Low ionospheric reactions on tropical depressions prior hurricanes

The variance in complexity of both physical and chemical processes occurring in the atmosphere during a tropical depression/cyclone, directly and indirectly causes changes at different altitude domains and geographical locations. Some of these processes occur in the ionosphere and have been the epicenter of previous studies. Global positioning technology has been employed in computing the total electron content variance during high ionosphere analyses. Such atmospheric related works are of immense significance as it is known that tropical depressions have a tendency to develop into tropical cyclones or, after all, recede to the unperturbed state. This therefore instigates the question on whether there exist any indications from which we can predict the time evolutions of atmospheric parameters after depression. Unfortunately, it becomes quite difficult to answer this question since little analysis has been undertaken on the behavior of low ionosphere.

A group of Serbian researchers investigated the long term low ionosphereic perturbations that last more than the lighting induced ones during such periods. They conducted in-depth studies on the effects of time of tropical depression beginning and its geographical position with respect to considered low ionospheric part to detectability of the low ionospheric disturbances. Their work is now published in the research journal, Advances in Space Research.

The research team commenced their experiments by sending very low frequency radio signals emitted by a NAA transmitter located in the United States of America and receiving them using a similar receiver located in Serbia so as to detect around 41 tropical depressions before the registered hurricanes. For this purpose, they developed a procedure for exceeding any long-term signal amplitude deviations during the considered periods around the tropical depression beginning, over three days before the day of depression. The research team considered subsamples with regard to the recorded time of tropical depression beginning, and analyzed sudden ionospheric disturbances with respect to time period and location of tropical depression beginning.

The authors observed that for all the cases studied, 88% of them recorded low ionosphereic perturbations. They also noted that signal deviations were detected during all the three time periods of daytime, nighttime and periods when solar terminator affects medium within signal propagate and they were repeatable. The researchers also found out that deviations in signal amplitudes were detected at different times in relation to tropical depression beginning, thereby indicating a possibility of detecting sudden ionospheric disturbances as precursor of tropical depressions.

The study by Aleksandra Nina and colleagues has successfully presented the pioneering work that investigates sudden disturbances in the low ionosphere in the period around tropical depression beginning. Most important, they have revealed that responses of the low ionosphere that lasts at least several tens of minutes in the periods of several hours around tropical depression beginning for near 90% of the considered cases, tend to indicate a large possibility of connection between the troposphere and low ionosphere in the periods around tropical depression beginning. The work presented a significant step towards comprehending and predicting hurricane formations.