Field Line Resonance Research Articles

Ground observations of high‐latitude Pc3‐4 ULF waves

A detailed study has been undertaken of Pc3‐4 waves recorded on the ground with the IMAGE magnetometer array (56° < Λ < 76°) during January and March 1998. We focus only on daytime events exhibiting high coherence (>0.6) across the entire station array. Most of these had well‐defined wave packet appearance in time series records and a clear peak in power spectra. Their occurrence and frequency suggest the waves are generated by the upstream ion‐cyclotron resonance mechanism, with no evidence of generation by the Kelvin‐Helmholtz instability. For each event the amplitude, phase, coherence, ellipticity, azimuth angle, and degree of polarization across the ground array were examined. The coherence length, azimuthal wave number, and hence the apparent wave propagation velocity were thus determined, with emphasis on the precision and significance of these measurements. It was found that these daytime Pc3‐4 pulsations usually have maximum amplitude near the magnetopause projection, meridional coherence lengths of order 1.5–2.0 × 103 km, and low azimuthal wave numbers during morning hours, averaging around −4.0 (indicating westward propagation). Over 80% of events propagated poleward and westward, with average equivalent ground velocity of 41 km/s N43°W for the H component. About 24–30% of the events are higher harmonics of field line resonances. There is no evidence that the remaining events arise from cavity modes or localized modulated electron precipitation. The observations instead suggest a mechanism involving mode coupling and field‐guided propagation. In this model, fast mode waves in the Pc3‐4 range entering near the subsolar point propagate earthward and due to the inhomogeneity of the magnetosphere couple to the field‐guided Alfvén mode. At certain latitudes, standing oscillations are established at harmonics of the local resonant frequency, while at other latitudes traveling waves convey energy to low altitudes. The expected L dependence of wave power and travel time agree well with observed amplitude and phase profiles.

Theory of dispersive shear Alfvén wave focusing in Earth's magnetosphere

It is shown that perpendicular gradients in shear Alfvén wave (SAW) dispersion regulate the localization of wave power on nightside geomagnetic L‐shells where narrow Field Line Resonances (FLRs) form. We estimate the timescale for this process, ω0tc = 1/, and demonstrate that it is analogous to optical wave focusing. Here, β is the gradient in the global wave dispersion parameter across L‐shells in the equatorial plane, and α is the gradient in the SAW eigenfrequency. It is demonstrated that dispersive SAWs with wave numbers and frequencies in a certain range, are subject to magnetospheric focusing onto L‐shells where they reach large amplitude and are expected to dissipate. Our theory addresses a class of arc scales that are comparable to the electron inertial length near the ionosphere, or the ion gyroradius near the equatorial plane. We further demonstrate that when the gradient in the SAW frequency reverses across the edges of auroral density cavities, it naturally traps dispersive SAWs and focuses them down to the inertial scale.

Pseudo-field line resonances in ground Pc5 pulsation events

Abstract. In this work we study four representative cases of Pc5 ground pulsation events with discrete and remarkably stable frequencies extended at least in a high-latitude range of ~20°; a feature that erroneously gives the impression for an oscillation mode with "one resonant field line". Additionally, the presented events show characteristic changes in polarization sense, for a meridian chain of stations from the IMAGE array, and maximize their amplitude at or close to the supposed resonant magnetic field shell, much like the typical FLR. Nevertheless, they are not authentic FLRs, but pseudo-FLRs, as they are called. These structures are produced by repetitive and tilted twin-vortex structures caused by magnetopause surface waves, which are probably imposed by solar wind pressure waves. The latter is confirmed with in-situ measurements obtained by the Cluster satellites, as well as the Geotail, Wind, ACE, and LANL 1994-084 satellites. This research effort is largely based on two recent works: first, Sarafopoulos (2004a) has observationally established that a solar wind pressure pulse (stepwise pressure variation) produces a twin-vortex (single vortex) current system over the ionosphere; second, Sarafopoulos (2004b) has studied ground events with characteristic dispersive latitude-dependent structures and showed that these are associated with twin-vortex ionosphere current systems. In this work, we show that each pseudo-FLR event is associated with successive and tilted large-scale twin-vortex current systems corresponding to a magnetopause surface wave with wavelength 10-20RE. We infer that between an authentic FLR, which is a spatially localized structure with an extent 0.5RE in the magnetospheric equatorial plane, and the magnetopause surface wavelength, there is a scale factor of 20-40. A chief observational finding, in this work, is that there are Pc5 ground pulsation events showing two gradual and latitude dependent phase-shifts of 180°, at the same time.

An investigation of the field-aligned currents associated with a large-scale ULF wave using data from CUTLASS and FAST

Abstract. On 14 December 1999, a large-scale ULF wave event was observed by the Hankasalmi radar of the SuperDARN chain. Simultaneously, the FAST satellite passed through the Hankasalmi field-of-view, measuring the magnetic field oscillations of the wave at around 2000km altitude, along with the precipitating ion and electron populations associated with these fields. A simple field line resonance model of the wave has been created and scaled using the wave's spatial and temporal characteristics inferred from SuperDARN and IMAGE magnetometer data. Here the model calculated field-aligned current is compared with field-aligned currents derived from the FAST energetic particle spectra and magnetic field measurements. This comparison reveals the small-scale structuring and energies of the current carriers in a large-scale Alfvén wave, a topic, which at present, is of considerable theoretical interest. When FAST traverses a region of the wave involving low upward field-aligned current densities, the current appears to be carried by unstructured downgoing electrons of energies less than 30eV. A downward current region appears to be carried partially by upgoing electrons below the FAST energy detection threshold, but also consists of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies <30eV, with the hotter upgoing electrons presumably representing those upgoing electrons which have been accelerated by the wave field above the low energy detection threshold of FAST. A stronger interval of upward current shows that small-scale structuring of scale ~50km has been imposed on the current carriers, which are downgoing magnetospheric electrons of energy 0-500eV.

Heavy ion mass loading of the geomagnetic field near the plasmapause and ULF wave implications

The structure of the density discontinuity across the plasmapause is often based on electron and H+ density profiles with the contribution of heavy ions (He+, O+ etc) neglected. Electron and ion density measurements in this region may differ significantly due to the presence of heavy ions and it is important for the intercomparison of different datasets to understand these differences. Dynamics Explorer (DE‐1) magnetic field and plasma composition data have been used to compare heavy ion responses across the plasmapause and to calculate the mass loaded ion density (ρ) profiles. To illustrate this we investigate mass loading through radial profile variations in the Alfven velocity (VA). Results show that the gradient in ρ and VA across the plasmapause is modified when mass loading due to multiple heavy ion species is included, particularly in the presence of the O+ torus. Application to ultra‐low frequency (ULF) field line resonance is used as an example where the contribution from heavy ions smoothes out the expected ULF wave resonant frequency discontinuity at the plasmapause.

Energy deposition by Alfvén waves into the dayside auroral oval: Cluster and FAST observations

We report in situ observations from the Cluster and FAST spacecraft showing the deposition of energy into the auroral ionosphere from broadband ULF waves in the cusp and low‐latitude boundary layer. A comparison of the wave Poynting flux with particle energy and flux at both satellites indicates that energy transfer from the broadband waves to the plasma occurs through field‐aligned electron acceleration, transverse ion acceleration, and Joule heating. These processes are shown to result in precipitating electron fluxes sufficient to drive bright aurora and cause outflows of energized electrons and O+ ions from the ionosphere into the low‐latitude boundary layer. By solving an eigenmode equation for Alfvén waves in the observed plasma environment, it is shown that the broadband waves observed at Cluster and FAST are dispersive Alfvén waves. It is demonstrated that these waves have wavelengths perpendicular to the geomagnetic field extending from significant fractions of an L shell down to ion gyroradii and electron inertial lengths and wave frequencies in the plasma frame from 1 mHz up to 50 mHz. These waves are shown to have wavelengths along the geomagnetic field of the order of the field line length between the ionosphere and the equatorial plane and become field line resonances (FLRs) when on closed field lines. It is shown that the inclusion of nonlinear and/or nonlocal kinetic effects is required in the description of these waves to account for accelerated particles observed. On the basis of the wave polarization and spectral properties observed from Cluster and FAST it is speculated that these waves are generated through the mode conversion of surface Alfvén waves driven by tailward flows in the low‐latitude boundary layer.

A statistical analysis of low‐frequency magnetic pulsations at cusp and cap latitudes in Antarctica

We report a statistical analysis of low‐frequency magnetic variations (magnetic pulsations, 0.8–7 mHz) at South Pole (74°S corrected geomagnetic latitude) and Terra Nova Bay (80°S) during 1996. The results show that at South Pole (at cusp latitudes) the pulsation power exhibits two maxima during the day, one in the local premidnight and the other in the morning. The first maximum disappears when the analysis is restricted to northward interplanetary magnetic field conditions (Bz > 1 nT), suggesting that it might be associated to substorm phenomena. During closed magnetospheric conditions, when the cusp is expected to be located poleward with respect to the station, the spectral and polarization characteristics of pulsations between 1 and 3 mHz suggest that resonant oscillations of the outermost closed field lines commonly occur at South Pole in the local morning. At Terra Nova Bay, in the polar cap, the pulsation power is much lower and its diurnal variation is characterized by a single maximum, which occurs around local noon, when the station approaches the cusp. The corresponding polarization pattern indicates that Terra Nova Bay is always located poleward with respect to resonant field lines.

Density enhancement in plasmasphere‐ionosphere plasma during the 2003 Halloween Superstorm: Observations along the 330th magnetic meridian in North America

On October 29–31, 2003, the ground observations of field line resonance signals and the total electron content (TEC) along the 330th magnetic meridian recorded extraordinary density variations in both the magnetosphere and the ionosphere. In the magnetosphere, the density decreased at outer L shells due to strong convection, whereas it increased significantly in the afternoon sector at L ≤ 4. In the ionosphere, a strong positive storm occurred at low latitudes, and storm enhanced density was also observed at approximately 1400 LT in mid‐latitude regions. The density enhancements in both the magnetosphere and ionosphere coincided with intervals of southward IMF and high‐speed solar wind, consistent with the scenario that the eastward electric field imposed on the ionosphere led to a positive storm which might contribute to the dense plasmaspheric drainage plume. These results demonstrate that the ionosphere can be an important factor modulating the density variations in a storm‐time plasmasphere.

ROCSAT observation of the field line resonance effect in a plasma pulsation at topside ionosphere

During the recovery phase of the 29–31 October 2003 storm, a plasma pulsation event of large flow oscillation perpendicular to the field line was observed by ROCSAT‐1 in a circular orbit at 600 km topside ionosphere. The flow undulation amplitude grows large as ROCSAT moves around the southernmost latitudes of the orbital coverage around the dusk region from dip latitude −50° to −53° to reach the largest observed oscillation amplitude of 380 m/s that is equivalent to an electric field of 18 mV/m. The undulation amplitude then decreases as ROCSAT moves back toward low latitudes in the northbound leg. A reversal of polarization and changes in polarization ellipticity are also observed as ROCSAT traverses across −50° dip latitude. The variation in wave amplitude together with the change in polarization suggests that a field line resonance (FLR) effect in the plasma pulsation has been observed along the ROCSAT orbit traversing different dip latitudes of magnetic shells. This event is different from past spacecraft observations of the Pi2 pulsation events manifested in the magnetospheric or plasmaspheric cavity resonance. The ROCSAT observation of the FLR effect will be for the first time to observe the plasma pulsation directly in space at a location just above the ionosphere where the induced ionospheric currents (or flows) have been manifesting the FLR effect observed on the ground. Our observation also indicates that a strong coupling exists between the global compressional eigenmodes and the excited transverse modes, as proposed in a model by Kivelson and Southwood [1986].

Nonlinear electron heating by resonant shear Alfvén waves in the ionosphere

Ionospheric electron heating by resonant standing shear Alfvén waves in Earth's magnetosphere is investigated. It is demonstrated that in field line resonances (FLRs), electron heating by Alfvén waves produces ionization and large changes in the ionospheric Pedersen conductivity. This leads to a strong feedback effect on the FLR amplitude, along with narrow localization in latitude. Analysis and computer simulations performed with a 2D finite element MHD code, indicate that the primary mechanisms responsible for variations in the electron temperature are ohmic heating by the electron component of the Pedersen current, and electron cooling due to ionization losses and collisions with neutrals. It is shown that electron heating can be quantitatively more important than direct collisional ionization by precipitating electrons. The latter can reduce dissipation losses by at most a factor of two.

Coordinated polar spacecraft, geosynchronous spacecraft, and ground-based observations of magnetopause processes and their coupling to the ionosphere

Abstract. In this paper, we present in-situ observations of processes occurring at the magnetopause and vicinity, including surface waves, oscillatory magnetospheric field lines, and flux transfer events, and coordinated observations at geosynchronous orbit by the GOES spacecraft, and on the ground by CANOPUS and 210° Magnetic Meridian (210MM) magnetometer arrays. On 7 February 2002, during a high-speed solar wind stream, the Polar spacecraft was skimming the magnetopause in a post-noon meridian plane for ~3h. During this interval, it made two short excursions and a few partial crossings into the magnetosheath and observed quasi-periodic cold ion bursts in the region adjacent to the magnetopause current layer. The multiple magnetopause crossings, as well as the velocity of the cold ion bursts, indicate that the magnetopause was oscillating with an ~6-min period. Simultaneous observations of Pc5 waves at geosynchronous orbit by the GOES spacecraft and on the ground by the CANOPUS magnetometer array reveal that these magnetospheric pulsations were forced oscillations of magnetic field lines directly driven by the magnetopause oscillations. The magnetospheric pulsations occurred only in a limited longitudinal region in the post-noon dayside sector, and were not a global phenomenon, as one would expect for global field line resonance. Thus, the magnetopause oscillations at the source were also limited to a localized region spanning ~4h in local time. These observations suggest that it is unlikely that the Kelvin-Helmholz instability and/or fluctuations in the solar wind dynamic pressure were the direct driving mechanisms for the observed boundary oscillations. Instead, the likely mechanism for the localized boundary oscillations was pulsed reconnection at the magnetopause occurring along the X-line extending over the same 4-h region. The Pc5 band pressure fluctuations commonly seen in high-speed solar wind streams may modulate the reconnection rate as an indirect cause of the observed Pc5 pulsations. During the same interval, two flux transfer events were also observed in the magnetosphere near the oscillating magnetopause. Their ground signatures were identified in the CANOPUS data. The time delays of the FTE signatures from the Polar spacecraft to the ground stations enable us to estimate that the longitudinal extent of the reconnection X-line at the magnetopause was ~43° or ~5.2 RE. The coordinated in-situ and ground-based observations suggest that FTEs are produced by transient reconnection taking place along a single extended X-line at the magnetopause, as suggested in the models by Scholer (1988) and Southwood et al. (1988). The observations from this study suggest that the reconnection occurred in two different forms simultaneously in the same general region at the dayside magnetopause: 1) continuous reconnection with a pulsed reconnection rate, and 2) transient reconnection as flux transfer events. Key words. Magnetospheric physics (Magnetopause, cusp and boundary layers; Magnetosphere-ionosphere interactions; MHD waves and instabilities)

Particle simulation of Alfvén waves excited at a boundary

A particle-in-cell (PIC) code has been developed that is capable of describing the propagation of compressional and shear Alfvén waves excited from a boundary. The code is used to elucidate the properties of Alfvén wave cones radiated from sources having transverse scale comparable to the electron skin depth. Good agreement between theoretical predictions and simulation results is found over a wide range of frequencies. An investigation has been undertaken of the effect of hot ions on the Alfvén wave cones. The PIC simulations demonstrate that as the ion temperature is increased there is a reversal in the cone angle. The reversal implies that there is a cross-field focusing of the shear Alfvén waves. This is a feature which is presently being considered in studies of field-line resonances in the earth’s magnetic field. The PIC results also illustrate the damping of shear modes due to the Doppler-shifted cyclotron resonance with hot ions.

Effects of ionospheric damping on MHD wave mode structure

We calculate the mode structure of magnetospheric MHD waves on a meridional plane. We have added the effect of ionospheric dissipation to the three-dimensional dipole field MHD model of Lee and Lysak (1999); this model allows a realistic Alfven speed profile for the plasmasphere and realistic boundary conditions at the outer boundaries that vary with respect to local time. Using power spectra and plots of spatial mode structure, we show that the two-dimensional transverse modes on the dipolar meridian are strongly affected by ionospheric damping, but the compressional modes are not. The location of field line resonances spreads wide as the damping increases, but the compressional mode structure remains stable.

Fine structure in ionospheric Alfvén resonator spectra observed at low latitude (L = 1.3)

A fine spectral structure in the electromagnetic background noise at 0.1 to 6 Hz is observed at a low latitude station. Its frequency scale falls roughly within the range of low‐latitude field‐line resonances (FLR), but the reported structure reveals a much higher quality factor than is usually found in FLR observations. It is interpreted here in terms of coupling between the ionospheric Alfvén resonator (IAR) and conventional field line resonances.

Separating spatial and temporal variations in auroral electric and magnetic fields by Cluster multipoint measurements

Abstract. Cluster multipoint measurements of the electric and magnetic fields from a crossing of auroral field lines at an altitude of 4RE are used to show that it is possible to resolve the ambiguity of temporal versus spatial variations in the fields. We show that the largest electric fields (of the order of 300mV/m when mapped down to the ionosphere) are of a quasi-static nature, unipolar, associated with upward electron beams, stable on a time scale of at least half a minute, and located in two regions of downward current. We conclude that they are the high-altitude analogues of the intense return current/black auroral electric field structures observed at lower altitudes by Freja and FAST. In between these structures there are temporal fluctuations, which are shown to likely be downward travelling Alfvén waves. The periods of these waves are 20-40s, which is not consistent with periods associated with either the Alfvénic ionospheric resonator, typical field line resonances or substorm onset related Pi2 oscillations. The multipoint measurements enable us to estimate a lower limit to the perpendicular wavelength of the Alfvén waves to be of the order of 120km, which suggests that the perpendicular wavelength is similar to the dimension of the region between the two quasi-static structures. This might indicate that the Alfvén waves are ducted within a wave guide, where the quasi-static structures are associated with the gradients making up this waveguide.

Cross‐field energy transfer of a body Alfvén wave propagating along and across a pressure‐balanced structure

We conduct hybrid numerical simulations with particle protons and fluid electrons of a low‐frequency, planar (body) and shear Alfvén wave imbedded in a smoothly varying cross‐field pressure‐balanced structure which provides a wave‐speed gradient. We consider wave propagation directions starting at 90°, which resembles the case of a surface wave, and less than 90° with respect to the gradient direction. We find that the planar Alfvén wave undergoes resonant absorption. When the propagation direction is less than 90°, we show that there are resonant field lines which can actually lose wave energy to other neighboring resonant field lines, which is a situation that has not been encountered in previous work with surface waves. A consequence of this process is an overall faster development time for smaller scales perpendicular to the magnetic field through phase mixing and potentially faster dissipation of these generated scales in coronal and solar wind plasma.

Electron spin resonance studies on magnetoresistive (La 0.67Ca 0.33)MnO 3 compound

Electron spin resonance (ESR) measurements have been carried out on colossal magneto-resistive (La 0.67Ca 0.33)MnO 3 thin films deposited on single-crystalline alumina substrate by laser ablation technique. Both, the resonance field value and line shape in ESR spectra were observed to exhibit strong temperature dependence between 4 and 300 K. Significant absorptions appeared at low fields as well. The ESR spectra at low temperature were seen to consist of overlapping low-energy spin wave resonance (SWR) modes excited across opposite surfaces of the film. The SWR data were analyzed by using a magnetic energy density including usual Zeeman and exchange terms contained both in bulk and surface anisotropy energies. The magnetic parameters, such as magnetization, exchange stiffness, relaxation and exchange anisotropy energy, have been deduced as a function of temperature between 4 and 300 K by fitting the simulated spectra to the experimental ones. It was found that quite strong exchange anisotropy is induced consistent with spins canting to give smaller magnetization due to the Dzyaloshinsky–Morya (D–M) interaction in perhaps spin glass-like phase at low temperature.

Ground/satellite signatures of field line resonance: A test of theoretical predictions

During June–July 2002 the low‐altitude (h ∼ 400 km) Challenging Minisatellite Payload (CHAMP) satellite passed approximately every 2nd day close to the South European Geomagnetic Array (SEGMA, 1.56 < L < 1.88) during daytime hours. We present here the analysis of a Pc3 geomagnetic pulsation event observed simultaneously in space and at the ground array during the conjunction of 6 July 2002. Both compressional and transverse oscillations were identified in CHAMP magnetic measurements. A close correspondence between the compressional component and the ground signals is observed. The behavior of the CHAMP azimuthal component shows evidence for the occurrence of a field line resonance at L ≅ 1.6. The frequency of these azimuthal oscillations is ∼20% higher than the frequency of both the compressional oscillation and the ground pulsations. Such a difference is explained in terms of a sort of Doppler shift caused by the fast movement of the satellite across the resonance region where the phase signal changes rapidly. A further analysis verifies for the first time by space measurements the theoretical pattern of the wave polarization sense in the resonance region. The comparison with corresponding SEGMA measurements also provides an unprecedented direct confirmation of the well‐known 90° rotation of the ULF wave polarization ellipse through the ionosphere.

Multi-instrument observations of ULF wave-driven discrete auroral arcs propagating sunward and equatorward from the poleward boundary of the duskside auroral oval

An investigation into how discrete frequency fast mode waves propagating sunward from the magnetotail may drive field line resonant Alfvén waves on closed field lines on the afternoon flank of the Earth’s magnetosphere is presented. Due to the morphology of the magnetic field lines on the flank, higher latitude poleward boundary auroral field lines map deeper into the magnetotail, implying that phase lags from mode coupling to sunward propagating fast mode waves in this stretched field geometry can overcome the standard poleward phase propagation of typical field line resonances, producing resonant Alfvén wave-driven discrete auroral arcs that propagate equatorward on closed field lines. A comparison is made between the predicted results and magnetometer, all sky camera and meridian scanning photometer observations of multiple east–west aligned discrete auroral arcs that propagate equatorward and sunward from the poleward boundary of the dayside auroral oval between 1400 and 1500 UT (approximately 1600 to 1700 MLT) on 3 January 1998. From this comparison, the conclusion is reached that the observations are consistent with the theoretical picture of discrete auroral arcs being driven by Alfvénic waves on closed field lines, the Alfvén waves being excited by discrete frequency fast mode waves propagating sunward from the magnetotail in the flank waveguide of the outer magnetosphere.

Monitoring the plasmapause using geomagnetic field line resonances

This paper discusses the use of ground magnetometer data to derive plasma mass density profiles of the dayside plasmapause region with spatial and temporal resolution in the range 0.15–0.4 RE and 20–60 min. This is achieved using cross‐phase techniques to identify field line resonance signatures that are not apparent in power spectra. Under quiet conditions, mass density profiles do not show a distinct plasmapause and closely resemble electron density profiles for similar conditions. Under more active conditions the plasmapause can be clearly identified, and its width can be resolved in about 20% of the cases. Spatial integration effects smooth the mass density profiles near the plasmapause boundaries, while comparison of the mass and electron densities allows estimates of the heavy ion mass loading. Temporal variations in the plasmapause position and plasmaspheric density depletions are readily resolved. Sudden changes in solar wind conditions cause a redistribution of plasma within ∼20 min, probably in response to penetration of the magnetospheric electric field into the plasmasphere. Field line resonances occur daily and provide a useful tool for investigating the plasmapause region, especially in conjunction with VLF whistler and in situ particle and imaging experiments. Furthermore, the extensive existing suites of magnetometer data permit retrospective studies of focus intervals.