bluuebunny/crossref_metadata_2025 huggingface dataset:
Dawn and dusk sector comparisons of small‐scale irregularities, convection, and particle precipitation in the high‐latitude ionosphere
Authors: Greenwald Raymond A. , Shepherd Simon G. , Sotirelis Thomas S. , Ruohoniemi J. Michael , Barnes Robin J.
Published: October 2002

Small‐scale ionospheric irregularities and auroral precipitation are common features of the auroral ionosphere, but their spatial association has not been examined on global scales. In this paper, we compare electron and ion precipitation from individual passes of the Defense Meteorological Satellite Program (DMSP) spacecraft with concurrent observations of ionospheric irregularities and plasma convection from the Northern Hemisphere component of the Super Dual Auroral Radar Network (SuperDARN). Because of the nature of the DMSP orbits and the spatial resolution of the SuperDARN measurements these comparisons have been limited to the dusk and postdawn sectors and spatial dimensions greater than ∼100 km. We have found that the SuperDARN radars generally observe ionospheric irregularities over a greater latitudinal extent than the DMSP satellites observe particle precipitation. Specifically, ionospheric irregularities are observed both equatorward and poleward of the convection reversal boundary (CRB) in the dawn and dusk sectors, whereas particle precipitation is only observed equatorward of the CRB. Under conditions where the radars can detect the true equatorward boundary of the irregularities, they are observed to extend equatorward of the particle precipitation. Both irregularities and particle precipitation expand equatorward with increasing geomagnetic activity, and there is evidence that precipitation regions with higher energy flux are associated with regions of stronger sunward convection. These results suggest that SuperDARN can provide a coarse determination of the auroral‐oval position that will complement measurements with optics and particle detectors. More importantly, they demonstrate the spatial relationships between precipitation, electric fields, and ionospheric irregularities that result from the electrical coupling between the magnetosphere and ionosphere and the dependence of ionospheric plasma instabilities on the ionospheric electric field and precipitation‐induced electron‐density gradients.
Wave‐particle interactions associated with nongyrotropic distribution functions: A hybrid simulation study
Authors: Convery P. D. , Schriver D. , Ashour‐Abdalla M. , Richard R. L.
Published: October 2002

Nongyrotropic plasma distribution functions can be formed in regions of space where guiding center motion breaks down as a result of strongly curved and weak ambient magnetic fields. Such are the conditions near the current sheet in the Earth's middle and distant magnetotail, where observations of nongyrotropic ion distributions have been made [Frank et al., 1994]. Here a systematic parameter study of nongyrotropic proton distributions using electromagnetic hybrid simulations is made. We model the observed nongyrotropic distributions by removing a number of arc length segments from a cold ring distribution and find significant differences with the results of simulations that initially have a gyrotropic ring distribution. Model nongyrotropic distributions with initially small perpendicular thermalization produce growing fluctuations that diffuse the ions into a stable Maxwellian‐like distribution within a few proton gyro periods. The growing waves produced by nongyrotropic distributions are similar to the electromagnetic proton cyclotron waves produced by a gyrotropic proton ring distribution in that they propagate parallel to the background magnetic field and occur at frequencies on the order of the proton gyrofrequency. The maximum energy of the fluctuating magnetic field increases as the initial proton distribution is made more nongyrotropic, that is, more highly bunched in perpendicular velocity space. This increase can be as much as twice the energy produced in the gyrotropic case.
Suppression of equatorial spread F by sporadic E
Authors: Stephan Andrew W. , Colerico Marlene , Mendillo Michael , Reinisch Bodo W. , Anderson David
Published: October 2002

We have examined quantitatively the influence a low‐latitude, premidnight sporadic E layer might have on the daily and hourly development of equatorial spread F (ESF). In particular, we calculated changes in the flux tube–integrated Pedersen conductivity as it affects the growth rate of the Rayleigh‐Taylor instability, which governs the initial development of ESF. We find that the growth rate is lowered by an order of magnitude with a density of 1 × 106 cm−3 in a slab from 115 to 120 km. Since sporadic E layers observed after dusk do not regularly reach these values, they are not a likely source of the daily variability in ESF. However, even a mild enhancement in the postsunset E region could lead to a significant suppression of ESF if it also inhibits the upward plasma drift of the prereversal enhancement, a key variable in the growth rate of the equatorial spread F instability. Thus, consistent with the nature of an instability, the second‐order effect (suppressed upward drift) is more important than the first‐order cause (reduced F region to E region conductivity) of inhibited ESF onset.
Transport of photoelectrons in the nightside magnetosphere
Authors: Khazanov G. V. , Liemohn M. W.
Published: October 2002

Kinetic modeling results are analyzed to examine the transport of photoelectrons through the nightside inner magnetosphere. Two sources are considered, those on the dayside from direct solar illumination and those across the nightside from light scattered by the upper atmosphere and geocorona. A natural filter exists on the nightside for the dayside photoelectrons. Coulomb collisions erode the distribution at low energies and low L shells, and magnetospheric convection compresses the electrons as they drift toward dawn. It is shown that for low‐activity levels a band of photoelectrons forms between L = 4 and 6 that extends throughout the nightside local times and into the morning sector. For the scattered light photoelectrons the trapped zone throughout the nightside is populated with electrons of E < 30 eV. At high L shells near dawn, convective compression on the nightside yields an accelerated population with electrons at energies up to twice the ionospheric energy maximum (that is, roughly 1200 eV for dayside photoelectrons and 60 eV for scattered light electrons). Modeled energy and pitch angle distributions are presented to show the features of these populations.
Low‐frequency intraseasonal variations of the wintertime very high latitude mesopause regions
Authors: Sivjee G. G. , Walterscheid R. L.
Published: October 2002

We have performed a spectral analysis of three fall and winter airglow data sets each for South Pole Station and for Eureka, Canada (80°N). These analyses show significant spectral features in the mesopause region at very high latitudes that are similar in period to low‐frequency intraseasonal (IS) variations seen in analysis of length of day, atmospheric angular momentum variations, and outgoing long wave radiation, as well as in height or wind fields up to 10 hPa (∼32 km). These are fairly broadband spectral features centered approximately at 17, 23, and 45 days. The 17‐day oscillation we find is more prominent than that found in the data sets reflecting variations mainly in the troposphere and stratosphere. This oscillation is most likely associated with the second symmetric s = 1 free Rossby mode. The data from South Pole Station should not reflect this mode, as such, because the spectral components that produce airglow variations should vanish at the pole unless they are zonally symmetric; however, free oscillations having zonal wave number zero with such long periods should not exist. We suggest that, at least at South Pole Station, the 17‐day oscillation is driven by interactions between the global Rossby mode and stationary wave number one planetary waves. In addition to the low‐frequency oscillations that have been reported in the IS literature, we see a wave in the 12–14 day period range. At South Pole Station that wave may also reflect forcing by a global free mode.
Spectral characteristics of UHF radar aurora
Authors: Jackel B. J. , Moorcroft D. R. , Foster J. C. , Schlegel K.
Published: October 2002

Radar observations at 440 (Millstone Hill) and 933 MHz (European Incoherent Scatter (EISCAT)) and at both small and large magnetic aspect angle have been used to study the spectral characteristics of backscatter from the auroral E region. The predominantly single‐peaked power spectra were found to have very similar shapes at both frequencies and at both small and large aspect angle. Spectral moments were estimated directly from the autocorrelation functions (ACFs) and were also characterized in terms of a correlation time (related to spectral width) and a decay exponent which quite accurately described the observations. For the most part, spectral shapes were found to be intermediate between Gaussian and Lorentzian, at both large and small magnetic aspect angles. Similar ACF shapes have been observed at much larger wavelengths at HF [Villain et al., 1996], suggesting that this may be a widespread and possibly fundamental characteristic of radar auroral backscatter. In addition to confirming and extending a number of previous observations on UHF spectral characteristics, correlation times were found to decrease sharply near an aspect angle of 3°, having a nearly constant value at all larger aspect angles. This behavior is consistent with a simplified picture of the energy balance for these large aspect angle waves.
Geotail observations of low‐frequency waves and high‐speed earthward flows during substorm onsets in the near magnetotail from 10 to 13 RE
Authors: Sigsbee K. , Cattell C. A. , Fairfield D. , Tsuruda K. , Kokubun S.
Published: October 2002

Geotail electric and magnetic field data from five substorms were used to examine the relationship between low‐frequency waves and high‐speed earthward flows at radial distances between 10 and 13 RE. Strong compressional fluctuations of the magnetic field in the Pi2 frequency range (0.007–0.03 Hz) were observed during the 26 April 1995 substorm and the other four substorms studied, in association with high‐speed earthward flows and magnetic field dipolarizations. However, the maximum earthward flow was generally observed 30 s to a few minutes after the start of the dipolarizations and magnetic field fluctuations in the Pi2 frequency range. Waves near the ion gyrofrequency (∼0.1–1.0 Hz) and the lower hybrid frequency (∼5–16 Hz) were also observed during all five substorms. The maximum amplitudes at these frequencies were not observed until after the start of the magnetic field dipolarization and earthward flow, which does not appear to be consistent with local substorm initiation by a cross‐field current driven instability. Recent work has shown the importance of high‐speed earthward flow bursts as drivers of substorm activity. However, we found that the earthward kinetic energy flux was much smaller than the Poynting flux or the thermal energy. This is consistent with the idea that currents driven by thermal pressure gradients and magnetic field changes are responsible for a major part of the substorm current wedge.
Effects of molecular ions on the collisional Rayleigh‐Taylor instability: Nonlinear evolution
Authors: Sekar R. , Kherani E. A.
Published: October 2002

The nonlinear evolution of collisional Rayleigh‐Taylor (CRT) instability in the F region of ionosphere is studied for two ions‐electron (O+, NO+, and e−) plasma. The enhancements in NO+ are found inside the O+ depletions. During linear phase of instability, NO+ concentration is mainly controlled by chemical process while during nonlinear phase, it is controlled by CRT induced transport process. In spite the dominant chemical loss during initial phase, significant NO+ ions could survive owing to their shielding by O+ depletions. The seeding of long wave perturbation, the O+ scale height below the base and prereversal enhancement play important role in supplying significant NO+ ions onto the topside which could possibly match the observations.
The life of a CME and the development of a MIR: From the Sun to 58 AU
Authors: Richardson J. D. , Paularena K. I. , Wang C. , Burlaga L. F.
Published: October 2002

Large coronal mass ejections (CMEs) have major effects on the structure of the solar wind in the heliosphere. The plasma and magnetic field can be compressed ahead of CMEs to form merged interaction regions (MIRs). One signature of some CMEs that can be used to track them through the heliosphere is an enhanced alpha abundance. We use this signature and a one‐dimensional MHD model to track a CME from an active region on the Sun to Wind (1 AU), then to Ulysses (5.3 AU) and finally to Voyager 2 (58 AU). The arrival times of the CME ejecta at the three spacecraft are identified primarily by the increased alpha abundance. The observed plasma profiles and timing are close to those predicted by propagating data outward (from Earth to Ulysses and from Ulysses to Voyager 2) using the MHD model and give us confidence we are observing the same events at all three spacecraft. This event and a subsequent CME bracket a developing MIR and thus allow us to track the development of this structure.
A self‐consistent model of the interacting ring current ions and electromagnetic ion cyclotron waves, initial results: Waves and precipitating fluxes
Authors: Khazanov G. V. , Gamayunov K. V. , Jordanova V. K. , Krivorutsky E. N.
Published: October 2002

Initial results from a newly developed model of the interacting ring current ions and ion cyclotron waves are presented. The model is based on the system of two kinetic equations: one equation describes the ring current ion dynamics, and another equation describes wave evolution. The system gives a self‐consistent description of the ring current ions and ion cyclotron waves in a quasilinear approach. These equations for the ion phase space distribution function and for the wave power spectral density were solved on a global magnetospheric scale under nonsteady state conditions during the 2–5 May 1998 storm. The structure and dynamics of the ring current proton precipitating flux regions and the ion cyclotron wave‐active zones during extreme geomagnetic disturbances on 4 May 1998 are presented and discussed in detail.
Saturn's E ring: A dynamical approach
Authors: Juhász Antal , Horányi Mihály
Published: October 2002

Dust particles in Saturn's faint E ring are expected to exhibit complex single‐particle dynamics. The optical depth in this region is very low; hence collective effects (dust‐dust collisions, waves, and self‐gravity) are negligible. There are four embedded moons in this ring that can serve as sources (ejecta generation via bombardment) and sinks (dust‐moon collision) of dust. Grains exposed to magnetospheric plasmas swiftly erode owing to sputtering, and because of their electrostatic charging, their motion is determined by electrodynamic forces, in addition to radiation pressure, drag, and gravity. As grains erode, their charge‐to‐mass ratio is increasing; hence the importance of electromagnetic forces steadily grows through the lifetime of a grain. The particles and fields environment plays a fundamental role in shaping the E ring. Here we present a detailed dynamical model of the motion of charged dust particles. We combine a large number of single‐particle orbits as a function of six free parameters (the radii of smallest and largest grains escaping any of the moons, the relative dust flux from each of the moons, and the exponent of the assumed power law size distribution of the grains generated in impacts onto the moons). Using a Monte Carlo approach, we determine the best combination of these parameters to match the ground‐based observations. The fit provides an excellent agreement between our computer model and several other ground‐based and spaceborne observations. The model will be tested in situ by the Cassini Cosmic Dust Analyzer instrument, due to arrive at Saturn in 2004.
A detailed study of equatorial electrojet phenomenon using Ørsted satellite observations
Authors: Jadhav Geeta , Rajaram Mita , Rajaram R.
Published: October 2002

Detailed analysis of the scalar magnetic field data from Ørsted satellite for quiet days from April 1999 to March 2000 has been undertaken to study the equatorial electrojet (EEJ) phenomenon. An objective technique has been adopted for the identification of the EEJ from the satellite data and estimation of the standard parameters associated with it. EEJ strength computed using the satellite data and simultaneous ground magnetic observatory data, for the Indian and American sectors, correlate very well authenticating the method used. Estimated zonal variation in the EEJ parameters such as peak current intensity (J0), and total current (I+) are broadly consistent with the earlier observations. We, however, observe that the width of the EEJ varies considerably with longitude, a feature not seen in the Pogo data. The study shows that the EEJ axis (center of EEJ) closely follows the dip equator at altitude of 106 km, but there is a small departure that undergoes diurnal variation, with a minimum at noon. The globally averaged EEJ amplitude follows the expected diurnal pattern. Principal component analysis technique reveals that first four components can explain around two thirds of the electrojet variability. The first component, which contributes a little over 30% to the observed variance, could be identified with the global variation of the EEJ emanating from the day‐to‐day variability of the migrating tides. The second and fourth components, which account for around 15 and 10% of the variance, respectively, are driven by forcing that depends on whether the location of the EEJ in that sector is in the Northern or Southern Hemisphere. The third component provides maximum contributions wherever the geomagnetic and dip equators are sufficiently close, accounting for 12.5% of the variance. The remaining components could be associated with contribution of nonmigratory tides or other unknown mechanisms. Thus the present study suggests that besides conductivity, atmospheric tidal modes play important role in defining the zonal variability of the EEJ current system.
HF radar observation of field‐aligned currents associated with quiet time transient flow bursts in the magnetosphere
Authors: Munsami V. , Pinnock M. , Rodger A. S.
Published: October 2002

The present study reveals the ionospheric signature of low level near‐Earth flow burst activity in the central plasma sheet during a relatively quiet magnetosphere. Flow bursts were observed by Geotail in the near‐Earth plasma sheet at (X = −17 RE, Y = −6 RE)GSM, between 0300 and 0400 UT on 9 March 1997. Simultaneous observations of the solar wind conditions by the Wind satellite located 235 RE upstream, and of the southern auroral ionosphere, by the Sanae‐Halley Super Dual Auroral Radar Network (SuperDARN) HF radar pair, offered a unique opportunity to investigate both the solar wind trigger and the related auroral ionospheric signature of these flow bursts. Although the prevailing solar wind conditions conformed to that expected for a quiescent magnetosphere, the solar wind dynamic pressure showed stepwise decreases, which gave rise to a near instantaneous observation of tailward flow bursts. Also, peak values in the local potential variation associated with the ionospheric convection during the quiet solar wind conditions exceeded 10 kV and therefore are not associated with viscous processes. We believe that the drop in solar wind pressure may have allowed the magnetosphere to relax into a lower metastable energy configuration. The associated reconfiguration of the magnetotail allows for the release or redistribution of stored magnetic energy, via magnetic reconnection, with the consequent observation of tailward flow bursts. The observations of earthward flow bursts, however, are thought to be a natural bimodal response of the magnetosphere to solar wind variations. Concurrent with these flow bursts in the magnetotail, we observed the development of convection vortices near the conjugate midnight auroral ionosphere, which are consistent with the flow of field‐aligned currents. The spatial structure and the temporal locale of the convection vortices are suggestive of a small substorm current wedge.
Rigidity dependence of the 11‐year variation of the cosmic ray intensity from 0.6 to 50 GV at the Earth in two 22‐year cycles
Authors: Lockwood J. A. , Debrunner H.
Published: October 2002

We have determined the time variation of the rigidity dependence of the galactic cosmic ray modulation from 0.6 to 50 GV at the Earth during two complete 22‐year solar cycles from 1960 to 2001. During three of the four cosmic ray intensity decrease periods the modulation can be described by the modulation function M = exp (KP−δ) with δ = 1.1 ± 0.1 and K varying to account for the magnitude of the modulation. The intensity decrease period from 1987 to 1990 was different with δ = 0.8 ± 0.1. The same modulation function M = exp (KP−δ), with δ = 1.15 ± 0.10, also fitted the four intensity recovery phases of the two 22‐year modulation cycles. Thus there was no change in the rigidity dependence of the 22‐year cosmic ray modulation with the change in solar magnetic field polarity.
Wind and ACE observations during the great flow of 1–4 May 1998: Relation to solar activity and implications for the magnetosphere
Authors: Farrugia C. J. , Popecki M. , Möbius E. , Jordanova V. K. , Desai M. I. , Fitzenreiter R. J. , Ogilvie K. W. , Matsui H. , Lepri S. , Zurbuchen T. , Mason G. M. , Lawrence G. R. , Burlaga L. F. , Lepping R. P. , Dwyer J. R. , McComas D.
Published: October 2002

Using magnetic field, plasma, and energetic particle data from Wind and ACE, we analyze interplanetary features associated with the first strongly geoeffective interval in the rising phase of solar cycle 23, which affected Earth on 1–4 May 1998. As shown by Skoug et al. [1999], the configuration consisted of a compound stream made up of an interplanetary coronal mass ejection (ICME) containing a magnetic cloud and being trailed by a hot, faster flow. In addition, we find that the front boundary of the ICME is a rotational discontinuity and the leading edge of the fast stream has a zero normal magnetic field component and is followed by a magnetic field which is strongly enhanced (by a factor of ∼4) and whose fluctuations lie in a plane approximately parallel to the leading edge. Energetic particle and composition observations confirm that the field lines of the magnetic cloud were connected to at least two different flare sites in the same active region. We infer a lower limit for the size of the solar footprint of the connected flux tube of 0.02 Rs2, i.e., ∼1010 km2. A dramatic weakening of the halo electron distribution occurred during 3 May at the time when other experimenters have documented the presence of prominence material. We hypothesize that the solar wind halo population was scattered by enhanced frequency of Coulomb collisions in the dense and very cold plasma. We discuss our energetic particle observations in terms of local acceleration at interplanetary shocks and field discontinuities as well as in terms of acceleration in flares and CME‐driven shocks. We also compare, in a specific formulation, the power and energy input of the May 1998 configuration to the magnetosphere with other much studied geoeffective events. We find that the power input far exceeded that in all previous geoeffective events in our sample and attribute this to the fact that the 1–4 May 1998 event consisted of a compound stream structure with an unprecedented power input during a ∼3‐hour burst of high‐speed flow on 4 May. A statistical survey using the OMNI database for the 6‐year period 1995–2000 confirms these inferences and indicates further approximate saturation levels for energy and power input of 10 J m−2 and 0.3 mW m−2, only exceeded in exceptional events such as May 1998 and July 2000. The solar energetic particle event at the leading edge of the fast stream might be the only advance warning the Earth would receive of the approach of a configuration of such a concentrated geoeffective potential.
Observation of the mesospheric and lower thermospheric 10‐hour wave in the northern polar region
Authors: Wu Q. , Killeen T. L. , McEwen D. , Solomon S. C. , Guo W. , Sivjee G. G. , Reeves J. M.
Published: October 2002

A 10‐hour wave in the mesospheric and lower thermospheric neutral winds and temperature was observed at northern high‐latitude stations Resolute (74.9°N) and Eureka (80.1°N) by two Fabry‐Perot interferometers, a Michelson interferometer, and a CCD spectrograph. The vertical wavelength of the wave was estimated to be ∼53 km. An interstation wave phase comparison yielded a zonal wave number close to five. The neutral wind wave amplitude increased with altitude up to 97 km and varied little with latitude. Our observations suggest that the 10‐hour wave is likely to be a result of the semidiurnal tide and the quasi‐2‐day wave nonlinear interaction and unlikely to be a Lamb wave. The OH emission and temperature observations are also consistent with this interpretation.
On the origin of kilometric continuum
Authors: Green James L. , Sandel B. R. , Fung Shing F. , Gallagher D. L. , Reinisch B. W.
Published: October 2002

The Radio Plasma Imager (RPI) and the extreme ultraviolet (EUV) imager instruments on the polar orbiting IMAGE satellite are providing new observations of the source region of kilometric continuum. Observations by the IMAGE instruments are now able to clarify some aspects of the source of this emission and put some previous observations into a new perspective. The IMAGE observations show that kilometric continuum is generated at the plasmapause, from sources in or very near the magnetic equator, within a bite‐out region of the plasmasphere. It is not known if the bite‐out region is a necessary condition for the generation of kilometric continuum. From ray‐tracing calculations in a model plasmasphere having a bite‐out feature, it is found that a kilometric continuum source can produce a relatively narrow emission cone that is largely confined to the longitudinal extent of the bite‐out structure. Since the bite‐out structure is observed to corotate with the plasmasphere, so would the beamed kilometric continuum. In addition, the observed narrow latitudinal extent of the emission cone does not appear to be a propagation effect but is perhaps produced by the emission mechanism, such as the linear or nonlinear mode‐conversion processes at the upper hybrid resonance proposed by a number of investigators. The processes by which bite‐out structures are produced in the plasmasphere are not completely understood at this time.
Currents in auroral substorms
Authors: Gjerloev J. W. , Hoffman R. A.
Published: October 2002

We present a model of the three‐dimensional current system in the nighttime sector during the expansion phase through early recovery phase for bulge‐type aurora substorms. The height‐integrated horizontal ionospheric currents are calculated from models of the height‐integrated conductivity and convection electric field previously published [Gjerloev and Hoffman, 2000a, 2000b, 2001] that were based on a set of data from the Dynamics Explorer 2 satellite, while the field‐aligned currents are deduced from the divergence of the horizontal currents. These model‐derived field‐aligned currents are then compared with the field‐aligned currents determined from magnetometer data from the same data set used to develop the conductance and electric field models. The comparison shows good agreement, thereby serving as an independent verification of the entire self‐consistent substorm model. We find that the feeding as well as the drainage of the substorm current wedge component of the westward electrojet is principally due to an imbalance of the downward region 0 and upward region 1 current sheets under which the electrojet is flowing. We do not find evidence of a simple downward current postmidnight and upward current premidnight. The eastward electrojet, which almost entirely originates on the dayside, is partly drained by a poleward current flowing across the Harang region into the westward electrojet at ∼19–21 magnetic local time (MLT) and partly by net upward field‐aligned currents located just prior to midnight.
Toroidal wave frequency at L = 6–10: Active Magnetospheric Particle Tracer Explorers/CCE observations and comparison with theoretical model
Authors: Takahashi Kazue , Denton Richard E. , Gallagher Dennis
Published: October 2002

Magnetospheric standing Alfvén waves are guided along the ambient magnetic field, and their frequency depends on the mass density of the plasma distributed along the field lines. These properties allow us to use Alfvén waves to map time‐dependent phenomena between space and ground and to estimate the mass density. In this paper we present a statistical study of the spatial variation of the fundamental frequency fT1 of toroidal‐mode standing Alfvén waves in the L range from 6 to 10, where L indicates the maximum geocentric distance on the field line. The data used for this analysis are energetic particle flux anisotropy (proxy of transverse electric field) and magnetic field measurements from the Active Magnetospheric Particle Tracer Explorers /Charge Composition Explorer (CCE) spacecraft. Using CCE data covering 4 years, we obtained ∼5000 20‐min intervals containing a clear signature of toroidal waves. The median fT1 is 6–10 mHz at L = 7 and decreases to 4–8 mHz at L = 9. The frequency tends to be lower at noon than at midnight. The observed frequencies are compared with numerically derived frequencies using an empirical mass density model [Gallagher et al., 2000] and a magnetic field model [Tsyganenko, 1989]. We found a good agreement for 1200–2400 magnetic local time (MLT) but a large discrepancy near 0300 MLT. This may indicate that the flux tubes at this local time are more heavily loaded than specified in Gallagher et al.'s [2000] model.
Cyclotron resonances of ions with obliquely propagating waves in coronal holes and the fast solar wind
Authors: Hollweg Joseph V. , Markovskii S. A.
Published: October 2002

There is a growing consensus that cyclotron resonances play important roles in heating protons and ions in coronal holes where the fast solar wind originates and throughout interplanetary space as well. Most work on cyclotron resonant interactions has concentrated on the special, but unrealistic, case of propagation along the ambient magnetic field, B0, because of the great simplification it gives. This paper offers a physical discussion of how the cyclotron resonances behave when the waves propagate obliquely to B0. We show how resonances at harmonics of the cyclotron frequency come about, and how the physics can be different depending on whether E⟂ is in or perpendicular to the plane containing k and B0 (k is wave vector, and E⟂ is the component of the wave electric field perpendicular to B0). If E⟂ is in the k‐B0 plane, the resonances are analogous to the Landau resonance and arise because the particle tends to stay in phase with the wave during the part of its orbit when it is interacting most strongly with E⟂. If E⟂ is perpendicular to the k‐B0 plane, then the resonances depend on the fact that the particle is at different positions during the parts of its orbit when it is interacting most strongly with E⟂. Our main results are our equations (10), (11), and (13) for the secular rate of energy gain (or loss) by a resonant particle and the unfamiliar result that ions can resonate with a purely right‐hand circularly polarized wave if the propagation is oblique. We conclude with some speculations about the origin of highly obliquely propagating ion resonant waves in the corona and solar wind. We point out that there are a number of instabilities that may generate such waves locally in the corona and solar wind.
Relative recovery of galactic and anomalous cosmic rays at 1 AU: Further evidence for modulation in the heliosheath
Authors: McDonald F. B. , Klecker B. , McGuire R. E. , Reames D. V.
Published: October 2002

For solar cycle 22 the large‐scale interplanetary disturbances produced by the intense solar activity of March/June 1991 had a long‐term effect on the recovery of galactic cosmic rays throughout the heliosphere that persisted for almost 5 years. At 1 AU, the recovery of 13 MeV/nucleon anomalous cosmic ray oxygen (ACR O+) is much more rapid than that of 265 MeV/nucleon galactic cosmic ray helium (GCR He), consistent with previous observations in the distant heliosphere [McDonald et al., 2000] and strengthening the concept that the region of the heliosheath plays an important role in the modulation of galactic cosmic rays. A comparison of the time histories of GCR He and ACR O+ at 1 and 44 AU observations suggest the recovery moves from the distant heliosphere inward toward 1 AU for this particular phase of the heliomagnetic cycle. There is a very low relative modulation potential, Φ, between 1 and 70 AU of 116 ± 6 MV for GCR He at solar minimum using the force field approximation. When combined with the small radial intensity gradients in the distant heliosphere, a much lower modulation potential is implied between 1 AU and the termination shock at solar minimum than had been assumed previously. There is no effect on the 13 MeV/nucleon ACR O+ intensity as the inclination of the heliospheric neutral current sheet decreases from 32° to its minimum value of 8°.
Ionospheric currents and field‐aligned currents generated by dynamo action in an asymmetric Earth magnetic field
Authors: Le Sager P. , Huang T. S.
Published: October 2002

To investigate the influence of the magnetic field configuration on large‐scale ionospheric electrodynamics, a geomagnetic field coordinate system based on Euler potentials is built for three magnetic field configurations: dipole, tilted dipole, and a revision of the International Geomagnetic Reference Field (IGRF). The two‐dimensional ionospheric dynamo equation is expressed in this framework under the assumptions of equipotential field lines and conservation of current, including horizontal ionospheric conduction current and interhemispheric magnetic‐field‐aligned current. Equinoctial symmetric conductivity and neutral wind distributions are used to isolate the effects of the magnetic asymmetry about the geographic equator. In the case of the IGRF only, the coupling along realistic field lines permits the reproduction of an equinox local time shift between horizontal‐current foci at all universal times. This is likely to explain the shift between the focus local times of Sq vortices that has been observed at equinox and at all universal times. The asymmetry due to nondipolar geomagnetic field distortions is found to be as efficient as conductivity and neutral wind asymmetries, which have been previously modeled, in driving Birkeland currents with an order of magnitude of 10−8 A m−2. The magnetic field asymmetry also turns out to be as important as wind and conductance asymmetries to specify the Birkeland current pattern.
Three‐dimensional spatial structures of midlatitude type 1 Es irregularities
Authors: Chu Yen‐Hsyang , Wang Chien‐Ya
Published: October 2002

Concurrent type 1 and type 2 radar returns from sporadic E (Es) irregularities detected by the Chung‐Li VHF radar are analyzed and discussed in this paper. For the present case the mean Doppler velocities of the type 1 echoes lasting ∼8 min are between ∼250 and 302 m/s, smaller than those observed in equatorial and auroral regions. The backscatter from type 1 irregularities is greater than that from type 2 irregularities by ∼6–8 dB. Interferometry analysis indicates that the echo patterns of the type 1 and type 2 irregularities projected in mutually orthogonal planes are striation‐like, strongly suggesting that the plasma structure associated with the irregularities is in a form of thin slab. The plasma slab with thickness of ∼1–2.5 km locates in the height range from ∼104 to 107 km. The horizontal extents of the type 1 irregularities in the striation‐like echoing region range from 12.7 to 6.8 km in the east‐west direction and from at least 7.2 to 5.1 km in the north‐south direction. The observational evidences that the slab structure has sharp edges in lateral edges and the type 1 irregularities drift in the meridional direction favor the slab model proposed by Haldoupis et al. [1996] to account for the excitation of the midlatitude type 1 irregularities. However, the other observations, including considerably large zonal extent of type 1 irregularities in the plasma slab and wide coverage of zonal polarization electric field, are unfavorable to the slab model. In addition, we present conclusive evidence in this article to show that the range variation of backscatter from Es irregularities may result directly from the zonal bulk movement of the Es irregularities in the echoing region rather than from the height variation of the irregularities. Therefore great caution should be taken in analyzing the dynamic behavior of Es irregularities from range‐time‐intensity plot.
Extension of the Harris magnetic field model to obtain exact, two‐dimensional, self‐consistent X point structures
Authors: Brittnacher M. , Whipple E. C.
Published: October 2002

Harris [1962] developed a one‐dimensional current sheet model in a magnetic field that was self‐consistent with a collisionless Maxwellian plasma. This model has served as a starting point for many current sheet studies. Kan [1973] showed how this approach could be extended to two dimensions, but there has been little further application of this extension to magnetospheric problems. We have followed Kan's technique and give exact analytic expressions for plasma and magnetic field structures in the vicinity of a two‐dimensional magnetic X point. The plasma is Maxwellian with arbitrary electron to ion mass ratio, but in the present stage of work the flow is constrained to be perpendicular to the plane containing the X point. The model is flexible in that it contains parameters that should allow it to be adapted to particular observational geometries and to be extended further using perturbation techniques to higher dimensions.
Winds and shears in the mesosphere and lower thermosphere: Results from four decades of chemical release wind measurements
Authors: Larsen M. F.
Published: October 2002

Since 1958, over 400 chemical tracer measurements have been made of the wind profiles in the upper mesosphere and lower thermosphere. The measurements cover a wide range of latitudes, longitudes, seasons, and local times. The results from the analysis of the data set show that a wind maximum in the altitude range between 100 and 110 km is a consistent feature of the observations at midlatitudes and low latitudes. The wind speed associated with the maximum exceeds 100 m s−1 in over 60% of the observations. Large shears are found, especially on the bottomside of the wind maximum, and the observed shears are often large enough to be either unstable or approaching instability. The wind maximum and shear features are characteristics of the overall data set and are not tied to particular observation sites, particular seasons, or special conditions.
A model of the near magnetosphere with a dawn‐dusk asymmetry 1. Mathematical structure
Authors: Tsyganenko N. A.
Published: October 2002

A new empirical magnetic field model has been developed, representing the variable configuration of the inner and near magnetosphere for different interplanetary conditions and the ground disturbance levels. This paper describes the mathematical structure of the model, while the results of fitting it to a new set of spacecraft data are presented in a companion paper. The general approach remains the same as in the earlier T96 model, but the mathematical description of all major sources of the magnetospheric field now applies recently developed new methods. In particular, the field deformation technique is extensively used, making it possible to realistically and flexibly represent the fields of the cross‐tail current, the ring current, and the Region 1 and 2 Birkeland currents. The new model ring current includes not only the axisymmetric component but also a partial ring current with field‐aligned closure currents, a feature absent in earlier data‐based models. The field of the cross‐tail current includes two modules whose current densities vary along the Sun‐Earth line with different rates. The cross‐tail current sheet warps in two dimensions in response to the geodipole tilt, its inner edge shifts along the Sun‐Earth line with growing disturbance, and its thickness varies along and across the tail. Birkeland currents of Regions 1 and 2 vary in response to interplanetary conditions, so that at ionospheric altitudes they shift in latitude and change their distribution in local time. The magnetospheric boundary is specified using a most recent empirical model [Shue et al., 1998]; its size is controlled by the solar wind ram pressure, and its shape also varies in response to changes of the Earth's dipole tilt angle. The model magnetopause ensures a full confinement of the fields of all sources inside the magnetopause, regardless of its shape and size. The model also includes an interplanetary magnetic field–controlled interconnection field, allowing a finite normal Bn at the magnetopause and hence open magnetospheric configurations.
Observations of persistent Leonid meteor trails 3. The “Glowworm”
Authors: Drummond Jack D. , Grime Brent W. , Gardner Chester S. , Liu Alan Z. , Chu Xinzhao , Kelley Michael C. , Kruschwitz Craig , Kane Timothy J.
Published: October 2002

A spectacular, well‐observed Leonid meteor of visual magnitude −14.3 appeared on 17 November 1998 and left a lingering trail, dubbed the Glowworm, that was well studied. From a location on Kirtland Air Force Base, near Albuquerque, New Mexico, we obtained CCD images of the trail from 94 to 203 s after the meteor and recorded a video with an intensified camera for even longer. From information obtained with a sodium lidar half an hour after the meteor, we have determined that a gravity wave with a vertical wavelength of 2.4 km was responsible for the right‐angled appearance of the trail. The trail ended abruptly at 85 km, and its uppermost altitude may have been no greater than 91 km. We designate the Glowworm a Type I trail: one that is wide (1 km), cloudy in appearance, has high diffusion rates (800 m2 s−1), high total line emission rates (1.5×1018 photons m−1 s−1), and is optically thicker than Type II trails. The lower parts of the Diamond Ring, another Leonid lingering trail that appeared 38 min earlier than the Glowworm, define the Type II trails, which appear as narrow, optically thinner parallel trails, with low diffusion rates (12 m2 s−1) and total line emission rates (1–3×1016 photons m−1 s−1). No explanation is offered for the two orders of magnitude difference in these quantities. The Glowworm meteor produced infrasound [ReVelle and Whitaker, 1999], from which a meteoroid mass estimate of 522 g was made. We compare our photometry to a detailed numerical modeling of the shape of the trail and emission from the Glowworm made by Zinn et al. [1999], who find that the largest contributors to emission recorded by our CCD and video cameras are atmospheric O2 vibrational bands. Compared to our measurements, their calculated emission is too high by two orders of magnitude, but since most of O2 emission may be absorbed by atmospheric O2 before it reaches the ground, this may indeed be the primary contributor to the observed flux. Although the calculations of Zinn et al. lead to a hollow cylinder appearance which may be appropriate for the Glowworm, it is not pronounced enough to account for the complete darkness between the parallel structures seen in Type II trails. An upper limit to backscattering from dust of 3.7 × 10−5 of the expected return was found from directing a 180 W copper vapor laser at the Glowworm.
Transequatorial propagation of the Pc1 emission on 23 October 1997
Authors: Potapov Alexandre S. , Polyushkina Tatyana N. , Afraimovich Eduard L. , Lipko Yuri V. , Hayashi Kanji
Published: October 2002

A Pc1 emission event with uncommon propagation characteristics was revealed in the study of the search‐coil magnetometer data from two Russian midlatitude observatories separated by 4000 km. Dynamic spectra of the emission showed pronounced structure with a repetition period of about 130 s. By comparing the wave form plots for the two locations we found an unusually large phase difference as much as nearly 180 between the plotted envelopes. We discuss a most probable model assuming that the signal received at one of the two observatories must propagate from the source region in the opposite hemisphere.
A statistical investigation of traveling convection vortices observed by the west coast Greenland magnetometer chain
Authors: Clauer C. Robert , Petrov V. G.
Published: October 2002

We report on the results of a statistical investigation of high‐latitude traveling ionospheric convection vortices measured by the west coast Greenland magnetometer chain during the year 1996. The events were selected using a data‐fitting procedure which identifies vortex‐like structures in the ionospheric current distribution. The procedure identified about 22,150 vortices during 1996. For each vortex event, we compute the average value of the current and the average northward and eastward speed, and the standard deviations of the parameters are also computed. We investigate the local time and solar wind dependence of the vortex strength and movement. The solar wind speed appears to be the primary factor which correlates best with the field‐aligned current strength related to individual ionospheric vortices. Increasing solar wind speed increases the vortex occurrence, as well as vortex intensity in all dayside local time sectors. The dependence is the strongest in the noon sector, followed by the morning and evening sectors. The interplanetary magnetic field (IMF) Bz component has a nearly comparable influence on vortex strength as the solar wind speed; however, this is observed primarily in the noon sector. The final factor which we have found to correlate with vortex occurrence and strength is the standard deviation of the magnetic field.
Interplanetary magnetic field Bx asymmetry effect on auroral brightness
Authors: Shue J.‐H. , Newell P. T. , Liou K. , Meng C.‐I. , Cowley S. W. H.
Published: October 2002

We derive Northern Hemispheric auroral brightness patterns using images from Polar Ultraviolet Imager Lyman‐Birge‐Hopfield long band emissions (∼170.0 nm). Pixels of these images are binned by the X and Y components of the interplanetary magnetic field (IMF) under the combinations of two seasonal (summer and winter) and two IMF Bz (northward and southward) conditions. Hemispheric auroral power is estimated from these images by an integration of auroral brightness over 60°–90° MLAT for all local times. It is found that the hemispheric auroral power is higher for negative Bx than for positive Bx under similar By conditions when the IMF is southward. This Bx asymmetry of the hemispheric auroral power is significant both in summer and winter. For the northward IMF the hemispheric auroral power is low and the Bx asymmetry is not statistically present in both summer and winter. Recent studies have shown that hemispheric auroral power for negative By is higher than that for positive By. In summary, hemispheric auroral power reaches its highest level when all the components of the IMF are negative.
Parallel potential driven by a kinetic Alfvén wave on geomagnetic field lines
Authors: Tikhonchuk V. T. , Rankin R.
Published: October 2002

The generation of parallel electric fields by a shear Alfvén wave standing along closed magnetic field lines in Earth's magnetosphere is considered under the condition that the electron bounce period is smaller than the wave period. The effects of the inhomogeneity of the magnetic field, plasma density, and electron temperature are analyzed in terms of the magnitude of the parallel potential, its location, and the distribution function of the current‐carrying electrons. The theory demonstrates a possible mechanism for keV electron energization in the polar magnetosphere.