bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiation from large space structures in low Earth orbit with induced alternating currents
Authors: Hastings D. E. , Barnett A. , Olbert S.
Published: February 2008

Large conducting space structures in low Earth orbit will have a nonnegligible motionally induced potential across their structures. The induced current flow through the body and the ionosphere causes the radiation of Alfvén and lower hybrid waves. This current flow is taken to be ac, and the radiated power is studied as a function of the ac frequency. The current may be ac because of either inductive coupling from the power system on the structure or active modulation. A space station‐like structure and tether are studied. For the space station structure the radiation impedance is particularly high for frequencies in the tens of kilohertz range, which suggests that the space station may be an efficient source of lower hybrid waves. The tether is also shown to be a generator of VLF waves up to source ac frequencies in the megahertz range. The implications for these two structures are discussed.
Ion acoustic instabilities excited by injection of an electron beam in space
Authors: Okuda H. , Ashour‐Abdalla M.
Published: February 2008

It is shown by means of numerical simulations that an injection of a tenuous suprathermal electron beam into a fully ionized plasma from spacecraft can lead to an excitation of ion acoustic waves. The return current carried by the ambient electrons has a drift speed that is much slower than the beam speed, so that these electrons can excite the ion waves via wave‐particle interaction. Numerical simulations confirm the excitation of ion acoustic waves and the acceleration of ions near the spacecraft producing a high‐energy tail.
Simultaneous radial and pitch angle diffusion in the outer electron radiation belt
Authors: Chiu Y. T. , Nightingale R. W. , Rinaldi M. A.
Published: February 2008

A solution of the bimodal (radial and pitch angle) diffusion equation for the radiation belts is developed with special regard for the requirements of satellite radiation belt data analysis. In this paper, we use this solution to test the bimodal theory of outer electron belt diffusion by confronting it with satellite data. Satellite observations, usually over finite volumes of (L, t) space, are seldom sufficient in space‐time duration to cover the relaxation to equilibrium of the entire radiation belt. Since time scales of continuous data coverage are often comparable to that of radiation belt disturbances, it is therefore inappropriate to apply impulsive semi‐infinite time response solutions of diffusion theory to interpret data from a finite window of (L, t) space. Observational limitations indicate that appropriate solutions for the interpretation of satellite data are general solutions for a finite‐volume boundary value problem in bimodal diffusion. Here we test such a solution as the prime candidate for comprehensive radiation belt dynamic modeling by applying the solution and developing a method of analysis to radiation belt electron data obtained by the SCATHA satellite at moderate geomagnetic activity. The results and the generality of our solution indicate its promise as a new approach to dynamic modeling of the radiation belts.
Jovian decametric arcs and Alfvén currents
Authors: Staelin David H. , Garnavich Peter M. , Leblanc Yolande
Published: February 2008

The Voyager Planetary Radio Astronomy (PRA) experiment revealed prominent “arcs” when the radio intensity at 1–40 MHz was displayed in time‐frequency coordinates. These data are consistent with multiple currents flowing along longitudinally separated Jovian magnetic flux tubes, each current radiating conically at angles up to ∼90°. The spacings between adjacent PRA arcs, and also between adjacent S burst arc segments observed at the Nançay Observatory, were analyzed and found to be so brief, less than 1–3 min, that either these currents and any associated Alfvén waves survive for at least a few circulations of Jupiter by Io (a hypothesis independently supported by the observations of arcs at all Io phase angles) or each such Alfvén current can radiate at several different cone angles simultaneously (a hypothesis independently supported by the observation of such behavior in the S burst arcs).
Ratios of photoelectron to EUV ionization rates for aeronomic studies
Authors: Richards P. G. , Torr D. G.
Published: February 2008

This study reveals that the ratios of the photoelectron to EUV ionization rates are not constant but depend on the degree of attenuation of the solar EUV flux and on the transport of photoelectrons. At high altitudes in the absence of photoelectron transport, the O+ and N2+ ionization rate ratios are about 0.35, but they increase with increasing optical depth to such an extent that in the vicinity of the ionization peak, photoelectron impact ionization is as important as photoionization for O+ and N2+. The O2+ ratio is about half that of O+ at high altitudes and also increases with increasing optical depth but reaches a peak of about 0.4. We present simple formulae which mimic the attenuation behavior of the ionization ratios. Transport effects become important above about 250 km where the ratios vary by a factor of 2 depending on the presence or absence of photoelectrons from the conjugate ionosphere. In addition to the photoelectron to EUV ionization ratios, we present photodissociative branching ratios for O2 and N2. These photodissociative ratios are also a function of the degree of attenuation of the EUV flux. In the region where attenuation is not important, the N+ to N2+ ratio is 0.14, and the O+ to O2+ ratio is 0.22. There is a factor of 2 uncertainty in our calculated ratios on account of uncertainties in the solar EUV flux spectrum and also uncertainties in the electron impact cross sections.
Study of a conjugate Pc 5‐6 magnetic and absorption pulsation event at subauroral latitudes in the morning sector
Authors: Oksman J. , Rosenberg T. J. , Lanzerotti L. J. , Maclennan C. G. , Singer H. J.
Published: February 2008

An interval of long‐period hydromagnetic wave activity occurring in the decays phase of a magnetic storm has been studied using ground‐based magnetic field and cosmic noise absorption data recorded at L ≲ 4.2. During an ∼90‐min interval of decreasing ring current following the storm maximum, a hydromagnetic pulsation train in the Pc 5‐6 range emerged (frequencies decreasing monotonically from ∼3 mHz to ∼1 mHz), accompanied by similar oscillations of the cosmic radio noise signal. There is some evidence in the data for the wave frequencies to be ∼0.2 mHz lower at L ∼ 3.0‐3.2 than at L ∼ 3.8‐4.2. The low wave frequencies are inconsistent with resonant oscillations of the field lines within the L shell range and probable magnetosphere plasma density conditions of observation. The waves were observed to propagate westward over at least 3 hours of local time, near dawn. An azimuthal wave number m ≈ 9 and a westward phase velocity varying from ∼2.5 to ∼7.5 km s−1 at 55° magnetic latitude characterized the event. The low azimuthal phase velocities make it unlikely that the waves were generated by the Kelvin‐Helmholtz instability on the magnetopause. These Pc 5‐6 waves appear to be odd mode, locally driven nonresonant oscillations. The ULF wave generation and propagation were most likely located on geomagnetic field lines in the range 3.5 ≲ L ≲ 6 in the region of hot storm time plasma outside the plasmapause where a gradient in particle precipitation flux was also observed. Neither the drift mirror instability nor the compressional drift wave instability appears to be a likely source for these waves. Modulation by the ULF waves of the pitch angle diffusion of energetic electrons injected into the outer radiation zone during the magnetospheric storm is suggested to account for the cosmic noise absorption pulsations observed outside the plasmapause. Irregular plasma density gradients in this region, inferred from the propagation characteristics of whistler mode VLF signals, may have caused the observed H‐D plane wave polarization variations.
Absolute differential and integral electron excitation cross sections for atomic oxygen, 5, revised values for the ³P → ³S0 (1304 Å) and ³P → ³D0 (989 Å) transitions below 30 eV
Authors: Gulcicek E. E. , Doering J. P.
Published: February 2008

A new apparatus designed specifically for the study of electron scattering from unstable species at very low incident energies has been constructed and put into operation. Remeasurement of the O I ³P → ³S0 (1304 Å) differential and integral cross sections (DCS and ICS) at energies below 30 eV has provided new values for the DCS and ICS for this cross section as well as for the ³P → ³D0 (989 Å) transition. These new values replace the cross sections reported by Vaughan and Doering (1987) which were measured in the original apparatus designed for high incident energy work. The two sets of measurements agree at incident energies 30 eV and above. However, the new values for the ³P → ³S0 transition at 20 and 16.5eV are larger than the previous measurements by a factor of 2 to 3. Extension of the work on this transition down to 13.87 eV shows that the cross section remains large to within a few eV of threshold, in agreement with the theoretical predictions of Rountree and Henry (1972) and Smith (1976). Similar behavior is observed for the ³P → ³D0 (989 Å) transition down to 20 eV.
Role of equatorial ionization anomaly in the initiation of equatorial spread F
Authors: Raghavarao R. , Nageswararao M. , Sastri J. Hanumath , Vyas G. D. , Sriramarao M.
Published: February 2008

A comparative study is made of the changes in the latitudinal structure of the F region electron density at fixed altitudes in the Indian equatorial region on days with and without postsunset onset of equatorial spread F, using (N‐h) profile data of Ahmedabad (dip latitude 18.6°N), Waltair (dip latitude 10.6°N), and Kodaikanal (dip latitude 1.5°N). It is found that on spread F days the ratio of the electron density in the altitude region 270 ‐ 300 km between Ahmedabad and Waltair showed a sudden enhancement starting at 1700 LT by a factor of 8 to 30 (at 1900 LT) from a near‐constant value of about 2 during the daytime. No such enhancement of the density ratio was evidenced on days without spread F. The enhancement of the electron density ratio prior to the onset of spread F is interpreted as an intensification of the northern crest of the equatorial anomaly, with the ionization in the bottomside F region as far north as 9° from the dip equator participating in the crest intensification process. The rapid intensification of the ionization anomaly engenders a similar augmentation of the neutral anomaly around sunset hours. This in turn creates a localized cell of altitude dependent equatorward neutral wind that aids further intensification of the crests of both the anomalies and augmentation of ionization in the magnetic field line tube passing through the height of maximum plasma density of the F2 layer over the equator. The net result of these coupling processes is a weakening of the ambient transequatorial wind (particularly during northern winter months) and reduction of the north‐south asymmetry of the ionization anomaly crests, a condition favorable for the onset of spread F (Maruyama and Matuura, 1984). The present study thus indicates a significant role of the anomaly intensification at altitudes much below the F region peak and associated neutral dynamics in the initiation of spread F.
Solar flare protons and alpha particles during the last three solar cycles
Authors: Goswami J. N. , McGuire R. E. , Reedy R. C. , Lal D. , Jha R.
Published: February 2008

Event‐integrated fluxes of protons and alpha particles in solar‐flare‐associated particle events during solar cycle 21 (1976–1986) are determined from data obtained by detectors on board the IMP‐7 and IMP‐8 satellites. Sixty‐three solar particle events with proton fluence (E > 10 MeV) > 107 cm−2 were identified from October 1972 to March 1987. The average omnidirectional flux of protons with kinetic energy > 10 MeV for cycle 21, 64 cm−2s−1, is lower than the corresponding number for cycle 20 (92 cm−2s−1) based on satellite data and for the cycle 19 (378 cm−2s−1) based on lunar sample data. Six large events contributed 70% of the total proton fluence during solar cycle 21. Several events in early 1981 with high proton fluences could account for much of the high 56Co radioactivities observed in the small‐sized Salem meteorite. The event‐averaged alpha‐particle to proton ratio in the energy interval 1–10 MeV/nucleon varies from 0.006 to 0.05, with an average value of 0.02 for the whole cycle. The events during solar cycle 21 are characterized by softer spectra for both protons and alpha particles compared to those in earlier solar cycles. No definitive correlation exists between cycle‐averaged solar flare proton fluxes and peak sunspot numbers. A comparison with long‐term (million year) averaged data for these parameters, obtained from lunar sample data, shows that the contemporary solar flare proton spectra are characterized by softer spectra (lower R0 values). A similar comparison cannot be made for the mean long‐term averaged flux, as the contemporary average suffers from uncertainty due to statistics of single events. For this reason, the solar energetic particle activity for the last two solar cycles cannot be considered as representative of average solar particle activity over longer time periods.
The Jovian aurora: Electron or ion precipitation?
Authors: Waite J. H. , Clarke J. T. , Cravens T. E. , Hammond C. M.
Published: February 2008

High signal‐to‐noise spectra of the Jovian aurora at UV wavelengths obtained using the International Ultraviolet Explorer Observatory (including the brightest Jovian aurora observed to date) are used to study the existence of sulfur and oxygen emissions which would be associated with the precipitation of energetic heavy ions in the upper Jovian atmosphere. Model calculations of heavy ion precipitation and corresponding estimates of the associated sulfur and oxygen UV emissions carried out in the preceding companion paper of Horanyi et al. suggest emission values for 1304‐Å O I emission that are at least 50 times larger than the upper limit values set by the IUE observations reported here. On the other hand a possible emission feature of S II at 1256 Å is comparable to the theoretically predicted emission intensity. Earlier X ray observations and in situ plasma observations have indicated the existence of energetic heavy ion precipitation in the Jovian auroral zone. Based on the IUE observations reported here, we suggest a scenario where heavy ion auroral energy deposition is concentrated at altitudes below the homopause (i.e., >300 keV/nucleon) and electrons with energies of 10 to 30 keV are responsible for the bulk of the observable UV and EUV emissions since they deposit their energy above the methane‐absorbing layer defined by the homopause.
Solar EUV measurements at Venus based on photoelectron emission from the Pioneer Venus Langmuir Probe
Authors: Brace L. H. , Hoegy W. R. , Theis R. F.
Published: February 2008

The photoelectron current from the Pioneer Venus Langmuir probe has provided measurements of the solar extreme ultraviolet flux at Venus since 1979. This current is the product of the photoelectric yield of the collector and the solar spectrum at wavelengths short enough to cause emission. Calculations show that approximately 51% of the emission is due to Lyman α (1216 Å), 46% is produced by wavelengths between 550 and 1100 Å, and less than 3% is due to wavelengths longer than Lyman α. Thus the Langmuir probe provides a direct measure of the total solar EUV flux, including most of the wavelengths that produce the Venus ionosphere and heat and excite neutrals in the thermosphere. The measurement technique is described, and the daily average measurements of photocurrent obtained between 1979 and 1987 are presented. The photocurrents exhibit variations related to the solar cycle and solar rotation, as well as a major 7.2‐month periodicity. We present three indices of EUV based on the measurements: (1) the photoemission current itself, (2) the total EUV flux, and (3) an F10.7‐like solar index. These are compared with related measurements made simultaneously at Earth. Since the measurements are made at Venus, they are more useful for many Pioneer Venus investigations than are Earth or Earth satellite observations of the Sun. These data may also help solar physicists track the intensity of EUV emission regions on the Sun while they are not visible from the Earth. The EUV flux profile of a solar flare event is also illustrated. In the future the method also could be applied on a comet mission to obtain the incident solar EUV flux, to measure the EUV extinction profiles of the cometary atmosphere, and to sample directly the dust and gas environment of the comet through the ionization the dust and gas produce when they impact the collector.
Observational and theoretical evidence for anisotropies in Saturn's magnetosphere
Authors: Richardson John D. , Eviatar Aharon
Published: February 2008

Plasma observations in Saturn's inner magnetosphere imply that ion distributions are highly anisotropic, with T⊥ > T∥. Recent improvements in Voyager plasma data analysis routines enable this hypothesis to be tested directly. The assumption of longitudinal symmetry in the ion flux tube content requires anisotropies (T⊥/T∥ ‐ 1) of about 2 and 5 in the protons and heavy ions, respectively. Observations are consistent with anisotropies in this range. Calculations of Coulomb time scales show that isotropization and energy diffusion time scales of water group ions are longer than the residence time of these ions, whereas similar calculations for protons show that isotropization, energy diffusion, and energy loss times are shorter than the proton loss time. Thus water group ions should be highly anisotropic and non‐Maxwellian, whereas proton distributions will approximate isotropic Maxwellians. A steady state kinetic equation is solved analytically for the distribution of perpendicular velocities for H2O+ picked up in Saturn's inner magnetosphere. The velocity distribution thus derived has a broad peak centered at about the corotation energy. We conclude that observational and theoretical evidence both support the hypothesis that large temperature anisotropies exist in Saturn's inner magnetosphere.
Magnetospheric substorms: An equivalent circuit approach
Authors: Liu Z. X. , Lee L. C. , Wei C. Q. , Akasofu S.‐I.
Published: February 2008

The solar wind‐magnetosphere‐ionosphere coupling is studied using an equivalent circuit model, in which the energy transfer from the solar wind to the inner magnetosphere and the ionosphere is assumed to consist of two parts: (1) the direct coupling between the solar wind and the polar ionosphere through the tail magnetopause; and (2) the indirect coupling through the magnetotail plasma sheet. The direct coupling leads to a highly enhanced level for region 1 field‐aligned currents and the auroral electrojet, while the indirect coupling through the plasma sheet leads to the occurrence of impulsive components, which are assumed to result from the current interruption of the tail circuit. A set of circuit parameters is carefully determined on the basis of the observed plasma parameters. The circuit model thus constructed can successfully reproduce several observed behaviors of the magnetosphere during major storms and substorms. The model predicts that the maximum potential ϕmax across the plasma sheet after the onset of substorm can be 2–3 times the potential (ϕCT) imposed at the tail magnetopause. The impulsive variations in the energy (WRC) stored in the ring current belt are found to be much smaller than the variations in the auroral electrojet and the region 1 field‐aligned currents.
Polar cap diurnal temperature variations: Observations and modeling
Authors: McCormac F. G. , Killeen T. L. , Burns A. G. , Meriwether J. W. , Roble R. G. , Wharton L. E. , Spencer N. W.
Published: February 2008

A diurnal variation in thermospheric temperature in the geomagnetic polar cap at solar minimum (January 1987) has been observed using high spectral resolution measurements of the O(¹D) emission line made with the Fabry‐Perot interferometer located at Thule, Greenland (76°32′N, 68°45′W, Λ = 86). The temperature maximum in the diurnal cycle occurs at 1600 UT, which is very near the magnetic local noon period in the northern hemisphere. Data from the wind and temperature spectrometer on the Dynamics Explorer 2 satellite show a similar diurnal variation in the northern polar cap thermospheric temperature for near solar maximum conditions. In an attempt to find the mechanisms responsible for the variation, we have used the diagnostics package developed for use with the NCAR thermospheric general circulation model (TGCM), which allows the tracking of individual parcels of gas in both time and space by interpolation through the model grid. At each of the predicted loci of the parcels we have examined the physical factors which influence the thermal balance by decomposing the thermodynamic equation into its constituent terms. By tracing the trajectory of a parcel backward in time and space from the location of Thule (i.e., well inside the geomagnetic polar cap), we have been able to show, for a model run with input parameters pertaining to day 314, 1976 (i.e., several weeks from solstice near solar minimum), that the observed diurnal temperature variation is attributable to the following factors: (1) the degree of solar heat input that a parcel experiences en route to the polar cap, which is dependent on whether or not it crosses the solar terminator, (2) the route a parcel takes through the polar cusp, i.e., whether the parcel skirts the edge or passes directly through the center determines the total quantity of heat added by soft particle impact within the cusp, (3) the time duration between maximum heat input from the combination of solar, cusp, auroral and Joule sources and the time of arrival overhead at Thule. In addition, we have shown (1) that the hot geomagnetic polar cap reported by Hays et al. (1984) is primarily due to transport of heat into the polar cap by parcels that were heated by auroral, Joule and cusp sources, and (2) that the hydrodynamical variations in the winter high‐latitude regions from solar maximum to solar minimum are insufficient to mask the thermodynamical effects related to the offset of the geographic and geomagnetic poles.
A test particle model of pickup ions at comet Halley
Authors: Luhmann J. G. , Fedder J. A. , Winske D.
Published: February 2008

A test particle treatment is used to investigate some of the details of the pickup cometary ions observed at comet Halley. The effects of the large‐scale magnetic and motional electric fields, as described by an MHD model of the comet, produce the characteristic V shape seen in Giotto observations in simulated energy‐time spectrograms. It is demonstrated that scattering produced by the addition of magnetic field fluctuations can obscure the tendency of the large‐scale field to deflect energetic ions picked up in the outer coma from the tail axis. The fact that the V is so clearly observed in the Giotto spectrogram thus suggests that although scattering must be invoked to explain the isotropic pitch angle distributions and highest energy ions, it does not have a major effect on the overall pickup ion spectrum.
Nature and the nonlinear evolution of electrostatic waves associated with the AMPTE solar wind releases
Authors: Omidi N. , Akimoto K. , Gurnett D. A. , Anderson R. R.
Published: February 2008

To understand the generation and the nonlinear evolution of the electrostatic waves observed during the AMPTE (Active Magnetosphere Particle Tracer Explorers) solar wind releases, a detailed investigation is conducted. Previous linear studies have indicated that two distinct sets of instabilities may be responsible for the generation of these waves. One set consists of ion acoustic type instabilities which are insensitive to the presence of a background magnetic field, while the other group corresponds to the modified two‐stream instabilities and requires the solar wind flow to be across the ambient magnetic field. In order to establish which set of instabilities are more viable for the generation of the observed electrostatic waves a detailed linear Vlasov theory has been conducted by numerically solving the full electromagnetic dispersion relation. In addition, both the plasma wave and the magnetic field measurements by the IRM (Ion Release Module) spacecraft were used to correlate the frequency and the power of the observed waves with the magnitude and the direction of the solar wind magnetic field. The results of these analyses indicate that the ion acoustic type instabilities have growth rates that are an order of magnitude or more larger than those of the modified two‐stream instabilities. Similarly, the observations show no correlation between the magnitude and the direction of the ambient magnetic field on the one hand and the wave frequency and amplitude on the other hand, thus indicating that the ion acoustic type instabilities are the likely generation mechanism. In order to investigate the nonlinear evolution of these instabilities and discern their role in the coupling of the released ions to the solar wind, one‐dimensional full particle as well as fluid electron electrostatic simulations have been performed. The results show that both the solar wind protons and the released ions can be heated and accelerated in the directions oblique to the solar wind flow velocity.
The effects of plasma sheet boundary flow and plasma mantle flow on the ion tearing instability
Authors: Lakhina G. S. , Schindler K.
Published: February 2008

The effect of plasma bulk flow on the ion tearing mode occurring in the Earth's plasma sheet is investigated. The flow is assumed to be confined to the region of open field lines and aligned with the magnetic field. An explicit dispersion relation is obtained for two kinds of Alfvén Mach number profiles idealizing the plasma mantle flow and the plasma sheet boundary flow. In both cases, the growth rate as well as the region of unstable wave numbers are increased as compared to the case without plasma bulk flow. The effect of plasma mantle flow on the growth rate increases as the inner boundary of the mantle approaches the plasma sheet. For the case of plasma sheet boundary flow, the growth rate increases rapidly as the layer where shear flows are concentrated moves toward the center of the sheet. Possible consequences of these results for the dynamic properties of the magnetotail are discussed.
Response of magnetosphere‐ionosphere current systems to changes in the interplanetary magnetic field
Authors: Rostoker G. , Savoie D. , Phan T. D.
Published: February 2008

Theoretical studies of the solar‐terrestrial interaction often are carried out assuming that steady state conditions are applicable and with the a priori assumption that any specific change in the input conditions will produce magnetospheric and ionospheric responses which are repeatable. In this paper we shall first demonstrate that the variability of the interplanetary magnetic field (IMF) is sufficiently large that, under normal circumstances, the assumption of steady state conditions is questionable. We shall further show that the response of the magnetosphere‐ionosphere current systems to changes in interplanetary conditions is dependent on the prior state of the system. In this respect it will be shown that the strength of the driven system currents which grow in response to a southward turning of the IMF is dependent on the level of magnetospheric activity prior to that southward turning (as quantified by the AL index of geomagnetic activity).
Determination of thermospheric temperatures and neutral densities at Arecibo from the ion energy balance
Authors: Burnside R. G. , Sulzer M. P. , Walker J. C. G.
Published: February 2008

We use incoherent scatter measurements of ion and electron temperature and electron density to infer exospheric neutral temperatures and thermospheric atomic oxygen densities from the ion energy balance equation. Measurements obtained at four periods in 1985 and 1986, during the recent solar cycle minimum, are examined. At all seasons, the MSIS‐86 empirical model of the thermosphere is found to overestimate daytime exospheric temperatures at Arecibo by an average of about 50°K. We find that daytime concentrations of H+ are not negligible at Arecibo at solar cycle minimum. Unless the presence of light ions is accounted for when analyzing autocorrelation functions measured by the incoherent scatter technique, significant errors can be made in determining the ion temperature and inferring the exospheric temperature. Deduced atomic oxygen densities are in good agreement with the MSIS model if it is adjusted to match the observed neutral temperatures and the commonly used value of the O+‐O collision cross section is increased by 70%.
Diurnal radio aurora variations at 50 Mhz measured by the bistatic auroral radar system radars
Authors: André Dieter , McNamara Allen G. , Wallis Don D. , McIntosh Bruce A. , Hughes Terry J. , Sofko George J. , Koehler James A. , McDiarmid Don R. , Prikryl Paul , Watermann Juergen
Published: February 2008

Mean Doppler velocity and intensity data obtained from the normal operating mode (alternating single pulse/double pulse mode as also used by STARE) of the BARS radars from selected intervals between March 1986 and July 1987 have been used to describe typical morphological characteristics and diurnal variations in the backscatter. Comparisons with the MARIA magnetometers at Gillam and Eskimo Point verify that the BARS drift velocities obtained by “standard” merging (for which one assumes that the radar Doppler shifts depend on the cosine of the angle between the electron Hall drift and the radar look direction) are in qualitative agreement with the equivalent currents. Three basic time intervals could be identified: (1) an evening interval between 2100 and 0200 UT (1500 to 2000 MLT) with westward E×B drift, which occurs only under disturbed conditions, and for which the Nipawin backscatter intensity is very low most of the time; (2) a breakup interval (dominated by eastward drift but including westward turnings) which under disturbed conditions can extend from 0200 to 1000 UT (2000 to 0400 MLT), but which occurs even under relatively quiet conditions when no backscatter precedes or follows it, in which case it lasts about 2 hours around 0600 UT; (3) an interval with oscillations around a mainly eastward drift; backscatter in this interval does not seem to depend on the occurrence of the previous two types and lasts from 1000 up to 1500 UT (0400 to 0900 MLT). No backscatter has yet been measured during the rest of the day (1500 to 2100 UT or 0900 to 1500 MLT). The period from 2130 UT, May 28, to 1430 UT, May 29, 1987, is used as a sample day during which all three backscatter types occurred.
Ducted whistler propagation outside the plasmapause
Authors: Carpenter D. L. , Šulić D. M.
Published: February 2008

A study has been made of the conditions under which lightning whistlers are observed after propagating along geomagnetic field aligned paths or “ducts” located outside the plasmapause. The study was based in part upon results previously obtained from Antarctic whistler recordings in 1963 and in part upon new data from 45 days of observations at Siple Station, Antarctica (L ≃ 4.3) in 1977 and in 1982. Propagation beyond the plasmapause was found, as expected, to be rare in comparison to propagation within the nearby outer plasmasphere. However, detectable propagation beyond the plasmapause near dawn was found to occur on at least one path on roughly one half of the days studied. The path equatorial radii of propagation tended to cluster in two locations, one at the plasmapause outer edge, and the other in a belt separated from the plasmapause by a region of low activity of order 0.5 RE in extent. The outer edge of the belt was at L ≃ 5.5–6. The probability of whistler detection at any L value outside the plasmapause up to ≃ 6–7 was found to increase with local time across the dayside of the Earth; maximum activity was observed after noon. Whistler activity beyond the plasmapause was least common in the 1800–2400 MLT sector. Spatial variations in lightning source activity may be responsible for large variations with longitude in whistler activity outside the plasmapause and for the many cases in which only a small fraction of whistlers within a given period exhibit evidence of plasmatrough propagation. Ionospheric processes, such as wave damping, defocusing in the mid‐latitude trough, scattering by 10 to 100‐m irregularities, and focusing within 50 to 100‐km blobs, can explain many features of the observed spatial distribution of whistler paths beyond the plasmapause and also the relatively low amplitude of the associated whistlers as compared to plasmasphere events. The large differences in whistler activity between afternoon and premidnight appear to be due to spatial differences in perturbing magnetospheric electric field activity and in electron density in the plasma trough region. The experimental findings serve to make a relatively sharp distinction between the plasmasphere and the region beyond from both the propagation and wave‐particle‐interaction points of view.
On the acceleration of electrons and positive ions in the same direction along magnetic field lines by parallel electric fields
Authors: Hultqvist Bengt
Published: February 2008

It is demonstrated that the simultaneous observations on Viking of upward field‐aligned fluxes of energetic ions and electrons of energies in the same range may be due to acceleration in field‐aligned electric fields, the ions in an upward directed parallel dc field and the electrons in a downward directed parallel field which is fluctuating but appears as quasi‐static for the electrons as long as they are in the acceleration region. Most of the upward accelerated electrons leave the limited acceleration region in an effective half period of the fluctuating field when it points downward. The fluctuating field has only a small effect on the ions.
A numerical model for auroral precipitation
Authors: Swift Daniel W.
Published: February 2008

A numerical simulation model is used to show that the velocity distribution function of electrons responsible for the discrete aurora is modified by two processes: (1) In the region above the ionosphere, electrons are accelerated by a dc potential drop that extends from the plasma sheet to the topside ionosphere. In this region the number of particles per unit length in a magnetic flux tube increases with altitude. The simulations also show that electrons will be accelerated to higher energies on field lines with low plasma density. (2) When the accelerated electron beam encounters the ionospheric plasma, wave‐particle interactions occur. When the plasma density is a function of the coordinates perpendicular to the magnetic field, upper and lower hybrid waves are excited, which heat the electrons perpendicular to the magnetic field. Upper hybrid waves produce rapid perpendicular heating, which leads to large perpendicular gradients in the upward flowing parts of the electron distribution function, which in turn would enhance growth rates in auroral kilometric radiation. Linearized analysis of growth rates of upper and lower hybrid waves are presented to assess the likelihood that upper hybrid waves play a significant role in auroral processes.
Generation of electron conical distributions by upper hybrid waves in the Earth's polar region
Authors: Wong H. K. , Menietti J. D. , Lin C. S. , Burch J. L.
Published: February 2008

The heating of electron populations through resonant wave‐particle interaction with upper hybrid turbulence is examined. It is first demonstrated that upper hybrid waves are easily excited by a population of warm electrons with a loss cone type distribution in a background of cold electron plasma. Quasi‐linear theory is then used to show that the upper hybrid turbulence can preferentially heat the warm electrons in the perpendicular direction. In analogy to ion conic formation, this perpendicular heating process is a viable mechanism of generating electron conical distributions observed in the Earth's midaltitude polar regions.
Electric potential structures and propagation of electron beams injected from a spacecraft into a plasma
Authors: Singh Nagendra , Hwang K. S.
Published: February 2008

The propagation of electron beams injected from a spacecraft into an ambient plasma and the associated potential structures are investigated by one‐dimensional Vlasov simulations. For moderate beams, for which the time average spacecraft potential (Φsa) lies in the range Te ≪ eΦsa ≲ WB, where Te is the electron temperature in energy units and WB is the average beam energy, a double layer forms near the beam head which propagates into the ambient plasma much more slowly than the initial beam velocity. The double layer accelerates the ambient electrons, which carry the return current to the spacecraft. The double layer formation is being reported for the first time. For weak beams, for which |eΦsa| ≲ Te, the beam propagates with the initial beam velocity, and no double layer formation occurs. On the other hand, for strong beams for which eΦsa > WB, the bulk of the beam is returned to the spacecraft, and the main feature of the potential structure is a sheath formation with an intense electric field limited to distances <10λd near the spacecraft surface. These features of the potential structures are compared with those seen in laboratory and space experiments on electron beam injections.
A one‐dimensional multispecies magnetohydrodynamic model of the dayside ionosphere of Venus
Authors: Shinagawa H. , Cravens T. E.
Published: February 2008

The behavior of the plasma and magnetic field in the dayside ionosphere of Venus was studied for both time‐dependent and steady state conditions, using a one‐dimensional multispecies MHD model. The coupled continuity, momentum, and Maxwell's equations were solved simultaneously for several ion species. The multispecies MHD model includes the momentum coupling terms for all ion‐ion and ion‐neutral interactions. The present model is more complete than the “one‐major‐ion” model of Shinagawa et al. (1987). The results of the MHD model and the “one‐major‐ion” model are compared; they indicate that the one‐major‐ion treatment was a fairly good approximation. The magnetic field in the lower ionosphere is maintained by the magnetic flux transported downward from the magnetic barrier region by plasma motion, and the typical time scale for the decay of the magnetic field is about several hours. Two new cases are presented: (1) steady state conditions for the magnetized ionosphere, and (2) inclusion of ion loss due to horizontal transport in the magnetized region. For steady state conditions, the electric field in the ionosphere should be independent of altitude in a one‐dimensional model. Above 160 km, the convective motion contributes to the electric field, while below 160 km the ohmic resistivity term makes the main contribution to the electric field. The effects of the divergence of the horizontal plasma velocity were approximately included by solving a simplified horizontal ion momentum equation. The resulting calculated profiles of the magnetic field and the electron density agree much better with the observations at high altitudes than those without the ion loss terms. These results indicate the importance of the horizontal transport processes in the ionosphere of Venus at high altitudes.
Exact solutions and low‐frequency instability of the adiabatic auroral arc model
Authors: Cornwall John M.
Published: February 2008

The adiabatic auroral arc model couples a kinetic theory parallel current driven by mirror forces to horizontal ionospheric currents; the resulting equations are nonlinear. We develop some exact stationary solutions to these equations, some of them based on the Liouville equation, with both latitudinal and longitudinal spatial variations. These Liouville equation exact solutions are related to stability boundaries of low‐frequency instabilities such as Kelvin‐Helmholtz, as study of a simplified model shows.
On the existence of the 11‐year cycle in solar activity before the Maunder Minimum
Authors: Attolini M. R. , Cecchini S. , Castagnoli G. Cini , Galli M. , Nanni T.
Published: February 2008

The existence of the 11‐year cycle in solar activity before the Maunder minimum is clearly demonstrated with cosmogenic 10Be in polar ice during 1180‐1500 A.D. For that interval a periodicity of 11.4 ± 0.2 years is found with a high significance level. Indication of a cyclicity that resembles the Hale magnetic cycle is also observed at a lower significance level. A highly variable cyclicity in the band 9.5‐11.5 years is also found in the record of historical aurorae which appears to be well correlated with the 10Be cyclicity for the same time interval. It is concluded that the Schwabe, or 11‐year, cycle and the Hale magnetic cycle were present before and after the Maunder minimum, even though it is not possible to understand the variability of the cycle.
The polarization of escaping terrestrial continuum radiation
Authors: Gurnett D. A. , Calvert W. , Huff R. L. , Jones D. , Sugiura M.
Published: February 2008

Plasma wave measurements from the DE 1 (Dynamics Explorer 1) spacecraft are used to determine the polarization of an escaping terrestrial continuum radiation event that occurred on March 2, 1982. The source of the radiation was determined by direction finding to be located near the magnetic equator on the nightside of the Earth at a radial distance of about 2.8‐3.5 RE. The radiation was emitted in two meridional beams, one north and the other south of the magnetic equator. Polarization measurements using the two orthogonal electric antennas on DE 1 show that the radiation is right‐hand polarized with respect to an outward directed E plane normal in the northern hemisphere and left‐hand polarized in the southern hemisphere. Comparisons with the local magnetic field show that both the northern and southern hemisphere beams are propagating in the L‐O mode at the spacecraft. The mode of propagation has also been confirmed using measurements of the E plane normal angle and ellipse ratio. Because the angle between the magnetic field and the E plane normal rotates through perpendicular as the radiation propagated from the source to the spacecraft, mode coupling effects must be evaluated when considering the mode of propagation in the source. Estimates of the spatial gradients over the ∼ 1‐RE distance between the source and the spacecraft indicate that the radiation has not reached the region of limiting polarization. Therefore the mode of propagation must be the same at the source and at the spacecraft: i.e., the L‐O mode. These observations support the linear conversion model of Jones in which the radiation is produced by coupling from intense upper hybrid resonance emissions near the plasmapause. This conversion mechanism predicts that continuum radiation generated in the vicinity of the plasmapause should be emitted primarily in the L‐O mode. Remote‐sensing analyses based on Jones’ model yield source locations in agreement with those derived by the direction‐finding method.
Snapshots of high‐latitude electrodynamics using Viking and DMSP F7 observations
Authors: Marklund G. T. , Blomberg L. G. , Stasiewicz K. , Murphree J. S. , Pottelette R. , Zanetti L. J. , Potemra T. A. , Hardy D. A. , Rich F. J.
Published: February 2008

Simultaneous observations by the Viking and the DMSP F7 satellites have been used as input to a new method to obtain snapshot pictures of the auroral electrodynamics. In particular, an “instantaneous” global equipotential (or convection) pattern is calculated from distributions of field‐aligned current and conductivity which are qualitatively consistent with the Viking auroral imager data and quantitatively consistent with magnetic field and particle data from the two satellites. This convection pattern, which is of the normal two‐cell type, with a weak dusk cell and a strong, elongated crescent‐shaped dawn cell (consistent with positive interplanetary magnetic field By), agrees well with the Viking electric field data. The model and the observed potential profiles agree nicely along the entire Viking orbit except for two intervals above acceleration regions where deviations are to be expected (due to parallel electric fields). These regions are characterized by U‐shaped potential minima, upward field‐aligned currents, upgoing ion beams, and relatively intense auroral kilometric radiation. Thus, the model results are consistent with the Viking observations not only on a global scale but also on the scale of the auroral acceleration regions. The corresponding convection in the magnetosphere is obtained from a simple projection to the equatorial plane of the deduced two‐cell convection pattern. From this the location of the plasmapause is inferred.
The influence of conductivities consistent with field‐aligned currents on high‐latitude convection patterns
Authors: Blomberg L. G. , Marklund G. T.
Published: February 2008

The influence on the high‐latitude ionospheric convection of conductivities associated with upward field‐aligned currents is investigated. Potential patterns are calculated from a given distribution of field‐aligned currents and a conductivity model. The resulting patterns are shown to be modified considerably by including a coupling term between the conductivity and the field‐aligned current in the conductivity model. The clockwise rotation of the entire potential pattern is reduced when the conductivity enhancement coincides with the regions of upward field‐aligned current. Also, the electric field within these regions turns out to be rather insensitive to changes in the magnitude of the current. In regions of downward current or when the current‐dependent conductivity is excluded, there is on the other hand an almost linear relationship between current and electric field. Although the particles producing the conductivity enhancement may not be the same as those carrying the major part of the field‐aligned current, it is clear from observations that there is a positive correlation between upward current and conductivity. Therefore, the simple relationship used in this study is believed to reflect rather well the principal features of the current‐conductivity coupling, which is of importance to the modeling of ionospheric electrodynamics.
Electrodynamic patterns in the polar region during periods of extreme magnetic quiescence
Authors: Hoffman R. A. , Sugiura M. , Maynard N. C. , Candey R. M. , Craven J. D. , Frank L. A.
Published: February 2008

The electrodynamic patterns in the polar region during periods of extreme magnetic and auroral quiescence are analyzed for the existence of region 1 and region 2 field‐aligned current systems, convection patterns, and electron precipitation boundaries. Within the capabilities of identification, the large‐scale region 1 and region 2 currents are usually absent during quiet periods, except in the cusp region, though a seasonal effect is observed with lowest probabilities of identifying these regions occurring in the winter hemisphere. Field‐aligned currents continue to be present, but as small‐scale structures distributed through the auroral oval and over the entire polar cap. Convection patterns are always found in the summer hemisphere, though with large‐amplitude irregularities superimposed, but their probability for existence decreases significantly around the winter solstice. Structured electron precipitation extends over the entire polar region independent of season. The likelihood of finding separate regions of diffuse and structured electron precipitation is fairly small except in the cusp. No consistent relationship exists between the high‐latitude boundary of >35‐keV electron precipitation and boundaries for lower‐energy electrons. The extreme quiescent periods for the northern hemisphere seem to exist when the interplanetary magnetic field has a northward component and a high probability of a sunward component, producing minimal merging sites for the northern polar cap field lines.