Plasmaspheric Wind Research Articles

Ion Outflow and Escape in the Terrestrial Magnetosphere: Cluster Advances

AbstractCluster was the first mission in the terrestrial magnetosphere to involve four spacecraft in a tetrahedral configuration, providing three‐dimensional measurements of the space plasma parameters. Cluster was also equipped with a very comprehensive instrumentation, allowing the measurement of the ion populations outflowing from the ionosphere, their circulation in the magnetosphere, and their eventual escape to outer space. The observations of the outflowing and escaping ion populations performed by Cluster are reviewed and the most prominent results highlighted. These show the dominance in the magnetotail lobes of cold plasma outflows originating from the polar caps. For the energetic heavy ion outflow, the cusps constitute the main source. Their transport and acceleration through the polar cap into the lobes and then into the plasma sheet has been characterized. The dependence of the polar outflow on the solar wind parameters and on the geomagnetic activity has been evaluated for both cold ion populations and heavy energetic ions. For the latter, outflow has been observed during all periods but an increase by two orders of magnitude has been shown during extreme space weather conditions. This outflow is adequate to change the composition of the atmosphere over geological timescales. At lower latitudes, the existence of a plasmaspheric wind, providing a continuous leak from the plasmasphere, has been demonstrated. The general scheme of the outflowing ion circulation in the magnetosphere or escape, and its dependence on the IMF conditions, has been outlined. However, several questions remain open, waiting for a future space mission to address them.

Improving Predictions of the 3D Dynamic Model of the Plasmasphere

In this perspective paper, we review and discuss different ways that can be used to improve the predictions of the models of the plasmaspheric region. The density of the background cold plasma and the plasmapause position are very important to determine the formation and propagation of waves and interactions with the other regions of the magnetosphere. Improvement of predictions includes refinement of the forecast of the geomagnetic indices that influence the density and the temperature of the particles in some models. Progress is also necessary for the understanding of the physical processes that affect the position of the plasmapause and its thickness since this boundary is not always very sharp, especially during low geomagnetic activity. These processes include the refilling after geomagnetic storms and substorms, the links with the ionosphere, and the expanding plasmaspheric wind during prolonged quiet periods. Using observations fromin situsatellites like Van Allen Probes (EMFISIS and HOPE instruments), empirical relations can be determined to improve the dependence of the density and the temperature as a function of the radial distance, the latitude, and the magnetic local time, inside and outside the plasmasphere. This will be the first step for the improvement of our 3D dynamic SWIFF plasmaspheric model (SPM).

Recent highlights from Cluster, the first 3-D magnetospheric mission

Abstract. The Cluster mission has been operated successfully for 14 years. During this time period, the evolution of the orbit has enabled Cluster to sample many more magnetospheric regions than was initially anticipated. So far, the separation of the Cluster spacecraft has been changed more than 30 times and has ranged from a few kilometres up to 36 000 km. These orbital changes have enabled the science team to address a wide variety of scientific objectives in key regions of Earth's geospace environment: the solar wind and bow shock, the magnetopause, polar cusps, magnetotail, plasmasphere and the auroral acceleration region. Recent results have shed new light on solar wind turbulence. They showed that the magnetosheath can be asymmetric under low Mach number and that it can contain density enhancement that may affect the magnetosphere. The magnetopause was found to be thinner and to have a higher current density on the duskside than on the dawnside. New methods have been used to obtain characteristic of the magnetotail current sheet and high-temporal-resolution measurements of electron pitch angle within flux transfer events (FTEs). Plasmaspheric wind has been discovered, and the refilling of the plasmasphere was observed for the first time over a very wide range of L shells. New models of global electric and magnetic fields of the magnetosphere have been obtained where Cluster, due to its polar orbit, has been essential. Finally, magnetic reconnection was viewed for the first time with high-resolution wave and electron measurements and acceleration of plasma was observed during times of varying rate of magnetic reconnection. The analysis of Cluster data was facilitated by the creation of the Cluster Science Data System (CSDS) and the Cluster Science Archive (CSA). Those systems were implemented to provide, for the first time for a plasma physics mission, a long-term public archive of all calibrated high-resolution data from all instruments.

Detection of a plasmaspheric wind in the Earth's magnetosphere by the Cluster spacecraft

Abstract. Plumes, forming at the plasmapause and released outwards, constitute a well-established mode for plasmaspheric material release to the Earth's magnetosphere. They are associated to active periods and the related electric field change. In 1992, Lemaire and Shunk proposed the existence of an additional mode for plasmaspheric material release to the Earth's magnetosphere: a plasmaspheric wind, steadily transporting cold plasmaspheric plasma outwards across the geomagnetic field lines, even during prolonged periods of quiet geomagnetic conditions. This has been proposed on a theoretical basis. Direct detection of this wind has, however, eluded observation in the past. Analysis of ion measurements, acquired in the outer plasmasphere by the CIS experiment onboard the four Cluster spacecraft, provide now an experimental confirmation of the plasmaspheric wind. This wind has been systematically detected in the outer plasmasphere during quiet and moderately active conditions, and calculations show that it could provide a substantial contribution to the magnetospheric plasma populations outside the Earth's plasmasphere. Similar winds should also exist on other planets, or astrophysical objects, quickly rotating and having an atmosphere and a magnetic field.

Investigating the southern daytime midlatitude trough's relation with the daytime Weddell Sea Anomaly during equinoxes

A detailed study with statistical investigations was conducted on the morning topside ionosphere for the 1996/1997 equinoctial period utilizing Defense Meteorological Satellite Program data and coupled thermosphere‐ionosphere plasmasphere wind simulations. Highly structured plasma densities, including a daytime Weddell Sea Anomaly (WSA; about 0900 LT) over the southeastern Pacific and localized high‐latitude plasma enhancements (about 0800 LT) in the Australian sector, were tracked. Poleward and equatorward winds, respectively, were strongest there. High‐resolution regional surface maps and field‐aligned profiles tracked their thermal structures, O+ and H+ composition, vertical drifts (VZ, m/s) and horizontal plasma flows {FY; [i+/(cm2s)]}. A daytime heavy‐ion (Ni‐O+) stagnation trough [(54.5 ± 4.5)°S; geomagnetic] appeared equatorward of both the WSA [60.0 ± 4.0)°S (center), 185–310°E; geographic] and these enhancements [>(62.0 ± 3.5)°S; 30–180°E]. Meanwhile, the plasmapause developed poleward of the WSA and equatorward of these enhancements. Plasma convection became significantly increased by enhanced electron temperatures nearby upflows underlying localized plasma enhancements. Upflow velocities, modeled as about 694.5 m/s, shows close similarities to the actual direct measurements of about 700 m/s. Both slow and enhanced plasma convections promoted stagnation events and trough formation. However, regular trough features became significantly altered by the development of a downward dipping poleward wall in the WSA region and by the absence of regular trough walls in the region of localized plasma enhancements. Statistically, the trough and localized plasma enhancement locations varied linearly with magnetic activity and showed an equatorward movement with increasing activity. A rate of (0.026 ± 0.002)°S/nT for the trough and (0.067 ± 0.003)°S/nT for the localized plasma enhancements were modeled. This higher (approximately two times) rate of movement indicates the localized plasma enhancements' more responsiveness.

An investigation of the northern hemisphere midlatitude nighttime plasma density enhancements and their relations to the midlatitude nighttime trough during summer

This study has utilized regional surface maps and field‐aligned latitudinal profiles derived from multiinstrument DMSP‐F15 data. It investigates northern midlatitude plasma enhancements in the nighttime topside ionosphere during summer and their relation to the midlatitude heavy ion stagnation trough. The tracked midlatitude summer enhancements showed significant longitudinal variations. Their best development occurred in the 50°N–60°N/110°E–170°E (geographic) region. Proven by Coupled Thermosphere‐Ionosphere Plasmasphere wind velocity simulations, equatorward winds were strongest there. This region is the northern hemisphere equivalent of the Weddell Sea Anomaly's (WSA) locality. There, northern summer plasma enhancements resembled the nighttime WSA, a southern summer equinoctial phenomenon, and thus were named as WSA‐like feature. Their plasma density increased mainly owing to the mechanical or direct effects of the equatorward winds. However, as results show, the WSA‐like feature was an ordinary northern midlatitude nighttime enhancement occurring in summer. This was further confirmed by the WSA‐like feature's relation to the trough and by the trough's movement. Over the northern hemisphere, the summer trough appeared at (62.461 ± 2.93)°N (geomagnetic), poleward of the WSA‐like feature and other midlatitude plasma enhancements, and moved equatorward with a rate of (0.097 ± 0.04)°N/nT when magnetic activity increased. As our previous WSA investigation indicates, the southern summer trough develops at lower latitudes, at (47.1564 ± 2.16)°S, equatorward of the nighttime WSA and moves poleward with a rate of (0.0497 ± 0.003)°S/nT with increasing magnetic activity under the combined influence of the nighttime WSA and South Atlantic Magnetic Anomaly.

Supersonic neutral winds and neutral streams in the thermosphere‐ionosphere‐plasmasphere system

Model studies were conducted to determine the conditions that might give rise to supersonic neutral winds in the high‐latitude thermosphere and that can produce neutral streams in the polar wind and plasmasphere. For the supersonic neutral wind study, we used a time‐dependent, three‐dimensional, global thermosphere‐ionosphere model. Simulations were conducted for different seasonal conditions, solar activity levels, cross‐polar‐cap potentials, and convection pattern shapes. Supersonic neutral winds were found to occur at altitudes above 200 km for cross‐polar‐cap potentials greater than about 150 kV, although most of the supersonic winds occurred between 300 and 600 km. The Mach number of the neutral winds was typically less than 1.5, and the supersonic winds were generated within 3 h after enhanced plasma convection. These winds were primarily located in the midnight‐dawn sector of the polar cap, and their dusk‐to‐dawn extent was of the order of 150 km. Also, during enhanced convection, the ion outflow in the polar wind increases because of the elevated plasma temperatures, and this leads to enhanced ion‐neutral charge exchange reactions, and hence, to the production of neutral streams in the polar wind (Neutral Polar Wind). Likewise, on closed magnetic field lines, the upflowing ionospheric ions can charge exchange with the background neutrals, and this produces neutral streams in the plasmasphere (Neutral Plasmasphere Wind). Coupled models of the ionosphere‐polar wind‐exosphere and ionosphere‐plasmasphere‐exosphere were used to elucidate the basic characteristics of these neutral streams. An important result is that the neutral O streams typically do not have sufficient energy to escape in both the polar wind and plasmasphere, and they rain down on the global thermosphere (Neutral Rain). The downward streaming neutral O atoms pass through the exobase at 500–600 km and provide a source of momentum and energy for the thermosphere. The results presented here correspond to a focused modeling study that was presented at the Ionospheric Effects Symposium (May 2008).

Recent Progress in Physics-Based Models of the Plasmasphere

We describe recent progress in physics-based models of the plasmasphere using the fluid and the kinetic approaches. Global modeling of the dynamics and influence of the plasmasphere is presented. Results from global plasmasphere simulations are used to understand and quantify (i) the electric potential pattern and evolution during geomagnetic storms, and (ii) the influence of the plasmasphere on the excitation of electromagnetic ion cyclotron (EMIC) waves and precipitation of energetic ions in the inner magnetosphere. The interactions of the plasmasphere with the ionosphere and the other regions of the magnetosphere are pointed out. We show the results of simulations for the formation of the plasmapause and discuss the influence of plasmaspheric wind and of ultra low frequency (ULF) waves for transport of plasmaspheric material. Theoretical models used to describe the electric field and plasma distribution in the plasmasphere are presented. Model predictions are compared to recent Cluster and Image observations, but also to results of earlier models and satellite observations.

Plasma plume circulation and impact in an MHD substorm

We investigate the fate of a plasmaspheric plume generated by a discrete period of southward interplanetary magnetic field (IMF) to assess its contribution to plasma sheet and ring current pressure and compare with that for other sources. We use test particle motions in Lyon‐Fedder‐Mobarry (LFM) global circulation model fields. The inner magnetosphere is simulated with the Comprehensive Ring Current Model (CRCM) model of Fok and Wolf, driven by the transpolar potential developed by the LFM magnetosphere. A variant of the Ober plasmasphere model is embedded within the models and driven by them. Global circulation is stimulated by a period of southward IMF embedded within a long interval of northward IMF. This leads to the production of a well‐defined plasmaspheric plume, enhancing the plasma density sunward of the plasmasphere. Test particles are launched with the properties of plasmaspheric ions on the L = 6.6 RE shell and weighted with densities as specified by the Ober model, as it responds to convection imposed by CRCM. Particles are tracked until they are lost from the system downstream or into the atmosphere, using the Delcourt full equations of motion, implemented for finite element fields. Results are compared with earlier computations of polar and auroral wind outflows. The plume produces an enhanced flow of plasma ∼10 times the normal polar wind global fluence. However, we find that most of the “plasmaspheric wind” is lost from the magnetosphere such that its contribution to the ring current energy density is comparable to that of the normal polar wind for this type of event.

Smooth electron density transition from plasmasphere to the subauroral region

Upper hybrid resonance (UHR) noise band and the nonthermal continuum (NTC) radiation were routinely observed by the radio plasma imager (RPI) onboard the IMAGE satellite when it transited the plasmasphere and subauroral regions. The lower cutoff frequencies of the UHR band and the NTC radiation provide an estimate of the electron plasma frequencies along the satellite orbit. A steep electron density gradient, which defines the plasmapause, was commonly observed when IMAGE transited from the plasmasphere to the subauroral region or vice versa. It appears however that, on many occasions, the electron density transition from the plasmasphere to the subauroral region is smooth without a clear signature of the plasmapause. Such smooth transitions can occur at various magnetic local times. The events presented in this study were observed after geomagnetic activities had been quiet for 2 or more days with Kp primarily less than 3. The smooth density transitions from the plasmasphere to the subauroral region may be an observational evidence of the suggested plasmaspheric wind.

Convective instabilities in the plasmasphere

According to observations, the averaged slope of the equatorial plasma density distribution is nearly constant ( d ln n / d L = - 0.724 ) during very quiet geomagnetic activity from L = 2 to L > 7 , and even smaller ( d ln n / d L = - 1.5 ) during more typical quiet activity. These slopes are steeper than the threshold values for which magnetostatic plasma density distributions in the equatorial regions become unstable with respect to transverse and translational interchange motion in the Earth's gravitational and magnetic fields. When the effects of a centrifugal force due to corotation of the plasma with the angular velocity of the Earth are taken into account, these convective instability thresholds are significantly affected, at least in the vicinity of and beyond geosynchronous orbit. Nevertheless, for L < 6.6 , neither instability criterion is satisfied (i.e. corotating plasma should be convectively stable inside geosynchronous orbit) for the observed density slope as well as for that corresponding to a rigidly corotating plasmasphere in magnetostatic equilibrium. This conclusion holds under the assumption that the unperturbed magnetic field lines are straight (i.e. not curved, as are geomagnetic field lines). However, this assumption is not realistic and the effects associated with the magnetic curvature are largely dominant over the effects of the effective gravity in the equatorial regions of the plasmasphere and of the magnetosphere in general. When the effect of magnetic field tension (due to the curvature of the dipole magnetic field lines, and B-field gradient) is properly taken into account, the thermal plasma confined in the Earth's dipole magnetic field cannot remain in magnetostatic equilibrium but becomes convectively unstable for smaller L, i.e. much deeper inside the equatorial plasmasphere. In other words, the equatorial region of the plasmasphere becomes convectively unstable at much lower altitudes when the curvature of dipole magnetic field lines is taken into account. Thus the existence of a plasmaspheric wind is supported by our theoretical result: i.e. the equatorial plasmasphere is not in magnetostatic/barometric/diffusive equilibrium, but is convectively unstable at L = 2 and beyond. The picture of a static equatorial plasmasphere seems to be at odd, even in a saturated stage following a long period of quiet geomagnetic conditions.

Hydrostatic equilibrium and convective stability in the plasmasphere

It has been shown that no hydrostatic/barometric model for a rotating plasmasphere is able to fit the equatorial electron density distributions observed by the ISEE satellite for L<8, following prolonged periods of quite magnetic conditions. Indeed, it has been found that all these saturated plasma density profiles are characterized by scale heights which are independent of L, while those corresponding to any hydrostatic/barometric models are always increasing functions of L. Furthermore, all calculated barometric equatorial density profiles have a minimum value at an equatorial distance given by L o =(GM E / Ω 2R 3 E ) 1/3=6.6( Ω E / Ω) 2/3, depending on the value of the angular rotational speed, Ω: L o≤6.6 for Ω/Ω E≥1. The position of this minimum is independent of the particle velocity distribution function (VDF) assumed (i.e. on the energy spectrum of the ions and electrons); the value of the minimum density depends on the temperature and density at the low altitude reference, as well as on the kappa index of the Lorentzian VDF assumed to calculate the barometric model. There is no evidence for such a minimum value in the observed equatorial density profiles of ISEE. This is considered as a second indication that the plasmasphere is not in hydrostatic/barometric equilibrium. Next, it is shown that for L> L o all barometric models are convectively unstable with respect to interchange and quasi-interchange. Therefore, it is not surprising that observed density profiles do not fit any of the profiles corresponding to corotating barometric models. A final indication that the plasmasphere is not in hydrostatic equilibrium but in a state of continuous hydrodynamic expansion comes from the fact that the total plasma pressure at the outer edge of the magnetosphere is lower than the kinetic pressure predicted by barometric models at large radial distances. As a consequence of this pressure unbalance, the plasmasphere is expected to expand continuously like the solar corona. Evidence for such a plasmaspheric wind had already been inferred earlier from the study of the ion density profiles obtained from the OGO-5 mission.

Plasma transport in the plasmasphere

The plasmasphere is filled with very low energy plasma upwelling from the topside ionosphere. The field-aligned distribution of this thermal ionospheric plasma is controlled by the gravitational and centrifugal potential distribution. There are two extreme types of hydrostatic plasma distribution in this field-aligned potential : the Diffusive Equilibrium distribution and the Exospheric Equilibrium distribution corresponding respectively to a saturated and to an almost empty magnetic flux tube. As a result of pitch angle scattering by Coulomb collisions an increasing number of ions escaping from the ionosphere are stored on trapped orbits with mirror points at high altitudes in the low density region. As a result of collisions the field-aligned density distribution gradually changes from exospheric equilibrium with a highly anisotropic pitch angle (cigar like) distribution to a diffusive equilibrium with a nearly isotropic pitch angle distribution. It is shown that the suprathermal ions become anisotropic much more slowly than ions of energies smaller than 1 eV. The Coulomb collision times have been estimated for flux tubes at different L values. A numerical simulation of the flux tube refilling process has been presented. The diurnal variation of the equatorial plasma density has been illustrated for plasma elements convected along drift paths which have a large dawn- dusk asymetry. The formation of a Light Ion Trough is discussed. Finally, evidence has also been given for the existence of a ‘plasmaspheric wind’ corresponding to a slow subsonic and continuous radial expansion of the plasma stored in the plasmasphere.