An Ephemeral Red Arc Appeared at 68° MLat at a Pseudo Breakup During Geomagnetically Quiet ConditionsOyama, S.; Shinbori, A.; Ogawa, Y.; Kellinsalmi, M.; Raita, T.; Aikio, A.; Vanhamäki, H.; Shiokawa, K.; Virtanen, I.; Cai, L.; Workayehu, A. B.; Pedersen, M.; Kauristie, K.; Tsuda, T. T.; Kozelov, B.; Demekhov, A.; Yahnin, A.; Tsuchiya, F.; Kumamoto, A.; Kasahara, Y.; Matsuoka, A.; Shoji, M.; Teramoto, M.; Lester, M.
doi: 10.1029/2020JA028468pmid: N/A
Various subauroral optical features have been studied by analyzing data collected during periods of geomagnetic disturbances. Most events have been typically found at geomagnetic latitudes of 45–60°. In this study, however, we present a red arc event found at geomagnetic 68° north (L ≈ 7.1) in the Scandinavian sector during a period of geomagnetically quiet conditions within a short intermission between two high‐speed solar wind events. The red arc appeared to coincide with a pseudo breakup at geomagnetic 71–72°N and a rapid equatorward expansion of the polar cap. However, the red arc disappeared in approximately 7 min. Simultaneous measurements with the Swarm A/C satellites indicated the appearance of the red arc at the ionospheric trough minimum and a conspicuous enhancement of the electron temperature, suggesting the generation of the arc by heat flux. Since there are meaningful differences in the red arc features from already‐known subauroral optical features such as the stable auroral red (SAR) arc, we considered that the red arc is a new phenomenon. We suggest that the ephemeral red arc may represent the moment of SAR arc birth associated with substorm particle injection, which is generally masked by bright dynamic aurorae.
Comparison of Electron Density and Temperature From the CSES Satellite With Other Space‐Borne and Ground‐Based ObservationsYan, Rui; Zhima, Zeren; Xiong, Chao; Shen, Xuhui; Huang, Jianping; Guan, Yibing; Zhu, Xinghong; Liu, Chao
doi: 10.1029/2019JA027747pmid: N/A
In this paper we provide a comprehensive comparison of in situ electron density (Ne) and temperature (Te) measured by Langmuir probe (LAP) on board the China Seismo‐Electromagnetic Satellite (CSES), with nearly simultaneous measurements from the Swarm B satellite, incoherent scatter radar (ISR) at Millstone Hill, as well as predictions from empirical models including the incoherent scatter radar ionospheric model (ISRIM) for Millstone Hill and International Reference Ionosphere model (IRI‐2016). Results reveal that the global distributions and their relative variations of Ne/Te from CSES and Swarm are quite consistent during conjunction periods of the two satellites, although the absolute values of Swarm Ne are proportionally larger than that of CSES. The large‐scale ionospheric structures, such as the equatorial ionization anomaly (EIA), the longitudinal wave number (WN3/4), the Weddell Sea anomaly (WSA), the northern midlatitude summer nighttime anomaly (MSNA), and the midlatitude ionospheric trough (MIT), are well represented by the CSES measurements. For the temporal variation over Millstone Hill station, CSES Ne at nightside shows some different characteristics from the predictions of IRI and ISRIM, possibly due to the influences of MIT and midlatitude arc (MLA) that are often observed at latitudes of Millstone Hill. Our results suggest that the CSES in situ plasma parameters are reliable with a high scientific potential for investigation of geophysics and space physics.
Long‐Term Dropout of Relativistic Electrons in the Outer Radiation Belt During Two Sequential Geomagnetic StormsWu, H.; Chen, T.; Kalegaev, V. V.; Panasyuk, M. I.; Vlasova, N. A.; Duan, S.; Zhang, X.; He, Z.; Luo, J.; Wang, C.
doi: 10.1029/2020JA028098pmid: N/A
On 31 January 2016, the flux of >2 MeV electrons observed by Geostationary Operational Environmental Satellite (GOES)‐13 dropped to the background level during a minor storm main phase (−48 nT). Then, a second storm (−53 nT) occurred on 2 February; during the 3 days after its main phase, the flux remained at background level. Using data from various instruments on the GOES, Polar Operational Environmental Satellites (POES), Radiation Belt Storm Probes (RBSP), Meteor‐M2, and Fengyun‐series spacecraft, we study this long‐term dropout of MeV electrons during two sequential storms of similar magnitude under lightly disturbed solar wind conditions. Observations from low‐altitude satellites show that the fluxes decreased first at higher L‐shells and then gradually propagated inward. Moreover, the fluxes were almost completely lost and dropped to the background level at L > 5, while the fluxes at 4 < L < 5 were partly lost, as observed by RBSP and low‐altitude satellites. Finally, observations show that on 5 February, only the fluxes at L > 5.5 recovered, while the fluxes at 4 < L < 5 did not return to the prestorm levels. These observations indicate that the loss and recovery processes developed first at higher L‐shells. Phase space density (PSD) analysis shows that radial outward diffusion was the main reason for the dropout at higher L‐shells. Regarding electron enhancement, stronger inward diffusion was accompanied by ultra‐low‐frequency (ULF) wave activities at higher L‐shells, and chorus waves observed at outer L‐shells provided conditions for relativistic electron flux recovery to the prestorm levels.
Resonant Whistler‐Electron Interactions: MMS Observations Versus Test‐Particle SimulationBehar, E.; Sahraoui, F.; Berčič, L.
doi: 10.1029/2020JA028040pmid: N/A
We present a novel technique to analyze VDF, which allows to capture their fine structure including wave‐particles resonances. By applying the technique to magnetospheric multiscale (MMS) data, the simultaneous observation of characteristic three‐dimensional (3‐D) signatures in the electron velocity distribution function (VDF) and intense quasi‐monochromatic waves in the terrestrial magnetosheath is investigated. The intense wave packets are characterized and modeled analytically as quasi‐parallel circularly polarized whistler waves and applied to a test‐particle simulation in view of gaining insight into the signature of the wave‐particle resonances in velocity space. Both the Landau and the cyclotron resonances were evidenced in the test‐particle simulations. The location and general shape of the test‐particle signatures do account for the observations, but the finer details, such as the symmetry of the observed signatures, are not matched, indicating either the limits of the test‐particle approach or a more fundamental physical mechanism not yet grasped. Finally, it is shown that the energization of the electrons in this precise resonance case cannot be diagnosed using the moments of the distribution function, as done with the classical E · J “dissipation” estimate.
The Global Distribution of Ultralow‐Frequency Waves in Jupiter's MagnetosphereManners, H.; Masters, A.
doi: 10.1029/2020JA028345pmid: N/A
Jupiter's giant magnetosphere is a complex system seldom in a configuration approximating steady state, and a clear picture of its governing dynamics remains elusive. Crucial to understanding how the magnetosphere behaves on a large scale are disturbances to the system on length‐scales comparable to the cavity, which are communicated by magnetohydrodynamic waves in the ultralow‐frequency band (<1 mHz). In this study we used magnetometer data from multiple spacecraft to perform the first global heritage survey of these waves in the magnetosphere. To map the equatorial region, we relied on the large local‐time coverage provided by the Galileo spacecraft. Flyby encounters performed by Voyager 1 and 2, Pioneer 10 and 11, and Ulysses provided local‐time coverage of the dawn sector. We found several hundred events where significant wave power was present, with periods spanning ∼5–60 min. The majority of events consisted of multiple superposed discrete periods. Periods at ∼15, ∼30, and ∼40 min dominated the event‐averaged spectrum, consistent with the spectra of quasi‐periodic pulsations often reported in the literature. Most events were clustered in the outer magnetosphere close to the magnetopause at noon and dusk, suggesting that an external driving mechanism may dominate. The most energetic events occurred close to the planet, though more sporadically, indicating an accumulation of wave energy in the inner magnetosphere or infrequent impulsive drivers in the region. Our findings suggest that dynamics of the system at large scales is modulated by this diverse population of waves, which permeate the magnetosphere through several cavities and wave guides.
Role of Bottom‐Side Density Gradient in the Development of Equatorial Plasma Bubble/Spread F Irregularities: Solar Minimum and Maximum ConditionsAbdu, M. A.; Kherani, E. A.; Sousasantos, J.
doi: 10.1029/2020JA027773pmid: N/A
From the analysis of Digisonde data over Brazilian equatorial and low‐latitude sites, we investigate the relative importance of the different parameters driving the generation of rising bubble‐type and bottom‐type spread F (SF) irregularities. Data for the complete month of October 2001, a solar maximum epoch (F10.7 = 210), and that of October 2008, an extended solar minimum period (F10.7 = 70), are analyzed to examine the SF intensity and occurrence rate as a function of the evening prereversal vertical drift velocity and the corresponding F layer heights and the bottom‐side density gradient. While the SF at the equatorial site is indicative of both the bottom‐side irregularities and rising bubbles, the SF at the low latitude represents exclusively the latter. Comparison of the results, from the two epochs, reveals a large decrease in the intensity and occurrence rate of plasma bubbles, with a decrease in solar flux. But a notable increase in these characteristics is observed in the case of bottom‐side SF. It is found that a larger (steeper) density gradient of the F layer bottom side that exists in the low solar flux condition is responsible for an enhanced Raleigh‐Taylor instability growth, counterbalancing a reduction in this rate that may arise from a smaller prereversal vertical drift and lower layer height that also characterize the low solar flux condition. Thus, the role of the bottom‐side density gradient in the ESF instability growth has been identified for the first time in terms of its ability to explain the contrasting irregularity features as observed during solar flux maximum and minimum years.
Effects of IMF By on Ring Current Asymmetry Under Southward IMF Bz Conditions Observed at Ground Magnetic Stations: Case StudiesKumar, Sandeep; Veenadhari, B.; Chakrabarty, D.; Tulasi Ram, S.; Kikuchi, T.; Miyoshi, Y.
doi: 10.1029/2019JA027493pmid: N/A
In this paper, we have evaluated the role of interplanetary magnetic field (IMF) By on the asymmetry of the ring current during the main phase of geomagnetic storms. The mean H variations have been calculated using 31 ground magnetic stations over magnetic latitudes of 09–45° following the methodology of Li et al. (2011, https://doi.org/10.1029/2011JA016886). Further, the magnetic local time (MLT) variations in the H component at these stations w.r.t. the mean H were investigated for three cases of geomagnetic storms with varying southward IMF Bz and IMF By conditions. Significant ring current asymmetries were observed during the main phase of geomagnetic storms. The primary role of IMF Bz on the asymmetry of the ring current is observed from these cases. More importantly, the investigation brings out for the first time, the additional role of IMF By in influencing the MLT distribution of ring current observed at ground magnetic stations. Under southward IMF Bz conditions, it is shown based on SuperDARN and AMPERE data that IMF By can alter the MLT distribution of ring current under suitable conditions. The timescales of IMF By also play very important role in determining the asymmetry in the ring current. Under steady convection state, IMF By can rotate the convection cells based on its polarity, which in turn can change the MLT distribution of ring current observed by low‐latitude ground stations. This investigation, thus, brings out the important role of IMF By on the asymmetric MLT distribution of ring current under southward IMF Bz.