arXiv subjects

Yukinaga Miyashita

Publications and source records attributed to Yukinaga Miyashita.

6 recordsLinked to original sources

arXiv2026

Extreme, transient bursts of energy in the auroral ionosphere. I. Predictive radar tracking

Three-metre Farley-Buneman irregularities observed by the \textsc{icebear} VHF radar organize into clusters whose apparent motion follows the electric field mapped from the magnetosphere. We track these clusters automatically: each is bounded by an $α$-shape at every time step, consecutive frames are associated by an optimal assignment combining shape overlap with a predicted displacement. Births, deaths, splits, and mergers are monitored, and each trajectory is reduced to per-segment velocities by piecewise linear regression. Tracked speeds are validated against in-situ ion drifts measured by DMSP F16 during two conjunctions in May 2021. Across four years of disturbed conditions, the speed distribution of 74,517 tracked clusters agrees with Swarm A cross-track ion drifts to within a factor of two in probability density for all speeds between 300 and 4000 m/s, and the radar-tracked speed distribution continues as a power law well beyond the noise limit imposed on Swarm by spacecraft attitude jitter. Binning by geomagnetic activity yields a parameterization of the field dispersion conditional on threshold exceedance, $σ^2 = 3.68 \times 10^5$ (SME/100 nT)$^{0.353}$~m$^2$ s$^{-2}$, equivalent to 30 to 60 mV/m across the observed activity range, which supplies the amplitude statistics entering the variance term of height-integrated Joule dissipation. During the 10 May 2024 super-storm, on closed field lines equatorward of the dayside cusp, we retrieved an upper-tail sample of this distribution, finding a cluster moving at $11,240\pm660$ m/s and implying a field of approximately 560 mV/m. Together with the unstable fraction of a space weather model's grid volume, our parameterization can in future close the sub-grid contribution to the storm-time heating budget in the auroral ionosphere.

physics.space-ph↗

arXiv2026

Extreme, transient bursts of energy in the auroral ionosphere. II. A magnetotail dipolarization event

We report ground-based coherent VHF radar observations of extreme turbulent field-structures detected in coincidence with a magnetospheric substorm-associated magnetotail dipolarization. The field-structures are observed by the ICEBEAR radar, in the form of Farley-Buneman (FB) waves in the auroral electrojets, and the field-structures themselves move an order of magnitude faster than the saturation speed of the underlying FB waves, implying transient electric field sources up to 330 mV/m in strength. The field-structures are identified and automatically tracked using an unsupervised clustering & tracking algorithm, applied to clutters of ICEBEAR radar backscatter targets, a method that turns the Doppler radar into a tracking radar capable of measuring the ionospheric ExB-drift by proxy. We place this finding in a coordinated multi-instrument context. Three THEMIS spacecraft observed the dipolarization event in-situ in the near-Earth plasma sheet. In the ionosphere, Swarm A, crossing through the guilty auroral arc at the onset of the dipolarization event, recorded clear signatures of propagating Alfvén waves threading the relevant flux tube. We interpret the ICEBEAR transients as the natural ionospheric foot signature of a shear Alfvén pulse launched by the bipolar space-charge (Hall) electric field of the thinned current sheet, with amplification along the converging flux tube, partial reflection at the ionospheric boundary, and spatial sharpening by precipitation-produced Pedersen-conductance gradients on the auroral arc edges. A one-dimensional wave-transmission analysis recovers the observations. Our results elucidate a tightly controlled coupling between magnetotail processes and meter-scale auroral plasma turbulence, and demonstrate the capability of ICEBEAR to resolve extreme, transient electric-field enhancements in the ionosphere.

physics.space-ph↗

arXiv2026

Contemporaneous Appearances of Auroral Spiral and Transpolar Arc: Polar UVI Observations and Global MHD Simulations

A local vortex-structured aurora and a large-scale transpolar arc (TPA) were contemporaneously observed by the Polar ultraviolet imager (UVI) during the late recovery phase of a substorm, and the interplanetary magnetic field (IMF) BY and BZ were negative and negative-to-positive. The TPA grew along the dawnside auroral oval from the nightside to the dayside, and an auroral spiral and several spots were located azimuthally near the poleward edge of the nightside auroral oval. Both auroras had tailward elongated source regions with scales of ~30 RE (spiral) and more than ~45 RE (TPA). To examine their magnetospheric/ionospheric field-aligned current (FAC) profiles, we performed global magnetohydrodynamic (MHD) simulations, using two different types of code: Block-Adaptive-Tree Solar-wind Roe Upwind Scheme (BATS-R-US) and improved REProduce Plasma Universe (REPPU). Both MHD simulations reproduced the tailward elongated TPA-associated FAC structures. The spiral-associated FAC intensity was, however, approximately three orders of magnitude weaker than the TPA-associated FAC intensity. Only improved REPPU simulations replicated faint but continuous poleward extending streak-like structures without evident FACs, instead of the auroral spiral. Geomagnetic field measurements showed that the spiral had upward (from the ionosphere to the magnetosphere) FACs, and its appearance might be accompanied by ultra-low-frequency Pc5 waves. Our results suggest that 1) a local-scale spiral might be formed with much weaker magnetotail FACs than global-scale TPA-associated FACs, although the spiral source region is elongated tailward, and 2) a solar wind-magnetosphere-ionosphere coupling system with minimal or no significant substorm effects is required to form the spiral with the weak magnetotail FACs.

physics.space-ph↗

arXiv2025

Eastward Transients in the Dayside Ionosphere II: A Parallel-plate Capacitor-Like Effect

During the 23 April 2023 geospace storm, we observed chorus wave-driven, energetic particle precipitation on closed magnetic field lines in the dayside magnetosphere. Simultaneously and in the ionosphere's bottom-side, we observed signatures of impact ionization and strong enhancements in the ionospheric electric field, via radar-detection of meter-scale turbulence, and with matching temporal characteristics as that of the magnetospheric observations. We detailed this in a companion paper. In the present article, we place those observations into context with the dayside ionosphere, and describe a remarkably similar event that took place during the May 2024 geospace superstorm. In both cases, fast, eastward-moving electric field structures were excited equatorward of the ionospheric cusp, on closed magnetic field-lines -- observations that challenge existing modes of explanation for electrodynamics in the cusp-region, where most such observations are interpreted in the context of poleward-moving auroral forms. Instead, primarily eastward-moving electric field structures were associated with turbulent Hall currents that are perhaps characteristically excited during geospace storms by wave-particle interactions near magnetospheric equator or by proton precipitation characteristics in the cusp, forming a `parallel-plate capacitor-like effect'. We propose that transient eastward electrodynamic bursts in the dayside ionosphere might be a common, albeit previously unresolved, feature of geomagnetic storms.

physics.space-ph↗

arXiv2025

A Characteristic Signature of Magnetospheric Wave-Particle Interactions Found in the Turbulent E-region

Plasma waves in the magnetosphere scatter electrons, causing them to precipitate into Earth's atmosphere, imparting their temporal characteristics to diffuse auroras. In a case study of conjugate radar and satellite observations, we demonstrate a close and unprecedented association between enhanced electrostatic cyclotron harmonic wave activity in the magnetosphere and the appearance of meter-scale plasma turbulence a few seconds later in the lower ionosphere on nearby magnetic field lines. Such direct structuring of the ionosphere carries implications for our understanding of space weather.

physics.space-ph↗

arXiv2023

Auroral Morphological Changes to the Formation of Auroral Spiral during the Late Substorm Recovery Phase: Polar UVI and Ground All-Sky Camera Observations

The ultraviolet imager (UVI) of the Polar spacecraft and an all-sky camera at Longyearbyen contemporaneously detected an auroral vortex structure (so-called "auroral spiral") on 10 January 1997. From space, the auroral spiral was observed as a "small spot" (one of an azimuthally-aligned chain of similar spots) in the poleward region of the main auroral oval from 18 h to 24 h magnetic local time. These auroral spots were formed while the substorm-associated auroral bulge was subsiding and several poleward-elongated auroral streak-like structures appeared during the late substorm recovery phase. During the spiral interval, the geomagnetically north-south and east-west components of the geomagnetic field, which were observed at several ground magnetic stations around Svalbard island, showed significant negative and positive bays caused by the field-aligned currents related with the aurora spiral appearance. The negative bays were reflected in the variations of local geomagnetic activity index (SML) which was provided from the SuperMAG magnetometer network at high latitudes. To pursue the spiral source region in the magnetotail, we trace each UVI image along field lines to the magnetic equatorial plane of the nightside magnetosphere using an empirical magnetic field model. Interestingly, the magnetotail region corresponding to the auroral spiral covered a broad region from Xgsm ~ -40 to -70 RE at Ygsm ~ 8 to 12 RE. The appearance of this auroral spiral suggests that extensive areas of the magnetotail (but local regions in the ionosphere) remain active even when the substorm almost ceases, and geomagnetic conditions are almost stable.

physics.space-ph↗