Abstract
Abstract. Powerful high-frequency (HF) radio waves can be used to efficiently modify the upper-ionospheric plasmas of the F region. The pressure gradient induced by modulated electron heating at ultralow-frequency (ULF) drives a local oscillating diamagnetic ring current source perpendicular to the ambient magnetic field, which can act as an antenna radiating ULF waves. In this paper, utilizing the HF heating model and the model of ULF wave generation and propagation, we investigate the effects of both the background ionospheric profiles at different latitudes in the daytime and nighttime ionosphere and the modulation frequency on the process of the HF modulated heating and the subsequent generation and propagation of artificial ULF waves. Firstly, based on a relation among the radiation efficiency of the ring current source, the size of the spatial distribution of the modulated electron temperature and the wavelength of ULF waves, we discuss the possibility of the effects of the background ionospheric parameters and the modulation frequency. Then the numerical simulations with both models are performed to demonstrate the prediction. Six different background parameters are used in the simulation, and they are from the International Reference Ionosphere (IRI-2012) model and the neutral atmosphere model (NRLMSISE-00), including the High Frequency Active Auroral Research Program (HAARP; 62.39° N, 145.15° W), Wuhan (30.52° N, 114.32° E) and Jicamarca (11.95° S, 76.87° W) at 02:00 and 14:00 LT. A modulation frequency sweep is also used in the simulation. Finally, by analyzing the numerical results, we come to the following conclusions: in the nighttime ionosphere, the size of the spatial distribution of the modulated electron temperature and the ground magnitude of the magnetic field of ULF wave are larger, while the propagation loss due to Joule heating is smaller compared to the daytime ionosphere; the amplitude of the electron temperature oscillation decreases with latitude in the daytime ionosphere, while it increases with latitude in the nighttime ionosphere; both the electron temperature oscillation amplitude and the ground ULF wave magnitude decreases as the modulation frequency increases; when the electron temperature oscillation is fixed as input, the radiation efficiency of the ring current source is higher in the nighttime ionosphere than in the daytime ionosphere.
Highlights
The ultralow-frequency (ULF) waves with a frequency in the range of 0.1–10 Hz, which exist extensively in the terrestrial space, are associated with numerous intriguing space physical problems, including magnetosphere–ionosphere– atmosphere coupling and radiation belt modeling
An analytical mathematical deduction by Vartanyan (2015) based on Eqs. (18) and (19) has revealed the relation between the horizontal half-width of the spatial distribution of the δTe and the radiation efficiency of the ring current source to some extent. This deduction is carried out under the following assumptions: firstly, the geomagnetic field is in the z direction and the ionosphere is uniform, with parameters such as electron density and Alfvén speed constant; secondly, the wave frequency is much lower than the ion cyclotron frequency; thirdly, the ring current source driven by modulated HF heating only radiates the ULF waves in the x direction, which is perpendicular to the ambient geomagnetic field
Based on the HF heating model and the model of ULF wave generation and propagation, we investigate the effects of the background profiles and the modulation frequencies on the process of modulated HF heating in the ionospheric F region and the subsequent generation and propagation of the artificially generated ULF waves
Summary
The ultralow-frequency (ULF) waves with a frequency in the range of 0.1–10 Hz, which exist extensively in the terrestrial space, are associated with numerous intriguing space physical problems, including magnetosphere–ionosphere– atmosphere coupling and radiation belt modeling. In a series of experiments conducted from 2009 to 2010 at HAARP, artificial ULF and lower ELF signals generated by the modulated heating ionospheric F region in the absence of electrojets were received on the ground far away from the heating facility, and the dependence on the heating conditions differs from the low-frequency waves generated by modulating the ionospheric currents (Papadopoulos et al, 2011a, b; Eliasson and Papadopoulos, 2012) This artificial generation of ULF waves relates to the oscillating diamagnetic drift current in the upper ionosphere due to the modulated heating, which is based on the ICD (ionospheric current drive) theory proposed by Papadopoulos et al (2007).
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