The Northern and Southern lights, also known as auroras, are as varied as the colours they display in the night sky. Discrete auroras are the kind that typically grace our desktops and calendar covers, and that are produced a few thousand kilometres above Earth’s surface. By contrast, pulsating auroras that are created tens of thousands of kilometres away, in the equatorial region of the magnetosphere — the area around Earth that is dominated by the planet’s magnetic field. For decades, it has been suggested that pulsating auroras are the result of interactions between magnetospheric electrons and electromagnetic waves called chorus waves that send electrons careering towards Earth’s atmosphere along magnetic-field lines1–3. In a paper in Nature, Kasahara et al.4 report direct evidence for this process using observations both from Earth’s surface and from a spacecraft positioned on a field line.
Because magnetic fields are invisible to the human eye, the prediction of where a field line hits Earth and where that same field line exists out in space — a task known as magnetic-field-line mapping — is extremely difficult5. Luckily, electrons that move around Earth tend to follow these field lines closely. When these particles interact with chorus waves, they can be directed into a region of the upper atmosphere called the ionosphere, where they often generate auroral light. This allows us instantly to see the footprint of the associated field lines.
In addition, if we have an observation platform at a precise location out in space, we can detect the chorus waves that caused the electrons to head towards the atmosphere and see fluctuations in the electron population that arise from the oscillation of the waves. The trick is to get the ground-based and space-based observations to line up at the right time and place, and to have instruments sensitive enough to view both processes simultaneously. This feat has eluded observers ever since the theory of pulsating-aurora generation was developed6,7.
The first challenge is to have an instrument capable of making the in situ measurement of electrons in space at the required resolution. The Arase spacecraft8, launched by the Japan Aerospace Exploration Agency in late 2016, carries just such an electron detector, which was designed to observe electrons within a narrow window around a magnetic-field line. The spacecraft is also equipped with instruments to detect chorus waves. Kasahara and colleagues analysed data from the spacecraft to uncover fluctuations in the electron population and the associated chorus waves.
The next obstacle was to determine where the field line threading the position of the spacecraft would hit the ground. Magnetic-field models are now sophisticated enough to be able to inform researchers of the approximate location of a field-line footprint in Earth’s atmosphere, generally with higher accuracy when the level of geomagnetic activity (magnetic storms) is low. In the vicinity of this footprint, Kasahara et al. looked for a corresponding pulsating-aurora signal — namely, variations in auroral-light intensity that matched the fluctuations in the electron population. They identified such a signal in measurements from an all-sky imager based in central Canada9, which essentially records black-and-white video of the hemispherical view of the sky above (see Figure 2 of the paper4).
Thanks to Kasahara and colleagues, we can see the complete process of pulsating-aurora generation for the first time: the fluctuations in an electron population out in space; the chorus waves responsible for these fluctuations; and the variations in auroral-light intensity from the ground (Fig. 1). The last part is somewhat analogous to watching an image on an old-fashioned television, where the ionosphere is the ‘screen’ onto which electrons are projected. Despite this simple picture, researchers are aware that the ionosphere probably changes the incoming signal — a detail that will no doubt be scrutinized in future studies.
Kasahara et al. carried out an analysis in which they correlated the electron fluctuations and chorus waves in space with the pulsating-aurora signals seen by the all-sky imager on the ground. This step revealed the precise location in the atmosphere in which the field-line footprint resides. Such a technique has incredible potential to test and refine our current magnetic-field models by comparing the modelled footprint location to the observed location. In the future, magnetic-field-line mapping might well rely on a similar methodology to gain insight into magnetic topology — the structure and linkage of field lines.
There is one caveat, however: clear skies are required to see and measure the pulsating-aurora signals, so Earth’s terrestrial weather needs to cooperate. Furthermore, the chorus waves contain components of different frequency that interact with magnetospheric electrons in different ways depending on the energy of the particles. This affects which particles end up travelling down to Earth’s atmosphere. These details are directly related to geomagnetic activity and have not yet been fully quantified. There is still a rich body of research to be carried out regarding the mysterious pulsating auroras.
Nature 554, 302-303 (2018)