Refine
Language
- English (61)
Is part of the Bibliography
- yes (61)
Keywords
- radiation belts (10)
- magnetosphere (8)
- electrons (6)
- Radiation belts (4)
- Van Allen Probes (4)
- chorus waves (3)
- inner magnetosphere (3)
- model (3)
- pitch angle (3)
- wave-particle interactions (3)
Data assimilation aims to blend incomplete and inaccurate data with physics-based dynamical models. In the Earth's radiation belts, it is used to reconstruct electron phase space density, and it has become an increasingly important tool in validating our current understanding of radiation belt dynamics, identifying new physical processes, and predicting the near-Earth hazardous radiation environment. In this study, we perform reanalysis of the sparse measurements from four spacecraft using the three-dimensional Versatile Electron Radiation Belt diffusion model and a split-operator Kalman filter over a 6-month period from 1 October 2012 to 1 April 2013. In comparison to previous works, our 3-D model accounts for more physical processes, namely, mixed pitch angle-energy diffusion, scattering by Electromagnetic Ion Cyclotron waves, and magnetopause shadowing. We describe how data assimilation, by means of the innovation vector, can be used to account for missing physics in the model. We use this method to identify the radial distances from the Earth and the geomagnetic conditions where our model is inconsistent with the measured phase space density for different values of the invariants mu and K. As a result, the Kalman filter adjusts the predictions in order to match the observations, and we interpret this as evidence of where and when additional source or loss processes are active. The current work demonstrates that 3-D data assimilation provides a comprehensive picture of the radiation belt electrons and is a crucial step toward performing reanalysis using measurements from ongoing and future missions.
The investigation of heavy ions dynamics and properties in the Earth's magnetosphere is still an important field of research as they play an important role in several space weather aspects. We present a statistical survey of the average ion mass in the dayside magnetosphere made comparing plasma mass density with electron number density measurements and focusing on both spatial and geomagnetic activity dependence. Field line resonance frequency observations across the European quasi-Meridional Magnetometer Array, are used to infer the equatorial plasma mass density in the range of magnetic L-shells 1.6-6.2. The electron number density is derived from local electric field measurements made on Van Allen Probes using the Neural-network-based Upper-hybrid Resonance Determination algorithm. The analysis is conducted separately for the plasmasphere and the plasmatrough during favorable periods for which both the plasma parameters are observed simultaneously. We found that throughout the plasmasphere the average ion mass is similar or equal to 1 amu for a wide range of geomagnetic activity conditions, suggesting that the plasma mainly consist of hydrogen ions, without regard to the level of geomagnetic activity. Conversely, the plasmatrough is characterized by a variable composition, highlighting a heavy ion mass loading that increases with increasing levels of geomagnetic disturbance. During the most disturbed conditions, the average radial structure shows a broad maximum around 3-4 Earth radii, probably correlated with the accumulation of oxygen ions near the plasmapause. Those ions are mostly observed in the post-dawn and pre-dusk longitudinal sectors.
Pitch Angle Scattering of Sub-MeV Relativistic Electrons by Electromagnetic Ion Cyclotron Waves
(2019)
Electromagnetic ion cyclotron (EMIC) waves have long been considered to be a significant loss mechanism for relativistic electrons. This has most often been attributed to resonant interactions with the highest amplitude waves. But recent observations have suggested that the dominant energy of electrons precipitated to the atmosphere may often be relatively low, less than 1 MeV, whereas the minimum resonant energy of the highest amplitude waves is often greater than 2 MeV. Here we use relativistic electron test particle simulations in the wavefields of a hybrid code simulation of EMIC waves in dipole geometry in order to show that significant pitch angle scattering can occur due to interaction with low-amplitude short-wavelength EMIC waves. In the case we examined, these waves are in the H band (at frequencies above the He+ gyrofrequency), even though the highest amplitude waves were in the He band frequency range (below the He+ gyrofrequency). We also present wave power distributions for 29 EMIC simulations in straight magnetic field line geometry that show that the high wave number portion of the spectrum is in every case mostly due to the H band waves. Though He band waves are often associated with relativistic electron precipitation, it is possible that the He band waves do not directly scatter the sub-megaelectron volts (sub-MeV) electrons, but that the presence of He band waves is associated with high plasma density which lowers the minimum resonant energy so that these electrons can more easily resonate with the H band waves.
Space radiation is one of the main concerns for human space flights. The prediction of the radiation dose for the actual spacecraft geometry is very important for the planning of long-duration missions. We present a numerical method for the fast calculation of the radiation dose rate during a space flight. We demonstrate its application for dose calculations during the first and the second sessions of the MATROSHKA-R space experiment with a spherical tissue-equivalent phantom. The main advantage of the method is the short simulation time, so it can be applied for urgent radiation dose calculations for low-Earth orbit space missions. The method uses depth-dose curve and shield-and-composition distribution functions to calculate a radiation dose at the point of interest. The spacecraft geometry is processed into a shield-and-composition distribution function using a ray-tracing method. Depth-dose curves are calculated using the GEANT4 Monte-Carlo code (version 10.00.P02) for a double-layer aluminum-water shielding. Aluminum-water shielding is a good approximation of the real geometry, as water is a good equivalent for biological tissues, and aluminum is the major material of spacecraft bodies.
Storm Time Depletions of Multi-MeV Radiation Belt Electrons Observed at Different Pitch Angles
(2019)
During geomagnetic storms, the rapid depletion of the high-energy (several MeV) outer radiation belt electrons is the result of loss to the interplanetary medium through the magnetopause, outward radial diffusion, and loss to the atmosphere due to wave-particle interactions. We have performed a statistical study of 110 storms using pitch angle resolved electron flux measurements from the Van Allen Probes mission and found that inside of the radiation belt (L* = 3 - 5) the number of storms that result in depletion of electrons with equatorial pitch angle alpha(eq) = 30 degrees is higher than number of storms that result in depletion of electrons with equatorial pitch angle alpha(eq) = 75 degrees. We conclude that this result is consistent with electron scattering by whistler and electromagnetic ion cyclotron waves. At the outer edge of the radiation belt (L* >= 5.2) the number of storms that result in depletion is also large (similar to 40-50%), emphasizing the significance of the magnetopause shadowing effect and outward radial transport.
Electromagnetic ion cyclotron (EMIC) waves play an important role in the dynamics of ultrarelativistic electron population in the radiation belts. However, as EMIC waves are very sporadic, developing a parameterization of such wave properties is a challenging task. Currently, there are no dynamic, activity-dependent models of EMIC waves that can be used in the long-term (several months) simulations, which makes the quantitative modeling of the radiation belt dynamics incomplete. In this study, we investigate Kp, Dst, and AE indices, solar wind speed, and dynamic pressure as possible parameters of EMIC wave presence. The EMIC waves are included in the long-term simulations (1year, including different geomagnetic activity) performed with the Versatile Electron Radiation Belt code, and we compare results of the simulation with the Van Allen Probes observations. The comparison shows that modeling with EMIC waves, parameterized by solar wind dynamic pressure, provides a better agreement with the observations among considered parameterizations. The simulation with EMIC waves improves the dynamics of ultrarelativistic fluxes and reproduces the formation of the local minimum in the phase space density profiles.
Predicting the electron population of Earth's ring current during geomagnetic storms still remains a challenging task.
In this work, we investigate the sensitivity of 10 keV ring current electrons to different driving processes, parameterised by the Kp index, during several moderate and intense storms.
Results are validated against measurements from the Van Allen Probes satellites. Perturbing the Kp index allows us to identify the most dominant processes for moderate and intense storms respectively.
We find that during moderate storms (Kp < 6) the drift velocities mostly control the behaviour of low energy electrons, while loss from wave-particle interactions is the most critical parameter for quantifying the evolution of intense storms (Kp > 6). Perturbations of the Kp index used to drive the boundary conditions at GEO and set the plasmapause location only show a minimal effect on simulation results over a limited L range.
It is further shown that the flux at L & SIM; 3 is more sensitive to changes in the Kp index compared to higher L shells, making it a good proxy for validating the source-loss balance of a ring current model.
The ratio of the proton ring velocity (VR) to the local Alfven speed (VA), in addition to proton ring distributions, plays a key factor in the excitation of magnetosonic waves at frequencies between the proton cyclotron frequency fcp and the lower hybrid resonance frequency fLHR in the Earth's magnetosphere. Here we investigate whether there is a statistically significant relationship between occurrences of proton rings and magnetosonic waves both outside and inside the plasmapause using particle and wave data from Van Allen Probe-A during the time period of October 2012 to December 2015. We also perform a statistical survey of the ratio of the ring energy (ER, corresponding to VR) to the Alfven energy (EA, corresponding to VA) to determine the favorable conditions under which magnetosonic waves in each of two frequency bands (fcp < f ≤ 0.5 fLHR and 0.5 fLHR < f < fLHR) can be excited. The results show that the magnetosonic waves in both frequency bands occur around the postnoon (12–18 magnetic local time, MLT) sector outside the plasmapause when ER is comparable to or lower than EA, and those in lower-frequency bands (fcp < f ≤ 0.5 fLHR) occur around the postnoon sector inside the plasmapause when ER/EA > ~9. However, there is one discrepancy between occurrences of proton rings and magnetosonic waves in low-frequency bands around the prenoon sector (6–12 MLT) outside the plasmapause, which suggests either that the waves may have propagated during active time from the postnoon sector after being excited during quiet time, or they may have locally excited in the prenoon sector during active time.
We present the dependence of the magnetosonic wave amplitudes both outside and inside the plasmapause on the solar wind and AE index using Van Allen Probe-A spacecraft during the time period of 1 October 2012 to 31 December 2015, based on a correlation and regression analysis. Solar wind parameters considered are the southward interplanetary magnetic field (IMF B-S), solar wind number density (N-SW), and bulk speed (V-SW). We find that the wave amplitudes outside (inside) the plasmapause are well correlated with the preceding AE, IMF B-S, and N-SW with time delays, each corresponding to 2-3 h (3-4 h), 4-5 h (3-4 h), and 2-3 h (8-9 h), while the correlation with V-SW is ambiguous both inside and outside the plasmapause. As measured by the correlation coefficient, the IMF B-S is the most influential solar wind parameter that affects the dayside wave amplitudes both outside and inside the plasmapause, while N-SW contributes to enhancing the duskside waves outside the plasmapause. The AE effect on wave amplitudes is comparable to that of IMF B-S. More interestingly, regression with time histories of the solar wind parameters and the AE index preceding the wave measurements outside the plasmapause shows significant dependence on the IMF B-S, N-SW, and AE: the region of peak coefficients is changed with time delay for IMF B-S and AE, while isolated peaks around duskside remain gradually decrease with time for N-SW. In addition, the regression with magnetosonic waves inside the plasmapause shows high coefficients around prenoon sector with preceding IMF B-S and V-SW.
Statistical Analysis of Hiss Waves in Plasmaspheric Plumes Using Van Allen Probe Observations
(2019)
Plasmaspheric hiss waves commonly observed in high‐density regions in the Earth's magnetosphere are known to be one of the main contributors to the loss of radiation belt electrons. There has been a lot of effort to investigate the distributions of hiss waves in the plasmasphere, while relatively little attention has been given to those in the plasmaspheric plume. In this study, we present for the first time a statistical analysis of the occurrence and the spatial distribution of wave amplitudes and wave normal angles for hiss waves in plumes using Van Allen Probes observations during the period of October 2012 to December 2016. Statistical results show that a wide range of hiss wave amplitudes in plumes from a few picotesla to >100 pT is observed, but a modest (<20 pT) wave amplitude is more commonly observed regardless of geomagnetic activity in both the midnight‐to‐dawn and dusk sector. By contrast, stronger amplitude hiss occurs preferentially during geomagnetically active times in the dusk sector. The wave normal angles are distributed over a broad range from 0° to 90° with a bimodal distribution: a quasi‐field‐aligned population (<20°) with an occurrence rate of <60% and an oblique one (>50°) with a relative low occurrence rate of ≲20%. Therefore, from a statistical point of view, we confirm that the hiss intensity (a few tens of picotesla) and field‐aligned hiss wave adopted in previous simulation studies are a reasonable assumption but stress that the activity dependence of the wave amplitude should be considered.