A muon hodoscope doesn’t just count how many muons hit it — it also tells you which direction they’re coming from, with pretty good accuracy. That turns out to be really important.
When a coronal mass ejection (CME) from the Sun arrives, it doesn’t block cosmic rays evenly from all directions. Instead, it distorts their paths in complicated, uneven ways because of the tangled magnetic fields around it.
This leads to effects like:
- particles being pushed or compressed at the shock front
- some directions being shielded more than others
- particles being scattered and redirected
In short: the changes depend a lot on direction and particle energy.
What different detectors see
- Neutron monitors (which detect lower-energy cosmic rays) see a big, simple drop in counts. That’s because those lower-energy particles are blocked fairly uniformly.
- Muon hodoscopes, on the other hand, detect higher-energy particles (roughly 10–100 GeV). These particles:
- can push through the CME more easily
- don’t decrease much in total number
- but do change direction noticeably
So what actually happens?
Instead of seeing a big drop in muon counts, muon hodoscopes see:
- fewer muons from some directions
- more muons from others
This creates a shifting pattern in the sky — almost like the cosmic rays are being “pushed sideways” as the CME passes.
Why this matters
- This directional shift can show up hours before the overall drop seen by neutron monitors.
- That makes muon hodoscopes useful as early warning systems for space weather.
Typical effects
- Total muon count: small change (about 1–5% drop)
- Directional changes: much larger (10–20% differences depending on direction)
- Neutron monitors: bigger overall drop (about 3–15%)
Bottom line
- Neutron monitors tell you “how much cosmic radiation decreased”
- Muon hodoscopes tell you “how the cosmic radiation flow is being redirected”
And that directional information gives you an earlier and more detailed picture of what the CME is doing.
If you want to learn useful things about how coronal mass ejections (CMEs) affect muons, just counting the total number of muons with a simple detector isn’t enough. What really matters is where in the sky the muons are coming from, because CMEs don’t affect all directions equally—they change the muon rate differently depending on direction.
A simple and inexpensive way to get this directional information is to build a basic detector (called a hodoscope) using two layers. Each layer has two wide strips that detect passing particles. The strips in the top layer are arranged at right angles to the strips in the bottom layer.
This setup creates four possible combinations when a muon passes through both layers—essentially dividing the sky into four rough directions.
When a muon goes through, the pair of strips that detect it tells you which of those four directions it came from. The direction measurement isn’t very precise, but this simple four-direction detector is about the easiest and cheapest way to see how muon counts change depending on direction during CME activity.