Magnetospheric Oscillations Research Articles

Ionospheric ULF oscillations driven from above Arecibo

Understanding the coupling between the surface of the sun, the solar corona, the solar wind, the Earth's magnetosphere and ionosphere is a major focus of today's space research. Over 20 years ago, detailed observations of the ionospheric electron density at Arecibo revealed short period fluctuations (3 minutes), but their cause was never determined. Also, in the past 20 years a number of papers have reported observations of discrete (1–5 mHz) waves at persistent “magic frequencies” in magnetometer data and radar observations of ionospheric drift. Such observations have been generally interpreted as standing waves excited by magnetospheric cavity compressional modes. This assertion has been recently challenged by suggestions that these magic frequencies are not standing waves, but are due to inherent oscillations in the solar wind. Here we show that a subset of discrete 1.7 mHz oscillations in the solar wind plasma density are likely transmitted via a passive magnetospheric oscillation, and are observed as small amplitude (0.1%), discrete 1.7 mHz oscillations in the ionospheric plasma line using the extremely sensitive ionospheric measurements over Arecibo, Puerto Rico. We present over three hours of data using the electron plasma line component of the Arecibo radar incoherent scatter radar spectrum, solar wind number density from the WIND satellite, and magnetic field from the GOES satellite. This paper demonstrates a link between the solar wind and low latitude ionosphere.

Slow magnetosonic oscillations withm>> 1 in a dipole magnetosphere with rotating plasma

[1] A mechanism of excitation of azimuthal small-scale (m 1) slow magnetosonic (SMS) oscillations by the external currents in the near-equatorial region of the terrestrial magnetosphere is proposed. A theoretical study of the structure and spectrum of such oscillations is performed in the model of the magnetosphere with the dipole magnetic field and rotating plasma. In the direction along field lines of the geomagnetic field the oscillations are waves standing between the magnetically conjugated points. The amplitude of the magnetospheric SMS oscillations with m 1 decreases from the equatorial plane to the ionosphere at scales much lower then the field line length. So one is not able to observe the SMS-oscillation field in the vicinity of the ionosphere and on the surface of the Earth. In the direction across the magnetic shells, the standing SMS waves have a structure typical for resonant oscillations. Latitudinal distributions of eigenfrequencies of several first harmonics of SMS oscillations with the polarization similar to the polarization of poloidal and toroidal Alfven waves in the plane perpendicular to the field lines are drawn. INDEX

Multiple cusps during an extended northward IMF period with a significant By component

On 21–22 March 2001, four cusp‐like regions were observed consecutively in about five hours by all four Cluster spacecraft when the interplanetary magnetic field (IMF) was northward with a significant By component. All four cusp‐like encounters were characterized by turbulent magnetic fields, high‐density plasma, and plasma flow significantly slower than the magnetosheath level. The cusp‐like regions are associated with thermalized, bidirectional distributed plasma electrons. The first encountered cusp is the main cusp; the other three cusp‐like regions are temporal effects. The normal velocities vn at boundary interfaces (exit from the cusp) are found to be almost three times larger than that at boundary interfaces (entry into the cusp). The boundary normal, velocity, and timing analysis for six clear boundaries of the last three cusps obtained by all four spacecraft indicates that they are most likely one boundary shifting in the dawn‐dusk direction between the dayside magnetosphere/trapping region and the cusp region. So the Cluster spacecraft have been observing the same cusp, and it appeared as four cusp‐like regions due to possible magnetospheric oscillations. Oscillations with a period of 22 min are observed by the Cluster spacecraft in the high‐latitude region, which is in agreement with the cold‐dense plasma sheet fluctuations (20 min period) observed by the Geotail satellite. Multiple cusps observed by Cluster and the wavy‐like structures in the dusk low‐latitude boundary layer observed by Geotail simultaneously suggest the whole magnetosphere is oscillating during northward IMF.

Electrodynamics of a substorm‐related field line resonance observed by the Polar satellite in comparison with ground Pi2 pulsations

Observations from the Polar satellite during a substorm on 29 August 1996 are used to characterize electrodynamic properties of a nightside field line resonance (FLR). A negative H bay and Pi2 pulsations were simultaneously observed at midlatitude SAMNET ground stations (L = 2.6 − 4.5, ∼0000 MLT). During the pulsations, Polar was moving outbound on a 0330 MLT meridian and crossing L shells from 4 to 5.3. While in the central plasma sheet and outside the plasmasphere, Polar recorded toroidal standing waves for ∼20 min starting simultaneously with the ground pulsations. The magnetic‐field‐aligned Poynting flux associated with the FLR was ≤4 μW m−2. Whereas on the ground at L ≤ 4, the Pi2 frequency remained nearly constant and was similar for all stations, the toroidal frequency of the in situ FLR (L ≥ 4) decreased as the satellite crossed field lines with increasing L value. Although this L dependence of frequency of in situ FLR has been inferred in previous studies using multiple satellites at different local times, the 29 August 1996 event directly confirms this frequency dependence for substorm‐related FLR over a large L range on the same meridian. Furthermore, ground (L ≤ 4) and satellite (L ≥ 4) observations allow for the possibility that the oscillations were manifestations of two different magnetospheric oscillation modes, resulting from the same initial magnetospheric disturbance.

Observations of discrete, global magnetospheric oscillations directly driven by solar wind density variations

In this paper we present six events in which both the time series and the spectral content of solar wind number density fluctuations and magnetospheric magnetic field observations were highly correlated for intervals ranging from a few to twelve hours. The fluctuations were periodic and occurred at discrete frequencies which often matched the f = 1.3, 1.9, 2.6, and 3.4 mHz oscillations that have been attributed to global magnetospheric MHD cavity and/or waveguide modes. We also observed significant power in the sub‐mHz region, with frequencies as low as f = 0.1 mHz. We show that these fluctuations were first observed in the solar wind, far upstream from the Earth, and argue that the convected density perturbations slowly alter the size of the magnetospheric cavity leading to the appearance of multiple, discrete magnetospheric oscillations. We argue that for these events the discrete frequencies were an inherent property of the solar wind and were not related to a possible cavity or waveguide mode of the magnetosphere. We then show that the density fluctuations, when converted into length scales, organize into scale sizes of L = 23, 30, 45, and 80–100 RE. Finally, we speculate on a possible solar source for the periodic solar wind structures.

Global magnetospheric-ionospheric oscillations initiated by a solar wind pressure impulse

We present simultaneous observations of long‐period (∼1 hour) global magnetospheric‐ionospheric oscillations with multiple spacecraft and ground‐based instruments. These oscillations occurred following a major increase in the solar wind dynamic pressure. When the solar wind pressure impulse impinged on the magnetosphere, oscillations of energetic plasma particles in the ring current region were excited. The strongest oscillations came from electrons in the energy range of 75–225 keV with periods of 55–80 min. Ground magnetometers measured periodic (∼1 hour) enhancements of ionospheric currents from the auroral to equatorial latitudes. However, the solar wind pressure and IMF in this event did not show variations similar to the energetic particle oscillations and/or ground magnetometer deviations. We suggest that the magnetosphere has an intrinsic period of ∼1 hour during quiet times. The magnetospheric oscillations with this period can be initiated by a solar wind pressure impulse. After the initiation the solar wind no longer plays a role in the subsequent evolution of the magnetospheric oscillations.

Association between quiet‐time Pi2 pulsations, poleward boundary intensifications, and plasma sheet particle fluxes

Pi2 pulsations have been observed to occur at low latitudes during extremely quiet conditions. We have found an excellent correlation between an occurrence of such a Pi2 train and high‐latitude Pi2 pulsations and bay‐like disturbances in ground magnetic field of the type associated with poleward boundary intensifications (PBIs). The Pi2s and the PBIs that are inferred to occur simultaneously, show a ∼30 min repetition rate. They also exhibit an excellent correlation with energetic particle enhancements in the near Earth tail plasma sheet. These associations demonstrate the large‐scale nature of the magnetosphere‐ionosphere disturbance that gives rise to PBIs and shows that the disturbance may have important effects on energetic plasma sheet particles. They show for the first time that PBIs are associated with clearly observable Pi2 pulsations at low latitudes. Our observations are consistent with the possibility that PBIs are part of a large‐scale magnetospheric oscillation with ∼30 min period.

Coordinated ground-based and Cluster observations of large amplitude global magnetospheric oscillations during a fast solar wind speed interval

Abstract. We present magnetospheric observations of very large amplitude global scale ULF waves, from 9 and 10 December 2000 when the upstream solar wind speed exceeded 600 km/s. We characterise these ULF waves using ground-based magnetometer, radar and optical instrumentation on both the dawn and dusk flanks; we find evidence to support the hypothesis that discrete frequency field line resonances (FLRs) were being driven by magnetospheric waveguide modes. During the early part of this interval, Cluster was on an outbound pass from the northern dusk side magnetospheric lobe into the magnetosheath, local-time conjugate to the Canadian sector. In situ magnetic fluctuations, observed by Cluster FGM, show evidence of quasi-periodic motion of the magnetosheath boundary layer with the same period as the ULF waves seen on the ground. Our observations represent the first simultaneous magnetometer, radar and optical observations of the characteristics of FLRs, and confirm the potential importance of ULF waves for magnetosphere-ionosphere coupling, particularly via the generation and modulation of electron precipitation into the ionosphere. The in situ Cluster measurements support the hypothesis that, during intervals of fast solar wind speed, the Kelvin-Helmholtz instability (KHI) can excite magnetospheric waveguide modes which bathe the flank magnetosphere with discrete frequency ULF wave power and drive large amplitude FLRs. Paper submitted to the special issue devoted to "Cluster: First scientific results", Ann. Geophysicae, 19, 10/11/12, 2001.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; MHD waves and instabilities; solar wind-magnetosphere interactions)

High-latitude radiation and resonant enhancements of the Jovian synchrotron emissions

A model describing (1) short time scale increases in Jovian synchrotron radiation and (2) emissions from high Jovian latitudes is presented. In the present model resonant interaction between oblique whistler waves and relativistic electrons diffuses the energy and the pitch angle of electrons which bounce along the Jovian magnetic field. The resonant interaction violates the first two adiabatic invariants and the energy diffusion results in hardening of the electron spectrum and in intensification of the synchrotron radiation. This model complements the radial diffusion process which (a) violates the third adiabatic invariant of a seed population via interaction with low-frequency magnetospheric oscillations and (b) increases the flux of energetic electrons at low L-shells over long time scales by conserving the first adiabatic invariant. The high-latitude emissions indicate also acceleration of bouncing electrons and radiation in the region of the strongest magnetic field. The seed particles and whistler waves determine the form of the electron distribution tail. This tail enhances the high-latitude synchrotron radiation observed at Earth. The resulting emissivities of the nonthermal radio waves in the ordinary/extraordinary modes of propagation, the polarization and the intensity spectrum observed at Earth due to these high-energy populations are calculated.

Magnetohydrodynamic simulations of accretion disks around a weakly magnetized neutron star in strong gravity

We carried out two dimensional high-resolution magnetohydrodynamic (MHD) simulations of an accretion disk around a weakly magnetized neutron star. General relativistic effects are taken into account by using pseudo-Newtonian potential of Paczýnski, B., P.J. Witta (1980). When magnetic loops connect the neutron star and the accretion disk, the twist injection from the disk or from the rotating neutron star triggers expansion of the loops. Since the expanding magnetic loops prevent inflow toward the magnetic poles of the neutron star, disk matter accumulates on the boundary between the magnetosphere and the disk. Magnetic reconnection taking place in the loops creates a channel along which the disk matter can accrete and unloads the magnetosphere. This process produces quasi-periodic variation of the accretion flow in the innermost region of the disk. We found two kinds of oscillations. One is the magnetospheric oscillation regulated by magnetic reconnection. The other is the radial disk oscillation. The typical frequency of the oscillations is 100 Hz to 2 kHz. Furthermore, we predict that QPO sources inevitably accompany X-ray flares by magnetic reconnection and bipolar outflows of hot X-ray emitting plasma similar to the optical jets in protostars.

A model for the reflection of Alfvén waves at the source region of the Birkeland current system: The tau generator

The source region of the Birkeland current system, also called the generator of the auroral current circuit, is probably located in the outer equatorial magnetosphere on closed magnetic field lines. Alfvén waves have often been suggested as carriers for field‐aligned currents, momentum, and electromagnetic energy which is converted to particle kinetic energy and ohmic heat at the polar end of the auroral flux tubes. In this report we present a model for the interaction of Alfvén waves with the plasma in the generator region: the tau generator. We show that existing concepts like current and voltage generators correspond to extreme values of the intrinsic parameter τ. A frequency dependent reflection coefficient can be found which has considerable effect on the evolution of magnetospheric oscillations. Furthermore, we discuss the physical significance of the parameter τ and relate it to some measure for the effective height of the generator region.

Range finding of Alfvén oscillations and direction finding of ion-cyclotron waves by using the ground-based ULF finder

Abstract. A new approach to the problem of direction and distance finding of magnetospheric ULF oscillations is described. It is based on additional information about the structure of geoelectromagnetic field at the Earth's surface which is contained in the known relations of the theory of magnetovariation and magnetotelluric sounding. This allows us to widen the range of diagnostic tools by using observations of Alfvén oscillations in the Pc 3–5 frequency band and the ion-cyclotron waves in the Pc 1 frequency band. Preliminary results of the remote sensing of the magnetosphere at low-latitudes using the MHD ranger technique are presented. The prospects for remote sensing of the plasmapause position are discussed.

Gamma-ray bursts from planet-magnetosphere systems around neutron stars

We studied a planet-magnetosphere interaction model for γ-ray bursts with the picture of a current circuit which consists of the magnetosphere and the surface of a neutron star. The motion of planets or comets in the magnetosphere works as the battery in the circuit system. The physical condition on the surface of the neutron star is important to make a closed current circuit inducing good conversion from the kinetic energy of a planet to that of the magnetosphere oscillation. An old and cooled neutron star with the temperature ⋍ 10 eV can prepare the condition for closing the current circuit in the surface of the neutron star. The planet-magnetosphere system is unstable to feedback instability related to the variability of the γ-ray bursts.

A new method for detecting the projection of magnetospheric oscillations into the ionosphere

In situ magnetospheric observations and the corresponding data collected at the foot of the field line in the ionosphere can together form an invaluable tool for studying the processes involved in magnetosphere‐ionosphere coupling provided the mappings between various regions in the magnetosphere and ionosphere are precisely known. A multivariate analysis technique is used to identify spatially localized modes of oscillations in the ionospheric plasma velocities monitored at F region heights by the Goose Bay HF radar. The temporal variations of these modes are compared with the magnetic fluctuations recorded by the AMPTE CCE satellite which was within the magnetic latitude‐longitude region monitored by the radar. It is possible to clearly identify spatially localized modes in the ionosphere with temporal variations that track the azimuthal oscillations observed in the magnetosphere. This analysis approach offers a powerful method for establishing mappings between different regions of the magnetosphere and ionosphere under widely varying background conditions. The method, which is applied here exclusively to the mapping problem, can also be used to identify the natural modes of magnetospheric oscillation.

Gamma-ray bursts from planet-magnetosphere systems around neutron stars

We studied a planet-magnetosphere interaction model for γ-ray bursts with the picture of a current circuit which consists of the magnetosphere and the surface of a neutron star. The motion of planets or comets in the magnetosphere works as the battery in the circuit system. The physical condition on the surface of the neutron star is important to make a closed current circuit inducing good conversion from the kinetic energy of a planet to that of the magnetosphere oscillation. An old and cooled neutron star with the temperature ≃10 eV can prepare the condition for closing the current circuit in the surface of the neutron star

Explorer 45 (S³-A) observations of the magnetosphere and magnetopause during the August 4-6, 1972, magnetic storm period

The Explorer 45 (S³-A) satellite performed extensive field and particle measurements in the heart of the magnetosphere during the double magnetic storm period of August 4–6, 1972. Both the ground level magnetic records and the magnetic field deformations measured along the orbit by the satellite indicated the existence of only a moderate ring current. This was confirmed by the measurements of the total proton energy density by the on-board particle detectors, which showed a maximum energy density less than the densities observed during the December 1971 and June 1972 magnetic storms. The plasmapause in the noon quadrant was eroded continuously from the onset of the first storm at the beginning of August 4 to an altitude below L = 2.07 at about 1800 hours on August 5. Throughout the entire orbit during which the second sudden commencement occurred a large amount of low-frequency electric and magnetic field noise was encountered. The most remarkable observation during this orbit was the contraction of the magnetopause to distances inside the satellite location at L = 5.2. The magnetic field reversed direction and increased in magnitude to more than 500 γ, the particle detectors encountered huge fluxes of streaming protons, and the field detectors measured intense broad band noise. Following the period of magnetospheric boundary crossings, Explorer 45 encountered a series of large-amplitude magnetospheric oscillations, seen both in the dc and low-frequency field measurements and in the intensities of the particle fluxes. As is true for other storms, a region of detached plasma or a taillike structure was encountered in the dusk hours during the development of the second main phase, and a nose structure in the proton ring current distribution was observed, which has been shown to be a characteristic feature in the dusk hours during the ring current development.

Periods of the Magnetospheric Oscillations and the Plasma Density

Periods of the Magnetospheric Oscillations and the Plasma Density

Diurnal Variations in the Power Spectrum and Polarization of Telluric Currents at Conjugate Points, L = 2·6

The power spectrum of telluric currents, at the sub-auroral latitude where L = 2�6, shows a pronounced diurnal variation. The variation is contributed mainly by a strong peak at about 40 s, which shows a large cyclic fluctuation in amplitude and also a tendency to shift in frequency. The amplitude is larger and the period somewhat shorter during the day-time. The 40-s peak is also subject to appreciable random fluctuation in period. The spectrum shows shorter periods (7-14 s), and longer periods (several hundred seconds) in addition to the 40-s peak. There is a general shift to slightly longer periods at the time of auroral onset. The spectra are compared with those obtained at other latitudes. A period of about 40 s appears to have a wide distribution in latitude. It can be followed from mid latitudes to latitudes in the auroral zone and polewards of the auroral zone. The ignificant variation with geomagnetic latitude seems to be the relative amount of power associated ith anyone period rather than in the period itself. The observations are not readily econciled with theoretical estimates of magnetospheric oscillation in the transverse and compressional modes.