Medium-scale traveling ionospheric disturbances over Taiwan observed with HF Doppler sounding 2. Aeronomy

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. [Figure not available: see fulltext.]