摘要:

In this paper we investigate plasma wave electric and magnetic fields in the vicinity of the magnetopause by using recent measurements from the ISEE 1 and 2 spacecraft. Strong electric and magnetic field turbulence is often observed at the magnetopause. The electric field spectrum of this turbulence typically extends over an extremely large frequency range, from less than a few hertz to above 100 kHz, and the magnetic field turbulence typically extends from a few hertz to about 1 kHz. The maximum intensities usually occur in the magnetopause current layer and plasma boundary layer. Somewhat similar turbulence spectra are also sometimes observed in association with flux transfer events and possible ‘inclusions’ of boundary layer plasma in the magnetosphere. In addition to the broad‐band electric and magnetic field turbulence, narrow‐band electrostatic emissions are occasionally observed near the electron plasma frequency in the vicinity of the magnetopause. Two possible plasma instabilities, the electrostatic ion‐cyclotron instability and the lower‐hybrid‐drift instability, are considered the primary candidates for explaining the broad‐band electric field turbulence. The narrow‐band electrostatic emissions near the local electron plasma frequency are believed to be either plasma oscillations or electrostatic waves near the upper‐hybrid

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07043

年代:1979

数据来源: WILEY

摘要:

The Millstone Hill radar, Westford, Massachusetts (Λ = 56°), has been upgraded for observations in support of the international magnetosphere study by means of the addition of a 150‐foot‐diameter fully steerable antenna. This antenna allows measurements ofFregion ion drifts, from whichFregion electric fields can be derived, using the existing UHF (68‐cm wavelength) radar. The paper describes attempts to measure the electric field in the auroral zone over the interval 60° ≤ Λ ≤ 75°. This effort entails making observations to the north of Millstone at very low elevation angles over periods of ≥24 hours. To secure estimates of both the E‐W and the N‐S components of the drifts, the measurements are made in two directions separated by ±15° from the magnetic meridian, and to achieve the desired coverage in latitude, measurements must be made at two elevation angles for each of these azimuths. The separation in time and space of the two components of the drift that are observed introduces a difficulty in interpreting the results. This has been overcome by matching the observations, in a least mean squares sense, by analytical functions describing the components of the electric field, each of which has 102 degrees of freedom. The results obtained thus represent the variation of the auroral electric fields over 24 hours local time smoothed with respect to both latitude and time. The utility of this approach is illustrated using sample results for one moderately quiet day, and the resolution and accuracy of th

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07059

年代:1979

数据来源: WILEY

摘要:

Positive ions in the energy range E = 3–50 eV have been measured on the dayside at geosynchronous orbit with the University of California at San Diego auroral particles experiment on ATS 6. It is found that the near‐equatorial plasma trough contains a warm plasma component with a relatively stable temperature κT≃11 eV and a low “isotropic component” of density (determined from fluxes near α=90°) typically in the range 0.1–0.4 ion/cc. The pitch angle distributions of these warm ions are typically field‐aligned, indicating an ionospheric origin, and have pitch angle half widths in the range 20°–40°. Pancake (flux peaks at α = 90°) and conical (sharp peaks at α = 10°–40°) distributions are also observed. Abrupt enhancements of the plasma density to 1–10 ions/cc, or more, are seen in the late afternoon and last from several minutes to a few hours. These regions of enhanced density are typically cooler than the ambient trough plasma, are often associated with pancake pitch angle distributions, and are believed to be plasma clouds convected to the spacecraft from the vicinity of the du

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07075

年代:1979

数据来源: WILEY

摘要:

The correlation of Pc 1 and IPDP pulsations with particle precipitation shows that the waves are left (L) polarized when observed simultaneously with precipitation. This is consistent with theoretical understanding of the generation of the pulsations as an L wave in the magnetosphere and the L wave propagation through the ionosphere with horizontal ducting of the R wave. It is suggested that L polarization of Pc 1 and IPDP locates the ULF source field line. Three events studied were IPDP correlated with riometer absorption. The observations strongly support the inward convecting source model for the generation of IPDP. On the other hand, the origin of the particles (presumably electrons) responsible for the riometer absorption is still an open question. Finally, a very long duration Pc 1 (HM emission) event is presented where L polarized wave packets were very closely correlated with auroral light bursts measured at 4278 Å wavelength

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07091

年代:1979

数据来源: WILEY

摘要:

As Voyager 1 moved out of the dawnside of the earth's magnetosphere on September 5, 1977 (at position (−2.6, −16.5, 1.1) earth radii in GSE), it crossed the magnetopause apparently seven times, despite the high spacecraft speed of 11 km/s. Normals to the magnetopause and their associated error cones were well determined for each of the crossings through the use of a minimum variance analysis of the internal magnetic field. The oscillating nature of the ecliptic plane component of these normals indicates that most of the multiple crossings were due to a wavelike surface disturbance moving tailward along the magnetopause. We modeled the wave, which was aperiodic, as a sequence of sine waves with amplitudeAi, wavelength λi, and speedVi. These quantities were determined for two pairs of intervals from the measured slopes, occurrence times, and relative positions of six magnetopause crossings. The average amplitude wasA=2100−500+3800km, and the wavelengths were of the order of 47,000−12,000+30,000km. The wave speed was approximately 340−95+210km/s, and typical periods were in the neighborhood of 170±60 s. The magnetopause thickness was estimated to lie in the range 300–700 km with higher values possible. The estimated amplitude of these waves was obviously small compared to their wavelengths; this conclusion is independent of any bulk normal motion of the magnetopause that might have

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07099

年代:1979

数据来源: WILEY

摘要:

Vertical bubble velocities in equatorial spreadFhave been investigated analytically byOtt[1978],Ossakow and Chaturvedi[1978], andHudson[1978], all of whom found a proportionality of the vertical velocity to bubble depletion density. If the depletion density increases with height for a convectively growing bubble formed at the bottom of theFlayer, then the vertical bubble velocity should increase with height also. In this paper we present VHF radar observations of equatorial spreadFthat support the latter conclusion.

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07107

年代:1979

数据来源: WILEY

摘要:

Observations of the aurora borealis were undertaken during the period August 10–20, 1976, from Fort Smith, Northwest Territories, Canada. Two auroral substorm ‘breakup’ events which occurred in the Alberta sector during this period were photographically documented and have been studied together with the associated polar magnetic substorm events. It is found that significant westward‐directed ionospheric current flow occurs for perhaps 2 or 3 minutes prior to the appearance of the auroral breakup surge form. This current appears to lie parallel to the bright arc which forms before onset of the breakup phase. Such an observation is important relative to the timing of the onsets both of auroral and polar magnetic substorms, and also may have critical implications relative to a theoretical understanding of the sequence of events leading to the auroral substorm

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07113

年代:1979

数据来源: WILEY

摘要:

Using data from the JHU/APL experiments on the Imp 7 and 8 spacecraft, we have examined several energetic (Ep≳ 0.3 MeV,Ee≳ 0.2 MeV) particle bursts associated with substorm events in the magnetotail. During these events the spacecraft were within ∼10REof the nominal neutral sheet (ZSM≈ 0) and within ∼20REof each other. Individual proton and electron bursts observed by both spacecraft do not always coincide nor does magnetotail activity correlate strongly with auroral activity on time scales less than ∼1 hour. This may be a consequence of the magnetic connection of the spacecraft to the region producing the energetic particles. Strong proton anisotropies are observed during most events. Several of the bursts exhibit rapid onsets and slow, near‐exponential decays with repetitive injections. Combining magnetic field and particle measurements on Imp 8, we can determine the pitch angle distributions with a time resolution of ∼10 s. During intense particle bursts the 0.3‐MeV protons exhibit unidirection or bidirectional anisotropies along the magnetic field. At other times, the protons exhibited anisotropies indicating motion from dawn to dusk across field lines. Both types of proton anisotropies persist for minutes, suggesting that the mechanism generating them must be active for at least as long. The energetic electrons tend not to be anisotropic, at least to the same degree as the protons. The data suggest the presence of small localized acceleration regions in the magnetotail which are observable when magnetically connected to the spacecraft. Little convincing evidence is found for a single neutral line extending across the width o

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07123

年代:1979

数据来源: WILEY

摘要:

High‐energy (E>0.3 MeV) proton increases within the magnetosphere both at synchronous orbit (6.6RE) and in the plasma sheet (∼18RE) have been studied. Measurements at 6.6REreveal that most (>80%) substorms have no associated injections of>0.3‐MeV protons (above the typical ambient level of 5 × 10³ (cm² s sr MeV)−1). Those relatively rare substorms which produce large fluxes of high‐energy particles show behavior ranging from drift echo to nondrift echo types of enhancements. The drift echo events are characterized by brief, well‐defined pulses of protons which are observed to drift azimuthally about the earth several times before dispersing. The nondrift echo events exhibit clear flux enhancements, but they do not show very evident pulsed behavior. The relative occurrence probability of high‐energy proton enhancements at 6.6REshows a strong positive correlation both with solar wind speed and withKp. A substantial correlation with southward interplanetary magnetic field (IMF) is also found. Very similarKp, solar wind, and IMF dependences are found for plasma sheet proton (Ep>0.5 MeV) enhancements above a threshold of ∼10 (cm² s sr MeV)−1as measured at 18RE. The close similarity of differential energy spectra,Kp, solar wind, and IMF dependences at synchronous orbit and in the plasma sheet suggests a common acceleration source. Bursts of energetic protons (Ep∼ 1 MeV) are also often observed simultaneously in data obtained in the interplanetary medium when proton enhancements are seen at 6.6RE. A model depicting the relationship between proton events in various magnetospheric regions is presented. It is suggested that large transient (<1 min) induced electric fields exist within the plasma sheet at the time of those substorm onsets that occur during periods of high solar wind speed and southward IMF. Such electric fields then produce both the proton pulses seen at synchronous orbit and the previously reported plasma sheet ‘impulsive’ tailward flowing proton bursts. We relate these initial proton populations directly to the substorm onset acceleration mechanism. The more frequently observed type of plasma sheet proton enhancements which is observed in the expanding plasma sheet is, in the present model, attributed to previously injected protons from the outer radiation zone filling the plasma sheet volume during the

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07138

年代:1979

数据来源: WILEY

摘要:

This paper presents the results of a statistical analysis of substorm‐associated plasma flow in the sector of the magnetotail within (−18 RE>XSM>−35 RE;|YSM| ≤ 12 RE; |dZSM| ≤2.5 RE). This ‘substorm analysis’ used data from the Los Alamos Scientific Laboratory plasma instruments on the Imp 6 and Imp 8 spacecraft and was keyed to substorms identified primarily in magnetograms from individual auroral zone stations. We find that a sudden establishment or enhancement of flow in a persisting direction is observed at the onset of about one half of occurring substorms. About 80 percent of these sudden changes produce plasma flow in the tailward direction and only 20 percent produce earthward flow. These results seem more consistent with a neutral line model of substorms (e.g., the tearing theory) that predicts tailward plasma flow tailward of a near‐earth ‘substorm neutral line’ than with a rarefaction wave model that does not envision neutral line formation and that predicts solely earthward plasma flow in the plasma sheet during substorms. We also repeated the ‘thinning analysis’ previously used by other authors, that is keyed to occurrence of plasma sheet thinning during periods when the AE index is enhanced, and for which there is no restriction on dZSM. This analysis did not reveal the preponderance of earthward flow during plasma sheet thinning that the other authors reported. Rather, it showed nearly equal occurrence of earthward and tailward flows, with some preference for tailward flo

ISSN:0148-0227    

DOI:10.1029/JA084iA12p07155

年代:1979

数据来源: WILEY