摘要:

We examined the orientation and structure of 14 current sheets that were observed in sector boundary regions during the primary mission of Helios 2 (January through April 1976). We found that the field rotation through the current sheets (which are not null sheets) tends to occur at large angles with respect to the ecliptic plane. Most of the observed structures (which had typical thickness ranging between 1 × 104and 6 × 104km) can be interpreted as tangential discontinuities oriented at large angles with respect to the ecliptic plane. We also discuss the implications of a comparison between present results and the global configuration of the sector boundary surface. In particular, if we assume that the sheet normals are the same as the local normals to the global surface, our results do not support any solar cycle modulation for the orientation of boundarie

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00607

年代:1982

数据来源: WILEY

摘要:

Magnetic clouds are defined as regions with a radial dimension ≈ 0.25 AU (at 1 AU) in which the magnetic field strength is high and the magnetic field direction changes appreciably by means of rotation of one component ofB∽nearly parallel to a plane. The magnetic field geometry in such a magnetic cloud is consistent with that of a magnetic loop, but it cannot be determined uniquely. Forty‐five clouds were identified in interplanetary data obtained near earth between 1967 and 1978; at least one cloud passed the earth every 3 months. Three classes of clouds were identified, corresponding to the association of a cloud with a shock, a stream interface, or a CME. There are approximately equal numbers of clouds in each class, and the field and plasma parameters in each class are similar, suggesting that the three types of clouds might be different manifestations of a single phenomenon (e.g., a coronal transient). Interface‐associated clouds may have been swept up by corotating streams. Shock‐associated clouds move faster than the other two types, which are basically slow flows. The magnetic pressure inside the clouds is higher than the ion pressure, and the sum is higher than the pressure of the material outside of the cloud. This implies that the magnetic clouds were expanding even at 1 AU, and the average expansion speed is estimated to be of the order of half the ambient Alf

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00613

年代:1982

数据来源: WILEY

摘要:

We examine data on particle events resulting from three solar flares, two of which generated interplanetary shocks, to determine the effect of the shocks on the population of protons at energies ≳ 30 MeV. In both cases, the arrival of the shock marks the start of the decay phase of the particle event, and changes in particle anisotropies are related to features in the shock structure. We conclude that each shock is a source of protons with energies ≳ 30 MeV, which propagate along the interplanetary magnetic field lines and are observed at large distances (1 AU) away from the shock. A model in which these protons are accelerated by the shocks rather than being stored in or contained behind the shocks is shown to be consistent with the observati

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00625

年代:1982

数据来源: WILEY

摘要:

Enhanced fluxes of low‐energy electrons backstreaming from the earth's bow shock have been identified at ISEE 3. When present, these fluxes modify ambient solar wind electron velocity distributionsf(υ) in characteristic ways that depends on whether ISEE 3 is near the edge of, or within the interior of the earth's electron foreshock. Near the edge, energy peaks inf(υ) are observed. Such distributions should be locally unstable to electron plasma oscillations. Well within the interior of the foreshock, enhanced fluxes of electrons with energies up to the maximum detected by the Los Alamos electron analyzer (∼1 keV) are observed over the full backward hemisphere. These electrons can be modelled with an asymptotic power law distribution having index in the range 4 ≲pbs≲ 6. At intermediate energies (∼20–50 eV), twin angular peaks are observed centered on the magnetic field directionBˆ. Also observed at these times are depressions inf(υ) at energies less than ∼20 eV that are centered onBˆ. Such distributions having a perpendicular temperature greater than their parallel temperature may be locally unstable to the generation of whistler waves. Analysis of a particularly clean example of connection to the bow shock is consistent with the possibility that the observed electron fluxes emerge from the forward foot of the electron heating region within the bow shock where the electron density and temperature are larger than that of the unperturbed upstream solar wind by a factor of ∼1.2. This analysis also indicates that the electrostatic potential within the forward foot of the shock is between ∼5 and 50 V more positive than that within plasma far upstream at ISEE 3. However, these interpretations depend on the assumption of nearly scatter‐free propagation, which may not hold. Using parameters derived from model fits to measured velocity distributions, we estimate ∼10−4of the incident solar wind electrons are accelerated by the bow shock back into the upstream region with

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00632

年代:1982

数据来源: WILEY

摘要:

This report describes a new class of upstream wave with relatively high frequencies (∼1 Hz in the spacecraft frame of reference) and small amplitudes (1/2;γ peak to peak) compared with the more common larger amplitude (ΔB/B∼ 1), low‐frequency (0.03 Hz) upstream waves. These waves were first noted in association with beams of ions reflected back upstream at the bow shock, appearing in conjunction with some but not all of such beams. Further work confirms the association of the waves with the beams: the presence of a beam appears to be a necessary condition for the observation of the waves. Beam presence is, however, not a sufficient condition for existence of the waves and no clear correlation has been found between beam properties and the presence or absence of the waves. We have used characterizations of the beams provided by the ISEE 1 and 2 LEPEDEA plasma analyzers along with wave profiles obtained by the ISEE 1 and 2 magnetometers in probing these relationships. In addition, measurements made by the magnetometers were used to determine the intrinsic properties of these waves, using the simultaneous two point measurements to calculate and eliminate Doppler shifting effects. Results demonstrate that these are right‐hand elliptically polarized whistler mode waves with plasma rest frame frequencies of 20–100 times the proton gyrofrequency and wavelengths ∼100 km. The waves tend to propagate in the plane formed by the local IMF direction and the solar wind at angles ∼45° with respect to the magnetic field direction. Group velocities estimated from cold plasma theory for waves of these frequencies traveling in the observed directions are often not sufficient to allow the waves to reach the spacecraft at the times of observation. This argues against the possibility that the waves could have propagated upstream to the spacecraft position from the

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00643

年代:1982

数据来源: WILEY

摘要:

We calculate the current contributed by ions trapped in the vicinity of a magneticXline. TheXline is formed from the superposition of a dipole and a uniform, antiparallel magnetic field. The system (Dungey's open model for the magnetosphere) has an electric field with equipotential magnetic field lines and with an asymptotic behavior such that at a great distance the electric field is uniform in the dawn‐to‐dusk direction. In the magnetic neutral regionEis directed along theXline toward dusk. Ions incident within about one gyrolength from theXline are captured and accelerated along theXline to large kinetic energies by the electric field. Most ions escape from the neutral region by Speiser's ejection mechanism into spiral orbits along open magnetic field lines. The neutral region ion current is azimuthal and varies very nearly as cos ϕ on the dayside as predicted by Alfven, whereфis the GSM longitude, but is negligible on the nightside in this zero‐order calculation. Ion energization rates and escaping pitch angle distributions are estimated. The effect of the neutral region current is to extend the magnetic neutral line into a neutral sheet in the north‐south direction. This leads to a positive feedback mechanism whereby the greater vertical extent of the neutral region enhances ion capture, ensuring a still larger current. Thus a magnetopauselike structure will be rapidly formed for even very small incident plasma

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00651

年代:1982

数据来源: WILEY

摘要:

When thecE × B/B² drift velocity is large enough to exceed the particle gyro velocity, adiabatic theory becomes more complicated and less useful: more complicated because there are five drifts in addition to theE × Bgradient, and line curvature drifts and three more terms in the parallel equation of motion; less useful because the second and third invariantsJand Φ are no longer valid in mirror geometry due to the rapid drift across field lines that destroys any semblance of periodicity in the bounce motion. (Approximate periodicity is necessary to have an adiabatic invariant.) But the special case of a rapidly rotating rigid magnetic field, withE∥= 0, is an exception. If the field at any time is merely a rotation of that at an earlier time, the drift and parallel equations simplify, and even better, the second invariant is again valid. The drift equation now has two rather than five additional terms—a Coriolis drift and a centrifugal drift. Just as in a slowly or nonrotating mirror system, the second invariant now provides a (rapidly rotating) drift shell to which the guiding center is confined. The particle kinetic energy in the rotating frame, minus the centrifugal potential, is an exact constant of motion in a rigid rotator. This constant is well known but does not by itself put limits on radial motion toward or away from the rotation axis, hence it does not limit particle energy changes. But the drift shell does limit and make periodic any radial excursions, and this is why previous studies of particle motion, in particular examples of rigid rotators, have shown that particles experience no steady energy gain

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00661

年代:1982

数据来源: WILEY

摘要:

With an array of collimated cadmium telluride spectrometers on a spinning polar‐orbiting satellite (P78‐1) a study has been made of the energetic bremsstrahlung X ray (>21‐keV) patterns following substorms. From an altitude of ∼600 km, X rays emanating from a wide range of magnetic local times can be observed and the local time profiles mapped on a given pass of the satellite across either polar cap. Pronounced longitude variations in the X ray intensities and hence in the precipitating electron fluxes were observed in the daytime sector at various delay times after the onset of substorms near local midnight. The longitude profile features were found to persist over time periods of at least a few minutes. Longitude variations in the energy spectra of the bremsstrahlung X rays were also obtained, and in several cases the spectra were found to be harder at later local times after midnight, although no consistent trends were evident. Coordinated measurements of electrons from the SCATHA satellite at near‐synchronous orbit revealed enhancements in the fluxes of trapped electrons on the dayside at the times of the X ray events, indicating a correlation between the trapped and precipitating electron fluxes. Additional information on the longitude profiles of electron precipitation was obtained from riometer measurements in the Danish chain, which also provided complete time histories at the station longitudes. The longitude distributions of energetic electron precipitation as inferred from satellite X ray intensity and from riometer absorption measurements show very good agreement. The riometer absorption at the Narssarssuaq station with anLvalue of 7.3, comparable to the SCATHA values, was found to be very similar in time profile to the trapped electron fluxes measured at SCATHA, showing a close association between the trapped and precipitating fluxes of

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00671

年代:1982

数据来源: WILEY

摘要:

By using averaged data from ATS 6, ion energy and pitch angle distribution functions were examined for a magnetically quiet day (July 18, 1974). The data showed that for both field‐aligned and perpendicular fluxes, the populations had a mixture of characteristic energies. It was found that over three different energy bands in the range 3–600 eV the distribution functions could be fairly well approximated by Maxwellian distributions with temperatures in the ranges 3–10 eV, 30–50 eV, and approximately 70 eV in energy bands of 3–30 eV, 30–140 eV, and 140–600 eV, respectively. Pitch angle distributions were found to vary with the local time; strong field‐aligned particle fluxes were measured in the midnight and afternoon sectors, minor field‐aligned components persisted to some extent at all times, especially at low energies (E<10 eV). The pitch angle distributions and energy distributions were used to compute total ion density, assuming that the proton was the dominant ion species. In the morning sector (0300–0600 LT) a change of field‐aligned fluxes to pancake distributions peaking at pitch angles near 90° was seen. By using the assumption that the plasma was corotating with the satellite, we have examined pitch angle scattering mechanisms responsible for the observed transformation of pitch angle distribution. It was found that a magnetic noise of power spectral densityb²≲10−3γ²/Hz belonging to electromagnetic ion cyclotron mode (Lmode) near the ion cyclotron frequency could be very effective in trapping the field‐aligned f

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00681

年代:1982

数据来源: WILEY

摘要:

Thermal plasma parameters derived by the mutual impedance experiment on GEOS are described. The experiment is well suited to the measurement of the electron density and temperature of the outer plasmasphere (when kTe/Ne<1.6 eV/cm³). This investigation of the whole set of data supplied by GEOS 1 (4

ISSN:0148-0227    

DOI:10.1029/JA087iA02p00695

年代:1982

数据来源: WILEY