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Pc1 geomagnetic pulsations correspond to electromagnetic ion cyclotron (EMIC) waves in the magnetosphere and are excited there with frequencies of 0.2–5 Hz. The instantaneous longitudinal extent of Pc1 waves on the ground has not been estimated yet. In this study, we analyze the Pc1 pulsations observed at seven longitudinally-distributed ground stations at subauroral latitudes at ∼60° magnetic latitude for 1 year from July 2018 to June 2019. The hourly occurrence rates of Pc1 pulsations at all 7 stations have a peak (14%–39.6%) in the post-noon sector and a local minimum (4.1%–8.1%) at midnight. The average frequencies become highest (0.6–1.1 Hz) after midnight and lowest (0.3–0.5 Hz) after noon at all 7 stations. An increasing tendency of total Pc1 occurrence with respect to magnetic latitude was observed. Based on these observations, we obtained a peak of probability distribution of the instantaneous Pc1 longitudinal extent as ∼82.5° with a half maximum at ∼114°, though this probability distribution can be affected by the limitation of the number of the stations. We also made model calculations on the possible longitudinal extent using artificial random Pc1 waves with fixed extents. The comparison of the model results with the observation suggests longitudinal extent of 70°–86° comparable to the peak of probability distribution (∼82.5°). A superposed epoch analysis shows that the longitudinal extent of Pc1 waves tends to increase during recovery phase of geomagnetic storms.

This Polar Science special issue, “SuperDARN/Studies of Geospace Dynamics - Today and Future,” originated from an international SuperDARN (Super Dual Auroral Radar Network) annual workshop held in Japan in June 2019, and is focused on studies of geospace dynamics particularly related to SuperDARN. Its purpose is to overview recent wide and active research, new scientific results and future perspectives mainly through, but not limited to, the scientific papers presented at the workshop. This special issue is an opportunity to commemorate a quarter century since the establishment of SuperDARN in 1995 and to contribute to the further development of geospace sciences and relevant technology. Thirteen valuable papers have been published covering a wide variety of scientific and technical topics. © 2021

We examined the growth and propagation of fine-structured electromagnetic ion cyclotron (EMIC) waves related to time-varying density irregularities using multipoint measurement data observed by Arase, Van Allen Probe A, and two ground-based induction magnetometers (Gakona and Dawson) during a field line conjunction event on April 18, 2019. We analyzed the wave data obtained by the aforementioned spacecraft and stations, and found that the appearance of fine structures in the observed EMIC waves clearly coincided with the ambient electron density irregularities in the magnetosphere, which can cause periodic wave growth and waveguiding on their propagation. Furthermore, we found that the latitudinal widths of the EMIC wave activity region and the wave propagation duct were ∼185 km and less than 80 km at an auroral altitude of 100 km, respectively. We also found thermal ion heating (urn:x-wiley:00948276:media:grl63318:grl63318-math-0001 eV/q) during the EMIC wave activity.

type:Journal Article, We present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar (PANSY) at Syowa Station (SYO; ‐69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System (MAARSY) at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03‐07 UT on March 21, 2017, just after the arrival of corotating interaction region (CIR) in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler‐mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐80 km altitude. Cosmic noise absorption (CNA) measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55‐70 km measured with an MF radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, CNA, and PMWE, can be explained by the interaction between the high‐speed solar wind containing CIRs and the magnetosphere.

An analysis of noise attenuation during 80 solar flares between 2013 and 2017 was carried out at frequencies 8–20 MHz using 34 Super Dual Auroral Radar Network radars and the EKB ISTP SB RAS radar. The attenuation was determined on the basis of noise measurements performed by the radars during the intervals between transmitting periods. The location of the primary contributing ground sources of noise was found by consideration of the propagation paths of radar backscatter from the ground. The elevation angle for the ground echoes was determined through a new empirical model. It was used to determine the paths of the noise and the location of its source. The method was particularly well suited for daytime situations, which had to be limited for the most part to only two crossings through the D region. Knowing the radio path was used to determine an equivalent vertical propagation attenuation factor. The change in the noise during solar flares was correlated with solar radiation lines measured by GOES/XRS, GOES/EUVS, SDO/AIA, SDO/EVE, SOHO/SEM, and PROBA2/LYRA instruments. Radiation in the 1 to 8 Å and near 100 Å are shown to be primarily responsible for the increase in the radionoise absorption, and by inference, for an increase in the D and E region density. The data are also shown to be consistent with a radar frequency dependence having a power law with an exponent of −1.6. This study shows that a new data set can be made available to study D and E regions.

We present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar at Syowa Station (SYO; −69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03–07 UT on 21 March 2017, just after the arrival of corotating interaction region in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐ to 80‐km altitude. Cosmic noise absorption measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55–70 km measured with an medium‐frequency radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, cosmic noise absorption, and PMWE, can be explained by the interaction between the high‐speed solar wind containing corotating interaction regions and the magnetosphere.

Chorus waves, among the most intense electromagnetic emissions in the Earth’s magnetosphere, magnetized planets, and laboratory plasmas, play an important role in the acceleration and loss of energetic electrons in the plasma universe through resonant interactions with electrons. However, the spatial evolution of the electron resonant interactions with electromagnetic waves remains poorly understood owing to imaging difficulties. Here we provide a compelling visualization of chorus element wave–particle interactions in the Earth’s magnetosphere. Through in-situ measurements of chorus waveforms with the Arase satellite and transient auroral flashes from electron precipitation events as detected by 100-Hz video sampling from the ground, Earth’s aurora becomes a display for the resonant interactions. Our observations capture an asymmetric spatial development, correlated strongly with the amplitude variation of discrete chorus elements. This finding is not theoretically predicted but helps in understanding the rapid scattering processes of energetic electrons near the Earth and other magnetized planets.