摘要:

Occultation by a local elevation on the surface of constant magnetic field is proposed as a new interpretation for the unusual properties of Neptune high‐latitude emission. Abrupt changes in intensity and polarization of this broadband smooth radio emission were observed as the Voyager 2 spacecraft passed near the north magnetic pole before closest approach. The observed sequence of cutoffs with polarization reversal would not occur during descent of the spacecraft through regular surfaces of increasing magnetic field. The sequence can be understood in terms of constant‐frequency (constant‐field) surfaces that are not only offset from the planet center but are locally highly distorted by an elevation that occults the outgoing extraordinary‐mode beam. The required occulter is similar to the field enhancement observed directly by the magnetometer team when Voyager reached lower altitude farther to the west. We present evidence that the sources of the high‐latitude emission are located near the longitude of the minimum‐Banomaly associated with the dipole offset and that the local elevation of constant‐Bsurfaces extends eastward from the longitude where it is directly measured by the magnetometer to the longitude where occultation of the remote radio source is observed. Together, the radio and magnetometer experiments indicate that the constant‐frequency surfaces are distorted by an elevation that extends 0.3 rad in the longitudinal direction. On the 462 kHz surface, about 1.1RNfrom the dipole center, the local elevation must be at least 0.45RNabove the undisturbed (dipole

ISSN:0148-0227    

DOI:10.1029/91JA02390

年代:1992

数据来源: WILEY

摘要:

Preliminary theoretical and computational analyses of the Combined Release and Radiation Effects Satellite (CRRES) magnetospheric barium releases are presented. The focus of the studies is on the evolution of the diamagnetic cavity which is formed by the barium ions as they expand outward, and on the structuring of the density and magnetic field during the expansion phase of the releases. Two sets of simulation studies are discussed. The first set is based upon a two‐dimensional ideal MHD code and provides estimates of the time and length scales associated with the formation and collapse of the diamagnetic cavity. The second set uses a nonideal MHD code; specifically, the Hall term is included. This additional term is critical to the dynamics of sub‐Alfvénic plasma expansions, such as the CRRES barium releases, because it leads to instability of the expanding plasma. We performed detailed simulations of the G4 and G10 releases. In both cases the expanding plasma rapidly structured: the G4 release structured at timet≲ 3 s and developed scale sizes ∼ 1‐2 km, while the G10 release structured at timet≲ 22 s and developed scale sizes ∼ 10‐15 km. We also find that the diamagnetic cavity size is reduced from those obtained from the ideal MHD results because of the structure. On the other hand, the structuring allows the formation of plasma blobs which appear to free stream across the magnetic field; thus, the barium plasma can propagate to larger distances transverse to the magnetic field than the case where no structuring occurs. Finally, we also discovered a new normal mode of the system which may be excited at the leading edge of the expanding barium plasma. This mode is a magnetic drift wave which propagates azimuthally around the barium cloud in the frequency

ISSN:0148-0227    

DOI:10.1029/91JA02144

年代:1992

数据来源: WILEY

摘要:

Vector magnetic fields at geosynchronous orbit were measured during 1980‐1984 using the operational GOES 2, GOES 5, and GOES 6 spacecraft magnetometers. We corrected these spacecraft measurements for offsets due to spacecraft state and then used these field estimates to create a data base with 1‐min resolution. Hourly quiet field values were calculated for these years from this data base using the ground‐based geomagnetic index criteriaAE<120 nT and |Dst|<20 nT. These quiet field components, rotated into dipoleHVDcoordinates, were approximated by the first two coefficients of a two‐dimensional Fourier series in time of day and season. These Fourier harmonics provide a compact method of approximating the quiet synchronous field at any time of day and any season. The quiet geosynchronous field components, to first order, are given by mean values of about 90 nT, −60 nT, and 5 nT; and sinusoidal diurnal amplitudes of about 21 nT, 5 nT, and 5 nT, respectively, forH,V, andDwhere the spacecraft magnetometer was located near the geomagnetic meridian. The second harmonic diurnal amplitudes and the first and second harmonic seasonal amplitudes are typically of the order of a few nanoteslas or less except for theDcomponent, which exhibits a larger seasonal variation. Furthermore, a one‐dimensional Fourier series in time of day was used to study the quiet field dependence on solar wind dynamic pressure,Pd, by indexing the measurements into five pressure ranges during 1980. TheHcomponent of the quiet field increased 4.6 nT from 80.2 to 84.8 nT in its mean amplitude and 20.8 nT from 11.9 to 32.7 nT in its first harmonic amplitude forPdincreasing from 0.71 × 10−8to 3.31 × 10−8dyn/cm². These quietHmeasurements, including the pressure dependence, are compared with a first‐order field model (Mead, 1964) superimposed with a tail current, resulting in magnetospheric currents (magnetopause and tail) in agreement with previous model values. The measured field pressure dependence and the Mead model suggest a tail current de

ISSN:0148-0227    

DOI:10.1029/91JA02135

年代:1992

数据来源: WILEY

摘要:

Recently, it has been reported that small‐scale (λ ∼ 0.1‐2 km) density irregularities occur during 100‐Hz electric field bursts in the nightside ionosphere of Venus. The correlation of field and plasma fluctuations suggests that a local plasma instability may be responsible for the turbulence. In this paper we provide a detailed analysis of the lower‐hybrid‐drift instability as a mechanism to generate the observed irregularities. We develop a fully electromagnetic theory that is relevant to the finite β plasma in Venus’ ionosphere and include collisional effects (e.g., electron‐ion, electron‐neutral, and ion‐neutral collisions). The key features of the analysis that favor this instability are (1) it is a flute mode and propagates orthogonal to the ambient magnetic field, (2) it is a relatively short wavelength mode and the Doppler‐shifted frequency can be ≳ 100 Hz, (3) it can produce both electric field and density fluctuations, as well as magnetic field fluctuations in a finite β plasma, and (4) it is most unstable in low‐β plasmas (i.e., β ≲ 1) so that it is likely to occur in the low‐density, high‐magnetic‐field ionospheric holes. These features are

ISSN:0148-0227    

DOI:10.1029/91JA02404

年代:1992

数据来源: WILEY

摘要:

The Nimbus 7 satellite has measured the solar total irradiance from November 1978 to July 1991 (153 months). These measurements are important both in solar physics and for climate change. To insure that the Nimbus 7 measurements are capturing the true behavior of the Sun, it is essential that the properties of the radiometer and its changes over time be understood. The calibration of the radiometer can be viewed as a process of removing instrumental influences from the raw measurements, leaving the experimenter with an estimate of solar variability. In this paper the changing radiometer pointing, the zero offsets, the stability of the gain, the temperature sensitivity, and the influences of other platform instruments are all examined and their effects on the measurements considered. Only the question of relative accuracy (not absolute) is examined. The resulting derived solar irradiances are compared to previous analyses of the Nimbus 7 radiometer and to the Solar Maximum Mission (SMM) measurements. Compared to previous analyses, the newly derived values are higher and somewhat less variable than the older values. Removal of the off‐axis pointing errors and a new treatment of the zero offsets are the major reasons for the changes. Compared to the SMM measurements, both agree quite well so long as any solar activity is present. When the Sun becomes quiet, so its irradiance variability is less than the Nimbus 7 radiometer resolution, the comparison to the SMM results breaks down. Between 1980 and 1988 the correlation of the daily values is 0.83, compared to 0.62 using previously published values from both satellites. The monthly means have a correlation of 0.90, indicating that about 80% of the longer‐term variance is in common. In 1980 when both satellites were operating without problems, Nimbus 7 was 0.3155% higher on average. For May 1984 to December 1988, Nimbus 7 was 0.3255% higher. Therefore if the Nimbus 7 satellite continues to operate properly until a period of about 1 year after the launch of UARS with its active cavity radiometer irradiance monitor (ACRIM), it appears that the differences between Nimbus 7, SMM ACRIM, and UARS ACRIM can be measured to within a few hundredths of a percent. A self‐consistent set of solar irradiance measurements from several satellites over nearly two solar cycles appears fea

ISSN:0148-0227    

DOI:10.1029/91JA02488

年代:1992

数据来源: WILEY

摘要:

Simultaneous ground‐based observations of narrowband and broadband VLF radio waves and of cloud‐to‐ground lightning were made at widely spaced locations during the 1987 Wave‐Induced Particle Precipitation (WIPP) campaign, conducted from Wallops Island, Virginia. Based on these observations, the first case study has been made of the relationships among located cloud‐to‐ground (CG) lightning flashes, whistlers, and associated ionospheric effects during a substorm particle injection event. This event took place 2 days after the strongest geomagnetic storm of 1987, during a reintensification in geomagnetic activity (Kp= 5) that did not affect the high rate of whistlers observed at Faraday Station, Antarctica (L= 2.46). At the time of the injection event, several intense nighttime thunderstorms were located over Long Island and the coast of New England, between 400 km northwest and 600 km north of the region geomagnetically conjugate to Faraday. About two thirds of the CG flashes that were detected in these thunderstorms during the hour following the injection event onset were found to be causatively associated with whistlers received at Faraday. During the same period the amplitude of the 24.0‐kHz signal from the NAA transmitter in Cutler, Maine, propagating over the thunderstorm centers toward Wallops Island was repeatedly perturbed in a manner characteristic of previously reported VLF signatures of transient and localized ionization enhancements atDregion altitudes. Though such enhancements may have been caused by whistler‐induced burst electron precipitation from the magnetosphere, the data in this case are insufficient to establish a clear connection between the NAA amplitude perturbations and the Faraday Station whistlers. In view of the proximity of the NAA great circle path to the storm center, heating of the lower ionosphere by intense radiation from lightning may also have played a role in the observed VLF perturbations. The onset of each of the NAA signal perturbation events coincided with an intense cluster of radio atmospherics. Detailed temporal variations in the ELF (0.3–3 kHz) and VLF (3–30 kHz) power of similar “sferic clusters” correlated well with variations in the power of simultaneous “anomalous” optical events (AOEs) observed by a down‐looking photodiode detector on a rocket at alt

ISSN:0148-0227    

DOI:10.1029/91JA02583

年代:1992

数据来源: WILEY

摘要:

We solve a linear numerical model of Alfvén waves reflecting from the high‐latitude ionosphere, both to better understand the role of the ionosphere in the magnetosphere/ionosphere coupling process and to compare model results with in situ measurements. We use the model to compute the frequency‐dependent amplitude and phase relations between the meridional electric and the zonal magnetic fields due to Alfvén waves. These relations are compared with measurements taken by an auroral sounding rocket flown in the morningside oval and by the HILAT satellite traversing the oval at local noon. The sounding rocket's trajectory was mostly parallel to the auroral oval, and it measured enhanced fluctuating field energy in regions of electron precipitation. The rocket‐measured phase data are in excellent agreement with the Alfvén wave model, and the relation between the modeled and the measured amplitudes is fair, leading us to conclude that the rocket‐measured fields are dominated by interfering Alfvén waves, forming a standing wave pattern. The field amplitudes measured by HILAT are related by the height‐integrated Pedersen conductivity Σp, indicating that the measured field fluctuations were due mainly to structured field‐aligned current systems. A reason for the relative lack of Alfvén wave energy in the HILAT measurements could be the fact that the satellite traveled mostly perpendicular to the oval and therefore quickly traversed narrow regions of electron precipitation and associated wave activity. Alternatively, the lower velocity and eastward flight path of the rocket lead to Doppler‐shifted frequencies ofLshell aligned current structures which are much smaller than HILAT would measure and which in fact may be below our range of interest (0.1–1 Hz), leaving Alfvén waves as the dominant source of fluctuating field energy in the roc

ISSN:0148-0227    

DOI:10.1029/91JA02300

年代:1992

数据来源: WILEY

摘要:

Modern models of thermospheric composition and temperature and of excitation and radiative transfer processes are used to simulate the O I 130‐nm emission from Mars measured by the Mariner 9 ultraviolet spectrometer. We use the Mars thermospheric general circulation model calculations (MTGCM) of Bougher et al. (1988a) and the Monte Carlo partial frequency redistribution multiple scattering code of Meier and Lee (1982). We find that the decline in atomic oxygen through the daylight hours predicted by the MTGCM cannot be reconciled with the excess afternoon brightness seen in the data. Oxygen concentrations inferred from the data show a positive gradient through the day, in agreement with the original analysis by Strickland et al. (1973), although the absolute amounts are somewhat less because we use a larger photoelectron impact excitation and a somewhat larger solar flux in the 130‐nm triplet. In addition, the data suggest that the oxygen abundance increases toward high southerly latitudes, in contrast with the MTGCM prediction of high values in the northern (winter) hemisphere. It appears that solar forcing alone cannot account for the observed characteristics of the Martian thermosphere and that wave and tidal effects may profoundly affect the structure, winds, and composit

ISSN:0148-0227    

DOI:10.1029/91JA02489

年代:1992

数据来源: WILEY

摘要:

The ionospheric responses to the solar eclipse of September 23, 1987, in the equatorial anomaly crest region have been investigated by using ionospheric vertical sounding, VLF propagation delay time, and differential Doppler shift data observed at Chungli, which is located near the northern equatorial anomaly crest region. It has been found that temporal variations of theF1layer andDregion are mainly controlled by local solar radiation. Quantitative analysis of the variations of theF1layer critical frequency, ƒ0F1, shows that electrons are removed from theF1layer through ionic recombination. However, the temporal variations of ƒ>0F2and electron density above 200 km show that the variations of theF2layer around the equatorial anomaly region are controlled not by local solar radiation but by solar radiation at the equator. The fountain effect plays an important role even during the solar eclipse. The VLF propagation time delay is controlled by the variations of average path obscuration. Atmospheric gravity waves produced by the moving bow wave front of the solar eclipse are found with a period around 17‐23 min and wavelength about 293

ISSN:0148-0227    

DOI:10.1029/91JA02409

年代:1992

数据来源: WILEY

摘要:

Experimental investigations of large‐scale modifications created by a high‐power HF beam (effective radiated power of ∼100 MW, frequency of 3–5 MHz) in the nighttime ionosphere above Arecibo Observatory are presented. The modifications consist of field‐aligned temperature enhancements (δTe/Te0∼ 2–4) and density depletions (|δne|/ne0∼ 25–50%) and have length scales along the geomagnetic field of tens of kilometers. A two‐stage time evolution of the modifications is documented; they start from a broad and symmetric perturbation and develop into a narrow, northward shifted universal steady state on a time scale of 15–30 min. It is found that nonlinear refraction, i.e., the self‐consistent bending of theOmode HF beam across the geomagnetic field lines and the reorientation of the HF reflection surface to a geometry parallel to the geomagnetic field, is the key process involved in the generation of the large modifications. Preconditioning results showing the immediate onset of the narrow late‐stage behavior provide dramatic confirmation of the underlying nonlinear mechanism sustaining the large modifications. Highlights of quantitative comparisons of these observations with a transport model including nonlinear refraction have been given in a previous public

ISSN:0148-0227    

DOI:10.1029/91JA02403

年代:1992

数据来源: WILEY