Tides are universal phenomena and often play essential roles in planetary and galactic systems wherever gradients in gravitational attraction are important. As the Earth's sole natural satellite, the Moon and its gravitational interaction with the Earth have attracted extensive research and curiosity over several hundred years. Lunar tides have since been well-known to affect the first three states of matter in the Earth-Moon system: solid Earth tides, liquid ocean tides, and neutral gas-dominated atmospheric tides, which are common in all planetary-moon systems. At Earth, these lunar tides mainly have semidiurnal and semimonthly periods, which influence the Earth-Moon system by impacting many natural cycles that influence our daily life.

In fully ionized plasmas, the fourth state of matter, one may expect that tidal effects would be negligible because the gravity force is extremely weak compared to the electromagnetic force in the Earth-Moon space environment.

Recently, an extensive database of data collected over the past 40 years from various spacecraft was compiled, containing observations of the outer boundary location of the plasmasphere. Known as the plasmapause, which this region may be regarded as the surface of a “plasma ocean” surrounding Earth. Using this database, we report the first identification of lunar tides on the surface of this plasma ocean. In order to rule out the possibility of data anomalies and ensure the reliability of the results, the database was divided into two sub-datasets analyzed in three different ways. In all cases, similar and consistent tidal signals were observed. Regarding the characteristics of the lunar plasmaspheric tide, we found that this signal possesses distinct diurnal (and monthly) periodicities, surprisingly different from the semidiurnal (and semimonthly) variations dominant in the atmosphere, ocean, and solid Earth tides. Interestingly, the lunar plasmaspheric tide forms a plasmasphere bulge that is offset 90 degrees ahead of the Earth-Moon axis, which is significantly different from the ocean high tide.

After confirming that this lunar signal exists, the essential question is how does such a lunar tide with such an offset and diurnal/monthly periods occur in the plasmasphere? It is known that the motion of the low energy charged particles and the electric field are essential in determining the position of plasmapause. The electric field in the inner magnetosphere is composed of the steady corotation electric field that is determined by Earth’s magnetic moment and rate of rotation, and the varying magnetospheric convection electric field that is controlled by solar wind and geomagnetic activity. Thus, we used independent observations from NASA Van Allen Probes observations to check the electric field and found observational evidence for the existence of lunar tidal variations in the distribution of the radial electric field. The periodicity of this lunar tidal signal in the radial electric field is also diurnal and monthly, which is similar to that of the lunar tide wave in the plasmapause but with a phase difference of 180º. Using two plasmapause formation models (“Zero Parallel Force surface” and “Last Closed Equipotential”), we showed that the perturbed electric field is capable of generating the observed lunar tide in the plasmapause. In other words, we show that the electromagnetic force couples with gravity to give rise to the lunar plasmaspheric tide and can explain the observable periodic tidal features.

As for how the lunar phase adjusts the radial electric field, one possibility is that neutral winds in the ionosphere, which are modulated by the lunar phase and can generate electric potential differences that map along magnetic field lines threading the plasmasphere, perturb the radial electric field, and thus modulate the plasmapause position. This result further advances our understanding of the Moon-atmosphere-magnetosphere coupling process through gravitation, with important participation of the electromagnetic force.

Our discovery of this plasma tidal effect with distinct characteristics may indicate a fundamental interaction mechanism in the Earth-Moon system that has not been previously considered. Furthermore, reflected by this plasma tide, the plasma flow in the entire Moon-Earth space exists as a persistent background variation of the magnetosphere and can modulate Moon-Earth space dynamics continuously, although the observed perturbations caused by the lunar tide are often relatively small compared to those arising from solar and geomagnetic activities. Since this plasma tide effect appears to be predictably fundamental, it may be expected not only in the plasmasphere, but over a much wider set of phenomena. Whether the magnetic field or plasma lunar tides seen in space are related to the Earth’s crustal and oceanic tide is also a question worthy of discussion in future. The configuration and structure of Earth’s cold plasma in relation to the magnetosphere are not unique, with similar structures being found at other planets and astrophysical objects, hence this plasma tide observed at Earth may be a generic phenomenon found throughout the cosmos. Therefore, the finding of this lunar tidal effect in the plasmasphere not only extends our knowledge of the Earth-Moon system, but also opens new perspectives for further studies of tidal interactions with charged particles in other planetary and larger scale systems.