Plasmaspheric Density Research Articles

Migration of Fast Magnetosonic Waves in the Magnetosphere With a Plasmaspheric Plume

AbstractPlasmaspheric density structures are considered to control the propagation trajectories of fast magnetosonic (MS) waves in the inner magnetosphere. However, whether the plasmaspheric plume can effectively alter the propagation of MS waves remains unknown. Based on the analytical model of plasma density, ray tracing simulations are performed to investigate the propagation of exactly perpendicular MS waves in the equatorial plane in the magnetosphere containing a plasmaspheric plume. We find that plasmatrough and plume MS waves propagating toward the plasmaspheric plume can be reflected into the plasmaspheric core by the plume, then potentially migrating globally and thus quasi‐trapped inside the plasmaspheric core. The simulations also indicate that lower‐frequency MS waves approaching the plasmaspheric plume are more easily reflected and quasi‐trapped inside the plasmaspheric core. Our findings illustrate a previously unexplored way that plasmatrough MS waves could access and be trapped inside the plasmaspheric core via azimuthal plasmaspheric density structures.

Effects of Plasma Density on the Spatial and Temporal Scale Sizes of Plasmaspheric Hiss

AbstractThe effects of plasma density on the plasmaspheric hiss amplitude and its spatial and temporal scale sizes are investigated using data from Van Allen Probes A and B. The plasmaspheric hiss amplitude and scale sizes are found to be more affected by the small‐scale density variations than the overall density profile of the plasmasphere. In detail, our results suggest that both the hiss wave amplitude and its scale sizes are (a) related to the total electron density when the density is less than ∼1,000 cm−3; (b) independent of the total electron density when the density is greater than ∼1,000 cm−3; and (c) better correlated with the detrended density (removing the background plasmasphere density profile from the total electron density) regardless of the density value.

Filtering of Magnetosonic Waves by Mesoscale Plasmaspheric Density Interfaces

AbstractMagnetosonic waves inside and outside the plasmasphere differ statistically in occurrence rate, frequency, and intensity. How the density interface separates magnetosonic waves inside and outside the plasmasphere remains not fully understood. Here we report an experimental test made with the Van Allen Probes mission from the plasmaspheric plume through the low‐density channel to the plasmaspheric core. Our linear instability analysis and two‐dimensional full‐wave modeling support that the magnetosonic waves propagate from elsewhere to the channel, undergo reflection and transmission at the flanking plasmaspheric density interfaces and eventually exhibit drastic differences in intensity and frequency coverage between neighboring regions. Such a mesoscale (tens of wavelength wide) interface with a strong refractive index gradient allows the transformation of incident waves to surface waves and consequently filters waves in both frequency and orientation. This unexpected filtering pattern could commonly occur at the plasmaspheric boundary and eventually affect the global distribution of magnetosonic waves.

Inner Belt Wisp Precipitation Measured by ELFIN: Regimes of Energetic Electron Scattering by VLF Transmitter Waves

AbstractMan‐made very low frequency (VLF) transmitter waves play a critical role in energetic electron scattering and precipitation from the inner radiation belt, a type of which is called wisp precipitation. Wisps exhibit dispersive energy‐versus‐L spectra due to the evolution of electron cyclotron resonance conditions with near‐monochromatic VLF transmitter waves. Here, we report on such observations of inner belt wisp precipitation events with full pitch angle resolution in the energy range of 50 to ∼500 keV as measured by Electron Loss and Fields Investigation (ELFIN) at L < ∼2 between March 2021 and April 2022. Statistical observations (82 events) reveal occasional (18 events) wisp precipitation events with local bounce‐loss‐cone electron flux enhancements, which provide new information compared with flux enhancements measured in previous studies only in the drift loss cone. Based on magnetic field and plasmaspheric density models, quasilinear theory, and detailed pitch angle distributions of wisps from ELFIN, we have estimated the wisp electron bounce‐averaged pitch angle diffusion coefficients to be of the order of 10−4 to 10−2 s−1. These are several orders of magnitude larger than the diffusion rates calculated from models using global statistical averages of VLF transmitter wave power. When using our estimated diffusion coefficients to deduce the associated local transmitter wave amplitudes near the equator, based on quasilinear calculations from a transmitter‐induced electron diffusion model, we find these wave amplitudes to be >1 mV/m. Although probable overestimates, such inferred wave amplitudes exceed the theoretical threshold amplitude for nonlinear interactions, strongly suggesting that it is necessary to include nonlinear effects for an accurate evaluation of energetic electron scattering by transmitter waves.

A New Mechanism for Early-Time Plasmaspheric Refilling: The Role of Charge Exchange Reactions in the Transport of Energy and Mass Throughout the Ring Current-Plasmasphere System.

Cold H+ produced via charge exchange reactions between ring current ions and exospheric neutral hydrogen constitutes an additional source of cold plasma that further contributes to the plasmasphere and affects the plasma dynamics in the Earth's magnetosphere system; however, its production and associated effects on the plasmasphere dynamics have not been fully assessed and quantified. In this study, we perform numerical simulations mimicking an idealized three-phase geomagnetic storm to investigate the role of heavy ion composition in the ring current (O+ vs. N+) and exospheric neutral hydrogen density in the production of cold H+ via charge exchange reactions. It is found that ring current heavy ions produce more than 50% of the total cold H+ via charge exchange reactions, and energetic N+ is more efficient in producing cold H+ via charge exchange reactions than O+. Furthermore, the density structure of the cold H+ is highly dependent on the mass of the parent ion; that is, cold H+ deriving from charge exchange reactions involving energetic O+ with neutral hydrogen, populates the lower L-shells, while cold H+ deriving from charge exchange reactions involving energetic N+ with neutral hydrogen populates the higher L-shells. In addition, the density of cold H+ produced via charge exchange reactions involving N+ can be peak at values up to one order of magnitude larger than the local plasmaspheric density, suggesting that solely considering the supply of cold plasma from the ionosphere to the plasmasphere can lead to a significant underestimation of plasmasphere density.

Quantifying Radiation Belt Electron Loss Processes at L

We present a comprehensive analysis of the processes that lead to quasilinear pitch-angle-scattering loss of electrons from the L<4 region of the Earth's inner magnetosphere during geomagnetically quiet times. We consider scattering via Coulomb collisions, hiss waves, lightning-generated whistler (LGW) waves, waves from ground-based very-low-frequency (VLF) transmitters, and electromagnetic ion cyclotron (EMIC) waves. The amplitude, frequency, and wave normal angle spectra of these waves are parameterized with empirical wave models, which are then used to compute pitch-angle diffusion coefficients. From these coefficients, we estimate the decay timescales, or lifetimes, of 30keV to 4MeV electrons and compare the results with timescales obtained from in-situ observations. We demonstrate good quantitative agreement between the two over most of the L and energy range under investigation. Our analysis suggests that the electron decay timescales are very sensitive to the choice of plasmaspheric density model. At L<2, where our theoretical lifetimes do not agree well with the observations, we show that including Coulomb energy drag (ionization energy loss) in our calculations significantly improves the quantitative agreement with the observed decay timescales. We also use an accurate model of the geomagnetic field to provide an estimate of the effect that the drift-loss cone has on the theoretically calculated electron lifetimes, which are usually obtained using an axisymmetric dipole field.

Using VLF Transmitter Signals at LEO for Plasmasphere Model Validation

AbstractThis study presents analysis of very low frequency (VLF) transmitter signal measurements on the Very‐Low‐Frequency Propagation Mapper (VPM) CubeSat in low‐Earth orbit. Six months of satellite operation provided good data coverage, used to build global statistical maps of VLF power distribution. The power distribution above four powerful transmitters is used as input for ray tracing to study signal propagation to the conjugate hemisphere in two plasmaspheric density models. The ray tracing results are further compared with VPM measurements to determine which model provides better agreement with observations. As ray propagation largely depends on the background plasma density distribution, this indirect method can be used for plasmaspheric density model validation as an alternative to multipoint in situ plasma measurements that may not be readily obtainable. In addition, it can be used to investigate Landau damping and ducted versus non‐ducted propagation of VLF signals.

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.

Investigation of Small‐Scale Electron Density Irregularities Observed by the Arase and Van Allen Probes Satellites Inside and Outside the Plasmasphere

AbstractIn situ electron density profiles obtained from Arase in the night magnetic local time (MLT) sector and from RBSP‐B covering all MLTs are used to study the small‐scale density irregularities present in the plasmasphere and near the plasmapause. Electron density perturbations with amplitudes &gt;10% from background density and with time‐scales less than 30‐min are investigated here as the small‐scale density irregularities. The statistical survey of the density irregularities is carried out using nearly 2 years of density data obtained from RBSP‐B and 4 months of data from Arase satellites. The results show that density irregularities are present globally at all MLT sectors and L‐shells both inside and outside the plasmapause, with a higher occurrence at L &gt; 4. The occurrence of density irregularities is found to be higher during disturbed geomagnetic and interplanetary conditions. The case studies presented here revealed: (1) The plasmaspheric density irregularities observed during both quiet and disturbed conditions are found to coexist with the hot plasma sheet population. (2) During quiet periods, the plasma waves in the whistler‐mode frequency range are found to be modulated by the small‐scale density irregularities, with density depletions coinciding well with the decrease in whistler intensity. Our observations suggest that different source mechanisms are responsible for the generation of density structures at different MLTs and geomagnetic conditions.

Local Heating of Oxygen Ions in the Presence of Magnetosonic Waves: Possible Source for the Warm Plasma Cloak?

AbstractIn the energy regime between the plasmasphere (a few eV) and the ring current (greater than 1 keV), there exists another magnetospheric particle population with energies from a few eV to a few keV, the origins of which are debated. Studies explore generation mechanisms for warm plasma energies in the inner magnetosphere through two observed phenomena: the warm plasma cloak and the oxygen torus. The relations between these two populations are unclear. Recent data reveal local heating of cold H+ and He+ ions to warm plasma energies by magnetosonic waves. In this study, we report first observations of thermal O+ heating by magnetosonic waves and link the heating to a possible formation mechanism for the warm plasma cloak. The O+ heating is observed by different plasmaspheric density profiles, including density channels. We observe that O+ heating always occurs with thermal H+ and He+ heating. We investigate the harmonic structure of the observed magnetosonic waves and find intense O+ heating is accompanied by discrete heavy ion gyroharmonics. We suggest that locally heated thermal ions to 100s eV by magnetosonic waves along the plasmapause could provide a possible mechanism for warm plasma cloak generation.

Determining Plasmaspheric Density From the Upper Hybrid Resonance and From the Spacecraft Potential: How Do They Compare?

AbstractThe plasmasphere is a critical region of the magnetosphere. It is important for the evolution of Earth's radiation belts. Waves in the plasmasphere interior (hiss) and vicinity (electromagnetic ion cyclotron, chorus) help control the acceleration and loss of radiation belt particles. Thus, understanding the extent, structure, content, and dynamics of the plasmasphere is crucial to understanding radiation belt losses. The Van Allen Probes mission uses two methods to determine the total plasma density. First, the upper hybrid resonance frequency can provide electron density; this determination is the most accurate and robust. However, it requires significant analysis and is challenging during geomagnetically active times: It becomes difficult to interpret the wave spectrum, and the amount of available data is severely limited. Second, the spacecraft potential is a proxy for the plasma density. These high‐resolution measurements are available with high duty cycle. However, environmental effects can limit the accuracy of this method. The relation between spacecraft potential and density is empirical, requiring an independent density measurement and repeated checks. We perform a quantitative comparison of these two in situ techniques during the first 3.5 years of the Van Allen Probes mission. We show how to calibrate potential‐based density measurements using only publicly available wave‐derived densities to provide high‐fidelity results even if upper hybrid measurements are sparse or unavailable. We quantify the level of uncertainty to expect from potential‐derived density data. Our approach can be applied to any in situ spacecraft mission where reliable absolute density and spacecraft potential data are available.

VLF Transmitters and Lightning‐Generated Whistlers: 1. Modeling Waves From Source to Space

AbstractWaves from nine major ground‐based very low frequency (VLF) transmitters are modeled from their sources to 660 km altitude with a full‐wave code, which reliably treats transionospheric attenuation, and then ray‐and‐power traced throughout the plasmasphere. Lightning‐generated whistlers, previously modeled at 660 km altitude, are ray‐and‐power traced throughout the plasmasphere as well. The resulting profiles of electric and magnetic fields, including wave normal angles, are organized by value. Two versions of a realistic plasmaspheric density model are used, and ducted as well as nonducted propagation are treated. Results are compared to empirical models based on near‐equatorial measurements by Van Allen Probes. A companion paper will evaluate resonant interactions of these waves with radiation belt electrons.

Development of a 3‐D Plasmapause Model With a Back‐Propagation Neural Network

AbstractSeveral empirical models have been previously developed to study the characteristics of the global plasmasphere. A three‐dimensional solar wind‐driven global dynamic plasmapause model was developed in this study using a back‐propagation neural network based on multisatellite measurements. Our database contains 37,859 plasmapause crossing events from 4 January 1995 to 31 December 2015 covering all 24 magnetic local times (MLTs) from 66° to 79° magnetic latitudes. This model is parameterized by solar wind speed , interplanetary magnetic field , SYM‐H, and indices with smooth MLTs and latitude dependence. As the first 3‐D empirical model, this model corresponds to the true plasmapause structure and is useful for observing space weather and other applications especially for predicting the shape of the plasmapause by forecasting the input parameters. Moreover, the proposed model is not only highly sensitive to these parameters, but also to their changes over days, seasons, and years; this means that the diurnal, seasonal, and annual variations of the parameters can be captured by the model. Therefore, this model can provide a more accurate plasmapause position compared with previous models and can also be used as a basic reference to develop other models such as a global plasmaspheric density model.

Decay of Ultrarelativistic Remnant Belt Electrons Through Scattering by Plasmaspheric Hiss

AbstractUltrarelativistic electron remnant belts appear frequently following geomagnetic disturbances and are located in‐between the inner radiation belt and a reforming outer belt. As remnant belts are relatively stable, here we explore the importance of hiss and electromagnetic ion cyclotron waves in controlling the observed decay rates of remnant belt ultrarelativistic electrons in a statistical way. Using measurements from the Van Allen Probes inside the plasmasphere for 25 remnant belt events that occurred between 2012 and 2017 and that are located in the region 2.9&lt;L&lt;4.0, we demonstrate that, in general, the observed decay rates and their scaling with electron energy are in good agreement with theoretical lifetimes of ultrarelativistic electrons calculated for hiss‐induced loss based on recent statistics of hiss waves and plasmaspheric density. A slight variation of electron lifetimes with energy is noticed for several peculiar events. We show that this behavior is apparently unrelated to inward radial diffusion. Moreover, we show that simultaneously observed electromagnetic ion cyclotron waves in the hydrogen band, despite their very low time‐averaged intensity of about 9 pT, can sufficiently increase electron scattering at low pitch angles as compared with hiss alone to account for the observations.

Solar Rotation Period Driven Modulations of Plasmaspheric Density and Convective Electric Field in the Inner Magnetosphere

AbstractThis paper presents the first analysis of Van Allen Probes measurements of the cold plasma density and electric field in the inner magnetosphere to show that intervals of strong modulation at the solar rotation period occur in the locations of the outer plasmasphere and plasmapause (~0.7 RE peak‐to‐peak), in the large‐scale electric field (~0.24 mV/m peak‐to‐peak), and in the cold plasma density (~250 to ~70 cm−3 peak‐to‐peak). Solar rotation modulation of the inner magnetosphere is more apparent in the declining phase of the solar cycle than near solar maximum. The periodicities in these parameters are compared to solar extreme ultraviolet irradiance, solar wind dawn‐dusk electric field, and Kp. The variations in the plasmapause location at the solar rotation period anticorrelate with solar wind electric field, magnetospheric electric field, and Kp, but not with extreme ultraviolet irradiance, indicating that convective erosion is the dominant physical process controlling the plasmapause at these timescales.

Simulations of Van Allen Probes Plasmaspheric Electron Density Observations

AbstractWe simulate equatorial plasmaspheric electron densities using a physics‐based model (Cold PLasma, CPL; used in the ring current‐atmosphere interactions model) of the source and loss processes of refilling and erosion driven by empirical inputs. The performance of CPL is evaluated against in situ measurements by the Van Allen Probes (Radiation Belt Storm Probes) for two events: the 31 May to 5 June and 15 to 20 January 2013 geomagnetic storms observed in the premidnight and postmidnight magnetic local time (MLT) sectors, respectively. Overall, CPL reproduces the radial extent of the plasmasphere to within a mean absolute difference of L. The model electric field responsible for E × B convection and the parameterization of geomagnetic conditions (under the Kp‐index and solar wind properties) implemented by CPL did not account for localized enhancements in the duskward electric field during increased activity. Rather, it was found to be largely dependent on the measure of the quiet time background. This property indicates that the agreement between these simulations and observations does not account for the complete set of physical processes during extreme (strong or weak) geomagnetic conditions impacting the plasmasphere. Nevertheless, at the presented resolution of the model CPL does provide good agreement in reproducing Radiation Belt Storm Probes observations of plasmaspheric density and plasmapause location.

Coincident Observations by the Kharkiv IS Radar and Ionosonde, DMSP and Arase (ERG) Satellites, and FLIP Model Simulations: Implications for the NRLMSISE‐00 Hydrogen Density, Plasmasphere, and Ionosphere

AbstractThis paper reports the results of ionosphere and plasmasphere observations with the Kharkiv incoherent scatter radar and ionosonde, Defense Meteorological Satellite Program, and Arase (ERG) satellites and simulations with field line interhemispheric plasma model during the equinoxes and solstices of solar minimum 24. The results reveal the need to increase NRLMSISE‐00 thermospheric hydrogen density by a factor of ~2. For the first time, it is shown that the measured plasmaspheric density can be reproduced with doubled NRLMSISE‐00 hydrogen density only. A factor of ~2 decrease of plasmaspheric density in deep inner magnetosphere (L ≈ 2.1) caused by very weak magnetic disturbance (Dst &gt; −22 nT) of 24 December 2017 was observed in the morning of 25 December 2017. During the next night, prominent effects of partially depleted flux tube were observed in the topside ionosphere (~50% reduced H+ ion density) and at the F2‐layer peak (~50% decreased electron density). The likely physical mechanisms are discussed.

Determining Plasmaspheric Densities from Observations of Plasmaspheric Hiss

AbstractA new method of inferring electron plasma densities inside of the plasmasphere is presented. Utilizing observations of the electric and magnetic field wave power associated with plasmaspheric hiss, coupled with the cold plasma dispersion relation, permits calculation of the plasma density. This methodology yields a density estimate for each frequency channel and time interval where plasmaspheric hiss is observed and is shown to yield results that are generally in agreement with densities determined via other methods. A statistical calibration is performed against the density from the upper hybrid line, accounting for both systematic offsets and distribution scatter in the hiss‐inferred densities. This calculation and calibration methodology provides accurate density estimates, both statistically and for individual events. These calibrated calculated densities are not subject to the same upper limit as densities inferred via other methodologies, thus permitting density estimates to be extended to lower L shells. This is of particular interest given that fpe/fce ratios indicate favorable conditions for efficient pitch‐angle and energy diffusion in this region. Since hiss is almost always observable inside of the plasmasphere, the hiss‐inferred densities are available for the majority of time periods, with 79% data coverage for L &lt; 4. This compares to 33–37% data coverage for other methods of inferring plasma densities. Due to the high‐accuracy of these hiss‐inferred densities and their plentiful availability, this methodology provides a viable alternative of calculating event‐specific densities, and therefore diffusion coefficients, as opposed to relying on empirical models for periods when densities from other sources are not available.

Reconstruction of Plasmaspheric Density Distributions by Applying a Tomography Technique to Jason‐1 Plasmaspheric TEC Measurements

AbstractWe have developed a tomography algorithm and applied it to a plasmaspheric total electron content data set from Jason‐1 satellite (~1,336 km), which has a Global Positioning System receiver onboard. To invert the measured plasmaspheric total electron content into vertical distribution of electron density, we adopted a multiplicative algebraic reconstruction technique with an assumed initial density distribution. The reconstruction of the plasmaspheric density distribution was performed on Indian (70°E − 90°E), Pacific (200°E − 220°E), and Atlantic (320°E − 340°E) longitudinal planes during the periods of high solar activity (F10.7 &gt; 100) and low geomagnetic activity (Ap &lt; 12) from 2002 to 2005. The reconstructed density distribution displays general climatological characteristics of the plasmasphere. For all three longitudinal sectors, the reconstructed density distributions show weak diurnal variations being greater during daytime (09–15 LT) than nighttime (21–03 LT). In Atlantic sector, the reconstructed plasmaspheric density exhibits an annual anomaly (higher density in December than in June), which was not apparent in other longitude sectors. By fitting the reconstructed density profiles, we derived empirical functions of equatorial plasmaspheric density profiles for 4 months (March, June, September, and December) in the three longitude sectors. The parameters associated with these functions were found to exhibit diurnal variation and annual anomaly characteristics.

Understanding the global dynamics of the equatorial ionosphere in Africa for space weather capabilities: A science case for AfrequaMARN

The equatorial region of the Earth's ionosphere is one of the most complex ionospheric regions due to its interactions, instabilities, and several unresolved questions regarding its dynamics, electrodynamics, and physical processes. The equatorial ionosphere overall spans three continents with the longest region being that over the African continent. Satellite observations have demonstrated that very large differences exist in the formation of ionospheric irregularities over the African sector compared with other longitudinal sectors. This may be a consequence of the symmetric shape of the magnetic equator over the continent and the lack of variability in latitude. In this paper, we propose a science campaign to equip the African sector of the magnetic equator with ground-based instruments, specifically magnetometers and radars. The network of radars proposed is similar in style and technique to the high-latitude SuperDARN radar network, while the magnetometers will form an array along the equatorial belt. These two proposed space physics instruments will be used to study this region of the equatorial ionosphere over a long interval of time, at least one solar cycle. The deployment of an array of magnetometers (AfrequaMA) and a radar network (AfrequaRN) in the African sector of the magnetic equator is jointly called the Africa Equatorial Magnetometer Array and Radar Network (AfrequaMARN), which will provide simultaneous observations of both electric and magnetic variations over the African sector. We also examine the possible science questions such a magnetometer array and radar network would be able to address, both individually and in conjunction with other space-based and ground-based instrumentation. The proposed projects will clearly improve our understanding of the dynamics of the equatorial ionosphere and our understanding of its role in balancing the large-scale ionospheric current system, and will contribute to our ability to adequately model ionospheric and plasmaspheric densities. It will also enhance our understanding of global ionospheric processes, which will improve the space weather capabilities of the African and international space science communities.