bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation of auroral power with the polar cap index
Authors: Liou K. , Carbary J. F. , Newell P. T. , Meng C.‐I. , Rasmussen O.
Published: May 2003

We compare auroral power (AP) as derived from a global imager with the polar cap (PC) index for the periods of December 1996 through January 1997 (winter) and June 1997 through July 1997 (summer). The auroral power is inferred from auroral luminosity in the N2 Lyman‐Birge‐Hopfield bands (λ ∼ 160–180 nm) acquired from the ultraviolet imager on board the ISTP/GGS program's Polar satellite in the Northern Hemisphere. The PC index (1‐min time resolution) is based on the horizontal component of the magnetometer recordings from a single near‐geomagnetic polar station at Thule (85.4° corrected geomagnetic latitude). A fair correlation exists between the hemispheric auroral power and PC (r = 0.70), with higher correlation for winter (r = 0.77) compared to summer (r = 0.69). Also, PC correlates with nightside auroral power much better than with dayside auroral power. The best AP‐PC correlation took place in the pre‐dawn and post‐dusk sectors. The seasonal and diurnal effects are a result of competition between the ionospheric convection currents and field‐aligned currents manifested by seasonal and diurnal variations in ionospheric conductivity. Finally, a comparison of the AP‐PC correlation between substorm and non‐substorm times indicates that substorm electrojets contribute little, if not at all, to the PC index.
A parametric study of the influence of ion and electron properties on the excitation of electromagnetic ion cyclotron waves in coronal mass ejections
Authors: Dasso Sergio , Gratton Fausto T. , Farrugia Charles J.
Published: May 2003

Interplanetary coronal mass ejections (ICMEs) often possess a negative proton thermal anisotropy, Ap = T⟂,p/T∥,p − 1 < 0 (T∥, T⟂: parallel and perpendicular temperatures, respectively) so that right‐hand polarized electromagnetic ion cyclotron waves (EICWs) may be amplified by a kinetic instability [Farrugia et al., 1998a]. However, in view of the low proton beta of ICMEs, several physical parameters, besides Ap, need to be in the right range to excite this instability with significant growth rates. In this paper we present a parametric study of EICWs aimed at identifying those parameters which are most influential in fostering the emission of these waves in ICME scenarios. We analyze here the influence of: (1) thermal and suprathermal protons, (2) thermal alpha particles (αs), and (3) thermal electrons. We solve the dispersion relation of EICWs including protons, αs and electrons, all modeled with bi‐Maxwellian distribution functions, and a minority population of suprathermal protons using a kappa function for the velocity component along the field. For physical regimes of ICMEs we find that the instability depends critically on the values of the following parameters: proton beta, proton thermal anisotropy, relative abundance of the suprathermal protons, α‐to‐proton relative abundance, α‐to‐proton temperature ratio, α particle thermal anisotropy, electron‐to‐proton temperature ratio, and thermal anisotropy of electrons. The effect of these parameters on the instability is either direct (when they increase the number of resonant particles) or indirect (when they decrease the phase speed of the wave so that more particles can resonate). Data surveys on EICWs should take into account the whole set of parameters indicated here, since the expected level of wave excitation results from their combined action. The study may be useful in understanding the considerable level of magnetic fluctuations observed in interplanetary CMEs by the Wind spacecraft.
Cyclotron acceleration of radiation belt electrons by whistlers
Authors: Trakhtengerts V. Y. , Rycroft M. J. , Nunn D. , Demekhov A. G.
Published: May 2003

After reviewing briefly the theory of the gyroresonant interactions between a quasimonochromatic whistler‐mode wave and energetic electrons trapped in the magnetosphere, we extend this theory to consider such interactions for a natural whistler arising from a lightning discharge in the Earth's atmosphere. It is shown that, near the equatorial plane of the magnetosphere, whistler components above the nose frequency can accelerate energetic electrons. This acceleration takes place when the gyroresonant electrons are trapped by the wave field. The acceleration rate in this regime is much greater than is stochastic acceleration in the untrapped regime. It is not accompanied by pitch angle scattering which characterizes the untrapped regime. For example, at L = 3, a gyroresonant electron with an energy of ∼6 keV and a pitch angle of 45° could have its energy increased by ∼24% to 7.4 keV and its pitch angle changed to 70° after a single interaction with a whistler whose frequency changes from 1/3 to 1/2 the equatorial gyrofrequency. Highly anisotropic distributions of van Allen radiation belt electrons with “pancake” pitch angle distributions can result from such an acceleration.
Interstellar magnetic field effects on the termination shock, heliopause, and bow shock: Aligned MHD flow
Authors: Ratkiewicz Romana , McKenzie James F.
Published: May 2003

In the paper we examine the numerical results for the case of aligned interstellar MHD flow with fixed boundary conditions for the solar wind and the interstellar plasma whose magnetic field strength is varied between 1.0 to 3.0 μG. The results for aligned flow show how variations of the upstream Alfvén Mach number can cause substantial changes in the location of the bow shock and heliopause near the stagnation line. The heliopause experiences a squeezing effect, which increases with decreasing Alfvén Mach number. At a critical magnetic field strength the structure of the heliospheric boundary changes dramatically, moving outward from the Sun by virtue of a sucking effect. The purpose of the paper is to explain the mechanisms which can give rise to a sucking effect of the heliopause.
A new method for reconstruction of the vertical electron density distribution in the upper ionosphere and plasmasphere
Authors: Stankov Stanimir M. , Jakowski Norbert , Heise Stefan , Muhtarov Plamen , Kutiev Ivan , Warnant Rene
Published: May 2003

Ground‐based ionosphere sounding measurements alone are incapable of reliably modeling the topside electron density distribution above the F layer peak density height. Such information can be derived from Global Positioning System (GPS)‐based total electron content (TEC) measurements. A novel technique is presented for retrieving the electron density height profile from three types of measurements: ionosonde (foF2, foE, M3000F2, hmf2), TEC (GPS‐based), and O+‐H+ ion transition level. The method employs new formulae based on Chapman, sech‐squared, and exponential ionosphere profilers to construct a system of equations, the solution of which system provides the unknown ion scale heights, sufficient to construct a unique electron density profile at the site of measurements. All formulae are based on the assumption of diffusive equilibrium with constant scale height for each ion species. The presented technique is most suitable for middle‐ and high‐geomagnetic latitudes and possible applications include: development, evaluation, and improvement of theoretical and empirical ionospheric models, development of similar reconstruction methods utilizing low‐earth‐orbiting satellite measurements of TEC, operational reconstruction of the electron density on a real‐time basis, etc.
Particle simulation of plasma response to an applied electric field parallel to magnetic field lines
Authors: Omura Y. , Heikkila W. J. , Umeda T. , Ninomiya K. , Matsumoto H.
Published: May 2003

We study response of thermal plasmas to an induction electric field via one‐dimensional particle simulations. The induction electric field is assumed to be uniform in space and constant in time. Because of acceleration of electrons and ions in the opposite directions, there arise counter streaming electrons and ions that cause the Buneman instability. Depending on the ratio of the ion temperature Ti to the electron temperature Te, responses to the electric field are different. For a case with hot ions (Ti ≫ Te) the Buneman instability leads to formation of large isolated electrostatic potentials which trap some electrons to move with ions. For a case with colder ions (Ti ≪ Te) the Buneman instability is taken over by excitation of ion acoustic waves, which diffuse the low‐energy part of the accelerated electrons to stabilize the instability. However, a substantial part of the electrons are grouped together at the high‐energy part, forming a distinct bump in the electron distribution. In the present simulations we have found that the induction electric field can form an electron beam along the magnetic field line. Since the electron beam leaves the region of the induction electric field and moves into an unperturbed plasma, the accelerated electrons can cause a bump‐on‐tail instability. This can lead to formation of electrostatic solitary waves as frequently observed by the GEOTAIL spacecraft in the plasma sheet boundary layer (PSBL). The persistent observation of the electrostatic solitary waves indicates their association with the induction electric field that results from meandering motion of the current sheet in the magnetotail.
Which is a significant contributor for outside of the plasmapause, an ionospheric filling or a leakage of plasmaspheric materials?: Comparison of He II (304 Å) images
Authors: Yoshikawa I. , Yamazaki A. , Yamashita K. , Takizawa Y. , Nakamura M.
Published: May 2003

The EUV imagery of the plasmasphere was taken by the He II (304 Å) scanner on 20–21 September 1998 after the first success on 9–10 September [Nakamura et al., 2000]. Total amount of the plasma seen in the outside of the plasmapause is inconsistent between the first and the second images, although geomagnetic and solar conditions are almost the same. These plasmas are directly filled from the ionosphere or continuously leak from the plasmasphere. We conclude that the contribution of the plasma leakage from the plasmasphere is as significant as the contribution of the direct filling from the ionosphere even under a quiet/moderate geomagnetic condition.
Controlled precipitation of radiation belt electrons
Authors: Inan U. S. , Bell T. F. , Bortnik J. , Albert J. M.
Published: May 2003

First‐order estimates indicate that the lifetime of energetic (a few MeV) electrons in the inner radiation belts (e.g., near L = 2) may be significantly reduced by in situ injection of whistler mode waves at radiated power levels of a few kW at frequencies of a few kHz. Our estimates are based on previously published results concerning the effect on the electron lifetimes of VLF signals from ground‐based VLF transmitters operating in the 17–23 kHz range. Waves at lower frequencies (a few kHz) can drive diffusion rates that are higher by a factor of as much as ∼30 and can also be efficiently stored in the magnetospheric cavity, resulting in additional effective enhancement of wave power density of a factor of ∼16. This wave power enhancement is also expected to enhance the MeV electron diffusion rates.
Empirical model of proton fluxes in the equatorial inner magnetosphere: 2. Properties and applications
Authors: Milillo A. , Orsini S. , Delcourt D. C. , Mura A. , Massetti S. , De Angelis E. , Ebihara Y.
Published: May 2003

The most probable magnetospheric condition during solar minimum is investigated by using the empirical model of the equatorial proton fluxes presented by Milillo et al. [2001]. The model consists of several segments that reproduce the effects of different physical processes. Those segments describing particle injection and diffusion in the equatorial magnetosphere are considered. The model provides an overall picture of the magnetosphere plasma distributions, thus allowing estimates of the major physical parameters. From this statistical distribution we reconstruct the pattern of the electric field equipotential lines on the equatorial plane. We compute drift paths of test particles at different initial energies and positions, and we focus on both open and closed trajectories and examine through energy‐time spectrograms typical features, like dawn‐dusk asymmetries of particle transport or flux depletions at specific energies (ion spectral gaps). Some selected parameters of the model are properly varied in order to investigate its possible capability to reproduce different magnetospheric conditions.
Quasi‐steady drift paths in a model magnetosphere with AMIE electric field: Implications for ring current formation
Authors: Chen Margaret W. , Schulz Michael , Lu Gang , Lyons Larry R.
Published: May 2003

Recent measurements show that magnetospheric convection electric fields during the main phases of magnetic storms are much more complicated in spatial structure than the electric fields that have generally been used to model formation of the stormtime ring current. To investigate the transport effects of such more realistic stormtime electric fields on magnetospheric charged particles, we map the Assimilative Model of Ionospheric Electrodynamics (AMIE) electric potential functions analytically with a simple magnetic field model at selected times of interest during several magnetic storms from 1997–1998 and during the extremely large storm of July 2000. We calculated corresponding contours of constant Hamiltonian (kinetic plus potential energy) for first invariant values that range from 0 to 30 MeV/G (representative of equatorially mirroring cold plasma, ring current, and radiation‐belt ions and electrons). These equatorial contours would constitute drift paths if the electric field were truly constant in time. We thus calculated how far along such quasi‐drift paths the corresponding particles would have drifted after specific amounts of time, and we compare these quasi‐drift characteristics with those obtained from a simple semiempirical model of the convection electric field. We find considerable variability among stormtime equatorial quasi‐drift paths, reflecting the known variability of AMIE equipotentials. Patterns of equatorial quasi‐drift in the simplified electric field model are (by construction) symmetric about the dawn‐dusk meridian. During the main phases of storms, the equatorial electric field derived from AMIE tends to be strongest in an MLT sector several hours wide on the night side. This concentration of AMIE equipotentials provides a channel for rapid transport (requiring ∼20–30 min) for ions with first invariant values representative of the ring current population from the nightside neutral line to low L values (∼3–4) near dusk, where the partial ring current forms. During the extremely large “Bastille Day” storm of 15 July 2000 (minimumDst= −300 nT) the drift patterns derived from AMIE show penetration of ions to as low asL∼ 2. This deep penetration of ring current ions could help to account for the very strong ring current that was observed during this storm. The quasi‐steady state drift properties help us anticipate the implications of a more realistic electric field model for the particle drifts that lead to the formation of the stormtime ring current.
Tidal signatures and aliasing in temperature data from slowly precessing satellites
Authors: Oberheide J. , Hagan M. E. , Roble R. G.
Published: May 2003

Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME‐GCM) data are used to simulate tidal patterns in satellite‐borne observations of the mesosphere and lower thermosphere (MLT) region. The results are given for the measurement geometry of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, though the technique is applicable to any satellite‐based instrument. TIME‐GCM tides are sampled along the instrument measurement track and analyzed in terms of ascending/descending orbit node differences. Our simulations show a significant aliasing, e.g., of migrating and nonmigrating diurnal and semidiurnal components into tidal fields of observed zonal wavenumbers s′ ≥ 0. Above the mesopause, the model tides induce a 10 K bias in daily means of zonally averaged temperatures. An estimate of the tide‐induced vertical displacement of air parcels is given, assuming tidal motions are adiabatic. Corresponding signatures are expected to be present in MLT observations of chemical species. Addressing tidal‐sampling issues is of general interest for comparing satellite observations with models, climatologies, and other measurements.
On resonantly excited MHD waves in the magnetotail
Authors: Erdélyi R. , Taroyan Y.
Published: May 2003

The resonant flow instability (RFI) and the Kelvin‐Helmholtz instability (KHI) are investigated as possible wave‐generating mechanisms in the mantle‐like boundary layer of the Earth's magnetotail where all equilibrium quantities transit continuously from magnetosheath values to values more characteristic of the tail lobe. It is shown that as in the case of a sharp interface, the KHI requiring high flow speeds in the magnetosheath is unlikely to be operative under typical conditions. The RFI which is physically distinct from the KHI may appear at lower flow speeds owing to the inhomogeneity of the mantle‐like boundary layer. It is shown that the RFI can be important when the variation length‐scale of the flow velocity is smaller than the variation length‐scales of other equilibrium quantities such as the density and the magnetic field strength. Interpretation in terms of the wave energy flux is presented and the applicability to the magnetotail is discussed.
Signs of magnetic helicity in interplanetary coronal mass ejections and associated prominences: Case study
Authors: Ruzmaikin Alexander , Martin Sara , Hu Qiang
Published: May 2003

We study the distribution of magnetic helicity in interplanetary coronal mass ejections (CMEs) separated into magnetic clouds and remnants of the prominence structures for three cases with identified remnants of prominences. The magnetic configurations of magnetic clouds and prominence remnants can satisfactorily be described by the flux rope approximation. It is shown that the magnetic helicities in both parts of the CMEs have the same sign. This sign is consistent with the dominance of CMEs with negative helicity in the northern hemisphere and positive in the southern hemisphere. The results are discussed in the context of different models of CME eruption at the Sun.
Simultaneous EISCAT Svalbard radar and DMSP observations of ion upflow in the dayside polar ionosphere
Authors: Ogawa Y. , Fujii R. , Buchert S. C. , Nozawa S. , Ohtani S.
Published: May 2003

Regions where dayside field‐aligned (FA) ion upflows occur are identified, and the relative occurrences and characteristics are compared. The study is based on ∼170 simultaneous events observed with the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and spacecraft from the DMSP. We found that ion upflows occur not only in the cusp and cleft (the low‐altitude portion of the low‐latitude boundary layer), which traditionally have been regarded as regions of ion upflow, but also in the region connected to the mantle. Ion upflows are less frequently seen in the boundary plasma sheet (BPS) and are very rarely seen in the central plasma sheet at high latitude on the dayside. Almost all of the events in which the average FA ion velocity is >100 m s−1 are associated with relatively high soft electron precipitation (differential energy flux of electrons at 100 eV > 107 eV cm−2 s−1 sr−1 eV−1), although soft electron precipitation with similarly high flux exists also in the BPS, where the ion velocities are mostly <100 m s−1. These results indicate that soft particle precipitation is the predominant energy source driving ion upflow in the topside ionosphere, but it triggers ion upflow effectively not in the BPS, only in the other high‐latitude regions on the dayside.
Pc5 micropulsation power at conjugate high‐latitude locations
Authors: Ballatore Paola
Published: May 2003

The micropulsation power, integrated over the Pc5 frequency range, has been calculated for the horizontal component of the geomagnetic field at two high‐latitude conjugate locations: Dumont D'Urville (corrected geomagnetic coordinates: 80.61°S, 235.76°E) and Mould Bay (corrected geomagnetic coordinates: 80.85°N, 272.65°E). Because of the different distances between the geographic and the geomagnetic poles in each hemisphere, the comparison between the Pc5 power observed at Dumont D'Urville and at Mould Bay shows the relative importance of geomagnetic and solar illumination effects in driving low‐frequency micropulsation activity. In particular, similarities observed at the two sites can be explained in terms of their common geomagnetic characteristics, while differences can be attributed to the different sunlight or solar zenith angle configurations. Results show that the local summer Pc5 power is statistically higher in the northern hemisphere than in the southern one. This hemispherical difference is smaller for the local equinoxes, and it is only very slight or absent for local winters. These findings are interpreted in terms of the proportionality between the Pc5 power and the ionospheric conductance, which is higher at Mould Bay owing to more permanent and direct sunlight conditions during local summers and equinoxes. Thus the different geographic coordinates affect the Pc5 power at the two considered sites so much so that their effect is visible regardless of the geomagnetic similarities. However, the influence of the geomagnetic activity on Pc5 power is found to be more significant than these geographical effects or than the seasonal effects. In fact, for Kp < 2 the difference in simultaneous observations at Mould Bay and at Dumont D'Urville is <∣10∣ nT2 with an occurrence >70%. The magnetic local time modulation of Pc5 power is similar in both hemispheres, since it is determined by the equivalent geomagnetic characteristics and regardless of possible geographical differences. In particular, the occurrence of higher Pc5 power observed at Mould Bay than at Dumont D'Urville is not localized in one specific magnetic local time sector. The present observations are compared with previous results regarding solar illumination effects on geomagnetic activity and on auroral brightness. Finally, no solar cycle effect is observed on either the Pc5 power level, on its hemispherical dependence, or on its daily modulation.
Relationship between ionospheric conductivity and intensity of the daytime region 1 field‐aligned current in geomagnetically quiet conditions
Authors: Yamamoto T. , Ozaki M. , Inoue S.
Published: May 2003

The relationship between the region 1 field‐aligned current (FAC) intensity and the ionospheric conductivity in geomagnetically quiet conditions has been believed to be useful to determine whether the FAC is driven by a voltage generator or a current generator. This paper, however, shows that at least for daytime (0800–1600 MLT) FACs, the current generator has the same characteristic of “linear relationship between the current intensity and the conductivity” as the voltage generator. This conclusion is obtained due to the fact that the sum of Pedersen conductivities, maintained by solar EUV ionization, at two conjugate points on the northern and southern ionospheres is approximately independent of the solar zenith angle at either of these points. Notably, the pressure‐gradient‐driven model for the generation of the region 1 FAC from the low‐latitude boundary layer is then consistent with observations that the current intensity increases linearly with the height‐integrated Pedersen conductivity.
Simulations of the thermospheric response to the diffuse aurora using a three‐dimensional high‐resolution model
Authors: Parish H. F. , Walterscheid R. L. , Jones P. W. , Lyons L. R.
Published: May 2003

Recent observations within the substorm recovery phase postmidnight diffuse aurora reveal phenomena that are not well understood. Strong neutral winds are seen within the postmidnight diffuse aurora in measurements from the series of Atmospheric Response in Aurora (ARIA) campaigns, which show a great deal of vertical structure and large vertical shears. The ARIA observations also consistently show a peak in the wind magnitude between around 110 and 120 km, which has not been well explained, with a strong shear below the peak that is sometimes found to be unstable, with a Richardson number < 0.25. A three‐dimensional (3‐D) high‐resolution model developed recently at UCLA has been used to perform simulations to try to understand the poorly explained phenomena found in the ARIA measurements. Model simulations show that large zonal and meridional winds can be produced in the region of the postmidnight diffuse aurora using auroral forcing parameters within the range of observations. A wind maximum is produced in the 110–120 km altitude range, within the same height range as the maxima found in the ARIA observations, using auroral forcing alone. When sufficiently high vertical resolution is used for auroral forcing parameters within the range of observations, the region below the peak may be unstable, with a Richardson number < 0.25. When simulations are performed with auroral forcing based on measurements from the ARIA I campaign, and background winds and tides are introduced from the Coupled Thermosphere–Ionosphere Plasmasphere (CTIP) model [Fuller‐Rowell et al., 1987; Bailey et al., 1997], the basic features of the vertical structure of the ARIA I wind measurements are reproduced, although the magnitude of the peak around 115 km altitude is somewhat smaller than that observed. The 115 km peak is likely produced by a combination of auroral and tidal forcing processes. The vertical structure of the winds is sensitive to the phases of the tidal modes. Improvements in the ability of the model to simulate the observations from the ARIA campaign may be due to the use of the 3‐D rather than a 2‐D high‐resolution model.
Analyses on the geometrical structure of magnetic field in the current sheet based on cluster measurements
Authors: Shen C. , Li X. , Dunlop M. , Liu Z. X. , Balogh A. , Baker D. N. , Hapgood M. , Wang X.
Published: May 2003

The geometrical structure of the magnetic field is a critical character in the magnetospheric dynamics. Using the magnetic field data measured by the Cluster constellation satellites, the geometrical structure including the curvature radius, directions of curvature, and normal of the osculating planes of the magnetic field lines within the current sheet/neutral sheet have been investigated. The results are (1) Inside of the tail neutral sheet (NS), the curvature of magnetic field lines points towards Earth, the normal of the osculating plane points duskward, and the characteristic half width (or the minimum curvature radius) of the neutral sheet is generally less than 2 RE, for many cases less than 1600 km. (2) Outside of the neutral sheet, the curvature of magnetic field lines pointed northward (southward) at the north (south) side of NS, the normal of the osculating plane points dawnward, and the curvature radius is about 5 RE ∼ 10 RE. (3) Thin NS, where the magnetic field lines have the minimum of the curvature radius less than 0.25 RE, may appear at all the local time between LT 20 hours and 4 hours, but thin NS occurs more frequently near to midnight than that at the dawnside and duskside. (4) The size of the NS is dependent on substorm phases. Generally, the NS is thin during the growth and expansion phases and grows thick during the recovery phase. (5) For the one‐dimensional NS, the half thickness and flapping velocity of the NS could be quantitatively determined. Therefore the differential geometry analyses based on Cluster 4‐point magnetic measurements open a window for visioning the three‐dimensional static and dynamic magnetic field structure of geomagnetosphere.
Electron density distribution over the northern polar region deduced from IMAGE/radio plasma imager sounding
Authors: Nsumei P. A. , Huang X. , Reinisch B. W. , Song P. , Vasyliunas V. M. , Green J. L. , Fung S. F. , Benson R. F. , Gallagher D. L.
Published: May 2003

Electron density Ne distributions in the northern polar region are determined from 1.4 to 5.0 RE at invariant latitudes greater than 70° using the radio plasma imager (RPI) on the IMAGE satellite over the period from June 2000 to November 2001. An empirical Ne model in this region is derived as a function of radial distance and geomagnetic activity. A power law with a power index of −5.09 ± 0.03 describes the variation of Ne with radial distance. The Ne appears to increase exponentially with the Kp index. The altitude dependence of the empirical model at low geomagnetic activity is qualitatively consistent with previous models derived from the ISIS 1 and the Dynamic Explorer 1 spacecraft.
2.5‐D fluid simulations of the solar wind interacting with multiple dipoles on the surface of the Moon
Authors: Harnett Erika M. , Winglee Robert M.
Published: May 2003

Initial two‐dimensional (2‐D) MHD simulations indicated that mini‐magnetospheres can form around magnetic anomalies on the surface of the Moon but required magnetic field strengths at 100 km above the surface an order of magnitude larger than in situ measurements. Modeling the lunar magnetic anomalies with multiple dipoles in 2.5‐D MPD simulations inflates the size of the mini‐magnetospheres for only small increases in the magnitude of the total magnetic field. Multiple dipoles increase the lateral distance over which solar wind plasma is held off the surface. This extended magnetic field geometry inflates the mini‐magnetosphere by inhibiting fluid flow within the shock region. With multiple dipoles, a mini‐magnetosphere will form with magnetic field magnitudes smaller than the lower limit for a single dipole. These results indicate that the higher order moments of the anomalous magnetic fields play a significant role in deflecting the solar wind and determining the size and shape of the mini‐magnetosphere.
Hybrid simulations of solar wind interaction with magnetized asteroids: Comparison with Galileo observations near Gaspra and Ida
Authors: Blanco‐Cano X. , Omidi N. , Russell C. T.
Published: May 2003

We use a two‐dimensional hybrid code (fluid electrons and particle ions) to simulate the interaction of the solar wind with a magnetized asteroid. Understanding the nature of this interaction and of the signatures such as whistler waves that can be generated in the perturbed region can help us to establish the strength of magnetization of the asteroid. We model the asteroid magnetic field as a dipole with different strengths and consider different geometries between the interplanetary magnetic field (IMF) and the solar wind flow. We find that the characteristics of the wake change dramatically with asteroidal magnetic moment. When the magnetic moment is small a whistler wake is generated. Stronger dipolar moments generate a wake with magnetosonic characteristics. Galileo data showed the existence of magnetic perturbations near asteroids Gaspra and Ida. In contrast, no magnetic signature was observed by the NEAR‐Shoemaker spacecraft in the vicinity of asteroid 433 Eros. Previous works have compared Galileo observations with Hall‐MHD simulations. In this work we compare the results of hybrid simulations with Galileo observations near Gaspra and Ida. Our simulations show that when the IMF is perpendicular to the solar wind flow, the interaction of the plasma with a magnetized asteroid generates a perturbation downstream of the asteroid that is formed by whistler and magnetosonic waves. This perturbation resembles in some ways the signature observed by Galileo in the vicinity of Gaspra. However, the discrepancies found between observations and simulated signatures lead us to conclude that the perturbation near Gaspra was not generated by the interaction of the solar wind with a magnetized asteroid. In the case of Ida the IMF is at 45° to the flow, and our simulation results differ from observations. The wave wake forms upstream of the asteroid at a distance that is small compared with observations and has a small size. Therefore we believe that the signature observed near Ida was not generated by the interaction of the solar wind with the asteroid. In both cases the observed signatures are linearly polarized resembling the magnetic discontinuities that are commonly found in the solar wind and in contrast to the nearly circularly polarized signatures of the simulations.
Modeling the inner plasma sheet protons and magnetic field under enhanced convection
Authors: Wang Chih‐Ping , Lyons Larry R. , Chen Margaret W. , Wolf Richard A. , Toffoletto Frank R.
Published: May 2003

In order to understand the evolution of the protons and magnetic field in the inner plasma sheet from quiet to disturbed conditions, we incorporate a modified version of the Magnetospheric Specification Model (MSM) with a modified version of the Tsyganenko 96 (T96) magnetic field model to simulate the protons and magnetic field under an increasing convection electric field with two‐dimensional (2‐D) force balance maintained along the midnight meridian. The local time dependent proton differential fluxes assigned to the model boundary are a mixture of hot plasma from the mantle and cooler plasma from the low latitude boundary layer (LLBL). We previously used this model to simulate the inner plasma sheet under weak convection corresponding to a cross polar cap potential drop (ΔΦPC) equal to 26 kV and obtained 2‐D quiet time equilibrium for proton and magnetic field that agrees well with observations. We start our simulation for enhanced convection with this quiet time equilibrium and time‐independent boundary particle sources and increase ΔΦPC steadily from 26 to 146 kV in 5 hours. Simulations are also run separately to steady states by keeping ΔΦPC constant after it is increased to 98 and 146 kV. The magnitudes of proton pressure, number density, and temperature and their increase from quiet to moderate activity (ΔΦPC = 98 kV) are consistent with most observations. Our simulation at high activity (ΔΦPC = 146 kV) underestimates the observed pressure and temperature. This disagreement indicates possible dependence of the boundary particle sources on activity and possible effects of solar wind dynamic pressure enhancements that have not yet been included in our simulation. The simulated equatorial pressures and temperatures show stronger enhancement on the dusk side than on the dawn side as convection is increased, while density profiles show an increase on the dawn side and a decrease on the dusk side. The simulated proton flow speed at the equatorial plane increases with enhancing convection while the overall flow direction does not change significantly, a result of enhancement in both the earthward electric drift and the azimuthal diamagnetic drift. The equatorial magnetic field strength decreases more in the near‐Earth plasma sheet than at larger radial distances as ΔΦPC increases, resulting in an increasing flat radial profile with enhancing convection. The feedbacks from diamagnetic drift and magnetic fields to increasing convection are found to restrain the pressure increase. Based on the good agreement between our results and observations at moderate activity, our magnetic field indicates that the plasma and magnetic field in the plasma sheet can be in a state far from possible force balance inconsistency during periods of moderately enhanced convection. A scale analysis of our results shows that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet for moderate activity.
Fine structure of the diamagnetic cavity boundary in comet Halley
Authors: Israelevich P. L. , Ershkovich A. I. , Gombosi T. I. , Neubauer F. M. , Cohen O.
Published: May 2003

We calculated the electric current density in the diamagnetic cavity boundary layer (DCBL) using magnetic field data obtained during the Giotto mission to comet Halley. This current possesses both the component perpendicular to the local magnetic field and the parallel component. The perpendicular current is responsible for the screening of the diamagnetic cavity boundary from the field in the magnetic barrier. This current is supported by the electric field tangential to the boundary. The behavior of the parallel electric current component resembles the Alfvén wings which arise due to the interaction of the magnetized plasma flow with a conducting obstacle. However, the electric current connecting the two wings does not flow through the whole volume of the obstacle. On the contrary, the wings are connected by the perpendicular current in the DCBL. In the inner portion of the DCBL this current diverges producing the parallel current component whose direction is opposite to that in the outer portion of the DCBL. In order to support such a current system at the diamagnetic cavity boundary, the potential drop across DCBL should be as small as 0.5V. Such a small value of the potential drop agrees with the penetration depth of cometary ions from the cavity to the magnetic barrier: it is equal to the ion gyroradius and therefore is not affected by any significant electric field normal to the diamagnetic cavity boundary.
Low‐energy neutral atoms observed near the Earth
Authors: Wilson G. R. , Moore T. E. , Collier M.
Published: May 2003

During the summer of 2000 the LENA instrument on the IMAGE spacecraft collected a unique data set of low‐energy, neutral atom emissions. The data suggests an ever‐present flux of low energy (<50 eV) oxygen neutrals in the near‐Earth environment, typically below 4000 km altitude. Because of two different patterns in the images made during a number of spacecraft perigee passes we infer at least two main sources for these particles. One (possibly from the auroral zone) produces a high degree of variability from one image to the next and creates more energetic particles, and another (possibly part of the hot oxygen geocorona) gives less variability within a sequence of images and lower energy particles. Overall, the measured neutral fluxes increase with increasing magnetic activity. During Southern Hemisphere winter the total neutral flux rates vary diurnally with peak intensity near 0740 UT and a minimum near 1940 UT. Near perigee, neutral fluxes are often seen coming from a broad range of directions so that a localized source, such as the auroral zone, alone cannot produce it. Processes affecting low‐energy oxygen neutrals, such as gravitational deflection or spacecraft ram deflection, cannot broaden a localized source enough to match the data.
Fast type‐I waves in the equatorial electrojet: Evidence for nonisothermal ion‐acoustic speeds in the lower E region
Authors: St.‐Maurice J.‐P. , Choudhary R. K. , Ecklund Warner L. , Tsunoda Roland T.
Published: May 2003

There is plenty of evidence to suggest that the phase velocity of large amplitude irregularities produced by the modified two‐stream and the gradient‐drift instabilities are the same as the threshold speed, namely, the nominal ion‐acoustic speed in the modified two‐stream case. In this context, it is rather puzzling to note that the phase velocity of type‐I waves in the equatorial electrojet is known to easily exceed 400 m/s during strong electrojet conditions. This is puzzling because the ion‐acoustic speed of the medium is expected to be 100 m/s slower than these observations. Explaining the observations as an increase in the nominal ion‐acoustic speed through much higher neutral temperatures than expected or through electron heating by plasma waves is problematic at best. The first explanation violates everything we know about the neutral atmospheric temperature near the mesopause, while in the latter case, we only have to recall the emerging view that electron heating is done, at high latitudes, by parallel wave fields and that there is no evidence for the existence of such fields in the equatorial case. By contrast, we show that, contrary to what is normally assumed, electron thermal fluctuations cannot be neglected in the theoretical treatment of the instability when the ion collision frequency becomes large compared to the wave frequency and the wave aspect angle is small. These electron thermal fluctuations are caused by electron adiabatic heating and cooling effects related to the wave dynamics. When the electron thermal fluctuations are included in the calculations the derived instability threshold speeds match the upper limit reached by the observations. The increase becomes detectable at 108 km altitude and increases rapidly with decreasing altitude to become roughly 1.5 times as large as the isothermal ion‐acoustic speed below 100 km altitude. We show in this paper that the new threshold speed provided by the nonisothermal threshold calculations provides an excellent match for the largest vertical type‐I phase speeds that were observed during a very strong daytime electrojet event.
Slow drift mirror modes in finite electron‐temperature plasma: Hydrodynamic and kinetic drift mirror instabilities
Authors: Pokhotelov Oleg A. , Sandberg Ingmar , Sagdeev Roald Z. , Treumann Rudolf A. , Onishchenko Oleg G. , Balikhin Michael A. , Pavlenko Vladimir P.
Published: May 2003

We develop a fully kinetic linear theory of the drift mirror (DM) instability accounting for arbitrary particle velocity distribution functions including nonzero electron temperature effects and plasma pressure anisotropy. In the quasi‐hydrodynamic limiting case the theory reproduces the results obtained for the ion mirror instability. However, for the very low frequency electron DM modes which can develop in a nonuniform plasma of nonzero electron temperature, Te ≠ 0, such an equivalence does not exist. We refer to these modes as slow drift mirror (SDM) modes in order to distinguish them from the conventional ion‐DM mode. Two new instabilities, one hydrodynamic and one kinetic, that lead to the growth of SDM modes are found in the fully kinetic regime. The first instability develops for values of the ion anisotropy lower than required for the classical ion‐DM instability. The second instability occurs when the conditions for the ion‐DM instability are satisfied as well. The frequency of the SDM mode is less than the frequency of the ion‐DM mode, ω ≪ ωDM ∼ ωn, where ωn is the density gradient‐drift frequency of the ions. However, when the electron temperature is of the order of the parallel ion temperature, the SDM instability growth rate may become comparable or even higher than that of the ion‐DM instability. The free energy necessary for these new instabilities is taken from two sources. The main source is the energy stored in the ion pressure anisotropy. The second source is the electron pressure gradient which builds up in a plasma of nonzero electron temperature.
An exact solution for the dispersive Alfven switch‐on shock
Authors: Hamilton R. L. , Wu C. C.
Published: May 2003

An exact solution for the dispersive Alfven switch‐on shock is found in the case of weak nonlinearity, weak dispersion, and weak damping. Two singular and apparently nonphysical solutions are found as well. The switch‐on shock solution is shown to reduce, in the appropriate limits, to two documented special cases of the shock: one case being a description of the planar shock without dispersion and the other being an asymptotic description of the circularly polarized wave standing upstream from the shock in the dispersive case. The solution developed here provides the complete shock structure for either case. This exact solution may serve as a convenient basis for an analytical study of the stability of dispersive switch‐on shocks.
Consequences of proton and alpha anisotropies in the solar wind: Hybrid simulations
Authors: Gary S. Peter , Yin Lin , Winske Dan , Ofman Leon , Goldstein Bruce E. , Neugebauer Marcia
Published: May 2003

Alfvén fluctuations propagating away from the Sun in the solar corona and solar wind transfer energy via cyclotron resonances to ions of successively larger charge‐to‐mass ratios. This can yield T⟂/T∥ > 1 for each ion species, where the subscripts refer to directions relative to the background magnetic field. If these anisotropies become sufficiently large, they drive electromagnetic ion cyclotron instabilities. This paper describes two‐dimensional hybrid simulations of a collisionless, homogeneous, magnetized plasma to study the consequences of scattering by enhanced field fluctuations from such instabilities. The ions in the simulations consist of majority protons and minority alpha particles with initial bi‐Maxwellian velocity distributions and representative solar wind parameters including a nonzero alpha/proton relative speed. The simulations show that both helium and proton cyclotron instabilities reduce the driving anisotropy, reduce initial differences between the proton and alpha particle anisotropies, and, as a new result, usually reduce initial alpha/proton speeds. These results are somewhat different from theoretical predictions of ion scattering by interaction with outward propagating Alfvén‐cyclotron waves but are consistent with observations from Ulysses.
Relationship between electric field and currents in the ionosphere and the geomagnetic Sq field
Authors: Takeda M. , Iyemori T. , Saito A.
Published: May 2003

Relationships of the geomagnetic Sq field to the electric field, conductivity, and currents in the ionosphere were studied from the correlation of the Sq amplitude in the Y‐component with that of the ionospheric electric field and Hall currents. The electric field was obtained from the observation by the MU radar from 1989 to 2001, and the currents were estimated from the obtained field and the conductivity calculated by using the electron density profile obtained from the IRI 90 model. Estimated daily amplitude of the ionospheric Hall currents fairly well explains that of the Sq field. It was shown that variation of the Sq amplitude is mainly caused by the local ionospheric conductivity even if the variation is treated by each season separately. Another interesting feature is that the Sq amplitude tends to be small in winter for the same estimated Hall ionospheric currents. This is caused either by the seasonal variation of Hall conductivity to the Pedersen conductivity causing the variation of the contribution of the Hall currents to the total currents or by the asymmetry of the neutral wind through the magnetic effect of the ionospheric currents driven by the wind or of the field‐aligned currents flowing between the both hemispheres driven by the asymmetric dynamo action in the ionosphere.
Change of the plasma sheet ion composition during magnetic storm development observed by the Geotail spacecraft
Authors: Nosé Masahito , McEntire Richard W. , Christon Stephen P.
Published: May 2003

The present study aims to investigate how and where ions of ionospheric origin are accelerated to the ring current energy (a few tens to a few hundreds of keV) and how they are supplied to the ring current. We examined the plasma sheet ion composition during magnetic storm development, using energetic (9–210 keV) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. We selected two magnetic storms, that is, the 16–17 May 2000 storm and the 25 December 1998 storm, for which the energy density ratios of O+/H+ and He+/H+ in the plasma sheet were calculated from the EPIC/STICS data. These magnetic storms had a minimum of the SYM‐H index (the 1‐min Dst index) less than −50 nT and a duration of the main phase shorter than 6 hours. We obtained the following results: (1) Both the O+/H+ and He+/H+ energy density ratios were anticorrelated with the SYM‐H index (∣r∣ = 0.73–0.88); (2) The O+/H+ energy density ratio was rather constant at ∼0.1 before storms, but reached 0.3–1.0 at the storm maximum; and (3) The He+/H+ energy density ratio increased from 0.01–0.02 before storms to 0.04–0.1 at the storm maximum. These ion composition changes are comparable to those in the ring current, which have been reported by previous studies, indicating that ions of ionospheric origin are possibly convected to the ring current via the plasma sheet. A close inspection of ion energy spectra revealed that the observed ion composition changes can be attributed to the mass‐dependent acceleration of ions by the dawn‐to‐dusk electric field in the current sheet and the additional transport of ionospheric ions into the plasma sheet.