---
title: "How does the atmosphere make neutrinos all around us? | Sandbox Physics"
description: "Cosmic-ray cascades in the air make neutrinos with a predictable flavor mix. They arrive from overhead, from below and from the horizon, over many distances."
canonical_url: "https://sandboxphysics.com/stories/neutrinos/atmospheric-neutrino-production/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/neutrinos/atmospheric-neutrino-production/"
last_modified: "2026-10-10"
---

[2026 Nobel Prize in Physics](https://sandboxphysics.com/nobel/2026/physics/)

# How does the atmosphere make neutrinos all around us?

When a cosmic ray hits the air, it sets off a cascade of particles. Pions and muons in the cascade decay into neutrinos that arrive from overhead, from below and from the horizon. A detector has to model that pattern before it can test for oscillations.

Sandbox Physics · 2026-10-09 · Updated 2026-10-10 · First ideas

## The incoming cosmic ray is only the beginning

High-energy protons and other nuclei arrive from space and hit nuclei in the air. Each collision makes secondary particles, which can collide again or decay. The growing spray of particles is called an atmospheric shower.

Charged pions and kaons in the shower are important parents of atmospheric neutrinos. Their decays also make muons, which decay in turn and add more neutrinos. The whole atmosphere becomes a particle source that covers the Earth.

These neutrinos are called atmospheric because they are made in the air. The cosmic ray that started the shower may have come from a distant astrophysical accelerator, but the neutrino itself was born in our atmosphere. That difference matters in searches for neutrinos from cosmic sources.

Sources: [1] · [2] · [3]

## Follow one pion through two decays

A positive pion can decay into a positive muon and a muon neutrino. If that muon also decays, it produces a positron, an electron neutrino and a muon antineutrino. A negative pion gives the same chain with particles and antiparticles swapped.

Count the products of this complete chain and you get two muon-flavor neutrinos or antineutrinos for each electron-flavor one. That is where the familiar rough two-to-one ratio comes from, at relatively low energies and before any oscillation.

The ratio comes from counting particles in a simplified chain. The products carry different energies, and a detector’s energy bin does not collect whole chains one at a time. So the ratio describes the mix at the source; it does not predict exactly twice as many muon events.

$$
\begin{aligned}\pi^+&\longrightarrow\mu^++\nu_\mu\\\mu^+&\longrightarrow e^++\nu_e+\bar\nu_\mu\end{aligned}
$$

One full chain, from a positive pion through the muon decay, gives two muon-flavor particles and one electron-flavor particle, counting neutrinos and antineutrinos together. The count says nothing about their energies or how likely each is to be detected.

Sources: [1] · [2] · [4]

1. 01 **Cosmic ray in air**

   Collisions make pions and other parent particles

2. 02 **Decay or interact**

   Energy and air density decide which comes first

3. 03 **Travel to the detector**

   Direction sets the distance; flavor can change on the way

What the air produces, what happens on the way and what the detector registers are separate steps. The two-to-one count from one full pion–muon chain does not fix the ratio of detected events.

## At higher energies, the parents do not all get time to decay

Relativistic time dilation stretches a fast muon’s lifetime as seen from the Earth, so it may reach the ground before it decays. A muon that never decays in the air takes away the electron-flavor neutrino and one of the muon-flavor contributions from the chain.

Pions and kaons can also collide before they decay. Which happens first depends on energy and on the density of the air. A nearly horizontal path through thin air gives them a different chance to decay than a downward path into denser layers.

Detailed flux calculations therefore need the incoming cosmic-ray spectrum, models of particle production, the structure of the atmosphere and the Earth’s magnetic field. Measurements of atmospheric muons check the same cascade physics. Some factors cancel in the flavor ratio, but the ratio still depends on these inputs.

Sources: [1] · [5] · [2]

## A neutrino from below was made in the air on the far side of the Earth

At the GeV energies that mattered for the first atmospheric-oscillation studies, most neutrinos cross the Earth without being absorbed. A detector therefore receives neutrinos made in the air overhead and neutrinos made in the air on the other side of the planet.

Neutrinos from overhead travel a relatively short way. Those from below can cover thousands of kilometres before interacting near the detector. The arrival direction lets an experiment compare travel distances without moving the detector or an artificial source.

The flux is not the same from every direction. Geomagnetic effects, the geometry of the cascade, the energy and the detector’s acceptance all shape the expected angular distribution. At much higher energies the Earth also absorbs neutrinos, as IceCube measurements later showed.

Sources: [5] · [6] · [7]

## A natural source became a test of how neutrinos travel

A detector sees the charged particles that neutrino interactions produce, not the flux that left the atmosphere. To compare flavors, an analysis needs interaction cross sections, particle identification and efficiencies. To compare directions, it also has to relate each neutrino’s direction to the direction of its visible products.

In 1998, Super-Kamiokande used these flavor and direction patterns to test for oscillations. The key deficit appeared in upward-going muon-flavor events, which had travelled long distances; the electron-like events gave a complementary comparison.

Atmospheric neutrinos later played two roles: a signal for studying neutrino properties and a background in searches for high-energy cosmic neutrinos. Both start from the same question: what did the atmosphere produce, and how did the journey and the detector turn it into what was observed?

Sources: [6] · [1] · [3] · [4]

## What we learned

Atmospheric neutrinos come from particle decays in cosmic-ray cascades. Physics explains both their flavor mix at the source and their directions, but the simple two-to-one count needs refining before it can be compared with detected events.

## Try it in the Lab

[Atmospheric neutrino zenith experiment](https://sandboxphysics.com/labs/atmospheric-neutrino-zenith/) [Cherenkov rings](https://sandboxphysics.com/labs/cherenkov-rings/)

## Primary sources & revision

1. [Honda et al. · Atmospheric flux with a model calibrated to muon data (2007)](https://arxiv.org/abs/astro-ph/0611418)
2. [Gaisser & Honda · Flux of Atmospheric Neutrinos (2002)](https://arxiv.org/abs/hep-ph/0203272)
3. [IceCube Collaboration · Evidence for High-Energy Extraterrestrial Neutrinos (2013)](https://arxiv.org/abs/1311.5238)
4. [Particle Data Group · Neutrino Masses, Mixing, and Oscillations (2024)](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-neutrino-mixing.pdf)
5. [Honda et al. · A three-dimensional atmospheric-neutrino flux calculation (2004)](https://arxiv.org/abs/astro-ph/0404457)
6. [Super-Kamiokande Collaboration · Evidence for Oscillation of Atmospheric Neutrinos (1998)](https://arxiv.org/abs/hep-ex/9807003)
7. [IceCube Collaboration · Multi-TeV neutrino cross section using Earth absorption (2017)](https://arxiv.org/abs/1711.08119)

First published 2026-10-09; last revised 2026-10-10. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication.

[← Previous: Why can a neutrino change flavor?](https://sandboxphysics.com/stories/neutrinos/neutrino-flavor-oscillations/) [Next: Super-Kamiokande: the neutrinos that took the long way →](https://sandboxphysics.com/stories/neutrinos/super-k-zenith-angle/)

## Keep reading

Neutrinos · Reading path

### Understand flavor change and mass

1. [Article **Before anyone saw oscillations, how did the mixing idea take shape?**](https://sandboxphysics.com/stories/neutrinos/pontecorvo-maki-nakagawa-sakata-mixing/)
2. [Article **Why can a neutrino change flavor?**](https://sandboxphysics.com/stories/neutrinos/neutrino-flavor-oscillations/)
3. [You are here **How does the atmosphere make neutrinos all around us?**](https://sandboxphysics.com/stories/neutrinos/atmospheric-neutrino-production/)
4. [Article **Super-Kamiokande: the neutrinos that took the long way**](https://sandboxphysics.com/stories/neutrinos/super-k-zenith-angle/)
5. [Article **Is finding a missing flavor the same as finding a new one?**](https://sandboxphysics.com/stories/neutrinos/neutrino-appearance-and-disappearance/)
6. [Article **2015: how did two very different experiments establish flavor change?**](https://sandboxphysics.com/stories/neutrinos/nobel-2015-two-neutrino-evidence-paths/)
7. [Article **Neutrinos have mass. Why does oscillation not tell us how much?**](https://sandboxphysics.com/stories/neutrinos/oscillations-and-mass/)
8. [Article **Can the Earth help tell which neutrino mass comes first?**](https://sandboxphysics.com/stories/neutrinos/earth-matter-neutrino-mass-ordering/)

Neutrinos · Topic

### How IceCube found neutrinos from space

Which events and checks turned a few flashes in the ice into a discovery?

1. [Article **2026 Nobel Prize in Physics: why look for neutrinos in Antarctic ice?**](https://sandboxphysics.com/stories/neutrinos/nobel-physics-2026/)
2. [Article **What can a neutrino tell us that light cannot?**](https://sandboxphysics.com/stories/neutrinos/high-energy-neutrinos-astronomy-window/)
3. [Article **A huge detector, a few events: reading neutrino scales**](https://sandboxphysics.com/stories/neutrinos/neutrino-energy-distance-and-counts/)
4. [Article **How does a faint signal become a neutrino discovery?**](https://sandboxphysics.com/stories/neutrinos/neutrino-discovery-evidence-chain/)
5. [Article **Read IceCube’s discovery paper, one figure at a time**](https://sandboxphysics.com/stories/neutrinos/icecube-original-paper-figure-guide/)
6. [You are here **How does the atmosphere make neutrinos all around us?**](https://sandboxphysics.com/stories/neutrinos/atmospheric-neutrino-production/)
7. [Article **IceCube’s 28 events: finding high-energy neutrinos from space**](https://sandboxphysics.com/stories/neutrinos/icecube-2013-evidence/)
8. [Article **TXS 0506+056: one alert, two separate lines of evidence**](https://sandboxphysics.com/stories/neutrinos/txs-0506-056-multimessenger/)
9. [Article **NGC 1068: what does a neutrino excess establish?**](https://sandboxphysics.com/stories/neutrinos/ngc-1068-neutrino-source-evidence/)
10. [Article **One neutrino sky, three different questions**](https://sandboxphysics.com/stories/neutrinos/neutrino-sky-diffuse-galactic-point-sources/)
11. [Article **When can a few neutrino events become evidence?**](https://sandboxphysics.com/stories/neutrinos/few-neutrino-events-statistical-evidence/)
12. [Article **Open a real neutrino event file and read it carefully**](https://sandboxphysics.com/stories/neutrinos/reading-public-neutrino-event-data/)

Neutrinos · Topic

### Three kinds of neutrino and how they mix

How did physicists find a second and a third kind of neutrino, and then show that the three flavors mix?

1. [Article **Why did physicists need a second kind of neutrino?**](https://sandboxphysics.com/stories/neutrinos/brookhaven-muon-neutrino/)
2. [Article **DONUT: catching a tau before it disappears**](https://sandboxphysics.com/stories/neutrinos/donut-tau-neutrino/)
3. [You are here **How does the atmosphere make neutrinos all around us?**](https://sandboxphysics.com/stories/neutrinos/atmospheric-neutrino-production/)
4. [Article **Super-Kamiokande: the neutrinos that took the long way**](https://sandboxphysics.com/stories/neutrinos/super-k-zenith-angle/)
5. [Article **Before anyone saw oscillations, how did the mixing idea take shape?**](https://sandboxphysics.com/stories/neutrinos/pontecorvo-maki-nakagawa-sakata-mixing/)
6. [Article **Why can a neutrino change flavor?**](https://sandboxphysics.com/stories/neutrinos/neutrino-flavor-oscillations/)
7. [Article **Is finding a missing flavor the same as finding a new one?**](https://sandboxphysics.com/stories/neutrinos/neutrino-appearance-and-disappearance/)
8. [Article **How do you make a beam of particles you cannot steer?**](https://sandboxphysics.com/stories/neutrinos/making-an-accelerator-neutrino-beam/)
9. [Article **K2K and MINOS: test flavor change with a distance you choose**](https://sandboxphysics.com/stories/neutrinos/k2k-minos-controlled-baselines/)
10. [Article **OPERA: tau appearance and a corrected clock anomaly**](https://sandboxphysics.com/stories/neutrinos/opera-tau-appearance-and-timing-correction/)