A new type of the series of quasi-periodic (QP) very low frequency (VLF) emissions in frequency range of 1-5 kHz, and not associated with geomagnetic pulsations, has been discovered at auroral ...latitudes (L = 5.3) during the Finnish VLF campaign (held in December 2011). At least five unusually spectacular events, each with a duration of several hours, have been observed during the night under conditions of quiet geomagnetic activity (Kp = 0-1), although QPs usually occur during the daytime. Contrary to the QP emissions typically occurring during the day, the spectral structure of these QP events represented an extended, complicated sequence of repeated discrete rising VLF signals. Their duration was about 2-3 min each, with the repetition periods ranging from ~1 min to ~10 min. Two such nighttime non-typical events are reported in this paper. The fine structure of the separated QP elements may represent a mixture of the different frequency band signals, which seem to have independent origins. It was found that the periodic signals with lower frequency appear to trigger the strong dispersive upper frequency signals. The temporal dynamics of the spectral structure of the QPs studied were significantly controlled by some disturbances in the solar wind and interplanetary magnetic field (IMF). This finding is very important for future theoretical investigations because the generation mechanism of this new type of QP emissions is not yet understood.
Unexpected short patches of natural VLF emissions at
f
> 5 kHz have been observed at the ground station of Kannuslehto (KAN,
L
~ 5.5) in Northern Finland. In contrast with usual VLF emissions ...(e.g., chorus, hiss, and quasiperiodic emissions) these high-frequency bursty-patches are observed at frequencies higher than half of the equatorial electron gyro-frequency of the
L
shell of KAN. Moreover, most of these waves reached frequencies above the local equatorial electron gyrofrequency at
L
= 5.5. Thus, they cannot be attributed to the classical theory of electron-cyclotron interaction. We present a review of VLF bursty-patches at KAN during winters 2011–2021. These emissions have rarely been observed as they are usually hidden by sferics originating from lightning discharges. Therefore, a special numeric filtering technique was used to reduce noise from sferics. VLF bursty-patches typically occur as sequences of short right-hand polarized bursts separated by a few minutes and lasting several hours. Here, we discuss the spectral structure of long-lasting bursty-patches (6 + hours) and the properties of individual patches. We established two categories: (1) “
triggered-like”
hiss-like bursts at
f
~ 4–7 kHz with a very abrupt onset and detected under quiet geomagnetic conditions, and (2) “
dash-like”
emissions at
f
> 6 kHz that resemble narrowband hiss and observed under moderate activity. Even though VLF bursty-patches in winters 2011–2021 were observed under weak or slightly disturbed magnetic activity, their annual cyclical occurrence was similar to variations in solar activity. The nature of these VLF patches has not been established yet, but they appear to be generated at L shells lower than that of KAN. Their exact generation region and propagation behavior remain unknown, with further theoretical and experimental research being required.
A thorough investigation of short-period oscillations in the Earth’s magnetic field as a fundamental natural process of the magnetospheric plasma began in Russia after V.A. Troitskaya established two ...oscillatory regimes in the geomagnetic field, namely, the regimes of continuous (
Pc
) and irregular pulsations (
Pi
). For studying these pulsations, 19 stations recording the telluric currents were installed during the International Geophysical Year (IGY, 1957–1959) on Troitskaya’s initiative. One of these stations was the Borok station. Subsequently, Borok has become the basic site for investigating geomagnetic pulsations and the main center for studying the short-period pulsations (SPPs) in the Earth’s magnetic field. This is the Borok scientific station where the key fundamental regularities of different types of geomagnetic pulsations were established. Troitskaya led and actively participated these works. Troitskaya organized and conducted the first complex geomagnetic observations in the world at the conjugate points Sogra (Arkhangelsk region, Russia) and Kerguelen (Indian Ocean). These studies were initially tested at the Borok observatory, where it was established that the wave packets of
Pc
1 geomagnetic pulsations are alternately observed in the northern and southern hemispheres in contrast to the other pulsation types which simultaneously occur in both hemispheres. The studies carried out at Borok promoted the establishment of a new direction in geophysics—diagnostics of the state of the magnetosphere based on the ground observations of geomagnetic pulsations. The analysis of simultaneous observations of the geomagnetic pulsations at polar latitudes of the Arctic and Antarctic was also for the first time conducted at the Borok observatory. This analysis revealed the main characteristics of wave phenomena at the geomagnetic poles and in the vicinity of the projection of the dayside polar cusp. Thus, for the first time in the world, Troitskaya and her Borok colleagues established the key patterns of the oscillatory regimes in the geomagnetic field of the Earth. This laid the basis for the further experimental and theoretical investigations which have shown that SPPs play a leading role in the dynamics of the magnetospheric plasma. In this paper we also list of 60 of Troitskaya’s main publications.
Results are presented from an analysis of auroral hiss bursts and geomagnetic pulsations recorded at the Lovozero observatory. The dynamics of the region on the Earth’s surface illuminated by the ...auroral hiss is compared to that of the aurora accompanying its occurrence. Events when hiss bursts and pulsations occur simultaneously with the emergence of an aurora or an increase in their brightness are identified. A mechanism of a possible connection between the observed phenomena is proposed on the basis of the analysis.
The response of strong magnetic storms in variations of the electric field of the near-ground atmosphere of the midlatitudes are studied according to data from long-term observations of aeroelectric ...(
Ez
) and geomagnetic fields at the midlatitude Borok Geophysical Observatory. For the period 1998–2015, 19 strong and very strong magnetic storms with a minimal
Dst
index of < –100 nT and a maximal index value of
Kp
> 7, which correspond to undisturbed meteorological conditions of the lower atmosphere (i.e., good weather conditions), are identified at Borok Observatory. The effect of a magnetic storm on changes in the electric field of the near-surface atmosphere is shown to be more efficiently manifested in the daytime around noon. A statistically significant variation in the strength of the aeroelectric field was found. It was characterized by an increase in the
Ez
value over a time interval of ±4 h relative to the time of the minimum of the
Ds
t variation of the magnetic storm.
This paper reviews individual cases of the relationships between variations of solar wind parameters and variations of the DC vertical atmospheric electric field,
E
z
, and current density,
J
z
, ...measured at ground level in the Arctic, at the S. Siedlecki Polish Polar Station Hornsund, Spitsbergen (Svalbard, Norway), and at the mid-latitude S. Kalinowski Geophysical Observatory in Swider (Poland). A considerable number of events from Hornsund confirmed previous observations of regularity of effects related to the station’s position against the location of the potential bays of ionospheric convection and polar electrojets, observed in other polar locations, as well as effects of other polar cap current systems. This allowed us to conclude that the physical dependence of ground-level
E
z
and
J
z
on solar wind changes produce measurable effects which do not require statistical analysis to be observed. We can also expect that the dependence does exist, especially in strongly disturbed circumstances, e.g., following solar flares and Earth-directed coronal mass ejections, at middle latitudes. However, further investigations of these physical relationships by this approach are practically almost impossible since a very large number of variable parameters simultaneously affect the recorded lower atmospheric variables. In addition, results of quantitative analysis of predicted and observed effects are not satisfactory. Future research studies require more efficient ways of investigation by theoretical treatment and modelling work using existing and novel observational data besides taking advantage of scientific progress in magnetospheric physics.
We investigate a non-typical very low frequency (VLF) 1–4 kHz hiss representing a sequence of separated noise bursts with a strange "mushroom-like" shape in the frequency–time domain, each one ...lasting several minutes. These strange afternoon VLF emissions were recorded at Kannuslehto (KAN, = 67.74° N, λ = 26.27° E; L ∼ 5.5) in northern Finland during the late recovery phase of the small magnetic storm on 8 December 2013. The left-hand (LH) polarized 2–3 kHz "mushroom caps" were clearly separated from the right-hand (RH) polarized "mushroom stems" at the frequency of about 1.8–1.9 kHz, which could match the lower ionosphere waveguide cutoff (the first transverse resonance of the Earth–ionosphere cavity). We hypothesize that this VLF burst sequence could be a result of the modulation of the VLF hiss electron–cyclotron instability from the strong Pc5 geomagnetic pulsations observed simultaneously at ground-based stations as well as in the inner magnetosphere by the Time History of Events and Macroscale Interactions during Substorms mission probe (THEMIS-E; ThE). This assumption is confirmed by a similar modulation of the intensity of the energetic (1–10 keV) electrons simultaneously observed by the same ThE spacecraft. In addition, the data of the European Incoherent Scatter Scientific Association (EISCAT) radar at Tromsø show a similar quasi-periodicity in the ratio of the Hall-to-Pedersen conductance, which may be used as a proxy for the energetic particle precipitation enhancement. Our findings suggest that this strange mushroom-like shape of the considered VLF hiss could be a combined mutual effect of the magnetospheric ULF–VLF (ultra low frequency–very low frequency) wave interaction and the ionosphere waveguide propagation.
The event (24 November 2006, ∼0400–0500 UT) of the simultaneous observations of Pc5 ULF geomagnetic pulsations, electron precipitation (CNA riometer absorption), and whistler‐mode chorus, as well as ...solar wind (SW) and IMF parameters have been analyzed based on the data from IMAGE magnetometers, Finnish riometer array, and temporal VLF station. The visible correlation between the simultaneous occurrence of several minutes scale patches of chorus and pulsating CNA enhancements was found. The dynamic spectra of the riometer data showed a maximum at ∼3.5 mHz in the first half‐hour interval and at ∼2.0 mHz in the second one, while the ULF pulsation spectra exhibit these two maxima in both intervals simultaneously. In the first time interval, the Pc5 pulsations at ∼3.5 mHz demonstrated the typical FLR feature. The SW dynamic pressure fluctuations showed a broad (1.5–3.5 mHz) spectral maximum in the first interval; however, in the second one, the simultaneous oscillations at ∼2.0 mHz were observed in SW pressure and in IMF Bz. The similar ∼2.0 mHz peak has been found in the spectra of Pc5 pulsations from auroral zone to the equator, in riometer absorption, and in VLF chorus power. We suggest that the modulation of particle precipitation and whistler‐mode chorus patches was caused by the 2.0 mHz compressional component of Pc5 poloidal geomagnetic pulsations driven in the magnetosphere by SW dynamic pressure and IMF Bz disturbances.
Night-time VLF (very low frequency) chorus bursts lasting about one hour have been recorded at Finnish temporal station Kannuslehto (CGM: 64.2°; 107.9°, L = 5.3) during two VLF campaigns (on 25 ...February-4 March 2008 and 27 March-17 April 2011). The chorus bursts were associated with substorm development. They were accompanied by riometer absorption enhancements, which occurred simultaneously within as large longitude areas as from pre-midnight (Sodankylä, ~22:00 MLT) to the late morning (Tixie, ~03:00 MLT and Gakona, ~08:00 MLT) longitudes. It was found that the pre-midnight chorus observed on the ground occurred simultaneously with VLF chorus emissions recorded in the late morning on the low-altitude DEMETER satellite crossing the similar geomagnetic latitudes on the opposite local time sector. For the first time some evidence of simultaneous chorus burst generation in the global longitudinal scale was found (from pre-midnight to the late morning) by using direct comparison with satellite data as well as using non-direct indicator-azimuthally extended riometer absorption enhancements.
Polar substorms include substorms observed at geomagnetic latitudes above 70° MLAT in the absence of simultaneous negative magnetic bays at lower latitudes, that is, substorms on the compressed ...contracted auroral oval. The general morphological features of polar substorms are considered based on the example of individual events registered on Svalbard arch. It is shown that polar substorms, like “classical” substorms, are characterized by the formation of a substorm current wedge and a steplike movement to the pole after the onset of a substorm, generation of
Pi
2 geomagnetic pulsations, and an increase of the
PC
-index of the polar cap before the onset of the substorm. At the same time, there are certain differences between polar substorms and “classical” substorms; namely, they start on more distant L-shells, develop in the region of a contracted auroral oval, occur at earlier pre-midnight hours, and generate only at low solar wind speeds and weakly disturbed geomagnetic conditions. It has been suggested that polar substorms may be a specific type of “classical” substorms that develop in the evening sector under magnetically quiet or weakly disturbed conditions when the auroral oval is concracted. The source of polar substorms may also be a local intensification of previously existing substorms in the post-midnight sector.
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