bluuebunny/crossref_metadata_2025 huggingface dataset:
Percolation of charged particle orbits in two‐dimensional irregular magnetic fields and its effect in the magnetospheric tail
Authors: Arzner K. , Scholer M. , Treumann R. A.
Published: October 2002

This paper considers the motion of charged particles in irregular, statistically homogeneous, two‐dimensional magnetic fields, a situation which might arise in multiple reconnecting current sheets and in two‐dimensional MHD turbulence. The (test) particle dynamics is investigated with respect to the existence of a kinetic energy threshold, beyond which the trajectories become unbounded and effective diffusion occurs. Reformulating the Lorentz force kinematics in terms of an effective potential, the threshold problem reduces to a two‐dimensional continuum percolation problem. Assuming that any realizations ofAy(x,z) and −Ay(x,z) appear with equal probability, a rigorous condition for unbounded trajectories in terms of the motion invariants is derived:, withthe kinetic energy andPythe canonical momentum in the invariant direction. Its validity is verified by means of Monte‐Carlo simulations, and the effect of an additional electric field on particle acceleration is discussed. The percolation criterion allows conclusions about the mobility and origin of energetic particles in turbulent regions of the magnetospheric tail plasma sheet.
Dependence of equatorial bubbles and bottomside spread F on season, magnetic activity, and E × B drift velocity during solar maximum
Authors: Whalen James A.
Published: October 2002

Equatorial bubbles occur sporadically, so that the resulting scintillation that disrupts transionospheric RF propagation is unpredictable. Furthermore, this randomness prevents the systematic observation of the formation of bubbles necessary for making progress in their prediction. However, recent results indicate that a necessary condition for bubble formation is bottomside spread F (BSSF); that BSSF is a function of the F region dynamo electric field; and that this field is dependent on magnetic activity and on season. This work measures these conditions using an array of ionospheric sounders in conjunction with other spread F observations, all of which are ground‐based, in the western American sector, during solar maximum. Three spread F levels are observed: total BSSF and strong BSSF, both measured near the dip equator, and macroscopic bubbles measured at the equatorial anomaly. The three independently measured thresholds form a hierarchy in which macroscopic bubbles are a subset of strong BSSF, which is a subset of total BSSF. Each level decreases as a linear function of Kp averaged in the 6 hours preceding observation during March, April, September, and October, and during November–February; however, during May–August the levels are small and are independent of Kp. As a result, the probability of occurrence of each level, most importantly of macroscopic bubbles, is a quantitative function of Kp and season. The seasonal variation of Kp dependence indicates that the suppression of irregularities results from the decrease in the maximum prereversal vertical drift velocity of the equatorial F layer and therefore from the decrease in the eastward electric field.
Correlative study of neutral winds and scintillation drifts measured near the magnetic equator
Authors: Valladares C. E. , Meriwether J. W. , Sheehan R. , Biondi M. A.
Published: October 2002

Measurements of the thermospheric neutral wind at Arequipa, Peru, and observations of the drift of the irregularities at Ancon, Peru, are used to study the coupling that exists between ions and neutrals at equatorial latitudes and the variability of this coupling as a function of the occurrence of scintillations. This study is based on data collected at the Arequipa and Ancon stations between 1996 and 1998. Our comparative analysis indicates that the relative wind‐drift values vary depending on season and the solar flux level. We found that during the equinoxes and low solar flux values, the averaged zonal drift is larger than the wind by 15 m s−1, but for solar flux values above 130 units, the average wind exceeds the drift values by 10 to 20 m s−1. We suggest that the occurrence of larger equinoctial drifts can be explained by the existence of altitude gradients in the zonal wind during that season. During the June solstice the zonal wind seems to exceed the irregularity drift by ∼10–20 m s−1 independent of the solar flux. We also find that the meridional wind shows a modest dependence on the scintillation activity during the June solstice. During scintillation events and between 2000 and 2400 LT the averaged meridional wind observed to the south and north of Arequipa exceeds their corresponding no‐scintillation values by 20 m s−1. We present likely explanations of this effect.
Magnetospheric signature of an ionospheric traveling convection vortex event
Authors: Moretto T. , Hesse M. , Yahnin A. , Ieda A. , Murr D. , Watermann J. F.
Published: October 2002

We analyze Geotail observations of magnetic field and flow oscillations in the dayside magnetosphere that occurred in direct association with an event of traveling convection vortices observed in the northern high‐latitude ionosphere. Geotail was positioned on a magnetic flux tube that mapped very close to the centers of the driving field‐aligned currents in the ionosphere. These are the first in situ observations from the source region of such events. The flow signature is seen clearly in the cold, dense plasma population, which the satellite encounters during the event, in addition to a hot magnetospheric (plasma sheet‐like) population. This report will present the observations and briefly discuss their implications for our understanding of the traveling convection vortex (TCV) signature. We interpret the event in terms of a boundary wave at the magnetopause. Our analysis suggests that strong gradients in the magnetic field strength or plasma density in the inner magnetosphere should play an important role in this process.
A model‐derived storm time asymmetric ring current driven electric field description
Authors: Ridley A. J. , Liemohn M. W.
Published: October 2002

Low‐latitude ionospheric and near‐Earth magnetospheric electric fields are calculated from model results of the storm time asymmetric ring current. These fields are generated from subauroral field‐aligned currents out of the ionosphere in the midnight sector and into the ionosphere on the dayside. The currents balance the divergence of the asymmetric ring current, which is the dominant component of the ring current during main phase and early recovery phase of magnetic storms. The basic shape of the electric potential pattern is described, both in the ionosphere and in the magnetosphere. It is found that intense ring current injection events can create potential differences up to 200 kV and can create local electric fields in the nightside magnetosphere >5 mV m−1. The magnitudes and locations of the most intense electric fields are quite consistent with observations of subauroral (ionospheric) and near‐Earth (magnetospheric) electric fields during magnetic storms. In addition, a relationship between the magnitude of the low‐latitude electric potential and theDst* index is described. This relationship can be used to predict the location and size of strong low‐latitude electric fields and currents. The presented electric field results are derived from model output, such that there is no feedback of the calculated fields back into the model.
Global MHD modeling of the impact of a solar wind pressure change
Authors: Keller Kristi A. , Hesse Michael , Kuznetsova Maria , Rastätter Lutz , Moretto Therese , Gombosi Tamas I. , DeZeeuw Darren L.
Published: October 2002

A sudden increase in the solar wind dynamic pressure compresses the magnetosphere and launches compressional waves into the magnetosphere. The global response of the magnetosphere, including the ionosphere and the location of the field‐aligned current (FAC) generation, to a step increase in the solar wind density has been studied using a global three‐dimensional adaptive MHD model. As the density increase propagated along the flanks of the magnetopause, a two‐phased response was seen in the ionosphere. The first response was an increase in FACs near the polar cap. For this response we found the location of FACs to lie just inside the magnetosphere. The second response was an increase in FACs at lower latitudes. The increase in FACs was in the same direction as region 1 currents. For the second response we found the location of FACs to fall well within the magnetosphere.
Radar observations of magnetospheric activity during extremely quiet solar wind conditions
Authors: Walker A. D. M. , Baker K. B. , Pinnock M. , Dudeney J. R. , Rash J. P. S.
Published: October 2002

During a period of extremely quiet solar wind conditions from 8 to 10 March 1997, strong activity was observed by the Southern Hemisphere Auroral Radar Experiment Super Dual Auroral Radar Network radars in the Antarctic premidnight ionosphere. This activity took the form of quasiperiodic flow bursts with ionospheric drift velocities exceeding 2 km s−1. Data from the Satellite Experiments Simultaneous with Antarctic Measurements (SESAME) automated geophysical observatories in Antarctica and Defense Meteorological Satellite Program and Polar satellites are used with the radar data to study the convection flow in the southern polar ionosphere at the time of these flow bursts. The study shows that the bursts occurred with an approximate period of 12 min. Their direction was westward, and they were superimposed on a background westward flow. In the premidnight sector this is interpreted as a flow associated with dipolarization of the magnetotail tail field. There is a band of strong particle precipitation associated with the flow bursts. The location suggests that they occur deep in the magnetotail and cannot be associated with any lobe reconnection. They are at a latitude near the region where a viscously driven convection cell is expected to exist, and their sense is that of the return convection flow in such a cell. The results suggest that there is an internal magnetospheric mechanism for sporadic energy release in the magnetotail that need not be associated with changes in solar wind reconnection on the magnetopause.
Solar activity in extended cycles
Authors: Li K. J. , Yun H. S. , Liang H. F. , Gu X. M.
Published: October 2002

It is well known that the first sunspot groups of a new cycle appear before sunspot minimum and continue to emerge for 12 or more years, ending after the following minimum. The period of time over which activity belonging to a specific cycle is observed is called the “extended activity cycle.” A criterion for differentiating sunspot groups into their proper solar cycles during the overlapping, transitional period is proposed. The monthly sunspot group numbers during each extended activity cycle are constructed to show characteristics of solar activity for them. On the basis of the defining characteristics we estimate the conventional start of the beginning time of sunspot cycle 24 (the next cycle) to occur ∼2007.2 ± 1 year, at the 68% percent level of confidence.
Comparison of three magnetopause prediction models under extreme solar wind conditions
Authors: Yang Y.‐H. , Chao J. K. , Lin C.‐H. , Shue J.‐H. , Wang X.‐Y. , Song P. , Russell C. T. , Lepping R. P. , Lazarus A. J.
Published: October 2002

A database, the largest one to date, of magnetosheath encounters by geosynchronous satellites during 1986–1992 and 1999–2000 and upstream observations by Wind, IMP 8, or Geotail in the solar wind are used to estimate the forecasting capability of the models of Chao et al. [2001], Shue et al. [1998], and Petrinec and Russell [1996]. For each of the 1‐min resolution data points obtained by GOES spacecraft, we check the following two things: if the magnetosheath was observed by the spacecraft, and if each of the three models predicted a magnetosheath encounter by the spacecraft. Three parameters are defined to quantify the models' forecasting capability: probability of prediction (PoP), probability of detection (PoD), and false alarm rate (FAR). A higher PoP and PoD with a lower FAR imply a better forecasting model. In the 1986–1992 period we found that most of the magnetosheath encounters observed at 6.6 RE are detected. In particular, the Chao et al. [2001] model predicts the lowest FAR compared with those of the other two models. We have also studied the magnetosheath encounters made by GOES spacecraft for the period from 1999 to 2000 using Wind and Geotail as the solar wind monitor. This independent database of magnetosheath encounters during 1999–2000 confirms our previous anticipations. The PoD of Petrinec and Russell [1996] model (94%) is much higher than those of Shue et al. [1998] (74%) and Chao et al. [2001] (84%) models. The Chao et al. [2001] model has a higher PoP than the other two models. The values of FAR for Chao et al. [2001], Shue et al. [1998], and Petrinec and Russell [1996] models are 25, 27, and 40%, respectively.
Birkeland current system key parameters derived from Iridium observations: Method and initial validation results
Authors: Anderson B. J. , Takahashi K. , Kamei T. , Waters C. L. , Toth B. A.
Published: October 2002

The Iridium satellites in 780 km altitude, circular polar orbits provide continuous global monitoring of the Birkeland current system via engineering magnetometer data. These data have been used to characterize basic features of the global field‐aligned currents (FACs) with a time window of 45 min and a time step of 15 min. The three sigma magnetometer data noise threshold is 93 nT on average. The fraction of measurements above the noise is used to provide one measure of the location of the auroral FACs. Measures are also presented for the mean latitude and equatorward/poleward extent of the region 1/region 2 FAC system. The equatorward latitude of region 1/region 2 FACs is anticorrelated with Kp, r = −0.68. Indices are presented for the net FAC intensity in terms of the eastward (westward) magnetic perturbation in the northern (southern) hemisphere by analogy with the AE, AU, and AL indices. The Iridium system indices show high correlation with the quick look auroral electrojet indices both in individual cases and statistically, r = +0.73 between their logarithms. Results are presented for two storms, 22–23 September 1999, Dst minimum approximately −160 nT, and 21–22 October 1999, Dst minimum approximately −230 nT, reflecting that intensification and equatorward expansion of the global FACs occur in response to southward IMF. Enhanced dynamic pressure promotes more rapid equatorial expansion, 10° in 1.5 hours for the September storm, for which the dynamic pressure was enhanced, 15–20 nPa, at southward IMF turning, as opposed to the October case, 13° over ∼8 hours, for which the southward turning occurred during nominal dynamic pressure, ∼5 nPa. In both storms the current intensity decreases to prestorm levels within an hour when the IMF turns northward or nearly horizontal, at the beginning of storm recovery. The key parameters are a useful means of accessing the Iridium system data for preliminary analyses, and the initial results provide motivation for future analyses to quantify the accuracy and reliability of products derived from the Iridium system data.
Ion outflow and associated perpendicular heating in the cusp observed by Interball Auroral Probe and Fast Auroral Snapshot
Authors: Bouhram M. , Dubouloz N. , Malingre M. , Jasperse J. R. , Pottelette R. , Senior C. , Delcourt D. , Carlson C. W. , Roth I. , Berthomier M. , Sauvaud J.‐A.
Published: October 2002

The spatial properties of ionospheric ion outflows associated with perpendicular heating processes in the cusp are studied using a conjunction study from two satellites and ground radar systems. Low‐energy outflowing ions are measured in a wide longitudinal range, between 13,000 and 19,000 km in altitude, over the dayside polar cap by the Hyperboloid experiment aboard the Interball Auroral Probe (AP). These observations are related to conjugate convection field measurements by the Saskatoon‐Kapuskasing pair of the Super Dual Auroral Radar Network (SuperDARN). Data analysis suggests that outflowing ions originate from a wide magnetic local time range associated with both the dayside cusp region and the dayside cleft region. A direct cusp crossing by the Fast Auroral Snapshot (FAST) satellite at 2000‐km altitude shows a correlation between transverse ion heating in a thin latitudinal region (∼1.3°) and the presence of broadband extremely low frequency (BBELF) turbulence, in addition to more intense electrostatic waves near and just above the lower hybrid (LH) frequency. This event is unusual because on the basis of the statistical survey of Freja and FAST data, most of the ion‐heating events in the midaltitude auroral zone are correlated with enhanced emissions in the BBELF range. Furthermore, the electron population has energies that are too small to drive LH waves unstable. This event offers the opportunity to analyze the contribution of cusp magnetospheric ion injections to the heating of the ambient H+ and O+ ions. Kinetic instability calculations demonstrate that LH waves are destabilized by the ring distributions, which result from injections of high‐energy magnetosheath ions. Calculations show that a preheating mechanism by BBELF turbulence near the ion gyrofrequencies is also required so that LH heating is able to occur. The altitude dependence of the LH perpendicular heating is then analyzed by modeling the transport of the injected magnetospheric ions along magnetic field lines. It is shown that LH heating acts as an additional process from ∼2000 up to 10,000 km in altitude. In addition, trajectory calculations show that the low‐energy outflowing H+ and O+ ions observed along Interball AP orbit in the polar cap are heated inside the cusp at altitudes extending up to 15,000 km. These results are then assembled to construct a possible heating scenario inside the cusp for this data set. The contribution of the different energization mechanisms to the ion heating as a function of altitude is then discussed.
Effects of interstellar magnetic field B and constant flux of neutral H on the heliosphere
Authors: Ratkiewicz Romana , Ben‐Jaffel Lotfi
Published: October 2002

A treatment showing that the local interstellar medium (LISM) bow shock (BS) may exist and the heliosphere is tilted has recently been given by Ben‐Jaffel et al.[2000] and used in an attempt to interpret combined Voyager UVS data and Hubble Space Telescope spectra of the Ly α sky background emission as plausible evidence for the existence of BS. However, the authors stressed the need for a more accurate description of the interface region in order to interpret the available data confidently. In this paper we use a more sophisticated Three‐Dimensional Time‐Dependent Magnetohydrodynamic with Neutral Particles (3DT MHD + N) model of the interaction between the solar wind and the interstellar medium with LISM neutral particles included. We report a parametric study of the effect of the LISM magnetic field strength and inclination angle in the presence of the neutral H atoms on the heliosphere and the heliospheric boundary. Ninety model cases that have been analyzed facilitate in building a diagnostic tool either to interpret archive data on the sky background radiation field or to stimulate new observations related to the interface, observations that may capture some of the fine structure revealed by the new model. Because the inclination angle and strength of the interstellar magnetic field induce unambiguous signatures in the heliospheric structure, our model calculations may serve as a lever to constrain the nature of the LISM from observational data.
Outflow from the ionosphere in the vicinity of the cusp
Authors: Valek P. W. , Perez J. D. , Jahn J.‐M. , Pollock C. J. , Wüest M. P. , Friedel R. H. W. , Moore T. E. , Peterson W. K.
Published: October 2002

Low‐energy ion outflows from the ionosphere are known to be an important source of plasma for the magnetosphere. The objective of this study is to describe the spatial relationships between low‐energy ionospheric outflows and the cusp. The analysis is similar to that used in superposed epoch analyses, except that a spatial reference is used instead of a temporal one. Data from the Thermal Ion Dynamics Experiment (TIDE) supported by data from the Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) and Hydra, during low‐altitude, perigee passes over the South Pole by the Polar spacecraft are used. The presence of isotropic ions from the magnetosheath is used to define the equatorward boundary of the cusp. By using this magnetospheric boundary as a reference point instead of invariant latitude, the temporal variability of the cusp location with respect to the Earth is removed. Poleward of this boundary in the cusp proper, observations show ionospheric outflow in addition to the isotropic magnetosheath ions. A technique used to separate the ion distributions originating from the two sources is presented and applied in order to calculate moments of the outflowing ion distribution. Ionospheric outflow is found to occur in the cleft region that extends ≈1.5° equatorward of the low‐latitude cusp boundary and in the cusp that extends ≈5° poleward of this boundary. Both the parallel and perpendicular temperatures of the outflowing ions are found to be approximately a factor of 3 larger on the equatorward side of this cusp boundary, i.e., in the cleft.
Hill model of transpolar potential saturation: Comparisons with MHD simulations
Authors: Siscoe G. L. , Erickson G. M. , Sonnerup B. U. Ö. , Maynard N. C. , Schoendorf J. A. , Siebert K. D. , Weimer D. R. , White W. W. , Wilson G. R.
Published: October 2002

We present a comparison between a simple but general model of solar wind‐magnetosphere‐ionosphere coupling (the Hill model) and the output of a global magnetospheric MHD code, the Integrated Space Weather Prediction Model (ISM). The Hill model predicts transpolar potential and region 1 currents from environmental conditions specified at both boundaries of the magnetosphere: at the solar wind boundary, electric field strength, ram pressure, and interplanetary magnetic field direction; at the ionospheric boundary, conductance and dipole strength. As its defining feature, the Hill model predicts saturation of the transpolar potential for high electric field intensities in the solar wind, which accords with observations. The model predicts how saturation depends on boundary conditions. We compare the output from ISM runs against these predictions. The agreement is quite good for non‐storm conditions (differences less than 10%) and still good for storm conditions (differences up to 20%). The comparison demonstrates that global MHD codes (like ISM) can also exhibit saturation of transpolar potential for high electric field intensities in the solar wind. We use both models to explore how the strength of solar wind‐magnetosphere‐ionosphere coupling depends on the strength of Earth's magnetic dipole, which varies on short geological timescales. As measured by power into the ionosphere, these models suggest that magnetic storms might be considerably more active for high dipole strengths.
Simultaneous total electron content and all‐sky camera measurements of an auroral arc
Authors: Kintner P. M. , Kil H. , Deehr C. , Schuck P.
Published: October 2002

We present an example of Global Positioning System (GPS) derived total electron content (TEC) and all‐sky camera (ASC) images that show increases of TEC by ∼10 × 1016 electrons m−2 (10 TEC units) occurring simultaneously with auroral light in ASC images. The TEC example appears to be an E region density enhancement produced by two discrete auroral arcs occurring in the late morning auroral oval at 1000 LT. This suggests that GPS signal TEC measurements can be used to detect individual auroral arcs and that individual discrete auroral arcs are responsible for some high‐latitude phase scintillations. The specific auroral feature detected was a poleward moving auroral form believed to occur in the polar cap where the ionosphere is convecting antisunward. The magnitude of the rate of change of TEC (dTEC/dt) is comparable to that previously reported. However, the timescales associated with the event, the order of 1 min, suggest that the data sampling technique commonly used by chain GPS TEC receivers (averaging and time decimation) will undersample E region TEC perturbations produced by active auroral displays. The localized nature of this example implies that L1 ranging errors of at least 1.6 m will be introduced by auroral arcs into systems relying on differential GPS for navigation or augmentation. Although the TEC and auroral arcs presented herein occurred in the late morning auroral oval, we expect that the effects of discrete auroral arcs on GPS TEC and subsequent ranging errors should occur at all local times. Furthermore, GPS receivers can be used to detect individual discrete arcs.
On the generation of cusp HF backscatter irregularities
Authors: Moen J. , Walker I. K. , Kersley L. , Milan S. E.
Published: October 2002

HF radar observations, 630.0 nm cusp auroral emission, and an ionospheric tomography scan are compared to investigate the mechanism by which radar cusp echoes are generated. A collocation is reported between the equatorward boundaries of broad spectral width HF radar backscatter and 630.0 nm cusp auroral precipitation. In addition, the radio tomographic image reveals spatial structures in ionospheric electron density that map closely to the precipitation. However, the large‐scale horizontal gradient appears to be insufficiently steep to account alone for the rapid growth of irregularities by the gradient drift mechanism. It is proposed that the initial source of the decameter‐scale features responsible for the backscatter may result from fine structure within the precipitation itself, or alternatively, from the cascade down of unstable intermediate‐scale gradients created in the plasma convecting poleward from the source region. The latter process may explain the ongoing backscatter found poleward of the auroral band.
Quiet time magnetotail dynamics and their implications for the substorm trigger
Authors: Ohtani S. , Yamaguchi R. , Nosé M. , Kawano H. , Engebretson M. , Yumoto K.
Published: October 2002

The present study seeks to systematically examine the fast plasma flow in the plasma sheet at geomagnetically quiet time. The study uses plasma measurements made by the Geotail satellite in the midnight sector at x > −50 RE during quiet intervals with a total duration of 446 hours. Comparison with the results of previous studies suggests that the occurrence frequency of the perpendicular flow velocity depends more clearly on the radial distance than on geomagnetic activity. Two extreme events were selected for detailed studies; they occurred on 23 November and 10–11 December 1994. In the 23 November event, Geotail was located ∼37 RE from Earth and observed a fast tailward flow ( ∼ −1250 km s−1) with a strongly southward magnetic field (BZ ∼ −8.9 nT). The signature indicates that a near‐Earth neutral line was formed earthward of the satellite and the reconnection reached the lobe magnetic field. On the ground, however, only weak (<100 nT) magnetic disturbances were observed at high (∼75°) latitudes but not at auroral zone stations. The result strongly suggests that lobe reconnection is not sufficient for the global development of a substorm. The 10–11 December 1994 event is very similar to the 23 November 1994 event except that Geotail observed a fast ( ∼ +1500 km s−1) earthward flow rather than a tailward flow, along with the dipolarization of the local magnetic field. It is asserted that the near‐Earth substorm process, that is, tail current disruption, controls the development of a substorm. The fast plasma flow may set a favorable condition for this process to proceed, and therefore the substorm may tend to develop following generation of the fast flow, but the result of the present study indicates that near‐Earth reconnection does not necessarily trigger the global substorm.
Influence of the aerodynamic drag on the motion of interplanetary ejecta
Authors: Vršnak Bojan , Gopalswamy Nat
Published: October 2002

A simple semi‐empirical model for the motion of interplanetary ejecta is proposed to advance the prediction of their arrival times at Earth. It is considered that the driving force and the gravity are much smaller than the aerodynamic drag force. The interaction with the ambient solar wind is modeled using a simple expression for the acceleration = −γ(υ−w), where w = w(R) is the distance‐dependent solar wind speed. It is assumed that the coefficient γ decreases with the heliocentric distance as γ = αR−β, where α and β are constants. The equation of motion is integrated numerically to relate the Earth transit time and the associated in situ velocity with the velocity of coronal mass ejection. The results reproduce well the observations in the whole velocity range of interest. The model values are compared with some other models in which the interplanetary acceleration is not velocity dependent, as well as with the model where the drag acceleration is quadratic in velocity = −γ2(υ − w)∣υ − w∣.
Middle ultraviolet and visible spectrum of SO2 by electron impact
Authors: Ajello J. M. , Hansen D. L. , Beegle L. W. , Terrell C. A. , Kanik I. , James G. K. , Makarov O. P.
Published: October 2002

Electron‐impact‐induced fluorescence spectra of SO2 in the middle ultraviolet and visible wavelength regions (200–600 nm) have been measured in the laboratory using a crossed beam experiment at three electron impact energies. The emission spectra at 8, 18, and 98 eV exhibit a broad and continuous emission region extending from 225 to near 600 nm with a peak emission close to 330 nm. The quasicontinuous SO2 bands arise primarily from direct excitation of SO2. At 18 and 98 eV, simultaneous excitation and dissociation of SO2 produces distinct vibrational bands from SO and from atomic emission lines from S I, S II, O I, and O II that are superimposed on the SO2 electronic transitions. The laboratory spectra were compared to green/violet color ratios obtained at Io by the Galileo Orbiter Solid State Imaging experiment. The laboratory spectra were also applied to the Cassini Imaging Subsystem to determine which filter combinations are particularly sensitive to electron energy, if the atmospheric gas present in the auroral atmosphere is solely or primarily SO2.
Ring current intensity estimated from low‐altitude proton observations
Authors: Søraas F. , Aarsnes K. , Oksavik K. , Evans D. S.
Published: October 2002

Using data from the low‐altitude polar orbiting satellite NOAA‐15 the proton precipitation rate into the upper atmosphere can be monitored with a 100‐min time resolution. From the total power of precipitating protons in the midnight/evening local time sector, a parameter Q(t) is used as a proxy for estimating the energy injection rate into the ring current (RC) due to energetic protons. The injection rate Q(t) is not based upon solar wind parameters but directly on the observed proton precipitation rate. This is done under the assumption that the loss cone acts as a “splash catcher” for the protons injected into the RC. The protons in the loss cone thus do not represent a loss from the RC but are, in fact, a measure of the proton injection rate into the RC. Using the Burton equation and Q(t) as a measure for the true energy injection rate, a value proportional to the energy content in the RC due to energetic protons can be calculated. Assuming a decay constant for the ions in the RC and that the magnetic field depression is proportional to the RC energy content, a RC index can be determined. When Dst is corrected for magnetopause and tail currents, the linear correlation coefficient between the corrected Dst* and the RC index is between 0.8 and 0.9; thus ∼70–80% of the variance in this corrected Dst* can be accounted for. The high correlation strongly indicates that Q(t) is a measure of the true energy injection rate and that Dst* is a measure of the energy content in the RC. Observations of proton precipitation measured by low‐altitude polar‐orbiting satellites thus have the potential for deriving a space‐based Dst index in near real time that is not influenced by magnetic fields generated by magnetopause, field‐aligned, and tail currents.
Pickup ions near Mars associated with escaping oxygen atoms
Authors: Cravens T. E. , Hoppe A. , Ledvina S. A. , McKenna‐Lawlor S.
Published: October 2002

Ions produced by ionization of Martian neutral atoms or molecules and picked up by the solar wind flow are expected to be an important ingredient of the Martian plasma environment. Significant fluxes of energetic (55–72 keV) oxygen ions were recorded in the wake of Mars and near the bow shock by the solar low‐energy detector (SLED) charged particle detector onboard the Phobos 2 spacecraft. Also, copious fluxes of oxygen ions in the ranges 0.5–25 and 0.01–6 keV/q were detected in the Martian wake by the Automatic Space Plasma Experiment with Rotating Analyzer (ASPERA) instrument on Phobos 2. This paper provides a quantitative analysis of the SLED energetic ion data using a test particle model in which one million ion trajectories were numerically calculated. These trajectories were used to determine the ion flux as a function of energy in the vicinity of Mars for conditions appropriate for Circular Orbit 42 of Phobos 2. The electric and magnetic fields required by the test particle model were taken from a three‐dimensional magnetohydrodynamic (MHD) model of the solar wind interaction with Mars. The ions were started at rest with a probability proportional to the density expected for exospheric hot oxygen. The test particle model supports the identification of the ions observed in channel 1 of the SLED instrument as pick‐up oxygen ions that are created by the ionization of oxygen atoms in the distant part of the exosphere. The flux of 55–72 keV oxygen ions near the orbit of the Phobos 2 should be proportional to the oxygen density at radial distances from Mars of about 10 Rm (Martian radii) and hence proportional to the direct oxygen escape rate from Mars that is an important part of the overall oxygen loss rate at Mars. The modeled energetic oxygen fluxes also exhibit a spin modulation as did the SLED fluxes during Circular Orbit 42.
Ultralow‐frequency magnetohydrodynamics in boundary‐constrained geomagnetic flux coordinates
Authors: Proehl Jeffrey A. , Lotko William , Kouznetsov Igor , Geimer Shireen D.
Published: October 2002

A new method is described for constructing geomagnetic flux coordinates, constrained by magnetospheric and ionospheric boundary surfaces. The technique is especially useful for computing boundary value solutions in a geometry defined by a realistic, three‐dimensional geomagnetic field with boundary conditions specified on surfaces oriented arbitrarily relative to the background magnetic field. An adaptable algorithm for calculating the metric tensor for a general class of geomagnetic flux coordinate systems is presented. Application to ultralow‐frequency wave propagation in the magnetosphere is illustrated by solving the equations of linear, one‐fluid magnetohydrodynamics for the driven field line resonance in a dipolar geomagnetic field bounded by a spherical ionosphere. A novel diagnostic for energy flow in the driven resonance problem shows that MHD wave power flows “radially” inward as a compressional wave from the boundary driver to the resonant flux surface, is diverted azimuthally at the resonant surface, and becomes field‐aligned in the resonant surface by coupling to the magnetically guided Alfvén wave at locations where the shear mode is in azimuthal phase quadrature with the compressional driver.
Response of interplanetary glow to global variations of hydrogen ionization rate and solar Lyman α flux
Authors: Bzowski Maciej , Summanen Tuula , Ruciński; Daniel , Kyrölä Erkki
Published: October 2002

The aim of the paper is to investigate the influence of the solar cycle variations of the radiation pressure and time‐ and heliolatitude‐dependent ionization rate on the spatial distribution and temporal evolution of the interplanetary hydrogen Lyman α glow, observed from 1 AU. We identify the variables affecting the distribution and evolution of the glow intensity and derive a parametric formula that links quite accurately the evolution of the intensities from arbitrary lines of sight with the variations of solar radiation pressure and net ionization rate. We illustrate that the radiation pressure μ, ecliptic ionization rate βecl, and ionization rate anisotropy qion can be derived with a reasonable accuracy from time series of ratios Q of the glow intensities from selected pairs of lines of sight because μ, βecl, qion, and Q are approximately related to each other by simple linear formulae. Further, we show the range of the north‐south anisotropy in the glow that can be expected despite the north‐south symmetry of the ionization rate. Finally, we qualitatively discuss the sensitivity of our results to other phenomena possibly affecting the distribution and evolution of the solar Lyman α backscatter glow, including the influence of the heliospheric interface and of the radiative transport correction.
Effects of fast and slow solar wind on the correlations between interplanetary medium and geomagnetic activity
Authors: Ballatore Paola
Published: October 2002

The coupling between interplanetary parameters and geomagnetic activity has been investigated. In particular, the correlations between the interplanetary medium and the geomagnetic indices Kp and Dst have been calculated for different ranges of solar wind speed. The correlation coefficients obtained for data points corresponding to solar wind slower than 550 km/s are equal or slightly higher than the global correlations. The observations show generally lower correlation coefficients for solar wind speeds faster than 550 km/s. These results suggest that at high solar wind speeds the processes responsible for the energy transfer between the interplanetary medium and the magnetosphere saturate. In addition, the influence of internal magnetospheric plasma physics on the geomagnetic activity may be larger for the faster solar wind intervals. In the context of the deterministically chaotic approximations we discuss how the threshold at ∼550 km/s might represent the break of the order in the interplanetary‐geomagnetic coupling system, so that the linear correlations or the correlations with a relatively weak departure from linearity are significant mostly during the slower solar wind.
Particle tomography of the inner magnetosphere
Authors: Korth H. , Thomsen M. F. , Glassmeier K.‐H. , Phillips W. S.
Published: October 2002

In a previous study we performed a statistical analysis of the geosynchronous plasma environment from measurements of the magnetospheric plasma analyzer instruments aboard the Los Alamos geosynchronous spacecraft. In that study the mean spin‐averaged particle fluxes were determined as a function of local time and Kp index, averaged over an entire year of observations. Particles on open drift trajectories should cross geosynchronous orbit twice during their drift from the nightside plasma sheet, through the inner magnetosphere, and out to the dayside magnetopause. The ratio of incoming plasma sheet phase space density and outgoing dayside phase space density of every drift path thus contains information about particle losses during the drift through the near‐Earth region. For ions the losses are largely caused by charge exchange reactions with hydrogen atoms. Applying tomographic inversion techniques, we use the observed statistical losses inside the geosynchronous orbit region to infer the spatial distribution of exospheric neutral hydrogen. The particle drift paths from the nightside of geosynchronous orbit to the dayside are calculated from electric and magnetic field models. To test the sensitivity of the tomography to the convection model, we invert the geosynchronous particle observations using various field model combinations and compare the results. We find that the neutral hydrogen densities obtained from the inversion disagree with existing models of the exosphere, mainly in the near‐Earth region. These differences are due to lower‐than‐expected losses of lower‐energy particles that nominally drift through the inner region and/or particle sources not considered in the study.
Anisotropy regulation and plateau formation through pitch angle diffusion of solar wind protons in resonance with cyclotron waves
Authors: Tu C.‐Y. , Marsch E.
Published: October 2002

In this paper the velocity distribution functions (VDFs) of protons measured by Helios in fast solar wind are analyzed in the framework of quasilinear theory (QLT). Evidence is presented that the shape of the central isodensity contours in velocity space and the temperature anisotropy of the core part of the VDFs can be explained by wave‐induced plateau formation according to QLT. The plateau is formed by protons that are in resonance with cyclotron waves, which are assumed to propagate both outwardly and inwardly at phase speeds following from the plasma dispersion relation. For the proton VDFs measured near 0.3 AU in fast low‐beta solar wind, the theoretical predictions of QLT, using the cold plasma dispersion relation, agree well with the in situ observations. For the proton VDFs measured near 1 AU in fast high‐beta wind, the predictions of QLT, using again the cold plasma dispersion relation, only give an upper limit for the anisotropy. Yet, considering thermal effects in the dispersion relation, a better agreement between the theory based on resonant ion diffusion and the observations is obtained. For nondispersive waves a simple relation between the ion thermal speed parallel to the magnetic field and the ion‐temperature anisotropy is derived, which is shown to be consistent with the anisotropy of the heavy O+5 ion as observed on the Solar and Heliospheric Observatory (SOHO), as well as with the anisotropy predicted numerically by a hybrid simulation of the ion‐temperature regulation by waves.
Cross polar cap potentials measured with Super Dual Auroral Radar Network during quasi‐steady solar wind and interplanetary magnetic field conditions
Authors: Shepherd S. G. , Greenwald R. A. , Ruohoniemi J. M.
Published: October 2002

We have analyzed Super Dual Auroral Radar Network (SuperDARN) data between February 1998 and December 2000 to determine the statistical characteristics of the total variation in the high‐latitude ionospheric electric potential, or cross polar cap potential, ΦPC. Periods are chosen to satisfy the criteria that (1) the solar wind and interplanetary magnetic field (IMF) are quasi‐stable for ≥40 min and (2) sufficient SuperDARN data exist to adequately determine ΦPC. A total of 9464 individual 10‐min periods satisfying the first criteria are analyzed. A subset of 2721 periods satisfy both criteria, of which 1638 are considered high‐confidence periods. The resulting data set shows that for quasi‐steady solar wind and IMF, ΦPC (1) is nonlinear in the expression for the effective interplanetary electric field EKL, (2) saturates at high values of EKL, and (3) is highly variable for any given value of EKL. These results indicate that simple formulations involving the upstream solar wind and IMF conditions are inadequate to describe the instantaneous ΦPC and that the inclusion of internal and coupling processes between the magnetosphere and ionosphere may be necessary.
Signatures of traveling convection vortices in ground magnetograms under the equatorial electrojet
Authors: Trivedi N. B. , Sibeck D. G. , Zesta E. , Santos J. C. , Yumoto K. , Kitamura T. , Shinohara M. , Dutra S. L. G.
Published: October 2002

Traveling convection vortices (TCVs) are transient events in high‐latitude ground magnetograms that can be interpreted as evidence for localized vortical flows in the high‐latitude ionosphere. Their centers travel eastward or westward at geomagnetic latitudes near 72° to 74°. TCVs have been attributed to various transient interactions at the magnetopause but also to processes occurring deeper within the magnetosphere. Each interaction mechanism should launch shear Alfvén waves that produce high‐latitude ground signatures but also compressional waves that should produce signatures at lower latitudes. Zesta et al. [1999], Moretto et al. [1997], Ridley et al. [1998], and Lühr et al. [1998] report detailed analyses for several high‐latitude TCVs. In this paper, we present evidence for corresponding signatures at Belem, São Luis, and Teresina and other stations under or nearby the equatorial electrojet. The equatorial signatures are most pronounced when the high‐latitude signatures are (1) longitudinally extended, (2) isolated, and (3) attain greatest amplitudes.
HF pump‐induced large scale radial drift of small scale magnetic field‐aligned density striations
Authors: Koloskov A. V. , Leyser T. B. , Yampolski Yu. M. , Beley V. S.
Published: October 2002

A powerful high frequency (HF) electromagnetic pump wave transmitted into the ionospheric F region excites filamentary density irregularities stretched along the geomagnetic field. We present experimental results of small scale (5–15 m across the geomagnetic field) magnetic field‐aligned striations (FAS) excited using the Sura HF pump facility, Russia. The FAS were observed by resonant HF scatter in bistatic mode utilizing continuous wave broadcasting transmissions from near Moscow and reception with the UTR‐2 radio telescope as well as in nearly monostatic mode using a pulsed radar transmitter of Kharkov National University and reception at UTR‐2. The radar scatter showed that the previously observed split Doppler spectra in scattered continuous wave sounding is a result of pump‐induced large scale outward radial drift of the FAS during HF pumping. For sufficiently short pump pulses the radial drift reverses from outward during pumping to inward after pump‐off as the FAS decay. This drift reversal does not occur after longer pump pulses. The drift speed amounts to 2–4 m/s.
Relative contributions of H+ and O+ to the ring current energy near magnetic storm maximum
Authors: Greenspan M. E. , Hamilton D. C.
Published: October 2002

Close to the peak of a magnetic storm, the ratio of the O+ to the H+ contribution to the ring current energy, U(O+)/U(H+), depends on both solar EUV flux and storm size. This ratio provides a lower limit on the importance of the ionospheric plasma source compared to the solar wind source since only the ionosphere supplies O+ while both the solar wind and the ionosphere supply H+. We have used measurements from the CHEM ion spectrometer on the Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE spacecraft to assess U(O+)/U(H+) near storm maximum. CHEM measured equatorial ion composition over the energy per charge range 1.5–300 keV/e for more than 4 years, from solar minimum through much of the rising phase of Solar Cycle 22. Thus our data set is large, including 67 magnetic storms with minimum Dst values less than −50 nT. To estimate the O+ and H+ contributions to the ring current energy, we have multiplied local measurements of each ion's energy density along the CCE trajectory by the appropriate dipole L shell volume and summed over L values from two to seven. We have used a bilinear regression to assess the dependence of U(O+)/U(H+) on solar EUV flux, parameterized by F10.7, and storm size, parameterized by minimum Dst value Dstmin. We have found that both F10.7 and Dstmin are important (and nearly independent) predictors of U(O+)/U(H+). Only four storms of 67 had U(O+)/U(H+) > 1. Two of the four were fairly small, with |Dstmin| < 100 nT but with high F10.7 values. This shows that O+ can sometimes contribute the majority of the ring current energy during small storms when the solar EUV flux is high (i.e., near solar maximum) as well as during very large storms throughout the solar cycle.