bluuebunny/crossref_metadata_2025 huggingface dataset:
Polar UVI observations of dayside auroral transient events
Authors: Vorobjev V. G. , Yagodkina O. I. , Sibeck D. G. , Liou K. , Meng C.‐I.
Published: February 2004

We analyze Polar Ultraviolet Imager (UVI) observations of auroral transient events (ATEs) in the dayside Northern Hemisphere. During 5 winter months in 1996 and 1997, we found 31 prenoon ATEs but only 13 afternoon events. Prenoon and afternoon event characteristics differ. Prenoon ATEs generally appear as bright spots of auroral luminosity in the area from 0800 to 1000 magnetic local time (MLT) and 74.5° and 76.5° corrected geomagnetic latitude (CGL). Bright aurorae then quickly expand westward and poleward, accompanied by high‐latitude magnetic impulsive events (MIE) and traveling convection vortices (TCV). Afternoon ATEs usually appear as a sudden intensification of aurorae in the area from 1400 to 1600 MLT and 75.5° to 78.5° CGL. Within 15‐20 min the bright band of luminosity extends eastward to reach 2000–2100 MLT at 70°–72° CGL. Although midlatitude and low‐latitude ground magnetograms in the evening sector record increases in the horizontal component of the magnetic field, no corresponding features occur at stations in the morning sector. Afternoon ATEs correspond to abrupt changes in the interplanetary magnetic field (IMF) orientation, but not to significant variations of the solar wind dynamic pressure, indicating that the auroral transient events occur as part of the magnetospheric response to abrupt changes in the foreshock geometry.
A 16‐moment solar wind model: From the chromosphere to 1 AU
Authors: Lie‐Svendsen Øystein , Leer Egil , Hansteen Viggo H.
Published: February 2004

We present a solar wind fluid model extending from the chromosphere to Earth. The model is based on the gyrotropic approximation to the 16‐moment set of transport equations, in which we solve for the density, drift speed, temperature parallel and perpendicular to the magnetic field, and transport of parallel and perpendicular thermal energy along the magnetic field (heat flux). The solar wind plasma is created dynamically through (photo) ionization in the chromosphere, and the plasma density in the transition region and corona is computed dynamically, dependent on the type of coronal heating applied, rather than being set arbitrarily. The model improves the description of proton energy transport in the transition region, where classical heat conduction is only retrieved in the collision‐dominated limit. This model can serve as a “test bed” for any coronal heating mechanism. We consider heating of protons by a turbulent cascade of Alfvén waves in rapidly expanding coronal holes. The resulting high coronal proton temperatures lead to a downward proton energy flux from the corona which is much smaller than what classical transport theory predicts, causing a very low coronal density and an extremely fast solar wind with a small mass flux. Only when some of the wave energy is forcibly deposited in the lower transition region can a realistic solar wind be obtained. Because of the poor proton heat transport, in order to produce a realistic solar wind any viable heating mechanism must deposit some energy in the transition region, either directly or via explicit heating of coronal electrons.
Ion composition aspects of magnetotail plasma flows
Authors: Lennartsson O. W.
Published: February 2004

Recent archiving of energetic (<18 keV/e) ion composition data from the ISEE 1 satellite has created a 4.5‐year World Wide Web based set well suited for statistical investigations of magnetospheric plasmas inside of 23 RE, especially with regard to signatures of solar (He++) or terrestrial (O+) origins. This is an application that investigates tail ion flows in the 10–23 RE radial range, specifically the (spacecraft spin plane) GSE x and y flows of H+, He++, and O+ ions in the 0.1–16 keV/e energy range. The plasma sheet is divided into (1) “high,” (2) “intermediate,” and (3) “low” latitude regions based on magnetic field GSE elevation angle λ and ion beta (sum of H+, He++, He+, and O+ ions), such that regions 1 and 2 both have
,
, with region 1 being at β < 0.1 and region 2 being at β > 0.1, and region 3 has

and β > 0.1. A fourth region has remaining plasma sheet samples (30%). In region 1 the most common drift is within ±30° of the tailward magnetic field direction for all ions, including, somewhat surprisingly, the He++ ions (about half the time). Tailward drift speeds (<100 km s−1) are substantially smaller than thermal speeds (∼500 km s−1) for both He++ and H+ ions. Only the O+ ions have a net field‐aligned (and tailward) distribution
. On average, the He++/H+ number density and mean energy ratios are ∼4% and ∼4, respectively, indicating a mostly solar origin of both species. The second most common drift of He++ and H+ ions (∼20%) is aligned near the earthward field direction and has the largest speeds (>100 km s−1). It also has velocity dispersion, however, and seems to be associated with the leading edge of intermittent bursts of ions expanding earthward from the downtail plasma sheet. Superimposed on these drifts is E × B drift of varying speed and direction, which has a net average of the order of 10 km s−1 toward local midnight, away from the tail flanks. Comparing times (2‐hour intervals) of northward and southward interplanetary magnetic field, drift speeds are some 50% larger in the southward case, and thermal speeds are 10–20% larger. In region 2, drifts are more random in direction and somewhat slower, whereas thermal speeds are 30–50% larger than in region 1. Only the O+ ions maintain dominantly tailward (field‐aligned) drift. The average E × B drift is reversed, away from local midnight, but still of the order of 10 km s−1. In region 3, average E × B drift is sunward and away from midnight. Sunward drift occurs ∼2.5 times more often than tailward drift with northward interplanetary magnetic field and almost 8 times more often with southward interplanetary magnetic field. Comparing all four regions, tailward flows of H+ and He++ ions in region 1 can account for some but only part (≤30%) of the sunward flows at lower latitude, implying that solar ions enter the plasma sheet both tailward and sunward of the ISEE 1.
Solar wind magnetic fluctuation spectra: Dispersion versus damping
Authors: Stawicki Olaf , Gary S. Peter , Li Hui
Published: February 2004

Solar wind magnetic fluctuation power spectra at frequencies ƒ < 1 Hz are commonly observed to have the approximate power law dependence ƒ−5/3. These observations may be described by Kolmogorov diffusion in wavenumber space which defines what is called the inertial range. At wavelengths shorter than the inertial range, spectra often are observed to have steeper power laws. This intermediate wavelength regime is sometimes called the dissipation range because it has been suggested that this steeper slope is caused by collisionless damping of Alfvén and magnetosonic waves. Here it is argued that, at intermediate wavenumbers, Alfvén fluctuations are suppressed by proton cyclotron damping, so the observed power spectra are likely to consist of weakly damped magnetosonic/whistler waves which have an increased wavenumber diffusion rate due to their dispersion. Numerical calculations at βp ≲ 2.5 with a model representing this picture yield fluctuation spectra with steep power laws at intermediate wavenumbers and with sharp cutoffs due to electron cyclotron damping at still shorter wavelengths. The term “dispersion range” is more appropriate to describe steep power law spectra in the intermediate wavenumber regime.
High‐latitude magnetic reconnection in sub‐Alfvénic flow: Interball Tail observations on May 29, 1996
Authors: Avanov L. A. , Smirnov V. N. , Waite J. H. , Fuselier S. A. , Vaisberg O. L.
Published: February 2004

The Interball Tail spacecraft crossed the high‐latitude magnetopause near the cusp region under northward interplanetary magnetic field (IMF) conditions on May 29, 1996, with magnetic local time and magnetic latitude of ∼73 hours and ∼65.4°, respectively. Under these IMF conditions the Interball Tail spacecraft observed quasi‐steady reconnection in progress and evidence for a relatively stable reconnection site at high latitudes. Sunward plasma flow observed by Interball Tail and a determination of the tangential stress balance indicated that reconnection was occurring poleward of the Earth's magnetic cusp, above the spacecraft's location. At these high latitudes the gasdynamic model of the solar wind/magnetosphere interaction indicates that the magnetosheath flow should be super‐Alfvénic, and therefore that the reconnection site should have propagated tailward. However, the spacecraft observed sub‐Alfvénic flow in the magnetosheath region adjacent to the magnetopause current layer near the reconnection site indicating that the reconnection site may have moved in the sunward direction. These observations suggest that the region of sub‐Alfvénic flow and stable, quasi‐steady reconnection extend to very high latitudes under northward IMF conditions. It is shown that the thickness of the magnetopause current layer for this event (estimated as ∼1600 km) is consistent with that found for reconnection at the dayside magnetopause.
A comparison of Pi2 pulsations in the inner magnetosphere and magnetic pulsations at geosynchronous orbit
Authors: Kim K.‐H. , Takahashi K. , Lee D.‐H. , Lin N. , Cattell C. A.
Published: February 2004

A mechanism proposed for Pi2 pulsations in the inner (L = 2–5) magnetosphere is fast‐mode waves trapped in the plasmasphere. The presence of the trapped waves can be examined using multisatellite observations in the inner and outer magnetosphere. Until now, there were few simultaneous observations of Pi2 pulsations in the different magnetospheric regions. In this study we examine two Pi2 pulsations observed in the inner magnetosphere from the Active Magnetospheric Particle Tracer Explorer s/Charge Composition Explorer (AMPTE/CCE) and compare them to magnetic variations observed from the geostationary GOES 5 and 6 satellites. The first and second Pi2s were observed when CCE was located at L ∼ 3.9 and 5.0, respectively, and in the premidnight sector (∼2000–2100 magnetic local time). During both events, CCE observed strongly compressional oscillations. At geosynchronous orbit, pulsations were excited simultaneously at the onset times of the compressional oscillations at CCE. However, the transverse (by) component dominated, and its period differed from those of the compressional oscillations at CCE. This suggests that the oscillations at CCE and at geosynchronous orbit are excited by a common broadband source but that their generation mechanisms may be different (i.e., a trapped fast mode for the compressional oscillations at CCE and a field line resonance for the transverse oscillations at geosynchronous orbit).
Global MHD simulation of the magnetosphere for November 24, 1996
Authors: Slinker S. P. , Fedder J. A. , Ruohoniemi J. M. , Lyon J. G.
Published: February 2004

Using Wind‐measured solar wind data, we have simulated the Earth's magnetosphere and ionosphere for the period 1930–2330 UT on November 24, 1996. The simulation model is a global, three‐dimensional, MHD formulation. This event is the focus of a Geospace Environment Modeling (GEM) substorm challenge. The event features a strongly northward interplanetary magnetic field (IMF) period, followed by a sudden rotation to steadily southward. About 80 min after the Wind‐observed north‐south transition a substorm was observed by the Polar‐visible imaging system (VIS). There was good data coverage throughout this period, and the modeling community was challenged to simulate the event. During the northward period the simulation produces a small open polar cap at high magnetic latitudes, especially on the dawnside, where a polar arc was observed by the Polar‐VIS and was reproduced in the simulation results. The simulation also shows the ionospheric response to the southward transition in the IMF propagate from the day side to the night side in only a few minutes, consistent with Super Dual Auroral Radar Network (SuperDARN) data. Later, the simulation also produces a substorm, but it occurs nearly a half hour earlier than was observed. Numerical experiments were performed by altering the solar wind IMF to investigate the sensitivity of substorm onset timing to this parameter. The model substorms occur spontaneously and do not show a strong correlation between an IMF‐induced convection reduction and the onset. The initial event in the expansion onset of the simulation substorm is the beginning of fast magnetic reconnection at ∼ 20 RE in the stressed magnetotail.
An improved model of ionospheric electric potentials including substorm perturbations and application to the Geospace Environment Modeling November 24, 1996, event
Authors: Weimer D. R.
Published: February 2004

An improved model of ionospheric electric potentials/convection patterns is presented here. This model will produce potentials for any desired level of interplanetary magnetic field (IMF), solar wind velocity, number density, and dipole tilt angle, as well as an optional value for the AL index. The following improvements have been made: (1) Terms for solar wind electric field and dynamics pressure have been added. (2) Nonlinear changes in the potentials as the magnitude of the IMF increases, owing to the decreasing effective width of the solar wind coupling region, are now allowed. (3) The lower boundary of the potential patterns is now variable according to the conditions, rather than fixed at an arbitrary location, resulting in fewer coefficients in the spherical harmonic expansion and smoother patterns. (4) The influence of magnetospheric substorms, or nightside processes, on the potential patterns is now included as a perturbation using the AL index as an optional controlling parameter. The utility of the new substorm component is demonstrated with the event on November 24, 1996, which had been chosen for the “Geospace Environment Modeling (GEM) Substorm Challenge.” By an examination of the potential patterns over the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain, it is shown that the IMF alone is not sufficient to predict the electric potential patterns when there are substorms present, demonstrating the influence of the substorm “unloading” component. This model can reproduce the very complex changes that occur in the potential patterns as the IMF orientation rotates in the GSM Y‐Z plane, and it is useful for scientific studies as input for other models or validation of computer simulation results. There is also a practical application of predicting “space weather” in the magnetosphere, on the basis of upstream solar wind/IMF measurements. The later application will become more effective when accurate methods become available for predicting the AL indices.
Global simulation of the Geospace Environment Modeling substorm challenge event
Authors: Raeder J. , McPherron R. L. , Frank L. A. , Kokubun S. , Lu G. , Mukai T. , Paterson W. R. , Sigwarth J. B. , Singer H. J. , Slavin J. A.
Published: February 2004

We use a global model of Earth's magnetosphere and ionosphere to simulate the Geospace Environment Modeling (GEM) substorm challenge event of November 24, 1996. We compare our results to International Monitor for Auroral Geomagnetic Effects (IMAGE) ground magnetometer data, assimilative mapping of ionospheric electrodynamics (AMIE) polar cap potential and field aligned current patterns, Polar Visible Imaging System (VIS) estimates of the polar cap magnetic flux, GOES 8 geosynchronous magnetometer data, IMP 8 magnetometer data, and Geotail plasma and magnetic field data. We find generally good agreement between the simulation and the data. The modeled evolution of this substorm generally follows the phenomenological near‐Earth neutral line model. However, reconnection in the tail is very localized, which makes establishing a causal relation between tail dynamics and auroral dynamics difficult, if not impossible. We also find that the model results critically depend on the parameterization of auroral Hall and Pedersen conductances and anomalous resistivity in the magnetosphere. For many combinations of parameters that enter these parameterizations, no substorm develops in the model, but instead the magnetosphere enters a steady convection mode. The main deviation of the model from the data is excessive convection, which leads to a strong, driven westward electrojet in the growth phase, only partial tail loading, and a reduced recovery phase. Possible remedies are a better model for auroral conductances, an improved anomalous resistivity model, and a more realistic treatment of the ring current.
An investigation of the influence of data and model inputs on assimilative mapping of ionospheric electrodynamics
Authors: Lu G. , Richmond A. D. , Ruohoniemi J. M. , Greenwald R. A. , Hairston M. , Rich F. J. , Evans D. S.
Published: February 2004

The Geospace Environment Modeling (GEM) substorm challenge event of November 24, 1996, has been used as a test case to investigate the influence of different data and model inputs on the assimilative mapping of ionospheric electrodynamics (AMIE) outputs. During that period the interplanetary magnetic field (IMF) went from northward to southward and then returned to northward. In the later part of the day a moderate substorm with AL ∼ −600 nT took place. The AMIE convection patterns derived from the Super Dual Auroral Radar Network (SuperDARN) data alone are generally similar to those derived using ground magnetometer alone, especially during the relatively stable southward IMF period. However, some differences are found during the northward IMF period and during the substorm; namely, the reversed convection configuration near local noon imaged by SuperDARN is absent in the magnetometer observations while the strong convection on the nightside recorded by the magnetometers during the substorm expansion phase is not seen by the radars. Different conductance models do not seem to have a big effect on the large‐scale distributions of ionospheric convection and Joule heating, but they do alter the cross polar cap potential drop and the hemispheric integrated Joule heating rate by nearly a factor of 2. When the AMIE‐derived electric potential drop was compared with the in situ measurements along the satellite track, it is found that AMIE underestimated the potential drop by 20 kV, amounting to a 22% underestimation. The analysis of the AMIE results based on the SuperDARN and ground magnetometer data reiterates the view that the response of ionospheric convection to a sudden IMF southward turning is global and nearly simultaneous.
Modeling ring current proton precipitation by electromagnetic ion cyclotron waves during the May 14–16, 1997, storm
Authors: Jordanova V. K. , Farrugia C. J. , Thorne R. M. , Khazanov G. V. , Reeves G. D. , Thomsen M. F.
Published: February 2004

We study mechanisms contributing to proton precipitation from the ring current during the May 14–16, 1997, geomagnetic storm. This storm was caused partly by Bz< 0 fields in the sheath region behind an interplanetary shock and partly by the magnetic cloud driving the shock. The storm was characterized by a maximum Kp=7− and a minimum Dst=−115 nT and had a distinctive two‐phase decay related to the passage of the ejection at the Earth. We model the ring current development caused by adiabatic drifts and losses due to charge exchange, Coulomb collisions, wave‐particle interactions, and atmospheric collisions at low altitudes. The nightside magnetospheric inflow is simulated using geosynchronous Los Alamos National Laboratory data, whereas the dayside free outflow corresponds to losses through the dayside magnetopause. We calculate the equatorial growth rate of electromagnetic ion cyclotron waves with frequencies between the oxygen and helium gyrofrequencies and their integrated wave gain as the storm progresses. The regions of maximum wave amplification compare reasonably well to satellite observations. A time‐dependent global wave model is constructed, and the spatial and temporal evolution of precipitating proton fluxes during different storm phases is determined. We find that the global patterns of proton precipitation are very dynamic: located at larger L shells during prestorm conditions, moving to lower L shells as geomagnetic activity increases during storm main phase, and receding back toward larger L shells with storm recovery. However, the most intense fluxes are observed along the duskside plasmapause during the main and early recovery phase of the storm and are caused by plasma wave scattering. This study is relevant to the analysis of the anticipated new data sets from the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) and Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) missions.
The profile of the hydrogen Hβ emission line in proton aurora
Authors: Lummerzheim D. , Galand M.
Published: February 2004

Models of hydrogen‐proton transport in proton aurora predict the line profile of the hydrogen emissions from specified incident proton precipitation. We are using a model that includes collisional angular redistribution which leads to upward moving proton and hydrogen fluxes. For ground‐based observation in the magnetic zenith this causes a small Doppler broadening toward the red in the line profile. The precipitating energetic hydrogen atoms are responsible for the prominent Doppler shift toward the blue. The resulting line profile has thus both a widened red and a widened blue wing. Using a spectrometer with sufficient spectral resolution to distinguish the red‐shifted wing of the line from the instrumental line broadening, we obtain Hβ line profiles (486.1 nm). Comparing the predicted line shapes to our observations, we find the red‐shifted wing is due to upward moving hydrogen as predicted by the angular redistribution in the model calculations. The shape of the blue‐shifted wing, rather than the location of the peak of the blue‐shifted line profile, is a suitable indicator of the mean energy of the precipitating proton flux.
The quantitative relationship between auroral brightness and solar EUV Pedersen conductance
Authors: Shue J.‐H. , Newell P. T. , Liou K. , Meng C.‐I.
Published: February 2004

A quantitative relationship between auroral brightness and solar EUV produced Pedersen conductance is established by using Lyman‐Birge‐Hopfleld long and short bands of auroral emissions from Polar Ultraviolet Imager (UVI). We used a conductance model characterized by solar zenith angle, solar F10.7 radio flux, and the local magnetic field to estimate solar EUV Pedersen conductance for each pixel of the images. The hourly average IMP 8 solar wind data set is used to derive corresponding solar wind conditions. Pixels of the auroral brightness in Polar UVI images have been binned in terms of magnetic latitude and local time. We relate the auroral brightness to the conductance and find that a positive correlation occurs in the early morning sector, indicating increasing brightness with increasing conductance. An anticorrelation appears in the premidnight region, supporting the previously reported suppression of the discrete aurora occurrence in sunlight. The correlation becomes stronger for the southward interplanetary magnetic field (IMF) condition than for the all Bz condition in the early morning and premidnight sectors, indicating that the conductance effect on the auroral brightness becomes significant when the IMF is southward. The afternoon auroral bright spot is not eminent when the ionospheric conductance is low.
Origins of energetic ions in the cusp
Authors: Trattner K. J. , Fuselier S. A. , Peterson W. K. , Chang S.‐W. , Friedel R. , Aellig M. R.
Published: February 2004

Recent studies reported on a new magnetospheric phenomenon called cusp energetic particle (CEP) events. It has been suggested that these energetic particles with significant fluxes up to several hundred keV/e are accelerated locally in the cusp. An alternative explanation for the energetic particle events is that they are accelerated at the quasi‐parallel bow shock, then transported downstream and enter the cusp along newly reconnected field lines or some other solar wind entry mechanism. It is well known that shock‐accelerated ions have characteristic abundance ratios, composition ratios relative to the solar wind composition, temperatures, and spectral dependency on solar wind conditions. These parameters are used to distinguish between a local (cusp) and remote (bow shock) acceleration. Our results show that average values of these parameters in the cusp are comparable to those at quasi‐parallel shocks. Furthermore, changes in some of these parameters indicate a solar wind entry process that depends on energy and mass. No local acceleration is required to explain the observed CEP events up to 150 keV/e. For ions above 150 keV/e the magnetosphere itself may be responsible for the observed cusp fluxes.
Polar/Toroidal Imaging Mass‐Angle Spectrograph observations of suprathermal ion outflow during solar minimum conditions
Authors: Peterson W. K. , Collin H. L. , Yau A. W. , Lennartsson O. W.
Published: February 2004

We present observations of the magnitude and variability of escaping suprathermal ions in the energy per charge range of 15 eV/e to 33 keV/e. The data were obtained from the Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) on the Polar spacecraft from April 1996 to September 1998 over the Earth's southern Polar cap during solar minimum conditions. The net outflow rates of ionospheric ions derived from this data set are significantly different from those inferred from analysis of similar data obtained at higher altitudes from the Dynamics Explorer (DE) 1 satellite. The data present a clear picture of the seasonal variation of ion outflow as a function of solar illumination (i.e., season). We conclude that the differences between the present results and previous DE 1 estimates of the magnitude of escaping suprathermal ions can be explained by energization of the H+ component of the Polar wind above the 6000–8000 km altitude region, where the Polar data were acquired. We also note that seasonal variations in He+ outflow presented here are not as large as those reported previously.
Influence of interplanetary magnetic field on global auroral patterns
Authors: Shue J.‐H. , Newell P. T. , Liou K. , Meng C.‐I
Published: February 2004

We produce average patterns of global auroral brightness using images from the Polar Ultraviolet Imager (UVI). UVI measures auroral brightness at two wavelength bands within the N2 Lyman‐Birge‐Hopfield emission spectra, allowing a determination of average energy and total precipitating energy flux. The auroral patterns are organized by both season (to show the effects of solar UV flux on auroral patterns) and interplanetary magnetic field (IMF) orientation. One global pattern for each of eight IMF clock angles in the GSM YZ plane is presented. It is shown that the dependence of global auroral patterns on IMF changes with the season. The afternoon bright spot is more distinct from the nightside aurora for B>y < 0 than for By > 0 under large northward IMF or small IMF Bz conditions during the summer. The average position of the duskside aurora varies with the sign of IMF By during the summer. However, its location remains almost the same during the winter. Northern Hemisphere auroral power is greater for By < 0 than for By > 0. The By effect on auroral power is more pronounced when the southward IMF is larger.
Polar observations of the time‐varying cusp
Authors: Rae I. J. , Lester M. , Milan S. E. , Fritz T. A. , Grande M. , Scudder J. D.
Published: February 2004

We present a multi‐instrument case study of the signatures associated with the cusp observed by the Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE), Hot Plasma Analyzer (HYDRA) and Magnetic Fields Experiment (MFE) instruments onboard the Polar satellite between 0900 and 1300 UT on October 29, 1996. Characteristic signatures of the midaltitude cusp include ions of energies 102–104 eV, electrons of energies 10–103 eV, and a structured depression in the local magnetic field. During this cusp pass, ion fluxes in the 1–20 keV range were modulated quasi‐periodically on timescales of between 12 and 20 min, while energy‐dispersed ion count rates were observed close to the open‐closed field line boundary. The particle populations on the cusp field lines are dominated by H+ and He++ ions (characteristic of magnetosheath‐like plasma), and on closed field lines dominated by uniform fluxes of high‐energy (50–200keV) protons, which are not pulsed. The pulsed particle events during this case study occur during both southward and northward interplanetary magnetic field (IMF), but differ slightly in nature. After the IMF rotated northward, Polar still observed the pulsed particle events, but magnetosheath‐like and closed field line particles coexisted on the same magnetic flux tubes. It is not evident as to how this mix of plasma can exist on the same field line during northward IMF. HYDRA also observed higher‐frequency pulsing, hypothesized to be caused by low‐latitude transient magnetic reconnection. The simultaneous observation of two different frequency components has not been previously reported in spacecraft observations of the midaltitude cusp. Two further cusp passes during similar conditions are presented. The short‐period pulsed particle signatures were observed in all three examples, the longer‐period pulsing observed in two. The events differed in duration, most likely owing to the effect of IMF By on the location of the reconnection site, and hence the location of the polar cusp.
The storm‐substorm relationship: Ion injections in geosynchronous measurements and composite energetic neutral atom images
Authors: Reeves G. D. , Henderson M. G.
Published: February 2004

We have analyzed isolated and storm time ion injections using geosynchronous particles, energetic neutral atom (ENA) data, and Dst. There are both surprising similarities between the two classes of events as well as important differences that bear directly on the relationship between storms and substorms. The average geosynchronous ion responses during the growth phase, at onset, and in the ≈15 min following onset are nearly identical in intensity, spectral hardness, and temporal profile. ENA observations confirm that similarity and additionally show that the two classes of injections span nearly the same extent in local time. The two classes of injections differ primarily in the subsequent behavior of the ion fluxes. For the isolated injections the fluxes return to preevent levels within about an hour, and exhibit the expected westward drift and dispersion. For the storm time injections the fluxes remain elevated for at least several hours following the initial injection. Additionally, the ENA observations show new evidence that the region of new particle injections expands eastward (opposite to the ion drift direction) to encompass most of the nightside. Within 3 hours, ENA emissions are observed coming from most of the inner magnetosphere but have still not formed a symmetric, trapped distribution. Within those same 3 hours Dst decreased an average of 40 nT with the initial decrease observed in the same hour as the initial injection. The isolated injections did not produce a measurable Dst signature. These results show that despite many remarkable similarities, storm time ion injection events are different from isolated injection events
Diffusion of 155 to 430 keV protons in the Earth's radiation belt
Authors: Walt M. , Voss H. D. , Lev‐Tov S. J. , Mobilia J. , Jahn J.‐M.
Published: February 2004

Pitch angle distributions and energy spectra of 155–430 keV trapped protons between L=4 and L=5.5 are examined to study pitch angle and radial diffusion rates during a period of low magnetic activity. To improve the sensitivity to pitch angle diffusion, the measurements were made near the loss cone where angular gradients in the distribution function are large. In this region the phase space density is such that both radial diffusion and pitch angle diffusion lead to flux increases while charge exchange leads to proton loss. However, the conventional prescriptions for the diffusion coefficients predict too much diffusion at L=4, implying growth in the proton fluxes while observations show loss. At L=5, theory and observation both show slight growth, and at low pitch angles diffusion in pitch angle is necessary to compensate for charge‐exchange losses. Although Dst was low during the 13 days preceding the measurements, the proton belt had not reached a steady state with diffusion sources matching charge‐exchange losses.
Ring current ion composition during solar minimum and rising solar activity: Polar/CAMMICE/MICS results
Authors: Pulkkinen T. I. , Ganushkina N. Yu. , Baker D. N. , Turner N. E. , Fennell J. F. , Roeder J. , Fritz T. A. , Grande M. , Kellett B. , Kettmann G.
Published: February 2004

This paper shows statistical results of the ring current ion composition and its variability as a function of solar cycle and magnetospheric activity for 3 < L < 8. Spin‐averaged energetic particle (1–200 keV) measurements from Polar are combined with geomagnetic indices as well as solar wind and interplanetary observations from the Wind spacecraft during a period from September 1996 to March 1999. The statistics are performed both for time‐averaged values for all periods as well as for peak flux values during geomagnetic storms (defined as Dst < −50 nT) that occurred during this period. The average O+ energy density increases by about a factor of 5 during the rising phase of the solar cycle from the minimum values in 1996, while the average values of H+ and He show variability but no consistently increasing trend. The O+ flux is small (below 10%) compared with the hydrogen flux, and the average energy density ranges from a few percent at solar minimum to ∼10% at high solar activity time in early 1999. The O+ flux is typically smaller than the He+ flux, reaching comparable values only during the latter part of the period when the solar activity increased. Analogously, the energy densities of O+ and He+ are about equal during 1996 and 1997, whereas the O+ energy density is about twice the He+ energy density during the higher solar activity period in 1998 and early 1999.
MeV magnetosheath ions energized at the bow shock
Authors: Chang S.‐W. , Scudder J. D. , Kudela K. , Spence H. E. , Fennell J. F. , Lepping R. P. , Lin R. P. , Russell C. T.
Published: February 2004

A causal relationship between midlatitude magnetosheath energetic ions and bow shock magnetic geometry was previously established for ion energy up to 200 keV e−1 for the May 4, 1998, storm event. This study demonstrates that magnetosheath ions with energies above 200 keV up to 1 MeV simply extend the ion spectrum to form a power law tail. Results of cross‐correlation analysis suggest that these ions also come directly from the quasi‐parallel bow shock, not the magnetosphere. This is confirmed by a comparison of energetic ion fluxes simultaneously measured in the magnetosheath and at the quasi‐parallel bow shock when both regions are likely connected by the magnetic field lines. We suggest that ions are accelerated at the quasi‐parallel bow shock to energies as high as 1 MeV and subsequently transported into the magnetosheath during this event.
Three‐dimensional artificial neural network model of the dayside magnetopause
Authors: Dmitriev A. V. , Suvorova A. V.
Published: February 2004

Artificial neural networks (ANN) from the package NeuroShell 2 are applied to the modeling of the dayside magnetopause. The model data set is based on magnetopause crossings and on the corresponding hour‐averaged measurements of the solar wind plasma and the interplanetary magnetic field. The ANN model represents the three‐dimensional shape and size of the dayside magnetopause as a function of the solar wind dynamic pressure and theBzandBycomponents of the interplanetary magnetic field. The model describes the dynamics of the magnetic cusp and global asymmetry of the dayside magnetopause under a wide range of external conditions. Small variations in the size of the magnetopause as a function of the absolute value ofByare presented quantitatively in the form of an analytic expression. The ANN model shows three regimes of magnetopause dynamics that are controlled by theBzcomponent: a pressure balance regime (Bz0 nT), a reconnection regime (0>Bz>−10 nT), and a regime of reconnection saturation (Bz<−10 nT). Also, three different regimes of magnetopause dynamics at the subsolar point are described by a modified pressure balance equation obtained from the ANN model.
Propagation sense of low‐frequency MHD waves in the magnetosheath observed by Geotail
Authors: Matsuoka Ayako , Southwood David J. , Kokubun Susumu , Mukai Toshifumi
Published: February 2004

Using Geotail data from the months of December 1994 and January 1995, we have investigated transverse fluctuations in the magnetic field and in the plasma velocity in the magnetosheath. In the frequency range below about 10 mHz, a change in the magnetic field is often linearly correlated with a change in plasma velocity, satisfying the Walén relation expected from the theory of MHD wave propagation. We found well‐correlated events in a wide range of radial distances of the magnetosheath, −42 RE < GSM‐X < 6 RE, where RE is the radius of the Earth. The correlation is sometimes positive and sometimes negative, depending upon the angle between the magnetic field and the plasma flow direction. The results indicate that, in a frame moving with the plasma, the waves propagate unidirectionally in the same direction as the plasma flow in a draping field. In other words, the wave energy is always propagating simultaneously antisunward and outward. Moreover, our results suggest two possibilities for the origin of the signals in the magnetosheath: the magnetopause or the nose region of the bow shock.
The cross‐polar potential drop and its correlation to the solar wind
Authors: Eriksson S. , Ergun R. E. , Carlson C. W. , Peria W.
Published: February 2004

The cross‐polar potential drop Φpc as derived from the FAST satellite is used to study the average magnetospheric response to changes in the solar wind as monitored by the Wind spacecraft. The coupling of the solar wind with the magnetosphere is examined using the interplanetary magnetic field (IMF) BZ, the model reconnection electric field vBtsink(θ/2) for κ=3, 4, and the Akasofu‐Perreault ε parameter. Initial results comprising 37 cases of Φpc show one major response of the magnetosphere to the solar wind forcing at 15 min time lag followed by two minor pulses at 55 min and 105 min, respectively, during times when the IMF polarity was mainly southward and the geomagnetic activity was moderate to low. Φpc shows a very good correlation with typical models of the reconnection electric field at 15 min time lag, reaching a maximum linear correlation coefficient of r=0.95 for vBtsin3(θ/2). In order to reach an understanding of the importance of individually calculated correlation coefficients, we introduce the statistical bootstrap algorithm of Efron and Tibshirani [1993], which allows us to estimate a correlation coefficient standard error. In defining a quality measure based on this method, the significance coefficient s, we are able to interpret a resulting correlation coefficient time lag series in terms of a linear prediction filter similar to earlier techniques. The results on the magnetospheric response for Φpc are further compared with those obtained using the geomagnetic indices Dst, SYM‐H, and ASY‐H. The similar magnetospheric response to the solar wind electric field for Φpc and ASY‐H at time delays of more than 40 min together with a high correlation coefficient between Φpc and ASY‐H suggest that these magnetospheric parameters couple to one another. The average dynamic response of the ionospheric convection to the solar wind electric field is in essential agreement with those reported by Klimas et al. [1994] on a normal magnetospheric mode with a recurrence frequency of 50 min.
Conditions for the validity of the incompressible assumption for the ballooning instability in the long‐thin magnetospheric equilibrium
Authors: Miura Akira
Published: February 2004

Conditions under which the compressibility can be neglected for the magnetospheric ballooning instability, which arises in the shear Alfvén branch, are clarified in the context of ideal magnetohydrodynamic plasmas and stochastic plasmas by using the normal mode analysis and the energy principle. An expansion in the small parameter, which is equal to the ratio of the ∇⊥B0 scale length to the field line curvature radius, shows that the incompressible assumption is valid for the ballooning instability with long‐thin perturbations in the low‐frequency regime and in the long‐thin magnetospheric equilibrium, in which the field line curvature radius is much larger than the ∇⊥B0 scale length. When the long‐thin assumption for the equilibrium is not satisfied near the equator, the calculation of the energy functional for a trial function shows that the strongly localized ballooning mode is essentially incompressible if the plasma β at the equator is much larger than 6/Γ, where Γ is the ratio of specific heats. For the stochastic plasmas near the equator the strongly localized ballooning mode is essentially incompressible irrespective of the β value. These results justify the incompressible assumption made in a previous ballooning stability analysis for the long‐thin magnetospheric equilibrium. It is suggested that before the substorm onset, the near‐Earth plasma sheet becomes more taillike, and the long‐thin assumption for the equilibrium becomes more likely to be satisfied on average, and thus the near‐Earth plasma sheet becomes more favorable to the onset of the ballooning instability without the strong stabilizing influence of the compressibility.
Substorm currents associated with magnetotail magnetic dipolarization: Geotail observations
Authors: Nakai H. , Kamide Y.
Published: February 2004

Three typical events of magnetic field dipolarization in the near‐Earth magnetotail observed by the Geotail satellite are critically studied. During these events, Geotail was in the region R= 10–18 RE in the midnight sector. Transient, not step‐like negative, perturbations in the By component were observed at Geotail just after the boundary crossings from the southern tail lobe to the plasma sheet, suggesting the existence of double sheet currents: a pair of earthward and tailward field‐aligned sheet currents with the earthward current on the poleward side. Geotail simultaneously observed an intense northward electric field, which indicates that a dynamo region for the double sheet current exists tailward of the spacecraft. Examining magnetograms at ground stations, it is found that the ionospheric foot points of Geotail were collocated with the region of the enhanced westward electrojet. On the basis of these observations, we propose a new substorm current model in which a double sheet current system is coupled with the conventional substorm wedge current system.
Cosmic ray survey to Antarctica and coupling functions for neutron component near solar minimum (1996–1997): 1. Methodology and data quality assurance
Authors: Villoresi G. , Dorman L. I. , Iucci N. , Ptitsyna N. G.
Published: February 2004

During the 1996–1997 austral summer, a 3NM‐64 neutron monitor was operated on board the ship Italica to record cosmic ray neutron intensities in ocean areas from Italy to Antarctica and back. Two bare BF3 counters also measured the flux of thermalized neutrons. The main purpose of this research was the determination of an accurate latitude curve of cosmic ray nucleonic intensity during solar minimum to be used for obtaining the coupling function appropriate for this phase of solar cycle. Accuracy was achieved through reliable instrumentation and proper control of the experiment and by determining and applying for the first time all corrections, for (1) changes in vertical atmospheric mass column, by taking into account wind effect; (2) oscillations of the ship produced by sea roughness; (3) atmospheric temperature changes; (4) primary cosmic ray temporal variations including north–south asymmetry; and (5) temporal variations in the cosmic ray east–west effect, caused by asymmetric shielding on the ship. In this paper we describe the experiment, present the complex procedure of data reduction which was required for the survey data, evaluate the intensity changes produced by primary cosmic ray temporal variations, correct the data for meteorological effects, and present the “fully corrected” daily values of neutron intensity as a function of threshold rigidity. Several independent tests are applied to evaluate the quality of the “fully corrected” data and to determine their residual statistical uncertainty. In two companion papers, all meteorological effects are investigated in detail, geomagnetic effects are evaluated, and coupling functions are computed.
Variations of the atmospheric electric field in the near‐pole region related to the interplanetary magnetic field
Authors: Frank‐Kamenetsky A. V. , Troshichev O. A. , Burns G. B. , Papitashvili V. O.
Published: February 2004

Variations in the atmospheric, near‐surface vertical electric field component Ez measured at the Russian Antarctic station Vostok in 1998 are analyzed in conjunction with changes of the interplanetary magnetic field (IMF). A total of 134 days were selected which satisfied the “fair weather” conditions, that is, days with absence of high winds, falling or drifting snow, clouds, and electric field “pollution” from the station's power plant. It is shown that the average diurnal variation of Ez for these days follows the global geoelectric field “fair ‐ weather” diurnal variation: the “Carnegie” curve, which describes the global electric circuit formed by the thunderstorm activity occurring mostly over equatorial regions. The Ez diurnal variation shows strong seasonal dependence: it is maximal (∼40% of the average daily magnitude) in summer but gradually reduces through the equinoctial months and is almost negligible during the austral winter. Ez at Vostok is strongly affected by variations in both the IMF By and Bz components. The influence of By is dominant during geomagnetic daytime hours (1100–1400 UT at Vostok): Ez increases with By in the range from −10 to +10 nT. The IMF Bz effect is mainly seen at dawn (Ez decreases with Bz) and dusk (Ez increases with Bz).
Solar cycle lengths and climate: A reference revisited
Authors: Laut Peter , Gundermann Jesper
Published: February 2004

An article published by Friis‐Christensen and Lassen [1991] appeared to indicate an association between solar cycle lengths (SCLs) and climate. It attracted worldwide attention and has since been extensively referred to. We here present an updated analysis using a recent temperature reconstruction with the time period of comparison considerably expanded. The correlation is found to be weak. In the light of this new result we analyze the question how the article by Friis‐Christensen and Lassen was able to create the impression of a ‘strikingly good agreement’, as the authors described it. We show that the main reason is an unacceptable mixing of filtered and nonfiltered data in the graphical representation. Hereby, an artificial agreement of the solar data with the global warming since 1970 was established. The article by Friis‐Christensen and Lassen has created and still creates confusion both in scientific and public discussions on climate change. We have therefore found it relevant to deliver the present analysis.