Radio Plasma Imager Research Articles

Optimization of Radial Diffusion Coefficients for the Proton Radiation Belt During the CRRES Era

AbstractProton flux measurements from the Proton Telescope instrument aboard the CRRES satellite are revisited, and used to drive a radial diffusion model of the inner proton belt at 1.1 ≤ L ≤ 1.65. Our model utilizes a physics‐based evaluation of the cosmic ray albedo neutron decay (CRAND) source, and coulomb collisional loss is driven by a drift averaged density model combining results from the International Reference Ionosphere, NRLMSIS‐00 atmosphere and Radio Plasma Imager plasmasphere models, parameterized by solar activity and season. We drive our model using time‐averaged data at L = 1.65 to calculate steady state profiles of equatorial phase space density, and optimize our choice of radial diffusion coefficients based on four defining parameters to minimize the difference between model and data. This is first performed for a quiet period when the belt can be assumed to represent steady state. Additionally, we investigate fitting steady state solutions to time averages taken during active periods where the data exhibits limited deviation from steady state, demonstrated by CRRES measurements following the March 24, 1991 storm. We also discuss a way to make the optimization process more reliable by excluding periods of variability in plasmaspheric density from any time average. Lastly, we compare our resultant diffusion coefficients to those derived via a similar process in previous work, and diffusion coefficients derived for electrons from ground and in situ observations. We find that higher diffusion coefficients are derived compared with previous work, and suggest more work is required to derive proton diffusion coefficients for different geomagnetic activity levels.

Evolution of mass density and O+ concentration at geostationary orbit during storm and quiet events

AbstractWe investigated mass density ρm and O+ concentration ηO+≡nO+/ne (where nO+ and ne are the O+ and electron density, respectively) during two events, one active and one more quiet. We found ρm from observations of Alfvén wave frequencies measured by the GOES, and we investigated composition by combining measurements of ρm with measurements of ion density nMPA,i from the Magnetospheric Plasma Analyzer (MPA) instrument on Los Alamos National Laboratory spacecraft or ne from the Radio Plasma Imager instrument on the Imager for Magnetopause‐to‐Aurora Global Exploration spacecraft. Using a simple assumption for the He+ density at solar maximum based on a statistical study, we found ηO+ values ranging from near zero to close to unity. For geostationary spacecraft that corotate with the Earth, sudden changes in density for both ρm and ne often appear between dusk and midnight magnetic local time, especially when Kp is significantly above zero. This probably indicates that the bulk (total) ions have energy below a few keV and that the satellites are crossing from closed or previously closed to open drift paths. During long periods that are geomagnetically quiet, the mass density varies little, but ne gradually refills leading to a gradual change in composition from low‐density plasma that is relatively cold and heavy (high‐average ion mass M ≡ ρm/ne) to high‐density plasma that is relatively cold and light (low M) plasmasphere‐like plasma. During active periods we observe a similar daily oscillation in plasma properties from the dayside to the nightside, with cold and light high‐density plasma (more plasmasphere‐like) on the dayside and hotter and more heavy low‐density plasma (more plasma sheet‐like) on the nightside. The value of ne is very dependent on whether it is measured inside or outside a plasmaspheric plume, while ρm is not. All of our results were found at solar maximum; previous results suggest that there will be much less O+ at solar minimum under all conditions.

Evaluating the diffusive equilibrium models: Comparison with the IMAGE RPI field‐aligned electron density measurements

AbstractThe diffusive equilibrium models that are widely used by the space physics community to describe the plasma densities in the plasmasphere are evaluated with field‐aligned electron density measurements from the radio plasma imager (RPI) instrument onboard the IMAGE satellite. The original mathematical form of the diffusive equilibrium model was based on the hydrostatic equilibrium along the magnetic field line with the centrifugal force and the field‐aligned electrostatic force as well as a large number of simplifying approximations. Six free parameters in the mathematical form have been conventionally determined from observations. We evaluate four sets of the parameters that have been reported in the literature. The evaluation is made according to the equatorial radial distance dependence, latitudinal dependence at a given radial distance, and the combined radial and latitudinal dependences. We find that the mathematical form given in the diffusive equilibrium model is intrinsically incompatible with the measurements unless another large number of free parameters are artificially introduced, which essentially changes the nature of a theoretical model to an empirical model.

Magnetospheric electron‐velocity‐distribution function information from wave observations

The electron‐velocity‐distribution function was determined to be highly non‐Maxwellian and more appropriate to a kappa distribution, with κ ≈ 2.0, near magnetic midnight in the low‐latitude magnetosphere just outside a stable plasmasphere during extremely quiet geomagnetic conditions. The kappa results were based on sounder‐stimulated Qn plasma resonances using the Radio Plasma Imager (RPI) on the IMAGE satellite; the state of the plasmasphere was determined from IMAGE/EUV observations. The Qn resonances correspond to the maximum frequencies of Bernstein‐mode waves that are observed between the harmonics of the electron cyclotron frequency in the frequency domain above the upper‐hybrid frequency. Here we present the results of a parametric investigation that included suprathermal electrons in the electron‐velocity‐distribution function used in the plasma‐wave dispersion equation to calculate the Qn frequencies for a range of kappa and fpe/fce values for Qn resonances from Q1 to Q9. The Qn frequencies were also calculated using a Maxwellian distribution, and they were found to be greater than those calculated using a kappa distribution with the frequency differences increasing with increasing n for a fixed κ and with decreasing κ for a fixed n. The calculated fQn values have been incorporated into the RPI BinBrowser software providing a powerful tool for rapidly obtaining information on the nature of the magnetospheric electron‐velocity‐distribution function and the electron number density Ne. This capability enabled accurate (within a few percent) in situ Ne determinations to be made along the outbound orbital track as IMAGE moved away from the plasmapause. The extremely quiet geomagnetic conditions allowed IMAGE/EUV‐extracted counts to be compared with the RPI‐determined orbital‐track Ne profile. The comparisons revealed remarkably similar Ne structures.

Field‐aligned distribution of the plasmaspheric electron density: An empirical model derived from the IMAGE RPI measurements

We present a newly developed empirical model of the plasma density in the plasmasphere. It is based on more than 700 density profiles along field lines derived from active sounding measurements made by the radio plasma imager on IMAGE between June 2000 and July 2005. The measurements cover all magnetic local times and vary from L = 1.6 to L = 4 spatially, with every case manually confirmed to be within the plasmasphere by studying the corresponding dynamic spectrogram. The resulting model depends not only on L‐shell but also on magnetic latitude and can be applied to specify the electron densities in the plasmasphere between 2000 km altitude and the plasmapause (the plasmapause location itself is not included in this model). It consists of two parts: the equatorial density, which falls off exponentially as a function of L‐shell; and the field‐aligned dependence on magnetic latitude and L‐shell (in the form of invariant magnetic latitude). The fluctuations of density appear to be greater than what could be explained by a possible dependence on magnetic local time or season, and the dependence on geomagnetic activity is weak and cannot be discerned. The solar cycle effect is not included because the database covers only a fraction of a solar cycle. The performance of the model is evaluated by comparison to four previously developed plasmaspheric models and is further tested against the in situ passive IMAGE RPI measurements of the upper hybrid resonance frequency. While the equatorial densities of different models are mostly within the statistical uncertainties (especially at distances greater than L = 3), the clear latitudinal dependence of the RPI model presents an improvement over previous models. The model shows that the field‐aligned density distribution can be treated neither as constant nor as a simple diffusive equilibrium distribution profile. This electron density model combined with an assumed model of the ion composition can be used to estimate the time for an Alfven wave to propagate from one hemisphere to the other, to determine the plasma frequencies along a field line, and to calculate the raypaths for high frequency waves propagating in the plasmasphere.

Magnetospheric radio tomographic imaging with IMAGE and Wind

[1] Recent theoretical studies have shown the feasibility and potential scientific value of radio tomographic imaging of Earth's magnetosphere by measuring Faraday rotation and phase difference (or group delay) of coherent radio wave signals. On 15 August 2000, a 6 W linearly polarized 828 kHz signal transmitted by the Radio Plasma Imager (RPI) on the IMAGE spacecraft was clearly detected by WAVES X and Z antennas on Wind spacecraft. Following our previous analysis of the path-integrated product change of the magnetic field and plasma density based on the spin rate measurement, we report here Faraday rotation measured from absolute antenna orientation using the phase difference between the spin-phase modeled RPI signal and the WAVES X- and Z-antenna received RPI signals. The new approach gives Faraday rotation without the mod (π) ambiguity. The average electron density extracted along a typical signal propagation path over a 1 hour measurement window agrees well with empirical models of the northern polar region derived from years of measurements. Finally, we demonstrate preliminary 2-D radio tomographic imaging of magnetospheric plasma density using the Faraday rotation measurement.

Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 1. Observations and interpretation

[1] A survey of echoes detected in 2004–2005 during pulse transmissions from the Radio Plasma Imager (RPI) instrument on the IMAGE satellite has revealed several new features of sounder generated whistler mode (WM) echoes and has indicated ways in which the echoes may be used for remote sensing of the Earth's plasma structure at altitudes <5000 km. In this paper we describe the frequency versus travel time (f − t) forms of the WM echoes as they appear on RPI plasmagrams and discuss qualitatively their raypaths and diagnostic potentials. Based on their reflection mechanism, the WM echoes can be classified as: magnetospherically reflected (MR), specularly reflected (SR), or backscattered (BS). The MR echoes are reflected at altitudes where the local lower hybrid frequency (flh) is equal to the transmitted pulse frequency f, a phenomenon familiar from both theory and passive recordings of WM wave activity. The SR echoes (previously reported in a higher frequency range) are reflected at the Earth-ionosphere boundary, either with wave vector at normal incidence or, more commonly (and unexpectedly, due to ray bending in the layered ionosphere), at oblique incidence. The BS echoes are the result of scattering from small scale size plasma density irregularities close to IMAGE. The echoes are described as discrete, multipath, and diffuse, depending upon the amount of travel-time spreading caused by the presence of field aligned density irregularities (FAIs) along echo raypaths. The WM echoes described in this paper have been observed at altitudes less than 5,000 km and at all latitudes and at most MLTs. The diagnostic potential of these phenomena for remotely studying the distribution of plasma density and composition along the geomagnetic field line B0, as well as the presence of FAIs of varying scale sizes, is enhanced by the tendency for SR and MR echoes to be observed simultaneously along with the upward propagating signals from a spatial distribution of communication VLF transmitters. We believe that our findings about WM propagation and echoing in an irregular medium have important implications for the connection between WM waves and the Earth's radiation belts. In a companion paper by Sonwalkar et al. (2011), we employ ray tracing and refractive index diagrams in quantitative support of this paper and also present two diagnostic case studies of plasma density, ion effective mass, and ion composition along B0.

Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 2. Sounding of electron density, ion effective mass (meff), ion composition (H+, He+, O+), and density irregularities

[1] A companion paper by Sonwalkar et al. (2011) provided new details of whistler mode radio sounding of the altitude range below ∼5000 km by the Radio Plasma Imager (RPI) instrument on the IMAGE satellite. That paper presented frequency-vs- group time delay records of echoes whose raypaths either 1) reversed direction through refraction at altitudes above the ionosphere where the wave frequency was approximately equal to the local lower hybrid resonance frequency flh (magnetospherically reflected or MR echoes), or 2) returned to IMAGE from reflection points along the sharp lower boundary of the ionosphere at ∼90 km (obliquely incident (OI) or normally incident (NI) specularly reflected (SR) echoes). The MR and OI echo paths were shown to form narrow loops, while the NI echo followed the same raypath down and back. Furthermore, the echoes were found to be discrete or broadened in time delay either by multipath propagation or by scattering from field aligned irregularities (FAIs). We begin with a direct interpretive approach, employing a combination of refractive index diagrams, ray tracings, and a plasma density model to predict the detailed frequency-vs-time properties of echoes detected when the sounder is operated over a wide range of whistler mode frequencies (typically 6 kHz to 63 kHz) and the satellite is either above or below the altitude of the maximum flh along the geomagnetic field line B0 in the upper ionosphere. We then consider the inverse problem, estimation of the parameters of the prevailing plasma density model from the observed echo properties. Thanks to variations in the sensitivity of the various echo forms to the altitude profiles of electron density and ion effective mass meff, we use the observed frequency-vs- group time delay (tg − f) details of simultaneously received MR and SR echoes to infer the properties of a diffusive equilibrium model of the plasma, including estimates of the ion composition in the important transition region from the O+-dominated ionosphere to the light ion regime above. Our results on electron density and ion composition measurements are in general agreement with those obtained from in situ measurements on the IMAGE and DMSP-F15 satellites, with bottomside sounding results from nearby Ionosondes, and with values obtained from the IRI-2007 model. We also demonstrate a method of estimating the scale sizes and locations of FAIs located along or near WM echo paths.

Impedance characteristics of an active antenna at whistler mode frequencies

We use the radio plasma imager (RPI) on the IMAGE satellite to investigate the impedance characteristics of an active electric antenna in space plasma at whistler mode frequencies. A dedicated experiment was carried out on 21–22 September 2005 for two orbits in the plasmasphere. The input impedance characteristics of the dipole antenna submerged in plasma is determined at whistler frequencies. The results are consistent with a physical model in which the antenna is negatively charged to a large voltage and the plasma around each antenna element forms an ion sheath that varies with time in its radius. Within the plasmasphere, these sheaths are a part of the antenna‐plasma system and represent a capacitive component of the tuned antenna circuit. It is shown that, inside the plasmasphere, the RPI antenna capacitance varied from 430 to 480 pF. The plasma sheath formed around the antenna in the plasmasphere increases its capacitance by 20%–30% with respect to the near–free space capacitance (364 pF). Comparison of these values to model calculations shows good agreement with a difference smaller than 5%. Measurements of the antenna input resistance showed that, inside the plasmasphere, its value was between 200 and 500 Ω, varying considerably with changes in the ambient electron density and cyclotron frequency. The measured antenna input resistance is compared to model calculations.

IMAGE and DMSP observations of a density trough inside the plasmasphere

We report coordinated observations of a density trough within the plasmasphere using the measurements from the radio plasma imager (RPI) and extreme ultraviolet imager (EUV) on the IMAGE satellite and the measurements from DMSP F‐15. The density trough inside the plasmasphere with a width of ∼0.7 RE in terms of L shell (from L ∼ 2.3 to L ∼ 3.0) was observed in situ by RPI when IMAGE traversed the plasmasphere in ∼2130 magnetic local time (MLT) sector. The plasmasphere images taken by the IMAGE EUV instrument confirm that the density trough is inside the plasmasphere. A 2‐D electron density image constructed from the RPI active sounding measurements reveals that the density trough extends along the magnetic field from the IMAGE orbit to at least 41° magnetic latitude. Meanwhile, the DMSP‐F15 satellite, circling the Earth at about 850 km altitude, detected a light ion density trough at the same time on the same L shells and similar MLT sector. The coordinated observations with the IMAGE and DMSP‐F15 satellite demonstrate, for the first time, that the density trough is a low‐density plasmaspheric structure extending from the plasmasphere to the topside ionosphere along the geomagnetic field lines.

The nightside‐to‐dayside evolution of the inner magnetosphere: Imager for Magnetopause‐to‐Aurora Global Exploration Radio Plasma Imager observations

It took about 12 h for the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite to fly from the outbound pass to inbound pass of every inner magnetosphere crossing. This provides a unique opportunity to study the nightside‐to‐dayside evolution of the corotating inner magnetosphere when the outbound pass is in the nighttime while the inbound pass is in the daytime because a flux tube observed on the nightside may be observed again on the dayside in such situation. The differences between the two observations may be caused by the evolution of the flux tube. By analyzing both the passive and active sounding measurements from the IMAGE Radio Plasma Imager, we study two cases concerning this evolution. One, under a quiet geomagnetic condition, shows a typical evolution process during which the plasmapause was observed on both the nightside and the dayside. The other, during the recovery phase of a magnetic storm, shows a different inner magnetospheric structure in which the distinct plasmapause observed on the nightside becomes unidentifiable on the dayside as the density in the nearly empty nightside plasmatrough increases to a level similar to that of the plasmasphere. It is shown that the evolutions of the inner magnetosphere in both cases were primarily controlled by the fast plasma refilling of the flux tubes from the ionosphere as the flux tubes drift from the nightside to the dayside. In the former case the fast refilling was confined inside L = ∼6.3, while in the latter case the fast refilling extended to at least L = 10. The present observations provide an example for fast refilling as a possible cause of the smooth density transition from the plasmasphere to the subauroral region and demonstrate the importance of the plasmasphere‐ionosphere coupling in controlling the structures of the inner magnetosphere.

Forced Precessions of the IMAGE Spacecraft

IMAGE (Imager for Magnetopause-to-Aurora Global Exploration), NASA’s first Medium-class Explorer (MIDEX) mission, was led by Southwest Research Institute, successfully captured global neutral atom images of the Earth’s inner magnetosphere and far and extreme ultraviolet images of the aurora and plasmasphere, respectively, and performed radio sounding of various magnetospheric boundaries and regions, during the period May 2000 to December 2005. During the in-orbit checkout and deployment phase of IMAGE, two significant spacecraft attitude modes meriting further study became apparent. The first was an unanticipated forced precession of the core body driven by the spin-plane wire antennas of the Radio Plasma Imager (RPI), with an initial period on the order of 20 minutes. The second was a slower precession of the entire system, driven by gravity gradient acting on the extended wire spin plane and having a period of approximately 105 days. Star Trackers onboard the spacecraft enabled the periods of both motions to be very well constrained by attitude observations. Both motions are analyzed and calculated rates are compared to rates observed in star-tracker data for the actual system.

Comparison of kilometric continuum latitudinal radiation patterns with linear mode conversion theory

Using 80 observations of kilometric continuum (KC) radiation patterns detected by the Radio Plasma Imager on the IMAGE spacecraft, the latitudinal variation of these radiation patterns is compared with the predictions of linear mode conversion theory (LMCT). Six of these comparisons are presented in this paper. Because the location and shape of the plasmapause at the source cannot be simultaneously determined with all of the KC observations, we estimated the minimum frequency‐dependent beaming angle possible based on a statistical upper limit plasma density versus L‐shell profile. This density versus L‐shell profile is equated to the 90th percentile plasma density versus L‐shell curve derived from over 40,000 in situ measurements of the upper hybrid resonance frequency made over the first 3 years of the IMAGE mission. For all cases, starting at frequencies approximately &gt;150 kHz, the model beam center was found to lie just within or outside the radiation pattern. Unrealistic peak densities are required at the equatorial source L‐shell locations which are necessary to force the beam centers to lie deep within the observed radiation patterns. We present this as strong evidence that LMCT cannot be used to explain KC. In order to explain the latitudinal location of the high‐frequency end of these KC radiation patterns a theory is required in which these frequencies are beamed at equatorial angles that are much smaller than the predictions of LMCT. Hashimoto et al. (2005), analyzing one KC radiation pattern detected by CRRES, reached the same conclusion. We present one case in which the source location was inferred by equating the harmonic spacing in the KC radiation pattern with the model electron cyclotron frequency at the source. The LMCT model beams, for this source location, were found to lie outside of the radiation pattern at all frequencies, again consistent with Hashimoto et al. (2005). In addition a survey of KC observations by IMAGE in MLT is presented in which the probability of occurrence peaks in both the dawn and evening sectors.

Polar cap electron density distribution from IMAGE radio plasma imager measurements: Empirical model with the effects of solar illumination and geomagnetic activity

We present a statistical study of the relative importance of solar illumination and geomagnetic activity dependences of the electron density (Ne) distribution in the polar cap magnetosphere based on 5 years of electron density measurements made by the radio plasma imager (RPI) on board the IMAGE spacecraft. This study covers a geocentric distance of R = 1.4–5.0 RE, and the polar cap is defined by an empirical boundary model that takes into account the dynamic nature of the location and size of the polar cap. The RPI Ne data show that the electron density distribution within the polar cap depends on the geocentric distance, R, geomagnetic activity level, e.g., measured by the Kp index, and solar illumination (solar zenith angle) at the footprints of the geomagnetic field lines. Our analysis of RPI Ne data shows that although an increase in geomagnetic activity leads to an enhanced Ne, the enhancement is found to be altitude‐dependent such that it is most pronounced at higher altitudes and less significant at lower altitudes. At geocentric distance of R = 4.5 RE, an increase in the geomagnetic activity level from Kp &lt; 2 to ∼5 results in an Ne increase by a factor of ∼5. On the other hand, the observations show a strong solar illumination control of Ne at lower altitudes and not at higher. RPI Ne data show that at geocentric distance of about 2 RE in the polar cap, the average Ne is larger on the sunlit side than on the darkside by a factor of 3–4 for both quiet and disturbed conditions. At geocentric distance of R ≈ 2.5 RE the effects of these two factors on Ne appear to be comparable. Similar to previous polar cap density models, the new empirical model of Ne developed in this study takes the form of a power law. While in the previous Ne functional representations the power index is a constant, the power index in our representation of Ne distribution is a function of Kp and solar zenith angle.

Ground magnetometer observation of a cross‐phase reversal at a steep plasmapause

The cross‐phase technique employs ground‐based magnetometer data in order to determine the resonance frequency of a geomagnetic field line. Typically, a positive cross‐phase maximum identifies the field line resonance frequency, but occasionally, a negative cross‐phase maximum is observed and is believed to be a feature of the steep density gradient at the plasmapause. For a few hours during the local morning of 14 May 2001 the cross‐phase maximum, observed using two pairs of ground‐based magnetometer stations from the European sector, with midpoints at L = 3.16 and L = 3.34, reversed polarity from positive to negative. All other British Geological Survey, Sub‐Auroral Magnetometer Network, and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer array station pairs examined between L = 2.39 and L = 6.54 showed a positive cross‐phase maximum throughout the day. The Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite made an excellent close magnetic conjunction with these ground‐based magnetometer arrays on this day, and data from the IMAGE Radio Plasma Imager instrument show a very steep plasmapause in this region during this UT interval. IMAGE Extreme Ultraviolet Imager global plasmasphere images show that the plasmapause moved outward through the day, passing through the region of the observed negative cross‐phase maxima. This rare observation of a negative cross‐phase maximum occurs at the location of a plasmapause with a gradient steeper than r−8 and thus is in agreement with theory. The two cross‐phase peak polarity reversals are explained by the evolution of the local density profile.

Proton cyclotron echoes and a new resonance observed by the Radio Plasma Imager instrument on the IMAGE satellite

revealed in echo time delay versus frequency forms that arrive at multiples of the local proton cyclotron period tp. Lower-altitude (<4000 km) versions of two of these proton cyclotron (PC) forms were previously observed in the topside ionosphere. A new resonance, apparently confined to altitudes above � 7000 km, was observed at a frequency � 15% above the electron cyclotron frequency fce. We believe that PC echoes and the new resonance are driven by a variety of mechanisms, but only exceptionally strong echoes in the whistler-mode domain are discussed in detail. Those echoes indicate that peak excitation of the protons occurs as a transient event at the beginning of each rf pulse. We infer that there is spatial bunching of accelerated protons during the initial formation of an electron sheath around the positive-voltage antenna element. The gyrating protons then produce a series of electrostatic pulses at multiples of tp. The most efficient proton excitation is expected to occur at frequencies near and below the proton plasma frequency fpp. In contrast to these echoes, the discrete PC echoes above fce and near fZ show evidence of thermal-mode wave propagation at the rf frequency of the sounder pulses, while the new resonance above fce suggests the existence of a ringing phenomenon in the plasma that is unique to altitudes above � 7000 km.

Enhanced high‐altitude polar cap plasma and magnetic field values in response to the interplanetary magnetic cloud that caused the great storm of 31 March 2001: A case study for a new magnetospheric index

The magnetospheric electron number density and the magnetic field strength near 8 RE over the polar cap increased dramatically after the arrival of an interplanetary magnetic cloud on 31 March 2001. These parameters were determined with high accuracy from the plasma resonances stimulated by the Radio Plasma Imager (RPI) on Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) near apogee during both quiet (30 March 2001) and disturbed (31 March 2001) days. The quiet day and disturbed day values were each compared with magnetospheric magnetic field and electron density models; good agreement was found with the former but not the latter. The magnetospheric response was also expressed in terms of the ratio of the electron plasma frequency fpe to the electron cyclotron frequency fce, which is proportional to the ratio of the electron gyroradius to the Debye radius. Simultaneous Wind measurements of the solar wind magnetic field strength, speed, and plasma density were used to calculate the solar wind quasi‐invariant QI. This index is equivalent to the ratio of the solar wind magnetic pressure to the solar wind ram pressure or to the inverse of the magnetic Mach number squared. These nondimensional quantities, QI and fpe/fce, have fundamental meanings in the solar wind MHD regime and in the relation between electric and magnetic forces on electrons in the magnetosphere, respectively. During the large 31 March 2001 storm, IMAGE was at the right place at the right time so as to enable comparisons between RPI fpe/fce and Wind QI determinations. Both QI and fpe/fce formed maxima during 6‐hour observing intervals during this storm that were found to be highly correlated (87%) with a magnetospheric time lag of about 3 hours for fpe/fce. These results, based on a detailed case study of this important event, suggest that the plasma parameter fpe/fce may serve as a useful magnetospheric index.

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.

Extreme polar cap density enhancements along magnetic field lines during an intense geomagnetic storm

Sounding measurements from the radio plasma imager (RPI) on the IMAGE satellite are used to derive electron number density distributions along magnetic field lines in the polar cap magnetosphere during an intense magnetic storm. It is shown that electron densities along magnetic field lines in the polar cap magnetosphere were greatly enhanced on both the dayside and nightside during the storm, compared to the electron density profiles measured during periods of lower geomagnetic activities. The electron density enhancements were observed extending to 7 Earth radii (RE) in altitude on the dayside, with the electron density value reaching about 10 cm−3 at 7 RE altitude. The observed density enhancements were likely due to the enhanced cleft ion fountain during the storm although some of nightside density enhancements might be caused by the increased ion outflows locally in the polar cap. The strongest electron density enhancements observed on the dayside are possibly further associated with storm‐time transport of plasma from the midlatitude ionosphere and plasmasphere to high latitudes, which manifests as a plasma plume intruding to dayside high latitudes as seen from total electron content (TEC) maps. With an enhanced source population supplied by the plasma plume, acceleration and heating processes in the dayside cusp/auroral region may produce a large flux of outflowing plasma along magnetic field lines while the outflowing plasma is convected anti‐sunward toward the polar cap. These processes lead to strongly enhanced cleft ion fountain and thus greatly raised electron densities at magnetospheric altitudes in the polar cap. The present study captures an event of a massive redistribution of the magnetospheric and ionospheric plasma during a geomagnetic storm caused by extreme solar wind/interplanetary magnetic field (IMF) conditions.

Empirical specification of field‐aligned plasma density profiles for plasmasphere refilling

Sounding measurements from the radio plasma imager (RPI) on the IMAGE satellite are used to derive electron density distributions along magnetic field lines in the nightside plasmasphere and plasmatrough for three midlatitude passes. These passes occurred during (1) a magnetic storm, (2) a prolonged quiet time, and (3) a sudden commencement of a storm, respectively. It is found that the density profiles of filled (in the inner plasmasphere) and depleted (in the plasmatrough or outer plasmasphere) flux tubes have different field line dependence. A multivariant least squares fit with a simple analytical function is used to model the density profiles. The fitting parameters in the function define the field line dependence of a density profile, i.e., the steepness of the density profile at high latitudes and the flatness at low latitudes. In each pass the density profiles along the filled and depleted flux tubes can be well modeled with the selected functional form, with two different sets of fitting parameter values for filled and depleted flux tubes. For the three passes examined, the fitting parameter values are not sensitive to the geomagnetic activity for the inner plasmasphere density profiles but vary slightly for the plasmatrough or outer plasmasphere density profiles from case to case. The equatorial densities extrapolated from the measured density profiles approximately have a power law relation with L values. The results suggest that the selected function has potential of being able to construct realistic global empirical plasmasphere/plasmatrough models. Furthermore, it is now feasible to empirically determine the density profiles along the depleted flux tubes for plasmasphere refilling studies.