Plasmaspheric Plasma Research Articles

Magnetospheric response to solar wind forcing: ultra-low-frequency wave–particle interaction perspective

Abstract. Solar wind forcing, e.g., interplanetary shock and/or solar wind dynamic pressure pulses impacting Earth's magnetosphere, manifests many fundamental important space physics phenomena, including producing electromagnetic waves, plasma heating, and energetic particle acceleration. This paper summarizes our present understanding of the magnetospheric response to solar wind forcing in the aspects of radiation belt electrons, ring current ions and plasmaspheric plasma physics based on in situ spacecraft measurements, ground-based magnetometer data, magnetohydrodynamics (MHD) and kinetic simulations. Magnetosphere response to solar wind forcing is not just a “one-kick” scenario. It is found that after the impact of solar wind forcing on Earth's magnetosphere, plasma heating and energetic particle acceleration started nearly immediately and could last for a few hours. Even a small dynamic pressure change in interplanetary shock or solar wind pressure pulse can play a non-negligible role in magnetospheric physics. The impact leads to generation of a series of waves, including poloidal-mode ultra-low-frequency (ULF) waves. The fast acceleration of energetic electrons in the radiation belt and energetic ions in the ring current region response to the impact usually contains two contributing steps: (1) the initial adiabatic acceleration due to the magnetospheric compression, (2) followed by the wave–particle resonant acceleration dominated by global or localized poloidal ULF waves excited at various L-shells. Generalized theory of drift and drift–bounce resonance with growth- or decay-localized ULF waves has been developed to explain in situ spacecraft observations. The wave-related observational features like distorted energy spectrum, “boomerang” and “fishbone” pitch angle distributions of radiation belt electrons, ring current ions and plasmaspheric plasma can be explained in the framework of this generalized theory. It is worth pointing out here that poloidal ULF waves are much more efficient at accelerating and modulating electrons (fundamental mode) in the radiation belt and charged ions (second harmonic) in the ring current region. The results presented in this paper can be widely used in solar wind interacting with other planets such as Mercury, Jupiter, Saturn, Uranus and Neptune and other astrophysical objects with magnetic fields.

Energy Exchange Between Electromagnetic Ion Cyclotron (EMIC) Waves and Thermal Plasma: From Theory to Observations

The cold plasmaspheric plasma, the ring current and the radiation belts constitute three important populations of the inner magnetosphere. The overlap region between these populations gives rise to wave-particle interactions between different plasma species and wave modes observed in the magnetosphere, in particular, electromagnetic ion cyclotron (EMIC) waves. These waves can resonantly interact with multiple particle species, being an important loss process for both ring current ions and radiation belt electrons, as well as a cold plasma heating mechanism. This mini-review will focus on the interaction between EMIC waves and cold and thermal plasma, specifically the role of EMIC waves in cold and thermal electron and ion heating. It will discuss early theoretical results in conjunction with numerical modelling and recent satellite observations, and address outstanding problems and controversies in this field.

Key elements of auroral substorm development and their relationship to recent observations of detached sub-auroral phenomena including STEVE-like emissions

Since Akasofu introduced the concept of the substorm in 1964, numerous refinements have been added to the phenomenological model of how auroral substorms develop. Here, we review several of the most important of these including; polar cap patches, intensifications of the Polar Activated Band (PAB) at the poleward edge of the bulge, the development of new arcs poleward, the generation of auroral streamers, the evolution of streamers into auroral torches and omega bands, the association of streamers with particle injection and BBF activity in the tail, and wedgelet-like magnetic perturbations on the ground. We also review the concept of “contact breakups” which are breakups that appear to be triggered by the arrival of streamer-like forms in the equatorward regions of the oval. Finally, we remind the community that the addition of detached sub-auroral emissions was made to the phenomenological picture of substorm develop in the late 1960s along with the concept of auroral streamers. Newer observations of detached sub-auroral (STEVE-like emissions) from the Viking/UVI and POLAR/VIS imagers are presented which confirm that they are east-west aligned bands of emission that separate away from the equatorward edge of the auroral oval in response to intensifications of the poleward edge of the bulge and subsequent streamer production. We propose a new model for the formation of detached sub-auroral (STEVE-like) emissions in which a non-linear growth of a SAPS-driven instability (e.g. like the shear-flow/ballooning instability) at the plasmapause results in the disruption of the boundary separating hot plasma sheet particle from the cold plasmaspheric plasma. It is proposed that the resulting intermixing of plasma populations leads to both the observed STEVE-like emissions and also provides a source of cold plasma on open drift paths that can feed the long-lived drainage plumes that have recently been discovered.

Proton Auroras Equatorward of the Oval as a Manifestation of the Ion-Cyclotron Instability in the Earth’s Magnetosphere (Brief Review)

Different types of proton auroras observed by the IMAGE satellite equatorward of the proton aurora oval are briefly reviewed. These auroras are caused by the precipitation of energetic protons from the Earth’s magnetosphere during the development of the ion-cyclotron instability. In addition to the previously considered types of proton auroras (spots, evening arcs, and dayside flashes), a new type is described: longlasting proton auroras on the dayside. The scheme of interrelation between different proton auroras equatorward of the oval with the distribution of cold plasmaspheric plasma is given.

Investigating the Development of Abnormal Subauroral Ion Drift (ASAID) and Abnormal Subauroral Polarization Stream (ASAPS) During the Magnetically Active Times of September 2003

AbstractThis study investigates two recently reported subauroral phenomena: the abnormal subauroral ion drift (ASAID) appearing as an inverted SAID and the shielding‐E—SAID structure depicting a SAID feature on the poleward side of a small eastward or antisunward flow channel that is the shielding electric (E) field's signature. We have analyzed polar cross sections, constructed with multi‐instrument Defense Meteorological Satellite Program data, for the development of these subauroral phenomena. New results show the features of abnormal subauroral polarization stream (ASAPS) and ASAID‐ASAPS comprised by a narrow ASAID embedded in a wider ASAPS. We have identified undershielding, perfect shielding, and overshielding events. Our observational results demonstrate SAPS development during undershielding, the absence of subauroral flow channel during perfect shielding, and ASAID/ASAPS and shielding‐E—SAID/SAPS development during overshielding. The appearance of an ASAID‐ASAPS structure together with a pair of dayside‐nightside eastward auroral flow channels implies the intensification of region 2 field‐aligned currents via the westward traveling surge and thus the strengthening of overshielding conditions. From the observational results presented we conclude for the magnetically active time period studied that (i) the shielding E field drove the wider ASAPS flow channel, (ii) the ASAID‐ASAPS structure's narrow antisunward flow channel developed due to the injections of hot ring current ions in a short‐circuited system wherein the hot ring current plasma was closer to the Earth than the cold plasmaspheric plasma, and (iii) overshielding created this hot‐cold plasma configuration via the development of a plasmaspheric shoulder.

Occurrence of EMIC waves and plasmaspheric plasmas derived from THEMIS observations in the outer magnetosphere: Revisit

AbstractWe have statistically studied the relationship between electromagnetic ion cyclotron (EMIC) waves and cold plasmaspheric plasma (Nsp) in the L range of 6–12 using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data for 2008–2011. The important observational results are as follows: (1) Under quiet geomagnetic conditions (Kp ≤ 1), the maximum occurrence rate of the hydrogen (H) band EMIC waves appears in the early morning sector (0600–0900 MLT) at the outermost region (L= 10–12). (2) Under moderate and disturbed conditions (Kp ≥ 2), the H‐band occurrence rate is higher in the morning‐to‐early‐afternoon sector for L > 10. (3) The high‐occurrence region of helium (He) band waves for Kp ≤ 1 varies from L = 7 to 12 in radial distances along the local time (i.e., at L ∼ 7 near noon and at L= 8–12 near late afternoon). (4) The He‐band waves for Kp ≥ 2 are mainly localized between 1200 and 1800 MLT with a peak around 1500–1600 MLT at L= 8–10. (5) Nsp is much higher for the He‐band intervals than for the H‐band intervals by a factor of 10 or more. The He‐band high occurrence appears at a steep Nsp gradient region. (6) The morning‐afternoon asymmetry of the normalized frequency seen both in H‐band and He‐band is similar to the asymmetric distribution of Nsp along the local time. These observations indicate that the cold plasma density plays a significant role in determining the spectral properties of EMIC waves. We discuss whether a morning‐afternoon asymmetry of the EMIC wave properties can be explained by the spatial distribution of cold plasmaspheric plasma.

Mass loading at the magnetopause through the plasmaspheric plume

AbstractAn investigation of the transport of the plasmaspheric plasma to the dayside magnetopause under disturbed geomagnetic conditions associated with plasmaspheric plumes is conducted by numerical simulations using the Dynamic Fluid‐Kinetic (DyFK) model. The simulations present the field‐aligned density distributions and kinetic features of multiple ion species (H+/He+/O+) in a plasmaspheric flux tube that corotates while convecting toward the dayside magnetopause. The simulations show that the plasmaspheric plume can provide an equatorial magnetopause plasma density which is up to 3 orders of magnitude higher than that under the typical conditions. With the effects of wave‐particle interaction included, such mass loading through the plasmaspheric plume is even stronger. The local reconnection rate at the location where the plasmaspheric plume mass loads at the dayside magnetopause may decrease by 90%, depending on the geomagnetic activities, the outer plasmaspheric density, and the level and spatial span of the wave‐particle interaction.

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.

Statistical analysis of the plasmaspheric plume at the magnetopause

During times of enhanced magnetospheric convection, cold dense plasma from the plasmasphere can form a plume extending sunward toward the dayside magnetopause. A statistical study is presented of cold high‐density plasmaspheric plasma at the magnetopause. Observations from the Time History of Events and Macroscale Interactions (THEMIS) spacecraft show the plume to be present at the dayside magnetopause during 12.5% (148 of 1184) of the crossings. Its most common location in magnetic local time when contacting the magnetopause is at 13.6 h. The magnetopause crossings show evidence for reconnection in 68% of events with the plume while only 47% of events without plume. Although the plume is more likely to be observed at the magnetopause when reconnection is occurring, the typical reconnection jet velocity is lower for plume events than nonplume events, indicating the presence of the plume may be slowing the efficiency of the reconnection process.

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.

Fast acceleration of inner magnetospheric hydrogen and oxygen ions by shock induced ULF waves

The interaction between interplanetary shocks and the Earth's magnetosphere manifests in many important space physics phenomena including particle acceleration. We investigated the response of the inner magnetospheric hydrogen and oxygen ions to a strong interplanetary shock impinging on the Earth's magnetosphere. Both hydrogen and oxygen ions are found to be heated/accelerated significantly with their temperature enhanced by a factor of two and three immediately after ∼1 min and ∼12 min of the shock arrival respectively. Multiple energy dispersion signatures of ions were found in the parallel and anti‐parallel direction to the magnetic field immediately after the interplanetary shock impact. The energy dispersions in the anti‐parallel direction preceded those in the parallel direction. Multiple dispersion signatures can be explained by the flux modulations of local ions (rather than the ions from the Earth's ionosphere) by ULF waves. It is found that the energy spectrum from 10 eV to ∼40 keV are highly correlated with the cross product of observed ULF wave electric and magnetic field (V = (E × B)/B2), which indicate that both cold plasmaspheric plasma and hot thermal ions (10 eV to ∼40 keV) are accelerated and decelerated with the various phases of ULF wave electric field. We then demonstrate that ion acceleration due to the interplanetary shock compression on the Earth's magnetic field is rather limited, whereas the major contribution to acceleration comes from the electric field carried by ULF waves via drift‐bounce resonance for both the hydrogen and oxygen ions. The integrated hydrogen and oxygen ion flux with the poloidal mode ULF waves are highly coherent (>0.9) whereas the coherence with the toroidal mode ULF waves is negligible, implying that the poloidal mode ULF waves are much more efficient in accelerating hydrogen and oxygen ions in the inner magnetosphere than the toroidal mode ULF waves. The duration of high coherence for oxygen ions with the poloidal mode ULF wave is longer than that for hydrogen ions, indicating that oxygen ions can be heated/accelerated more efficiently by the poloidal mode ULF wave induced by the interplanetary shock.

Symmetry and asymmetry of ionospheric weather at magnetic conjugate points for two midlatitude observatories

Variations of the ionospheric weather W-index for two midlatitude observatories, namely, Grahamstown and Hermanus, and their conjugate counterpart locations in Africa are studied for a period from October 2010 to December 2011. The observatories are located in the longitude sector, which has consistent magnetic equator and geographic equator so that geomagnetic latitudes of the line of force are very close to the corresponding geographic latitudes providing opportunity to ignore the impact of the difference of the gravitational field and the geomagnetic field at the conjugate points on the ionosphere structure and dynamics. The ionosondes of Grahamstown and Hermanus provide data of the critical frequency (foF2), and Global Ionospheric Maps (GIM) provide the total electron content (TECgps) along the magnetic field line up to the conjugate point in the opposite hemisphere. The global model of the ionosphere, International Reference Ionosphere, extended to the plasmasphere altitude of 20,200km (IRI-Plas) is used to deliver the F2 layer peak parameters from TECgps at the magnetic conjugate area. The evidence is obtained that the electron gas heated by day and cooled by night at the summer hemisphere as compared with the opposite features in the conjugate winter hemisphere testifies on a reversal of plasma fluxes along the magnetic field line by the solar terminator. The ionospheric weather W-index is derived from NmF2 (related with foF2) and TECgps data. It is found that symmetry of W-index behavior in the magnetic conjugate hemispheres is dominant for the equinoxes when plasma movement along the magnetic line of force is imposed on symmetrical background electron density and electron content. Asymmetry of the ionospheric storm effects is observed for solstices when the plasma diffuse down more slowly into the colder winter hemisphere than into the warmer summer hemisphere inducing either plasma increase (positive phase) or decrease (negative phase of W-index) in the ionospheric and plasmaspheric plasma density.

Characteristics of field line resonance type geomagnetic pulsations and variations of plasmaspheric plasma density distribution

The period of field line resonance (FLR) type geomagnetic pulsations depends on the length of the field line and on the plasma density in the inner magnetosphere (plasmasphere), where field lines are closed. Here as FLR period, the period belonging to the maximum occurrence frequency of the occurrence frequency spectrum (equivalent resonance curve) of pulsations has been considered. The resonance system may be replaced by an equivalent resonant circuit. The plasma density would correspond to the ohmic load. The plasma in the plasmasphere originates from the ionosphere, thus FLR period, occurrence frequency are also affected by the maximum electron density in the ionosphere. The FLR period has shown an enhancement with increasing F region electron density, while the occurrence frequency indicated diminishing trend (possible damping effect). Thus, the increased plasma density may be the cause of the decreased occurrence of FLR type pulsations in the winter months of solar activity maximum years (winter anomaly).

Plasmaspheric Density Structures and Dynamics: Properties Observed by the CLUSTER and IMAGE Missions

Plasmaspheric density structures have been studied since the discovery of the plasmasphere in the late 1950s. But the advent of the Cluster and Image missions in 2000 has added substantially to our knowledge of density structures, thanks to the new capabilities of those missions: global imaging with Image and four-point in situ measurements with Cluster. The study of plasma sources and losses has given new results on refilling rates and erosion processes. Two-dimensional density images of the plasmasphere have been obtained. The spatial gradient of plasmaspheric density has been computed. The ratios between H+, He+ and O+ have been deduced from different ion measurements. Plasmaspheric plumes have been studied in detail with new tools, which provide information on their morphology, dynamics and occurrence. Density structures at smaller scales have been revealed with those missions, structures that could not be clearly distinguished before the global images from Image and the four-point measurements by Cluster became available. New terms have been given to these structures, like “shoulders”, “channels”, “fingers” and “crenulations”. This paper reviews the most relevant new results about the plasmaspheric plasma obtained since the start of the Cluster and Image missions.

SAPS measurements around the magnetic equator by CRRES

Enhancements of convection electric fields during two substorms have been analyzed using CRRES satellite data measured in the premidnight inner magnetosphere. The electric field, related to subauroral polarization streams (SAPS), begins to increase within 30 sec after the substorm onset, indicating a quicker response of convection in the inner magnetosphere to substorms than has been reported (∼10 min) before. A prompt response of the ion pressure and the following decrease in the cold plasma density supports the fact that the electric field enhances just after the substorm onset and drives accelerations of energetic ions and plasmapause erosion. The SAPS electric field enhances between the earthward edges of the ring current and plasmasheet, and the plasmapause coincides with the earthward edge of the electron plasmasheet. The plasmapause location deviates from the stagnation point, and the SAPS electric field penetrates into the plasmasphere, driving a sunward plasma drift of the plasmaspheric plasma.

Electrostatic electron cyclotron harmonic waves observed by the Akebono satellite near the equatorial region of the plasmasphere

Abstract Analysis of the plasma wave observation data provided by the plasma waves and sounder experiment (PWS) on board the Akebono satellite frequently reveals the presence of electrostatic electron cyclotron harmonic (ESCH) waves in the low-latitude region (MLAT < 45°) of the plasmasphere within an altitude range from about 3000 km to the apogee of the satellite (initial apogee was 10,500 km). Even at moderate or low geomagnetic activity, intense ESCH waves often appear near the equatorial region of the plasmasphere above the upper hybrid resonance (UHR) frequency at the lowest harmonic number branch of the f Qn ESCH waves. We identified these plasma waves as the equatorial plasmasphere f Qn waves (EP-f Qn). The spectra of the EP-f Qn waves are characterized by a narrow band structure and by a strong nature, with a wave intensity that ranges from 3.46 × 10-8 to 3.31 × 10-4 V/m. The maximum intensity is nearly coincident with the upper limit of the PWS receiver in the low-gain mode. Statistical analysis results reveal that the EP-f Qn waves are observable in all the local time sectors; however, the occurrence probability shows a clear enhancement in the early morning sector of 01–03 MLT in the plasmasphere. The EP-f Qn wave activities are suppressed within a period of strong magnetic disturbances as well as solar minimum phase. The linear dispersion relation analysis using a two-component plasma model reveals that supra-thermal plasma with the energy of about 750 eV and with a large temperature anisotropy (A = T-perp/T-parallel–1 > 40) must be present in order to realize an appearance of a positive growth rate at the observed frequency and propagation angle of the ESCH waves. Since the hot plasma with such a high anisotropy has not been detected, the validity of the present two-component plasma model remains an open question. The occurrence feature of the ESCH waves showed that there is a constant activation or a constant flow-in of free energy to generate the strong plasma instability of ESCH waves near the post-midnight sector of the plasmasphere. The existence of ESCH waves revealed that the nature of the plasmaspheric plasma is more turbulent and active than has been believed.

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.

Chung Park, pioneer of magnetosphere–ionosphere coupling research

Chung Park (1938–2003) was a true pioneer of magnetosphere–ionosphere coupling research. During a short career at Stanford University that began in 1970 and ended in 1981, he wrote seminal papers on several topics. Using ground-based whistler data, he was the first to demonstrate experimentally that day-side upward ion flow from the mid-latitude ionosphere was sufficient to maintain the night-time ionosphere. He made the only measurements to date of longitudinally localized drainage of significant quantities of plasmaspheric plasma into the underlying ionosphere during a period of enhanced convection activity. He pioneered in demonstrating the presence at ionospheric heights of geophysically important electric fields that originate in the troposphere in thunderstorm centers. He cooperated in a unique study of the guidance of whistler-mode waves by field-aligned density irregularities (ducts) in the magnetosphere. Park provided unique observational data on nonlinear wave–particle interaction processes such as: (i) the development of sidebands during the injection of whistler-mode waves from Siple, Antarctica, and (ii) the mysterious whistler precursor phenomenon. Today, in spite of the several decades that have elapsed since his work, Park's early findings remain cornerstones of our understanding of magnetosphere–ionosphere coupling processes. Some of his later studies of non-linear magnetospheric wave–particle interaction phenomena have stirred lively debate, and today remain relevant to a number of topics in space plasma wave research.

Two-dimensional drift velocities from the IMAGE EUV plasmaspheric imager

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) Mission extreme ultraviolet (EUV) imager observes He + plasmaspheric ions throughout the inner magnetosphere. Limited by ionizing radiation and viewing close to the sun, images of the He + distribution are available every 10 min for many hours as the spacecraft passes through apogee in its highly elliptical orbit. As a consistent constituent at about 15%, He + is an excellent surrogate for monitoring all of the processes that control the dynamics of plasmaspheric plasma. In particular, the motion of He + transverse to the ambient magnetic field is a direct indication of convective electric fields. The analysis of boundary motions has already achieved new insights into the electrodynamic coupling processes taking place between energetic magnetospheric plasmas and the ionosphere. Yet to be fulfilled, however, is the original promise that global EUV images of the plasmasphere might yield two-dimensional pictures of mesoscale to macroscale electric fields in the inner magnetosphere. This work details the technique and initial application of an IMAGE EUV analysis that appears capable of following thermal plasma motion on a global basis.

Electron density in the magnetosphere

Observations of the electron density ne based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument (PWI) are available for March 1996 to September 1997, during which time the Polar orbit sampled all MLT values three times. In a previous study, we modeled the electron density dependence along field lines as ne = ne0(Rmax/R)α, where ne0 is the equatorial electron density, Rmax ≈ LRE is the maximum geocentric radius R to any point on the field line, and α = αmodel = 8.0 − 3.0 log10ne0 + 0.28(log10ne0)2 − 0.43(Rmax/RE), for all categories of plasma (plasmasphere and plasmatrough). (In the formula for αmodel, ne0 is expressed in cm−3.) Here, we illustrate the field line dependence using several example events. We show that the plasmapause is much more evident on the large radius portion of the orbit and that at R ∼ 2 RE the electron density tends to level out at large Rmax to a constant value ∼100 cm−3. We also present an example of plasmaspheric plasma extending out to at least L ∼ 9 on the dawnside during particularly calm geomagnetic conditions (as indicated by low Kp). Then we present the average equatorial profiles of ne0 versus Rmax for plasmasphere and plasmatrough. Our average plasmasphere profile is found to have values intermediate between those based on the models of Carpenter and Anderson and Sheeley et al. The plasmatrough equatorial density ne0 scales with respect to Rmax like Rmax−3.4, but in the region for which our plasmatrough data is most reliable (L ≤ 6), it is well fit by the Rmax−4.0 scaling of Sheeley et al. or the Rmax−4.5 scaling of Carpenter and Anderson. We present a simple interpretation for the field line dependence of the density. For large ne0, such as occurs in the plasmasphere, α is close to zero on average (implying that ne is roughly constant along field lines). When ne0 decreases, so does ne at R = 2 RE, but the value there does not decrease much below 100 cm−3. (It is unclear if this value is an absolute lower density limit because most often the upper hybrid resonance emission disappears at R ∼ 2 RE because fp/fce < 1, where fp is the plasma frequency and fce is the electron cyclotron frequency.) Finally, we examined the dependence of α and the density at the equator and at R ∼ 2 RE on the average 〈Kp〉 (Kp averaged with a 3‐day timescale). There is no clear dependence of the average α − αmodel on 〈Kp〉 or on MLT. In the plasmasphere, ne0 decreases with respect to increasing 〈Kp〉.