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We analyzed intense geomagnetically induced currents (GICs) recorded during a complex space weather event observed on 23–24 April 2023. Two geomagnetic storms characterized by SYM/H intensities of −179 nT and −233 nT were caused by southward interplanetary magnetic field (IMF) Bz component of −25 nT in the sheath fields, and −33 nT in the magnetic cloud (MC) fields, respectively. GIC observations were divided into two local time sectors: nighttime (1700–2400 UT on 23 April) GICs observed during the interplanetary sheath magnetic storm, and morning sector (0200–0700 UT on 24 April) GICs observed during the MC magnetic storm. By using the direct measurements of GICs on several substations of Karelian-Kola power line (located in the north-west portion of Russia) and gas pipeline station near Mäntsälä (south of Finland), we managed to trace the meridional profile of GIC increases at different latitudes. It was shown that the night sector GIC intensifications (∼18–42 A) occurred in accordance with poleward expansion of the westward electrojet during a substorm. On the other hand, the intense morning sector GICs (∼12–46 A) were caused by Ps 6 magnetic pulsations. In addition, a strong local morning GIC (∼44 A) was associated with a local substorm-like disturbance caused by a high-density solar wind structure, possibly a coronal loop portion of an interplanetary coronal mass ejection. • Intense GICs (∼12–46A) detected during two magnetic storms on 23–24 April 2023. • Night sector GIC bursts occurred in accordance with substorm expansion. • GIC bursts in the morning sector were caused by Ps6 magnetic pulsations. • Maximal GIC was associated with the local substorm-like intensification caused by coronal loop portion of an ICME. • This is the first time that a coronal loop has been shown to be geoeffective. [ABSTRACT FROM AUTHOR]

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 (fo F2). 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 fo F2 were observed at daytime during quiet or moderately disturbed space weather conditions. In most cases (about 80%), the coherence between the fo F2 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. [ABSTRACT FROM AUTHOR]

Geomagnetic storms and Forbush decreases (FD) on Earth are primarily caused by interplanetary coronal mass ejections (ICMEs) and stream/corotating interaction regions (SIRs/CIRs) originating in the Sun, which are propagated as a low-energy plasma disturbance through the interplanetary magnetic field (IMF). In this paper, we study the variations of the solar-wind parameters (solar wind velocity, plasma density, and IMF-B z component) and the Earth's disturbance storm-time index (D st) in relation to cosmic ray flux measurements from 8 neutron monitor stations distributed over Canada, Russia, Finland, and Greenland, during 3 intense geomagnetic storms occurred during the 24th solar cycle (March 16–18, 2015, June 21–23, 2015, and September 7–9, 2017). The wavelet analysis of the cosmic ray intensity reveals the clear evolution of the classical two-step FD with a peak period of approximately 2.1 h. The correlation-delay analysis shows a very strong correlation (∼0.9) between the relative count rate changes in cosmic ray intensity and the indices of solar wind speed and D st. We obtain similar time-delay responses to the solar wind speed for all the cases (∼4 h), but large discrepancies are seen for the D st index between the storms. We, therefore, recommend not using the D st index for predicting Forbush decreases. Finally, we employ the resulting delay times to parameterise the Forbush decreases in terms of the solar wind, and we obtain a predictive model with R2 parameter of an approximate value of 0.8. Moreover, we observe a possible dependence on solar wind proton density which modulates the magnitude of Forbush decreases under similar solar wind speed conditions. Our results verify the suitability of using solar wind parameters to predict Forbush decreases in the cosmic ray flux. • A deep Forbush decrease was recorded in the galactic cosmic ray flux, showing a 6% decrease over all the selected stations. • Correlation-delay analyses with space weather and geomagnetic indices have shown that V sw and D st index have significant correlation with cosmic ray flux, with values above 0.9 for the storms of March and June 2015, and about 0.85 for the storm of September 8, 2017. • A possible dependence on solar wind proton density has been observed, which modulates the magnitude of Forbush decreases under similar solar wind velocity conditions. [ABSTRACT FROM AUTHOR]

Abstract: Non-typical long lasting quasi-periodic (QP) VLF emissions have been recorded in Northern Finland at L∼5.3 during the recent Finnish VLF campaign held in December 2011. Contrary to the typical daytime QP emissions, the night-time and early morning (00–05UT) event reported here for the first time is a sequence of 1.5–3.5kHz noise bursts lasting for several tens of seconds with an unusually long repetition period which gradually decreases from ∼700s to ∼50s. These QP emissions were observed under conditions of very quiet geomagnetic activity (K p =0). In spite of that, the interplanetary magnetic field generally had a small southward component, and a high-latitude substorm occurred on the night-side. After this substorm, the repetition period of the VLF bursts suddenly dropped from ∼200s to∼60s and the spectral structure of QP wave changed. We attribute these QP emissions to auto-oscillations of the cyclotron instability of the Earth's radiation belts. According to the theory, the repetition period of the QP should be inversely proportional to the flux of the gyroresonant energetic electrons. Thus the increased flux of energetic electrons injected by the substorm probably led to the decreasing QP repetition periods. [Copyright &y& Elsevier]

Energetic charged particle precipitation associated with solar wind perturbations causes enhanced high-frequency radiowave absorption in the high-latitude ionosphere. This study models 38.2 MHz cosmic noise absorption (CNA) by utilising measurements from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi, Finland obtained during solar cycle 23 (1996–2009) associated with the passage of interplanetary coronal mass ejections (ICMEs) past Earth; ICMEs are a major driver of enhanced geomagnetic activity. Superposed epoch analysis suggests that the r6absorption vs. time profile depends on whether ICME arrival occurs in the day-time (10–14 MLT) or night-time (22-02 MLT) for IRIS, with peak absorption occurring ~2–3 h ahead of ICME arrival or ~4 h after ICME arrival, respectively. We determine which combinations of solar wind and IMF parameters show the best correlation with the absorption associated with day-time or night-time arriving ICMEs using superposed epoch analysis and the least squares estimation method. Various combinations of solar wind parameters (including bulk velocity v, density n, and the interplanetary magnetic field north and south components Bz and the SYMH geomagnetic index), have been ranked to obtain the best coupling function for the absorption associated with day- and night-time arriving ICMEs. The absorption for day-time events is found to correlate closely with the solar wind dynamic pressure, SYMH, and the northward direction of the Bz while the absorption for night-time events is most closely related to the direction of the Bz and SYMH. The coupling functions are found to model the observed absorption successfully, with correlation coefficients of ~0.7–0.8 between the observed and modelled absorption. • We have studied cosmic noise absorption (CNA) during disturbed periods associated with Interplanetary Coronal Mass Ejections (ICMES). • We analysed radio ionospheric opacity meter (Riometer) measurements for the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi, Finland for CNA covering solar cycle 23 (i.e. 1996-2009). • We showed that the response of the ionosphere is different depending on the time that the ICME occurs. • We utilised the ICME catalogue produced by one of the authors (Richardson and Cane) to identify ICME periods within solar cycle 23. • The absorption vs. time profile associated with ICMEs is found to depend on whether the ICME starts in the daytime (10–14 MLT) or at night (22-02 MLT) for IRIS (Figs. 3–5). • We used various combinations of solar wind and IMF parameters and ranked them (see Tables 2 and 3) to obtain the best coupling parameter for day- and night-time absorption. • The daytime ICME model profile is found to correlate closely with solar wind dynamic pressure, SYMH and northward B z , while the night-side ICME model is most closely related to B z and SYMH. [ABSTRACT FROM AUTHOR]