bluuebunny/crossref_metadata_2025 huggingface dataset:
Empirical polar cap potentials
Authors: Boyle C. B. , Reiff P. H. , Hairston M. R.
Published: February 2004

DMSP satellite plasma flow data from 1987–1990 are used to derive empirical models of the polar cap potential for quasi‐steady interplanetary magnetic field (IMF) conditions. The large data set, due to the high duty cycle and nearly Sun synchronous DMSP orbits, allowed very stringent data selection criteria. The analysis indicates that a good description of the unskewed (Heppner Maynard pattern A) steady state polar cap potential is ΦA = 10−4v2+11.7B sin3 (θ/2) kV, where v is the solar wind velocity in kilometers per second, B is the magnitude of the interplanetary magnetic field in nanoteslas, and θ = arccos (Bz/|B|)GSM. The IMF‐dependent contribution to the cross polar cap potential does not depend significantly on solar wind pressure. Functional forms for the potential do benefit from inclusion of an IMF independent term proportional to the solar wind flow energy. Best fits to IMF‐independent contributions to the steady state polar cap potential yield ∼16 kV for vsw = 400 kilometers per second. During steady IMF the total unskewed polar cap potential drop is shown to be approximately ΦA = 16.5 + 15.5 Kp kV. The distribution of potential around the polar cap is examined as a function of magnetic local time. A sinusoidal distribution is an excellent description of the distribution, and more complex forms are not justified by this data set. Analysis of this data set shows no evidence of saturation of the polar cap potential for large |MF|. A simple unified description of the polar cap potential at all magnetic local times (MLT) and IMF, Φ(IMF, MLT) = −4.1 + 0.5 sin ((2π/24) MLT + 0.056 + 0.015 Byeff) (1.1 × 10–4 v2 + 11.1 B sin3 (θ/2)) kV, is generated, where Byeff is By (−BY) in the northern (southern) hemisphere. If IMF data is unavailable, the polar cap potential is well described by ΦA(Kp, MLT) = − 4.1 + 1/2 sin ((2π/24) MLT + ϕHM)(16.4 + 15.2 Kp) kV, where ÖϕHM is a small phase correction of (−0.054, −0.031, 0.040) for Heppner‐Maynard convection patterns (BC, A, DE), respectively.
The 100 Hz electric fields observed on Pioneer Venus Orbiter and a case against the whistler hypothesis for them
Authors: Cole K. D. , Hoegy W. R.
Published: February 2004

Without invoking the whistler hypothesis, but assuming they are not man‐made it is shown (1) that the 100 Hz electric fields observed on the Pioneer Venus orbiter in the lower ionosphere (below 160 km, at night) can be so intense that they would produce unrealistic heating and depletion of the ionosphere, if they existed on a planetary scale; and (2) assuming the signal is not electrostatic, their accompanying magnetic field b at high intensity would be greater than the ambient field. Similar conclusions follow from invoking linear theory at whistler frequencies to explain the signal. Nonlinear theory would be needed to describe the propagation, if whistlers are the cause of the high‐intensity signals, but this would not overcome the heating problem. Previous conclusions based on linear theory would be invalid for these high‐intensity signals. In addition, for the most intense electric fields at 129 km altitude the ionization produced by the intensely heated electrons would be excessive. It is concluded that the 100 Hz signals can not be due to whistlers, or electromagnetic waves of planetary scale, and therefore these signals could not be used to support the ease for lightning on Venus. These arguments favor the hypothesis that the 100 Hz electric fields are highly localized to the spacecraft.
Simulations of the low‐latitude midnight temperature maximum
Authors: Fesen C. G.
Published: February 2004

To investigate possible sources of the low‐latitude midnight temperature maximum (MTM), a series of simulations was made with the National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). The MTM is a tidal phenomenon and thus is ultimately due to the thermospheric tidal sources: in situ thermal excitation; ion‐neutralmomentum coupling; and the tidal waves arising in the lower atmosphere which penetrate the thermosphere. In the TIEGCM, the development of the MTM is due to the upward propagating semidiurnal tides. The model simulations suggest that the observed seasonal variation of the MTM, in which the MTM occurs earlier and more strongly in the local summer hemisphere, may be due to the interaction of the 2,2 and 2,3 modes: in summer, the two modes reinforce each other, and in winter they offset each other. This study further suggests that much of the variability observed in the MTM may be due to the variability in the upward propagating tides. The semidiurnal modal mix impinging on the lower thermosphere may vary on a day‐to‐day basis, depending on the background mean temperature and winds.
Electrodynamic tether currents in the day/night ionosphere: Correlations during the Plasma Motor Generator mission
Authors: Wilson Thomas L. , Chlouber Dean , Jost R. Jerry
Published: February 2004

Ground‐based radar observations during NASA's Plasma Motor Generator (PMG) electrodynamic tether mission are reported, which indicate the existence of ion‐acoustic or soliton wave packets produced as ionospheric excitations by the orbiting PMG tether system. These appear to be the first such radar measurements of traveling ionospheric disturbances propagating along geomagnetic field lines during an active orbiting ionospheric tether‐plasma experiment. The presence of such excitations gives added importance to earlier studies of the plasma radiation modes involved in tether electrodynamics and the part they play in the ionospheric energy balance mechanisms. In addition, tether‐current measurements studied during postflight data analysis show a pronounced current decrease when the spacecraft passed from daylight to darkness, which may be related to current limiting. The significance of this observed day/night correlation in conjunction with the soliton‐like wave packet radiation modes calls for a detailed investigation of the PMG tether‐current measurements in terms of local and global interactions of the PMG with Earth's ionosphere, the initial work for which we show here. This includes an analysis of the radiation impedance due to Alfvén and lower hybrid bands, ion‐acoustic (soliton) modes, and whistler waves. A decomposition of the measured tether currents into Hall, Pedersen, Cowling, and Birkeland conductivities is presented.
Spread F and the structure of equatorial ionization depletions in the southern anomaly region
Authors: Sales G. S. , Reinisch B. W. , Scali J. L. , Dozois C. , Bullett T. W. , Weber E. J. , Ning P.
Published: February 2004

Combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator. In Chile, as part of the MISETA campaign in fall 1994, a CCD‐enhanced all‐sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11°S. Concurrently, a Digisonde collocated with the photometer monitored the appearance of spread F. In between the ionograms, the sounder operated as a Doppler interferometer identifying the locations of F layer irregularities associated with the spread F. They were found to lie inside the emission depletion regions. The HF sounder, requiring orthogonality with the field‐aligned F layer irregularities to generate the spread structure, tracked these irregularities inside the emission depletion bands as they drifted eastward. Ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site. Model calculations indicate that the sounder rays encounter orthogonality with the Earth's magnetic field within the depletion bubble southward from the site, consistent with the local dip angle. The combination of optical images with HF radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a Doppler interferometer. The Digisonde measurements and ray tracing show for the first time that the spread F signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion.
Cosmic ray decreases: 1964–1994
Authors: Cane H. V. , Richardson I. G. , von Rosenvinge T. T.
Published: February 2004

We have studied 30 years (1964–1994) of neutron monitor data in order to understand the principle mechanisms causing short‐term (< 20‐day duration) cosmic ray decreases seen at Earth. By examining the characteristics of associated low energy (<200 MeV) particle enhancements in combination with the neutron monitor data, we have determined the responsible solar wind disturbances for 153 of the 180 ≥ 4% decreases. The vast majority (86% of the 153 events) are caused by coronal mass ejections and the shocks that they generate. The ejecta is intercepted only when the solar event originates within 50° of the Sun's central meridian. For more distant events, only the shock is intercepted at Earth. We present a list of all 180 events seen in the years 1964–1994 together with the associated solar event, when this can be determined, and some details about the solar wind structures based on in situ solar wind data, if available. This list represents a compendium of major solar wind disturbances affecting a large section of the inner heliosphere over this time period. We also discuss enhanced daily variations in some events which are related to radial gradients caused by strong disturbances inside the Earth's orbit.
Theory of nearly perpendicular electrostatic plasma waves and comparison to Freja satellite observations
Authors: Seyler C. E. , Wahlund J.‐E.
Published: February 2004

The linear and nonlinear two‐fluid nature of oblique electrostatic plasma waves is explored and clarified. It is found that two distinct wave regimes exist corresponding to propagation angles greater than or less than
with respect to the perpendicular direction of the magnetic field. Propagation angles greater than ε correspond to either the electrostatic ion cyclotron wave or the shorter wavelength oblique ion acoustic wave, which are termed the fast ion cyclotron and fast ion acoustic waves respectively. Angles less than ϵ correspond to the inertial Alfvén wave which is called the slow ion cyclotron wave and in the short wavelength limit is called the slow ion acoustic wave. The slow ion‐acoustic wave is found to have a predominately ion‐Boltzmann response. Initial value simulations having fast or slow ion cyclotron wave initial conditions of modest amplitudes result in nonlinear steepening and subsequent breakup into fast or slow ion acoustic waves respectively. This mechanism of ion acoustic wave generation may partially explain the origin of ion acoustic activity detected by the Freja satellite within regions of ion and electron acceleration.
Interaction of the solar wind with the local interstellar medium: A multifluid approach
Authors: Zank G. P. , Pauls H. L. , Williams L. L. , Hall D. T.
Published: February 2004

A distinct fluid description for neutral hydrogen originating from either the interstellar medium, the heliosheath, or the solar wind has been developed to describe the detailed interaction of the solar wind with the local interstellar medium (LISM) [Williams et al., 1995]. Such a multifluid description for the hydrogen serves to capture the highly anisotropic nature of the neutral distribution. The neutral multifluid is coupled to a hydrodynamic plasma through charge exchange and the time‐dependent model is solved in two spatial dimensions. This approach is used (1) to elucidate the nature of the solar wind ‐ LISM interaction more carefully and precisely than has been done previously; (2) to identify precisely the role played by the three identifiably distinct neutral populations in determining the global structure and dynamics of the heliosphere (it is found, for example, that the heliopause is weakly time‐dependent for the two‐shock model); (3) to contrast these results with those obtained from the simpler, computationally more efficient model developed by Pauls et al. [1995]; and (4) to investigate the differences in global heliospheric structure and neutral properties between a two‐shock (which results from a supersonic interstellar wind impinging on the heliosphere) and a one‐shock (i.e., for a subsonic interstellar wind) model. In particular, observational criteria are presented that may allow one to distinguish between a one‐shock and a two‐shock heliosphere.
Long‐term and solar cycle variation of the ring current
Authors: Vennerstrøm S. , Friis‐Christensen E.
Published: February 2004

The long‐term and solar cycle variation of geomagnetic activity since the beginning of the century is examined. It is demonstrated that the daily average of the horizontal magnetic perturbation of a single middle‐ to low‐latitude observatory is highly correlated with the daily average Dst. Using three such observatories, we construct an almost unbroken time series of daily values of the ring current activity from 1902–1986, denoted Hm. Comparing Hm with aa, we find that the gross features of the century scale variations of aa and Hm are similar: the ring current activity has increased significantly since the beginning of the century. Examining the solar cycle variation of Hm and aa, we find significant differences. Hm is better correlated with the sunspot number and is relatively larger in the beginning of the solar cycle, while aa is relatively larger in the late descending phase. Hm reveals a dual‐peak structure with two peaks close to solar maximum. We suggest that geomagnetic activity during the solar cycle can be divided into three peaks. The first peak occurs shortly before sunspot maximum and is seen with largest amplitude in the ring current activity. The second peak occurs about 2 years after sunspot maximum and is statistically the largest peak. The third peak occurs late in the descending phase of the solar cycle and is seen with larger amplitude in substorm activity than in ring current activity.
Flux ropes, interhemispheric conjugacy, and magnetospheric current closure
Authors: Kivelson M. G. , Khurana K. K. , Walker R. J. , Kepko L. , Xu D.
Published: February 2004

Flux ropes are thought to be formed of twisted flux tubes that surround a unidirectional field. Such structures are often observed in the magnetotail of Earth, generally following the onset of the expansion phase of a substorm. They appear to lie in or near the neutral sheet with a dawn to dusk orientation of the central axis. Observations do not define the linkage of these structures to other parts of the magnetosphere‐ionosphere system, although there is evidence that they sometimes contain ions whose source is the ionosphere. Here we consider the current that flows through flux ropes, with an emphasis on the current closure paths. We discuss how flux ropes link to the ionosphere, and what ionospheric signatures are expected, and we attempt to identify how flux rope current relates to other magnetospheric currents. Our analysis is based on Galileo flux rope observations and on the results of a magnetohydrodynamic simulation of the magnetosphere when the interplanetary magnetic field is southward but tilted in the dawn‐dusk direction. We suggest that the ionospheric signatures have a predictable dawn‐dusk, north‐south asymmetry, and we speculate that the flux rope currents close through the substorm current wedge.
Effects of wave superposition on the polarization of electromagnetic ion cyclotron waves
Authors: Denton Richard E. , Anderson Brian J. , Ho George , Hamilton Douglas C.
Published: February 2004

Using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft, we make a detailed comparison between the observed polarization properties of electromagnetic ion cyclotron (EMIC) waves and those predicted by theory. The polarization can be described by three parameters: the ellipticity ε, the ratio of parallel (to the background magnetic field B0) magnetic fluctuations δBz to the major axis component of the elliptical perturbation in the perpendicular plane δBmajor, and the phase angle between δBz and δBmajor. On the basis of the plasma parameters observed during EMIC events, we have calculated the linear properties of the theoretical modes and compared these to the observations. The result is that two and in some cases, three of the observed polarization properties are inconsistent with the assumption that the waves result from a single linear mode. We use a simple model with two constituent waves in various azimuthal orientations (around B0) and temporal phase relations and show that the distribution of observed polarization properties can be understood as resulting from the superposition of more than one mode. When there is superposition, the instantaneous polarization characteristics of the fluctuations do not reliably reflect the constituent wave properties and the minimum variance direction cannot be associated with a wave vector direction. Nonetheless, we have shown that the constituent wave properties can be inferred from the distribution of observed properties. For superposition of two waves with only slightly dissimilar characteristics, the constituent wave ε is approximately the median observed ε, ε, and the constituent θkB (angle between the wave vector k and B0) is approximately given by tan θkB = δBz/δBmajor/ε, with the overbar on δBz/δBmajor again indicating a median value.
Mode conversion into non‐MHD waves at the Alfvén layer: The case against the field line resonance concept
Authors: Bellan Paul M.
Published: February 2004

According to ideal magnetohydrodynamics (MHD), a magnetic field line in an inhomogeneous plasma becomes “resonant” when the condition ω = k‖vA is satisfied on the field line; here ω is the frequency of a perturbation, k| is the component of the wavenumber parallel to the magnetic field, and υA is the local Alfven velocity on the field line. This concept of field line resonance (FLR) has long been used to explain observed magnetospheric pulsations and, in particular, recent models have proposed that magnetospheric FLRs are excited by compressional MHD waves trapped in a surrounding cavity or waveguide. We present here a reexamination of this problem using an analysis based directly on the Maxwell‐Lorentz equations, a more fundamental description of plasma behavior than MHD. Our analysis shows that the ω = k‖υA layer is actually the location where an incident compressional MHD wave mode converts into a non‐MHD inertial electron Alfvén wave. We find that all fields are finite and regular at the ω = k‖υA layer and conclude that field line resonance does not exist in reality and so is an artifact of ideal MHD.
Observations of magnetic reconnection at the lobe magnetopause
Authors: Gosling J. T. , Thomsen M. F. , Le G. , Russell C. T.
Published: February 2004

Measurements made by the Fast Plasma Experiment on ISEE 2 provide direct evidence for the rereconnection of the open field lines of the tail lobes with the interplanetary magnetic field at the lobe magnetopause. The evidence, obtained on seven different dates at locations well removed from the noon‐midnight plane, consists primarily of observations of accelerated magnetosheath plasma within the lobe magnetopause current layer. These accelerations were either sunward or tailward, depending on the position of the satellite relative to the rereconnection site. Such accelerations have been observed in both the northern and the southern hemispheres, on both the dawn and the dusk sides of the magnetosphere, and both sunward and tailward of the dawn‐dusk terminator. Except for one event, all of the lobe magnetopause reconnection events examined here occurred at times of large local field shears at the magnetopause; these large field shears were associated primarily with large x and y magnetic field components rather than with the z component. In each case the magnetosheath plasma crossing into the magnetopause current layer was heated as well as accelerated. Often, this newly heated and accelerated plasma, which was confined to the current layer where the field rotated from its magnetosheath orientation to its magnetospheric orientation, coexisted as a double‐ion beam with unaccelerated mantle plasma that had entered the magnetosphere earlier upstream from the rereconnection site. The change in velocity experienced by the newly entered plasma within the current layer was usually in reasonable accord with stress balance at the magnetopause. Apparent accelerations and decelerations observed upstream from the magnetopause in two of these events were associated with secondary, field‐aligned ion populations that appeared to result from reflection of magnetosheath ions at the magnetopause.
Structure of plasma sheet in magnetotail: Double‐peaked electric current sheet
Authors: Hoshino M. , Nishida A. , Mukai T. , Saito Y. , Yamamoto T. , Kokubun S.
Published: February 2004

The structure of the plasma sheet in the distant magnetotail observed by the Geotail satellite is examined. We found that the observed structure of the plasma sheet is often different from the standard Harris‐type plasma sheet [Harris, 1962]. The observed structure can be expressed as a double‐peaked current sheet which has a pair of localized electric currents away from the neutral sheet, i.e., a geometrically thick zero magnetic field region attached to the thin boundary layer with a large magnetic field gradient. This type of the plasma sheet is often observed in the distant magnetotail at radial distances of −50 > XGSM/RE > −125. We discuss a possible model to explain the formation of the double‐peaked current sheet in terms of large‐scale magnetic reconnection associated with slow shocks.
On the focusing of whistler wave field intensity in the quasi‐resonant propagation regime
Authors: Lundin B. , Krafft C.
Published: February 2004

The propagation of whistler mode wave packets in a cold magnetospheric plasma is investigated in the quasi‐resonant regime (near the oblique lower hybrid resonance) using the ray approximation. The analysis is essentially simplified by introducing a special curvilinear orthogonal coordinate system (resonant coordinate system (RCS)), which is naturally adapted to the description of the propagation of quasi‐electrostatic waves in a cold plasma, when the group velocity and the wave vector tend to be orthogonal to each other in the deep quasi‐electrostatic limit. It is shown that in an overdense plasma of the Earth plasmasphere, the parameters of the RCS are independent of the space density distribution of the background plasma. The general expressions for the Hamiltonian and whistler wave field intensity focusing law in a deep quasi‐resonant regime are derived.
Energetic helium isotopes trapped in the magnetosphere
Authors: Chen Jiasheng , Guzik T. Gregory , Wefel John P. , Pyle K. Roger , Cooper John F.
Published: February 2004

Helium nuclei have been measured within the magnetosphere (L ≤ 6) during the 1990/1991 CRRES mission over an energy range of ∼40–100 MeV nucleonl−1. The Office of Naval Research 604 instrument resolves helium isotopes with a mass resolution of ∼0.1 amu. Each helium isotope can be represented by a power law energy spectrum, and the energy spectrum of the 3He is different from that of 4He; that is, the 3He/4He ratio is energy dependent. In the energy range 51–86 MeV/nucleon the 3He/4He ratio is 7.4 ± 2.6 for L = 1.15–1.3 and 2.2±0.6 for L = 1.8–2.15. All observed helium events at L < 2.65 show pitch angle distributions consistent with equatorially trapped ions, whereas the distributions at larger L values are more uniform, within a factor of 2. For 2.15 < L < 6, as L decreases, the 3He/4He ratio increases and the helium energy spectrum softens. A possible origin of the geomagnetically trapped helium isotopes at L <2.15 is proton interactions in the residual atmosphere. The CRRES satellite was operational during the large geomagnetic storm on March 24, 1991, and we have divided the analysis into prestorm and poststorm time periods. Following the storm, the magnetosphere became more dynamic, and at L ∼ 2.3 an enhanced helium flux was observed. Over the energy range of 51–86 MeV nucleonl−1 both the average 3He and 4He fluxes at L = 2.15–2.65 increased by a factor of 6 after the storm relative to the prestorm period, while at L = 1.8–2.15 the average 3He and 4He fluxes decreased by factors of 3 and 11, respectively, following the storm. The implications of these observed results are discussed.
Reconnection in the near‐Earth plasma sheet: A three‐dimensional model
Authors: Wiechen H. , Büchner J. , Otto A.
Published: February 2004

We present results of three‐dimensional MHD simulations of reconnection in the near‐Earth plasma sheet. We use two‐dimensional start configurations which include the Earth's dipole field explicitly and three‐dimensional resistivity profiles. These equilibria are stable against large‐scale tearing instability, that is, the mode with the maximum growth rate. For nonhomogeneous resistivity profiles, however, the system may become unstable against nonlinear, resistive modes. We discuss onset and nonlinear evolution of reconnection due to these instabilities. One result of our three‐dimensional simulations is that earthward flow can be found even tailward of the reconnection region. This confirms a typical feature of previous two‐dimensional simulations of reconnection in the near‐Earth plasma sheet. The nonlinear resistive instabilities found in our simulations could be relevant for phenomena like pseudobreakups.
Solar wind streamer belt structure
Authors: Crooker N. U. , Burton M. E. , Siscoe G. L. , Kahler S. W. , Gosling J. T. , Smith E. J.
Published: February 2004

In the course of 2 days in October 1978, ISEE 3 encountered 14 magnetic field reversals, five pronounced and several lesser high‐beta regions called “plasma sheets,” two distinct entropy regimes, two heat flux dropouts (HFDs), and planar magnetic structure led by a shock wave. The complexity of the interval afforded an excellent opportunity to determine the relationships between these streamer belt structures, their geometry and topology, and the resulting implications for the structure of the heliospheric current sheet (HCS). None of the results of three tests applied to the data support a wavy or folded HCS hypothesis. In particular, a test for true magnetic polarity using primarily heat flux data indicated that the field reversals occurred in a predominantly toward sector with three pockets of true away polarity and three of false away polarity. The results are consistent with a view in which current sheets bound intertwined flux ropes, some of which coil back on themselves, possibly as remnants of transient coronal activity. A comparison between joint occurrences of the various structures showed that the most intense plasma sheets in the interval coincided with true polarity reversals, that nearly every reversal, whether true or false, corresponded to at least a weak plasma sheet, and that plasma sheets occurred within HFD intervals. The plasma sheet/HFD relationship was statistically confirmed in a search for plasma sheets in published HFDs.
Experimental evaluation of the Lundquist number for the Earth's magnetopause and magnetotail
Authors: Cattell C. A.
Published: February 2004

A realistic appraisal of the Lundquist number based on experimental determination of both the resistivity and the scale sizes is necessary for determining the validity of MHD theories and numerical simulations of important dynamical processes, such as reconnection, within the Earth's magnetosphere. On the basis of previously published ISEE 1 and Geotail observations of low‐frequency waves, identified as lower hybrid drift waves, and various experimental estimates of scale sizes, the Lundquist number is calculated for the magnetotail plasma sheet and the magnetopause of the Earth. This calculation is made using the quasi‐linear expression for the anomalous resistivity. If nonlinear effects and/or additional wave modes are important, the Lundquist number would be reduced. For typical values of the magnetic field, density, temperature, wave electric field amplitude, and scale size, it is shown that the Lundquist number ranges from the order of tenths to tens for the active, thinned near‐Earth plasma sheet and for the deep tail and ranges from the order of tenths to one hundred for the magnetopause.
Synoptic maps of polar caps for stable interplanetary magnetic field intervals during January 1992 geospace environment modeling campaign
Authors: Lyons L. R. , Lu G. , de la Beaujardière O. , Rich F. J.
Published: February 2004

Observations from January 27–28, 1992, when four polar‐orbiting DMSP satellites were in operation, are used to construct synoptic maps of convective flows and particle regions within the polar ionosphere. We chose periods of relative steady interplanetary magnetic field (IMF) and use data from multiple satellite passes during each period to obtain unprecedented two‐dimensional coverage. For strongly negative IMF By, convection patterns are similar to those that have been previously obtained but also show significant IMF Bz effects. For Bz » |By|, we see strong sunward flow near the pole, but antisunward convection on open field lines at lower latitudes on the nightside. The particle observations reveal the two‐dimensional structure of the cusp/mantle region and the existence of a strong polar arc within the circular convection cell that has a negative electric field divergence on open field lines. The particle observations also readily identify the magnetic separatrix when a satellite crosses the inner edge of the cusp or the boundary between the plasma sheet and polar rain. However, on the morningside and often on the afternoonside, we find a distinct and well‐defined region of soft‐electron and magnetosheath‐like ion precipitation [(the “soft‐electron zone” (SEZ)] lying between the plasma sheet and the region of polar rain. Separatrix identification is uncertain at local times where the SEZ is present; however, it appears that large portions of the SEZ are on open field lines. Dawn‐dusk displacements of the open field line regions in the two polar caps are seen that are consistent with IMF By effects. Additionally, the dayside separatrix is observed to be further poleward for positive than for negative IMF Bz, whereas Bz is not observed to have a significant effect on the location of the nightside separatrix. Also, we consistently find larger cross polar‐cap potential drops in the southern hemisphere than in the northern hemisphere. This suggests that there are significant (tens of kV) magnetic‐field‐aligned potential drops along high‐altitude regions of open field lines where the magnetic field is weak and particle motion strongly violates the guiding center approximation.
Testing the accuracy of magnetospheric model field line mapping
Authors: Pulkkinen T. I. , Tsyganenko N. A.
Published: February 2004

We present a new method to test the statistical accuracy of field‐aligned mappings by using a large magnetospheric data set and apply the method to the Tsyganenko [1989] model. The obtained errors in the field‐aligned mappings are generally small in the auroral zone outside the midnight sector but become larger on the nightside and within the polar cap. The model tail field is overstretched near midnight and the model field lines flare out toward the flanks and toward high Z values too much. The results also imply that there is a shear layer between the plasma sheet and lobe that could be indicative of the region 1 field‐aligned current system. The large‐scale mapping properties suggest that the magnetotail field is asymmetric with respect to the noon‐midnight meridian: the model better represents the evening‐sector field configuration, whereas the morning‐sector model field lines map too far down the tail.
The detection of dust grains by a wire dipole antenna: The Radio Dust Analyzer
Authors: Meuris Peter , Meyer‐Vernet Nicole , Lemaire Joseph F.
Published: February 2004

The possibility of a wire dipole antenna to study the characteristics of dust grains is examined. Charged dust grains passing by the antenna induce an electric potential change for the time of the flyby. These “waveforms” are studied as a function of the characteristics of the dust grain (its charge and velocity vector) and the plasma parameters. The thermal noise level due to flyby, emission, and impacts of the ambient plasma electrons is calculated and compared with the magnitude of the dust signal. Qualitative analysis yields a minimum detectable grain size, the minimum antenna sensitivity required, and the number of detectable events per time unit for cometary, planetary, and interplanetary environments. We call this recently proposed dust‐detection technique Radio Dust Analyzer (RDA).
Giant pulsations: An explanation for their rarity and occurrence during geomagnetically quiet times
Authors: Chisham G.
Published: February 2004

It is generally agreed that giant pulsations (Pgs) are the result of a particle instability that occurs inside the magnetosphere rather than the consequence of an external stimulus. Previous studies have suggested that protons with energies ∼5–30 keV play a role in Pg excitation. It is shown that protons with energies ∼5–30 keV, injected into the inner magnetosphere on the nightside, will only drift westward around the Earth on enclosed paths if the E × B drifts due to the magnetospheric convection and corotation electric fields are small. This is the case when the magnetosphere is quiet. If the E × B drifts are large, as is the case for more disturbed times, then their influence may overcome that of the gradient‐curvature drift for these lower energy protons, detrapping them from their enclosed paths and allowing them to follow convective paths to the dayside magnetopause. At these times, the lower energy protons which may be an important factor in Pg generation will not reach the early morning sector where Pgs occur, This phenomena can explain the rarity and occurrence during quiet times of Pgs. It can also explain the quashing of Pg activity during substorms and the tendency for Pgs to occur on successive days, 24 hours apart. A similar reasoning can also explain why radially polarized waves with large azimuthal wave numbers, thought to be generated by the bounce resonance mechanism, are frequently observed in the afternoon/evening sector of the magnetosphere but occur infrequently in the morning sector.
Impulsive plasmoid penetration of a tangential discontinuity: Two‐dimensional ideal and Hall magnetohydrodynamics
Authors: Huba J. D.
Published: February 2004

Impulsive plasmoid penetration of a tangential discontinuity (e.g., the magnetopause) is studied using two‐dimensional ideal and Hall magnetohydro‐dynamics. The model assumes a unidirectional magnetic field and describes the plasma and field dynamics transverse to the field; this is an idealized model and not typical of the magnetopause. Two magnetic field configurations are studied: the parallel field case where the magnetic fields in the magnetosheath and magnetosphere are in the same direction and the antiparallel field case where they are in opposite directions. The evolution of the plasmoid is symmetric about its propagation direction in ideal MHD. One new ideal MHD result is that after the plasmoid penetrates the discontinuity it undergoes vortex shedding. The Hall MHD results are similar to those obtained by Savoini et al. [1994]. The major similarity is that the plasmoid penetration process is asymmetric. The Hall antiparallel field case, although asymmetric, evolves very much like the ideal MHD case. However, the Hall parallel field case evolves much differently from the ideal MHD case. The plasmoid undergoes a substantial deformation in the direction orthogonal to the propagation direction and does not penetrate as far into the magnetosphere. We suggest that the Hall term can be an important mechanism to provide asymmetric plasmoid propagation through the discontinuity and that Hall MHD can capture important macroscopic effects observed in hybrid simulations that are not described by ideal MHD.
Comparison of models and measurements at Millstone Hill during the January 24–26, 1993, minor storm interval
Authors: Buonsanto M. J. , Codrescu M. , Emery B. A. , Fesen C. G. , Fuller‐Rowell T. J. , Melendez‐Alvira D. J. , Sipler D. P.
Published: February 2004

Results from four first‐principle models are compared with Millstone Hill incoherent scatter radar and Fabry‐Perot interferometer measurements taken during January 24–26, 1993, a period which included a minor geomagnetic storm. The models used in this study are the thermosphere ionosphere electrodynamics general circulation model (TIEGCM) with and without forcings from the assimilative mapping of ionospheric electrodynamics (AMIE) technique, the coupled thermosphere ionosphere model (CTIM), and the field line interhemispheric plasma (FLIP) model. The present study is the first time the AMIE inputs have been used in the TIEGCM model. TIEGCM and CTIM both underestimate the neutral temperature because of an underestimation of the Joule heating rate. An increase in the high latitude Joule heating would modify the thermospheric circulation. This could result in increases in N2 and O2 density above Millstone Hill, which would decrease the AMIE TIEGCM peak electron density (NmF2) to agree better with the observations, but would result in poorer agreement between CTIM and the data. The FLIP model NmF2 is a little low compared to the data, perhaps because of an inadequacy of the mass spectrometer incoherent scatter (MSIS) 86 model composition or the H+ flux in the model. Good agreement is obtained between atomic oxygen density [O] given by MSIS and [O] obtained from the radar data using a heat balance equation, provided an O+–O collision frequency factor of 1.3 is used. While the TIEGCM underestimates the electron and ion temperatures, the FLIP model reproduces major features of the data, apart from a large nighttime enhancement in Te. During the minor storm interval the observed neutral winds show alternating equatorward surges and abatements apparently due to passage of traveling atmospheric disturbances (TADs) seen in the model results. These are associated with a late evening increase observed in NmF2 accompanied by a large increase in F2 peak height (hmF2). These perturbations in NmF2 and hmF2 are not reproduced by the TIEGCM or CTIM. The NmF2 increase may be due to a decrease in O+ recombination rate caused by the higher hmF2, combined with compressional effects of a TAD and an enhanced downward flux of O+ ions.
Geometry of Saturn's magnetopause model
Authors: Maurice S. , Engle Irene M. , Blanc M. , Skubis M.
Published: February 2004

We report on an idealized three‐dimensional model of Saturn's magnetopause. Using the subsolar point distance R0 = 24 RS, we provide an analytical parameterization of the magnetopause model described by Maurice and Engle [1995]. This representation of Saturn's magnetopause is consistent with Voyager 1 magnetopause crossing observations. The magnetopause shape is then scaled to 17 RS ≤ R0 ≤ 45 RS and applied to Pioneer 11 and Voyager 2 observations. Results highlight variations of R0 during magnetopause crossings (over a few hours) and/or between inbound and outbound crossings (over a few days). Finally, we extend the parameterization of Saturn's magnetopause to nonzero dipole tilt angles. The model is intended for use in support of the Cassini mission.
Particle flows observed in ejecta during solar event onsets and their implication for the magnetic field topology
Authors: Richardson I. G. , Cane H. V.
Published: February 2004

We investigate the magnetic field topology of ejecta (the interplanetary manifestations of solar mass ejections) by using observations of energetic particle anisotropies made by the Goddard Space Flight Center instruments on the ISEE 3, Helios 1, and Helios 2 spacecraft at ≤1 AU from the Sun. In particular, we investigate particle flows observed following the onsets of 39 solar energetic particle events which occurred when the observing spacecraft was located in a preexisting ejecta. To infer when the spacecraft was located in an ejecta, we examine a wide range of indicators of the presence of ejecta, including magnetic cloud signatures, bidirectional energetic ion flows, bidirectional solar wind electron heat fluxes, and depressed proton plasma temperatures. We find that inside ejecta, the particle arrival direction in the ecliptic plane varies widely from event to event, with at least 30% of the events showing flows from the east of the Sun. In contrast, in 60 similar events observed outside ejecta, the particle arrival directions are consistent with flows from the Sun along near‐Parker spiral field lines in >90% of cases. We also discuss multiple‐spacecraft observations of three particle events which illustrate the contrasting particle flows observed during the same event by spacecraft in different solar wind structures. We conclude that the observations are consistent with the presence of nonspiral magnetic fields, including looped magnetic field lines connected to the Sun, in at least a subset of ejecta.
HF radar observations of decaying artificial field‐aligned irregularities
Authors: Hysell D. L. , Kelley M. C. , Yampolski Y. M. , Beley V. S. , Koloskov A. V. , Ponomarenko P. V. , Tyrnov O. F.
Published: February 2004

Bistatic HF radar scatter from magnetic field‐aligned irregularities created by the Sura heater near Nizhny Novgorod, Russia, has been detected and analyzed at the UTR 2 decametric radio telescope near Kharkov, Ukraine. The decay phase of the irregularities exhibits two decay time constants in general. The shorter time constant, which is only sometimes seen, is consistent with classical diffusive dissipation theory for small‐scale, field‐aligned waves. The longer time constant, which is almost always present, is much too long to be accounted for by diffusion. It is more likely associated with the decay of large‐ and intermediate‐scale plasma turbulence. Three‐wave nonlinear interaction with turbulent modes is shown to be capable of generating the field‐aligned irregularities.
Survey of flux transfer events observed with the ISEE 1 spacecraft: Rotational polarity and the source region
Authors: Kawano H. , Russell C. T.
Published: February 2004

We have surveyed transient bipolar perturbations of the magnetic field component normal to the magnetopause, identifiable as flux transfer events (FTEs), from nine seasons during which the ISEE 1 spacecraft completely precessed through the dayside magnetosheath and magnetosphere. Our results indicate that FTEs tend to occur more frequently at postnoon local times, rather than the expected prenoon local times if these events had been associated with pressure pulses generated in the foreshock. The amplitude of both the magnetosheath FTEs and the magnetospheric FTEs tends to decrease, whereas the duration of the FTEs tends to increase, with increasing distance from the magnetopause. The occurrence rate of FTEs tends to increase with decreasing distance from the magnetopause. These features strongly indicate a magnetopause origin for FTEs in both regions. The rotational polarity of the magnetic field perturbations in a plane roughly parallel to the equator reverses across the noon meridian and across the magnetopause. This feature is consistent with an interpretation in which FTEs are generated on the dayside magnetopause and subsequently move antisunward along the magnetopause.
High electron temperature associated with the prereversal enhancement in the equatorial ionosphere
Authors: Oyama K.‐I. , Abdu M. A. , Balan N. , Bailey G. J. , Watanabe S. , Takahashi T. , de Paula E. R. , Batista I. S. , Isoda F. , Oya H.
Published: February 2004

High electron temperature in the equatorial ionization anomaly region detected first by Kyokko satellite and later observed frequently by Hinotori satellite are found to be closely associated with the ionization crests of the anomaly. This phenomenon, called the equatorial electron temperature anomaly, is found to occur predominantly in the equinoctial months and to become enhanced with the increase in solar activity. It is mainly a nighttime phenomenon and shows maximum temperature enhancement at around 2100 LT. Using a theoretical model, a mechanism for its occurrence is presented. The mechanism is based on the plasma transport in the evening equatorial ionosphere resulting from the sunset electrodynamical processes.
Comment on the ponderomotive self‐action of Alfvén waves
Authors: Guglielmi Anatol V.
Published: February 2004

The nonlinear equation descriptive of the ponderomotive self‐action of standing Alfvén waves is presented. Solutions to this equation possess a peculiar form, even though the wave amplitudes are relatively small. It is shown that the period of oscillations is diminished with a rise of amplitude. Preliminary results suggest that the heavy (light) ions are accumulated at the nodes (antinodes) of the nonlinear standing wave. It is concluded that account must be taken of the ponderomotive self‐action in studies of Alfvén oscillations of the magnetosphere.
A statistical survey of auroral solitary waves and weak double layers: 2. Measurement accuracy and ambient plasma density
Authors: Eriksson Anders I. , Mälkki Anssi , Dovner Per Ola , Boström Rolf , Holmgren Gunnar , Holback Bengt
Published: February 2004

Solitary electrostatic waves (SWs) were frequently observed by the Viking satellite in the auroral plasma. Previously, Mälkki et al. [1993] reported occurrence statistics and voltage data for these structures. In this paper, we make a detailed study of the measurement process in order to establish what further knowledge can be extracted from the data. The possibilities that SWs are spacecraft‐generated pulses or signatures of rectification of hypothetical wave packets are investigated and rejected. We analyze Langmuir probe data from several SW events and show that densities are at least as low as previously reported (a few cm−3). In order to assess the quality of the SW observations in this very tenuous plasma, we use a semiempirical model of the complete probe‐spacecraft‐plasma system, including nonlinearities in the probe sheaths. The usefulness of the model is shown by its ability to model details in the measured fluctuations. It is found that the errors in the voltage (electric field) measurements of solitary waves are very small, while the density fluctuation estimates may be severely affected by spurious signals. Therefore details in the shape of the observed probe current signature cannot be used to study the detailed shape of the density depletion. This limits the possibility of identifying weak double layer (WDL) characteristics in SWs, as effects of time evolution result in signatures similar to those of net potential drops.