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We present an event based on Reimei satellite observations in the low‐altitude midnight auroral region, showing that intense and clear energy‐dispersed electron precipitations, repetitively generated by field‐aligned accelerations due to dispersive Alfvén waves, were modulating inverted‐V electrons. These Alfvénic electrons had peak energies equal to or slightly larger than those of the inverted‐Vs and were associated with the filamentary auroral forms rapidly streaming at the poleward edge of a broad discrete arc. This arc was caused by the inverted‐V accompanied by ion depletions produced by quasi‐electrostatic parallel potential drop. Assuming instantaneous electron accelerations over a wide energy range in a single location and a simple time‐of‐flight effect for the energy‐time dispersions, the Alfvénic source distances were estimated 1,500 ± 500 km above the satellite altitude of ∼676 km, a lower bound since the interaction locations are realistically distributed in altitudinally extended regions. The electron characteristics in detailed energy‐pitch angle distributions obtained at high time resolution can be categorized into: (a) original inverted‐V fluxes energized by quasi‐electrostatic upward electric field, (b) accelerated and decelerated/reduced inverted‐V fluxes, (c) field‐aligned energy‐dispersed precipitations accelerated by dispersive Alfvén waves, and (d) upwelling secondary components effectively produced by the field‐aligned precipitations particularly at energies of a few tens of eV. This event is useful to reveal the interactions between the inverted‐V and Alfvénic electrons and their related ionospheric effects in the magnetosphere‐ionosphere coupling processes. The detailed energy‐pitch angle distributions presented here provide constraints for models of these interactions and processes. Field‐aligned Alfvénic accelerations of electrons repetitively occurred at the poleward edge of broad auroral arc in the nightside ovalEnergy‐dispersed electron signatures modulated the inverted‐V electron distributions and implied short (1,000–2,000 km) source distancesThe Alfvénic electrons were associated with repetitive filamentary auroral forms streaming at the edge of the discrete arc Field‐aligned Alfvénic accelerations of electrons repetitively occurred at the poleward edge of broad auroral arc in the nightside oval Energy‐dispersed electron signatures modulated the inverted‐V electron distributions and implied short (1,000–2,000 km) source distances The Alfvénic electrons were associated with repetitive filamentary auroral forms streaming at the edge of the discrete arc

We made observations of magnetic field variations in association with pulsating auroras with the magneto‐impedance sensor magnetometer (MIM) carried by the Loss through Auroral Microburst Pulsations (LAMP) sounding rocket that was launched at 11:27:30 UT on 5 March 2022 from Poker Flat Research Range, Alaska. At an altitude of 200–250 km, MIM detected clear enhancements of the magnetic field by 15–25 nT in both the northward and westward components. From simultaneous observations with the ground all‐sky camera, we found that the footprint of LAMP at the 100 km altitude was located near the center of a pulsating auroral patch. The auroral patch had a dimension of ∼90 km in latitude and ∼25 km in longitude, and its major axis was inclined toward northwest. These observations were compared with results of a simple model calculation, in which local electron precipitation into the thin‐layer ionosphere causes an elliptical auroral patch. The conductivity within the patch is enhanced in the background electric field and as a result, the magnetic field variations are induced around the auroral patch. The model calculation results can explain the MIM observations if the electric field points toward southeast and one of the model parameters is adjusted. We conclude that the pulsating auroral patch in this event was associated with a one‐pair field‐aligned current that consists of downward (upward) currents at the poleward (equatorward) edge of the patch. This current structure is maintained even if the auroral patch is latitudinally elongated. Magneto‐impedance (MI) sensor was carried by a sounding rocket and first applied to magnetic field measurement in spaceMagnetic field variations were observed when the footprint of the payload was located near the center of an auroral patchThe magnetic field variations are thought to be caused by field‐aligned currents flowing at the edges of the pulsating auroral patch Magneto‐impedance (MI) sensor was carried by a sounding rocket and first applied to magnetic field measurement in space Magnetic field variations were observed when the footprint of the payload was located near the center of an auroral patch The magnetic field variations are thought to be caused by field‐aligned currents flowing at the edges of the pulsating auroral patch

We present the simultaneous and conjugated auroral emission and particle data obtained by a low‐altitude polar‐orbiting micro‐satellite, Reimei, for elucidating their latitudinal distributions and variations in the nightside auroral oval. Here are reported a few notable examples of the Reimei observations with high time and spatial resolutions, namely ∼120 msec. and ∼1.2 km × 1.2 km for multispectral auroral images and 40 msec. for energy‐pitch angle distributions of electrons and ions with energies of 10 eV–12 keV, respectively. The auroral images show various fine‐scale auroral activities characterized by the following types of auroral forms and variations: faint bands, streaming multiple arcs, shearing arcs, and vortices/curls, which are typical of the latitudinal properties of auroras. The particle analyzer simultaneously observed various properties of electron energy‐pitch angle and latitudinal distributions, and their temporal variations, each of which corresponds to a type of the auroral activities. Their features are summarized below. Reimei repetitively observed inverted‐V signatures of low‐energy (<1 keV) field‐aligned electrons in addition to the higher‐energy (several keV) diffuse electrons in low‐latitude auroral oval. In more active regions at higher latitudes, the dominant energy flux responsible for the multiple‐arc emissions was carried by the well‐known inverted‐V electron precipitation. The rapidly rotating vortices or so‐called curls of fine‐scale discrete auroras near the poleward boundary of the auroral oval were closely associated with the significant energy fluxes of spiky field‐aligned electron bursts with energy‐time dispersions produced by dispersive Alfvén waves. Plasma observations with auroral imaging in nightside auroral oval present clear characteristics of several types of particle accelerationsThe image‐particle data reveal correlations and latitudinal properties/dependence of plasma dynamics with the forms and motions of aurorasEnergy‐pitch angle distributions are crucial for elucidating the particle acceleration processes recognized in energy‐time spectrograms Plasma observations with auroral imaging in nightside auroral oval present clear characteristics of several types of particle accelerations The image‐particle data reveal correlations and latitudinal properties/dependence of plasma dynamics with the forms and motions of auroras Energy‐pitch angle distributions are crucial for elucidating the particle acceleration processes recognized in energy‐time spectrograms

The demonstrative space plasma physics communities in the world have been conducting intensive research and development activities for instrument miniaturization because compact and lightweight sensors are required for installation on micro/small satellites and CubeSats in ongoing and future space exploration missions. Recently, we designed a novel type of space plasma instrument capable of simultaneous energy analyses of low‐energy (∼eV to several 10s of keV) electrons and ions with a single sensor head consisting of triple‐dome analyzer with triple‐layer collimator for covering 360° field‐of‐view similar to that of the prevailing top‐hat type. Two separate but correlated electrostatic fields, produced by a single high‐voltage supply unit in double gaps of the triple‐dome/layer structures, are used for electron/ion energy analyses. In the breadboard model (BBM) designed here, the energy/angular resolutions and the analyzer constants (ratios of measured particle energy to applied voltage) are approximately 9.0%/3.5° and 4.0, and 10%/4° and 4.4 for the electron and ion energy analyses, respectively. We also fabricated the first BBM of the triple‐dome type of electrostatic analyzer for experimental performance tests in electron/ion beamline facility. According to the experimental data by the two‐dimensionally position‐sensitive particle detector, the calibrated properties of energy/angular analyses and the other sensor performance show good agreement with numerical design results. The performance of the triple‐dome analyzer would satisfy observation requirements, as represented by measurements of three‐dimensional velocity distributions of electrons and ions in future missions using spin‐type microsatellites/CubeSats under stringent resource conditions. We designed an electrostatic analyzer with a 360° field‐of‐view for simultaneous electron and ion measurements from eV/q to 10s of keV/qThe sensor consists of triple domes following a deflecting collimator ring with possibilities of future application and modificationsThe experimental results in our beamline calibration facility showed good agreement with instrumental properties by the numerical design We designed an electrostatic analyzer with a 360° field‐of‐view for simultaneous electron and ion measurements from eV/q to 10s of keV/q The sensor consists of triple domes following a deflecting collimator ring with possibilities of future application and modifications The experimental results in our beamline calibration facility showed good agreement with instrumental properties by the numerical design

We conducted developmental experiments using avalanche photodiode (APD) for electron measurements applicable in future space plasma explorations. Electrons with energies of 10 eV to a hundred keV could be detected by applying “floating” voltages up to +5 kV to APD to achieve wide‐energy electron measurements. We detected 10‐eV electrons from an electron source as energized electrons of an energy of approximately 5 (5.01) keV due to the 5‐keV electrostatic acceleration (=Source energy: 0.01 keV + Acceleration energy: 5 keV; hereafter, (0.01 + 5) keV) with an energy resolution of ∼4.6 keV by simplified assumption and calculation based on peak energy and full width at half maximum of the pulse height distribution. The energy resolution for (5 + 0)‐keV electrons, ∼5.3 keV in our experiments, was equivalent to the previous results. On the other hand, we concluded that for (5 + 5) keV, the energy resolution was significantly improved to 1.3 keV compared with that for (5 + 0) keV. For (1 + 5)‐keV electrons, the energy distribution showed a broadening toward lower energies than that for (6 + 0) keV because the backscattered electrons produced secondary electrons at the APD chassis surface, which were accelerated to 5 keV by the floating voltage to be detected by the APD. We verified that the floating‐mode APD can contribute to space plasma measurements as a detector with characteristics different from those of microchannel plates. This is the first achievement to establish advanced in‐situ observation techniques based on APD for wide‐energy (a few eV to a hundred keV) electron measurements in space. We conducted fundamental developments of floating‐mode avalanche photodiode (APD) for reducing lowermost measurable energy of electrons in spaceExperimental results indicate that the floating‐mode APD applied by +5 kV as floating voltage can detect 10‐eV electronsWe identified contaminations by secondary electrons that were produced by backscattered electrons and accelerated by the floating voltage We conducted fundamental developments of floating‐mode avalanche photodiode (APD) for reducing lowermost measurable energy of electrons in space Experimental results indicate that the floating‐mode APD applied by +5 kV as floating voltage can detect 10‐eV electrons We identified contaminations by secondary electrons that were produced by backscattered electrons and accelerated by the floating voltage