A. Ieda

Statistical analysis of the plasmoid evolution with Geotail observations

Plasmoids in the Earths magnetotail were studied statistically, using low energy particle (LEP) and magnetic field (MGF) data from the Geotail spacecraft. Their evolution along the tail axis from XGSM′ = −16 to −210 RE was investigated with 824 plasmoid events. Their dependence on YGSM′ was studied as well to derive the three-dimensional structure of the plasmoids. (The coordinates are aberrated to remove the average effects of the orbital velocity of the Earth about the Sun.) We defined a plasmoid as a structure with rotating magnetic fields and enhanced total pressure. In the near tail (XGSM′ ≥ −50 RE), there was a tendency for the plasmoids to be observed in the premidnight sector around the tail axis (|YGSM′ − 3| ≤ 10 RE), while they were observed widely (|YGSM′ ≤ 20 RE) in the middle tail (−50 > XGSM′ ≥ −100 RE) and in the distant tail (−100 RE > XGSM′). The plasmoids expanded in the ±YGSM′ direction with typical velocities of ±130 ± 100 km/s in the near tail. This strongly supports the view that plasmoids are initially formed at the near-Earth neutral line which has a limited extent in the YGSM′ direction. The plasmoids accelerated in the downtail direction from 400 ± 200 km/s to 700 ± 300 km/s from the near to the middle tail. Then, it is suggested that they decelerated to 600 ± 200 km/s as they traveled to the distant tail. The ion temperature inside plasmoids was 4.5 ± 2 keV in the near and middle tail, and then rapidly decreased to 2 ± 1 keV from the middle to the distant tail region. The ion temperature in the distant tail was 2 times higher than the values deduced previously. Typical plasmoid dimensions were estimated to be 10 RE (length) × 40 RE (width) × 10 RE (height) in the middle and distant tail. The energy carried by each plasmoid was ∼2 × 1014 J in the middle tail, half of which was lost from the middle to the distant tail. Inside plasmoids, the thermal energy flux exceeded the bulk energy flux and Poynting flux. The energy released tailward in the course of a substorm was estimated to be roughly 1015 J.

Plasmoid ejection and auroral brightenings

Geotail plasma and magnetic field observations of 24 plasmoids between 21 and 29 RE have been compared with Polar ultraviolet observations of auroral brightenings. Both single and multiple plasmoids almost always corresponded to brightenings, but the brightenings were sometimes weak and spatially limited and did not always grow to a global substorm. Even a case where a plasmoid event occurred with fast postplasmoid flow corresponded to a weak brightening but no substorm. Some brightenings did not correspond to plasmoids, but these brightenings were observed away from the longitude of Geotail, indicating that plasmoids have a small longitudinal extent in the near tail. The plasmoids were occasionally observed before the brightenings but more frequently were observed 0–2 min after the brightenings, with the delays probably due to the transit time to the observation point. It seems likely that formation of a near-Earth neutral line causes each brightening in the polar ionosphere, but these formations do not always lead to a full-fledged substorm. What additional circumstance causes development of a full, large-scale substorm remains an open question.

Geotail observations of plasma sheet ion composition over 16 years: On variations of average plasma ion mass and O+ triggering substorm model

[1] We examined long-term variations of ion composition in the plasma sheet, using energetic (9.4-212.1 keV/e) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. EPIC/STICS observations are available from 17 October 1992 for more than 16 years, covering the declining phase of solar cycle 22, all of solar cycle 23, and the early phase of solar cycle 24. This unprecedented long-term data set revealed that (1) the He + /H + and O + /H + flux ratios in the plasma sheet were dependent on the F10.7 index; (2) the F10.7 index dependence is stronger for O + /H + than He + /H + ; (3) the 0 + /H + flux ratio is also weakly correlated with the ∑Kp index; and (4) the He 2+ /H + flux ratio in the plasma sheet appeared to show no long-term trend. From these results, we derived empirical equations related to plasma sheet ion composition and the F10.7 index and estimated that the average plasma ion mass changes from ∼1.1 amu during solar minimum to ∼2.8 amu during solar maximum. In such a case, the Alfven velocity during solar maximum decreases to ∼60% of the solar minimum value. Thus, physical processes in the plasma sheet are considered to be much different between solar minimum and solar maximum. We also compared long-term variation of the plasma sheet ion composition with that of the substorm occurrence rate, which is evaluated by the number of Pi2 pulsations. No correlation or negative correlation was found between them. This result contradicts the O + triggering substorm model, in which heavy ions in the plasma sheet increase the growth rate of the linear ion tearing mode and play an important role in localization and initiation of substorms. In contrast, O + ions in the plasma sheet may prevent occurrence of substorms.

Statistical visualization of Earth's magnetotail during substorms by means of multidimensional superposed epoch analysis with Geotail data

A multidimensional superposed epoch analysis of plasma and magnetic field data from the Geotail spacecraft was used to visualize the time evolution of plasmoids in the Earths magnetotail statistically. The substorm events were identified by Pi2 events at midlatitude ground stations when Geotail was in the downtail region from -30 R E to -200 R E . Plasma density, velocity, temperature, and magnetic field data were sorted and analyzed according to the Pi2 onset time. We selected 156 substorm events that were either a single onset or the first onset of multiple onsets. In the statistical visualization, we found that plasma acceleration does not take place in a small region around X ∼ -30 R E , but occurs in a more widely spread region extending from X ∼ -30 R E down to X ∼ -90 R E . The spatial structure of the plasmoid is characterized by a bilatitudinal structure where the fast plasma flow in the plasma sheet boundary stretches from the equator and a relatively slow plasma sheet flow is encountered with preexisting plasma sheet populations. Finite By fields are created in the inner plasma sheet boundary, and the total pressure is enhanced inside the plasmoid. The evolution of the plasma and magnetic fields as well as the deduced parameters, such as the number flux of plasma or energy fluxes, agreed well with a magnetic reconnection model.

Ground‐based ELF/VLF chorus observations at subauroral latitudes—VLF‐CHAIN Campaign

We report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) of 17–25 February 2012 at subauroral latitudes at Athabasca (L=4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising-tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling-tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty-patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions.

Theory, modeling, and integrated studies in the Arase (ERG) project

Understanding of underlying mechanisms of drastic variations of the near-Earth space (geospace) is one of the current focuses of the magnetospheric physics. The science target of the geospace research project Exploration of energization and Radiation in Geospace (ERG) is to understand the geospace variations with a focus on the relativistic electron acceleration and loss processes. In order to achieve the goal, the ERG project consists of the three parts: the Arase (ERG) satellite, ground-based observations, and theory/modeling/integrated studies. The role of theory/modeling/integrated studies part is to promote relevant theoretical and simulation studies as well as integrated data analysis to combine different kinds of observations and modeling. Here we provide technical reports on simulation and empirical models related to the ERG project together with their roles in the integrated studies of dynamic geospace variations. The simulation and empirical models covered include the radial diffusion model of the radiation belt electrons, GEMSIS-RB and RBW models, CIMI model with global MHD simulation REPPU, GEMSIS-RC model, plasmasphere thermosphere model, self-consistent wave–particle interaction simulations (electron hybrid code and ion hybrid code), the ionospheric electric potential (GEMSIS-POT) model, and SuperDARN electric field models with data assimilation. ERG (Arase) science center tools to support integrated studies with various kinds of data are also briefly introduced.

Approximate forms of daytime ionospheric conductance

The solar zenith angle (SZA) dependence of the conductance is studied and a simple theoretical form for the Hall-to-Pedersen conductance ratio is developed, using the peak plasma production height. The European Incoherent Scatter (EISCAT) radar observations at Tromso (67 MLAT) on 30 March 2012 were used to calculate the conductance. The daytime electric conductance is associated with plasma created by solar extreme ultraviolet radiation incident on the neutral atmosphere of the Earth. However, it has been uncertain whether previous conductance models are consistent with the ideal Chapman theory for such plasma productions. We found that the SZA dependence of the conductance is consistent with the Chapman theory after simple modifications. The Pedersen conductance can be understood by approximating the plasma density height profile as being flat in the topside E region and by taking into account the upward gradient of atmospheric temperature. An additional consideration is necessary for the Hall conductance, which decreases with increasing SZA more rapidly than the Pedersen conductance. This rapid decrease is presumably caused by a thinning of the Hall conductivity layer from noon toward nighttime. We expressed this thinning in terms of the peak production height in the Chapman theory.

The Optical Mesosphere Thermosphere Imagers (OMTIs) for network measurements of aurora and airglow

The Optical Mesosphere Thermosphere Imagers (OMTIs) currently consist of eight all‐sky cooled‐CCD imagers and several interferometers and spectrometers. They are making routine observations of aurora and airglow in Japan, Australia, Indonesia, and Canada. Here we show recent results of OMTIs particularly from the two Canadian stations at Resolute Bay (RSB) and Athabasca (ATH). At RSB, we observe polar‐cap plasma patches almost always during southward IMF periods. From two‐dimensional cross‐correlation analyses, we determine velocity vectors of the patches, which indicates the ionospheric convection vector, showing high correlation with the IMF‐By and‐Bz variations. At ATH, we often observe isolated proton arcs and Stable Auroral Red (SAR) arcs, which are located equatorward of the auroral oval. The appearance of the isolated proton arcs is highly correlated with the Pc 1 geomagnetic pulsations measured simultaneously at ATH, suggesting interactions between the electromagnetic ion cyclotron (EMIC) waves an...

Visualization tool for three-dimensional plasma velocity distributions (ISEE_3D) as a plug-in for SPEDAS

This paper introduces ISEE_3D, an interactive visualization tool for three-dimensional plasma velocity distribution functions, developed by the Institute for Space-Earth Environmental Research, Nagoya University, Japan. The tool provides a variety of methods to visualize the distribution function of space plasma: scatter, volume, and isosurface modes. The tool also has a wide range of functions, such as displaying magnetic field vectors and two-dimensional slices of distributions to facilitate extensive analysis. The coordinate transformation to the magnetic field coordinates is also implemented in the tool. The source codes of the tool are written as scripts of a widely used data analysis software language, Interactive Data Language, which has been widespread in the field of space physics and solar physics. The current version of the tool can be used for data files of the plasma distribution function from the Geotail satellite mission, which are publicly accessible through the Data Archives and Transmission System of the Institute of Space and Astronautical Science (ISAS)/Japan Aerospace Exploration Agency (JAXA). The tool is also available in the Space Physics Environment Data Analysis Software to visualize plasma data from the Magnetospheric Multiscale and the Time History of Events and Macroscale Interactions during Substorms missions. The tool is planned to be applied to data from other missions, such as Arase (ERG) and Van Allen Probes after replacing or adding data loading plug-ins. This visualization tool helps scientists understand the dynamics of space plasma better, particularly in the regions where the magnetohydrodynamic approximation is not valid, for example, the Earth’s inner magnetosphere, magnetopause, bow shock, and plasma sheet.

A Statistical Study of Pressure Changes in the Near-Earth Magnetotail Associated with Substorm Expansion Onsets

We have statistically studied substorm-associated evolution of the near-Earth magnetotail to understand the substorm triggering mechanism. In the present chapter we focus on changes in the total and plasma pressures. We find that energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization, i.e., at –12 > X > –18 R e than in the surrounding regions. Unlike previously reported results, the plasma pressure increases in association with the initial dipolarization at X > –12 R e, the increase largely contributed by high-energy particles. This result suggests that the rarefaction wave scenario proposed in the current disruption model is questionable.