You are listening to cosmic rays. Particles from exploded stars and other galaxies hit Earth's atmosphere all the time. Neutron monitors at 25 places around the world count the fragments that reach the ground. Here every monitor is one piano key: the more it counts, the more often and the harder its key is struck.
What you hear
- One key per station. Keys on the left belong to stations near the poles and play low notes. Keys on the right belong to stations closer to the equator and play high notes.
- A normal day is a calm, irregular sprinkle of notes, like rain. A station that counts a little more than it did over the past day plays more often and louder. If it counts less, it plays more quietly and less often.
- When the Sun erupts and a cloud of magnetised gas (a coronal mass ejection) sweeps past Earth, it shields us from part of the cosmic rays. Within hours, counts drop by a few percent all over the world. This is called a Forbush decrease, and the piano falls quiet, sometimes for days.
- A few times per solar cycle the Sun itself accelerates particles hard enough for them to reach the ground. In such a ground level enhancement the polar keys on the left suddenly start hammering.
What you see
- The keys are tinted by how far each station is from its usual level: orange means more particles than usual, blue means fewer. The white line on each key is the same value as a needle. A key lights up when it is struck.
- The picture below the keys is the last 24 hours, with the newest minute at the top. Each column is one station. Times are in UTC.
- Click or drag in the history to jump there. While NMDB's live data are paused, the page plays the most recent published day as an endless loop at one minute per second, so one pass takes 24 minutes.
Controls
| Scale | Which notes the keys play. The default spreads A minor pentatonic over three octaves, so neighbouring stations sometimes share a note. The other scales give every station its own step. |
| Voice | Piano, a plucked harp, or a soft bell. |
| Octave − / + | Moves everything down or up by an octave. |
| Density | How many notes there are overall. |
| Sensitivity | How strongly small changes in the counts change the number of notes. Turned to zero, every station plays at the same rate. |
| Reverb, Delay | Room and echo. |
| MIDI out | Sends the notes to a synthesizer or music software over Web MIDI (Chrome, Edge, Firefox). The internal sound can be switched off. |
For the detector community
Data. Real-time 1-minute data from the Neutron Monitor Database (NMDB): revised values where available, otherwise original, corrected for atmospheric pressure and detector efficiency. While NMDB's public real-time access is suspended, a server job fetches the most recent published 24 h once a night, and the page plays that day as a loop. The header shows its date.
Detectors. Primary cosmic rays (about 90 % protons, about 9 % helium nuclei, the rest heavier nuclei) with energies from about 1 GeV to a few tens of GeV start hadronic cascades in the atmosphere. Neutron monitors record the nucleonic component that reaches the ground. In the standard NM64 design, incoming hadrons produce evaporation neutrons in a lead producer. These neutrons are thermalised in polyethylene and counted in BF3 or 3He proportional counters. Count rates depend strongly on altitude: high stations such as JUNG1, SOPO and DOMC count more and respond more strongly.
Key order: cutoff rigidity. The geomagnetic field deflects charged particles. Rigidity (momentum per charge, in GV) determines how strongly. The vertical cutoff rigidity Rc of a site is roughly the minimum rigidity a particle needs to arrive there from the zenith. It is near 0 GV at the poles, where the atmosphere sets the effective threshold at roughly 1 GV, and rises to more than 8 GV at ATHN and MXCO. The keyboard is ordered by Rc, which makes it a coarse spectrometer: modulation effects are strongest in the low-rigidity part of the spectrum, so Forbush decreases are usually deepest and GLEs strongest on the left. The Rc values used here are approximate and do not track the secular change of the geomagnetic field.
From counts to notes.
| Smoothing | 15-minute trailing mean of the 1-minute counts (at least 5 valid minutes in the window). |
| Deviation d | Smoothed rate relative to the median of the station's own smoothed rate over the last 24 h, in %. The colour scale saturates at ±2 %. |
| Strike rate | λ = 0.35 s⁻¹ · density · exp(k · d), with k = sensitivity (default 0.4, so +1 % gives about 1.5× the rate) and d clamped to ±8 %. Strikes are drawn independently per station as a Poisson process, evaluated every 20 ms. |
| Velocity | Random 0.45–1.0, scaled by (1 + d/4). |
| Pitch, pan | Rc rank, mapped to scale steps and stereo position from left to right. |
| Gaps | A station with no value in the last 30 minutes stays silent. |
What the baseline hides. The reference is a running 24-hour median, not a long-term baseline. The piano therefore plays departures from the past day, not absolute flux. Slow variations such as the 11-year solar cycle drop out completely. A Forbush decrease that lasts longer than about half a day gradually pulls the median down with it, so a deep, long event sounds less dramatic than it is, and its recovery can sound like a slight excess. Faster structure does come through: the diurnal anisotropy (a few tenths of a percent) can be heard as a slow daily wave whose phase differs from station to station, and the residual barometric effect of storms shows up wherever the pressure correction is imperfect.
Acknowledgements
We acknowledge the NMDB database (www.nmdb.eu), founded under the European Union's FP7 programme (contract no. 213007), for providing data. The data of each neutron monitor are provided by the institute that operates it.