bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical modeling of Alfvén waves observed by the Polar spacecraft in the nightside plasma sheet boundary layer
Authors: Streltsov A. V. , Lotko W. , Keiling A. , Wygant J. R.
Published: October 2002

Results from a numerical study of localized, large‐amplitude electromagnetic disturbances measured by the Polar satellite in the nightside plasma sheet are presented. It is shown that these disturbances can be explained in terms of shear Alfvén waves generated in the magnetotail and interacting with a microturbulent layer at low altitudes. The plasma microturbulence/anomalous resistivity self‐consistently included in the nonlinear, two‐fluid MHD model causes the wave reflection and defines its structure and amplitude in the lower magnetosphere. It also causes the dissipation of the wave energy inside the turbulent/resistive layer, which leads to the creation of parallel electric field and parallel electron acceleration. Thus plasma turbulence connects electromagnetic structures measured by Polar at radial distance 4–6 RE with bright aurora simultaneously observed by the Polar Ultraviolet Imager at the ionospheric footprint of the corresponding magnetic field lines.
Reducing heliospheric magnetic flux from coronal mass ejections without disconnection
Authors: Crooker N. U. , Gosling J. T. , Kahler S. W.
Published: October 2002

Until 1995, disconnection at the Sun was thought to be the only solution to the problem of balancing the magnetic flux coronal mass ejections (CMEs) added to the heliosphere, in spite of the fact that the expected solar wind signature of disconnection was rare. Disconnection was pictured as merging between open field lines to create completely disconnected U‐shaped structures or merging between closed field lines to create, in two dimensions, completely disconnected plasmoids. Since 1995, both theoretical and observational studies have made important contributions toward a revision of this solution. On the basis of a synthesis of these studies, we suggest that the primary flux balancing mechanism is not complete disconnection but rather merging between closed and open fields, which we call “interchange reconnection.” Disconnection on closed fields plays a role but, in three dimensions, only as a partial process that prevents expansion of arcade loops into the heliosphere by creating helical fields still connected to the Sun. We suggest that interchange reconnection occurs in the leg of a CME until it is completely open, long after its leading edge has departed from the vicinity of the Sun, and we discuss preliminary findings from Ulysses testing this view. Further, we suggest that interchange reconnection in CMEs may be driven by the large‐scale circulation of open field lines required by the Fisk model.
Dynamo circuits for magnetopause reconnection
Authors: Siebert K. D. , Siscoe G. L.
Published: October 2002

From the perspective of electric circuit theory a question regarding the existence of magnetopause reconnection arises because in circuit theory reconnection is a load and a load needs a dynamo to supply it. However, among current systems that have been identified that thread the presumed regions of magnetopause reconnection, none also treads a dynamo. Yet there is overwhelming observational evidence that magnetopause reconnection occurs. This paper attempts to reconcile the circuit theory viewpoint with observations of reconnection. The procedure is first to show that results of MHD simulation can be translated into circuit theory language. Then MHD simulation is applied to show that magnetopause reconnection is associated with current systems that close through the bow shock or the magnetosheath, which from the viewpoint of circuit theory are dynamos. Thus the reconciliation is simply that reconnection current systems are different from the standard set of current systems that have traditionally been associated with the magnetopause.
Correlation of magnetic field intensities and solar wind speeds of events observed by ACE
Authors: Owens Mathew J. , Cargill Peter J.
Published: October 2002

The relationship between the magnetic field intensity and speed of solar wind events is examined using ∼3 years of data from the ACE spacecraft. No preselection of coronal mass ejections (CMEs) or magnetic clouds is carried out. The correlation between the field intensity and maximum speed is shown to increase significantly when |B| > 18 nT for 3 hours or more. Of the 24 events satisfying this criterion, 50% are magnetic clouds, the remaining half having no ordered field structure. A weaker correlation also exists between southward magnetic field and speed. Sixteen of the events are associated with halo CMEs leaving the Sun 2 to 4 days prior to the leading edge of the events arriving at ACE. Events selected by speed thresholds show no significant correlation, suggesting different relations between field intensity and speed for fast solar wind streams and ICMEs.
Simulating the lunar geomagnetic variations
Authors: Stening R. J. , Carmody C. , Du J.
Published: October 2002

The equivalent circuit method is used to simulate the lunar semidiurnal geomagnetic tides. Samples of observations of the lunar geomagnetic tide are also presented, derived from analyses of hourly values of the horizontal component of the magnetic field recorded at several observatories. These are compared to the results of the simulations at different seasons. The winds used in the simulation are derived from the Global Scale Wave Model (GSWM) and from those determined by Forbes and Gillette [1982]. The calculations are only carried out for a lunar age of zero (new moon), but this still enables a comparison to be made with the observed phases of the lunar geomagnetic tide. A mostly fair agreement is obtained both for the amplitude and phase of the lunar geomagnetic tide. The agreement is generally better when winds from the Global Scale Wave Model are used rather than those from the earlier Forbes and Gillette model. The July simulation gives poorer agreement: there is corroborating evidence that the phase of the winds near the equator in July should be about 3 hours earlier than that presently given by the GSWM. The situation in December is different from other seasons both in the simulation and the observations, but agreement is difficult to achieve on account of the year‐to‐year variability in this season.
Auroral records from Canada 1769–1821
Authors: Broughton Peter
Published: October 2002

Over 1000 previously unused Canadian auroral observations from manuscript sources between 1771 and 1819 are examined. They are especially reliable because they are part of meteorological registers which simultaneously contain information on cloudiness and are significant because they add substantially to the scarce database of historical records, especially from high geomagnetic latitudes (57° < Φ1800 < 70°). The statistics derived from these data show an interval of low auroral activity from about 1785 to 1815, a decade earlier than the Dalton solar activity minimum, superimposed on a fairly regular periodicity of about 11 years and seasonal variations. The relationship between the auroral occurrence frequency and solar activity from 1795 to 1815 is similar to what is normally seen at lower latitudes suggesting that the auroral oval contracted more than usual during the Dalton minimum. The data support the type of relationship found by Ohl, which directly relates the frequency of auroral occurrence at high latitudes during the declining portion of one solar cycle to the sunspot number at the following maximum and the rapidity with which the solar activity returns to a maximum.
Adiabatic energization in the ring current and its relation to other source and loss terms
Authors: Liemohn M. W. , Kozyra J. U. , Clauer C. R. , Khazanov G. V. , Thomsen M. F.
Published: October 2002

The influence of adiabatic energization and deenergization effects, caused by particle drift in radial distance, on ring current growth rates and loss lifetimes is investigated. Growth and loss rates from simulation results of four storms (5 June 1991, 15 May 1997, 19 October 1998, and 25 September 1998) are examined and compared against the y component of the solar wind electric field (Ey,sw). Energy change rates with and without the inclusion of adiabatic energy changes are considered to isolate the influence of this mechanism in governing changes of ring current strength. It is found that the influence of adiabatic drift effects on the energy change rates is very large when energization and deenergization are considered separately as gain and loss mechanisms, often about an order of magnitude larger than all other source or loss terms combined. This is true not only during storm times, when the open drift path configuration of the hot ions dominates the physics of the ring current, but also during quiet times, when the small oscillation in L of the closed trajectories creates a large source and loss of energy each drift orbit. However, the net energy change from adiabatic drift is often smaller than other source and loss processes, especially during quiet times. Energization from adiabatic drift dominates ring current growth only during portions of the main phase of storms. Furthermore, the net‐adiabatic energization is often positive, because some particles are lost in the inner magnetosphere before they can adiabatically deenergize. It is shown that the inclusion of only this net‐adiabatic drift effect in the total source rate or loss lifetime (depending on the sign of the net‐adiabatic energization) best matches the observed source and loss values from empirical Dst predictor methods (that is, for consistency, these values should be compared between the calculation methods). While adiabatic deenergization dominates the loss timescales for all Ey,sw values, there is a characteristic decrease in the loss lifetime for higher Ey,sw, seen computationally and observationally. It is shown that these shorter loss lifetimes at higher Ey,sw are caused primarily by enhanced dayside flow out. The simulation results show that, on average, flow out is a larger energy loss term than charge exchange (the next biggest loss term for the ring current) for all Ey,sw values.
A large‐scale traveling ionospheric disturbance during the magnetic storm of 15 September 1999
Authors: Shiokawa K. , Otsuka Y. , Ogawa T. , Balan N. , Igarashi K. , Ridley A. J. , Knipp D. J. , Saito A. , Yumoto K.
Published: October 2002

Using a comprehensive data set and model calculations, we have investigated a prominent large‐scale traveling ionospheric disturbance (LSTID) observed in Japan (∼37°–16° MLAT) on 15 September 1999, during a recovery phase of sequential storms. The LSTID was detected at 2300–2400 LT (1400–1500 UT) as an enhancement of the 630‐nm airglow intensity (50→350 R), a decrease in the F layer virtual height (at 2 MHz, 360→200 km), an enhancement of foF2 (6→8 MHz), and an enhancement of GPS total electron content (∼1.0 × 1016 m−2). Multipoint and imaging observations of these parameters show that the LSTID moved equatorward over Japan with a velocity of ∼400–450 m/s. From a comparison with the Sheffield University Plasmasphere‐Ionosphere Model (SUPIM) we conclude that an enhancement (250–300 m/s) of poleward neutral wind (that is propagating equatorward) caused these observational features of the LSTID at midlatitudes. To investigate generation of the LSTID by auroral energy input, we have used auroral images obtained by the Polar UVI instrument, magnetic field variations obtained at multipoint ground stations, and the empirical Joule heating rate calculated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. Intense auroral energy input was observed at 0800–1100 UT (4–6 hours before the LSTID), probably causing equatorward neutral wind at lower latitudes. It is likely that the poleward wind pulse that caused the observed LSTID was generated associated with the cessation of this equatorward wind. The effect of Lorentz force is also discussed.
Joule heating patterns as a function of polar cap index
Authors: Chun Francis K. , Knipp Delores J. , McHarg Matthew G. , Lacey James R. , Lu Gang , Emery Barbara A.
Published: October 2002

Previous work by Chun et al. [1999] has shown that the polar cap (PC) index can be used as a proxy indicator of the integrated Joule heating rate in the Northern Hemisphere. However, knowledge of the spatial distribution of Joule heating is also important. The PC index is a single magnetometer station‐derived index which measures the level of geomagnetic activity in the polar cap. A negative PC index corresponds to a condition related to lobe‐merging convection conditions in the polar cap, near‐zero PC indicates a quiet polar cap, and a positive PC indicates geomagnetically active periods. In this study we developed average patterns of Joule heating as a function of PC using 56 days (∼12,800 individual patterns) of Assimilative Mapping of Ionospheric Electrodynamics data from various case studies. With PC ranging from −3 to 8, we divide the Joule heating patterns into PC bins of 1.0, finding that there is a clear spatial evolution of Joule heating from negative PC to positive PC. When PC is negative, Joule heating on average is constrained to the high‐latitude dayside. As PC goes to zero, Joule heating disappears. When PC increases in the positive direction, Joule heating intensifies throughout the auroral oval, with primary heating occurring along the dawn/dusk flanks. Further analysis reveals that it is primarily changes in the electric potential and not the Pedersen conductance that accounts for Joule heating shifting from the auroral zone to the high‐latitude dayside when PC goes negative. We also find that the cross polar cap potential is linearly proportional to the PC index, consistent with previous studies. Finally, we compare the hemispheric energy budget between Joule heating and electron particle energy. We find that globally, Joule heating is roughly 4 times that of particle energy during geomagnetically active times (positive PC). During quiet times (zero PC), Joule heating and particle energy are equal contributors. When lobe‐merging conditions occur (negative PC), Joule heating again dominates over particle energy except during the winter season, when polar cap conductivity is driven more by particle precipitation.
Prediction of solar activity: Role of long‐term variations
Authors: Kane R. P.
Published: October 2002

From the near‐zero values during the Maunder Minimum (1645–1715), the maxima Rz(max) of smoothed sunspot numbers reached values of ∼140 by cycle 0–1 (∼1750) and then had long‐term fluctuations (high values for 3–5 cycles, followed by low values for 3–5 cycles, differing by as much as 50 units), on which random fluctuations were superposed. There is some indication of a ∼100‐year oscillation that might have peaked near cycle 20 at Rz(max) values near ∼150. If this oscillation persists, the cycles in the next 50 years or so may have lower and lower R(max), perhaps dropping below 100, and recovering thereafter. The aa(min) values, and to a lesser degree, the Rz(min) values, are also expected to follow the same pattern.
Altitude variations of cosmic ray induced production of aerosols: Implications for global cloudiness and climate
Authors: Yu Fangqun
Published: October 2002

The indirect radiative forcing of atmospheric aerosols is sensitive to particle size and concentration, which are influenced significantly by nucleation processes. Via its role in aerosol formation, cosmic ray may affect the cloud condensation nuclei abundance and hence the global cloud properties and climate. Systematic variations in ionization rates due to the modulation of cosmic ray radiation by the solar cycle are sufficient to cause notable variations in aerosol production, and we find that the signs of such variations are altitude‐dependent. Our study indicates that an increase in cosmic ray fluxes generally leads to an increase in particle production in the lower troposphere but a decrease in particle production in the upper troposphere. The main reason of such an altitude‐dependent influence is that the dependence of particle production rate on ionization rate is a complex function of ionization rate itself, as well as precursor gas concentration and ambient conditions. The implications of altitude variations of cosmic ray‐induced aerosol production on global cloudiness and climate are discussed. In addition to the reported positive correlation between cosmic ray variations and low cloudiness, our analysis reveals that high cloudiness may be anti‐correlated with cosmic ray variations if volcano and El Niño impacts are excluded. The observed different correlations between cosmic ray variations and low, middle and high cloud anomalies appear to be consistent with the predicted different sensitivities of particle production to cosmic ray changes at different altitudes. A systematic change in global cloudiness may change the atmosphere heating profile, and if confirmed, may provide the external forcing needed to reconcile the different surface and troposphere temperature trends. Much more work is needed to understand how and how much the cosmic ray variations will affect the global cloud properties and climate.
Properties of the magnetic field in the Jovian magnetotail
Authors: Kivelson Margaret G. , Khurana Krishan K.
Published: October 2002

Magnetometer data acquired as the Galileo Orbiter apoapsis rotated from dawn to dusk across the magnetotail of the Jovian magnetosphere between late 1995 and the end of May 2000 are used to characterize the magnetic field and the distribution of magnetic pressure in the inner part of the Jovian magnetotail. The distances probed extend to ∼150RJor roughly 3 times the distance to the nose of the magnetopause, analogous to distances within 30REin the magnetotail of Earth. The magnetic pressure in the center of the plasma sheet is typically almost an order of magnitude smaller than the lobe pressure, which therefore is roughly equal to the peak thermal plasma pressure in the plasma sheet. The lobe magnetic pressure decrease with radial distance can be described roughly as a power law with an exponent of −2.74, and the lobe field magnitude decreases with distance to the −1.37. In comparing radial variations of the lobe magnetic fields of Jupiter and Earth we argue that rescaled lengths based on nominal magnetopause standoff distances and rescaled field magnitudes based on planetary dipole field strengths are required. Comparison of rescaled fits suggest that the power law dependence on radial distance should not be extrapolated beyond 150RJand that beyond this distance the trend may become asymptotically constant. Systematic asymmetries of the field structure and magnetic pressure across the midnight meridian in the region beyond 25RJdowntail are notable, with the flux tubes being less stretched (with larger equatorialBz) near dusk than near dawn. The lobe pressure attains its minimum value in the dusk sector where the plasma sheet magnetic pressure maximizes. We argue that only a small fraction of the magnetic flux remaining in the lobes in the magnetotail beyond 100RJcloses across the plasma sheet, but the structure of open and closed flux tubes may be nonuniform across the tail. We discuss briefly mechanisms that may lead to the observed asymmetry.
Analytic model for the electrostatic potential jump across collisionless shocks, with application to Earth's bow shock
Authors: Kuncic Zdenka , Cairns Iver H. , Knock Stuart
Published: October 2002

Existing observational analyses demonstrate that the electrostatic potential jump across collisionless shocks (Δϕ′ in the de Hoffmann–Teller frame) strongly influences the behavior of electrons. The empirical relations inferred from these data provide crucial constraints on shock‐related phenomena, such as foreshock electron beams and radio emission. However, a theoretical interpretation of these relations, especially their parameter dependences, is necessary. We present an analytic model for Δϕ′ which incorporates the full dependences on solar wind and shock conditions. The model involves three assumptions for the spatially varying number density n(x), magnetic field B(x), and perpendicular and parallel electron temperatures (Te⟂(x) and Te∥(x) across the shock: (1) n(x) ∝ B(x) in a piecewise linear fashion; (2) Te⟂(x) ∝ B(x); and (3) ΔTe∥ ∝ ΔTe⟂. Empirical and theoretical arguments for these assumptions exist for moderate to strong shocks, breaking down for weak shocks (assumption 3) and shocks where Te increases by a factor >4 (assumption 2). The model is qualitatively and quantitatively consistent with the known empirical relations between Δϕ′ and the jumps in Te and B across the shock, providing a theoretical basis for them. We use the model to test for a possible correlation between Δϕ′ and the change in the normal component of the ion ram energy, ΔEram. We find that the relation between Δϕ′ and ΔEram is only approximately linear over a range of shockfront locations for constant solar wind conditions, with the relation depending strongly on the angle θbu,1 between the upstream flow speed u1 and magnetic field B1 and on the upstream ratio u1/vA,1 of flow speed to Alfvèn speed. Our model provides a natural interpretation for the spread of observational data around the best linear fit suggested by Hull et al. [2000].
Three‐dimensional velocity structure of the polar wind
Authors: Demars H. G. , Schunk R. W.
Published: October 2002

A three‐dimensional, time‐dependent, fluid model was used to investigate the velocity structure of the global ionosphere–polar wind system. Each simulation involved following a large number of high‐latitude flux tubes (over 1000) as they moved under the influence of convection and corotational electric fields. A flux tube typically traversed various high‐latitude regions during the course of a simulation, including the subauroral ionosphere, dayside and nightside auroral ovals, and polar cap. Heating due to auroral electron precipitation was included in the simulations, but “nonclassical” heating mechanisms at high altitudes, such as wave‐particle interactions, were not. The model results show the variation of H+ and O+ drift velocities with respect to magnetic latitude and longitude over the northern polar region, as well as variations with altitude and universal time (UT). A simulated geomagnetic storm of moderate intensity was introduced at a particular UT, and the effect of variable geomagnetic activity levels on the two major ion species was studied. Equivalent simulations were performed for four geophysical cases, corresponding to summer and winter solstices at both solar maximum and minimum. These simulations reveal the seasonal and solar cycle dependence of the ion velocities for both quiet and geomagnetically active times. The H+ and O+ populations responded to the varying geophysical conditions in dramatically different ways. These responses provide insight into the role played by the polar wind in the larger and inclusive system of fields and plasmas referred to as the Sun‐Earth connection. The self‐consistent high‐resolution results obtained here reveal structure in the polar wind that cannot be resolved by simpler studies using velocity profiles based on averaged experimental data. The results of this investigation were compared with those of relevant observational studies.
Successive CMEs and complex ejecta
Authors: Burlaga L. F. , Plunkett S. P. , St. Cyr O. C.
Published: October 2002

We identified three sets of successive halo CMEs directed toward Earth (two or more CMEs observed within 1–4 days in the corona) and the corresponding flows and magnetic fields at 1 AU. Each set of successive halo CMEs was associated with complex ejecta at 1 AU. Complex ejecta were defined by Burlaga et al. [2001] as fast (>600 km/s) flows that are neither corotating flows nor magnetic clouds, which move past Earth during a day or more. They correspond to the transient flows discussed in the literature prior to the discovery of magnetic clouds. We infer that in each case the successive CMEs merged en route from the Sun to the Earth to form complex ejecta. In general, the components of the complex ejecta corresponding to individual CMEs cannot be identified, suggesting that the merging process is nonlinear and irreversible, and memory of the conditions near the sun is lost in the process. The identity of the individual CMEs is gradually lost as the interaction proceeds within 1 AU. In some cases, the interaction can result in what appears to be a single stream in the speed profile at 1 AU. Nevertheless, given observations of successive halo CMEs, one can possibly predict the arrival of complex ejecta or more complicated transient flow systems at 1 AU. In some cases, complex ejecta related to multiple CMEs can be associated with a single source region, an active region.
Cyclotron amplification of whistler‐mode waves: A parametric study relevant to discrete VLF emissions in the Earth's magnetosphere
Authors: Pasmanik D. L. , Demekhov A. G. , Nunn D. , Trakhtengerts V. Y. , Rycroft M. J.
Published: October 2002

We study the amplification of ducted whistler‐mode waves in the Earth's magnetosphere by the cyclotron instability for different types of energetic electron distributions in velocity space. Particular attention is paid to the comparison between cases of smooth distribution functions and those with sharp gradients (“steps”) in velocity space, which arise naturally owing to interactions with noise‐like VLF emissions. We show that step‐like features greatly favor the amplification of whistler‐mode waves propagating along the Earth's magnetic field line and hence the generation of narrowband VLF emissions even if the electron anisotropy is only moderate. The results obtained are discussed in light of observations of discrete VLF emissions from the magnetosphere, in particular those of Bell et al. [2000].
Field line resonances in a stretched magnetotail: CANOPUS optical and magnetometer observations
Authors: Wanliss J. A. , Rankin R. , Samson J. C. , Tikhonchuk V. T.
Published: October 2002

Ground‐based observations support the existence of 1–4 mHz field line resonances (FLRs) at auroral latitudes. The low frequencies suggest that many nightside FLRs are excited on stretched magnetic field lines in the inner plasma sheet. Data from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometers and photometers on 21 February 1993 indicate a FLR with a peak at 2.4 mHz, and that of 31 January 1997 indicate a peak at 1.3 mHz. In this paper, models of FLRs and models of stretched magnetotail topologies deduced from optical data are compared with the observations. The FLR model is independently constrained by a parametric near‐Earth magnetotail model that allows us to make direct comparisons with the observed FLR frequency. For the two case studies mentioned above, we are able to show that the theoretical FLR spectrum, computed on stretched field lines, is comparable to the low‐frequency experimental observations.
High‐latitude ionospheric convection models derived from Defense Meteorological Satellite Program ion drift observations and parameterized by the interplanetary magnetic field strength and direction
Authors: Papitashvili Vladimir O. , Rich Frederick J.
Published: October 2002

A series of new high‐latitude ionospheric convection models have been constructed using Defense Meteorological Satellite Program (DMSP) thermal ion drift measurements. The models are obtained by sorting cross polar cap electrostatic potentials into magnetic latitude/magnetic local time bins. A regression analysis of the potentials in each bin is then implemented for establishing the relationships to the interplanetary magnetic field (IMF) for three seasons: summer, winter, and equinox. A linear modeling formula for the ionospheric electrodynamics (LIMIE) yields a convection response to the average solar wind (i.e., the “quasi‐viscous” interaction) and to changes in the IMF By, Bz ≤ 0, and Bz > 0 components. The modeled convection is a superposition of the first two parameters with either the IMF Bz ≤ 0 or the Bz > 0 component. A global model is created by fitting the regression analysis results to a spherical harmonic function. The resulting DMSP‐based ionospheric convection model (DICM) is fully parameterized by the IMF strength and direction. With this model, ionospheric convection patterns can be generated for any IMF configuration during quiet to moderate geomagnetic conditions. We compare the DICM model with other available high‐latitude convection patterns organized by the IMF. The new elements in DICM are its quasi‐viscous and separate IMF‐dependent terms for both the northern and southern polar regions, which are not explicitly found in other ionospheric convection studies. The DICM's seasonal dependence and interhemispheric symmetry/asymmetry features show that the summer cross‐polar potentials are 10–15% smaller than the winter potentials. The latter is in agreement with the seasonal dependence of field‐aligned currents and with the voltage‐current relationship required for the proper magnetosphere‐ionosphere coupling.
New parametric decays of proton beam–plasma electromagnetic waves
Authors: Gomberoff L. , Gomberoff K. , Brinca A. L.
Published: October 2002

Ion beam–plasma interactions are the source of wave activity in several space environments. Therefore the study of these waves and their parametric decays are very important in space physics. Thus we study parametric decays of right‐hand polarized proton beam–plasma waves including the effect of the beam. It is shown that there are new instabilities due to the beam and the associated ion acoustic waves. The branch of the linear dispersion relation corresponding to the beam has negative energy so that whenever this branch participates in a decay, the corresponding instability should lead to an explosive instability. Some explosive behavior has indeed been found in simulation results involving right‐hand polarized waves. Since the free‐energy source of the linear beam–plasma instabilities is the beam energy, one expects the beam speed to decrease as the waves grow and experience nonlinear decays. Therefore we study the effect of varying beam velocity in the parametric decays. It is shown that there are instabilities which depend on the beam velocity in such a way that they are stabilized as the beam decelerates.
Multifrequency measurements of HF Doppler velocity in the auroral E region
Authors: Makarevitch R. A. , Koustov A. V. , Sofko G. J. , André D. , Ogawa T.
Published: October 2002

HF measurements in Prince George, British Columbia (Canada), at five radar frequencies between 9.3 and 15.7 MHz are considered to study the Doppler velocity of E region coherent echoes. One event showing a regular variation of velocity with radar frequency, slant range, and azimuth of observations is analyzed in detail. For this event, plasma drifts were in access of 700 m s−1, but the observed velocities were below 250 m s−1 since measurements were performed at large flow angles (L shell angles 45°< ϕ< 100°). We show that measured Doppler velocity depends on irregularity scale but only within the Farley‐Buneman (F‐B) instability cone (45°< ϕ< 75°). We demonstrate that maximum velocities measured at the highest radar frequency are ∼1.3 times larger than those at the lowest frequency. We also show that for observations inside the instability cone, the velocity magnitude strongly decreases with aspect angle and the rate of the decrease is scale sensitive. The effect can be described by the fluid theory formula if the nominal electron collision frequencies are replaced by anomalous collision frequencies that are ∼5 times larger. However, for observations outside the F‐B instability cone (75°< ϕ< 100°), the Doppler velocity does not show significant variation with aspect angle. For these directions, velocity change with flow angle was insignificant, very similar at all radar frequencies, and not consistent with the expected “cosine” law. The implications of the measurements on the theory of electrojet instabilities and the processes of coherent echo formation are discussed.
Signatures of wave‐ion interactions in the solar wind: Ulysses observations
Authors: Gary S. Peter , Goldstein Bruce E. , Neugebauer Marcia
Published: October 2002

Theory and simulations predict that resonant wave‐particle scattering by enhanced magnetic fluctuations yields characteristic signatures in properties of solar wind ions. Here Ulysses/Solar Wind Observations Over the Poles of the Sun (SWOOPS) instrument measurements of protons and alpha particles are examined to seek three types of these signatures: (1) a statistical constraint on the proton temperature anisotropy imposed by the electromagnetic proton cyclotron instability; (2) a correlation between the alpha anisotropy and the alpha/proton relative flow speed driven by electromagnetic alpha/proton instabilities; and (3) relationships among the proton anisotropy, the alpha anisotropy, and the alpha/proton speed induced by the cascade of long‐wavelength Alfvén fluctuations to ion cyclotron resonances at shorter wavelengths. No signatures of alpha/proton instabilities are found. However, the first example of the proton anisotropy constraint in the slow solar wind is demonstrated, and a theoretical prediction for Alfvén/cyclotron wave acceleration in the fast wind is similar to a statistical lower bound observed on the alpha/proton speed.
Generation of the fast solar wind: A review with emphasis on the resonant cyclotron interaction
Authors: Hollweg Joseph V. , Isenberg Philip A.
Published: October 2002

In situ measurements of the solar wind and remote observations of coronal holes have strongly implicated the resonant interaction with ion cyclotron waves as the responsible mechanism for heating and accelerating coronal hole ions to generate the fast solar wind. We review the current observational and theoretical knowledge of this mechanism and the progress that has been made in modeling the solar wind properties that result from this interaction. We begin by examining the observational and theoretical motivations for the continued study of this mechanism, including a brief historical review of these ideas. We then discuss the interplay of the resonance condition and the wave dispersion relation, which determines which ions can exchange energy with the waves. The physical basis for the interaction is then described, and we derive simple expressions for the ion response to the resonant wave dissipation. The complicated topic of oblique propagation is dealt with next, including how the resonant interaction operates for obliquely propagating waves. The plasma response to the resonant dissipation is often approximated by treating the ion populations as fluids, and we examine the solar wind models which incorporate various versions of this interaction. We then present a sample model that illustrates many of the properties and shortcomings of the current fluid results in this area. However, the resonant interaction is most accurately treated with a kinetic description of the ions and the recent attempts to construct kinetic models are explored. We close with a discussion of the many observational and theoretical gaps in our understanding that remain, including a presentation of alternative mechanisms and some speculations on future developments in this field.
Quantitative test of the cavity resonance explanation of plasmaspheric Pi2 frequencies
Authors: Denton R. E. , Lee D. H. , Takahashi K. , Goldstein J. , Anderson R.
Published: October 2002

A Pi2 wave event was observed by the CRRES satellite on day 48 (17 February) of 1991, 1220–1250 UT [Takahashi et al., 2001]. Using the electron density profile measured by CRRES on its outward pass (during which the wave event was observed), and assuming that the ions are all H+, we calculated the theoretical plasmaspheric cavity resonance frequencies using a two‐dimensional MHD simulation in dipole geometry (azimuthal symmetry). Our theoretical frequencies are in good agreement with the observed frequencies, and this supports the cavity resonance mechanism as an explanation for the observations. The observed and theoretical harmonic frequency ratios f2/f1 and f3/f1 were in rough agreement with earlier studies. Comparison of the outbound and inbound legs of the CRRES orbit on 17 February 1991 shows that the density on the outbound leg at magnetic local time (MLT) = 20–23.4 was different than that on the inbound leg at MLT = 23.4–4.5. On the outbound leg an enhanced density region beyond the region of the nominal plasmapause (which may have been part of a plasmaspheric tail [Sandel et al., 2001]) played an important role in determining the cavity resonance frequencies. Observations by the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft have revealed that there is often a great deal of azimuthal structure in the plasmasphere (including such tails). Such structure may play an important role in determining observed Pi2 frequencies.
Multisatellite observations of MeV ion injections during storms
Authors: Lorentzen K. R. , Mazur J. E. , Looper M. D. , Fennell J. F. , Blake J. B.
Published: October 2002

We report here on the formation of new ion radiation belts observed in connection with several solar energetic particle events and large geomagnetic storms in 1998 and 2000. We use observations from the Polar spacecraft, the highly elliptical orbit (HEO) 1997‐068, and the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), to study details of the inner zone radiation belt at high and low altitudes. We focus specifically on the four International Solar Terrestrial Physics events of August and September 1998 and April and July 2000. In several events we find new 2–15 MeV proton belts at various locations between L = 2.0 and L = 3.5. The low‐altitude SAMPEX observations revealed features not visible at high altitudes, such as radiation belts with multiple peaks in L shell. During the July 2000 event, energetic helium and iron were observed at L ∼ 2, suggesting a solar energetic particle source for these injected ions. We compare observations of these new belts and remark on the significant differences from event to event.
Theory of magnetic connectivity in the solar corona
Authors: Titov Vyacheslav S. , Hornig Gunnar , Démoulin Pascal
Published: October 2002

Although the analysis of observational data indicates that quasi‐separatrix layers (QSLs) of magnetic configurations have to play an important role in solar flares, the corresponding theory is only at an initial stage so far. In particular, there is still a need of a proper definition of QSLs based on a comprehensive mathematical description of magnetic connectivity. Such a definition is given here by analyzing the mapping produced by the field lines which connect photospheric areas of positive and negative magnetic polarities. It is shown that magnetic configurations may have regions, where cross sections of magnetic flux tubes are strongly squashed by this mapping. These are the geometrical features that can be identified as the QSLs. The theory is applied to quadrupole configuration to demonstrate that it may contain two QSLs combined in a special structure called hyperbolic flux tube (HFT). Both theoretical and observational arguments indicate that the HFT is a preferred site for magnetic reconnection processes in solar flares.
Jovian S burst drift rates and S burst/L burst interactions in a phase‐bunching model
Authors: Willes A. J.
Published: October 2002

A nonlinear feedback mechanism for the generation of Jovian S bursts is reexamined in the light of recent observations of S burst phase coherence. In this model, phase‐coherent radiation is generated by phase‐bunched electrons within interaction regions drifting along the Io flux tube. It is shown that the same mechanism, operating in two separate regimes, can produce both constant‐frequency (zero‐frequency‐drift) and rapid‐frequency‐drift emissions. Evaluation of S burst drift rates reveals that several unexplained properties of S bursts, namely, their complex morphologies and interactions with narrowband L bursts, are consistent with a phase‐bunching model.
Observations of bow shock and magnetopause crossings from geosynchronous orbit on 31 March 2001
Authors: Ober Daniel M. , Thomsen M. F. , Maynard N. C.
Published: October 2002

On 31 March 2001, significant enhancements in both the strength of the interplanetary magnetic field and the solar wind dynamic pressure pushed the magnetopause and bow shock inside of geosynchronous orbit. We present here observations of the unshocked solar wind observed with the Los Alamos magnetospheric plasma analyzer on geosynchronous satellite 1994‐084 near local noon. The magnetosheath was observed intermittently at geosynchronous satellites 1991‐080, 1994‐084, and LANL‐01A, across much of the dayside magnetosphere. Comparisons with theory of the observed bow shock and magnetopause locations at geosynchronous orbit near noon are in reasonable agreement. Using nearly simultaneous observations of the magnetopause and bow shock locations at 0451 UT, we have tested models for the thickness of the magnetosheath and flaring of the magnetopause at that time. Using the least flared of the magnetopause models, the estimated thickness of the magnetosheath would be thicker than would be predicted by either the gas dynamic solution of Spreiter et al. [1966] or the MHD simulations of Cairns and Lyon [1996]. Alternatively, placing the location of the magnetopause at the nose of the magnetosphere in conformance with the thickness of the magnetosheath from Cairns and Lyon [1996] requires less flaring on the flanks of the magnetosphere than currently present in the empirical models. The models also have difficulty predicting the other observed magnetopause crossings far from local noon apparently due to excessive flaring of the models for the conditions on this day.
Pickup ion acceleration by turbulent field‐aligned electric fields in the slow low‐latitude solar wind
Authors: le Roux Jakobus A. , Zank Gary P. , Matthaeus William H.
Published: October 2002

Recent spacecraft observations reveal the continuous presence of energetic tails in solar wind and pickup ion spectra in the quiet, slow low‐latitude solar wind. To explain this phenomenon, we take into account growing evidence from observations, theory, and simulations that MHD turbulence in the solar wind has a dominant two‐dimensional (2‐D) component. Of special significance is that 2‐D MHD turbulence simulations feature turbulent field‐aligned electric fields due to turbulent convection of 2‐D magnetic field fluctuations. Such electric fields have been observed in the low‐latitude solar wind. We explore pickup ion propagation and acceleration between the Sun and Earth with a newly developed kinetic numerical model. It includes a random distribution of large‐scale field‐aligned electric field fluctuations consistent with observations in the low‐latitude solar wind. The main result is the qualitative reproduction of observed accelerated interstellar pickup He+spectra at Earth. We find that a spectral “knee” connects the pickup ion core with the accelerated “tail.” Like observations, the simulated tail is approximately a power lawf(v) ∝v−4.5until it reaches particle speeds ∼10 times the solar wind speed, where a rollover occurs. The energetic tail particles originate closer to the Sun, where acceleration is most efficient. The knee forms because the local pickup ion source particles are abundant but weakly accelerated. Further predictions, such as large knees for pickup ion species formed from interstellar neutrals with large ionization cavities, and the absence of knees in accelerated spectra of inner source pickup ions are also discussed.
Three‐dimensional particle simulations of collisionless magnetic reconnection
Authors: Zeiler A. , Biskamp D. , Drake J. F. , Rogers B. N. , Shay M. A. , Scholer M.
Published: October 2002

Three‐dimensional (3‐D) particle simulations are performed in a double current layer configuration to investigate the stability of current sheets and boundary layers which develop during magnetic reconnection of antiparallel fields in collisionless plasma. The strong current layers that develop near the x line remain surprisingly laminar, with no evidence of turbulence and associated anomalous resistivity or viscosity. Neither the electron shear flow instabilities nor kink‐like instabilities, which have been observed in these current layers in earlier simulations, are present. The sharp boundary layers which form between the inflow and outflow regions downstream of the x line are unstable to the lower hybrid drift instability. The associated fluctuations, however, do not strongly impact the rate of reconnection. As a consequence, magnetic reconnection in the 3‐D system remains nearly two dimensional.
Thin current sheets in the magnetotail and the loss of equilibrium
Authors: Birn Joachim , Schindler Karl
Published: October 2002

Using two‐dimensional magnetohydrostatic equilibium theory, together with flux, entropy, and topology conservation, we demonstrate that a finite boundary deformation of magnetotail equilibria can lead to locally infinite current density enhancement, that is, the formation of a singular current sheet. Equilibrium configurations that satisfy the constraints cease to exist when the boundary deformation exceeds the critical value. This provides a strong argument for the onset of instability or the loss of equilibrium, regardless of the dissipation mechanism. The major feature causing the current density increase appears to be the localized increase of the boundary flaring angle, rather than the absolute magnitude of the boundary deformation. Such boundary perturbations might be caused by a solar wind pressure variation or suitable changes of the boundary electric field. Catastrophe points can be reached even for relatively small perturbations of the boundary. In those cases the excess energy associated with the thin current sheet is small. Nevertheless, a major disruption may be triggered when the underlying configuration becomes unstable upon the local breakdown of ideal MHD.
Solar flares not producing sudden phase advances
Authors: Kaufmann Pierre , Rizzo Piazza Liliana , Fernandez José Henrique , Rocha da Silva Márcia
Published: October 2002

We present a study of solar flares that do not produce disturbances in the low terrestrial ionosphere, detectable in very low frequency (VLF) low ionosphere propagation as sudden phase advances (SPA). We selected only flares with larger optical H‐α importance (equal or larger than 1), occurred near the cycle 22 solar maximum (1987–1989), at times when VLF data were available for long‐distance propagation paths entirely illuminated by the Sun. For the 463 optical solar flares selected, we found that 18.8% had no measurable effects on the lower ionosphere. Inversely, all measured SPAs did have a solar counterpart, optical (such a subflare) and/or X‐ray fluxes. Among the H‐α flares that do not produce measurable disturbances in the low terrestrial ionosphere, there is a surprising selection for events occurred at the solar limbs. On the other hand, the X‐ray flux for the H‐α flares selected, obtained from GOES for the bands 0.5–4 Å and 1–8 Å exhibited a rather scattered correlation with the SPAs amplitudes. GOES X‐rays fluxes for H‐α flares not producing SPAs extend over 2–3 orders of magnitude. These results may suggest real physical distinctions between events. Possible explanations suggest the existence of a directional trend for the soft X‐ray produced in flares possibly combined with the blockage of ionizing X rays behind the solar limbs.