March 18, 2025
Imagine having a system which can accurately predict with a lead time of one hour, the vagaries of the ionosphere in the lower latitude regions like India, how powerful will be planning in the aviation and satellite navigation sectors. Appreciating the complex nature of the tropical ionosphere, especially near the magnetic equator, this dream has so far been unmaterialized until the National Atmospheric Research Laboratory (NARL) came up with a solution in the form of a prediction algorithm by using long term ionosonde data from three low latitude stations (Trivandrum, Sriharikota and Gadanki) in India.
Equatorial plasma bubbles (EPBs) are north-south aligned vertical wedge like depleted electron density structures, which are formed by naturally occurring plasma instability in the equatorial ionosphere. They adversely affect satellite-based communication and navigation systems, high frequency terrestrial communication including the Over-The-Horizon radar applications, and GNSS radio occultation measurements for atmospheric and ionospheric parameters. Many industries/agencies engaged in satellite tracking, aviation, mining, agriculture, and construction are affected by the EPBs.
EPBs are formed after the sunset by the Rayleigh-Taylor instability, but their prediction remained a challenge. A new and robust technique has been developed at NARL to predict the formation of EPB. The technique is based on the physics of localized upwelling of the F layer that takes place prior to the sunset (Figure 1).
Figure 1:The evening equatorial ionosphere, depicting upward motion of the F layer base due to PRE along with localized upwelling. The localized regions, where the upwelling of the F layer base is higher than their neighbourhood, are the sites for EPB formation.
Upwelling has been characterized by the second time derivative of the base height of the F layer (d2h′F/dt2) observed by ionosonde. This, in a way, represents upward acceleration of the F layer ascent (dV/dt) which has a direct bearing on the growth of the instability. It is shown that when the acceleration exceeds a threshold of 0.01 m s-2, EPB is formed. Using large dataset covering diverse solar flux and geomagnetic conditions from three Indian low latitude stations, namely, Trivandrum, Sriharikota and Gadanki, it is demonstrated that prediction for the formation of EPB can be made about an hour in advance with an accuracy of 99.86%, which is unprecedented and remarkable (Figure 2).
Figure 2:Quadrature chart showing statistics of prediction and occurrence of overhead EPB. Positive (negative) X-axis represents prediction for the formation of overhead EPB (no overhead EPB) and positive (negative) Y-axis represents observation of overhead EPB (drifted EPB and no EPB).
The investigation led to the conclusion that longitudinally distributed ground/space-based sensors monitoring the base of the F layer could provide a viable means for predicting EPB formation in a broader longitude zone for practical purposes. This new prediction technique can greatly improve satellite-based navigation applications.
For more details refer to: Patra, A. K., & Das, S. K. (2025). Prediction of equatorial plasma bubble formation using ionosonde observations from India. AGU Advances, 6, e2024AV001323.https://doi.org/10.1029/2024AV001323
This novel algorithm has implications to improve satellite-based navigation applications wherein navigation signals are affected by electron density fluctuations.