An investigation into the spectral parameters of ultra-low-frequency (ULF) waves in the polar caps and magnetotail

The present case study is focused on fluctuations at ∼ 1.5 mHz observed on open field lines in both of the polar caps in ground-based geomagnetic data and in the electron concentration in the Northern Hemisphere ionosphere. Coherent pulsations with a relatively narrow narrowband-like spectra and a higher fraction of transversal components in the total spectral power are also observed by the Cluster satellites in the magnetotail magnetic field. Interestingly, the pulsations in the magnetotail started after pulsations over a similar frequency range observed in the solar wind dynamic pressure and interplanetary magnetic field (IMF) had been switched off. This suggests evidence of an internal resonant magnetotail mode which is normally masked by a higher-amplitude broadband ultra-low-frequency (ULF) “noise” of extra-magnetospheric origin.publishedVersio

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

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

Even moderate geomagnetic pulsations can cause fluctuations of foF2 frequency of the auroral ionosphere

Abstract The ionosonde at the Sodankylä Geophysical Observatory (SOD; 67∘ N, 27∘ E; Finland) routinely performs vertical sounding once per minute which enables the study of fast ionospheric variations at a frequency of the long-period geomagnetic pulsations Pc5–6/Pi3 (1–5 mHz). Using the ionosonde data from April 2014–December 2015 and colocated geomagnetic measurements, we have investigated a correspondence between the magnetic field pulsations and variations of the critical frequency of radio waves reflected from the ionospheric F2 layer (foF2). For this study, we have developed a technique for automated retrieval of the critical frequency of the F2 layer from ionograms. As a rule, the Pc5–6/Pi3 frequency band fluctuations in foF2 were observed at daytime during quiet or moderately disturbed space weather conditions. In most cases (about 80 %), the coherence between the foF2 variations and geomagnetic pulsations was low. However in some cases (specified as “coherent”) the coherence was as large as γ²≥0.5. The following conditions are favorable for the occurrence of coherent cases: enhanced auroral activity (6 h maximal auroral electrojet (AE) ≥800 nT), high solar wind speed (V>600 km/s), fluctuating solar wind pressure and northward interplanetary magnetic field. In the cases when the coherence was higher at shorter periods of oscillations, the magnetic pulsations demonstrated features typical for the Alfvén field line resonance

Nighttime Pc3 pulsations: MM100 and MAGDAS observations

Abstract In this paper, we present a statistical and case analysis of nighttime Pc3 pulsations observed from middle to equatorial latitudes during the year 2003. We found two groups of nighttime Pc3 pulsations. Pc3s of the first group are in fact the nightside counterpart of morning Pc3 pulsations with large azimuthal scales slowly attenuating toward midnight. Such night signatures of morning Pc3 waves are observed during the periods of fast solar wind ( $$V>500\,\hbox {km/s}$$ V > 500 km/s ). The second type is the locally generated night Pc3 pulsations. They can be observed under moderate solar wind velocities. Maximal occurrence rates and amplitudes for these pulsations are recorded at middle geomagnetic latitudes near the local magnetic midnight. Probably, they are associated with auroral activations or local non-substorm bursty processes. Graphical abstract An example of a nighttime Pc3 pulsation Graphical abstract Empirical probability density function of the solar wind speed for the intervals with the two types of night Pc3 pulsations and for all the intervals analyzed