摘要:

We have developed a comprehensive model to study the dynamics and energetics of the ionosphere of Titan. We solved the one‐dimensional, time‐dependent, coupled continuity and momentum equations for several ion species, together with single ion and electron energy equations, in order to calculate density, velocity, and temperature profiles. Calculations were carried out for several cases corresponding to different local times and configurations of the Titan‐Saturn system. In our model the effects of horizontal magnetic fields were assumed to be negligible, except for their effect on reducing the electron and ion thermal conductivities and inhibiting vertical transport in the subram region. The ionospheric density peak was found to be at an altitude of about 1100 km, in accordance with earlier model calculations. The ionosphere is chemically controlled below an altitude of about 1500 km. Above this level, ion densities differ significantly from their chemical equilibrium values due to strong upward ion velocities. Heat is deposited in a narrow region around the ionospheric peak, resulting in temperature profiles increasing sharply and reaching nearly constant values of 800‐1000°K for electrons and 300°K for ions in the topside, assuming conditions appropriate for the wake region. In the subram region magnetic correction factors make the electron heat conductivities negligible, resulting in electron temperatures increasing strongly with altitude and reaching values in the order of 5000°K at our upper boundary located at 2200 km. Ion chemical heating is found to play an important role in shaping the ion energy balance in Titan's

ISSN:0148-0227    

DOI:10.1029/93JA02286

年代:1994

数据来源: WILEY

摘要:

Meridian scanning photometer measurements taken in the magnetic postdawn sector at Longyearbyen, Svalbard, between 0300 and 0630 UT on December 29, 1981, are analyzed in conjunction with particle and field data retrieved during two near passes of the Dynamics Explorer 2 (DE 2) satellite. The interval included a sudden storm commencement (SSC) at 0455 UT. Pre‐SSC optical and particle measurements showed a system of arcs that are spaced at ∼1.1° intervals in magnetic latitude, embedded within the region 1 current system and span the convection reversal. The softer particle precipitation appears to have a source near the flanks of the magnetotail while the harder, more equatorward precipitation originates closer to Earth. During the SSC period the entire sky brightened, with enhanced 630.0‐nm emissions extending from the northern horizon to south of magnetic zenith; intense but spatially separated 557.7‐nm emissions dominated the southern horizon. DE 2 detected more than an order of magnitude increase and near isotropization of ring current electron fluxes, enhanced precipitation from the plasma sheet and significantly decreased auroral zone convection. Region 1/region 2 currents remained, with wavelike structures superposed. A dual timescale response to the SSC is consistent with ground and satellite measurements. On few minute travel timescales for hydromagnetic waves to pass through the system, magnetospheric particles accelerate and precipitate to increase the ionospheric conductivity. Global, field‐aligned currents change more slowly. To maintain similar field‐aligned currents with higher ionospheric conductances requires reduced electric fields. After 0520 UT the optical emissions settled into stable, but latitudinally separated, bands of 630.0‐ and 557.7‐nm emissions characteristic of cleft and plasma sheet precipitati

ISSN:0148-0227    

DOI:10.1029/92JA03027

年代:1994

数据来源: WILEY

摘要:

Broadband noise enhancements in the frequency range of ∼1.4‐4.8 MHz have been observed with a ground‐based receiver located at Two Rivers, Alaska (near Fairbanks). During the 5‐month period from November 1991 to March 1992, eight broadband noise enhancements were recorded. A correlation is observed between the radio noise enhancements and magnetic activity recorded with the magnetometer in College, Alaska (45 km away). Initial examination of college ionosonde data also suggests that sporadic E is associated with the termination of some of the events. The enhancements are characterized by a low‐frequency cutoff at ∼1.4 MHz and a decrease in intensity near ∼2.8 MHz, which roughly correspond to 1 and 2 times the ionospheric electron cyclotron frequency, respectively. Although some discrete man‐made signals are observed to be enhanced during the events, the bulk of the broadband enhancements may be composed of natural signals. If natural, the observed spectral shape is consistent with calculations of synchrotron radiation combined with cyclotron absorption; in this case, these signals would be diagnostic of the hardness of the auroral electron

ISSN:0148-0227    

DOI:10.1029/93JA01806

年代:1994

数据来源: WILEY

摘要:

This study assesses the impact of new O(3P) photoionization and N2photoabsorption cross sections, and O+oscillator strengths and transition probabilities, on O II 834‐Å airglow calculations. The 834‐Å intensities computed using the new emission parameters are in good agreement with rocket measurements obtained in 1978 and 1980. The present study does not support a suggested reduction in the N2photoabsorption cross section based on an earlier analysis of the rocket data. This study also explores the problem of determiningFregion electron densities from satellite limb scans of the O+834‐Å emission. Our results indicate that electron density profiles inferred from limb scans are not necessarily unique; estimates ofNmF2andhmF2can vary by at least a factor of 2 and 50 km, resp

ISSN:0148-0227    

DOI:10.1029/93JA02283

年代:1994

数据来源: WILEY

摘要:

The aspect angle dependence of the Doppler velocity of radar auroral echoes was studied by means of a comparison between the signals received simultaneously on four 50‐MHz CW links. In most of the events, the observed Doppler velocity decreased with increasing aspect angle but the rate of decrease was lower than that predicted by linear theory. The velocity decrease was about a factor of 0.4 between geometrical aspect angles of 1.5° and 4.0° and 0.6 to 0.7 between 3.2° and 5°. A model for the Doppler velocity behavior of the radar auroral signals is proposed, combining both propagation and scattering effects. Allowance is made for “ray spreading” arising from processes like refraction and diffraction during propagation. In addition, height integration over the scattering volume is taken int

ISSN:0148-0227    

DOI:10.1029/93JA02665

年代:1994

数据来源: WILEY

摘要:

As part of the NASA CRRES chemical release program in September 1990, two Ba and also one each Sr and Ca canisters of a boron‐titanium thermite mixture, which vaporizes the element on ignition, were released near perigee after dusk in the South Pacific to study the critical velocity effect proposed by Alfvén. The critical velocities of these three elements are 2.7, 3.5, and 5.4 km/s respectively, all well below the orbital velocity of 9.4 km/s. On September 10, 1990, a Sr and Ba pair (G‐13, or critical ionization velocity (CIV) I) was released near Rarotonga at ∼515 km altitude in a background electron density of 3.4 × 106cm−3. On September 14, 1990, G‐14 or CIV II released a Ca and Ba pair west of New Caledonia near 595 km at an electron density of 1.5 × 106cm−3. Ions of all three elements were observed with low‐light level imagers from two aircraft after they had transited up the magnetic field lines into the sunlight. Emissions from the spherically expanding neutral gas shells below the solar terminator, observed with cameras filtered for the Ba+ion line at 4554 Å and also in unfiltered imagers for ∼15 s after release, are probably due to excitation by hot electrons created in the CIV process. The ions created clearly lost much of their energy, which we now show can be explained by elastic collisions: Ba++ O. Inventories of the observed ions indicate yields of 0.15% and 1.84% for Ba in the first and second experiments, 0.02% for Sr and 0.27% for Ca. Ionization from all the releases continued along the satellite trajectory much longer (>45 s) than expected for a CIV process. The ion production along the satellite track versus time typically shows a rapid rise to a peak in a few seconds followed by an exponential decrease to a level essentially constant rate. The characteristic distances for CIV I and II are 47 and 62 km, respectively. We interpret the early time rise and exponential fall to be due to CIV ionization, of 0.014% (CIV I) and 0.40% (CIV II) for the Ba releases. The later ions produced at a constant rate probably have origins from other such processes as stripping and associative ionization collisions with atmospheric constituents primarily O, and charge exchange with O+, He+, and H+. We suggest that the much larger Ba ionization rate in CIV II than CIV I is due to the fact that the release occurred in the peak Ca density where hot electrons w

ISSN:0148-0227    

DOI:10.1029/93JA01921

年代:1994

数据来源: WILEY

摘要:

Observations of electrons during a sounding rocket flight from Sondrestromfjord, Greenland, in 1985 showed numerous instances of electron time‐of‐flight dispersion, that is, a coherent structure in which fast electrons arrived before slow electrons. The instrumentation measured many pitch angles simultaneously with high time resolution, providing a detailed view of the temporal evolution of the electron signature at rocket altitudes which was indicative of an impulsive injection of electrons over a range of energies. A model utilizing an extended, quasi‐static (static on a timescale of 1 s), parallel electric field was developed to investigate a possible mechanism for the injection of these electrons. The model is found to fit the data well only when a background is added which cannot be self‐consistently generated by the model, although other parameters used in the model are in general agreement with previous reports. This leads to the conclusion that electron dispersion is more probably generated by a wave acceleration me

ISSN:0148-0227    

DOI:10.1029/93JA01745

年代:1994

数据来源: WILEY

摘要:

A thermal oscillating two‐stream instability as the generation mechanism of electron Bernstein/upper hybrid waves by theo‐mode HF heater wave in the high‐latitude ionosphere is investigated. The purely growing decay mode is described by the fluid equations, while the kinetic equations are employed to derive the coupled mode equations of the electron Bernstein/upper hybrid sidebands. It is found that the instability can be excited in the altitude regions both above and below the upper hybrid resonance layer. The parametric excitation of electron Bernstein waves is in general not effective. On the other hand, the upper hybrid sidebands of the instability excited in the “below” region have been considered by Leyser (1991) to be the pumps for the parametric excitation of downshifted maximum (DM) lines of the stimulated electromagnetic emissions (SEEs) observed in the ionospheric heating experiments. The detail analysis of the proposed work shows that the instability zone of upper hybrid waves below the upper hybrid resonance layer becomes very small when the heater frequency fnof;0is operated near three times of the local electron cyclotron frequency fnof;c. Thus the source wave of the DM lines is effectively suppressed. This result explains the phenomenon of quenching of the DM feature in the SEE spectrum as fnof;0is adjusted near 3fnof;cin the ionospheric modification ex

ISSN:0148-0227    

DOI:10.1029/93JA02668

年代:1994

数据来源: WILEY

摘要:

Narrow‐band, whistlerlike magnetic events distinguished by nearly monochromatic signals decreasing in frequency with time have been observed for the first time at midlatitudes in the ELF band. Measurements performed during September 3 to October 5, 1985 at Table Mountain, California (34.4°N, 117.7°W), show that the frequency and dispersion characteristics of these events are similar to events detected at auroral latitudes (Heacock, 1974), including a narrow‐band magnetic signal monotonically decreasing in frequency from 120 to 60 Hz over a 40 s interval with a mean center frequency of approximately 90 Hz. No echoes were observed. Maximum amplitudes of the magnetic signals ranged from just above the approximately 1 pT Hz−1/2floor of the ambient background to roughly 20 pT Hz−1/2. The polarization was predominantly linear in the geographic east‐west direction. The midlatitude ELF whistlers reported here have a significantly lower average daily rate of occurrence than those reported for auroral latitudes. However, as with the high‐latitude events, they displayed an occurrence rate that is maximum during local daytime. Following Heacock (1974), it is suggested that a possible source for these events is whistler mode lion roars occurring in field‐aligned ducts of enhanced cold plasma densities in the distant magnetosheath. For favorable configurations of these ducts extending from the magnetosheath into the polar cusp, the waves may propagate to the Earth through the cusp acting as a waveguide. Although lightning is usually considered to be the dominant source of ELF noise in the Earth ionosphere cavity, magnetosheath ELF noise coupled into the cavity at high latitudes may represent an additional source. The fractional intensities of the natural ELF noise power within the cavity that are generated by this mechanism are presently unknown. However, a determination of these fractional intensities would be useful in the interpretation of the Schumann resonances as a measure of the totality of global lightning, particularly in the case of measurements made at high latitudes in the vicinity of the cusp‐ionosphere coupling region. Similarly, high‐latitude detection of very weak, artificially generated ELF signals, such as by HF heating of the polar electrojet, may be affected by the proximity of such a local ionospher

ISSN:0148-0227    

DOI:10.1029/93JA02103

年代:1994

数据来源: WILEY

摘要:

The presence of anisotropic plasma distributions, trapped at the Earth's magnetic equator, has consequences for the electric field structure which must exist in equilibrium along the magnetic field line. Data from SCATHA and Dynamics Explorer 1 indicate that the core ion distributions at the magnetic equator can be well described as bi‐Maxwellian distributions, with a perpendicular temperature an order of magnitude larger than the parallel temperature. A collisionless model is developed for the variation in plasma parameters, following the forms developed by Whipple (1977). If the core electron anisotropy is low, the resulting electric field of ∼0.1 µV m−1is pointed away from the equator. Under these conditions the self‐consistent electric field will not overcome the effects of magnetic trapping. The resulting potential distribution results in a local maximum in total plasma density at the equator. Only when the electron distribution is primarily field‐aligned can there be a density minimum at the equator. Comparisons are made between this model and the observed variations in DE 1 plasma parameters wit

ISSN:0148-0227    

DOI:10.1029/93JA02776

年代:1994

数据来源: WILEY