bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of electron beam generated waves resulting in transverse ion heating and filamentation of the plasma
Authors: Singh N. , Loo S. M. , Wells B. E. , Lakhina G. S.
Published: January 2004

We present here a systematic simulational study on electron beam driven waves and their consequences in terms of plasma electrodynamics. The study is performed by using three‐dimensional particle‐in‐cell code, parallelized to simulate a large volume of plasma. Our simulation shows that an initial electron beam of finite radius with beam velocity along the ambient magnetic field triggers a series of events in the evolution of the waves and the plasma. In the initial stage (t ≤ 200 ωpo−1, ωpo being the electron plasma frequency with the total electron density n0), high frequency waves near ω ∼ ωpo are driven. These waves progressively disappear giving way to the dominance of lower hybrid (LH) waves. The phase of the lower hybrid waves lasts over the time interval
. In this time period the LH waves stochastically accelerate ions transverse to B0, and the beam electrons are scattered outside the initial beam volume to occupy the entire volume of the simulation plasma. The ion acceleration leads to the formation of elongated tail in the perpendicular velocity distribution. The large‐amplitude LH waves are seen to undergo a parametric decay into resonance cone waves at frequencies ω < Ωi, the ion cyclotron frequency. Such extremely low‐frequency (ELF) waves are the electrostatic version of the inertial Alfven waves. The phase of the strong LH waves is followed by a stage in which HF waves with frequencies ω ∼ ωpo appear again, but in this stage they are strongly modulated by the already present ELF waves and other low‐frequency waves in the frequency range near the ion cyclotron frequency Ωi and its harmonics. Beginning with the LH wave stage and continuing into the late stages of ELF waves, the plasma density is highly filamented and the filaments oscillate with the ELF frequencies. The relevance of our results to the observations on plasma waves from satellites is brought out.
Wave activity in Europa's wake: Implications for ion pickup
Authors: Volwerk M. , Kivelson M. G. , Khurana K. K.
Published: January 2004

Intense wave power at low frequencies (near and below the cyclotron frequencies of heavy ions) was present in Europa's wake during the E11 and E15 flybys. The fluctuations are mainly transverse to the background magnetic field, and wave characteristics indicate that they are ion cyclotron waves driven by positively charged pickup ions. In both flybys there is evidence, from the wave polarization, for pickup of negatively charged chlorine ions. The pickup rate inferred for the E15 flyby, when the moon is near the center of the Jovian current sheet, is larger than that for the E11 flyby, when the moon is outside the Jovian current sheet. The wave power does not provide absolute pickup density values, because the waves are observed in regions where their growth has not yet saturated. Low‐frequency (< K+ gyrofrequency) magnetohydrodynamic waves are also present at the edges of the wake region. We identify signatures in the magnetic field that are reminiscent of interchange/ballooning of mass‐loaded flux tubes from the wake/pickup region expanding into less dense ambient medium.
An explanation of the ion cloud morphology in the CRRES plasma injection experiments
Authors: Delamere P. A. , Swift D. W. , Stenbaek‐Nielsen H. C.
Published: January 2004

Optical observations of the CRRES plasma injection experiments revealed a highly structured ion cloud. In an attempt to understand these structures, we have developed a three‐dimensional hybrid code to study the interaction between an injected plasma cloud and an ambient magnetized plasma. The primary advantage of this code is a seamless interface between a small plasma cloud, treated with fully kinetic particles, and an MHD fluid (ambient plasma). The results of the simulation are in good agreement with the observations. According to the simulation, a very dense ion core was formed as the neutral barium photoionized. The core polarized and moved in the initial injection direction via E × B drift. The simulation also provided insight into a structured secondary neutral cloud created by Ba‐Ba+ charge exchange. Specifically, the ion core split into two parts along the magnetic field, creating a nonuniform distribution of charge‐exchanged neutrals. This structured ion core also leads to a herringbone structure in the trailing ion cloud. The code can be applied to similar problems including comets, coronal mass ejections, the Io‐Jupiter interaction, and other plasma injection experiments.
Interaction of two long‐wavelength modes in the nonlinear numerical simulation model of equatorial spread F
Authors: Sekar R. , Kherani E. A. , Rao P. B. , Patra A. K.
Published: January 2004

In order to understand the evolution of equatorial spread F structures and their dynamics an investigation was carried out with the perturbation consisting of a single wavelength mode and superposition of two modes. The investigation revealed that the depleted region always moves upward with a single wavelength mode, while the superposition of two modes gives rise to low‐level plasma depletion which moves downward even when the the ambient plasma motion is upward, in addition to well‐developed upward moving plasma bubbles and downdrafting enhancements with varying degrees of development. Further, the evolution of well‐developed plasma bubbles with scale size corresponding to a shorter‐wavelength mode is possible even with very small (0.5%) amplitude of perturbation when it rides over a long‐wavelength mode with large (5%) amplitude of perturbation. The longer‐wavelength mode develops to form a lower envelope over which multiple plumes with varying degree of depletions ride, with the separation between the plumes decided by the short‐wavelength perturbation subject to the amplitude of the former being larger in comparison with the latter. The rising multiple plumes and the descending structure along with a downward moving streak as observed by the Indian Mesosphere‐Stratosphere‐Troposphere (VHF) radar can qualitatively be understood on the basis of the interaction between the two modes which gives rise to depletions with varying growth rates and a rapidly descending enhancement followed by a downward moving weakly depleted region depending upon the wave parameters of the two modes.
EISCAT Svalbard Radar observations of ionospheric plasma dynamics in relation to dayside auroral transients
Authors: Moen J. , van Eyken A. P. , Carlson H. C.
Published: January 2004

The first EISCAT Svalbard Radar (ESR) and ground optical campaign was carried out in January 1998 to cover a variety of dayside auroral activities in the 1000–1400 magnetic local time sector. The current paper focuses on 1 hour of combined radar, meridian scanning photometer, and ground magnetometer observations near the cusp convection throat on January 23, 1998. The radar was pointing north, at a fixed elevation of 45°, in the scan plane of the photometer. Several plasma dynamical features were revealed by this simplest possible convection mode of the ESR, such as spontaneous movement of the cusp inflow region and cusp‐related ionization blobs pushing through the ambient incompressible plasma. Multiple morning discrete arcs were found to be aligned with convection streamlines equatorward of the flow reversal, while plasma in the polar cap inflow region streamed perpendicularly across the optical cusp boundary, as earlier anticipated.
A statistical study of transient event motion at geosynchronous orbit
Authors: Sanny J. , Berube D. , Sibeck D. G.
Published: January 2004

The geosynchronous GOES 5 and GOES 6 satellites frequently observe transient events marked by magnetic field strength increases and bipolar magnetic field signatures lasting several minutes. In this study we report a survey of 87 events observed simultaneously by both GOES spacecraft (for a total of 174 individual observations) from August to December 1984. Events detected in the prenoon sector outnumbered those in the postnoon sector by about a 3 to 1 ratio. The distribution of the events versus local time exhibited a significant prenoon peak like the distribution of magnetic impulse events observed in high‐latitude ground magnetometers. A cross‐correlation analysis of the two GOES data sets indicated lags that range from 0 to over 2 min, with the majority of the events moving antisunward. The short lags correspond to azimuthal speeds of hundreds of kilometers per second, greater than flow speeds in the magnetosheath, but less than fast mode waves. The short lags may indicate that the events move primarily latitudinally and/or that transient events are seldom localized, but rather occur over extended, if not global, regions. Investigations of event occurrence versus interplanetary magnetic field (IMF) Bz, event motion versus IMF By, and correspondence between upstream plasma data and the events all indicate that pressure pulses are the likely source of many of the events. About 27% of the events with simultaneous solar wind data were preceded by sharp reversals in one or more IMF components, and nearly all of this particular group of events occurred in the dawn sector. This suggests that the pressure pulses may be commonly generated in the foreshock/bow shock region, since the prenoon magnetopause lies generally behind the quasi‐parallel bow shock where such pulses are thought to be triggered by IMF discontinuities. Finally, several events in the data set were also observed by the AMPTE/CCE. These are presented as case studies.
A quantitative study of ionospheric density gradients at midlatitudes
Authors: Vo H. B. , Foster J. C.
Published: January 2004

The subauroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F region ionospheric density structuring due to the space weather effects of magnetospheric disturbance electric fields. Communications and navigation systems relying on transionospheric propagation must be able to compensate for the effects of the sharp changes (>10X) in total electron content (TEC) associated with the ionospheric trough and storm time disturbance effects at midlatitudes. The Millstone Hill incoherent scatter radar database has been used to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at middle and subauroral latitudes as a function of solar cycle, local time, and level of geomagnetic activity. More than 11,000 radar elevation scans covering >20° of latitude and altitudes between 150 and 750 km have been used to identify the characteristics of the density gradient near the equatorward edge of the ionospheric trough in a variety of circumstances spanning 20 years and two solar cycles. Pronounced density gradients can be identified in ∼35% of the Millstone Hill scans, and we present a statistical characterization of average magnitude and location for these steepest TEC gradients. In some cases (especially near noon) the equatorward edge of the trough lies poleward of our observational field of view, and gradients associated with phenomena other than the trough contribute to our statistics. On most days the trough appears in the radar scans between 1600 and 2000 magnetic local time (MLT). Larger TEC gradients occur at solar maximum and when the background TEC is higher. The steepest gradients occur in an environment of high TEC (at solar maximum and adjacent to regions of storm‐enhanced density (SED)), when the processes which generate the trough are strongest (high Kp). High gradient values occur in the sunlit sector, with maximum values of TEC gradient (∼10 TEC/deg latitude, with 1 TEC unit = 1016 el m−2) found in the postnoon ionosphere. Mean solar maximum TEC gradient at 1600 MLT is 3–4 TEC/deg for Kp < 2, increases linearly with Kp for Kp from 2 to 4, and is nearly constant at a value of 7–8 TEC/deg for Kp from 4 to 6. SED, the bulk redistribution of F region plasma by disturbance electric fields, can result in TEC > 100 over New England and TEC gradients of ∼50 TEC/deg.
Viewing corotating interaction regions globally using energetic neutral atoms
Authors: Kóta J. , Hsieh K. C. , Jokipii J. R. , Czechowski A. , Hilchenbach M.
Published: January 2004

We present model simulations to estimate the flux of 25–100 keV energetic neutral atoms (ENAs) that can be expected from charge exchange between the ion populations accelerated at corotating interaction regions and the slow neutral atoms penetrating from the local interstellar matter. The extended structure of a corotating interaction region (CIR) evolving in space is a major plasma feature in the inner heliosphere that contains a wealth of information on shock development and charged‐particle acceleration. The detection of energetic neutral atoms originating from CIRs, with directional, mass, and energy resolutions, is a way to view CIRs remotely. This global view in ENAs may provide important information complementary to in situ particle and plasma and field observations. We apply a three‐dimensional numerical code to model CIRs and simulate the transport of energetic particles accelerated at the forward and reverse shocks of the CIR. Our simulation results are applied to the ongoing ENA observations by the Solar and Heliospheric Observatory (SOHO) and that anticipated by Cassini. We find that a CIR origin of the ENAs detected by the high suprathermal time‐of‐flight sensor of the charge, element, and isotope analysis system on SOHO cannot be ruled out. Our simulations also reveal that the concentration of local interstellar He atoms focused by the Sun's gravitation contributes significantly to the production of ENAs in the inner regions of the quiet heliosphere.
Symplectic integration: A new approach to tracing charged particle motion in the geomagnetic field
Authors: Yugo H. , Iyemori T.
Published: January 2004

A new integration technique, symplectic method, is introduced and applied for the tracing of a charged particle motion in a dipole magnetic field. This method is an integral technique for the Hamilton's system by the repetition of canonical transformation, and has been tested in celestial mechanics in recent years. Compared with the standard Runge‐Kutta method (4th order), the numerical error accumulation is much smaller for the symplectic method. An effective potential is introduced and used to discuss an abrupt transition between trapping and un‐trapping regions.
Measurement of the latitudinal distributions of total electron content during equatorial spread F events
Authors: Valladares C. E. , Basu S. , Groves K. , Hagan M. P. , Hysell D. , Mazzella A. J. , Sheehan R. E.
Published: February 2004

We have constructed latitudinal profiles of the total electron content (TEC) using measurements from six GPS receivers conducted during 1998. The TEC profiles have been divided into two groups: One corresponds to days when plumes or equatorial spread F (ESF) develops, and the second group portrays days of no‐ESF condition. The presence/absence of ESF is based on the signature of the coherent echoes measured by the Jicamarca Unattended Long‐Term Investigation (JULIA) radar and records of scintillations from two sites spaced in latitude. One scintillation station is located near the magnetic equator (Ancon) and the other 12° southward (Antofagasta). The TEC profiles display the typical day‐to‐day and seasonal variability seen at low latitudes. During the equinoxes, we observed quite often the crests of the anomaly located between 12° and 20° away from the magnetic equator and a trough in‐between. The monthly distribution of the appearance of the anomaly and the local time of their appearance are in very good agreement with the reported variability of the upward vertical drifts and the current theory of the equatorial fountain effect. During the equinoxes and the December solstice, the TEC anomaly is observed almost every day, sometimes when there is no ESF activity. Nevertheless, fine inspection of the TEC latitudinal profiles suggests the existence of a close relationship between the temporal evolution of the TEC profiles near sunset and the onset of ESF. We have examined the TEC latitudinal distributions in two different ways. First, we calculated time difference profiles using the distributions corresponding to 1800 and 2000 LT. Second, we used a parameterization of the TEC distributions obtained at 2000 LT. The first method indicates quite drastic increases of the crest values and sharp decreases near the trough during ESF days. In contrast, during days of no ESF there exist almost uniform TEC decreases at all latitudes. The second method displays a preferred high crest/trough ratio (>2), small TEC values at the trough, and large latitudinal integrated values during ESF events.
New evidence for the brightness and ionization of blue starters and blue jets
Authors: Wescott E. M. , Sentman D. D. , Stenbaek‐Nielsen H. C. , Huet P. , Heavner M. J. , Moudry D. R.
Published: January 2004

Blue jets and blue starters are partially ionized luminous cones of primarily blue light that propagate upward out of the top of thunderstorms at speeds of order 100 km s−1. Blue jets propagate up ∼40 km, but blue starters, which resemble blue jets, terminate abruptly after only a few kilometers of upward travel. Theories on the origin of blue jets have proposed that they are due to either positive or negative streamers or runaway electrons. Quantitative analysis of new multi‐instrument observations of a blue starter from an aircraft during the Energetics of Upper Atmospheric Excitation by Lightning, 1998 (EXL98) campaign of July 1998, shows that the ionization accounts for ∼3% of the observed intensity. Quantitative analysis of a remarkable color photograph of a blue jet taken from Réunion Island in the Indian Ocean shows that the minimum optical energy deposition was ∼0.5 MJ. The same photograph shows details of streamers never before seen.
Spectral solar UV irradiance data for cycle 21
Authors: DeLand Matthew T. , Cebula Richard P.
Published: January 2004

The Nimbus 7 Solar Backscatter Ultraviolet (SBUV) instrument, which began taking data in November 1978, was the first instrument to make solar UV irradiance measurements covering both the minimum and maximum activity levels of a solar cycle. The currently archived irradiance data set was processed with an instrument characterization which fails to completely account for sensor degradation in the later part of the data record, thus limiting the accuracy of estimated long‐term solar activity variations and the scientific value of the data. In this paper, we describe an improved Nimbus 7 SBUV spectral irradiance data set, which utilizes a more accurate model for instrument sensitivity and treats other time‐dependent problems in the archived data. Estimated long‐term irradiance changes during solar cycle 21 are 8.3(±2.6)% at 205 nm, and 4.9(±1.8)% at 240 nm. The revised Nimbus 7 SBUV irradiance data are in good agreement with predictions of solar cycle variations from the Mg II index proxy model. These solar irradiance changes are also consistent with overlapping irradiance data from the declining phase of solar cycle 21 measured by the Solar Mesosphere Explorer (SME). The Nimbus 7 SBUV irradiance data represent the earliest component of a 20+ year continuous record of solar spectral UV activity.
Three‐dimensional structuring characteristics of high‐latitude plasma patches
Authors: Gondarenko N. A. , Guzdar P. N.
Published: January 2004

A three‐dimensional nonlinear study of the turbulence and sheared flow, generated by the gradient drift instability with ion inertial effects in the high‐latitude plasma patches, is presented. Numerical simulations demonstrate quasi‐two‐dimensional turbulence with irregularities aligned along the magnetic field direction. Power spectra of the velocities in the plane transverse to the magnetic field and the density irregularities display anisotropy in the turbulence. Secondary Kelvin‐Helmholtz instability of the gradient drift elongated vortices tends to isotropize the spectra. Ion‐neutral collisions determine the level of the shear flow and therefore the saturation level of density and potential fluctuations. The irregularities penetrate through the entire patch due to the nonlinear development of the instability and do not remain localized on the edges of the plasma patch. The computed spectra and spatial structures, observed by DE 2 satellite in the midnight‐noon direction, are in good agreement. The inertial terms unify the gradient drift and Kelvin‐Helmholtz instabilities, the sources of small‐scale structures generated in the polar cap plasma patches, and give rise to irregularity characteristics which are in a good agreement with general features of the observed structures.
A comprehensive survey of plasmasphere refilling at geosynchronous orbit
Authors: Su Yi‐Jiun , Thomsen Michelle F. , Borovsky Joseph E. , Lawrence David J.
Published: January 2004

Plasmasphere refilling is one of the most fundamental examples of ionosphere‐magnetosphere coupling. Geosynchronous low‐energy ion measurements are particularly useful in addressing this issue. In this survey we collect almost 11 satellite years of data, from January 1990 to September 2000, from Los Alamos magnetospheric plasma analyzers on board five satellites. The data are sorted according to the level of magnetospheric activity, local time, season, and solar cycle phase for a comprehensive examination of the early‐time refilling rate. These statistical results indicate that the early‐time refilling rate increases with increasing geomagnetic activity. Additionally, the early‐time refilling rate at low solar activity is about a factor of 1.5 – 2 higher than that at high solar activity. This may be related to the higher ionospheric neutral hydrogen density at solar minimum. Moreover, when the geomagnetic activity index is higher than 3, the early‐time refilling rate is observed to be slightly higher during the seasonal equinox periods than at solstice, but no seasonal variation is found at low geomagnetic activity. The late‐time refilling rate is also examined during intervals of prolonged geomagnetic quiet. The refilling process is not linear as a function of time: The early‐time refilling rate ranges between 2.5 and 6.5 cm−3 d−1, while the late‐time refilling rate is found to be ∼ 10 – 25 cm−3 d−1. The late‐time refilling rate is also anticorrelated with solar activity. The plasmasphere at geosynchronous orbit is observed to reach a maximum density at ∼ 100 – 200 cm−3.
Three‐dimensional numerical simulation of MHD waves observed by the Extreme Ultraviolet Imaging Telescope
Authors: Wu S. T. , Zheng Huinan , Wang S. , Thompson B. J. , Plunkett S. P. , Zhao X. P. , Dryer M.
Published: January 2004

We investigate the global large amplitude waves propagating across the solar disk as observed by the SOHO/Extreme Ultraviolet Imaging Telescope (EIT). These waves appear to be similar to those observed in Hα in the chromosphere and which are known as “Moreton waves,” associated with large solar flares [Moreton, 1960, 1964]. Uchida [1968] interpreted these Moreton waves as the propagation of a hydromagnetic disturbance in the corona with its wavefront intersecting the chromosphere to produce the Moreton wave as observed in movie sequences of Hα images. To search for an understanding of the physical characteristics of these newly observed EIT waves, we constructed a three‐dimensional, time‐dependent, numerical magnetohydrodynamic (MHD) model. Measured global magnetic fields, obtained from the Wilcox Solar Observatory (WSO) at Stanford University, are used as the initial magnetic field to investigate hydromagnetic wave propagation in a three‐dimensional spherical geometry. Using magnetohydrodynamic wave theory together with simulation, we are able to identify these observed EIT waves as fast mode MHD waves dominated by the acoustic mode, called magnetosonic waves. The results to be presented include the following: (1) comparison of observed and simulated morphology projected on the disk and the distance‐time curves on the solar disk; (2) three‐dimensional evolution of the disturbed magnetic field lines at various viewing angles; (3) evolution of the plasma density profile at a specific location as a function of latitude; and (4) computed Friedrich's diagrams to identify the MHD wave characteristics.
Ionospheric topography maps using multiple‐wavelength all‐sky images
Authors: Makela Jonathan J. , Kelley Michael C. , González Sixto A. , Aponte Nestor , McCoy Robert P.
Published: February 2004

We outline a technique to create three‐dimensional topographic maps of the Earth's ionosphere. Using all‐sky images at 630.0 and 777.4 nm taken with the Cornell All‐Sky Imager (CASI) while located at the Arecibo Observatory, we can estimate the maximum density (Nm) and the height (Hm) of the F layer over a 1000 × 1000 km area. This is possible because to first order, the 777.4 nm emission, produced by radiative recombination, is proportional to the integral of the square of the plasma density, whereas the 630.0 nm line, produced by charge exchange and dissociative recombination, depends on both plasma height and density. Using the neutral atmosphere given by the Mass Spectrometer Incoherent Scatter (MSIS‐86) model and electron densities from the international reference ionosphere 1995 (IRI‐95) model, we show that the estimates given by these maps are good to within 5% of the values used as input into the models. These errors are slightly larger (10%) when extreme gradients in the height of the F layer are present. We apply our technique to two different nights in 1999. In one example these maps show a steeply rising ridge of ionization stretching equatorward of the Caribbean site, punctuated by a series of parallel ridges and valleys. We compare these observations with previous work at Arecibo during very high magnetic activity. In our case we find no evidence for particle precipitation and agree with Sahai et al. [1981a] that spatial variations may have affected the earlier study. Another example shows the Appleton anomaly much farther north than normal. Instability processes are indicated in the former case, while physical mechanisms associated with a magnetic storm are explored in the latter case.
Ionospheric Alfvén resonator revisited: Feedback instability
Authors: Pokhotelov Oleg A. , Khruschev V. , Parrot M. , Senchenkov S. , Pavlenko V. P.
Published: January 2004

The theory of ionospheric Alfvén resonator (IAR) and IAR feedback instability is reconsidered. Using a simplified model of the topside ionosphere, we have reanalyzed the physical properties of the IAR interaction with magnetospheric convective flow. It is found that in the absence of the convective flow the IAR eigenmodes exhibit a strong damping due to the leakage of the wave energy through the resonator upper wall and Joule dissipation in the conductive ionosphere. It is found that maximum of the dissipation rate appears when the ionospheric conductivity approaches the “IAR wave conductivity” and becomes infinite. However, the presence of Hall dispersion, associated with the coupling of Alfvén wave modes with the compressional perturbations, reduces the infinite damping of the IAR eigenmodes in this region and makes it dependent on the wavelength. The increase in the convection electric field leads to a substantial modification of the IAR eigenmode frequencies and to reduction of the eigenmode damping rates. For a given perpendicular wavelength the position of maximum damping rate shifts to the region with lower ionospheric conductivity. When the convection electric field approaches a certain critical value, the resonator becomes unstable. This results in the IAR feedback instability. A new type of the IAR feedback instability with the lowest threshold value of convection velocity is found. The physical mechanism of this instability is similar to the Čerenkov radiation in collisionless plasmas. The favorable conditions for the instability onset are realized when the ionospheric conductivity is low, i.e., for the nighttime conditions. We found that the lowest value of the marginal electric field which is capable to trigger the feedback instability turns out to be nearly twice smaller than that predicted by the previous analysis. This effect may result in the decrease of the critical value of the electric field of the magnetospheric convection that is necessary for the formation of the turbulent Alfvén boundary layer and appearance of the anomalous conductivity in the IAR region.
Examination of the three‐cycle quasiperiodicity of geomagnetic indices in relation to prediction of the size of cycle 23
Authors: Kane R. P.
Published: January 2004

From the study of the Ap and aa geomagnetic indices for 1932 onward, Ahluwalia [1998] reported a three‐cycle quasiperiodicity, with lower Ap(min) and aa(min) values in cycles 17, 20, 23. An examination of aa indices for periods extending back to 1868 (and further back to 1844, using the Helsinki equivalent aa index data set) did not show the three‐cycle pattern, though a possible periodicity of ∼25–30 years may exist. The more overwhelming variation was an uptrend that had a minimum near cycle 14 (∼1900) and may have peaked recently. The long series of sunspot number maxima (cycles 0–22, 1750 onward, but not reliable before cycle 8) showed two long‐term swings, one of ∼90 years and another of ∼100 years, the latter peaking in recent cycles and indicating a probable downtrend in the coming 100 years. The sunspot number maxima series had an excellent correlation with aa(min) series back to 1868 (and even after including the estimated values back to 1844), which is being used for prediction of the size of the present cycle 23. Various predictions for cycle 23, which started in 1996 and seems to have peaked recently at a smoothed sunspot number of ∼121, are discussed.
Response of ionospheric convection to changes in the interplanetary magnetic field: Lessons from a MHD simulation
Authors: Maynard N. C. , Siscoe G. L. , Sonnerup B. U. Ö. , White W. W. , Siebert K. D. , Weimer D. R. , Erickson G. M. , Schoendorf J. A. , Ober D. M. , Wilson G. R.
Published: January 2004

Characteristics of magnetospheric and high‐latitude ionospheric convection pattern responses to abrupt changes in the interplanetary magnetic field (IMF) orientation have been investigated using an MHD model with a step function reversal of IMF polarity (positive to negative BY) in otherwise steady solar wind conditions. By examining model outputs at 1 min intervals, we have tracked the evolution of the IMF polarity reversal through the magnetosphere, with particular attention to changes in the ionosphere and at the magnetopause. For discussion, times are referenced relative to the time of first contact (t = 0) of the IMF reversal with the subsolar nose of the magnetopause at ∼ 10.5 RE. The linear change in large‐scale ionospheric convection pattern begins at t = 8 min, reproducing the difference pattern results of Ridley et al. [1997, 1998]. Field‐aligned current difference patterns, similarly derived, show an initial two‐cell pattern earlier, at t = 4 min. The current difference two‐cell pattern grows slowly at first, then faster as the potential pattern begins to change. The first magnetic response to the impact of the abrupt IMF transition at the magnetopause nose is to reverse the tilt of the last‐closed field lines and of the “first”‐open field lines. This change in tilt occurs within the boundary layer before merging of IMF with closed magnetospheric field lines starts. In the case of steady state IMF BY, IMF field lines undergo merging or “changing partners” with other IMF field lines, as they approach the nose and tilt in response to currents. When the BY reversal approaches the magnetopause nose, IMF field lines from behind the reversal overtake and merge with those in front of the reversal, thus puncturing the reversal front and uncoupling the layer of solar wind plasma in the reversal zone from the magnetosphere. The uncoupled layer propagates tail ward entirely within the magnetosheath. Merging of closed magnetospheric field lines with the new polarity IMF begins at t = 3 min and starts to affect local currents near the cusp 1 min later. While merging starts early and controls the addition of open flux to the polar cap, large‐scale convection pattern changes are tied to the currents, which are controlled in the boundary layers. The resulting convection pattern is an amalgamation of these diverse responses. These results support the conclusion of Maynard et al. [2001b], that the small convection cell is driven from the opposite hemisphere in BY‐dominated situations.
Kinetic study of the mirror mode
Authors: Génot V. , Schwartz S. J. , Mazelle C. , Balikhin M. , Dunlop M. , Bauer T. M.
Published: January 2004

The linear Vlasov dispersion is solved to reveal the kinetic properties of the mirror mode. The existence of the torsional component of both the magnetic and velocity perturbations are kinetic features that are not explained by MHD theory. A parameter study is then employed to clarify the behavior of these particular components under different plasma conditions. The key parameters are the ion temperature anisotropy and the plasma to magnetic pressure ratio which act in similar ways to control the value of the torsional (or Alfvénic) components. Data from the Active Magnetospheric Particle Tracer Explorers mission show that under most magnetosheath plasma conditions the magnetic torsional component is negligible, whereas the velocity one is always larger or comparable to the other components. This experimental illustration also shows that there exist elliptically polarized mirror modes, contrary to what our, and all previous, analytical treatments predict.
Temporal variations of geocoronal and heliospheric X‐ray emission associated with the solar wind interaction with neutrals
Authors: Cravens T. E. , Robertson I. P. , Snowden S. L.
Published: January 2004

X‐ray emission due to charge transfer collisions between heavy solar wind ions and neutrals has been predicted to exist both in the heliosphere and in the geocorona. The heliospheric X‐ray emission can account for roughly half of the observed soft X‐ray background intensity. It was also suggested that temporal variations in the heliospheric and geocoronal soft X‐ray intensities will result from solar wind variations. In this paper, a simple model of the charge exchange X‐ray emission mechanism is combined with measured solar wind parameters as a function of time and used to generate predictions of the temporal variation of the X‐ray intensity observed at Earth for the time periods 1990–1993 and 1996–1998. Measured solar wind proton fluxes are also directly compared with the “long‐term enhancement” part of the soft X‐ray background measured by the Röntgen Satellite (ROSAT). A significant positive correlation exists, which supports the existence of X‐ray emission associated with the solar wind interaction with either interstellar neutrals and/or with geocoronal neutral hydrogen.
Inferring E region electron density profiles at Jicamarca from Faraday rotation of coherent scatter
Authors: Hysell D. L. , Chau J. L.
Published: February 2004

A new technique for measuring E region plasma density profiles in the equatorial electrojet using a bistatic coherent scatter radar is described. The technique utilizes the Faraday rotation of the obliquely and coherently scattered signal. Plasma density versus altitude is inferred from the rate of Faraday rotation as a function of range and elevation angle. A narrow beam width is required to minimize returns from unwanted azimuths, but this can be achieved in a bistatic experiment using relatively small antenna arrays with widely spaced elements. We give a sample time sequence of daytime electron density profiles that were measured with the new technique at altitudes between 95 and 110 km. Scatter from pure two‐stream waves makes it possible to measure both the bottomside and topside density profiles during the day. The importance of this new technique becomes evident when one realizes that only a few rocket flights have provided density profiles through these altitudes at the magnetic equator; the region has been inaccessible to any remote sensing technique until now.
The correlation between solar energetic particle peak intensities and speeds of coronal mass ejections: Effects of ambient particle intensities and energy spectra
Authors: Kahler S. W.
Published: January 2004

The correlation of peak intensities of solar energetic particle (SEP) events with the speeds of the associated coronal mass ejections (CMEs) is understood to be a result of SEP acceleration at shocks driven by the CMEs. However, the peak SEP intensities associated with CMEs of a given speed vary over ∼ 4 orders of magnitude. We examine a database of 71 E > 10 MeV SEP events observed with the GOES satellite to determine whether enhanced ambient SEP intensities at the times of the CMEs and/or variations among SEP event spectra contribute to the large range of peak SEP event intensities. A statistical analysis shows that enhanced ambient SEP intensities may be a contributing factor to the range of SEP events of higher peak intensities, probably by providing sources of energetic seed particles for the shock acceleration process. Another factor is the variation of energy spectra among the SEP events, which generally have harder spectra with increasing peak intensities. The observed increase of peak SEP intensities and hardening of peak SEP spectra with increasingly westward solar source regions is only a minor factor in the range of SEP peak intensities in the CME speed correlation.
Prediction of Earth arrival times of interplanetary southward magnetic field turnings
Authors: Horbury T. S. , Burgess D. , Fränz M. , Owen C. J.
Published: January 2004

ACE magnetic field and velocity data, taken in L1 halo orbit, are used to predict the arrival time at the Wind spacecraft of sharp southward turnings of the interplanetary magnetic field. If such events are assumed to be tangential discontinuities, their orientations can be deduced from the cross product of the field immediately upstream and downstream of them. It is shown that such an assumption results in better predictions of arrival times than using minimum variance or simply assuming a sunward normal. Of the 70% of events observed at ACE that could be identified at Wind, 93% of arrival times were predicted within 10 min using the cross product, compared to 56% using minimum variance and 67% assuming a sunward normal. The most important factor controlling the likelihood of observing events at Wind was the angle of the discontinuity normal from the flow direction, although weak dependencies on solar wind speed, flow‐perpendicular separation, and normal‐perpendicular separation were also found. These results suggest that the cross product is an accurate estimator of discontinuity orientations and that using L1 monitor data, it would be possible to predict arrival times of around 86% of southward turnings at Earth with an accuracy of 10 min.
Using steady state MHD results to predict the global state of the magnetosphere‐ionosphere system
Authors: Ridley A. J. , De Zeeuw D. L. , Gombosi T. I. , Powell K. G.
Published: January 2004

Using magnetospheric MHD codes for space weather prediction has many advantages over other techniques including the specification of the entire magnetospheric state instead of some subsection, such as the ionospheric electric potential pattern. Time‐dependent MHD simulations are computationally expensive and therefore nonfeasible to continuously run in real time. This study focuses on determining whether a quasi steady state magnetospheric configuration, determined by an MHD code, can be used as a prediction for the true state of the magnetosphere during relatively steady interplanetary magnetic field (IMF) intervals. To determine the feasibility of this, the University of Michigan MHD code was run to a nearly steady solution using the average IMF By and Bx and solar wind conditions observed on March 19–20, 1999. The IMF Bz was set to nearly the minimum observed during this period. Ground‐based magnetograms were simulated using the model output, which were then compared with the actual magnetograms for the given stations, using the root‐mean‐square (RMS) difference to determine the error. The RMS differences were compared to the RMS variation of the data. Different ionospheric conductance models were used to investigate possible sources of error. The model results within the polar cap and at lower latitudes showed that the model was reproducing the global structure of the region 1 currents accurately. It is further noted that the RMS differences leave room for significant improvement in the specification.
Extreme Ultraviolet Explorer spectra of the Io plasma torus: Improved spectral resolution and new results
Authors: Herbert Floyd , Gladstone G. Randall , Ballester Gilda E.
Published: January 2004

Spectral images of the Io plasma torus obtained from ∼165 hours of observation by the Extreme Ultraviolet Explorer (EUVE) spacecraft during the 124 encounter of the Galileo spacecraft with the torus and at an earlier epoch in 1996 are analyzed by a spectral‐resolution‐enhancing deconvolution algorithm in order to estimate averaged torus properties. Because of our analysis's improved spectral resolution, we are better able to separate lines arising from different ion species as compared to earlier EUVE studies. We exploit this capability to estimate emission‐weighted whole‐torus averages of electron temperature and ion fraction. Comparing the 1996 results for the torus to those of 1999, we estimate that (1) the electrons were hotter (5.4 versus 4.6 eV), (2) the luminosity was higher (375 versus 245 GW in the 350–730 Å region), (3) the mass was lower (by 10–15%), (4) the sulfur/oxygen ratio was higher (0.8 versus 0.5), and (5) the ionization level was greater (1.2 versus 0.8 for [S++]/[S+], 0.22 versus 0.15 for [O++]/[O+]) in the 1996 observation than during 124. Considering the constraints of formation, chemistry, transport, and mass loss, the torus has fewer degrees of freedom than observables, so these two snapshots give us clues as to how torus characteristics may covary. For example, our data are consistent with the hypothesis that electron temperature and density are anticorrelated. We also confirm that the Io‐correlated enhancement of torus brightness discovered by Voyager still persists. However, we raise new doubts that the torus's dawn‐dusk brightness asymmetry is the result of torus mass loss that is controlled by the total mass of the torus. We also estimate corrections to the electron impact collision strengths for many EUV multiplets of interest for understanding the torus and other sulfur and oxygen plasmas of astrophysical interest, finding, for example, that the O+/S++ 484 Å feature is about an order of magnitude brighter than that predicted by available electron collision strength data.
Passage through lo's ionospheric plasmas by the Galileo spacecraft
Authors: Frank L. A. , Paterson W. R.
Published: January 2004

On February 22, 2000, the Galileo spacecraft passed by the moon Io at a closest approach distance of 206 km. This altitude was sufficiently low that the plasma analyzer observed the three‐dimensional ion velocity distributions as functions of energy/charge (E/Q) at the top of Io's ionosphere. The ionospheric ion distributions consisted of two populations, a warm distribution with density and temperature of ∼8000 cm−3 and 10,000 K and a cool population with 3000 cm−3 and 2300 K. This cooler temperature is in the range of those observed remotely for some volcanic plumes. The bulk speed of these ions was 2 km s−1 with respect to Io's surface. In order to identify the mass/unit charge (M/Q) of the ionospheric ions, the E/Q spectra of the pickup ions in this region were examined. The most probable M/Q of the primary population of these ions was inferred to be 64. Remote spectroscopic observations of Io's surface and atmosphere suggest that these ions are S2+ and/or SO2+, although SO+ and SO3+ are not eliminated as possibilities. There is some evidence for lesser densities of heavier ions such as S3+ and S4+. The densities, temperatures, and bulk flow velocities of torus ions were measured as the spacecraft approached Io from the upstream direction. A combination of fits to the E/Q spectra observed with Galileo, the determinations of the M/Q of the primary ions with the plasma instrumentation on this spacecraft during other passages, and the previous identification of the primary ions with Voyager 1 plasma and remote observations find the following primary composition for the unperturbed torus near Io: O++ (50 cm−3, 30 eV), O+ (200 cm−3, 30 eV), S++ (400 cm−3, 90 eV), and S+ (100 cm−3, 90 eV). At a radial distance from Io of ∼19,700 km (10.9 Io radii), changes in the torus ion density, temperature, and bulk flow velocity provide evidence that a cloud of neutral gases is co‐orbiting with Io and is providing a substantial interaction with the torus ions.
Emission profiles of neutral oxygen and sulfur in Io's exospheric corona
Authors: Wolven B. C. , Moos H. W. , Retherford K. D. , Feldman P. D. , Strobel D. F. , Smyth W. H. , Roesler F. L.
Published: January 2004

Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) observations of Io acquired in 1997 [Roesler et al., 1999] provided the first simultaneous spatially resolved measurements of emission from neutral sulfur and oxygen, the dominant atomic species in Io's exospheric corona. Previous measurements of Io's corona relied primarily on sunlight resonantly scattered from sodium, a trace element in Io's atmosphere, and required measurement during mutual satellite eclipses to obtain the necessary spatial resolution. We present here spatial profiles of Io's extended emissions derived from observations spanning the time period from October 1997 to February 2000. The STIS Far Ultraviolet Multi‐Anode Microchannel Array (FUV‐MAMA) detector permits measurement of the emissions with a spatial resolution of ∼0.05 Io radii out to distances of ∼20 Io radii. Useful measurements are limited to ∼10 Io radii owing to the low signal‐to‐noise ratio of the extended emission features. The coronal emission profiles vary considerably in slope and intensity and are generally brighter for Io west (duskside) of Jupiter. Emission profiles obtained near western elongation are relatively symmetric about Io; profiles obtained in other orbital positions display varying degrees of asymmetry, with enhanced emissions and generally steeper slopes in the downstream direction relative to the plasma flow. The downstream‐upstream profile asymmetry is thought to be caused by higher electron densities in Io's plasma wake. While the coverage of the data is limited in both Jovian System III coordinates and geocentric phase, the intensities of emission from regions both near Io and in the extended corona vary with System III longitude in a near‐simultaneous fashion, suggesting local torus electron density as the probable source of this modulation. The observed ratio of oxygen to sulfur emission is relatively constant in time, perhaps reflecting the stoichiometric ratio of the SO2 source molecules. Eclipse and posteclipse observations on February 25, 2000, show a dramatic increase in profile emission brightness and slope, suggesting a dynamic response by a sublimation‐supported component of Io's SO2 atmosphere and associated atomic species.