Magnetic Field Signatures Research Articles

Simultaneous satellite and ground observations of transient events near the morningside magnetopause

We present observations of transient events on April 13, 1996, using simultaneous measurements in the solar wind, near the magnetopause, at geosynchronous orbit, and on the ground at high latitudes. On this day, Geotail passed outbound through the morningside magnetopause. When Geotail was in the magnetosphere, a clear bipolar perturbation in the magnetic field component normal to the nominal magnetopause and transient outbound (from the magnetosphere to the magnetosheath)/inbound (from the magnetosheath to the magnetosphere) magnetopause crossings were observed. When Geotail was in the magnetosheath, inbound/outbound crossings were observed. During the bipolar magnetic field signature, Geotail observed asymmetric bipolar (less inward/more outward) flow normal to the nominal magnetopause accompanied by sunward flow. Inward flow was observed when the total magnetic field of the event was enhanced, whereas outward flow was observed when the total magnetic field was depressed. We observed that some of the transient outbound/inbound crossings show significantly different signatures in the plasma flow and magnetic field, that is, weak inward flow and enhanced magnetic field before the outbound crossing and strong outward flow and decreased magnetic field after the inbound crossing. The field and flow variations during the transient outbound/inbound crossings are qualitatively similar to those during the bipolar magnetic field signature. These observations suggest that the bipolar field signature and the transient magnetopause crossings would be similar phenomena on the magnetopause. We discuss whether the observed field and flow variations during the transient events are caused by inward/outward magnetopause motion due to compression/expansion of the magnetosphere by comparing the data sets obtained from different regions of the solar wind‐magnetosphere system.

Analytical Modeling of Magnetic and Gravity Signatures of Unexploded Ordnance

The dominant cost and time driver in unexploded ordnance (UXO) cleanup is the necessity of digging (excavating) “false alarm” gèophysical anomalies. As many as 75% of all the anomalies investigated are false alarms, e.g., scrap metal, ordnance debris, cans, wire, etc. The effort to develop capability to discriminate UXO anomalies from false alarm anomalies is a driver for the development of geophysical model-based signature modeling. The signature modeling is important for forward signature prediction and ultimately for geophysical inversion of buried object parameters from measured data. Researchers are actively developing forward and inverse modeling capability for time-domain and frequency-domain electromagnetic induction, ground penetrating radar, magnetic, and gravity anomaly signatures for realistic UXO shapes. This paper documents progress in developing forward gravity and total magnetic field analytical solutions. Gravity and magnetic methods are passive and share similarity in mathematical formalism. If both gravity and magnetic signatures of UXO can be detected in practice, the potential exists for an integrated bulk density determination as a discriminant. Fundamental concepts of the gravity and magnetic methods as applied to detection and potential discrimination of buried UXO are discussed. Analytical solutions for the gravity and total magnetic field signatures of prolate spheroid models of UXO are developed. The total magnetic field solution is a multipole expansion approximation that includes the dipole and octupole contributions, with the quadrupole contribution zero due to symmetry of the spheroid. The gravity solution is a full analytical solution for the field exterior to a spheroid. Examples and results of application of the solutions to spheroid UXO modeling are presented, along with implications for detection and discrimination. The examples include the effects of spheroid orientation on the signature characteristics. Total magnetic field anomalies of UXO are generally detectable to at least the maximum predicted penetration depths for typical soil types and low background magnetic noise. Gravity anomalies for [Formula: see text] projectiles and larger UXO are detectable at very shallow depths (i.e., just below the surface), but only the very largest UXO (e.g., [Formula: see text] projectiles) will produce detectable anomalies for burial depths approaching [Formula: see text].

Physics of Coronal Mass Ejections: A New Paradigm of Solar Eruptions

The prevailing framework of understanding coronal mass ejections (CMEs) and, indeed, solar eruptions in general is the hypothesis that the quasi-static changes in the photospheric magnetic field increases the magnetic energy in the corona and causes sudden release of the stored energy. However, this hypothesis, which may be called the `storage-release' paradigm, has yet to produce a quantitative model of CMEs and their heliospheric consequences. Recently, a new theory has been proposed to explain the physics of CMEs. This theory posits that the initial structure is a magnetic flux rope that is ultimately connected to the solar dynamo in the convection zone and that magnetic energy propagating from the source along the submerged magnetic structure enters the corona and drives the eruption. Specifically, the theory describes CMEs as the dynamical response of coronal flux ropes to the `injected' poloidal flux and predicts that CMEs evolve into interplanetary magnetic clouds (MCs). In a recent series of studies, the physics-based theory was shown to correctly describe the observed dynamics of a class of CMEs and the properties of MCs, providing the first unified description of the CME-MC dynamics. The apparent success of the theory suggests a new paradigm in which CMEs are viewed as a relaxation process in response to the increased magnetic energy propagating from the dynamo. The motion of a flux rope and its magnetic energy is everywhere given by \(\tilde \upsilon \lesssim 1\), where \(\tilde \upsilon \equiv \upsilon /V_M {\text{ and }}V_M \) is the local characteristic speed. It is predicted that the injected poloidal flux can cause a subtle but distinct signature in the tangential field at the base of the corona. The tangential field, which is difficult to observe, and the poloidal magnetic energy have been neglected in previous theories. This paper reviews the current CME research, compares the physics of the two competing paradigms, and suggests new observable magnetic field signatures.

Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures

Observations of the varying orientations of comet tails led to the suggestion of the existence of the solar wind--a continuous outflow of ionized material from the Sun. It is now well established that gas from comets is ionized by several processes and joins the solar wind, forming an ion (plasma) tail that points away from the Sun. The plasma environments of three comets have been measured in situ, but only in the upstream direction or less than 8,000 km downstream of the nucleus. Here we report a fortuitous crossing by a spacecraft of the plasma tail of comet Hyakutake (C/1996 B2), at a distance of more than 3.8 astronomical units (550 million kilometres) from its nucleus. This surpasses the tail length of 2 AU determined for the Great March Comet of 1843 (C/1843 D1). Our measurements reveal that, at this distance, the tail of comet Hyakutake was a structured entity at least 7 million kilometres in diameter.

Interplanetary Magnetic Field Line Mixing Deduced from Impulsive Solar Flare Particles.

We have studied fine-scale temporal variations in the arrival profiles of approximately 20 keV nucleon-1 to approximately 2 MeV nucleon-1 ions from impulsive solar flares using instrumentation on board the Advanced Composition Explorer spacecraft at 1 AU between 1997 November and 1999 July. The particle events often had short-timescale ( approximately 3 hr) variations in their intensity that occurred simultaneously across all energies and were generally not in coincidence with any local magnetic field or plasma signature. These features appear to be caused by the convection of magnetic flux tubes past the observer that are alternately filled and devoid of flare ions even though they had a common flare source at the Sun. Thus, we have used the particles to study the mixing of the interplanetary magnetic field that is due to random walk. We deduce an average timescale of 3.2 hr for these features, which corresponds to a length of approximately 0.03 AU.

Broadband (ULF-VLF) surface impedance measurements using MIMDAS

Electromagnetic radiation from distant, discrete, ground-to-cloud lightning return-strokes can be recorded as atmospherics (spherics), using both electric and magnetic field antennas. The Earth-ionosphere waveguide filters this radiation into the ULF-VLF radio bands from 3 Hz to 30 kHz with a notch centred at approximately 2 kHz. The relationship between the orthogonal horizontal electric and magnetic field signatures at the receiver is directly related to the surface impedance of the Earth at that point. Transforming the transient fields to the frequency domain allows the calculation of the surface impedance over the frequency range determined by the sample rate and record length. From this, resistivity versus depth models of the Earth can be determined. This paper demonstrates that a discrete spheric can be considered as a broadband vertically polarised plane-wave source suitable for magnetotelluric (MT) style geophysical surveying, and that the data collected could be used to deduce the near-surface resistivity profile. The results from measurements made with MIM Exploration’s proprietary distributed acquisition system (MIMDAS) are presented. Multi-station data were acquired in a telluric profiling mode (without a magnetic field reference) to demonstrate spatial coherence and correlation with conventional MIMDAS MT pseudosections and IP inversion models. The advantages of the technique include the broad band of measurement and rapid data-acquisition time for reliable statistics. Valid data were collected between 24 kHz and less than 200 Hz and while stacking improves data integrity, useful information may be extracted from less than 5 ms of a 6 s record, corresponding to a single strong spheric. This technique has potential value in near-surface geotechnical applications and as an aid in removing statics from conventional continuous-MT survey data.

Non-linear inversion for the hydrostatic structure of the solar interior

We present the results of a non-linear inverse analysis for the hydrostatic, spherically symmetric component of the solar internal structure using the observed p-mode frequencies. The iterative non-linear inversion technique used here is based on the succesive Born approximation description of solar p-modes developed by Roxburgh & Vorontsov. This description can give a high resolution of regions of rapid variation of seismic parameters with depth (e.g., the base of the convection zone), and accounts accurately for the strong influence of gravity perturbations on low-degree modes which penetrate deep into the solar core. The inversion procedure is non-linear; the eigenfrequency equation obtained from the Born approximation is solved by iteration. The particular target of our inverse analysis is to achieve the highest possible resolution of the region near the base of the solar convection zone, searching for possible signatures of penetrative convection, element diffusion and/or strong magnetic fields. The results of the global inversion obtained with solar p-mode frequencies provided by the recent high-quality observational data (GONG, SOI/MDI, GOLF) are presented and discussed.

THECOSMICMICROWAVEBACKGROUND ANDPARTICLEPHYSICS

▪ Abstract In forthcoming years, connections between cosmology and particle physics will become increasingly important with the advent of a new generation of cosmic microwave background (CMB) experiments. Here, we review a number of these links. Our primary focus is on new CMB tests of inflation. We explain how the inflationary predictions for the geometry of the Universe and primordial density perturbations will be tested by CMB temperature fluctuations, and how the gravitational waves predicted by inflation can be pursued with the CMB polarization. The CMB signatures of topological defects and primordial magnetic fields from cosmological phase transitions are also discussed. Furthermore, we review current and future CMB constraints on various types of dark matter (e.g. massive neutrinos, weakly interacting massive particles, axions, vacuum energy), decaying particles, the baryon asymmetry of the Universe, ultra-high-energy cosmic rays, exotic cosmological topologies, and other new physics.

Horizontal electrojet associated with the sun-aligned arcs

Analysis of two strong sun-aligned arcs over the Canadian Eureka Observatory (89° CGM) near the north magnetic pole and accompanying related ground magnetometer measurements has identified an electrojet current of ∼1×10 4 A flowing within the arcs in a sunward direction. The electrojet current was carried by low energy electrons created by impacting precipitation drifting at E× B/B 2 velocity within the arcs, where E is the dawn-to-dusk electric field. One of the arcs moved rapidly in a dawn to dusk direction. The measured arc velocity was 365 m/s. This agrees well with the velocity of 380 m/s inferred from the magnetic field signature of the electrojet current. This study suggests that such an electrojet is present whenever a polar arc is set up. However, a few conditions are required to observe clear ground magnetic signatures of the electrojet: (1) quiet local magnetic conditions; (2) a single sun aligned arc near or moving across zenith; and (3) arc excitation by ≥1 keV electrons.

Numerical analysis of global ionospheric current system including the effect of equatorial enhancement

Abstract. A modeling method is proposed to derive a two-dimensional ionospheric layer conductivity, which is appropriate to obtain a realistic solution of the polar-originating ionospheric current system including equatorial enhancement. The model can be obtained by modifying the conventional, thin shell conductivity model. It is shown that the modification for one of the non-diagonal terms (Σθφ) in the conductivity tensor near the equatorial region is very important; the term influences the profile of the ionospheric electric field around the equator drastically. The proposed model can reproduce well the results representing the observed electric and magnetic field signatures of geomagnetic sudden commencement. The new model is applied to two factors concerning polar-originating ionospheric current systems. First, the latitudinal profile of the DP2 amplitude in the daytime is examined, changing the canceling rate for the dawn-to-dusk electric field by the region 2 field-aligned current. It is shown that the equatorial enhancement would not appear when the ratio of the total amount of the region 2 field-aligned current to that of region 1 exceeds 0.5. Second, the north-south asymmetry of the magnetic fields in the summer solstice condition of the ionospheric conductivity is examined by calculating the global ionospheric current system covering both hemispheres simultaneously. It is shown that the positive relationship between the magnitudes of high latitude magnetic fields and the conductivity is clearly seen if a voltage generator is given as the source, while the relationship is vague or even reversed for a current generator. The new model, based on the International Reference Ionosphere (IRI) model, can be applied to further investigations in the quantitative analysis of the magnetosphere-ionosphere coupling problems.Key words. Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions)

Evidence for a global disturbance with monochromatic pulsations and energetic electron bunching

We present data from a number of ground stations and satellites that reveal an example of a previously unreported type of global event. The event is characterized by the occurrence of monochromatic pulsations of widely varying frequencies in different regions of local times and L shells. The pulsations appear to be modulated with a ∼45 min periodicity. Simultaneously, energetic particle fluxes observed at geosynchronous orbit (with energies resulting in a drift period of ∼45 min) appear to become phase bunched and the pulsations are observed to occur coincident with minima in the particle bunching. Corresponding data from GOES 5 show an increase in the compressional component of the magnetic field with this period. Data from South Pole Station show fluctuations in the east–west component of the magnetic field at half this period, along with a weak auroral signature and riometer absorptions. We conclude that the data show the existence of a global disturbance with a compressional magnetic field signature, and we suggest that this compression induces a radial electric field, based on Faraday's law. Phase bunching of energetic electrons (due to the induction electric field) and monochromatic pulsations are consequences of the global disturbance, although the mechanism responsible for exciting the pulsations is not clear.

Numerical analysis of global ionospheric current system including the effect of equatorial enhancement

A modeling method is proposed to derive a two-dimensional ionospheric layer conductivity, which is appropriate to obtain a realistic solution of the polar-originating ionospheric current system including equatorial enhancement. The model can be obtained by modifying the conventional, thin shell conductivity model. It is shown that the modification for one of the non-diagonal terms (Σθφ) in the conductivity tensor near the equatorial region is very important; the term influences the profile of the ionospheric electric field around the equator drastically. The proposed model can reproduce well the results representing the observed electric and magnetic field signatures of geomagnetic sudden commencement. The new model is applied to two factors concerning polar-originating ionospheric current systems. First, the latitudinal profile of the DP2 amplitude in the daytime is examined, changing the canceling rate for the dawn-to-dusk electric field by the region 2 field-aligned current. It is shown that the equatorial enhancement would not appear when the ratio of the total amount of the region 2 field-aligned current to that of region 1 exceeds 0.5. Second, the north-south asymmetry of the magnetic fields in the summer solstice condition of the ionospheric conductivity is examined by calculating the global ionospheric current system covering both hemispheres simultaneously. It is shown that the positive relationship between the magnitudes of high latitude magnetic fields and the conductivity is clearly seen if a voltage generator is given as the source, while the relationship is vague or even reversed for a current generator. The new model, based on the International Reference Ionosphere (IRI) model, can be applied to further investigations in the quantitative analysis of the magnetosphere-ionosphere coupling problems.Key words. Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions)

Disturbances in Mercury's magnetosphere: Are the Mariner 10 “substorms” simply driven?

In addition to providing the first in situ evidence of a magnetosphere at Mercury, the first flyby by Mariner 10 inspired reports of Earth‐like substorms. While the small scales at Mercury should make the substorm timescale there much shorter than it is at the Earth, these early interpretations may have too readily assumed that the substorm requirement of an energy storage and release phase occurs. Instead, its size should make Mercury's magnetosphere especially prone to disturbances that are purely “driven” by the changing external boundary conditions on the magnetosphere imposed by the solar wind. These result simply from the magnetosphere's relatively unhindered reconfiguration in response to the varying interplanetary parameters, including the IMF southward component. In this paper we demonstrate that the “disturbed” structure observed outbound from closest approach during the first Mariner 10 flyby can alternately be explained as a consequence of a typical period of rotating IMF. We use an appropriately modified IMF‐dependent terrestrial magnetosphere model scaled for Mercury, together with an assumed, reasonable IMF time series, to reproduce the magnetic field signature during the disturbed outbound pass segment. This result suggests that rapid restructuring of the small magnetosphere in response to changing local interplanetary conditions may dominate the magnetospheric dynamics at Mercury. Future Mercury magnetosphere missions should be instrumented to distinguish between this driven magnetospheric dynamism and any internal Earth‐like substorm processes. These results also remind us that driven reconfigurations must always be considered in studies of transients in the terrestrial magnetosphere.

Dayside auroral dynamics: South Pole ‐ AMPTE/CCE observations

Coordinated observations were made between the South Pole Station 630‐nm all‐sky imager and the Active Magnetospheric Particle Tracer Explorer (AMPTE) CCE satellite when it was at 9 RE apogee in the midday sector in an effort to understand the variety of dynamical changes at the magnetopause that map to the dayside aurora. On June 18, 1988, three events were observed. The first, at 1150 MLT, was a local event showing a rotation of the magnetic field and the sudden appearance of low‐energy electrons indicating the satellite entering the magnetosheath or low‐altitude boundary layer. This event was preceded by a type of poleward expansion of the aurora which had been previously interpreted as a possible signature/footprint of a flux transfer event (FTE). Although the magnetic field signatures at the satellite are not characteristic of an FTE, the event could be a magnetic field change produced by an FTE, which after some delay propagated to the vicinity of the satellite. The foot of the field line containing the satellite was at some distance from the south pole meridian. The second event, starting at 1610 UT (1240 MLT), was a clear case of field erosion during which the satellite entered the magnetosheath characterized by a reduction in energetic electrons, an increase in soft electrons, and a decrease in the magnetic field strength accompanied by an equatorward motion of the aurora. The third event, in the afternoon at about 1900 UT (1530 MLT), showed similar changes in the electron fluxes and similar equatorward motion of the aurora. However, this event was associated with an increase in the magnetic field at the satellite. Such a field increase could be characteristic of a pressure pulse. However, there was a corresponding equatorward motion of the aurora, which suggests that the event was also a flux erosion event in which the fringing fields of the region 1 current system, which normally cause a field depression at the subsolar region, created ah increase in the field at the CCE satellite near the late afternoon side. The latitudinal (equatorward) movements of the aurora for all three events were strongly correlated with the global AE index which was dominated by stations in the dawn/dusk sectors.

The effect of magnetosheath plasma flow on flux transfer events produced by the onset of merging at a single X line

We present predictions on the signatures of flux transfer events (FTEs) at the dayside magnetopause by the single X line MHD model based on the subsolar environment: initially in the magnetosheath with background flow northward parallel to the dayside magnetopause. In conformity with our previous investigation [Ku and Sibeck, 1997] we choose realistic parameteres for the simulation with the ratio of ρmsh/ρsph =10, Bmsh/Bsph = 0.5, and Tmsh/Tsph = 0.175 along the magnetosheath and magnetospheric sides. The localized resistivity turns on near the low northern latitude of dayside magnetopause, and the initial magnitude of background magnetosheath plasma flow is set , or equivalent to 30% of magnetosheath Alfvén velocity. Simulation results produce signatures of FTEs similar to the previous case without the background magnetosheath flow [Ku and Sibeck, 1997] but different in some respects. Events moving opposite to this flow slow down but intensify, whereas events moving in the direction of the magnetosheath flow accelerate but weaken. Similarly, the satellite observes no significant signatures of FTEs in the magnetosphere. All the FTEs exhibit asymmetric bipolar magnetic field signatures normal to the magnetopause. Scatterplots versus the plasma density reveal that steep changes for the temperature kT, the magnetic field Bz, and the Alfvén velocity occur in the magnetospheric side immediately adjacent to the magnetopause.

INTERBALL-1 plasma sheet encounters and three-dimensional MHD modeling results

We use magnetic field observations by the fluxgate magnetometer FGM-I during two substorm related INTERBALL-1 plasma sheet encounters to discuss signatures of reconnection in the Earth's magnetotail. In the first case the satellite stayed northward of the current sheet; in the second case for most of the substorm time it stayed in the northern lobes until it approached the plasma sheet and, finally, crossed the current sheet. In order to determine whether the observed magnetic field signatures can be understood in the framework of a resistive magnetohydrodynamic (MHD) approach to reconnection, we generated data using a three-dimensional numerical MHD model of the tail. In the model, reconnection takes place due to a localized finite resistivity. Comparing the modeled data with the measured ones, we find that, although the substorm-related dipolarization-like evolution of the B x and the B z field components can be well understood in terms of a resistive MHD approach to reconnection with dipolarization starting before the substorm ground onset and geostationary injection tailward of the satellite position, all attempts failed to explain the observed transient dawnward drop of the shear component B y by a resistive MHD model of three-dimensional reconnection. We conclude that the dawnward drop has to be caused by effects beyond the resistive MHD approach.

Observations of flux rope − associated particle bursts with GEOTAIL in the distant tail

Abstract. Geotail energetic particle, magnetic field data and plasma observations (EPIC, MGF and CPI experiments) have been examined for a number of energetic particle bursts in the distant tail (120Re<|XGSM|< 130 Re), associated with moving magnetic field structures, following substorm onsets. The features obtained from this data analysis are consistent with the distant magnetotail dynamics determined first by ISEE3 observations and explained in terms of the neutral line model. At the onset of the bursts, before plasma sheet entrance, energetic electrons appear as a field-aligned beam flowing in the tailward direction, followed by anisotropic ions. Within the flux rope region, suprathermal ions exhibit a convective anisotropy, which allows determination of the plasma flow velocity, assuming that the anisotropy arises from the Compton-Getting effect. The velocities thus determined in the plasma sheet are estimated to be 200–650 km/s, and compare favourably with the velocities derived from the CPI electron and proton experiment. The estimated length of magnetic field structures varies between 28 and 56 Re and depends on the strength of the westward electrojet intensification. Finally, the three structures reported here show clear magnetic field signatures of flux rope topology. The existence of a strong magnetic field aligned approximately along the Y-axis and centred on the north-to-south excursion of the field, and the bipolar signature in both By and/or Bz components, is consistent with the existence of closed field lines extending from Earth and wrapping around the core of the flux rope structure.Key words. Magnetospheric configuration and dynamics · Magnetotail

Geotail observations of energetic ion species and magnetic field in plasmoid‐like structures in the course of an isolated substorm event

On January 15, 1994, the ion spectrometer high energy particle ‐ low energy particle detector (HEP‐LD) on the Japanese spacecraft Geotail observed five quasi‐periodic energetic ion bursts in the deep tail (X=−96 RE). These bursts were associated with plasmoid‐like structures in the magnetic field components. In. addition, three multiple TCR groups were identified in the interval. The observations in the distant tail occurred during a time interval of substorm activity which also produced multiple injections in the geosynchronous orbit region. The HEP‐LD observations show that Bz bipolar plasmoid‐like structures are associated with tailward flowing particle bursts. However, earthward flowing particle bursts are predominantly associated with bipolar signatures in By. In addition, an oxygen burst was seen in the back of a plasmoid (postplasmoid) which showed both By and Bz bipolar magnetic field signatures. The oxygen burst lasted for 23 min, and the density ratio (O/H) reached 15% for the HEP‐LD energy range (in the same plasmoid, this ratio was approximately 1% before the oxygen burst). The oxygen burst exhibited a strong beam‐like structure which occupied only 6 ∼ 7% of the full solid angle (4π). We suggest that energized oxygen ions of ionospheric origin travel downtail in the narrow postplasmoid‐plasma sheet which trails the plasmoid. Furthermore, we suggest that the magnetosphere dissipated larger quantities of energy during this very intense substorm event by ejecting multiple relatively small plasmoids rather than through the formation and ejection of a single large plasmoid.

Internal structure of flux transfer events produced by the onset of merging at a single X line

We present a new, higher‐order, two‐dimensional, magnetohydrodynamic model for the detailed internal structure of bubbles produced by the onset of merging along a single subsolar X line. The key feature of the model is to implement a nonlinear filter to postprocess results provided by the pseudospectral element method. The devised nonlinear filter (1) eliminates only the high‐frequency oscillations, (2) retains conservation laws, and (3) enforces a total variation diminished criterion to guarantee the elimination of spurious oscillations near discontinuities. For realistic parameters, the model predicts that satellites which remain in the magnetosheath will observe asymmetric bipolar magnetic field signatures normal to the magnetopause, magnetic field strength increases, density and temperature decreases, and velocities opposite the motion of the events. Satellites in the magnetosheath which enter the events observe asymmetric bipolar signatures normal to the magnetopause, magnetic field strength decreases, densities intermediate between those in the magnetosheath and magneto‐sphere, temperature increases, and velocities in the direction of event motion. Satellites initially located just outside the the magnetopause observe greatly depressed densities and enhanced temperatures (but no significant bipolar signatures normal to the magnetopause) followed by velocity jets of magnetosheath‐like plasma on northward magnetospheric magnetic field lines. Satellites in the magnetosphere which enter the events observe depressed magnetic field strengths, enhanced densities and temperatures, and accelerated flows surrounding a core region of accelerated magnetosheath‐like plasma. Neither they nor satellites which remain in the magnetosphere observe any significant bipolar signatures normal to the nominal magnetopause.

Substorms, tail flows and plasmoids

The GEOTAIL spacecraft has made comprehensive magnetic field and plasma observations in the Earth's magnetotail at radial distances of 10–210 Re. Data from the GEOTAIL spacecraft have led to the following conclusions regarding the relationship between substorms and field and plasma behaviors. Substorms produce systematic and repeatable signatures in magnetic field and plasmas in various radial distances in the magnetotail. A repeatable signature for substorms is tailward plasma flows with a southward magnetic field structure. These are often associated with plasmoids, as seen in the local magnetic field variations. Examples are presented of signatures seen at different distances from the Earth. From a statistical survey of the data, it is shown that magnetic reconnection takes place in the expansion phase at radial distances of 20–30 Re. Plasmoids are created there and travel tailward at high speeds. Earthward flows in the recovery phase appear only inside 100 Re. Plasmoids are dominant substorm signatures beyond 100 Re.