Magnetospheric Cavity Research Articles

Energy deposition in the ionosphere through a global field line resonance

Abstract. We present an interval whereby we can estimate the energy dissipation in the ionosphere through an externally-driven field line resonance. In this paper, we utilise an interval described in general by Rae et al. (2005), where the global magnetospheric cavity was shown to be energised via a high solar wind speed stream. Using the ground-based instrumentation available, we estimate the spatial extent of the generated pulsations to be at least 10° in latitude and 65° in longitude, a sizeable fraction of the dusk-sector ionosphere. Using a fortuitous conjunction with the Polar spacecraft, we compare point measurements of the net downward Poynting vector to the estimated Joule heating rate in the ionosphere, and find that model values of the Pedersen conductance are reasonable. In the interval of interest, we estimate the total dissipation rate during a global field line resonance to be comparable to that reported in substorm studies. Previous studies have estimated the total energy deposition via field line resonance to be up to 4% of that deposited during a small substorm. However, in this paper we find that the total energy deposited via Joule heating may actually be 30% or more of the energy deposited in the ionosphere during a substorm cycle using a conservative estimate of the pulsation duration.

Global Pc5 wave activity observed using SuperDARN radars and ground magnetometers during an extended period of northward IMF

Observations are presented of long-lived global Pc5 ULF wave activity observed at a wide range of local times. The event was monitored in the high latitude ionosphere (∼60–80° magnetic latitude) by several SuperDARN HF radars and 5 magnetometer chains in Scandinavia, Greenland, Canada, Alaska and Russia. The event coincided with a protracted period (∼36 h) of northward interplanetary magnetic field (IMF). The study focuses on 4 h during which distinct dawn/dusk asymmetries in the wave characteristics were observed with multiple field line resonance (FLR) structures observed in the dawn flank at 1.7, 2.6, 3.3, 4.2 and 5.4 mHz and compressional oscillations in the dusk flank at 1.7 and 2.3 mHz. The data indicated an anti-sunward propagation in both the dawn and dusk flanks and a low azimuthal m number (∣ m∣∼6) suggesting a generation mechanism external to the Earth's magnetosphere. A sudden increase in the solar wind dynamic pressure followed by a period of strongly northward, B z dominated IMF, coincides with the observations and also a large increase in Pc5 wave power observed in the dawn flank. The observed enhancements in the wave activity and FLR structures are thought to be due to a Kelvin–Helmholtz driven waveguide mode. Additionally, there is no evidence that the frequencies of the FLRs are intrinsic to the solar wind. It thus seems that the frequencies were determined by the dimensions of the magnetospheric cavity.

Protoplanet magnetosphere interactions

Aims.In this paper, we study a simple model of an orbiting protoplanet in a central magnetospheric cavity, the entry into such a cavity having been proposed as a mechanism for halting inward orbital migration.

Plasma Morphology at Mars. Aspera-3 Observations

A total of about of 400 orbits during the first year of the ASPERA-3 operation onboard the Mars Express spacecraft were analyzed to obtain a statistical pattern of the main plasma domains in the Martian space environment. The environment is controlled by the direct interaction between the solar wind and the planetary exosphere/ionosphere which results in the formation of the magnetospheric cavity. Ionospheric plasma was traced by the characteristic “spectral lines” of photoelectrons that make it possible to detect an ionospheric component even far from the planet. Plasma of solar wind and planetary origin was distinguished by the ion mass spectrometry. Several different regions, namely, boundary layer/mantle, plasma sheet, region with ionospheric photoelectrons, ray-like structures near the wake boundary were identified. Upstream parameters like solar wind ram pressure and the direction of the interplanetary electric field were inferred as proxy from the Mars Global Surveyor magnetic field data at a reference point of the magnetic pile up region in the northern dayside hemisphere. It is shown that morphology and dynamics of the main plasma domains and their boundaries are governed by these factors as well as by local crustal magnetizations which add complexity and variability to the plasma and magnetic field environment.

Long-period magnetic disturbances or Pc5 events aboard INTERBALL-Auroral and POLAR

This paper discusses observations of dynamic magnetic field variations on auroral field lines at mid and high altitudes, utilizing a conjunction of POLAR and INTERBALL-Auroral spacecraft in the dusk magnetosphere on January 11, 1997. Both spacecraft indicate the presence of periodic magnetic variations in the Pc5 frequency range in the field-aligned current region 1. The satellites are believed to be closely on the same field line or within a flux tube (radius 130 km of its projection in the ionosphere). The polarization analysis shows an interesting event: a rotation of the main polarization axis by about 90° between POLAR and INTERBALL. When the two spacecraft are very close to a common field line, the difference between their observed magnetic fields might arise from a rotation of the field between the two altitudes. We cannot exclude different polarization properties arising from different horizontal positions (although small) of both satellite with respect to local plasma boundary. Frequency analysis of both solar wind density (WIND) and INTERBALL magnetic field showed that the solar wind pressure variation contains the same spectrum (2.4–4.4 mHz). It appears that the solar wind pressure variations are sufficient for a parametric mechanism of excitation of compressional disturbances propagating along the magnetopause and within the magnetospheric cavity.

A statistical correlation of Pc5 pulsations and solar wind pressure oscillations

The SHARE high frequency (HF) radar in Antarctica is used to compare ionospheric plasma flow oscillations in the Pc5 frequency range with low-frequency oscillations in the solar wind pressure measured by the ACE spacecraft. Ten different days in 2000 and 2001 are analysed with respect to different frequencies and geomagnetic latitudes. Both data sets are bandpass filtered and a complex demodulation technique is used to calculate the correlation in each band. On a number of occasions the wave packet structure of the Pc5 pulsations is in good or excellent agreement with the wave packet structure of the solar wind pressure oscillations. This strongly suggests that the oscillations were directly driven by the solar wind. Particularly good correlation is found in the frequency band 0.8–1.2 mHz. Pulsations in this frequency range are hard to reconcile with the magnetospheric cavity mode model. We conclude that, at least on some occasions, Pc5 pulsations may be directly driven and the magnetosphere cavity/waveguide then assumes a more passive role.

Solar System Magnetospheres

This article proposes a short review of our present knowledge of solar system magnetospheres, with the purpose of placing the study of Saturn’s magnetosphere in the context of a comparative approach. We describe the diversity of solar system magnetospheres and the underlying causes of this diversity: nature and magnetization state of the planetary obstacle, presence or not of a dense atmosphere, rotation state of the planet, existence of a system of satellites, rings and neutral gas populations in orbit around the planet. We follow the “russian doll” hierarchy of solar system magnetospheres to briefly describe the different objects of this family: the heliosphere, which is the Sun’s magnetosphere; the “elementary” magnetospheres of the inner planets, Earth and Mercury; the “complex” magnetospheres of the giant planets, dominated by planetary rotation and the presence of interacting objects within their magnetospheric cavities, some of which, like Ganymede, Io or Titan, produce small intrinsic or induced magnetospheres inside the large one. We finally describe the main original features of Saturn’s magnetosphere as we see them after the Voyager fly-bys and before the arrival of Cassini at Saturn, and list some of the key questions which Cassini will have to address during its four-year orbital tour.

Solar wind-magnetosphere–ionosphere coupling at Jupiter

A fundamental property of planetary magnetospheres, which varies from planet to planet, is the dynamical influence of either the solar wind and its embedded interplanetary magnetic field (IMF) or of the rotation of the planet. Jovian magnetospheric dynamics are mainly dominated by the combination of rapid planetary rotation and the outflow of material originating from the moon Io, orbiting deep within the magnetospheric cavity. The main auroral oval in Jupiter’s polar ionosphere appears fixed with respect to the planet, an indication of planetary control, and is understood to be associated with large-scale magnetosphere–ionosphere coupling, and the transfer of angular momentum from the ionosphere to the middle magnetosphere plasma. We will first discuss the origin of this main auroral oval, and the theoretical model which followed. We will also summarise recent developments of the theoretical model to include the effects of precipitation-induced enhancements of the ionospheric Pedersen conductivity in regions of upward field-aligned currents. Jupiter’s polar auroral emissions, which include all auroral emission lying poleward of the main auroral oval, are ordered by magnetic local time, indicating potential external control by the solar wind. We have recently considered from a theoretical standpoint what the effects of pulsed dayside magnetic reconnection will be at Jupiter. This will generate a twin-vortical flow pattern in the ionosphere across the open-closed field line boundary, associated with a bi-polar (i.e. upward and downward) field-aligned current pair. Here, we discuss the conditions under which such currents may be carried in either magnetospheric or cusp plasmas, and summarise the consequences for auroral emissions at UV and X-ray wavelengths.

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].

Moon–magnetosphere interactions: a tutorial

The interactions between the Galilean satellites and the plasma of the Jovian magnetosphere, acting on spatial scales from that of ion gyroradii to that of magnetohydrodynamics (MHD), change the plasma momentum, temperature, and phase space distribution functions and generate strong electrical currents. In the immediate vicinity of the moons, these currents are often highly structured, possibly because of varying ionospheric conductivity and possibly because of non-uniform pickup rates. Ion pickup changes the velocity space distribution of energetic particles, f( v), where v is velocity. Distributions become more anisotropic and can become unstable to wave generation. That there would be interesting plasma responses near Io was fully anticipated, but one of the surprises of Galileo's mission was the range of effects observed at all of the Galilean satellites. Electron beams and an assortment of MHD and plasma waves develop in the regions around the moons, although each interaction region is different. Coupling of the plasma near the moons to the Jovian ionosphere creates auroral footprints and, in the case of Io, produces a leading trail in the ionosphere that extends almost half way around Jupiter. The energy source driving the auroral signatures is not fully understood but must require field aligned electric fields that accelerate elections at the feet of the flux tubes of Io and Europa, bodies that interact directly with the incident plasma, and at the foot of the flux tube of Ganymede, a body that is shielded from direct interaction with the background plasma by its magnetospheric cavity.

Moon?magnetosphere interactions: a tutorial

The interactions between the Galilean satellites and the plasma of the Jovian magnetosphere, acting on spatial scales from that of ion gyroradii to that of magnetohydrodynamics (MHD), change the plasma momentum, temperature, and phase space distribution functions and generate strong electrical currents. In the immediate vicinity of the moons, these currents are often highly structured, possibly because of varying ionospheric conductivity and possibly because of non-uniform pickup rates. Ion pickup changes the velocity space distribution of energetic particles, f ( v ), where v is velocity. Distributions become more anisotropic and can become unstable to wave generation. That there would be interesting plasma responses near Io was fully anticipated, but one of the surprises of Galileo's mission was the range of effects observed at all of the Galilean satellites. Electron beams and an assortment of MHD and plasma waves develop in the regions around the moons, although each interaction region is different. Coupling of the plasma near the moons to the Jovian ionosphere creates auroral footprints and, in the case of Io, produces a leading trail in the ionosphere that extends almost half way around Jupiter. The energy source driving the auroral signatures is not fully understood but must require field aligned electric fields that accelerate elections at the feet of the flux tubes of Io and Europa, bodies that interact directly with the incident plasma, and at the foot of the flux tube of Ganymede, a body that is shielded from direct interaction with the background plasma by its magnetospheric cavity.

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.

Controlled precipitation of radiation belt electrons

First‐order estimates indicate that the lifetime of energetic (a few MeV) electrons in the inner radiation belts (e.g., near L = 2) may be significantly reduced by in situ injection of whistler mode waves at radiated power levels of a few kW at frequencies of a few kHz. Our estimates are based on previously published results concerning the effect on the electron lifetimes of VLF signals from ground‐based VLF transmitters operating in the 17–23 kHz range. Waves at lower frequencies (a few kHz) can drive diffusion rates that are higher by a factor of as much as ∼30 and can also be efficiently stored in the magnetospheric cavity, resulting in additional effective enhancement of wave power density of a factor of ∼16. This wave power enhancement is also expected to enhance the MeV electron diffusion rates.

Dynamical response of the magnetosphere‐ionosphere system to a solar wind dynamic pressure oscillation

On 28 February 1998, four quasi‐periodic pressure pulses with an amplitude of a few nPa detected by ACE gave rise to periodic compressions of the magnetosphere with period of about 14 min. In concert with periodic compressed and expanded states of the magnetosphere forced directly by the pressure variation, a coherent geomagnetic field fluctuation with the same period appeared on a global scale and was recorded at stations located from polar to equatorial regions. Most ground‐level geomagnetic field signatures on the dayside can be interpreted as the result of a global ionospheric current system, like the global Pc5 event examined by Motoba et al. [2002]. In the afternoon polar ionosphere covered with the dense magnetometer stations, a vortical current structure associated with pressure‐induced field‐aligned currents (FACs) is centered at 72° ± 1° and consists of a counterclockwise (clockwise) vortex in response to positive (negative) changes of solar wind pressure oscillation. Although the vortical current signatures are unclear in the morning sector, each afternoon vortex could pair with the morning one with opposite rotation. During this event, the interplanetary magnetic field (IMF) remained steady with a strong southward orientation (−10 nT or less). In addition to the pressure‐induced FAC system, the steady southward IMF drives the dayside Region 1 (R1) current system, resulting in the familiar large‐scale two‐cell convection pattern in the ionosphere observed by SuperDARN radars. The SuperDARN convection patterns indicated that the ionospheric convection reversal boundary (CRB) in the afternoon was located in the range of 73° ∼ 77°N around 15 MLT. The ionospheric footprint of the pressure‐induced FAC in the afternoon was found to be 1.5° ± 1.1° ∼ 4.0° ± 1.4° equatorward of the CRB. This suggests that the pressure‐induced FAC is started inside the R1 current system originating from the outer magnetospheric boundary layer. We argue that the paired FAC system responsible for the global geomagnetic fluctuations on the ground arises from the oscillatory large‐scale dynamical convection originating well inside the closed field lines in direct response to the quasi‐periodic pressure variations, not from the localized undulations on the magnetopause nor from global eigenmode oscillations of the magnetospheric cavity.

Excitation of field line resonances by MHD waves originating in the solar wind

Recent observations suggest that in some circumstances a field line resonance may be driven by discrete oscillations in the solar wind. A theoretical model of excitation of magnetohydrodynamic (MHD) field line resonances by waves originating in the solar wind is presented. Such waves occur in the solar wind although their source is not understood. The propagation of an MHD wave in the solar wind is followed through the bow shock and magnetosheath. It is shown how the properties of the magnetospheric cavity are functions of longitude. The incident wave is captured at a longitude where its azimuthal wave number has a value allowing matching to the cavity oscillation. There is significant amplification at the bow shock and at the magnetopause. It is found that the signal penetrates the cavity and excites a field line resonance on the flanks of the magnetosphere in the morning and afternoon sectors. Rather than behaving as a waveguide, the cavity behaves more like an optical filter.

Observations of magnetospheric plasmas by the radio plasma imager (RPI) on the image mission

The Imager for Magnetopause-to-Aurora Global Exploration satellite (IMAGE, see http://image.gsfc.nasa.gov.), launched on March 25, 2000, is the first mission dedicated to observing the global-scale structure and dynamics of the magnetosphere. Remote-sensing instruments on IMAGE are designed to observe the magnetopause, ring current, plasmasphere, polar cusp, and the auroral region, in order to reveal the morphologies and interactions among these inter-connected regions. In particular, the RPI, a digital radio sounder with direction-finding capabilities, can transmit radio pulses from 3 kHz to 3MHz (corresponding to plasma densities of 0.1–10 5 cm −3) to probe different magnetospheric plasmas. Using two 500-m tip-to-tip orthogonal dipole antennas in the spin plane and a 20-m tip-to-tip spin-axis dipole antenna, signals reflected at remote plasma regions are received as echoes. The RPI measures the echo amplitude, phase, range, polarization, Doppler shift, and angle of arrival as a function of sounding frequency. Advanced digital techniques allow echoes of low-power transmitted signals to be detected over magnetospheric distances. With a nominal 2-minute IMAGE spin period in a highly inclined (90°) elliptical orbit (apogee of 8 R E), the RPI (with a nominal 1-min resolution) situated in the magnetospheric cavity observes the inner and outer magnetospheric boundaries. Its observations can reveal the electron density profiles and dynamics of key magnetospheric plasma regions. This paper highlights the science objectives of the RPI on IMAGE and presents some early RPI observations.

Modulation of Jupiter's main auroral oval emissions by solar wind induced expansions and compressions of the magnetosphere

It has recently been suggested that the jovian ‘main auroral oval’ is associated with the region of upward-directed field-aligned currents in the circuit formed through angular momentum exchange between the atmosphere–ionosphere and the equatorial iogenic magnetospheric plasma. It has also been suggested that the luminosity of these emissions should be modulated primarily by the dynamic pressure of the solar wind, which causes expansions and compressions of the magnetosphere and thereby changes the angular velocity of the magnetospheric plasma and the strength of the coupling currents. Here we present a quantitative demonstration of these effects. We consider an initial empirically based middle magnetosphere field and plasma configuration extending to 50 R J, which expands outwards to 70 R J, or is compressed inwards to 30 R J, due e.g. to sudden changes in the dynamic pressure of the solar wind. Changes in the angular velocity of the plasma are computed from the flux-preserving motions of the field lines using conservation of angular momentum. The initial configuration produces a band of aurora in which the luminosity falls over ∼1° latitude from ∼200 kR at the poleward boundary to ∼20 kR . Expansion of the current sheet to 70 R J produces a significant increase in luminosity over this range, with the largest effect occurring at the poleward boundary where a thin (∼50– 100 km ) arc-like structure of MR intensity is formed. The initial emission is reduced by about an order of magnitude for a compression to 40 R J, and essentially to zero for a compression to 35 R J, when the middle magnetosphere plasma is brought to near rigid corotation over the whole radial range such that the coupling current system is essentially switched off. Further compression to 30 R J then induces significant super-corotation, with a reversal in sense of the magnetosphere–ionosphere coupling current system. In this case the middle magnetosphere field-aligned currents reverse to downward throughout, such that we would not expect them to be associated with bright electron-induced auroras. However, upward-directed ‘return’ currents then flow in the poleward region outside of the middle magnetosphere, which may be associated with an auroral oval lying poleward of the usual ‘main oval’ location. A ‘Hill-type’ equilibrium will re-form on time scales of a few tens of hours, and with it, the main oval auroras will re-emerge, corresponding to the changed size of the magnetospheric cavity.

A theory of field line resonance in a dipole‐like axisymmetric magnetosphere

A self‐consistent field line resonance problem in the model of an axisymmetric dipole‐like magnetosphere is solved. The spatial structure not only of resonance Alfven waves but also of the magnetosonic wave exciting them was obtained in this paper. The field structure of the magnetosonic wave has been determined both inside the magnetospheric cavity and in the solar wind region near the magnetosphere. This made it possible to compare the amplitudes of magnetosonic waves incident from the solar wind on the magnetosphere and the resonance Alfven oscillations which they excite in the magnetosphere. It is shown that as magnetosonic oscillations penetrate deep into the magnetosphere, their amplitude decreases several orders of magnitude. The amplitude of Alfven oscillations excited inside the magnetosphere is comparable with that of magnetosonic waves in the solar wind region. The amplitude of resonance Alfven oscillations on internal magnetic shells is significantly larger than that of resonance oscillations excited in the transition layer of the magnetosphere.

Long‐period magnetospheric‐ionospheric perturbations during northward interplanetary magnetic field

In this paper we present observations of long‐period magnetospheric‐ionospheric perturbations during northward interplanetary magnetic field (IMF). On November 10–11, 1998, the IMF was northward for 29 hours. The solar wind and IMF parameters were relatively steady. After the IMF had been northward for 14 hours, strong ionospheric velocity (or electric field) perturbations were observed by the Iceland West HF radar in the postmidnight/dawn sector. The velocity perturbations showed periods in the range 50–60 min, with a mean value ∼54 min. The Super Dual Auroral Radar Network observed periodic changes of nightside ionospheric convection. For each cycle a large‐scale convection cell formed around 0200 magnetic local time (MLT) near magnetic latitude 70° and grew for ∼30 min. The convection cell then moved eastward with a mean velocity of ∼1.4 km s−1. The final position of the cell focus was around 0600 MLT near magnetic latitude 77°. Ground magnetometers recorded weak magnetic perturbations with periods 50–60 min in both the premidnight and postmidnight sectors. The GOES 8 satellite also observed magnetospheric magnetic field perturbations with similar periods at L ∼ 6 on the nightside. The satellite and the key radars which observed the convection oscillations are nearly magnetically conjugate, so the good temporal correlation between the satellite and radar data indicates that the magnetospheric and ionospheric perturbations have a common source. We propose that the magnetospheric‐ionospheric perturbations originated within the magnetosphere. The transformation of the magnetospheric tail from an extended to a more dipolar shape during northward IMF is associated with 40–60 min period global tail oscillations which ultimately result in the generation of field‐aligned currents and nightside ionospheric convection vortices near X = −10 RE. If the proposed mechanism is correct, it will be possible to infer the magnetospheric cavity shape from the measured periodicity of the tail oscillations during northward IMF.

Radio sounding in space: magnetosphere and topside ionosphere

Modern sounding techniques have been developed for the space-borne exploration of Earth's magnetosphere and topside ionosphere. Two new satellite instruments will use the advanced techniques of the ground-based Digisondes. The Radio Plasma Imager (RPI), a low-frequency sounder with 500-m dipole antennas designed to sweep from 3 kHz to 3 MHz, will be part of NASA's IMAGE mission to be launched in February 2000 into an elliptical orbit with an altitude at apogee of 7 R e. While in the magnetospheric cavity, RPI will receive echoes from the magnetopause and the plasmasphere and will measure the direct response of the magnetosphere's configuration to changes in the solar wind. With three orthogonal dipole antennas (two 500-m tip-to-tip antennas in the spin plane used for transmission and reception, one 20-m antenna along the spin axis for reception only) the arrival angle of returning echoes can be determined with high accuracy. The other instrument is the TOPside Automated Sounder (TOPAS), which was originally conceived for the Ukrainian WARNING mission with a launch date in 2001. Using one antenna for transmission and three orthogonal 10-m antennas for reception, TOPAS will be able to determine the arrival angle of ionospheric echoes and their wave polarization. It will then be possible to automatically scale the topside ionograms and calculate the electron density profiles in real time. Operating as a high-frequency radar, TOPAS will for the first time measure topside plasma velocities by tracking the motions of plasma irregularities.