Weichao Tu - Academia.edu (original) (raw)
Papers by Weichao Tu
Journal of Geophysical Research: Space Physics
During the 9 March 2018 event with two consecutive interplanetary shocks compressing the dayside ... more During the 9 March 2018 event with two consecutive interplanetary shocks compressing the dayside magnetosphere, the azimuthal mode structure and frequency spectrum of ultra low frequency magnetic pulsations are resolved using a cross‐spectral analysis based on high‐fidelity multi‐probe Magnetospheric Multiscale Mission (MMS) magnetometer data. The results based on the MMS 4 and MMS 3 pair of measurements show that shock arrival leads to low mode ( |m|≤3 vertmvertle3\vert m\vert \le 3vertmvertle3 ) magnetic fluctuations in the Pc4‐5 regimes, and smaller spatial scale fluctuations implied by the dominant high mode numbers are observed after both shock signatures hit and passed the magnetosphere. Detailed evolution of the mode structure is also shown for the first shock to reveal the development of high mode structure from a bump‐on‐tail distribution at m≈20 mapprox20m\approx 20mapprox20 to a dominant peak at m≈50 mapprox50m\approx 50mapprox50 in about 10 min. In addition, an interesting change of sign in m mmm from negative to positive is observed as MMS crosses ∼11 MLT pre‐noon, which is consistent with the picture of wave generation by dayside magnetopause compression and then anti‐sunward propagation. For both shocks, the contribution of higher frequency waves (Pc‐4 range compared with Pc‐5) to the total wave power is found to be negligible before and after the shock impact, but it becomes more significant during the shock impact.
nonstorm time enhancement of relativistic electrons
2018 2nd URSI Atlantic Radio Science Meeting (AT-RASC), 2018
Regarding radiation belt dropouts, no previous statistical study using electron phase space densi... more Regarding radiation belt dropouts, no previous statistical study using electron phase space density (PSD) to reveal the real loss. Thus, we conduct a statistical study of radiation belt dropouts. If PSD drops by factor >5 within a period less than 8 hours, we call it a dropout event. Based on 4 years of VAP data, we get the dropout distribution as a function of μ, K, and L* and the relationship with solar wind parameters and geomagnetic parameters. The results show that high L* dropouts cover wider μ, K range. The low L* dropouts mainly follow the influence regions due to EMIC wave. Few dropouts at low μ, K due to injection while there are many dropouts with low μ, high K, which is likely due to EMIC waves. Dropouts occurring without EMIC influence are company with stronger solar wind and magnetic activity.
Journal of Geophysical Research: Space Physics, 2020
Angular response functions are derived for four electron channels and six proton channels of the ... more Angular response functions are derived for four electron channels and six proton channels of the SEM-2 MEPED particle telescopes on the POES and MetOp satellites from Geant4 simulations previously used to derive the energy response. They are combined with model electron distributions in energy and pitch angle to show that the vertical 0 • telescope, intended to measure precipitating electrons, instead usually measures trapped or quasi-trapped electrons, except during times of enhanced pitch angle diffusion. A simplified dynamical model of the radiation belt electron distribution near the loss cone, as a function of longitude, energy, and pitch angle, that accounts for pitch angle diffusion, azimuthal drift, and atmospheric backscatter is fit to sample MEPED electron data at L = 4 during times of differing diffusion rates. It is then used to compute precipitating electron flux, as function of energy and longitude, that is lower than would be estimated by assuming that the 0 • telescope always measures precipitating electrons.
Journal of Geophysical Research: Space Physics, 2020
Using the global Lagrangian version of the piecewise parabolic method‐magnetohydrodynamic (PPMLR‐... more Using the global Lagrangian version of the piecewise parabolic method‐magnetohydrodynamic (PPMLR‐MHD) model, we simulate two consecutive storms in December 2015, a moderate storm on 14–15 December and a strong storm on 19–22 December, and calculate the radial diffusion coefficients (DLL) from the simulated ultralow frequency waves. We find that even though the strong storm leads to more enhanced Bz and Eφ power than the moderate storm, the two storms share in common a lot of features on the azimuthal mode structure and power spectrum of ultralow frequency waves. For both storms, the total Bz and Eφ power is better correlated with the solar wind dynamic pressure in the storm initial phase and more correlated with AE index in the recovery phase. Bz wave power is shown to be mostly distributed in low mode numbers, while Eφ power spreads over a wider range of modes. Furthermore, the Bz and Eφ power spectral densities are found to be higher at higher L regions, with a stronger L dependence in the Bz spectra. The estimated DLL based on MHD fields shows that inside the magnetopause, the contribution from electric fields is larger than or comparable to that from magnetic fields, and our event‐specific MHD‐based DLL can be smaller than some previous empirical DLL estimations by more than an order of magnitude. At last, by validating against in situ observations from Magnetospheric Multiscale spacecraft, our MHD results are found to generally well reproduce the total Bz fields and wave power for both storms, while the Eφ power is underestimated in the MHD simulations.
Geophysical Research Letters, 2018
Relativistic electron flux in the Earth's radiation belt are observed to drop by orders of magnit... more Relativistic electron flux in the Earth's radiation belt are observed to drop by orders of magnitude on timescales of a few hours. Where do the electrons go? This is one of the most important outstanding questions in radiation belt studies. Here we perform statistical analysis on the radiation belt dropouts based on four years of electron phase space density data from Van Allen Probes. Our results show that the dropouts at larger * regions have higher occurrence, and cover a wider range in and (the first the second adiabatic invariants) compared to those at low * regions. By comparing the statistical distribution of the dropout occurrence and ratio with the minimum resonant energy curve by EMIC waves, we find that EMIC wave scattering is the dominant loss mechanism at low * regions, while the dropouts at high * are due to a combination of EMIC wave scattering and outward radial diffusion associated with magnetopause shadowing, with outward radial diffusion leading to larger loss than EMIC wave scattering. The radiation belt dropouts at high * regions also have strong , dependence. The electron variation at low and low is dominated by fast injection or convection followed by fast decay, leading to very low occurrence of electron dropouts. However, at high , low and other high regimes, electrons suffer from abrupt dropouts and show high occurrence of dropouts at the high regimes due to a combination of the two loss mechanisms.
Journal of Geophysical Research: Space Physics, 2019
major storm with the use of different magnetic field models • Dipole-reconstructed PSD is contami... more major storm with the use of different magnetic field models • Dipole-reconstructed PSD is contaminated by artificial isolated peaks that can be wrongly interpreted as an effect of local acceleration • Non physical PSD peaks vanish when a realistic magnetic field model is used, emphasizing the importance of betatron acceleration
Journal of Geophysical Research: Space Physics, 2014
To effectively study loss due to hiss-driven precipitation of relativistic electrons in the outer... more To effectively study loss due to hiss-driven precipitation of relativistic electrons in the outer radiation belt, it is useful to isolate this loss by studying a time of relatively quiet geomagnetic activity. We present a case of initial enhancement and slow, steady decay of 700 keV-2 MeV electron populations in the outer radiation belt during an extended quiescent period from ∼15 December 2012 to 13 January 2013. We incorporate particle measurements from a constellation of satellites, including the Colorado Student Space Weather Experiment (CSSWE) CubeSat, the Van Allen Probes twin spacecraft, and Time History of Events and Macroscale Interactions during Substorms (THEMIS), to understand the evolution of the electron populations across pitch angle and energy. Additional data from calculated phase space density, as well as hiss and chorus wave data from Van Allen Probes, help complete the picture of the slow precipitation loss of relativistic electrons during a quiet time. Electron loss to the atmosphere during this event is quantified through use of the Loss Index Method, utilizing CSSWE measurements at low Earth orbit. By comparing these results against equatorial Van Allen Probes electron flux data, we conclude the net precipitation loss of the outer radiation belt content to be greater than 92%, suggesting no significant acceleration during this period, and resulting in faster electron loss rates than have previously been reported.
Earth's outer radiation belt is a relativistic electron environment that is hazardous to spac... more Earth's outer radiation belt is a relativistic electron environment that is hazardous to space systems. It is characterized by large variations in the electron flux, which are controlled by the competition between source, transport, and loss processes. One of the central questions in outer radiation belt research is to resolve the relative contribution of radial diffusion, wave heating, and loss to the enhancement and decay of the radiation belt electrons. This thesis studies them together and separately. Firstly, we develop an empirical Fokker-Planck model that includes radial diffusion, an internal source, and finite electron lifetimes parameterized as functions of geomagnetic indices. By simulating the observed electron variations, the model suggests that the required magnitudes of radial diffusion and internal heating for the enhancement of energetic electrons in the outer radiation belt vary from storm to storm, and generally internal heating contributes more to the enhance...
Radial diffusion is one of the most important acceleration mechanisms for radiation belt electron... more Radial diffusion is one of the most important acceleration mechanisms for radiation belt electrons, which is due to the drift-resonant interactions with large-scale fluctuations of the magnetosphere's magnetic and electric fields (Pc4 and Pc5 ranges of ULF waves). A key step in radial diffusion simulations is to quantify the radial diffusion coefficient, which is related to the power spectral density and global mode structure of the ULF waves. However, difficulties in determining the global properties of ULF waves have guided researchers towards specifying empirical forms of the diffusion coefficient, introducing additional uncertainties in the radiation belt studies. In order to quantify the radial diffusion, we run the global MHD simulations to obtain the mode structure and power spectrum of the ULF waves and validate the simulation results with available satellite measurements, such as GOES and THEMIS measurements. The calculated diffusion coefficient is shown to be dominated...
Journal of Geophysical Research: Space Physics, 2014
The sudden loss of energetic protons in the inner radiation belt has been observed during geomagn... more The sudden loss of energetic protons in the inner radiation belt has been observed during geomagnetic storms. It is hypothesized that this sudden loss occurs because of changes in the geomagnetic field configuration which lead to a breakdown of the first adiabatic invariant, μ, in a process called magnetic field line curvature scattering or μ scattering. Comparison of observations to various analytic model predictions for μ scattering induced loss has, however, yielded discrepancies. To better understand how well the analytic models predict the proton loss, test particle simulations are carried out for various magnetic field configurations. Although our simulation results agree well with the analytic models for single μ-scattering events, the results after cumulative μ scattering can show significant disagreement with the theoretical predictions based on analytic models. In particular, we find the assumption that protons with predicted initial δμ/μ > 0.01 or ε > 0.1 are ultimately lost overestimates the proton loss. Based on the test particle simulation results, we develop a new empirical model, called the "ε-onset" model, to predict the minimum value of ε at which all protons of a given pitch angle and energy can be assumed to be lost due to μ scattering. By applying our ε-onset model as the variable cutoff condition between trapping and detrapping, we obtain very good agreement between theoretical predictions and the simulation results for a range of Kp, suggesting that the ε-onset model can potentially serve as an easy-to-use and more reliable predictor of inner belt proton loss due to μ scattering than the previously used fixed-valued cutoff conditions. TU ET AL.
Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (RE... more Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (REPTile) on board the Colorado Student Space Weather Experiment (CSSWE) CubeSat mission, which was launched into a highly inclined (65°) low Earth orbit, are analyzed along with measurements from the Relativistic Electron and Proton Telescope (REPT) and the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the Van Allen Probes, which are in a low inclination (10°) geo-transfer-like orbit. Both REPT and MagEIS measure the full distribution of energetic electrons as they traverse the heart of the outer radiation belt. However, due to the small equatorial loss cone (only a few degrees), it is difficult for REPT and MagEIS to directly determine which electrons will precipitate into the atmosphere, a major radiation belt loss process. REPTile, a miniaturized version of REPT, measures the fraction of the total electron population that has small enough equatorial pitch angles to reach the altitude of CSSWE, 480 km × 780 km, thus measuring the precipitating population as well as the trapped and quasi-trapped populations. These newly available measurements provide an unprecedented opportunity to investigate the source, loss, and energization processes that are responsible for the dynamic behavior of outer radiation belt electrons. The focus of this paper will be on the characteristics of relativistic electrons measured by REPTile during the October 2012 storms; also included are long-term measurements from the Solar Anomalous and Magnetospheric Particle Explorer to put this study into context.
Journal of Geophysical Research: Space Physics, 2015
We used the fluxgate magnetometer data from Combined Release and Radiation Effects Satellite (CRR... more We used the fluxgate magnetometer data from Combined Release and Radiation Effects Satellite (CRRES) to estimate the power spectral density (PSD) of the compressional component of the geomagnetic field in the ∼1 mHz to ∼8 mHz range. We conclude that magnetic wave power is generally higher in the noon sector for quiet times with no significant difference between the dawn, dusk, and the midnight sectors. However, during high Kp activity, the noon sector is not necessarily dominant anymore. The magnetic PSDs have a very distinct dependence on Kp. In addition, the PSDs appear to have a weak dependence on McIlwain parameter L with power slightly increasing as L increases. The magnetic wave PSDs are used along with the Fei et al. (2006) formulation to compute D B LL [CRRES] as a function of L and Kp. The L dependence of D B LL [CRRES] is systematically studied and is shown to depend on Kp. More significantly, we conclude that D E LL is the dominant term driving radial diffusion, typically exceeding D B LL by 1-2 orders of magnitude.
Journal of Geophysical Research: Space Physics, 2010
Based on SAMPEX/PET observations, the rates and the spatial and temporal variations of electron l... more Based on SAMPEX/PET observations, the rates and the spatial and temporal variations of electron loss to the atmosphere in the Earth's radiation belt were quantified using a drift diffusion model that includes the effects of azimuthal drift and pitch angle diffusion. The measured electrons by SAMPEX can be distinguished as trapped, quasi-trapped (in the drift loss cone), and precipitating (in the bounce loss cone). The drift diffusion model simulates the low-altitude electron distribution from SAMPEX. After fitting the model results to the data, the magnitudes and variations of the electron lifetime can be quantitatively determined based on the optimum model parameter values. Three magnetic storms of different magnitudes were selected to estimate the various loss rates of ∼0.5-3 MeV electrons during different phases of the storms and at L shells ranging from L = 3.5 to L = 6.5 (L represents the radial distance in the equatorial plane under a dipole field approximation). The storms represent a small storm, a moderate storm from the current solar minimum, and an intense storm right after the previous solar maximum. Model results for the three individual events showed that fast precipitation losses of relativistic electrons, as short as hours, persistently occurred in the storm main phases and with more efficient loss at higher energies over wide range of L regions and over all the SAMPEX-covered local times. In addition to this newly discovered common feature of the main phase electron loss for all the storm events and at all L locations, some other properties of the electron loss rates, such as the local time and energy dependence that vary with time or locations, were also estimated and discussed. This method combining model with the low-altitude observations provides direct quantification of the electron loss rate, a prerequisite for any comprehensive modeling of the radiation belt electron dynamics.
Journal of Geophysical Research, 2012
Whether energetic electrons (10s of keV) in the magnetosheath can be directly transported into th... more Whether energetic electrons (10s of keV) in the magnetosheath can be directly transported into the magnetosphere and further energized through radial diffusion is significant in understanding the physical mechanisms for producing the radiation belt electrons (>100s of keV) in the magnetosphere. In this study, we analyze more than two hundred magnetopause crossing events using the energetic electron and magnetic field measurements from Geotail and compare the flux and phase space density (PSD) of the energetic electrons on both sides of the magnetopause. It is found that for most of the events (>70%), the fluxes and PSDs of energetic electrons in the magnetosheath are less than those in the magnetosphere, suggesting that the energetic electrons in the magnetosheath cannot be a direct source sufficient for the energetic electrons inside the magnetosphere. In fact, our analysis suggests a possible leakage of the energetic electrons from inside to outside the magnetopause. By investigating the average energetic electron flux distribution in the magnetosheath, we find that the energetic electron fluxes are higher near the bow shock and the magnetopause than in between. The high energetic electron flux near the bow shock can be understood as due to energization of electrons when they go through the bow shock. The relatively low flux of the energetic electrons in between indicates that it is difficult for the energetic electrons to travel from the bow shock to the magnetopause and vice versa, possibly because the energetic electrons near the bow shock and the magnetopause are all on open magnetic field lines and these two relatively intense energetic electron populations in the magnetosheath rarely get mixed.
Journal of Geophysical Research: Space Physics
During the 9 March 2018 event with two consecutive interplanetary shocks compressing the dayside ... more During the 9 March 2018 event with two consecutive interplanetary shocks compressing the dayside magnetosphere, the azimuthal mode structure and frequency spectrum of ultra low frequency magnetic pulsations are resolved using a cross‐spectral analysis based on high‐fidelity multi‐probe Magnetospheric Multiscale Mission (MMS) magnetometer data. The results based on the MMS 4 and MMS 3 pair of measurements show that shock arrival leads to low mode ( |m|≤3 vertmvertle3\vert m\vert \le 3vertmvertle3 ) magnetic fluctuations in the Pc4‐5 regimes, and smaller spatial scale fluctuations implied by the dominant high mode numbers are observed after both shock signatures hit and passed the magnetosphere. Detailed evolution of the mode structure is also shown for the first shock to reveal the development of high mode structure from a bump‐on‐tail distribution at m≈20 mapprox20m\approx 20mapprox20 to a dominant peak at m≈50 mapprox50m\approx 50mapprox50 in about 10 min. In addition, an interesting change of sign in m mmm from negative to positive is observed as MMS crosses ∼11 MLT pre‐noon, which is consistent with the picture of wave generation by dayside magnetopause compression and then anti‐sunward propagation. For both shocks, the contribution of higher frequency waves (Pc‐4 range compared with Pc‐5) to the total wave power is found to be negligible before and after the shock impact, but it becomes more significant during the shock impact.
nonstorm time enhancement of relativistic electrons
2018 2nd URSI Atlantic Radio Science Meeting (AT-RASC), 2018
Regarding radiation belt dropouts, no previous statistical study using electron phase space densi... more Regarding radiation belt dropouts, no previous statistical study using electron phase space density (PSD) to reveal the real loss. Thus, we conduct a statistical study of radiation belt dropouts. If PSD drops by factor >5 within a period less than 8 hours, we call it a dropout event. Based on 4 years of VAP data, we get the dropout distribution as a function of μ, K, and L* and the relationship with solar wind parameters and geomagnetic parameters. The results show that high L* dropouts cover wider μ, K range. The low L* dropouts mainly follow the influence regions due to EMIC wave. Few dropouts at low μ, K due to injection while there are many dropouts with low μ, high K, which is likely due to EMIC waves. Dropouts occurring without EMIC influence are company with stronger solar wind and magnetic activity.
Journal of Geophysical Research: Space Physics, 2020
Angular response functions are derived for four electron channels and six proton channels of the ... more Angular response functions are derived for four electron channels and six proton channels of the SEM-2 MEPED particle telescopes on the POES and MetOp satellites from Geant4 simulations previously used to derive the energy response. They are combined with model electron distributions in energy and pitch angle to show that the vertical 0 • telescope, intended to measure precipitating electrons, instead usually measures trapped or quasi-trapped electrons, except during times of enhanced pitch angle diffusion. A simplified dynamical model of the radiation belt electron distribution near the loss cone, as a function of longitude, energy, and pitch angle, that accounts for pitch angle diffusion, azimuthal drift, and atmospheric backscatter is fit to sample MEPED electron data at L = 4 during times of differing diffusion rates. It is then used to compute precipitating electron flux, as function of energy and longitude, that is lower than would be estimated by assuming that the 0 • telescope always measures precipitating electrons.
Journal of Geophysical Research: Space Physics, 2020
Using the global Lagrangian version of the piecewise parabolic method‐magnetohydrodynamic (PPMLR‐... more Using the global Lagrangian version of the piecewise parabolic method‐magnetohydrodynamic (PPMLR‐MHD) model, we simulate two consecutive storms in December 2015, a moderate storm on 14–15 December and a strong storm on 19–22 December, and calculate the radial diffusion coefficients (DLL) from the simulated ultralow frequency waves. We find that even though the strong storm leads to more enhanced Bz and Eφ power than the moderate storm, the two storms share in common a lot of features on the azimuthal mode structure and power spectrum of ultralow frequency waves. For both storms, the total Bz and Eφ power is better correlated with the solar wind dynamic pressure in the storm initial phase and more correlated with AE index in the recovery phase. Bz wave power is shown to be mostly distributed in low mode numbers, while Eφ power spreads over a wider range of modes. Furthermore, the Bz and Eφ power spectral densities are found to be higher at higher L regions, with a stronger L dependence in the Bz spectra. The estimated DLL based on MHD fields shows that inside the magnetopause, the contribution from electric fields is larger than or comparable to that from magnetic fields, and our event‐specific MHD‐based DLL can be smaller than some previous empirical DLL estimations by more than an order of magnitude. At last, by validating against in situ observations from Magnetospheric Multiscale spacecraft, our MHD results are found to generally well reproduce the total Bz fields and wave power for both storms, while the Eφ power is underestimated in the MHD simulations.
Geophysical Research Letters, 2018
Relativistic electron flux in the Earth's radiation belt are observed to drop by orders of magnit... more Relativistic electron flux in the Earth's radiation belt are observed to drop by orders of magnitude on timescales of a few hours. Where do the electrons go? This is one of the most important outstanding questions in radiation belt studies. Here we perform statistical analysis on the radiation belt dropouts based on four years of electron phase space density data from Van Allen Probes. Our results show that the dropouts at larger * regions have higher occurrence, and cover a wider range in and (the first the second adiabatic invariants) compared to those at low * regions. By comparing the statistical distribution of the dropout occurrence and ratio with the minimum resonant energy curve by EMIC waves, we find that EMIC wave scattering is the dominant loss mechanism at low * regions, while the dropouts at high * are due to a combination of EMIC wave scattering and outward radial diffusion associated with magnetopause shadowing, with outward radial diffusion leading to larger loss than EMIC wave scattering. The radiation belt dropouts at high * regions also have strong , dependence. The electron variation at low and low is dominated by fast injection or convection followed by fast decay, leading to very low occurrence of electron dropouts. However, at high , low and other high regimes, electrons suffer from abrupt dropouts and show high occurrence of dropouts at the high regimes due to a combination of the two loss mechanisms.
Journal of Geophysical Research: Space Physics, 2019
major storm with the use of different magnetic field models • Dipole-reconstructed PSD is contami... more major storm with the use of different magnetic field models • Dipole-reconstructed PSD is contaminated by artificial isolated peaks that can be wrongly interpreted as an effect of local acceleration • Non physical PSD peaks vanish when a realistic magnetic field model is used, emphasizing the importance of betatron acceleration
Journal of Geophysical Research: Space Physics, 2014
To effectively study loss due to hiss-driven precipitation of relativistic electrons in the outer... more To effectively study loss due to hiss-driven precipitation of relativistic electrons in the outer radiation belt, it is useful to isolate this loss by studying a time of relatively quiet geomagnetic activity. We present a case of initial enhancement and slow, steady decay of 700 keV-2 MeV electron populations in the outer radiation belt during an extended quiescent period from ∼15 December 2012 to 13 January 2013. We incorporate particle measurements from a constellation of satellites, including the Colorado Student Space Weather Experiment (CSSWE) CubeSat, the Van Allen Probes twin spacecraft, and Time History of Events and Macroscale Interactions during Substorms (THEMIS), to understand the evolution of the electron populations across pitch angle and energy. Additional data from calculated phase space density, as well as hiss and chorus wave data from Van Allen Probes, help complete the picture of the slow precipitation loss of relativistic electrons during a quiet time. Electron loss to the atmosphere during this event is quantified through use of the Loss Index Method, utilizing CSSWE measurements at low Earth orbit. By comparing these results against equatorial Van Allen Probes electron flux data, we conclude the net precipitation loss of the outer radiation belt content to be greater than 92%, suggesting no significant acceleration during this period, and resulting in faster electron loss rates than have previously been reported.
Earth's outer radiation belt is a relativistic electron environment that is hazardous to spac... more Earth's outer radiation belt is a relativistic electron environment that is hazardous to space systems. It is characterized by large variations in the electron flux, which are controlled by the competition between source, transport, and loss processes. One of the central questions in outer radiation belt research is to resolve the relative contribution of radial diffusion, wave heating, and loss to the enhancement and decay of the radiation belt electrons. This thesis studies them together and separately. Firstly, we develop an empirical Fokker-Planck model that includes radial diffusion, an internal source, and finite electron lifetimes parameterized as functions of geomagnetic indices. By simulating the observed electron variations, the model suggests that the required magnitudes of radial diffusion and internal heating for the enhancement of energetic electrons in the outer radiation belt vary from storm to storm, and generally internal heating contributes more to the enhance...
Radial diffusion is one of the most important acceleration mechanisms for radiation belt electron... more Radial diffusion is one of the most important acceleration mechanisms for radiation belt electrons, which is due to the drift-resonant interactions with large-scale fluctuations of the magnetosphere's magnetic and electric fields (Pc4 and Pc5 ranges of ULF waves). A key step in radial diffusion simulations is to quantify the radial diffusion coefficient, which is related to the power spectral density and global mode structure of the ULF waves. However, difficulties in determining the global properties of ULF waves have guided researchers towards specifying empirical forms of the diffusion coefficient, introducing additional uncertainties in the radiation belt studies. In order to quantify the radial diffusion, we run the global MHD simulations to obtain the mode structure and power spectrum of the ULF waves and validate the simulation results with available satellite measurements, such as GOES and THEMIS measurements. The calculated diffusion coefficient is shown to be dominated...
Journal of Geophysical Research: Space Physics, 2014
The sudden loss of energetic protons in the inner radiation belt has been observed during geomagn... more The sudden loss of energetic protons in the inner radiation belt has been observed during geomagnetic storms. It is hypothesized that this sudden loss occurs because of changes in the geomagnetic field configuration which lead to a breakdown of the first adiabatic invariant, μ, in a process called magnetic field line curvature scattering or μ scattering. Comparison of observations to various analytic model predictions for μ scattering induced loss has, however, yielded discrepancies. To better understand how well the analytic models predict the proton loss, test particle simulations are carried out for various magnetic field configurations. Although our simulation results agree well with the analytic models for single μ-scattering events, the results after cumulative μ scattering can show significant disagreement with the theoretical predictions based on analytic models. In particular, we find the assumption that protons with predicted initial δμ/μ > 0.01 or ε > 0.1 are ultimately lost overestimates the proton loss. Based on the test particle simulation results, we develop a new empirical model, called the "ε-onset" model, to predict the minimum value of ε at which all protons of a given pitch angle and energy can be assumed to be lost due to μ scattering. By applying our ε-onset model as the variable cutoff condition between trapping and detrapping, we obtain very good agreement between theoretical predictions and the simulation results for a range of Kp, suggesting that the ε-onset model can potentially serve as an easy-to-use and more reliable predictor of inner belt proton loss due to μ scattering than the previously used fixed-valued cutoff conditions. TU ET AL.
Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (RE... more Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (REPTile) on board the Colorado Student Space Weather Experiment (CSSWE) CubeSat mission, which was launched into a highly inclined (65°) low Earth orbit, are analyzed along with measurements from the Relativistic Electron and Proton Telescope (REPT) and the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the Van Allen Probes, which are in a low inclination (10°) geo-transfer-like orbit. Both REPT and MagEIS measure the full distribution of energetic electrons as they traverse the heart of the outer radiation belt. However, due to the small equatorial loss cone (only a few degrees), it is difficult for REPT and MagEIS to directly determine which electrons will precipitate into the atmosphere, a major radiation belt loss process. REPTile, a miniaturized version of REPT, measures the fraction of the total electron population that has small enough equatorial pitch angles to reach the altitude of CSSWE, 480 km × 780 km, thus measuring the precipitating population as well as the trapped and quasi-trapped populations. These newly available measurements provide an unprecedented opportunity to investigate the source, loss, and energization processes that are responsible for the dynamic behavior of outer radiation belt electrons. The focus of this paper will be on the characteristics of relativistic electrons measured by REPTile during the October 2012 storms; also included are long-term measurements from the Solar Anomalous and Magnetospheric Particle Explorer to put this study into context.
Journal of Geophysical Research: Space Physics, 2015
We used the fluxgate magnetometer data from Combined Release and Radiation Effects Satellite (CRR... more We used the fluxgate magnetometer data from Combined Release and Radiation Effects Satellite (CRRES) to estimate the power spectral density (PSD) of the compressional component of the geomagnetic field in the ∼1 mHz to ∼8 mHz range. We conclude that magnetic wave power is generally higher in the noon sector for quiet times with no significant difference between the dawn, dusk, and the midnight sectors. However, during high Kp activity, the noon sector is not necessarily dominant anymore. The magnetic PSDs have a very distinct dependence on Kp. In addition, the PSDs appear to have a weak dependence on McIlwain parameter L with power slightly increasing as L increases. The magnetic wave PSDs are used along with the Fei et al. (2006) formulation to compute D B LL [CRRES] as a function of L and Kp. The L dependence of D B LL [CRRES] is systematically studied and is shown to depend on Kp. More significantly, we conclude that D E LL is the dominant term driving radial diffusion, typically exceeding D B LL by 1-2 orders of magnitude.
Journal of Geophysical Research: Space Physics, 2010
Based on SAMPEX/PET observations, the rates and the spatial and temporal variations of electron l... more Based on SAMPEX/PET observations, the rates and the spatial and temporal variations of electron loss to the atmosphere in the Earth's radiation belt were quantified using a drift diffusion model that includes the effects of azimuthal drift and pitch angle diffusion. The measured electrons by SAMPEX can be distinguished as trapped, quasi-trapped (in the drift loss cone), and precipitating (in the bounce loss cone). The drift diffusion model simulates the low-altitude electron distribution from SAMPEX. After fitting the model results to the data, the magnitudes and variations of the electron lifetime can be quantitatively determined based on the optimum model parameter values. Three magnetic storms of different magnitudes were selected to estimate the various loss rates of ∼0.5-3 MeV electrons during different phases of the storms and at L shells ranging from L = 3.5 to L = 6.5 (L represents the radial distance in the equatorial plane under a dipole field approximation). The storms represent a small storm, a moderate storm from the current solar minimum, and an intense storm right after the previous solar maximum. Model results for the three individual events showed that fast precipitation losses of relativistic electrons, as short as hours, persistently occurred in the storm main phases and with more efficient loss at higher energies over wide range of L regions and over all the SAMPEX-covered local times. In addition to this newly discovered common feature of the main phase electron loss for all the storm events and at all L locations, some other properties of the electron loss rates, such as the local time and energy dependence that vary with time or locations, were also estimated and discussed. This method combining model with the low-altitude observations provides direct quantification of the electron loss rate, a prerequisite for any comprehensive modeling of the radiation belt electron dynamics.
Journal of Geophysical Research, 2012
Whether energetic electrons (10s of keV) in the magnetosheath can be directly transported into th... more Whether energetic electrons (10s of keV) in the magnetosheath can be directly transported into the magnetosphere and further energized through radial diffusion is significant in understanding the physical mechanisms for producing the radiation belt electrons (>100s of keV) in the magnetosphere. In this study, we analyze more than two hundred magnetopause crossing events using the energetic electron and magnetic field measurements from Geotail and compare the flux and phase space density (PSD) of the energetic electrons on both sides of the magnetopause. It is found that for most of the events (>70%), the fluxes and PSDs of energetic electrons in the magnetosheath are less than those in the magnetosphere, suggesting that the energetic electrons in the magnetosheath cannot be a direct source sufficient for the energetic electrons inside the magnetosphere. In fact, our analysis suggests a possible leakage of the energetic electrons from inside to outside the magnetopause. By investigating the average energetic electron flux distribution in the magnetosheath, we find that the energetic electron fluxes are higher near the bow shock and the magnetopause than in between. The high energetic electron flux near the bow shock can be understood as due to energization of electrons when they go through the bow shock. The relatively low flux of the energetic electrons in between indicates that it is difficult for the energetic electrons to travel from the bow shock to the magnetopause and vice versa, possibly because the energetic electrons near the bow shock and the magnetopause are all on open magnetic field lines and these two relatively intense energetic electron populations in the magnetosheath rarely get mixed.