Magnetospherically trapped ions as a source of magnetosheath energetic ions
TL;DR: In this article, the authors used three-dimensional ion spectra from the ISEE 1 for a magnetopause crossing on Nov. 10, 1977 to construct a 3D distribution function in the magnetosphere and in the sheath.
Abstract: It has been suggested that energetic ions observed in the magnetosheath may be due to the direct leakage of trapped magnetospheric ions. To test this hypothesis, three-dimensional ion spectra from the energetic particle experiment on ISEE 1 for a magnetopause crossing on Nov. 10, 1977 are utilized to construct three-dimensional distribution functions in the magnetosphere and in the sheath. Using the observed magnetic field, a simple one-dimensional model of the magnetopause is developed. Ions are then followed in the model, starting in the magnetosphere, through the magnetopause and ending up in the sheath. Using Liouville's Theorem a model sheath distribution function is then built up by following the magnetospheric distribution function through the model fields. The model distribution is then compared with the observed sheath distribution. For this case it is found that the main features of the observed ions in the sheath are consistent with direct leakage and with no energization or de-energization processes, and an inward-pointing normal component is required. The energetic particles mapped in this case apparently follow a flux tube which does not penetrate the magnetopause where local tangential electric fields have been reported.
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01 Aug 1982
TL;DR: In this article, the authors present a detailed analysis of plasmas and fields in the vicinity of the dayside magnetopause using the International Magnetospheric Study by the ISEE 1 and 2 spacecraft and conclude that dayside reconnection is the dominant contributor under usual conditions.
Abstract: The physical processes which give rise to convection in the earth's magnetosphere have been the subject of active debate for many years. Most of the discussion has centered on two basic pictures, one in which closed magnetospheric flux tubes are transported from dayside to nightside in a boundary layer around the flanks of the magnetosphere by a ‘viscouslike’ process occurring at the magnetopause and the other in which open flux tubes are transported over the poles of the earth after reconnection has taken place with the interplanetary magnetic field. These processes may coexist on a continuous basis, and the question then arises as to their relative contributions to usual total cross-magnetospheric voltages of ∼40–100 kV. The first detailed observations of plasmas and fields in the vicinity of the dayside magnetopause have recently been made during the International Magnetospheric Study by the ISEE 1 and 2 spacecraft and are discussed in this paper in relation to this question. Observations which relate to the occurrence of ‘quasi-steady’ as well as impulsive flux transfer event (FTE) reconnection are reviewed in detail in the paper, together with measurements of the properties of the boundary layer on the magnetospheric flanks. Particular emphasis is given to the interpretation of these data in terms of the physical processes occurring. It is argued that the ISEE observations of quasi-steady reconnection are indeed compatible with the process playing a major role in magnetospheric dynamics. The observed frequency of these events at the magnetopause indicates that they are associated either with infrequent intervals of intense magnetospheric convection or with more frequent contributions of lesser intensity (i.e., ∼40 kV whenever IMF Bz is negative). The latter seems the more likely situation at the present time. In addition, a rough estimate of ∼20 kV is made for the contribution due to FTE's, this figure having the nature of a lower limit. It is therefore argued that the in situ ISEE observations pertaining to reconnection processes are consistent with the view that these provide a major contribution to magnetospheric flows. A preliminary picture is suggested from our inferences based on ISEE data in which quasi-steady subsolar reconnection occurs essentially continuously in a band ∼2 hours LT wide when IMF Bz ≤ 0, centered often near the noon meridian, while at other local times, reconnection also occurs but in an unsteady, sporadic manner. Examination of the unsteady flows observed in the boundary layers on the flanks of the magnetosphere yields voltages of typically ∼5–20 kV for dusk and dawn layers combined. Similar values are obtained from studies of low-altitude spacecraft observations, although there is some disagreement in the literature concerning the exact identification of the boundary layer in such data. This latter topic is also reviewed. It is also argued that part, at least, of the boundary layer flow could occur on open field lines and indeed that some of the boundary layer observations correspond closely to what one would expect for the magnetospheric counterpart of magnetosheath FTE's. Finally, recent analyses of the variation of the transpolar voltage (measured at low altitudes) with solar wind conditions suggest an average ∼30-kV contribution from the boundary layers, a rather higher value than is indicated by the in situ measurements. However, all these estimates suggest that the layers generally provide a small but sometimes significant contribution to the magnetospheric voltage. We conclude that dayside reconnection is the dominant contributor under usual conditions, in agreement with conclusions reached previously on the basis of less direct information.
578 citations
01 Feb 1984
TL;DR: Flux transfer events (FTEs) observed on both the interior and exterior of the dayside magnetopause region by the ISEE 1 and 2 spacecraft are noted to be a feature of the magnetosphere region covered by the spacecraft when the magnetic field in the magnetosheath has a southward component.
Abstract: Flux transfer events (FTEs), observed on both the interior and exterior of the dayside magnetopause region by the ISEE 1 and 2 spacecraft, are noted to be a feature of the magnetopause region covered by the spacecraft when the magnetic field in the magnetosheath has a southward component, but not when it is northward. During periods of southward magnetosheath field, the average number and recurrence time of FTE signatures/magnetopause crossing are similar to those observed in the magnetopause interior, implying that the magnetosheath and magnetosphere FTEs are aspects of the same physical phenomenon. It is speculated that FTEs may provide the dominant means of flux transfer required for the driving of geomagnetic disturbances.
418 citations
TL;DR: In this paper, the moments of the distribution function were calculated from two-dimensional or three-dimensional data for five passes of the magnetopause region at 20 and 40 deg northern latitudes, and the different observed magnetic field signatures were consistent with expectations for encounters of the flux tubes at different relative locations.
Abstract: Plasma and magnetic field data from ISEE 1 and 2 are examined for 5 passes of the magnetopause region at 20 and 40 deg northern latitudes, and are presented in terms of moments of the distribution function, calculated from two-dimensional or three-dimensional data. Flux transfer events are characterized by a mixture of magnetosheath and magnetospheric particles, which supports the hypothesis that flux transfer events represent encounters of reconnected flux tubes. An excess pressure appears to be balanced by the tension of the ambient magnetic field lines as they are draped around the reconnected flux tube, and the different observed magnetic field signatures are consistent with expectations for encounters of the flux tubes at different relative locations. It is suggested that increased flow speeds are caused by continued reconnection at the low-latitude boundaries of the flux tubes.
379 citations
TL;DR: In this paper, the authors review the nonlinear dynamics of charged particles in the magnetotail configuration, focusing on the relationships between the dynamics and physical observables, and propose a new framework of understanding magnetospheric plasma properties.
Abstract: An important region of the earth's magnetosphere is the nightside magnetotail, which is believed to play a significant role in energy storage and release associated with substorms. The magnetotail contains a current sheet which separates regions of oppositely directed magnetic field. Particle motion in the collisionless magnetotail has been a long-standing problem. Recent research from the dynamical point of view has yielded considerable new insights into the fundamental properties of orbits and of particle distribution functions. A new framework of understanding magnetospheric plasma properties is emerging. Some novel predictions based directly on nonlinear dynamics have proved to be robust and in apparent good agreement with observation. The earth's magnetotail may serve as a paradigm, one accessible by in situ observation, of a broad class of boundary regions with embedded current sheets. This article reviews the nonlinear dynamics of charged particles in the magnetotail configuration. The emphasis is on the relationships between the dynamics and physical observables. At the end of the introduction, sections containing basic material are indicated.
189 citations
18 Mar 2013
119 citations
References
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TL;DR: In this article, satellite magnetometer data at a magnetopause penetration can be used to determine the vector normal to the magnetopsause current layer and the magnetic-field component along this normal.
Abstract: It is shown how satellite magnetometer data at a magnetopause penetration can be used to determine the vector normal to the magnetopause current layer and the magnetic-field component along this normal. According to theory such a component is a measure of the amount of field reconnection at magnetopause. Results from 22 Explorer 12 boundary penetrations are presented indicating normal-field components of less than 5 γ in two-thirds of the cases. Measured field variations within the current layer are presented to demonstrate the existence of two fundamentally different types of boundary structure, the rotational and the tangential discontinuity. The former of these permits a nonzero normal field component, whereas the latter does not. The rotational discontinuity seems to occur predominantly during magnetic storms and two of these cases, involving substantial normal-field components, provide compelling evidence that field reconnection takes place during the storm main phase. Finally, the calculated normal vector is compared with the normal to the surface of the Mead-Beard magnetosphere model.
1,090 citations
01 Dec 1978
Abstract: The magnetic field profiles across the magnetopause obtained by the ISEE-1 and -2 spacecraft separated by only a few hundred kilometers are examined for four passes. During one of these passes the magnetosheath field was northward, during one it was slightly southward, and in two it was strongly southward. The velocity of the magnetopause is found to be highly irregular ranging from 4 to over 40 km s-1 and varying in less time than it takes for a spacecraft to cross the boundary. Thicknesses ranged from 500 to over 1000 km.Clear evidence for reconnection is found in the data when the magnetosheath field is southward. However, this evidence is not in the form of classic rotational discontinuity signatures. Rather, it is in the form of flux transfer events, in which reconnection starts and stops in a matter of minutes or less, resulting in the ripping off of flux tubes from the magnetosphere. Evidence for flux transfer events can be found both in the magnetosheath and the outer magnetosphere due to their alteration of the boundary normal. In particular, their presence at the time of magnetopause crossings invalidates the usual 2-dimensional analysis of magnetopause structure. Not only are these flux transfer events probably the dominant means of reconnection on the magnetopause, but they may also serve as an important source of magnetopause oscillations, and hence of pulsations in the outer magnetosphere. On two days the flux transfer rate was estimated to be of the order of 2 × 1012 Maxwells per second by the flux transfer events detected at ISEE. Events not detectable at ISEE and continued reconnection after passage of an FTE past ISEE could have resulted in an even greater reconnection rate at these times.
985 citations
TL;DR: The magnetic field profiles across the magnetopause obtained by the ISEE-1 and -2 spacecraft separated by only a few hundred kilometers are examined for four passes as discussed by the authors, during which the magnetosheath field was northward, during one pass it was slightly southward, and in two it was strongly southward.
Abstract: The magnetic field profiles across the magnetopause obtained by the ISEE-1 and -2 spacecraft separated by only a few hundred kilometers are examined for four passes. During one of these passes the magnetosheath field was northward, during one it was slightly southward, and in two it was strongly southward. The velocity of the magnetopause is found to be highly irregular ranging from 4 to over 40 km s-1 and varying in less time than it takes for a spacecraft to cross the boundary. Thicknesses ranged from 500 to over 1000 km.
934 citations
747 citations
TL;DR: The early ISEE orbits provided the opportunity to study the magnetopause and its environs only a few Earth radii above the subsolar point with unprecedented temporal resolution as discussed by the authors.
Abstract: The early ISEE orbits provided the opportunity to study the magnetopause and its environs only a few Earth radii above the subsolar point Measurements of complete two-dimensional ion and electron distributions every 3 or 12 s, and of three-dimensional distributions every 12 or 48 s by the LASL/MPI instrumentation on both spacecraft allow a detailed study of the plasma properties with unprecedented temporal resolution This paper presents observations obtained during four successive inbound orbits in November 1977, containing a total of 9 magnetopause crossings, which occurred under widely differing orientations of the external magnetic field The main findings are: (1) The magnetosheath flow near the magnetopause is characterized by large fluctuations, which often appear to be temporal in nature (2) Between ∼01 and ∼03R E outside the magnetopause, the plasma density and pressure often start to gradually decrease as the magnetopause is approached, in conjunction with an increase in magnetic field strength These observations are in accordance with the formation of a depletion layer due to the compression of magnetic flux tubes (3) In cases where the magnetopause can be well resolved, it exhibits fluctuations in density, and especially pressure and bulk velocity around average magnetosheath values The pressure fluctuations are anticorrelated with simultaneous magnetic field pressure changes (4) In one case the magnetopause is characterized by substantially displaced electron and proton boundaries and a proton flow direction change from upwards along the magnetopause to a direction tranverse to the geomagnetic field These features are in agreement with a model of the magnetopause described by Parker (5) The character of the magnetopause sometimes varies strongly between ISEE-1 and -2 crossings which occur ∼ 1 min apart At times this is clearly the result of highly non-uniform motions There are also cases where there is very good agreement between the structures observed by the two satellites (6) In three of the nine crossings no boundary layer was present adjacent to the magnetopause More remarkably, two of the three occurred while the external magnetic field had a substantial southward component, in clear contradiction to expectations from current reconnection models (7) The only thick (low-latitude) boundary layer (LLBL) observed was characterized by sharp changes at its inner and outer edges This profile is difficult to reconcile with local plasma entry by either direct influx or diffusion (8) During the crossings which showed no boundary layer adjacent to the magnetopause, magnetosheath-like plasma was encountered sometime later Possible explanations include the sudden formation of a boundary layer at this location right at the time of the encounter, and a crossing of an ‘inclusion’ of magnetosheath plasma within the magnetosphere (9) The flow in the LLBI is highly variable observed directions include flow towards and away from the subsolar point, along the geomagnetic field and across it, tangential and normal to the magnetopause Some of these features clearly are nonstationary The scale size over which the flow directions change exceeds the separation distance (several hundred km) of the two spacecraft
175 citations