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This chapter reviews fundamental properties and recent advances of diffuse and pulsating aurora. Diffuse and pulsating aurora often occurs on closed field lines and involves energetic electron precipitation by wave-particle interaction. After summarizing the definition, large-scale morphology, types of pulsation, and driving processes, we review observation techniques, occurrence, duration, altitude, evolution, small-scale structures, fast modulation, relation to high-energy precipitation, the role of ECH waves, reflected and secondary electrons, ionosphere dynamics, and simulation of wave-particle interaction. Finally we discuss open questions of diffuse and pulsating aurora.

This paper reviews our current understanding of auroral features that appear poleward of the main auroral oval within the polar cap, especially those that are known as Sun-aligned arcs, transpolar arcs, or theta auroras. They tend to appear predominantly during periods of quiet geomagnetic activity or northwards directed interplanetary magnetic field (IMF). We also introduce polar rain aurora which has been considered as a phenomenon on open field lines. We describe the morphology of such auroras, their development and dynamics in response to solar wind-magnetosphere coupling processes, and the models that have been developed to explain them.

The sporadic E (Es) layer is an ionospheric layer of very high density in the E region. Prediction of the Es layer generation is not yet possible. To understand the generation mechanisms, the structures of the Es layer and their evolution were studied. Data a ultra-dense global navigation satellite system (GNSS) network over Japan operated by SoftBank, rate-of-total electron content (ROTI) maps with 0 . 05 ∘ × 0 . 05 ∘ resolutions in the latitude and longitudes (approximately 5 × 5 km at the E region altitudes) were derived. Complex fine horizontal structures of the sporadic E (Es) layer which occurred over Japan in the daytime on 17 May 2024 were imaged. Fine-scale structures of the Es layer and their temporal evolution from generation to decay were elucidated. The complex structures, chain of small patches, vortices, and ripples were observed. The scale size and lifetime were similar to those predicted by high spatial resolution numerical simulations. Future studies include the statistics of occurrences of different complex shapes, scale sizes of them, area of the Es layer, and lifetime. Graphical Abstract

Polar cap airglow patches have been known as regions of enhanced 630.0 nm airglow detected by ground-based all-sky imagers at the polar cap latitudes well inside the main auroral oval. Although they were already recognized almost four decades ago as counterparts of polar cap (plasma density) patches, such airglow observations had not been utilized extensively for the studies of ionospheric structures and/or magnetosphere-ionosphere coupling processes in the polar cap. In the last two decades, following the development of highly-sensitive airglow imagers equipped with cooled CCD (Charge Coupled Device) cameras, it has become possible to visualize the dynamical temporal evolution and complicated spatial structure of airglow patches with improved signal-to-noise ratio. Such a progress has enabled us not only to use airglow patches as tracers for plasma convection in the polar cap but also to understand the generation of small-scale plasma irregularities in the ionospheric F region. In addition, recent observations demonstrated a case in which an airglow patch was accompanied by an intense flow channel and corresponding field-aligned current structure along its edges. This implies that airglow patches can signify magnetosphere-ionosphere coupling process in the region of open field lines at the polar cap latitudes, serving as a remote sensing tool just like auroras do. Further studies showed an association of airglow patches with the intensification of aurora on the nightside (Poleward Boundary Intensification: PBI and/or streamer) leading to the expansion phase onset of substorms. This paper reviews such recent progresses in the researches of airglow patches obtained by combining data from all-sky airglow imagers, radars and low-altitude satellite observations in the polar cap.

The phenomenon known as strong thermal emission velocity enhancement (STEVE) is a purple/mauve arc-shaped atmospheric glow observed at lower latitudes of the auroral oval on the duskside. Simultaneous observations using a ground-based camera and a low-altitude satellite have shown that STEVE is accompanied by rapid westward ion flows. Such fast ion flows are termed the subauroral ion drift (SAID) or subauroral polarization stream (SAPS). Similarly, an eastward fast ion flow known as the dawnside auroral polarization stream (DAPS) is observed within the Region 1 current on the dawnside. If the optical phenomenon triggered by SAID/SAPS corresponds to STEVE, a comparable optical phenomenon should be driven by DAPS. Thus far, however, such a phenomenon has not been reported. This study discovers, for the first time, a purple-colored optical phenomenon characterized by the fast eastward ion flows, a possible signature of DAPS, occurring poleward of the bright green arc in the post-midnight sector. We present color all-sky images obtained by a ground-based commercial digital camera, along with wide-coverage optical measurements and in-situ data from low-altitude satellites. The results imply that this glow requires not only a high-speed ion flow but also its sharp latitudinal gradient at the boundary between the Region 1 and 2. Graphical Abstract

Previous studies have shown the occurrence of short-duration plasma flow bursts, typically lasting a few minutes, within the cusp region, often accompanied by the poleward motion of the 630-nm aurora. In this study, we investigated the ion heating characteristics associated with a large-scale plasma flow burst, which coincides with a poleward expansion of the 630-nm aurora. We analyzed data from simultaneous observations using an all-sky imager at Svalbard, the EISCAT Svalbard Radar (ESR), and the SuperDARN radar at Hankasalmi. Additionally, we examined magnetic perturbation data from the Swarm satellites as complementary data to assess the presence of a significant east–west component in the plasma flow. The plasma flow burst was detected by the ESR at approximately 10:40 UT on 8 December 2016, indicated by an increase in the F -region ion temperature ( T i ) due to frictional heating arising from ion–neutral velocity differences. Coinciding with the arrival of the front of the plasma flow burst within the ESR’s field of view (accurate to within 1 min), the poleward boundary of the cusp electron precipitation, as observed in the 630-nm aurora, passed through the ESR’s field of view by moving poleward, seemingly drawn by the front of the plasma flow burst. The altitude profiles of T i revealed a local maximum at an altitude of approximately 150 km, where the temperature increased from ~ 1200 to ~ 1880 K within 4 min. At altitudes between 190 and 300 km T i was lower than this local maximum T i prior to the plasma flow burst, but rapidly increased to approximately match the local maximum T i . However, following this rapid increase, the T i enhancement at these altitudes appeared to be suppressed, with the estimated rate of suppression being −80 and −220 K/min. Based on the investigation through two-dimensional simplified numerical simulations, we suggest that the suppression of T i is likely due to the formation of a relatively fast neutral flow, driven by increased ion drag associated with ionization enhancement resulting from low-energy electron precipitation. Graphical Abstract

Mt. Asama, Nagano Prefecture, Japan, erupted at 11:02 UT on September 1st, 2004. Ionospheric disturbances associated with this eruption were detected using TEC data and HF Doppler observations. It has already been reported that the TEC data obtained from GNSS receivers revealed that N-shaped disturbances in TEC with a period of 76 s occurred 12 min after the eruption. In the HF Doppler sounding observations, on the other hand, a spiky variation in the Doppler frequency with a frequency of about 10 mHz was observed 9–11 min after the eruption. Ray tracing calculations of the acoustic waves confirmed that both N-shaped disturbances in TEC and the spiky variations in the Doppler frequency were generated by the acoustic wave due to the eruption of Mt. Asama. Compared to earthquakes, volcanic eruptions are treated as point sources of atmospheric waves. Therefore, the acoustic waves produced by the present eruption were considered to behave as shock waves. In the HF Doppler observation, subsequent to the spiky signature, a longer-period wavy disturbance appeared. The frequency of these long-period disturbances is about 4 mHz, which is almost the same as the coseismic ionospheric disturbances. The cause of these long-period disturbances is considered to be gravity waves due to the partial transformation of the eruption energy or the acoustic resonance between the ground and lower ionosphere. Although both are typical causes of disturbance associated with earthquakes, it was difficult to determine which was the primary cause in the present case. In order to clarify the cause of wavy disturbances in the present event, it is necessary to investigate atmospheric waves associated with volcanic eruptions in detail using additional observation data, such as ground-based infrastructure sound networks. Graphical Abstract

The explosive eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15 January 2022 generated atmospheric waves traveling around the Earth, which caused ionospheric disturbances on various spatio-temporal scales. A HF Doppler sounding system in Japan detected characteristic ionospheric disturbances showing periodic oscillations in the Doppler frequency with a period of ~ 4 min. In this study, such periodic oscillations were examined by comparing Doppler frequency data with Total Electron Content data obtained by Global Navigation Satellite System. The observed periodic oscillations in the Doppler frequency were characterized by a sawtooth or S-letter shaped variation, implying the passage of the traveling ionospheric disturbances through the reflection points of the HF Doppler sounding system. It was also found that the periodic oscillations occurred prior to the arrival of the tropospheric Lamb wave excited by the Tonga eruption. From the total electron content data, the traveling ionospheric disturbances causing the periodic oscillations were excited by the tropospheric Lamb waves at the conjugate point in the southern hemisphere, namely, the electric field perturbations due to the Lamb waves in the southern hemisphere mapped onto the sensing area of the HF Doppler sounding system in the northern hemisphere along the magnetic field lines. The periodic oscillations were observed only in the path between Chofu transmitter and Sarobetsu receiver, whose the radio propagation path is almost aligned in the north–south direction. This suggests that the traveling ionospheric disturbance has a structure elongating in the meridional direction. The variation in the Doppler frequency was reproduced by using a simple model of the propagation of the traveling ionospheric disturbances and the resultant motion of the reflection point. As a result, the vertical motion of the reflection point associated with the periodic oscillations was estimated to be about 1 km. It is known that 4-min period variations are sometimes observed in association with earthquakes, which is due to resonances of acoustic mode waves propagating between the ground and the lower ionosphere. Therefore, a similar resonance structure in the southern hemisphere is a plausible source of the traveling ionospheric disturbances detected in the northern hemisphere.

The activity of citizen scientists who capture images of aurora borealis using digital cameras has recently been contributing to research regarding space physics by professional scientists. Auroral images captured using digital cameras not only fascinate us, but may also provide information about the energy of precipitating auroral electrons from space; this ability makes the use of digital cameras more meaningful. To support the application of digital cameras, we have developed artificial intelligence that monitors the auroral appearance in Tromsø, Norway, instead of relying on the human eye, and implemented a web application, “Tromsø AI”, which notifies the scientists of the appearance of auroras in real-time. This “AI” has a double meaning: artificial intelligence and eyes (instead of human eyes). Utilizing the Tromsø AI, we also classified large-scale optical data to derive annual, monthly, and UT variations of the auroral occurrence rate for the first time. The derived occurrence characteristics are fairly consistent with the results obtained using the naked eye, and the evaluation using the validation data also showed a high F1 score of over 93%, indicating that the classifier has a performance comparable to that of the human eye classifying observed images.

We introduce a network observation of anomalous long-distance propagation of aeronautical navigation (NAV) very high-frequency (VHF) radio waves due to sporadic E layer (Es). The system has been operative since May 2019 at 6 stations in Japan. The receiver consists of a log-periodic antenna, air-band filter, software-defined radio and small computer which is capable of recording the strength of radio signals in the frequency band from 98 to 118 MHz with a temporal resolution of 2 s. The receiver covers not only the NAV band including VHF omnidirectional radio range (VOR), instrument landing system localizer (ILS LOC) and ground-based augmentation system VHF data broadcast (GBAS VDB) from 108 to 118 MHz, but also broadcasting channels from 98 to 108 MHz. Soon after the start of the full operation of the network observation, a strong Es event was detected by an ionosonde in Tokyo during the daytime on July 4, 2019, in which foEs (critical frequency of Es) was sometimes higher than 15 MHz. The VHF radio wave monitoring system detected multiple signatures of Es anomalous propagation (EsAP) at all the stations extending from Okinawa to Hokkaido. At some stations, the EsAP signatures continued for a few hours with some brief intervals of disappearance. The observed correspondence between the enhancement of foEs and the occurrence of anomalous propagation confirmed that an extreme electron density enhancement within Es caused the anomalous long-distance propagation of VHF NAV signals. The data from this network observation can be browsed at http://gwave.cei.uec.ac.jp/cgi-bin/vor/vhf.cgi in near real-time basis. This near real-time monitoring capability allows people in the aeronautical operation community such as air navigation service providers, pilots, and airline engineers to check the propagation environment of VHF NAV signals online, which contributes to a mitigation of ionospheric space weather impacts on the aeronautical navigation systems. Not only that the current method for detecting Es in a wide area can be used to visualize the spatial distribution of Es in two-dimensional fashion through a combination of other observation techniques such as ionosondes and total electron content (TEC) measurements using Global Navigation Satellite System (GNSS) signals.