Characterization of Mid-Latitude Ultralow-Frequency Ionospheric Currents Excited by Artificial Modulation

Powerful high-frequency (HF) radio waves can modulate the ionosphere and lead to the current generation. Along these lines, in this work, a numerical model for the investigation of ultralow-frequency (ULF) currents excited by radio waves heating the ionosphere in mid-latitudes was established. The impact of both neutral winds and pressure gradient was taken into account by the proposed model. Combining the ionospheric background data of Beijing, the characterization of ULF ionospheric currents (1 Hz) was numerically calculated and discussed. From our analysis, it was demonstrated that the electron temperature relaxation time gradually increased with increasing the altitude during the day and night. The temperature relaxation time of 1 s was attributed to an altitude of 140 km during the day and 180 km at night. The maxima of neutral wind term current was 22 nA
$\cdot \text{m}^{-2}$
at daytime, corresponding to the position 102 km. Interestingly, at nighttime, the maximum value of the current was reduced to 0.032 nA
$\cdot \text{m}^{-2}$
, corresponding to 112 km. The electronic pressure gradient term current exhibited a maximum value of 8 nA
$\cdot \text{m}^{-2}$
at the position 104 km during the day, while, at nighttime, the maxima of the current was 0.19 nA
$\cdot \text{m}^{-2}$
, corresponding to the position near 224 km. It was also proved that the properties of the neutral wind term current are strongly dependent on the ionospheric conductivity but are constrained by the electron temperature and the relaxation time. In addition, the current induced by the electronic pressure gradient term is mainly correlated with the electronic Hall conductivity and the electron temperature relaxation time.