Ultralow Frequency Waves Research Articles

Properties of frequency distribution of Pc5-range pulsations observed with the Ekaterinburg decameter radar in the nightside ionosphere

A statistical study of waves in the magnetosphere registered with the midlatitude coherent decameter radar located near Ekaterinburg (EKB), Russia is presented. The radar monitors ionospheric flow velocities whose small-scale variations are evoked by magnetosphere ultra low frequency (ULF) waves of Pc5-range. Data from 16 events observed during 7 months in 2014 and 2015 underwent wavelet analysis. Frequencies of the oscillating components were compared with Alfvén eigenfrequencies, which were inferred from THEMIS and Van Allen spacecraft data. The comparison showed that only a minor part of the oscillations registered with the radar in the nightside ionosphere could be attributed to the Alfvén mode. A majority of the waves have lower frequencies, which do not show dependence on Alfvén eigenfrequency of a field line.

Electron Drift Resonance in the MHD‐Coupled Comprehensive Inner Magnetosphere‐Ionosphere Model

AbstractRelativistic electrons in the outer radiation belt are highly dynamic and respond to interplanetary solar wind structures interacting with the Earth's magnetic field. A known mechanism dictating electron dynamics is the drift‐resonant interaction with ultralow frequency (ULF) waves. The present work simulates the ring current and radiation belt electron populations in the bounce‐averaged, kinetic Comprehensive Inner Magnetosphere‐Ionosphere model coupled with the Block Adaptive Tree Solar Wind Roe‐type Upwind Scheme global magnetospheric magnetohydrodynamic (MHD) code using an idealized ULF wave solar wind density driver. ULF waves generated with 10 min periods (at 1.67 mHz frequencies) in the MHD model are characterized and the corresponding energization of electrons and radial transport of electron phase space density is presented. The drift‐resonant electron energy is determined in the simulation and is consistent with the electron resonance conditions in dipolar magnetic fields. The present results will be an important component of understanding inner magnetospheric dynamics and how these inner magnetospheric populations interact with ULF waves resulting from interplanetary solar wind structures.

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

One of the most important issues in space physics is to identify the dominant processes that transfer energy from the solar wind to energetic particle populations in Earth’s inner magnetosphere. Ultra-low-frequency (ULF) waves are an important consideration as they propagate electromagnetic energy over vast distances with little dissipation and interact with charged particles via drift resonance and drift-bounce resonance. ULF waves also take part in magnetosphere-ionosphere coupling and thus play an essential role in regulating energy flow throughout the entire system. This review summarizes recent advances in the characterization of ULF Pc3-5 waves in different regions of the magnetosphere, including ion and electron acceleration associated with these waves.

Waves from the Sun: to the 100th anniversary of V.A. Troitskaya’s birth

It has been one hundred years since the birth of the outstanding scientist Professor V.A. Troitskaya. Her remarkable achievements in solar-terrestrial physics are widely known. For many years, Valeria A. Troitskaya was the President of the International Association of Geomagnetism and Aeronomy. This article deals with only one aspect of the multifaceted creative activity of V.A. Troitskaya. It relates to the problem of sources of ultra-low frequency (ULF) electromagnetic oscillations and waves outside Earth’s magnetosphere. We were fortunate to work under the leadership of V.A. Troitskaya on this problem. In this paper, we briefly describe the history from the emergence of the idea of the extramagnetospheric origin of dayside permanent ULF oscillations in the late 1960s to the modern quest made by ground and satellite means for ULF waves excited by solar surface oscillations propagating in the interplanetary medium and reaching Earth.

Low‐Energy (<200 eV) Electron Acceleration by ULF Waves in the Plasmaspheric Boundary Layer: Van Allen Probes Observation

AbstractWe report observational evidence of cold plasmaspheric electron (<200 eV) acceleration by ultralow frequency (ULF) waves in the plasmaspheric boundary layer on 10 September 2015. Strongly enhanced cold electron fluxes in the energy spectrogram were observed along with second harmonic mode waves with a period of about 1 min which lasted several hours during two consecutive Van Allen Probe B orbits. Cold electron (<200 eV) and energetic proton (10–20 keV) bidirectional pitch angle signatures observed during the event are suggestive of the drift‐bounce resonance mechanism. The correlation between enhanced energy fluxes and ULF waves leads to the conclusions that plasmaspheric dynamics is strongly affected by ULF waves. Van Allen Probe A and B, GOES 13, GOES 15, and MMS 1 observations suggest that ULF waves in the event were strongest on the duskside magnetosphere. Measurements from MMS 1 contain no evidence of an external wave source during the period when ULF waves and injected energetic protons with a bump‐on‐tail distribution were detected by Van Allen Probe B. This suggests that the observed ULF waves were probably excited by a localized drift‐bounce resonant instability, with the free energy supplied by substorm‐injected energetic protons. The observations by Van Allen Probe B suggest that energy transfer between particle species in different energy ranges can take place through the action of ULF waves, demonstrating the important role of these waves in the dynamical processes of the inner magnetosphere.

Identifying Ultra Low Frequency Waves in the Lunar Plasma Environment Using Trajectory Analysisand Resonance Conditions.

Recent studies show that localized crustal magnetic fields on the lunar surface can reflect a significant portion of the incoming solar wind protons. These reflected ions can drive a wide range of plasma waves. It is difficult to determine the intrinsic properties of low-frequency waves with single-spacecraft observations, which can be heavily Doppler shifted. We describe a technique to combine trajectory analysis of reflected protons with the Doppler shift and resonance conditions to identify ultralow-frequency waves at the Moon. On 31 January 2014 plasma waves were detected by one of the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes as it approached the lunar wake; these waves were not detected by the second ARTEMIS probe located upstream in the undisturbed solar wind. The observed waves had a frequency below the local ion cyclotron frequency and had right-hand circular polarization in the reference frame of the Moon. By solving the Doppler shift and the cyclotron resonance equations, we determined the conditions for reflected ions to excite the observed waves. Simulated trajectories of reflected ions correspond to ARTEMIS ion observations and support the hypothesis that reflected ions are the primary driver of the waves. By combining trajectory analysis with the resonance conditions, we identify scenarios where ions that satisfy the resonance conditions are present in the right location to generate the observed waves. Using this method, we can uniquely identify the observed waves as upstream propagating right-hand polarized waves, subject to the assumption that they are generated by cyclotron resonance with ions.

Волны от Солнца: к 100-летию со дня рождения В.А. Троицкой

It has been one hundred years since the birth of the outstanding scientist Professor V.A. Troitskaya. Her remarkable achievements in solar-terrestrial physics are widely known. For many years, Valeria Alekseevna was the President of the International Association of Geomagnetism and Aeronomy. This article deals with only one aspect of the multifaceted creative activity of V.A. Troitskaya. It relates to the problem of sources of ultra-low frequency (ULF) electromagnetic oscillations and waves outside Earth’s magnetosphere. We were fortunate to work under the leadership of V.A. Troitskaya on this problem. In this paper, we briefly describe the history from the emergence of the idea of the extramagnetospheric origin of dayside permanent ULF oscillations in the late 1960s to the modern quest made by ground and satellite means for ULF waves excited by solar surface oscillations propagating in the interplanetary medium and reaching Earth.

Ultra low frequency waves at Venus: Observations by the Venus Express spacecraft

The generation of waves with low frequencies (below 100 mHz) has been observed in the environment of most bodies in the solar system and well studied at Earth. These waves can be generated either upstream of the body in the solar wind by ionization of planetary exospheres or ions reflected from a bow shock or in the magnetosheath closer to the magnetic barrier. For Mars and Venus the waves may have special importance since they can contribute to the erosion of the ionopause and by that enhance atmospheric escape. While over the past years many case studies on wave phenomena observed at Venus have been published most statistical studies have been based on magnetic observations only. On the other hand the generation mechanisms and transport of these waves through the magnetosphere can only be quantified using both magnetic and particle observations. We use the long time observations of Venus Express (2006–2014) to determine the predominant processes and transport parameters. First we demonstrate the analysis methods in four case studies, then we present a statistical analysis by determining transport ratios from the complete Venus Express dataset. We find that Alfvenic waves are very dominant (>80%) in the solar wind and in the core magnetosheath. Fast waves are observed mainly at the bow shock (around 40%) but also at the magnetic barrier where they may be most important for the energy transfer into the ionosphere. Their occurrence in the magnetotail may be an artifact of the detection of individual plasma jets in this region. Slow mode waves are rarely dominating but occur with probability of about 10% at the bow shock and in the pile-up-region. Mirror mode waves have probability <20% in the magnetosheath slightly increasing towards the pile-up-boundary.

Investigation of ∼ 20–40 mHz ULF waves and their driving mechanisms in Mercury's dayside magnetosphere

Abstract. Ultra-low-frequency (ULF) waves in the ∼ 20–40 mHz range are frequently observed in the Mercury magnetosphere using Mercury Surface Space Environment Geochemistry, and Ranging (MESSENGER) magnetic field data. The majority of these waves have very similar characteristics to the waves likely driven by Kelvin–Helmholtz (KH) ULF waves (which are retained as a subset of the wave events studied in this paper) identified in a previous study. Significant ULF wave activity is observed in the dawn sector of the magnetosphere. This indicates that Mercury KH waves may be more common between 6 and 12 magnetic local time than previously predicted and that magnetospheric ULF waves in the frequency band ∼ 20–40 mHz can be used as a detection tool for Hermean KH waves.

The Role of Solar Wind Structures in the Generation of ULF Waves in the Inner Magnetosphere

The plasma of the solar wind incident upon the Earth’s magnetosphere can produce several types of geoeffective events. Among them, an important phenomenon consists of the interrelation of the magnetospheric–ionospheric current systems and the charged-particle population of the Earth’s Van Allen radiation belts. Ultra-low-frequency (ULF) waves resonantly interacting with such particles have been claimed to play a major role in the energetic particle flux changes, particularly at the outer radiation belt, which is mainly composed of electrons at relativistic energies. In this article, we use global magnetohydrodynamic simulations along with in situ and ground-based observations to evaluate the ability of two different solar wind transient (SWT) events to generate ULF (few to tens of mHz) waves in the equatorial region of the inner magnetosphere. Magnetic field and plasma data from the Advanced Composition Explorer (ACE) satellite were used to characterize these two SWT events as being a sector boundary crossing (SBC) on 24 September 2013, and an interplanetary coronal mass ejection (ICME) in conjunction with a shock on 2 October 2013. Associated with these events, the twin Van Allen Probes measured a depletion of the outer belt relativistic electron flux concurrent with magnetic and electric field power spectra consistent with ULF waves. Two ground-based observatories apart in 90∘ longitude also showed evidence of ULF-wave activity for the two SWT events. Magnetohydrodynamic (MHD) simulation results show that the ULF-like oscillations in the modeled electric and magnetic fields observed during both events are a result from the SWT coupling to the magnetosphere. The analysis of the MHD simulation results together with the observations leads to the conclusion that the two SWT structures analyzed in this article can be geoeffective on different levels, with each one leading to distinct ring current intensities, but both SWTs are related to the same disturbance in the outer radiation belt, i.e. a dropout in the relativistic electron fluxes. Therefore, minor disturbances in the solar wind parameters, such as those related to an SBC, may initiate physical processes that are able to be geoeffective for the outer radiation belt.

Charged particle behavior in localized ultralow frequency waves: Theory and observations

AbstractThe formation and variability of the Van Allen radiation belts are highly influenced by charged particles accelerated via drift‐resonant interactions with ultralow frequency (ULF) waves. In the prevailing theory of drift resonance, the ULF wave amplitude is assumed independent of magnetic longitude. This assumption is not generally valid in Earth's magnetosphere, as supported by numerous observations that point to the localized nature of ULF waves. Here we introduce a longitude dependence of the ULF wave amplitude, achieved via a von Mises function, into the theoretical framework of ULF wave‐particle drift resonance. To validate the revised theory, the predicted particle signatures are compared with observational data through a best fit procedure. It is demonstrated that incorporation of nonlocal effects in drift‐resonance theory provides an improved understanding of charged particle behavior in the inner magnetosphere through the intermediary of ULF waves.

Pi2 Pulsations During Extremely Quiet Geomagnetic Condition: Van Allen Probe Observations

A ultra low frequency (ULF) wave, Pi2, has been reported to occur during periods of extremely quiet magnetospheric and solar wind conditions. And no statistical study on the Pi2 has been performed during extremely quiet conditions, using satellite observations to the author’s knowledge. Also Pi2 pulsations in the space fluxgate magnetometers near perigee failed to attract scientist’s attention previously. In this paper, Pi2 pulsations detected by the Van Allen probe satellites (VAP-A &amp; VAP-B) were investigated statistically. During the period from October 2012 to December 2014, ninety six Pi2 events were identified using VAP when Kp = 0 while using Kakioka (KAK, L = 1.23) as a reference ground station. Seventy five events had high coherence between VAP-Bz and H components at KAK station. As a result, it was found that 77 % of the events had power spectra between 5 and 12 mHz, which differs from the regular Pi2 band range of from 6.7 to 25 mHz. In addition, it was shown that it is possible to observe Pi2 pulsations from space fluxgate magnetometers near perigee. Twenty two clean Pi2 pulsations were found where L &lt; 4 and four examples of Pi2 oscillations at different L shells are presented in this paper.

Interaction of the solar wind with comets: a Rosetta perspective.

The Rosetta mission provides an unprecedented possibility to study the interaction of comets with the solar wind. As the spacecraft accompanies comet 67P/Churyumov–Gerasimenko from its very low-activity stage through its perihelion phase, the physics of mass loading is witnessed for various activity levels of the nucleus. While observations at other comets provided snapshots of the interaction region and its various plasma boundaries, Rosetta observations allow a detailed study of the temporal evolution of the innermost cometary magnetosphere. Owing to the short passage time of the solar wind through the interaction region, plasma instabilities such as ring--beam and non-gyrotropic instabilities are of less importance during the early life of the magnetosphere. Large-amplitude ultra-low-frequency (ULF) waves, the ‘singing’ of the comet, is probably due to a modified ion Weibel instability. This instability drives a cross-field current of implanted cometary ions unstable. The initial pick-up of these ions causes a major deflection of the solar wind protons. Proton deflection, cross-field current and the instability induce a threefold structure of the innermost interaction region with the characteristic Mach cone and Whistler wings as stationary interaction signatures as well as the ULF waves representing the dynamic aspect of the interaction.This article is part of the themed issue ‘Cometary science after Rosetta’.

Geomagnetic activity and local time dependence of the distribution of ultra low-frequency wave power in azimuthal wavenumbers, &amp;lt;i&amp;gt;m&amp;lt;/i&amp;gt;

Abstract. The azimuthal wavenumber m of ultra low-frequency (ULF) waves in the magnetosphere is a required parameter in the calculations of the diffusion rates of energetic electrons and protons in the magnetosphere, as electrons and protons of drift frequency ωd have been shown to radially diffuse due to resonant interaction with ULF waves of frequency ω = mωd. However, there are difficulties in estimating m, due to lack of multipoint measurements. In this paper we use magnetic field measurements at geosynchronous orbit to calculate the cross-spectrogram power and phase differences between time series from magnetometer pairs. Subsequently, assuming that ULF waves of a certain frequency and m would be observed with a certain phase difference between two azimuthally aligned magnetometers, the fraction of the total power in each phase difference range is calculated. As part of the analysis, both quiet-time and storm-time distributions of power per m number are calculated, and it is shown that during active times, a smaller fraction of total power is confined to lower m than during quiet times. It is also shown that in the dayside region, power is distributed mostly to the lowest azimuthal wavenumbers m = 1 and 2, whereas on the nightside it is more equally distributed to all m that can be resolved by the azimuthal separation between two spacecraft.

Cosmic radio noise absorption in the high‐latitude ionosphere during solar wind high‐speed streams

AbstractThe effect of solar wind high‐speed streams (HSSs) on energetic particle precipitation at auroral and subauroral latitudes (L = 3.8–5.7) is studied by using cosmic noise absorption (CNA) data measured by the Finnish riometer chain during 95 HSS events occurring between 2006 and 2008. The data are divided into “long” and “short” HSS events, depending on whether the maximum solar wind speed is reached within 24 h or later after the arrival of the corotating interaction region (CIR) at the bow shock. We find that CNA is more frequent during long events and extends to subauroral latitudes, unlike during short events. CNA is observed for at least 4 days after the CIR arrival during long events, and only 3 days during short events. In addition, CNA is divided into three categories, depending on whether it is associated with local substorm activity, with ultralow frequency (ULF) wave activity, or neither of these. Substorm‐type CNA dominates in the 21–06 magnetic local time (MLT) sector, while ULF‐type CNA dominates from morning to afternoon and follows the daily SYM‐H index variations. This indicates the importance of having energetic (E &gt; 30 keV) electrons available for interacting with the very low frequency (VLF) chorus waves, which may be modulated by ULF waves. Finally, we find a correlation between ULF‐type CNA on the dayside and substorm activity on the nightside with a 30 min delay, suggesting that substorm injections energize electrons, which then drift to the morning and dayside, where they precipitate into the ionosphere.

A multispacecraft event study of Pc5 ultralow‐frequency waves in the magnetosphere and their external drivers

AbstractWe investigate a quiet time event of magnetospheric Pc5 ultralow‐frequency (ULF) waves and their likely external drivers using multiple spacecraft observations. Enhancements of electric and magnetic field perturbations in two narrow frequency bands, 1.5–2 mHz and 3.5–4 mHz, were observed over a large radial distance range from r ~ 5 to 11 RE. During the first half of this event, perturbations were mainly observed in the transverse components and only in the 3.5–4 mHz band. In comparison, enhancements were stronger during the second half in both transverse and compressional components and in both frequency bands. No indication of field line resonances was found for these magnetic field perturbations. Perturbations in these two bands were also observed in the magnetosheath, but not in the solar wind dynamic pressure perturbations. For the first interval, good correlations between the flow perturbations in the magnetosphere and magnetosheath and an indirect signature for Kelvin‐Helmholtz (K‐H) vortices suggest K‐H surface waves as the driver. For the second interval, good correlations are found between the magnetosheath dynamic pressure perturbations, magnetopause deformation, and magnetospheric waves, all in good correspondence to interplanetary magnetic field (IMF) discontinuities. The characteristics of these perturbations can be explained by being driven by foreshock perturbations resulting from these IMF discontinuities. This event shows that even during quiet periods, K‐H‐unstable magnetopause and ion foreshock perturbations can combine to create a highly dynamic magnetospheric ULF wave environment.

Growth rate measurement of ULF waves in the ion foreshock

AbstractWe report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock. We employ the two identical ARTEMIS spacecraft orbiting the Moon to characterize crescent‐shaped reflected ion beams and relatively monochromatic ULF waves. The event presented here features spacecraft separation of ∼2.5 Earth radii (0.9 ± 0.1 wavelengths) in the solar wind flow direction along a nearly radial interplanetary magnetic field. The ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are also within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from resonant beam instability theory and prior satellite measurements. These results will inform future missions near bow and interplanetary shocks as well as future nonlinear studies related to turbulence and dissipation in the heliosphere.

Comparing and contrasting dispersionless injections at geosynchronous orbit during a substorm event

AbstractParticle injections in the magnetosphere transport electrons and ions from the magnetotail to the radiation belts. Here we consider generation mechanisms of “dispersionless” injections, namely, those with simultaneous increase of the particle flux over a wide energy range. In this study we take advantage of multisatellite observations which simultaneously monitor Earth's magnetospheric dynamics from the tail toward the radiation belts during a substorm event. Dispersionless injections are associated with instabilities in the plasma sheet during the growth phase of the substorm, with a dipolarization front at the onset and with magnetic flux pileup during the expansion phase. They show different spatial spread and propagation characteristics. Injection associated with the dipolarization front is the most penetrating. At geosynchronous orbit (6.6 RE), the electron distributions do not have a classic power law fit but instead a bump on tail centered on ∼120 keV during dispersionless electron injections. However, electron distributions of injections associated with magnetic flux pileup in the magnetotail (13 RE) do not show such a signature. We surmise that an additional resonant acceleration occurs in between these locations. We relate the acceleration mechanism to the electron drift resonance with ultralow frequency waves localized in the inner magnetosphere.

Second harmonic poloidal waves observed by Van Allen Probes in the dusk‐midnight sector

AbstractThis paper presents observations of ultralow‐frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred 2 days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the premidnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single‐spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be ∼100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (∼0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy‐dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift‐bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift‐bounce resonance mechanism with free energy fed by the inward radial gradient of ∼80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit.

Van Allen Probes observation of a 360° phase shift in the flux modulation of injected electrons by ULF waves

AbstractWe present Van Allen Probe observation of drift‐resonance interaction between energetic electrons and ultralow frequency (ULF) waves on 29 October 2013. Oscillations in electron flux were observed at the period of ∼450 s, which is also the dominant period of the observed ULF magnetic pulsations. The phase shift of the electron fluxes (∼50 to 150 keV) across the estimated resonant energy (∼104 keV) is ∼360°. This phase relationship is different from the characteristic 180° phase shift as expected from the drift‐resonance theory. We speculate that the additional 180° phase difference arises from the inversion of electron phase space density (PSD) gradient, which in turn is caused by the drift motion of the substorm injected electrons. This PSD gradient adjusts the characteristic particle signatures in the drift‐resonance theory, which indicates a coupling effect between the magnetotail and the radiation belt and helps to better understand the wave‐particle interaction in the magnetosphere.