1) Electron Distributions in Kinetic Scale Field Line Resonances: A Comparison of Simulations and Observations.
Magnetic field lines emerge from the southern polar region of the Earth, curve upward and over in a circular fashion, then enter the northern polar region. These field lines can be populated with plasma and radiation belt particles. Field line resonances are wave-like motions of these field lines that resemble the motion of guitar strings, with favored pitches or frequencies that depend on their length and the properties of the plasma. These waves are important for coupling the upper atmosphere to electron populations in space, and for radiation belt dynamics. Comparing computer simulations of electrons with observations from Van Allen Probes, shows how field line resonances can cause particles and energy to be lost to the upper atmosphere. Improved understanding of this mechanism helps explain geomagnetic storm development.
Damiano P.A., Chaston C.C., Hull A.J., and Johnson J.R. (2018). Electron Distributions in Kinetic Scale Field Line Resonances: A Comparison of Simulations and Observations. Geophysical Research Letters. DOI: 10.1029/2018GL077748
2) Modeling the Depletion and Recovery of the Outer Radiation Belt During a Geomagnetic Storm: Combined MHD and Test Particle Simulations.
One way in which Van Allen Probes aids in the understanding of radiation belt formation is by providing observations to guide simulation model development. If a simulation model has accurately included the necessary physical processes then the simulation results should match the observations. Researchers can then assess how the various mechanisms contribute. Often, the outer radiation belt nearly disappears at the onset of a storm, only to come back afterwards, sometimes stronger. In order to understand such behavior, one such storm, the 2013 St. Patrick's Day Storm, was simulated and the results compared to Van Allen Probes observations. A global magnetohydrodynamic (MHD) code was used to simulate the electric and magnetic fields of the storm, which were then used to move and accelerate electron test particles. The results show how at the onset of the storm the existing belt is annihilated and swept into interplanetary space, and that during the course of the storm a new, stronger belt is created. The high‐resolution simulations show how lower energy electrons can become trapped and energized within bursty confined flow channels that erupt near Earth during the course of the storm. This is a critical new development in understanding radiation belt formation.
Sorathia K. A., Ukhorskiy A Y, Merkin V. G., Fennell J. F., and Claudepierre S G (2018). Modeling the Depletion and Recovery of the Outer Radiation Belt During a Geomagnetic Storm: Combined MHD and Test Particle Simulations Journal of Geophysical Research: Space Physics. DOI: 10.1029/2018JA025506
3) Understanding the Driver of Energetic Electron Precipitation Using Coordinated Multisatellite Measurements.
Energetic electrons can move from space down into the Earth's upper atmosphere and cause chemical changes in the atmosphere leading to ozone reduction. One physical process that can cause such electrons to be redirected down into the atmosphere is when they interact with a plasma wave. In this study Van Allen Probes provided wave measurements at high altitude in space, and a satellite called MetOp‐01 provided measurements of downward going electrons just above the atmosphere. At the same time that MetOp‐01 saw downward going electrons, a Van Allen Probes satellite observed strong electromagnetic ion cyclotron (EMIC) waves. The evidence shows that the observed electron precipitation is primarily driven by these EMIC waves. The electron precipitation was seen at very high relativistic energies, and also down to low auroral energies. These observations raise an interesting question because, while existing theories can explain how EMIC drive high energy electrons into the atmosphere, they cannot explain the low‐energy electron precipitation. A proper explanation will have to wait for improved theories or more observations.
Capannolo L., Li W, Ma Q, Zhang X.-J., Redmon R. J., et al. (2018). Understanding the Driver of Energetic Electron Precipitation Using Coordinated Multisatellite Measurements Geophysical Research Letters . DOI: 10.1029/2018GL078604
1) Determining Plasmaspheric Densities from Observations of Plasmaspheric Hiss.
The Earths’s plasmasphere is a region of dense, cool, low-energy plasma in space just above the Earth’s atmosphere near the Equator, and just inside the Earth’s radiation belts. The plasmasphere is populated by a variety of waves that depend on this dense cool plasma, and these waves impact the adjacent radiation belts. In order to understand the nature of these waves, and their impact on the radiation belts, scientists need to know the density of the plasma in the plasmasphere. Current methods of inferring this density only work 33% of the time. The authors have developed a new method based on the characteristics of plasmaspheric hiss, a kind of wave frequently seen in the plasmasphere. They used the previous method to calibrate the new method, which now works 79% of the time. This provides much better understanding of the conditions responsible for sculpting the Earth’s radiation belts.
Hartley D. P., Kletzing C A, De Pascuale S., Kurth W S, and ík O. (2018). Determining Plasmaspheric Densities from Observations of Plasmaspheric Hiss. Journal of Geophysical Research: Space Physics. DOI: 10.1029/2018JA025658
2) Evidence of Microbursts Observed Near the Equatorial Plane in the Outer Van Allen Radiation Belt.
Microbursts are a short impulsive increases of electrons precipitating into the upper atmosphere from the outer Van Allen radiation belts. They are believed to be an important process for radiation belt losses. One possible source of microbursts is scattering of trapped radiation belt electrons by a plasma wave called chorus. Chorus waves can both accelerate and scatter electrons into the atmosphere. Since microbursts have been previously observed by high-altitude balloons and low Earth orbiting spacecraft, there has been little evidence that directly link chorus waves and the microbursts generated. This study shows observations by the Van Allen Probes spacecraft of microbursts and the waves that cause them deep inside the outer radiation belt. The Van Allen Probes are configured to extensively study the wave and particle environment in the magnetosphere, which allows us to understand the physics of microbursts in detail. This unique perspective enables us to understand how these electrons were transported by the chorus wave, and compare it to a hypothesized quasi-linear diffusion model. The results indicate that the observed transport of microburst electrons was not consistent with the hypothesized diffusion model. This will motivate improvements of models of microbursts and our understanding of these processes.
Shumko Mykhaylo, Turner Drew L, O'Brien T P, Claudepierre Seth G., Sample John, et al. (2018). Evidence of Microbursts Observed Near the Equatorial Plane in the Outer Van Allen Radiation Belt. Geophysical Research Letters. DOI: 10.1029/2018GL078451
3) Nonlinear drift resonance between charged particles and ultra-low frequency waves: Theory and Observations.
In Earth's Van Allen radiation belts, ultra‐low frequency waves play a crucial role in accelerating charged particles via a process named drift resonance. When such a resonance occurs, an electron moves with the wave electric field like a surfer, and is accelerated in the process. In previous studies of drift resonance, a small wave approximation is used. In this study, the theory is extended into the large wave regime, and the characteristics of the accelerated electrons are calculated. Such newly predicted electron characteristics are found to agree with electron observations from NASA's Van Allen Probes. This provides the first identification of drift resonance in the large wave regime and highlights the importance of nonlinear effects in ULF wave‐particle interactions within the Van Allen radiation belts..
Li Li, Zhou Xu-Zhi, Omura Yoshiharu, Wang Zi-Han, Zong Qiu-Gang, et al. (2018). Nonlinear drift resonance between charged particles and ultra-low frequency waves: Theory and Observations. Geophysical Research Letters. DOI: 10.1029/2018GL079038
1) A Census of Plasma Waves and Structures Associated With an Injection Front in the Inner Magnetosphere.
The Electric Field and Waves instrument on Van Allen Probes observes many kinds of waves that can accelerate radiation belt electrons, or scatter them into the Earth’s atmosphere where they are lost. Among these waves are a variety of electric field structures that can only be identified in high time resolution observations that are only sporadically available. Since these high time resolution observations are usually triggered, briefly, when large signals are detected, there is a bias toward large events. This study used a rare interval when high resolution electric field data were available for 45 consecutive minutes to perform a more balanced survey. The wave properties found using this unbiased data are considerably different than those found previously, with significant implications for wave‐particle interactions that can lead to radiation belt formation and loss.
Malaspina, D. M., A. Ukhorskiy, X. Chu, and J. Wygan (2018). A Census of Plasma Waves and Structures Associated With an Injection Front in the Inner Magnetosphere. Journal of Geophysical Research: Space Physics, 123, 4. DOI: 10.1002/2017ja025005
2) Electrostatic Steepening of Whistler Waves.
Electric fields can accelerate electrons in space, causing the formation of radiation belts around Earth. Van Allen Probes observed whistler waves with substantial electric field power at harmonics of the whistler wave fundamental frequency. At the same time the ambient electrons were observed to be a mix of two different populations, one hot and one cold. Simulations show electric fields forming at such harmonics due to an interaction with a mixture of hot and cold electrons. This is a fundamental process that produces substantial electric fields, and explains one mechanism for electron acceleration in space.
Vasko, I. Y., O. V. Agapitov, F. S. Mozer, J. W. Bonnell, A. V. Artemyev, V. V. Krasnoselskikh, and Y. Tong (2018). Electrostatic Steepening of Whistler Waves. Physical Review Letters, 120, 19. DOI: 10.1103/PhysRevLett.120.195101
3) Electron nonlinear resonant interaction with short and intense parallel chorus wave-packets.
The interaction of electrons with chorus waves is responsible for major changes in the radiation belts. This interaction is traditionally estimated using a simple approximation that these waves are small. Recent Van Allen Probes observations of a large number of very intense chorus waves challenge this traditional description. A more complex formulation taking into account that the waves are large enough to interact with themselves produces similar effects, but occurring at a much faster rate. These waves accelerate radiation belt electrons much more quickly.
Mourenas D., Zhang X.-J., Artemyev A. V., Angelopoulos V, Thorne R M, et al (2018). Electron nonlinear resonant interaction with short and intense parallel chorus wave-packets. Journal of Geophysical Research: Space Physics. DOI: 10.1029/2018JA025417
1) Evidence for competing theories of pulsating aurora.
Pulsating aurora are flickering patches of diffuse light that appear in the night sky in the polar regions. This type of aurora is caused by the “flickering” precipitation of energetic electrons originating from high altitude and colliding with the upper atmosphere. The precipitating electrons are scattered Earthward by some type of wave. Two different types of waves have been proposed by two different teams. Mozer (2017) provides evidence that Time Domain Structures (a kind of wave) are responsible. Nishimura (2018) published a comment pointing to evidence from their earlier work that chorus waves are responsible, and Mozer (2018) replied. These studies examined a series of events to see which kind of waves were present when pulsating aurora were seen. Both comments try to look more carefully at the events in question, providing additional observations and analysis in an attempt to divine the wave mode responsible for producing pulsating aurora.
Mozer, F. S., O. V. Agapitov, A. Hull, S. Lejosne, and I. Y. Vasko (2017), Pulsating auroras produced by interactions of electrons and time domain structures, J. Geophys. Res. Space Physics, 122, 8604–8616, DOI: 10.1002/2017JA024223.
Nishimura, Y., Bortnik, J., Li, W., Angelopoulos, V., Donovan, E. F., & Spanswick, E. L. (2018). Comment on Pulsating auroras produced by interactions of electrons and time domain structures” by Mozer et al. Journal of Geophysical Research: Space Physics, 123, 2064–2070. DOI: 10.1002/2017JA024844
2) Electrons are injected farther inward toward the Earth because of an interaction with the upper atmosphere.
The interaction between the Sun and the Earth’s magnetic field often transmits significant energy to the electrons and protons in space. As a result, these particles move closer to the Earth and are “injected” into the near Earth space environment. Since the 1970s, scientists have theorized that electrons and protons with the same initial energy and starting from the same location will travel the same distance toward the Earth. Yet recent observations from the Van Allen Probes reveal that this is not the case! Electrons appear to be systematically injected “closer” than protons. So what happens along the way to make electrons approach Earth at closer distances? To answer this question, the authors combine observations from both space and the ground. They show that a localized source of electric field in the upper atmosphere called a SubAuroral Polarization Stream (“SAPS”) is always present during these injections. They argue that a SAPS acts like a marathon aid station for electrons in that it provides them with additional energy. As a result, electrons “move faster” and move in closer to Earth.
Lejosne, S., Kunduri, B. S. R., Mozer, F. S., & Turner, D. L. (2018). Energetic electron injections deep into the inner magnetosphere: A result of the subauroral polarization stream (SAPS) potential drop. Geophysical Research Letters, 45. DOI: 10.1029/2018GL077969
1) Van Allen teams up with other missions to study complex event
Van Allen teams up with the Magnetospheric Multiscale (MMS), Geotail, and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) missions to get a global picture of how charged particles are injected into near-Earth space. Waves of electrons and ions are energized as they crash into near-Earth space, sometimes during events called substorms. Single spacecraft might see just electrons or protons, or both, and they might see strong or weak signatures. Turner et al. (2018) used the ten spacecraft from these four missions (and 6 additional non-NASA spacecraft) to observe a series of such events. They were able to construct a coherent, but complex picture. Five weak injections of electrons built up to a stronger simultaneous injection of electrons and ions and a substorm. The complex picture built up from these observations exhibits details that help explain the physical mechanisms behind these events.
Turner, D. L., Fennell, J. F., Blake, J. B.,Claudepierre, S. G., Clemmons, J. H.,Jaynes, A. N.,Reeves, G. D. (2017). Multipoint observations of energetic particle injections and substorm activity during a conjunction between Magnetospheric Multiscale (MMS) and Van Allen Probes. Journal of Geophysical Research: Space Physics,122,11,481-11,504 Available at: https://doi.org/10.1002/2017JA024554
2) More than one way to create radiation belts.
Observations and simulations of two different radiation belt enhancements point to two different mechanisms. Van Allen Probes observed a significant electron flux increase during a geomagnetic storm during 17–18 March 2013, and also when there was no storm during 19–20 September 2013. By observing what waves were present and simulating the effects of these waves, the authors were able to conclude that the March enhancement was mostly do to electrons resonating with chorus waves, and that the September enhancement was mostly caused by ultralow-frequency waves diffusing electrons inward toward the Earth. Both mechanisms operate together, and either may dominate, depending on conditions.
Ma, Q., et al. (2018), Quantitative evaluation of radial diffusion and local acceleration processes during GEM challenge events, Journal of Geophysical Research, Available at: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017JA025114.
3) Radiation belt dropouts due to broadband electromagnetic waves.
Recent Van Allen Probes observations reveal the presence of broadband electromagnetic waves in the inner magnetosphere during the main phase of geomagnetic storms. These waves are made up of electric and magnetic fields oscillating roughly 1 to 100 times a second. That these waves were present in the radiation belts came as a surprise. They were observed to be very intense during the build up of geomagnetic storms. Radiation belt electrons can bounce back and forth in space from near the north pole to near the south pole. If this bouncing motion is as fast as the wave, perhaps both oscillating 5 times a second, then the wave can push the electrons out of the radiation belt. Radiation belt dropouts can be explained by this mechanism.
Chaston, C. C., Bonnell, J. W., Wygant, J. R., Reeves, G. D., Baker, D. N., & Melrose, D. B. (2018). Radiation belt dropouts and drift-bounce resonances in broadband electromagnetic waves. Geophysical Research Letters, 45. Available at: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017GL076362
Chaston, C. C., Bonnell, J. W., Kletzing, C. A., Hospodarsky, G. B., Wygant, J. R., & Smith, C. W. (2015). Broadband low-frequency electromagnetic waves in the inner magnetosphere. Journal of Geophysical Research: Space Physics, 120, 8603–8615. Available at: https://doi.org/10.1002/2015JA021690
1) Three-Step Buildup of the 17 March 2015 Storm Ring Current: Implication for the Cause of the Unexpected Storm Intensification
Geomagnetic storms are caused by gusts of faster denser solar wind, with an embedded southward magnetic field. These gusts of solar wind are caused by events on the Sun such as flares, coronal mass ejections, and coronal holes. Geomagnetic storms feature a buildup of energetic plasma in near-Earth space that produces magnetic disturbances on Earth and can energize the Earth’s radiation belts. While these events are driven by the solar wind, they are not simple, since the effect of previous solar wind events lingers in near Earth space and affects the later response. A recent paper by Keika et al. shows how a storm on March 15, 2017 evolved in three steps. Each storm intensification depended on how hot the pre-existing near Earth plasma was. This result shows details of how each gust of solar wind can have a greater effect than the previous one by building upon the effects of previous ones. This explains why some storms are bigger than others, even when the solar wind is blowing similarly.
Keika Kunihiro, Seki Kanako, é Masahito, Miyoshi Yoshizumi, Lanzerotti Louis J., et al.. Journal of Geophysical Research: Space Physics , Date: 01/2018 DOI: 10.1002/2017JA024462 Available at: http://onlinelibrary.wiley.com/wol1/doi/10.1002/2017JA024462/full
2) Pulsating aurora from electron scattering by chorus waves
Pulsating aurorae are blinking patches of light tens to hundreds of kilometers across that appear in the sky at high-latitudes in both hemispheres, usually between midnight and dawn. Multiple patches often cover the entire sky. This auroral pulsation is generated by the intermittent precipitation of energetic electrons arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. Observations by the Japanese Arase mission, a Van Allen Probes partner, offer the first direct observational evidence of this interaction. They observed energetic electrons being scattered by chorus waves into alignment with the magnetic field, which they then follow down to the Earth. This observation is made possible by a high angular-resolution (3.5°) electron sensor MEP-e, which can observe electrons very closely aligned with the magnetic field. The pulsating aurora that resulted from these precipitating electrons was observed by cameras on the ground. This completes the chain of causation from chorus waves, to the scattering of electrons into alignment with the magnetic field, to blinking patches of light in the sky, solving the mystery of this perplexing auroral phenomenon.
S. Kasahara, y. Miyoshi, S. Yokota, T. Mitani, Y. Kasahara, S. Matsuda, A. Kumamoto, A. Matsuoka, Y. Kazama, H. U. Frey, V. Angelopoulos, S. Kurita, K. Keika, K. Seki & I. Shinohara. Nature , Date: 02/2018 DOI: 10.1038/nature25505 Available at: https://www.nature.com/articles/nature25505
1) Van Allen Probes completes third cycle around the Earth
Over the past 5 years the apogee (high point) of the Van Allen Probes orbit made 3 full circles around Earth. This provided unprecedented observational coverage of radiation belt activity during more than 90 geomagnetic storms. By revisiting the same spacial location at different levels of geomagnetic activity the Probes enable scientists to build a systematic picture of how radiation belts evolve during storms.
2) Whistler mode chorus waves cause radiation belt losses
Microbursts are highly-structured 1/10 second bursts of energetic electrons into Earth’s atmosphere, where they are absorbed. Microbursts are considered to be a major mechanism for radiation belt electrons to be lost. Chorus waves are produced by plasma in near Earth space. They appear as repeated “chirps” that usually rise in pitch, one after another in a semi-random pattern. According to leading theories, microbursts are caused by whistler chorus waves. Two separate studies recently confirmed this theory using Van Allen Probes spacecraft at high altitude to observe the waves with their electric and magnetic field instruments, and a CubeSat in low Earth orbit to detect microbursts. Cubesats are small satellites about the size of a loaf of bread. Both CubeSats, FIREBIRD II and AC6, saw microbursts similar to the pattern of the chorus waves seen at higher altitude, for two separate events. Establishing this theory is a major step in predicting the coming and going of intense radiation belts.
Breneman, A. W., Crew, A., Sample, J., Klumpar, D., Johnson, A., Agapitov, O.,…Kletzing, C. A. (2017). Observations directly linking relativistic electron microbursts to whistler mode chorus: Van Allen Probes and FIREBIRD II. Geophysical Research Letters, 44, 11,265–11,272. https://doi.org/10.1002/2017GL075001
Mozer, F. S., Agapitov, O. V., Blake, J. B. & Vasko, I. Y. (2017). Simultaneous Observations Of Lower Band Chorus Emissions At The Equator And Microburst Precipitating Electrons In The Ionosphere. Geophysical Research Letters, 44. https://doi.org/10.1002/2017GL076120
3) Excess electron energy is converted into high frequency wave energy
Chorus waves appear as repeated “chirps”, usually rising in pitch, in space plasmas. The Van Allen Probes electric and magnetic field instruments observed chorus waves in the vicinity of Earth’s radiation belts using a special high resolution mode. They saw a different kind of higher frequency wave called Langmuir waves appear embedded within the chorus “chirps”. Scientists used wave theory and observations of electrons inside the waves to conclude that the chorus waves made electron beams, which then produced the Langmuir waves. This understanding of a basic plasma interaction observed in near Earth space can be applied to plasmas throughout the universe.
Li, J., Bortnik, J., An, X., Li, W., Thorne, R. M., Zhou, M., ... Spence, H. E. (2017). Chorus wave modulation of Langmuir waves in the radiation belts. Geophysical Research Letters, 44, 11,713–11,721. https://doi.org/10.1002/2017GL075877
4) New Comprehensive Model of Proton Inner Radiation Belt
The inner radiation belt is made of very energetic protons. They can be difficult to measure because the highest energy protons can produce noise in all energy channels of an instrument. Van Allen’s REPT instrument helps overcome the noise problem with an advanced coincidence logic design. Using a deep understanding of the instrument response and an unprecedented 5 years of quality observations, scientists were able to construct a more accurate model of the energy and location of inner belt protons. Spacecraft operators now have a better tool to understand this hazardous environment.
Selesnick, R. S., Baker, D. N., Kanekal, S. G., Hoxie, V. C., & Li, X. (2018). Modeling the proton radiation belt with Van Allen Probes Relativistic Electron-Proton Telescope data. Journal of Geophysical Research: Space Physics, 123. https://doi.org/10.1002/2017JA024661
5) Chorus waves observed to travel along magnetic field lines between the Earth and space intact.
In space plasmas chorus waves appear as repeated “chirps”, usually rising in pitch. Chorus elements were observed by a ground-based station in Kannuslehto in Northern Finland using two large 30 foot loop antennas. At nearly the same time they were observed by electric and magnetic wave receivers aboard Van Allen Probe A, which was much higher up near the Earth’s radiation belts. They were identified as belonging to the same traveling waves because they exhibited the same pattern of elements. There was a slight delay of 1.3 seconds indicating the travel time. What surprised observers was that they had travelled down from space following a magnetic field line, and reflected back up to the spacecraft, without loosing their shape.
Demekhov, A. G., Manninen, J., Santolík, O., & Titova, E. E. (2017). Conjugate groundspacecraft observations of VLF chorus elements. Geophysical Research Letters, 44, 11,735—11,744. https://doi.org/10.1002/2017GL076139