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  1. Abstract

    Although many substorm‐related observations have been made, we still have limited insight into propagation of the plasma and field perturbations in Pi2 frequencies (∼7–25 mHz) in association with substorm aurora, particularly from the auroral source region in the inner magnetosphere to the ground. In this study, we present conjugate observations of a substorm brightening aurora using an all‐sky camera and an inner‐magnetospheric satellite Arase atL ∼ 5. A camera at Gakona (62.39°N, 214.78°E), Alaska, observed a substorm auroral brightening on 28 December 2018, and the footprint of the satellite was located just equatorward of the aurora. Around the timing of the auroral brightening, the satellite observed a series of quasi‐periodic variations in the electric and magnetic fields and in the energy flux of electrons and ions. We demonstrate that the diamagnetic variations of thermal pressure and medium‐energy ion energy flux in the inner magnetosphere show approximately one‐to‐one correspondence with the oscillations in luminosity of the substorm brightening aurora and high‐latitudinal Pi2 pulsations on the ground. We also found their anti‐correlation with low‐energy electrons. Cavity‐type Pi2 pulsations were observed at mid‐ and low‐latitudinal stations. Based on these observations, we suggest that a wave phenomenon in the substorm auroral source region, like ballooning type instability, play an important role in the development of substorm and related auroral brightening and high‐latitude Pi2, and that the variation of the auroral luminosity was directly driven by keV electrons which were modulated by Alfven waves in the inner magnetosphere.

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    Free, publicly-accessible full text available October 1, 2024
  2. Abstract

    A specialized ground‐based system has been developed for simultaneous observations of pulsating aurora (PsA) and related magnetospheric phenomena with the Arase satellite. The instrument suite is composed of (a) six 100 Hz sampling high‐speed all‐sky imagers (ASIs), (b) two 10 Hz sampling monochromatic ASIs observing 427.8 and 844.6 nm auroral emissions, (c) a 20 Hz sampling fluxgate magnetometer. The 100 Hz ASIs were deployed in four stations in Scandinavia and two stations in Alaska, which have been used for capturing the main pulsations and quasi 3 Hz internal modulations of PsA at the same time. The 10 Hz sampling monochromatic ASIs have been operative in Tromsø, Norway with the 20 Hz sampling magnetometer. Combination of these multiple instruments with the European Incoherent SCATter (EISCAT) radar enables us to detect the low‐altitude ionization due to energetic electron precipitation during PsA and further to reveal the ionospheric electrodynamics behind PsA. Since the launch of the Arase satellite, the data from these instruments have been examined in comparison with the wave and particle data from the satellite in the magnetosphere. In the future, the system can be utilized not only for studies of PsA but also for other classes of aurora in close collaboration with the planned EISCAT_3D project.

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    Free, publicly-accessible full text available August 1, 2024
  3. Abstract Erebus volcano, Antarctica, with its persistent phonolite lava lake, is a classic example of an evolved, CO 2 -rich rift volcano. Seismic studies provide limited images of the magmatic system. Here we show using magnetotelluric data that a steep, melt-related conduit of low electrical resistivity originating in the upper mantle undergoes pronounced lateral re-orientation in the deep crust before reaching shallower magmatic storage and the summit lava lake. The lateral turn represents a structural fault-valve controlling episodic flow of magma and CO 2 vapour, which replenish and heat the high level phonolite differentiation zone. This magmatic valve lies within an inferred, east-west structural trend forming part of an accommodation zone across the southern termination of the Terror Rift, providing a dilatant magma pathway. Unlike H 2 O-rich subduction arc volcanoes, CO 2 -dominated Erebus geophysically shows continuous magmatic structure to shallow crustal depths of < 1 km, as the melt does not experience decompression-related volatile supersaturation and viscous stalling. 
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  4. Abstract

    South Pole Station, Antarctica (SPA, magnetic latitude = −74.5°, magnetic local time (MLT) = UT–3.5 h), is a unique observatory which can capture daytime auroral forms throughout austral winter season. We have studied the properties and origin of ultralow‐frequency (ULF) range modulation of daytime diffuse aurora, using data acquired on June 23, 2017 by multi‐instrument measurements at SPA and in situ measurements in the dayside outer magnetosphere. At 1500–1600 UT, monochromatic Pc5‐range pulsations (period ∼10 min) emerged in the midday diffuse auroral region. The sequential 2‐D images reveal that the auroral pulsations result from the repetitive formation of faint, diffuse auroral patches, propagating poleward at a speed of ∼1.5 km s−1. Interestingly, no obviously similar magnetic pulsations were found at SPA. The results differ fundamentally from the ground optical and magnetic signatures expected for a standing field line resonance. On the other hand, the co‐located riometer and VLF receiver observed clearly synchronized pulsations, suggesting that tens‐of‐keV electrons interact with modulated chorus waves and then are scattered down to the auroral pulsation region. During the same interval, the THEMIS‐D spacecraft detected corresponding Pc5 oscillations in the dayside outer magnetosphere (9–10REand ∼15 MLT). The compressional component of the magnetospheric Pc5 waves, presumably driven by an external source, exhibited a good correspondence to the daytime Pc5 auroral pulsations. The simultaneous SPA–THEMIS observations highlight the role of compressional Pc5 pulsations in the dayside outer magnetosphere in determining the periodicity of daytime high‐latitude diffuse auroral pulsations.

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  5. Abstract

    Pc3 range frequency (22–100 mHz) auroral pulsations often occur at daytime high latitudes, equatorward of the cusp/cleft and typically map to the dayside outer magnetosphere. In this paper we present simultaneous observations of compressional Pc3 magnetic pulsations in the dayside outer magnetosphere that occurred in direct association with daytime Pc3 auroral pulsations at South Pole Station (−74.4° magnetic latitude). The pulsations were almost identical at the two locations, and their correlation was clearest when the magnetospheric pulsations were highly monochromatic. Lower‐band chorus waves and keV electron fluxes were also modulated in the Pc3 range, likely by the compressional magnetic pulsations. The common Pc3 frequency in the magnetosphere and aurora matched well with the predicted frequency of upstream ultralow frequency waves. These results provide the first compelling evidence for the direct dayside outer magnetosphere‐ionosphere linkage between upstream‐generated compressional Pc3 waves, Pc3 range modulations of chorus waves and keV electrons, and Pc3 auroral pulsations.

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