We know that geomagnetic storms, like the recent one that took place in April this year, disturb the ionosphere affecting satellite communication systems on Earth. But, even during the absence of geomagnetic activity, some large-amplitude planetary-scale atmospheric waves can impose significant variability on the mesosphere and the ionosphere, according to a recent study. The study can show the path towards tracking day-to-day variations of trans-ionospheric signals used by the Global Navigation Satellite System. 

The Earth’s atmosphere is divided into different layers, namely, the troposphere, the stratosphere, the mesosphere, and the thermosphere. The partially ionized part of the mesosphere and thermosphere together form the ionosphere. Due to the presence of the charged particles and the Earth’s magnetic field, various electrodynamic processes take place in the ionosphere. 

Neutral particles, which are large in numbers in comparison to ionised particles, play a very important role in these processes. The atmospheric layers are generally coupled through various dynamical processes. One of such process is atmospheric waves which cause significant day-to-day variability in the ionosphere. These waves can be observed in a wide spectrum from small-scale gravity waves to the largest-scale planetary waves.

Attempting to study the quantum of influence of atmospheric waves on neutral-ion coupling in the mesosphere and thermosphere, a team of scientists from the Indian Institute of Geomagnetism (IIG), an autonomous institute of the Department of Science and Technology (DST), examined a strong planetary-scale wave of period close to two days (referred as quasi 2-day wave) during January 2015 at low latitudes. Such a wave generally occurs as transient bursts lasting for a few weeks.

Using medium frequency (MF) radar and meteor radar observations of the mesosphere, the team assessed the variability of the neutral atmosphere and ionosphere imposed by this wave and found an exact mirroring of the disturbance across the two regions. A special feature of this wave confirmed in this report was its westward propagating nature producing three alternating crests and troughs (corresponding to a zonal wavenumber-3 wave) in major atmospheric parameters around the globe. 

Analysis of geomagnetic data from multiple stations at low latitudes showed ionospheric oscillations with similar zonal characteristics and periods that matched the mesospheric wave signature. The finding of the exact reflection of the mesospheric quasi-2-day planetary wave in the geomagnetic field records obtained globally on this occasion was the highlight of this study. 

These similarities in wave characteristics observed in the mesosphere and in the ionospheric E-region dynamo region (a process in the ionosphere through which moving charged particles generate currents which in turn create a magnetic field) have indicated the uninterrupted propagation of the wave into the ionosphere. The research showed that the wave during this event was strong enough to disturb the neutral winds in the mesosphere and the thermosphere, thereby modulating the E-region dynamo (a process in the ionosphere through which moving charged particles generate currents which in turn create a magnetic field). It also showed that the system in this region (referred to as solar quiet current -Sq), finally manifested in the ground geomagnetic records as variation at periods corresponding to the quasi 2-day wave.

The study published in the Journal of Geophysical Research - Space Physics thus demonstrates that large-amplitude planetary-scale waves originating in the lower and middle atmosphere impose significant variability at ionospheric heights, and hence majorly contribute to the atmosphere-ionosphere coupling. 

Such studies on the influence of atmospheric dynamics on the ionosphere could help in determining the sources of quiet-time space weather. These studies also find applications in understanding the sources of the day-to-day variations of the trans-ionospheric signals used by the Global Navigation Satellite System and over-the-horizon radars.

Publication link:  https://doi.org/10.1029/2022JA031098

Figure: The red curve displays the quasi-2-day oscillations in the geomagnetic field over Kourou (5°N, 52°W), while the blue curve represents the quasi-2-day wave in the mesospheric wind over Kolhapur (16°N, 74°E). The two stations are well separated in longitude (~120°), yet the wave signatures observed are very similar. This suggests that the same westward propagating wave-3 mode is present in both the mesosphere and the ionosphere