Radio astronomy began with the mysterious noise caught by the antenna of Karl Jansky and the rhythmic pulses that Jocelyn Bell Burnell playfully called 'little green men'. But the true planetary radio beacon lit up in 1955, when receivers accidentally stumbled upon Jupiter's decametric radiation — it cut off sharply at 40 MHz. Like a beacon fading at a specific wavelength. This cutoff, predicted by Maxwell's equations, directly revealed the planet's magnetic field: the frequency at which the electron cyclotron maser — radiation from plasma twisted by the magnetic field — falls silent is exactly proportional to the field. Since then, astronomers dreamed of finding such a beacon in brown dwarfs — substellar objects too light for thermonuclear fusion but still generating their own dynamo. And now, in the binary system of methane dwarfs J1019+65 (cold brown dwarfs with methane in their atmosphere), the interferometer LOFAR finally caught the long-sought signal.

Three radio telescopes — LOFAR, uGMRT, and VLA — listened to the system's polarized whisper for a total of over 70 hours. They captured the circular component of the radiation (Stokes V), which is precisely what the maser produces. The frequency at which the maser falls silent (the cyclotron frequency $ν_c$ ≈ 2.8 B, in MHz per gauss) directly indicates the field strength B at the magnetic poles. For J1019+65, silence occurs around 200–300 MHz — this yields a field of just 126 gauss, only nine times stronger than Jupiter's, even though the dwarf itself rotates much faster. This number fits precisely into the dynamo scaling of the Elsasser number (ratio of magnetic to Coriolis forces), proposed back in 1979: the field saturates when magnetic force balances the Coriolis force. Other models — thermal, based on mixing — overestimated predictions by two to four orders of magnitude, leading astronomers to search for kilogauss fields that do not exist.

The dwarf beacon does not simply fall silent — it ceases to be a beacon of a large-scale dipole field and turns into a chaos of coronal loops. The high-frequency radiation detected for decades from other brown dwarfs originates not in a global magnet, but in compact spots — analogues of solar faculae, only with fields of hundreds of gauss. This means that cold dwarfs, lying at the very edge of star formation, have two independent radio faces: a calm, predictable one carrying information about the whole object's field, and a flaring, variable one showing local magnetic weather. Numerical simulations already hint that large- and small-scale fields saturate independently. Now, new observations are needed: GMRT Band-2 can directly catch the spectral break, while simultaneous monitoring at centimeter wavelengths can track the 'coronal' variability. If even a dozen such methane beacons are found, astronomers will construct the first empirical dynamo diagram for substellar masses.

Thus, seven decades after the first planetary radio beacon, another cold object traced the sky with a polarized beam. Its silence at high frequencies said more than the loudest signal. The Elsasser law, long considered merely an elegant hypothesis, gained experimental confirmation. And while future telescopes like SKA prepare to scan exoplanet magnetospheres, we already have a calibration point: 126 gauss at a distance of 23.3 parsecs — a guiding beacon for cosmic field cartographers.