bluuebunny/crossref_metadata_2025 huggingface dataset:
ULF wave damping in the auroral acceleration region
Authors: Fedorov E. , Pilipenko V. , Engebretson M. J.
Published: February 2004

The width of a resonant peak in the spatial distribution of the ULF Pc5 pulsation amplitude at high latitudes is controlled by the dominant dissipation mechanism of Alfven waves in the ionosphere‐magnetosphere system. Among possible mechanisms, mostly Joule dissipation and dispersive effects are considered to terminate the growth and narrowing of a resonant peak pumped by an external source. We estimate the width of the resonance which may be provided by the field‐aligned electric field in a thermal, mirror‐confined collisionless plasma. We consider a simple case, in which a potential drop is confined within a narrow region above the ionosphere, called the auroral acceleration region. Theoretical estimates show that the mirror‐force mechanism can dominate over the ionospheric dissipation and dispersive effects in auroral regions with a sufficiently large field‐aligned potential drop. Therefore ground‐based monitoring of the resonance width (e.g., with the gradient method) may be used as an indicator of the occurrence of large‐scale parallel electric fields in the outer magnetosphere.
Current‐produced magnetic field effects on current collection
Authors: Khazanov G. V. , Stone N. H. , Krivorutsky E. N. , Liemohn M. W.
Published: February 2004

Current collection by an infinitely long, conducting cylinder in a magnetized plasma, taking into account the magnetic field of the collected current, is discussed. A region of closed magnetic surfaces disconnects the cylinder from infinity. Owing to this, the collected current depends on the ratio between this region and the plasma sheath region and, under some conditions, current reduction arises. It is found that the upper bound limit of current collection is reduced due to this change of magnetic field topology. The effect can be substantial even if the orbit‐limited model of current collection is valid. This model is used to find the reduction of the total current collected by a cylinder (e.g., a bare tether). It is shown that this effect strongly depends on plasma density. The results are applied to a tether system in the ionosphere, and it is found that current reduction can be significant for long tethers in typical day side ionospheric conditions.
Determination of the effects of substorms on the storm‐time ring current using neural networks
Authors: Munsami V.
Published: February 2004

A controversy exists as to whether the injection of particles into the ring current is caused by the induced electric field associated with the substorm expansive phase or by the enhanced convection electric field that is directly driven by the solar wind. To investigate the causal storm‐substorm relationship, artificial neural networks are used to examine the correlation between the westward auroral electrojet index (AL) and the ring current index (Dst). We use the hourly solar wind number density n, the solar wind velocity V, the interplanetary magnetic field (IMF) magnitude B, and the southward component of the IMF, Bs, to predict separately the hourly Dst and AL indices. For the first time we show, using the prediction residuals, the significant UT dependent effects of the substorm current wedge on Dst, as a consequence of the sparse distribution of only four stations that are used to compute the Dst index. When none of the Dst stations is found beneath a typical substorm current wedge, during the main phase of a magnetic storm, we find no correlation of Dst residuals with AL residuals. However, if a Dst. station is found beneath a substorm current wedge, during the main phase of a magnetic storm, then a correlation of Dst residuals with AL residuals is observed. Moreover, the mean correlation of Dst residuals with AL residuals over all phases of a magnetic storm is only 2.4%. We therefore suggest that the expected effect of substorm expansive phase activity, in the development of the storm‐time ring current, is not detectable in the Dst index, but that the Dst index is contaminated whenever a Dst station passes beneath the substorm current wedge.
Ring current heating of the thermal electrons at solar maximum
Authors: Liemohn M. W. , Kozyra J. U. , Richards P. G. , Khazanov G. V. , Buonsanto M. J. , Jordanova V. K.
Published: February 2004

To quantify the energy input to the thermal electrons due to Coulomb collisional degradation of hot ions in the inner magnetosphere, the heating rate is calculated from the results of a time‐dependent kinetic ring current model. The large June 4–7, 1991, storm during the last solar maximum, when the hot O+ content is maximal, is chosen for this study. Modeled electron heat fluxes into the topside ionosphere reach 1011 eV cm−2 s−1 on the dusk side, a large value that will certainly have an impact on the density, temperature, and composition of the upper ionosphere and thermosphere. Comparable maximum values of heat inputs to the inner magnetospheric thermal plasma are expected to arise again during storms of the present solar maximum. The calculated heating rates from the ring current simulations are compared directly with detailed ionospheric modeling results for the Millstone Hill field line (L = 3). It is seen that heating from the ring current is more than adequate to account for the nightside topside heat input necessary to obtain the observed electron temperatures during this storm, even taking into account the limitations of the comparison. The reason for this is the abundance of O+ in the ring current at energies of a few tens of keV deep in the inner magnetosphere.
MHD turbulence in Saturn's magnetosheath downstream of a quasi‐parallel bow shock
Authors: Cattaneo M. B. Bavassano , Moreno G. , Russo G. , Richardson J. D.
Published: February 2004

The origin of the MHD turbulence downstream of a quasi‐parallel shock is investigated using plasma and magnetic field data taken by Voyagers 1 and 2 during their outbound traversals of the Saturn's magnetosheath. The characteristics of the waves observed in that region are compared with those of the waves present (1) downstream of a quasi‐perpendicular shock and (2) in the foreshock. The last comparison supplies direct evidence that the upstream waves, filling the foreshock, are the main source of the magnetosheath turbulence, immediately downstream of a quasi‐parallel shock.
North‐south flows observed in the outer heliosphere at solar minimum: Vortex streets or corotating interaction regions?
Authors: Goldstein M. L. , Roberts D. A. , Burlaga L. F. , Siregar E. , Deane A. E.
Published: February 2004

During the last two solar minima in the distant heliosphere the equatorial heliospheric plasma velocity oscillated perpendicular to the ecliptic plane with an approximately 26‐day period in the distant heliosphere. Two explanations have been proposed: compressive interactions between streams and velocity shear interactions that produce a Kármán vortex street. The latter interpretation has been challenged on the basis that the velocity jumps are supersonic, thereby suppressing the Kelvin‐Helmholtz (K‐H) instability. Here we examine this issue using a time‐dependent compressible magnetohydrodynamics code solved in spherical coordinates in the two‐dimensional r – θ plane. We conclude that supersonic flow does suppress small‐scale instabilities and that the classic Kármán vortex street cannot be excited. Both velocity shear layers and stream interactions can, however, produce signatures in density, velocity, and magnetic field that resemble the observations. In particular, we find north‐south variations of the flow velocity with a period that is approximately half that of the period of the variation in flow speed: a result insensitive to the thickness of the velocity shear layers. A depletion in density (and magnetic field magnitude) relative to the expected Parker value is predicted by the simulations that generate the north‐south flow via velocity shear. The Voyager spacecraft observed a similar depletion in the outer heliosphere during the last two solar minima. When the effective tilt of the plasma sheet is increased, corotating interaction regions produce shock waves and other complex time‐dependent evolution. We conclude that at solar minimum the observed north‐south oscillations are a robust phenomenon that can form from either the interaction of fast and slow solar wind streams or from velocity shear. Which mechanism dominates is a consequence of the degree of tilt of the heliospheric current sheet, the magnitude of the velocity shear, and other physical parameters. However, the depletions seen in density and magnetic flux in the Voyager data suggest that velocity shear in the outer heliosphere at solar minimum may be the dominant cause of the observed north‐south flow patterns.
Scales of heliospheric current sheet coherence between 1 and 5 AU
Authors: Crooker N. U. , Kahler S. W. , Gosling J. T. , Larson D. E. , Lepping R. P. , Smith E. J. , De Keyser J.
Published: February 2004

The structure of the heliospheric current sheet (HCS) at Ulysses and at Wind is compared during the period of near‐radial alignment in 1998. Electron heat flux and magnetic field data are used to determine true magnetic field polarity. During the solar rotation period surrounding alignment there was considerable disagreement between the HCS crossings at Wind and those predicted from the corresponding source surface map, both in number and in location, consistent with the disordered, temporally varying solar wind at this ascending phase of the solar cycle. Despite this complexity the four crossings closest to the time of radial alignment at Wind were successfully identified in Ulysses data with use of a one‐dimensional hydrodynamic code. Further, minimum variance analysis for the first two crossings, which were separated by only 16 hours at Wind, indicated coherent propagation of a large‐scale warp in the HCS. Analysis of the local structure of the HCS on the four crossings, however, revealed a high level of variability both from case to case and from one spacecraft to the other. For example, the third crossing at Wind was a single‐sheet crossing adjacent to structures with fields folded back on themselves and a brief period of counter‐streaming electrons implying a transient structure. At Ulysses multiple sheets were encountered. At the fourth crossing Wind passed through a counterstreaming event with a flux rope signature containing a south pointing axis, while Ulysses passed through a flux tube with little field rotation and northward pointing field. The results are consistent with the view that the heliospheric current sheet is coherent as a global structure but highly variable in local structure over angular distances of a few degrees.
Voyager 2 observations of helium abundance enhancements from 1–60 AU
Authors: Wang C. , Richardson J. D.
Published: February 2004

The Plasma Science Experiment on the Voyager 2 spacecraft recently detected helium abundance enhancements (HAEs) near 60 AU in which the He++ ions comprise more than 10% of the solar wind density. These large values may result from solar events related to the rising phase of solar cycle 23. Large He++/H+ density ratios are probably the best signature for tracking coronal mass ejections (CMEs) through the outer heliosphere for the subset of CMEs with this property. We analyze the high‐resolution spectra for the Voyager 2 mission (from 1977 to 1999, 1–60 AU) and find that HAE events are observed ∼1.1% of the time with He++/H+ abundance ratios as high as 0.35. The long‐term variation of HAEs is related to the solar cycle. HAEs occur in all solar wind regimes but are associated with higher speeds, lower proton temperatures, and higher magnetic fields than the ambient solar wind. HAEs are generally not associated with interplanetary shocks. The He++/H+ temperature ratio increases with distance from the Sun, is larger when He++/H+ density ratios are large, and is smaller when proton densities are high.
Spatial structure of the cusp/cleft ion fountain: A case study using a magnetic conjugacy between Interball AP and a pair of SuperDARN radars
Authors: Dubouloz N. , Bouhram M. , Senior C. , Delcourt D. , Malingre M. , Sauvaud J.‐A.
Published: February 2004

The spatial structure of low‐energy ion outflows associated with heating processes in the dayside cusp/cleft region is investigated by using a magnetic conjugacy between Interball Auroral Probe (AP) and the Saskatoon‐Kapuskasing pair of the Super Dual Auroral Radar Network (SuperDARN). The interplanetary magnetic field during this event is characterized by By < Bz < 0 components, which breaks the symmetry of morning and afternoon convection cells relative to the noon meridian. As a result, plasma convection over the afternoon polar cap is not antisunward, but almost azimuthally oriented. The three‐dimensional thermal ion distributions measured by the Hyperboloid experiment on board Interball AP are used as input of numerical simulations in order to investigate the spatial structure of the ion heating processes. The simulations include the complete guiding center motion of ions under the effect of gravity, geomagnetic field, and convection field measured by SuperDARN radars. In contrast to the classical cleft ion fountain picture, we demonstrate that the observed ions originate from a wide latitudinal interval and that the heating region likely coincides with the polar cusp. The numerical simulations allow us to reconstruct the downstream (relative to convection) boundary of the heating region, as well as ion distributions along this boundary. Ions are found to cross this boundary and to exit the heating region over a broad range of altitudes (up to at least 15,000 km) with an average altitude increasing with ion mass, their average pitch angle increasing with altitude in agreement with altitude cumulative heating scenarios.
Simultaneous ground based observations of an auroral arc in daytime/twilighttime O I 630.0 nm emission and by incoherent scatter radar
Authors: Pallamraju D. , Baumgardner J. , Chakrabarti S. , Pedersen T. R.
Published: February 2004

We present the ground‐based oxygen 630.0 nm daytime optical measurements of a discrete auroral arc from Sondre Stromfjord, Greenland. The optical measurements were made using an imaging echelle spectrograph built at Boston University. We show that the auroral optical signature extracted from the blue‐sky background agrees closely in both space and time with the aurorally enhanced electron densities at 200 km altitude obtained simultaneously by the incoherent scatter radar. The dayglow measurements are also in good agreement with the integrated emission rates modeled using the measured Ne, Te, and Ti profiles from the radar. The results reported in this paper demonstrate the potential of this spectrograph to observe aurora during daytime, and it promises to be a valuable complement to the existing tools for the investigation of upper atmospheric phenomena.
Relation between optical emissions, particles, electric fields, and Alfvén waves in a multiple rayed arc
Authors: Hallinan Thomas J. , Kimball J. , Stenbaek‐Nielsen H. C. , Lynch K. , Arnoldy R. , Bonnell J. , Kintner P.
Published: February 2004

Velocities of rays in auroral arcs were used to infer the perpendicular electric fields above the acceleration region. Using rocket measurements of electron energy as a proxy for the high‐altitude potential, the high‐altitude perpendicular electric fields were calculated and found to be in good agreement with those derived from the ray motions. Additionally, a 0.6 Hz oscillating electric field at high altitude was postulated on the basis of the passing rays. Such a field was also calculated from the electron energy measurements and was found to be closely related to an Alfvén wave measured on the payload following a delay of 0.8 s. The measured electron energy flux agreed well with the auroral luminosity down to scale sizes of about 10 km. The combination of ground‐based imaging and the measured energy flux also allowed a determination of the lower border altitude of the arcs. They were found to be somewhat higher (130 km) than expected on the basis of the electron energy. A tall rayed arc with a lower border height of 170 km was associated with a burst of suprathermal electrons on the poleward edge of the aurora.
Damping standing Alfvén waves in the magnetosphere
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2004

Using the WKB approximation, we analyze the phase and energy structure of damped Alfvén wave structures in a magnetospheric field configuration. In the WKB limit the real part of the eigenfrequency is determined by the field line length while the damping rate is determined by the ionospheric conductivity. Expressions are worked linking the damping decrement to the ionospheric damping, and it is shown that the damping effect of each ionosphere adds linearly to the damping decrement. We end with a critique of the use of the WKB solution for Alfvénic wave structures and indicate where such solutions may be unreliable.
Current‐induced magnetic field effects on bare tether current collection: A parametric study
Authors: Khazanov G. V. , Stone N. H. , Krivorutsky E. N. , Gamayunov K. V. , Liemohn M. W.
Published: February 2004

The physical process of current collection by a “bare wire” electrodynamic tether in space is considered. The study uses an improved model that takes into account the resistance of the wire and the magnetic shielding induced by current flow in the tether. The plasma density, ne, electron temperature, Te, tether length, L, tether radius, rw and the angle of the geomagnetic field to the tether (90°‐α) were all used as parameters. It is shown, for certain tether configurations and parameter values, that magnetic shielding reduces the collected current. In general, any parametric change that increases tether current, and hence, the strength of the current‐induced magnetic field relative to the strength of the electric field between the tether and the ambient plasma, will increase the shielding effect. Tether current is increased directly with tether collection area (which depends on L and rw), plasma conductivity (which depends on ne and Te), and the motional emf along the tether (which increases with L and the angle 90°‐α). It turns out that, as any of these parameters change so as to cause the overall tether current to increase, the overestimate of current that results from ignoring the magnetic shielding effect becomes correspondingly greater. Moreover, it is shown that a tether system in the thruster (or motor) mode suffers greater current reduction from magnetic shielding than does the same tether deployed in the generator mode. Finally, it is shown that, for certain tether system configurations combined with particular values of the governing plasma parameters, current‐induced magnetic shielding can significantly reduce the collected current and, therefore, system efficiency. For example, in the case of an electrodynamic tether system in the thruster mode under conditions of ne=1.67×106 cm−3, α=60°, rw=2.5 mm, and Em=34 V/km, magnetic shielding will reduce the collected current 10% at a point L=0.65 km along the tether (4.3 A instead of 4.8 A) and this increases to more than 40% at L= 1.3km (9.6 A instead of 13.4A).
Heating of the low‐latitude solar wind by dissipation of turbulent magnetic fluctuations
Authors: Smith Charles W. , Matthaeus W. H. , Zank Gary P. , Ness Norman F. , Oughton Sean , Richardson John D.
Published: February 2004

We test a theory presented previously to account for the turbulent transport of magnetic fluctuation energy in the solar wind and the related dissipation and heating of the ambient ion population. This theory accounts for the injection of magnetic energy through the damping of large‐scale flow gradients, such as wind shear and compression, and incorporates the injection of magnetic energy due to wave excitation by interstellar pickup ions. The theory assumes quasi‐two‐dimensional spectral transport of the fluctuation energy and subsequent dissipation that heats the thermal protons. We compare the predictions of this theory with Voyager 2 and Pioneer 11 observations of magnetic fluctuation energy, magnetic correlation lengths, and ambient proton temperatures. Near‐Earth Omnitape observations are used to adjust for solar variability, and the possibility that high‐latitude effects could mask possible radial dependences is considered. We find abundant evidence for in situ heating of the protons, which we quantify, and show that the observed magnetic energy is consistent with the ion temperatures.
Relation between cusp and mantle in MHD simulation
Authors: Siscoe G. L. , Erickson G. M. , Sonnerup B. U. Ö. , Maynard N. C. , Siebert K. D. , Weimer D. R. , White W. W.
Published: February 2004

A global MHD simulation illustrates the relative contributions to magnetotail structure of two mantle models. One model is the data‐based picture that Rosenbauer et al. [1975] originally put forth showing the cusp to be the source of the mantle. In this model the cusp is populated by magnetosheath particles that enter it as a consequence of magnetic reconnection on the day side magnetopause. The other model, which derives from MHD considerations, notes that since the mantle lies on open magnetic field lines, the solar wind should be able to enter it along the tail magnetopause downwind from the cusp. In the simulation results presented here, both modes of plasma entry into the mantle occur and their relative contributions are seen. The mantle, identified in the simulation as the volume of the tail occupied by a slow‐mode expansion wave filled with magnetosheath plasma, lies mostly on streamlines that originate in the dayside cusp (cusp model), but the bulk of plasma inflow comes directly from the magnetosheath (magnetosheath model). The demarcation between streamlines originating in the cusp (cusp model) and streamlines originating in the solar wind (magnetosheath model) is sharp. We call this demarcation the fluopause. The fluopause is the downwind extension of the solar wind stagnation streamline. At 100 RE downtail the fluopause lies several RE inside the magnetopause identified by the peak in the electrical current that defines the magnetotail boundary. The spatial separation between the fluopause and the magnetopause defines the volume to which the magnetosheath model refers. Whereas the magnetopause clearly marks the start of the slow‐mode expansion wave that defines the mantle, all MHD parameters and their slopes are continuous across the fluopause. The fluopause is only determinable by following streamlines. Streamlines in the mantle that originate in the solar wind (magnetosheath model) do not converge on the plasma sheet and, so, are not a likely source of particles for the plasma sheet. Streamlines in the mantle that originate in the cusp (cusp model) do converge on the plasma sheet and, so, are likely candidates for the particles that supply the plasma sheet. At their earthward ends, streamlines that originate in the cusp reach low altitudes and might, in nature, be populated with ionospheric plasma. An implication of the present result is that the inner portions of mantle encounters that have been identified in IMP 8 and Geotail data refer to plasma that has spent part of its life in the dayside cusp and possibly in the ionosphere. The latter possibility might account for Geotail observations that identify O+ as a common constituent of deep‐mantle plasma.
Model/data comparison of F region ionospheric perturbation over Millstone Hill during the severe geomagnetic storm of July 15–16, 2000
Authors: Pavlov A. V. , Foster J. C.
Published: February 2004

We compare measurements of the ionospheric F region at Millstone Hill during the severe geomagnetic storm of July 15–16, 2000, with results from a time‐dependent mathematical model of the Earth's ionosphere and plasmasphere. Use of [O] correction factors with the Mass Spectrometer Incoherent Scatter (MSIS‐86) model was found to bring the modeled and measured F region main peak electron densities into agreement. We have evaluated the value of the additional heating rate that should be added to the normal photoelectron heating in the electron energy equation in the nighttime plasmasphere region above 5000 km to bring the measured and modeled electron temperatures into agreement. We conclude that anomalously low values of the F peak altitude seen as a G condition on ionograms and observed on July 16 exist in the ionosphere due to a decrease in the production rates of oxygen ions resulting from low values of atomic oxygen density. The resulting effect of vibrationally excited N2 and O2 on NmF2 is the decrease of the calculated daytime F region main peak electron density by up to a factor of ∼ 3. The modeled electron temperature is very sensitive to the electron density, and this decrease in electron density results in an increase in the calculated daytime electron temperature of ∼ 930 K at the F region main peak altitude, giving closer agreement between the measured and modeled electron temperatures. The model without the inclusion of vibrationally excited N2 and O2 does not reproduce both the daytime and nighttime electron densities and temperatures. Inclusion of vibrationally excited N2 and O2 brings the model and data into better agreement. Without the contribution of vibrationally excited N2 and O2 in the loss rate of O+(4S) ions, the model calculations result in a very large disagreement between the measured and modeled electron density altitude profiles during the G condition daytime period on July 16. In order to bring the modeled and measured electron density altitude profiles into agreement, it would be necessary to decrease the MSIS‐86 model [O] by a factor of 5.5. It is unlikely that that the MSIS‐86 atomic oxygen number density correction factor is so large. This finding provides an additional argument for the inclusion of the effects of vibrationally excited N2 and O2 in ionospheric models.
Stability properties of high‐pressure geotail flux tubes
Authors: Horton W. , Wong H. V. , Van Dam J. W. , Crabtree C.
Published: February 2004

Kinetic theory is used to investigate the stability of ballooning interchange modes in the high‐pressure geotail plasma. A variational form of the stability problem is used to compare new kinetic stability results with MHD, fast MHD, and Kruskal and Oberman [1958] stability results. Two types of drift modes are analyzed: A kinetic ion pressure gradient drift wave with a frequency given by the ion diamagnetic drift frequency ω*pi and a very low frequency mode |ω| ≪ ω*pi, ωDi that is often called a convective cell or the trapped particle mode. In the high‐pressure geotail plasma a general procedure for solving the stability problem in a 1/β expansion for the minimizing δB‖ is carried out to derive an integral differential equation for the kinetically valid displacement field ξψ for a flux tube. The plasma energy released by these modes is estimated in the nonlinear state. The role of these instabilities in substorm dynamics is assessed in the substorm scenarios described by Maynard et al. [1996].
Observations of plasma sheet dynamics earthward of the onset region with the Geotail spacecraft
Authors: Frank L. A. , Paterson W. R. , Sigwarth J. B. , Mukai T.
Published: February 2004

During the onset of an auroral substorm at about 0406 UT on August 14, 1996, the position of the Geotail spacecraft was located very near the corresponding region of instability in the equatorial plane as determined with a global model of the geomagnetic fields which provided the mapping from the auroral ionosphere. The Earth Camera on board the Polar spacecraft was used to acquire the global auroral images. The magnetic fields in the ring current were sufficiently stretched by the equatorial current sheet such that the radial position of the Geotail spacecraft at 10.2 RE was earthward of the onset region. Earthward flows at speeds in the range of 200 km/s for a period of several minutes were coincident with the initial brightening of a localized region of the previously existing auroral oval. This auroral luminosity subsequently declined within several minutes without a well‐developed expansive phase; that is, a “pseudo‐onset” occurred. Partial dipolarization of the magnetic fields occurred with this onset as well as large‐scale magnetic fluctuations which continued through the brightening of another spot on the auroral oval at 0413 UT. These fluctuations ceased with the end of a well‐developed expansive phase at about 0424 UT, when the auroral luminosities entered their recovery phase. At the beginning of the short expansive phase at 0420 UT the magnetometer on GOES 8 at a radial distance of 6.6 RE recorded a major dipolarization of the magnetic fields as an earthward moving injection front of hot plasma passed across the Geotail position. The present observations of the onset phenomena earthward of the instability region are combined with previously reported measurements of plasmas and magnetic fields at distances tailward of the instability in the ring current in order to begin sketches of the relationship among several dynamical features of the onset region.
Magnetic cloud Bs events and their dependence on cloud parameters
Authors: Zhao X. P. , Hoeksema J. T. , Marubashi K.
Published: February 2004

It had been found that the duration and intensity of the southward interplanetary magnetic field within magnetic clouds (the magnetic cloud Bs event) correlate linearly with the ecliptic latitude of the cloud's central axial field, the one of eight characteristic parameters of expanding magnetic clouds. On the basis of the parameter list determined using the expanding magnetic cloud model for all clouds to be examined, this work examines the dependence of magnetic cloud Bs events on the other seven parameters as well as the ecliptic latitude of the cloud's central axial field. The correlation of the duration and intensity of magnetic cloud Bs events with the ecliptic latitude of the central axial field hats been confirmed by the new parameter list with correlation coefficients higher than our earlier study. It is found among the eight parameters that in addition to the ecliptic latitude, the strength of the central axial field, the cloud's bulk speed, and the relative impact distance of the spacecraft to the cloud's central axis are the other parameters that are closely associated with the duration and intensity of magnetic cloud Bs events. There are most probable values for most of the eight parameters of expanding magnetic clouds. In predicting the duration and intensity of magnetic cloud Bs events using the expanding cloud model these most probable values may be used to replace those model parameters that are unavailable from solar observations. This provides a possibility to predict the duration and intensity of magnetic cloud Bs events based on a few parameters that are available from solar observations.
Response of the upper atmosphere to auroral protons
Authors: Galand M. , Fuller‐Rowell T. J. , Codrescu M. V.
Published: February 2004

A three‐dimensional, time‐dependent, coupled model of the thermosphere and ionosphere has been used to assess the influence of proton auroral precipitation on Earth's upper atmosphere. Statistical patterns of auroral electron and proton precipitation, derived from DMSP satellite observations, have been used to drive the model. Overall, electrons are the dominant particle energy source, with protons contributing ∼ 15% of the total energy. However, owing to the offset of the proton auroral oval toward dusk, in certain spatial regions protons can carry most of the energy. This is the case particularly at the equatorward edge of the dusk sector and at the poleward edge of the dawn sector of the auroral oval. The increase in Pedersen conductivity raises the average Joule heating by ∼ 10%, so raising the E and F region temperature by as much as 7%. The enhanced E region ionization also drives stronger neutral winds in the lower thermosphere through ion drag, which alters the temperature structure through transport, adiabatic heating, and adiabatic cooling. The neutral wind velocity modifications in the E region can reach 40% in some sectors. In addition, the upwelling of neutral gas raises the N2/O ratio, depleting the F region and so reducing the ion‐drag driven winds in this region. This study illustrates the modest yet significant impact of auroral proton precipitation on the upper atmosphere.
Introduction to special section: Proton precipitation into the atmosphere
Authors: Galand Marina
Published: February 2004

Energetic protons, precipitating into the atmosphere, interact with the ambient neutrals, leading to excitation, ionization, elastic scattering, and, for MeV protons, dissociation. In addition, a proton can capture an electron, producing an energetic H atom, which has enough energy to interact, in turn, with the ambient neutrals. This H atom can also get stripped of its electron, becoming a proton again. Because of these charge‐changing reactions, the incident proton beam penetrating the atmosphere becomes a mixture of protons and H atoms [see, e.g., Rets, 1989].
Conjugate observations of electromagnetic ion cyclotron waves
Authors: Bräysy T. , Mursula K.
Published: February 2004

We compare electromagnetic ion cyclotron (EMIC) waves observed by the electric field instrument on Polar to simultaneous Pc1 pulsations detected at the subauroral Sodankylä station. We studied 44 passes where Polar was within ±1.5 hours in magnetic local time (MLT) from Sodankylä during a Pc1 pulsation event. In 70% of these passes a matching EMIC wave was observed in space. This shows that ground Pc1s indeed originate at high altitudes in the magnetosphere and have their source field line within a rather limited (±1–1.5 hours) MLT range. It also limits horizontal ducting of waves in the longitude direction to less than ∼1000–1500 km, and it gives a lower limit of ∼3 hours on the MLT extent of a typical EMIC wave band in space. Out of 69 EMIC events, 75% were observed at Sodankylä. This restricts the scale of ionospheric modification on the amplitude and frequency of waves. In several passes, two or three EMIC waves corresponding to different frequency branches were observed simultaneously. We discuss the location, frequency, amplitude, and ground visibility of the EMIC waves of these three branches separately. The results emphasize the high‐latitude dayside as a source of EMIC waves observed on the ground.
Studies of relativistic electron injection events in 1997 and 1998
Authors: Blake J. B. , Selesnick R. S. , Baker D. N. , Kanekal S.
Published: February 2004

The acceleration of magnetospheric electrons to multi‐MeV energies has been a topic of much interest and study over the last several years. To further investigate this acceleration process, Polar and SAMPEX data from 1997 and 1998 were used to examine the general, repeating characteristics of the relativistic electron injection events that occurred over the 2‐year period. Around the intensity peak, usually in the L range from 3.2 to 4.5, it was found that (1) during the main phase of the storm, there is a dropout of the relativistic electron fluxes; (2) fluxes increase as recovery begins; (3) the electrons are most sharply peaked in pitch angle at the beginning of the event and flatten with time; (4) the rate at which the flux increases slows with increasing energy; and (5) strong pitch angle scattering is intimately associated with relativistic electron injection events. A recirculation or resonant heating process coupled with rapid radial diffusion or a melding of both would be consistent with the observations.
Two encounters of the substorm onset region with the Geotail spacecraft
Authors: Frank L. A. , Sigwarth J. B. , Paterson W. R. , Kokubun S.
Published: February 2004

The Earth Camera on board the Polar spacecraft provided high‐resolution images for 112 auroral substorms when the Geotail spacecraft was in the nighttime sector of Earth's magnetotail during the period May 1 through September 30, 1996. The purpose of the study was to investigate the dynamical behavior of the plasmas and magnetic fields at the Geotail spacecraft when it was positioned within the central plasma sheet prior to and during the onset of a substorm. In addition, the position of the Geotail spacecraft was required to be within 20 min in local time and several degrees in magnetic latitude of the equatorial position of the magnetic field lines threading the auroral onset region as viewed with the Polar camera and as mapped with a global magnetic field model. For this study these onset mapping locations were required to be earthward of the Geotail spacecraft. Only two substorms, on July 2 and September 4, satisfied these restraints which were imposed in order to observe the dynamics of the central plasma sheet. The radial distances for the Geotail locations for these two substorms were 11.7 and 15.0 RE, respectively. Before the onsets there was a sustained period of earthward flow of plasma into the extraterrestrial ring current at average speeds of ∼50 km s−1. Then, for a period of several minutes before the auroral onset, this plasma flow became stagnant and was accompanied, in part, by fluctuations in components of the magnetic field. At substorm onset a tailward high‐speed flow of plasmas at 200–300 km S−1 began which subsequently subsided within several minutes. The magnetic fields in this tailward directed pulse of plasma flows exhibited a substantial southward component which is accounted for by magnetic merging of field lines at locations inside of the Geotail spacecraft orbit and thus in the ring current or earthward edge of the plasma sheet. Overall, these fortuitous observations support the merging of magnetic field lines at substorm onset in the plasmas with high‐energy densities in the ring current but within a very restricted range of local time, several tens of minutes or less.
Statistical visualization of Earth's magnetotail during substorms by means of multidimensional superposed epoch analysis with Geotail data
Authors: Machida S. , Ieda A. , Mukai T. , Saito Y. , Nishida A.
Published: February 2004

A multidimensional superposed epoch analysis of plasma and magnetic field data from the Geotail spacecraft was used to visualize the time evolution of plasmoids in the Earth's magnetotail statistically. The substorm events were identified by Pi2 events at midlatitude ground stations when Geotail was in the downtail region from −30 RE to −200 RE. Plasma density, velocity, temperature, and magnetic field data were sorted and analyzed according to the Pi2 onset time. We selected 156 substorm events that were either a single onset or the first onset of multiple onsets. In the statistical visualization, we found that plasma acceleration does not take place in a small region around X ∼ −30 RE, but occurs in a more widely spread region extending from X ∼ −30 RE down to X ∼ −90 RE. The spatial structure of the plasmoid is characterized by a bilatitudinal structure where the fast plasma flow in the plasma sheet boundary stretches from the equator and a relatively slow plasma sheet flow is encountered with preexisting plasma sheet populations. Finite By fields are created in the inner plasma sheet boundary, and the total pressure is enhanced inside the plasmoid. The evolution of the plasma and magnetic fields as well as the deduced parameters, such as the number flux of plasma or energy fluxes, agreed well with a magnetic reconnection model.
Low‐latitude plasma mantle in the near‐Earth magnetosphere: Geotail observations
Authors: Taguchi S. , Kishida H. , Mukai T. , Saito Y.
Published: February 2004

This report provides evidence for the appearance of the plasma mantle at low latitudes, ZGSM ∼0. A detailed analysis of Geotail observations on January 20, 1996, clarifies the characteristics of the plasma flow in the low‐latitude mantle. The tailward flow speed in the mantle is 200–400 km s−1, smaller than the flow speed (≳400 km s−1) in the adjacent magnetosheath, and flow and field perturbations satisfy the criteria for a rotational discontinuity. The velocity component parallel to the mantle magnetic field line is positively correlated with the ion number density, consistent with a boundary layer interpretation. The component of the velocity perpendicular to the magnetic field lines is predominantly poleward at speeds up to 150 km s−1. This poleward convection can be interpreted as being driven by a J × B force operating on the field lines.
Decrease of the first Schumann resonance frequency during solar proton events
Authors: Roldugin V. C. , Maltsev Y. P. , Petrova G. A. , Vasiljev A. N.
Published: February 2004

The variations of the first‐order Schumann resonance frequency in Kola peninsula are studied for four solar proton events on November 4 and 6, 1997 and on May 2 and May 6, 1998. It is found that the frequency decreases by about 0.15 Hz during the peaks of the proton penetrations. Such a decrease is in agreement with modeling calculations. A very intensive solar X ray burst preceding the proton flare on November 6 caused an increase of the resonance frequency. The decrease of amplitude about of 0.2 pT and the decrease of the resonance bandwidth about of 0.2 Hz are observed in the November events.
Solar wind signatures associated with magnetic clouds
Authors: Blanco‐Cano X. , Bravo S.
Published: February 2004

Magnetic clouds are commonly observed in association with bidirectional fluxes of electrons and ions, and in some cases also with shocks and helium enrichments [Gosling, 1990; Richardson, 1997]. In this work we use satellite data in the solar wind near Earth to study 71 magnetic clouds observed between 1967 and 1984 in order to find possible differences in the characteristics of clouds when they are or they are not associated with shocks and with helium enhancements. In our sample we find that 51% of the clouds were preceded by an interplanetary transient shock and that 65% were associated with a helium enhancement observed before, inside, or after the cloud. To investigate the characteristics of magnetic clouds we consider the spatial profiles of magnetic field and plasma parameters (bulk speed, density, temperature, β, and pressure) inside them, observed as temporal variations as clouds pass plasma and magnetic field detectors on board satellites in near‐Earth orbit. The mean magnitudes of these parameters inside the cloud were also considered. We find that the profiles of the magnetic field strength and plasma parameters are different depending on whether the cloud is associated or not with a shock and/or a helium enrichment. The mean values of magnetic field strength, bulk speed, temperature, and pressure are higher for shock‐associated clouds than for clouds without a shock. Mean values of bulk speed, density, temperature, and β are different for clouds associated with helium enrichments and for clouds without this signature. Mean values of the magnetic field strength and pressure are very similar for both types of clouds. Some of these results were previously obtained by other authors.
Magnetopause reconnection induced by magnetosheath Hall‐MHD fluctuations
Authors: Belmont G. , Rezeau L.
Published: February 2004

Any penetration of solar wind plasma into the magnetosphere would be precluded if the plasma were strictly frozen in the magnetic field. Such transfers cannot therefore be modeled in the frame of ideal MHD: they demand nonideal effects and are likely to involve reconnection at the magnetopause. The strong magnetic fluctuations observed at this boundary have been suggested for a long time to be responsible for these phenomena. The present study revisits two questions: what is the origin of the strong magnetopause fluctuations and by which mechanisms can they allow for reconnection? It is first confirmed that the preexisting magnetosheath fluctuations can be the primary cause of the strong magnetopause fluctuations. The phenomenon invoked in a preceding paper to explain their amplification, known as “Alfven resonance,” is ruled out and shown to be an artifact of ideal MHD. The amplification at the boundary is instead explained by a nonresonant mode conversion (due to the magnetopause gradient), followed by a trapping of the resulting Alfven wave in the boundary (due to the magnetic field rotation). The trapped Alfven wave has strong amplitude and its finite frequency is responsible for a departure from ideal MHD associated with reconnection distributed all over the magnetopause surface. We evidence that reconnected magnetic flux, driven by the incident magnetosheath waves, is able in this way to penetrate the magnetosphere, and we show how a local reconnection rate can be estimated. This result should be the starting point for a new approach of the magnetopause transfer problem since, in this scenario, reconnection occurs without external electrostatic electric field, as in stationary X point models, and without any local instability, as in tearing type models.
Two distinct substorm onsets
Authors: Mishin V. M. , Saifudinova T. , Bazarzhapov A. , Russell C. T. , Baumjohann W. , Nakamura R. , Kubyshkina M.
Published: February 2004

At times, substorms occur in a particular sequence, where a clear growth phase is followed by a first onset at lower latitudes and a second one involving all latitudes between 60° and 70°. The second onset is followed by a clear recovery phase. In the present paper we present nine such sequences in the form of a superposed epoch analysis. Comparison with solar wind data shows that the second onset occurs when the interplanetary magnetic field turns northward. Determination of the change in the open polar cap magnetic flux using the magnetogram inversion technique method indicates that the first onset occurs during an interval of continuous loading of the tail with new open flux merged at the dayside, Hence, despite showing clear ionospheric signatures of substorm expansion, this onset likely involves the reconnection of closed plasma sheet field lines only, while during the second onset and expansion phase, reconnection clearly proceeds from closed plasma sheet to open lobe field lines.