Effects of E-region electric fields on F-region parameters at the magnetic equator

The effects of E-region electric fields on F-region parameters like height of constant electron density contours (h_N) and semi-thickness, at an equatorial station, Trivandrum, are investigated. The E-region east-west electric field (E_y) has been deduced using the horizontal magnetic field values from the ground magnetograms. It is found that the semi-thickness parameter and h_N follow closely the E-region electric field (E_y) variations in the forenoon and noon periods. In the afternoon, it is shown that there is close association between h_N and E_y, when the electric field variations are large. On a day-to-day basis also, h_N and the E-region electric field show very good correlation in the forenoon and noon hours. It is also shown that higher levels are affected more than the lower levels in the F-region by E_y.     

The winter anomaly in ionospheric absorption and the D-region ion chemistry

A detailed analysis of the D-region ion composition measurements performed by Zbinden et al. (1975), during a winter day of high ionospheric absorption, has been carried out. The study examines the interactive mesosphere-D-region processes which occur in such anomalous conditions and their implication for water cluster ion chemistry. Two clustering regimes for NO⁺ have been observed in the data. Association with N₂ is identified as the dominant process below 76 km. Between 76 and 78 km altitude the effective loss rate of NO⁺ drops by two orders of magnitude. Above 77 km, the three-body reaction NO⁺ + CO₂+M→NO⁺CO₂+M seems to be the main NO⁺ loss. A mesospheric temperature profile could be derived from the ion composition data. This indicates the presence of a strong inversion above 76 km altitude. The wavelike structure obtained, is shown to be consistent with in situ winter temperature measurements. The sharp suppression of the N₂ association reaction could, thus, be explained by an increase in the collisional break-up of the NO⁺N₂ ion because of the enhanced temperature. In conclusion, our study indicates that, besides the increase in the production of NO⁺ and O₂⁺, due to an enhancement in the minor ionizable constituents, an additional thermal mesosphere-D-region interaction seems necessary to explain this winter anomalous ion composition data.     

The high latitude circulation and temperature structure of the thermosphere near solstice

The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCAR's thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F-region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E-region ionosphere.

The results of several TGCM runs are used to show that at F-region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid.

The F-region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer (DE−2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s⁻¹) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s⁻¹). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE−2 satellite measurements.     

Observation of a thin Es-layer by the eiscat radar

High resolution E-region measurements carried out on 16 November 1983 using the EISCAT incoherent scatter radar are presented. The experiment was monostatic with a vertical radar beam, and it was based on a Barker-coded four-pulse code on one frequency channel and Barker-coded single pulses on three channels. The basic integration time was 15 s and the spatial resolution 450 m. The results reveal a short-lived but intense thin sporadic E-layer at 18:00–18:06 U.T. at an altitude of about 106 km. Both before and during the event, downward ion velocities of the order of 100 m s⁻¹ are observed above this height. A convergent null in the vertical ion speed is occasionally seen at the layer altitude. The layer occurrence is associated with auroral arcs drifting across the radar beam. Simultaneous observations of the STARE radar show an ionospheric electric field of 25–30 mV m⁻¹. The field always has a westward component, which is in accordance with the observed downward plasma flow. Most of the time when the layer is intense, the field points into the NW-sector. Theoretically, this field direction should create convergent vertical plasma motion. Therefore it is suggested that the observed E_s-layer is created by the action of the auroral electric field rather than by the wind shear mechanism.     

An auroral F-region study using in situ measurements by the Atmosphere Explorer-C satellite

On 14 July 1974 the Atmosphere Explorer-C satellite flew through an aurora at F-region altitudes just after local midnight. The effects of the particle influx are clearly evident in the ion densities, the 6300 Å airglow, and the electron and ion temperatures. This event provided an opportunity to study the agreement between the observed ion densities and those calculated from photochemical theory using in situ measurements of such atmospheric parameters as the neutral densities and the differential electron energy spectra obtained along the satellite track. Good agreement is obtained for the ions O₂⁺, NO⁺ and N₂⁺ using photochemical theory and measured rate constants and electron impact cross sections. Atomic nitrogen densities are calculated from the observed [NO⁺]/[O₂⁺] ratio. In the region of most intense electron fluxes (20 erg cm⁻² sec⁻¹) at 280 km, the N density is found to be between 2 and 7 × 10⁷ cm⁻³. The resulting N densities are found to account for approx. 60% of the production of N⁺ through electron impact on N and the resonant charge exchange of O⁺(²P) with N(⁴S). This reaction also provides a significant source of O(¹S) in the aurora at F-region altitudes. In the region of intense fast electron influx, the reaction with atomic nitrogen is found to be the main loss of O⁺(²P).     

Electron excitation and auroral emission parameters

Auroral luminosities of the main emission lines in the aurora have been calculated for excitation by an isotopic primary electron flux with spectra of the form J(E) = AE exp (−E/E₁) + B(E₂)E exp (−E/E₁). The variation of emissions from O and N₂⁺ with height are shown, as are the variations of column integrated intensities and pertinent intensity ratios with the characteristic energy E₂, this leading to a method of estimating the electron spectrum from ground observation.     

Gamma ray spectrum of the crab nebula in the multi TeV region

The THEMISTOCLE array of 18 Cherenkov detectors which has a 3 TeV gamma energy threshold, has detected a signal from the Crab nebula at a 5.8 standard deviation level. Information on the energy spectrum is obtained in the range 3–15 TeV. The integrated flux can be fitted with the form, Φ (> E) = (3.7 ± 0.5) × 10^-12 (E/5)-^{−1.5 ± 0.20} cm⁻² s⁻¹ (E in TeV) compatible with the extrapolation of results at lower energies. The Crab signal is used to measure the angular resolution of the multi-telescope technique. The value obtained is 2.3 mr (0.15°) in agreement with the results of simulations, and confirms the interest of this new method for multi-TeV gamma-ray detection.     

The cosmic ray primary composition in the “knee” region through the EAS electromagnetic and muon measurements at EAS-TOP

The evolution of the cosmic ray primary composition in the energy range 10⁶–10⁷ GeV (i.e. the “knee” region) is studied by means of the e.m. and muon data of the Extensive Air Shower EAS-TOP array (Campo Imperatore, National Gran Sasso Laboratories). The measurement is performed through: (a) the correlated muon number (N_μ) and shower size (N_e) spectra, and (b) the evolution of the average muon numbers and their distributions as a function of the shower size. From analysis (a) the dominance of helium primaries at the knee, and therefore the possibility that the knee itself is due to a break in their energy spectrum (at E_k^He=(3.5±0.3)×10⁶ GeV) are deduced. Concerning analysis (b), the measurement accuracies allow the classification in terms of three mass groups: light (p,He), intermediate (CNO), and heavy (Fe). At primary energies E₀≈10⁶ GeV the results are consistent with the extrapolations of the data from direct experiments. In the knee region the obtained evolution of the energy spectra leads to: (i) an average steep spectrum of the light mass group (γ_p,He>3.1), (ii) a spectrum of the intermediate mass group harder than the one of the light component (γ_CNO2.75, possibly bending at E_k^CNO≈(6–7)×10⁶ GeV), (iii) a constant slope for the spectrum of the heavy primaries (γ_Fe2.3–2.7) consistent with the direct measurements. In the investigated energy range, the average primary mass increases from lnA=1.6–1.9 at E₀1.5×10⁶ GeV to lnA=2.8–3.1 at E₀1.5×10⁷ GeV. The result supports the standard acceleration and propagation models of galactic cosmic rays that predict rigidity dependent cut-offs for the primary spectra of the different nuclei. The uncertainties connected to the hadronic interaction model (QGSJET in CORSIKA) used for the interpretation are discussed.     

Comparison of high-energy galactic and atmospheric tau neutrino flux

We compare the tau neutrino flux arising from the galaxy and the earth atmosphere for 10³E/GeV10¹¹. The intrinsic and oscillated tau neutrino fluxes from both sources are calculated. The intrinsic galactic ν_τ flux (E10³ GeV) is calculated by considering the interactions of high-energy cosmic-rays with the matter present in our galaxy, whereas the oscillated galactic ν_τ flux is coming from the oscillation of the galactic ν_μ flux. For the intrinsic atmospheric ν_τ flux, we extend the validity of a previous calculation from E10⁶ GeV up to E10¹¹ GeV. The oscillated atmospheric ν_τ flux is, on the other hand, rather suppressed. We find that, for 10³E/GeV5×10⁷, the oscillated ν_τ flux along the galactic plane dominates over the maximal intrinsic atmospheric ν_τ flux, i.e., the flux along the horizontal direction. We also briefly mention the presently envisaged prospects for observing these high-energy tau neutrinos.     

A numerical investigation of thermosphere-ionosphere interaction over Millstone Hill

The upper thermosphere and F-region ionosphere system at 43°N is modelled for equinox and moderate solar conditions via a series of iterative calculations employing a thermospheric wind model and a one-dimensional ionospheric model which are mutually coupled. Several feedback loops within the system involving F₂-layer peak height, F₂-layer peak density, zonal wind, meridional wind, and Coriolis force are investigated to better understand the interactive aspect of ionosphere-thermosphere coupling. The interplay of primary importance involves the night-time ascent/descent of the F-layer due to equatorward/poleward neutral winds, the resulting changes in ion drag presented to the meridional and zonal wind fields, and the Coriolis force modification of the ion drag coupling. Wind shear and plasma profile shape are not significantly coupled. For magnetically undisturbed conditions, self-consistent treatment of these effects modifies a non-interactive “control” calculation by 20–35 m s⁻¹ in the wind field. During geomagnetically disturbed periods interactive processes play a more crucial role in determining thermospheric and ionospheric storm responses. Our calculations reveal wind enhancements of up to 100 m s⁻¹ associated with the lifting and negative-phase depletion of the F-region for prolonged magnetic disturbance conditions, the former mechanism accounting for a major portion of the effect.     

Flux limits on ultra high energy neutrinos with AMANDA-B10

Data taken during 1997 with the AMANDA-B10 detector are searched for a diffuse flux of neutrinos of all flavors with energies above 10¹⁶ eV. At these energies the Earth is opaque to neutrinos, and thus neutrino induced events are concentrated at the horizon. The background are large muon bundles from down-going atmospheric air shower events. No excess events above the background expectation are observed and a neutrino flux following E⁻², with an equal mix of all flavors, is limited to E²Φ(10¹⁵ eV < E < 3 × 10¹⁸ eV)  0.99 × 10⁻⁶ GeV cm⁻² s⁻¹ sr⁻¹ at 90% confidence level. This is the most restrictive experimental bound placed by any neutrino detector at these energies. Bounds to specific extraterrestrial neutrino flux predictions are also presented.     

Current-driven dust-acoustic instability in a collisional plasma

The excitation of the dust-acoustic instability in a collisional dusty plasma is investigated. For conditions similar to those of recent laboratory experiments with neutral gas pressures of 100mTorr, it is found that zero-order electric fields E₀ 10 Vm⁻¹ are sufficient for the growth of perturbations with centimeter wavelengths. Much larger wavelengths generally require larger values of E₀. Free electrons in the dusty plasma have a stabilizing effect, which can be very pronounced at the longest wavelengths.     

Space–time foam and cosmic-ray interactions

It has been proposed that propagation of cosmic-rays at extreme-energy may be sensitive to Lorentz-violating metric fluctuations (“foam”). We investigate the changes in interaction thresholds for cosmic-rays and gamma-rays interacting on the CMB and IR backgrounds, for a class of stochastic models of space–time foam. The strength of the foam is characterized by the factor (E/M_P)^a, where a is a phenomenological suppression parameter. We find that there exists a critical value of a (dependent on the particular reaction: a_crit3 for cosmic-rays, 1 for gamma-rays), below which the threshold energy can only be lowered, and above which the threshold energy may be raised, but at most by a factor of two. Thus, it does not appear possible in this class of models to extend cosmic-ray spectra significantly beyond their classical absorption energies. However, the lower thresholds resulting from foam may have signatures in the cosmic-ray spectrum. In the context of this foam model, we find that cosmic-ray energies cannot exceed the fundamental Planck scale, and so set a lower bound of 10⁸ TeV for the scale of gravity. We also find that suppression of p→pπ⁰ and γ→e⁻e⁺ “decays” favors values aa_crit. Finally, we comment on the apparent non-conservation of particle energy–momentum, and speculate on its re-emergence as dark energy in the foamy vacuum.     

The cosmic ray energy spectrum between 10 and 10 eV

The energy spectrum of cosmic rays with primary energies between 10¹⁴ eV and 10¹⁶ eV has been studied with the CASA-MIA air shower array. The measured differential energy spectrum is a power law (dj/dE ∝ E^−y) with spectral indices γ of 2.66±0.02 below approximately 10¹⁵ eV and 3.00±0.05 above. A new method is used for measuring primary energy derived from ground-based data in a compositionally insensitive way. In contrast with some previous reports, the “knee” of the energy spectrum does not appear sharp, but rather a smooth transition over energies from 10¹⁵ eV to 3.0 × 10¹⁵ eV.     

Impact cratering of granular mixture targets made of H2O Ice–CO2 Ice–pyrophylite

Experiments related to impacts onto three-component targets which could simulate cometary nucleus or planetary regolith cemented by ices are presented here. The impact velocities are from 133 to 632 m s⁻¹. The components are powdered mineral (pyrophylite), H₂O ice, and CO₂ ice mixed 1:1:0.74 by mass. The porosity of fresh samples is about 0.48. Two types of the samples were studied: nonheated samples and samples heated by thermal radiation. Within the samples a layered structure was formed. The cratering pattern strongly depended on the history of the samples. The craters formed in nonheated targets had regular shapes. The volume was easy to be determined and it was proportional to impact energy E. The crater depth scales as E^0.5. Impacts on the thermally stratified target led to ejection of a large amount of material from the loose sub-crustal layer. For some particular interval of impact velocity a cratering pattern can demonstrate unusual properties: small hole through the rigid crust and considerable mass transfer (radially, outward of the impact point) within sub-crustal layer.     

Atmospheric expansion through Joule heating by horizontal electric fields

Incoherent scatter measurements made along a magnetic field line into aurora during a period of high electric field in the recovery phase of a substorm show (1) considerably increased electron densities well above the normal F-region maximum, and (2) field-aligned plasma drifts that increase with altitude. A model invoking atmospheric expansion through Joule heating by the horizontal electric field driving the auroral electrojet is used to explain the observations. From this study it is concluded that during magnetically disturbed periods (1) Joule heating by the auroral electrojet raises the neutral temperature and density in the auroral zone ionosphere at F-region heights, (2) ionization formed by the aurora is transported upward by the expanding atmosphere, at times producing an appreciable increase in lower exospheric plasma densities on the field lines containing the aurora, and (3) combined satellite, radar, and optical observations during periods of aurora and high electric field could provide measured F-region collision frequencies.     

A further study of the method of identifying absorption line systems in QSO spectra

The method of identifying absorption line systems in QSO spectra (Cui et al. 1983; Chen et al. 1983) is further developed here. Certain limitations of the method and their improvements are discussed. Certain other problems requiring further study are pointed out. The improved method is applied to PKS 0528-250, and gives two new absorption line systems Z_a = 0.065 and 0.0345 in addition to the four systems Z_a = 2.8110, 2.8130, 2.5275, 2.1410, consistent with the systems A₁A₂, B, C of Norton et al. (1980). However, the systems D₁, D₂, E, F and G of Chen and Norton (1984) are not recovered. The reason for this discrepancy is discussed.     

Photon heating of the atmosphere at F-region heights

The transfer of energy from an ionizing photon to the atoms and molecules of the neutral gas in the F-region of the atmosphere is investigated. It is found that photoionization heating should be divided into two parts: (1) photoelectron heating associated with the slowing down of the fast photoelectrons formed by photoionization; and (2) reaction heating associated with the chemical reactions undergone by the ions formed in the photoionization process. The photoelectron heating will take place near the time and place of photoionization while the reaction heating will occur at the time and place of the ionic reactions.

Photoelectron and reaction heating rates per unit column are computed for the daytime, and reaction heating rates per unit column are computed for the nighttime. It is concluded that: (1) chemical reactions at night lead to a small but significant amount of F-region heating; and (2) reaction heating during the day is nearly proportional to the cosine of the solar zenith angle except near sunrise and sunset.     

Small-scale auroral arc distortions

Small-scale spatially periodic distortions of auroral forms have been studied utilizing low-light level television observations made at various locations in the Northern and Southern Hemispheres. The most commonly observed features were folds and vortex-like curl formations. The curls, identified here with the Kelvin-Helmholtz instability due to fluid shear, invariably had a counterclockwise rotational shape and motion when viewed in a direction anti-parallel to the Earth's magnetic field. The typical measured wavelength (5 km) and measured growth rate (4.2 sec⁻¹) were used to evaluate the Kelvin-Helmholtz dispersion relation for the apparent shear ω_s = ∂ ν_x/ ∂y (28 sec⁻¹). The apparent horizontal velocities of both folds (0–5 km/sec) and curls (0–22 km/sec) were invariably observed to be counterclockwise with respect to the multiple arc centre when viewed antiparallel to B. Consistent agreement between rotational shape and rotational motion suggests that the apparent growth rate and the apparent horizontal velocities closely approximate the actual values. If the shear results from E×B drifts in a space charge field, the calculated value for ω_s, implies an unneutralized electron density 0–1 cm⁻³ and a ΔE across the arc element 500mV/m. The velocity measurements indicate that the ΔE values for individual elements can combine to produce transient electric fields at the edges of multiple arcs as high as 1000 mV/m.     

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2−41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST /STIS spectra of a high-velocity interstellar cloud ( v _LSR=+80 km s⁻¹) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s⁻¹ (FWHM). The high-velocity cloud absorption is observed in the lines of C  ii , O  i , Si  ii , Si  iii , Si  iv and S  iii . Limits can be placed on the amount of S  ii and Fe  ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C  ii and O  i , suggests that this high-velocity gas is warm ( T _k∼10³–10⁴ K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si  iv . This sightline also intercepts two other high-velocity clouds that produce weak absorption features at v _LSR=+113 and +130 km s⁻¹ in the STIS spectra.