Displays

Whistler-mode chorus emissions are generated at the equator in the parallel direction to the magnetic field, and propagate toward higher latitudes changing the wave normal angle gradually to oblique directions. Interaction between the wave and energetic electrons through Landau resonance becomes effective in the oblique propagation. As observed from the guiding center of a Landau resonant electron moving with the parallel phase velocity, the wave phase becomes stationary. With the perpendicular wave number and the deviation of the particle position from the guiding center, the electron see a wave phase of a right-handed circulary polaraized wave, which causes efficient acceleration by the perpendicular component of the wave electric field [1]. The interaction time between the resonant electron and the wave packet is maximized with the frequency close to half the cyclotron frequency, because the parallel phase velocity becomes nearly equal to the parallel group velocity. The efficient acceleration of resonant electrons causes damping of the wave at half the cyclotron frequency. Although our previous model assumed that the nonlinear wave damping was due to the parallel wave electric field in the presence of the gradient of magnetic field [2], we have confirmed that the nonlinear trapping due to the perpendicular components of the wave fields plays the major role in the electron acceleration and resultant wave damping in the nonuniform magnetic field [3]. In addition to the nonlinear damping, propagation characteristics of upper and and lower band chorus wave packets are much different. The Gendrin angle, at which the group velocity takes the parallel direction, exists only for the lower band chorus, while the group velocity of the upper band chorus takes highly oblique directions [4], and this difference enhances separation of the two bands in space. A single chorus element can be generated at the equator forming a long-lasting rising tone emission covering half the cyclotron frequency. As the wave packet propagates away from the equator, it splits into lower band and upper band wave packets because of the nonlinear damping through Landau resonance at half the cyclotron frequency, and the wave packets propagate in different directions.

References 
[1] Omura, Y., Hsieh, Y.-K., Foster, J., et al., (2019),  Cyclotron acceleration of relativistic electrons through Landau resonance with obliquely propagating whistler-mode chorus emissions, J. Geophys., Res.: Space Physics, 124, 2795–2810.
[2] Omura, Y., Hikishima, M., Katoh, Y., et al.. (2009), Nonlinear mechanisms of lower band and upper band VLF chorus emissions in the magnetosphere, J. Geophys. Res., 114, A07217.
[3] Hsieh, Y.-K., & Omura, Y. (2018), Nonlinear damping of oblique whistler mode waves via Landau resonance, J. Geophys. Res.: Space Physics,123, 7462–7472. 
[4] Hsieh, Y.-K, & Omura, Y. (2017). Study of wave-particle interactions for whistler mode waves at oblique angles by utilizing the gyroaveraging method, Radio Science, 52, 1268–1281.

How to cite: Omura, Y. and Hsieh, Y.-K.: Generation of lower band and upper band whistler-mode chorus emissions and associated electron acceleration in the inner magnetosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2143, https://doi.org/10.5194/egusphere-egu2020-2143, 2020.

Electromagnetic ion cyclotron (EMIC) waves are found to be most prevalent during geomagnetic storms and solar wind pressure pulses which provide the necessary free energy for the wave growth. However, they have also been regularly observed
in the absence of these two drivers. These non-storm time and non-pressure pulse EMIC events are very well associated with individual night side injections during substorms. However, not all substorm injections elicit wave activity. Our study aims to determine which substorm trigger wave activity. EMIC events excited during substorm injections are examined and various plasma parameters that are responsible for wave growth are studied. We find that injections that are associated with EMIC waves are also associated with enhanced high latitude ionospheric convection, which are manifestations of strong magnetospheric electric fields. The convective signatures occur at local times similar to those of the observed wave activity.

How to cite: Bhanu, R., Sibeck, D., Ruohoniemi, M., Kunduri, B., Halford, A., Reeves, G., and Reddy, V.: Ion injection triggered EMIC wave activity and its association with enhanced convection periods , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8921, https://doi.org/10.5194/egusphere-egu2020-8921, 2020.

Plasmasphere is a torus of cold plasma surrounding the Earth and is a very dynamic region. Its dynamics is driven by space weather. Having an accurate model of the plasmasphere is very important for wave-particle interactions and radiation belt modeling. In recent years, feedforward neural networks (NNs) have been successfully applied to reconstruct the global plasmasphere dynamics in the equatorial plane [Bortnik et al., 2016, Zhelavskaya et al., 2017, Chu et al., 2017]. These neural network-based models have been able to capture the large-scale dynamics of the plasmasphere, such as plume formation and the erosion of the plasmasphere on the night side. However, NNs have one limitation. When data is abundant, NNs perform really well. In contrast, when the coverage is limited or non-existent, as during geomagnetic storms, NNs do not perform well. The reason is that since these data are underrepresented in the training set, NNs cannot learn from the limited number of examples. This limitation can be overcome by employing physics-based modeling during such intervals. Physics-based models perform stably during high geomagnetic activity time periods if initialized and configured correctly. In this work, we show the combined approach to model the global plasmasphere dynamics that utilizes advantages of both neural network- and physics-based modeling and produces accurate global plasma density reconstruction during extreme events. We present examples of the global plasma density reconstruction for a number of extreme geomagnetic storms that occured in the past including the Halloween storm in 2003. We validate the global density reconstructions by comparing them to the IMAGE EUV images of the He+ particles distribution in the Earth’s plasmasphere for the same time periods.

How to cite: Zhelavskaya, I., Aseev, N., Shprits, Y., and Spasojevic, M.: A combined neural network- and physics-based approach for modeling the plasmasphere dynamics , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16970, https://doi.org/10.5194/egusphere-egu2020-16970, 2020.

Interplanetary coronal mass ejections (ICMEs) can dramatically affect electrons in the outer radiation belt. Electron energy flux and location varies over a range of timescales during these events, depending on ICME characteristics. This highly complex response means that electron flux within the outer radiation belt and precipitation into the upper atmosphere during ICMEs is not yet fully understood. This study analyses the electron response to two ICMEs, which occurred near the maximum of Solar Cycle 24. Both ICMEs had leading shocks and sheaths, followed by magnetic flux ropes in the ejecta. The magnetic field in these flux ropes rotated throughout the events, with opposite rotation in each event. The field rotated from south to north during the first event, while the second event had rotation from north to south. Data from Van Allen Probes were used to study electron flux variation in the outer radiation belt, while POES data were used for electron precipitation into the upper atmosphere. Qualitative analysis of these data was carried out in order to characterise the temporal and spatial variations in electron flux and precipitation throughout these two events, with particular focus on the effects of the sheath and rotating magnetic field in the ICME ejecta. In both events, we observe enhanced precipitation at mid-latitudes during the southward portion of the ejecta, with greater enhancements taking place in lower energy electron populations. By contrast, flux of outer radiation belt electron populations differs significantly between the two ICMEs, highlighting the complexity of the electron flux response to these space weather events.

How to cite: George, H. E., Kilpua, E., Osmane, A., Asikainen, T., Rodger, C. J., Kalliokosi, M., and Palmroth, M.: Electron Flux and Precipitation During ICME Case Studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5002, https://doi.org/10.5194/egusphere-egu2020-5002, 2020.

Particle precipitation originated from the magnetosphere provides important energy source to the upper atmosphere, leading to ionization and enhancement of conductivity, which in turn changes the electric potential in the MI system to influence the plasma convection in the magnetosphere. In this study, we simulate ring current particle precipitation caused by several important loss mechanisms, including electron precipitation due to whistler wave scattering, ion precipitation due to EMIC wave diffusion and field line curvature scattering. These physical mechanisms are implemented in the kinetic ring current model via diffusion equation with associated pitch angle diffusion coefficients. The precipitation is subsequently input to a two-stream transport model at the top of ionosphere in order to examine its impact on the ionsopheric conductivity. It is found that during intense storm time, electron precipitation of tens of keV dominates in the dawn sector and leads to significant enhancement of conductivity at low altitudes. On the other hand, proton precipitation on the nightside mostly occurs for energy below 10 keV, and contributes to ionization above 100 km, resulting in enhancement of conductivity there. Consequently, the height profile of both Pedersen and Hall conductivity exhibits two layers, potentially complicating the current closure in the ionosphere system.

How to cite: Yu, Y., Tian, X., Zhu, M., and Pr, S.: Modeling particle precipitation and effects on the ionospheric conductivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3181, https://doi.org/10.5194/egusphere-egu2020-3181, 2020.

The energetic particles of the outer radiation belt are highly variable in space, time and energy, due to the complex interplay between various mechanisms that contribute to their energization and/or loss. Previous studies have focused on the influence of solar wind and magnetospheric processes on the electron population dynamics, showing that the eventual effect of the various interplanetary drivers results from different combinations of IMF and solar wind parameters. Yet, all of these studies were limited in temporal, spatial and energy coverage. In this work, we take advantage of a large dataset, which includes multipoint measurements of electron fluxes covering a large energy range and various orbits (e.g. Van Allen Probes, GOES, HIMAWARI, SREM monitors, etc.), as well as approximately the whole solar cycle 24 to deduce specific interplanetary parameter schemes that drive enhancements or depletions of relativistic electrons in the outer radiation belt. Our study also investigates parameters which are correlated to the Solar Energetic Particle (SEP) environment with the long-term goal of connecting the two sets of results for coherent merging of environment models.

This work is supported by ESA’s Science Core Technology Programme (CTP) under contract No. 4000127282/19/IB/gg.

How to cite: Katsavrias, C., Nasi, A., Papadimitriou, C., Aminalragia-Giamini, S., Sandberg, I., Jiggens, P., and Daglis, I. A.: Identification of interplanetary parameter schemes which drive the variability of the magnetospheric radiation environment , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-632, https://doi.org/10.5194/egusphere-egu2020-632, 2020.

The electron population inside Earth’s outer radiation belt is highly variable and typically linked to geomagnetic activity such as storms and substorms. These variations can differ with radial distance, such that the fluxes at the outer boundary are different from those in the heart of the belt. Using data from the Proton Electron Telescope (PET) on board NASA’s Solar Anomalous Magnetospheric Particle Explorer (SAMPEX), we have examined the correlation between electron fluxes at all L's within the radiation belts for a range of geomagnetic conditions, as well as longer-term averages. Our analysis shows that fluxes at L≈2-4 and L≈4-10 are well correlated within these regions, with coefficients in excess of 80%, however, the correlation between these two regions is low. These correlations vary between storm-times and quiet-times. We examine whether, and to what extent this correlation is related to the level of enhancement of the outer radiation belt during geomagnetic storms, and whether the plasmapause plays any role defining the different regions of correlated flux.

How to cite: Walton, S., Forsyth, C., Rae, I. J., Watt, C., Horne, R., Thompson, R., Rodger, C., Clilverd, M., and Walach, M.: How Coherent are Flux Variations in the Outer Van Allen Radiation Belt?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20204, https://doi.org/10.5194/egusphere-egu2020-20204, 2020.

Drifting electron holes (DEHs), manifesting as sudden but mild dropout in electron flux, are a common phenomenon seen in the Earth's magnetosphere. It manifests the change of the state of the magnetosphere. However, previous studies primarily focus on DEHs during geomagnetically active time (e.g., substorm). Not until recently have quiet time DEHs been reported. In this paper, we present a systematic study on the quiet time DEHs. BeiDa Imaging Electron Spectrometer (BD-IES) measurements from 2015 to 2017 are investigated. Twenty-two DEH events are identified. The DEHs cover the whole energy range of BD-IES (50–600 keV). Generally, the DEHs are positively dispersive with respect to energy. Time-of-flight analysis suggests the dispersion results from electron drift motion and gives the location where the DEHs originated from. Statistics reveal the DEHs primarily originated from the postmidnight magnetosphere. In addition, superposed epoch analysis applied to geomagnetic indices and solar wind parameters indicates these DEH events occurred during geomagnetically quiet time. No storm or substorm activity could be identified. However, an investigation into nightside midlatitude ground magnetic records suggests these quiet time DEHs were accompanied by Pi2 pulsations. The DEH-Pi2 connection indicates a possible DEH-bursty bulk flow (BBF) connection, since nightside midlatitude Pi2 activity is generally attributed to magnetotail BBFs. This connection is also supported by a case study of coordinated magnetotail observations from Magnetospheric Multiscale spacecraft. Therefore, we suggest the quiet time DEHs could be caused by magnetotail BBFs, similar to the substorm time DEHs.

How to cite: Liu, Z.-Y., Zong, Q.-G., and Zou, H.: Drifting Electron Holes Occurring During Geomagnetically Quiet Times: BD-IES Observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1987, https://doi.org/10.5194/egusphere-egu2020-1987, 2020.

We revisit the typical interplanetary shock event on November 7, 2004, with high resolution total electron content (TEC) measurements obtained by the distributed Global Navigation Satellite System (GNSS) receivers. TEC impulses were observed after the IP shock impinged on the dayside agnetosphere at ~18:27 UT. In view of the similarity of the wave form and the time-delay characteristics, the TEC impulses were regarded as responses to the IP shock, despite the small amplitude (in the order of 0.4 TECU). Particularly, the peak of the TEC impulse was first observed by the receivers located around 120°W geographic longitude (corresponding to noon magnetic local time), while receivers at both sides recorded the impulse sequentially afterwards. From the timedelay of the TEC impulse, we derive the propagation velocity of the shock induced pulse. The angular velocity of the pulse is estimated to be ~2 degree per second, which is in the same order as the propagation speed of a typical shock pulse in the magnetosphere. Our results present global observational features of the shock pulse and provide new aspects to understand the ionospheric-magnetospheric dynamics in response to IP shocks.

How to cite: Chen, X., Li, Q., Zong, Q., and Hao, Y.: A New Approach to Monitoring the Interplanetary Shock Induced Pulse: TEC Measurements by the GNSS Receiver Network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6462, https://doi.org/10.5194/egusphere-egu2020-6462, 2020.

Radial diffusion is one of the dominant physical mechanisms that drives acceleration andloss of the radiation belt electrons due to wave-particle interactions with ultra-low frequency (ULF) waves, which makes it very important for radiation belt modeling and forecasting.  We investigate the sensitivity of several parameterizations of the radial diffusion including Brautigam and Albert (2000), Ozeke et al. (2014), Ali et al. (2016), and Liu et al. (2016) on long-term radiation belt modeling using the Versatile Electron Radiation Belt (VERB) code.  Following previous studies, we first perform 1-D radial diffusion simulations.  To take into account effects of local acceleration and loss, we perform additional 3-D simulations, including pitch-angle, energy and mixed diffusion.

How to cite: Drozdov, A., Allison, H., Shprits, Y., and Aseev, N.: A Comparison of Radial Diffusion Coefficients in 1-D and 3-D Long-Term Radiation Belt Simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17757, https://doi.org/10.5194/egusphere-egu2020-17757, 2020.

Modeling and observations have shown that energy diffusion by chorus waves is an important source of acceleration of electrons to relativistic energies. By performing long‐term simulations using the three‐dimensional Versatile Electron Radiation Belt (VERB-3D) code, we test how the latitudinal dependence of chorus waves can affect the dynamics of the radiation belt electrons. Results show that the variability of chorus waves at high latitudes is critical for modeling of megaelectron volt (MeV) electrons. We show that, depending on the latitudinal distribution of chorus waves under different geomagnetic conditions, they cannot only produce a net acceleration but also a net loss of MeV electrons. Decrease in high‐latitude chorus waves can tip the balance between acceleration and loss toward acceleration, or alternatively, the increase in high‐latitude waves can result in a net loss of MeV electrons. Variations in high‐latitude chorus may account for some of the variability of MeV electrons.

Our simulation results also show that the position of the plasmapause plays a significant role in the dynamic evolution of relativistic electrons. The magnetopause shadowing effect is included by using last closed drift shell (LCDS), and it is shown to significantly contribute to the dropouts of relativistic electrons at high L*.

How to cite: Wang, D., Shprits, Y., Zhelavskaya, I., Drozdov, A., Aseev, N., Effenberger, F., Castillo, A., and Cervantes, S.: Controlling Effect of Wave Models and Plasma Boundaries on the Dynamic Evolution of Relativistic Radiation Belt Electrons, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17946, https://doi.org/10.5194/egusphere-egu2020-17946, 2020.

We present a statistical study of energy-dependent and L shell-dependent inner boundary of the outer radiation belt during 37 isolated geomagnetic storms using observations from Van Allen Probes from 2013 to 2017. There are mutual transformations between "V-shaped" and "S-shaped" inner boundaries during different storm phases, resulting from the competition among electron loss, radial transport and local acceleration. The radial position, onset time, E_st (the minimum energy at L_st where the inner boundary starts to exhibit an S-shaped form), and the radial width of S-shaped boundary (ΔL) are quantitatively defined according to the formation of a reversed energy spectrum (electron flux going up with increasing energies from hundreds of keV to ~1 MeV) from a kappa-like spectrum (electron flux steeply falling with increasing energies). The case and statistical results present that (1) The inner boundary has repeatable features associated with storms: the inner boundary is transformed from S-shaped to V-shaped form in several hours during the storm commencement and main phase, and retains in the V-shaped form for several days until it evolves into S-shaped during late recovery phase; (2) ΔL shows positive correlation with SYM-H index; (3) The duration of the V-shaped form is positively correlated with the storm intensity and the duration of the recovery phase; (4) The minimum energy E_st are mainly distributed in the range of 100-550 keV. All these findings have important implications for understanding the dynamics of energetic electrons in the slot region and the outer radiation belt during geomagnetic storms.

How to cite: Shi, X., Ren, J., and Zong, Q.: The dynamics of the inner boundary of the outer radiation belt during geomagnetic storms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19607, https://doi.org/10.5194/egusphere-egu2020-19607, 2020.

The evolution of chorus waves is important in the inner magnetosphere since it is closely related to the loss and acceleration of radiation belt electrons. In this study, we develop neural-network-based models for upper-band chorus (UBC; 0.5 f_ce < f <  f_ce ) waves and lower-band chorus (LBC; 0.05 f_ce < f < 0.5 f_ce) waves, where f_ce is the equatorial electron gyrofrequency. We establish a root-mean-square amplitude database for both UBC and LBC using Van Allen Probe levels 2 and 3 data products from the EMFISIS payload between October 1, 2012 and January 14, 2018. Based on the database, we construct an artificial neural network with corresponding L, magnetic local time, magnetic latitude, solar wind parameters and geomagnetic indices on different time windows as model inputs. Additionally, we adopt several different feature selection techniques to determine the most important features of magnetospheric chorus waves, reduce training or running time and improve the model accuracy. Our study suggests that the model results using the machine learning technique have the great potential to highly improve current understanding of the radiation belt dynamics.

How to cite: Guo, Y., Ni, B., Wang, D., Shprits, Y., Fu, S., Cao, X., and Gu, X.: A Neural Network Model of Three-dimensional Magnetospheric Chorus Waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19053, https://doi.org/10.5194/egusphere-egu2020-19053, 2020.

Electron pitch angle distribution (PAD) is a critical parameter in the study of the dynamics of the radiation belt electrons. It is well known that solar wind pressure has an impact on the PAD of the geomagnetically trapped electrons. Using the Van Allen Probes' data, we find that the MeV electron PAD at 4.5<L*<5.5 became narrowing (PAD is mainly concentrated at 90 degree) for over three days during a prolonged enhancement of the solar wind number density on November 27-30, 2015. During that period, the EMIC waves are observed by Van Allen Probe-A and ground stations on the afternoon and dusk MLTs at L>4. Meanwile, the precipitations of tens of keV protons and MeV electrons are observed by POES satellites. Additionally, there is a growing dip in electron phase space density at L*~5, indicating a local loss caused by the wave-particle interaction. The narrowing of the electron PAD is energy-dependent and the PAD is more anisotropic for electrons with higher energy, which is consistent with the wave-particle interaction with the EMIC waves. Furthermore, previous studies have shown that high solar wind density can lead to a hot and dense plasma sheet. The inward penetration of a dense plasma-sheet down to 4 Re has been confirmed by THEMIS spacecraft. We suggest that the overlap of the plasma sheet and the plasmasphere provide a favorable condition for exciting EMIC waves and the loss of small pitch angle electrons by EMIC waves can lead to the electron PAD narrowing. 

How to cite: Xie, L., Xiong, Y., Fu, S., and Pu, Z.: Ultra-relativistic electron’s pitch angle distribution narrowing associated with the solar wind density enhancement, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13017, https://doi.org/10.5194/egusphere-egu2020-13017, 2020.

Much evidence has indicated that charge exchange with the neutral atoms is an important loss mechanism of the ring current ions, especially during the slow recovery phase of a geomagnetic storm. Most of the studies, however, were focused on the global effect of the charge exchange on the ring current decay. The effect on different magnetic local times and L shells has not been achieved. In this study, based on the in-situ energetic ion data (Level 3) from RBSPICE onboard two Van Allen Probes, we study the contribution of the charge exchange, calculated from the differential flux of ions, to the local ring current decay at different magnetic local times and radial distance. Results indicate that the charge exchange effect on the ring current decay shows clear MLT and L dependence. Our study provides important information of spatial distribution of the ring current loss evolution, which could be as a reference during the ring current modeling.

How to cite: Li, S. and Luo, H.: MLT Dependence of Contribution of Charge Exchange Loss to the Storm Time Ring Current Decay: Van Allen Probes Observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21176, https://doi.org/10.5194/egusphere-egu2020-21176, 2020.

Magnetospheric chorus is known to play a significant role in the acceleration and loss of radiation belt electrons. Interactions of chorus waves with radiation belt particles are commonly evaluated using quasi-linear diffusion codes that rely on statistical models, which might not accurately provide the instantaneous global wave distribution from limited in-situ wave measurements. Thus, a novel technique capable of inferring wave amplitudes from POES particle measurements, with an extensive coverage of L-shell and magnetic local time, has been established to obtain event-specific, global dynamic evolutions of chorus waves. This study, using 5 years of POES electron data, further improves the technique, and enables us to subsequently infer the chorus wave amplitudes for all useful data points (removing the electrons which were in the drift loss cone) and to construct the global distribution of lower-band chorus wave intensity. The results obtained from the improved technique reproduce Van Allen Probes in-situ observations of chorus waves reasonably well and reconstruct the major features of the global distribution of chorus waves. We demonstrate that such a data-based, dynamic model can provide near-real-time estimates of chorus wave intensity on a global scale for any time period when POES data are available, which cannot be obtained from in-situ wave measurements by equatorial satellites alone, but is crucial for quantifying the  dynamics of the radiation belt electrons.

How to cite: Zhang, Y., Ni, B., Gu, X., Shprits, Y., Fu, S., Cao, X., and Xiang, Z.: Global Morphology of Lower-band Chorus Wave Intensity Reconstructed Using multi-year POES Electron Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19284, https://doi.org/10.5194/egusphere-egu2020-19284, 2020.

Corotation electric field is important in the inner magnetosphere topology, which was usually calculated by assuming 24h corotation period. However, some studies suggested that plasmasphere corotation lag exists which leads to the decrease of corotation electric field. In this study, we use electric field measurements from Van Allen Probes mission from 2013 to 2017 to statistically calculate the distribution of large-scale electric field in the inner magnetosphere. A new method is subsequently developed to separate corotation electric field from convection electric field. Our research shows electric field is inversely proportional to the square of L, and, with the assumption of dipole magnetic field, the rotation period of plasmasphere is estimated as 27h, consistent to the results by Sandel et al. [2003] and Burch et al. [2004] with EUV imaging of the plasmasphere. Based on the research, a new empirical model of innermagnetospheric corotation electric field was estibalished, which is significant for a more accurate understanding the large-scale electric field in the inner magnetosphere.

How to cite: Liu, W. and Zhang, Z.: Calculation of corotation electric field based on Van Allen Probes measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7657, https://doi.org/10.5194/egusphere-egu2020-7657, 2020.

The tens of keV ion populations observed in the ring current region at L~ 3- 7, generally have pancake-shaped pitch angle distributions (PADs), that is, peaked at 90 degrees. These pancake PADs are formed due to a combination of betatron and Fermi acceleration when they are transported from the tail plasma sheet, where the major ring current plasma originates. However, in this study, by using the Van Allen Probe observations from 2012 to 2018 on the dayside, unexpectedly we have found that about 5% of the time, protons with energies of ~30 to 50 keV show two distinct populations according to their PADs, having an additional population of field-aligned ions overlapping with the original pancake population. The newly appearing field-aligned populations have higher occurrence rates at ~12-16 MLT during geomagnetically active times. In particular, we have studied eight such events in detail and traced back these ions to their source regions according to the energy-dependent dispersion signatures caused by the differences in drift velocities. We found that the source regions are located around 12 to 18 MLT which coincides with the high occurrence rate region of 12-16 MLT. Based on the ionospheric and LANL geosynchronous observations of these eight events, it is suggested that these energetic ions with field-aligned PADs most probably are accelerated in the post-noon sector in association with ionospheric disturbances that are triggered by tail injection. These results provide evidence of another important source of the ring current ions.

How to cite: Yue, C., Bortnik, J., Zou, S., Nishimura, Y., and Foster, J. C.: Episodic occurrence of field-aligned energetic ions on the dayside , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1294, https://doi.org/10.5194/egusphere-egu2020-1294, 2020.

In the Earth's inner magnetosphere, charged particles can be accelerated and transported by ultralow frequency (ULF) waves via drift resonance. We investigate the effects of magnetospheric convection on the nonlinear drift resonance process, which provides an inhomogeneity factor S to externally drive the pendulum equation that describes the particle motion in the ULF wave  field. The S factor, defined as the ratio of the driving amplitude to the square of the pendulum trapping frequency, is found to vary with magnetic local time and as a consequence, oscillates quasi-periodically at the particle drift frequency. To better understand the particle behavior governed by the driven pendulum equation, we carry out simulations to obtain the evolution of electron distribution functions in energy and L-shell phase space. We find that resonant electrons can remain trapped by the low-m ULF waves under strong convection electric  field, whereas for high-m ULF waves, the electrons trajectories can be significantly modified. More interestingly, the electron drift frequency is close to the nonlinear trapping frequency for intermediate-m ULF waves, which corresponds to chaotic motion of resonant electrons. These  findings shed new light on the nature of particle coherent and diffusive transport in the inner magnetosphere.

How to cite: Zhou, X., Li, L., Omura, Y., Zong, Q., Fu, S., Rankin, R., and Degeling, A.: Roles of magnetospheric convection on nonlinear drift resonance between electrons and ULF waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2034, https://doi.org/10.5194/egusphere-egu2020-2034, 2020.

Previous studies have demonstrated that the field line resonance (FLR) frequencies detected on closed magnetospheric field lines can be used to estimate the plasma mass density in the inner magnetosphere. This method, also known as “normal-mode magnetoseismology,” can act as a virtual instrument that turns spacecraft measurements of magnetic and/or electric field into plasma mass density, which is a fundamental physical quantity that is difficult to measure directly but important to investigations involving the MHD timescales, reconnection rates, or instability/wave growth rates.

In this study, we use normal-mode magnetoseismology to help investigate the characteristics of the oxygen torus, which is the narrow region of enhanced O+ density in the vicinity of the plasmapause that may form during the storm recovery phase. The formation of the oxygen torus is still an outstanding question, and the geomagnetic mass spectrometer effect and the direct ring current heating of the ionosphere have been proposed as two possible causes. We identify the location and timing of oxygen torus occurrence by examining the FLR-inferred plasma mass densities in Magnetospheric Multiscale (MMS) and Van Allen Probes (RBSP) observations and compare them with the charge densities derived from the upper hybrid resonance frequency detected by the respective plasma wave experiments on the spacecraft. We find that, while MMS and RBSP could both observe clear enhancements of heavy ions during a magnetic storm, the degree and the width of O+ enhancement can vary with location. The timing of oxygen torus occurrence may differ from storm to storm. In RBSP measurements, we also compare the bulk densities with the partial densities of low-energy ions detected by the HOPE instrument. While the average ion mass can be greater for 30 eV – 1 keV ions than that for the bulk plasma in the oxygen torus, it is evident that the majority of the ions in the oxygen torus are below 30 eV, confirming the need to examine the bulk mass and charge densities through electromagnetic sounding methods.

How to cite: Wilcox, B., Chi, P., Takahashi, K., and Denton, R.: Normal-mode Magnetoseismology as a Virtual Plasma Mass Density Instrument and Its Use in Investigation of Oxygen Torus during Magnetic Storms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12323, https://doi.org/10.5194/egusphere-egu2020-12323, 2020.

The SuperDARN Hokkaido Pair (HOP) of radars data with special operation modes are used to study the wavy variations of plasma flow embedded in larger-scale, fast flow structures at subauroral latitudes (SAPS). Because of the limited number of examples studied so far, their generation mechanism is not fully understood yet. In this paper we focus mainly on the events on Sep 08, 2017 and Aug. 26, 2018. Both events occurred near the peak of large geomagnetic storms. These events were registered by the SuperDARN radars with higher temporal resolution (3 and 12 seconds respectively) camping beams. Using both camping beam data and 2-dimensional data (with 1 to 2 min temporal resolution) enable us to examine the period, wavelength and propagation speed of these wave structures. In addition, using the data with the new fitting algorithm (fitacf Ver. 3) we have more extended coverage of the echo regions. We notice that  both events were observed during geomagnetic storms (minimum Dst: -124 nT and -174 nT) and the wave structures have limited spatial extent in magnetic local time. On the other hand, there are several differences between these events such as period, propagation speed and geomagnetic latitude. Their possible generation mechanisms will be discussed.

How to cite: Nishitani, N., Hori, T., and Teramoto, M.: Multi-event analysis of SAPS Wave Structures observed by the SuperDARN Hokkaido Pair of radars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4357, https://doi.org/10.5194/egusphere-egu2020-4357, 2020.

In the inner magnetosphere there are sharp plasma boundaries that can cause resonance cavities. The four Cluster satellites move in a string-of-pearls configuration at perigee (at L=4-5) so that they are spatially well separated but their separation is still short that at least some of them are simultaneously at different positions inside the cavity. In the presence of cavity resonance of a half wavelength, all spacecraft inside the cavity observe the same wave mode in the same phase. In this talk we analyze and present a number of cavity resonances observed by the Cluster spacecraft. Typical observed mode frequencies are between 4 - 14 mHz, depending on the size of the cavity. It appears that the occurrence of cavity resonances is well correlated with changes in geomagnetic activity and they are quite common. They tend to occur at 12-16 MLT and 21-23 MLT. These ULF waves may have a significant impact on radiation belt particles as they cover a large L shell range.

How to cite: Laakso, H.: Presence of cavity resonances in the inner magnetosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4580, https://doi.org/10.5194/egusphere-egu2020-4580, 2020.

Poynting flux energy is deposited from the magnetosphere in high latitudes, and measures the electromagnetic energy transmitted between the magnetosphere and the ionosphere. Little attention has been paid on the seasonal variation of the longitudinal pattern of the Poynting flux. Here, using long-term measurements of the ion drifts and the magnetic field by the DMSP satellite in the topside ionosphere, a statistical investigation of the longitudinal distributions of the Poynting flux in polar region during quiet times is conducted. Both case study and statistics show that there is a local maximum in downward Poynting flux in the pre-noon sector. Generally, the maximum is centered around geographic longitude of 120° west and geographic latitude of 80°, meaning that the total energy transferred into the ionosphere is the greatest in this region. The longitudinal distribution of the Poynting flux also exhibit clear seasonal variations with the longitudinal asymmetry the most significant in norther summer. The results could provide some new sights in future investigations of magnetosphere-ionosphere coupling in the polar region with observations and simulations.

How to cite: Zhang, X.-X., Yu, C., Wang, W., and He, F.: Variations of Poynting Flux in the Northern Hemisphere during Quiet Times, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6312, https://doi.org/10.5194/egusphere-egu2020-6312, 2020.