The amount of cosmic rays reaching Earth isn’t constant — it changes over time. One major reason is the activity of the Sun.
The Solar Wind Shields Galactic Cosmic Rays #
The Sun constantly emits the solar wind, a stream of charged particles (protons, electrons, and others) that carries magnetic field throughout the solar system. Cosmic rays arriving from outside the galaxy (galactic cosmic rays) are partly deflected by this field, making it harder for them to reach the inner solar system.
The 11-Year Solar Cycle #
Solar activity rises and falls in an roughly 11-year cycle, tracked by the number of sunspots (the solar cycle). During periods of high solar activity — when the solar wind is strong — fewer galactic cosmic rays reach Earth. Conversely, cosmic ray levels tend to rise during quiet periods of solar activity.
Forbush Decreases #
A Forbush decrease is a sudden drop in the cosmic rays reaching Earth, lasting several days, triggered by a solar flare or coronal mass ejection (CME). A large eruption on the Sun’s surface releases plasma and a strong magnetic field into interplanetary space; as this passes by Earth, it temporarily shields the planet from galactic cosmic rays.
The largest Forbush decreases on record have reduced cosmic ray intensity by several tens of percent.
Connecting to Everyday Cosmic Ray Observation #
Running a ground-based detector like OSECHI over an extended period can reveal the gradual variation tied to the solar cycle, as well as the temporary dip that follows a major solar flare. In the field of space weather, cosmic ray data like this serves as one of the tools used to monitor solar activity.
References #
- Potgieter, M. S., “Solar Modulation of Cosmic Rays,” Living Reviews in Solar Physics (2013). arXiv:1306.4421