A case study of the spectral parameters of ULF fluctuations before substorms with no evident trigger in the interplanetary space

Abstract

Abstract. Our recent study (Yagova et al., 2017) shows that ultralow frequency (ULF) pulsations are seen in ground-based magnetic and luminosity data in the polar cap for a few hours preceding isolated non-triggered substorms. Such pulsations are characterised by a high level of coherence. There might be two possible origins of these pulsations: ULF disturbances in the solar wind or processes inside the magnetosphere, which lead to a substorm generation. In this case, the first scenario should be considered as a substorm trigger. To understand the role of the mentioned mechanisms in developing of pulsations, a detailed case study has been carried out. It is shown that before a weak substorm, fluctuations with the central frequency 1.5 mHz are observed in the both polar caps in ground-based geomagnetic data. Coherent pulsations with the same main frequency are also seen in the magnetotail magnetic field, in addition to simultaneous electron concentration in the ionosphere, while fluctuations in electron temperature and electron flux in the magnetosphere have a slightly different main frequency. These pulsations appeared after ULF activity in solar wind and interplanetary magnetic field became inappreciable.