Influence of Substorm Activity on the Formation of Ultra Low Frequency Noise Emissions in the Frequency Range of 0–7 Hz

The results of the study of simultaneous observations of midlatitude ultra low frequency (ULF) noise emissions in the frequency range of 0–7 Hz and disturbances in the night sector of an auroral oval are presented. Data from observations of the magnetic field at the midlatitude Borok Observatory ( L = 2.8) and 1-min data of the AL index are used for the analysis. It is shown that two kinds of ULF noise emission are observed simultaneously in the hertz range primarily in the summer season: one looks like a diffuse spot, and the other one is characterized by the occurrence of spectral resonance structures determined by the influence of the ionospheric Alfvén resonator. Diffuse spots are recorded mostly in the evening sector of the magnetosphere, and they precede the observation of spectral resonance structures, which are detected primarily at about midnight. Comparison of the observation intervals for diffuse spots with the dynamics of the AL index showed that diffuse spots form during substorm activity on the night side of the Earth in 80% of cases. It is established that the interval between the onset of substorm disturbances and the onset of the observation of diffuse spots is ∼60 min in most cases. It is assumed that the formation of diffuse spots is related to characteristics of the dynamics of protons injected from the magnetospheric tail in substorms and to the appearance of plasmaspheric plumes in the evening sector.