Magnetic Field Signatures Research Articles

Observations of earthward and tailward propagating flux rope plasmoids: Expanding the plasmoid model of geomagnetic substorms

A survey of IMP 8 magnetometer data for plasmoid signatures during magnetospheric intervals from 1981 through 1983 found 16 plasmoids and 37 traveling compression regions as well as two earthward propagating flux ropes and 19 south‐north bipolar lobe signatures. The properties of these relatively near‐Earth plasmoids, traveling compression regions, and earthward propagating flux ropes and a qualitative model for their formation are presented. The plasmoids have estimated sizes, durations, magnetic field signatures, downtail velocities, and substorm associations very similar to those of the plasmoids identified in ISEE 3 deep‐tail observations. The occurrence frequency of these near‐Earth plasma sheet plasmoids is significantly smaller than that of plasmoids found in the mid‐ and deep tail with ISEE 3. The earthward propagating flux ropes are characterized by a south‐north bipolar turning in the GSM Bz component, are localized near the noon‐midnight meridional plane, and are strongly correlated with interplanetary magnetic field Bz north and small isolated high latitude geomagnetic substorms. These events are also apparently very rare and/or spatially localized. We propose that these structures are “proto‐plasmoids,” i.e., plasmoids for which near‐Earth magnetic reconnection stopped before all the closed plasma sheet field lines were reconnected. The proto‐plasmoids are then “trapped” inside closed magnetic field lines and propagate earthward owing to the effect of the distant X‐line's earthward plasma flow. We suggest that the two different “types” of plasmoids are due to the different energy states of the magnetosphere during periods of southward and northward interplanetary magnetic field.

Energetic particle, plasma and magnetic field signatures of a poloidal pulsation in Jupiter's magnetosphere

During its February 1992 inbound journey into Jupiter's magnetosphere, Ulysses made several near-encounters with (and actual traversals of) the Jovian magnetodisk. In at least one such event (investigated here) the anticipated variations in energetic particle fluxes, plasma density, magnetic field intensity, and magnetic field direction at the planetary rotation period were further modulated (strongly and quasi-periodically) at a higher frequency, as if the magnetodisk and magnetic-equatorial surface were “flapping” relative to the position of the spacecraft. This additional modulation occurred around 14: 00–19: 00 U.T. on 7 February (Day 38) 1992, with Ulysses at a nominal (dipolar) magnetic latitude ≈ 11°–22° and at a zenocentric distance r ≈ 18–22 R j. The quasi-period 2π/Ω revealed by autocorrelation analysis of the energetic-proton data (and confirmed by visual examination of plotted data from the various instruments) was about 6.8 ks (≈1.9 h). Oscillations in energetic particle fluxes and plasma density were typically 180° out-of-phase with those in magnetic field intensity. The 6.8 ks period is consistent with oscillation of the magnetic shell of radius r ≈ 21 R j at the second poloidal harmonic, which is the mode that would correspond to a “flapping” of the magnetodisk and (more generally) of the magnetic-equatorial surface.

High‐time resolution measurements of upstream magnetic field and plasma conditions during flux transfer events at the Earth's dayside magnetopause

We present preliminary results of a study of upstream magnetic field and plasma conditions measured by IRM during flux transfer events observed at the Earth's magnetopause by CCE. This study was designed to determine the importance of various upstream factors in the formation of bipolar magnetic field signatures called flux transfer events (FTEs). Six FTE encounters were examined. Three cases were when the two satellites were on similar magnetic field lines. Preliminary investigation showed that fluctuations occurred in the Bz component of the interplanetary magnetic field (IMF) resulting in a southward field preceding the FTE in all three of these cases. In two of these cases, the changes were characterized by a distinct rotation from a strong southward to a strong northward field. There were also accompanying changes in the dynamic and thermal pressure in the solar wind immediately before the FTE was encountered. Examination of the 3d plasma distributions showed that these pulses were due to the addition of energetic upstreaming foreshock particles, There were no consistent changes in either Bz or the plasma pressure at IRM for the three events when the satellites were not connected by the IMF.

Observational test of hot flow anomaly formation by the interaction of a magnetic discontinuity with the bow shock

The formation of a hot flow anomaly (HFA) observed near the Earth's bow shock appears to be due to the interaction between the bow shock and an impinging discontinuity in the upstream plasma. Recent single‐particle and two‐dimensional hybrid numerical studies have suggested that such an interaction will produce an HFA only if the motional electric field in the ambient plasma points toward the discontinuity, thereby focusing shock‐reflected ions into it. We perform a test of this electric field orientation for a set of nine HFA events observed by the ISEE spacecraft and described previously in the literature. The principal difficulty with the test is the determination of the normals to the discontinuities. Application of a minimum variance analysis produces discontinuity normals which suggest that the majority of the discontinuities were probably tangential, even though it is doubtful that the conditions for validity are very well satisfied for these events. Under the assumption that the discontinuities were tangential, the predicted electric field orientation is found on at least one side of all nine observed HFAs (on the trailing edge of seven and the leading edge of five, and on both sides of three events). Further, there is evidence that asymmetries in the observed magnetic field signatures are related to the orientation of the motional electric field: The events in which the electric field points toward the discontinuity on both sides tend to be those with fairly symmetric flanking magnetic field enhancements. Two‐dimensional hybrid simulations appear to confirm the association between the magnetic field signature and the electric field orientations, and they show further that rotational discontinuities appear to be less effective at producing distinct HFA‐like signatures than tangential discontinuities.

Magnetic Field Signatures Near Galileo's Closest Approach to Gaspra

Two large magnetic field rotations were recorded by the spacecraft Galileo 1 minute before and 2 minutes after its closest approach to the asteroid Gaspra. The timing and the geometry of the field changes suggest a connection with Gaspra, and the events can be interpreted as the result of the draping of the solar wind field around a magnetospheric obstacle. Gaspra's surface field is inferred to be within an order of magnitude of Earth's surface field, and its magnetic moment per unit mass is in the range observed for iron meteorites and highly magnetized chondrites. The location of the magnetic signatures suggests that perturbations are carried by waves in the magnetosonic-whistler mode with wavelengths between electron and ion gyro radii.

Magnetic field properties of the intermediate transition of the Venus ionosheath

We report new results of a statistical analysis of the electric and magnetic field data of the Pioneer Venus Orbiter (PVO) on the intermediate transition of the Venus ionosheath. This transition marks the outer boundary of an expansion region along the flanks of the inner ionosheath, and is identified by simultaneous observations of weak plasma fluxes and characteristic 30 kHz electric field bursts measured, respectively, with the plasma analyzer and the electric field detector. In many cases, the magnetic field signature across the intermediate transition is characterized by a sudden decrease in intensity and an abrupt rotation so that the direction is more nearly aligned with the Sun‐Venus line. In other cases the 30 kHz bursts occur outside a region of enhanced magnetic field reminiscent of the magnetic barrier external to the Venus dayside ionopause. In these cases there is also a rotation of the magnetic field toward the Sun‐Venus line associated with the 30 kHz bursts, but, in general, the conditions change more gradually. These variations may result from the expansion of the shocked solar wind at and near the magnetic polar regions of the Venus ionopause around which the draped interplanetary magnetic field lines slip over the planet. The expansion of the ionosheath plasma may result from local heating processes associated with a friction‐like interaction between the shocked solar wind and the topside ionospheric plasma.

Initial signatures of magnetic field and energetic particle fluxes at tail Reconfiguration: Explosive growth phase

Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetometer and Medium Energy Particle Analyzer (MEPA) data are used to examine the initial signatures of tail field reconfiguration observed in the near‐Earth magnetotail (< 9 RE). Sixteen events are selected preliminarily from 9 months (January‐September 1985) of magnetometer data according to two criteria, that is, an unambiguous commencement of tail field reconfiguration and a sharp recovery of the north‐south (H) component. The second criterion requires that the satellite was close to the onset region of current disruption. Although these strict criteria result in the small number of events, the magnetic and particle flux signatures of the events are considered to be informative concerning the mechanism of substorm onsets. It is found that these tail reconfiguration events are classified into two types: Type I and Type II. In Type I events a current disruption starts in a flux tube that is inward (earthward/equatorward) of the spacecraft, and consequently, the spacecraft is immersed in a hot plasma region expanding from inward (earthward/equatorward). The other type (Type II) is characterized by a distinctive interval (explosive growth phase) just prior to the local commencement of tail reconfiguration. The duration of this interval is typically 1 min, much shorter than that of the so‐called growth phase. During this interval the north‐south magnetic (H) component is depressed sharply, and the flux of energetic ions increases outward (tailward/poleward) of the spacecraft, suggesting that the cross‐tail current is explosively enhanced. It is also found that the radial magnetic (V) component changes with a distinctive phase relationship relative to the north‐south component, which can also be explained in terms of the explosive enhancement in the cross‐tail current intensity just prior to the current disruption. This enhancement is inferred to be a local process, rather than a result of a current disruption which has occurred somewhere else, although it is possible that the commencement of the H recovery observed is not exactly simultaneous with a substorm onset. The present results contribute significantly to modeling efforts regarding the triggering mechanism of substorms in the magnetotail.

ISEE‐1 and ‐2 observations of an isolated diamagnetic event: An earthward‐moving plasma bulge or a tail‐aligned flux rope?

This paper reports a transient strong diamagnetic event observed with the ISEE‐1 and ‐2 spacecraft in the near‐Earth magnetotail. Magnetic signatures associated with the event resemble those of flux rope events found in the distant magnetotail. The present event should be distinguished from those in several previous reports based on the following points: (1) The event is observed in the near‐Earth tail (X ∼−17 RE), not in the distant tail. (2) The tail field changes from a stressed configuration to a more dipolar one in the course of the event, and the plasma data suggest that the heating and the resultant expansion of the plasma take place simultaneously. (3) The minimum of the magnetic field strength BT is ∼1 nT; such a strong depression is rarely observed in the near‐Earth tail. (4) An earthward plasma jet (300 to 500 km/s) continues for about 3 min, including a period of very weak magnetic field; the duration of the jet is exceptionally long compared to previously reported high‐speed flows in the plasma sheet. These magnetic field and plasma signatures are discussed in terms of two models: an earthward‐moving plasma bulge and a tail‐aligned flux rope.

Resolution of magnetic flux tubes on the Sun

MAGNETIC flux at the surface of the Sun is predominantly concentrated in discrete areas with kilogauss field strengths1. Except for sunspots, these areas are too small to have been resolved by conventional observations. These magnetic flux tubes are an essential part of the physics of the activity and heating of the outer atmosphere of the Sun and other late-type stars2, but although their average properties have been studied in considerable detail3,4, direct observations of them have been lacking because of turbulence in the Earth's atmosphere, which limits resolution to ∼400 km. Using a newly developed technique of speckle inter-ferometry5, we have obtained simultaneous direct observations of the white-light and magnetic field signature of flux tubes. Individual flux tubes are seen, with resolved diameters of ∼200 km and continuum brightness contrast of at least +30%. Magnetic features larger than 300 km in size tend, however, to be darker than their surroundings.

Modeling the magnetic signature of impact‐generated plasma clouds

A theoretical model has been developed to evaluate the magnetic field disturbances due to plasma clouds formed by hypervelocity (68 km/s) impacts of milligram mass dust particles on the Giotto spacecraft at comet P/Halley. The model is partly analytical and assumes unmagnetized ions and frozen‐in electrons as suggested by the high time‐resolution magnetic field observations. It treats the cloud development after an initial phase with unfrozen plasma ending a few milliseconds after impact. Fitting the model to the observed magnetic field signature of the impacts allows the determination of the center‐of‐mass velocity and the expansion velocity as well as the total number of particles in the expanding cloud, which leads to an estimate for the mass of the dust particle.

Test of Closed‐Loop Degaussing Algorithm on a Minesweeper Engine

ABSTRACTA closed‐loop degaussing technique for MCM (mine countermeasure) class vessels has been developed and tested on a minesweeper engine. The system was designed to accurately predict and degauss the off‐board magnetic field signatures from measurements taken on‐board a non‐magnetic hull naval vessel. Calibration procedures for an on‐board to off‐board signature prediction algorithm have been investigated. An onboard sensor configuration, that requires only four single axis gradiometers, has been shown to accurately predict the magnetic signatures of the full‐scale engine.

Magnetic field structures and related phenomena at quasi-parallel shocks

Under “quasi-parallel” conditions, when the upstream magnetic field is nearly aligned with the shock normal, collisionless shocks exhibit unsteady, complicated behaviour due primarily to the relative ease of particle motion from the downstream, shocked state to the upstream foreshock region. Perhaps the most straight-forward manifestation of the resultant feedback system can be found in the variety of magnetic field signatures which accompany such shock transitions. The rather inefficient coupling between incident and backstreaming populations introduces a multiplicity of length scales, and a corresponding pattern of magnetic structures. This paper summarizes and orders these field structures, from the low amplitude, nearly sinusoidal, low frequency waves observed far upstream to the large amplitude turbulent “pulsations” which are more traditionally associated with the shock itself.

Ion composition measurements within magnetospheric flux transfer events

Impulsive reconnection near the subsolar magnetopause is a fundamental process of interaction between the magnetosphere and the solar wind flow. Flux Transfer Events (FTEs) appear to be a manifestation of this process. It is therefore important to fully characterize the FTE so as to better understand the physics of the interaction process. We use magnetic field signatures from the AMPTE/CCE spacecraft to identify two magnetospheric FTEs. Electron distributions measured in the plane of the magnetopause inside these FTEs complement previously reported electron measurements. We report, for the first time, ion composition measurements in the energy range ∼0 to 20 keV/e within an FTE. We find that the ion distributions, the ion composition, and the flow velocities are unique to the FTE and unlike either the adjacent magnetosphere, the nearby boundary layer, or the nearby magnetosheath. The H+, He+, and He2+ distribution functions in the FTEs have reversed temperature anisotropies and the relative He2+ abundance is depressed with respect to either the magneosheath or the low latitude boundary layer.

Hypervelocity dust particle impacts observed by the Giotto Magnetometer and Plasma Experiments

We report thirteen very short events in the magnetic field of the inner magnetic pile‐up region of comet Halley observed by the Giotto magnetometer experiment together with simultaneous plasma data obtained by the Johnstone plasma analyzer and the ion mass spectrometer experiments. The events are due to dust impacts in the milligram range on the spacecraft at the relative velocity between the cemetery dust and the spacecraft of 68 km/sec. They are generally consistent with dust impact events derived from spacecraft attitude perturbations by the Giotto camera [Curdt and Keller, private communication]. Their characteristic shape generally involves a sudden decrease in magnetic field magnitude, a subsequent overshoot beyond initial field values and an asymptotic approach to the initial field somewhat reminiscent of the magnetic field signature after the AMPTE releases in the solar wind. These observations give a new way of analyzing ultra‐fast dust particles incident on a spacecraft.

Synchrotron Radiation from Energetic Electrons Emitted by AGN: A Probe for Magnetic Fields in External Galaxies

Shock acceleration of protons in the central region of active galaxies can energize them to Lorentz factors as high as 108 (Sikora et al., 1987), and these can subsequently produce a host of other relativistic particles including pions, e+e– pairs and neutrons. The luminosities of each species are expected to be of the same order of magnitude. Rapid decay of the pions leads to the secondary production of photons and pairs with energies of around 109 − 1011 MeV. The electrons and positrons can escape the compact central region and interact with the microwave background forming a pair cascade, and can also emit synchrotron radiation in the magnetic field. The neutrons do not interact with the field, and a significant fraction of them can escape the central region of a galaxy (Kirk and Mastichiadis, 1989). They can travel until they decay, producing protons and electrons in outer regions of the galaxy. Their decay time of γnτn gives a typical length for decay of about 1 kpc for the most energetic neutrons. The synchrotron radiation of these decay product electrons is examined in Baring (1989, in preparation), and it produces definite signatures of galactic magnetic fields. Magnetic fields of 1μG imply synchrotron emission in the X-ray and soft gamma-ray range for maximum Lorentz factors of γe = 1010, with a continuum extending down to much lower energies. It is observed that cooler neutrons deposit electrons at smaller radii, and these electrons are cooler (in a decay γe ~ γn). Hence the radiation would be cooler at smaller radii. This provides a diagnostic for the magnetic field: estimates of the field strength are possible from cut-offs that are expected in spectra from galactic halos. The injection of energetic electrons via neutron decay is found to yield a sharp cut-off in the injection distribution at γm = γe ~ r/τnc at radius r. Below this, no electrons are injected since they are produced in decays at smaller radii. This implies a low energy cutoff of ωm = γ2mBmec2 in the spectrum at given radius. Typically for r = 10 pc and a field of 1μG, the cutoff is at 10−8mec2 in the far infra-red. At larger radii, this low energy cutoff rapidly increases to X-ray energies. This cut-off provides a good way to measure the magnetic field strength and obtain its spatial dependence. In practice the situation is complicated by the superposition of different regions within the galaxy along the line of sight. Ways in which the observations could be deconvolved are discussed in Baring (1989, in preparation). A real possibility exists for detecting these signatures of energetic neutron emission from central regions of nearby galaxies and using the electron synchrotron spectra to spatially map galactic fields.

A user's guide to locating flux transfer events in low‐altitude satellite measurements: An S3‐2 case study

Although flux transfer events (FTEs) have been observed frequently at high altitudes near the magnetopause their ionospheric signatures remain elusive. Vestiges of FTEs and other magnetopause phenomena have possibly been detected in transient magnetic field perturbations measured on the ground at cusp latitudes. Direct observations of FTEs by instrumentation on polar orbiting satellites have not yet been forthcoming. We have developed a methodology for use by investigators based on expected, simultaneous measurements of plasma drifts (electric fields), field‐aligned currents (magnetic fields) and precipitating particle fluxes. To test and illustrate this methodology we present the results of a search through the available S3‐2 data for electric and magnetic field signatures from the mid‐day sector displaying irregular structures of scales 100 to 200 km, consistent with ionospheric mapping of FTEs. Out of 36 orbits between December 1975 and April 1976 we found five possible events. An event selected for detailed analysis was observed approximately at 2219 UT on January 5, 1976 during an equatorward crossing of the northern dayside auroral region at about 1030 MLT. The qualitative electric and magnetic field fluctuations expected during a satellite encounter with an FTE at ionospheric altitudes are compared with single component measurements for S3‐2. Although an encounter with a fast stream in the plasma flow poleward of the cusp cannot be excluded, the data are consistent with expectations for the S3‐2 spacecraft having made a near center crossing of an ionospheric footprint of an FTE with a diameter of about 150 km moving eastward along the polar cap boundary with a velocity of 1.3 km/s. Criteria for distinguishing between FTEs and fast streams from satellite measurements of two components of the electric and magnetic field are presented.

The magnetospheric response to 8‐minute period strong‐amplitude upstream pressure variations

This paper documents a series of brief, strong (Δp/p = 1), dynamic pressure oscillations that occurred in the region upstream of the Earth's bow shock during a period of radial interplanetary magnetic field (IMF). The analyzed set of oscillations, which may be either intrinsic solar wind or bow shock‐related phenomena, recur approximately every 8–10 min, and their magnetic field signatures occur nearly simultaneously over great distances transverse to the Earth‐Sun line. The pressure oscillations appear to drive tailward‐moving magnetopause surface wavelets. In turn, the surface wavelets can be identified as hydromagnetic waves with strong compressional components in the outer magnetosphere and as quasi‐periodic variations in electron precipitation and high‐latitude ground pulsations. We use observations by spacecraft in the outer dayside magnetosphere to predict geosynchronous and subsolar magnetic field strengths, the location of the subsolar magnetopause, the solar wind dynamic pressure, and variations in the energetic magnetospheric ion flux.

Substorm variations in the magnitude of the magnetic field: AMPTE/CCE observations

By using AMPTE/CCE energetic particle data, 167 ion injections that are essentially dispersionless over a nominal energy range of 25 keV to 285 keV have been cataloged. The ion, electron, and magnetic field signatures of these events are the kind normally associated with substorms in the near‐Earth magnetotail. We have examined the total magnetic field strength variations over the course of these events to identify the dependence on dipole latitude. The change in is poorly correlated with magnetic latitude. We interpret this result as indicating that during periods of substorm activity, the latitudinal position of the current sheet varies significantly within the 32° wedge centered on the dipole equator traversed by CCE. On the other hand, the changes in are well correlated with the local field properties themselves. This study therefore indicates that even in the near‐Earth magnetotail out to 8.8 RE (CCE apogee), the local field measurements are a better guide to the determination of one's position relative to the current sheet during a substorm than the magnetic latitude. Statistical relationships between Δ and both and |Bx| (in GSM coordinates) are consistent with the changes in that one would expect from a disruption/diversion of the cross‐tail current. The radial gradient of the reduction of the cross‐tail current during substorms appears to be negative, although the evidence is not conclusive. Finally, the local time variation of the statistical relationships is consistent with Birkeland current segments of the substorm current wedge which have a large extent in local time.

Boundary layer dynamics in the description of magnetospheric substorms

Magnetospheric substorm activity has been previously explained primarily as the effects of the episodic unloading of energy stored in the Earth's magnetotail. The physics of the unloading process has been associated with magnetic field line reconnection, with one dominant school of thought contending that reconnection occurs close to the Earth (i.e., inside of ∼15 RE). This near‐Earth neutral line model seeks to explain plasma and field characteristics observed in the magnetotail by the development and ultimate disappearance downtail of a plasmoid involving the population of the plasma sheet antiearthward of the near‐Earth neutral line. In this paper we present a study of a significant number of substorm expansive phase events where the phenomenology at ionospheric levels is clearly defined in terms of onset time and locale of the substorm disturbed region. Through the detailed presentation of a subset of these events it is demonstrated that the magnetotail particle and field observations associated with a substorm expansive phase can equally well be explained through the spatial movement of the boundary layers and central plasma sheet in the magnetotail. The sweeping of these regions past the satellite, even in the absence of temporal variations within the various regions, can lead to a set of plasma flow observations typical of what is observed in the magnetotail during substorm activity. We further demonstrate that filamentary currents in the plasma sheet boundary layer can produce the magnetic field signatures presently attributed to retreating plasmoids in the magnetotail. The major strength of this emphasis on boundary layer dynamics is that it provides a framework which naturally encompasses both the driven system and loading‐unloading system which are recognized to be operative simultaneously during magnetospheric substorm activity.

A theory of patchy and intermittent reconnections for magnetospheric flux transfer events

We propose that the elbow‐shaped flux transfer events (FTEs) of Russell and Elphic (1979) are initiated by a specific normal mode of three‐dimensional tearing of the magnetopause current sheet, resulting in a three‐dimensional component reconnection driven by the solar wind. The patchy nature of the proposed three‐dimensional tearing reconnection is determined by the structure of the three‐dimensional tearing on the dayside magnetopause, leading to isolated reconnection sites; the intermittent nature is governed externally by the fluctuation of the interplanetary magnetic field (IMF) and internally regulated by the ionospheric line‐tying effect. The proposed 3DT reconnection can occur simultaneously with the single x line intermittent or quasi‐steady reconnection if the IMF drapes over the dayside magnetopause, which takes place when the solar wind speed exceeds the MHD fast‐mode speed. Magnetic field signatures of the 3DT reconnection are consistent with the elbow‐shaped FTE signatures. Electric fields associated with the elbow‐shaped FTEs can be expected to be spiky, peaking near the center of the localized FTE reconnection sites. The spiky electric field of FTEs is expected to produce channels of enhanced magnetospheric convection. The observed multiple field‐aligned current sheets and auroral arcs fanning out from the cusp region along enhanced convection channels can be expected as consequences of the proposed 3DT reconnection on the dayside magnetopause. The polar cap potential due to the elbow‐shaped FTEs is predicted to have a dependence on the IMF polar angle somewhat different from that of the single x line reconnection.