摘要:

Magnetic field observations during the first ISEE‐3 pass into a previously unexplored region of the geotail (−80 to −220 Re) are discussed. The tail structures customarily detected closer to Earth are also identifiable in the distant tail field data: two lobes separated by a plasma sheet and embedded neutral sheet. A plasma sheet boundary layer, characterized by small diamagnetic field decreases and the presence of large amplitude (5 nT p‐p) electromagnetic waves, is often, but not always present. It is found that the tail maintains a well‐ordered magnetic field structure out to −220 Re. Near‐earth (−40 to −80 Re), the magnetic field in the plasma sheet is directed southward only 5% of the time, but this percentage gradually increases with distance until it is southward 50% the time at X = −220 Re. The implications of the above observations for reconnection models are discussed. Significant Byfields are detected within the neutral sheet and the field rotation often takes place primarily in the X‐Y plane. These transverse fields may be associated with tailward moving plasmoids or with a

ISSN:0094-8276    

DOI:10.1029/GL011i001p00001

年代:1984

数据来源: WILEY

摘要:

Using plasma electron and magnetic field measurements from ISEE 3, 220 REfrom earth, we find that the magnetotail at that distance is a coherent structure that evidently waves about through distances comparable to its own lateral scale size. For about one‐third of the time it was inside the magnetotail, ISEE 3 was in the plasma sheet. During quiet times the plasma sheet is apparently quite thin, but in response to geomagnetic activity it expands, becoming filled with hot plasma flowing tailward at speeds sometimes exceeding 1000 km/sec, and forces the magnetotail cross‐section itself to expand. The plasma sheet’s expansion is delayed typically by about 30 minutes from the onset of the associated geomagnetic activity (often a clearly identified isolated substorm). The magnetic field in the newly‐expanded plasma sheet usually exhibits a few‐minute steep northward excursion followed by a more prolonged (and often steep) southward excursion. We believe these to be the signatures of arrival of a plasmoid formed and released near the earth at the onset of the corresponding geomagnetic activity. The discreteness of these plasma releases through the magnetotail and their close association with onsets of geomagnetic activity at earth suggest that they are consequences of spontaneous release, probably by magnetic reconnection, of energy and plasma earlier stored in the ma

ISSN:0094-8276    

DOI:10.1029/GL011i001p00005

年代:1984

数据来源: WILEY

摘要:

This letter presents a detailed experimental analysis of the electric fields and electron flow velocities at a rotational magnetopause discontinuity. It is shown for this event that the magnetosheath plasma velocity is sharply decelerated on entry into the magnetosphere in very good agreement with a theoretical magnetohydrodynamic fit. This type of event is remarkable because the quantityE·Jis negative during the magnetopause crossing. This case represents one of two types of valid solution to the MHD boundary conditions. It differs from previously published examples of reconnective magnetopause geometries in that energy is being transferred locally from kinetic energy into electromagnetic energy at the time of these measurements

ISSN:0094-8276    

DOI:10.1029/GL011i001p00008

年代:1984

数据来源: WILEY

摘要:

The collisionless tearing mode in a neutral sheet is studied in the presence of ion temperature anisotropy using Vlasov description for both ions and electrons. It is found that the growth rate of the instability is significantly enhanced if Ti⊥/Ti∥>1 where the symbols ⊥ and ∥ refer to the directions perpendicular and parallel to the equilibrium magnetic field. For typical magnetotail parameters with modest temperature anisotropy, it is shown that the linear e‐folding time is reduced to a small fraction of the time delays believed to precede the onset of reconnection. This enhancement of the growth rate is due to the Lorentz force acting on the ions that cross the neutral plane, traversing beyond the conventional electron tear

ISSN:0094-8276    

DOI:10.1029/GL011i001p00012

年代:1984

数据来源: WILEY

摘要:

It is shown that finite amplitude electrostatic ion cyclotron (EIC) waves can parametrically decay into daughter EIC waves and kinetic Alfvén waves. Since the current associated with the latter is d.c. in the EIC time scale, it can linearly excite EIC waves when the amplitude of the Alfvén waves is sufficiently high. This provides a mechanism for the simultaneous existence of EIC and Alfvén fluctuations. Application of the theory in the auroral zone of the ionosphere is discuss

ISSN:0094-8276    

DOI:10.1029/GL011i001p00016

年代:1984

数据来源: WILEY

摘要:

Phase velocities of plasma waves near the lower hybrid frequency were measured with an interferometer composed of two spatially separated electron density probes. The plasma waves were produced in the F‐region ionosphere by an argon ion beam. By calculating the normalized cross spectrum of the plasma waves a coherency of .98 was estimated along with a maximum phase difference of π/3 radians between the two probes. This implies that the wavelength was 6 meters compared to an O+gyroradius of 3.8 meters and that the phase velocity was 45 km/sec compared to an ion beam velocity of 12.4 km/sec. These numbers compare favorably with recent predictions of a nonresonant mode produced by a dense ion be

ISSN:0094-8276    

DOI:10.1029/GL011i001p00019

年代:1984

数据来源: WILEY

摘要:

Magnetic field data observed by MAGSAT at altitude between 250‐550 km clearly showed that a pair of vertical currents appears in the dayside polar cap when the interplanetary magnetic field (IMF) has a strong northward component. The currents were approximately symmetric with respect to the noon meridian, and flowed into the ionosphere on the duskside and away from it on the dawnside. These currents were very clearly seen on May 10, 1980, when the IMF was kept strongly northward all day long. The horizontal magnetic field along 10 successive orbits on the day indicated the signature. The distribution of the vertical magnetic field observed along the same orbits was reasonably explained in terms of the ionospheric Hall currrent which was produced when the vertical current flowed into or away from the ionosphere. The existence of such vertical currents was also confirmed by the statistical analysis over the whole MAGSAT data perio

ISSN:0094-8276    

DOI:10.1029/GL011i001p00023

年代:1984

数据来源: WILEY

摘要:

We have examined all energetic medium nuclei (carbon, nitrogen, and oxygen) flux increases observed by the satellites IMP 7 and IMP 8 at 1 AU during Bartels rotations 1906‐1974. After removing flare‐related increases, the remaining 145 "events" were compared to interplanetary magnetic field and solar wind parameters. We have discovered a class of flux enhancements in which the ion increases occur close to the onset of magnetic sector boundary crossings. We interpret this observation as a facilitated access to 1 AU of energetic ions from the corona or chromosphere via the magnetic sector structure. It appears that this access is more significant for medium than for lighter nuclei, suggesting a possible charge‐ or rigidity‐dependent transport me

ISSN:0094-8276    

DOI:10.1029/GL011i001p00027

年代:1984

数据来源: WILEY

摘要:

A study of the plasma and electric field measurements of the PVO in the Venus ionosheath near the terminator region is presented. A drastic decrease of the particle flux intensity measured with the plasma instrument is encountered in the inner ionosheath before the inbound crossing of the ionopause. The outer boundary of the region of weak plasma fluxes is consistent with the presence of a rarefaction wave as reported previously from the Mariner 5, Venera and PVO plasma measurements in the Venus wake. Near that boundary there are, in addition, appreciable changes in the electric field signals detected with the PVO electric field detector. These observations suggest the existence of local plasma wave activity associated with turbulent flow conditions in that region.

ISSN:0094-8276    

DOI:10.1029/GL011i001p00031

年代:1984

数据来源: WILEY

摘要:

The long‐wave features of the gravity field of Venus differ from those of the Earth's field not only in their strong positive correlation with topography, but also in their gentler spectral slope. These properties are inconsistent with generation of the gravity field by plate tectonics or by processes at great depths; they are consistent with generation by a mantle convective system supporting the broad features in topography with an effective compensation depth of about 450 k

ISSN:0094-8276    

DOI:10.1029/GL011i001p00035

年代:1984

数据来源: WILEY