bluuebunny/crossref_metadata_2025 huggingface dataset:
Millstone Hill incoherent scatter observations of auroral convection over 60° ≤Λ≤75°, 1. Observing and data reduction procedures
Authors: Evans J. V. , Holt J. M. , Wand R. H.
Published: February 2008

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 of F region ion drifts, from which F region 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 the method are discussed.
Observations of warm plasma in the dayside plasma trough at geosynchronous orbit
Authors: Horwitz J. L. , Chappell C. R.
Published: February 2008

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 dusk plasmaspheric bulge region.
Further laboratory measurements of the beam‐plasma discharge
Authors: Bernstein W. , Leinbach H. , Kellogg P. J. , Monson S. J. , Hallinan T.
Published: February 2008

Further laboratory measurements of the beam‐plasma discharge (BPD) produced when an energetic electron beam traverses an initially neutral gas are presented. The experimental data indicate that the critical beam current Ic at energy V, magnetic field B, and system length L required for ignition obeys the empirical relationship Ic ∝ V3/2/B0.7 PL at pressures P < 2 × 10−5 torr. This relationship appears to primarily describe the accumulation of ambient plasma density, collisionally produced by the beam itself, to a critical value where ωpe² ≈ ωce² at which ignition occurs. Additional measurements of the narrow‐band cyclotron‐related waves (fce < f < 1.4fce) observed at beam currents below BPD threshold do not clearly establish whether this instability is a necessary precursor to the BPD. The relevance of these experiments to past and planned electron beam experiments in space is briefly discussed.
Morphological features of the winter anomaly in ionospheric absorption of radio waves at middle latitudes
Authors: Sato Teruo
Published: February 2008

The spatial extent and duration of the winter anomaly, i.e., anomalous and large enhancement in ionospheric absorption of HF and MF waves occurring on groups of winter days at middle latitudes, are examined on the basis of fmin data at 18 stations in USSR, Japan, and North America during the periods November–February in 1971–1972 and 1972–1973, considering that fmin enhancement represents an increased ionospheric absorption of HF and MF waves. The relationship between the phenomena of winter anomaly and geomagnetic storm is also examined. It is found that the winter anomaly occurs simultaneously in an extensive daytime area at middle latitudes, at least in some scattered region over the area, whose longitudinal extent reaches at most ∼100°. The southern boundary of the anomaly seems to be controlled geomagnetically and located below 30° in geomagnetic latitude. Also it is shown that the winter anomaly seems to be associated with geomagnetic storms, mainly sc (sudden commencement) storms, because outstanding enhancements in wave absorption begin usually 1–3 days after the beginning of an individual sc storm and last for more than a few days with decreasing magnitude. It is suggested that the winter anomaly is ascribed to ionization enhancements in the lower D region due to precipitating electrons from the outer atmosphere, mainly from the slot region, associated with geomagnetic storms.
Composition of the nighttime ionospheric F1 region near the magnetic equator
Authors: Anderson D.N. , Rusch D.W.
Published: February 2008

Satellite and rocket measurements of ion and electron density indicate that near the magnetic equator, in the nighttime (dusk) ionospheric F1 region, NO+ is often the dominant ion up to altitudes of 250 to 300 km. To study this region we theoretically calculate NO+, O2+, and O+ ion densities by solving the coupled, time‐dependent, nonlinear ion continuity equations numerically and include the effects of production, loss and transport by diffusion and
drift. Incorporation of a realistic vertical drift model which includes a prereversal enhancement in upward drift, produces NO+, O2+, and O+ density profiles which are consistent with the observations. The chemical reaction O2+ + N(4S) → NO+ + O is an important one and acts as a sink for O2+ ions and a source of NO+ as the F layer moves upward under the action of drift. Both the observations and calculations show that where NO+ is the dominant ion, the electron and ion densities are nearly constant in altitude, i.e., the plasma scale height becomes very large. The implications of this on the growth of large scale irregularities are briefly discussed.
Light ion concentrations and fluxes in the polar regions during magnetically quiet times
Authors: Hoffman J.H. , Dodson W.H.
Published: February 2008

Ion concentrations, flow velocities, and fluxes, measured by the ion mass spectrometer on the Isis 2 satellite for periods around the solstices and equinoxes of 1971 and 1972, have been used in a study of the polar wind morphology during magnetically quiet times. The results confirm the major tenets of polar wind theories; namely, that an upward flow of both H+ and He+ ions exists at all times over the entire polar region; H+ fluxes are of the order of 108 ions/cm2 s in both summer and winter polar regions. O+ is the dominant ion specie at 1400‐km altitude in regions of strong H+ flows, and H+ is depleted over the entire region where the polar wind flows. H+ fluxes tend to be lower in winter than in summer, whereas He+ fluxes are a factor of 10 higher in winter and tend to follow the neutral helium concentration near the F2 maximum. The latter agree well with model calculations of flux in the winter but lie above the model values by a factor of 2 in the summer. H+ fluxes also agree well with ion flow models.
Relationship between auroral electrons fluxes and field aligned electric potential difference
Authors: Fridman M. , Lemaire J.
Published: February 2008

A kinetic theory is used to model the field‐aligned distribution of auroral electrons along dipole magnetic field lines. Analytical formulae are obtained for the particle flux (J∥) and energy flux (ε) as functions of V, the electric potential difference between the equatorial source region and the ionosphere where the accelerated electrons are precipitated and detected by rocket instrumentation. For 1 ≪eV/Eo( ≪500, ε is nearly proportional to V², and J) is proportional to V (Eo) is the parallel thermal energy of the electrons; in the plasmasheet, Eo(=0.2–0.6 keV). The observed values for the field‐aligned potential drop (V) usually range within the limits defined by this double inequality (i.e., 0.6 kV ≪V ≪100 kV). The ohmic‐ like behavior of an auroral magnetic flux tube (J) ∝ V) and the constancy of ε/V² has been found experimentally by Lyons et al. (1979). Using an asymptotic expansion for the kinetic electron precipitation flux and energy flux we have obtained a useful formula which relates the Lyons‐Evans‐Lundin constant K(=εobs/Vobs²) to the auroral electron density and temperature in the source region (i.e., in the plasmasheet).
Ion heating by strong electrostatic ion cyclotron turbulence
Authors: Lysak R.L. , Hudson M.K. , Temerin M.
Published: February 2008

A theory of the ion heating due to electrostatic ion cyclotron (EIC) waves in the auroral zone is presented. Due to the slowly convecting nature of the EIC mode, quasi‐linear plateau formation cannot stabilize the waves, and growth occurs until the nonlinear mechanisms of ion resonance broadening and electron trapping provide saturation. The large amplitude and coherent nature of the resulting wave imply that quasi‐linear theory provides only a lower limit to the ion heating. An upper bound on the heating rate is derived using a time‐average model of ion dynamics in the coherent waves. The effects of ion heating in the presence of the magnetic gradient force and parallel electric fields are considered, with the result that perpendicular energies over 100 eV are easily attainable from a 1 eV source plasma. Perpendicular heating in the absence of a parallel electric field yields conical ion distributions, which in the presence of an electric field become field‐aligned beams.
Nonlinear standing Alfvén wave current system at Io: Theory
Authors: Neubauer F.M.
Published: February 2008

We present a nonlinear analytical model of the Alfvén current tubes continuing the currents through Io (or rather its ionosphere) generated by the unipolar inductor effect due to Io's motion relative to the magnetospheric plasma. We thereby extend the linear work by Drell et al. (1965) to the fully nonlinear, sub‐Alfvénic situation also including flow which is not perpendicular to the background magnetic field. The following principal results have been obtained: (1) The portion of the currents feeding Io is aligned with the Alfvén characteristics at an angle θA = tan−1 MA to the magnetic field for the special case of perpendicular flow where MA is the Alfvén Mach number. (2) The Alfvén tubes act like an external conductance ΣA = 1/(µ0VA(1 + MA² + 2MA sin θ)1/2) where VA is the Alfvén speed and θ the angular deviation from perpendicular flow towards the direction of Alfvén wave propagation. Hence the Jovian ionospheric conductivity is not necessary for current closure. (3) In addition, the Alfvén tubes may be reflected from either the torus boundary or the Jovian ionosphere. The efficiency of the resulting interaction with these boundaries varies with Io position. The interaction is particularly strong at extreme magnetic latitudes, thereby suggesting a mechanism for the Io control of decametric emissions. (4) The reflected Alfvén waves may heat both the torus plasma and the Jovian ionosphere as well as produce increased diffusion of high‐energy particles in the torus. (5) From the point of view of the electrodynamic interaction, Io is unique among the Jovian satellites for several reasons: these include its ionosphere arising from ionized volcanic gases, a high external Alfvénic conductance ΣA, and a high corotational voltage in addition to the interaction phenomenon with a boundary. (6) We find that Amalthea is probably strongly coupled to Jupiter's ionosphere while the outer Galilean satellites may occasionally experience super‐Alfvénic conditions.
Atomic nitrogen and oxygen ions in the daytime high‐latitude F region
Authors: Schunk R.W. , Raitt W.J.
Published: February 2008

We have extended our high‐latitude, ionospheric, dynamic model to include N+ in addition to the ions NO+, O2+, N2+, and O+. The ion He+ was also included but altitude profiles of this ion were obtained from our previous polar wind study. We have further improved our model by updating the various chemical reaction rates and by including the latest solar EUV fluxes measured by the Atmosphere Explorer satellites, the most recent MSIS model of the neutral atmosphere (N2, O2, O, and He) and the latest empirical model of atomic nitrogen. The improved model was used to study the solar cycle, seasonal, and geomagnetic activity variations of the daytime high‐latitude F layer. Both zonal and meridional convection electric fields were considered. Without allowance for electric fields, the peak O+ and N+ densities varied by an order of magnitude and the altitudes of the peaks varied by about 100 km over the range of geophysical conditions studied. Convection electric fields can also produce about an order of magnitude change in the O+ and N+ densities. These electric field induced changes could either assist or oppose the solar cycle, seasonal, and geomagnetic activity variations depending on the ionospheric conditions. In general, N+ was the second most abundant ion in the upper F region, but there were cases when He+ was more abundant than N+ even though He+ was in a state of outflow. Also, we speculate that at times, N+ can be the dominant ion in the upper F region.
First negative band system of nitrogen in the night sky over Arecibo during geomagnetic storms
Authors: Meriwether John W. , Walker James C.G.
Published: February 2008

Improvements in the airglow instrumentation at the Arecibo Observatory have permitted measurements of the brightness of the airglow emission from N2+ ions at 4278‐Å with sensitivity and time resolution much greater than those achieved when we first detected this emission in 1976. We report here the results of observations of this emission during magnetic storms in April and May of 1978. During this time surface brightnesses as high as 0.8 R were measured. The new measurements reveal fluctuations in brightness of the order of 20 per cent in times of about 30 minutes, which we tentatively attribute to fluctuations in the energetic particle flux that excites the emission. We also found a strong correlation between brightness and geomagnetic activity that was not apparent in our earlier, less sensitive measurements. The surface brightness at 4278‐Å increases at a rate of 0.4 to 0.5 R per 100 γ with increasing depression of Dst. Attempts to measure the rotational temperature of the emission were thwarted by contamination from molecular oxygen Herzberg bands, which we find to be significant at the very low levels of nitrogen emission which appear in the night sky at our latitude (18° north geographic, 30° geomagnetic).
Theoretical interpretation of drift wave type fluctuations at the plasmapause
Authors: Shukla P.K. , Bujarbarua S.
Published: February 2008

It is shown that an electrostatic pump near the upper hybrid resonance frequency can induce a new macroscopic parametric instability. The latter could be one of the candidates for the excitation of the low‐frequency drift wave type fluctuations at the plasmapause.
A theoretical study of the effects of quiet‐time electromagnetic drifts on the behavior of thermal plasma at mid‐latitudes
Authors: Murphy J.A. , Bailey G.J. , Moffett R.J.
Published: February 2008

Starting with a depleted protonosphere, the behavior of O+ and H+ at L=.2 is examined theoretically to determine the influence of a quiet time diurnal electromagnetic drift with a 24‐hour period. The conditions are appropriate to sunspot maximum. Large changes in the H+ tube content at L=3.2 occur as the protonosphere is replenished. In the absence of the electromagnetic drift, such changes would imply field‐aligned proton fluxes up to the order of 1010 cm−2 s−1; the actual field‐aligned proton flux is of the order of 108 cm−2 s−1. The combined effect of diurnal drifts and frequency of magnetic storms can produce multiple peaks in the diurnal behavior of the H+ tube content at L=3.2. As the drift lowers the altitude of the O+ layer during daytime, the values of NmF2 decrease owing to the change in the linear loss coefficient. At night, cross‐L gradients in NmF2 are significant for the conditions considered, and the outward drift increases NmF2 values at L=3.2. When the protonosphere is sufficiently replenished, the inward drift causes downward proton fluxes at L=3.2.
Multistation observations of SITECs over a full solar cycle
Authors: Buonsanto M.J. , Titheridge J.E.
Published: February 2008

Total electron content records from four southern hemisphere stations show 43 SITECs (sudden increases of the total electron content) in the period 1966–1977. Relating these to solar flare data shows that the probability that a given flare will produce a SITEC increases with the peak 10.7‐cm solar flux; with the value of cos χ, where χ is the solar zenith angle; and with decreasing central distance of the flare. The sizes of SITECs also increase significantly with the solar flux. SITECs are generally larger at lower latitudes in summer but show little latitudinal variation in winter. This pattern occurs at both solar maximum and solar minimum and is explained by seasonal changes in cos χ and in ionospheric loss rates. SITEC size versus latitude, however, is more variable during magnetically disturbed periods. Relative loss rates at different stations are obtained from the observed rise rates of corresponding SITECs. Results show that rapid ionospheric recombination (due to molecular ions) is more important at higher latitudes in summer and at lower latitudes in winter. This reflects primarily seasonal and latitudinal changes in the ratio N2/O, at heights above approximately 150 km. Similar variations are revealed in an analysis of five SITECs recorded at two northern hemisphere stations. SITEC rise rate measurements therefore provide a useful tool for studying differences in atmospheric composition at different sites.
Solar wind ion injections in the morning auroral oval
Authors: Carlson C.W. , Torbert R.B.
Published: February 2008

Energy versus time dispersion of precipitating ion bursts was observed during two rocket flights in the morning auroral oval. Individual bursts lasted about 20 seconds and repeated on a time scale of 100 seconds. The dispersion is interpreted as energy‐dependent time‐of‐flight delay which provides identification of ion charge‐to‐mass ratio as well as injection source distance. The data are consistent with injections of magnetosheath ions with injection distances ranging from 7 to 19 earth radii. Individual events extended over a region of at least several degrees of magnetic longitude and 0.3 degrees of invariant latitude. It is suggested that these events are associated with turbulent plasma entry into the magnetospheric entry layer.
Evidence of an ionospheric reflecting layer below the classical D region
Authors: Rasmussen J.E. , Kossey P.A. , Lewis E.A.
Published: February 2008

A series of direct and ionospherically reflected low‐frequency pulses, recorded over a typical 24‐hour period in December 1972, are presented for both normal and rotated polarizations. Examination of the pulse wave forms strongly suggests the simultaneous presence of two discrete but just barely resolved reflections from different heights in the daytime ionosphere. The upper reflection, whose height varied from about 72 to 78 km as the solar zenith angle changed, is identified with the classical D region, caused primarily by Lyman α radiation. The lower reflection was at a nearly constant altitude of about 63 km from shortly after sunrise to shortly before sunset. The ionization responsible for the low‐altitude reflection may be caused by cosmic rays and photodetachment, a mechanism previously proposed by other writers in connection with the hypothetical C layer of the lower daytime ionosphere. An ionization model which closely reproduces the observed reflected wave forms was developed using full wave theory and spectral analysis. The model is a 6‐km thick layer with a uniform conductivity of 1.8 × 10−7 mhos/m, corresponding to an estimated electron density of 60–160 el/cm³ in the 63‐ to 69‐km region of the daytime ionosphere.
Interhemispheric transport induced by neutral zonal winds in the F region
Authors: Heelis R.A. , Hanson W.B.
Published: February 2008

Reliable measurements of the interhemispheric ion transport velocity have shown a large longitudinal variability at equinox. The roles of zonal and meridional F region neutral winds have been evaluated in a simple way to account for the observed longitude dependence. The magnetic declination and relative displacement of the geographic and geomagnetic equators are both important considerations. It is shown that the zonal wind makes a strong contribution to the observed interhemispheric transport velocities near equinox.
Dayside cusp auroral morphology related to nightside magnetic activity
Authors: Sandholt P.‐E. , Henriksen K. , Deehr C.S. , Sivjee G.G. , Romick G. \J. , Egeland A.
Published: February 2008

Meridian‐scanning photometer (MSP) measurements of atmospheric optical emissions made in Longyearbyen, Svalbard, in the winter of 1978–1979 show two major types of daytime auroras associated with the magnetospheric cusp region. A steady, subvisual arc extending over 1.5° to 5° of invariant latitude characterized by a predominance of 6300‐Å emission is a permanent feature of the cusp region. Additionally, short‐lived, relatively narrow arcs and bands appear occasionally within this region; in these transient auroral forms the [O I] 5577‐Å line and the N2+ 1 NG bands are the strong optical emissions. Detailed correlation of the cusp optical emission and simultaneous geomagnetic measurements from both day and nightside stations show that the cusp, as defined by the 6300‐Å arcs, moves equatorward during magnetically disturbed conditions. This equatorward drift of the cusp region is directly proportional to the maximum negative excursion of the horizontal disturbance magnetic field in the meridian, which is approximately 180° displaced in longitude from the dayside cusp; (while the local magnetic disturbance on Svalbard is remarkably similar to nightside observations, it is approximately a factor of 5 less intense, making it difficult to correlate cusp position with local magnetic activity during relatively weak magnetic substorms). From three days of simultaneous data it may be seen that an increase in cusp auroral activity is characterized by the appearance of discrete auroral arcs and bands that occur in the prenoon and postnoon periods. The most significant result from these measurements is the absence of any detectable delay between the enhancement in cusp auroral activity and the commencement of magnetic disturbance at a station situated under maximum electrojet current on the nightside.
Rigidity dependence of cosmic ray scintillations in the 50‐ to 300‐GV range
Authors: Bergamasco L. , Osborne A.R. , Alessio S. , Cini G.
Published: February 2008

We present results of the spectral analysis of the fluctuations of cosmic rays recorded by different underground muon detectors (h=7–76 meters water equivalent). The experimental power spectra, compared with the predictions of diffusion theory in the nonlinear closure approximation, suggest the persistence of solar modulation up to rigidities of ∼300 GV and the existence of nonlinear interactions between particles and the interplanetary magnetic field.
Effect of a horizontal wind on 1.27‐µm auroral emission from O2 (′Δg) molecules
Authors: Rees M.H. , Roble R.G.
Published: February 2008

A horizontal wind blowing through a region where auroral bombardment produces metastable O2 (′Δg) molecules transports the excited species downstream. Absence of collisional deactivation and a long radiative lifetime result in 1.27‐µm radiation that may be observed several hundred kilometers downwind at locations entirely devoid of energetic particle bombardment and the usual prompt emissions characteristic of aurora. These circumstances can lead to large 1.27 µm/4278 Å emission rate ratios as well as large absolute emission rates of 1.27 µm, and thus help to account for some unusual observations reported in the literature.
A possible magnetic wake of Titan: Pioneer 11 observations
Authors: Jones D. E. , Tsurutani B. T. , Smith E. J. , Walker R. J. , Sonett C. P.
Published: February 2008

The characteristics of the magnetic signature measured by Pioneer 11 during the several hour interval around the crossing of Titan's L shell have been found to be consistent with plausible models of the interaction of a 200 km/s corotating magnetized plasma with a conducting or magnetized object. The observed manner in which the magnetic fluctuations varied throughout the interval, the average magnetosheath/ambient field amplitude ratio, and the occurrence of a field minimum at the time of closest approach to the axis of the extended Titan tail are all consistent with the detection of a magnetic wake roughly 145 RT downstream from Titan. In addition, values of the plasma mass density derived from the interaction geometry are consistent with an upper limit inferred from in situ plasma measurements obtained during the outbound leg of Pioneer's trajectory at Saturn. Although two magnetic events of similar duration occurred within 10 hours and on either side of the Titan related event, they coincided closely with probable passage of the spacecraft through tail and/or ring current systems. In addition, the character of the Titan interval appears to differ from the other two in a number of other important aspects.
Saturn's atmospheric temperature structure and heat budget
Authors: Orton Glenn S. , Ingersoll Andrew P.
Published: February 2008

The effective temperature of Saturn from 30°S to 10°N is 96.5±2.5 K. This value is 1.9 K higher than our preliminary estimate (Ingersoll et al., 1980). The atmospheric mole fraction of H2 relative to H2 + He is 90±3%. This value is derived by comparing infrared and radio occultation data (Kliore et al., this issue) for the same latitude. The high value of the effective temperature suggests that Saturn has an additional energy source besides cooling and contraction. The high mole fraction of H2 suggests that separation of heavier He toward the core may be supplying the additional energy. Atmospheric temperatures in the 60‐ to 600‐mbar range are 2.5 K lower within 7° of the equator than at higher latitudes. An almost isothermal layer exists between 60 and 160 mbar at all latitudes.
Temperatures and optical depths of Saturn's rings and a brightness temperature for Titan
Authors: Froidevaux Lucien , Ingersoll Andrew P.
Published: February 2008

The Pioneer Saturn infrared radiometer viewed Saturn's rings at 20‐ and 45‐µm wavelength under several conditions of illumination. The data are analyzed to infer radial locations of major ring boundaries, temperatures and temperature gradients, and normal optical depths. Error bounds on the above inferred quantities are given. Most ring boundaries are defined to ±0.01 Rs(1 Rs≡6 × 104 km) and are in good agreement with those inferred from the imaging photopolarimeter experiment. Temperatures generally decrease with radial distance from the planet. A significant temperature gradient exists from the colder north (unilluminated) side of the rings to the warmer south side. The gradient appears to be steepest on the south side. Ring optical depths are greater than some previously published values and are approximately 0.1 for the Cassini division and the C ring. In addition, the C ring optical depth decreases towards the planet. The temperature drop during eclipse is ≳10 K, implying low thermal inertia for the ring particles. Titan's 45‐µm brightness temperature is 75±5 K, in good agreement with earth‐based observations.
Properties of the longitudinal dielectric function: An application to the auroral plasma
Authors: Dusenbery P. B. , Kaufmann R. L.
Published: February 2008

The longitudinal dielectric function is used to study purely electrostatic plasma waves. The present paper is concerned with linear waves in a spatially homogeneous, collisionless, uniformly magnetized plasma. The plasma is described by a two temperature, streaming Maxwellian distribution. Sample plasma parameters are selected to model the auroral ionosphere at rocket altitudes, and wave numbers are selected to produce resonance with energetic auroral electrons. The longitudinal dielectric function is investigated in detail as the plasma density is varied to produce an interaction between upper hybrid and electron cyclotron waves. The strongest wave growth we have found to date in the auroral ionosphere is associated with this interaction.
Measurements of heavy ions in the low‐altitude regions of the outer zone
Authors: Blake J. B. , Fennell J. F. , Hovestadt D.
Published: February 2008

Measurements of heavy ions in the low‐altitude outer zone are discussed in this paper. The data were acquired with a heavy‐ion sensor aboard the S3‐2 satellite in the time period between December 24, 1975, and March 3, 1976. At L=3.25, 0.35 ≤B ≤0.25, and E >250 keV/nucleon, mirroring fluxes of 370 cm−2 s−1 sr−1 He ions and 0.25 cm−2 s−1 sr−1 CNO ions were observed. Thus the CNO/He ratio was ≈ 6.8 × 10−5. Equatorial measurements (Spjeldvik and Fritz, 1978c; Hovestadt et al., 1978) gave a much larger He/CNO ratio; clearly, magnetospheric processes strongly discriminate against populating the low‐altitude regions with ions of increasing mass in the energy range of hundreds of keV. In addition, a comparison of the S3‐2 data with those from Injun 5 acquired in January 1969 in the same region of the magnetosphere indicated that the low‐altitude CNO/He ratio is strongly time dependent.
The relative importance of the interplanetary electric field and magnetospheric substorms on partial ring current development
Authors: Clauer C. Robert , McPherron Robert L.
Published: February 2008

During periods of substorm activity, midlatitude geomagnetic data often reveal a depression in the northward component of the magnetic field in a limited region near the dusk meridian. This magnetic signature has usually been attributed to a partial ring current which forms as a result of the substorm activity. It is intriguing, however, that not all substorms are associated with the presence of a partial ring current. Results of a study which examines the relationship between substorm activity and the formation of the partial ring current will be presented. Using midlatitude magnetic data, the partial ring current may be parameterized by the maximum strength of the field depression which develops near the dusk meridian. This parameter has been related to time integral of the AE, AU and AL indices as well as to the dawn‐to‐dusk component of the interplanetary electric field. The results show little relation of the partial ring current parameter to the measures of substorm activity but a high correlation with the interplanetary electric field. An investigation of timing relationships is also reported. A comparison of the onset times of auroral zone activity and partial ring current growth shows that the partial ring current is as likely to begin to develop before the onset of auroral zone activity as after. A southward turning of the interplanetary magnetic field however, consistently precedes the establishment of a partial ring current. These results are interpreted to indicate that enhancement of the fractional solar wind emf applied across the magnetosphere plays a direct causative role in the development of the partial ring current.
The solar wind interaction with Venus: Pioneer Venus observations of bow shock location and structure
Authors: Slavin J. A. , Elphic R. C. , Russell C. T. , Scarf F. L. , Wolfe J. H. , Mihalov J. D. , Intriligator D. S. , Brace L. H. , Taylor H. A. , Daniell R. E.
Published: February 2008

Pioneer Venus observations are used in conducting a study of the location and structure of the Venus bow shock. The trace of the shock in the solar wind aberrated terminator plane is nearly circular at an altitude of 1.38RVindependent of interplanetary magnetic field orientation with an extrapolated subsolar height of 0.38RV. Gas dynamic relations and scaling of the terrestrial analogue are used to determine the effective impenetrable obstacle altitude from the mean shock surface with the conclusion that it lies beneath the observed height of the ionopause. The short‐term variability in shock position is similar to that found at the earth, while over the long‐term bow shock, altitude varies by up to ∼35% in phase with the solar cycle owing to causes other than changing solar wind Mach number. In contrast to ionopause position, which is shown to be well determined by external pressure measurements, bow shock altitude is found to be only weakly dependent upon ionopause height and solar wind dynamic pressure. These results are interpreted in terms of interactions with exospheric neutrals and/or lack of complete deflection of the incident solar wind by currents induced in the ionosphere modifying the flow about Venus from that associated with a tangential discontinuity obstacle of nearly constant radius. The downstream bow shock is smaller in diameter than that of terrestrial case despite the larger Mach cone angle at 0.72 AU most probably due to the smaller relative size of the Venus magnetotail. A brief survey of shock structure with Pioneer Venus instrumentation shows general agreement as to the time and location of the shock crossings with a transition layer thickness of the order of the ion inertial length scale. The observed variation in bow shock structure and the foreshock with upstream parameters was similar to that seen at the earth.
The onset and development of Kelvin‐Helmholtz instability at the Venus ionopause
Authors: Wolff R. S. , Goldstein B. E. , Yeates C. M.
Published: February 2008

We investigate the Kelvin‐Helmholtz instability at the Venus ionopause resulting from the flow of the (shocked) solar wind tangential to the ionopause for the case where the interplanetary field is oriented normal to the direction of flow. It is found that gravity stabilizes the long wavelength perturbations, and the finite thickness of the boundary layer stabilizes short wavelength modes. The magnetic ‘gyroviscosity’ due to finite Larmor radius effects either destabilizes the boundary or stabilizes it according to whether the solar wind electric field points away from or toward the ionosphere. For solar wind and ionosphere plasma parameters consistent with Pioneer Venus observations we find that the instabilities with the greatest growth rates (shortest growth times) have wavelengths of ∼50–150 km and growth times of ∼0.5 to several seconds. In addition, we show how distortion of the ionopause by Kelvin‐Helmholtz instability might lead to the formation of magnetic ‘flux ropes’ inside the ionosphere as well as ionospheric ‘bubbles’ embedded in the solar wind.
Empirical models of the electron temperature and density in the Venus ionosphere
Authors: Theis R. F. , Brace L. H. , Mayr H. G.
Published: February 2008

Empirical models of the electron temperature and density of the Venus ionosphere have been derived from Pioneer Venus measurements acquired between December 10, 1978, and August 9, 1979. This time interval allowed periapsis to scan a full range of solar zenith angles (SZA) and local times. The models describe the average ionosphere conditions in the vicinity of periapsis (18°N) and from 155‐km altitude to an upper boundary altitude, which varies with solar zenith angle. The upper boundary is determined by the average ionopause height, which rises steadily from approximately 340 km at 0° SZA to almost 1100 km at 110° SZA. The electron density in the Venus ionosphere is characterized by nearly uniform values across the dayside followed by a sharp decrease in the vicinity of the terminator (90° SZA). Considerable amounts of ionization persist throughout the nightside ionosphere, a fact which requires that the ionization either be transported to the nightside from the dayside or produced locally by particle precipitation. The electron temperature is characterized by very high temperatures on the dayside followed by a gradual decrease beyond the terminator with increasing solar zenith angle, although significantly elevated temperatures exist throughout the nightside. A great deal of large and small scale structure, evident in both the Ne and Te measurements, is not reflected in the model, although the distribution of these irregularities is readily revealed by plotting the ratio of the individual measurements to the model values at each point. These plots show that the irregularities in Te are greatest at low altitudes and in the nightside ionosphere.
Morphology of the Venus ultraviolet night airglow
Authors: Stewart A. I. F. , Gérard J.‐C. , Rusch D. W. , Bougher S. W.
Published: February 2008

Images of the nightside of Venus in the (0,1)δ band of nitric oxide have been obtained by the Pioneer Venus orbiter ultraviolet spectrometer (OUVS). The emission, which is produced by radiative association of N and O, shows a bright spot reaching ∼5 kR and located at 2 a.m. local solar time just south of the equator. The emitting layer is at 111±7‐km altitude. A one dimensional vertical transport model shows that the hemispheric average brightness of 0.8 kR is consistent with the orbiter neutral mass spectrometer (ONMS) measurements of N and O near 167 km, and that the altitude of the emitting layer is consistent with the eddy mixing model proposed to explain the dayside helium profile measured by the bus neutral mass spectrometer. In the model, N reaches a peak of 7 × 108 cm−3 at 114 km, and O reaches a peak of 2.6 × 1011 cm−3 at 106 km. There is a fair degree of consistency between the ONMS, OUVS, and other airglow measurements, except as regards the local time dependence.
Composition of the Venus lower atmosphere from the Pioneer Venus Mass Spectrometer
Authors: Hoffman J. H. , Hodges R. R. , Donahue T. M. , McElroy M. B.
Published: February 2008

Data from the Pioneer Venus sounder probe neutral mass spectrometer confirm that the major constituents of the lower atmosphere of Venus are CO2 and N2, with the latter having a mass abundance of about 3%. The data allow the order of up to 1000 ppm of water vapor and 500 ppm of SO2 and COS below the clouds, but lower bounds for the abundances of these compounds cannot be established because they appeared in large amounts as lingering products of the reaction of cloud droplets deposited on the gas inlet leak to the mass spectrometer. The presence of several minor constituents on Venus is more certain. Of particular significance is the discovery that there is a hundred‐fold excess of nonradiogenic argon and neon on Venus over terrestrial abundances in addition to a small deficit of radiogenic 40Ar. The isotopic ratios of carbon, oxygen, and nonradiogenic argon are similar to those on earth. Other minor constituents detected on Venus include helium and ethane, and there is strong, but presently inconclusive, evidence for H2S. Through laboratory simulation it has been shown that the measured pattern of volatile release during the evaporation of cloud materials blocking the inlet leak correspond to the volatiles produced when a leak is coated with an 85% concentration solution of H2SO4 and immersed in a Venus atmosphere simulator.
Structure and circulation of the Venus atmosphere
Authors: Schubert G. , Covey C. , Genio A. Del , Elson L. S. , Keating G. , Seiff A. , Young R. E. , Apt J. , Counselman C. C. , Kliore A. J. , Limaye S. S. , Revercomb H. E. , Sromovsky L. A. , Suomi V. E. , Taylor F. , Woo R. , von Zahn U.
Published: February 2008

Pioneer Venus has revealed important new features of the structure and the circulation of Venus' atmosphere. The temperature decreases from nearly 750 K at the surface to about 180 K at about 100 km. Above 100 km, there is a marked contrast between the day‐side and the night‐side thermal structures. On the day side there is a thermosphere in which temperatures increase with height to an exospheric temperature of about 300 K. On the night side there is a ‘cryosphere’ in which temperatures decrease with height to an exospheric temperature of about 100 K. The atmosphere is stably stratified from the highest altitudes down to about 28 km except for a layer in the clouds, between about 50 and 55 km, which is nearly adiabatic. Between about 20 and 28 km, the lapse rate is also nearly adiabatic while there is evidence for stable stratification between about 10 and 20 km. Horizontal thermal contrasts are of the order of 1–2% in the deep atmosphere and 100% in the upper atmosphere. At and below the clouds, temperatures generally decrease with latitude on constant pressure surfaces; above the clouds, between about 70 and 90 km, there is a reversed zonally averaged latitudinal temperature gradient. The dominant circulation of the atmosphere above the lowest one or two scale heights is a zonal retrograde motion with 100 m/s winds at 60 km altitude. There is also a superrotation of the atmosphere at altitudes of 150 km and above. Low latitude height profiles of the zonal wind have alternating layers of high and low shear which correlate with structure in the vertical profiles of static stability. Advection of heat by the large zonal winds helps maintain the relatively small longitudinal thermal contrasts throughout the atmosphere below the clouds. Latitudinal temperature and pressure contrasts are consistent with a zonally rotating atmosphere in approximate cyclostrophic balance. Meridional winds below 60 km vary in speed from a few to about 10 m/s; the winds are poleward at the cloud tops. A cloud level Hadley cell driven by solar heating combines with the zonal circulation to produce a cloud top polar vortex. Eddies in the form of convective cells, small‐scale gravity waves, and planetary scale waves are found throughout the atmosphere. Eddies, as well as mean meridional circulations, may be important in the transport of energy and momentum. Venus' atmospheric circulation is not steady despite the planet's small obliquity and nearly circular orbit.
Cloud and haze properties from Pioneer Venus polarimetry
Authors: Kawabata K. , Coffeen D. L. , Hansen J. E. , Lane W. A. , Sato Makoto , Travis L. D.
Published: February 2008

Analysis of linear polarization data obtained by the Pioneer Venus Orbiter Cloud Photopolarimeter experiment indicates that the visible clouds at low and mid‐latitudes are composed predominantly of 1 µm radius H2SO4 droplets, an identification made previously by using earth‐based observations. Mixed within and extending above this main visible cloud is an extensive haze of submicron‐sized particles. These haze particles have a refractive index of 1.45±0.04 at λ ≃550 nm, an effective radius of 0.23±0.04 µm, and a size distribution with an effective variance of 0.18±0.1. The polarization of the bright regions poleward of about 55° latitude is produced almost entirely by this submicron haze. The submicron haze has been found to exhibit large spatial and temporal variations. In January 1979 the haze vertical optical thickness in the polar region at λ=365 nm was τh ∼0.8 above the main cloud of l‐µm particles. By comparison, the optical thickness of the haze above the main cloud at low latitudes was typically 1 order of magnitude smaller, τh ∼0.06, however haze mixed within the cloud contributed ∼18% of the total scattering cross section per unit volume at λ=365 nm. More recent observations indicate that there are major changes of the haze on time scales of hundreds of days (e.g., the optical thickness of the polar cap haze is smaller by a factor of 2–3 in October 1979 than in January 1979). Substantial diurnal variations exist at low latitudes, with a greater amount of haze near the morning terminator than near the noon meridian. The global distribution of haze will be monitored during the extended Pioneer Venus mission to permit analysis of long time scale variations as well as correlation with characteristics of the atmospheric dynamics deduced from ultraviolet images of the cloud tops.
The thermal balance of venus in light of the Pioneer Venus Mission
Authors: Tomasko M. G. , Smith Peter H. , Suomi V. E. , Sromovsky L. A. , Revercomb H. E. , Taylor F. W. , Martonchik D. J. , Seiff A. , Boese R. , Pollack J. B. , Ingersoll A. P. , Schubert G. , Covey C. C.
Published: February 2008

Instruments flown on the Pioneer Venus orbiter and probes measured many of the properties of the atmosphere of Venus which control its thermal balance and support its high surface temperature. Estimates based on orbiter measurements place the effective radiating temperature of Venus at 228±5 K, corresponding to an emission of 153±13 W/m², and the bolometric Bond albedo at 0.80±0.02, corresponding to a solar energy absorption of 132±13 W/m². Uncertainties in these preliminary values are too large to interpret the flux difference as a true energy imbalance. A mode of submicron particles is suggested as an important source of thermal opacity near the cloud tops to explain the orbiter and probe thermal flux measurements. Comparison of the measured solar flux profile with thermal fluxes computed from the measured temperature structure and composition shows that the greenhouse mechanism explains essentially all of the 500 K difference between the surface and radiating temperatures of Venus. Precise comparison of the observed and computed value of this difference is hindered by uncertainties in the local variability of H2O and in the thermal opacity of CO2 and H2O at high temperature and pressure. The directly measured thermal flux profiles at the small probe sites are surprisingly large and variable in the lower atmosphere. Observed zonal and meridional circulation are qualitatively as required to produce the observed uniformity of temperature structure. However, the present lack of quantitative estimates of the horizontal and vertical dynamical heat transports implied by these measurements is a significant gap in the understanding of the thermal balance of the atmosphere of Venus.
Greenhouse models of Venus' High surface temperature, as constrained by Pioneer Venus measurements
Authors: Pollack James B. , Toon Owen B. , Boese Robert
Published: February 2008

Recent measurements conducted from the Pioneer Venus probes and orbiter have provided a significantly improved definition of the solar net flux profile, the gaseous composition, temperature structure, and cloud properties of Venus' lower atmosphere. Using these data, we have carried out a series of one‐dimensional radiative‐convective equilibrium calculations to determine the viability of the greenhouse model of Venus' high surface temperature and to assess the chief contributors to the greenhouse effect. New sources of infrared opacity include the permitted transitions of SO2, CO, and HCl as well as opacity due to several pressure‐induced transitions of CO2. We find that the observed surface temperature and lapse rate structure of the lower atmosphere can be reproduced quite closely with a greenhouse model that contains the water vapor abundance reported by the Venera spectrophotometer experiment. Thus the greenhouse effect can account for essentially all of Venus' high surface temperature. The prime sources of infrared opacity are, in order of importance, CO2, H2O, cloud particles, and SO2, with CO and HCl playing very minor roles.