Magnetic field vibrates like a drum

NASA satellites detect the first standing waves at the outer hull of the earth's magnetic field

When a particularly fast, short solar storm strikes the earth's magnetic field, it causes standing waves in the magnetopause (blue) and inside the magnetosphere (green). © E. Masongsong / UCLA, M. Archer / QMUL, H. Hietala / UTU
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Protective vibration: When a violent solar storm strikes the Earth's magnetic field, its outer boundary swings like an eardrum - it forms standing waves. Such wave patterns in the magnetopause were theoretically predicted 45 years ago. But only now they have been able to demonstrate using NASA's THEMIS satellites clearly, as researchers report in the journal "Nature Communications".

Thanks to the complex layers of the terrestrial magnetosphere, we are protected from the cosmic rays and the solar wind. Only a small fraction of the high-energy particles can penetrate into the upper atmosphere in strong solar storms - and creates magnificent auroras. But the impact of such a plasma tower is not without its mark on the magnetopause, the outer boundary zone of the magnetic field: explosive contacts between magnetic field lines cause electron fountains to fly far into space, as researchers recently discovered.

Theoretically predicted: standing wave in the magnetopause. © Martin Archer

The secret of the standing waves

The collision with the solar wind causes the entire boundary layer of the magnetic field to oscillate. Like on a membrane, surface waves spread on it. According to a theory postulated forty-five years ago, these waves can take on a particular form under certain conditions: "Earlier studies of magnetopause dynamics suggested that so-called standing waves could spread on the solar side, " explains Kort co-author Ferdinand Plaschke from the Institute for Space Research in Graz.

Such standing waves come about when the vibrations propagate from the day-side to the poles and are then reflected there by the ionosphere. If the frequencies of this back-and-forth movement match, peaks and troughs of the waves seem to remain in the same place. Also, the string of a guitar or the membrane of a drum can form such standing waves.

Cosmic "Schlegel" meets magnetic field

The problem, however: So far, no scientist has been able to detect such standing waves in the magnetopause. "Faced with 45 years of unsuccessful search, there have been speculations that these eardrum-like vibrations may not exist, " says first author Martin Archer of Queen Mary University in London. He and his team have now re-evaluated the magnetic field data from NASA's THEMIS satellites. display

In fact, they came to fruition thanks to perfect cosmic timing. Because in August 2007, the THEMIS satellites recorded the impact of a particularly violent, rapid solar storm on the Earth's magnetic field. This high-speed jet hurled large amounts of high-energy particles onto the magnetopause innerhalb within just 100 milliseconds, acting like a mallet hitting a tense tympanic membrane.

Finally proven

The deciding factor is that the recorded data show that the magnetopause did not just form chaotic waves during this impact. Instead, very low-frequency oscillations developed, whose behavior indicated the formation of standing waves, as the researchers report. In other words, as predicted 45 years ago in theory, the outer boundary of the magnetic field swung like an eardrum.

The Earth's magnetic field as a drum. Martin Archer

"This is the first direct observation that shows that this mechanism does exist, " Archer and his team state. "This proves that the plasma boundary can capture the energy of surface waves in the form of such eigenmodes." However, how often such standing waves arise in the magnetic pause and what conditions are necessary is still unknown. (Nature Communications, 2019; doi: 10.1038 / s41467-018-08134-5)

Source: Queen Mary University of London, Austrian Academy of Sciences

- Nadja Podbregar