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A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Space Science".

Deadline for manuscript submissions: closed (30 November 2023) | Viewed by 17635

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Special Issue Editors

Prof. Dr. Qiugang Zong

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Guest Editor

School of Earth and Space Science, Peking University, Beijing 100871, China
Interests: heliospheric physics; cosmic ray; interstellar gas; interstellar magnetic field; radiation belt

Prof. Dr. Zejun Hu

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Guest Editor

Center for Space Physics and Astronomy, Polar Research Institute of China, Shanghai 200136, China
Interests: space weather; aurora physics

Dr. Zhonghua Yao

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Guest Editor

Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Interests: planetary aurorae; particle energization and X-ray emission at Jupiter; magnetic reconnection and magnetic dipolarization in planetary magnetospheres

Dr. Jianjun Liu

E-Mail Website
Guest Editor

Center for Space Physics and Astronomy, Polar Research Institute of China, Shanghai 200136, China
Interests: aurora and ionospheric convection research; space weather

Special Issue Information

Dear Colleagues,

The auroras are very amazing universal phenomena, widely present in solar system planets and particularly those with global magnetic fields and atmospheres, e.g., Earth, Jupiter, Saturn, Neptune, Uranus. They are believed to be powered by the interaction of solar wind with the planetary system and produced through the collision of precipitating charged particles and upper atmosphere.

There are still, however, crucial gaps in our knowledge about the physical mechanisms and dynamic processes of the various types of auroras—for instance, the acceleration mechanisms of precipitating electrons for discrete auroras, the mechanisms responsible for pitch angle scattering of electrons and protons responsible for diffuse aurora, the physical processes related to the spatiotemporal structuring of discrete and diffuse auroras, and so on. The developments of aurora observations have promoted the explanations of auroral phenomenology, but they often fall short in key aspects and lack a unified theoretical framework. Furthermore, while some similar auroral phenomena have been identified in other planets, there are also significant differences. Based on the similarities and differences among planets, we could potentially assess the basic ideas of auroral processes. Many new satellite programs and observation programs have been implemented recently, providing unprecedented opportunities to further explore the physical mechanisms behind auroras.

Potential topics include but are not limited to:

Auroral morphology;
Auroral instruments;
Auroral acceleration region;
Auroras in the planetary system;
Auroral particles precipitation;
Wave–particle interaction related to auroras;
Substorms and storms auroras.

Prof. Dr. Qiugang Zong
Prof. Dr. Zejun Hu
Dr. Zhonghua Yao
Dr. Jianjun Liu
Guest Editors

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Keywords

auroral physics
discrete and diffuse auroras
auroral substorms
shock aurora
wave-particle interaction
ULF wave
day-side and night-side reconnections
kelvin-helmholtz instability

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Published Papers (10 papers)

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Research

12 pages, 14170 KiB

Open AccessArticle

Variations in Pulsating Aurora Emission in 337 nm and 391 nm Nitrogen Spectral Lines during Geomagnetic Substorms

by Pavel Klimov, Vera Nikolaeva, Alexander Belov, Boris Kozelov, Alexei Murashov, Alexei Roldugin and Sergei Sharakin

Universe 2023, 9(10), 441; https://doi.org/10.3390/universe9100441 - 30 Sep 2023

Cited by 1 | Viewed by 1438

Abstract

Spectroscopic measurements of aurora emissions provide valuable insights into the altitude of electron atmospheric penetration and their maximum energy. To achieve this, the photometers used in the PAIPS (Pulsating Aurora Imaging Photometers System) project are equipped with spectrometers. These spectrometers enable the measurement of auroral emissions in narrow spectral lines with a temporal resolution of milliseconds. In this study, we present two cases of PsA (Pulsating Aurora) measurements in the 337 nm and 391 nm spectral lines. We demonstrate that during quiet geomagnetic conditions the ratio of night sky emissions in these bands is close to one and significantly increases during substorms. We propose and implement a special procedure for estimating this ratio. Our findings reveal that the intensity of emissions in both spectral lines correlates with the AL index of geomagnetic activity. However, the ratio between the emissions fluctuates around constant values over time and does not undergo significant changes throughout the entire PsA event, which can last for more than an hour. Full article

(This article belongs to the Special Issue Auroral Physics)

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11 pages, 6309 KiB

Open AccessArticle

Conjunction Ground Triangulation of Auroras and Magnetospheric Processes Observed by the Van Allen Probe Satellite near 6 Re

by Boris V. Kozelov and Elena E. Titova

Universe 2023, 9(8), 353; https://doi.org/10.3390/universe9080353 - 29 Jul 2023

Viewed by 1128

Abstract

Conjunction observations of auroras with electron distributions and broadband electrostatic fluctuations on Van Allen Probe A satellite in the equatorial region are considered. Using triangulation measurements, the energy spectra of the precipitating electrons in the rayed auroral structures were determined for the 17 March 2015 event. A comparison of the spectra of precipitating electrons in the auroral rays with satellite measurements of electrons in the equatorial region related to the aurora showed their agreement. The concomitance between Van Allen Probe A broadband electric waves and auroral variations measured by the ground-based auroral camera was observed on 17 March 2015. This suggests that broadband electrostatic waves may be responsible for electron precipitation, leading to the formation of rayed structures in the aurora. Full article

(This article belongs to the Special Issue Auroral Physics)

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12 pages, 1106 KiB

Open AccessArticle

Aurora Classification in All-Sky Images via CNN–Transformer

by Jian Lian, Tianyu Liu and Yanan Zhou

Universe 2023, 9(5), 230; https://doi.org/10.3390/universe9050230 - 15 May 2023

Cited by 7 | Viewed by 1865

Abstract

An aurora is a unique geophysical phenomenon with polar characteristics that can be directly observed with the naked eye. It is the most concentrated manifestation of solar–terrestrial physical processes (especially magnetospheric–ionospheric interactions) in polar regions and is also the best window for studying solar storms. Due to the rich morphological information in aurora images, people are paying more and more attention to studying aurora phenomena from the perspective of images. Recently, some machine learning and deep learning methods have been applied to this field and have achieved preliminary results. However, due to the limitations of these learning models, they still need to meet the requirements for the classification and prediction of auroral images regarding recognition accuracy. In order to solve this problem, this study introduces a convolutional neural network transformer solution based on vision transformers. Comparative experiments show that the proposed method can effectively improve the accuracy of aurora image classification, and its performance has exceeded that of state-of-the-art deep learning methods. The experimental results show that the algorithm presented in this study is an effective instrument for classifying auroral images and can provide practical assistance for related research. Full article

(This article belongs to the Special Issue Auroral Physics)

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13 pages, 14387 KiB

Open AccessArticle

The Response of Auroral-Oval Waves to CIR-Driven Recurrent Storms: FY-3E/ACMag Observations

by Zhi-Yang Liu, Wei-Guo Zong, Qiu-Gang Zong, Jin-Song Wang, Xiang-Qian Yu, Yong-Fu Wang, Hong Zou, Sui-Yan Fu, Chao Yue, Ze-Jun Hu and Jian-Jun Liu

Universe 2023, 9(5), 213; https://doi.org/10.3390/universe9050213 - 28 Apr 2023

Cited by 1 | Viewed by 1371

Abstract

Alfven-branch waves provide an efficient means for transporting energy into the auroral oval. Here, we report observations of these waves obtained by the Fengyun-3E (FY-3E)/ACMag instruments, which are designed to detect three-dimensional AC magnetic fields in the 0.05–25 Hz band. The observations suggest that broadband waves are a permanent feature of the auroral oval, although their amplitude and locations vary with the global state of the magnetosphere. We primarily focus on the data obtained from 10 July 2021 to 26 August 2021, during which a series of recurrent storms driven by solar wind corotating interaction regions (CIRs) occurred. Analysis of the observations shows that the auroral-oval waves grow in amplitude (1–3 orders of magnitude) and shift to lower latitude (∼10°) immediately following the decrease in the SYM-H index in each storm. Further investigation reveals the response of the auroral-oval waves has a time scale equal to or less than FY-3E’s effective revisiting time, which is about 45 min. The observations presented in this paper confirm that the FY-3E/ACMag instruments provide a high-resolution monitor of the auroral-oval waves and could further our understanding of auroral physics. Full article

(This article belongs to the Special Issue Auroral Physics)

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19 pages, 11851 KiB

Open AccessArticle

Classification and Distribution of the Dayside Ion Upflows Associated with Auroral Particle Precipitation

by Yao Yu, Ze-Jun Hu, Hong-Tao Cai and Yi-Sheng Zhang

Universe 2023, 9(4), 164; https://doi.org/10.3390/universe9040164 - 29 Mar 2023

Viewed by 1437

Abstract

Two important phenomena of the solar wind–magnetosphere–ionosphere coupling are auroral particle precipitation and the formation of ions flowing upward from the ionosphere. They have opposite transport directions of energy and substance. Based on the observations of particle precipitation and ion drift from the DMSP F13 satellite in January and July 2005, the ionospheric ion upflows in dayside auroral oval (0600–1800 MLT) can be divided into five types according to the velocity of ion upflows and the spectrum characteristics of auroral particle precipitation, and the distribution for different types of ion upflows is studied. The results show that the ion upflows mainly occur in the geomagnetic latitude (MLAT) range of 70–80°.The main magnetospheric source region of ion upflows (type A and D) caused by the accelerated electron (mainly the soft electron) corresponds to Low Latitude Boundary Layer (LLBL) and Cusp, and ion upflows of type B and C (related to the process of ambipolar diffusion caused by electron acceleration) mainly occur in LLBL and Boundary Plasma Sheet (BPS), while ion upflows of type E without electron acceleration mainly occur in the central plasma sheet (CPS).The dawn–dusk asymmetry is obvious in the winter season, with the ion upflows mainly occurring on the dawn/dusk side ionosphere. However, the ion upflows in summer mainly occur at the magnetic noon, with a symmetric distribution centered at the magnetic noon. The occurrence of ion upflow in winter is significantly higher than that in summer, and it is significantly enhanced during the period of moderate geomagnetic activity. The upward region expands to the lower latitude when the geomagnetic activity is enhanced. The effect of interplanetary magnetic field (IMF) components has also been studied in this paper. When IMF Bx is negative, the upflow occurrence increases in the region of 1500–1800 MLT and 0600–0900 MLT, with the MLAT range below 70°. The direction of IMF By may lead to the high-incidence area reverse at the prenoon or postnoon region. The occurrence of ion upflows with the MLAT range below 75° increases significantly when IMF is southward. Type A ion upflow has the highest velocity of ion upflows, followed by type E, and type D has the lowest. The average velocity of ion upflows in winter is significantly higher than that in summer. Full article

(This article belongs to the Special Issue Auroral Physics)

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11 pages, 2840 KiB

Open AccessArticle

The Dynamics of Earth’s Cusp in Response to the Interplanetary Shock

by Jie Ren, Qiugang Zong, Suiyan Fu, Huigen Yang, Zejun Hu, Xiaoxin Zhang, Xuzhi Zhou, Chao Yue, Lynn Kistler, Patrick Daly, Elena Kronberg and Robert Rankin

Universe 2023, 9(3), 143; https://doi.org/10.3390/universe9030143 - 8 Mar 2023

Cited by 1 | Viewed by 1719

Abstract

The Earth’s magnetospheric cusp, a region with an off-equatorial magnetic field minimum, is an important place which directly transports plasma and energy from the solar wind into the magnetosphere and ionosphere. Its magnetic topology and charged particles therein are known to respond to the solar wind and the interplanetary magnetic field. However, its dynamics in response to the interplanetary (IP) shock are still unknown, due to lack of direct spacecraft observations. This study first reports the observations of the cusp’s motion under the drive of an IP shock and both strong electric fields and outflowing energetic ions in the moving cusp. After an IP shock arrival on 7 September 2017, triple cusps were observed by Cluster C4 when it was crossing the high-altitude northern polar region to the sub-solar magnetosphere. The multiple cusps had a one-to-one correspondence with the dayside magnetosphere compression and relaxation detected by THEMIS E, indicating that one cusp moved back and forth three times due to the IP shock’s impact. In the moving cusp, there were strong impulsive electric fields with a peak of up to ∼40 mV/m and an ionospheric source population of upward propagating ions (O

^{+}

, He

^{+}

and H

^{+}

) with energies extending to MeV. However, the outflowing ions outside the cusp had energies of no more than 1 keV. An enhancement of energetic O

^{+}

appeared inside the cusp with the flux ratio of O

^{+}

^{+}

increasing from 10 keV to ∼ MeV, which implies the efficient acceleration of O

^{+}

. These observations are shown to be consistent with the prompt acceleration by the impulsive electric fields, which is mass-dependent. This finding suggests a new acceleration mechanism for cusp energetic ions, especially for O

^{+}

. Full article

(This article belongs to the Special Issue Auroral Physics)

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11 pages, 2799 KiB

Open AccessArticle

Automated Classification of Auroral Images with Deep Neural Networks

by Zhiyuan Shang, Zhonghua Yao, Jian Liu, Linli Xu, Yan Xu, Binzheng Zhang, Ruilong Guo and Yong Wei

Universe 2023, 9(2), 96; https://doi.org/10.3390/universe9020096 - 12 Feb 2023

Cited by 5 | Viewed by 1846

Abstract

Terrestrial auroras are highly structured that visualize the perturbations of energetic particles and electromagnetic fields in Earth’s space environments. However, the identification of auroral morphologies is often subjective, which results in confusion in the community. Automated tools are highly valuable in the classification of auroral structures. Both CNNs (convolutional neural networks) and transformer models based on the self-attention mechanism in deep learning are capable of extracting features from images. In this study, we applied multiple algorithms in the classification of auroral structures and performed a comparison on their performances. Trans-former and ConvNeXt models were firstly used in the analysis of auroras in this study. The results show that the ConvNeXt model can have the highest accuracy of 98.5% among all of the applied algorithms. This study provides a direct comparison of deep learning tools on the application of classifying auroral structures and shows promising capability, clearly demonstrating that auto-mated tools can help to minimize the bias in future auroral studies. Full article

(This article belongs to the Special Issue Auroral Physics)

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18 pages, 3813 KiB

Open AccessArticle

Temperature Variations in the Mesosphere and Lower Thermosphere during Geomagnetic Storms with Disparate Durations at High Latitudes

by Guanchun Wei, Jianyong Lu, Wenbin Wang, Yufeng Tian, Jingyuan Li, Shiping Xiong, Meng Sun, Fuzhen Shen, Zheng Li, Hua Zhang, Jingqi Cui, Chaolei Yang, Jingrui Yao, Shuwen Jiang, Zhixin Zhu and Jingye Wang

Universe 2023, 9(2), 86; https://doi.org/10.3390/universe9020086 - 5 Feb 2023

Cited by 2 | Viewed by 1513

Abstract

Using the temperature data observed from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), we investigate the response of the mesosphere and lower thermosphere (MLT) to two medium geomagnetic storms with disparate durations, on 20 April 2018 and 10 April 2022. The high-latitude MLT temperature increase in the Southern hemisphere can reach 40 K during April 2018 geomagnetic storm with a longer duration (Kp values greater than 4 for 15 h), while the temperature variations are less than 10 K for the April 2022 event (Kp values greater than 4 for 6 h). To investigate the different temperature responses to disparate geomagnetic storm durations and understand what physical process results in this difference, we simulated the two events using the thermosphere ionosphere mesosphere electrodynamics general circulation model (TIMEGCM). The simulations show that more particles and energy input in longer-duration geomagnetic storms produce larger ion drag force and pressure gradient force at ~130 km, and then the enhanced two forces cause faster horizontal wind, leading to larger horizontal divergence. Subsequently, the stronger downward vertical wind is transported to the MLT region (below 110 km) and ultimately makes greater temperature increases through adiabatic heating/cooling and vertical advection. Therefore, the effects of the storm’s duration on the MLT temperature are also important. Full article

(This article belongs to the Special Issue Auroral Physics)

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14 pages, 11446 KiB

Open AccessArticle

Automatic Identification of Aurora Fold Structure in All-Sky Images

by Qian Wang, Haonan Fang and Bin Li

Universe 2023, 9(2), 79; https://doi.org/10.3390/universe9020079 - 1 Feb 2023

Cited by 3 | Viewed by 1949

Abstract

Identification of small-scale auroral structures is key to searching for auroral events. However, it is impracticable for humans to manually select sufficient aurora events for statistical analysis, and it is also challenging for computers because of the non-rigid shape and fluid nature of auroras. Fold structure is the most common type of auroral small-scale structure, and its appearance is indicative of a variety of auroral events. This paper proposes a small-scale aurora structure identification framework to automatically detect aurora fold structures. First, the location and shape of auroras are identified based on a deep learning segmentation network. Then, the skeleton of the auroral shape is extracted to represent the trajectories of auroras. Finally, the proposed skeleton-based fold identification module (SFIM) can detect the aurora fold structure. To evaluate the effectiveness of the proposed method, we built an aurora fold structure sample dataset, namely F-dataset, containing 2000 images at 557.7 nm obtained by the all-sky imagers at Yellow River Station (YRS), Ny-Ålesund, Svalbard. Experimental results show that automatic identification can achieve good consistency with human perception. Statistical analysis of over 30,000 images shows that the fold occurrence has a distinct double-peak distribution at pre-noon and post-noon. Full article

(This article belongs to the Special Issue Auroral Physics)

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14 pages, 5127 KiB

Open AccessArticle

Transient Flashes in Saturn’s UV Aurora: An Analysis of Hubble Space Telescope 2013–2017 Campaigns and Cassini Magnetic Field Measurements

by Tianshu Qin, Sarah V. Badman, Joe Kinrade and Alexander Bader

Universe 2022, 8(11), 602; https://doi.org/10.3390/universe8110602 - 17 Nov 2022

Viewed by 1712

Abstract

We examined Hubble Space Telescope images of Saturn’s northern UV aurora in 2013–2017, identified 29 short-lived flashes, and examined simultaneous magnetometer data collected by the Cassini orbiter. When observation cadence permitted, a flash lifetime of 4–17 min (subject to exposure time-related uncertainties), and [...] Read more.

^{°}

). These transient flashes are identified in both the presence and absence of Saturn’s main auroral oval, indicating the lack of dependence on the main emission power. The concurrent magnetic field pulsations generally form a sawtooth shape, and the local field strength experiences a change of 0.2 to 2.0 nT (depending on the distance of Cassini). The quasiperiodic pulsation events were all detected when the spacecraft was in the southern hemisphere with conjugate flashes in northern aurora, suggesting these events occur on closed field lines, and typically showing a sudden transition to a less lagging, more southward magnetic field configuration. We also found the ionospheric footprint of the spacecraft must be close to the region of flashes for magnetic field pulsations to be detected, implying a localised rather than global driving process. Full article

(This article belongs to the Special Issue Auroral Physics)

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