Field Pulsations Research Articles

The effect of density inhomogeneity on standing Alfven wave structure

At geomagnetic latitudes well within the plasmasphere (~25°–40°), ionospheric O + ions have a significant effect on hydromagnetic wave frequencies, waveforms and wave mode coupling. We apply a physically realistic plasmasphere model to obtain the eigenfunctions of the toroidal mode standing waves in a dipole field. Ion mass density inhomogeneities along the field lines (e.g. ionospheric O +, hemispheric asymmetry, or equatorial enhancements) result in a confinement of the wave fields within any significant density increase. Near latitude 30° the fundamental toroidal mode electric field has its largest values well away from the equatorial plane. The effects of the ionospheric F-region need to be considered when mapping pulsation fields and evaluating hydromagnetic wave coupling at low latitudes. The irregular spacing of the toroidal mode harmonic frequencies near latitude 30° is a direct consequence of the Alfvén velocity inhomogeneity over a substantial portion of the field line length.

Magnetic pulsations at the quasi‐parallel shock

The plasma and field properties of large‐amplitude magnetic field pulsations upstream from the quasi‐parallel region of the Earth's bow shock are examined in high time resolution using data from ISEE 1 and 2. The relative timing of the magnetic field profiles observed at the two spacecraft shows that some of the pulsations are convecting antisunward across the spacecraft while others are brief out/in motions of the bow shock across the spacecraft. Pulsations with both timing signatures are the site of slowing and heating of the solar wind plasma. The ions tend to be only weakly heated in the convecting pulsations, while within the out/in pulsations the ion heating can be quite substantial but variable. This variation occurs not only from pulsation to pulsation but also from point to point within a given pulsation. In general, the hottest distributions within the out/in pulsations tend to occur in regions of lower density and field strength. Magnetic pulsations bear a number of similarities to previously identified hot diamagnetic cavity events as well as to more durable crossings of the quasi‐parallel shock itself. These various phenomena may be different manifestations of the same basic physical processes, in particular the coupling of coherently reflected ions to the solar wind beam.

Magnetospheric plasma density diagnosis from gradient measurements of geomagnetic pulsations

It is shown that gradient measurements of geomagnetic pulsation fields along a meridian can be used as a method of magnetospheric plasma density diagnosis. The gradient method is applied to observations of Pc4 pulsations at five stations spaced about 100 km apart on a meridian through the British Isles. Values are calculated for equatorial plasma density which compare favourably with those obtained from whistler measurements with the advantage that there is no requirement to calculate the L-shell of the field line for the top of which the plasma density is found.

Observation by space-borne detectors of electric fields and hydromagnetic waves in the ionosphere over an earthquake centre

Abstract Vertically directed quasistatic electric fields of 3–7 mV m−1 and magnetic field pulsations with amplitude 3 nT at frequencies of the order of 1 Hz have been observed on the Intercosmos-Bulgaria-1300 satellite in the near-equatorial ionosphere over an earthquake centre (φ ∗ = 3.39° S and λ ∗ = 177.43° E , M = 4.8) on 21 January 1982

Restoration of the meridional structure of geomagnetic pulsation fields from gradient measurements

The usefulness of gradient measurements in the investigation of the resonance structure of geomagnetic pulsation fields along a meridian is extended. Starting with a simple resonance model and synchronous observations of Pc4 pulsations at five stations spaced about 100 km apart on a meridian, the gradient method is used to restore the structure of the pulsation field on a significant part of the meridian (about 1000 km) adjacent to the observation points.

The relationship between ULF geomagnetic pulsations and ionospheric Doppler oscillations: Derivation of a model

Several papers have been written about the observed relationship between ground based magnetometer pulsation measurements and simultaneous oscillations of Doppler frequency shifts in ionospherically reflected radio frequency echoes. In this paper we derive the mechanisms which relate these observations for the vertical incidence case at low to mid latitudes. We investigate the effects of oscillating electric and magnetic fields at ionospheric heights on the phase path of the reflected radio wave, which gives rise to the Doppler shifts. We identify three main mechanisms at work in the ionosphere, which are applicable at all latitudes; however, our numerical computations do not take account of certain high‐latitude effects. By quantifying the electric and magnetic pulsation fields for a partially reflected downcoming Alfvén wave, we derive a quantitative phase and amplitude relationship between the rate of change of phase path on Doppler velocity and the pulsation magnetic field measured on the ground. A surprising result is that the dominant mechanism is not necessarily the vertical component of bulk plasma movement, as has been previously suggested. In many cases, the dominant mechanism is compression and rarefaction of plasma frozen onto the field lines as they oscillate under the action of the field‐aligned component of the pulsation magnetic field.

Auroral activities and long-period geomagnetic pulsations: 2. Ps5 pulsations following auroral breakup in the premidnight hours.

Bright auroral forms several hundreds of km in spatial extent were observed repeatedly (a few to about 15min, periodicity) in the premidnight hours, following auroral breakups. The auroral forms drifted towards the northwest or the southwest, and were related to the magnetic field fluctuations on the ground in a similar period range. The leading front of the drifting auroral form has a discrete auroral (vortexchain) feature. After the passage of discrete forms pulsating aurora appears. From the comparison with magnetic field perturbations at the geosynchronous orbit satellites GOES 5 and GOES 6, these auroral forms are likely to correspond to the depression of the magnetic total intensity. A possible source of these bright auroral forms is hot plasmas injected from the plasma sheet. The concurrent magnetic pulsations, here called Ps5, would be produced by the fluctuating electric field and conductivity in the ionosphere due to the electron precipitation into the auroral forms. Hot plasma clouds, quasi-periodically injected during the course of a substorm, are related to the auroral and magnetic field pulsations. A steep injection front of hot plasma in the magnetosphere is likely related to discrete auroras at the ionosphere.

Simultaneous observation of Pc 3–4 pulsations in the solar wind and in the Earth's magnetosphere

The equatorially orbiting AMPTE (Active Magnetospheric Particle Tracer Explorers) CCE and IRM satellites have made numerous observations of Pc 3–4 magnetic field pulsations (10‐ to 100‐s period) simultaneously at locations upstream of the earth's bow shock and inside the magnetosphere. These observations show solar wind/interplanetary magnetic field (IMF) control of two categories of dayside magnetospheric pulsations: (a) Harmonically structured, azimuthally polarized pulsations are commonly observed from L = 4 to 9 in association with upstream waves. Their periods (including harmonics) appear to be governed by local resonant conditions. Varying levels of broadband compressional wave power are associated with these pulsations, most prominently near local noon, (b) More monochromatic compressional pulsations are clearly evident on occasion, with periods identical to those observed simultaneously in the solar wind. Harmonically structured, azimuthally polarized pulsations also occur in conjunction with these compressional pulsations, but again display frequencies characteristic of local resonant conditions. As in earlier studies there is clear control of the occurrence of pulsations both inside and outside the magnetosphere by the cone angle of the IMF, and harmonic pulsation amplitude increases when the solar wind velocity increases. The observations reported here are consistent with a high‐latitude (cusp) entry mechanism for wave energy related to harmonically structured pulsations.

Mechanisms for observed total electron content pulsations at mid latitudes

Total electron content variations in the Pc3– Pc4 range of frequencies of the order of 4 parts in 10 4 have been reported in apparent correlation with simultaneous ground based magnetic pulsation observations. By means of a term-by-term analysis of the continuity equation for electrons, the plausibility of various mechanisms is investigated. The most likely explanation is in terms of localized increases in the electron density at F-region heights caused by the field-aligned (compressional) component of the pulsation magnetic field. The analysis predicts a tendency for the amplitude of the TEC pulsations to vary in antiphase with ground-based measurements of the north-south component of the pulsation field.

Degeneration of an isotropic random field of electric charge pulsations in electrodynamics

The conditions under which an isotropic field of electric pulsations can exist in an electrically charged medium are indicated, with allowance in Ohm's law for the convection and diffusion of charged particles and their drift in the electric field. Mean parameter distributions are found for isotropic random fields and equations are obtained for correlation moments containing electric pulsations. The equations are analyzed with respect to the two-point correlations between the pulsations in the electric charge densities (the limit of single-point correlations is reached, invariant relations are obtained, and particular solutions are constructed). The degeneration laws of the isotropic fields of the electric parameters as a result of diffusion and drift. processes are found. Field degeneration due only to drift particle motion is studied under arbitrary initial conditions. Such field degeneration if there is no diffusion mechanism is a fundamental feature of turbulent electro-hydrodynamic motions.

A statistical study of wave poynting vectors measured during long‐period magnetospheric pulsations at geostationary orbit

Electric and magnetic field data measured by the electron beam experiment and the magnetometer on board the geostationary satellite GEOS 2 were used to investigate wave Poynting vectors associated with long‐period (150–600 s) magnetospheric pulsations. A total of 3580 vectors were calculated for pulsations occurring during 186 days in the dayside magnetosphere. The ratio between the electric and magnetic field wave amplitudes was in general well above the local Alfvén speed and was found to increase with increasing wave frequency. The fraction of electric field pulsations for which magnetic wave components could also be identified was therefore larger for the low‐ and smaller for the high‐frequency events in the range 1.67–6.67 mHz. Poynting fluxes were found to have values between 10−10 and 10−5 W/m². For most pulsations with periods between 400 and 600 s the part of the vectors perpendicular to the ambient magnetic field had an inward component and was directed toward the nose of the magnetosphere in the prenoon and afternoon sectors. The behavior of the corresponding components of the pulsations with a 150‐ to 300‐s period was not as clear. For all events observed in the winter season (between November 1, 1978, and February 28, 1979), 59% of the vectors had a field‐aligned component directed into the northern hemisphere, whereas 41% of the vectors were directed southward. The frequency dependence of the E/B ratio, the magnitudes of the Poynting vectors, and their directional distributions are consistent for most events with the picture of standing shear Alfvén waves caused by solar wind driven surface waves on the outer boundaries of the magnetosphere.

Ionospheric propagation of ULF-waves

Features of mid-latitude ionospheric propagation of geomagnetic pulsations are studied. It is shown that the orientation of the horizontal component of the incident wave vector with reference to the geomagnetic meridian has a strong influence on the amplitude transformation and on the angle of rotation of the polarization plane. The effect of different conductivities of the Earth's crust and the contribution of Hall currents to the total pulsation field in the case of an inclined geomagnetic field are also considered.

The spatial structure of different ULF pulsation types: A review of STARE radar results

The Scandinavian Twin Auroral Radar Experiment (STARE) has proved useful in estimating the ionospheric electric fields of several types of ULF pulsation. These usually involve the propagation of hydromagnetic waves along the geomagnetic field lines. Reflection at the auroral ionosphere boundary can then result in standing wave resonances. Because of its good spatial resolution and coverage of this boundary, STARE provides unique data on the spatial variations of the pulsation fields. Background theoretical aspects and STARE observations are presented for three types of pulsation: impulsive or ringing Pc 5, monochromatic resonant Pc 5, and pulsations thought to be generated by instabilities in the ring current particle population. Since the ionosphere is resistive, it acts as an energy sink for the pulsations, and estimates of the ionospheric Joule heating are presented. Finally, some suggestions for future work are given.

Ionospheric electric field pulsations: A comparison between VLF results from an ionospheric heating experiment and STARE

Very low‐frequency radio waves can be generated in the lower ionosphere by periodically modulating the conductivity, and thus natural currents, using high power HF radio waves. In conjunction with the heating facility near Tromsø, Norway, such waves have been generated and detected on the ground using two orthogonal receiving antennas so that the polarization ellipse of the VLF signal could be reconstructed. Changes in the strength and direction of the electric field driving the modulated current are reflected in variations in the size and orientation of the VLF ellipse. During a natural Pc 5 pulsation event on October 16, 1981, a close correlation was found between the VLF signal and the electric field pulsations observed by the STARE auroral radars. This first comparison between the Heating and STARE experiments shows that VLF signals from modulated ionospheric heating experiments can provide a sensitive, high time resolution indicator of the ionospheric electric field.

Impulse‐excited pulsations during the July 29, 1977, event

The propagation of a geomagnetic sudden impulse (si) and the magnetic field pulsations excited by it in the magnetosphere is traced from the bow shock in the solar wind, through the magnetosphere, to the ground. Within the magnetosphere the impulse appears as a compressive magnetohydrodynamic (MHD) impulse that travels rapidly (∼1500 km/s) tailward. A resonant oscillation observed both in space and on the ground is excited near geostationary orbit. The effect of the si is enhanced by a factor of at least 5 on the ground near the geomagnetic equator. We suggest that discontinuities in the solar wind may be a more important source of exciting dayside pulsations than has been commonly assumed.

Pulsation research during the IMS

Workers in the pulsation field entered the period of the International Magnetospheric Study (IMS) with a lot of theories that needed testing. Because of this and the wealth of new data available from the IMS observational campaigns, many new observational results have emerged from the IMS. The experience of the IMS shows us that the most fruitful method of working has been small‐scale collaborations, between either a few individuals or a few groups with complementary data sets. The larger, more formally organized workshops do not seem to have been as successful in producing results, though they may have fostered useful contacts. In the years to come I hope and expect to see more of this sort of small‐scale collaboration; especially as the initial results obtainable from a single data set are now published, experimenters should turn to seeing how their data can be used in conjunction with other data sets. I also feel that the time is ripe for more theoretical work. The IMS has left us with a lot of new observations that need explanation.

Relationship between ionospheric electric field and ground magnetic pulsations in the PC 3 domain at midlatitude

At middle latitudes, measurements of electric field fluctuations in the F region, in the Pc 3 frequency range is difficult. In France, with the incoherent scatter velocity measurements, three experiments have been performed. Technical performances and difficulties are described. Specific signal processing methods are presented, and pulsations in the ionosphere are described in time and in frequency form. They are identified by comparison with ground magnetic measurements through the use of autospectrum and coherency coefficient estimations. In the ionosphere we have found few events with amplitude waves higher than the detection level. When there was correlation between ionosphere and ground it was possible to infer the impedance of the ionosphere plus ground for the transfer between the electric field in the F region and the magnetic field on the ground. The deduced value of this impedance is approximatively 10 Ω. According to Hughes and Southwood's theory this means that the signals have a very short horizontal spatial scale.

Excitation of radioauroral pulsations and penetration of PiC waves through the ionosphere

Radioauroral pulsations at T~ 4.5 s have been observed during a substorm. They appear as poleward propagating waves. The most intense Pi magnetic pulsations occur at T~ 9s. Using the transformation theory of hydromagnetic waves through the ionosphere it is concluded that the radioauroral pulsations have been generated by the electric field pulsations associated with intense poleward propagating hydromagnetic waves. Polarization characteristics of magnetic pulsations observed on the ground are also shown to be in agreement with this interpretation.

An observed characteristic of concurrent magnetic field and electron precipitation pulsations

In each case for a collection of seven large-amplitude events, spectral peaks in the 1 to 10 mHz range for electron precipitation pulsations were observed to correspond to spectral peaks for concurrent magnetic field pulsations. The converse is not true, i.e. a magnetic field pulsation spectral peak need not correspond to a precipitation spectral peak, and in fact there may not even be a concurrent precipitation pulsation event. The data are consistent with occurrences of interactions between a portion of the hydromagnetic wave spectrum and trapped electrons in the equatorial region of the field line of the site.

Some properties of geomagnetic field pulsations in the 0.1–1.0-Hz frequency range: A quantitative description and comparison with the satellite and ground-based observations

By using an image-dipole magnetic field model for a variety of plasma density profiles we have studied the latitude effect of the 0.1–1.0-Hz hydromagnetic wave propagation in the Earth's magnetosphere. On comparing the results of signal group delay time calculations for dipole and model magnetic fields with ground and satellite observations we obtain some propagation characteristics of Pc1s and localize the regions of their generation. Our results show that most high-latitude Pc1 events are generated in the outer magnetosphere in accordance with ground and satellite observations and theoretical considerations. The non-dipole geometry of the geomagnetic field in the outer magnetosphere (at geomagnetic latitudes φ 0 > 66°, L > 6) has a significant effect on the hydromagnetic wave propagation.