bluuebunny/crossref_metadata_2025 huggingface dataset:
On the role of charge exchange in the formation of the Martian magnetic pileup boundary
Authors: Chen Y. , Cloutier P. A. , Crider D. H. , Mazelle C. , Rème H.
Published: February 2004

In the magnetic pileup boundary (MPB) of Mars a steep increase in the magnitude of the magnetic field is observed in the data from the magnetometer on board Mars Global Surveyor, and the growth of the field is more precipitous than common solar wind‐ionosphere models predict. Analysis of both Venus and Mars magnetic field data has strongly implied that plasma pressure is exchanged for magnetic pressure in the magnetic pileup field. To explain this, effects of the Martian exosphere must be included. Then charge exchange between the shock‐compressed solar wind protons, which dominate the thermal pressure, and neutral exospheric atoms (mainly H and O) should play a crucial role in the formation of the MPB, as is also inherently required by the conservation of momentum flux. To test this idea, a model of the solar wind interaction with the Martian upper atmosphere, with charge exchange included, is constructed to reproduce the observed magnetic field of the MPB of Mars. The simulation results show that charge exchange is able to account for the sudden buildup of the magnetic field in the Martian MPB, with the implication that the same mechanism produces the magnetic field increase at Venus and comets.
Solar wind plasma parameters on Ulysses: Detailed comparison between the URAP and SWOOPS experiments
Authors: Issautier Karine , Skoug R. M. , Gosling J. T. , Gary S. P. , McComas D. J.
Published: February 2004

The presence of several instruments measuring plasma parameters aboard Ulysses made possible an extensive comparison between them. In this paper, we focus on solar wind electron parameters measured by the Unified Radio and Plasma (URAP) wave receiver, using the thermal noise spectroscopy method, and by the Solar Wind Observations Over the Poles of the Sun (SWOOPS) electron and ion analyzers. We compare the data sets acquired by both experiments during the ecliptic path in 1990/1991 and the first pole‐to‐pole fast transit of Ulysses in 1994/1995, near the maximum and minimum of solar activity, respectively. From the comparison, we find very good agreement between the total density derived by both experiments; the average offset between the two data sets differs by less than 5%. We find a systematic offset of 5% to 10% in the core electron temperature values depending on the solar wind flow speed. We present detailed observations obtained during the crossing of the stream interface on March 28, 1995, by Ulysses, when it returned to the high‐speed wind region in the northern hemisphere. We find that the core electron temperature increases around 40% at the interface, which is in agreement with the statistical result obtained using IMP plasma measurements at 1 AU. We point out that SWOOPS provides reliable ion diagnostics in addition to suprathermal (halo) electron parameters and yields fundamental information on particle velocity distributions, whereas the improved quasi‐thermal noise method provides total electron densities and core electron temperatures accurately.
Three‐fluid Ohm's law
Authors: Song P. , Gombosi T. I. , Ridley A. J.
Published: February 2004

We develop a three‐fluid theory, including electrons, ions, and neutral particles and their collisions, to describe the interaction between the magnetosphere, ionosphere, and thermosphere. We derive Ohm's law including the collisions between ions, electrons, and neutrals. In particular, our analysis includes the effects of electron‐ion collisions. The electron and ion equations are solved in a completely coupled manner. The complete form of three‐fluid Ohm's law in the plasma frame is obtained for a steady‐state, uniform, three‐fluid flow. This form of Ohm's law can be simplified to obtain the ideal MHD frozen‐in condition when all collisions are neglected and the generalized Ohm's law when the collisions with neutrals are neglected. In our form the neutral velocity in the conventional collisional Ohm's law is replaced by the plasma velocity. More physical insights can be obtained from this form of Ohm's law in the context of magnetosphere‐ionosphere coupling. This form describes the magnetosphere‐ionosphere coupling without involvement of the neutral wind. The ionospheric velocity continuously deviates from the electric drift velocity from high altitudes to low altitudes. In addition, the plasma momentum equation for steady‐state uniform three‐fluid flow is derived. It describes ionosphere‐thermosphere coupling. The Ohm's law in the neutral wind frame, or the conventional form of Ohm's law, can be derived by combining the Ohm's law in the plasma frame with the momentum equation. The conductivities become the same as the conventional ones under a few approximations. Combination of the two forms of Ohm's law provides the relationship between the electric field and the ionospheric velocity. When the neutral wind stays still, this relationship can be used to describe magnetosphere‐ionosphere coupling.
Atmospheric effects of precipitating energetic hydrogen atoms on the Martian atmosphere
Authors: Kallio Esa , Barabash Stas
Published: February 2004

The Martian atmosphere is under the influence of an intense flux of precipitating energetic (≲1 keV) hydrogen atoms. In the solar wind and in the magnetosheath, fast hydrogen atoms are produced by charge exchange between solar wind protons and the hydrogen corona. Atmospheric effects of the precipitating hydrogen atoms are thus manifestations of the direct interaction between solar wind protons and the planetary neutrals. A three‐dimensional Monte Carlo simulation model has been developed to study different atmospheric effects of the precipitating hydrogen atoms. The model is used to calculate the altitude profiles of the energy deposition rates, the ion production rates, and the photon emission rates at different solar zenith angles under low solar activity conditions. The peak loss and production rates under typical solar wind conditions caused by precipitating hydrogen atoms are estimated to be ∼1% of the corresponding peak values due to extreme ultraviolet radiation but comparable or larger than effects of H+ and O+ precipitation at low altitudes. The results indicate that a substantial part of the incoming particle and energy flux is scattered back from the Martian atmosphere.
Midcourse Space Experiment/Ultraviolet and Visible Imaging and Spectrographic Imaging limb observations of combined proton/hydrogen/electron aurora
Authors: Strickland D. J. , Bishop J. , Evans J. S. , Majeed T. , Cox R. J. , Morrison D. , Romick G. J. , Carbary J. F. , Paxton L. J. , Meng C.‐I.
Published: February 2004

Simultaneous measurements of auroral limb H Lyman α, H Balmer α, H Balmer β, N2+ 1 NG 391.4‐nm, and N2 2 PG 337.1‐nm emissions excited by combined proton/hydrogen/electron precipitation are reported. The data were recorded by the Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) spectrographic imagers on the Midcourse Space Experiment (MSX) satellite on November 10, 1996, while viewing a diffuse emission region during a limb‐scanning data collection event. Energy fluxes associated with the electron and proton/hydrogen precipitation components are estimated with one‐dimensional fits to selected limb profiles using a transport‐theoretic model. Spectral radiances (110–900 nm) are also presented. The data presented here are but one example of the large number of MSX/UVISI data sets that have been collected offering opportunities for scientific investigations of auroral emission phenomena and for retrieval of composition and particle precipitation parameters from optical remote sensing data.
Radio signatures of the origin and propagation of coronal mass ejections through the solar corona and interplanetary medium
Authors: Reiner M. J. , Kaiser M. L. , Bougeret J.‐L.
Published: February 2004

During a 16‐day period from February 5 to 20, 2000, a series of decametric‐to‐kilometric wavelength type II and type III radio events was observed by the WAVES radio experiment on board the Wind spacecraft. These radio events were related to observed coronal mass ejections (CMEs) and their associated flares. Each of the solar eruptive events was initiated by an intense, complex type III radio burst, which occurred within minutes of the liftoff on the CME. Some of the CMEs produced decametric‐hectometric (D‐H) type II radio emissions, which, when their frequency drift rates were sufficiently well defined, were used to provide a speed estimate. The complex type III and D‐H type II radio emissions gave an indication of the presence of a CME well before the CME was first observed in the coronagraph images. This series of CMEs also generated interplanetary (kilometric) type II radio emissions that tracked the CME‐associated shock through the interplanetary medium and established the terrestrial connection. Thus the various radio emissions associated with these solar eruptive events provided a global view of each entire Sun‐Earth connection event, from the initiation and liftoff of the CME at the Sun, to the propagation of the CME‐associated shock through the solar corona and interplanetary medium, to its arrival at 1 AU. Finally, we show that simultaneous Wind/Ulysses observations of the interplanetary type II radio emissions on February 9–10 provide important information on the nature of the type II emission, on the type II source locations, and on the radiation characteristics of the type II emissions. For example, these simultaneous observations clearly indicate that the sporadic nature of the type II radiation was intrinsic to the radio source region.
Io's equatorial spots: Morphology of neutral UV emissions
Authors: Retherford Kurt D. , Moos H. Warren , Strobel Darrell F. , Wolven Brian C. , Roesler Fred L.
Published: February 2004

The first observations of Io with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) showed that the brightest ultraviolet emissions come from localized regions near Io's equator, designated “equatorial spots.” This paper presents a detailed study of the location, shape, and brightness of the equatorial spots in near‐monochromatic images obtained using STIS in the first‐order long‐slit spectroscopy mode. This study provides evidence that the equatorial emissions are linked to the interaction between the Jovian magnetosphere and Io's atmosphere. The morphology of the equatorial spots reported here provides additional information on the nature of this complex electrodynamic interaction. We find the following principal results: the locations of the equatorial spots are correlated with the Jovian magnetic field orientation at Io, but with a relation that is not 1:1; the equatorial spots are centered 10°–30° longitude downstream from Io's sub‐Jovian longitude; the brightness of the emissions in this data set is correlated with Io's distance from the plasma torus centrifugal equator; and the anti‐Jovian equatorial spots are ∼20% brighter than the sub‐Jovian equatorial spots.
Particle injections with auroral expansions
Authors: Liou K. , Meng C.‐I , Newell P. T. , Lui A. T. Y. , Reeves G. D. , Belian R. D.
Published: February 2004

We compare the onset of dispersionless energetic particle injections, observed as a sudden increase of energetic (tens to hundreds of keV) electron and ion fluxes on a timescale of ∼1 min, with the start of auroral breakups. A total of 34 dispersionless injections observed by Los Alamos National Laboratory (LANL) satellites are analyzed, and their corresponding auroral breakups are determined with global auroral images acquired from the Polar ultraviolet imager. An important finding is that dispersionless injections can actually be associated with substorm intensification. The injection time at LANL relative to the start of auroral breakups varies from −2 to 8 min and can sometimes be more than 10 min. The average lag time for the injections compared to the auroral breakups is 1.8 min with a standard deviation of 2.5 min. It is suggested that particle energization must take place in the magnetotail ∼l min earlier than the start of the explosive auroral substorm onset, while the delay of the injections at LANL is due to a propagation effect. An implied average earthward injection boundary is estimated to be ∼ 6.9 – 9.2 RE. Further analysis of the delay time indicates that the transport of substorm injections is associated with the enhancement of convection electric field by a factor of ∼5, corresponding to an earthward convection flow speed of 5 – 120 km s−1. Dispersionless injections can take place in a fairly wide magnetic local time (MLT) region from 2000 to 0100 MLT with a peak at 2200 MLT, where auroral breakups occur most frequently. More importantly, dispersionless injections have ionospheric footprints clustered around the location of auroral breakup within ±1 hour of MLT, further supporting the concept of the close relationship between the substorm injections and the auroral breakups.
Polar/Toroidal Imaging Mass‐Angle Spectrograph survey of earthward field‐aligned proton flows from the near‐midnight tail
Authors: Lennartsson O. W. , Trattner K. J. , Collin H. L. , Peterson W. K.
Published: February 2004

Data from the Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) ion mass spectrometer on the Polar satellite, covering 15 eV e−1 to 33 keV e−1 in energy and essentially 4 π in view angles, are used to investigate the properties of earthward (sunward) field‐aligned flows of ions, especially protons, in the plasma sheet‐lobe transition region near local midnight. A total of 142 crossings of this region are analyzed at 12‐s time resolution, all in the Northern Hemisphere, at R(SM) ∼4–7 RE, and most (106) in the poleward (sunward) direction. Earthward proton flows are prominent in this transition region (>50% of the time), typically appearing as sudden “blasts” with the most energetic protons (∼33 keV) arriving first with weak flux, followed by protons of decreasing energy and increasing flux until either (1) a new “blast” appears, (2) the flux ends at a sharp boundary, or (3) the flux fades away within a few minutes as the mean energy drops to a few keV. Frequent step‐like changes (<12 s) of the flux suggest that perpendicular gradients on the scale of proton gyroradii are common. Peak flux is similar to central plasma sheet proton flux (105–106 (cm2 sr s keV/e)−1) and usually occurs at E ∼ 4–12 keV. Only the initial phase of each “blast” (∼1 min) displays pronounced field alignment of the proton velocity distribution, consistent with the time‐of‐flight separation of an otherwise more or less isotropic distribution (at Polar altitude) with dƒ/dv < 0. The temporal dispersive signatures are often consistent with a source at R(SM) ≤ 30 RE. No systematic latitudinal velocity dispersion is found, implying that the equatorial plasma source is itself convecting. In short, the proton “blasts” appear as sudden local expansions of central plasma sheet particles along reconfigured (“dipolarized”) magnetic field lines.
The global efficiency of relativistic electron production in the Earth's magnetosphere
Authors: Baker D. N. , Kanekal S. G. , Blake J. B. , Pulkkinen T. I.
Published: February 2004

Data from sensors on board the Polar, Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), and other Earth‐orbiting spacecraft show that high‐energy electron intensities vary in a global, coherent way throughout the outer trapping zone of the terrestrial magnetosphere. Long‐term measurements of very energetic trapped electrons provide a comprehensive view of the large variability of intensities on timescales less than 1 day to times corresponding to the 11‐year sun‐spot cycle. The average density of relativistic electrons within the outer magnetospheric trapping region (2.5<L<6.5) is calculated in this paper during a 7‐year observing period. The total energy content and power dissipation in this relativistic electron component are then compared on a yearly basis to the solar wind energy that impacts the magnetosphere during the same time. The overall efficiency of relativistic electron production is thereby computed and is found consistently to be ∼1% throughout the entire observing interval. Such efficiencies are important and potentially useful in scaling electron acceleration in other, more remote solar, planetary, and cosmic systems. The results presented here suggest that a range of different processes are involved in producing very energetic electrons in various astrophysical plasmas.
Electrodynamics of the poleward auroral border observed by Polar during a substorm on April 22, 1998
Authors: Ober Daniel M. , Maynard N. C. , Burke W. J. , Peterson W. K. , Sigwarth J. B. , Frank L. A. , Scudder J. D. , Hughes W. J. , Russell C. T.
Published: February 2004

Observations from Polar during a substorm on April 22, 1998, are used to specify electrodynamic characteristics of the high‐latitude auroral boundary on the nightside. Polar was moving equatorward near invariant latitude 72°, 2305 magnetic local time as it crossed the auroral boundary near the end of the substorm's expansion phase. This boundary was marked by severe east‐west plasma flow shears, a reversal of the in‐track electric field component, and multiple field‐aligned currents. Harmonizing ground measurements with auroral images and in situ particle and field data from Polar reveals five electrodynamic features of the boundary. (1) A 20‐min delay occurred between substorm onset and when the total magnetic flux in the polar cap began to decrease. This represents the time that elapsed before reconnection of open lobe flux began along a near‐Earth X‐line. (2) The reconnection electric field at the ionospheric projection of the X‐line ranged between 20 and 70 mV m−1. Reconnection was intermittent, turning on and off at different locations. (3) Electric and magnetic field structures observed by Polar suggest that Alfvén waves propagating along the auroral boundary carried a double‐layer current. Downward Poynting flux was observed at the poleward auroral boundary associated with these currents. (4) Magnetic and electric field oscillations with periods of ∼90 s were detected on open field lines beginning ∼4 min before Polar entered the auroral oval. Oscillations with similar frequencies were observed both on the ground near Polar's magnetic footprint and at geosynchronous orbit. This indicates that the oscillations represent a large‐scale phenomenon occurring over a large portion of the nightside magnetosphere. Coupling on open field lines derives from fringing fields associated with ionospheric closure of DP 1 currents. (5) Upward flowing hydrogen and oxygen ions were detected at and equatorward of the auroral boundary. Perpendicularly accelerated O+ ions detected in the immediate vicinity of the boundary can be explained by direct acceleration by the ambient electric field perpendicular to the local magnetic field. Equatorward of the boundary, O+ distributions were typical of ion conics.
Polar Ultraviolet Imager observations of global auroral power as a function of polar cap size and magnetotail stretching
Authors: Newell Patrick T. , Liou Kan , Sotirelis Thomas , Meng Ching‐I.
Published: February 2004

The Polar Ultraviolet Imager (UVI) instrument can quantitatively determine important magnetospheric descriptors, notably including substorm onset time and global auroral output. Previous research has related the input variables of the magnetospheric system, namely solar wind parameters, to various output variables. However, a complex system such as the magnetosphere includes, in addition to inputs and outputs, state variables. Polar cap flux and magnetotail stretching are two such that can be estimated from the Defense Meteorological Satellite Program (DMSP) series satellites. We herein determine that both polar cap flux, Φc, and the magnetotail stretching index, b2i, do correlate well with 40‐min averages of nightside auroral power observed by UVI. There were a total of n = 638 distinct 40‐min intervals within which b2i, Φc, and nightside auroral power could be determined. The correlations with premidnight auroral power were r = 0.72 for Φc and r = −0.76 for b2i. The multiple‐correlation coefficient of these two variables with nightside auroral power was 0.81. These sample correlations are far better than the sample correlations of solar wind input variables to nightside auroral power. Thus accurate space weather forecasting can demonstrably benefit greatly by monitoring current magnetospheric state variables (nowcasting), rather than attempting to reproduce output variables solely from solar wind inputs. Attempts to predict substorm onsets were less successful. Although the average polar cap flux prior to onset is larger than normal, the difference is not large enough to have significant predictive capability. Specifically, polar cap flux averaged 404 ± 133 and 422 ± 148 MWb for the entire years of 1996 and 1997, respectively, while the polar cap flux at the time of substorm onset averaged 455 ± 143 MWb.
Numerical model of the plasma sheath generated by the plasma source instrument aboard the Polar satellite
Authors: Singh N. , Leung W. C. , Moore T. E. , Craven P. D.
Published: February 2004

The plasma sheath generated by the operation of the Plasma Source Instrument (PSI) aboard the Polar satellite is studied by using a three‐dimensional particle‐in‐cell (PIC) code. When the satellite passes through the region of low‐density plasma, the satellite charges to positive potentials as high as 40–50 V, owing to the photoelectron emission. In such a case, ambient core ions cannot accurately be measured or detected. The goal of the onboard PSI is to reduce the floating potential of the satellite to a sufficiently low value so that the ions in the polar wind become detectable. When the PSI is operated, ion‐rich xenon plasma is ejected from the satellite, such that the floating potential of the satellite is reduced and is maintained at ∼2 V. Accordingly, in our three‐dimensional PIC simulation we considered that the potential of the satellite is 2 V as a fixed bias. Considering the relatively high density of the xenon plasma in the sheath (∼10–103 cm−3), the ambient plasma of low density(<1 cm−3) is neglected. In the simulations the electric fields and plasma dynamics are calculated self‐consistently. We found that an “apple”‐shape positive potential sheath forms surrounding the satellite. In the region near the PSI emission a high positive potential hill develops. Near the Thermal Ion Dynamics Experiment detector away from the PSI, the potentials are sufficiently low for the ambient polar wind ions to reach it. In the simulations it takes only about a couple of tens of electron gyroperiods for the sheath to reach a quasi steady state. This time is approximately the time taken by the heavy Xe+ ions to expand up to about one average Larmor radius of electrons from the satellite surface. After this time the expansion of the sheath in directions transverse to the ambient magnetic field slows down because the electrons are magnetized. Using the quasi steady sheath, we performed trajectory calculations to characterize the detector response to a highly supersonic polar wind flow. The detected ions' velocity distribution shows significant deviations from a shifted Maxwellian in the ambient polar wind population. The deviations are caused by the effects of electric fields on the ions' motion as they traverse the sheath.
Characterization of the active current system around the TSS‐IR satellite through magnetic field measurements taken by the Tethered Magnetometer instrument
Authors: Mariani F. , Orsini S. , Candidi M. , Marcucci M. F. , Acuña M. , Musmann G. , Ness N. F.
Published: February 2004

In this paper we present the measurements and interpretations of the Tethered Magnetometer (TEMAG) experiment flown on the Tethered Satellite Mission Reflight TSS‐IR. Data were taken during the active operating phase when current steps of variable amplitude were injected along the electrically conducting tether. The magnetic field shows clear signatures in the components and amplitude, dependent upon the orientation of the magnetometer boom with respect to the spacecraft ram velocity vector. In the eight events studied, current effects could be observed in the magnetic field data only when the boom was pointing toward the ram side hemisphere of the satellite. The picture emerging from the observed results suggests the formation of a current sheet on the ram side, possibly associated with the aerodynamic shock and compression of the geomagnetic field ahead of the satellite. Although the limited data do not allow full description of the phenomenology, they place important observational constraints on any theoretical model.
Investigation of the conjugacy between auroral breakup and energetic electron injection
Authors: Lazutin L. L. , Borovkov L. P. , Kozelova T. V. , Kornilov I. A. , Tagirov V. R. , Korth A. , Stadsnes J. , Ullaland S.
Published: February 2004

Auroral TV observations at Loparskaya, Kola Peninsula, and CRRES energetic electron and proton measurements during a moderate isolated substorm at 2000–2100 UT on February 13, 1991, are compared. The CRRES was at the apogee of orbit 494 in the outer radiation belt near the magnetic equator. CRRES footprint projection estimated by using a Tsyganenko magnetic field model was situated at the same latitudes several degrees eastward of active aurora during the substorm breakup or the intensification. The intensification consists of several activations of 1‐min duration, and this fine spatial and temporal structure is important for the conjugacy investigations. It is shown that injected energetic electrons have been accelerated at the same time as one of the auroral activations at the same latitude and approximately the same azimuthal extended region. The lack of enhanced electrons during other activations suggests that the acceleration region has a sharp equatorial boundary.
Cusp field‐aligned currents and ion outflows
Authors: Strangeway R. J. , Russell C. T. , Carlson C. W. , McFadden J. P. , Ergun R. E. , Temerin M. , Klumpar D. M. , Peterson W. K. , Moore T. E.
Published: February 2004

On September 24 and 25, 1998, the Polar spacecraft observed intense outflows of terrestrial ions in association with the passage of an interplanetary shock and coronal mass ejection. The orbit of the Fast Auroral Snapshot (FAST) Explorer was in the noon‐midnight meridian during this ion outflow event, and FAST passed through the day side cusp region at ∼4000 km altitude every 2.2 hours. FAST was therefore able to monitor the ion outflows subsequently observed by Polar. We show that while the outflows were more intense after the shock passage, the overall particle and field signatures within the cusp region were qualitatively similar both before and after the shock passage. FAST observations show that the cusp particle precipitation marks the lower latitude leg of a pair of field‐aligned currents and further, that both field‐aligned current sheets appear to be on open field lines. Moreover, the polarity of the cusp currents is controlled by the polarity of the interplanetary magnetic field (IMF) y‐component, such that the magnetic field perturbation associated with the pair of cusp currents is in the same direction as the IMF By. This is a consequence of the reconnection of cusp‐region field lines at the magnetopause, with the flux transport resulting in electromagnetic energy being transmitted along field lines to the ionosphere as Poynting flux. We show that this Poynting flux can be as high as 120 mW m−2 (120 ergs cm−2 s−1) at FAST altitudes (∼500 mW m−2 at ionospheric altitudes), presumably because of the strong IMF By (∼40 nT), and is the dominant energy input to the cusp‐region ionosphere. Furthermore, we find that the peak ion outflow flux is correlated with the peak downward Poynting flux, although only a few passes through the cusp centered around the time of the shock passage were used to determine this correlation. The energy carried by Poynting flux is dissipated as heat within the ionosphere, through Joule dissipation. The heating will tend to increase the ionospheric scale height, allowing greater access of ionospheric ions to the altitudes where transverse ion heating via ELF waves can occur. Thus electromagnetic energy supplied by the transport of reconnected magnetic flux is the essential first step in a multistep process that enhances the outflow of ionospheric plasma in the dayside cusp.
Collisional interactions of precipitating energetic neutral atoms with upper‐atmospheric particles in the low‐latitude region
Authors: Asamura K. , Mukai T. , Saito Y. , Kazama Y. , Machida S.
Published: February 2004

Altitude profiles of energetic neutral atom (ENA) fluxes precipitating into the upper atmosphere are simulated numerically by using a Monte Carlo method with source magnetospheric protons distributed between L shells 4 and 7, taking account of collisional interactions with atmospheric particles. Numerical results are in reasonable agreement with the profile of ∼10keV neutral atom fluxes recently measured under a geomagnetically quiet condition. Through collisional interactions, ENAs can be converted to ions and then the ionized ENAs can be reneutralized and repeat this cycle. Therefore the precipitating ENAs lose their original directionality because of gyromotions of the ionized ENAs. However, the source information is still seen in ENA data measured at low altitudes where collisions are significant, because their pitch angles are almost conserved with the forward scattering approximation for each collision in the case of parallel geomagnetic fields at low altitudes. On the other hand, a part of ENAs precipitating into the atmosphere returns to the magnetosphere. These reflected ENAs act as a secondary source that cannot be neglected in ENA data measured in the magnetosphere.
Concerning the local time asymmetry of Pc5 wave power at the ground and field line resonance widths
Authors: Glassmeier Karl‐Heinz , Stellmacher Martin
Published: February 2004

A statistical analysis of transverse wave activity in the Pc 5 frequency band using satellite magnetic field data from a 6 month interval shows a symmetric distribution along the geostationary orbit in the magnetosphere, whereas observations on the ground display an asymmetric distribution with regard to the noon sector with maximum wave activity on the morningside. As a possible cause of these different local time variations of the wave activity, the ionospheric screening effect is discussed, which depends on the radial scale lengths or resonance width of the waves as well as on their azimuthal wave number. It is suggested in this study that a local time variation of the resonance width of the waves is caused by a variation of the radial gradient of the plasma density along the geostationary orbit, leading to a slowly varying efficiency of the ionospheric screening effect with local time. Local time variations of the resonance width thus will be of equal importance to explain the observed asymmetry as, for example, local time variations of wave sources causing local time variations of the azimuthal wave number.
Cosmic ray survey to Antarctica and coupling functions for neutron component near solar minimum (1996–1997): 2. Determination of meteorological effects
Authors: Iucci N. , Villoresi G. , Dorman L. I. , Parisi1 M.
Published: February 2004

In this paper we present an analysis of meteorological effects influencing the counting rate of a standard cosmic ray NM‐64 neutron monitor and bare neutron counters used during the latitude survey performed in 1996–1997 from Italy to Antarctica and back. In particular, we determine for the first time the influence of the Bernoulli effect on the evaluation of atmospheric mass and the effect of sea state strength on neutron counting rate, by using data recorded during the long period spent in the Antarctic region (vertical cutoff rigidity Rcp ≤ 1 GV). Atmospheric absorption coefficients have been computed accurately for the Antarctic region, as well as at different cutoff rigidities by combining previous findings together with our determinations at Rcp ≤ 1 GV and Rcp = 6.2 GV. We also estimate the temperature effect on the NM‐64 counting rate by using the sea level temperature during the expedition and the vertical distribution data of atmospheric temperature measured by satellites. Data corrections for all meteorological effects are discussed.
Geomagnetic negative sudden impulse due to a magnetic cloud observed on May 13, 1995
Authors: Takeuchi T. , Araki T. , Luehr H. , Rasmussen O. , Watermann J. , Milling D. K. , Mann I. R. , Yumoto K. , Shiokawa K. , Nagai T.
Published: February 2004

A geomagnetic negative sudden impulse (SI−) observed on May 13, 1995, was examined using magnetic field data from ETS VI and GOES 7 in the magnetosphere and ground geomagnetic observation networks. The SI− was caused by a sudden decrease in the solar wind dynamic pressure at the front boundary of a magnetic cloud embedded in a density enhancement region. The amplitude and the fall time at Kakioka geomagnetic observatory (magnetic latitude 26.9°, magnetic longitude 208.3°) were 26 nT and 10 min, respectively. Although SI− has been considered to be the mirror image of geomagnetic positive sudden impulse (SI+) or sudden commencement (SC), we found the polarization distribution of the SI− consistent with that of SC. We suggest that the contribution from the longitudinal movement of a twin‐vortex ionospheric current system is dominant to produce the polarization of SC and SI−. We also discuss the relationship between the angle at which discontinuities in the solar wind impinge upon the magnetosphere and the geomagnetic response.
Stratification of the F2 layer observed in Southeast Asia
Authors: Lynn K. J. W. , Harris T. J. , Sjarifudin M.
Published: February 2004

This paper describes the latitudinal variation in F2 stratification [Balan and Bailey, 1995] as observed by a number of oblique and vertical ionosondes operating in Southeast Asia during 1997. Stratification of the F2 layer was seen at dip latitudes from 4°S to 18°S on the southern side of the magnetic equator but did not occur at the closest reflection point to the magnetic equator (dip latitude = 2.3°N). The observed transient cusp (vertical ionosonde) or additional nose (oblique ionosonde) was defined as an F3 layer or an F1.5 layer depending on whether it occurred above or below the layer which maintained continuity with the normal F2 layer peak. Within the zone of occurrence, the transient layer was commonly seen as an F3 layer at reflection points closest to the magnetic equator but invariably as an F1.5 layer at reflection points farther from the magnetic equator. These observations suggest that the distortion in the equatorial electron density profile associated with the phenomenon moved toward the base of the F2 layer as magnetic field lines descended with increasing latitude. Stratification of the F2 layer commenced at the same local time (e.g., 0845 LT in November 1997) throughout the longitudinal range of coverage and was associated with a rapid rise in F2 layer height following sunrise. The stratification ended at times varying from 1300 LT to sunset and was associated with a fall in the height of F2 peak electron density. The region of maximum F2 layer stratification lay between the magnetic equator and the peak of the southern equatorial anomaly.
The state transition model of the substorm onset
Authors: Tanaka T.
Published: February 2004

The substorm mechanism is investigated from a resistive magnetohydrodynamic simulation under the assumption that the magnetotail becomes more diffusive as it goes further downtail. The simulation uses the finite volume total‐variation diminishing scheme on an unstructured grid system to evaluate the magnetosphere‐ionosphere coupling effect more precisely and to reduce the numerical viscosity in the near‐Earth plasma sheet. The calculation started from a stationary solution under a northward interplanetary magnetic field (IMF) condition with non‐zero IMF By. After a southward turning of the IMF the simulation results show the progress of plasma sheet thinning in the magnetosphere, together with increases in the size of the auroral oval and in the magnitude of the field aligned current (FAC) in the polar ionosphere. This thinning is promoted by the drain of closed flux from the plasma sheet occurring under the enhanced convection. In this stage the reclosure process in the plasma sheet which determines the flux piling up from the midtail to the near‐Earth plasma sheet is not so effective, since it is still controlled by the remnant of northward IMF. The substorm onset occurs as an abrupt change of pressure distribution in the near‐Earth plasma sheet and an intrusion of convection flow into the inner magnetosphere. After the onset the simulation results reproduce both the dipolarization in the near‐Earth tail and the near‐Earth neutral line (NENL) at the midtail, together with plasma injection into the inner magnetosphere and an enhancement of the nightside FAC. Dipolarization is hastened by a northward re‐turning of the IMF, indicating that it is triggered off through the breakdown of dynamic stress balance in the near‐tail plasma sheet established under the convection‐controlling tail thinning. It is concluded that the direct cause of the onset is the dipolarization, which is not a mere pileup of the flux ejected from the NENL but the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self‐organizing criticality. In this idea the NENL is a topologically controlled diffusive process and not directly responsible for the onset, although it constitutes an important part in the convection system necessary for the state transition. After the onset the simulation results show a thickening of the plasma sheet and a tailward retreating of the NENL. Under a continuously southward IMF this state with a thickened plasma sheet shows a signature of the steady magnetospheric convection event.
Propagation characteristics of auroral kilometric radiation observed by the MEMO experiment on Interball 2
Authors: Parrot M. , Lefeuvre F. , Rauch J. L. , Santolìk O. , Mogilevski M. M.
Published: February 2004

The MEMO experiment is a part of the Interball 2 wave consortium. It is connected to a total of six electric and nine magnetic independent sensors. It provides waveforms associated with the measurement of two to five components in the three frequency bands: ELF (5–1000 Hz), VLF (1–20 kHz), and LF (20–250 kHz). Waveforms of three magnetic components and one electric component recorded during observations of auroral kilometric radiation (AKR) allow a detailed study of the characteristics of these emissions. In particular, the wave normal directions of AKR relative to the Earth's magnetic field are determined using several methods: the classical methods based on the plane wave approximation [Means, 1972] and the wave distribution function method which represents the evaluation of the wave energy density distribution with respect to the angular frequency and the wave normal direction(s). One event is fully analyzed in this paper. It is shown that AKR propagates with a polarization quasi‐circular (ellipticity value ∼ 0.9), a right polarization (i.e., R‐X mode), and wave normals weakly oblique (∼30°).
Poleward progressing quasiperiodic disturbances at cusp latitudes: The role of wave processes
Authors: Pilipenko V. A. , Fedorov E. N. , Engebretson M. J. , Papitashvili V. O. , Watermann J. F.
Published: February 2004

Observations from various magnetometer networks, including Magnetomer Array for Cusp and Cleft Studies (MACCS), Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS), the Greenland Coastal Chain, and the U.S./Russia Antarctic Array, made on January 5, 1995, show an event of very long period pulsations (30‐ to 40‐min period) in the cusp region. Pulsations were driven by quasiperiodic Interplanetary Magnetic Field (IMF) By variations which were observed by the Wind spacecraft. Disturbances of this type are commonly interpreted as poleward moving, east–west oriented ionospheric currents (e.g., intensification of the DPY current system), stimulated by reconnection processes at the dayside magnetopause. However, the temporal/spatial, ground‐based structure of such disturbances can better be described if this picture is augmented by the inclusion of transient wave processes, for example, distortions of the Alfven phase front, transmitting a disturbance from an assumed reconnection region to the ionosphere. In general, this type of disturbance, which we suggest calling PDPY6 pulsations, is shown to be a manifestation of the modulation of the high‐latitude ionosphere electrodynamics by a large‐scale Alfven wave in the solar wind under favorable IMF orientation.
On the elastic and inelastic collisions between precipitating energetic hydrogen atoms and Martian atmospheric neutrals
Authors: Kallio Esa , Barabash Stas
Published: February 2004

Properties of elastic and inelastic processes in the Martian upper atmosphere caused by precipitating hydrogen atoms have been studied. The energetic (≲ 1 keV) neutral atoms, ENAs, are produced by the charge exchange process between the solar wind protons and the atoms in the Martian exosphere. The process gives a mechanism to deposit energy to the Martian atmosphere in addition to the well‐known EUV radiation. The energy deposition is studied by both a microscopic and a macroscopic model. The microscopic approach is based on Monte Carlo (MC) simulations which make it possible to study individual collisions and trajectories. The macroscopic approach utilizes the continuous slowing down approximation (CSDA). The CSDA equations are derived starting from the first principles in order to analyze the role of the backscattered particles. The MC and CSDA models are used to study how the energy deposition depends on the energy of precipitating ENAs and on the solar zenith angle. The role of the adopted scattering cross‐section models is analyzed, as well as differences between elastic and inelastic collisions. MC simulations suggest that the backscattered ENAs may play an important role in the energy deposition process. Comparisons between MC and CSDA models also illustrate that the CSDA model can give a relatively good approximation for the energy deposition profiles with a proper choice of the free parameters.
Compressional Pc5 pulsations as sloshing in the plasma sheet
Authors: Vaivads A. , Haerendel G. , Baumjohann W. , Nakamura R. , Kucharek H. , Georgescu E. , Klecker B. , Kistler L. M.
Published: February 2004

In the morningside plasma sheet close to the equatorial plane, satellites often observe magnetic field amplitude variations on timescales of several minutes (Pc5 range). Here we present an event study which shows that in some cases these variations can be a result of magnetospheric regions with different magnetic field amplitudes sloshing back and forth past the satellite. The sloshing is caused by toroidal Alfvén oscillations.
Properties of large electric fields in the plasma sheet at 4–7 RE measured with Polar
Authors: Keiling A. , Wygant J. R. , Cattell C. , Johnson M. , Temerin M. , Mozer F. S. , Kletzing C. A. , Scudder J. , Russell C. T.
Published: February 2004

Measurements from the Polar satellite provide evidence for large electric field structures in the plasma sheet at geocentric distances of 4–7 RE‐ These structures had amplitudes perpendicular to the ambient magnetic field that can exceed 100 m V m−1 (6 s averaged). Two years (from May 1, 1996, to April 30, 1998) of electric field data (EZ component, approximately along GSE z) were surveyed. The distribution in invariant latitude (ILAT) and magnetic local time (MLT) of large perpendicular electric field events (defined as ≥20 mV m−1 for a 6‐s average) delineates the statistical auroral oval with the majority of events occurring in the nightside centered around midnight and a smaller concentration around 1500 MLT. The magnitude‐versus‐altitude distribution of the electric fields between 4 and 7 RE in the nightside could be explained by models which assume either shear Alfvén waves propagating into regions of larger background magnetic fields or electrostatic structures being mapped quasi‐statically along equipotential magnetic field lines. In addition, this survey yielded 24 very large amplitude events with |E⊥| ≥100 mV m−1 (6 s averaged), all of which occurred in the nightside. In the spacecraft frame, the electric field structures occurred on timescales ranging from 10 to 60 s. About 85% of these events occurred in the vicinity of the outer boundary of the plasma sheet; the rest occurred in the central plasma sheet. The polarity of the electric fields was dominantly perpendicular to the nominal plasma sheet boundary. For a large fraction of events (≤50%) the ratios of electric and magnetic fields in the period range from 10 to 60 s were consistent with Alfvén waves. Large Poynting flux (up to 2.5 ergs cm−2s−1) dominantly directed downward along the background magnetic field was associated with 21 events. All 24 events occurred during geomagnetic disturbances such as magnetic substorms. A conjugate study with ground stations for 14 events (out of the 24 events) showed that these structures occurred during times of rapid changes in the H component (or X component) of magnetometer data. For most events this time corresponded to the expansion phase; two events occurred during a quick recovery of the negative H bay signature. Thus there is evidence that large electromagnetic energy transfer processes in the plasma sheet occur during the most dynamic phase of geomagnetic disturbances. From the statistical analysis it was found that Polar observed events larger than 100 mV m−1 (50 mV m−1) in the plasma sheet between 2100 and 0300 MLT with a 2–4% (15%) probability per crossing. These probabilities will be compared to the probability of substorm occurrence during Polar plasma sheet crossings.
Optimization of the neural‐network geomagnetic model for forecasting large‐amplitude substorm events
Authors: Gavrishchaka Valeriy V. , Ganguli Supriya B.
Published: February 2004

Artificial neural networks (NN) have been used to model solar wind‐driven auroral electrojet dynamics, with emphasis on the reliable real‐time forecasting of auroral electrojet activity (theAEindex) from solar wind input. Practical limitations of the NN‐based models used earlier are clarified. These include the inability to accurately predict large‐amplitude substorm events, which is the most important feature for many applications. A novel technique for improving predictions is suggested based on application‐specific threshold mapping and symbolic encoding of theAEindex. This approach allows us to disregard relatively unimportant details of small‐amplitude perturbations and effectively improve forecasting of large‐amplitude events. Results from our new model imply that application‐oriented optimization of the real‐time substorm forecasting system can be an important factor in the overall improvement of the prediction accuracy.