Ultra-Low Frequency (ULF) waves are waves with a period between roughly 1 and 1000 seconds that are usually measured with magnetometers on the Earth's surface or on spacecraft. They are, however, not exclusively found in near-Earth space, but also around other planets and in interplanetary space.
The study of ULF waves found its commencement with the observation of a magnetic storm in 1859 at the Kew Observatory by Steward. In early days the ULF waves were only observable from ground magnetometers. No information about the source for these global oscillations could be found, but a classification could be made. It was found that some waves were quasi-sinusoidal and continuous (Pc) whereas others did not have a well-defined frequency and these were called irregular pulsations (Pi).
These waves are oscillations of the (Earth's) magnetic field lines, that can either be propagating or standing waves. On the closed dipole field lines, there can be (harmonic) standing oscillations similar to the oscillations of a violin string. These are field line resonances, which fall into the Pc-5 category, and their frequency can be used e.g. to estimate the ion mass density at the magnetic equator.
In the space age, in-situ measurements could be made to address the sources and/or characteristics of these waves, but it was quickly recognized, in the late 1970s, that to do this properly, multi-spacecraft measurements were needed. This was first realized with the launch of ISEE 1 and 2 in 1977.
In the magnetotail other ULF waves can be generated by explosive phenomena such as magnetic reconnection. Because of the special planar topology of the magnetotail, there can be various eigen modes of the tail that can be excited (e.g. magnetotail flapping). Otherwise, because of the fast flows created by reconnection, instabilities on the boundaries of the flow channel can generate the Kelvin-Helmholtz instability and create ULF waves.
Such waves can be well studied by the multi-spacecraft missions in near-Earth space such as Cluster, THEMIS, and MMS. With the multipoint measurements, characteristics such as the propagation velocity and the spatial and temporal evolution of the ULF waves can be determined.
ULF waves are not limited to the Earth’s magnetosphere, also in other places in our solar system these waves have been observed. Unfortunately, in these cases there are usually only single spacecraft measurements.
The Venus Express mission measured ion cyclotron waves (Pc-5) in orbit around Venus. Although Venus does not have its own internal magnetic field, in its neighbourhood in the solar wind, these waves can be generated by pickup of ions that are created by ionization of neutrals in the extended exosphere.
One particularly interesting case was the singing comet, where Rosetta measured waves between 40 and 100 mHz around comet 67P/Churyumov-Gerasimenko, created by an, up to then, unmeasured phenomenon, that the large gyro radius of the picked-up ions strum the magnetic field lines around the comet and make then resonate. In this case, the lander Philae was also equipped with a magnetometer and 2 point measurements of these "singing waves" were possible during the descent phase.
ESA's mission to Mecury, BepiColombo, will investigate the Hermian magnetosphere, which is very dynamic. From earlier missions it is known that there is strong ULF activity from reconnection, but also from ion cyclotron waves which are, most likely, created by ion pickup in the solar wind. Here the two modules MMO and MPO will deliver 2-point measurements of the magnetosphere, and only during specific conjunctions of the spacecraft also of the ULF waves.
The upcoming ESA mission to Jupiter, JUICE, which will orbit the Jovian moon Ganymede at the end of its mission will measure ULF waves in the enormous Jovian magnetosphere. But at the end of its mission, in orbit around Ganymede, JUICE will study the small magnetosphere of this moon, where it is known that e.g. field line resonances occur.