Plasma Sheet Thinning Research Articles

Comment on “ionospheric signature of plasma sheet thinning prior to a substorm” by J.R. Dudeney and A.S. Rodger

In a recent paper, Dudeney and Rodger (1988) have presented a study of a substorm expansive phase which they use to attempt to distinguish between the near-Earth neutral line model for substorms (cf. Hones, 1984) and the boundary layer model for substorms proposed by Rostoker and Eastman (1987). One of the features of the event studied by Dudeney and Rodger is a Pi2 pulsation which, the authors contend, is not accompanied by a substorm expansive phase. The purpose of this Comment is two-fold. In the first place I wish to demonstrate that the evidence that the Pi2 pulsation alluded to by the authors is not accompanied by a substorm expansive phase is far from compelling. In the second instance, reasons for rejecting the boundary layer model as an explanation for the event under consideration are shown to be without foundation. which is connected to the magnetosphere by fieldaligned currents at its eastern and western edges (viz. a current wedge). In this case, it is clear that to the equatorward side of the ionospheric electrojet, the magnetic signature is a negative Z-component bay. A 10 nT Z-component bay is quite consistent with the development of a westward electrojet of peak strength 150 nT whose center is 10” poleward of the observing station (e.g. Halley). (Note that Fig. 1 shows a westward electrojet for the Northern Hemisphere, so that the corresponding signature for a negative Z-component bay would be a positive Z-component bay at the Southern Hemisphere station of Halley.) Based on these considerations, I would suggest that the authors have no strong reason to believe that their 00: 50 U.T. pulsation is not accompanied by an expansive phase onset or intensification.

Ionospheric signature of plasma sheet thinning prior to a substorm

A case study of the behaviour of the poleward edge of the main F-region trough in the premidnight sector associated with a substorm is presented. It is found that the poleward edge moves rapidly equatorwards prior to the onset of the substorm as determined from ground magnetometer signatures. It is well known that before magnetic midnight the poleward edge is produced by soft precipitation from the inner plasma sheet. The data are therefore interpreted as the ground signature of the process of plasma sheet thinning and dipolarisation of the near-Earth geomagnetic field on the nightside, as described by the Near Earth Neutral Line model of substorm behaviour. The competing Plasma sheet Boundary Layer model does not appear to be consistent with our data in this case.

Multipoint measurements of magnetotail dynamics

Multipoint spacecraft measurements have played an important role in documenting the morphology of magnetospheric substorms. Widely separated measurements have quite clearly delineated many large scale features such as, (1) plasma sheet thinning associated with the onset of substorms, (2) energy storage and release in the magnetotail associated with the growth and expansion phase of substorms, (3) the earthward and tailward field aligned currents associated with the dawn and dusk portions of the substorm current wedge and (4) the deep tail response that follows substorms detected near synchronous orbit. Smaller scale phenomena that must be observed on finer time scales and detected in more limited spatial regions have been studied by ISEE 1 and 2 and a number of spacecraft near geosynchronous orbit. Injection fronts that are sometimes thought to propagate earthward toward synchronous orbit are difficult to detect and observed time delays may be due at least partially to longitudinal expansions of the substorm current wedge. Measurements near the equatorial current sheet where substorm onsets are often thought to originate, frequently reveal rapid time variations yet nearby spacecraft are seldom available to help determine whether these variations are due to local turbulence or more coherent behavior on a somewhat larger scale. High time resolution measurements from the Cluster and ISTP programs should help rectify these ambiguities.

Influence of plasma flow on the structure of plasma sheet - theoretical model and its application to magnetospheric substorm processes.

Under a given flow pattern and some assumptions, we have solved ideal and viscous MHD equaons and got 2-dimensional, time-independent plasma sheet models. Our results show that plasma flow has a great influence on the structure of the Earth's magnetotail plasma sheet. Ignoring plasma flow, we find the thickness of plasma sheet remains a constant about 5 Re. Considering the plasma flow, we find when M, the Mach number, is greater than (1.2/γ) 1/2 corresponding to the supersonic flow, the plasma sheet becomes thinner; when M is smaller than (1.2/γ) 1/2 corresponding to the subsonic flow, the plasma sheet becomes thicker. We find that the anomalous viscosity can affect the thickness of the plasma sheet also. Applied to magnetospheric substorm processes, our models can explain some main observational results, such as plasma sheet thinning and thickening. On the other hand, it can explain some contradictions of satellite data analysis made by different authors.

Comet P/Giacobini-Zinner electron and H2O(+) column densities from ICE and ground-based observations

An H2O(+) spatial mission profile, extracted from an optical CCD spectrogram obtained during the ICE/Giacobini-Zinner encounter, is compared to the electron-density profile that was deduced from in situ measurements by the radio experiment aboard ICE. The electron column density along a line of sight has two components, one from the spherically symmetric coma, and the second from a thin plasma sheet, whenever it is along the line of sight. The deduced electron column-density profile agrees well with the observed H2O(+) emission profile. It is concluded that the electrons and the H2O(+) ions are distributed similarly 9600 km tailward from the cometary nucleus, that the ratio of number densities of H2O(+) ions to electrons is about 1/4 at this point, and that the width of the plasma sheet is about 16,000 km. 33 references.

Field‐aligned plasma flow in MHD simulations of magnetotail reconnection and the formation of boundary layers

We have used our two‐dimensional resistive MHD code to study details of the plasma flow in two classes of magnetotail reconnection models: (1) near‐earth reconnection as initiated by the sudden occurrence of anomalous dissipation, which leads to plasmoid formation and ejection consistent with phenomenological magnetospheric substorm models, and (2) distant reconnection which is forced to occur near a given location by a nonuniform inflow of plasma, energy, and magnetic flux through the high‐latitude boundaries modeling the possible effects of plasma entry through the plasma mantle, which expands until it reaches the plasma sheet. Near‐earth reconnection produces predominantly tailward flow, which occupies a major part of the plasma sheet cross section in z. Except for narrow regions close to the neutral sheet, this flow is largely field aligned. Due to the fact that the onset of this fast flow closely coincides with plasma sheet thinning, the observation at a fixed satellite location off the central neutral sheet looks like the crossing of a flow boundary layer when the satellite exits from the plasma sheet into the lobe. The distant reconnection process showed several features consistent with observations in the near and distant tail: the formation of a distant neutral sheet, rather than a single neutral line with the possibility of the occurrence of multiple neutral lines and quasi‐stagnant magnetic islands, and the splitting of the earthward flow into two boundary layers, or “separatrix layers,” inside the magnetic separatrix that limits the region of closed magnetic flux tubes.

An MHD simulation of the interaction of the solar wind with the outflowing plasma from a comet

The interaction between the solar wind and the outflowing plasmas from a comet has been studied by using a two‐dimensional time‐dependent magnetohydrodynamic (MHD) simulation. The model reproduced several features of the comet‐solar wind interaction predicted by earlier theories and observed on the recent cometary probes. These include the formation of the contact surface and the cometary magnetotail. For a constant interplanetary magnetic field (IMF) the cometary plasma captures field lines which drape over the comet to form an antiparallel magnetic field configuration in the tail and a thin plasma sheet. Eventually, tail magnetic reconnection begins to occur at several points. When the IMF orientation is reversed dayside magnetic reconnection occurs at the subsolar point and a large disturbance propagates down the tail.

A statistical study of plasma sheet dynamics using Isee 1 and 2 energetic particle flux data

During magnetospheric substorms, satellites embedded in the plasma sheet often detect transient dropouts of plasma and energetic particle fluxes, a phenomenon generally interpreted as indicating the exit of the satellite into the magnetospheric lobe due to a plasma sheet thinning. In order to determine the large‐scale dynamics of the near‐earth plasma sheet during substorms, three satellite years of ISEE 1 and 2 energetic particle flux data (1.5 and 6 keV), corresponding to 461 particle flux dropouts, have been analyzed. The principal results show that flux dropouts can be observed anywhere in the nightside plasma sheet, independent of the satellite's, geocentric distance (for R > 12 RE), magnetic local time (except near the magnetospheric flanks) and estimated distance to the neutral sheet. Furthermore, flux dropouts can be observed for any combination of the AE index value and the satellite's distance to the neutral sheet, which shows that the plasma sheet is dynamic even during weak magnetospheric disturbances. Substorms during which the satellites, though situated in the plasma sheet, did not detect any flux dropout, have also been examined, and it is found that the plasma sheet thickness can locally remain unaffected by substorm development for AE index values up to at least 1000 nT. The predictions of the two major plasma sheet thinning models, i.e., the near‐tail X‐type magnetic neutral line formation model and the MHD rarefaction wave propagation model, are compared to the experimental results, and it is concluded that neither model can account for all of the observations; plasma sheet dynamics are more complex. Phenomenologically, this study suggests that multiple pinching of the plasma sheet and/or large‐amplitude three‐dimensional plasma sheet oscillations are important in plasma sheet dynamics.

Particle dynamics of the plasma sheet boundary layer

The plasma sheet boundary layer dynamics during substorms are analyzed using ISEE 1 and 2 energetic particle flux data. Considering the plasma sheet boundary layer as a tangential discontinuity and thus its movements as perpendicular to the local magnetic field direction, the differential timing of the two spacecraft data sets is used to calculate the boundary layer velocity and thickness. The velocity values are deduced to be of the order of 40 km/s and boundary thickness varies from a few tens of km to more than 4 R E with an average value of ∼5000 km. For a majority of the boundaries studied the boundary layer appears simultaneously in all energy channels; within it significant spectral variations are observed and the spectrum is harder at plasma sheet recovery. The experimental data are discussed in the framework of the actual plasma sheet thinning models.

A simulation study of forced reconnection processes and magnetospheric storms and substorms

We simulate, on the basis of a two‐dimensional incompressible MHD code, the plasma dynamics of a long magnetotail (∼200 RE) under a constant and time‐varying driving force delivered from the solar wind. It is found that under a constant driving force the magnetic fields in the magnetotail tend to reconnect impulsively and the formation of X line and plasmoids occurs intermittently and repeatedly every 2–4 hours. Under a time‐varying driving force the post‐driven phase of a plasmoid formation and the spontaneous reconnection process are also studied. Our results show that magnetic reconnection in the magnetotail during magnetospheric substorms and storms is basically a driven process. A complete sequence of the event, going from the gradual pile‐up of magnetic flux to the formation and the antiearthward convection of the new X line and to the ejection of plasmoids from the antiearthward end of the magnetotail, requires the presence of the driving force. The simulation results may also be useful in unifying the different interpretations (1) of the plasmoid formation and plasma sheet thinning and (2) of the relative importance of magnetotail processes with respect to the direct solar wind effects during substorms.

Numerical studies on magnetotail formation and driven reconnection

The formation of the magnetotail configuration under the influence of the solar wind plasma flow entering the magnetosphere and the associated driven reconnection in the magnetotail region are studied numerically by means of a two‐dimensional time‐dependent nonlinear resistive MHD code. The initial magnetic field is generated by a two‐dimensional dipole, and the initial plasma pressure is assumed uniform. As the solar wind plasmas move in the tailward direction, the magnetic field lines are stretched so that a two‐dimensional tail‐like configuration is formed. Plasma sheet thinning is observed during the further development of tail formation. When the magnetic field line is stretched and hence the neutral sheet current is enhanced enough, x‐type reconnection is triggered in the presence of resistivity. Plasmoid is formed in the downstream region and is accelerated along the sun‐earth line as a consequence of reconnection. A moderate earthward and strong tailward plasma jetting is also observed. Dependence of the simulation results on the resistivity and the solar wind energy flux is discussed in detail. The reconnection rate does not depend on resistivity, but the resistivity influences the onset time of reconnection. In contrast, the reconnection rate is very sensitive to the incoming solar wind speed. A broad current sheet is formed in the neutral sheet region in the present case rather than a pair of sharply peaked current sheets, which were observed in the previous simulation studies in which the initial configuration was one‐dimensional.

Magnetotail plasma observations during the 1054 UT substorm on March 22, 1979 (CDAW 6)

The paper reports ISEE 1 and 2 plasma observations in the plasma sheet near 15 RE radial distance for the 1054 substorm on March 22, 1979. Noteworthy features are plasma sheet thinning and the brief appearance of rapid tailward flows after substorm onset, strong earthward flows (superimposed on dawnward convection) at the beginning of plasma sheet expansion and a quieting of all plasma properties at substorm recovery. Much of the time, however, plasma flows are small and switch directions. In particular, no strong flows are observed during neutral sheet crossings. Two‐dimensional velocity distribution functions show a complex behavior, with several plasma components often coexisting and at times streaming in opposite directions. The interpretation of the observations in terms of the formation of a neutral line earthward of and close to ISEE at substorm onset is tempting, but three‐dimensional spatial effects as well as rapid temporal changes have to be invoked to obtain consistency.

Electric fields in the plasma sheet and plasma sheet boundary layer

Data from the spherical double probe electric field experiment on ISEE 1 have been used to study a number of plasma sheet/lobe boundary crossings during intervals selected for the Coordinated Data Analysis Workshop 6 on March 22 and 31, 1979. These crossings took place during periods of substorm occurrence. They could be identified by keV plasma measurements and by using the electric field probes as a reference for measurements of the spacecraft potential. Typical spacecraft potentials in the plasma sheet are +10 V to +20 V. Because of decreased plasma density in the lobes the satellite potential increases here to +20 V to +30 V. Strong electric fields, with a dominant dawn‐to‐dusk component, are observed throughout the boundary layer outside the plasma sheet both for contracting and expanding motions of the plasma sheet and for different magnetic field directions. Characteristic amplitudes and durations are 5–10 mV m−1 and 5–15 min. The corresponding E × B vectors are always toward the plasma sheet. The spacecraft were situated both north and south of the plasma sheet edge, in such boundary layers, and in two or three cases the spacecraft were most likely on the earthward side of an X line. These latter cases coincided with the maximum of the expansion phase of substorms. Prior to these substorms, during plasma sheet thinning, onsets of sunward convection were observed by an electric field experiment on GEOS 2 in the geostationary orbit.

ISEE-1 and 2 observations of field-aligned currents in the distant midnight magnetosphere

Magnetic field measurements obtained in the nightside magnetosphere by the co-orbiting ISEE-1 and 2 spacecraft have been examined for signatures of field-aligned currents (FAC). Such currents are found on the boundary of the plasma sheet both when the plasma sheet is expanding and when it is thinning. Plasma sheet boundary layer current structure and substorm associated dynamics can be determined using the two spacecraft, although for slow traversals of the FAC sheet the spatial/temporal ambiguity is still an issue. We often find evidence for the existence of waves on the plasma sheet boundary, leading to multiple crossings of the FAC sheet. At times the boundary layer FAC sheet orientation is nearly parallel to the X-Z GSM plane, suggesting ‘protrusions’ of plasma sheet into the lobes. The boundary layer current polarity is, as expected, into the ionosphere in the midnight to dawn local time sector, and outward near dusk. Current sheet thicknesses and velocities are essentially independent of plasma sheet expansion or thinning, having typical values of 1500 km and 20–40 km/s respectively. Characteristic boundary layer current densities are about 10 nanoamps per square meter.

Implications of solar flare dynamics for reconnection in magnetospheric substorms

From observations of two-ribbon solar flares, we present a new line of evidence that magnetic reconnection is of key importance in magnetospheric substorms. We infer that in substorms reconnection of closed field lines in the near-Earth thinned plasma sheet both initiates and is driven by the overall MHD instability that drives the tailward expulsion of the reconnected closed field (0 loops). The general basis for this inference is the longstanding notion that two-ribbon flares and substorms are essentially similar phenomena, driven by similar processes. We give an array of observed similarities that substantiate this view. More specifically, our inference for substorms is drawn from observations of filament eruptions in two-ribbon flares, from which we conclude that the heart of the overall instability consists of reconnection and eruption of the closed magnetic field in and around the filament. We propose that essentially the same overall instability operates in substorms. Our point is not that the magnetic field configuration or the microphysics in substorms is identical to that in two-ribbon flares, but that the overall instability results from essentially the same combination of reconnection and eruption of closed magnetic field.

Associations of geomagnetic activity with plasma sheet thinning and expansion: A statistical study

Associations of geomagnetic activity in the auroral zone with thinnings and expansions of the magnetotail plasma sheet are examined statistically in this paper. We first identified many plasma sheet thinnings and expansions in plasma and particle data from VELA satellites and from OGO 5 without reference to ground magnetic data. These events were grouped according to the location of the detecting satellite in the magnetotail. For each such group the times of thinning or expansion were then used as fiducial times in a superposed‐epoch analysis of the geomagnetic AL index values that were recorded in 8‐hour intervals centered on the event times. The results show that many plasma sheet thinnings and expansions are related to discrete negative bay structures that are the classical signature of substorms. Furthermore, they support earlier findings that plasma sheet thinning and expansion at the VELA orbit (r ≈ 18 RE) tend to be associated with the onset of the auroral zone negative bay and the beginning of its subsidence, respectively. Earthward of r ≈ 13–15 RE, plasma sheet expansion occurs near the time of the onset of the negative bay, again in agreement with earlier findings. A large fraction of plasma sheet expansions to half thicknesses of ≳ 6 RE at the VELA orbit are associated not with a baylike geomagnetic disturbance but with subsidence of a prolonged interval of disturbance. The study also shows that many plasma sheet expansions are related simply to generally enhanced geomagnetic activity showing no baylike or other distinctive features.

A multisatellite study of the plasma sheet dynamics at substorm onset

A multi‐satellite study has been conducted on the temporal relationship between energetic particle injections observed at the geostationary orbit onboard GEOS‐2 and decreases of thermal plasma sheet particle fluxes ("plasma sheet thinnings") observed in the more distant geomagnetic tail onboard the ISEE‐1 and ‐2 spacecraft. A case by case analysis as well as a statistical study of 100 events recorded from February to May 1979 and 1980 show that particle injection and particle flux decrease are detected to within less than 5 minutes of each other, for an average intersatellite distance of 15.7 RE. The observed spread in Δt corresponds best to Alfvén wave transit times. These particle phenomena are observed at the onset of the auroral zone magnetic bays. The results reported here suggest that the dynamics of the inner and outer boundaries of the plasma sheet are closely related to each other and controlled by large scale processes that develop at substorm onset inside the magnetospheric tail and result in a cross‐tail current disruption.

On the microphysics of the magnetotail reconnection region

We shall review the theories of the tearing mode instability and its implications for the reconnection process in the magnetotail. It is shown that in the thin plasma sheet whose width is comparable to the ion inertial length, the Hall current effect is important. The Hall current effect has not been taken into account so far in the collisionless tearing mode theory. It is argued that the previous collisionless tearing mode theory would not be complete, since the neglect of the Hall current effect leads to an artificial exclusion of the vorticity excitation term from the basic governing equations of the instability.

Numerical simulation of the interaction of the plasma sheet with the lobes of the Earth's magnetotail

Codes involving one and two spatial dimensions and three velocity dimensions are used to model the earth's magnetotail. It is shown that the magnetotail can become inflated as a consequence of low‐energy plasma convection toward the neutral plane. The two‐dimensional code shows the development of small‐scale turbulence. However, the growth of the turbulence appears limited and does not lead to substantial dissipation. A one‐dimensional model was used to simulate a particle population consisting of plasma sheet ions and cold lobe plasma. An applied convection electric field causes plasma sheet thinning and later expansion, consistent with observed substorm morphology. During the course of a simulated substorm the cold particles are heated and merge with the plasma sheet population. The magnetic field energy shows a slight increase, followed by a more rapid decrease as all of the cold particles are swept into the magnetotail. The code exhibits a conversion of both magnetic field energy and energy supplied by the convection electric field into particle energy. The simulations suggest that much of the magnetotail substorm morphology may be a simple consequence of an increase, followed by a decrease, in the convection electric field, without the requirement of any magnetospheric size scale plasma instability or other disruptive processes. It is also concluded that the presence of the convection electric field and a continuing replenishment of low‐energy particles in the magnetotail lobes are both necessary for maintenance of the magnetotail.

Multiple crossings of a very thin plasma sheet in the Earth's magnetotail

High‐resolution magnetic field, plasma, and energetic particle data from the IMP 8 spacecraft have been used to study multiple crossings of the earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. IMP 8 is found to traverse the entire thickness of the β > 1 plasma sheet in times that are as short as 10 s. Such traversals recur on a time scale of several minutes, and they are associated with high velocity plasma flows that are usually directed tailward but are occasionally directed earthward for brief intervals. We argue that these observations can be explained by rapid oscillations of a plasma sheet that is only a few thousand kilometers thick, a dimension that is comparable to the gyroradius of energetic protons. When ions of 50 keV and 290 keV are detected at a location on tail lobe field lines near the plasma sheet, they are often seen to be moving in the dawn to dusk direction and may be undergoing serpentine cross tail motion. Differences in the angular distributions of the two energies indicate that the higher energy ions are preferentially located on field lines deeper in the tail lobe. This result is consistent with ion acceleration at a neutral line with subsequent drift in a cross‐tail electric field that moves the lower energy particles closer to the equatorial plane. A neutral line acceleration model is also supported by our observation that tailward streaming energetic electrons are occasionally present at the lobe‐plasma sheet interface.