Contribution of ULF Wave Activity to the Global Recovery of the Outer Radiation Belt During the Passage of a High-Speed Solar Wind Stream Observed in September 2014

Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high-speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground- and space-based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L-shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period

Solar wind conditions leading to efficient radiation belt electron acceleration: A superposed epoch analysis

Determining preferential solar wind conditions leading to efficient radiation belt electron acceleration is crucial for predicting radiation belt electron dynamics. Using Van Allen Probes electron observations (>1 MeV) from 2012 to 2015, we identify a number of efficient and inefficient acceleration events separately to perform a superposed epoch analysis of the corresponding solar wind parameters and geomagnetic indices. By directly comparing efficient and inefficient acceleration events, we clearly show that prolonged southward Bz, high solar wind speed, and low dynamic pressure are critical for electron acceleration to >1 MeV energies in the heart of the outer radiation belt. We also evaluate chorus wave evolution using the superposed epoch analysis for the identified efficient and inefficient acceleration events and find that chorus wave intensity is much stronger and lasts longer during efficient electron acceleration events, supporting the scenario that chorus waves play a key role in MeV electron acceleration

Solar wind conditions leading to efficient radiation belt electron acceleration: A superposed epoch analysis

Determining preferential solar wind conditions leading to efficient radiation belt electron acceleration is crucial for predicting radiation belt electron dynamics. Using Van Allen Probes electron observations (>1 MeV) from 2012 to 2015, we identify a number of efficient and inefficient acceleration events separately to perform a superposed epoch analysis of the corresponding solar wind parameters and geomagnetic indices. By directly comparing efficient and inefficient acceleration events, we clearly show that prolonged southward Bz, high solar wind speed, and low dynamic pressure are critical for electron acceleration to >1 MeV energies in the heart of the outer radiation belt. We also evaluate chorus wave evolution using the superposed epoch analysis for the identified efficient and inefficient acceleration events and find that chorus wave intensity is much stronger and lasts longer during efficient electron acceleration events, supporting the scenario that chorus waves play a key role in MeV electron acceleration

On the relation between radiation belt electrons and solar wind parameters/geomagnetic indices: Dependence on the first adiabatic invariant and L

The relation between radiation belt electrons and solar wind/magnetospheric processes is of particular interest due to both scientific and practical needs. Though many studies have focused on this topic, electron data from Van Allen Probes with wide L shell coverage and fine energy resolution, for the first time, enabled this statistical study on the relation between radiation belt electrons and solar wind parameters/geomagnetic indices as a function of first adiabatic invariant μ and L*. Good correlations between electron phase space density (PSD) and solar wind speed, southward IMF Bz, SYM-H and AL indices are found over wide μ and L* ranges, with higher correlation coefficients and shorter time lags for low-μ electrons than high-μ electrons; the anti-correlation between electron PSD and solar wind proton density is limited to high-μ electrons at high L*. The solar wind dynamic pressure has dominantly positive correlation with low-μ electrons and negative correlation with high-μ electrons at different L*. In addition, electron PSD enhancements also correlate well with various solar wind/geomagnetic parameters, and for most parameters this correlation is even better than that of electron PSD while the time lag is also much shorter. Among all parameters investigated, AL index is shown to correlate the best with electron PSD enhancements, with correlation coefficients up to ~0.8 for low-μ electrons (time lag ~ 0 day) and ~0.7 for high-μ electrons (time lag ~ 1-2 days), suggesting the importance of seed and source populations provided by substorms in radiation belt electron PSD enhancements

Variability of the pitch angle distribution of radiation belt ultrarelativistic electrons during and following intense geomagnetic storms: Van Allen Probes observations

Fifteen months of pitch angle resolved Van Allen Probes Relativistic Electron-Proton Telescope (REPT) measurements of differential electron flux are analyzed to investigate the characteristic variability of the pitch angle distribution of radiation belt ultrarelativistic (>2MeV) electrons during storm conditions and during the long-term poststorm decay. By modeling the ultrarelativistic electron pitch angle distribution as sinnα, where α is the equatorial pitch angle, we examine the spatiotemporal variations of the n value. The results show that, in general, n values increase with the level of geomagnetic activity. In principle, ultrarelativistic electrons respond to geomagnetic storms by becoming more peaked at 90 pitch angle with n values of 2-3 as a supportive signature of chorus acceleration outside the plasmasphere. High n values also exist inside the plasmasphere, being localized adjacent to the plasmapause and exhibiting energy dependence, which suggests a significant contribution from electromagnetic ion cyclotron (EMIC) wave scattering. During quiet periods, n values generally evolve to become small, i.e., 0-1. The slow and long-term decays of the ultrarelativistic electrons after geomagnetic storms, while prominent, produce energy and L-shell-dependent decay time scales in association with the solar and geomagnetic activity and wave-particle interaction processes. At lower L shells inside the plasmasphere, the decay time scales τd for electrons at REPT energies are generally larger, varying from tens of days to hundreds of days, which can be mainly attributed to the combined effect of hiss-induced pitch angle scattering and inward radial diffusion. As L shell increases to L~3.5, a narrow region exists (with a width of ~0.5L), where the observed ultrarelativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, τd generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the sinnα function fitting and the estimate of decay time scale offers a convenient and useful means to evaluate the underlying physical processes that play a role in driving the acceleration and loss of ultrarelativistic electrons and to assess their relative contributions. Key Points Ultrarelativistic electron PADs vary dynamically during storms Ultrarelativistic electron fluxes show slow poststorm decays The analyses are useful to evaluate the underlying physic

Variability of the pitch angle distribution of radiation belt ultrarelativistic electrons during and following intense geomagnetic storms: Van Allen Probes observations

Fifteen months of pitch angle resolved Van Allen Probes Relativistic Electron-Proton Telescope (REPT) measurements of differential electron flux are analyzed to investigate the characteristic variability of the pitch angle distribution of radiation belt ultrarelativistic (>2MeV) electrons during storm conditions and during the long-term poststorm decay. By modeling the ultrarelativistic electron pitch angle distribution as sinnα, where α is the equatorial pitch angle, we examine the spatiotemporal variations of the n value. The results show that, in general, n values increase with the level of geomagnetic activity. In principle, ultrarelativistic electrons respond to geomagnetic storms by becoming more peaked at 90 pitch angle with n values of 2-3 as a supportive signature of chorus acceleration outside the plasmasphere. High n values also exist inside the plasmasphere, being localized adjacent to the plasmapause and exhibiting energy dependence, which suggests a significant contribution from electromagnetic ion cyclotron (EMIC) wave scattering. During quiet periods, n values generally evolve to become small, i.e., 0-1. The slow and long-term decays of the ultrarelativistic electrons after geomagnetic storms, while prominent, produce energy and L-shell-dependent decay time scales in association with the solar and geomagnetic activity and wave-particle interaction processes. At lower L shells inside the plasmasphere, the decay time scales τd for electrons at REPT energies are generally larger, varying from tens of days to hundreds of days, which can be mainly attributed to the combined effect of hiss-induced pitch angle scattering and inward radial diffusion. As L shell increases to L~3.5, a narrow region exists (with a width of ~0.5L), where the observed ultrarelativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, τd generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the sinnα function fitting and the estimate of decay time scale offers a convenient and useful means to evaluate the underlying physical processes that play a role in driving the acceleration and loss of ultrarelativistic electrons and to assess their relative contributions. Key Points Ultrarelativistic electron PADs vary dynamically during storms Ultrarelativistic electron fluxes show slow poststorm decays The analyses are useful to evaluate the underlying physic

Variability of the pitch angle distribution of radiation belt ultrarelativistic electrons during and following intense geomagnetic storms: Van Allen Probes observations

Fifteen months of pitch angle resolved Van Allen Probes Relativistic Electron-Proton Telescope (REPT) measurements of differential electron flux are analyzed to investigate the characteristic variability of the pitch angle distribution of radiation belt ultrarelativistic (>2MeV) electrons during storm conditions and during the long-term poststorm decay. By modeling the ultrarelativistic electron pitch angle distribution as sinnα, where α is the equatorial pitch angle, we examine the spatiotemporal variations of the n value. The results show that, in general, n values increase with the level of geomagnetic activity. In principle, ultrarelativistic electrons respond to geomagnetic storms by becoming more peaked at 90 pitch angle with n values of 2-3 as a supportive signature of chorus acceleration outside the plasmasphere. High n values also exist inside the plasmasphere, being localized adjacent to the plasmapause and exhibiting energy dependence, which suggests a significant contribution from electromagnetic ion cyclotron (EMIC) wave scattering. During quiet periods, n values generally evolve to become small, i.e., 0-1. The slow and long-term decays of the ultrarelativistic electrons after geomagnetic storms, while prominent, produce energy and L-shell-dependent decay time scales in association with the solar and geomagnetic activity and wave-particle interaction processes. At lower L shells inside the plasmasphere, the decay time scales τd for electrons at REPT energies are generally larger, varying from tens of days to hundreds of days, which can be mainly attributed to the combined effect of hiss-induced pitch angle scattering and inward radial diffusion. As L shell increases to L~3.5, a narrow region exists (with a width of ~0.5L), where the observed ultrarelativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, τd generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the sinnα function fitting and the estimate of decay time scale offers a convenient and useful means to evaluate the underlying physical processes that play a role in driving the acceleration and loss of ultrarelativistic electrons and to assess their relative contributions. Key Points Ultrarelativistic electron PADs vary dynamically during storms Ultrarelativistic electron fluxes show slow poststorm decays The analyses are useful to evaluate the underlying physic

Source and seed populations for relativistic electrons: Their roles in radiation belt changes

Strong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August-September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13-22 September, initiated by a short-lived geomagnetic storm and characterized by a long period of primarily northward interplanetary magnetic field (IMF), showed strong depletion of relativistic electrons (including an unprecedented observation of long-lasting depletion at geostationary orbit) while an immediately preceding, and another immediately subsequent, storm showed strong radiation belt enhancement. We demonstrate with these data that two distinct electron populations resulting from magnetospheric substorm activity are crucial elements in the ultimate acceleration of highly relativistic electrons in the outer belt: the source population (tens of keV) that give rise to VLF wave growth and the seed population (hundreds of keV) that are, in turn, accelerated through VLF wave interactions to much higher energies. ULF waves may also play a role by either inhibiting or enhancing this process through radial diffusion effects. If any components of the inner magnetospheric accelerator happen to be absent, the relativistic radiation belt enhancement fails to materialize. Key Points Source/seed energy electrons required to produce MeV radiation belt energization Substorm injections lead to VLF wave growth, producing MeV acceleration ULF waves may enhance loss/acceleration due to increased outward/inward diffusio

Enhancing Students` Speaking Skill through Task-Based Language Teaching (TBLT) at English Tadris Department of STAIN Kerinci

This paper reports a classroom action research which conducted in an EFL classroom. The problem of this study is that teaching and learning process tends to be monotonous due to the single method used by English teachers. The learners` speaking course is familiarized with English structures. It requires a communicative and constructive method such as TBLT. The purpose of this study is to describe the effectiveness of TBLT in enhancing students` speaking skill. 30 EFL learners at the seventh semester at STAIN Kerinci took a part in this study. The instruments used to collect the data were speaking test, observation, and field-note. The results of the study showed that there were 2 cycles needed to implement the method. The process of teaching and learning in the first cycle indicates that TBLT improved learners` speaking skill, though some problems were needed to be solved. Unlike the cycle I, the process in the cycle II was improved in term of learners` speaking score and their motivation to attend the course if compared with those in cycle I. So, it is concluded that TBLT is an appropriate method to improve learners` speaking skill

Source and seed populations for relativistic electrons: Their roles in radiation belt changes

© 2015. American Geophysical Union. All Rights Reserved. Strong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August-September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13-22 September, initiated by a short-lived geomagnetic storm and characterized by a long period of primarily northward interplanetary magnetic field (IMF), showed strong depletion of relativistic electrons (including an unprecedented observation of long-lasting depletion at geostationary orbit) while an immediately preceding, and another immediately subsequent, storm showed strong radiation belt enhancement. We demonstrate with these data that two distinct electron populations resulting from magnetospheric substorm activity are crucial elements in the ultimate acceleration of highly relativistic electrons in the outer belt: the source population (tens of keV) that give rise to VLF wave growth and the seed population (hundreds of keV) that are, in turn, accelerated through VLF wave interactions to much higher energies. ULF waves may also play a role by either inhibiting or enhancing this process through radial diffusion effects. If any components of the inner magnetospheric accelerator happen to be absent, the relativistic radiation belt enhancement fails to materialize