Entry Of Plasma Research Articles

Multi‐Instrument Analysis of the Formation and Segmentation of Tongue of Ionization Into Two Consecutive Polar Cap Patches

AbstractThis paper investigates the formation and segmentation of the tongue of ionization into two consecutive polar cap patches using multi‐instrument observations from 27 February 2014. We provide insights into how the interplanetary magnetic field (IMF) variations influence the formation and segmentation of these patches. Our findings reveal that the entry of dayside dense plasma into the polar cap is predominantly driven by the modified convection near the cusp region, which is controlled by the transition of IMF By or the sudden drop of IMF Bz. Furthermore, we observe a rapid north‐westward plasma flow within the patch segmentation region, accompanied by equatorward‐expanded and enhanced convection near the cusp region. This fast‐moving flow, approximately 1.5 km/s, is characterized by low density and high electron temperature and shows a signature of a Subauroral Polarization Stream. This suggests that the fast‐westward flow, in conjunction with the expansion and contraction of ionospheric convection, plays a crucial role in the segmentation of polar cap patches from the dayside plasma reservoir. This study provides a comprehensive observation of the evolution of polar cap patches, thereby advancing our understanding of the dynamic mechanisms governing patch formation and segmentation.

Discrete Aurora on the Nightside of Mars: Occurrence Location and Probability

AbstractThis paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons.

Development of a Turbulent Cascade behind the Bow Shock under Quiet Conditions in the Solar Wind

The bow shock crossing by the solar wind can lead in a number of cases to significant changes in the development of the turbulent cascade. Individual cases previously studied on the basis of experimental measurements of the characteristics of turbulence in the magnetosheath have not yet identified the factors that have the greatest influence on the modification of the turbulent cascade behind the bow shock. In this paper, we consider several observation cases of spectra of the compressible component in magnetosheath fluctuations on two satellites separated in space under calm conditions in the solar wind. This makes it possible to estimate the influence of the magnetosheath boundaries and the bow shock topology on the dynamics of a turbulent cascade when the plasma moves behind the bow shock. It is shown that there is a significant redistribution of energy in the turbulent cascade immediately behind the quasi perpendicular bow shock in the daytime part of the magnetosheath. This affects the magnetohydrodynamic scales, and the cascade properties are restored upon further propagation of the plasma towards the flanks. At the same time, behind the quasi parallel bow shock, the characteristics of the turbulent cascade upon the entry of plasma into the magnetosheath change only on subionic scales.

Solar Wind Ion Entry Into the Magnetosphere During Northward IMF.

Extended periods of northward interplanetary magnetic field (IMF) lead to the formation of a cold, dense plasma sheet due to the entry of solar wind plasma into the magnetosphere. Identifying the paths that the solar wind takes to enter the magnetosphere, and their relative importance has remained elusive. Any theoretical model of entry must satisfy observational constraints, such as the overall entry rate and the dawn‐dusk asymmetry observed in the cold, dense plasma sheet. We model, using a combination of global magnetohydrodynamic and test particle simulations, solar wind ion entry into the magnetosphere during northward IMF and compare entry facilitated by the Kelvin‐Helmholtz instability to cusp reconnection. For Kelvin‐Helmholtz entry we reproduce transport rates inferred from observation and kinetic modeling and find that intravortex reconnection creates buoyant flux tubes, which provides, through interchange instability, a mechanism of filling the central plasma sheet with cold magnetosheath plasma. For cusp entry we show that an intrinsic dawn‐dusk asymmetry is created during entry that is the result of alignment of the westward ion drift with the dawnward electric field typically observed during northward IMF. We show that both entry mechanisms provide comparable mass but affect entering plasma differently. The flank‐entering plasma is cold and dawn‐dusk symmetric, whereas the cusp‐entering plasma is accelerated and preferentially deflected toward dawn. The combined effect of these entry mechanisms results in a plasma sheet population that exhibits dawn‐dusk asymmetry in the manner that is seen in nature: a two‐component (hot and cold) dusk flank and hotter, broadly peaked dawn population.

Transport and entry of plasma clouds/jets across transverse magnetic discontinuities: Three‐dimensional electromagnetic particle‐in‐cell simulations

AbstractIn this paper we use three‐dimensional electromagnetic particle‐in‐cell simulations to investigate the interaction of a small Larmor radius plasma cloud/jet with a transverse nonuniform magnetic field typical to a tangential discontinuity in a parallel geometry. The simulation setup corresponds to an idealized, yet relevant, magnetospheric configuration likely to be observed at the magnetopause during northward orientation of the interplanetary magnetic field. The numerical simulations are adapted to study the kinetic effects and their role on the transport and entry of localized plasma jets similar to those identified inside the Earth's magnetosheath propagating toward the magnetopause. The simulations reveal the formation of a perpendicular polarization electric field inside the main bulk of the plasma cloud that enables its forward transport and entry across the transverse magnetic field. The jet is able to penetrate the transition region when the height of the magnetic barrier does not exceed a certain critical threshold. Otherwise, the forward transport along the injection direction is stopped before full penetration of the magnetopause. Moreover, the jet is pushed back and simultaneously deflected in the perpendicular plane to the magnetic field. Our simulations evidence physical processes advocated previously by the theoretical model of impulsive penetration and revealed in laboratory experiments.

Cassini observations of Saturn's southern polar cusp

AbstractThe magnetospheric cusps are important sites of the coupling of a magnetosphere with the solar wind. The combination of both ground‐ and space‐based observations at Earth has enabled considerable progress to be made in understanding the terrestrial cusp and its role in the coupling of the magnetosphere to the solar wind via the polar magnetosphere. Voyager 2 fully explored Neptune's cusp in 1989, but highly inclined orbits of the Cassini spacecraft at Saturn present the most recent opportunity to repeatedly study the polar magnetosphere of a rapidly rotating planet. In this paper we discuss observations made by Cassini during two passes through Saturn's southern polar magnetosphere. Our main findings are that (i) Cassini directly encounters the southern polar cusp with evidence for the entry of magnetosheath plasma into the cusp via magnetopause reconnection, (ii) magnetopause reconnection and entry of plasma into the cusp can occur over a range of solar wind conditions, and (iii) double cusp morphologies are consistent with the position of the cusp oscillating in phase with Saturn's global magnetospheric periodicities.

GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 2: Interhemispheric comparison

Abstract. During the ascending phase of solar cycle 24, a series of interplanetary coronal mass ejections (ICMEs) in the period 7–17 March 2012 caused geomagnetic storms that strongly affected high-latitude ionosphere in the Northern and Southern Hemisphere. GPS phase scintillation was observed at northern and southern high latitudes by arrays of GPS ionospheric scintillation and TEC monitors (GISTMs) and geodetic-quality GPS receivers sampling at 1 Hz. Mapped as a function of magnetic latitude and magnetic local time (MLT), the scintillation was observed in the ionospheric cusp, the tongue of ionization fragmented into patches, sun-aligned arcs in the polar cap, and nightside auroral oval and subauroral latitudes. Complementing a companion paper (Prikryl et al., 2015a) that focuses on the high-latitude ionospheric response to variable solar wind in the North American sector, interhemispheric comparison reveals commonalities as well as differences and asymmetries between the northern and southern high latitudes, as a consequence of the coupling between the solar wind and magnetosphere. The interhemispheric asymmetries are caused by the dawn–dusk component of the interplanetary magnetic field controlling the MLT of the cusp entry of the storm-enhanced density plasma into the polar cap and the orientation relative to the noon–midnight meridian of the tongue of ionization.

Plasma distribution in Mercury's magnetosphere derived from MESSENGER Magnetometer and Fast Imaging Plasma Spectrometer observations

AbstractWe assess the statistical spatial distribution of plasma in Mercury's magnetosphere from observations of magnetic pressure deficits and plasma characteristics by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The statistical distributions of proton flux and pressure were derived from 10 months of Fast Imaging Plasma Spectrometer (FIPS) observations obtained during the orbital phase of the MESSENGER mission. The Magnetometer‐derived pressure distributions compare favorably with those deduced from the FIPS observations at locations where depressions in the magnetic field associated with the presence of enhanced plasma pressures are discernible in the Magnetometer data. The magnitudes of the magnetic pressure deficit and the plasma pressure agree on average, although the two measures of plasma pressure may deviate for individual events by as much as a factor of ~3. The FIPS distributions provide better statistics in regions where the plasma is more tenuous and reveal an enhanced plasma population near the magnetopause flanks resulting from direct entry of magnetosheath plasma into the low‐latitude boundary layer of the magnetosphere. The plasma observations also exhibit a pronounced north‐south asymmetry on the nightside, with markedly lower fluxes at low altitudes in the northern hemisphere than at higher altitudes in the south on the same field line. This asymmetry is consistent with particle loss to the southern hemisphere surface during bounce motion in Mercury's offset dipole magnetic field.

Nonclassical Transport and Particle-Field Coupling: from Laboratory Plasmas to the Solar Wind

Understanding transport of thermal and suprathermal particles is a fundamental issue in laboratory, solar-terrestrial, and astrophysical plasmas. For laboratory fusion experiments, confinement of particles and energy is essential for sustaining the plasma long enough to reach burning conditions. For solar wind and magnetospheric plasmas, transport properties determine the spatial and temporal distribution of energetic particles, which can be harmful for spacecraft functioning, as well as the entry of solar wind plasma into the magnetosphere. For astrophysical plasmas, transport properties determine the efficiency of particle acceleration processes and affect observable radiative signatures. In all cases, transport depends on the interaction of thermal and suprathermal particles with the electric and magnetic fluctuations in the plasma. Understanding transport therefore requires us to understand these interactions, which encompass a wide range of scales, from magnetohydrodynamic to kinetic scales, with larger scale structures also having a role. The wealth of transport studies during recent decades has shown the existence of a variety of regimes that differ from the classical quasilinear regime. In this paper we give an overview of nonclassical plasma transport regimes, discussing theoretical approaches to superdiffusive and subdiffusive transport, wave–particle interactions at microscopic kinetic scales, the influence of coherent structures and of avalanching transport, and the results of numerical simulations and experimental data analyses. Applications to laboratory plasmas and space plasmas are discussed.

Magnetised Kelvin-Helmholtz instability in the intermediate regime between subsonic and supersonic regimes

The understanding of the dynamics at play at the Earth’s Magnetopause, the boundary separating the Earth’s magnetosphere and the solar wind plasmas, is of primary importance for space plasma modeling. We focus our attention on the low latitude flank of the magnetosphere where the velocity shear between the magnetosheath and the magnetospheric plasmas is the energetic source of Kelvin-Helmholtz instability. On the shoulder of the resulting vortex chain, different secondary instabilities are at play depending on the local plasma parameters and compete with the vortex pairing process. Most important, secondary instabilities, among other magnetic reconnection, control the plasma mixing as well as the entry of solar wind plasma in the magnetosphere. We make use of a two-fluid model, including the Hall term and the electron mass in the generalized Ohm’s law, to study the 2D non-linear evolution of the Kelvin-Helmholtz instability at the magnetosheath–magnetosphere interface, in the intermediate regime between subsonic and supersonic regimes. We study the saturation mechanisms, depending on the density jump across the shear layer and the magnetic field strength in the plane. In the presence of a weak in-plane magnetic field, the dynamics of the Kelvin-Helmholtz rolled-up vortices self-consistently generates thin current sheets where reconnection instability eventually enables fast reconnection to develop. Such a system enables to study guide field multiple-island collisionless magnetic reconnection as embedded in a large-scale dynamic system, unlike the classical static, ad hoc reconnection setups. In this regime, reconnection is shown to inhibit the vortex pairing process. This study provides a clear example of nonlinear, cross-scale, collisionless plasma dynamics.

Interhemispheric comparison of GPS phase scintillation at high latitudes during the magnetic-cloud-induced geomagnetic storm of 5–7 April 2010

Abstract. Arrays of GPS Ionospheric Scintillation and TEC Monitors (GISTMs) are used in a comparative scintillation study focusing on quasi-conjugate pairs of GPS receivers in the Arctic and Antarctic. Intense GPS phase scintillation and rapid variations in ionospheric total electron content (TEC) that can result in cycle slips were observed at high latitudes with dual-frequency GPS receivers during the first significant geomagnetic storm of solar cycle 24 on 5–7 April 2010. The impact of a bipolar magnetic cloud of north-south (NS) type embedded in high speed solar wind from a coronal hole caused a geomagnetic storm with maximum 3-hourly Kp = 8- and hourly ring current Dst = −73 nT. The interhemispheric comparison of phase scintillation reveals similarities but also asymmetries of the ionospheric response in the northern and southern auroral zones, cusps and polar caps. In the nightside auroral oval and in the cusp/cleft sectors the phase scintillation was observed in both hemispheres at about the same times and was correlated with geomagnetic activity. The scintillation level was very similar in approximately conjugate locations in Qiqiktarjuaq (75.4° N; 23.4° E CGM lat. and lon.) and South Pole (74.1° S; 18.9° E), in Longyearbyen (75.3° N; 111.2° E) and Zhongshan (74.7° S; 96.7° E), while it was significantly higher in Cambridge Bay (77.0° N; 310.1° E) than at Mario Zucchelli (80.0° S; 307.7° E). In the polar cap, when the interplanetary magnetic field (IMF) was strongly northward, the ionization due to energetic particle precipitation was a likely cause of scintillation that was stronger at Concordia (88.8° S; 54.4° E) in the dark ionosphere than in the sunlit ionosphere over Eureka (88.1° N; 333.4° E), due to a difference in ionospheric conductivity. When the IMF tilted southward, weak or no significant scintillation was detected in the northern polar cap, while in the southern polar cap rapidly varying TEC and strong phase scintillation persisted for many hours. This interhemispheric asymmetry is explained by the difference in the location of solar terminator relative to the cusps in the Northern and Southern Hemisphere. Solar terminator was in the immediate proximity of the cusp in the Southern Hemisphere where sunlit ionospheric plasma was readily convected into the central polar cap and a long series of patches was observed. In contrast, solar terminator was far poleward of the northern cusp thus reducing the entry of sunlit plasma and formation of dense patches. This is consistent with the observed and modeled seasonal variation in occurrence of polar cap patches. The GPS scintillation and TEC data analysis is supported by data from ground-based networks of magnetometers, riometers, ionosondes, HF radars and all-sky imagers, as well as particle flux measurements by DMSP satellites.

Kelvin-Helmholtz waves under southward interplanetary magnetic field

[1] The Kelvin-Helmholtz waves have been observed along the Earth's low-latitude magnetopause and have been suggested to play a certain role in the entry of solar wind plasma into Earth's magnetosphere. In situ observations of the KH waves (KHW) and, in particular, a nonlinear stage of the KH instability, i.e., rolled-up KH vortices (KHVs), have been reported to occur preferentially for northward interplanetary magnetic field (IMF). Using Cluster data, we present the first in situ observation of nonlinearly developed KHW during southward IMF. The analysis reveals that there is a mixture of less-developed and more-developed KHW that shows inconsistent variations in scale size and the magnetic perturbations in the context of the expected evolution of KH structures. A coherence analysis implies that the observed KHW under southward IMF appear to be irregular and intermittent. These irregular and turbulent characteristics are more noticeable than previously reported KHW events that have been detected preferentially during northward IMF. This suggests that under southward IMF KHVs become easily irregular and temporally intermittent, which might explain the preferential in situ detection of KHVs when the IMF is northward. MHD simulation of the present event shows that during southward IMF dynamically active subsolar environments can cause KHV that evolve with considerable intermittency. The MHD simulations appear to reproduce well the qualitative features of the Cluster observations.

Solar wind control of plasma number density in the near‐Earth plasma sheet

The plasma number density in the plasma sheet depends on the solar wind number density and the north‐south component of interplanetary magnetic field (IMF Bz) with the time lag of several hours. We examined such dependences as functions of (X, Y) coordinates in the near‐Earth plasma sheet by fitting observations of plasma sheet and solar wind to an empirical model equation. We explored shortest and longest response time lags to IMF Bz by optimizing the correlation coefficient of fit. Analyses were conducted separately for northward and southward IMF dominant conditions. The dependence of plasma sheet number density on solar wind number density is stronger in the near‐tail region (r > 20 RE) under the southward IMF dominant condition. The dependence has weak dawn‐dusk asymmetry under the southward IMF dominant condition, whereas it is stronger in the dusk flank under the northward IMF dominant condition. The dependence on IMF Bz is globally positive under the northward IMF dominant condition, whereas the dependence is negative in the near‐Earth premidnight region under the southward IMF dominant condition. Both shortest and longest time lags increase from the mid‐tail to the near‐Earth premidnight region under the southward IMF dominant condition. On the other hand, the shortest (longest) time lag increases antisunward (sunward) along flanks under the northward IMF dominant condition. These features can be explained in terms of reconnection and Kelvin‐Helmholtz diffusion mechanisms for entry of magnetosheath plasma into the plasma sheet and electric and magnetic drift transport in the plasma sheet.

Psoriasin, one of several new proteins identified in nasal lavage fluid from allergic and non-allergic individuals using 2-dimensional gel electrophoresis and mass spectrometry

BackgroundExtravasation and luminal entry of plasma occurs continuously in the nose. This process is markedly facilitated in patients with symptomatic allergic rhinitis, resulting in an increased secretion of proteins. Identification of these proteins is an important step in the understanding of the pathological mechanisms in allergic diseases. DNA microarrays have recently made it possible to compare mRNA profiles of lavage fluids from healthy and diseased patients, whereas information on the protein level is still lacking.MethodsNasal lavage fluid was collected from 11 patients with symptomatic allergic rhinitis and 11 healthy volunteers. 2-dimensional gel electrophoresis was used to separate proteins in the lavage fluids. Protein spots were picked from the gels and identified using mass spectrometry and database search. Selected proteins were confirmed with western blot.Results61 spots were identified, of which 21 were separate proteins. 6 of these proteins (psoriasin, galectin-3, alpha enolase, intersectin-2, Wnt-2B and hypothetical protein MGC33648) had not previously been described in nasal lavage fluids. The levels of psoriasin were markedly down-regulated in allergic individuals. Prolactin-inducible protein was also found to be down-regulated, whereas different fragments of albumin together with Ig gamma 2 chain c region, transthyretin and splice isoform 1 of Wnt-2B were up-regulated among the allergic patients.ConclusionThe identification of proteins in nasal lavage fluid with 2-dimensional gelelectrophoresis in combination with mass spectrometry is a novel tool to profile protein expression in allergic rhinitis and it might prove useful in the hunt for new therapeutic targets or diagnostic markers for allergic diseases. Psoriasin is a potent chemotactic factor and its down-regulation during inflammation might be of importance for the outcome of the disease.

Low-energy/high-charge-state ions in the polar ionosphere observed by AKEBONO/SMS

The suprathermal ion mass spectrometer (SMS) onboard the AKEBONO satellite has occasionally observed a broad mass peak in the ion mass spectrum at amu/ q ∼ 2.7. SMS is capable of measuring a wide range of ion mass (1–64 amu/ q) at low energy ( E < ∼100 eV) with good mass resolution (Δ M/ M ∼ 0.1). The events were observed at about 1 R e altitude, and mostly at the polar cusp region as identified by the on-board low-energy particle (LEP) instrument. The broad mass peak is consistent with high-charge state ions commonly seen in the solar wind such as C 6+, O 7+, and O 6+. Although it is difficult to identify the charge state of these ions definitively using SMS, which is a mass-per-charge analyzer, decomposition of the broad mass peak results in a reasonable ratio of O 7+/O 6+ to that in the solar wind. Statistical study shows that this type of events has been observed with an occurrence rate of about 10% near the dayside cusp region. Also, the occurrence rate is slightly higher when IMF B z is positive, suggesting the entry of the solar wind plasma into the polar ionosphere with weaker acceleration during northward IMF compared with southward IMF.

Observations of diverging field-aligned ion flow with the ESR

Abstract. We report on observations of a diverging ion flow along the geomagnetic field that is often seen at the EISCAT Svalbard radar. The flow is upward above the peak of the electron density in the F-region and downward below the peak. We estimate that in such events mass transport along the field line is important for the ionization balance, and that the shape of the F-layer and its ion composition should be strongly influenced by it. Diverging flow typically occurs when there are signatures of direct entry of sheath plasma to the ionosphere in the form of intense soft particle precipitation, and we suggest that it is caused by the ionization and ionospheric electron heating associated with this precipitation. On average, 30% of all events with ion upflow also show significant ion downflow below. Key words.Ionosphere (polar ionosphere; ionization mechanism; plasma temperature and density)

Airway microvascular extravasation and luminal entry of plasma

Extravasation of plasma from postcapillary venules is a specific in vivo response to inflammatory insults. In the nasal and bronchial airways, extravasated plasma has a widespread distribution in the lamina propria, between the epithelial cells and in the airway lumen. This feature, in combination with the fact that the process involves extravasation of bulk plasma, with all peptides and proteins of plasma, indicates that plasma exudation contributes to the dramatic change of the mucosal milieu that characterizes airway inflammation. Accordingly, this process is of key importance to conditions such as allergic rhinitis and asthma. The means by which extravasated plasma participates in mucosal defence is physiological in the sense that it may operate on the surface of the epithelium without impairing its function as an absorption barrier. The flow of plasma into the airway lumen may thus wash away unwanted material from inter-epithelial cell spaces, exuded binding proteins may bind unwanted solutes non-specifically and extravasated immunoglobulins may neutralize allergens. In addition to the role as defence mechanism, extravasated plasma components may act as important pro-inflammatory factors. Furthermore, experimental data as well as observations in natural disease suggest that luminal levels of plasma proteins can be employed as an accessible index reflecting to what degree the airway mucosa is affected by inflammatory processes.

Double cusp: Model prediction and observational verification

Recent modeling of the entry of solar wind plasma into the magnetosphere and ionosphere has adequately simulated the large‐scale particle precipitation features in the observed cusp, mantle, polar rain, and open‐field line low‐latitude boundary layer regions. The assumption of a simple dawn‐dusk electric field limited the models to the near‐noon region and southward interplanetary magnetic field (IMF) case. Here, we present an improved model that incorporates the electric field obtained from statistical convection patterns. When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of a double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower‐latitude cusp ions originate from low‐latitude magnetosheath, whereas the higher‐latitude ions originate from the high‐latitude magnetosheath. The lower‐latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp, as would be observed from a typical meridional trajectory of a polar‐orbiting satellite. The higher‐latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher‐latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. This prediction was subsequently confirmed in a large case study with Defense Meteorological Satellite Program (DMSP) data. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF By and small negative IMF Bz ,30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double‐cusp result is consistent with the following new statistical results: (1) the cusp latitudinal width increases with |IMF By| and (2) the cusp equatorward boundary moves to lower latitude with increasing |IMF By|.

Electromagnetic ion cyclotron waves in the high‐altitude cusp: Polar observations

High‐resolution magnetic field data from the Polar Magnetic Field Experiment (MFE) show that narrowband waves at frequencies ∼0.2–3 Hz are a permanent feature in the vicinity of the polar cusp. The waves have been found in the magnetosphere adjacent to the cusp (both poleward and equatorward of the cusp) and in the cusp itself. The occurrence of waves is coincident with depression of magnetic field strength associated with enhanced plasma density, indicating the entry of magnetosheath plasma into the cusp region. The wave frequencies are generally scaled by the local proton cyclotron frequency and vary between 0.2 and 1.7 times local proton cyclotron frequency. This suggests that the waves are generated in the cusp region by the precipitating magnetosheath plasma. The properties of the waves are highly variable. The waves exhibit both left‐handed and right‐handed polarization in the spacecraft frame. The propagation angles vary from nearly parallel to nearly perpendicular to the magnetic field. We find no correlation among wave frequency, propagation angle, and polarization. Combined magnetic field and electric field data for the waves indicate that the energy flux of the waves is guided by the background magnetic field and points downward toward the ionosphere.

Foreword [to Special Section on Insights From the Polar Spacecraft

The International Solar Terrestrial Physics (ISTP) program [Alexander and Nishida, 1984, p. 1] defined an ambitious goal: to develop a comprehensive, global understanding of the generation and flow of energy from the Sun through the Earth s space environment (geospace) and to define the cause‐and‐effect relationships between the physical processes that link different regions of this dynamic environment. Four ISTP spacecraft were launched to accomplish the objectives of the program, two into the solar wind, Solar and Heliospheric Observatory (SOHO) from the European Space Agency (ESA) [Fleck et al., 1995] and Wind [Russell, 1995] from NASA, and two within the magnetosphere, Geotail from the Institute of Space and Astronautical Science (ISAS) [Terasawa and Kamide, 1994] and Polar from NASA [Russell, 1995]. Within this fleet the Polar spacecraft has had the responsibility for measuring the entry of plasma into the Polar magnetosphere and the geomagnetic tail, the flow of plasma to and from the ionosphere, and the deposition of particle energy in the ionosphere and upper atmosphere [Hoffman et al., 1996]. Because of common science interests, the Polar Science Team looks with anticipation toward coordinated activities with the recently launched four Cluster spacecraft mission by ESA [Escoubet et al., 1997], which will become an integral part of the ISTP program.