摘要:

One hundred four measurements of the peak electron density in the dayside ionosphere of Venus (solar zenith angle (SZA)<80°), along with 11 Venera 9–10 measurements, were analyzed in order to establish the response of the dayside peak to the varying solar EUV flux over the maximum to minimum phase of solar cycle 21 (December 1979 to December 1986). The relationship of the peak electron density normalized to SZA = 0° and the EUV flux index was found to beNm=N150(FEUV/150)0.376, which is in agreement with theoretical predictions based on Chapman theory modified for photochemical processes. The corresponding dependence on solar zenith angle at any given value ofFEUVwas found to beNm(SZA) =N0cos0.511(SZA). The altitude of the peak was found to be independent of either SZA orFEUV. When the CO2+production ratePCO2+was computed using the Venus International Reference Atmosphere (VIRA) upper atmosphere model and the EUV flux computed from Hinteregger's (1981) formulae, it was found that the theoretically expected value of 0.5 for Δ log (Nm/Te0.275)/Δ log (PCO2+) was not obtained. Instead, when the assumption was used thatTe=Tnat the altitude of the peak, and the solar cycle corrections given in the VIRA model were applied, a log‐log slope of 0.454±0.010 was obtained, and a value of 0.485±0.009 resulted when the VTS3 Venus thermosphere model (Hedin et al., 1983) was used. This suggested that the assumptionTe=Tnis not satisfactory. When the electron temperatureTe*was extrapolated from the electron temperature probe model (Theis et al., 1980) toh= 140 km and a solar cycle variation of was assumed, the theoretically expected value of 0.500±0.010 was obtained for Δ log (Nm/Te0.275)/Δ log (PCO2+) forK= 0.30 when the VIRA model was used andK= 0.35 when the VTS3 model was employed. This result implies that the electron temperature at the altitude of the ionosphere main peak on Venus decreases by only about 25% from solar maximum to minimum, in contrast to a decrease of about 50‐75% at altitudes o

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13339

年代:1989

数据来源: WILEY

摘要:

The gasdynamic convected magnetic field model for predicting solar wind flow past a planetary magnetoionopause obstacle has been extended to three dimensions to apply to obstacles of nonaxisymmetric shape. The need for such an extension is of first‐order importance for Jupiter and Saturn because the effects of rapid spin, large size, and substantial ring current phenomena are believed to cause the magnetospheres of these planets to be significantly broader near the planetary equatorial plane than near the noon‐midnight polar meridian plane. Direct observational determination of such asymmetries for Jupiter and Saturn has not been possible, however, because only a limited amount of data are available from four spacecraft at Jupiter and three at Saturn, all of which followed flyby trajectories at fairly low latitudes. With the aid of the new three‐dimensional model, it is now possible to infer the degree of flattening of the magnetospheric cross sections from a knowledge of the locations of the low‐latitude magnetopause and bow shock crossings. In this paper, the computational procedures of the new model are described, and calculated results are presented for a number of magnetospheres of elliptic cross sections with values ranging from 1 to 2 for the ratio a/b of major (equatorial) to minor (polar) axes. This range is sufficient to include values appropriate to both Jupiter and Saturn. Comparisons of the model results with the locations of observed crossings of the magnetopause and bow shock directly provide an estimate of the degree of equatorial broadening of the magnetospheric cross section for each of these planets. For Jupiter, the results indicate a broadening to a/b∼1.75, a value consistent with previous estimates determined from independent calculations of the three‐dimensional magnetosphere shape formed by adding to the planetary dipole field the magnetic field of an equatorial current sheet selected so as to match the observed Jovian magnetic field near the equatorial plane. For Saturn, a similar comparison indicates a smaller broadening to a/b∼1.25. The determination is less certain than for Jupiter, however, because of the smaller amount and greater scatter of the a

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13353

年代:1989

数据来源: WILEY

摘要:

Electrostatic emissions measured by the Voyager 2 plasma wave detector (PWS) during the inbound crossing of the Uranian bow shock are shown to differ in some aspects from the waves measured during bow shock crossings at Jupiter and Saturn. The wave amplitudes in the foot of the bow shock at Uranus are in general much lower than those detected at the other outer planets due to the unusually enhanced solar wind ion temperature during the crossing. This reduces the effectiveness of wave‐particle interactions in heating the incoming electrons. Strong wave emissions are observed in the shock ramp that possibly arise from currents producing a Buneman mode instability. Plasma instrument (PLS) and magnetometer (MAG) measurements reveal a complicated shock structure reminiscent of computer simulations of high‐Mach number shocks when the effects of anomalous resistivity are reduced, and are consistent with high ion temperatures restricting the growth of electrostatic wa

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13367

年代:1989

数据来源: WILEY

摘要:

Solar wind thermal electrons are normally nearly isotropic, with a temperature anisotropy,T∥/T⊥, of 1.0 to 1.2. This study presents ISEE 3 observations of anomalous electron distributions for whichT⊥exceedsT∥in the solar wind near 1 AU. Twelve anomaly events were identified, lasting from 24 min to 6 hours. These events generally share the following characteristics: (1) high plasma density, (2) low solar wind speed, (3) magnetic field which is nearly transverse to the flow, and (4) low electron and ion temperatures. The processes of solar wind adiabatic expansion and isotropization via Coulomb collisions could be expected to lead to such anomalous anisotropies under conditions similar to those observed. However, these conditions actually produceT⊥>T∥for only a small fraction of the time, suggesting that other mechanisms are also important in regulating solar wind electron di

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13377

年代:1989

数据来源: WILEY

摘要:

Intense magnetosonic waves, originally propagating at the right angle with the interplanetary magnetic field, can excite a purely growing mode along the interplanetary magnetic field together with two symmetric magnetosonic sidebands propagating obliquely across the magnetic field. This instability process leads to the filamentation of the magnetosonic pump waves. These two excited magnetosonic sideband modes propagate together perpendicularly across the magnetic field and, meanwhile, form a standing wave pattern along the magnetic field. The thresholds of this filamentation instability can be exceeded in the solar wind environment. It is predicted that the density fluctuations produced by the filamentation instability along the interplanetary magnetic field have wavelengths greater than, at least, a few Earth radii. The polarization of the obliquely propagating magnetosonic waves excited by the filamentation instability is determined by the characteristics of the magnetosonic pump waves and the environmental plasmas.

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13387

年代:1989

数据来源: WILEY

摘要:

Electrostatic wave measurements from the Active Magnetospheric Particle Tracer Explorer Ion Release Module have been used to investigate the wave modes and their possible generation mechanisms in the Earth's bow shock and magnetosheath. It is demonstrated that electrostatic waves are present in the bow shock and magnetosheath with frequencies above the maximum frequency for Doppler‐shifted ion acoustic waves, yet below the plasma frequency. Waves in this frequency range are tentatively identified as electron beam mode waves. Data from 45 bow shock crossings are then used to investigate possible correlations between the electrostatic wave properties and the near‐shock plasma parameters. The most significant relationships found are anticorrelations with Alfven Mach number and electron beta. Mechanisms which might produce electron beams in the shock and magnetosheath are discussed in terms of the correlation study results. These mechanisms include acceleration by the cross‐shock electric field and by lower hybrid frequency waves. A magnetosheath “time of flight” mechanism, in analogy to the electron foreshock region, is introduced as another possible beam generation

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13397

年代:1989

数据来源: WILEY

摘要:

We analyze 127 one‐hour average samples of central plasma sheet ions and electrons in order to determine spectral characteristics of these magnetotail particle populations during periods of low geomagnetic activity (AE12RE. We find that, for even the longest periods sampled, the nearly isotropic central plasma sheet total ion and electron populations were measured to be continuous particle distributions from our lowest energy of tens of eV/e to a few hundred keV. The kappa distribution function (f∼ [1 +E/κEo]−κ−1, whereEo, the energy of the peak differential number flux (measured in particles/cm² s sr keV), is related to the temperature through κ, a constant) most often reproduces the observed differential energy spectra. Spectra dominated by a single kappa functional form are observed during 83 (99) hours for ions (electrons). Spectra which are not dominated by a single kappa functional form can usually be closely approximated by superposed kappa functional forms. For both ions and electrons κ is typically in the range 4–8, with a most probable value between 5 and 6, so that the spectral shape is distinctly non‐Maxwellian.EoiandEoeare highly correlated, whereas κiand κeare not correlated; κiis roughly proportional toEoi½, whereas κeis not correlated withEoe. We statistically investigate the importance of flux and energy contributions from extramagnetospheric sources by separately analyzing intervals when simultaneously measured interplanetary particle fluxes are either enhanced or at low levels. A linear superposition of plasma sheet fluxes and interplanetary fluxes that have entered the magnetosphere is observed. The presence of interplanetary particles does not affect the average values of plasma sheetEoor κ. We conclude that forAE<100 nT the nonthermal shape of plasma sheet particle distributions results from ongoing magnetospheric processes which are probably independent of geomagnetic act

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13409

年代:1989

数据来源: WILEY

摘要:

The simultaneous occurrence of locally resonant toroidal pulsations over a range ofLshells complicates the interpretation of observations of these waves made from elliptically orbiting spacecraft. A theoretical model is developed which shows that measurements of ULF wave frequencies from spacecraft may be distorted due to the combined effects of spacecraft motion and spatial gradients in wave frequency, phase velocity, and the oscillation onset time. Spectral broadening and frequency shifts due to sampling biases caused by spacecraft acceleration and nonlinear variations in wave frequency with position are also considered. For toroidal field line resonances observed from AMPTE/CCE, distortions due to spatial gradients in wave frequency are the most important. For commonly observed standing Alfvén waves, the measured frequency is lower than the true frequency on the outbound leg of the orbit while it is anomalously high on the inbound pass. The predicted frequency error for the AMPTE/CCE orbit ranges from ≈15% forL= 5 to ≈4% forL= 8. Comparison of fundamental mode frequency measurements made on 10 inbound and 9 outbound passes during low to moderateKp(2− to 4+) in the morning sector (0500 to 0900 MLT) confirms the predictions of the model. The average effective ringing time is estimated to be 1000±300 s. The agreement of this result with theoretical calculations of damping rates due to ionospheric Joule dissipation (Newton et al., 1978) indicates that phase mixing between adjacent toroidal oscillations is not the dominant damping mechanism. The variation of pulsation frequency withLis also used to examine the dependence of plasma mass density onr. The form ρ ∝r−qis found to fit the data reasonably well in the outer magnetosphere withqranging from 3.0 to 5.5 for individual passes andq= 4.0 determined from the combination of all

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13425

年代:1989

数据来源: WILEY

摘要:

Plasma flow data from the AE‐C, AE‐D and DE 2 satellites have been used to systematically study the distribution of the convection potential around the polar cap boundary under a variety of different interplanetary magnetic field (IMF) conditions. For either a “garden hose” (BxBy0) orientation of the IMF, the potential distribution is mainly affected by the sign ofBy. In the northern hemisphere, the zero potential line (which separates the dusk convection cell from the dawn cell) on the dayside shifts duskward asBychanges from positive to negative. But in the southern hemisphere, a dawnward shift has been found, although the uncertainties are large. The typical range of displacement is about ±1.5 hours MLT. Note that this shift is in the opposite direction from most simple schematic models of ionospheric flow; this reflects the fact that our “polar cap” boundary is typically more poleward than the flow reversal associated with the region 1 current system, which shifts in the opposite direction. Thus the enhanced flow region typically crosses noon. In most cases a sine wave is an adequate representation of the distribution of potential around the boundary. However, in a few cases the data favors (at the 80% confidence level) a steeper gradient near noon, more indicative of a “throat.” The potential drop at the duskside boundary is almost always greater than at the dawnside boundary. The dawn‐dusk potential offset is about 20% of the total polar cap potential drop forBZ≥1 nT, 10% forBZ∼0, and 1% forBZ≤−4 nT. A slight duskward shift of the pattern is observed as the IMF changes from “garden hose” to “ortho‐garden hose” conditions. Analytic equipotential contours, given our potential function as a boundary condition, are constructed for several IMF conditions. We find they agree reasonably well with the observed flow data, despite our simplifying assumptions of uniform conductivity and

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13447

年代:1989

数据来源: WILEY

摘要:

In this decade, a new technique for the study of ionospheric electrodynamics has been implemented in an evolving generation of high‐latitude HF radars. Coherent backscatter from electron density irregularities at F region altitudes is utilized to observe convective plasma motion. The electronic beam forming and scanning capabilities of the radars afford an excellent combination of spatial (∼50 km) and temporal (∼1 min) resolution of the large‐scale (∼106km²) convection pattern. In this paper, we outline the methods developed to synthesize the HF radar data into two‐dimensional maps of convection velocity. Although any single radar can directly measure only the line‐of‐sight, or radial, component of the plasma motion, the convection pattern is sometimes so uniform and stable that scanning in azimuth serves to determine the transverse component as well. Under more variable conditions, data from a second radar are necessary to unambiguously resolve velocity vectors. In either case, a limited region of vector solution can be expanded into contiguous areas of single‐radar radial velocity data by noting that the convection must everywhere be divergence‐free, i.e., ▽ ·v= 0. It is thus often possible to map velocity vectors without extensive second‐radar coverage. We present several examples of two‐dimensional velocity maps. These show instances ofLshell‐aligned flow in the dusk sector, the reversal of convection near magnetic midnight, and counterstreaming in the dayside cleft. We include a study of merged coherent and incoherent radar data that illustrates the applicability of these methods to ot

ISSN:0148-0227    

DOI:10.1029/JA094iA10p13463

年代:1989

数据来源: WILEY