Kyoto, Japan -- Dark matter's existence is all but certain -- astronomers believe it makes up about a quarter of the universe's total energy content -- yet its true identity has eluded us for decades. Two of the leading candidates for dark matter are the hypothetical particles ultralight axions and dark photons, which in the range studied here would be some 19 to 21 orders of magnitude lighter than the electron.

Conventional axion searches tend to involve converting them into photons with the help of strong laboratory magnets. However, research in a laboratory inherently limits the space over which such a field can be applied. A collaborative team of researchers from Kyoto University, Hiroshima University, and Nihon University realized that, by contrast, Earth's own magnetic field spans a scale no laboratory could match.

"We asked ourselves whether we could use the Earth itself as a giant detector in the search," says corresponding author Atsushi Taruya. "The Earth-ionosphere cavity acts as a natural resonator that amplifies electromagnetic waves right around the mass range we wanted to probe."

Kyoto, Japan -- The space around Earth is becoming increasingly crowded, with thousands of satellites and pieces of debris now sharing low Earth orbit. Even above this region -- at altitudes of several hundred kilometers -- the extremely thin upper atmosphere can slow satellites down, making accurate information on atmospheric density essential for predicting satellite motion and preventing collisions.

The upper atmosphere is composed of more than 99 percent electrically neutral atmosphere called the thermosphere, and thermospheric density is the term used to describe neutral atmospheric density at heights from 100 to 1000 kilometers above the Earth's surface. While the ionized gas of the ionosphere makes up less than 1 percent of the atmosphere, the effect of the ionized gas on radio wave propagation renders the ionosphere relatively easy to observe, while thermospheric observations pose a considerable challenge.

Nonetheless, developing observation methods for thermospheric density is essential both to further advancing the science of the upper atmosphere and for applications in space engineering. These needs inspired a team of researchers at Kyoto University to develop a new method of visualizing this elusive environment.

A recent study published in National Science Review has revealed a hidden population of high-frequency magnetohydrodynamic waves in the Sun’s polar corona. Using high-resolution Solar Orbiter observations, researchers found that these waves carry substantial energy, offering new clues to coronal heating and the acceleration of the fast solar wind.