bluuebunny/crossref_metadata_2025 huggingface dataset:
Annual and semiannual variations of the midlatitude ionosphere under low solar activity
Authors: Kawamura S. , Balan N. , Otsuka Y. , Fukao S.
Published: October 2002

The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135°E, 35°N) incoherent scatter observations and Sheffield University plasmasphere‐ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200–600 km altitudes measured by the radar under low solar activity (F10.7 ≤ 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (Uθ) and northward perpendicular plasma drift velocity (V⟂) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS‐86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio); the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to ∼400 km and shows comparatively weak annual and semiannual variations.
Four spacecraft measurements of the quasiperpendicular terrestrial bow shock: Orientation and motion
Authors: Horbury T. S. , Cargill P. J. , Lucek E. A. , Eastwood J. , Balogh A. , Dunlop M. W. , Fornacon K.‐H. , Georgescu E.
Published: October 2002

Measurements of the magnetic field at the four Cluster spacecraft, typically separated by ∼600 km, during bow shock crossings allow the orientation and motion of this structure to be estimated. Results from 48 clean and steady quasiperpendicular crossings during 2000 and 2001, covering local times from 0600 to 1700, reveal the bow shock normal to be remarkably stable, under a wide range of steady upstream conditions. Nearly 80% of normals lay within 10° of those of two bow shock models, suggesting that the timing method is accurate to around 10°, and possibly better, and therefore that four spacecraft timings are a useful estimator of the orientation and motion of quasiperpendicular bow shocks. These results show that models provide a good approximation to the bow shock surface and can therefore be used when four spacecraft data are not available. In contrast, only 19% of magnetic coplanarity vectors were within 10° of the model normal. The mean deviation of the coplanarity vector from the timing‐derived normal for shocks with θBN < 70° was 22 ± 4°. Typical shock velocities were ∼35 km s−1, although the fastest measured shock was traveling outbound at nearly 150 km s−1.
Using two ground stations to identify magnetospheric field line eigenfrequency as a continuous function of ground latitude
Authors: Kawano H. , Yumoto K. , Pilipenko V. A. , Tanaka Y.‐M. , Takasaki S. , Iizima M. , Seto M.
Published: October 2002

Ground observations of magnetospheric field line eigenfrequency are useful in continuously monitoring the magnetospheric plasma density. The amplitude‐phase gradient method (APGM) [Pilipenko and Fedorov, 1994] is a technique which is applied to H‐component magnetometer data from two ground stations aligned along a meridian; as a result, it yields the eigenfrequency and the resonance width as continuous functions of latitude. This feature (yielding the two quantities at any latitudes from two stations' data) is unique to APGM: Classic gradient methods yield the two quantities only at the midpoint of the two stations. This paper presents the first time APGM is applied to actual data: We apply it to two pairs of stations, independently, to test if the two results agree. The two pairs are made from three stations (one overlapped) almost aligned along a meridian, at L = 1.32, 1.36, and 1.39 (latitudinal separation: 1.51° and 1.06°). The data were taken when Pc3 pulsations took place. The resultant two latitude profiles of the eigenfrequency (resonance width) are very close to each other, demonstrating the consistency and usefulness of APGM. We also present a new technique to obtain one latitude profile of the eigenfrequency and that of the resonance width from any number of stations; we apply it to our three stations' data and show its consistency and usefulness. The application result unambiguously shows that the eigenfrequency at low latitudes increases with increasing latitude, which is ascribed to mass loading on field lines of ionospheric heavy ions. The result also indicates a large damping rate of the eigen‐oscillations.
Generation of electromagnetic ion cyclotron waves in the ionosphere by localized transverse dc electric fields
Authors: Peñano Joseph R. , Ganguli Gurudas
Published: October 2002

Localized dc electric fields directed perpendicular to the geomagnetic field are studied as a possible ionospheric source of electromagnetic ion cyclotron waves. The eigenvalue formulation of the problem is solved numerically and a class of unstable electromagnetic ion cyclotron eigenmodes are found that have properties consistent with some observations of broadband waves near auroral arcs. The frequency range of instability is comparable to the spectral width measured from photometer observations of broadband auroral flickering. Eigenmodes are associated with electric field polarization reversals and E/B ratios similar to that encountered by satellites and sounding rockets traversing auroral arcs. The dc electric field affects ion resonance conditions and allows for resonant interaction within the extent of the eigenfunction.
Three‐dimensional analyses of electric currents and pressure anisotropies in the plasma sheet
Authors: Kaufmann Richard L. , Lu Chen , Paterson W. R. , Frank L. A.
Published: October 2002

Long‐term averaged three‐dimensional (3‐D) databased magnetotail models have been created for many plasma and field parameters. A simple modeling technique that was used in earlier work is compared with a new more comprehensive procedure. Good agreement between the two methods was found when both produced stable results. The simpler method generated reliable models from any of the available data sets, while the more complex method did not. It is shown how analyses of the models can determine plasma and field parameters that are too small to be measured directly. The magnetic field data was used to calculate average electric currents flowing in the x and y directions. Full 3‐D distributions of jx and jy were determined from the models even though these currents were too small to measure directly with adequate accuracy. Changes of the electron and ion pressure anisotropies as a function of distance along an average magnetic field line then were analyzed. It was concluded that the electron anisotropy was created by an electric field with a parallel component near the neutral sheet. The parallel electric field is small but is required to maintain charge neutrality in the region containing guiding center electron and nonguiding center ion orbits. In addition to creating anisotropic electron distributions the presence of this parallel electric field violates the ideal MHD assumptions near the neutral sheet. The ion anisotropy suggests that the net effect of chaotic ion motion near the neutral sheet is to create a weak source cone distribution superimposed on a denser isotropic component.
Reconstruction of magnetic clouds in the solar wind: Orientations and configurations
Authors: Hu Qiang , Sonnerup Bengt U. Ö.
Published: October 2002

We develop a new approach for determining the orientation of the axis (z) of finite‐β magnetic flux ropes from single‐spacecraft data. It consists of an optimization procedure based on two‐dimensional magnetohydrostatic theory. From the requirement that the transverse pressure, Pt = p + Bz2/2μ0, be a function of the calculated magnetic potential, A, alone, an optimal z axis is found. Benchmark studies using analytical flux rope solutions show the feasibility of this axis‐determination approach. Two magnetic cloud events, on 18 October 1995 and 9 January 1997, observed by the Wind spacecraft at 1 AU, are examined. The time series of plasma and magnetic field data, collected when the magnetic structures move past the spacecraft, are utilized as spatial initial values for numerical integration of the nonlinear, plane Grad‐Shafranov equation. The recovered magnetic cloud structures show helical magnetic field configurations, representative of magnetic flux ropes, with invariance along their axes. Their cross sections consist of nested irregular loops of transverse field lines rather than the concentric circles of an axially symmetric model. In addition to their axis orientations, we obtain other quantitative features of the two magnetic clouds from their recovered cross sections, including their sizes (diameters ∼ 0.2 AU), their impact parameters (|y0| ≲ 0.01 AU), their maximum axial field strengths (20 and 14 nT, respectively), and the twist of field lines (∼ 1.30–2.68 turns AU−1) around the flux rope's axis. We find that errors in the orientation of the flux rope axis may result in significant variations in the main parameters, i.e., the impact parameter, as well as the size and shape of the cross section.
L dependence of energetic electron precipitation driven by magnetospherically reflecting whistler waves
Authors: Bortnik J. , Inan U. S. , Bell T. F.
Published: October 2002

Ray tracing analysis and a first‐order treatment of the wave frequency and electron energy dependence of gyroresonant pitch angle scattering reveals the L shell dependence of the time‐integrated energetic electron (>150 keV) precipitation flux in the drift loss cone due to a single cloud‐to‐ground lightning stroke. Primary features of the L dependence are determined by the ray paths of magnetospherically reflecting nonducted whistler waves, as well as the refraction of the wave normal vectors due to ionospheric horizontal electron density gradients at low latitudes. Calculations for lightning source latitudes of 25°, 35°, and 45°, with an assumed “average” ionospheric electron density profile, as well as an International Reference Ionosphere model profile, indicate that horizontal ionospheric electron density gradients can cause the whistler wave energy to focus near the geomagnetic equator on the first magnetospheric traverse, resulting in a spatially narrow precipitation signature at low L shells (1.4 < L < 1.65). The magnitude of this low‐L peak is found to be sensitively dependent on the local ionospheric density gradients. Subsequent magnetospheric reflections result in a broad precipitation peak at higher L shells (1.8 < L < 4.2), which is robustly present, with magnitude being relatively insensitive to lightning source‐latitude and ionospheric horizontal gradients. The calculated L dependences were found to be in excellent agreement with drift‐loss cone fluxes measured aboard the SAMPEX satellite.
Energetics of the ionospheric feedback interaction
Authors: Lysak Robert L. , Song Yan
Published: October 2002

The ionospheric feedback instability has been invoked as a possible mechanism for the formation of narrow auroral arcs. This instability can excite eigenmodes of both field line resonances and the ionospheric Alfvén resonator, producing narrow‐scale structures. Although the basic dispersion relation of this instability has been discussed for both of these cases, the energetics of this instability has not been discussed quantitatively and questions remain as to the nonlinear evolution of this instability. The free energy for this instability comes from the reduction of Joule heating due to the preexisting convection caused by the self‐consistent changes in ionization and conductivity due to Alfvénic perturbations on the ionosphere. In an active ionosphere, narrow‐scale Alfvén waves can be overreflected; i.e., the reflected wave can have a larger amplitude than the incident wave, with the extra energy coming from a local reduction of Joule heating. Recombination produces a damping of this instability, particularly for high background conductivity, indicating that this instability operates best in a dark background ionosphere. This feedback interaction produces narrow‐scale currents when strong gradients in the conductivity are produced, and effects from parallel resistivity or possibly kinetic effects will become important in its evolution. Theoretical constraints on low‐spatial resolution observations of the energy dissipated by precipitation as opposed to Joule heating will be discussed.
Absorption cross sections and solar photodissociation rates of deuterated isotopomers of methanol
Authors: Cheng Bing‐Ming , Bahou Mohammed , Lee Yuan‐Pern , Lee L. C.
Published: October 2002

Photoabsorption cross sections of CH3OH, CH3OD, CD3OH, and CD3OD are determined in the spectral region 165–220 nm using synchrotron radiation as a light source. Excitation of methanol isotopomers in this wavelength region breaks mainly the O–H or O–D bond to form H or D, respectively. Significantly smaller absorption cross sections near the threshold region are observed for CH3OD and CD3OD. These data are used to calculate corresponding photodissociation rates by the solar flux at 1 AU. The rates for CH3OD and CD3OD are only about 80% and 82% of those for CH3OH and CD3OH, respectively. The effective photo‐induced isotopic enrichment factors (per mil) are estimated to be 199‰ for CH3OD, −35‰ for CD3OH, and 154‰ for CD3OD. Such large enrichment factors are expected to induce detectable H/D fractionation in comets or elsewhere if the photo‐induced isotopic fractionation effect is important in controlling methanol chemistry.
Ion Joule heating as a function of electric field scale size
Authors: Walker David N. , Amatucci William E. , Ganguli Gurudas I. , Fernsler Richard F.
Published: October 2002

In an earlier work we described a laboratory‐based experimental series that demonstrated the transition from velocity‐shear‐driven ion cyclotron wave heating to ion Joule heating as a function of ion‐neutral collision frequency. As part of this effort, we demonstrated the ability to control the magnitude and direction of the electric field (or the sign of potential change) in space in a simulated ionospheric environment. In the results presented here we have calculated the electric field by differentiation of a fit to the potential profile and have used the full width at half maximum of this field as a marker for the size of the region. In the case of the ion temperature measurements, which are derived from energy analyzer data by conventional techniques, we demonstrate increased ion temperature consistent with increasing electric field region length. We also have been able to demonstrate a peak in the ion temperature as a function of collision frequency and the Joule heating rate. The data suggest that for approximately constant pressure and electric field strength, the normalized ion temperature increase will be limited for a given scale size. Finally, we argue that these observations are consistent with the notion of a limit to ion heating based on a comparison of the scale length of the region to the mean free path for ion‐neutral collisions.
Observation of the terrestrial bow shock in quasi‐electrostatic subshock regime
Authors: Balikhin M. A. , Nozdrachev M. , Dunlop M. , Krasnosel'skikh V. , Walker S. N. , Alleyne H. St. C. K. , Formisano V. , Andre M. , Balogh A. , Eriksson A. , Yearby K.
Published: October 2002

Electric field measurements made on board the four Cluster II satellites are used to estimate the spatial scale of the electrostatic potential in the front of the terrestrial bow shock in the quasi‐perpendicular regime. High‐amplitude electric fields (up to 70 mV m−1) were measured at the shock fronts encountered on 31 March 2001. The main change in the cross‐shock potential took place on spatial scales of a few electron inertial lengths. Such small scales in the electrostatic potential can be explained in the framework of the quasi‐electrostatic subshock concept. The high magnitude of the upstream magnetic field, which resulted in an unusually small β and moderate Mach numbers, created favorable conditions for the occurrence of quasi‐electrostatic subshocks in the shock front.
Energy analysis of substorms based on remote sensing techniques, solar wind measurements, and geomagnetic indices
Authors: Østgaard N. , Germany G. , Stadsnes J. , Vondrak R. R.
Published: October 2002

Observations from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on board the Polar satellite have been used to examine the energy deposition in the Northern Hemisphere by precipitating electrons for seven substorms during 1997. By combining the results from these two remote sensing techniques we derive the 5 min average electron energy distributions from 100 eV to 100 keV with a spatial resolution of ∼700 km. During growth phase we find that most of the energy is carried by electrons below 10 keV. The study shows that the maximum intensity of the substorm is mostly defined by the flux of electrons below 10 keV. In contrast, the electrons above 10 keV show a greater intensification at substorm onset and are more confined azimuthally during the expansion phase. By using the most appropriate parameterized methods to estimate the energy increase of the ring current (UR) and the Joule heating rate in both hemispheres (UJ) and by adding the rate of energy deposition in both hemispheres by precipitating electrons (UA), we estimate the total energy dissipation rate during substorms (UT). Our estimate of UA is a factor 2–4 larger than that reported in earlier studies. We find that the contributions to the total time‐integrated energy dissipation over the duration of the substorm, W(UT), from W(UR), W(UJ), and W(UA) on average are 15%, 56%, and 29%. Comparing W(UT) and the time‐integrated ϵ parameter, W(ϵ), which approximates the solar wind input due to dayside reconnection, we find that the ϵ parameter does not always provide enough energy into the magnetosphere to balance W(UT). An additional energy transfer mechanism is needed to balance the energy budget. We find that a viscous interaction that transfers 0.17% of the solar wind kinetic energy in addition to the energy transfer due to dayside reconnection, estimated by the ϵ parameter, is sufficient to balance the total energy dissipation UT.
A relation between the energy deposition by electron precipitation and geomagnetic indices during substorms
Authors: Østgaard N. , Vondrak R. R. , Gjerloev J. W. , Germany G.
Published: October 2002

Observations from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on board the Polar satellite have been used to derive the total energy dissipation (UA) in the Northern Hemisphere by electron precipitation in the energy range from 100 eV to 100 keV. Comparing with geomagnetic indices, we find that during substorms, UA is linearly related to the quick look AEQL1/2 and ALQL1/2 indices. The best correlation (0.86) is found between the energy flux above 10 keV and the ALQL index, which reflects that the energetic electron precipitation modulates the westward electrojet intensity by affecting the Hall conductance. The AUQL index which reflects the eastward electrojet intensity shows poor correlation with UA, either for soft or energetic electrons. This is consistent with an electric field dominance in the dusk sector and a minor role for auroral conductance in the eastward electrojet. On the basis of ionospheric electrodynamics, we argue that a nonlinear relation between UA and AE (and AL) is more appropriate than a linear relation. We show that the linear relations reported by others do not fit our data set and that they provide UA values which are too low. For the total electron energy flux (0.1–100 keV) we find the best fit to be UA[GW] = 4.4 ALQL1/2 − 7.6, with a correlation coefficient of 0.83, which is slightly better than that for the AEQL index (0.77). By time‐integrating the UA derived from ALQL, we obtain estimates of the total deposited energy during substorms within ±20% of the time‐integrated UA derived from UV and X rays.
On the bow shock θBn dependence of upstream 70 keV to 2 MeV ion fluxes
Authors: Meziane K. , Hull A. J. , Hamza A. M. , Lin R. P.
Published: October 2002

We present a statistical study of 216 energetic ion events observed just upstream of Earth's bow shock by the Wind spacecraft. The ions that compose this database range in energy from 27 keV to 2 MeV. Of particular interest is the effect of shock geometry on the properties of the energetic ions. Our approach is different from previous studies in that we only include upstream ion events associated with a local bow shock crossing. In this way we determine the shock geometry locally, rather than from extrapolations of remote upstream observations to a model shock source location, providing a better quantitative determination of the properties of the energetic ions in relation to shock geometry. Under typical interplanetary conditions in the absence of a preexisting population of ambient energetic ions (E ≥ 50 keV), we find that the ion energy spectrum is characterized by an energy cutoff at ∼200–330 keV. Neither the energy cutoff nor the energetic ion spectrum show any dependence on shock geometry as measured by the angle between the shock normal and magnetic field θBn. However, when an ambient population of energetic particles is present in the interplanetary medium, ion flux levels measured just upstream of the terrestrial shock reach a minimum near θBn ∼ 45° irrespective of energy. The energy spectrum spans up to ∼2 MeV. Moreover, ions with energies E ≥ 550 keV are observed only at θBn ≥ 45°. The flux levels of these more energetic ions increase, in an average sense, with increasing values of θBn. Lee's [1982] self‐consistent theory of ion diffusive shock acceleration appears to explain the energetic ion flux levels observed at θBn ≤ 40°. Upstream energetic ions with energies E ≥ 550 keV appear to be the result of shock drift acceleration.
On the occurrence of storm‐induced nighttime ionization enhancements at ionospheric middle latitudes
Authors: Belehaki A. , Tsagouri I.
Published: October 2002

Ionospheric observations from nine middle‐latitude stations are studied for five magnetic storms that occurred during September and October 2000. The correlation between various solar wind, magnetospheric and ionospheric parameters shows that the nighttime ionospheric response is strongly dependent on the conditions during which solar wind‐magnetosphere coupling occurred. Storms with initial compressive phase and rapidly evolving main phase have as a global effect the ionization depletion in the nightside at middle latitudes, independent of the storm intensity. These storms are caused by the abrupt dissipation of a large amount of energy input, resulting in the rapid expansion of the neutral composition disturbance zone equatorward, producing the observed negative effects in all middle latitude stations presented here. Gradually evolving geomagnetic storms, driven by slowing increasing southward IMF, result in the observation of positive effects at night in low to middle latitude stations. The weaker the intensity of the storm is, according to the Dst index, the more likely it is that one will observe nighttime ionization enhancements in subauroral latitudes as well. There are two competing mechanisms causing the observed effects; the expansion of the neutral composition disturbance zone results in negative effects, while downward plasmaspheric fluxes produce ionization enhancements at night. Gradually evolving storms are characterized by the restricted development of the neutral composition disturbance zone to higher latitudes, and the extent of its equatorward boundary depends on the intensity of the storm. During storms of this type, the role of plasmaspheric fluxes dominates at middle to low latitudes. Their effects are observable up to subauroral latitudes given that the neutral composition disturbance zone development is restricted to higher latitudes, as happens when the geomagnetic activity is of low or moderate intensity.
The nature of convectively unstable waveguide mode disturbances on the magnetospheric flanks
Authors: Wright Andrew N. , Mills Katharine J. , Longbottom Aaron W. , Ruderman Michael S.
Published: October 2002

Convectively unstable disturbances are often described in terms of propagating wave packets that grow and broaden. We extend such a description here and show how it recovers previous results and agrees with simulations. Applying these ideas to the magnetospheric flanks, we find that such wave packets will often not provide a driver of long enough duration for establishing a fully formed field line resonance (FLR). This suggest that the alternative description of the convective instability as a steady wave train radiated by a fixed source may be more appropriate for the flanks on some occasions. Whether the unstable waveguide modes prefer to exist as steady wave trains or convecting wave packets can have important implications for identifying the mechanisms through which the magnetosheath is coupled to the magnetosphere.
Relationship between multiple substorm onsets and the IMF: A case study
Authors: Cheng C.‐C. , Russell C. T. , Connors M. , Chi P. J.
Published: October 2002

The relationship between a series of Pi2 pulsations and their associated substorm onsets with the IMF is examined on 5 May 1999. The magnetograms from the CANOPUS array show the occurrence of three substorm onsets, accompanied by Pi2 bursts. Analysis of the auroral electrojet location and current intensity as deduced from the Churchill line in the CANOPUS array shows that the following onsets move poleward. ACE and Wind magnetic field data show that the first onset occurs during a period of weak southward interplanetary magnetic field (IMF) and that the second onset occurs about an hour later when the IMF has become slightly northward. The third onset occurs during strongly northward IMF. Magnetic disturbances at GOES 8 and GOES 10 confirm that each onset is accompanied by disturbances in the field like those expected from the substorm current wedge. Meanwhile, from the IMP 8 observations in the magnetotail, the decreases of the total and X components of the magnetic field imply that significant removal of magnetic flux from the tail has occurred after the third onset. Thus a single cycle of dayside reconnection transport of open flux and reconnection of that open flux may contain within it multiple onsets of tail reconnection and dipolarization of the night magnetosphere, some possibly not involving the open flux in the lobes.
On the proper interpretation of ionospheric conductance estimated through satellite photometry
Authors: Semeter Joshua , Doe Richard
Published: October 2002

As a consequence of the electron continuity equation, ionospheric conductance derived from photometric measurements of the aurora represents a root‐mean‐square over the field of view and exposure time of the photometer, and not a true average. The magnitude of the discrepancy between these estimates depends on the statistical variance of the electron density within the sampling window and is therefore strongly dependent on both the resolution of the sensor and the activity of the aurora. We use high‐resolution optical and incoherent scatter radar measurements to quantify the relationship among the instantaneous (“true”) conductance, the average conductance, and the photometrically derived conductance during an auroral substorm over Sondrestrom, Greenland. For a 36 s exposure time (typical for the ultraviolet imager (UVI) sensor on the Polar satellite), we show that the photometric estimate can be biased from the average value by 40% for Hall and 20% for Pedersen; the mean, in turn, can misrepresent the true conductance by more than 150% for Hall and 100% for Pedersen, owing to undersampling small‐scale variability. We develop two schemes for correcting (in the former case) and understanding (in the latter case) these effects using conjunctive ground‐based diagnostics.
Geosynchronous magnetic field temporal response to solar wind and IMF variations
Authors: Wing S. , Sibeck D. G. , Wiltberger M. , Singer H.
Published: October 2002

We investigate the time‐dependent response of the geosynchronous magnetic field to variations in the interplanetary magnetic field (IMF) orientation and sudden changes in the solar wind dynamic pressure. Because IMF discontinuities drape about the magnetopause, we expect the response to southward IMF turnings to occur nearly simultaneously over a wide range of local times. By contrast, the pressure variations launched by solar wind tangential discontinuities aligned with the spiral IMF into the magnetosheath should keep pace with the solar wind and generally strike the postnoon magnetopause first. Consistent with these predictions, we find that the magnetospheric magnetic field strength begins to diminish at all dayside local times within 4 to 5 min following the estimated arrival time of southward IMF turnings at the subsolar magnetopause, and that the nightside field begins to reconfigure within 12 min. By contrast, the response to pressure variations generally appears some 2.2 min after the arrival of the discontinuity in the noon‐dusk sector, 4.4 min later in the noon‐dawn sector, and 8.8 min later on the nightside.
Field‐aligned currents in the dayside cusp and polar cap region during northward IMF
Authors: Vennerstrøm S. , Moretto T. , Olsen N. , Friis‐Christensen E. , Stampe A. M. , Watermann J. F.
Published: October 2002

The field‐aligned currents in the dayside cusp and polar cap region are examined using magnetic data from the low‐altitude polar‐orbiting satellite Ørsted. The study is confined to cases where the interplanetary magnetic field (IMF) has a steady northward component and to a rather narrow region spanning ∼4 hours around magnetic noon. We examine individual passes using a maximum variance analysis method, and we complement, for a single event, with ground‐based data from the Greenland meridian chain of magnetometers. We suggest that when an east‐west component By of the IMF exists for positive IMF Bz, the two NBZ (northward Bz) field‐aligned currents that prevail over the polar region rotate to form the two field‐aligned currents equatorward and poleward of the east‐west flowing ionospheric DPY current in the dayside. The high accuracy of the Ørsted data makes it possible to uncover details not previously described.
Electrostatic electron cyclotron waves generated by low‐energy electron beams
Authors: Menietti J. D. , Santolik O. , Scudder J. D. , Pickett J. S. , Gurnett D. A.
Published: October 2002

We report the results of an investigation of waves observed by the Polar spacecraft at high altitudes and latitudes and at frequencies just above the cyclotron frequency. These observations are made frequently when the spacecraft is over the polar cap as well as near the dayside cusp and the nightside auroral region and for ratios of gyrofrequency to plasma frequency, fp/fg ∼ 1. We investigate the role of electron beams with E≲1 keV in the generation of these waves. Observed plasma parameters are used as input to a modification of the Waves in Homogeneous, Anisotropic Multi‐component Plasmas (WHAMP) computer code to place constraints on the free energy source and growth of these waves. We conclude these waves are an indicator of the presence of low‐energy electron beams and a cold electron component (E≲0.2 eV) in regions with fp/fg≲1.
Electron signatures and Alfvén waves
Authors: Andersson L. , Ivchenko N. , Clemmons J. , Namgaladze A. A. , Gustavsson B. , Wahlund J.‐E. , Eliasson L. , Yurik R. Yu.
Published: October 2002

We identify two distinct electron populations associated with Alfvén waves in the Freja data set using the high time resolution state of the art electron detector. One of the populations, detected together with an Alfvén wave, is field‐aligned and can be seen as trapped within the wave. The other electron population is detected before the wave and consists of electrons which have left the wave at a point with a velocity higher than the local Alfvén speed. In the paper, the electrons leaving wave are modeled for different density profiles and are compared with the observed data. Depending on the density profile, the model can produce the same energy‐time and pitch angle‐time dispersion that is observed in the Freja data. The conclusion of the paper is that the Alfvén wave can explain the observed particle signatures. It is shown that the Alfvén wave acceleration can create electron signatures similar to inverted‐V structures. The density distribution along a flux tube has an important role in the type of particle signatures that can be detected at low altitudes.
A non‐force‐free approach to the topology of magnetic clouds in the solar wind
Authors: Hidalgo M. A. , Cid C. , Viñas A. F. , Sequeiros J.
Published: October 2002

We present a new model for the topology of magnetic clouds at 1 AU that relates the magnetic field vector with the current density of the cloud without assuming the force‐free condition. In addition, the model is formulated in such a way that it can be fitted directly to the data without the need of initially determining the flux rope axis by minimum variance. The model provides both the magnetic field strength and the direction in just one fitting procedure. In addition to the orientation of its axis and the relative closest‐approach distance between the spacecraft and the cloud axis, the fit of the model to the experimental data allows an estimation of the current density of plasma inside the magnetic cloud. From our study on several clouds we conclude that this has values of the order of 10−12 C m−2 s−1. Compared with the force‐free model, although the improvement gained in the χ2 values is not very great, the fitting procedure is shorter and easier, and the number of parameters involved has been reduced from seven to five. Our model also lets us check the validity of force‐free approximation by the calculation of the value of the current density perpendicular to the flux rope, j⟂ = j × B/|B|. This component is assumed to be zero in the force‐free model, but for all analyzed cases we obtain a nonzero value. This finite value implies the existence of pressure gradients inside the cloud that cannot be explained with a force‐free approximation. In this paper, four cases are presented that have already been analyzed in the literature. We show that our model can explain the profiles of the magnetic field as a magnetic cloud passes a spacecraft.
Particle simulations of relativistic electron beam injection from spacecraft
Authors: Neubert Torsten , Gilchrist Brian E.
Published: October 2002

Linear accelerators producing relativistic (5 MeV) electron beams are now down to a size that allows them to be flown on spacecraft and sounding rockets. This opens up new opportunities for atmospheric/ionospheric modification experiments where the mesosphere and lower thermosphere regions can be perturbed down to 40‐km altitude. In this paper the relativistic electron beam injection process is investigated by means of three‐dimensional particle‐in‐cell simulations to determine the initial interaction of the beam with the spacecraft and the ambient plasma. The results indicate that relativistic beams are more stable than keV‐energy beams investigated in the past, allowing the injection and propagation of beams with currents several orders of magnitude higher than those for keV‐energy beams. The superior stability of relativistic beams is the result of a combination of effects including the higher relativistic electron mass, a lower beam density, and a smaller effect from spacecraft charging. Relativistic beams injected downward from spacecraft are therefore expected to deposit a large fraction of the energy in the middle atmosphere. In the high‐current limit (I > 100 A) the beam self‐fields are strong. In this regime a beam may propagate in the ion‐focused regime, where beam electrons expel ambient electrons to create a channel of ambient ions that space charge neutralize the beam. The establishment of the ion channel, however, creates significant turbulence and scattering.
A new perspective on plasma supply mechanisms to the magnetotail from a statistical comparison of dayside mirroring O+ at low altitudes with lobe/mantle beams
Authors: Seki K. , Elphic R. C. , Thomsen M. F. , Bonnell J. , McFadden J. P. , Lund E. J. , Hirahara M. , Terasawa T. , Mukai T.
Published: October 2002

Observations of tailward cold O+ beams (COBs) in the distant lobe/mantle shed new light upon plasma supply mechanisms to the magnetotail since their location up to a tailward distance of 210 RE is not explicable with a conventional view of magnetospheric dynamics. The COBs exist primarily in the mantlelike regions that correspond to the transport route of magnetic flux tubes reconnected at the dayside magnetopause, and thus it has been suggested that these high‐energy COBs in the distant lobe/mantle have originated from trapped O+ ions in the dayside magnetosphere. In order to examine the validity of this scenario the phase space density (PSD) of the COBs observed by Geotail is compared statistically with that of mirroring O+ ions around the cusp observed by FAST at low altitudes (400–4200 km) utilizing particle trajectory tracings in empirical magnetospheric models. The energy distribution of the averaged peak PSD of COBs is different at energies below and above ∼1 keV and thus suggests that more than one source contributes to the COBs. The mirroring O+ increases in quantity with increasing solar activity and suggest increment of trapped O+ ions in the dayside magnetosphere. A statistical comparison shows that the O+ PSD around the low‐altitude cusp is similar to that of COBs above ∼1 keV, while the COB PSD is typically higher than that of O+ at FAST at energies <∼1 keV. These results suggest that the trapped O+ in the dayside magnetosphere is a potential source of COBs at energies above 1 keV, while for COBs below 1 keV, polar O+ outflows from the cusp/cleft regions are the most probable source, as suggested by a conventional view.
Mean winds and tides in the Arctic mesosphere and lower thermosphere
Authors: Mitchell N. J. , Pancheva D. , Middleton H. R. , Hagan M. E.
Published: October 2002

A new meteor radar has been deployed at Esrange (68°N, 21°E) near Kiruna in Sweden. The radar has been used to measure winds and tides in the Arctic mesosphere/lower thermosphere region over the interval August 1999 to July 2000. Conspicuous features of the measured mean winds include a strong shear of up to 5 m s−1 km−1 in the summer zonal flow, resulting in eastward flows of in excess of 30 m s−1 at the upper heights observed (97 km). An equatorward meridional flow is observed in summer and winter, which reaches maximum values of ∼13 m s−1 in a summer jet. Poleward flow in comparison is weak and only occurs at the equinoxes. The monthly mean and seasonal mean behaviors of the amplitudes and phases of the 12‐ and 24‐hour tides are examined and compared to the Global Scale Wave Model‐98. The observations are generally very close to the model predictions for the 24‐hour tide and for the 12‐hour tide in winter. In other seasons, particularly summer, the observed 12‐hour tidal amplitudes are significantly larger than those predicted. In spring and summer the wavelength of the 12‐hour tide is significantly larger than that predicted.
ULF Pc5‐6 magnetic activity in the polar cap as observed along a geomagnetic meridian in Antarctica
Authors: Yagova N. V. , Lanzerotti L. J. , Villante U. , Pilipenko V. A. , Lepidi S. , Francia P. , Papitashvili V. O. , Rodger A. S.
Published: October 2002

Latitudinal and diurnal distributions of spectral power and spatial coherency parameters of the geomagnetic variations in the Pc5‐6 (1–6 mHz) frequency range are analyzed using data of magnetometer stations in Antarctica. The available stations give the possibility to form a latitude chain along the geomagnetic meridian 40°E stretching from magnetic latitude 69°S to 86°S. Long‐period ULF activity at polar cap latitudes is characterized by lower amplitudes and wider spectra with lower central frequencies as compared with typical auroral Pc5 pulsations. The meridional distribution of average Pc5‐6 spectral power is nonmonotonic and has a minimum near 80°. In general, the low‐frequency broadband ULF activities in the polar cap and at auroral latitudes seem to be decoupled. This long‐period ULF activity in the polar cap could be an image of wave activity in the tail lobes or the manifestation of turbulent component of the ionospheric convection at very high latitudes, but this requires further investigation.
Leptons with E > 200 MeV trapped in the Earth's radiation belts
Authors: Fiandrini E. , Esposito G. , Bertucci B. , Alpat B. , Battiston R. , Burger W. J. , Lamanna G. , Zuccon P.
Published: October 2002

For the first time, accurate measurements of electron and positron fluxes in the energy range 0.2–10 GeV have been performed with the Alpha Magnetic Spectrometer (AMS) at altitudes of 370–390 km in the geographic latitude interval ±51.7°. We describe the observed under‐cutoff lepton fluxes outside the region of the South Atlantic Anomaly. The separation in quasi‐trapped, long‐lifetime (O(10 s)), and albedo, short‐lifetime (O(100 ms)), components is explained in terms of the drift shell populations observed by AMS. A significantly higher relative abundance of positrons with respect to electrons is seen in the quasi‐trapped population. The flux maps as a function of the canonical adiabatic variables L, α0 are presented for the interval 0.95 < L < 3, 0° < α0 < 90° for electrons (E < 10 GeV) and positrons (E < 3 GeV). The results are compared to existing data at lower energies. The properties of the observed under‐cutoff particles are also investigated in terms of their residence times and geographical origin.
Geomagnetic negative sudden impulses: Interplanetary causes and polarization distribution
Authors: Takeuchi T. , Araki T. , Viljanen A. , Watermann J.
Published: October 2002

We made a study of the characteristics of geomagnetic negative sudden impulses (SI−s) identified in the midlatitude geomagnetic SYM indices and the causative structures in the solar wind using data from the Wind and ACE spacecraft. A total of 28 SI−s with an amplitude larger than 20 nT in the H component SYM index were found over the period 1995 through 1999, with 50% of them occurring in conjunction with a positive sudden impulse, SI+ (i.e., SI pair). In the SI pairs the amplitude of SI− was almost always larger than that of the preceding SI+. We attempted for the first time a classification of structures in the solar wind associated with SI−s. It is found that reverse shocks are not responsible for SI−s. Instead, SI−s are associated with varied structures such as tangential discontinuities at high‐low speed stream interfaces, front boundaries of interplanetary magnetic clouds, and trailing edges of heliospheric plasma sheets. There is no preferential association of SI−s in our sample with any particular type of solar wind structure. We investigated statistically the polarization characteristics of SI−s at high latitude. The sense of the polarization in the auroral zone tended to be clockwise in the afternoon and counterclockwise in the morning. The rotational sense reversed in the polar cap. The latitudinal reversal occurred in the range from 65° to 80°. Thus the polarization distribution of SI− is not opposite to but is consistent with that of SI+. 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−.
A magnetohydrodynamic model of plasma flow in the high‐altitude cusp
Authors: Taylor M. G. G. T. , Cargill P. J.
Published: October 2002

Using a quasi‐one‐dimensional (self‐similar) form of the magnetohydrodynamic equations, a model of the interaction of shocked magnetosheath plasma flow with an indented high‐altitude cusp is presented. The model applies only to a closed magnetopause. It is shown that the resulting plasma flow into the cusp indentation is highly dependent on the ratio of the plasma velocity to the magnetosonic wave speeds. If the magnetosheath velocity is in excess of the slow mode wave speed, the flow at the indentation is associated with a plasma compression accompanied by a minimal rotation of the flow into the cusp. The behavior of the density differs from that usually associated with self‐similar flows and is due to the relative phase between density, field, and flow associated with the slow mode wave. For flows in excess of the fast mode speed the flow rotates significantly into the cusp by means of a fast mode expansion wave. We propose that the slow mode wave model is applicable for the summertime cusp and that the fast mode one is applicable for the wintertime cusp.
Cusp aurora dependence on interplanetary magnetic field Bz
Authors: Fuselier S. A. , Frey H. U. , Trattner K. J. , Mende S. B. , Burch J. L.
Published: October 2002

The Imager for Magnetopause to Aurora Global Exploration (IMAGE) Spectrographic Imager (SI12) detects Doppler‐shifted Lyman alpha emissions created by charge exchange and de‐excitation of precipitating protons in the atmosphere. At high latitudes near local noon, emissions consistent with the location of the cusp foot point have distinct interplanetary magnetic field (IMF) dependence. For northward IMF a spot of emissions is observed poleward of the dayside auroral oval. By tracing the magnetic field lines from this spot using a model magnetosphere it is shown that the cusp foot point maps to a narrow region on the high‐latitude magnetopause where antiparallel magnetic reconnection may be occurring. As the IMF turns southward, the spot merges into the auroral oval, producing a broad region of intense emissions centered near local noon. By tracing the magnetic field lines from this broad region using a model magnetosphere it is shown that the cusp foot point maps to a relatively broad region on the dayside magnetopause where component magnetic reconnection may be occurring.
Modeling energetic particle injections in dynamic pulse fields with varying propagation speeds
Authors: Sarris Theodore E. , Li Xinlin , Tsaggas N. , Paschalidis N.
Published: October 2002

Dispersionless injections are a ubiquitous characteristic of substorms. They are defined as simultaneous enhancements in the fluxes of electrons and ions of different energies, and they are often observed near or inside geosynchronous orbit. We model dispersionless electron injections by considering the interaction of an earthward propagating electromagnetic pulse with the preexisting electron population. Such simulations have been performed previously [Li et al., 1993, 1998]; however, they assumed a constant propagation velocity for the transient fields. Observations have shown that substorm injections and associated magnetic signatures do not propagate at constant velocities, but rather slow down as they approach the inner magnetosphere. Between 4.5 and 6.6 RE the injection propagation speeds reach surprisingly low values, of the order of 24 km/s. Nonetheless, the injections still remain dispersionless [Reeves et al., 1996]. In our simulation we vary the pulse speed with the radial distance from the Earth to match the reported propagation speeds and demonstrate that dispersionless injections are achievable under such low propagation speeds. In particular, we simulate the dispersionless injections of 12 February 1991 measured at two radially displaced spacecraft (CRRES and LANL 1990–095), when they were both around local midnight.