Non-triggered auroral substorms and long-period (1–4 mHz) geomagnetic and auroral luminosity pulsations in the polar cap

Abstract

Abstract. A study is undertaken into parameters of the polar auroral and geomagnetic pulsations in the frequency range 1-4 mHz (Pc5/Pi3) during quiet geomagnetic intervals preceding auroral substorms and non-substorm background variations. Special attention is paid to substorms that occur under parameters of the interplanetary magnetic field (IMF) conditions typical for undisturbed days ("non-triggered substorms"). The spectral parameters of pulsations observed in auroral luminosity as measured by a meridian scanning photometer (Svalbard) in the polar cap and near the polar boundary of the auroral oval are studied and compared with those for the geomagnetic pulsations measured by the magnetometer network IMAGE in the same frequency range. It is found that Pc5/Pi3 power spectral density (PSD) is higher during pre-substorm time intervals than for non-substorm days and that specific variations of pulsation parameters ("substorm precursors") occur during the last 2-4 pre-substorm hours.

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

“…As was mentioned above, it is expected to find Pc5/Pi3 pulsations, similar to pre-substorm pulsations (Yagova et al, 2017), in the polar cap before the studied substorm. Indeed, ULF pulsations are observed in the ground magnetometer data stations located on Svalbard (from NAL to HOR) and at BJN and become indistinct at TRO station, situated on the mainland ( Figure 3).…”

“…As it was shown in (Yagova et al, 2017), geomagnetic and auroral luminosity (l = 557.7 nm), the specific pre-substorm pulsations in the frequency range 1-5 mHz (Pc5/Pi3) are observed in the polar cap for a few hours, preceding an isolated nontriggered substorm. The parameters quantifying their spectral content and spatial scale differ from those for typical polar cap Pi3s.…”

“…To quantify a shape of a PSD spectrum, the coefficients Li of the expansion of σ(F) dependence into Lejendre series were used, where σ=log(PSD) is and F=log(f) (Yagova et al, 2007;2010). As it was shown in Yagova et al, 2017, the parameter Q=-L2…”

“…The studied substorm can also be classed as isolated, since the maximum value of AE did not exceed 80 nT for at least 3 previous hours. The substorm perfectly fits in the dataset of selected days used in the paper (Yagova et al, 2017). According to that paper, it is expected to see specific pre-substorm ULF pulsations recorded during several pre-substorm hours, that are studied for this event together with fluctuations in the magnetotail, recorded by Cluster satellites.…”

“…One of them is a substorm developing without any evident external (i.e. from the Solar Wind / Interplanetary Magnetic Field (IMF)) trigger (a so called non-triggered substorm, see introduction Yagova et al, 2017).…”