Monthly Science Highlights

 

January 2018, February 2018, March 2018

 


 

 

March 2018

 

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, 8603–8615Available at: https://doi.org/10.1002/2015JA021690

 

 

February 2018

 

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

 

 

 

January 2018

 

 

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



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