Ultralow Frequency Pulsations Research Articles

Modelling of ULF Pc4 - Pc5 Pulsations with solar winds and geomagnetic storm for ULF earthquake precursor

The physical destruction and fatalities caused by earthquake events has compelled scientists to develop a method for predicting Earthquakes. It is almost impossible to detect earthquake events due to limited seismometer sensitivity; therefore, a non-seismological predictor was established by using the Ultra-Low Frequency (ULF) magnetic field as a potential earthquake precursor. ULF waves respond to magnetic activity that are transferred to Earth by solar winds, and become ultimately linked to earthquake events. The statistical correlation analysis is used between ULF Pc4 and Pc5 pulsation, solar wind parameters; IMF- component (Bz), solar wind speed (Vsw), solar wind pressure (Psw), solar wind input energy (IEsw), and near-equator geomagnetic storm (SYM/H ). Through a multiple regression modelled approach, the relationship between solar wind (SW) parameters (IMF-Bz, Vsw, Psw, IEsw) and near-equator geomagnetic index (SYM/H ) with ULF wave pulsations (Pc4 - Pc5) prior to an earthquake (EQ) event (magnitude, M = 3.0–6.0, depth, d< 100 km, epicentre distance from magnetometer station, r< 100 km) was determined. The solar wind and geomagnetic index parameters were obtained from OMNIWeb, where ULF pulsations (Pc4 - Pc5) were computed from the magnetometer Davao station (Philippines) (7.00oN, 125.40oE) located in a low latitude region. The ULF Pc5 band demonstrated the best fit for the ULF earthquake precursor model in relation to solar wind parameters (Vsw, Psw) and geomagnetic index (SYM/H ) with R2 and R2-adjusted values of (R2 =0.5011, R2-adj = 0.4940). The Vsw, Psw and SYM/H are major factors that contribute to the emission of ULF Pc5 prior to an earthquake, as determined by the stepwise regression method. This ULF earthquake precursor model is intended to address the problem seismologists face when predicting seismic events by assisting non-seismologists in locating the most effective EM-ULF wave bands in solar-terrestrial activities for detecting seismic events.

The Source, Significance, and Magnetospheric Impact of Periodic Density Structures Within Stream Interaction Regions

AbstractWe present several examples of magnetospheric ultralow frequency pulsations associated with stream interaction regions and demonstrate that the observed magnetospheric pulsations were also present in the solar wind number density. The distance of the solar wind monitor ranged from just upstream of Earth's bow shock to 261 RE, with a propagation time delay of up to 90 min. The number density oscillations far upstream of Earth are offset from similar oscillations observed within the magnetosphere by the advection timescale, suggesting that the periodic dynamic pressure enhancements were time stationary structures, passively advecting with the ambient solar wind. The density structures are larger than Earth's magnetosphere and slowly altered the dynamic pressure enveloping Earth, leading to a quasi‐static and globally coherent “forced‐breathing” of Earth's dayside magnetospheric cavity. The impact of these periodic solar wind density structures was observed in both magnetospheric magnetic field and energetic particle data. We further show that the structures were initially smaller‐amplitude, spatially larger, structures in the upstream slow solar wind and that the higher‐speed wind compressed and amplified these preexisting structures leading to a series of quasiperiodic density structures with periods typically near 20 min. Similar periodic density structures have been observed previously at L1 and in remote images, but never in the context of solar wind shocks and discontinuities. The existence of periodic density structures within stream interaction regions may play an important role in magnetospheric particle acceleration, loss, and transport, particularly for outer zone electrons that are highly responsive to ultralow frequency wave activity.

Calculating ultra-low-frequency wave power of the compressional magnetic field vs. &amp;lt;i&amp;gt;L&amp;lt;/i&amp;gt; and time: multi-spacecraft analysis using the Van Allen probes, THEMIS and GOES

Abstract. Ultra-low-frequency (ULF) pulsations are critical in radial diffusion processes of energetic particles, and the power spectral density (PSD) of these fluctuations is an integral part of the radial diffusion coefficients and of assimilative models of the radiation belts. Using simultaneous measurements from two Geostationary Operational Environmental Satellites (GOES) geosynchronous satellites, three satellites of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft constellation and the two Van Allen probes during a 10-day period of intense geomagnetic activity and ULF pulsations of October 2012, we calculate the PSDs of ULF pulsations at different L shells. By following the time history of measurements at different L it is shown that, during this time, ULF wave power is not enhanced uniformly throughout the magnetosphere but instead is mostly enhanced in the outer L shells, close to the magnetopause, and to a lesser extent in the inner magnetosphere, closer to the plasmapause. Furthermore, by using phase differences between two GOES geosynchronous satellite pairs, we estimate the daily-averaged distribution of power at different azimuthal wave numbers. These results can have significant implications in better defining the effect of radial diffusion in the phase space density of energetic particles for different wave numbers or L shell distributions of ULF power.

Van Allen Probes, THEMIS, GOES, and Cluster observations of EMIC waves, ULF pulsations, and an electron flux dropout

AbstractWe examined an electron flux dropout during the 12–14 November 2012 geomagnetic storm using observations from seven spacecraft: the two Van Allen Probes, Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐A (P5), Cluster 2, and Geostationary Operational Environmental Satellites (GOES) 13, 14, and 15. The electron fluxes for energies greater than 2.0 MeV observed by GOES 13, 14, and 15 at geosynchronous orbit and by the Van Allen Probes remained at or near instrumental background levels for more than 24 h from 12 to 14 November. For energies of 0.8 MeV, the GOES satellites observed two shorter intervals of reduced electron fluxes. The first interval of reduced 0.8 MeV electron fluxes on 12–13 November was associated with an interplanetary shock and a sudden impulse. Cluster, THEMIS, and GOES observed intense He+ electromagnetic ion cyclotron (EMIC) waves from just inside geosynchronous orbit out to the magnetopause across the dayside to the dusk flank. The second interval of reduced 0.8 MeV electron fluxes on 13–14 November was associated with a solar sector boundary crossing and development of a geomagnetic storm with Dst &lt; −100 nT. At the start of the recovery phase, both the 0.8 and 2.0 MeV electron fluxes finally returned to near prestorm values, possibly in response to strong ultralow frequency (ULF) waves observed by the Van Allen Probes near dawn. A combination of adiabatic effects, losses to the magnetopause, scattering by EMIC waves, and acceleration by ULF waves can explain the observed electron behavior.

Local time asymmetries and toroidal field line resonances: Global magnetospheric modeling in SWMF

AbstractWe present evidence of resonant wave‐wave coupling via toroidal field line resonance (FLR) signatures in the Space Weather Modeling Framework's (SWMF) global, terrestrial magnetospheric model in one simulation driven by a synthetic upstream solar wind with embedded broadband dynamic pressure fluctuations. Using in situ, stationary point measurements of the radial electric field along the 1500 LT meridian, we show that SWMF reproduces a multiharmonic, continuous distribution of FLRs exemplified by 180° phase reversals and amplitude peaks across the resonant L shells. By linearly increasing the amplitude of the dynamic pressure fluctuations in time, we observe a commensurate increase in the amplitude of the radial electric and azimuthal magnetic field fluctuations, which is consistent with the solar wind driver being the dominant source of the fast mode energy. While we find no discernible local time changes in the FLR frequencies despite large‐scale, monotonic variations in the dayside equatorial mass density, in selectively sampling resonant points and examining spectral resonance widths, we observe significant radial, harmonic, and time‐dependent local time asymmetries in the radial electric field amplitudes. A weak but persistent local time asymmetry exists in measures of the estimated coupling efficiency between the fast mode and toroidal wave fields, which exhibits a radial dependence consistent with the coupling strength examined by Mann et al. (1999) and Zhu and Kivelson (1988). We discuss internal structural mechanisms and additional external energy sources that may account for these asymmetries as we find that local time variations in the strength of the compressional driver are not the predominant source of the FLR amplitude asymmetries. These include resonant mode coupling of observed Kelvin‐Helmholtz surface wave generated Pc5 band ultralow frequency pulsations, local time differences in local ionospheric dampening rates, and variations in azimuthal mode number, which may impact the partitioning of spectral energy between the toroidal and poloidal wave modes.

Ionospheric irregularities during a substorm event: Observations of ULF pulsations and GPS scintillations

Plasma instability in the ionosphere is often observed as disturbances and distortions of the amplitude and phase of the radio signals, which are known as ionospheric scintillations. High-latitude ionospheric plasma, closely connected to the solar wind and magnetospheric dynamics, produces very dynamic and short-lived Global Positioning System (GPS) scintillations, making it challenging to characterize them. It is observed that scintillations in the high-latitude ionosphere occur frequently during geomagnetic storms and substorms. In addition, it is well known that Ultra Low Frequency (ULF) pulsations (Pi2 and Pi1B) are closely associated with substorm activity. This study reports simultaneous observations of Pi2 and Pi1B pulsations and GPS phase scintillations during a substorm using a newly designed Autonomous Adaptive Low-Power Instrument Platform (AAL-PIP) installed at the South Pole. The magnetic field and GPS data from the instruments appear to be associated in terms of their temporal and spectral features. Moreover, the scintillation events were observed near the auroral latitudes where Pi1B pulsations are commonly detected. The temporal, spectral and spatial association between the scintillation and geomagnetic pulsation events suggests that the magnetic field perturbations and enhanced electric fields caused by substorm currents could contribute to the creation of plasma instability in the high-latitude ionosphere, leading to GPS scintillations.

Resonant absorption with 2D variation of field line eigenfrequencies

Context. Resonant absorption, also known as field line resonance, can be used to describe coupling between fast and Alfven waves in non-uniform plasmas. Since the conditions for resonant absorption occur widely in astrophysics, it is applicable in many different contexts, all of which are united by their common physics. For example, resonant absorption is known to play a major role in the excitation of ultra-low frequency pulsations in the terrestrial magnetosphere and is also a leading explanation for the decay of fast kink oscillations of coronal loops. The occurrence of non-axisymmetric conditions in the magnetosphere, and observational evidence that coronal loops may possess fine transverse structure, highlight a need to consider equilibria that vary in two dimensions across the background magnetic field. Aims. We investigate the properties of resonant absorption when field line eigenfrequencies vary in two dimensions across the background magnetic field. We aim to place the theory on a firm mathematical footing and explore some of its key features. Methods. Using cold, linear, ideal MHD with a straight, uniform background magnetic field, we systematically obtain a complete analytic solution for behaviour at late times. This provides a framework from which the features of resonant absorption may be understood. The time-dependent problem is solved numerically, reproducing key features of the analytic solution. Results. Energy is deposited from a monochromatic fast wave as a phase mixing Alfven wave, in the vicinity of the resonant surface, at which the local field line eigenfrequency matches the frequency of the driver. A generalisation of the one dimensional phase mixing length to higher dimensions is suggested, and shown to successfully estimate the finest lengthscales in time-dependent simulations. The resonant Alfven wave is driven by gradients of the field aligned magnetic field perturbation, which is associated with the fast wave pressure. This leads to amplitude variations of the Alfven wave that can be used to reveal the spatial form of the fast wave.

Magnetopause surface oscillation frequencies at different solar wind conditions

Abstract. Statistical analyses of the magnetopause (MP) motion observed by THEMIS suggested that the MP oscillates preferably at some prominent (sometimes called "magic") frequencies, which were found to stand out also in ground-based and ionospheric measurements of geomagnetic ultra-low frequency pulsations. In this paper we present an extension to these statistical analyses of the observed MP oscillations examining their dependence on the prevalent interplanetary magnetic field (IMF), solar wind (SW) flow speed and cone angle conditions as well as their local time of occurrence. Our results show enhanced oscillation activity at these frequencies in the noon local time sector during periods of northward IMF, slow or moderate SW speed and low SW cone angle. This combination of conditions supports an interpretation in terms of standing Alfvénic Kruskal-Schwarzschild surface modes on the MP.

Electric and magnetic field observations of Pc4 and Pc5 pulsations in the inner magnetosphere: A statistical study

Ultralow frequency (ULF) waves in the Pc4 and Pc5 bands are ubiquitous in the inner magnetosphere and have significant influence on energetic particle transport. Investigating the source and characteristics of ULF waves also helps us better understand the interaction processes between the solar wind and the magnetosphere. However, owing to the limitation in instrumentation and spatial coverage, the distribution of ULF waves in local time and L shell in the inner magnetosphere has not been completely studied. The recent Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission provides unique opportunities to investigate the spatial distribution of ULF pulsations across different L shells with full local time coverage in the inner magnetosphere during solar minimum, with both electric and magnetic field measurements. Pc4 and Pc5 pulsations in the electric field observations are identified throughout 13 months of measurements, covering 24 h in local time. The pulsations are characterized as either toroidal or poloidal (including compressional) mode, depending on the polarization of the electric field. Subsequently, the pulsations' occurrence rate and wave power distributions in radial distance and local time are recorded. While the distributions of both Pc4 and Pc5 events vary greatly with radial distance and local time, Pc4 events are more frequently observed in the inner region around 5–6 RE and Pc5 events are more frequently observed in the outer region around 7–9 RE, which suggests that the field line resonance is an important source of the ULF waves. In the flank regions, the wave power is dominated by the toroidal mode, likely associated with the Kelvin‐Helmholtz (KH) instability. In the noon sector, the Pc5 ULF wave power is dominated by the poloidal mode, likely associated with the solar wind dynamic pressure disturbance. The KH instability plays an important role, suggested by our observations, during the solar minimum when the solar wind dynamic pressure is relatively weak. We also find that the contributions to the Pc5 ULF wave power from the external sources are larger than the contributions from the internal sources. These statistical results are important in characterizing Pc4 and Pc5 waves and also important for any efforts to model the transport of energetic particles in the magnetosphere.

Modulation of NTC frequencies by Pc5 ULF pulsations: Experimental test of the generation mechanism and magnetoseismology of the emitting surface

Nonthermal continuum (NTC) radiation is believed to be emitted by the conversion of an electrostatic wave into an electromagnetic one, which takes place at the Earth's magnetic equator. It is generally accepted that the frequency of the electrostatic wave at the source meets a local characteristic frequency placed in between two multiples of the electron cyclotron frequency, fce, which results in emission of a narrow band frequency element. In an event on 14 August 2003, we compare oscillations of the central frequency of distinct NTC frequency elements observed from Cluster orbiting near perigee, with simultaneous Pc5 Ultra Low Frequency (ULF) pulsations in the magnetic field observed from the same platform. The latter magnetic perturbations are interpreted as magnetohydrodynamic poloidal waves, where fundamental and second harmonic modes coexist. The NTC oscillation and the fundamental wave have similar periods, but are phase shifted by a quarter of period. From the correlation between both signals, and the proximity of the NTC source (localized via triangulation) with Cluster, we infer that the poloidal perturbations are spatially uniform between the source and the satellites. From the phase shift between signals, we conclude that the electrostatic wave which converts into NTC is mainly governed by the plasma density, affected by movements of the magnetic field lines. Furthermore, we demonstrate that the observations can be used to perform a magnetoseismology of the emitting surface. The results show a steepening of the plasmapause density profile near the satellites, which can be responsible for the generation of NTC emission.

Characterization of ultra low frequency (ULF) pulsations and the investigation of their possible source

Abstract. In this paper we present the results from the observation of ultra low frequency (ULF) pulsations in the Doppler velocity data from SuperDARN HF radar located at Goose Bay (61.94° N, 23.02° E, geomagnetic). Fourier spectral techniques were used to determine the spectral content of the data and the results show Pc 5 ULF pulsations (with a frequency range of 1 to 4 mHz) where the magnetic field lines were oscillating at discrete frequencies of about 1.3 and 1.9 mHz. These pulsations are classified as field lines resonance (FLR) since the 1.9 mHz component exhibited an enhancement in amplitude with an associated phase change of approximately 180° across a resonance latitude of 71.3°. The spatial and temporal structure of the ULF pulsations was examined by investigating their instantaneous amplitude which was calculated as the amplitude of the analytic signal. The results presented a full field of view which exhibit pulsations activity simultaneously from all beams. This representation shows that the peak amplitude of the 1.9 mHz component was observed over the longitudinal range of 13°. The temporal structure of the pulsations was investigated from the evolution of the 1.9 mHz component and the results showed that the ULF pulsations had a duration of about 1 h. Wavelet analysis was used to investigate solar wind as a probable source of the observed ULF pulsations. The time delay compared well with the solar wind travel time estimates and the results suggest a possible link between the solar wind and the observed pulsations. The sudden change in dynamic pressure also proved to be a possible source of the observed ULF pulsations.

Magnetospheric cavity modes driven by solar wind dynamic pressure fluctuations

We present results from Lyon‐Fedder‐Mobarry (LFM) global, three‐dimensional magnetohydrodynamic (MHD) simulations of the solar wind‐magnetosphere interaction. We use these simulations to investigate the role that solar wind dynamic pressure fluctuations play in the generation of magnetospheric ultra‐low frequency (ULF) pulsations. The simulations presented in this study are driven with idealized solar wind input conditions. In four of the simulations, we introduce monochromatic ULF fluctuations in the upstream solar wind dynamic pressure. In the fifth simulation, we introduce a continuum of ULF frequencies in the upstream solar wind dynamic pressure fluctuations. In this numerical experiment, the idealized nature of the solar wind driving conditions allows us to study the magnetospheric response to only a fluctuating upstream dynamic pressure, while holding all other solar wind driving parameters constant. The simulation results suggest that ULF fluctuations in the solar wind dynamic pressure can drive magnetospheric ULF pulsations in the electric and magnetic fields on the dayside. Moreover, the simulation results suggest that when the driving frequency of the solar wind dynamic pressure fluctuations matches one of the natural frequencies of the magnetosphere, magnetospheric cavity modes can be energized.

Solar wind driving of magnetospheric ULF waves: Pulsations driven by velocity shear at the magnetopause

We present results from global, three‐dimensional magnetohydrodynamic (MHD) simulations of the solar wind/magnetosphere interaction. These MHD simulations are used to study ultra low frequency (ULF) pulsations in the Earth's magnetosphere driven by shear instabilities at the flanks of the magnetopause. We drive the simulations with idealized, constant solar wind input parameters, ensuring that any discrete ULF pulsations generated in the simulation magnetosphere are not due to fluctuations in the solar wind. The simulations presented in this study are driven by purely southward interplanetary magnetic field (IMF) conditions, changing only the solar wind driving velocity while holding all of the other solar wind input parameters constant. We find surface waves near the dawn and dusk flank magnetopause and show that these waves are generated by the Kelvin‐Helmholtz (KH) instability. We also find that two KH modes are generated near the magnetopause boundary. One mode, the magnetopause KH mode, propagates tailward along the magnetopause boundary. The other mode, the inner KH mode, propagates tailward along the inner edge of the boundary layer (IEBL). We find large vortical structures associated with the inner KH mode that are centered on the IEBL. The phase velocities, wavelengths, and frequencies of the two KH modes are computed. The KH waves are found to be fairly monochromatic with well‐defined wavelengths. In addition, the inner and magnetopause KH modes are coupled and lead to a coupled oscillation of the low‐latitude boundary layer. The boundary layer thickness, d, is computed and we find maximum wave growth for kd = 0.5–1.0, where k is the wave number, consistent with the linear theory of the KH instability. We comment briefly on the effectiveness of these KH waves in the energization and transport of radiation belt electrons.

The effect of ultra-low frequency pulsations on tearing during deep drawing of cylindrical cups

In sheet metal forming there is an increasing demand for deep drawing of light weight components with complex shapes. However, thinner and more ductile materials are more susceptible to tearing during the process. It is known that greater draw depths can be achieved without tearing if the blankholder is subjected to a pulsating force. The present paper describes experimental investigations in which cylindrical-shaped cups are drawn with a pulsed blankholder force (PBHF) at ultra-low (<1 Hz) and low (1–10 Hz) frequencies. The tests were performed with blanks of steel (DC04) and of aluminium (5754-H111) over a range of draw ratios. The tests showed that tearing occurred within a well-defined part of the punch stroke. Of the two parameters, amplitude and frequency of the pulsed force, the latter had a more pronounced effect. For draw ratios resulting in a narrow working range of the static blankholder force, the process can be made more robust if the PBHF is applied at ultra-low frequencies. In addition to this, the PBHF must be synchronised in such a way that, during the identified critical tearing part of the stroke, the instantaneous PBHF is kept below the static tearing limit by synchronising the punch force and PBHF. Feasible values of the frequency depend on the length of the critical tearing part of the stroke and the upper and lower BHF limits of the process.

Ultralow frequency modulation of energetic particles in the dayside magnetosphere

Energetic electron and ion (electrons: 30 keV to 500 keV, protons: 30 keV to 1.5 MeV) flux variations associated with ultralow frequency (ULF) waves in the dayside magnetosphere were observed during the CLUSTER's perigee pass near 0900 MLT on Oct. 31, 2003. The ULF modulation terminated where higher frequency fluctuations appeared, as the CLUSTER spacecraft entered the plasmasphere boundary layer (PBL) where the plasma ion density was elevated. In the region from L ∼ 5.0 to 10, the periods of the ion flux modulation and the electron flux modulation are same but out‐of‐phase. The observed magnetic ULF pulsations are dominated by the toroidal mode, along with a relatively weaker poloidal wave. A 90° phase shift between the radial electric field and the azimuthal magnetic field indicates that dominating toroidal standing waves observed at the southern hemisphere are a fundamental harmonic. This study shows that the modulation of the electron flux is dominated by the toroidal mode in the region of L &gt; 7.5. The observations made in this analysis suggest the excitation of the energetic electron drift resonance at around 127 keV.

Spatial and temporal characteristics of poloidal waves in the terrestrial plasmasphere: a CLUSTER case study

Abstract. Oscillating magnetic field lines are frequently observed by spacecraft in the terrestrial and other planetary magnetospheres. The CLUSTER mission is a very suitable tool to further study these Alfvén waves as the four CLUSTER spacecraft provide for an opportunity to separate spatial and temporal structures in the terrestrial magnetosphere. Using a large scaled configuration formed by the four spacecraft we are able to detect a poloidal Ultra-Low-Frequency (ULF) pulsation of the magnetic and electric field in order to analyze its temporal and spatial structures. For this purpose the measurements are transformed into a specific field line related coordinate system to investigate their specific amplitude pattern depending on the path of the CLUSTER spacecraft across oscillating field lines. These measurements are then compared with modeled spacecraft observations across a localized poloidal wave resonator in the dayside plasmasphere. A detailed investigation of theoretically expected poloidal eigenfrequencies allows us to specify the observed 16 mHz pulsation as a third harmonic oscillation. Based on this we perform a case study providing a clear identification of wave properties such as an spatial scale structure of about 0.67 RE, the azimuthal wave number m≈30, temporal evolution, and energy transport in the detected ULF pulsations.

On the generation and evolution of aeroelectric structures in the surface layer

Ultralow frequency pulsations of electric field in the surface atmospheric layer were investigated under fair weather conditions. A new method of structural‐temporal analysis has been applied to the study of spatiotemporal structures of the electric field described previously by [Anisimov et al., 1994]. The method is based on exploration of the structural function by averaging the remote sensing data over respective temporal spans. This analysis allows quantitative estimations of spatial scales L ≃ 500 ‐ 103 m and temporal scales not less than τ = 10 min for the structural elements of the planetary boundary layer electricity; we call these recently examined elements “aeroelectric structures” (AES). Quasiperiodic sequences and high‐amplitude solitary AES have been recognized. Three‐dimensional structural‐temporal patterns are presented which directly characterize the level of electric energy perturbations connected with AES formation during night‐day evolution. A model of AES formation has been developed, taking into account the occurence of convective cells with respective turbulent air and space charge density distributions that are transferred by the wind over the ground and cause the electric field fluctuations at the points of observation. Therefore formation of such submesoscale structures can be explained by the redistribution of space charge within the surface layer, with the structures of the smallest scales coupled to the turbulent mixing of the ions and aerosols. In addition to the advection and turbulent mixing of space charge, we also consider the cooperative electroaerodynamic effects which might occur in a system of bipolar ion and aerosol particles under the influence of a terrestrial electric field. We have proposed an advanced model treating the AES formation as the result of instability arising in such a system, taking into account the dependence of the effective ion‐aerosol attachment coefficient on the external electric field strength.

Ultralow-frequency pulsations: Earth and Jupiter compared

Abstract Ultra-low frequency pulsations are one of the most important signatures of magnetospheric dynamics. Numerous observations of these waves have been reported with respect to the Earth magnetosphere, which led to a very detailed understanding of these waves. Some observations of ULF pulsations in the Jovian magnetosphere have been reported too. They open a new opportunity, that is they allow a study of ULF pulsations in terms of a comparative planetology. The main difference between both magnetospheric systems is probably the energy source. The solar wind is the main driver of those pulsations observed at Earth while the planet's rotational energy is a most important energy reservoir to be tapped at Jupiter. The current state of our knowledge of ULF pulsations in both magnetospheres is given with special emphazise given to a comparison between Earth and Jupiter.

STARE system looks at ULF magnetics

STARE (Scandinavian Twin Auroral Radar Experiment) has analyzed magnetospheric ultralow frequency (ULF) waves in the ionosphere since 1977. STARE data analysis recently discussed by J. J. Singer of the Air Force Geophysics Laboratory, Massachusetts, includes new explanations of the oscillations that occur in the shell structure of the geomagnetic field (Nature, May 5, 1983, p. 17).The ULF pulsations (periods from tens of seconds to 10 min) were thought to be standing hydromagnetic waves that resonate on geomagnetic field lines. Singer describes these waves as having the unusual character of periods that increase as a function of latitude. This phenomenon may clarify the nature of their source and of the characteristics between individual oscillating geomagnetic shells. Singer notes the argument supporting the standing‐wave theory as being the consistency of the wave periods with the time it takes Alfvén waves to travel along geomagnetic field lines between ionospheric reflection boundaries.

Initial observations of the artificial stimulation of ULF pulsations by pulsed VLF transmissions

Initial results are presented from an experiment to generate artificial ultra‐low‐frequency (ULF) hydromagnetic waves in the magnetosphere and ionosphere by means of specially modulated very‐low‐frequency (VLF) transmissions into the magnetosphere. The data were obtained predominantly during the interval September 1–7, 1973, when frequency‐shift‐keyed (FSK) VLF transmissions were made from Siple Station, Antarctica, and simultaneous VLF and ULF recordings were made at the conjugate point (Roberval, Quebec). The same 8‐hour transmission schedule was followed each day; it consisted primarily of intervals of FSK transmission (pulse lengths in the range 200 msec to 3.2 sec) interspersed with intervals of no transmission. All ULF pulsations with frequencies in the Pc 1 range (0.2 to 5 Hz) that occurred during the experiment started in an interval of FSK transmission and the average rate of occurrence of the ULF activity was about twice the rate observed on days with no transmissions.