Plasmaspheric Drainage Plume Research Articles

Effects of Cold Plasma on the Excitation of Internally Driven ULF Waves by Ring Current Ions Based On the Magnetosphere‐Ionosphere Coupled Model

AbstractInternally driven Pc4‐5 waves are excited in the plasmaspheric drainage plume (PDP) and near the plasmapause. The excitation of ultralow frequency (ULF) waves was investigated by using the magnetosphere‐ionosphere coupled model between Geospace Environment Modeling System for Integrated Studies‐Ring Current (GEMSIS‐RC) and GEMSIS‐POTential solver (GEMSIS‐POT). In order to investigate the effects of cold plasma on the wave excitation, the simulation code to describe the dynamics of cold plasma was included in the model. The model can reproduce the shrink of the plasmasphere on the nightside and the formation of the PDP on the dayside. First, fundamental Pc5 waves are excited through the drift resonance on the dayside. The waves are caused by positive energy gradient of ion phase space density (PSD) at 50–130 keV. Second harmonic waves (drift‐bounce resonance) are generated outside the plasmapause. These two types of ULF waves are also seen in the case of constant density. Unlike the case of constant density, localized eastward propagating Pc4 waves (drift resonance) are seen on the dawnside associated with the azimuthal density gradient. The azimuthal wave number of Pc4 waves is about 70 and anti‐earthward gradient of PSD about 10 keV contributes to wave growth. We also detect fundamental Pc4‐5 waves near the plasmapause on the nightside in the drift resonance with 100–150 keV ions. Simulation results suggest that the plasmapause has an effect to sustain the excitation of Pc4‐5 ULF wave through the drift resonance.

Polarization Properties of 3‐D Field Line Resonances

AbstractRecent work has shown that field line resonances (FLRs) can form with intermediate magnetic field polarizations, that is, between toroidal (azimuthal) and poloidal (radial). In this case, they are referred to as “3‐D”, given that the equilibrium and therefore associated wave solutions vary in all three dimensions. Such 3‐D variations in the medium can be expected in the magnetosphere in particular during the presence of a plasmaspheric drainage plume in the dusk flank. In this case, strong field‐aligned currents driven by FLRs can form along contours of Alfvén frequency which are very asymmetric azimuthally. In this study, we use magnetohydrodynamic (MHD) simulations performed in a background dipole magnetic field, to consider what the satellite signatures would be for a spacecraft observing such non‐toroidal FLRs under different magnetopause driving conditions. We consider how the 3‐D nature impacts the amplitude and phase relations traditionally expected for FLRs. We find that in the 3‐D FLR regions of intermediate polarization, the FLR has strong contributions from both the radial and azimuthal field components (velocity, magnetic or electric) and these are in or out of phase. We show how hodograms can be used to infer the FLR polarization in these cases as well as provide predictions for the variation of the FLR polarization with MLT across a plasmaspheric plume. We further show how the presence of a significant global fast mode can skew the resulting polarization estimates.

Concerning the interaction of a transmitted interplanetary impulse with a plasmaspheric drainage plume: First results from 3-D hybrid kinetic modeling

We present a new hybrid kinetic model to simulate the response of plasmaspheric drainage plumes to impulsive interplanetary pressure pulses. Since particle distributions attending the interplanetary pulses and in the drainage plume are non-Maxwellian, wave-particle interactions play a crucial role in energy transport within and outside the plumes. Finite gyroradius effects become important in mass loading of the transmitted impulse with the drainage plume ions. A forward-reverse shock structure develops from the initial step-like transmitted shock. First results show that the impulse causes strong deformations in the global structure of the plume. The anisotropic ion velocity distribution functions at the impulse front and inside the plume help us determine energy transport via wave-particle interactions throughout the Earth’s inner magnetosphere.

Control of ULF Wave Accessibility to the Inner Magnetosphere by the Convection of Plasma Density

AbstractDuring periods of storm activity and enhanced convection, the plasma density in the afternoon sector of the magnetosphere is highly dynamic due to the development of plasmaspheric drainage plume (PDP) structure. This significantly affects the local Alfvén speed and alters the propagation of ULF waves launched from the magnetopause. Therefore, it can be expected that the accessibility of ULF wave power for radiation belt energization is sensitively dependent on the recent history of magnetospheric convection and the stage of development of the PDP. This is investigated using a 3‐D model for ULF waves within the magnetosphere in which the plasma density distribution is evolved using an advection model for cold plasma, driven by a (VollandStern) convection electrostatic field (resulting in PDP structure). The wave model includes magnetic field day/night asymmetry and extends to a paraboloid dayside magnetopause, from which ULF waves are launched at various stages during the PDP development. We find that the plume structure significantly alters the field line resonance location, and the turning point for MHD fast waves, introducing strong asymmetry in the ULF wave distribution across the noon meridian. Moreover, the density enhancement within the PDP creates a waveguide or local cavity for MHD fast waves, such that eigenmodes formed allow the penetration of ULF wave power to much lower L within the plume than outside, providing an avenue for electron energization.

Magnetospheric ion influence at the dayside magnetopause

AbstractMagnetospheric ions have the potential to affect magnetic reconnection at the magnetopause. The results of a survey of magnetospheric ions near the magnetopause are reported here. Composition measurements from the Magnetospheric Multiscale (MMS) mission are used to determine the total mass density of all magnetospheric ions and distinguish two populations of magnetospheric ions: the warm plasma cloak and the plasmaspheric drainage plume. The warm plasma cloak can contain substantial O+, and the plasmaspheric plume can contain substantial He+. The results of the survey show that, for nominal magnetospheric activity, the warm plasma cloak and plasmaspheric plume will reduce the normalized reconnection rate at the magnetopause by greater than 20% only a few percent of the time.

Magnetospheric ion influence on magnetic reconnection at the duskside magnetopause

AbstractMagnetospheric ions from the ring current, warm plasma cloak, and the plasmaspheric drainage plume all interact with the dusk flank magnetopause. During periods of strong magnetospheric convection, these ions may contribute significantly to the magnetospheric mass density at the magnetopause. Observations from the Magnetospheric Multiscale mission Hot Plasma Composition Analyzer at the duskside magnetopause near reconnection X lines show that ions from the ring current and warm plasma cloak may have high mass densities. However, these mass densities are not as large as the mass density in the magnetosheath. The results suggest that except for possible influence from the plasmaspheric drainage plume, the other major magnetospheric ion populations do not greatly influence asymmetric reconnection at the duskside magnetopause.

An extensive survey of dayside diffuse aurora based on optical observations at Yellow River Station

AbstractBy using 7 years optical auroral observations obtained at Yellow River Station (magnetic latitude 76.24°N) at Ny‐Alesund, Svalbard, we performed the first extensive survey for the dayside diffuse auroras (DDAs) and acquired observational results as follows. (1) The DDAs can be classified into two broad categories, i.e., unstructured and structured DDAs. The unstructured DDAs are mainly distributed in morning and afternoon, but the structured DDAs predominantly occurred around the magnetic local noon (MLN). (2) The unstructured DDAs observed in morning and afternoon present obviously different properties. The afternoon ones are much stable and seldom show pulsating property. (3) The DDAs are more easily observed under geomagnetically quiet times. (4) The structured DDAs mainly show patchy, stripy, and irregular forms and are often pulsating and drifting. The drifting directions are mostly westward (with speed ~5 km/s), but there are cases showing eastward or poleward drifting. (5) The stripy DDAs are exclusively observed near the MLN and, most importantly, their alignments are confirmed to be consistent with the direction of ionospheric convection near the MLN. (6) A new auroral form, called throat aurora, is found to be developed from the stripy DDAs. Based on the observational results and previous studies, we proposed our explanations to the DDAs. We suggest that the unstructured DDAs observed in the morning are extensions of the nightside diffuse aurora to the dayside, but that observed in the afternoon are predominantly caused by proton precipitations. The structured DDAs occurred near the MLN are caused by interactions of cold plasma structures, which are supposed to be originated from the ionospheric outflows or plasmaspheric drainage plumes, with hot electrons from the plasma sheet. We suppose that the cold plasma structures for producing the patchy DDAs are in lumpy and are more likely from the plasmaspheric drainage plumes. The cold plasma structure for producing the stripy DDAs should be in wedge like and is generated by conveying the cold plasmas from lower L‐shell toward higher L‐shell with magnetospheric convection, and that for producing the irregular DDAs is resulted from deforming the wedge‐like structure by disturbance. The throat aurora is supposed to be projection of a newly opened flux of reconnection. In addition, we also found that structured DDAs correspond to structured electron precipitations in the ionosphere, which implies that the cold plasma structures in the magnetosphere are magnetically mapped to the ionosphere and act as a duct for producing the structured DDAs. We argue that we have presented some new observational results about DDA in this paper, which will be useful for fully understanding the DDAs.

Whistlers observed outside the plasmasphere: Correlation to plasmaspheric/plasmapause features

AbstractWhistlers observed outside the plasmasphere by Cluster have been correlated with the global plasmasphere using Imager for Magnetopause‐to‐Aurora Global Exploration‐Extreme Ultraviolet Imager (IMAGE‐EUV) observations. Of the 12 Cluster‐observed whistler events reported, EUV is able to provide global imaging of the plasmasphere for every event and demonstrates a direct correlation between the detection of lightning‐generated whistlers beyond the plasmapause and the presence of a global perturbation of the local plasmapause. Of these 12 correlated events, seven of the Cluster‐observed whistlers (or 58%) are associated with the Cluster spacecraft lying radially outward from a plasmaspheric notch. Two of the Cluster‐observed whistlers (17%) are associated with the low‐density region between the late afternoon plasmapause and the western wall of a plasmaspheric drainage plume. The final three Cluster‐observed whistler events (25%) are associated with a nonradial, nonazimuthal depletion in plasmaspheric He+ emission that are termed “notch‐like” crenulations. In one of these cases, the notch‐like crenulations appear to be manifestations entrained within the plasmasphere boundary layer of a standing wave on the surface of the plasmasphere. The correlated Cluster/IMAGE‐EUV observations suggest that the depleted flux tubes that connect the ionosphere to the low‐density regions of plasmaspheric trough and inner magnetosphere facilitate the escape of whistler waves from the plasmasphere.

Statistical storm time examination of MLT‐dependent plasmapause location derived from IMAGE EUV

AbstractThe location of the outer edge of the plasmasphere (the plasmapause) as a function of geomagnetic storm time is identified and investigated statistically in regard to the solar wind driver. Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) extreme ultraviolet (EUV) data are used to create an automated method that locates and extracts the plasmapause. The plasmapause extraction technique searches a set range of possible plasmasphere densities for a maximum gradient. The magnetic local time (MLT)‐dependent plasmapause results are compared to manual extraction results. The plasmapause results from 39 intense storms are examined along a normalized epoch storm timeline to determine the average plasmapause L shell as a function of MLT and storm time. The average extracted plasmapause L shell follows the expected storm time plasmapause behavior. The results show that during the main phase, the plasmapause moves earthward and a plasmaspheric drainage plume forms near dusk and across the dayside during strong convection. During the recovery phase, the plume rejoins the corotationally driven plasma while the average plasmapause location moves farther from the Earth. The results are also examined in terms of the solar wind driver. We find evidence that shows that the different categories of solar wind drivers result in different plasmaspheric configurations. During magnetic cloud‐driven events the plasmaspheric drainage plume appears at the start of the main phase. During sheath‐driven events the plume forms later but typically extends further in MLT.

Identifying the source region of plasmaspheric hiss

AbstractThe presence of the plasmaspheric hiss emission around the Earth has been known for more than 50 years, but its origin has remained unknown in terms of source location and mechanism. The hiss, made of whistler mode waves, exists for most of the time in the plasmasphere and is believed to control the radiation belt surrounding the Earth which makes its understanding very important. This paper presents direct observational evidence that the plasmaspheric hiss originates in the equatorial region of the plasmaspheric drainage plumes. It shows that the emissions propagate along the magnetic field lines and away from the equator in the plumes but toward the equator at lower L shells inside the plasmasphere. The observations also suggest that the hiss waves inside the plasmasphere are absorbed as they cross the equator.

Investigating storm‐enhanced density and polar tongue of ionization development during the 22 October 1999 great storm

AbstractWe investigate large‐scale plasma density increases occurring during the 22 October 1999 great storm and focus on storm‐enhanced density (SED) and polar tongue of ionization (TOI) development. Observations include two‐hourly Global Ionosphere Map series coupled with multi‐instrument in situ, space‐based, and ground‐based data plots, with Super Dual Auroral Radar Network two‐cell convection maps and with model‐generated neutral wind vector maps. Results demonstrate the equatorial electrojet events occurring in the Australian and American sectors, the high‐density plasma features, and their underlying plasma transportation processes. During the main phase, a series of four prompt penetration electric field (PPEF) events occurred with subauroral polarization stream E field development forming a plasmaspheric drainage plume. These E field events caused the repeated development of equatorial ionization anomaly (EIA), SED bulge, and SED plume during the local dusk‐midnight hours in those sectors that covered these local times. Showing a westward movement in accordance with their local time dependence, the EIA‐SED structure developed first in the American sector, later on over the Pacific, and finally in the Australian sector. The SED plume plasma found its way into the polar cap through the dayside cusp region, created in the north some large polar cap enhancements reaching up to seven times of the quiet time levels, and evolved in each hemisphere into a polar TOI. We speculate that the enhanced growth of EIA, transporting high‐density solar‐produced plasma to the SED bulge via strong net eastward E field effects, and the mechanical effects of equatorward neutral winds contributed to the polar TOI's increasingly better development.

Long‐lived plasmaspheric drainage plumes: Where does the plasma come from?

AbstractLong‐lived (weeks) plasmaspheric drainage plumes are explored. The long‐lived plumes occur during long‐lived high‐speed‐stream‐driven storms. Spacecraft in geosynchronous orbit see the plumes as dense plasmaspheric plasma advecting sunward toward the dayside magnetopause. The older plumes have the same densities and local time widths as younger plumes, and like younger plumes they are lumpy in density and they reside in a spatial gap in the electron plasma sheet (in sort of a drainage corridor). Magnetospheric‐convection simulations indicate that drainage from a filled outer plasmasphere can only supply a plume for 1.5–2 days. The question arises for long‐lived plumes (and for any plume older than about 2 days): Where is the plasma coming from? Three candidate sources appear promising: (1) substorm disruption of the nightside plasmasphere which may transport plasmaspheric plasma outward onto open drift orbits, (2) radial transport of plasmaspheric plasma in velocity‐shear‐driven instabilities near the duskside plasmapause, and (3) an anomalously high upflux of cold ionospheric protons from the tongue of ionization in the dayside ionosphere, which may directly supply ionospheric plasma into the plume. In the first two cases the plume is drainage of plasma from the magnetosphere; in the third case it is not. Where the plasma in long‐lived plumes is coming from is a quandary: to fix this dilemma, further work and probably full‐scale simulations are needed.

Electromagnetic ion cyclotron wave modeling during the geospace environment modeling challenge event

AbstractWe investigate the temporal evolution and the spatial distribution of electromagnetic ion cyclotron (EMIC) waves during the 8–11 June 2001 geomagnetic storm, one of the storms selected for study by the Geospace Environment Modeling program. Generations of EMIC waves in the H+, He+, and O+ bands are simulated using the kinetic ring current‐atmosphere interactions model with a self‐consistent magnetic field and a ray tracing code. Simulations show that strong wave gain occurs in the afternoon sector at L > 5 and overlaps with a high‐density plasmaspheric drainage plume. EMIC wave gain maximizes during the main phase and decreases in the recovery phase. We find that EMIC wave gain is stronger in the He+ band than in the other two bands in the inner magnetosphere, except the region of low L (< 3) where the H+ band is dominant due to an enhancement in the ring current anisotropy. Little wave gain is obtained for the O+ band. Comparison with in situ EMIC events and EMIC event proxies at five geosynchronous satellites shows consistence in the temporal and local time evolution of the wave distribution. Our simulations of the EMIC wave distribution also agree with proton aurora at subauroral latitudes observed from the Imager for Magnetopause‐to‐Aurora Global Exploration satellite.

Statistically measuring the amount of pitch angle scattering that energetic electrons undergo as they drift across the plasmaspheric drainage plume at geosynchronous orbit

AbstractUsing five spacecraft in geosynchronous orbit, plasmaspheric drainage plumes are located in the dayside magnetosphere and the measured pitch angle anisotropies of radiation belt electrons are compared duskward and dawnward of the plumes. Two hundred twenty‐six plume crossings are analyzed. It is found that the radiation belt anisotropy is systematically greater dawnward of plumes (before the electrons cross the plumes) than it is duskward of plumes (after the electrons have crossed the plumes). This change in anisotropy is attributed to pitch angle scattering of the radiation belt electrons during their passage through the plumes. A test database in the absence of plumes finds no equivalent change in the radiation belt anisotropy. The amount of pitch angle scattering by the plume is quantified, scattering times are estimated, and effective pitch angle diffusion coefficients within the plume are estimated. The pitch angle diffusion coefficients obtained from the scattering measurements are of the same magnitude as expected values for electromagnetic ion cyclotron (EMIC) waves at high electron energies (1.5 MeV); however, expected EMIC diffusion coefficients do not extend to pitch angles of 90° and would have difficulties explaining the observed isotropization of electrons. The pitch angle diffusion coefficients obtained from the scattering measurements are of the same magnitude as expected values for whistler mode hiss at lower electron energies (150 keV). Outward radial transport of the radiation belt caused by the pitch angle scattering in the plume is discussed.

Estimating the effects of ionospheric plasma on solar wind/magnetosphere coupling via mass loading of dayside reconnection: Ion‐plasma‐sheet oxygen, plasmaspheric drainage plumes, and the plasma cloak

Estimates are calculated for the storm time reduction of solar wind/magnetosphere coupling by the mass density ρm of the magnetospheric plasma. Based on the application of the Cassak‐Shay reconnection‐rate formula at the dayside magnetopause, a numerical factor M is developed to quantify the effect of ρm on the dayside reconnection rate. It is argued that the mass loading of dayside reconnection by ρm also makes reconnection more susceptible to shutoff by magnetosheath velocity shear: a formula is developed to estimate the shortening of the dayside reconnection X‐line by ρm. Surveys of plasmaspheric drainage plumes at geosynchronous orbit during high‐speed‐stream‐driven storms and coronal mass ejection (CME)‐driven storms are presented: in the surveys the CME‐driven storms are separated into sheath‐driven portions and magnetic‐cloud‐driven portions. The storm time mass density of the warm plasma cloak (ionospheric outflows into the electron plasma sheet) is obtained from Alfven‐wave analysis at geosynchronous orbit. A methodology is developed to extrapolate geosynchronous‐orbit plasma measurements to the dayside magnetopause. For each of the three plasmas, estimates of the fractional reduction of the total dayside reconnection rate vary, with typical values of tens of percent; i.e., solar wind/magnetosphere coupling is reduced by tens of percent during storms by oxygen in the ion plasma sheet, by the plasmaspheric drainage plume, and by the plasma cloak. Dependence of the reduction on the F10.7 solar radio flux is anticipated. Via these ionospheric‐origin plasmas, the magnetosphere can exert some control over solar wind/magnetosphere coupling. Pathways to gain a fuller understanding of the physics of the solar wind‐driven magnetosphere‐ionosphere system are discussed.

Energetic particle precipitation and the influence on the sub-ionosphere in the SED plume during a super geomagnetic storm

[1] In this paper, we present precipitation characteristic of energetic particles and the influence on the sub-ionosphere in the storm enhanced density (SED) plume in the event of the geomagnetic storm during March 31, 2001. With observations of the NOAA16 satellite, the peak of precipitating electron flux was located near the TEC peak in region of a SED plume recognized on two-dimensional GPS TEC maps. On the other hand, the peak of precipitating energetic proton flux was observed in the outer boundary of the SED plume. Those precipitations were associated with energetic ions/electrons injected into ring currents (RC) caused by a sequence of substorms. When the Digisonde at Millstone Hill was located in the SED plume, the minimum frequency of ionogram echoes (fmin) was observed to keep a high level. Considering the SED plume as a signature of a plasmaspheric drainage plume, the precipitation of energetic electrons is attributed to the RC-ELF hiss interaction in the plasmaspheric plume. Calculations of a simple sub-ionospheric model demonstrate that those energetic electrons can precipitate into the sub-ionosphere and cause sub-ionospheric ionization enhancements in the SED plume. As a result, the fmin observed by the Digisonde in the SED plume was kept at a high level. Therefore, the paper provides direct evidences that precipitating energetic RC electrons play a significant role in the coupling between magnetosphere and sub-ionosphere in SED plumes.

Relativistic‐electron dropouts and recovery: A superposed epoch study of the magnetosphere and the solar wind

During 124 high‐speed‐stream‐driven storms from two solar cycles, a multispacecraft average of the 1.1–1.5 MeV electron flux measured at geosynchronous orbit is examined to study global dropouts of the flux. Solar wind and magnetospheric measurements are analyzed with a superposed epoch technique, with the superpositions triggered by storm‐convection onset, by onset of the relativistic‐electron dropouts, and by recovery of the dropouts. It is found that the onset of dropout occurs after the passage of the IMF sector reversal prior to the passage of the corotating interaction region (CIR) stream interface. The recovery from dropout commences during the passage of the compressed fast wind. Relativistic‐electron‐dropout onset is temporally associated with the onset of the superdense ion and electron plasma sheet, with the onset of the extra‐hot ion and electron plasma sheet and with the formation of the plasmaspheric drainage plume. Dropout recovery is associated with the termination of the superdense plasma sheet and with a decay of the plasmaspheric drainage plume. When there is appreciable spatial overlap of the superdense ion plasma sheet with the drainage plume, dropouts occur, and when that overlap ends, dropouts recover. This points to pitch‐angle scattering by electromagnetic ion‐cyclotron (EMIC) waves as the primary cause of the relativistic‐electron dropouts, with the waves residing in the lumpy drainage plumes driven by the superdense ion plasma sheet. The drainage plume is caused by enhanced magnetospheric convection associated with southward (GSM) magnetic field after the IMF sector reversal. The superdense plasma sheet has its origin in the compressed slow wind of the CIR.

A statistical look at plasmaspheric drainage plumes

The properties of plasmaspheric drainage plumes are examined using cold‐plasma measurements in geosynchronous orbit. During high‐speed stream‐driven storms, 210 plume crossings are collected and statistically analyzed. Plumes that persist for 4 days are common, which was the duration of our search. Plumes weaken with age, becoming narrower in local time with plasma that becomes less dense. Cold‐plasma flow velocities are sunward in the plumes, with flow speeds decreasing as the storms progress. Plumes transfer typically 2 × 1026 ions/sec (1.2 ton/hr of protons) when they are young, and the rate of transport decreases with plume age. A total of approximately 2 × 1031 ions (34 tons of protons) are transported via plumes in the life of a storm. About half of the outer plasmasphere is drained in the first 20 hours of a storm. Large density fluctuations in the plumes indicate that the drainage plumes are lumpy, and large velocity fluctuations of the plasma flow indicate that the drainage plumes may be turbulent. Because of their persistence, drainage plumes are anticipated to be a regular feature of any ongoing geomagnetic storm.

Formation, dynamics, and impact of plasmaspheric plumes

Workshop on Plasmaspheric Drainage Plumes, Taos, New Mexico, 9–13 October 2006Plasmaspheric plumes result from erosion of the plasmasphere. The Institute of Geophysics and Planetary Physics (IGPP) Workshop on Plasmaspheric Drainage Plumes was convened in Taos, N. M., on 9–13 October 2006 to examine outstanding questions about the formation and dynamics of plumes, and the impact of plumes on the near‐Earth space environment (geospace). A second workshop on plasmaspheric drainage plumes is planned for late 2007.

Plasmaspheric drainage plume observed by the Polar satellite in the prenoon sector and the IMAGE satellite during the magnetic storm of 11 April 2001

During the early main phase of a geomagnetic storm on 11 April 2001, the Polar satellite was inside the magnetosphere in the prenoon sector (∼1000–1100 magnetic local times) and experienced a magnetopause crossing at L ≈ 6 because of the high solar wind dynamic pressure and strong southward interplanetary magnetic field (IMF). Just before the magnetopause crossing, Polar observed cold, dense plasma. That is, the cold, dense plasma was immediately adjacent to the compressed magnetopause. Using simultaneous observations by the IMAGE extreme ultraviolet (EUV) imager, we confirm that the cold, dense plasma observed by Polar is a plasmaspheric drainage plume extending outward from the plasmasphere to the magnetopause during the interval of high geomagnetic activity and strong southward IMF. We compare plasmaspheric mass densities determined from ground magnetometer data at L = 2.3 and 2.9 for a magnetically quiet time interval to mass densities determined during the magnetic storm time interval. We find no significant differences in the mass density between both intervals. These observations suggest that the sunward‐convecting plasmaspheric plasma observed at Polar is due to erosion of the outer layers of the plasmasphere beyond L = 2.9.