heliocentric potential, to capture the solar-cycle modulation of GCR. The heliocentric potential is a proxy for the integrated effect of the heliospheric magnetic field on GCR transport into the inner heliosphere. It is derived from neutron-monitor data (chiefly the Oulu Cosmic Ray Station in Finland and the Climax station historically) and is updated monthly ^[4].

A higher heliocentric potential corresponds to more solar activity, more interplanetary-field suppression, and lower GCR flux at Earth. CARI's lookup tables include a heliocentric-potential axis, so when you specify a departure date, the model retrieves the right modulation state for that month and applies the corresponding GCR flux to the calculation. CARI does not model individual SPEs; for those you need the per-event analysis discussed in our SPE guide.

Why this distinction matters operationally

• Baseline dose on any given flight is a GCR phenomenon. It changes slowly, predictably, with the solar cycle. CARI-7 captures it well.
• Event dose on any specific flight is an SPE phenomenon. It is unpredictable in detail, bounded, but real in magnitude. CARI-7 does not capture it; specialised codes do.
• For annual dose, GCR overwhelmingly dominates. SPE contribution averaged over a flying career is typically 1–3% of total dose for most aircrew, less for non-occupational fliers.
• For worst-case dose on a specific high-latitude long-haul leg during a major SPE, SPE contribution can briefly equal or exceed the GCR baseline.

How the two compare numerically

Property	Galactic cosmic rays (GCR)	Solar energetic particles (SEPs)
Source	Outside solar system (supernovae, AGN)	Sun (flares and CMEs)
Spectrum	Hard, power-law to TeV+	Soft, falling off rapidly above tens of MeV
Direction	Near-isotropic	Field-line-aligned from Sun
Temporal pattern	Steady (solar-cycle modulation only)	Episodic; single events lasting hours to days
Geomagnetic shielding	Effective at low latitudes	Much more effective at low latitudes (concentrates dose at poles)
Typical flight contribution	~ 5 µSv/hr at cruise	Variable: 0 normally; tens of µSv/hr at high latitudes during large events
Solar-cycle response	Anti-correlated (peak at solar min)	Correlated (peak at solar max)
CARI-7 handling	Modelled via heliocentric potential	Not modelled

What this means for interpreting a flight dose number

Any per-flight dose number in the literature or on a website (including ours) is, by default, a GCR-only number. It is the steady-state expected dose for that route at the appropriate solar-cycle phase. If you happened to fly during a major SPE, your actual dose was higher; if you did not, the number is a good estimate.

The right framing for a long-term flier: GCR is the predictable backbone that determines lifetime dose. SPEs add a long-tailed perturbation that matters in individual moments but contributes a small fraction of lifetime exposure. Both are real; their roles are different.

The atmospheric cascade

What actually delivers dose at flight altitudes is not the original cosmic-ray primary, which by the time it reaches FL370 has typically interacted with one or more atmospheric nuclei. The primary triggers a cascade of secondary protons, neutrons, pions, muons, and electromagnetic shower particles that propagates downward through the atmosphere. The cascade reaches peak particle number near 15–20 km altitude (the Pfotzer-Regener maximum); below that altitude the cascade gradually thins as particles lose energy to ionisation.

The dose at any altitude is the integrated effect of all cascade products that pass through a body at that altitude. Neutrons are particularly important because they have no electrical charge and propagate further through tissue, depositing energy via secondary nuclear interactions. The ICRP-103 radiation weighting factor for neutrons (w_R) varies from about 5 (for thermal neutrons) to about 20 (for fast neutrons in the MeV range), reflecting their high biological effectiveness per unit absorbed dose. CARI-7 handles this internally via the MCNPX transport calculation ^[1].

Why ground-level dose rate is so much lower

The atmospheric depth from FL370 to sea level is roughly 800 g/cm², about 4× the depth above FL370. This additional shielding attenuates the cascade by roughly two orders of magnitude. The cosmic-radiation contribution to background dose at sea level is about 0.33 mSv/yr (NCRP 160) ^[3]; the dose-rate at FL370 is roughly 100× the sea-level rate, integrating to the per-hour figures we use throughout the FlightRadiation guides.

The ground-level event distinction

A subset of solar particle events, those with enough high-energy protons to produce neutron cascades that reach ground-level detectors, are called Ground-Level Events (GLEs). About 70 GLEs have been recorded since systematic neutron monitoring began in the 1940s. GLEs are the events most relevant to in-flight dose because they have hard enough spectra to penetrate geomagnetic shielding and produce significant in-flight dose enhancements. Most NOAA-S-scale events are not GLEs; for in-flight dosimetry, the GLE catalogue maintained at the University of Oulu is the practical reference list of events that matter operationally ^[4].

How neutron-monitor data anchors the calculation

The world neutron-monitor network (Oulu, Climax historical, Moscow, Apatity, and several dozen others operated continuously since the late 1950s) provides the empirical anchor for GCR variability across the solar cycle. Neutron monitors at ground level detect the secondary neutrons produced by GCR primaries striking the atmosphere; the count rate is a sensitive proxy for the GCR flux at the top of the atmosphere modulated by the heliospheric magnetic field ^[4].

FAA CAMI uses neutron-monitor count rates from Oulu to derive the monthly heliocentric-potential values that CARI-7 consumes. The chain is: neutron monitor measures secondary count rate → empirical relationship converts to GCR primary flux → reduction to a single heliocentric-potential parameter → table lookup in CARI's MCNPX-generated dose grid. Each step in the chain has documented uncertainty; the headline ±25% on a CARI dose figure reflects the combined uncertainty across the chain.

Why GCR dose is stable across long timescales

One thing that makes GCR a useful baseline is that, over scales much longer than the 11-year solar cycle, GCR flux at Earth is remarkably stable. Cosmic-ray data extracted from cosmogenic nuclides in ice cores and tree rings indicate that GCR flux at Earth has varied by perhaps a factor of two over the last 10,000 years and shows long-term modulation correlated with grand solar minima (the Maunder Minimum, 1645-1715, being the best-known example). Over a human lifetime, the variation is dominated by the 11-year cycle and is bounded by roughly 30%. For dose-planning purposes the steady-state assumption built into CARI-7's monthly tables is appropriate.

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