bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanism of the OH‐initiated oxidation of methacrolein
Authors: Orlando John J. , Tyndall Geoffrey S. , Paulson Suzanne E.
Published: September 2002

The OH‐initiated oxidation of methacrolein, a major product of isoprene oxidation, has been studied in an environmental chamber using FT‐IR spectroscopy. Products observed (which account for more than 90% of the reacted carbon) were CO, CO2, hydroxyacetone, formaldehyde, and methacryloylperoxynitrate (MPAN). It is determined that the attack of OH on methacrolein occurs 55% of the time via addition to the double bond, and 45% via abstraction of the aldehydic hydrogen atom, in agreement with a previous study. The end products of the abstraction channel are identified and quantified for the first time, and the mechanism of their production discussed.
A surface chemistry model for nonreactive trace gas adsorption on ice: Implications for nitric acid scavenging by cirrus
Authors: Tabazadeh A. , Toon O. B. , Jensen E. J.
Published: September 2002

According to recent measurements HNO3 nearly saturates laboratory ice films in flow tubes for vapor pressures above 10−7 torr and temperatures below 230 K. In the upper troposphere the HNO3 vapor pressure is in the range of 1 to 3 × 10−8 torr and therefore the current laboratory estimates of saturation may not hold in the atmosphere. Here, with the aid of a surface chemistry model and laboratory data we estimate that the binding energy for HNO3 adsorbed on ice is about −14.2±0.2 kcal mol−1. Using the surface model, we explore the sensitivity of HNO3 scavenging efficiency to cirrus cloud temperature, ice number density, and HCl competitive co‐adsorption.
Radiative smoothing in zenith‐scattered skylight transmitted through optically thick clouds to the ground
Authors: von Savigny C. , Funk O. , Platt U. , Pfeilsticker K.
Published: September 2002

A ground‐based photometer was used to monitor the temporal variation of the zenith‐scattered skylight radiance (at λ=752 nm) with a temporal resolution of 2 Hz. In detail, eight data sets for different optically thick (τc ≥1/(1−g) with τc being the cloud's optical thickness and g the asymmetry factor for Mie scattering) cloud covers were analyzed with standard spectral methods. A general feature of all cloudy sky radiance power spectra is the presence of a remarkable scale break occurring at spatial scales that approximately correspond to the vertical cloud extension. This finding is consistent with theoretical estimates by Marshak et al. [1995] and Davis et al. [1997b] based on radiative smoothing by photon diffusion in vertically extended, optically thick and homogeneous clouds.
Dynamical range of the WINDMI model: An exploration of possible magnetospheric plasma states
Authors: Smith James P. , Thiffeault Jean‐Luc , Horton Wendell
Published: September 2002

This paper explores the dynamical range of the WINDMI model (Horton and Doxas, 1998]. Such low‐dimensional models provide us with the tools to understand the relationships of simple physical quantities within the magnetosphere (such as energy deposition, macroscopic currents, cross‐tail voltage, etc.) without the necessity of coping with the more complete but unwieldy models (MHD or particle codes, for example). The model is highly versatile: certain regions of the parameter space support stable fixed points, while others contain periodic states that exhibit period doubling, and sometimes chaos. States in each of these regimes (stable, periodic, chaotic) are investigated for their ability to accurately describe the observed properties of the magnetosphere‐ionosphere system. A brief discussion of applications of this model to current space physics problems is included.
On the formation of the storm‐time ring current belt
Authors: Sun W. , Akasofu S.‐I.
Published: September 2002

It is shown that the formation of the ring current belt during geomagnetic storms is not a result of an enhanced convection which is directly driven by the solar wind and that substorm processes are crucial in populating the ring current belt by O+ ions. In order to demonstrate our result, it is shown that the development of the ring current belt is closely related to the upward field‐aligned currents associated with magnetospheric substorms. Three periods were chosen for this particular study, based primarily on the availability of the needed data: March 17–19, 1978, March 20–21, 1990, and November 3–5, 1993. The correlation coefficients obtained were 0.33 between the corrected Dst and the directly driven component, indicating that the formation of the ring current belt is not a result of an enhanced convection. On the other hand, the correlation coefficient between the corrected Dst and the unloading component is 0.81. Therefore it may be concluded that the unloading component, or, more specifically, the magnetosphere‐ionosphere coupling process associated with the unloading component of substorms, plays an important role in the development of the ring current belt. Our result is consistent with observations that O+ ions are often a dominant component in the storm‐time ring current belt [Daglis, 1997].
Disintegration of trans‐Alfvénic shocks due to variable viscosity and resistivity
Authors: Markovskii S. A. , Skorokhodov S. L.
Published: September 2002

The evolution of magnetohydrodynamic shock waves depends on dissipation properties of the medium. The increase or decrease of dissipation coefficients results in a larger or smaller thickness of fast and slow shocks. This is not the case for trans‐Alfvénic shock waves (TASWs), i.e., such at which the flow velocity passes through the Alfvén velocity. In the present paper, it is shown that the variation of dissipation coefficients may cause disintegration of TASWs or diffusion‐like change of their upstream and downstream states. This mechanism is complementary to the accumulation of an Alfvén‐type perturbation inside the shock transition layer. It is also demonstrated that if this variation has a cyclic nature, the TASW undergoes oscillatory disintegration. In this process, it repeatedly transforms into another TASW and emits a nonlinear wave train consisting of shock and rarefaction waves. The implications for planetary bow shocks and shocks induced by CMEs are discussed. In the observational data the disintegration may show up as an unusual position of the bow shock under typical conditions of the solar wind and as a multiple, time‐varying structure of the CME leading edge.
Merged interaction regions observed by Voyagers 1 and 2 during 1998
Authors: Burlaga L. F. , Ness N. F.
Published: September 2002

A broad region of relatively intense magnetic fields (a “merged interaction region” (MIR)) was observed by Voyager 2 (V2) from approximately calendar day (DOY) 264–313, 1998, at a distance of 56.1 ± 0.2 AU and latitude −23.4°. From the average speed of the material in the MIR at V2, 450 ± 19 km/s, we estimate that the radial extent of the MIR was ≈ 13 AU. The average magnetic field strengthBat V2 was (0.10 ± 0.03) nT. A MIR was observed by Voyager 1 (V1) from ≈DOY 257–302, 1998, when the heliocentric distance of V1 wasR= 71.6 ± 0.2 AU and its heliographic latitude was 33.4°. The average magnetic field strength in this region at V1 was (0.05 ± 0.01) nT. If the MIRs observed by V1 and V2 left the Sun at the same time, then the mean speed at V1 was ∼1.3 times the mean speed at V2, i.e., ∼600 km/s. The radial extent of the MIR at V1 was ∼16 AU. The distribution of the azimuthal angles of the magnetic fields observed by V2 was double peaked, with both toward and away polarities, consistent with the location of V2 within the sector zone. The distribution of the azimuthal angles observed by V1 was single peaked with a positive polarity, consistent with the location of V1 above the sector zone. These distinctly different distributions of magnetic field directions suggests that V1 and V2 observed two related MIRs, rather than two parts of a single “global merged interaction region”.
Determination of the abundance of aluminum in the solar wind with SOHO/CELIAS/MTOF
Authors: Bochsler Peter , Ipavich Fred M. , Paquette John A. , Weygand James M. , Wurz Peter
Published: September 2002

The Al/Mg abundance ratio provides an excellent test case for investigating possible fractionation processes among low First lonization Potential (FIP) elements in the solar wind. Al and Mg are refractory elements; their abundance ratio has been well determined in solar system materials and inferences for the abundance ratio in the solar atmosphere are reliable. Al and Mg are at neighboring masses and have similar charge state properties in the solar corona; hence mass fractionation effects in the solar wind acceleration process and instrumental mass fractionation are minimal. From first observations during two relatively short periods, one recorded in coronal hole associated solar wind, the other in typical interstream solar wind, it is concluded that the solar wind ratio in both regimes is consistent with the solar system ratio. The Al/Mg ratio in interstream solar wind is 0.081±0.012, and in the sample of coronal hole associated solar wind it amounts to 0.076±0.011. A comparison of these results with the solar system ratio of 0.079±0.005 gives no indication for fractionation occurring among low FIP elements in the solar wind.
Can Kelvin‐Helmholtz instabilities of jet‐like structures and plumes cause solar wind fluctuations at 1 AU?
Authors: Parhi S. , Suess S. T. , Sulkanen M.
Published: September 2002

The long high‐latitude sampling of Ulysses provides the opportunity to study fine structures. At latitudes poleward of ∼ −60° the solar wind had fluctuations in velocity gradients which were attributed to “microstreams.” The data also suggested fluctuations characterized by magnetic plus thermal pressure balance structures (‘PBS’). At higher frequencies, MHD turbulence was observed and found to be less evolved than it is in the ecliptic but essentially independent of heliographic latitude. It is argued here that microstreams, PBS, and MHD turbulence could all be the remnants of mixing due to shear instabilities associated with plumes and other filamentary structures (“jets”) in coronal holes. To show this, we simulate a plume‐like jet in the presence of an ambient magnetic field. We find that the presence of the ambient field reduces the growth rate of the instability, but the shear between a jet and its ambient still becomes unstable to the MHD Kelvin‐Helmholtz instability when the shear speed is larger than the largest local magnetosonic speed, a condition probably satisfied for plumes.
Magnetic structure of the reconnection layer and core field generation in plasmoids
Authors: Karimabadi H. , Krauss‐Varban D. , Omidi N. , Vu H. X.
Published: September 2002

Plasmoids/flux ropes have been observed both at Earth's magnetopause as well as in the magnetotail. Magnetic field measurements of such structures often reveal that rather than a minimum in field strength at their centers as expected from a simple O‐type neutral line picture, they exhibit a strong core field. To address this issue, two‐dimensional (2‐D) and 3‐D hybrid simulations are used to investigate the magnetic structure of reconnection layer in general and the formation of the core field within plasmoids in particular. The reconnection layer in the magnetotail is found to be unstable to the fire hose instability. As a result, the region between the lobe and the central plasma sheet is nearly at the marginal fire hose condition. The magnetic signatures of single and multiple X line geometries are contrasted, and it is shown that the interaction of outflowing jets from neighboring X lines leads in general to a highly complex magnetic structure within a plasmoid. The large observed core fields are explained in terms of Hall‐generated currents which can naturally lead to core field strengths that even exceed the ambient lobe field in magnitude. Ion beta and the presence of a preexisting guide field are two important factors controlling the Hall‐generated fields. In particular, it is shown that the presence of the small ubiquitous cross‐tail field component in the magnetotail can under certain conditions lead to a strong unipolar plasmoid core field. There exist significant differences between core fields associated with plasmoids at the magnetopause and those in the tail. This is due to (1) high plasma beta in the magnetosheath and (2) the asymmetry in plasma density across the magnetopause. The former leads to smaller core fields at the magnetopause, whereas the latter leads to differences in the polarity and structure of core fields within magnetopause and magnetotail plasmoids. Such differences are illustrated through examples.
Magnetospheric screening of cosmic dust
Authors: Juhász Antal , Horányi Mihály
Published: September 2002

We investigate the effect of the terrestrial magnetosphere on the dynamics of cosmic dust. We show that the magnetosphere acts as a shield with an efficiency that depends on the size and the velocity of the incoming dust particles. We report on numerical simulations for dust grains of interplanetary, cometary, and lunar origin. We show that magnetospheric effects reduce the flux of interplanetary and lunar dust particles with radii a < 0.1 μm, but the shielding is much less effective for cometary dust grains owing to their high average approach velocity.
Drift ballooning mode in a kinetic model of the near‐Earth plasma sheet
Authors: Pritchett P. L. , Coroniti F. V.
Published: September 2002

In the near‐Earth plasma sheet the pressure gradient and magnetic curvature are both directed earthward. This region is thus potentially unstable to the ballooning mode provided that the plasma β becomes large enough. In this work a three‐dimensional electromagnetic full‐particle simulation is used to study the evolution of the near‐Earth plasma sheet under the influence of an external convection electric field. As the plasma β increases to ∼10 during the convection process, the excitation of a drift ballooning mode is observed whose real frequency and growth rate agree well with fluid predictions including the effect of the ion diamagnetic drift. The maximum intensity in the ballooning spectrum corresponds to an azimuthal mode number m ∼ 60. The spectrum is observed to evolve toward shorter wavelengths, but this process stops at the ion gyroradius scale. There is no sign of an explosive nonlinear stage of the instability. The bulk flows produced by the mode are only moderate (≲ 200 km/s), and the instability is not observed to produce any topological change in the plasma sheet configuration.
Reexamination of the connections between interplanetary magnetic field and Pc3 geomagnetic pulsations
Authors: Verö József , Zieger Bertalan
Published: September 2002

Data used in an earlier paper [Verő and Holló, 1978] are here reexamined to clarify problems which emerged concerning the previously found relationship between the interplanetary magnetic field and Pc3 geomagnetic pulsations. We show that Pc3 pulsation spectra are strongly affected by wave amplification in the vicinity of the local field line resonance period. Possibilities are presented for the suppression of this effect. The influence of the variability of the interplanetary magnetic field is analyzed, and it is shown that this is a major factor determining momentary pulsation activity. The possibility of exciting field line resonances by impulses imbedded into noise of the interplanetary magnetic field is considered.
Spatial relations of mantle and plasma sheet
Authors: Siscoe George , Kaymaz Zerefsan
Published: September 2002

This paper presents the first purely data‐based cross‐sectional maps of the magnetotail and surrounding magnetosheath showing the relative locations of the magnetosheath, the lobes, the mantles, and the plasma sheet. The maps are based on newly digitized speed, density, and temperature data from the Los Alamos National Laboratory (LANL) plasma instrument on IMP 8. They cover the 2‐year interval for which ISEE 3 and LANL plasma data overlap. They refer to the usual interplanetary magnetic field (IMF) |By| > IMF |Bz| situation and are given in the standard IMF By > 0 format. They reveal that the plasma sheet and the mantels overlap on the north‐dawn and south‐dusk flanks. Magnetically, these overlap regions have open‐like properties. The maps suggest that the plasma sheet extends to polar latitudes along the south‐dawn and north‐dusk magnetotail flanks for IMF By > 0. These segments also contain the flank expressions of the cross‐tail S and the x‐type nulls associated with the magnetosphere's magnetic separator line, both found in prior studies.
Energetic electron beams and trapped electrons at Io
Authors: Williams D. J. , Thorne R. M. , Mauk B.
Published: September 2002

We present results from a continuing study of the magnetic field‐aligned energetic electron beams and accompanying trapped electron distributions discovered during the Galileo satellite's passage through Io's cold, dense, low‐speed wake on December 7, 1995 [Williams et al., 1996]. A companion paper by Thorne et al. [this issue] presents an analysis and explanation of the evolution of the electron pitch angle distributions measured on approach to Io and describes the resulting energetic electron flow paths and adiabatically “forbidden” regions expected to exist around Io. In Io's wake, only bidirectional, field‐aligned electron beams are seen; no ion beams are observed. At energies >∼15 keV the measured beams represent an energy flow of ∼0.03 erg cm−2 s−1, and if they penetrate Jupiter's atmosphere, they can provide an energy deposition of ∼15 erg cm−2 s−1 at the foot of the Io flux tube. This is sufficient to stimulate observable aurora in Jupiter's atmosphere. Extrapolating the measured spectra to lower energies yields much higher values (e.g., ∼104 ergs cm−2 s−1 at 0.25 keV). The angular width of the measured trapped‐like electron distributions is independent of energy and varies across Io's wake in a manner consistent with the measured magnetic field variation. We conclude that these electrons are trapped in Io's magnetic field configuration. The narrowness of the beams and the simultaneous existence of an apparently unaccelerated trapped electron population provide evidence that the source region for the beams is close to Jupiter. A deconvolution of the detector response to the beams gives a beam angular half width of ∼6°, placing the formation of the beams at an altitude of ∼0.6–0.7 RJ. The slight energy dependence of the beam width provides a rough upper limit estimate of <∼1.8 (10)8 cm−3 for Io's neutral SO2 atmosphere in the flyby region. No proposed acceleration mechanism operating close to Io (neither double layers nor Fermi acceleration via propagating Alfven waves) is able to reproduce measured beam characteristics.
Survey of pancake‐shaped warm ion distributions at geosynchronous orbit
Authors: Ober Daniel M. , Thomsen M. F. , Gary S. R. , Gallagher D. L. , McComas D. J.
Published: September 2002

It has been proposed that the electromagnetic proton cyclotron instability is a strong source of heating for the anisotropic warm ions observed at geosynchronous orbit. We present here the results of a statistical study of pancake‐shaped warm ion distributions using a 1‐year interval of data observed with the Los Alamos magnetospheric plasma analyzer on the geosynchronous satellite 1994–084. Our results support previous findings that pancake‐shaped warm ion distributions occur more frequently on the dayside of the magnetosphere and occur very close to the plasmapause. We also confirm that the electromagnetic proton cyclotron instability is operating and is constraining the hot proton temperature anisotropy. However, our results indicate that the pancake‐shaped warm ion distributions observed at geosynchronous orbit are probably not generated by this instability but must be due to a different mechanism, possibly heating by lower hybrid waves.
A study of the propagation of Alfvén waves in the auroral density cavities
Authors: Génot V. , Louarn P. , Le Quéau D.
Published: September 2002

From the Viking and, more recently, the FAST spacecraft observations, it is known that the auroral acceleration regions correspond to relatively small‐scale density cavities (a few kilometers to a few tens of kilometers in a direction perpendicular to the magnetic field). In order to study how Alfvén waves can contribute to the auroral acceleration, we analyze their propagation in the presence of the sharp density gradients that characterize the edges of these cavities. It is shown that an Alfvén wave packet entering into the cavity quickly develops short perpendicular length scales in the region of density inhomogeneity. Transverse scales of the order of c/ωpe are reached after a propagation of a few tenths of second, which corresponds to ∼ 2000 km in the cavity. This contributes to the creation of significant parallel electric field in the region of density gradient. Its amplitude is enhanced by the formation, on the gradients, of strong space charges due to the ion polarization drift. As the auroral cavities are known to be strong current regions, the density gradients would thus be the sites of particularly powerful wave/particle energy transfer and consequently, if the incident flux of Alfvén waves coming from the magnetosphere is high enough, of the strongest particle acceleration.
Direct comparison of transverse ion acceleration mechanisms in the auroral region at solar minimum
Authors: Lund E. J. , Möbius E. , Klumpar D. M. , Kistler L. M. , Popecki M. A. , Klecker B. , Ergun R. E. , McFadden J. P. , Carlson C. W. , Strangeway R. J.
Published: September 2002

Previous studies have shown that most ion conies in the auroral zone are associated with either broadband extremely low frequency (BBELF) or electromagnetic ion cyclotron (EMIC) emissions. EMIC emissions, which are correlated with preferential acceleration of He+, are concentrated in the premidnight sector; BBELF emissions, which are found at all local times, are not associated with preferential acceleration of any species. We present a case study of five auroral passes which include ion conies associated with EMIC and BBELF waves at different locations. Our data indicate that EMIC waves can be locally important for accelerating outflowing He+ in the aurora. However, although this study's design exaggerates the contribution of EMIC waves to transverse ion acceleration, we find in each case that BBELF waves produce higher energy fluxes of upflowing ions. Therefore, events with BBELF emissions dominate the overall ion outflow.
Ionospheric convection and equivalent ionospheric currents in the dayside high‐latitude winter ionosphere
Authors: Lyatsky W. B. , Kustov A. V. , Sofko G. J. , Jacobsen B. , André D. , Cogger L. L.
Published: September 2002

Ionospheric convection inferred from Super Dual Auroral Radar Network (SuperDARN) HF radar measurements is compared with an equivalent ionospheric convection derived from ground magnetometer data in the dayside winter high‐latitude ionosphere. Although there was general agreement between observed convection patterns produced by radars and magnetometers, there were significant differences in details. The orientation of equivalent convection vectors inferred from magnetic data was often opposite to the convection vectors determined by the SuperDARN radars in the poleward part of the convection vortex structure, though the agreement was reasonable in its equatorward part. The magnitudes of convection vectors determined from radar data and those inferred from magnetometer data were often different. The observed differences are attributed to strong horizontal inhomogeneity in the ionospheric conductivity distribution for winter conditions. It is possible that magnetic disturbances in the dark high‐latitude ionosphere are strongly affected by field‐aligned currents at the terminator that separates regions of the sunlit highly conducting ionosphere and dark poorly conducting ionosphere.
Eigenmode analysis of field line oscillations interacting with the ionosphere‐atmosphere‐solid earth electromagnetic coupled system
Authors: Yoshikawa Akimasa , Itonaga Masahiro , Fujita Shigeru , Nakata Hiroyuki , Yumoto Kiyohumi
Published: September 2002

In order to understand the effect of ionosphere on the localized toroidal magnetic shell oscillation (field line oscillation), we have constructed a rectangular box model for the magnetosphere‐ionosphere‐solid earth system including the anisotropically conducting ionosphere and performed the eigenmode analysis of the magnetohydrodynamic (MHD) waves in the model. We have found that the eigenmodes of field line oscillation are effectively controlled by the height‐integrated Pedersen and Hall conductivities, meaning that the field‐aligned current (FAC) in the magnetosphere is closed to both the Pedersen current and the Hall current. The divergent Hall current, that is, the Hall current, which closes the FAC, is a newfound current that is driven by the Hall effect of the ionospheric inductive (rotational) electric field. The divergent Hall current brings into sharp relief by considering the ionosphere to be inductive, and it inevitably requires the electromagnetic coupling between the magnetosphere, ionosphere, atmosphere, and solid earth. The nature of the ionospheric current associated with the standing shear Alfvén mode is also analyzed in detail by using the concept of wave reflection and mode conversion at the inductive ionosphere. In particular, we emphasize that the magnetic flux sustained by the rotational Hall current, that is, accompanied by the radiation of fast magnetosonic wave to the magnetosphere and atmospheric poloidal magnetic wave to the atmosphere, should be taken into account for magnetosphere‐ionosphere coupling of FAC.
Plasma sheet access to geosynchronous orbit
Authors: Korth H. , Thomsen M. F. , Borovsky J. E. , McComas D. J.
Published: September 2002

One year's worth of magnetospheric plasma analyzer data from three Los Alamos geosynchronous satellites are used for a statistical study of proton and electron fluxes at geosynchronous orbit and their dependence on local time (LT) and geomagnetic activity level as measured by Kp. When displayed as a function of LT and Kp, the fluxes exhibit distinct boundaries, which are shown to be consistent with a combination of a global pattern of particle drift through the magnetosphere and loss processes mainly due to charge exchange of the ions and auroral precipitation of the electrons. A Hamiltonian energy conservation approach combined with the (U, B, K) coordinate transformation introduced by Whipple [1978] is used to calculate the theoretical position of the separatrix between open and closed drift trajectories (Alfvén layer) as a function of particle species, energy, local time, and geomagnetic activity level. The comparison of the theoretical boundaries with the observations confirms the predictions of plasma sheet access to the geosynchronous region. The analysis also provides independent statistical support for previously derived relationships between Kp and the strength of the global convection electric field.
Whistler mode waves in the magnetotail
Authors: Zhang Y. , Matsumoto H. , Kojima H.
Published: September 2002

Whistler mode waves in the magnetotail, especially the plasma sheet and the plasma sheet boundary layer, are frequently detected by a waveform capture on board Geotail satellite. The whistler mode waves with frequencies above 10 Hz are often narrowband and short‐lived. Statistical study shows that these whistler mode waves propagate in a direction quasi‐parallel to the ambient magnetic field with an average wave normal angle of 23°. This suggests that the electron cyclotron resonance is dominant for excitation of the whistler mode waves. Their frequencies range mainly from 0.05 to ∼ 0.5Ωe, where Ωe is the electron cyclotron frequency. Their average frequency is 0.21Ωe. Amplitudes of the whistler mode waves cover a range from a few picoteslas to ∼100 pT. Their mean amplitude is 44 pT or 10−2 B0, where B0 is the magnitude of the ambient magnetic field. One interesting feature is that the whistler mode waves propagate mostly in a direction either parallel or antiparallel to B0. The whistler mode waves are very likely excited by energetic electron beams, which arise from reconnections in the near/deep magnetotail. Another significant feature of the whistler mode waves is that the correlation between their amplitudes and phase speeds. A higher phase speed usually associates with a lower amplitude. Whistler mode waves in the near magnetotail (−100 RE < x <−10 RE) and the deep magnetotail (−210 RE < x < −100 RE) are very much the same in many respects. However, one significant difference is that the average electron resonant energy is higher in the near magnetotail (11 keV) than that in the deep magnetotail (1.6 keV). Observations of whistler mode waves may provide a way to monitor energetic electrons and processes of reconnection in the magnetotail.
High‐resolution radar measurements of turbulent structure in the low‐latitude E region
Authors: Patra A. K. , Rao P. B.
Published: September 2002

VHF radar observations with high range and time resolutions of 150 m and 2 s were made to study the large‐scale plasma turbulence in the low‐latitude E region over Gadanki. The observations are presented in the form of Doppler spectra, height‐time‐intensity (HTI) maps and height‐time‐Doppler velocity plots. The daytime E region is found to be unstable mostly in the lower part (<100 km) over a narrow altitude range and manifesting as discrete scattering layer, in contrast to large‐scale turbulent structures observed over Jicamarca. During nighttime, however, while thin layered structures similar to that of daytime are dominant in the lower part of the E region, the upper region is found to be unstable over a wide altitude range showing significant large‐scale turbulent structures that are of wave type, and also occasionally plume type resembling that of spread F. The HTI maps show that the large scale structures have vertical wavelengths ranging 1 to 2 km and periods of few tens of seconds. The Doppler velocities associated with the plume type structures are upward and wave‐type peturbations are downward with values mostly in the range of 50–100 m s−1 which are considerably lower to that observed at Jicamarca. The velocity spectral widths ranging 100–150 m s−1 are also found to be significantly lower compared to that reported for Jicamarca. Notwithstanding the differences noted above, the observations presented here seem to conform broadly to that reported for Jicamarca and also to that generated through the nonlocal nonlinear numerical simulation of the gradient drift instability.
Three‐dimensional plasma simulation of Io's interaction with the Io plasma torus: Asymmetric plasma flow
Authors: Saur Joachim , Neubauer Fritz M. , Strobel Darrell F. , Summers Michael E.
Published: September 2002

A three‐dimensional, stationary, two‐fluid plasma model for electrons and one ion species was developed to understand the local interaction of Io's atmosphere with the Io plasma torus and the formation of Io's ionosphere. Our model calculates, self‐consistently, the plasma density, the velocity and the temperatures of the ions and electrons, and the electric field for a given neutral atmosphere and imposed Io plasma torus conditions but assumes for the magnetic field the constant homogeneous Jovian field. With only photoionization in a pure SO2 atmosphere it is impossible to correctly model the plasma measurements by the Galileo spacecraft. With collisional ionization and photoionization the observations can be successfully modeled when the neutral atmospheric column density is Ncol = 6 × 1020 m−2 and the atmospheric scale height is H = 100 km. The energy reservoir of the Io plasma torus provides via electron heat conduction the necessary thermal energy for the maintenance of the collisional ionization process and thus the formation of Io's ionosphere. Anisotropic conductivity is shown numerically as well as analytically to be essential to understand the convection patterns and current systems across Io. The electric field is very greatly reduced, because the ionospheric conductances far exceed the Alfvén conductance ΣA, and also strongly twisted owing to the Hall effect. We find that the electric field is twisted by an analytic angle tan Θtwist = Σ2/(Σ1 + 2ΣA) from the anti‐Jupiter direction toward the direction of corotation for constant values of the Pedersen and Hall conductances Σ1 and Σ2 within a circle encompassing Io's ionosphere. Because the electron velocity is approximately equal to the E × B drift velocity, the electron flow trajectories are twisted by the same angle toward Jupiter, with E and B the electric and magnetic fields, respectively. Since Σ1 ∼ Σ2, the electron flow is strongly asymmetric during convection across Io, and the magnitude of this effect is directly due to the Hall conductivity. In contrast, the ions are diverted slightly away from Jupiter when passing Io. Large electric currents flow in Io's ionosphere owing to these substantially different flow patterns for electrons and ions, and our calculations predict that a total electric current of 5 million A was carried in each Alfvén wing during the Galileo flyby. We also find a total Joule heating rate dissipated in Io's ionosphere of P = 4.2 × 1011 W.
The Fe/O elemental abundance ratio in the solar wind as observed with SOHO CELIAS CTOF
Authors: Aellig M. R. , Hefti S. , Grünwaldt H. , Bochsler P. , Wurz P. , Ipavich F. M. , Hovestadt D.
Published: September 2002

Using data of the Charge Time‐of‐Flight (CTOF) mass spectrometer of the Charge, Element, and Isotope Analysis System (CELIAS) on board the Solar and Heliospheric Observatory (SOHO) from ∼80 days of observation around solar minimum we derive a value for the Fe/O abundance ratio for the inecliptic solar wind of 0.11 ± 0.03. Since Fe has a low first ionization potential (FIP) and O is a high‐FIP element, their relative abundance is diagnostic for the FIP fractionation process. The unprecedented time resolution of the CELIAS CTOF sensor allows a fine‐scaled study of the Fe/O ratio as a function of the solar wind bulk speed. On average, the Fe/O abundance ratio shows a continuous decrease by a factor of 2 with increasing solar wind speed between 350 and 500 km/s. This corresponds to the well‐known FIP effect dependence. Our value at ∼500 km/s agrees with the previously observed Fe/O ratio in the fast solar wind emerging from polar coronal holes whereas the value for speeds below 350 km/s is consistent with a remote abundance determination in the leg of a coronal streamer. The variability of the Fe/O abundance ratio is much larger in the slow than in the fast solar wind.
Upper Atmosphere Research Satellite observation of a Pi2 pulsation
Authors: Takahashi Kazue , Anderson Brian J. , Yumoto Kiyohumi
Published: September 2002

Satellites in low‐altitude orbits provide unique opportunities to study the ionospheric screening effects on ULF waves propagating from the magnetosphere. This paper presents the first report of observation of a Pi2 pulsation from the Upper Atmosphere Research Satellite (UARS) at an altitude of ∼580 km. The Pi2 pulsation occurred on November 18, 1993, when the satellite was over the Pacific near the three ground magnetometer sites, Chichijima (L = 1.14, MLT = 23), Ewa Beach (L = 1.17, MLT = 03), and Guam (L = 1.07, MLT = 23). On the ground the Pi2 was visible in the H (northward) component from 1337 to 1343 UT at all locations with the same period of 43 s but with an amplitude ranging from 2.1 nT (Guam) to 0.6 nT (Ewa Beach). At the satellite the same pulsation was detected in the H component with an amplitude of 1.7 nT. The transverse radial component and the parallel component at the satellite oscillated in phase, consistent with previous Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer observations at L > 2. When the ground signals are linearly interpolated to the geographic longitude and latitude of UARS, the orientation of the major axis of polarization agreed well with that observed by UARS, as is expected for a fast‐mode wave propagating through the ionosphere.
An evaluation of perpendicular diffusion models regarding cosmic ray modulation on the basis of a hydromagnetic description for solar wind turbulence
Authors: le Roux J. A. , Zank G. P. , Ptuskin V. S.
Published: September 2002

The theory for the perpendicular diffusion of cosmic rays is not well understood which hampers our understanding of cosmic ray modulation. In this paper, a spherically symmetric cosmic ray modulation model is used to evaluate three different theories for perpendicular cosmic ray diffusion in the ecliptic plane, subject to the limitation of negligible cosmic ray transport in the polar direction. In models for the perpendicular diffusion component, it is assumed that perpendicular diffusion due to large‐scale field line wandering dominates resonant perpendicular diffusion. To test these models, observations of anomalous and galactic helium obtained during a time of relatively small latitudinal gradients (1996) are used as a guideline. The spatial dependence of the parallel and perpendicular cosmic ray diffusion coefficients is determined completely theoretically using a promising hydromagnetic model for the transport of combined slab plus two‐dimensional turbulence in the solar wind [Zank et al., 1996]. The main result of the paper is that the nonperturbative model [Zank et al., 1998] yields the best results. Its success is determined by the following: (1) a perpendicular correlation length derived from two‐dimensional solar wind turbulence that is much larger than the parallel correlation length associated with slab turbulence; (2) a modest radial dependence of the radial diffusion coefficient in the outer heliosphere; and (3) a three interval rigidity dependence of the radial diffusion coefficient in the outer heliosphere with the smallest rigidity dependence in the middle interval and a strong rigidity squared dependence in the other two intervals. The nonperturbative model gives a tentative theoretical basis for the cosmic ray modulation simulations by Moraal et al. [1999], who found empirically that a similar spatial and three stage rigidity dependence for the radial diffusion coefficient is important for reproducing observed anomalous and galactic cosmic ray spectra.
Long‐period hydromagnetic waves at very high geomagnetic latitudes
Authors: Lanzerotti L. J. , Shono A. , Fukunishi H. , Maclennan C. G.
Published: September 2002

Low‐frequency (∼ 5 mHz) geomagnetic fluctuations are investigated at Antarctic ground‐based locations ranging from ∼ 71° to ∼ 80° geomagnetic latitude, and in the northern conjugate region. We show that a narrow band spectral feature at about this frequency often appears in the data, especially at the lower latitudes. We conclude that the demarcation in latitude between the appearance or not of the spectral tone indicates the location of the magnetopause, and that the ∼ 5 mHz frequency is the frequency of standing Alfvén waves on the last closed magnetosphere field lines.
Recovery signatures and occurrence properties of lightning‐associated subionospheric VLF perturbations
Authors: Sampath H. T. , Inan U. S. , Johnson M. P.
Published: September 2002

Subionospheric VLF signatures of lightning‐associated ionospheric disturbances observed at multiple sites in the central United States are used to investigate the occurrence properties and recovery signatures of characteristic perturbations exhibiting rapid onset followed by slow recoveries. The two different types of events, so‐called early/fast and lightning‐induced electron precipitation (LEP) [Inan et al., 1996a], are distinguished by the lack of presence of a few hundred millisecond delay between VLF event onsets and causative lightning discharges, respectively. Analysis of recovery signatures of the two types of events indicate subtle but distinct differences in the recovery rates. A majority of early/fast events were found to exhibit a more rapid initial recovery to preevent levels during the first 20 s of recovery, when compared to LEP events. This experimental evidence indicates that the physical nature of the ionospheric disturbance involved in the two classes of events are different, consistent with a recent theoretical suggestion [Inan et al., 1996c]. The occurrence properties of early/fast events observed by the Holographic Array for Ionospheric Lighting (HAIL) receivers indicate that ionospheric disturbances act primarily as forward scatterers.
Microstructure of Jovian decametric S bursts
Authors: Carr Thomas D. , Reyes Francisco
Published: September 2002

We have developed a method for displaying the spectral structure of Jupiter's decametric radio S bursts on timescales down to a few microseconds, 2 orders of magnitude finer than has been achieved elsewhere. By employing an extremely sensitive antenna (640 dipoles, at 26 MHz) and selecting only relatively weak S bursts that possess the simplest possible spectral shape, we identify frequently occurring structural features that we associate with localized emission centers. On timescales having better than about 30 μs resolution we find that the S burst baseband oscillation (and therefore the RF oscillation) is modulated to form distinct pulses, which we refer to as subpulses. Still finer time resolution reveals that within individual subpulses the baseband (and RF) oscillation often displays segments in which the usually drifting phase term abruptly becomes essentially constant and, after remaining so for 10 to 100 μs, abruptly resumes its random drift. It is these abruptly starting and ending segments of phase coherence that we attribute to isolated powerful centers of cyclotron maser wave amplification, which happen for brief intervals to be the only ones that are active. We believe that the more usual phase‐incoherent condition (i.e., one in which the instantaneous frequency drifts randomly within the emission band) is due to the fact that the resultant radiation is the sum of two or more components from neighboring wave amplification centers emitting at slightly different frequencies, with independently varying intensities. Possible models for the production of subpulses and phase coherent intervals are discussed.
Foreword [to Special Section on Interstellar Dust and the Heliosphere]
Authors: Frisch Priscilla C.
Published: September 2002

Dust grains in space provide intriguing tracers of fundamental processes governing the origin and evolution of matter in our galaxy. Dust grains are detected in a number of space environments. Interstellar dust grains (ISDG) are seen in diffuse and molecular interstellar clouds. They have now also been sampled directly by spacecraft within the solar system.
Evidence for Langmuir envelope solitons in solar type III burst source regions
Authors: Thejappa G. , Goldstein M. L. , MacDowall R. J. , Papadopoulos K. , Stone R. G.
Published: September 2002

We present, observational evidence for the generation of Langmuir envelope solitons in the source regions of solar type III radio bursts. The solitons appear to be formed by electron beams which excite either the modulational instability or oscillating two‐stream instability (OTSI). Millisecond data from the Ulysses Unified Radio and Plasma Wave Experiment (URAP) show that Langmuir waves associated with type III bursts occur as broad intense peaks with timescales ranging from 15 to 90 ms (6 – 27 km). These broad field structures have the properties expected of Langmuir envelope solitons, namely, the normalized peak energy densities, WL/neTe ∼ 10−5, are well above the modulational instability threshold; the spatial scales L, which range from 1 to 5 Langmuir wavelengths, show a high degree of inverse correlation with (WL/neTe)1/2; and the observed widths of these broad peaks agree well with the predicted widths of envelope solitons. We show that the orientation of the Langmuir field structures is random with respect to the ambient magnetic field, indicating that they are probably isotropic structures that have evolved from initially pancake‐like solitons. These observations suggest that strong turbulence processes, such as the modulational instability or the OTSI, stabilize the electron beams that produce type III bursts.
Remarks on the intrinsic timescale for reconnection on the dayside magnetopause
Authors: Uberoi C. , Lanzerotti L. J. , Maclennan C. G.
Published: September 2002

We show that with reasonable values of anomalous resistivity, surface‐wave‐induced magnetic reconnection at an interface between two plasma regions (such as the magnetosheath and the magnetopause) has an intrinsic timescale that can explain observations of a delay time between the southward turning of the interplanetary magnetic field and the onset of a flow transfer event at the magnetopause.
Long‐term negative trend in cosmic ray flux
Authors: Stozhkov Yuri I. , Pokrevsky Peter E. , Okhlopkov Victor P.
Published: September 2002

The cosmic ray fluxes in four consecutive solar activity minima (1964–1965, 1976–1977, 1987, and 1996–1997) are considered. The data obtained in long‐term stratospheric measurements and at ground level (neutron monitor and ionization chamber data) are used. The long‐term negative trend is derived from these experimental data. The value of the effect is δ≈−(0.01–0.09)% per year. The data on cosmogenic radioactive isotopes of 10Be and 14C which are produced by cosmic rays in the atmosphere also show the gradual decrease of their concentrations on the timescale of more than ∼104 years. The stratospheric measurements also propose that the cosmic ray spectrum becomes softer in the energy range E=0.1−1.5 GeV with the passage of time. The consideration of solar and interplanetary parameter changes, which could be responsible for the observed negative trend in cosmic ray flux, does not show any increase. The effect could be explained if supernova explosion had taken place in the nearby interstellar space. Such an explosion could occur about 104−5×l05 years ago at the distance 30–150 pc from the solar system.
Ground magnetic response to sudden changes in the interplanetary magnetic field orientation
Authors: Sitar R. J. , Clauer C. R.
Published: September 2002

Recent observations have shown that a discontinuity in the interplanetary magnetic field (IMF) can produce a discernible ground magnetic signature at cusp latitudes, whose characteristics closely resemble those of ionospheric traveling convection vortices. It has been suggested that a hot flow anomaly (HFA), produced by the interaction of certain tangential discontinuities with the bow shock, is the mechanism through which an IMF discontinuity can produce a ground magnetic response. Using solar wind plasma and IMF data from the IMP 8 satellite from 1992–1995, we have identified and categorized intervals during which IMF discontinuities are observed. Ground magnetometer observations from the Greenland coastal chains and the MAGIC array have been used to characterize any resulting ground response. The occurrence of ground signatures appears to be weakly related to the conditions necessary to produce HFAs.