Lost signal: How solar activity silenced earth's radiation

Researchers from HSE University and the Space Research Institute of the Russian Academy of Sciences analysed seven years of data from the ERG (Arase) satellite and, for the first time, provided a detailed description of a new type of radio emission from near-Earth space—the hectometric continuum, first discovered in 2017. The researchers found that this radiation appears a few hours after sunset and disappears one to three hours after sunrise. It was most frequently observed during the summer months and less often in spring and autumn. However, by mid-2022, when the Sun entered a phase of increased activity, the radiation had completely vanished—though the scientists believe the signal may reappear in the future. The study has been published in the Journal of Geophysical Research: Space Physics.

The Earth constantly emits radio waves—natural electromagnetic signals that occur in near-Earth space. Analysing these emissions helps scientists better understand how the Sun influences the magnetosphere, the region surrounding our planet where the magnetic field protects it from external forces.

Various types of radio emissions originate from within this region, one of which is the hectometric continuum (HMC), a narrowband natural radiation in the range of 600–1700 kHz—significantly lower than the broadcast frequencies of conventional radio stations. The sources of this radiation are located relatively close to Earth, at altitudes of about one to two Earth radii, where the magnetic field still governs the motion of charged particles. Such waves cannot be detected from the Earth's surface, as the dense layers of the ionosphere completely absorb them. Therefore, the HMC can only be observed with the help of spacecraft. For this reason, the hectometric continuum was discovered relatively recently—in 2017—by the Japanese ERG (Arase) satellite. Since then, the signal has been recorded only sporadically, and a complete picture of its behaviour has yet to be established.

To characterise the properties of the HMC and uncover the mechanism behind its occurrence, researchers from the Space Research Institute of the Russian Academy of Sciences and the HSE Faculty of Physics compiled all available satellite data to examine how this radiation evolved over time. To accomplish this, the scientists analysed about a thousand HMC observations collected between 2017 and 2023.

The results showed that the appearance of the signal is linked to processes occurring in the near-Earth plasma—a region filled with charged particles that move under the influence of Earth’s magnetic field and the solar wind. According to the authors, the hectometric continuum arises from a phenomenon known as double plasma resonance, in which two types of oscillations in the plasma coincide: the plasma’s natural oscillations and the rotation of electrons around Earth’s magnetic field lines. This resonance creates an instability that causes the plasma to emit radio waves. Such emission requires specific conditions—a certain plasma density and the presence of high-energy, or 'hot,' electrons.

The researchers found that the radiation occurs only at night and disappears one to three hours after sunrise. They explain this by noting that morning solar radiation increases the plasma density, disrupting the conditions necessary for radio wave generation. Similarly, the signal does not appear immediately after sunset but only a few hours later, once the ionosphere has cooled and restored the conditions required to excite the HMC.

In addition to the daily cycle, the radiation exhibits seasonal variations: it was observed more frequently in summer and less often in spring and autumn. Since mid-2022, the signal has disappeared. The scientists attribute this to the Sun entering a more active phase, during which its surface showed more sunspots, radio emissions at a wavelength of 10.7 cm increased, and ultraviolet radiation levels rose. These changes altered the structure of the plasma, eliminating the conditions necessary for HMC generation.

'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).

The study not only contributes to a better understanding of Earth’s magnetosphere but also opens the possibility of testing whether similar radio emissions occur on exoplanets. Such emissions could indicate the presence of a planetary magnetic field, an important factor for preserving an atmosphere and, potentially, supporting the existence of life.