bluuebunny/crossref_metadata_2025 huggingface dataset:
A 10 year intercomparison between collocated Special Sensor Microwave Imager oceanic surface wind speed retrievals and global analyses
Authors: Meissner T. , Smith D. , Wentz F.
Published: February 2004

To evaluate the scalar ocean surface wind speeds obtained from the Special Sensor Microwave Imager (SSM/I), we compare them over the time period from July 1987 through December 1997 with those from two global analyses: the National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Annual Reanalysis and the European Center for Medium‐Range Weather Forecasts (ECMWF)/Tropical Ocean‐Global Atmosphere Global Surface Analysis. We perform a statistical analysis for the whole globe and present time series analyses for selected geographical regions in connection with collocated wind speed difference maps. In order to evaluate further geographical biases observed in the SSM/I versus analyses comparisons we use wind speeds from the NASA scatterometer (NSCAT) for the 10 month period from September 1996 through June 1997 as a third data source. The value of the standard deviation for all collocated SSM/I ‐ ECMWF wind speed differences is 2.1 m s−1 and for all collocated SSM/I ‐ NCEP/NCAR reanalyis wind speed differences is 2.4 m s−1. When taking monthly or yearly averages in each pixel, which has the effect of cancelling out small timescale wind speed fluctuations, the values are between 0.8 and 1.2 m s−1, respectively. Global biases range between −0.05 and +0.55 m s−1 for the various SSM/I satellites. Our analysis allows us to identify regional biases for both the SSM/I and analyses winds. The NCEP/NCAR reanalysis wind speeds appear underestimated in the tropical Pacific and tropical Atlantic. ECMWF wind speeds appear underestimated near the southern Pacific islands NE of Australia. The analyses wind speeds are higher than the SSM/I wind speeds near the Argentinean coast. The SSM/I wind speeds appear high in the extratropical central and eastern Pacific and low in certain coastal regions with eastern boundary currents and in the Arabian Sea. The size of some of these biases are seasonally dependent.
Global and regional axial ocean angular momentum signals and length‐of‐day variations (1985–1996)
Authors: Ponte Rui M. , Stammer Detlef
Published: February 2004

Changes in ocean angular momentum M about the polar axis are related to fluctuations in zonal currents (relative component Mr) and latitudinal shifts in mass (planetary component MΩ). Output from a 1° ocean model is used to calculate global Mr, MΩ, and M time series at 5 day intervals for the period January 1985 to April 1996. The annual cycle in Mr, MΩ, and M is larger than the semiannual cycle, and MΩ amplitudes are nearly twice those of Mr. Year‐to‐year modulation of the seasonal cycle is present, but interannual variability is weak. The spectrum of M is red (background slope between ω−1 and ω−2) at subseasonal periods, implying a white or blue spectrum for the external torque on the ocean. Comparisons with previous studies indicate the importance of direct atmospheric forcing in inducing subseasonal M signals, relative to instabilities and other internal sources of rapid oceanic signals. Regional angular momentum estimates show that seasonal variability tends to be larger at low latitudes, but many local maxima exist because of the spatial structure of zonal current and mass variability. At seasonal timescales, latitudes ∼20°S–10°N contribute substantial variability to MΩ, while signals in Mr can be traced to Antarctic Circumpolar Current transports and associated circulation. Variability in M is found to be small when compared with similar time series for the atmosphere and the solid Earth, but ocean signals are significantly coherent with atmosphere‐solid Earth residuals, implying a measurable oceanic impact on length‐of‐day variations.
Last glacial benthic foraminiferal δ18O anomalies in the polar North Atlantic: A modern analogue evaluation
Authors: Bauch Dorothea , Bauch Henning A.
Published: February 2004

Modern processes are evaluated to understand the possible mechanisms behind last glacial benthic foraminiferal δ18O anomalies that occurred concurrent with meltwater events in the polar North Atlantic; such anomalies in the Nordic seas were recently interpreted to be caused by brine formation. Despite intensive sea‐ice production on circumarctic shelves, modern data show that brines ejected from sea‐ice formation containing low δ18O water do not significantly contribute to deep waters in the Arctic Ocean today. Assuming that this process was, nevertheless, responsible for δ18O anomalies in Nordic seas deep water during the last glaciation, a broad, shallow shelf area adjacent to the Nordic seas, such as the Barents Sea, had to be seasonally free of sea‐ice in order to serve as an area for brine formation. Another process which may explain δ18O‐depleted water at depth is found in the Weddell Sea today, where a low δ18O signal in deep waters originates from ice shelf interactions. If temperature were considered the main mechanism for the low benthic δ18O values, an increase of 4°C must have occurred in the deep water. An analogous situation with a reversed water temperature pattern due to a subsurface inflow of warm Atlantic water is found today in the eastern Arctic Ocean, and deep water warming is observed in the Greenland Gyre in the absence of deep convection. Because paleoproxy data also indicate an Atlantic water inflow into the Nordic seas during such benthic δ18O anomalies, temperature as a principal mechanism of changing δ18O cannot be excluded.
On the global oxygen anomaly and air‐sea flux
Authors: Garcia Hernan E. , Keeling Ralph F.
Published: February 2004

We present a new climatology of monthly air‐sea oxygen fluxes throughout the ice‐free surface global ocean. The climatology is based on weighted linear least squares regressions using heat flux monthly anomalies for spatial and temporal interpolation of historical O2 data. The seasonal oceanic variations show that the tropical belt (20°S‐20°N) is characterized by relatively small air‐sea fluxes when compared to the middle to high latitudes (40°–70°). The largest and lowest seasonal fluxes occur during summer and winter in both hemispheres. By means of an atmospheric transport model we show that our climatology is in better agreement with the observed amplitude and phasing of the variations in atmospheric O2/N2 ratios because of seasonal air‐sea exchanges at baseline stations in the Pacific Ocean than with previous air‐sea O2 climatologies. Our study indicates that the component of the air‐sea O2 flux that correlates with heat flux dominates the large‐scale air‐sea O2 exchange on seasonal timescales. The contribution of each major oceanic basin to the atmospheric observations is described. The seasonal net thermal (SNOT) and biological (SNOB) outgassing components of the flux are examined in relation to latitudinal bands, basin‐wide, and hemispheric contributions. The Southern Hemisphere's SNOB (∼0.26 Pmol) and SNOT (∼0.29 Pmol) values are larger than the Northern Hemisphere's SNOB (∼0.15 Pmol) and SNOT (∼0.16 Pmol) values (1 Pmol = 1015 mol). We estimate a global extratropical carbon new production during the outgassing season of 3.7 Pg C (1 Pg = 1015 g), lower than previous estimates with air‐sea O2 climatologies.
On the excitation of large aspect angle Farley‐Buneman echoes via three‐wave coupling: A dynamical system model
Authors: Hamza A. M. , Imamura H.
Published: January 2004

Linear and quasi‐linear theories of Farley‐Buneman waves predict that coherent scatter can only be observed at aspect angles of about 1° or less. Coherent echoes at large aspect angles, with values greater than the linear upper bound, are not very well understood theoretically. Radar observations have revealed the existence of such plasma irregularities at aspect angles as large as 10° and larger (Millstone Hill and European Incoherent Scatter). In this paper we suggest a possible mechanism capable to excite Farley‐Buneman waves at large aspect angles, which are otherwise damped according to the known conventional models. The proposed model is based on a nonlinear mode‐coupling mechanism channeling energy between waves at small aspect angles (linearly unstable) and ones at large aspect angles (linearly stable and damped). We studied the nonlinear coupling of Farley‐Buneman waves with significant parallel components. We are able to show that the interaction of the three modes can be very selective but very efficient at the same time. It is also found that the finite aspect angle effects have a profound impact on the saturation levels of Farley‐Buneman turbulence in the auroral region, a subject of further study indeed.
Processes associated with particle transport in corotating interaction regions and near stream interfaces
Authors: Intriligator D. S. , Jokipii J. R. , Horbury T. S. , Intriligator J. M. , Forsyth R. J. , Kunow H. , Wibberenz G. , Gosling J. T.
Published: February 2004

Pioneer and Ulysses observations of energetic particles and solar wind plasma and magnetic fields are analyzed to study particle transport in corotating interaction regions (CIRs) and near stream interfaces (SIs). Energetic particle diffusion with a lowered coefficient near the SIs as compared with the free solar wind may account for the energetic particle profiles in CIRs and for the modulation of Jovian electrons. Our analyses include examination of the magnetic power and variances perpendicular and parallel to the magnetic field for a range of wave numbers and for a range of conditions within two CIRs. Since the magnetic field immediately on either side of a SI, but near it, is not connected to either the forward shock or reverse shock, it is the cross‐field motion of the energetic particles that is relevant. Since the cross‐field transport is thought to be related to field line mixing, or random walk, we examined magnetic field fluctuations normal to the average field to determine whether any signature of reduced perpendicular particle transport could be found. Evidence consistent with reduced particle transport near the SI was indeed found when we examined time series of the 1‐min averages of the magnetic field components. A planar magnetic structure was associated with the trailing unshocked layer following the SIs (i.e., SI to SI + 12 hours) and also with the entire CIRs, but it was not present in the free solar wind. We have quantitatively examined the effects of shear and compression on particle transport and conclude that the effects of shear and compression reduce particle transport in CIRs and in the vicinity of SIs.
Ring current oxygen ions escaping into the magnetosheath
Authors: Zong Q.‐G. , Wilken B. , Fu S. Y. , Fritz T. A. , Korth A. , Hasebe N. , Williams D. J. , Pu Z.‐Y.
Published: January 2004

Storm‐related magnetospheric oxygen bursts were observed in the dayside magnetosheath during the coronal mass ejection on January 10, 1997. These singly charged oxygen ion events exhibited a clear antisunward flow. The oxygen ions are associated with a strong negative interplanetary magnetosheath field (IMF). The average Bz was almost − 50 nT, and the field projection in the x – y plane (GSE) was nearly constant in the Sun/dawn sector forming an angle of 45° relative to the Earth‐Sun axis. The magnetopause was identified as a rotational discontinuity by using the principal axis analysis (PAA) method. The three‐dimensional polar versus azimuthal angle distribution of the oxygen ions showed that the oxygen flow has a north to south velocity component. The observations suggest that the dayside reconnection process is generally steady. The energy dispersion can be explained with the time‐of‐flight (TOF) effect assuming oxygen ions are escaping from the magnetosphere along the reconnected field lines. The lack of hydrogen and helium ions during the observed oxygen bursts can be explained, as only oxygen ions are resupplied by the gradient drift in the inner magnetosphere because of their larger bounce periods with respect to hydrogen and helium ions. Therefore only oxygen ions are observed continuously in the magnetosheath. The estimated oxygen escape rate amounts to 0.61 × 1023 ions/s, about 33% of the input rate of the ring current. The observations imply that the stormtime ring current is asymmetric. A large amount of ring current oxygen ions escape from the magnetosphere into the magnetosheath.
Differentiating between colored random noise and deterministic chaos with the root mean squared deviation
Authors: Goode Brent , Cary John R. , Doxas I. , Horton W.
Published: January 2004

A method for distinguishing between data from a strange attractor and data from colored random noise is presented. For both types of data the apparent dimension, as measured by the scaling of the correlation function with length, is finite and noninteger for certain length scales. This would seem to indicate that such measurements by themselves are insufficient for concluding that the dynamics of the underlying system are low‐dimensional. To distinguish these two types of data, we have developed the variance growth test. The test looks for the increase in the variance, as measured by the average squared deviation, of a subset of data as the length of that subset is increased. For strange attractor data the variance saturates once the length of the subset exceeds the characteristic first return time. In contrast, the variance of colored random noise continues to increase with increasing subset length indefinitely, with a scaling law that is related to the apparent correlation dimension. Application of the method to the Bargatze data set shows that the AL index behaves like a deterministic dynamical system.
Observations of persistent Leonid meteor trails: 1. Advection of the “Diamond Ring”
Authors: Drummond Jack D. , Grime Brent W. , Gardner Chester S. , Liu Alan Z. , Chu Xinzhao , Kane Timothy J.
Published: January 2004

From a single image of a persistent trail left by a −1.5 magnitude Leonid meteor on November 17, 1998, the relative winds between 92.5 and 98 km altitude are derived, where the altitudes are determined by a sodium lidar. These are converted to true winds 82 sec after the appearance of the meteor by fixing the winds at 98 km to match the results of following the trail with the lidar for twelve minutes. The image and winds reveal a fine example of the effects of a gravity wave having a vertical wavelenth of 5.50 ± 0.02 km, a horizontal wavelength of 2650 ± 60 km, an intrinsic period of 19.5 ± 0.4 hours, and an observed period of 8.6 ± 0.1 hours. Effects of the gravity wave are still present in the wind field 70 min later.
Observations of persistent Leonid meteor trails: 2. Photometry and numerical modeling
Authors: Kruschwitz C. A. , Kelley M. C. , Gardner C. S. , Swenson G. , Liu A. Z. , Chu X. , Drummond J. D. , Grime B. W. , Armstrong W. T. , Plane J. M. C. , Jenniskens P.
Published: January 2004

During the 1998 Leonid meteor shower, multi‐instrument observations of persistent meteor trains were made from the Starfire Optical Range on Kirtland Air Force Base, New Mexico, and from a secondary site in nearby Placitas, New Mexico. The University of Illinois Na resonance lidar measured the Na density and temperature in the trains, while various cameras captured images and videos of the trains, some of which were observed to persist for more than 30 min. The Na density measurements allow the contribution of Na airglow to the observed train luminescence to be quantified for the first time. To do this, persistent train luminescence is numerically modeled. Cylindrical symmetry is assumed, and observed values of the Na density, temperature, and diffusivity are used. It is found that the expected Na luminosity is consistent with narrowband CCD all‐sky camera observations, but that these emissions can contribute only a small fraction of the total light observed in a 0.5–1 μ bandwidth. Other potential luminosity sources are examined, in particular, light resulting from the possible excitation of monoxides of meteoric metals (particularly FeO) and O2(b1∑g+) during reactions between atmospheric oxygen species and meteoric metals. It is found that the total luminosity of these combined processes falls somewhat short of explaining the observed brightness, and thus additional luminosity sources still are needed. In addition, the brightness distribution, the so‐called hollow cylinder effect, remains unexplained.
A dawn‐dusk density asymmetry in Earth's magnetosheath
Authors: Paularena K. I. , Richardson J. D. , Kolpak M. A. , Jackson C. R. , Siscoe G. L.
Published: January 2004

Magnetosheath data from IMP 8 and solar wind data from ISEE 1, ISEE 3, and WIND are used to investigate Earth's magnetosheath structure. Magnetosheath data are transformed into a coordinate system which allows data from regions with similar physical histories to be binned together. The results show a significant dawn‐dusk asymmetry in Earth's magnetosheath near solar maximum but not near solar minimum, with larger densities on the dawnside than on the duskside. We use a magnetohydrodynamic (MHD) simulation to model the Parker spiral interplanetary magnetic field (IMF) case; an asymmetry does develop in the same sense as observed in the data but with a somewhat smaller magnitude. Near solar minimum the observed density asymmetry is dependent on the IMF orientation, but this is not true near solar maximum, apparently ruling out both foreshock effects and different compressions/deflections by parallel and perpendicular shocks as causes. Earth's magnetosphere thus could be a contributor to this effect.
Influence of the rippling on the collisionless ion and electron motion in the shock front: A model study
Authors: Gedalin M.
Published: January 2004

Recent two‐dimensional hybrid simulations show that the shock is inhomogeneous along the shock front (rippled). Observations also provide some evidence of spatial inhomogeneity, not necessarily related to nonstationary features (waves). Recent analysis of an observed high Mach number shock has shown that its observed features are inconsistent with the assumption that it is one‐dimensional and stationary, although direct comparison of the two spacecraft measurements indicates good stationarity of its profile. We study the effects of the shock rippling alone on the collisionless motion of ions and electrons in a shock front on a simple model shock profile. We show that rippling may substantially affect ion motion, especially when the shock is rippled in the direction perpendicular to the main magnetic field. As a result, the downstream ion distribution becomes much more smooth and diffuse, which reduces the variations of the downstream ion pressure and may improve the shock stability. The smoothing is prompt and occurs at spatial scales substantially smaller than those required for the wave‐particle interaction to be significant. The required rippling scale is between the ion inertial length and upstream ion convective gyroradius, thus being significantly larger than the ramp width or spatial scale of the small‐scale structure. Electrons are much more sensitive to the rippling in the direction of the main magnetic field, which may help to partially fill the gap in the electron distribution which forms collisionlessly in the ramp.
Negative ionospheric storm coincident with DE 1‐observed thermospheric disturbance on October 14, 1981
Authors: Strickland D. J. , Daniell R. E. , Craven J. D.
Published: January 2004

The DE 1 far ultraviolet (FUV) imaging instrument recorded a large thermospheric disturbance over Europe and Russia on October 14, 1981, during a geomagnetic storm commencing near the end of the previous day. The disturbance is quantified in terms of O/N2, the column density ratio referenced to a fixed depth in N2. The ratio is derived from DE 1 dayglow observations with a filter whose signal transmission is dominated by O I 130.4‐nm emission. Within the disturbed region, O/N2 is less than that in surrounding regions, with minimum values being more than a factor of 2 smaller than undisturbed values. Images of O/N2 are compared to ground‐based ionosonde data from 13 sites, some within and others outside of the disturbed region. The data are in the form of Nmax, the maximum electron density, which in the presence of an F2 layer is equivalent to NmF2. The data exhibit negative ionospheric storm effects (reduced Nmax) within the disturbed region and normal behavior elsewhere. The degree of correlation between O/N2 and Nmax suggests that the approximate geographical extent of dayside regions experiencing negative ionospheric storm effects can be determined from global FUV imaging of the disturbed and surrounding regions. The analysis is extended to the following day to address the recovery of the disturbed region. The O/N2 ratio has returned to undisturbed values except in a smaller disturbance region containing weak reductions in O/N2. This is either a remnant of the strong disturbance from the day before or a newly formed disturbance from substorm activity several hours prior to the new observations. Only the ionosonde sites within this region recorded values of Nmax below their monthly median values.
An automated procedure for near‐real‐time Kp estimates
Authors: Takahashi Kazue , Toth Bruce A. , Olson John V.
Published: January 2004

The Kp index indicates geomagnetic disturbances in a simple manner. The index is derived from magnetic field data acquired at 11 ground stations distributed worldwide at subauroral latitudes (49°–62°), has values ranging from 00 to 90 in 28 steps, and is given at 3‐hour intervals. Kp is widely used to study the dynamic relationship between the solar wind and the magnetosphere, to empirically specify the location of the plasmapause and other plasma regions/boundaries, and also as input to various models of the magnetosphere and ionosphere. Currently, the official Kp index is delivered with a delay of many days, so it is not useful for near‐real‐time monitoring of the state of the magnetosphere. We have developed an algorithm to derive an estimated Kp, denoted Kpest, using magnetometer data from nine ground stations that can transmit data in near real time. The algorithm is fully automated and includes a data‐cleaning routine, a quiet‐day‐curve routine, and a routine to convert magnetic field deviations to Kpest. We have evaluated the performance of Kpest using archived magnetometer data and the official Kp. When data from all of the nine stations are available, the linear correlation coefficient between Kpest and Kp is 0.93. In addition, we find that a similarly high correlation between Kpest and Kp can occur when data from only one to three stations are used. We conclude that an automated procedure using data from a small number of ground stations can generate Kp estimates that are reliable in the context of near‐real‐time monitoring of space weather.
Determination of a proxy for F region meridional neutral winds using in situ satellite ion density measurements
Authors: Sultan P. J. , Rich F. J.
Published: January 2004

A simple parameter has been developed to describe the measured latitudinal profiles of the ion density at 840 km near the magnetic equator. This parameter, which we call the “asymmetry index,” is found to be well correlated with empirical neutral wind model output. Such a correlation makes it possible to use the index as a proxy representing the relative strength and direction of the transequatorial neutral wind. Topside ion densities are measured routinely by in situ sensors aboard Defense Meteorological Satellite Program (DMSP) spacecraft near 0600, 0900, 1800 and 2100 hours local time. DMSP data over the full range of solar activity levels and available local times are considered and compared to model neutral winds. While the shape of the observed profile (ni versus latitude) is controlled both by dynamics and by chemistry and varies as a function of season, longitude, local time, solar cycle, and geomagnetic activity, the neutral wind is clearly a significant factor. The profile asymmetry index is not a direct measurement of the instantaneous wind but rather represents the integrated effects of vertical and interhemispheric plasma transport due to the neutral wind over several hours. The index is shown to duplicate well (data/model correlation coefficient R ∼ 0.8) the longitudinal and seasonal morphology of the neutral wind blowing along the geomagnetic meridian for all available local times during solar maximum conditions, but only near the 0900 and 1800 LT meridians during solar minimum. These results suggest that during these times the meridional neutral wind is the dominant influence on the ion density profile shape. Thus the asymmetry index is a useful parameter for the study of the dynamics of the low‐latitude ionosphere. In particular, such a parameter would be important for use in operational ionospheric models that require a real‐time neutral wind input and in the verification and testing of future thermospheric wind models.
Improvements to the HAF solar wind model for space weather predictions
Authors: Fry C. D. , Sun W. , Deehr C. S. , Dryer M. , Smith Z. , Akasofu S.‐I. , Tokumaru M. , Kojima M.
Published: January 2004

We have assembled and tested, in real time, a space weather modeling system that starts at the Sun and extends to the Earth through a set of coupled, modular components. We describe recent efforts to improve the Hakamada‐Akasofu‐Fry (HAF) solar wind model that is presently used in our geomagnetic storm prediction system. We also present some results of these improvement efforts. In a related paper, Akasofu [2001] discusses the results of the first 2 decades using this system as a research tool and for space weather predictions. One key goal of our efforts is to provide quantitative forecasts of geoeffective solar wind conditions at the L1 satellite point and at Earth. Notably, we are addressing a key problem for space weather research: the prediction of the north‐south component (Bz) of the interplanetary magnetic field. This parameter is important for the transfer of energy from the solar wind to the terrestrial environment that results in space weather impacts upon society. We describe internal improvements, the incorporation of timely and accurate boundary conditions based upon solar observations, and the prediction of solar wind speed, density, magnetic field, and dynamic pressure. HAF model predictions of shock arrival time at the L1 satellite location are compared with the prediction skill of the two operational shock propagation models: the interplanetary shock propagation model (ISPM) and the shock‐time‐of‐arrival (STOA) model. We also show model simulations of shock propagation compared with interplanetary scintillation observations. Our modeling results provide a new appreciation of the importance of accurately characterizing event drivers and for the influences of the background heliospheric plasma on propagating interplanetary disturbances.
Solar cosmic ray events for the period 1561–1994: 2. The Gleissberg periodicity
Authors: McCracken K. G. , Dreschhoff G. A. M. , Smart D. F. , Shea M. A.
Published: January 2004

A total of 125 large fluence solar proton events identified from the nitrate deposition in ice core from Greenland for the period 1561–1950 are examined in an exploratory study of the geophysical information that will be available from such data in the future. These data have been augmented with ionospheric and satellite data for the period 1950–1994. There were five periods in the vicinity of 1610, 1710, 1790, 1870, and 1950, when large >30 MeV proton events with fluence greater than 2 × 109 cm−2 were up to 8 times more frequent than in the era of satellite observation. There is a well‐defined Gleissberg (approximately 80 year) periodicity in the large fluence proton events, with six well‐defined minima, two in close association with the Maunder and Dalton minima in sunspot number. The present “satellite” era is recognized as a recurrence of this series of minima. Comparison of the total solar proton production for the five Gleissberg cycles since 1580 shows that the cycle 1820–1910 was the most active followed by the cycle 1580–1660. The present Gleissberg cycle is one of the least effective in the production of solar proton events at Earth. It is shown that the solar and solar proton event data both indicate that the Maunder Minimum ended about 1700, 16 years before the commonly accepted date of 1716. It is proposed that the delayed “switch on” of aurorae after the Maunder Minimum is due to the changing nature of the solar corona from “Maunder Minimum” conditions to the more active conditions of the Gleissberg cycle, and a physical mechanism is proposed in which variations in the coronal densities modulate the efficiency of solar proton event production throughout the Gleissberg cycle. The “streaming limited fluence” for >30 MeV protons is estimated to be 6–8 × 109 cm−2, and the rapid decrease in the probability of occurrence of solar proton events observed in the vicinity of this fluence is proposed to be due to this effect.
Atmospheric effects of proton precipitation in the Martian atmosphere and its connection to the Mars‐solar wind interaction
Authors: Kallio Esa , Janhunen Pekka
Published: February 2004

Atmospheric effects of precipitating solar wind protons in the Martian atmosphere are studied. The proton flux to the atmosphere is derived from a newly developed global quasineutral hybrid simulation which includes solar wind H+ ions and planetary O+ ions. The motion of the precipitating particles in the atmosphere is followed, and the effects of collisions to atmospheric neutrals are studied by a collision‐to‐collision Monte Carlo algorithm. Maximum atmospheric effects are estimated by using a fully absorbing boundary condition in the hybrid model where all solar wind protons are allowed to precipitate into the atmosphere without reflection. The developed mass‐loaded hybrid code is found to reproduce many of the observed plasma and field features near Mars. When the vertical profiles of the energy deposition rates, CO2+ ionization rates, and Lyman alpha emission rates are calculated at different solar zenith angles, the maximum atmospheric effects on the dayside under average solar wind conditions are found to be typically a few percent of the effects of EUV radiation. On the nightside the proton precipitation is estimated to be intensive enough to be able to produce the measured ionospheric electron densities. The analysis illustrates that the atmospheric effects are strongly coupled with the global plasma interaction process between Mars and the solar wind.
Flow bursts and auroral activations: Onset timing and foot point location
Authors: Nakamura R. , Baumjohann W. , Brittnacher M. , Sergeev V. A. , Kubyshkina M. , Mukai T. , Liou K.
Published: February 2004

Flow burst events with a flux transfer rate exceeding 2 mV/m and with a duration of less than 10 min observed by Geotail are compared with auroral signatures obtained from the Polar ultraviolet imager. It is shown that all the flow bursts correspond either to localized auroral intensifications associated with small poleward expansions and pseudobreakups or to an activation starting at the poleward edge of the expanded auroral oval that develop equatorward toward the foot point of the satellite, including auroral streamers. Earthward flow bursts related to pseudobreakups and small expansions are observed mainly in the region earthward of 15 RE, more inward than those flows related to high‐latitude auroral activations and auroral streamers. Although most of these auroral activations precede the observations of the flow bursts by a few minutes, the activations that break up near the foot point of the satellite start typically within ±1 min of the onset of flow burst observation.
The location of low Mach number bow shocks at Earth
Authors: Fairfield D. H. , Iver H. Cairns , Desch M. D. , Szabo A. , Lazarus A. J. , Aellig M. R.
Published: January 2004

On April 26 and 27 and May 10–12, 1999, unusually low solar wind densities produced unusually low Alfven Mach numbers that moved the Earth's bow shock far out past its normal location. The shocks observed by the Wind spacecraft corresponded to shock subsolar distances of 45 and 42 RE, respectively, on these days which are the most distant locations at which the shock has ever been seen. Shock observations by three other spacecraft on these days along with 34 previously reported distant shocks are used to compare with the predictions of different models. A recent MHD bow shock model of Cairns and Lyon [1995] predicts the observed locations quite well as does a modified gasdynamic model of Farris and Russell [1994] if a new Mach‐number‐dependent shape parameter is used. A third model of Verigin et al. [1997] also predicts a shock shape and is also quite good. Bow shock predictions are limited by uncertainties in measurements of the very low densities and uncertainties in the position and shape of the magnetopause. Asymmetries in the shock shape caused by the interplanetary magnetic field direction and not accounted for by models are another likely source of uncertainty. These uncertainties make it impossible to clearly favor one theoretical model over another.
An empirical plasmasphere and trough density model: CRRES observations
Authors: Sheeley B. W. , Moldwin M. B. , Rassoul H. K. , Anderson R. R.
Published: January 2004

Combined Release and Radiation Effects Satellite (CRRES) sweep frequency receiver data were used to develop an empirical model of the plasmasphere and trough number density. The over 1000 CRRES orbits provided good statistical coverage of all local times between an L shell of 3 to 7. The CRRES density data were separated into plasmaspheric‐like and trough‐like by assuming a minimum density value for the plasmasphere as a function of L shell. For the plasmasphere the average number density (in cm−3) as a function of L shell (3 ≤ L ≤ 7) was found to be: np = 1390 (3/L)4.8 ± 440 (3/L)3.6. For the trough the average number density (in cm−3) as a function of L‐shell (3 ≤ L ≤ 7) and magnetic local time (0 ≤ LT ≤ 24) was found to be nt = l24 (3/L)4.0 + 36(3/L)3.5 cos({LT‐[7.7(3/L)2.0+12]}π/12) ± {78 (3/L)4.7 + 17 (3/L)3.7 cos[(LT ‐ 22)π/12]}. No clear dependence on magnetic activity was found for either density model. This empirical model is an improvement over earlier models in that it is continuous in local time and can be used to track densities based on refilling history. The model standard deviations are representative of either early time or late time refilling of the trough or newly filled or saturated plasmaspheric densities.
The role of proton precipitation in the excitation of auroral FUV emissions
Authors: Hubert B. , Gérard J.‐C. , Bisikalo D. V. , Shematovich V. I. , Solomon S. C.
Published: January 2004

Far ultraviolet remote sensing from a high‐altitude satellite is extensively used to image the global aurora, derive its energetics, and follow its dynamical morphology. It is generally assumed that the observed emissions are dominated by the interaction of the precipitated electrons with the thermospheric constituents. A model to calculate far ultraviolet emissions excited by auroral electrons and protons and the secondary electrons they generate has been used to calculate the volume excitation rate of the H I Ly‐α, O I 1304 and 1356 Å, N 11493 Å multiplets, and the N2 Lyman‐Birge‐Hopfield (LBH) bands. The characteristic energy and the energy flux are derived from the observed statistical distribution of precipitated protons and electrons. This model is applied to the midnight aurora, the noon cusp, and a proton‐dominated aurora for moderately disturbed conditions. We show that in the first two cases, direct electron impact dominates the vertically integrated emission rate over the proton component, although proton excitation plays an important role at some altitudes in the daytime cusp. In afternoon regions of the auroral zone near the auroral boundary, secondary electrons due to proton ionization are the main source of FUV emissions. The energy dependence of the efficiency of LBH band emission viewed from high altitude is calculated for electron and proton precipitations. Maps of the N2 LBH emission excited by both components are obtained, and regions of proton‐dominated auroral emission are identified. It is found that the distribution of the ratio of proton‐induced to electron‐induced brightness resembles maps of the ratio of the respective precipitated energy fluxes. Proton‐dominated FUV emissions are thus located in a C‐shaped sector extending from prenoon to midnight magnetic local times with a maximum proton contribution near the equatorward boundary of the statistical electron oval. The distribution of the Ly‐α/LBH intensity ratio is found to mimic the ratio of the proton flux/total energy flux, although it is insufficient by itself to accurately determine the relative fraction of auroral energy carried by the protons.
Hall effects on the generation of field‐aligned currents in three‐dimensional magnetic reconnection
Authors: Ma Z. W. , Lee L. C.
Published: January 2004

A three‐dimensional Hall MHD simulation is carried out to study the Hall effects on the generation of field‐aligned currents (FAC) and Alfvén waves by magnetic reconnection. The magnetic field is initially set up with an antiparallel component (Bz) with respect to the plane x = 0. The initial current flows in the y direction. Several interesting features emerge owing to the inclusion of the Hall term in the MHD equation. In Hall MHD, the magnetic field lines are frozen in the electron fluid, but not in the ion fluid. In the presence of a Hall term, a strong electron velocity in the − y direction is usually present in the region with a high current density J in the y direction. As a result, the reconnected magnetic field lines are bent and also shifted in the − y direction. Generated FACs break the dawn‐dusk symmetry, and the region of generated FACs tends to shift to the side with y < 0. The magnitude of generated FACs increases with increasing ion inertial length. We use the Walén relation to examine the presence of Alfvén waves. The ion and electron Walén ratios are defined as Ai ≡ Ωi‖/ΩA‖ and Ac ≡ Ωe‖/ΩA‖, respectively. Here, Ωi‖, Ωe‖, and ΩA‖ are the ion, electron, and Alfvén parallel vorticities, respectively. The ion and electron Walén relations are found to be different. It is also found that in the region J‖ < 0, Alfvén waves have a right‐hand polarization with Ai < 1, while in the region J‖ > 0, Alfvén waves have a left‐hand polarization with Ai > 1.
Maps of ionospheric field‐aligned currents as a function of the interplanetary magnetic field derived from Dynamics Explorer 2 data
Authors: Weimer D. R.
Published: February 2004

A new technique for mapping field‐aligned currents (FAC) with satellite magnetometer data has been used with Dynamics Explorer 2 measurements to produce an empirical model which maps the currents above the high‐latitude ionosphere as a function of the interplanetary magnetic field (IMF), solar wind velocity, solar wind density, and dipole tilt angle. This technique uses scalar magnetic Euler potentials, derived from integrating the measured magnetic deviations in much the same way as electric potentials are derived from integrating electric fields. This method works with any configuration of two‐dimensional distribution of the field‐aligned current, rather than assuming that the currents are in the form of infinite sheets or belts. The radial current density is found by a surface Laplacian operator on the scalar field. The maps of the FAC produced with this new technique are more quantitative and detailed than most of the preceding statistical diagrams, and they yield much insight into how the currents vary as the IMF clock angle changes, and how the field‐aligned current maps overlap the associated electric potential patterns. An optional component of the model shows changes in the currents associated with substorms, using the AL index as the controlling parameter. The most notable aspect of the substorm patterns is an increased region 0 current, which in addition to the region 2 current closes the majority of the current on the dusk side of the auroral surge. The results do not seem to agree with the traditional paradigm of the substorm current wedge closing through the ionosphere from dawn to dusk.
Impact of the new rate coefficients for the O atom vibrational deactivation and photodissociation of NO on the temperature and density structure of the terrestrial atmosphere
Authors: Sharma Ramesh D. , Roble Raymond G.
Published: January 2004

Recent laboratory work has arrived at a value of the rate coefficient for the room temperature O atom vibrational relaxation of NO(v=1) which is nearly one third the previously accepted value [Fernando and Smith, 1979]. Laboratory measurements have also established a new value for the rate coefficient for the photodissociation of NO which is ∼1.6 times the previously accepted value. Since the NO(v=1→v=0) emission, around 5.3 μm, is a very important cooling process in the lower terrestrial thermosphere, the new values of the two rate coefficients lead to a decrease in the amount of NO as well as the rate at which it cools. Using the global mean model [Roble, 1995] of the mesosphere, thermosphere, and ionosphere, we find that the new rate coefficients introduce large changes in the thermal and density structure of the atmosphere. The resulting model atmosphere appears unrealistic. We find that these large changes are moderated and the resulting neutral temperatures agree better with the Mass Spectrometer Incoherent Scatter (MSIS) model when the value of the CO2+O rate coefficient for excitation of the bending vibration is increased two fold to the value arrived at by modeling the 15 μm emission from CO2 from the lower terrestrial thermosphere. Finally, we describe the results of a number of runs with the thermosphere/ionosphere/mesosphere electrodynamics general circulation model (TIME‐GCM) to examine the global temperature, composition, and circulation changes that result from using the new rates for both NO and CO2.
Observations of simultaneous effects of merging in both hemispheres
Authors: Maynard N. C. , Burke W. J. , Sandholt P. E. , Moen J. , Ober D. M. , Lester M. , Weimer D. R. , Egeland A.
Published: January 2004

In this event study, we have compared electric field measurements acquired near magnetic noon during a rocket flight from the SvalRak range with solar wind and interplanetary magnetic field (IMF) observations. The cusp is spatially bifurcated relative to its source regions. The data indicate that many effects observed at northern high latitudes were driven by dayside merging in the Southern Hemisphere, probably near the dawn side of the cusp. Applying the antiparallel merging criterion of Crooker [1979], we show that complex ground‐based optical data are well ordered by considering that incoming interplanetary electric field phase fronts in the solar wind are tilted, allowing the two hemispheres to respond to the same elements of the solar wind stream at significantly different times. The data stream interacts first with the Southern Hemisphere at lag times significantly less than the simple advection time. Northern Hemisphere merging occurred later, the timing separation being related to the tilt and the strong IMF BX. Auroral emissions created by electrons injected from a Southern Hemisphere merging site may be located in close proximity to those from a Northern Hemisphere site, within the same all‐sky image. With proper lag times established for the two source regions, it is clear that variations of dayside auroral emissions occur in response to small changes in the interplanetary electric field. The bifurcation is driven by IMF BY, while BX accentuates differences in the timing of the interaction. The detailed harmonization of distinct auroral features with interplanetary drivers strongly supports the utility of the antiparallel merging criterion in estimating when and where the solar wind and the magnetosphere interact. A similar ordering of auroral effects and in situ data with interplanetary variations cannot be achieved if merging proceeded at lower latitudes along a continuous, tilted merging line passing through the subsolar region, as required by the component‐merging hypothesis. A consequence of merging limited to the high‐latitude regions is that the smaller convection cell is driven by merging in the opposite hemisphere. While these conclusions are based on our analysis of a single interval and need independent confirmation, the investigation opens new possibilities for understanding cusp electrodynamics, implying a much greater solar wind/IMF control of magnetosphere‐ionosphere phenomena than previously thought.
DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum
Authors: Huang C. Y. , Burke W. J. , Machuzak J. S. , Gentile L. C. , Sultan P. J.
Published: February 2004

The Defense Meteorological Satellite Program (DMSP) flights F9 and F10 crossed postsunset local time sectors approximately 14 times per day in Sun‐synchronous orbits at an altitude of ∼840 km. We have examined a large database of postsunset plasma density measurements acquired during ∼ 15,000 equatorial crossings made by DMSP F9 in 1989 and 1991 and DMSP F10 in 1991. On 2086 of these crossings equatorial plasma bubbles (EPBs) were observed as intervals of depleted and irregular plasma densities. We have analyzed these EPB events to determine their distributions with season, longitude (S/L), and levels of geomagnetic activity. The global S/L distributions of EPBs observed by the DMSP satellites are shown to be in general agreement with results from discrete ground‐based measurements. That is, the seasonal variations detected at 840 km in longitude bins hosting radar/scintillation observatories appear similar to those reported from the ground. Over the Atlantic sector where EPBs occur frequently, we found good agreement with predictions of a simple model proposed by Tsunoda [1985]. In the Pacific sector the frequency of EPB occurrence is considerably lower, and poor counting statistics preclude confident predictions regarding the absolute value of seasonal variations. We suggest that relatively large equatorial magnetic fields at Flayer altitudes in the Pacific (∼0.34 G) sector more strongly inhibit the growth of the Rayleigh‐Taylor instability than at Atlantic (∼0.25 G) longitudes. Contrary to general belief, we found that EPBs occurred regularly during geomagnetic storms, especially in the initial and main phases. EPB activity appears to have been suppressed from many hours to clays during storm recovery phases.
One‐ and two‐dimensional simulations of electron beam instability: Generation of electrostatic and electromagnetic 2fp waves
Authors: Kasaba Y. , Matsumoto H. , Omura Y.
Published: February 2004

We have performed computer simulations of the self‐consistent nonlinear evolution of electrostatic and electromagnetic 2fp waves excited by electron beams with electromagnetic particle code. In both one‐ and two‐dimensional periodic systems an electrostatic 2fp wave is generated at twice the wave number of forward propagating Langmuir waves by wave‐wave coupling. This wave grows with the forward propagating Langmuir wave in the nonlinear stage of the simulations. The electrostatic 2fp wave in the simulations is saturated at about −20 ∼ −30 dB of that of the Langmuir waves. It is larger than the value expected from observations in the terrestrial electron foreshock. The electromagnetic 2fp wave is only excited in two‐dimensional systems. The magnitude of the electromagnetic 2fp wave is correlated with the backward propagating Langmuir wave, not with the electrostatic 2fp wave. This result suggests that the electromagnetic 2fp wave is excited by the wave‐wave coupling of forward and backward propagating Langmuir waves. The typical power density estimated from a reasonable amplitude of Langmuir wave is of the same order or much weaker than the value typically observed around the electron foreshock.