Ultra-low Frequency Oscillation Research Articles

The influence of solar wind variability on magnetospheric ULF wave power

Abstract. Magnetospheric ultra-low frequency (ULF) oscillations in the Pc 4–5 frequency range play an important role in the dynamics of Earth's radiation belts, both by enhancing the radial diffusion through incoherent interactions and through the coherent drift-resonant interactions with trapped radiation belt electrons. The statistical distributions of magnetospheric ULF wave power are known to be strongly dependent on solar wind parameters such as solar wind speed and interplanetary magnetic field (IMF) orientation. Statistical characterisation of ULF wave power in the magnetosphere traditionally relies on average solar wind–IMF conditions over a specific time period. In this brief report, we perform an alternative characterisation of the solar wind influence on magnetospheric ULF wave activity through the characterisation of the solar wind driver by its variability using the standard deviation of solar wind parameters rather than a simple time average. We present a statistical study of nearly one solar cycle (1996–2004) of geosynchronous observations of magnetic ULF wave power and find that there is significant variation in ULF wave powers as a function of the dynamic properties of the solar wind. In particular, we find that the variability in IMF vector, rather than variabilities in other parameters (solar wind density, bulk velocity and ion temperature), plays the strongest role in controlling geosynchronous ULF power. We conclude that, although time-averaged bulk properties of the solar wind are a key factor in driving ULF powers in the magnetosphere, the solar wind variability can be an important contributor as well. This highlights the potential importance of including solar wind variability especially in studies of ULF wave dynamics in order to assess the efficiency of solar wind–magnetosphere coupling.

Multipoint visualization of ULF oscillations using the Super Dual Auroral Radar Network

We present a technique for the automatic detection of Pc5 (150 s to 600 s period) ultralow frequency (ULF) pulsations in ground and ionospheric backscatter from the Super Dual Auroral Radar Network (SuperDARN). This new technique enables rapid identification and visualization of ULF oscillations over the very wide geographical coverage of SuperDARN. The technique detects ULF oscillations using the Lomb-Scargle periodogram method, which provides a natural test for periodic behavior against the null hypothesis of a pure noise distribution. This does not require any interpolation across data gaps and is thus appropriate for SuperDARN data. We demonstrate the application of the technique to SuperDARN data for March 2014 and find that Pc5 pulsations are frequently observed by multiple radars simultaneously. A preliminary investigation using data from all Northern Hemisphere SuperDARN radars indicates that Pc5 pulsation activity occurs most often on the nightside of the magnetosphere, with a mean frequency of about 2 mHz.

Direct observation of an interaction in magnetosphere

Researchers have now directly observed an interaction between poloidal waves, a type of ultralow‐frequency oscillation of the Earth's geomagnetic fields, and ions in the ring current, a band of charged particles drifting around Earth in the magnetosphere above the equator. This so‐called drift resonance wave‐particle interaction has been studied theoretically, and some modes of the interaction (in particular the second harmonics, similar to higher‐frequency vibrations of a vibrating string) have been reported in spacecraft observations, but the first fundamental mode resonances of this interaction had not been clearly observed.

Observations of ULF waves in the solar corona and in the solar wind at the Earth's orbit

Signs were looked for that would indicate a possible connection between plasma velocity oscillations observed in the region of solar coronal holes and magnetic field oscillations as recorded in the interplanetary medium. The problem appears to be quite important since the presence of large-scale ULF waves in the solar wind can increase geoeffectiveness of high speed streams in the interplanetary plasma. Observations of solar oscillations in the FeI 6569Å spectral line in a coronal hole were taken as a basis. The measurements were carried out at the Horizontal Automated Solar Telescope of the Sayan Solar Observatory. High speed solar wind stream ejected from the coronal hole reached the Earth's orbit after approximately 60h. The spectra of solar oscillations were compared with those of ultra low frequency (ULF) oscillations of the interplanetary magnetic field (IMF) at libration point L1. The oscillations were recorded with the ACE magnetometer when the leading edge of the high speed stream, bringing increased ULF wave activity, reached the Earth. The spectra of solar oscillations had a sharp peak at about 3.4–3.6mHz. The spectrum of the solar wind ULF oscillations is much more complex, being formed by different sources. Nevertheless, ULF oscillations of the IMF often had peaks that were close in frequency to those of the solar oscillations. Analysis of the ULF wave spectra observed in the 92 high speed streams confirmed the presence of 3- and 5-min oscillations in the total wave spectrum. It is emphasized that the results cannot be regarded as proving a direct connection between solar oscillations and ULF waves at the Earth's orbit even though they do support such a possibility. Additional research is needed involving IMF wave trajectory calculations.

Evidence of accretion triggered oscillations in the pre-main-sequence interacting binary AK Sco

Abstract Pre-main sequence (PMS) binaries are surrounded by circumbinary discs from which matter falls on to both components. The material dragged from the circumbinary disc flows on to each star through independent streams channelled by the variable gravitational field. The action of the bar-like potential is most prominent in high eccentricity systems made of two equal mass stars. AK Sco is a unique PMS system composed of two F5 stars in an orbit with e = 0.47. Henceforth, it is an ideal laboratory to study matter infall in binaries and its role in orbit circularization. In this Letter, we report the detection of a 1.3 mHz ultra low-frequency oscillation in the ultraviolet light curve at periastron passage. This oscillation lasts 7 ks being most likely fed by the gravitational energy released when the stream's tails spiralling on to each star get in contact at periastron passage enhancing the accretion flow; this unveils a new mechanism for angular momentum loss during PMS evolution and a new type of interacting binary.

SuperDARN observations of the driver wave associated with FLRs

Ultra‐low frequency (ULF) field line resonances (FLRs) cause oscillations in F‐region plasma flows and can be detected in SuperDARN measured line‐of‐sight (l‐o‐s) velocities. In this paper, we characterize a ULF wave event with coordinated use of SuperDARN HF radars, optical instrumentation, ground based and space based magnetometers. On December 26, 2000 from 00:00–04:00 UT, the SuperDARN Pykkvibaer radar observes first and second harmonic FLR signatures at 0.8 mHz, while the Kodiak and Hankasalmi radars simultaneously observe the driver wave on open field lines at exactly the same 0.8 mHz frequency. These observations show that SuperDARN can provide a diagnostic of MHD wave propagation on open field lines and potentially be used to monitor MHD wave transmission across the magnetopause, and through the outer regions of the magnetosphere. MHD waves in the outer magnetosphere, which couple to FLRs, are seldom observed, and as far as we know this is the first report of a ground based observation of the driver wave. The observation of first and second harmonic FLRs in SuperDARN data is a unique and useful observation in the sense that it supports the theoretical body of work on expected behavior of FLRs, and their potential use in estimating magnetospheric properties such as density and magnetic topology. During the time interval of interest, Geotail is in the solar wind just outside the dawn flank region, and observes clear oscillations in the IMF Bz component at 0.8 mHz. High coherence is shown between the Geotail Bz oscillations and the radar Doppler velocities at 0.8 mHz, confirming that the 0.8 mHz FLR harmonic and the driver wave on open field lines is directly driven by the 0.8 mHz oscillation in the solar wind. Discrete ULF oscillations in the solar wind as direct drivers of ULF waves in the magnetosphere is a controversial topic, and the results reported here add to a growing body of evidence in support of direct solar wind drivers of ULF waves.

Relationship between the fluxes of relativistic electrons at geosynchronous orbit and the level of ULF activity on the Earth’s surface and in the solar wind during the 23rd solar activity cycle

We present the results of a cross-correlation analysis made on the basis of Spearman’s rank correlation method. The quantities to correlate are daily values of the fluence of energetic electrons at a geosynchronous orbit, intensities of ground and interplanetary ultra-low-frequency (ULF) oscillations in the Pc5 range, and parameters of the solar wind. The period under analysis is the 23rd cycle of solar activity, 1996–2006. Daily (from 6 h to 18 h of LT) magnetic data at two diametrically opposite observatories of the Intermagnet network are taken as ground-based measurements. The fluxes of electrons with energies higher than 2 MeV were measured by the geosynchronous GOES satellites. The data of magnetometers and plasma instruments installed on ACE and WIND spacecraft were used for analysis of the solar wind parameters and of the oscillations of the interplanetary magnetic field (IMF). Some results elucidating the role played by interplanetary ULF waves in the processes of generation of magneospheric oscillations and acceleration of energetic electrons are obtained. Among them are (i) high and stable correlation of ground ULF oscillations with waves in the solar wind; (ii) closer link of mean daily amplitudes of both interplanetary and ground oscillations with ‘tomorrow’ values of the solar wind velocity than with current values; and (iii) correlation of the intensity of ULF waves in the solar wind, normalized to the IMF magnitude, with fluxes of relativistic electrons in the magnetosphere.

Improvement of Heart Rate Variability by Eurythmy Therapy After a 6-Week Eurythmy Therapy Training

Eurythmy therapy (EYT) is a mind-body therapy used in anthroposophic medicine. Recently, the authors were able to show that at comparable workloads, EYT stimulated heart rate variability (HRV) whereas conventional ergometer training attenuated HRV. Furthermore, a long-term improvement of quality of life (QoL) and stress coping strategies by EYT could be shown. This study aimed to evaluate the long-term effects of EYT training on HRV. A total of 23 healthy women (mean age = 44.57 ± 8.04 years) performed 10 hours of EYT over a period of 6 weeks. Electrocardiograms were recorded before and after the EYT trial. HRV was quantified by the extent of high (HF), low (LF), very low (VLF), and ultra low frequency (ULF) oscillations of heart rate. Autonomic regulation was significantly changed following the EYT training compared with baseline. Especially the proportion referring to the total power (P) of HF/P and LF/P increased, whereas ULF/P and (ULF+VLF)/P decreased after the training period. EYT shifted the autonomic regulation proportionally referring to the total power mainly caused by changes of ULF and VLF components of HRV. The LF and HF spectral components were also decreased following EYT while their proportion in relation to the total variance of the power spectrum was increased. The proportional enhancement of the higher frequency and the decrease of the ULF and VLF components are probably an indicator of an improvement of autonomic regulation processes by more relaxed physical activity after the EYT training, thus supporting the plausibility of the improved QoL and better stress coping strategies.

Experimental evidence for direct penetration of ULF waves from the solar wind and their possible effect on acceleration of radiation belt electrons

The event of March 12–19, 2009, when a moderately high-speed solar wind stream flew around the Earth’s magnetosphere and carried millihertz ultralow-frequency (ULF) waves, has been analyzed. The stream caused a weak magnetic storm (D st min = −28 nT). Since March 13, fluxes of energetic (up to relativistic) electrons started increasing in the magnetosphere. Comparison of the spectra of ULF oscillations observed in the solar wind and magnetosphere and on the Earth’s surface indicated that a stable common spectral peak was present at frequencies of 3–4 mHz. This fact is interpreted as evidence that waves penetrated directly from the solar wind into the magnetosphere. Possible scenarios describing the participation of oscillations in the acceleration of medium-energy (E > 0.6 MeV) and high-energy (E > 2.0 MeV) electrons in the radiation belt are discussed. Based on comparing the event with the moderate magnetic storm of January 21–22, 2005, we concluded that favorable conditions for analyzing the interaction between the solar wind and the magnetosphere are formed during a deep minimum of solar activity.

Solar wind driving of magnetospheric ULF waves: Field line resonances driven by dynamic pressure fluctuations

Several observational studies suggest that solar wind dynamic pressure fluctuations can drive magnetospheric ultralow‐frequency (ULF) waves on the dayside. To investigate this causal relationship, we present results from Lyon‐Fedder‐Mobarry (LFM) global, three‐dimensional magnetohydrodynamic (MHD) simulations of the solar wind–magnetosphere interaction. These simulations are driven with synthetic solar wind input conditions where idealized ULF dynamic pressure fluctuations are embedded in the upstream solar wind. In three of the simulations, a monochromatic, sinusoidal ULF oscillation is introduced into the solar wind dynamic pressure time series. In the fourth simulation, a continuum of ULF fluctuations over the 0–50 mHz frequency band is introduced into the solar wind dynamic pressure time series. In this numerical experiment, the idealized solar wind input conditions allow us to study only the effect of a fluctuating solar wind dynamic pressure, while holding all of the other solar wind driving parameters constant. We show that the monochromatic solar wind dynamic pressure fluctuations drive toroidal mode field line resonances (FLRs) on the dayside at locations where the upstream driving frequency matches a local field line eigenfrequency. In addition, we show that the continuum of upstream solar wind dynamic pressure fluctuations drives a continuous spectrum of toroidal mode FLRs on the dayside. The characteristics of the simulated FLRs agree well with FLR observations, including a phase reversal radially across a peak in wave power, a change in the sense of polarization across the noon meridian, and a net flux of energy into the ionosphere.

ULF waves excited by negative/positive solar wind dynamic pressure impulses at geosynchronous orbit

When a solar wind dynamic pressure impulse impinges on the magnetophere, ultra‐low‐frequency (ULF) waves can be excited in the magnetosphere and the solar wind energy can be transported from interplanetary space into the inner magnetosphere. In this paper, we have systematically studied ULF waves excited at geosynchronous orbit by both positive and negative solar wind dynamic pressure pulses. We have identified 270 ULF events excited by positive solar wind dynamic pressure pulses and 254 ULF events excited by negative pulses from 1 January 2001 to 31 March 2009. We have found that the poloidal and toroidal waves excited by positive and negative pressure pulses oscillate in a similar manner of phase near 06:00 local time (LT) and 18:00 LT, but in antiphase near 12:00 LT and 0:00 LT. Furthermore, it is shown that excited ULF oscillations are in general stronger around local noon than those in the dawn and dusk flanks. It is demonstrated that disturbances induced by negative impulses are weaker than those by positive ones, and the poloidal wave amplitudes are stronger than the toroidal wave amplitudes both in positive and negative events. The potential impact of these excited waves on energetic electrons at geosynchronous orbit has also been discussed.

Large-scale fluctuations of PSBL magnetic flux tubes induced by the field-aligned motion of highly accelerated ions

Abstract. We present a comprehensive analysis of magnetic field and plasma data measured in the course of 170 crossings of the lobeward edge of Plasma Sheet Boundary Layer (PSBL) in the Earth's magnetotail by Cluster spacecraft. We found that large-scale fluctuations of the magnetic flux tubes have been registered during intervals of propagation of high velocity field-aligned ions. The observed kink-like oscillations propagate earthward along the main magnetic field with phase velocities of the order of local Alfvén velocity and have typical wavelengths ~5–20 RE, and frequencies of the order of 0.004–0.02 Hz. The oscillations of PSBL magnetic flux tubes are manifested also in a sudden increase of drift velocity of cold lobe ions streaming tailward. Since in the majority of PSBL crossings in our data set, the densities of currents corresponding to electron-ion relative drift have been low, the investigation of Kelvin-Helmholtz (K-H) instability in a bounded flow sandwiched between the plasma sheet and the lobe has been performed to analyze its relevance to generation of the observed ultra-low frequency oscillations with wavelengths much larger than the flow width. The calculations have shown that, when plasma conditions are favorable for the excitation of K-H instability at least at one of the flow boundaries, kink-like ultra-low frequency waves, resembling the experimentally observed ones, could become unstable and efficiently develop in the system.

Surface waves and field line resonances: A THEMIS case study

Using magnetic field and plasma observations from four of the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, a surface wave at the dawn flank of the magnetopause was identified on 25 April 2007. The wave had an amplitude of about 1 RE and propagated tailward with a velocity of about 190–240 km/s. Its azimuthal wavelength was in the range of 9–11 RE. Magnetosheath velocity values support the hypothesis that this surface wave was generated by the Kelvin‐Helmholtz instability. Simultaneously, an ultralow‐frequency (ULF) pulsation event was detected by the fifth THEMIS spacecraft deeper in the magnetosphere, at a distance of about 5–7 RE from the magnetopause. This ULF event showed all the signatures predicted for waves generated by the classical field line resonance process. Frequency and phases of the detected ULF oscillations were found to be in close agreement with the magnetopause surface periodic disturbances. We conclude that the observed ULF wave event was most likely directly generated by the magnetopause surface wave and thus represents one of the few known manifestations of the classical field line resonance process in space directly observed, a conclusion made possible due to the special configuration of the THEMIS mission and its five spacecraft.

Resonant absorption of ULF waves at Mercury's magnetosphere

Ultra low frequency (ULF) waves, which are assumed to be standing waves on the field, are observed by the Mariner 10 spacecraft at Mercury. These waves are oscillating at 38% of the proton gyrofrequency. It is well known that heavy ions, such as Na+, are abundant in Mercury's magnetosphere. Because the presence of different ion species has an influence on plasma dispersion characteristics near the ion gyrofrequencies, magnetospheric eigenoscillations observed at Mercury with frequency in the gyrofrequency range require a multi‐fluid treatment for the plasma. Thus ULF waves at Mercury may have a distinct difference from typical ULF oscillations at Earth, which are often described in terms of magnetohydrodynamics. By adopting a multi‐fluid numerical wave model, we examine how magnetic eigenoscillations occur in Mercury's magnetosphere. Because protons and sodium ions are the main constituents at Mercury, we assume an electron‐proton‐sodium plasma in our model. Our results show: (1) the observed ULF waves are likely compressional waves rather than standing oscillations such as field line resonances (FLRs), (2) FLRs at Mercury are expected to occur when the ion‐ion hybrid and/or Alfvén resonance conditions are satisfied, (3) the magnetic field of FLRs at Mercury's magnetosphere oscillates linearly in the east‐west (azimuthal) meridian when the frequency is located between two ion gyrofrequencies, and (4) the resonance frequency enables us to estimate the local heavy ion concentration ratio.

ULF Doppler oscillations of L = 2.5 flux tubes

Plasmaspheric ducts may execute Doppler oscillations driven by propagating ultra‐low frequency (ULF) waves. We examined about 100 such events recorded over 1 year under magnetically quiet conditions at L = 2.5 using artificially generated whistler‐mode VLF signals and ground magnetometers. Joint peaks in the VLF Doppler and magnetometer spectra occurred at the frequency expected for ULF waves generated by the ion‐cyclotron instability in the upstream solar wind. The VLF Doppler shifts are most likely due to radial motion of flux tubes of a few kilometers, driven by the east‐west electric field of propagating ULF waves. When the frequencies match, the incoming wave energy also couples to standing poloidal and azimuthal field line oscillations, producing field line resonance signatures in both the D and H components on ground‐based magnetometers. The phases of the VLF and ULF oscillations are consistent with ionospheric rotation of the downgoing ULF wave field. Since the scale size of VLF flux tubes is significantly smaller than for ULF flux tubes, VLF Doppler observations can provide more precise spatial information on ULF wave fields in the plasmasphere. Furthermore, it should be possible to use ULF oscillations to monitor the formation of quarter wavelength mode standing field line oscillations when the conjugate ionospheres have different conductivities.

New branches of natural ultra low-frequency electromagnetic oscillations of the ionospheric resonator

New branches of natural ultra low-frequency electromagnetic oscillations of the ionospheric resonator

The nature of sectorial-hemispheric asymmetry in the distribution of photospheric structures on the quiet Sun

The poorly studied, spatially coherent large-scale structures on the solar surface characterized by sectorial-hemispheric asymmetry may be produced by ultralow-frequency nonlinear oscillations of the Sun. This requires that (i) the wavevector of the oscillation be normal to the rotational axis of the Sun, (ii) the radial component of the wavevector be about a factor of five greater than the component tangential to the spherical surface, (iii) the Brunt-Vaisala frequency in the oscillation-trapping region correspond to a period of ≈22 yr, and (iv) the sectorial number of the spatially periodic component of the oscillation be equal to three (m=3).

Magnetospheric Field Line Resonances: A Comparative Planetology Approach

Planetary magnetospheres are natural laboratories for many interesting plasma physical processes which are difficult to study under normal laboratory conditions. Among the major processes occurring in space plasmas are the reconnection phenomenon and field line resonances. This paper deals with the second of these processes. A field line resonance is the resonant coupling between an isotropic mode and an anisotropic mode in a magnetized plasma. Field line resonances allow us to understand many features of ultra-low frequency oscillations in the terrestrial magnetosphere, that is resonant mode coupling is the current paradigm to explain geomagnetic pulsations. A brief historical introduction as well as a physical description of the field line resonance is given. Resonant mode coupling is discussed for the terrestrial, Hermean (Mercury), and Kronian (Saturn) magnetospheres, which represent natural laboratories with different conditions such as size of the laboratory, the background plasma density and composition, and the strength of the magnetic field. This comparative approach allows a deeper insight into the critical coupling problem than an isolated study of the terrestrial field line resonance phenomenon. Finally, resonant mode coupling between elastic wave modes in the solid Earth is briefly tackled and compared with the magnetospheric situation.

Ultralow frequency oscillator using a pendulum with crossed suspension wires

A pendulum using four suspension wires in an X formation is described which can provide an arbitrarily long period in one dimension. Design equations are derived, and tested experimentally. A number of applications are outlined, including a low-frequency vibration isolation system for a laser-interferometric gravity wave observatory.

On energy dependent modulation of the ULF ion flux oscillations observed at small pitch angles

The characteristics of the ultralow frequency oscillations in the ion fluxes observed at small pitch angles by the National Oceanic and Atmospheric Administration detector telescopes on board ATS 6 are again examined. The present report concentrates on the dramatic variation of the flux modulations detected in various energy channels during a single event which occurred on February 18, 1975. The wave amplitude is observed to be larger in a higher energy channel with energies from 100 keV to 150 keV and to decrease toward the lower energy channels. The lowest‐energy protons (25–33 keV) in general are seldom seen to be oscillating, but in this event they display a low‐amplitude oscillation which is 180° out of phase with the adjacent channel. Such energy dependent modulation of the flux oscillation is thought to be a consequence of the wave particle resonant interaction. However, the prediction of the bounce resonant interaction is not consistent with the observations of both the energy dependent variation of the flux amplitudes and a 180° change in the oscillation phase in the adjacent low‐energy channels that occurred in the February 18, 1975, event. Since the shape of the undisturbed particle distribution can also determine the variation of the particle perturbation at various energies, the first‐order particle distribution derived in a homogeneous plasma with a uniform magnetic field is examined without any specification of the wave mode. When the average particle distribution during the wave observation is used together with a parallel wave electric field that presumably causes the flux modulation at small pitch angles, a reasonable agreement is found between the variation of flux modulation derived from the slope of the average particle distribution and that from the experimental observation.