bluuebunny/crossref_metadata_2025 huggingface dataset:
Twelve‐hour tides in the winter northern polar mesosphere and lower thermosphere
Authors: Fisher Genene M. , Niciejewski Richard J. , Killeen Timothy L. , Gault William A. , Shepherd Gordon G. , Brown Stephen , Wu Qian
Published: October 2002

An investigation of the tidal oscillations near the winter northern polar mesopause has been carried out at Resolute Bay, Canada (74.9°N, 94.9°W). The daily, monthly, and annual variations of the tidal field were studied using the 1995/1996 and 1996/1997 data. Ground‐based measurements were obtained using a Fabry‐Perot interferometer with a circle to line interferometric optical system (FPI/CLIO) and the E Region Wind Interferometer (ERWIN). The FPI/CLIO observed the Doppler winds of the OH emission near 86 km altitude. The ERWIN provided simultaneous OH (near 86 km) and OI (near 97 km) wind measurements. Monthly data were compared to the Global Scale Wave Model (GSWM) and the Forbes/Vial Solar Semidiurnal Tidal Model predictions. Analysis of the 12‐hour oscillation near 86 and 97 km altitudes indicates that it is persistent and characteristic of a semidiurnal propagating tide. The semidiurnal tide was found to be highly variable in amplitude and phase from day to day. The observations presented provide a continuous and extensive data set for the winter season.
Fast neutron flux spectrum aboard Mars Odyssey during cruise
Authors: Feldman W. C. , Prettyman T. H. , Tokar R. L. , Byrd R. C. , Fuller K. R. , Gasnault O. , Longmire J. L. , Olsher R. H. , Storms S. A. , Thornton G. W. , Boynton W. V.
Published: October 2002

The energy spectrum of fast neutrons generated by interactions between galactic cosmic rays and the Mars Odyssey spacecraft during quiet interplanetary conditions is determined. It is given by Φn(En) = (0.071 ± 0.0044) En−(0.28±0.055) cm−2 s−1 MeV−1 for energies between 0.7 and 6 MeV. The estimated ratio of this component of spacecraft neutron background to the Martian leakage flux in Mars orbit is expected to amount to between 0.30 and 0.34. This fraction is sufficiently small so that the total background in orbit about Mars is expected to be dominated by Martian leakage neutrons reprocessed by the spacecraft. Nevertheless, the measured asymmetry properties of the Odyssey Neutron Spectrometer response function should allow separation of foreground and background neutron fluxes when in Mars orbit. The fast neutron flux between 0.7 and 6 MeV should only contribute 0.40 Roentgen Equivalent Man (rem) per year to the total radiation dose aboard a Mars Odyssey class spacecraft at solar maximum.
Localized ion outflow in response to a solar wind pressure pulse
Authors: Fuselier S. A. , Collin H. L. , Ghielmetti A. G. , Claflin E. S. , Moore T. E. , Collier M. R. , Frey H. , Mende S. B.
Published: October 2002

Neutrals created by charge‐exchange of low‐energy (∼10–100 eV) ion outflow from the ionosphere are imaged by the Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft. Comparing the neutral flux with changes in the solar wind during an ion outflow event on 24 June 2000 indicates that changes in solar wind density (and therefore dynamic pressure) are associated with episodic bursts of ion outflow. LENA images show that these bursts are confined to the duskside. Simultaneous images of the aurora from the IMAGE FUV Wideband Imaging Camera indicate that these episodic bursts on the duskside are associated with increased duskside auroral emissions. The pitch angle and charge exchange altitude for the ion outflow distributions are estimated by applying field line tracing in a model magnetic field and assuming that the outflow occurs in the auroral zone. The pitch angle information places constraints on the neutral atom images, indicating that the ion outflow observed on the duskside probably consists of high pitch angle conics and that field‐aligned ion outflow from other parts of the oval cannot be observed from the IMAGE spacecraft location.
Longitudinal ionospheric effects in the South Atlantic evening sector during solar maximum
Authors: de Paula E. R. , Souza J. R. , Fejer B. G. , Bailey G. J. , Heelis R. A.
Published: October 2002

Large‐scale horizontal gradients in ion density and vertical drift observed by the Atmospheric Explorer E satellite in the South Atlantic region (latitudes 10°S–20°S, longitudes 50°W–10°E) during the June solstice at solar maximum are presented and analyzed. These features occur during the nighttime period. The observations near 450‐km altitude show vertically downward ion drift velocities exceeding 120 m s−1 and depleted regions where the ion density is around 2 × 104 cm−3. It is shown, using values modeled by the Sheffield University Plasmasphere Ionosphere Model (SUPIM) along the satellite trajectory, that the large ion density depletions appear as a result of large downward ion drifts driven by large southward winds along the magnetic meridian and by diffusion. During others seasons such behavior is not observed by the AE‐E satellite, neither by SUPIM results. The roles played by the different physical processes responsible for the large downward drift velocities are investigated. The model results highlight the relationship between longitudinal variation of the ion densities and the location of the equatorial anomaly crest in the South Atlantic region.
Modeling the heliospheric current sheet: Solar cycle variations
Authors: Riley Pete , Linker J. A. , Mikić Z.
Published: October 2002

In this report we employ an empirically driven, three‐dimensional MHD model to explore the evolution of the heliospheric current sheet (HCS) during the course of the solar cycle. We compare our results with a simpler “constant‐speed” approach for mapping the HCS outward into the solar wind to demonstrate that dynamic effects can substantially deform the HCS in the inner heliosphere (≲5 AU). We find that these deformations are most pronounced at solar minimum and become less significant at solar maximum, when interaction regions are less effective. Although solar maximum is typically associated with transient, rather than corotating, processes, we show that even under such conditions, the HCS can maintain its structure over the course of several solar rotations. While the HCS may almost always be topologically equivalent to a “ballerina skirt,” we discuss an interval approaching the maximum of solar cycle 23 (Carrington rotations 1960 and 1961) when the shape would be better described as “conch shell”‐like. We use Ulysses magnetic field measurements to support the model results.
Climatology and storm time dependence of nighttime thermospheric neutral winds over Millstone Hill
Authors: Fejer B. G. , Emmert J. T. , Sipler D. P.
Published: October 2002

We use 630.0 nm nightglow Fabry‐Perot measurements over Millstone Hill from 1989–1999 to study the climatology and storm time dependence of the midlatitude thermospheric winds. Our quiet time wind patterns are consistent with results from earlier studies. We determine the perturbation winds by subtracting from each measurement the corresponding quiet time averages. The climatological zonal disturbance winds are largely independent of season and solar flux and show large early night westward and small late‐night eastward winds similar to disturbance ion drifts. The meridional perturbation winds vary strongly with season and solar flux. When the solar flux is low, the winter and equinox average meridional winds change from equatorward to poleward at ∼2200 LT, and the summer winds are equatorward throughout the night. The high solar flux meridional winds are poleward, with magnitudes increasing from dawn to dusk at all seasons. These disturbance winds patterns are in poor agreement with results from the empirical horizontal wind model, HWM‐93. The zonal and meridional disturbance winds show very large variations relative to their average values. We have also studied the time‐dependent response of the midlatitude thermospheric winds to enhanced magnetic activity. The early night westward winds build up to large amplitudes (about twice their climatological values) in ∼6 hours; the late‐night eastward winds are smaller and reach their peak values ∼3 hours after the increase in magnetic activity. The storm time dependence of the meridional winds is considerably more complex than that of the zonal winds, and it varies with season and solar flux. Following enhanced magnetic activity, equatorward winds are observed at all local times and seasons, but the increase of their amplitudes with storm time is fastest in the late local time sector. Near midnight, and when the solar flux is low, the meridional winds reverse from equatorward to poleward ∼6–10 hours after the increase in magnetic activity. This reversal is fastest (slowest) during December (June) solstice. At later local times, and for high solar flux conditions, the variation of the meridional disturbance winds is season independent. The observed storm time dependence partly explains the large variability of the disturbance winds.
Determining the size of lightning‐induced electron precipitation patches
Authors: Clilverd Mark A. , Nunn David , Lev‐Tov Sean J. , Inan Umran S. , Dowden Richard L. , Rodger Craig J. , Smith Andy J.
Published: October 2002

We analyze Trimpi signatures during 23 and 24 April 1994 at four sites on or near the Antarctic Peninsula (Palmer, Faraday, Rothera, and Halley) on subionospheric VLF signals received from four U.S. naval transmitters (NAA, NSS, NLK, and NPM). Electron precipitation patches are found to be large, i.e., ∼1500 km × 600 km, with the longer axis orientated east‐west. Calculations using a three‐dimensional Born scattering model, where patch densities are 1.5 electrons cm−3 above ambient at the center at ∼84 km altitude, provides results that are consistent with this picture. A high proportion (38%) of the Trimpi events were associated with strong lightning flashes in eastern United States. When lightning discharges had currents >65 kA (positive or negative), there was a >80% chance of seeing an associated Trimpi event. The chance of seeing any Trimpi events fell to near zero for discharges of <45 kA. The largest Trimpi perturbations occur when the center of the precipitation patch is 700–800 km from the receivers. This result is consistent with the modeling calculations for large patches. The equatorward edge of the precipitation patch was estimated to be at ∼60°S, close to the magnetic conjugate of the lightning. The close association of the equatorward edge of the precipitation patch with the conjugate location of the causative lightning is consistent with a quasi‐ducted whistler‐induced precipitation mechanism. Nonducted whistler‐induced precipitation mechanisms would predict a 5°–10° latitudinal gap between the lightning and the equatorward edge of the patch. However, the lack of observed whistlers at the time of the Trimpi events is consistent with the nonducted whistler mechanism and is not consistent with the quasi‐ducted mechanism, although the distances from duct exit point to receiver may have been too large (∼700–1000 km) for the signals to be detectable. Using the significantly larger patch dimensions determined in this study, it is estimated that lightning may well be 10–100 times more effective at depleting the radiation belts than hiss.
Models of two‐dimensional embedded thin current sheets from Vlasov theory
Authors: Schindler Karl , Birn Joachim
Published: October 2002

This paper uses the steady state Vlasov theory to obtain an explicit description of the properties of thin current sheets in two space dimensions. The method is general; in particular, it is not restricted to local Maxwellian distribution functions. The treatment includes chaotic particle motion, and the plasma is treated as quasi‐neutral. Such general equilibria are necessary for modeling observed structures in the Earth's magnetotail. In particular, this applies to thin current sheets embedded in wider sheets. Abandoning local Maxwellian distribution functions implies that the electric potential can no longer be transformed to zero such that additional efforts are required to include the electric field. Critical input parameters are the gyroscales of the different particle species and the ion/electron temperature ratio. The results illustrate how finite ion gyroscales affect the current sheet structure. Further aspects that are analyzed include the contributions of electrons and ions to the total current density, Hall currents, the occurrence of significant electric fields and the role of the ion/electron temperature ratio. The necessity of electric potentials in the self‐consistent structure of thin current sheets of the present type may provide a link between the presence or formation of thin current sheets and auroral arcs.
The release of near‐relativistic electrons from the Sun
Authors: Robinson I. M. , Simnett G. M.
Published: October 2002

Coronal flares are perhaps the most prolific source of low‐energy electrons (2–10 keV) detected in the interplanetary medium and are occasionally seen at higher energies ≥100 keV. We have searched 38–315 keV electron data from the Electron, Proton, and Alpha Monitor (EPAM) on the ACE spacecraft throughout 1998 for such events and focus upon three prolonged bursts of electrons on 5 May 1998. We find that certainly one and probably all three bursts originated in the corona several R⊙ above the photosphere. They were composed of multiple injections of electrons and were possibly quasi‐continuously released over almost 24 hours, periodically interrupted at ACE by changes in solar wind speed and magnetic connection to the Sun. We postulate that the presence of seed electrons in the corona from a chromospheric flare 3 days earlier played a role in the acceleration of these electrons to energies of ≥100 keV.
Instrumental recording of electrophonic sounds from Leonid fireballs
Authors: Zgrablić Goran , Vinković Dejan , Gradečak Silvija , Kovačić Damir , Biliškov Nikola , Grbac Neven , Andreić Željko , Garaj Slaven
Published: October 2002

Electrophonic meteor sounds, heard simultaneously with the meteor appearance, are a longstanding problem due to their nonintuitive nature. Previous investigations have been undermined by lack of instrumental recordings. Here we present the first instrumental detection of electrophonic sounds obtained during the observation of 1998 Leonids from Mongolia. Two Leonid fireballs of brightness −6.5m and −12m produced short, low‐frequency sounds, which were simultaneously recorded by microphones in a special setup and heard by different observers. Simultaneous measurements of electromagnetic ELF/VLF radiation above 500 Hz did not reveal any signal correlated to the electrophonic event. The lack of signal was explained by the low frequency of electrophones. We show that physical characteristics of Leonid electrophones cannot be completely explained by existing theories and that further theoretical refinement and observational work is needed. Finally, we tentatively suggest the possibility of stronger than expected coupling of fireballs with atmospheric charge dynamics and ionosphere.
D region reflection height modification by whistler‐induced electron precipitation
Authors: Rodger Craig J. , Clilverd Mark A. , Dowden Richard L.
Published: October 2002

We examine the electron density profile expected in the lower ionosphere due to a 0.2‐s whistler‐induced electron precipitation (WEP) burst with experimentally determined properties. The ionization rate in the lower ionosphere due to a single such WEP event has a height variation with a rather broad maximum, leading to additional electron densities of ∼5 electrons cm−3 stretching over altitudes of ∼75–92 km. For ambient nighttime conditions a single WEP burst with these parameters will lead to a significant electron density changes only for altitudes below ∼85 km. We go on to consider the cumulative response of the nighttime D region to a sustained series of WEP bursts observed through Trimpi perturbation activity on one night in the Antarctic. For altitudes >70 km, significant long‐term changes in electron densities due to WEP bursts can occur. The additional WEP‐produced ionization leads to increases in the high‐altitude electron densities, until a new equilibrium level is reached. Peak changes in electron density are ∼16 times ambient at 85 km and ∼7 times ambient at 90 km, occurring in the ∼15‐min period during which the WEP rate is at its peak (∼4.5 per min). The simulation suggests that electron density levels “settle” into an new quasi‐equilibrium state during the ∼3‐hour period where the ionization at 85‐km altitude is 10–12 times ambient due to WEP bursts occur at ∼3 min−1. The ionization changes produced by WEP bursts lead to lower reflection heights for VLF and LF radio waves (in the Earth‐ionosphere waveguide). While significant short‐term changes in reflection heights are likely, realistic long‐term changes in WEP occurrence rates do not appear likely to be able to explain the reported ∼2 km decrease in LF reflection heights observed during the last 35 years.
Properties of magnetic clouds and geomagnetic storms associated with eruption of coronal sigmoids
Authors: Leamon Robert J. , Canfield Richard C. , Pevtsov Alexei A.
Published: October 2002

We study 46 solar coronal eruptions associated with sigmoids seen in images from the Yohkoh Soft X‐ray Telescope (SXT). We relate the properties of the sigmoids to in situ measurements at 1 AU and geomagnetic storms. Our primary result is that erupting sigmoids tend to produce geoeffective magnetic clouds (MCs): 85% of the erupting sigmoidal structures studied spawned at least a “moderate” (|Dst| ≥ 50 nT) geomagnetic storm. A collateral result is that MCs associated with sigmoids do not show the same solar‐terrestrial correlations as those associated with filaments and, as such, form a distinct class of events. First, rather than reversing with the global solar dipole (at solar maximum), the leading field in MCs weakly (2:1) shows a solar cycle (Hale polarity) based correlation (reversing at solar minimum). Second, whereas the handedness of MCs associated with filament eruptions is strongly (95%) related to their launch hemisphere, that of MCs associated with sigmoid eruptions is only weakly (∼70%) so related. Finally, we are unaware of any model of the magnetic fields of sigmoids and their eruption that gives a useful prediction of the leading field orientation of their associated MC.
Interpreting observations of MF/HF radio emissions: Unstable wave modes and possibilities to passively diagnose ionospheric densities
Authors: Weatherwax A. T. , Yoon Peter H. , LaBelle J.
Published: October 2002

Recent satellite observations by the Wind spacecraft, as well as earlier observations by the ISIS 1 and 2 satellites, of MF/HF radio emissions have been attributed to upper hybrid/Z mode waves generated in the topside ionosphere at two and three times the local electron gyrofrequency fce. We calculate the growth rates of such emissions for various wave modes as a function of altitude in the topside ionosphere and for a variety of electron density profiles, employing a modified loss‐cone/horseshoe electron distribution function as the free energy source. These calculations represent the first application of this instability model to the topside ionosphere, in parallel to earlier efforts involving wave excitation in the bottomside of the F region. In addition to the Z mode, which is known to be unstable at points where the upper hybrid frequency matches cyclotron harmonics, the X mode can be directly generated near the first harmonic of the electron cyclotron frequency for certain density profiles. We further put forth how these emissions might be used as a remote diagnostic of ionospheric plasma properties.
On the linear theory of equatorial plasma instability: Comparison of different descriptions
Authors: Basu B.
Published: October 2002

Three theoretical descriptions, namely, the local description, the flux tube integrated description and the ballooning‐mode type description, of the equatorial plasma instability are presented for comparisons. On the basis of physical considerations it is argued that the ballooning‐mode type description is comparatively the most accurate and complete description of plasma instability in the equatorial ionosphere where the inhomogeneous plasma is embedded in a dipole magnetic field. The growth rates obtained from the ballooning‐mode type description are therefore the most accurate growth rates. In comparison, the local description overestimates the growth rates up to a certain altitude and then underestimates them, while the flux tube integrated description underestimates the growth rates at almost all altitudes where the growth rates are significant. This is shown for two sets of ionospheric plasma conditions. It is further pointed out that only the ballooning‐mode type description provides useful information on the wavelength dependence of the growth rates as well as on the spatial profiles of the excited modes. The localized spatial structures of the linear modes along the magnetic field line that are found in the ballooning‐mode type description are expected to impose topological constraints on the nonlinear evolution of the instability and thus to play an important role in the determination of the nonlinear state of the plasma.
Statistical survey of magnetic field and ion velocity fluctuations in the near‐Earth plasma sheet: Active Magnetospheric Particle Trace Explorers/Ion Release Module (AMPTE/IRM) measurements
Authors: Neagu Elena , Borovsky Joseph E. , Thomsen Michelle F. , Gary S. Peter , Baumjohann Wolfgang , Treumann Rudolf A.
Published: October 2002

Low‐frequency magnetic field and ion velocity fluctuations in the plasma sheet of Earth's magnetotail are surveyed using data from Active Magnetospheric Particle Trace Explorers/Ion Release Module (AMPTE/IRM) tail passes in 1985 and 1986. The data is divided into 259 ten‐minute‐long intervals, and the velocity and field fluctuation amplitudes and plasma parameters are determined for each interval. There is no significant dependence of the amplitude of the fluctuations on either the down‐tail distance or the distance from the neutral sheet. The amplitudes of both the field and velocity fluctuations are positively correlated with the level of geomagnetic activity as measured by the AE index. The strongest correlation is between the fluctuating velocities and the average ion velocities. These results suggest that geomagnetic activity is the global source of enhanced field and velocity fluctuations observed in the plasma sheet and that velocity shear instabilitiesmay be the proximate cause of these fluctuations.
Statistical study of auroral roar emissions observed at South Pole Station
Authors: LaBelle J. , Weatherwax A. T.
Published: October 2002

Auroral roar is a band‐limited natural auroral radio emission observed at ground level in the auroral zone at frequencies near 3 MHz and 4.5 MHz, close to 2 and 3 times the electron gyrofrequency. Using semiautomated data analysis techniques, we generate a database of frequencies, amplitudes, and universal times of 2fce‐auroral roar emissions observed at South Pole Station (74° invariant latitude) during 1999. These data confirm that auroral roar emissions are observed at ground level only during times when the ionosphere is not illuminated, and magnetic local time plays a significant role in controlling the occurrence rates of auroral roar emissions observed at ground level. Auroral roar occurrence rates and the ranges of auroral roar amplitudes and frequencies increase during times of high geomagnetic activity. The distribution of auroral roar frequencies is bimodal for large‐amplitude events, which we interpret as evidence that a substantial fraction of the auroral roar emissions observed at ground level originate in the topside ionosphere. Finally, superposed epoch analyses of occurrences of auroral roar relative to substorm onsets inferred from magnetometer and VLF data show that auroral roar emissions occur favorably preceding such events rather than after them, consistent with previous case study and anecdotal evidence.
Ionospheric response to the interplanetary magnetic field southward turning: Fast onset and slow reconfiguration
Authors: Lu G. , Holzer T. E. , Lummerzheim D. , Ruohoniemi J. M. , Stauning P. , Troshichev O. , Newell P. T. , Brittnacher M. , Parks G.
Published: October 2002

This paper presents a case study of ionospheric response to an interplanetary magnetic field (IMF) southward turning. It is based on a comprehensive set of observations, including a global network of ground magnetometers, global auroral images, and a SuperDARN HF radar. There is a clear evidence for a two‐stage ionospheric response to the IMF southward turning, namely, fast initial onset and slow final reconfiguration. The fast onset is manifested by nearly simultaneous (within 2 min) rise of ground magnetic perturbations at all local times, corroborated by a sudden change in the direction of line‐of‐sight velocity near local midnight and by the simultaneous equatorward shift of the auroral oval. The slow reconfiguration is characterized by the different rising rate of magnetic perturbations with latitudes: faster at high latitude than at lower latitudes. Furthermore, a cross‐correlation analysis of the magnetometer data shows that the maximum magnetic perturbation is reached first near local noon, and then spread toward the nightside, corresponding to a dayside‐to‐nightside propagation speed of ∼5 km/s along the auroral oval. Global ionospheric convection patterns are derived based on ground magnetometer data along with auroral conductances inferred from the Polar UV images, using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. The AMIE patterns, especially the residual convection patterns, clearly show a globally coherent development of two‐cell convection configuration following the IMF southward turning. While the foci of the convection patterns remain nearly steady, the convection flow does intensify with time and the cross‐polar‐cap potential drop increases. The overall changes as shown in the AMIE convection patterns therefore are fully consistent with the two‐stage ionospheric response to the IMF southward turning.
The Io current wing
Authors: Pontius Duane H.
Published: October 2002

Jupiter's satellite Io produces a standing magnetospheric disturbance, variously called the Alfvén wing or the Io flux tube, which carries current away from the satellite along the magnetic field and downstream. The physics governing this phenomenon remains controversial. This paper generalizes previous analytic treatments to include effects of plasma compressibility. The nonlinear MHD equations are shown to be equivalent to a nonlinear second‐order partial differential equation for a single scalar function of position in the plane transverse to the wing. This equation bears a strong resemblance to the velocity potential equation for two‐dimensional compressible flow in an ideal fluid. In certain limits the resemblance is exact, which permits approximate analytic solutions of the fluid case to be used to explore the properties of the compressible magnetofluid. The derivation allows translational symmetry in any direction, unlike the Alfvén‐wing model which requires symmetry along an Alfvén characteristic. The wing current is governed by an effective conductance that generalizes the Alfvén wing conductance but is flexible enough to maintain a constant value in regions with different plasma densities. Hence this formalism suggests how the local plasma can adjust to provide a continuous current along the wing, all the way from the dense torus, through the rarified high‐latitude regions, to Jupiter's ionosphere.
Sources of geomagnetic activity during nearly three solar cycles (1972–2000)
Authors: Richardson I. G. , Cane H. V. , Cliver E. W.
Published: October 2002

We examine the contributions of the principal solar wind components (corotating high‐speed streams, slow solar wind, and transient structures, i.e., interplanetary coronal mass ejections (CMEs), shocks, and postshock flows) to averages of theaageomagnetic index and the interplanetary magnetic field (IMF) strength in 1972–2000 during nearly three solar cycles. A prime motivation is to understand the influence of solar cycle variations in solar wind structure on long‐term (e.g., approximately annual) averages of these parameters. We show that high‐speed streams account for approximately two‐thirds of long‐termaaaverages at solar minimum, while at solar maximum, structures associated with transients make the largest contribution (∼50%), though contributions from streams and slow solar wind continue to be present. Similarly, high‐speed streams are the principal contributor (∼55%) to solar minimum averages of the IMF, while transient‐related structures are the leading contributor (∼40%) at solar maximum. These differences between solar maximum and minimum reflect the changing structure of the near‐ecliptic solar wind during the solar cycle. For minimum periods, the Earth is embedded in high‐speed streams ∼55% of the time versus ∼35% for slow solar wind and ∼10% for CME‐associated structures, while at solar maximum, typical percentages are as follows: high‐speed streams ∼35%, slow solar wind ∼30%, and CME‐associated ∼35%. These compositions show little cycle‐to‐cycle variation, at least for the interval considered in this paper. Despite the change in the occurrences of different types of solar wind over the solar cycle (and less significant changes from cycle to cycle), overall, variations in the averages of theaaindex and IMF closely follow those in corotating streams. Considering solar cycle averages, we show that high‐speed streams account for ∼44%, ∼48%, and ∼40% of the solar wind composition,aa, and the IMF strength, respectively, with corresponding figures of ∼22%, ∼32%, and ∼25% for CME‐related structures, and ∼33%, ∼19%, and ∼33% for slow solar wind.
A comparison of substorms occurring during magnetic storms with those occurring during quiet times
Authors: McPherron R. L. , Hsu T.‐S.
Published: October 2002

It has been suggested that there may be a fundamental difference between substorms that occur during magnetic storms and those that occur at other times. Baumjohann et al. [1996] presented evidence that there is no obvious change in lobe field in “quiet time” substorms but that “storm time” substorms exhibit the classic pattern of storage and release of lobe field energy. This result led them to speculate that the former are caused by current sheet disruption, while the latter are caused by reconnection of lobe flux. In this paper we examine their hypothesis with a much larger data set using definitions of the two types of substorms similar to theirs, as well as additional more restrictive definitions of these classes of events. Our results show that the only differences between the various classes are the absolute value of the lobe field and the size of the changes. When the data are normalized to unit field amplitude, we find that the percent change during storm time and non‐storm time substorms is nearly the same. The above conclusions are demonstrated with superposed epoch analysis of lobe field (Bt and Bz) for four classes of substorms: active times (Dst < −50 nT, mostly recovery phase), main phase substorms, non‐storm times (Dst > −25 nT), and quiet time substorms (no evidence of storm in Dst). Epoch zero for the analysis was taken as the main substorm onset (Pi2 onset closest to sharp break in AL index). Our results suggest that there is no qualitative distinction between the various classes of substorms, and so they are all likely to be caused by the same mechanism.
Asymmetry of auroral electron precipitations and its relationship to the substorm expansion phase onset
Authors: Stepanova M. V. , Antonova E. E. , Bosqued J. M. , Kovrazhkin R. A. , Aubel K. R.
Published: October 2002

The latitudinal asymmetry of accelerated electron precipitations is analyzed using AUREOL‐3 satellite data. It is found that the pattern of the precipitation of auroral electrons can be very asymmetric with the most powerful structures near the equatorial boundary of the auroral oval near midnight before substorm expansion phase onset. The presence of such a feature is interpreted as the existence of maximal magnetosphere‐ionosphere decoupling in the electron precipitation structure nearest the equator. It is argued that magnetosphere‐ionosphere decoupling can play the principal role in the localization of the substorm onset. The slow evolution of plasma pressure distribution under the influence of the electrostatic disturbances is analyzed. The threshold of the marginal instability is obtained. A scenario of substorm onset is suggested, which includes the development of localized dusk‐dawn electric field and the formation of thin sheets of downward accelerated electrons producing rayed arc and upward accelerated ions on the equatorial edge of the large‐scale electron acceleration region.
Identification of substorm expansive phase onsets
Authors: Rostoker Gordon
Published: October 2002

Over the years, many different particle and electromagnetic signatures have been invoked to identify the onset of what is called a “substorm.” In this paper I shall first clarify what is generally meant by the term “substorm onset,” and I shall show that some disturbances identified as substorm onsets should be treated with care. I shall review the substorm in terms of its two recognized characteristic components: directly driven and storage/release. I shall then discuss the importance of pseudobreakups in deciding what constitutes a legitimate substorm onset and, in doing so, will illustrate the subjective nature of present substorm onset identification techniques. I shall stress that substorm‐like disturbances can take place both near the equatorward edge of the midnight sector oval (mapping to the near‐Earth magnetotail inside ∼−12 RE) and near the poleward edge of the evening sector oval (mapping to regions much farther downtail). I shall present case studies that illustrate that some poleward edge disturbances have magnetic and auroral signatures that may be incorrectly interpreted as legitimate expansive phase onsets. On the basis of this study, I suggest that what may appear to be a lengthy series of closely spaced expansive phases may, in fact, be a single expansive phase followed by an arbitrarily long period of poleward border activity.
Pi2 onset time determination with information criterion
Authors: Higuchi T. , Ohtani S.‐I. , Uozumi T. , Yumoto K.
Published: October 2002

The present study proposes a new method to identify ground Pi2 onsets. The Pi2 wave is usually examined by applying a linear band‐pass filter or something equivalent, such as wavelet convolution, to ground magnetometer data, and its onset is determined based on the amplitude of the extracted wave component. The proposed method is distinguished from such conventional methods in terms of the following points. First the proposed method does not filter or smooth data, rather it analyzes the original data. Second the proposed method determines the Pi2 onset so that the intervals before and after the onset can be optimally described by different time series models. The optimal partition is determined by minimizing the Akaike information criterion (AIC). The time series models adopted are general enough to apply to any nonstationary time series irrespective of its waveform, frequency, amplitude, and signal‐to‐noise ratio. As a result, the proposed procedure is free from the choice of a threshold and is therefore objective. The procedure is applied to simulation data, and it is confirmed that the procedure can determine the onset with the precision of the data's time resolution. An application to magnetic field data from 210° magnetic meridian (MM) stations suggests that our procedure can be used for examining the latitudinal and longitudinal dependence of the onset time. The proposed procedure is expected to be useful for examining issues that have been difficult to address with conventional methods because of the precision required for determining Pi2 onset.
Lobe cell convection and field‐aligned currents poleward of the region 1 current system
Authors: Eriksson S. , Bonnell J. W. , Blomberg L. G. , Ergun R. E. , Marklund G. T. , Carlson C. W.
Published: October 2002

We present a case and statistical study of plasma convection in the Northern Hemisphere during summer conditions using electric field, magnetic field, and particle data taken during dawn‐dusk directed orbits of the FAST satellite. To our knowledge, this set provides the most comprehensive combination of data as yet presented in support of lobe cell convection from an ionospheric perspective this far from the noon sector. In particular, we study the current systems and convection patterns for all passes in July 1997 that show evidence for six large‐scale field‐aligned currents (FACs) rather than the usual system of four FACs associated with the region 1/region 2 current systems. A total of 71 passes out of 232 in the study had the extra pair of FACs. The extra pair of FACs in 30 of the 71 cases lies either on the dawnside or on the duskside of the noon‐midnight meridian, and their position is strongly correlated with the polarity of the IMF By (negative and positive, respectively). This is consistent with the IMF dependence of a three‐cell convection pattern of coexisting merging, viscous, and lobe‐type convection cells. The occurrence of the asymmetric FAC pair was also strongly linked to conditions of IMF |By/Bz| > 1. The extra pair of FACs in these cases was clearly associated with the lobe cell of the three‐cell convection system. The remaining 41 cases had the pair of FACs straddling the noon‐midnight meridian. The extra pair of FACs was often (20 cases out of 30) observed at magnetic local times more than three hours away from noon, rather than being confined to regions near noon and the typical location of the cusp. Such a current system consisting of a pair of FACs poleward of the nearest region 1 current is consistent with the IMF By‐dependent global MHD model developed by Ogino et al. [1986] for southward IMF conditions, as well as with other magnetospheric and ionospheric convection models that include the effects of merging occuring simultaneously at both low‐latitude dayside and high‐latitude lobe and flank magnetopause reconnection sites. Finally, the presence of the additional FACs and three‐cell convection well away from noon show that the entire dayside ionosphere is affected by IMF‐dependent processes, rather than only a limited region around noon.
Three‐dimensional MHD simulation of a loop‐like magnetic cloud in the solar wind
Authors: Vandas M. , Odstrčil D. , Watari S.
Published: October 2002

A magnetic cloud ejected from the Sun is simulated as a part of a toroid. Its evolution and propagation through interplanetary space are studied using three‐dimensional magnetohydrodynamic self‐consistent numerical simulations. The flux rope deforms as it moves from the Sun, and this deformation causes a hypothetical spacecraft to observe the flux rope two times at some locations; that is, it could be crossed at its apex and a flank. Simulated observations at the apex give all three signatures of a magnetic cloud following from its definition. The apex is rather flat and can be approximated by a prolate (in latitude) cylinder. At the flanks of the flux rope the temperature drop is not pronounced, but the magnetic field increase has a double‐peak profile.
Motion of auroral ion outflow structures observed with CLUSTER and IMAGE FUV
Authors: Kistler L. M. , Frey H. U. , Möbius E. , Mouikis C. , Quinn J. M. , Klecker B. , Rème H. , Bosqued J. M. , Dandouras I. , Sauvaud J. A. , Di Lellis A. M. , Formisano V. , Marcucci M. F. , Carlson C. W. , McFadden J. P. , Parks G. K. , McCarthy M. , Korth A. , Eliasson L. , Lundin R. , Paschmann G. , Popecki M. A. , Mende S. B. , Winningham J. D. , Fazakerley A. N.
Published: October 2002

During February 2001 the CLUSTER satellites recorded a number of perigee passes through the midnight auroral zone. We concentrate on one pass, on 23 February 2001, when structured outflow was observed. Simultaneous observations of the aurora were available from the FUV instrument on IMAGE. The features in the ion outflow observed by the Cluster Ion Spectrometry (CIS) experiment are compared with the auroral activity. Observations from the multiple CLUSTER spacecraft are used to determine the velocity of the outflow structures. We find a good correspondence between the observed ion outflow and the auroral arcs, with the highest energy outflow corresponding to the brightest arcs. The features at the equatorward edge, which are trapped precipitating ions, are stationary. In addition, the increased velocity structure at the poleward edge is also stationary. However, the bulk of the ion outflow structures, which are observed between these boundaries, are moving equatorward with a velocity of roughly 7 km/s, which corresponds to a velocity of 0.7 km/s at 100 km. One feature is observed moving poleward, at the same time that the auroral arc is expanding poleward. Comparisons with the motion of the auroral arcs and with the convection velocity measured by the EDI instrument on CLUSTER show that the motion of the structures in general agrees with the convective motion of the field lines.
Auroral structure at the cusp equatorward boundary: Relationship with the electron edge of low‐latitude boundary layer precipitation
Authors: Sandholt Per Even , Denig William F. , Farrugia Charles J. , Lybekk Bjørn , Trondsen Espen
Published: October 2002

We document the correspondence between different categories of auroral forms/activities and particle precipitation/plasma convection regimes in the 1100–1300 MLT/70°–77° MLAT sector during strongly negative interplanetary magnetic field By (−7 nT) and smaller (2 to −4 nT) Bz conditions. Ground observations of the aurora are combined with data from two overflights by the spacecraft DMSP F11 and F12 on 8 January 1999. We describe the dynamics of the aurorae in the midday sector corresponding to the particle precipitation regimes which in the literature are designated (1) the dayside extension of the central plasma sheet, (2) void, (3) the electron edge, (4) the low‐latitude boundary layer (LLBL), and (5) the plasma mantle. The aurorae observed in the corresponding latitude regimes are the following: (1) the pulsating, diffuse, green line‐dominated plasma sheet aurora (type 3), (2) a ∼100 km wide latitudinal gap in auroral emission, (3) and (4) the type 1 cusp aurora characterized by recurrent (T = 2–3 min) equatorward boundary intensifications (EBIs), and (5) poleward moving auroral forms. Special focus is placed on (1) the association between EBIs and the electron edge/ion cutoff observed at the equatorward boundary of the LLBL precipitation and (2) the gap in the auroral emission profile located on its equatorward side. The latter corresponds to the observed strongly depleted field–aligned electron fluxes (void), which in our view is due to the loss of plasma sheet particles along newly opened field lines. Thus, in this case study we document the auroral fine structure corresponding to the open LLBL, with its electron edge, as well as the signature of the escape of magnetospheric electrons along the adjacent field lines on its equatorward/inward side.
Solar cycle changes in coronal holes and space weather cycles
Authors: Luhmann J. G. , Li Y. , Arge C. N. , Gazis P. R. , Ulrich R.
Published: October 2002

Potential field source surface models of the coronal magnetic field, based on Mt. Wilson Observatory synoptic magnetograms, are used to infer the coronal hole sources of low‐heliolatitude solar wind over approximately the last three solar cycles. Related key parameters like interplanetary magnetic field and bulk velocity are also calculated. The results illustrate how the evolving contribution of the polar hole sources relative to that from low‐latitude and midlatitude active region hole sources can explain solar magnetic field control of long‐term interplanetary variations. In particular, the enduring consistent magnetogram record and continuous model displays produce a useful overview of the solar control of interplanetary cycles and trends that affect space weather.
Interplanetary magnetic field control of afternoon‐sector detached proton auroral arcs
Authors: Burch J. L. , Lewis W. S. , Immel T. J. , Anderson P. C. , Frey H. U. , Fuselier S. A. , Gérard J.‐C. , Mende S. B. , Mitchell D. G. , Thomsen M. F.
Published: October 2002

Data from the Far Ultraviolet Imager (FUV) on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite show that subauroral proton arcs appear in the afternoon sector during geomagnetically disturbed periods when the interplanetary magnetic field rotates either from south to north or from west to east and when the magnetosphere is moderately compressed. Time series of proton aurora images show that the proton emissions are generally aligned along the equatorward part of the auroral oval. However, when interplanetary magnetic field (IMF) Bz changes from negative to positive the auroral oval contracts toward higher latitudes while the ring current proton precipitation remains stationary, resulting in a separation of several degrees between the latitude of the new oval position and a subauroral proton arc in the afternoon sector. A similar effect occurs when IMF By rotates from negative to positive, in which case the oval in the afternoon sector retreats toward higher latitudes, again leaving a separation between the oval and the subauroral proton arc of several degrees. Comparisons with low‐altitude and geosynchronous satellite data show that the subauroral proton arc is caused by the precipitation of protons with energies from several keV to 30 keV and is likely associated with the existence of a plasmaspheric “drainage plume.” In contrast, the proton emissions along the main oval are caused by protons with energies generally less than 10 keV.
Measurement of Jupiter's decametric radio source parameters by the modulation lane method
Authors: Imai Kazumasa , Riihimaa Jorma J. , Reyes Francisco , Carr Thomas D.
Published: October 2002

In previously published works we have developed a model to explain the production of modulation lanes in the dynamic spectra of Jupiter's decametric emission. In this paper we use the model with newly available data to test the model and to measure emission source and beam parameters. Some of the data were wideband, providing for the first time the opportunity to verify the accuracy of the model in fitting the considerable lane curvature exhibited in such a case. Relatively precise measurements of the cone half‐angle of hollow‐cone beam and other source and beam parameters of Io‐B radiation were also made possible by the wideband data. Narrowband data enabled us to measure for the first time the beam cone half‐angle for non‐Io‐A radiation. The measured cone half‐angles for the two sources were both within ∼2° or 3° of 60°. This measurement is consistent with the long‐held idea that the sources Io‐B, Io‐A, and non‐Io‐A are due to the same rotating hollow‐cone beam and that the only difference between the latter two is the intensification of Io‐A radiation by the connection with Io in comparison with that of non‐Io‐A. We also measured the cone half‐angle for S bursts and found it to be less than that of the much more common L bursts.