bluuebunny/crossref_metadata_2025 huggingface dataset:
A 22‐year dependence in the size of near‐ecliptic corotating cosmic ray depressions during five solar minima
Authors: Richardson I. G. , Cane H. V. , Wibberenz G.
Published: September 2002

Evidence is presented for a 22‐year cycle in the amplitude of recurrent, near‐Earth, near‐Ecliptic, galactic cosmic ray modulations during the solar minimum periods in the mid‐1950s to mid‐1990s. These modulations are ∼50% larger during A > 0 epochs than during A < 0 epochs, where A is the direction of the solar global magnetic field. This difference is evident in all the cosmic ray data examined, including observations by neutron monitors and integral and differential particle data from the IMP 8 spacecraft. A change in the properties of the associated corotating streams does not appear to be responsible. Rather, the response of the cosmic rays to solar wind speed enhancements appears to be reduced in A < 0 minima. This pattern is inconsistent with a prediction from the drift model of cosmic ray modulation that recurrent modulations should be larger in A < 0 epochs. Other factors may contribute, such as an epoch‐dependence of the particle diffusion coefficient which may increase the effect of solar wind convection on the local cosmic ray density near Earth during A > 0 epochs. A change in the latitudinal GCR gradient in successive epochs is unlikely to be the sole reason for the change in depression size since (1) the proportionality constant between the recurrent depressions and latitudinal gradient depends on A, and (2) the cosmic ray density is not organized by crossings of the heliospheric current sheet.
Energetic electron butterfly distributions near Io
Authors: Thorne R. M. , Williams D. J. , Zhang L. D. , Stone S.
Published: September 2002

Pronounced variations in the energetic electron distribution observed by the Energetic Particle Detector during the Galileo flyby of Io are described as a quasi‐adiabatic response to the changing electric and magnetic field environment near the satellite. The energetic particle signatures can therefore be used to remotely sense the spatial distribution of electric and magnetic fields in the vicinity of Io. Electron pitch angle distributions evolve from a normal pancake distribution (peaked at 90° pitch angle) in the undisturbed torus to a butterfly distribution in the strong field depression near Io. The strongest flux depletions at 90° pitch angle result from a reduction in kinetic energy due to conservation of the first adiabatic invariant, as electrons are transported into the vicinity of Io. The magnitude of the flux depletion is related to the spectral index n of the electron energy spectrum (J ∼ E−n). Since the value of n tends to increase with increasing energy, the largest flux drop occurs at higher energy. In the low‐speed wake region downstream of Io, electrons exhibit an abrupt transition to a population which is consistent with trapping on bounce orbits within the magnetic depression near Io. This trapped population, which appears in the same spatial region as intense field‐aligned beams, is not a result of adiabatic transport from a source region upstream of Io. The phase space density of the “trapped” electron population is reduced, compared to the background torus, and particle tracing calculations in a realistic model environment near Io suggest that such electrons must be scattered into the region sampled by Galileo. Torus electrons with energies well above an MeV are excluded from a broad spatial region surrounding Io. This leads to a pronounced drop in the flux of penetrating particles near Io which allows the modest “trapped” electron population to be detected above the background level for energies up to 200 keV.
Analysis of the oxygen nightglow measured by the Hopkins Ultraviolet Telescope: Implications for ionospheric partial radiative recombination rate coefficients
Authors: Meléndez‐Alvira D. J. , Meier R. R. , Picone J. M. , Feldman P. D. , McLaughlin Brendan M.
Published: September 2002

The Hopkins Ultraviolet Telescope (HUT) recorded nighttime ultraviolet O+ spectra in the tropical ionosphere during the December 1990 Astro 1 space shuttle mission. We reexamine the HUT 911‐Å radiative recombination continuum spectra and the relative 911‐Å and 1356‐Å intensities in order to address questions raised in a previous study. First, we find that the radiative recombination of O+ to the ground state of O accounts for nearly all of the continuum emission. The need for a second component beyond the ionization threshold is eliminated by allowing for an instrumental wavelength shift. Second, we find that certain low‐count spectral features in some of the data are not due to a non‐Maxwellian energy distribution because of the fast electron thermalization in the nighttime F region. Instead, we show that these features are consistent with random fluctuations alone. Third, the best known O+ – O− mutual neutralization chemistry gives good agreement between theoretical and measured 1356/911 ratios, thereby demonstrating the consistency among the various determinations of the O− photodetachment cross section. We update the 911‐Å radiative recombination rate coefficient using recent photoionization cross section information, and the 1356‐Å rate coefficient by fitting HUT 1356/911 ratio data. The derived rate coefficient agrees very closely with our interpretation of independent 7774‐Å data. Both the three‐parameter fit to the 911‐Å radiative recombination continuum and the updated 911 and 1356‐Å radiative recombination rate coefficients improve the agreement between the model and measured 1356/911 ratios. The updated radiative recombination rate coefficients are 3.5 × 10−13 cm3 s−1 for 911‐Å, and 7.3 × 10−13 cm3 s−1 for 1356‐Å, both at 1160 K.
Hall‐MHD modeling of near‐Earth magnetotail current sheet thinning and evolution
Authors: Rastätter L. , Hesse M. , Schindler K.
Published: September 2002

The formation of thin current sheets is a common feature of the late substorm growth phase in the magnetotail of the Earth. The location and the structure of the evolving thin current sheet determine the onset and the dynamic phase of magnetic substorms. To determine the formation and further evolution of thin current sheets in the near‐Earth magnetotail, we employ a two‐fluid model of electrons and ions, the Hall‐MHD. We start from a two‐dimensional tail‐equilibrium model and apply a plasma inflow from the lobes to simulate the presubstorm loading process of the tail. Our results confirm recent 2.5‐dimensional particle and hybrid simulations which have shown thin current sheet formation, with the majority of the new current supported by the electrons. The Hall‐MHD simulations are extended to a current sheet thinning model with three‐dimensional driving conditions and different magnetic field boundary conditions. We investigated the changes to the tail magnetic and current structure brought about by the cross‐tail magnetic field components generated by Hall electric fields. For all these cases, the additional current is carried by the electrons, with very small effects of magnetic field and driving boundary conditions.
Probing Ganymede's magnetosphere with field line resonances
Authors: Volwerk M. , Kivelson M. G. , Khurana K. K. , McPherron R. L.
Published: September 2002

We report on the spectrum of field line resonances identified in data acquired by the Galileo spacecraft within Ganymede's magnetosphere on a relatively low latitude pass. We infer properties of the plasma distribution and its transport from the observed spectrum. The harmonic structure in the spectrum of the magnetometer data agrees very well with the frequencies predicted for resonances of a dipole field. The spectrum implies a density of 2 amu cm−3 near the equator on closed field lines of Ganymede's magnetic field inside of 2 RG (Ganymede radii). This density is significantly reduced relative to the local density of the Jovian plasma sheet near Ganymede (≤ 8 electrons cm−3, or ∼100 amu cm−3 for an average ion mass per charge of 20 amu and average charge of 1.5 electron charges). A shadowing effect of Ganymede for the flow of particles injected at a reconnection layer on the side of the moon downstream relative to the direction of torus plasma flow accounts for the marked density depletion. Implications for conducting paths near Ganymede's surface are considered.
Numerical simulation and analytical description of solar neutron transport in the Earth's atmosphere
Authors: Dorman L. I. , Valdés‐Galicia J. F. , Dorman I. V.
Published: September 2002

We present here results of a numerical simulation of small‐angle neutron multiscattering and attenuation in the Earth's atmosphere. A range of initial zenith angles, θo, and different atmospheric depths h are considered. We show that the angular distribution of neutrons remains symmetrical only for vertical arrival, θo = 0. For inclined arrival the distribution becomes asymmetric; this asymmetry grows with increasing initial zenith angle. The asymmetry of the angular distribution enhances the solar neutron refraction effect. It follows from our simulation that this effect is caused by the stronger attenuation of neutrons scattered to zenith angles larger than the arrival angle. This effect makes the effective zenith angle become smaller as the solar neutrons propagate downwards. The refraction angle of our simulation is not constant, but it is a growing function of θo and h. An analytical description of the problem is also performed starting from simple assumptions of the parameters involved in the solution. We modify this simple model in successive steps to take into account additional details of the solar neutron propagation, until it reaches reasonable agreement with our numerical simulation estimates.
The Dst geomagnetic response as a function of storm phase and amplitude and the solar wind electric field
Authors: Vassiliadis D. , Klimas A. J. , Valdivia J. A. , Baker D. N.
Published: September 2002

We examine the dependence of the Dst timescales on storm conditions and its implications for the storm‐substorm relationship. The growth, decay and oscillation timescales are expressed as functions of the storm magnitude and phase, and the solar wind electric‐field input VBs. Nonlinear, second‐order autoregressive moving average (ARMA) models are fit to 5‐min data and yield two timescales, an exponential decay with an average e‐folding time τ1 = −4.69 hours (−7.26 hours for the pressure‐corrected Dst(0)) and an inductive time τ2 = −0.81 hours (−0.05 hours for Dst(0)). Around these average values there is a systematic variation: (1) For most of the storm duration, τ1 is negative representing the rapid adjustment of the inner magnetosphere to the imposed electric field. (2) In the early main phase, however, τ1 = 5.29 hours (1.76 hours for Dst(0)), so the disturbance grows as a slow exponential. (3) During commencement and main phase, the timescales are complex conjugate and the response is oscillatory. Fast oscillations during storm commencement (period 1.13 hours: 8.48 min for Dst(0)) are a “ringing” response to interplanetary pressure enhancements. Slow oscillations in the main phase have an average period of 1.96 hours (1.55 hours for Dst(0)) and coincide with AL intensifications. The main phase can be separated into periods of oscillatory, fast decay (coincident with AL activity and probably due to injections) and monotonic slow decay (regular convection). (4) All timescales decrease with increasing interplanetary activity because high activity involves acceleration and loss of heavy ions with shorter lifetimes than protons. (5) Also, decay times are about twice as long during recovery than during main phase. (6) Similar dependences are found for the solar wind coupling coefficients. The models are similar to linear models in predictability and are stable with respect to perturbation in the initial conditions.
Substorm onset timing: The December 31, 1995, event
Authors: Ohtani S. , Creutzberg F. , Mukai T. , Singer H. , Lui A. T. Y. , Nakamura M. , Prikryl P. , Yumoto K. , Rostoker G.
Published: September 2002

The objective of the present study is to examine the timing of various onset‐associated signatures and address the cause‐and‐effect relationship between the formation of a near‐Earth neutral line (NENL) and the trigger of tail current disruption. An event selected for this study took place on December 31, 1995. In this event the Geotail satellite was located at X = −30.3 RE in the midnight sector at a local time between the GOES 8 and 9 geosynchronous satellites. The timing of the Geotail observation of a fast (950‐km/s) tailward convection flow accompanied with southward Bz (< −10 nT) indicates that the near‐Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was located closer to the onset meridian and observed a sudden recovery (dipolarization) of the local magnetic field but with a noticeable (≈1 min) delay from the ground onset. This delay can be interpreted in terms of the earthward expansion of tail current disruption initiated outside of geosynchronous orbit. The timing of all these features is consistent with the idea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to explain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, although it is possible that the reconnection process sets up a favorable condition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near‐Earth reconnection process as a major role in the substorm dynamics.
Three‐dimensional simulations of the ionospheric plasma transport in the presence of the structured field‐aligned flows
Authors: Gavrishchaka Valeriy V. , Ganguli Supriya B. , Guzdar Parvez N.
Published: September 2002

Recent observations from the Preja and FAST satellites as well as earlier observations from OGO 5, Heos 2, and DE 2 indicate the presence of fine structures in the field‐aligned plasma flows. We have simulated the effects of such structures on ionospheric plasma transport processes, using our three‐dimensional large‐scale, multimoment, multifluid model. The model solves the continuity, momentum and double adiabatic energy equations with anisotropic ion temperatures in the field line coordinates from 1500 km to 10 RE. It includes important physics of the ionosphere‐magnetosphere coupling region, such as the mirror force and generalized ambipolar electric field in a dipole magnetic field geometry. Freja satellite observations of the structured field‐aligned currents were used as input parameters in the simulation. It has been shown that in the presence of the field‐aligned current filaments with sufficiently small transverse scale sizes, the transverse transport can play a dominant role in the overall plasma response processes in the ionosphere‐magnetosphere coupling region. For example, the local changes in parallel flow velocity due to transverse transport, l min after the structured field‐aligned current application, may be > 30%. The plasma response time due to transverse transport has been found to be shorter than that due to parallel transport. The difference is more pronounced at higher altitudes.
Simultaneous Global Positioning System observations of equatorial scintillations and total electron content fluctuations
Authors: Beach Theodore L. , Kintner Paul M.
Published: September 2002

One aspect of the Global Positioning System (GPS) is the potential to conduct geophysical research, and worldwide networks of GPS receivers have been established to exploit this potential. Several research groups have begun using this global GPS data to study ionospheric total electron content (TEC) variations, also referred to as GPS phase fluctuations, as surrogates for ionospheric scintillations. This paper investigates the relationship between GPS amplitude scintillations and TEC variations for the same line of sight using observations from Ancón, Peru. These observations were taken under equatorial spread F conditions for three nights in April 1997. As expected, only when the spectrum of TEC fluctuations includes significant power at the Fresnel scale do scintillations appear. We also find that when the TEC fluctuation spectrum includes the Fresnel scale, the S4 scintillation index is roughly proportional to measures of TEC fluctuation for the weak scintillations observed. The proportionality constant varies from night to night, however, casting doubt on the ability to predict GPS S4 successfully from TEC fluctuation data alone. We also present a simple theoretical phase screen model and show that if a relationship between TEC fluctuation measures and S4 exists, that relationship depends on the power spectrum of phase variations at the screen. Unfortunately, the available TEC data, at 30 s per sample (with some aliasing apparently permitted), offer limited spectral information. A preliminary comparison of 1 s/sample data with the same data decimated to a 30 s/sample interval suggests, however, that the level of successful S4 prediction, based on TEC fluctuation measures alone, is comparable at either sample rate.
Spatial extent and dynamics of a thin current sheet during the substorm growth phase on December 10, 1996
Authors: Pulkkinen T. I. , Baker D. N. , Cogger L. L. , Frank L. A. , Sigwarth J. B. , Kokubun S. , Mukai T. , Singer H. J. , Slavin J. A. , Zelenyi L.
Published: September 2002

Multispacecraft observations are analyzed to examine the dynamics of thin current sheet evolution during the early phases of a substorm during the period 0630–0800 UT on December 10, 1996. The ground signatures showed two activations, one latitudinally localized at 0731 UT and another, larger expansion around 0800 UT. A local model consisting of two current sheets (a thin current sheet and a thicker plasma sheet) was developed utilizing two‐spacecraft magnetic field measurements within the magnetotail. Extension of the thin current sheet model to the inner magnetotail shows that during most of the growth phase, the thin current sheet intensified simultaneously and at the same rate in the mid‐magnetotail (∼30 RE) and near geosynchronous orbit. These results show that a thin current sheet extended from the near‐geostationary region to the midtail, and that the entire current sheet responded to the driving in a coherent way. Furthermore, timings suggest that the onset‐associated disturbances were observable in the midtail before the ground or geosynchronous signatures.
Depletion patterns and dust evolution in the interstellar medium
Authors: Jones A. P.
Published: September 2002

We review the use of elemental depletions in determining the composition of interstellar dust and present a new interpretation of the elemental depletion patterns for the dust forming elements in a range of diffuse cloud types. We discuss this within the context of dust processing in the interstellar medium and show that Si and Mg are selectively eroded from dust, with respect to Fe, as expected for a sputtering erosion process. However, we find that Si is preferentially and nonstoichiometrically eroded from dust with respect to Mg by some as yet unidentified process that may act in conjunction with grain sputtering. On this basis a new way of interpreting the depletions in terms of “continuous” dust processing through erosion in the interstellar medium is presented. The observed depletion patterns can then be understood in terms of a gradually changing grain chemical composition as the erosion of the atoms proceeds nonstoichiometrically in the low‐density interstellar medium. The stoichiometric erosion of multicomponent (e.g., core/mantle) single grains can qualitatively explain the observed depletions but is not consistent with the preferential erosion of Si from dust. We present suggestions for the usage of mineralogical terms within the context of interstellar and circumstellar dust mineralogy.
Solitary potential structures associated with ion and electron beams near 1RE altitude
Authors: Bounds Scott R. , Pfaff Robert F. , Knowlton Stephen F. , Mozer Forrest S. , Temerin Michael A. , Kletzing Craig A.
Published: September 2002

Small‐scale solitary electric potential structures are commonly observed on auroral field lines with the Polar Electric Field Instrument (EFI). This study focuses on observations of solitary structures in the southern hemisphere auroral zone at altitudes between 5500 and 7500 km. Some of the potential structures are similar to those observed previously by the S3‐3 and Viking satellites and are inferred to be negative potential pulses traveling upward along the auroral magnetic field lines, associated with upgoing ion beams and upward currents. The velocities of these “ion” solitary potential structures are estimated, using spaced EFI measurements, to be distributed within the range of ∼75 – 300 km s−1. In addition to these structures, a different type of solitary potential structure with.opposite polarity has been observed with faster propagation velocities. These faster structures (termed “electron” solitary potential structures) are distinguishable from the slower, ion solitary structures in that their distinctive bipolar electric field signature, common to both types of solitary structure, is reversed. The ultimate distinction for the electron solitary potential structures is that they are observed on auroral field lines in conjunction with magnetically field‐aligned upflowing electron beams. The electron solitary potential structures propagate up the field line in the same direction as the electron beam. An example is shown of the polarity reversal from ion to electron solitary potential structures coincident with a simultaneous shift from upgoing ion beams to upgoing electron beams.
Cyclotron resonance in coronal holes: 1. Heating and acceleration of protons, O5+, and Mg9+
Authors: Hollweg Joseph V.
Published: September 2002

The resonant heating and acceleration of protons and selected heavy ions in coronal holes are investigated by calculating trajectories of individual test particles under the influence of gravity, the electrostatic electric field, the mirror force, and the resonant acceleration due to interaction with dispersive ion cyclotron waves. The transverse heating due to the resonance is also included. We show in general terms how heavy ions can be more than mass proportionally heated, emphasizing that wave dispersion may play an important part in producing very hot heavy ions. We pay particular attention to the ultraviolet coronagraph spectrometer (UVCS) SOHO observation that the transverse temperature of O5+ is still increasing out to the outer limit of observation at ∼3.5 solar radii. Using both approximate analytical expressions and the trajectory calculations, we find that this observation can only be reproduced if the magnetic power spectrum falls off at least as steeply as k−2, where k is wavenumber. Surprisingly, this conclusion holds even when the power spectrum consists of two power laws, if the inner scale is proportional to the proton inertial length. Once the particles are heated transversely by the resonance, the mirror force provides the dominant outward acceleration and leads to heavy ions which flow faster than the protons. It is shown that it is possible to construct a model which gives reasonable agreement with the UVCS/SOHO data for both protons and O5+. Overall, we conclude that it is highly likely that the cyclotron resonance is responsible for heating protons and heavy ions in coronal holes. However, we also briefly discuss some data for Mg9+, which do not fit the overall picture.
The November 9, 1993, traveling convection vortex event: A case study
Authors: Zesta E. , Hughes W. J. , Engebretson M. J. , Hughes T. J. , Lazarus A. J. , Paularena K. I.
Published: September 2002

On November 9, 1993, at around 1715 UT, a strong and well‐structured traveling convection vortex (TCV) event occurred and was observed by the Magnetometer Array for Cusp and Cleft Studies (MACCS) and the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer chains. The Greenland chain, which is located farther to the east, observed only a very weak signature for the same event. We studied the propagation characteristics of the two‐dimensional vortical TCV current patterns. The TCV event is a series of four vortical currents propagating westward. We found that individual vortices propagate with different speeds. The vortices are created in the early postnoon region. They accelerate as they strengthen, and some decelerate, weaken, and disappear within the 6 hours of magnetic local time of the field of view of the ground magnetometers. The strongest, main vortex of the event accelerates until it moves out of the field of view and, more than likely, reaches well into the nightside. We studied the correlated solar wind and IMF signatures as observed by the IMP 8 spacecraft, sitting in the far dawnside and outside the bow shock. We found that the transient currents in the ionosphere are the result of sharp, short‐lived pressure pulses hitting the magnetopause during times of quiet and northward IMF that is primarily radial. We find that the pressure pulses are more than likely created just upstream of the bow shock by the interaction of the quasi‐parallel shock with IMF orientation changes and are not intrinsic features of the upstream solar wind. We also analyze the transient signatures in the inner magnetosphere by studying the magnetic field data in the GOES 6 and 7 satellites. We suggest that a series of five successive compression and depression peaks in the GOES magnetic field data are well correlated with the set of solar wind pressure pulses. We observe a propagation velocity of the transient event from the GOES 6 spacecraft to the GOES 7 spacecraft that agrees well with the propagation velocities that we calculate from the ground magnetometer stations.
Perkins instability revisited
Authors: Hamza A. M.
Published: September 2002

A thorough review of the Perkins instability is presented in this paper. We introduce two new elements to the earlier model presented by Perkins [1973], namely, the neutral wind velocity and gradients in the background field line integrated density and Pedersen conductivity, respectively. We derive the eigenfrequencies and growth rates for a new instability and recover the limiting cases discussed by Perkins [1973] and Miller [1997]. The presence of gradients in the background density and Pedersen conductivity reveals a new branch in the dispersion relation and can possibly enhance the Perkins instability growth rate. The need for horizontal gradients, as was pointed out by Basu et al. [1981], is essential for the generation of small‐scale and kilometer‐scale irregularities. The Perkins instability is shown to be enhanced by both the presence of a neutral wind and the background density and Pedersen conductivity.
An average image of proton plasma pressure and of current systems in the equatorial plane derived from AMPTE/CCE‐CHEM measurements
Authors: De Michelis Paola , Daglis Ioannis A. , Consolini Giuseppe
Published: September 2002

The present study attempts to visualize the global equatorial current systems and the proton pressure in the near‐Earth magnetosphere based on AMPTE/GCE‐CHEM measured proton distributions, which were sorted by the AE index (“quiet”: AE < 100 nT, “active”: 100 nT < AE < 600 nT). The data were averaged over 2 years (from January 1985 to June 1987) in order to obtain the necessary spatial resolution with statistical significance. The results provide an average image of proton plasma pressure, proton plasma anisotropy, and current systems as a function of geomagnetic activity. In particular, the changes of pressure anisotropy with local time and the noon‐midnight pressure asymmetry are studied and correlated to the current systems out of the equatorial plane that generate the closure circuits. Moreover, we identify and spatially locate two different current systems for the quiet period (the ring current and the inner portion of the quiet time cross‐tail current) and three different current structures for the active period (the ring current, the partial ring current, and the region 2 current).
Solar causes of the long‐term increase in geomagnetic activity
Authors: Stamper R. , Lockwood M. , Wild M. N. , Clark T. D. G.
Published: September 2002

We analyze the causes of the century‐long increase in geomagnetic activity, quantified by annual means of the aa index, using observations of interplanetary space, galactic cosmic rays, the ionosphere, and the auroral electrojet, made during the last three solar cycles. The effects of changes in ionospheric conductivity, the Earth's dipole tilt, and magnetic moment are shown to be small; only changes in near‐Earth interplanetary space make a significant contribution to the long‐term increase in activity. We study the effects of the interplanetary medium by applying dimensional analysis to generate the optimum solar wind‐magnetosphere energy coupling function, having an unprecedentedly high correlation coefficient of 0.97. Analysis of the terms of the coupling function shows that the largest contributions to the drift in activity over solar cycles 20–22 originate from rises in the average interplanetary magnetic field (IMF) strength, solar wind concentration, and speed; average IMF orientation has grown somewhat less propitious for causing geomagnetic activity. The combination of these factors explains almost all of the 39% rise in aa observed over the last three solar cycles. Whereas the IMF strength varies approximately in phase with sunspot numbers, neither its orientation nor the solar wind density shows any coherent solar cycle variation. The solar wind speed peaks strongly in the declining phase of even‐numbered cycles and can be identified as the chief cause of the phase shift between the sunspot numbers and the aa index. The rise in the IMF magnitude, the largest single contributor to the drift in geomagnetic activity, is shown to be caused by a rise in the solar coronal magnetic field, consistent with a rise in the coronal source field, modeled from photospheric observations, and an observed decay in cosmic ray fluxes.
Absolute solar 30.4 nm flux from sounding rocket observations during the solar cycle 23 minimum
Authors: Judge Darrell L. , McMullin Donald R. , Ogawa Howard S.
Published: September 2002

A transmission grating extreme ultraviolet (EUV) spectrometer, nominally identical to the Charge, Element, and Isotope Analysis System/Solar EUV Monitor (CELIAS/SEM) instrument on the Solar and Heliospheric Observatory (SOHO), has obtained accurate measurements of the integrated absolute solar extreme ultraviolet irradiance in an 8 nm band pass centered at 30.4 nm. The spectrometer also measured the EUV/soft X‐ray flux, but those data will be reported in a later paper. The instrument was launched on two sounding rocket flights from White Sands Missile Range, New Mexico, on June 26, 1996, and again on August 11, 1997, to provide a SOHO underflight calibration database in the EUV. The full disk solar 30.4 ± 40 nm fluxes measured by it on the above 2 days were 1.21 × 1010 and 1.42 × 1010 photons cm−2 s−1 at 1 AU, respectively. These measurements have an absolute 1σ uncertainty of 8.1%.
Distortion of the interplanetary magnetic field by three‐dimensional propagation of coronal mass ejections in a structured solar wind
Authors: Odstrcil D. , Pizzo V. J.
Published: September 2002

A three‐dimensional (3‐D) numerical magnetohydrodynamic model is used to investigate the temporal and spatial evolution of large‐scale solar wind (SW) disturbances. A tilted dipole outflow configuration is specified at the inner boundary near the Sun, and a structured, corotating SW flow with a monopolar interplanetary magnetic field (IMF) is established by dynamic relaxation between 0.14 and 1.04 AU. Time‐dependent variation of the pressure and velocity at the inner boundary is applied to simulate injection of a coronal mass ejection (CME) near the streamer belt. In this model, magnetic disturbances arise solely from distortion of the spiral IMF by the bulk motion of CME material; embedded magnetic structures, such as magnetic clouds or flux ropes, are not considered. Numerical results show that the motion and appearance of a CME are affected by its interaction with the velocity and density structure of the background SW. This 3‐D dynamic interaction also affects the orientation and magnitude of the IMF. Substantial excursions of the southward IMF component Bz are generated by three distinct mechanisms: shock compression of the IMF where the shock front is inclined to field lines, draping around the CME by mass flow convection, and distortion of the IMF by rarefaction waves trailing the CME. The geoeffectiveness of the disturbance depends on the initial position of the CME with respect to the streamer belt as well as on the position of Earth with respect to the pulse centerline.
Cyclotron resonance in coronal holes: 2. A two‐proton description
Authors: Hollweg Joseph V.
Published: September 2002

In a cold plasma, the ion‐cyclotron mode does not extend above the proton cyclotron frequency. As a consequence, for waves propagating outward from the Sun, only protons that are moving slower than the mean proton wind speed can resonate with this mode. Thus only roughly half of the proton distribution function can be in resonance at any instant of time. The proton distribution function is then expected to depart significantly from a bi‐Maxwellian, which is usually assumed to provide closure to a set of fluid equations. Here we consider the effects of the ion‐cyclotron resonance on protons in a coronal hole. We calculate the trajectories of individual protons in the electric, magnetic, and gravitational fields, and we include the resonant heating and acceleration for an average particle that is diffusing in phase space. For closure we consider two protons, which are proxies for the resonant and nonresonant halves of the distribution. Elementary arguments show that the two protons tend to approach nearly the same radial velocity. When the waves are dispersive, this means that the resonant wavenumber kres increases. For a power spectrum that is a power law in wavenumber, and if the dissipation is determined only by the resonant particles, then the resonant effects become very weak as kres becomes large and there is little heating or acceleration of the coronal plasma. On the other hand, if the dissipation is determined by a turbulent cascade, kres mainly controls the relative importance of resonant acceleration and resonant heating. Such models can yield good agreement with what is known about the behavior of protons in coronal holes.
Consequences of geomagnetic history on the high‐latitude thermosphere and ionosphere: Averages
Authors: Aruliah Anasuya L. , Müller‐Wodarg Ingo C. F. , Schoendorf Jacqueline
Published: September 2002

The thermospheric effects of “geomagnetic history” and the resulting ion‐neutral interactions are determined through the analysis of a long‐term database of high latitude neutral winds from Kiruna, Sweden, and simulations with the coupled thermosphere ionosphere plasmasphere model (CTIP). Three types of geomagnetic history are examined in detail with the data and the model: steady state conditions in which the Kp index for the current three hours is the same as the Kp index for the previous 3 hours; previously quiet conditions in which the Kp index for the current 3 hours is greater than the Kp index for the previous 3 hours; and previously active conditions in which the Kp index for the current 3 hours is less than the Kp index for the previous 3 hours. It is shown that during the hours of darkness at Kiruna, while the ionosphere responds immediately to changes in activity, the neutral gas can take between 3 and 6 hours to recover from the effects of any previous activity. Model simulations show that the rate of energy dissipation is also significantly dependent on geomagnetic history. For the previously active case the Joule heating and mechanical energy transfer rate are up to 4 times larger at certain latitudes as the steady state case. For the previously quiet case the heating rates are much smaller than the steady state case. There is a frequently made assumption that at high latitudes the mechanical energy transfer rate may be ignored as insignificant compared with the Joule heating rate. The results presented here show that this assumption is unreliable, particularly in the dusk sector and polar cap.
Monte Carlo simulation of runaway MeV electron breakdown with application to red sprites and terrestrial gamma ray flashes
Authors: Lehtinen N. G. , Bell T. F. , Inan U. S.
Published: September 2002

A three‐dimensional Monte Carlo model of the uniform relativistic runaway electron breakdown in air in the presence of static electric and magnetic fields is used to calculate electron distribution functions, avalanche rates, and the direction and velocity of avalanche propagation. We also derive the conditions required for an electron with a given momentum to start an avalanche in the absence of a magnetic field. The results are compared to previously developed kinetic and analytical models and our own analytical estimates, and it is concluded that the rates used in many early models [e.g., Lehtinen et al., 1997; Taranenko and Roussel‐Dupré, 1996; Yukhimuk et al., 1998; Roussel‐Dupré et al., 1998] are overestimated by a factor of ∼10. The Monte Carlo simulation results are applied to a fluid model of runaway electron beams in the middle atmosphere accelerated by quasi‐electrostatic fields following a positive lightning stroke. In particular, we consider the case of lightning discharges which drain positive charge from remote regions of a laterally extensive (> 100 km) thundercloud, using a Cartesian two‐dimensional model. The resulting optical emission intensities in red sprites associated with the runaway electrons are found to be negligible compared to the emissions from thermal electrons heated in the conventional type of breakdown. The calculated gamma ray flux is of the same order as the terrestrial gamma ray flashes observed by the Burst and Transient Source Experiment detector on the Compton Gamma Ray Observatory.
Modeling geomagnetically induced currents during different ionospheric situations
Authors: Viljanen Ari , Amm Olaf , Pirjola Risto
Published: September 2002

We use several realistic three‐dimensional models of ionospheric currents to calculate geomagnetically induced currents (GIC) in the Finnish high‐voltage power system. Of special interest are events during which the magnetic field changes rapidly and GIC are large. The geoelectric field driving GIC is determined with the complex image method, which is a fast and accurate tool for taking into account induction effects in the Earth. A detailed investigation is made applying a model of a westward traveling surge (WTS). It is capable of producing magnetic field variations and GIC which are of the same magnitude as the observed values. However, the WTS model yields too large time derivatives of the magnetic field. A much simpler line current model produces very realistic magnitudes of both the magnetic field and GIC. However, in contrast to WTS, it lacks the realistic spatial structure of the ground magnetic field. The requirement of accurate models of the Earth's conductivity is demonstrated by comparing a resistive and conductive structure in connection with a very rapid change of the magnetic field. Consideration of some other typical ionospheric events (Harang's discontinuity, omega band, pulsation) indicates that these phenomena probably cannot produce extremely large GIC.
Ring current activity during the early Bz < 0 phase of the January 1997 magnetic cloud
Authors: Jordanova V. K. , Torbert R. B. , Thorne R. M. , Collin H. L. , Roeder J. L. , Foster J. C.
Published: September 2002

The passage at Earth of the January 10–11, 1997, magnetic cloud induced a storm of moderate geomagnetic activity with Dst index reaching minimum values of about −83 nT. We study ring current formation during the early Bz negative phase of this magnetic cloud, using energetic particle data from three instruments on the Polar spacecraft and geosynchronous plasma data from the LANL spacecraft. We use our kinetic drift‐loss model to simulate the evolution of ring current H+, He+, and O+ ion distributions and associated aeronomical effects during this period. The results from two Volland‐Stern type magnetospheric electric field model formulations are compared: (1) Kp‐dependent and (2) interplanetary magnetic field (IMF) dependent. We demonstrate that while both electric field models reproduce well the main trends of ring current formation and decay during the storm, the IMF‐dependent model reproduces the rapidity of the main storm growth phase and its strength better. Comparing model results during the main phase of the storm with HYDRA, TIMAS, and CAMMICE data we find that the model reproduces very well the ring current distributions near dawn. The formation of the nose event, i.e., the rise of the 10–30 keV energy particles near dusk due to abruptly increased convection is, however, overestimated by the model. We compute plasmaspheric heating through Coulomb collisions as the storm evolves and find that maximum heating occurs initially on the nightside near L∼3.5 and subsequently moves earthward to L∼2.75, in agreement with Millstone Hill radar observations of midlatitude electron temperature enhancement on January 10. However, the magnitude of the energy transferred to plasmaspheric electrons through Coulomb collisions appears to be not sufficient to yield the observed elevated electron temperature at ∼0830 UT, suggesting that additional energy sources should be considered during this event.
An updated climatology of thermospheric neutral winds and F region ion drifts above Millstone Hill
Authors: Buonsanto M. J. , Witasse O. G.
Published: September 2002

Incoherent scatter radar data from 63 experiments during the period 1984–1997 have been used to update earlier published climatologies of ion drifts and neutral thermospheric winds at Millstone Hill. Data are binned according to season, solar activity, geomagnetic activity, and local solar time. The derived ion drift patterns are similar to those previously published, but we now find that the main effect of the increased magnetic activity is an enhancement of the usual nighttime west‐ward ion drift. A tidal decomposition of the winds from geomagnetically quiet conditions is carried out for each season/solar activity level to extract diurnal means, diurnal amplitudes and phases, and semidiurnal components. These winds are compared with previous results at Millstone Hill and the HWM‐93 model. Using our expanded wind database, statistically significant semidiurnal components are now found in both summer and winter, at both solar maximum and solar minimum. Our bin‐averaged results confirm earlier published findings that at all seasons the diurnal mean winds are more strongly equatorward and the diurnal amplitudes are stronger at solar cycle minimum than at solar cycle maximum. Diurnal amplitudes derived in the present work are larger than those found previously because we now use a smaller value for the O+, O collision frequency. Differences in the diurnal mean and diurnal amplitude between recent solar minimum data (1993–1997) and data from the previous solar minimum (1984–1986) are attributed to a difference in the EUV flux as inferred from Millstone Hill electron density data which one would not expect from the F10.7 index. This affects the ion drag and apparently also the relative importance of the EUV and high latitude heat sources.
Gravity wave interactions with mesospheric planetary waves: A mechanism for penetration into the thermosphere‐ionosphere system
Authors: Meyer Christian K.
Published: September 2002

This research is motivated by observations of oscillations in the thermosphere and ionosphere at periods close to those associated with planetary waves (PW) or “normal modes” of the lower atmosphere. Herein a 2‐D linear steadystate numerical model is used in conjunction with a hybrid gravity wave (GW) parameterization to examine the role of GW interactions in the penetration of westward propagating planetary waves from the mesosphere into the lower thermosphere. Results are presented for the quasi 2‐day and quasi 16‐day waves previously studied using the same model without explicit parameterization of GW influences. It is shown that the horizontal winds associated with the PWs result in modulation of upward propagating internal gravity waves, providing an insitu source of periodic forcing in the mesopause region. This forcing is shown to produce separate amplitude peaks in the PW horizontal wind fields in the lower thermosphere apart from the primary response in the mesosphere. Horizontal winds associated with the GW‐modified 2‐day wave extend well into the thermosphere up to 140 km, primarily at lower latitudes, whereas the GW‐modified 16‐day wave is enhanced in the 80–120 km altitude range. The phase structure of the modified waves are consistent with the concept of gravity wave filtering by a longitudinally varying planetary wave, with an anticorrelation between the phase of the mesospheric and thermospheric amplitude peaks. The roles of wave stress and eddy diffusion are analyzed and variations of the strength of the meridional GW source are considered.
An assessment of the role of ionospheric conductivity variations in determining the auroral electrojet characteristics
Authors: Akinari Nakagawa
Published: September 2002

A simple circuit model is used to examine how the conductivity variation of the ionosphere can affect the overall properties of circuits in the magnetosphere‐ionosphere coupling system. It is demonstrated that there exists a theoretical threshold in a current‐conductivity relation that distinguishes conductivity‐dominant electrojets from electric field‐dominant ones.
Evolution of kappa distributions under velocity space diffusion: A model for the observed relationship between their spectral parameters
Authors: Collier Michael R.
Published: September 2002

Velocity space diffusion acting on superthermal ions whose distributions resemble kappa functions (Maxwellian core with a power law tail) will cause the distributions to evolve so that the high‐energy spectral index κ is linearly related to core temperature with a constant of proportionality that (1) scales inversely with the distribution's characteristic energy, (2) is independent of the diffusion coefficient, and (3) has a positive intercept. This type of behavior has been observed in five different studies covering the magnetospheres of Earth, Jupiter, and Saturn as well as the solar wind. These results indicate the following: (1) velocity space diffusion frequently plays the dominant role in shaping particle distribution functions in many diverse space plasma environments and (2) hotter distributions with steeper spectral indices are “older” distributions in that they have evolved considerably from the source distribution.