The hectometric continuum occupies a frequency range of 600 - 1700 kHz and occurs at altitudes of one to two Earth radii, where Earth's magnetic field influences charged particle motion. Ground detection of this radiation is impossible due to absorption by the ionosphere; only satellite measurements can capture these signals.
Between 2017 and 2023, researchers documented about a thousand instances of hectometric continuum emissions. Data link the phenomenon to double plasma resonance, a process where natural plasma oscillations and the rotation of electrons coincide. This instability, requiring sufficient plasma density and energetic electrons, prompts the plasma to emit radio waves. The emission emerges a few hours after sunset and vanishes one to three hours after sunrise - solar heating raises the plasma density, halting wave generation, while nightfall eventually restores conditions for emissions to resume.
Seasonal patterns show greater emission frequency in summer and fewer events in spring and autumn. Since mid-2022, as the Sun entered a phase of heightened activity with more sunspots and increased ultraviolet output, emissions have ceased. The researchers note that, unlike other radio waves that intensify during solar flares, the hectometric continuum weakens or disappears altogether.
"Interestingly, unlike other radio signals that are amplified during bursts of solar activity - such as auroral kilometric radiation associated with auroras - the hectometric continuum, in contrast, diminishes. Therefore, we expect that it may reappear in a few years, when solar activity declines," comments Alexander Chernyshov, Associate Professor at the Joint Department of Space Physics with the Space Research Institute (RAS).
This research improves understanding of Earth's magnetosphere and may allow future searches for similar emissions from exoplanets as indicators of magnetic fields, atmospheres, and potential habitability.
Research Report:Hectometric Continuum Radiation Observations on Different Temporal Scales in Near-Earth Space
Related Links
National Research University Higher School of Economics
Solar Science News at SpaceDaily
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