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  1. Abstract

    Enhancements in electron density in the D‐region ionosphere attributed to the precipitation of high‐energy electrons, have previously been inferred from increases in cosmic radio noise absorption (CNA) using ground‐based riometers. However, there have been few studies of CNA observations at multi‐point stations distributed in longitudes. Thus, the spatio‐temporal development of the global distribution of CNA is not well understood. In this study, we investigated the longitudinal extent of CNA using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland. These stations are located encircling the earth at ∼60° north magnetic latitudes. We have conducted simultaneous observations of CNA at these stations since October 2017. Here we focus on seven substorms during a geomagnetic storm 25–28 August 2018 and study the spatio‐temporal development of the global distribution of CNA during these substorms. For all seven substorms, some stations observed CNA enhancements after the substorm onsets. In five cases, the CNA enhancements started around midnight and expanded eastward. The other two cases show westward and anti‐sunward development of CNA. The eastward expansion of CNA indicates the eastward drift of high‐energy electrons, which is the source of the CNA, due to gradient and curvature drift in the geomagnetic field. The westward expansion of CNA may correspond to westward expansion of the substorm injection region due to dawn‐to‐dusk electric fields. These results indicate that spatio‐temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere.

     
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    Free, publicly-accessible full text available January 1, 2025
  2. 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
  3. Abstract

    Artificial periodic inhomogeneity or API experiments were conducted at the HAARP facility in Gakona, Alaska, in October 2022. The experiments concentrated on measuring ionospheric irregularities induced in theE‐region. The irregularities exhibited characteristics regarding their occurrence altitudes, rise and fall times, and Doppler shifts comparable to results from experiments conducted previously at HAARP and elsewhere. The irregularities also occurred in discrete altitude bands. Seeking to quantify these results, we constructed a simple, one‐dimensional fluid model which includes the effects of HF wave heating (direct and indirect) together with electron and ion cooling and thermal conduction, ion production, loss, and diffusion. Critically, the model includes a potential solver and can represent the ambipolar electric field. The model produced API irregularities in three distinct altitude bands which decayed according to the ambipolar diffusion rate.

     
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  4. 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
  5. Free, publicly-accessible full text available September 20, 2024
  6. Electromagnetic waves excited by satellites and space debris moving through the earth’s plasma in low earth orbit can be detected in situ by a technique called Space Object Identification by Measurements of Orbit-Driven Waves (SOIMOW). Proximity measurements of space objects with plasma waves may allow tracking of space debris below the normal detection thresholds traditionally accomplished by optical telescopes and radar ranging sensors. SOIMOW uses in situ plasma receivers to identify space objects during orbital conjunctions. Satellites and other space objects moving through the near-earth ionosphere between 200 and 1000 km altitude become electrically charged by both electron collection and photo emission in sunlight. These hypersonic, charged objects excite a wide range of plasma waves. The SOIMOW technique has shown that electromagnetic plasma waves from known objects may be observed out to ranges of tens of kilometers, providing information on presence of the space objects. The SOIMOW concept has been demonstrated with the Radio Receiver Instrument (RRI) on the Swarm-E satellite. The amplitude, spectral, and polarization changes of the RRI data are consistent with electromagnetic, compressional Alfvén waves that are launched by charged space objects traveling across magnetic field lines. In addition, electrostatic waves at the space object can be generated by a lower hybrid drift or an ion acoustic wave instability. Both in situ electric field probes and remote detection of scattered satellite waves are being investigated to determine the location of orbiting objects. 
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    Free, publicly-accessible full text available September 19, 2024
  7. A sampling of the environment around inert space objects has determined, which linear and nonlinear waves are created by moving space debris of all sizes. Plasma waves excited by satellites and space debris moving through the Earth's plasma in low earth orbit have been measured with in situ electric field sensors on other satellites. These orbit driven plasma waves are of interest for proximity detection of space debris and sources of electrostatic and electromagnetic noise on spacecraft. Satellites and other space objects moving through the near-earth ionosphere between 200 and 1000 km altitude become electrically charged by both electron collection and photo emission in sunlight. These hypersonic, charged objects can excite a wide range of plasma waves. Measurements with the Radio Receiver Instrument (RRI) on the Swarm-E satellite have shown that electromagnetic plasma waves from known objects can be observed out to ranges of tens of kilometers. The amplitude, spectral, and polarization changes of the RRI data are consistent with electromagnetic, compressional Alfvén waves launched by charged space objects traveling across magnetic field lines. In addition, electrostatic lower hybrid waves or nonlinear ion acoustic pinned oscillations may have been self-generated and measured on the Swarm-E satellite. It is proposed that measurements of these waves with local electric field measurements or remote electromagnetic wave scatter may be useful to design systems for the location of orbiting objects. Spatial and temporal details of spacecraft charging are key to understanding the extent of waves associated with the object motion in space plasmas.

     
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    Free, publicly-accessible full text available September 1, 2024
  8. Free, publicly-accessible full text available August 19, 2024