Abstract
We analyzed horizontal velocities of medium-scale traveling ionospheric disturbances using HF Doppler sounding over Taiwan from January 2014 to January 2016. Meridional components of the observed horizontal velocities are in most cases larger than zonal components. In summer, poleward propagation prevails, whereas in winter, disturbances primarily propagate southward. Zonal components of the analyzed events are mostly eastward. Seasonal behaviors of the observed propagation directions are similar at other locations with HF Doppler sounding at middle and low latitudes. Horizontal velocities of the observed events are in the 50–400-ms−1 range. The upper limit is determined using the spacing between reflection points, making the uncertainty in velocity very large for events with velocities larger than this limit. The mean horizontal velocity is 210 ms−1. The range between the 20th and 80th percentile of horizontal velocities is 144–281 ms−1. Graphical abstract.
Highlights
Traveling ionospheric disturbances (TIDs) are wavelike perturbations of electron density that propagate in the ionosphere. Hines (1960) showed that they are usually caused by internal gravity waves (GWs) that propagate in the thermosphere
We present observations of Mesoscale traveling iono‐ spheric disturbances (MSTIDs)’ horizontal velocities over Taiwan using High frequency (HF) Doppler sounding
MSTIDs propagate in all directions, except the sector between ~180° and 300°
Summary
Traveling ionospheric disturbances (TIDs) are wavelike perturbations of electron density that propagate in the ionosphere. Hines (1960) showed that they are usually caused by internal gravity waves (GWs) that propagate in the thermosphere. We present observations of MSTIDs’ horizontal velocities over Taiwan using HF Doppler sounding. GWs propagating in the ionosphere cause an undulating electron density profile, which causes a variation in the reflection height, and a variation in the Doppler shift.
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