dawn chorus
Radio interference sometimes experienced at sunrise.
dawn chorus: radio noise that peaks at sunrise
Dawn chorus is a burst of broadband radio interference that affects long-wave and medium-wave reception during the first hour or two after sunrise, particularly in temperate latitudes during spring and autumn. The effect appears as a crackling or hissing noise that drowns out weak signals and gradually fades as the sun climbs higher. It was first documented systematically in the 1920s when long-wave broadcasting was new, and the name reflects its sudden, chorus-like arrival each morning.
The interference originates from natural ionospheric disturbances. As the sun rises, solar radiation ionizes the D-layer of the ionosphere unevenly, creating turbulent regions that emit radio noise across a wide frequency band, typically between 10 kHz and several MHz. The effect is strongest in the hour following local sunrise and can reduce signal-to-noise ratio by 10 to 20 dB or more, making distant stations briefly unreadable despite good propagation conditions otherwise.
Dawn chorus affects different frequency bands and locations with varying severity. Long-wave maritime navigation systems and medium-wave AM broadcast reception suffer noticeably in northern Europe and North America during the spring and autumn equinoxes, when the sun's angle creates the sharpest ionospheric gradients. Operators of VLF (very low frequency) receivers and radionavigation equipment encounter it regularly. Modern digital modes and narrowband reception techniques mitigate the effect somewhat by rejecting out-of-band noise, but cannot eliminate it entirely.
The phenomenon is distinct from other sunrise-related radio effects such as sudden ionospheric disturbances or solar flare events. Those are energetic phenomena that can last minutes to hours and affect HF propagation globally. Dawn chorus is predictable, geographically localized, and repeats daily in the same location. Radio operators working with long-wave systems simply factor it into their operating schedule, shifting critical communications to hours when the ionosphere is more stable.