Donghe Zhang

Morphological features of ionospheric response to typhoon

Couplings between the ionosphere and meteorological events have been studied widely. However, most of them are individual case studies or correlation analyses, and few are aiming at the full morphological features of the ionospheric response processes. In this paper, complete records of 24 strong typhoons from 1987 to 1992 were collected, and comparison was made with corresponding ionospheric HF Doppler shift data. The main purpose of the present work is to find the temporal evolution of these responses and their common features by the merit of the continuities of HF Doppler shift observation in time. On the basis of the statistical analyses, this paper reveals the common features of ionospheric responses to typhoon. A summary of these characteristics is as follows: ( 1) During the existing time of a typhoon, there are almost always medium-scale traveling ionospheric disturbances (TIDs) in the ionosphere, especially when a strong typhoon is landing or near the coast of a mainland. ( 2) These TIDs show quite clear periodicity and their periods vary with time and gradually grow longer. ( 3) After sunset, the wavelike disturbances with large magnitudes often excite the midlatitude spread-F. ( 4) The intense typhoon can cause the wavelike records of the Doppler shift to show the S-shaped echo tracing, which means that the amplitudes of those waves are sufficiently large, and ( 5) the sunrise-like phenomena often appear in nonsunrise time during the period the typhoon exists. The phenomena mentioned above are generally in agreement with the linear propagation theories of the acoustic-gravity waves (AGWs) in the atmosphere. A typhoon is surely one of the important ground sources of the wavelike disturbances in troposphere; this source is very effective especially when a typhoon is landing on or near a mainland coast. Of course, the morphological details of the ionospheric response to typhoon can by no means be completely identical every time. In this study, except for TIDs that almost always appear during all the typhoon events, the other common features are not seen every time. However, we are certain that the phenomena summarized above are statistically the manifestation of the ionospheric response to typhoon since they appear much more frequently during periods influenced by typhoon.

Mesoscale field‐aligned irregularity structures (FAIs) of airglow associated with medium‐scale traveling ionospheric disturbances (MSTIDs)

In this paper, we report the evolution (generation, amplification, and dissipation) of optically observed mesoscale field-aligned irregularity structures (FAIs) (similar to 150 km) associated with a medium-scale traveling ionospheric disturbance (MSTID) event. There have not been observations of mesoscale FAIs of airglow before. The mesoscale FAIs were generated in an airglow-depleted front of southwestward propagating MSTIDs that were simultaneously observed by an all-sky imager, a GPS monitor, and a digisonde around Xinglong (40.4 degrees N, 30.5 degrees magnetic latitude), China, on 17/18 February 2012. A normalized cross-correlation method has been used to obtain the velocities of mesoscale FAIs and MSTIDs. The mesoscale FAIs had an obvious northwestward relative velocity to main-body MSTIDs (about 87.0 m/s on average). The direction of this relative velocity was roughly parallel to the depleted fronts. Furthermore, the evolution of the mesoscale FAIs was mostly controlled by the intensity of the depleted fronts. Occurred in a highly elevated ionosphere that had a total electron content depletion associated with large negative airglow perturbations (-25%), the mesoscale FAIs grew rapidly when they experienced southeastward wind, which had a speed of about 100 m/s and were measured by a Fabry-Perot interferometer. A northeastward polarization electric field within a depleted airglow front can play a controlling role in the development of the mesoscale FAIs. The electric field can significantly elevate the ionosphere and move the mesoscale FAIs northwestward by the E x B drift. The processes for the generation and development of the polarization electric field and the mesoscale FAIs, however, need further study.

Hour-to-hour variability of the ionosphere: An application of the classical multidimensional scaling method

The classical multidimensional scaling (MDS) method is introduced and applied in the study of the hour-to-hour ionospheric variability based on the ionospheric foF2 observed at three ionosonde stations in East-Asia in 2002 and 2007. Results from the matrix eigen decompositions indicate that the annual part of the ionospheric variation is large in middle latitude and solar maximum period (2002) while low in the low latitude and solar minimum period (2007). The connectivity maps of the hour-to-hour ionospheric variability based on MDS method show some common diurnal features. The ionospheric connectivity between adjacent hours near noon hours and near midnight hours is high. The ionospheric connectivity between adjacent hours near sunrise hours and near sunset hours is poor, especially for the sunrise hours. Also there are latitudinal and solar activity dependences in this kind of connectivity. These results revealed from the ionospheric connectivity maps are useful physically and in practice for the ionospheric forecasting on the hour-to-hour scale.