Pc5 Pulsations Research Articles

Nighttime Pc3 pulsations: MM100 and MAGDAS observations

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}). 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 abstractAn example of a nighttime Pc3 pulsationGraphical abstractEmpirical 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.

Energy flux in 2‐D MHD waveguide in the outer magnetosphere

AbstractThe problems of large‐scale wave propagation and amplification in the outer magnetosphere are considered. Kelvin‐Helmholtz (KH) instability growth rate of the magnetospheric waveguide eigenmodes is investigated as a function of a coordinate along the magnetopause. The problem of solar wind MHD wave penetration into the waveguide is investigated for a broad range near Pc3 and Pc5 geomagnetic pulsation frequencies and realistic models of the magnetospheric waveguide. The expression for the waveguide eigenmode energy flux is obtained. This expression includes the effects of external wave penetration and mode amplification due to the KH instability, as well as losses due to dissipation in the vicinity of the Alfven resonance which are incorporated into the growth rate coefficient together with the instability.

Biotropic geomagnetic pulsations Pc1 resulting from the magnetic storm of March 17, 2015

Data obtained by a Hall sensor–based magnetic field detector have been used to record the spectral components of the geomagnetic field. An analysis of specific features of the spectral characteristics of geomagnetic fluctuations has indicated that the pulsations identified in the vicinity of the strong magnetic storm of March 17, 2015, are “pearl”-type fluctuations, i.e., biotropic geomagnetic Pc1 pulsations.

Multipoint spacecraft observations of long-lasting poloidal Pc4 pulsations in the dayside magnetosphere on 1–2 May 2014

Abstract. We use magnetic field and plasma observations from the Van Allen Probes, Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Geostationary Operational Environmental Satellite system (GOES) spacecraft to study the spatial and temporal characteristics of long-lasting poloidal Pc4 pulsations in the dayside magnetosphere. The pulsations were observed after the main phase of a moderate storm during low geomagnetic activity. The pulsations occurred during various interplanetary conditions and the solar wind parameters do not seem to control the occurrence of the pulsations. The most striking feature of the Pc4 magnetic field pulsations was their occurrence at similar locations during three of four successive orbits. We used this information to study the latitudinal nodal structure of the pulsations and demonstrated that the latitudinal extent of the magnetic field pulsations did not exceed 2 Earth radii (RE). A phase shift between the azimuthal and radial components of the electric and magnetic fields was observed from ZSM = 0.30 RE to ZSM = −0.16 RE. We used magnetic and electric field data from Van Allen Probes to determine the structure of ULF waves. We showed that the Pc4 magnetic field pulsations were radially polarized and are the second-mode harmonic waves. We suggest that the spacecraft were near a magnetic field null during the second orbit when they failed to observe the magnetic field pulsations at the local times where pulsations were observed on previous and successive orbits. We investigated the spectral structure of the Pc4 pulsations. Each spacecraft observed a decrease of the dominant period as it moved to a smaller L shell (stronger magnetic field strength). We demonstrated that higher frequencies occurred at times and locations where Alfvén velocities were greater, i.e., on Orbit 1. There is some evidence that the periods of the pulsations increased during the plasmasphere refilling following the storm.

Large-scale coordinated observations of Pc5 pulsation events

Abstract. HF (high-frequency) radars belonging to SuperDARN (Super Dual Auroral Radar Network) receive backscatter over substantial fields of view which, when combined, allow for simultaneous returns over extensive regions of the polar caps and midlatitudes. This makes them ideal instruments for the observation of pulsations in the Pc5 (1–5 mHz) frequency band. Relatively few pulsation events observed by multiple radars have been reported in the literature. Here we describe observations of three such events which extend over more than 120° of magnetic longitude in the Northern Hemisphere and one of which is also detected in the Southern Hemisphere. All three events show characteristics of field line resonances. In one case the pulsation has also been observed by magnetometers under or near the radar fields of view. The extensive longitudinal coverage allows accurate determination of azimuthal wave numbers. These are at the upper end of the lower values associated with external sources such as those in the solar wind. Such sources imply antisunward flow. However, the azimuthal wave number is negative, implying westward propagation at magnetic local times on both sides of noon, as would be expected from drift–bounce resonance with positive particles. Quiet conditions and a very low ring current during the events argue against this. The identification of the source of pulsations from a number of different mechanisms remains a problem of interest.

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

AbstractTo understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave‐particle interactions at subauroral latitudes, high‐time‐resolution (1/8 s) images of proton‐induced N2+ emissions were recorded using a new electron multiplying charge‐coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising‐tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. These results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

Наблюдения дрейфово-компрессионных волн с помощью среднеширотного декаметрового когерентного радара

Magnetospheric Pc5 pulsations observed on December 26, 2014 are analyzed. They were recorded in the nightside magnetosphere with a mid-latitude coherent decameter radar located near Ekaterinburg. It registers velocity variations in electric drift of ionospheric plasma caused by ULF waves in the magnetosphere. The westward direction of azimuthal propagation of wave coincides with the direction of magnetic drift of protons. A cross-wavelet analysis reveals that the frequency of oscillations depends on the wave number m, and the correlation between them is 0.90. The frequency increase from 2.5 to 5 mHz was followed by an increase in the absolute value m from 20 to 80. These features of the wave under study testify that it should be classified as a drift compressional mode which is typical for the ULF mode in kinetics. Existence conditions for it are the terminal pressure of plasma and its inhomogeneity across magnetic shells.

Modulation of the ionosphere by Pc5 waves observed simultaneously by GPS/TEC and EISCAT

Earlier studies demonstrated that the monitoring of the ionospheric total electron content (TEC) by global satellite navigation systems is a powerful method to study the propagation of transient disturbances in the ionosphere, induced by internal gravity waves. This technique has turned out to be sensitive enough to detect ionospheric signatures of magnetohydrodynamic waves as well. However, the effect of TEC modulation by ULF waves is not well examined as a responsible mechanism has not been firmly identified. During periods with intense Pc5 waves distinct pulsations with the same periodicity were found in the TEC data from high-latitude GPS receivers in Scandinavia. We analyze jointly responses in TEC variations and EISCAT ionospheric parameters to global Pc5 pulsations during the recovery phase of the strong magnetic storms on October 31, 2003. Comparison of periodic fluctuations of the electron density at different altitudes from EISCAT data shows that main contribution into TEC pulsations is provided by the lower ionosphere, up to ~150 km, that is the E-layer and lower F-layer. This observational fact favors the TEC modulation mechanism by field-aligned plasma transport induced by Alfven wave. Analytical estimates and numerical modeling support the effectiveness of this mechanism. Though the proposed hypothesis is basically consistent with the analyzed event, the correspondence between magnetic and ionospheric oscillations is not always perfect, so further studies need to be conducted to understand fully the TEC modulations associated with Pc5 pulsations.

Possible generation mechanisms for Pc1 pearl structures in the ionosphere based on 6 years of ground observations in Canada, Russia, and Japan

AbstractWe investigate pearl structures (amplitude modulations) of Pc1 pulsations simultaneously observed at Athabasca (ATH, 54.7°N, 246.7°E, L = 4.3) in Canada, Magadan (MGD, 60.1°N, 150.7°E, L = 2.6) in Russia, and Moshiri (MOS, 44.4°N, 142.3°E, L = 1.5) in Japan. From 6 years of ground observations, from 2008 to 2013, we selected 84 Pc1 events observed simultaneously at the longitudinally separated stations (ATH and MGD) and 370 events observed at the latitudinally separated stations (MGD and MOS), all with high coherence (>0.7) of Pc1 waveforms. We calculated the cross‐correlation coefficient (similarity: r) for the Pc1 pearl structures and found that more than half of the events in both pairs had low similarity (r < 0.7), indicating that most Pc1 waves exhibit different pearl structures at different stations. We found that high‐similarity Pc1 pearl structures (r > 0.7) at the longitudinally separated stations are concentrated from 6 to 15 UT when both stations are in the nighttime. The similarity of Pc1 pearl structures tends to show a negative correlation with the standard deviation of the polarization angle in both pairs. The observed repetition period of Pc1 pearl structures has a clear positive correlation with the repetition period estimated from Pc1 bandwidth by assuming beating of different frequencies. From these results, we suggest that ionospheric beating effect could be a dominant process for the generation of Pc1 pearl structures. Beating processes in the ionosphere with a spatially distributed ionospheric source can cause the different shapes of Pc1 pearl structures at different observation points during ionospheric duct propagation.

Equatorward evolution of auroras from the poleward auroral boundary

An all-sky imager installed at the midnight sector in Dawson City (66.0° in geomagnetic latitude) recorded the equatorward evolution of auroras from the auroral poleward boundary. The auroras evolved as shear layers expanding southeastward with velocities of 1–4km/s, referred to as N-S auroras, and occurred during the transient intensification of the convection electric fields in the nighttime magnetosphere, as inferred from an electron spectrogram at geosynchronous altitudes. A continuous increase in the inclination angle of the field lines and magnetic field perturbations associated with propagating ionospheric loop currents were observed in the auroral zone during the N-S auroras. Simultaneously, Pc4 pulsations were observed at low latitudes from night to day sectors. We conclude the following: (1) the N-S auroras are an auroral manifestation of the earthward drift of plasma sheet electrons in the equatorial plane associated with transient and localized convection electric fields; (2) the Pc4 pulsations are produced in the magnetosphere by plasma sheet ions in the plasmasphere. The localized convection fields produce a vortical motion of plasmas in the equatorial plane, which may initiate the N-S auroras and ionospheric loop currents in the auroral zone.

Equivalent currents associated with morning-sector geomagnetic Pc5 pulsations during auroral substorms

Abstract. Space and time variations of equivalent currents during morning-sector Pc5 pulsations (T ∼ 2–8 min) on 2 days (18 January and 19 February 2008) are studied in the context of substorm activity with THEMIS and MIRACLE ground-based instruments and THEMIS P3, P5, and P2 probes. These instruments covered the 22:00–07:00 magnetic local time during the analyzed events. In these cases abrupt changes in the Pc5 amplitudes, intensifications and/or weakenings, were recorded some minutes after auroral breakups in the midnight sector. We analyze three examples of Pc5 changes with the goal to resolve whether substorm activity can have an effect on Pc5 amplitude or not. In two cases (on 19 February) the most likely explanation for Pc5 amplitude changes comes from the solar wind (changes in the sign of interplanetary magnetic field Bz). In the third case (on 18 January) equivalent current patterns in the morning sector show an antisunward-propagating vortex which replaced the Pc5-related smaller vortices and consequently the pulsations weakened. We associate the large vortex with a field-aligned current system due to a sudden, although small, drop in solar wind pressure (from 1 to 0.2 nPa). However, the potential impact of midnight substorm activity cannot be totally excluded in this case, because enhanced fluxes of electrons with high enough energies (∼ 280 keV) to reach the region of Pc5 within the observed delay were observed by THEMIS P2 at longitudes between the midnight and morning-sector instrumentation.

A comprehensive analysis of the geomagnetic storms occurred during 18 February and 2 March 2014

The Geomagnetic storms are considered as one of the major natural hazards. Egyptian geomagnetic observatories observed multiple geomagnetic storms during 18 February to 2 March 2014. During this period, four interplanetary shocks successively hit the Earth’s magnetosphere, leading to four geomagnetic storms. The storm onsets occurred on 18, 20, 23 and 27 February. A non-substorm Pi2 pulsation was observed on 26 February. This Pi2 pulsation was detected in Egyptian observatories (Misallat and Abu Simbel), Kakioka station in Japan and Carson City station in US with nearly identical waveforms. Van Allen Probe missions observed non-compressional Pc4 pulsations on the recovery phase of the third storm. This Pc4 event is may be likely attributed to the decay of the ring current in the recovery phase.

Generation of resonant Alfvén waves in the auroral oval

Abstract. Many independent observations have shown that resonant field line Pc5 pulsations (typical periods of about several minutes) are preferably excited inside the auroral oval. Consideration of the most evident interpretation schemes shows that there is no simple explanation of this effect. Here we consider theoretically the generation of toroidal Pc5 Alfvén waves by fluctuating magnetospheric field-aligned currents. It is shown that the Alfvén wave latitudinal structure has the same features as in the case of the classical field line resonance driven by external sources: a peak localized in latitude and a 180° phase shift across the resonant magnetic shell. This model suggests an additional mechanism of ultra-low-frequency (ULF) wave excitation which can operate at auroral latitudes.

Features of Pc5 pulsations in the geomagnetic field, auroral luminosity, and Riometer absorption

Simultaneous morning Pc5 pulsations (f ~ 3–5 mHz) in the geomagnetic field, aurora intensities (in the 557.7 and 630.0 nm oxygen emissions and the 471.0 nm nitrogen emission), and riometer absorption, were studied based on the CARISMA, CANMOS, and NORSTAR network data for the event of January 1, 2000. According to the GOES-8 satellite observations, these Pc5 geomagnetic pulsations are observed as incompressible Alfven waves with toroidal polarization in the magnetosphere. Although the Pc5 pulsation frequencies in auroras, the geomagnetic field, and riometer absorption are close to one another, stable phase relationships are not observed between them. Far from all trains of geomagnetic Pc5 pulsations are accompanied by corresponding auroral pulsations; consequently, geomagnetic pulsations are primary with respect to auroral pulsations. Both geomagnetic and auroral pulsations propagate poleward, and the frequency decreases with increasing geomagnetic latitude. When auroral Pc5 pulsations appear, the ratio of the 557.7/630.0 nm emission intensity sharply increases, which indicates that auroral pulsations result from not simply modulated particle precipitation but also an additional periodic acceleration of auroral electrons by the wave field. A high correlation is not observed between Pc5 pulsations in auroras and the riometer absorption, which indicates that these pulsations have a common source but different generation mechanisms. Auroral luminosity modulation is supposedly related to the interaction between Alfven waves and the region with the field-aligned potential drop above the auroral ionosphere, and riometer absorption modulation is caused by the scattering of energetic electrons by VLF noise pulsations.

Sporadic Geomagnetic Pulsations at Frequencies of up to 15 HZ in the Magnetic Storm of November 7–14, 2004: Features of the Amplitude and Polarization Spectra and their Connection with Ion–Cyclotron Waves in the Magnetosphere

We study the dynamics of the geomagnetic-pulsation spectra at unusually high frequencies (including the frequencies exceeding the Schumann resonance frequency 8 Hz), which were detected for the first time at the Novaya Zhizn’ midlatitude station (the McIlwain parameter L = 2.6) at the time of a strong magnetic storm on November 07–14, 2004. To interpret the observed pulsation frequencies, we used the data from the NOAA low-orbit satellites which recorded localized precipitations of energetic protons (with energies of 30 to 80 keV) and calculations of the singlepass cyclotron amplification of electromagnetic ion–cyclotron waves. Amplitude and polarization characteristics of the radiation spectra at frequencies of up to 15 Hz at the Novaya Zhizn’ and Lovozero stations (L = 5.2) are compared. It is shown that the magnetic field oscillations in the frequency range 7–15 Hz correlate with proton precipitations and proton auroras at geomagnetic latitudes 50°–57° (L = 2.42–3.37). It is also shown that for a high anisotropy of the pitch-angle distribution of the ring-current protons at such low geomagnetic latitudes, the frequency spectrum of observed high-frequency radiation agrees well with the calculated location of the maximum of the single-pass cyclotron amplification of electromagnetic ion–cyclotron waves. Analysis of the data and calculation results has led to the conclusion that inherently the recorded signals are a high-frequency counterpart of the Pc1 pulsations and are due to the generation of ion–cyclotron waves in the magnetosphere at unusually low latitudes, which are probably stipulated by the shift of the plasma pause to these latitudes during a strong magnetic storm.

МГД-волновод во внешней магнитосфере и механизмы его возбуждения

The geomagnetic field and plasma inhomogeneities in the outer equatorial part of the magnetosphere are responsible for the existence of the channel with low Alfven speeds, which extends from the nose to the far flanks of the magnetosphere, both in the morning and evening sectors. This channel serves as a waveguide in the fast magnetosonic waves. Travelling along the waveguide (i.e., in the azimuthal direction), an eigenmode undergoes evolution. Parameters of the waveguide vary along the way of the wave propagation and the eigenmode “adapts” to these parameters. Conditions of the Kelvin-Helmholtz instability change due to the variation of the solar wind speed along the magnetopause. Conditions of the penetration of solar wind hydromagnetic waves into the magnetosphere change due to the same variation. The wave penetration process turns to the overreflection regime, which sharply amplifies the pump level of the magnetospheric waveguide. The fast mode propagating along the waveguide is accompanied by the Alfven resonance deep within the magnetosphere. Oscillation energy dissipation takes place in the vicinity of the Alfven resonance. Along the magnetic field lines, the Alfven resonance is a standing Alfven wave; thus it reaches the ionosphere and the Earth’s surface. At the same time, no fast waveguide modes localized in the low Alfven speed channel can be observed on the Earth. Waveguide oscillations evolution is investigated in this paper both analytically and numerically taking into account all of the aforementioned factors as the oscilla-tions propagate from the nose to the tail of the magnetosphere. Spectral composition and spatial structure of the oscillations are found. The theory allows for a description of Pc3 and Pc5 pulsations – the most important magnetospheric pulsations. As such it follows that Pc3 are localized on the dayside of the magnetosphere, whereas Pc 5 are localized in the dawn-dusk sectors – in full agreement with the observations.

The width of the frequency spectrum of Pc1 geomagnetic pulsations in quiet and disturbed conditions

The relation of the frequency spectrum width of Pc1 geomagnetic pulsations with magnetospheric parameters was theoretically studied. An analytical expression was obtained and numerical calculations were conducted. The formula for the increment (γ) was shown to include an important magnetospheric parameter, the ratio (V A/U ‖) of the Alfven velocity to the average velocity of energetic protons along the field line, which significantly affects the frequency spectrum width. The calculations show that the normalized width of the Pc1 frequency spectrum (x 0 = ωout/Ω i = V A/U ‖) decreases with decreasing (V A/U ‖). With increasing anisotropy of energetic protons A = T ⊥/T ‖ − 1, the normalized width of Pc1 frequency spectrum is also decreased for a fixed value of the parameter (V A/U ‖). These conclusions have been confirmed by analysis of spectrograms conducted in 2005–2010 of ground-based observations of Pc1 on the Scandinavian network of induction magnetometers. Analysis of these data showed that Pc1 pulsations were observed in a narrow frequency band of around 0.2–0.4 Hz, with a central frequency of oscillations in the series f ∼ 0.5–0.7 Hz, in more than 90% of cases in magnetically quiet conditions (Kp < 2). This corresponds to their generation beyond the plasmasphere. In more disturbed conditions (Kp ∼ 2–3), the central frequency of Pc1 oscillations was almost twice greater (∼ 1.0–1.2 Hz) and the spectral width was ∼ 0.5–0.7 Hz, which makes it possible to suggest that they can be generated within the plasmasphere.

Specific features of the generation of long-periodic geomagnetic pulsations in the event on June 25, 2008

The event of the generation of long-periodic geomagnetic Pc5 pulsations occurring on June 25, 2008 under undisturbed geomagnetic conditions is considered using data from ground-based and satellite observations. Two maxima in the frequency bands 1.9–2.2 and 2.9–3.3 mHz are revealed in the spectra of oscillations in the interplanetary medium, magnetosphere, and on the Earth. Pulsations with frequency 1.9–2.2 mHz were detected in all time sectors from low to high latitudes. The maximum amplitude of the oscillations was recorded in the longitude sector coinciding with position of the ion foreshock region. It is assumed that the event under consideration was caused both by variations of the solar wind dynamic pressure (amplified by the foreshock region) and by the direct penetration of ultra-low-frequency waves from solar wind to the magnetosphere.

An investigation of the nature of a Pc5 pulsation event using SuperDARN and magnetometer data

Pc5 pulsations are global magnetohydrodynamic events in the magnetosphere. We employed an Automated Pulsation Finder program to identify significant Pc5 pulsation events in SuperDARN radar data. The event presented here was visible in the Goose Bay, Saskatoon and Þykkvibaer high-frequency radars, located in the northern polar region. These observations were coordinated with magnetometers within their field of view. These two instrument types – radars and magnetometers – complement each other. These observations represent a significant fraction of the globe in longitude. Pulsation studies of this nature are rare in the literature. Combining these two instrument types, we investigated the nature of the pulsation and determined its qualitative polarisation characteristics. A complex demodulation technique was employed to determine amplitude and phase relationships between field components observed by the radars and magnetometers, which, in turn, afforded resolution of other characteristics of pulsations, such as wave number and phase velocity. The results are discussed in the context of the magnetohydrodynamic theory of magnetic pulsations, speculating on its generation mechanism. Investigation of this mechanism will be the subject of a future publication.

On the origin of burst Pc1 pulsations produced in interaction with an oblique interplanetary shock

We examined the features of bursts of unstructured Pc1 geomagnetic pulsations recorded with period in the range T=2–5s on 19 November 2007 using simultaneous observations by the geosynchronous satellites GOES-10, 11, 12, a constellation of high-apogee satellites THEMIS and by the CARISMA ground-based network of magnetometers. The pulsation excitation resulted from contact of an oblique interplanetary shock wave (ISW) with the magnetosphere. At geosynchronous orbit, we found eastward drift of the source of Pc1 bursts observed first by GOES-11 (~09MLT), then by GOES-12 (~13MLT) and, finally, by GOES-10 (~14MLT). Ground-based observatories with ~40° longitudinal separation observed the excitation of oscillations with a delay to the west and east as compared with the median Fort Simpson observatory. An increase in frequency, seen at the sharp leading edge of oscillations, lasted for about 150s. We determined the propagation velocity of the pulsations׳ source from the difference between the first observations of the pulsations by the satellites and at the Earth. In order to interpret the observed patterns of pulsation we considered different mechanisms such as: (1) Eastward drifting clouds of energetic electrons accelerated due to compression of the magnetosphere; (2) Plasmaspheric bulges (or detached plasma); (3) Magnetopause surface waves generated in the region of contact with the ISW and resulting in undulation of the region of developing the cyclotron instability.