摘要:

X rays in the energy band 0.2–3.0 keV have been detected coming from both polar regions of Jupiter. The observations were made in 1979 and 1981 by using the imaging proportional counter and high resolution imaging detectors on the Einstein X ray astronomy satellite. The measured flux density of ∼6×10−4cm−2s−1at earth corresponds to an X ray luminosity of ∼4×109W in the 0.2‐ to 3.0‐keV energy band. The energy spectrum of the X rays is extremely soft and can be characterized by a power law with an exponent of ∼2.3. Detector energy resolution is insufficient to distinguish a soft line spectrum from a continuum. However, the shape of the response and the observed X ray power indicate that the source of this auroral emission is not electron bremsstrahlung as on the earth, but is most probably line emission from O and S ions with energies between 0.03 and 4.0 MeV/nucleon precipitating from the outer boundary of the Io pl

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07731

年代:1983

数据来源: WILEY

摘要:

We have analyzed energetic protons in the energy range 30 to 500 keV and energetic electrons ≥75 keV obtained with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE‐1 during a plasma sheet crossing on March 26, 1978. The behavior of protons with energies of more than ∼100 keV is very different from that of the ∼30 to ∼100 keV protons which represent the suprathermal tail of the plasma sheet distribution. The more energetic ions appear on a time scale of several minutes following a northward turning of the tail magnetic field. At about the same time the plasma measurements show a velocity of ∼200 km/s in the tailward direction. This velocity enhancement is first seen at ISEE‐1 and later on at ISEE‐2, which is earthward of ISEE‐1. The temporal sequence of the energetic particle, magnetic field, and plasma observations and the proton and electron anisotropies are discussed in terms of acceleration near a magnetic neutral line which occurs well within the plasma sheet. In this framework the magnetic neutral line would move earthward, followed by a magnetic island. The extent of the neutral line would be limited to the dusk side of the tail. No disruption of the plasma sheet is observed as compared to large‐

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07742

年代:1983

数据来源: WILEY

摘要:

On the basis of three‐dimensional measurements of energetic particles and plasma from the ISEE 1 spacecraft, the occurrence of ion beams at the plasma sheet boundary in the magnetotail previously reported by Williams (1981) is examined in relation to substorm activity. In all seven cases studied it is found that ion beams streaming earthward and/or tailward are always present at the edge of the plasma sheet adjacent to the tail lobe. Ion beams penetrating into the plasma sheet region with no detectable density gradient are also observed. The observed ion beams may occur at high energies of tens of keV, or at medium energies of a few keV, or even at energies as low as tens of electron volts. Their intensities and energies do not have any obvious relationship to the substorm onset and expansion phases or the magnitude of the substorm disturbance as indicated by theAEindex. However, ion beams tend to be more intense and occur at higher energies when the plasma sheet is energized during the substorm recovery phase. The result suggests that the activation of the source for the streaming ions is not directly associated with any particular phase of a substor

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07753

年代:1983

数据来源: WILEY

摘要:

A statistical study of large electric field events, measured on ISEE 1 at geocentric distances of 3 to 23RE, has been performed in order to determine the occurrence frequency and spatial distribution of such events. Large electric fields were seen at all altitudes, with the lowest altitude events occurring on auroral field lines and the higher altitude events occurring on the high latitude boundary of the plasma sheet. The largest events had amplitudes that were approximately proportional to the square root of the local magnetic field strength, in agreement with expectations of a simple mapping model. The apparent concentration of big field events near local midnight is caused, at least in part, by a nonuniform frequency of satellite crossings through the plasma sheet boundary as a function of local time. Thus the spatial distribution and amplitudes of the large field events are not inconsistent with the mapping of low‐altitude electrostatic shocks to the plasma sheet boundary along equipotential magnetic field line

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07765

年代:1983

数据来源: WILEY

摘要:

Observations ofFregion electron density, electron and ion temperatures and ion drift velocity over the latitude interval 40°<Λ<70° have been made from Millstone Hill by means of incoherent scatter radar. We conclude that theFregion density trough that forms on the nightside of the earth may have more than one cause. Current computer models indicate that it can be produced as a consequence of the competing effects of convection and corotation that cause some flux tubes to remain for long periods in darkness in the premidnight sector, thereby allowing the ionisation in them to decay. Our observations show that troughs often are first seen in the late afternoon sector in regions of fast sunward convective flow and can move in latitude in association with substorms. We suggest that these troughs are produced by convection of low density regions from the nightside into daylight and the increased recombination rate that occurs in regions where the O+ions are forced through the neutral air at high speed (∼1 km/s). Troughs created in the dusk sector by this mechanism during substorms can be left as the ionospheric signature of the substorm or “fossil” when the region of fast convective flow moves poleward following the substorm. These extended E‐W troughs then are carried across the nightside of the earth by corotation. Relative to the surrounding regions, the trough changes little with time and finally is destroyed only when it is carried into sunlight on th

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07769

年代:1983

数据来源: WILEY

摘要:

Model predictions of the diurnal variations of plasma convection velocities and electron densities in the high‐latitude ionosphere were compared with Millstone Hill incoherent scatter radar observations for an equinox day on which there was moderate magnetic activity. On the observation day, three major morphological features were evident at 500 km, including a dayside high density region, a nightside mid‐latitude trough, and a region of slightly enhanced densities in the auroral zone. Although the dayside high density region was due to solar EUV radiation, it was not symmetrical about local noon (1000–1900 LT sector) owing to the effect of horizontal transport. The nightside mid‐latitude trough was the deepest, the widest, and reached its most equatorward latitude in the morning sector. The model was able to reproduce these two features quite accurately. In the dusk sector, the trough was filled in, and its latitudinal extent was restricted by a discrete auroral arc, a feature not included in the ionospheric model. Except for this arc region, the enhanced electron densities in the auroral zone were adequately described by the average precipitation fluxes used in the model. The observed plasma drift velocities were consistent with a two‐cell, asymmetric convection pattern with enhanced flow in the dusk sector. Outside the polar cap, the fall‐off of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. The convection pattern employed in the model included these features and had a 60 kV cross‐polar‐cap potential. Efforts to reproduce the observed behavior by using a larger cross‐polar‐cap potential (90 kV) or a symmetric pattern are also presented. These were generally less successful and demonstrate the sensitivity of the morphology of theFregion at high latitudes to the

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07783

年代:1983

数据来源: WILEY

摘要:

We have examined in detail the synchrotron emission by electrons of energy greater than a few TeV in the earth's magnetic field. The photon spectrum lies in the X ray and γ ray region. As the emission takes place in a narrow cone along the direction of the electron, the photons would be incident nearly along a straight line on a detector. This unique feature provides the signature to identify the electron unambiguously. The mean energy of the photons being proportional to the square of the electron energy allows us to determine the energy accurately. Though it may appear that one needs to know the arrival direction of electrons to obtain its energy, we have shown that an omnidirectional detector can be satisfactorily used to estimate the energy. We also show that the collecting power of the detector is a sensitive function of the area of the detectorA, the energy of electronE0, and the number of photons required to identify an electronnγ; asymptotically the collecting power is proportional toA1.43E0nγ−1.8. An instrument, with an energy threshold for the detection of photons, can be used to measure reliably the integral flux of electrons, even if it has limited energy resolution. We have calculated the event rate expected by using an ideal balloon‐borne detector capable of detecting above 20 keV at 4 g cm−2of atmospheric depth over Palestine, Texas, and compared with the expected rates using instruments based on currently available techniques of d

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07811

年代:1983

数据来源: WILEY

摘要:

Observations of the daily 1‐ to 8‐Å solar X ray emission from 1977 to 1981 are analyzed from a terrestrial viewpoint. A high‐resolution method of anharmonic frequency analysis is used to search for discrete frequencies in a finite time series and estimate the frequency, phase, and amplitude of each periodic component. The time series data are the broadband, full disk X ray measurements, primarily from the GOES 2 satellite, and are reduced from 3‐s data to a daily nonflare index representing background coronal emission. Intermediate‐term epochs are weakly stationary time intervals about 128 days long and correspond to major intervals of solar activity over about four and a half rotations. Time series of background X ray flux longer than about 128 days are nonstationary. Results suggest that near the boundary of an epoch the amplitude spectrum undergoes an episodic change in frequency, phase, and amplitude, after which a new spectrum appears that persists for the duration of the next epoch. Furthermore, those frequencies most closely corresponding to solar rotation are not normally distributed about a 27–28 day mean but occur at about 22,25,

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07823

年代:1983

数据来源: WILEY

摘要:

High time resolution (0.75 s) measurements obtained by the Pioneer 11 vector helium magnetometer inside Saturn's magnetosphere show quasi‐periodic waves to be present near the DioneLshell betweenL= 6.3 and 6.7. Although Dione was far from the spacecraft, the waves were observed when Pioneer was both inbound to, and outbound from, periapsis and are presumably associated indirectly with Dione. The waves have a characteristic period of 18 s and a typical amplitude of 5 nT. In the region in which the waves were observed, the Pioneer 11 plasma analyzer detected a peak plasma density associated with heavy ions presumably sputtered from Dione's surface and tentatively identified as O++. Subsequent Voyager observations in this inner torus appear to be consistent with 0+. The characteristic period of the waves is well below the proton gyroperiod. Because the heavy ions are hot (∼106K), the Alfvén phase speed and the ion thermal speeds are nearly the same. Ion cyclotron resonance of the waves with the dominant ions appears capable of generating the waves. Theoretical arguments based on the growth rates of the waves suggest that O++is more likely to be responsible than O+but that a resonance involving H+ions with energies of a few keV cannot be excluded. The existence of a pitch angle anisotropy associated with a loss cone is implied. For O++, a pitch angle anisotropy of ≃0.70, corresponding toT⊥/T∥≃ 1.7, is predicted. Pitch angle scattering of the heavy ions by the waves should cause precipitation with the possible production of an aurora near

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07831

年代:1983

数据来源: WILEY

摘要:

HAO white‐light K‐coronameter observations show that the inclination of the heliospheric current sheet at the base of the corona can be both large (nearly vertical with respect to the solar equator) or small during Carrington rotations 1660–1666 and even on a single solar rotation. We discuss Voyager 1 and 2 magnetic field observations of crossings of the heliospheric current sheet at distances from the sun of 1.4 and 2.8 AU. Two cases are considered: one in which the corresponding coronameter data indicate a nearly vertical (north–south) current sheet and another in which a nearly horizontal near‐equatorial current sheet is indicated. For the crossings of the vertical current sheet a variance analysis based on hour averages of the magnetic field data gave a minimum variance direction consistent with a steep inclination. The horizontal current sheet was observed by Voyager as a region of mixed polarity and low speeds lasting several days, consistent with multiple crossings of a horizontal but irregular and fluctuating current sheet at 1.4 AU. However, variance analysis of individual current sheet crossings in this interval, using 1.92‐s averages, did not give minimum variance directions consistent with a horizontal current sheet. We conclude that one cannot assume that the minimum variance direction will be the same as the normal to the current sheet when the analysis results are likely to be influenced by small‐scale variations or curvatures within or near the sheet proper. This influence may be more pronounced when the sheet is locally near

ISSN:0148-0227    

DOI:10.1029/JA088iA10p07837

年代:1983

数据来源: WILEY