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Abstract Very low frequency wave intensity variations measured by the Kannuslehto station, Finland in the frequency range 0–12 kHz between 2016 and 2020 are analyzed by the principal component analysis (PCA). As the analyzed ground-based measurements are basically continuous, the length of individual basis vectors entering into PCA is fundamentally arbitrary. To better characterize both long- and short-period variations, two PCAs with different lengths of the basis vectors are eventually performed. Specifically, either daily frequency–time spectrograms or individual frequency spectra are chosen as the PCA basis vectors. Analysis of the first three principal components shows substantial variations of the wave intensity due to seasonal and local time effects. Intensity variations related to the geomagnetic activity characterized by Kp and AE indices and standard deviation of the magnetic field magnitude are less significant. Moreover, PCA allows one to distinguish between nighttime and daytime Kannuslehto variations and study them independently. Solar and geomagnetic activity effects on the daytime and nighttime measurements are discussed. Wave intensity variations related to substorm occurrence are also analyzed.
Abstract We reveal previously unknown quasi-periodic (QP) very low frequency (VLF) emissions at the unusually high-frequency band of ∼ 7–12kHz by applying the digital filtering of strong atmospherics to the ground-based VLF data recorded at Kannuslehto station (KAN). It is located in northern Finland at L ∼ 5.5. The frequencies of QP emissions are much higher than the equatorial electron gyrofrequency at L ∼ 5.5. Thus, these emissions must have been generated at much lower L shells than KAN. Two high-frequency QP emission events have been studied in detail. The emissions were right-hand polarized waves indicating an overhead location of the exit area of waves in the ionosphere. In one event, the spectral–temporal forms of the emissions looked like a series of giant "bullets" due to the very abrupt cessation. Unfortunately, we could not explain such a strange dynamic spectral shape of the waves. In the second event, the modulation period was about 3min under the absence of simultaneous geomagnetic pulsations. The studied emissions lasted about 4h and were observed under the very quiet geomagnetic activity. The adequate mechanisms of the generation and propagation of the revealed high-frequency QP emissions have not yet been established. We speculate that studied QP emissions can be attributed to the auto-oscillations of the cyclotron instability in the magnetospheric plasma maser.
Abstract Using numerical filtering techniques allowing us to reduce noise from sferics, we are able to clearly study a new type of differently structured very low frequency (VLF) radio waves above f = 4 kHz at the ground station of Kannuslehto in northern Finland (KAN, MLAT = 64.4°N, L = 5.5). These emissions are intriguing, since they are detected at frequencies above half the electron gyrofrequency in the equatorial plane (fce) for the L-shell of Kannuslehto (fce ~ 5–6 kHz). They are commonly observed at Kannuslehto, but have also been infrequently reported at other stations, sometimes under different names. Their possible common origin and manner of propagation is still under investigation. This paper unifies the nomenclature by regrouping all these waves detected at frequencies higher than the local equatorial 0.5 fce at the L-shell of observation under the name of VLF bursty-patches. While these waves have different spectral features, they appeared mostly composed of hiss bursts with durations of a few seconds to several minutes. They also show periodic features with varying periodicity and shape. They are sometimes characterized by single bursts covering very large frequency ranges of several kHz. We also give a review of the different characteristics of VLF bursty-patches observed at Kannuslehto, which at the moment, is the station with the highest observation rate. We present recent observations between 2019 and 2021.
Abstract Very low frequency (VLF) auroral hiss at Kannuslehto (KAN), Finland, was analyzed with reference to the progress of 98 isolated magnetic substorms measured during the winter months of 2015–2018. Of these, 91 were accompanied by auroral hiss during the substorm growth phase. No auroral hiss was recorded during the expansion and recovery phases. We found that auroral hiss was observed under rising polar cap (PC) index, showing an increased solar wind energy input into the magnetosphere during the substorm growth phase. We also found that in 58 of the 65 events studied, KAN was located in the vicinity of enhanced field‐aligned currents (FACs) during auroral hiss occurrence. For the first time, it was established that auroral VLF hiss generation in the equatorial part of the auroral oval is a typical signature of a substorm growth phase.
Abstract The new type of daytime natural very low frequency (VLF) whistler mode emissions of the magnetospheric origin was found in the VLF observations at Kannuslehto station (L ∼ 5.5) in Northern Finland. The events occurred at the frequencies above 4–5 kHz even up to 15 kHz. These emissions have never been observed earlier in the spectrograms because they were hidden by strong impulsive atmospherics (sferics) in the same frequency range originating in lightning discharges. After filtering out the sferics, we surprisingly discovered completely new types of high frequency right-hand polarized peculiar VLF emissions with various unusual dynamic spectra. These new VLF emissions typically occur as a sequence of a number of short (∼1–3 min) burst-like structures or single short patches with the total duration up to a few hours. The spectral peculiarities of several long-lasting VLF events as well as individual short VLF patches are discussed. The dynamic spectra of these new VLF patches raise the question of the temporal and spatial details of the wave-particle interactions in the magnetospheric plasma. These emissions are observed only under quiet or weakly disturbed space weather conditions, but during small negative values of the Dst index, indicating the presence of a certain excess of the radiation belt electrons. Note, these high frequency VLF patches are observed at the frequencies much higher than the half of the equatorial electron gyrofrequency which is equal to ∼2.7 kHz at L ∼ 5.5. It seems that these emissions are generated deep in the magnetosphere, but the detailed nature, generation region, and propagation behavior of these newly discovered VLF emissions remain unknown. An appearance of high frequency VLF patches could be an indirect indicator of a local enhancement of electron fluxes in the radiation belt that are not directly measured and may occur even in the absence of visible ground-based geomagnetic disturbances. Further researches may shed new light on wave-particle interactions occurring in the Earth’s radiation belts.
Abstract Previous research has shown that the study of the global electrical circuit can be relevant to climate change studies, and this can be done through measurements of the potential gradient near the surface in fair weather conditions. However, potential gradient measurements can be highly variable due to different local effects (e.g., pollution, convective processes). In order to try to minimize these effects, potential gradient measurements can be performed at remote locations where anthropogenic influences are small. In this work we present potential gradient measurements from five stations at high latitudes in the Southern and Northern Hemisphere. This is the first description of new datasets from Halley, Antarctica; and Sodankyla, Finland. The effect of the polar cap ionospheric potential can be significant at some polar stations and detailed analysis performed here demonstrates a negligible effect on the surface potential gradient at Halley and Sodankyla. New criteria for determination of fair weather conditions at snow covered sites is also reported, demonstrating that wind speeds as low as 3 m/s can loft snow particles, and that the fetch of the measurement site is an important factor in determining this threshold wind speed. Daily and seasonal analysis of the potential gradient in fair weather conditions shows great agreement with the “universal” Carnegie curve of the global electric circuit, particularly at Halley. This demonstrates that high latitude sites, at which the magnetic and solar influences can be present, can also provide globally representative measurement sites for study of the global electric circuit.
Abstract The daytime lower ionosphere behaves as a solar X‐ray flare detector, which can be monitored using very low frequency (VLF) radio waves that propagate inside the Earth‐ionosphere waveguide. In this paper, we infer the lower ionosphere sensitivity variation over a complete solar cycle by using the minimum X‐ray fluence (FXmin) necessary to produce a disturbance of the quiescent ionospheric conductivity. FXmin is the photon energy flux integrated over the time interval from the start of a solar X‐ray flare to the beginning of the ionospheric disturbance recorded as amplitude deviation of the VLF signal. FXmin is computed for ionospheric disturbances that occurred in the time interval of December–January from 2007 to 2016 (solar cycle 24). The computation of FXmin uses the X‐ray flux in the wavelength band below 0.2 nm and the amplitude of VLF signals transmitted from France (HWU), Turkey (TBB), and U.S. (NAA), which were recorded in Brazil, Finland, and Peru. The main result of this study is that the long‐term variation of FXmin is correlated with the level of solar activity, having FXmin values in the range (1 − 12) × 10−7 J/m2. Our result suggests that FXmin is anticorrelated with the lower ionosphere sensitivity, confirming that the long‐term variation of the ionospheric sensitivity is anticorrelated with the level of solar activity. This result is important to identify the minimum X‐ray fluence that an external source of ionization must overcome in order to produce a measurable ionospheric disturbance during daytime.
Abstract Very low frequency (VLF) emissions of natural origin were identified for the first time by analyzing 1‐hr ground‐based magnetic field spectrograms in the 0.2‐ to 39‐kHz frequency range. Data were used from the Kannuslehto radio receiver (L‐shell ~5.5), recorded during different campaigns between 2006 and 2019. The spectrograms exhibit banded structures, which consist of several strip elements that vary in time and frequency over the event duration. Statistical analysis of 95 events shows that they are observed in the frequency range that extends from 2 to ~37 kHz, and mainly appearing above 16 kHz. The events span from 4 to 110 min and occur in the evening sector (~17–01 magnetic local time), mostly during quiet geomagnetic conditions. Furthermore, they are primarily left‐handed polarized and are associated with bursts of lightning‐related radio emissions such as sferics and tweeks.
Abstract To investigate longitudinal extent of electromagnetic wave activity, we report the first simultaneous ground‐based observations of magnetospheric ELF/VLF emissions at the following three longitudinally separated stations at auroral and subauroral latitudes: Athabasca, Canada (ATH; magnetic latitude: 61.3°N); Kannuslehto, Finland (KAN; 64.4°N); and Syowa Station, Antarctica (SYO; 70.5°S). The magnetic local time (MLT) separations of SYO‐KAN, ATH‐SYO, and ATH‐KAN, are 3, 8, and 11 h, respectively. Simultaneous observation data at these stations are available for a total of 48 days in 2012–2014. The simultaneous occurrence rates of ELF/VLF emissions are 9.8%, 2.5%, and 3.6% for SYO‐KAN, ATH‐SYO, and ATH‐KAN, respectively. We found that the simultaneous wave occurrence rate between two stations is higher in the morning‐dayside sector, indicating that the longitudinal extent of the emissions exhibits MLT dependence. When emissions are simultaneously observed at two stations, the average AE and |Dst| indices tend to be higher. Similarly, if the two stations are more separated in MLT, the average |Dst| index increases. These results suggest that the longitudinal extent of ELF/VLF emissions increases with increasing geomagnetic activity.
Abstract The subionospheric very low frequency (VLF) radio wave technique provides the possibility of investigating the response of the ionospheric D‐region to a diversity of transient and long‐term physical phenomena originating from above (e.g., energetic particle precipitation) and from below (e.g., atmospheric waves). In this study, we identify the periodicities that appear in VLF measurements and investigate how they may be related to changes in space weather and atmospheric activity. The powerful VLF signal transmitted from NAA (24 kHz) on the east coast of the United States, and received at Sodankylä, Finland, was analyzed. Wavelet transform, wavelet power spectrum, wavelet coherence, and cross‐wavelet spectrum were computed for daily averages of selected ionospheric, space weather, and atmospheric parameters from November 2008 until June 2018. Our results show that the significant VLF periods that appear during solar cycle 24 are the annual oscillation, semiannual oscillation, 121‐day, 86‐day, 61‐day, and solar rotation oscillations. We found that the annual oscillation corresponds to variability in mesospheric temperature and solar Lyman‐α (Ly‐α) flux and the semiannual oscillation to variability in space weather‐related parameters. The solar rotation oscillation observed in the VLF variability is mainly related to the Ly‐α flux variation at solar maximum and to geomagnetic activity variation during the declining phase of the solar cycle. Our results are important since they strengthen our understanding of the Earth’s D‐region response to solar and atmospheric forcing.
Abstract Diurnal very low frequency subionospheric radio wave phase measurements show a night‐to‐day transition pattern. During this sunrise transition a phase perturbation, which consist of a phase overshoot followed by a small phase recovery to normal daytime values, is often observed. The variability of the size of this sunrise phase perturbation and its maximum and end times were monitored to identify the associated physical causes. Very low frequency signal from the 22.1‐kHz UK transmitter (call sign GVT) recorded at Sodankylä, Finland, from 20 April 2010 to 31 December 2016 were used. The timing at the maximum of the phase perturbation period has an annual pattern that is well described by the seasonal variation of the sunrise time 28 km above the transmitter. Variations in ozone number density at 38–42‐km altitudes are better correlated (R = 0.7) with the sunrise phase perturbation variability than at any other altitudes below ~80 km, and exhibit higher correlation values than for atmospheric temperature. Our results show that the main characteristics of the observed very low frequency sunrise phase perturbation arise from shadowing of short‐wavelength solar UV flux from the D region ionosphere due to stratospheric ozone absorption.
Abstract Magnetospheric extremely low frequency/very low frequency (ELF/VLF) waves are plasma waves emitted from high‐energy electrons in the magnetosphere. These waves have received much attention, as they contribute to the acceleration and loss of relativistic electrons in the radiation belts through wave‐particle interactions. The longitudinal extent of ELF/VLF waves has not been well‐understood, although the extent is important in quantitative evaluation of relativistic electron variations. In this study, we analyzed data from continuous ground‐based simultaneous observations of ELF/VLF waves over a 2‐month period in November and December of 2017, using six loop antennas located at roughly equal intervals around the north geomagnetic pole at ∼60° magnetic latitudes. We estimated the longitudinal extent of magnetospheric ELF/VLF waves based on their occurrence rate. Our results showed that the ELF/VLF wave occurrence rate differed by twofold to threefold, depending on the longitudes of the observation sites. We explain this difference in terms of longitudinal differences in the ionosphere’s magnetic field intensity, possibly due to the electron loss that occurs during the bounce motion at longitudes of small magnetic field intensity. Based on our statistical analysis, we estimated the typical longitudinal extent of ELF/VLF waves as ∼76°. Time series analysis results showed that the large longitudinal extent of the ELF/VLF waves occurs frequently during the main phase of geomagnetic storms and is also associated with substorms represented by the auroral electrojet index.
Abstract Quasi-periodic (QP) emissions are a type of magnetospheric ELF/VLF waves characterized by a periodic intensity modulation ranging from tens of seconds to several minutes. Here, we present 63 QP events observed between January 2017 and December 2018. Initially detected at the VLF receiver in Kannuslehto, Finland (KAN, MLAT = 67.7°N, L = 5.5), we proceeded to check whether these events were simultaneously observed at other subauroral receivers. To do so we used the following PWING stations: Athabasca (ATH, MLAT = 61.2°N, L = 4.3, Canada), Gakona (GAK, MLAT = 63.6°N, L = 4.9, Alaska), Husafell (HUS, MLAT = 64.9°N, L = 5.6, Iceland), Istok (IST, MLAT = 60.6°N, L = 6.0, Russia), Kapuskasing (KAP, MLAT = 58.7°N, L = 3.8, Canada), Maimaga (MAM, MLAT = 58.0°N, L = 3.6, Russia), and Nain (NAI, MLAT = 65.8°N, L = 5.0, Canada). We found that: (1) QP emissions detected at KAN had a relatively longer observation time (1–10 h) than other stations, (2) 11.3% of the emissions at KAN were observed showing one-to-one correspondence at IST, and (3) no station other than IST simultaneously observed the same QP emission as KAN. Since KAN and IST are longitudinally separated by 60.6°, we estimate that the maximum meridional spread of conjugated QP emissions should be close to 60° or 4 MLT. Comparison with geomagnetic data shows half of the events are categorized as type II, while the rest are mixed (type I and II). This study is the first to clarify the longitudinal spread of QP waves observed on the ground by analyzing simultaneous observations over 2 years using multiple ground stations.
Abstract Magnetospheric Extremely Low‐Frequency/Very Low‐Frequency (ELF/VLF) waves have an important role in the acceleration and loss of energetic electrons in the magnetosphere through wave‐particle interaction. It is necessary to understand the spatiotemporal development of magnetospheric ELF/VLF waves to quantitatively estimate this effect of wave‐particle interaction, a global process not yet well understood. We investigated spatiotemporal development of magnetospheric ELF/VLF waves using 6 PWING ground‐based stations at subauroral latitudes, Exploration of energization and Radiation in Geospace and RBSP satellites, POES/MetOp satellites, and the RAM‐SCB model, focusing on the March and November 2017 storms driven by corotating interaction regions in the solar wind. Our results show that the ELF/VLF waves are enhanced over a longitudinal extent from midnight to morning and dayside associated with substorm electron injections. In the main to early storm recovery phase, we observe continuous ELF/VLF waves from ∼0 to ∼12 MLT in the dawn sector. This wide extent seems to be caused by frequent occurrence of substorms. The wave region expands eastward in association with the drift of source electrons injected by substorms from the nightside. We also observed dayside ELF/VLF wave enhancement, possibly driven by magnetospheric compression by solar wind, over an MLT extent of at least 5 h. Ground observations tend not to observe ELF/VLF waves in the post‐midnight sector, although other methods clearly show the existence of waves. This is possibly due to Landau damping of the waves, the absence of the plasma density duct structure, and/or enhanced auroral ionization of the ionosphere in the post‐midnight sector.
Abstract We compare for the first time two conjugate events showing simultaneous very low frequency (VLF) wave observations between the same ground station and spacecraft, at different geomagnetic conditions and on opposite sides of the magnetosphere. Waves were observed at Kannuslehto (MLAT = 64.4°N, L=5.46), Finland, and on board Arase (Exploration of energization and Radiation in Geospace, ERG) in the inner magnetosphere. Case 1 on 28 March 2017 shows quasiperiodic (QP) emissions and chorus simultaneously observed on the postmidnight side during the recovery phase of a storm, with sustained high solar wind speed and AE index. Case 2 on 30 November 2017 shows clear one‐to‐one correspondence of QP elements on the noonside during geomagnetic quiet time (Dst>10 nT and AE<100 nT). We present the characteristics of both cases, focusing on coherence and spatial extent of the waves, electron density, and magnetic field variations. We report that the magnetic field gradient plays a role in the changes of spectral features of the waves.
Abstract We show for the first time that elves can be produced by an unusual small‐scale continental spring‐time thunderstorm. The storm occurred in Central Europe, covered a very small area of ∼50 × ∼30 km and lasted only for ∼4 h on April 2, 2017. The fraction of intense positive cloud‐to‐ground lightning strokes was unusually high, reaching 55%, with a mean peak current of 64 kA. The peak currents of return strokes (RS) associated with elves exceeded ∼300 kA. Elves and their causative RS have been observed with different optical and electromagnetic recordings. Signatures of ionospheric disturbances indicating the presence of elves were found in measurements of displacement currents, ionospheric reflections of sferics and man‐made narrow‐band transmissions. All these electromagnetic observations coincide with four optical detections of elves and strongly suggest the occurrence of two more elves later in the decaying phase of the storm. Surprisingly, the same electromagnetic measurements indicate that other strong strokes did not produce any elves. Our simulation results show that the formation of an elve is not only determined by the high‐peak current of their causative strokes but that it is also controlled by the conductivity of the lightning channels and velocity of the current wavefront. We hypothesize that because of a lower conductivity of RS lightning channels and/or slower current waves only very strong strokes with peak currents above ∼300 kA might have been capable to produce observable elves during this thunderstorm.