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At least 109 records · Page 6
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NASA NTRS1980

The effects on the earth's magnetotail from shocks in the solar wind

A new, minimally diffusive MHD computer code has been used to simulate the time-dependent, 2D interaction of the solar wind with the earth's magnetosphere. The equilibrium with a simulated unmagnetized solar wind shows the basic features of the magnetosphere: bow shock, magnetopause, and long magnetotail. A simulated shock in the solar wind shows the presence of shock focussing in the midplane of the magnetotail. This focussing can be important in the energy budget of the plasma sheet and may have an important role in the initiation of substorms.

Lyon, J.↗
NASA NTRS1981

Field-aligned currents in the earth's magnetotail

Direct measurements of the plasma velocity distributions and simultaneous observations of magnetic fields are used to examine the character of field-aligned currents in the earth's magnetotail during the recovery phase of a magnetic substorm. Three contiguous field-aligned current sheets are identified at the interface between the magnetotail lobe and plasma sheet. Average current densities within these three current sheets are, in order of decreasing distance to the plasma sheet, +3.3 x 10 to the -9th, -1.3 x 10 to the -8th, and +1.1 x 10 to the -8th A/sq m, respectively.

Frank, L. A.↗
NASA NTRS1981

Ion streams in the magnetotail

Ion mass spectrometer observations of low-temperature streaming plasmas in the earth's magnetotail are reported. Measurements in the energy per charge range 0 to 17 keV/e were made at geocentric radial distances less than 23 earth radii from the ISEE 1 spacecraft. Ion streams of solar wind origin in the magnetotail boundary layer and of ionospheric origin in the tail lobes and plasma sheet are described. A statistical study of some of the characteristics of the streams allows the inference that the central tail lobe plasmas are primarily constituted of ion streams of ionospheric origin and that the ionosphere is a significant contributor to the hot plasmas that form the plasma sheet.

Sharp, R. D.↗
NASA NTRS1981

The distant bow shock and magnetotail of Venus - Magnetic field and plasma wave observations

An examination of the magnetic field and plasma wave data obtained by the Pioneer Venus orbiter in the wake region behind Venus discloses a well developed bow shock whose location is similar to that observed on previous missions in contrast to the dayside bow shock. Venus also has a well developed magnetotail in which the field strenght is enhanced over magnetosheath values and in which the magnetic field is aligned approximately with the solar wind direction. The boundary between magnetosheath and magnetotail is also marked by a change in the plasma wave spectrum.

Russell, C. T.↗
NASA NTRS1982

Evidence for a distant ( 8700 R sub J) Jovian magnetotail: Voyager 2 observations

A correlative survey of magnetometer (MAG) and Planetary Radio Astronomy (PRA) 1.2 kHz continuum radiation measurements from Voyager 2 provide evidence for at least eight distant Jovian magnetotail sightings occurring about once a month over the first 2/3 of 1981 at distances of approximately 5,000 to 9,000 R sub J. The occurrences of these events are in good agreement with prior Plasma Wave Science and Plasma Science identifications. Observations of these distant magnetotail, or tail filament, encounters appear most prevalent in both MAC and PRA data sets when the spacecraft was closest to the Jupiter-Sun axis at approximately 6,500 R sub J from the planet; the PRA events are also most intense during those times. A specific tail encounter occurring in mid-February 1981 is analyzed and shown to possess a remarkably symmetric magnetic field signature and to have a bipolar field structure in the central region. The bipolarity is characteristic of most of the eight events.

Lepping, R. P.↗
NASA NTRS1981

Energetic particles in the predawn magnetotail of Jupiter

A detailed account of the energetic electron and proton populations as observed with Voyager 1 and 2 during their passes through the dawn magnetotail of Jupiter is given. As in the case of Pioneer 10, a thin plasma sheet is found at the magnetic equator which was already well developed near 23 Jupiter radii. It is pointed out that the plasma sheet positions in the magnetotail can be represented by a distorted disk which rotates about the Jovian spin axis.

Schardt, A. W.↗
NASA NTRS1983

Exit of boundary layer plasma from the distant magnetotail

It is pointed out that the boundary layer plasma must somehow leave the magnetospheric system. It is proposed that the boundary layers on the dawn and dusk flanks are continued into the far magnetotail, becoming joined together far downstream, still on closed magnetic field lines. Within the combined layer there would be a dusk to dawn electric field for antisunward convection (as is the case for the boundary layers on the dawn and dusk flanks nearer to the earth). It is suggested that the boundary layer flow is so massive that the flow itself can generate the right electric field for continued flow. Most of the plasma in the mantle over the polar caps would also be convected out in a similar manner, rather feeding the plasma sheet. There would be a stagnation point inside the magnetotail, with any boundary or mantle plasma diffusing earthward of this point becoming the plasma sheet. It is deduced that no steady state solution is possible.

Heikkila, W. J.↗
NASA NTRS1984

Fine-scale structure of the Jovian magnetotail current sheet

New evidence concerning the detailed structure of the magnetic field in and near the Jovian tail sheet is presented, and nonunique temporal variations in the sheet structure are deduced. Data from Voyagers 1 and 2 are used to show that the local sheet structure, while tending to vary with time and distance from Jupiter, generally corresponds to one of four general types. The four types are classified using examples, and possible geometries associated with each type are identified. The locations of the different types of sheet crossings are pointed out and discussed. The possibility of reconnection occurring at the current sheet is discussed through analogy with the terrestrial magnetotail. It is concluded that the results support the view that reconnection occurs in the Jovian magnetotail as a time-dependent, and possibly spatially localized, process.

Behannon, K. W.↗
NASA NTRS1984

Composition measurements in the magnetotail between 60 R sub E and 240 R sub E

From January 1, 1983 to March 30, 1983 the ISEE-3 spacecraft was in the region of the magnetotail of the earth at distances from 60 to 240 R sub E. The ion composition instrument aboard ISEE-3 made ion composition and velocity measurements in two modes during this period. In the first mode the mass per charge (M/Q) range was 0.7 to 32 at velocities from 79 to 220 km per s. The M/Q range of the second mode was 1.4 to 6.0 at velocities from 300 to 620 km per s. Preliminary analysis of the data shows the magnetotail composition to have a significant solar wind-like component, with bulk velocities generally in the 300-600 km per s range.

Ogilvie, K. W.↗
NASA NTRS1984

Generation of broadband electrostatic noise by ion beam instabilities in the magnetotail

Particle data from ISEE 1 sampled in the earth's magnetotail show the presence of energetic ion beams in the boundary layer of the plasma sheet. A theory of instabilities driven by the beams is developed and compared with wave data sampled simultaneously to the particle data. It is concluded that the ion beams generate broadband electrostatic bursts of noise. The electrostatic noise correlates well with the occurrence of the beams, and the spectrum is consistent with that predicted from a negative energy beam instability under magnetotail conditions. The theory predicts that a spectrum of growing waves can be driven for frequencies from 0.001 omega(pe) up to omega(pe), the electron plasma frequency, with a spectral peak typically near 0.01 omega(pe) or lower, in agreement with the wave data. Furthermore, as one moves away from the source region perpendicular to the magnetic field, the high frequency components of the observed wave spectra are predicted to disappear gradually, leaving the low frequency part of the spectrum, also as is observed. Evidence is given for significant pitch angle scattering of the beams by the broadband electrostatic noise, leading to more isotropic ion distributions.

Grabbe, C. L.↗
NASA NTRS1985

Magnetotails of the terrestrial planets: A comparative study

Spacecraft observations have established that all of the terrestrial planets interact strongly with the solar wind and possess well developed magnetic tails. At Mercury, Earth, and possibly Mars the tail forms as a result of the solar wind dragging back field lines intrinsic to the planet. Venus differs dramatically from the other planets in that its magnetotail is composed of interplanetary field lines draped about the ionopause. Accordingly, the observations made at the terrestrial planets provide an opportunity to compare not only induced (Venus) magnetic tail properties with those of intrinsic field tails, but also the variation among intrinsic field tails ion the limits of no ionosphere (Mercury), weak intrinsic magnetic field (Mars), and strong magnetic field and ionosphere (Earth). A comparative investigation is made of terrestrial planet magnetotail structure and dynamics. The results are discussed in terms of the physical properties of these planets and their interactions with the solar wind.

Slavin, J. A.↗
NASA NTRS1985

The distant magnetotail's response to a strong interplanetary magnetic field By - Twisting, flattening, and field line bending

During an interval of strong interplanetary magnetic field (IMF) By, while ISEE 3 was in the distant magnetotail, the north lobe was observed south of the ecliptic plane. Lobe field lines were strongly bent in the direction of the IMF, and a dense boundary layer plasma was observed. During the interval, magnetopause normals pointed in the z direction, although ISEE 3 was near the dawnside ecliptic plane. The observations are interpreted in terms of field line bending within a twisted and flattened magnetotail.

Sibeck, D. G.↗
NASA NTRS1985

The boundary layers as the primary transport regions of the earth's magnetotail

A comprehensive survey of ISEE and IMP LEPEDEA plasma measurements in the earth's magnetotail reveals that the magnetospheric boundary layer and the plasma sheet boundary layer are the primary transport regions there. These plasma measurements also reveal various components of the plasma sheet, including the central plasma sheet and plasma sheet boundary layer. A significant new result reported here is that of cold- and hot-plasma components that are spatially co-present within the central plasma sheet. Such plasma components cannot be explained merely by temporal variations in spectra involving the entire plasma sheet. Contributions to a low temperature component of the plasma sheet enter directly from the boundary layer located along the magnetotail flanks. Field-aligned flows predominate within the plasma sheet boundary layer which is almost always present and is located near the high- and low-latitude border of the plasma sheet. The plasma sheet boundary layer comprises highly anisotropic ion distributions, including counter-streaming ion beams, that evolve into the hot, isotropic component of the plasma sheet. Tailward acceleration regions generate these ion beams with plasma input from the magnetospheric boundary layer. Antisunward-flowing ion beams, at E/q less than 1 kV and of ionospheric composition, are frequently observed in the plasma sheet boundary layer and in tail lobes. These ion beams are likely accelerated at low altitude over the polar cap and especially along auroral field lines.

Eastman, T. E.↗
NASA NTRS1986

The effect of different resistivity models on magnetotail dynamics

Two-dimensional, time-dependent MHD simulations of the interaction between the solar wind and the earth's magnetosphere have been performed to study magnetotail dynamics with varying forms of anomalous resistivity. In general, the resulting models conform to the neutral line model proposed for substorms with near-earth x points and high-speed tailward flows occurring in the magnetotail. However, in the case where Joule heating is included in the physical description of the system, the near-earth x point never moves far down tail, and high-speed tailward flows never stop. Only in the case where there is no Joule heating does the x point move down tail. Simultaneously, the high-speed tailward flows cease. These results indicate that the mechanism of energy dissipation can have an important effect on reconnection processes and the global magnetospheric dynamics.

Lyon, J. G.↗
NASA NTRS1986

Neutral sheet crossings by ISEE-3 in the distant magnetotail

Magnetic field data from ISEE-3 in the distant magnetotail at crossings of the field reversal (or neutral sheet) region were analyzed to determine the instantaneous direction of the normal component Bz at the crossing. A crossing identified as being almost always tailward of the steady-state X-line was selected. Data for 1 hr are discussed to illustrate difficulties. One particular smooth crossing shows that complicated microstructure can occur in times less than 1 min. Averaging over long times may eliminate essential information. Inspection of the magnetic field data at the highest resolution, however, shows that the direction of the plasma sheet flows and the sense of Bz across the neutral sheet do not always agree with the reconnection models. Rather, they indicate that the low latitude boundary layer may play a significant role in the dynamics of the magnetotail.

Heikkila, W. J.↗
NASA NTRS1986

The Jovian magnetotail

The current understanding of the plasma, magnetic field, and plasma wave characteristics of the near and distant Jovian magnetotail is discussed. The properties of these two regions are compared, with particular emphasis given to the data on the distant tail obtained by Voyager 2. The Jovian tail has many unusual properties, such as the large scale sausage-string shape of its outer boundary, but some of its properties resemble those of the earth's magnetotail, such as its central current sheet and a surrounding region resembling a plasma sheet consisting of hot ions. Data are presented that support the view that plasma in the Jovian tail plasma sheet comes from Jupiter.

Lepping, R. P.↗
NASA NTRS1986

The Venus magnetotail

The magnetic structure of the Venus magnetotail has been measured using 9 seasons of Pioneer Venus measurements in the distant wake region. The magnetotail region is about 4 Venus radii wide and about 3.2 Venus radii high at 12 Venus radii behind the planet. The tail contains about 2.3 Megawebers of flux, much of which closes across the tail. The J x B force associated with this cross tail component accelerates the tail plasma down the tail causing an ion loss rate of about 10 to the 26th ions per second. The Venus tail does respond to solar activity and contains significantly less magnetic flux at solar minimum.

Russell, C. T.↗
NASA NTRS1986

A review of broad-band electrostatic waves in the magnetotail

Recent results showing that ions beams are the cause of generating broad-band electrostatic waves in the magnetotail, as well as theoretical work in good agreement with the spectrum observed, is reviewed. The implications of this for the transport and other dynamics of the magnetotail are discussed.

Grabbe, Crockett L.↗