Yukitoshi Nishimura

Bio: Yukitoshi Nishimura is an academic researcher from Boston University. The author has contributed to research in topics: Substorm & Magnetosphere. The author has an hindex of 36, co-authored 207 publications receiving 4903 citations. Previous affiliations of Yukitoshi Nishimura include University of California, Los Angeles & Tohoku University.

The Space Physics Environment Data Analysis System (SPEDAS)

Abstract: With the advent of the Heliophysics/Geospace System Observatory (H/GSO), a complement of multi-spacecraft missions and ground-based observatories to study the space environment, data retrieval, analysis, and visualization of space physics data can be daunting. The Space Physics Environment Data Analysis System (SPEDAS), a grass-roots software development platform ( www.spedas.org ), is now officially supported by NASA Heliophysics as part of its data environment infrastructure. It serves more than a dozen space missions and ground observatories and can integrate the full complement of past and upcoming space physics missions with minimal resources, following clear, simple, and well-proven guidelines. Free, modular and configurable to the needs of individual missions, it works in both command-line (ideal for experienced users) and Graphical User Interface (GUI) mode (reducing the learning curve for first-time users). Both options have “crib-sheets,” user-command sequences in ASCII format that can facilitate record-and-repeat actions, especially for complex operations and plotting. Crib-sheets enhance scientific interactions, as users can move rapidly and accurately from exchanges of technical information on data processing to efficient discussions regarding data interpretation and science. SPEDAS can readily query and ingest all International Solar Terrestrial Physics (ISTP)-compatible products from the Space Physics Data Facility (SPDF), enabling access to a vast collection of historic and current mission data. The planned incorporation of Heliophysics Application Programmer’s Interface (HAPI) standards will facilitate data ingestion from distributed datasets that adhere to these standards. Although SPEDAS is currently Interactive Data Language (IDL)-based (and interfaces to Java-based tools such as Autoplot), efforts are under-way to expand it further to work with python (first as an interface tool and potentially even receiving an under-the-hood replacement). We review the SPEDAS development history, goals, and current implementation. We explain its “modes of use” with examples geared for users and outline its technical implementation and requirements with software developers in mind. We also describe SPEDAS personnel and software management, interfaces with other organizations, resources and support structure available to the community, and future development plans.

Identifying the Driver of Pulsating Aurora

Abstract: Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.

Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations

Abstract: [1] A critical, long‐standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all‐sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north‐south (N‐S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N‐S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre‐existing open‐closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward‐transport of the new plasma leads to a near‐Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere‐ionosphere electrodynamic coupling, which may play an important role in the motion of pre‐onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre‐onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well‐developed region 2 current system within the duskside convection cell, and also a high probability of diffuse‐appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

Statistical properties of plasmaspheric hiss derived from Van Allen Probes data and their Effects on radiation belt electron dynamics

Abstract: Author(s): Li, W; Ma, Q; Thorne, RM; Bortnik, J; Kletzing, CA; Kurth, WS; Hospodarsky, GB; Nishimura, Y | Abstract: ©2015. American Geophysical Union. Plasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550Hz), which was in frequent use, and that the hiss wave power frequently extends below 100Hz, particularly at larger L shells ( g ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum and find that the differences are up to a factor of ~5 and are dependent on energy and L shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100keV at L~5, although their effect is negligible at L≤3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics.

THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation

Abstract: [1] A statistical survey of plasma densities and electron distributions (0.5–100 keV) is performed using data obtained from the Time History of Events and Macroscale Interactions During Substorms spacecraft in near‐equatorial orbits from 1 July 2007 to 1 May 2009 in order to investigate optimum conditions for whistler mode chorus excitation. The plasma density calculated from the spacecraft potential, together with in situ magnetic field, is used to construct global maps of cyclotron and Landau resonant energies under quiet, moderate, and active geomagnetic conditions. Statistical results show that chorus intensity increases at higher AE index, with the strongest waves confined to regions where the ratio between the plasma frequency and gyrofrequency, fpe/fce, is less than 5. On the nightside, large electron anisotropies and intense chorus emissions indicate remarkable consistency with the confinement to 8 RE. Furthermore, as injected plasma sheet electrons drift from midnight through dawn toward the noon sector, their anisotropy increases and peaks on the dayside at 7 6) on the dayside. In addition, very isotropic distributions at a few keV, which may be produced by Landau resonance and contribute to the formation of the typical gap in the chorus spectrum near 0.5 fce, are commonly observed on the dayside. Citation: Li, W., et al. (2010), THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation, J. Geophys. Res., 115, A00F11, doi:10.1029/2009JA014845.

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

Abstract: The Electric and Magnetic Field Instrument and Integrated Science (EMFISIS) investigation on the NASA Radiation Belt Storm Probes (now named the Van Allen Probes) mission provides key wave and very low frequency magnetic field measurements to understand radiation belt acceleration, loss, and transport. The key science objectives and the contribution that EMFISIS makes to providing measurements as well as theory and modeling are described. The key components of the instruments suite, both electronics and sensors, including key functional parameters, calibration, and performance, demonstrate that EMFISIS provides the needed measurements for the science of the RBSP mission. The EMFISIS operational modes and data products, along with online availability and data tools provide the radiation belt science community with one the most complete sets of data ever collected.

Rapid local acceleration of relativistic radiation-belt electrons by magnetospheric chorus

Abstract: Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density, which are compelling evidence for local electron acceleration in the heart of the outer radiation belt, but are inconsistent with acceleration by inward radial diffusive transport. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth's outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

Radiation belt dynamics: The importance of wave-particle interactions

Abstract: [1] The flux of energetic electrons in the Earth's outer radiation belt can vary by several orders of magnitude over time scales less than a day, in response to changes in properties of the solar wind instigated by solar activity. Variability in the radiation belts is due to an imbalance between the dominant source and loss processes, caused by a violation of one or more of the adiabatic invariants. For radiation belt electrons, non-adiabatic behavior is primarily associated with energy and momentum transfer during interactions with various magnetospheric waves. A review is presented here of recent advances in both our understanding and global modeling of such wave-particle interactions, which have led to a paradigm shift in our understanding of electron acceleration in the radiation belts; internal local acceleration, rather than radial diffusion now appears to be the dominant acceleration process during the recovery phase of magnetic storms.

「Earth,Planets and Space」のオープンアクセス出版

Abstract: ▶ Gathers original articles on topics in earth and planetary sciences ▶ Coverage includes geomagnetism, aeronomy, space science, seismology, volcanology, geodesy and planetary science ▶ Official journal of the Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan, and The Japanese Society for Planetary Sciences

Scattering by chorus waves as the dominant cause of diffuse auroral precipitation

Abstract: Earth's diffuse aurora occurs over a broad latitude range and is primarily caused by the precipitation of low-energy (0.1-30-keV) electrons originating in the central plasma sheet, which is the source region for hot electrons in the nightside outer magnetosphere. Although generally not visible, the diffuse auroral precipitation provides the main source of energy for the high-latitude nightside upper atmosphere, leading to enhanced ionization and chemical changes. Previous theoretical studies have indicated that two distinct classes of magnetospheric plasma wave, electrostatic electron cyclotron harmonic waves and whistler-mode chorus waves, could be responsible for the electron scattering that leads to diffuse auroral precipitation, but it has hitherto not been possible to determine which is the more important. Here we report an analysis of satellite wave data and Fokker-Planck diffusion calculations which reveals that scattering by chorus is the dominant cause of the most intense diffuse auroral precipitation. This resolves a long-standing controversy. Furthermore, scattering by chorus can remove most electrons as they drift around Earth's magnetosphere, leading to the development of observed pancake distributions, and can account for the global morphology of the diffuse aurora.