Substorm processes have been studied in detail, and it is well known that interplanetary (IP) shock encountering the terrestrial magnetosphere causes global responses. However, how IP shock compression to the magnetosphere affects the development of an ongoing substorm remains uninvestigated. Herein, the simultaneous satellite and ground-based auroral evolutions associated with an IP shock impact on the magnetopause during an ongoing substorm on May 7th, 2005, were examined. The IMAGE satellite over the Southern Hemisphere captured the global development substorm, which was initiated at 17:38:47 UT. The poleward branch of the nightside auroral oval was fortuitously monitored by an all-sky camera at the Zhongshan Station (−74.5° magnetic latitude, ZHO) in Antarctica. The satellite imager observed continuous brightening and broadening of the nightside auroral oval after the IP shock arrival. The simultaneous ground-based optical aurora measurement displayed the intensification and expansion of a preexisting auroral surge poleward of the aurora oval. The geomagnetic field variations and the instantly increased PC indices indicated an elevated merging rate and enhanced the convection-related DP-2 currents. Therefore, this IP shock transient impact did not significantly change the ongoing development of the substorm, although it meets the magnetospheric precondition hypothesis.

Using THEMIS spacecraft observations from 2008 to 2010, we perform statistical study on the wide‐amplitude kinetic Hall‐like Alfvénic wave (KAW) pulse in the near‐Earth tail region of 8–12 RE. The KAW pulse is identified by Ez‐By pulse with parallel‐predominantly Poynting flux (|P///PT| > 0.7) and large Ez/By ratio (|∆Ez/∆By | > VA, VA is Alfvénic velocity). Three typical events are presented. Therein, the KAW pulse on 24 March 2009 at the magnetic dipolarization is accompanied by the ion/electron dispersed structure (IDS/EDS; time of flight effect from reconnection source). Statistical study shows that the near‐Earth KAW pulse has a typical temporal scale of ∼15–25 s. The KAW pulses are mainly distributed in the plasma sheet boundary layer (PSBL) and tail lobe regions of β < 0.1 (β is the ratio of thermal pressure to magnetic pressure). The KAW pule at the PSBL/lobe has a wide distribution in the dawn‐dusk direction (−10 RE < |Y| < 10 RE), with the maximum occurrence at the midnight (|Y| < 4 RE). The KAW pulses at the PSBL/lobe are predominantly earthward‐propagating. Only a small number of the KAW pulses are tailward‐propagating. The earthward‐propagating pulse has a higher Poynting flux (PT) than tailward‐propagating pulse. The KAW pulse occurs during quiet time (AE < 200 nT) as well as substorm interval (AE > 200 nT). The substorm‐interval pulse has a higher Poynting flux than the quiet‐time pulse. Finally, tail reconnection is likely the significant source of the near‐Earth KAW pulses at 8–12 RE.

Based on four‐point Magnetospheric Multiscale observation, we carefully analyze the vorticity field in the course of a reconnection jet on 27 June 2017. In this event, the convective electric field (Ec) is overwhelmed by the kinetic electric field (Ek). Accordingly, the ω‐field in the course of the BBF is dominated by kinetic vorticity (ωk). The ω‐field in the Ek‐dominated bursty bulk flow (BBF) is characterized by perpendicular anisotropy. Comparison of velocity‐curl vorticity ( ω=∇×V $\mathbf{\omega }=\nabla \times \mathbf{V}$ ) with E/B induced vorticity confirms a greater contribution to the BBF vorticity by E ( ωE=(B⋅∇)E/B2 ${\mathbf{\omega }}_{\mathbf{E}}=(\mathbf{B}\cdot \nabla )\boldsymbol{E}/{\boldsymbol{B}}^{2}$) than by B ( ωB=(E⋅∇)B/B2 ${\mathbf{\omega }}_{\mathbf{B}}=(\mathbf{E}\cdot \nabla )\mathbf{B}/{\boldsymbol{B}}^{2}$ ). Power spectrum density reveals that in the Ek‐dominated BBF, the Ec and Ek spectra have different power laws. Ec dominates the B‐spectrum while Ek dominates the E‐spectrum. The Ec(B) spectrum has a −5/3‐like slope below 0.25 Hz but a −3‐like above 0.25 Hz. The Ek(E) spectrum is −5/3‐like at the low‐frequency end (below 0.1 Hz) but −2‐like at the high‐frequency end (above 2 Hz). Within its medium frequency range, the spectrum is flat. Particularly, the Ek(E) spectrum exhibits the bump at 1–2 Hz in the frequency domain. The solitary/bipolar E‐spikes, with typical temporal scale of 0.3–1 s, are likely responsible for the E‐bump. Finally, we statistically analyze and compare the vorticity field in Ek‐dominated and Ec ‐dominated BBFs. The result demonstrates that the Ek‐dominated BBF tends to have stronger vorticity than the Ec ‐dominated BBF.

EDITORIAL article Front. Astron. Space Sci., 15 June 2022Sec. Space Physics https://doi.org/10.3389/fspas.2022.944040

We perform a case study on the evolution of the current sheet in different regions around the dipolarization front (DF), including magnetic‐dip preceding the DF, front at the DF, and magnetic pileup region (MPR) behind the DF based on magnetospheric multiscale (MMS) observation on July 31, 2017. In this event, MMS1 stays inside the current sheet during the whole bursty bulk flow (BBF) interval. Our analysis reveals that the cross‐tail current sheet at the DF is rolled up, signified by the depression (−V z /−B z ) at the dip and elevation (+V z /+B z ) at the front. The minimum variance analysis on the magnetic field method is applied to obtain the normal direction of the current sheet. The result confirms the roll‐up, that is, downward at the depressed current sheet and upward at the elevated current sheet. The current sheet roll‐up at the DF is asymmetric, with steeper elevation than depression. The elevation angle of the elevated current sheet is evaluated to be ∼30°. Strong duskward and predominantly perpendicular J spike (∼90 nA/m 2 ) concentrate at the interface between the dip and the front. The strength of the current of the J‐spike is about nine/three times the current at the dip/front. The front is characterized by positive E·J . In the dip/MPR, no such preference is seen. Ion/Electron pitch angle distributions exhibit significant and different evolutions in the roll‐up current sheet from dip to front, including their energy‐dependence and distributions. Finally, the roll‐up current sheet could decelerate BBF and change the flow structure. The potential significance of the roll‐up current sheet on BBF evolution is emphasized.

Using measurements from Cluster‐II space mission, we compared the characteristics of the fluctuations of the magnetic field magnitude and the Bz component in the current disruption (CD) regions. We used fast Fourier transform, statistical, and wavelet analysis, and wave surveyor technique on the multispacecraft measurements. Among the obtained results one can note the presence of spectral breaks in both the magnetic field magnitude and the Bz component at frequencies smaller or equal to the proton gyrofrequency. The numerical values of the spectral index for the magnetic field magnitude and for the Bz component are similar, and the nature of the turbulent processes is close to that of the homogeneous magnetohydrodynamic (MHD) (the spectral index varies from −2.00 to −1.31) on the large time scale and resembles Hall‐MHD (the spectral index varies from −2.33 to −2.99) on the smaller time scales. The kurtosis results are consistent with those of the spectral analysis. From wavelet technique, we detected powerful Pc4 and Pi1 pulsations, along with cascade features both for the magnetic field magnitude and for its z‐component. The dispersion ratios also indicate the presence of nonlinear energy cascade processes in CD regions. We have found that the pulsations observed for magnetic pressure are also present in thermal proton and electron pressures although their powers differ considerably. For the dynamic helium and oxygen pressures, and also for the thermal pressures of these components, only pulsations in the high‐frequency region are revealed.

Detailed properties of the spatially periodic auroral beads (ABs) that develop immediately prior to auroral substorm expansion onset on breakup auroral arcs pose stringent observational constraints on the substorm onset process. Theoretical predictions of the cross-field current instability (CCI) are evaluated in terms of these constraints. It is found that CCI satisfies these constraints associated with ABs, that is, their growth rates, wavelengths, and periods, by matching these characteristics well with the theoretical predictions of CCI. Several subtle trends in AB properties are revealed with CCI solutions applied to them. For a given observed wavelength in the ionosphere, waves with longer periods are generated at onset sites at longer downstream distances for both westward and eastward drifting ABs. Waves with higher growth rates are generated by higher drift speeds for a given observed period and for a given observed wavelength by onset sites at closer downstream distances. A distinct group of eastward drifting ABs having long periods and small wavelengths are related to onset sites with relatively high drift speeds and large downstream distances. These revelations indicate that the onset site can occur over a range of downstream distances. ABs possessing two different propagating directions may be accommodated. A two-dimensional equilibrium current sheet can be constructed from the adopted parameters for the current sheet used in CCI solution. Some misunderstandings of CCI are clarified as well. All these findings provide strong evidence for CCI as a viable substorm onset process.

Utilizing Magnetospheric Multiscale (MMS) observation in the tail plasma sheet, we study the vorticity field ( ω = ∇ × V ) of the plasma bulk (convective) velocity within the bursty bulk flow (BBF) in detailed. Two typical events are presented. In the event on 25 June 2017, E y is the main component. In the other event on 6 July 2017, E z is the main component. For both cases, the BBF electric field is dominated by the convective electric field ( Ec = − V × B ). Our case studies show clearly the existence of the convective vorticity field within the BBF. The vorticity field has prominent anisotropy (quantified by the anisotropic angle ( θ aa = arctan( ω ⊥ / ω ∥ )). More often, the BBF ω field has stronger perpendicular vorticity ( ω ⊥ ) than the parallel vorticity ( ω ∥ ). The dominance of vorticity by ω ⊥ ‐dominating BBF is confirmed in the statistical sense. In particular, event on 25 June shows the significant evolution of the ion flux energy with the strength of the ω field. The strong ω field corresponds to the ion flux enhancement at high energy (above 10 keV), while the weak ω field corresponds to the ion flux enhancement at medium energy (2–5 keV). Investigation of the subset of channel from fast plasma investigation partial moment measurement reveals that the ion behaviors in the strong and decayed BBFs are distinctly different. The channeled ions form the narrow band distribution in the strong BBF but the multiple‐layer distribution in the weak BBF. Finally, spectrum analysis indicates that the BBF ω ⊥ and ω ∥ have a similar scaling about −2.0 (below 0.2 Hz).

China's initial participation in the global monitoring of auroras for scientific and space weather investigations has been enabled by the successful launch of the Chinese Fengyun-3D satellite, which carries a wide-field auroral imager.

Utilizing ACE satellite observations from 1998 to 2009, we performed the elaborate study on the properties of the clock angle theta(CA) (arctan(By/Bz) (-90 degrees to 90 degrees) of the interplanetary magnetic field (IMF) in the solar wind at 1 AU. The solar wind with northward IMF (NW-IMF) and southward IMF (SW-IMF) are analyzed, independently. Statistical analysis shows that the solar wind with SW-IMF and NW-IMF has similar properties in general, including their durations, the IMF Bz and By components, and the IMF theta(CA). Then, the solar wind with NW-IMF (SW-IMF) is classified into five different temporal scales according to the duration of the NW-IMF (SW-IMF), i.e., very-short wind of 10-30 min, short-scale wind of 0.5-1 h, moderate-scale wind of 1-3 h, long-scale wind of 3-5 h, and super-long wind >5 h. Our analysis reveals that the IMF theta(CA) has a distinct decrease with increase of the temporal scale of the solar wind. Next, the solar wind is classified into two groups, i.e., the high-speed solar wind (>450 km/s) and the low-speed solar wind (<450 km/s). Our analysis indicates that the IMF theta(CA) depends highly on the solar wind speed. Statistically, high-speed solar wind tends to have larger IMF theta(CA) than low-speed solar wind. The evolutions of the solar wind and IMF with the solar activity are further studied, revealing no clear solar variation of the IMF theta(CA). Finally, we analyze the monthly variation of the IMF theta(CA). Superposed epoch result strongly suggests the seasonal variation of the IMF theta(CA). (C) 2019 Published by Elsevier Ltd on behalf of COSPAR.

We present simultaneous observations of aurorae at Jupiter from the Hubble Space Telescope and Hisaki, in combination with the in situ measurements of magnetic field, particles, and radio waves from the Juno Spacecraft in the outer magnetosphere, from similar to 80R(J) to 60R(J) during 17 to 22 March 2017. Two cycles of accumulation and release of magnetic flux, named magnetic loading/unloading, were identified during this period, which correlate well with electron energization and auroral intensifications. Magnetic reconnection events are identified during both the loading and unloading periods, indicating that reconnection and unloading are independent processes. These results show that the dynamics in the middle magnetosphere are coupled with auroral variability.

The current sheet in the Earth's magnetotail is a plasma region where many dynamic phenomena occur and has been attributed to be the origin of many magnetospheric disturbances. In addition, it is often viewed as a prototype of current sheets in other planets in our solar system as well as in astrophysical systems. The Earth's magnetotail has been surveyed by more satellite missions than at other planets, thus providing valuable knowledge that can be utilized in studies of other plasma systems. In this review, the basic characteristics of the current sheet are briefly discussed, followed by descriptions of some prominent structures and its motions. Its internal properties are also found to be influenced by the external solar wind magnetic field. A fundamentally important link between the Earth's magnetotail and the ionosphere is via magnetic-field-aligned currents, especially during substorm periods. Evaluation of this link brings to light the importance of kinetic processes in magnetospheric dynamics and some major discrepancies of the usually adopted fluid description of these phenomena. In particular, observations on the changes in current density and magnetic field during substorm activities in the magnetotail reveal very short timescales, indicating kinetic processes at play. The frozen-in condition that is implicitly assumed in fluid treatments of plasma dynamics is found to be invalid in these dynamic episodes. In addition, there is compelling evidence showing that observed features of the substorm current system are in agreement with the predictions by a kinetic approach and contradict the predictions by the fluid approach.

Flapping motion of the current sheets of planetary magnetotails is a common dynamic phenomenon. Previous studies of the Earth's magnetotail suggest that its flapping motion has two forms, that is, kink‐like flapping that can propagate as waves toward both flanks and steady flapping that moves up and down but does not propagate. Although some models have been proposed to explain the kink‐like flapping, its mechanism remains unclear. This paper surveys 87 flapping events statistically with respect to their flapping types, using the multipoint measurements of Cluster. The statistical results show that the up‐down steady flapping events tend to occur around the midnight region, and the kink‐like flapping events tend to occur near both flanks of the magnetotail. Thus, we propose that kink‐like flapping motion is causally related to steady flapping motion; that is, the up and down motion of steady flapping around the midnight region induces kink‐like flapping waves, which propagate toward both flanks of the magnetotail.

A kink‐like flapping event of Earth magnetotail current sheet, which consists of two frequency bands successively, is studied by the multipoint observations of Cluster. The multipoint analysis of Cluster observations demonstrates that the higher frequency band (period is about 10 min) has faster propagation velocity (about 30 km/s), shorter wavelength (about 3 RE), and smaller amplitude (1–1.5 RE). In contrast, the lower frequency band (period is about 22 min) shows slower propagation velocity (about 21 km/s), longer wavelength (about 4.4 RE), and larger amplitude (2–3 RE). Comparison with the flapping models demonstrates that the dispersion of theoretical models does not show consistency with the results of this event, which suggests that new or more advanced kink‐like flapping theories or models in the future have to consider the constraints of the dispersive properties demonstrated by this event.

Solar wind particles and ionospheric O + ions influence the near-Earth plasma sheet and inner magnetospheric composition. We studied the behavior of H + , O + and He + ions (9–210 keV) for intense and moderate geomagnetic storms of solar cycle 23 and 24. An average energy density < ε > of ions over a given interval and flux enhancement is estimated using observations from satellites at different L values, namely STICS sensor on-board Geotail spacecraft and HOPE spectrometer on-board Radiation Belt Storm Probes. It provides a comprehensive understanding of the energy density variation of H + , O + and He + ions with the strength of IMF Bz, Psw, intensity of storms and L value. Statistically, we observed that (1) In the plasma sheet region, during main phase of the intense geomagnetic storm, < ε O+/H+ > and < ε He+/H+ > enhances, (2) < ε O+/H+ > is well correlated with Psw (CC = 0.86) and IMF Bz (CC = 0.85), (3) < ε O+/H+ > shows higher correlation (CC = 0.73) with Kp than < ε He+/H+ > (CC = 0.65), indicating a fairly good dependence on the strength of geomagnetic activity, (4) < ε O+/H+ > and < ε He+/H+ > dependence on L value indicates that O + /H + and He + /H + is more pronounced near L = 3. It is a cumulative extension of the previous studies on ion composition change which is in accordance with the existing picture of the plasma sheet and inner magnetosphere.

Methods and approaches which can be used for the analysis of hydrodynamic and magnetohydrodynamic turbulent flows are chosen for this study. It is defined that the best methods for determination of turbulent process types are the methods of statistical physics. Within the statistical approach the fractal analysis (height of the maximum of probability density fluctuations of the studied parameters) and multifractal analysis (study of a power dependence of high order statistical moments and construction of multifractal spectrum) are considered. It is indicated that the statistical analysis of turbulent process properties can be supplemented with spectral studies (wavelet analysis).Physical processes in the transition regions of the magnetosphere: foreshock, shock, post-shock and magnetosheath are investigated using high frequency measurements by Cluster satellites. Extended self similarity analysis and structure function analysis demonstrate the presence of super-diffusion processes and the highest values of generalized diffusion coefficients observed in post-shock region.It can be noted that different approaches for the analysis of turbulent processes give similar results and indicate the presence of super-diffusion processes in the transition region of the Earth's magnetosphere. This fact must be taken into account when constructing quantitative models of a transfer process. Wavelet analysis shows the presence of cascade and inverse cascade processes in the Earth's magnetosheath. Good agreement with other studies and our new results contribute to improvement of our understanding of turbulence.

Solar wind energy is transferred to planetary magnetospheres via magnetopause reconnection, driving magnetospheric dynamics. At giant planets like Saturn, rapid rotation and internal plasma sources from geologically active moons also drive magnetospheric dynamics. In both cases, magnetic energy is regularly released via magnetospheric current redistributions that usually result in a change of the global magnetic field topology (named substorm dipolarization at Earth). Besides this substorm dipolarization, the front boundary of the reconnection outflow can also lead to a strong but localized magnetic dipolarization, named a reconnection front. The enhancement of the north-south magnetic component is usually adopted as the indicator of magnetic dipolarization. However, this field increase alone cannot distinguish between the two fundamentally different mechanisms. Using measurements from Cassini, we present multiple cases whereby we identify the two distinct types of dipolarization at Saturn. A comparison between Earth and Saturn provides new insight to revealing the energy dissipation in planetary magnetospheres.

A multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase. Conjugate observations of magnetic fields and plasma from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, of field‐aligned currents (FACs) from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellites, and from ground‐based magnetometers and aurora are all available. The comprehensive measurements allow us to further our understanding of the complicated causalities among dipolarization, FAC generation, particle acceleration, and auroral intensification. During the substorm expansion phase, the plasma sheet expanded and was perturbed leading to the generation of a slow mode wave, which modulated electron flux in the outer plasma sheet. During this current sheet expansion, field‐aligned currents formed, and geomagnetic perturbations were simultaneously detected by ground‐based instruments. However, a magnetic dipolarization did not occur until about 3 min later in the outer plasma sheet observed by THEMIS‐A spacecraft (THA). We believe that this dipolarization led to an efficient Fermi acceleration to electrons and consequently the cause of a significant auroral intensification during the expansion phase as observed by the All‐Sky Imagers (ASIs). This Fermi acceleration mechanism operating efficiently in the outer plasma sheet during the expansion phase could be a common explanation of the poleward auroral development after substorm onset. These results also show a good agreement between the upward FAC derived from AMPERE measurements and the auroral brightening observed by the ASIs.

Unipolar pulses of kinetic Alfven waves (KAW) are first observed in the near-Earth plasma sheet (NEPS) associated with dipolarizations during substorm expansion phases. Two similar events are studied with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations during substorms on 3 February 2008 and 7 February 2008. The unipolar pulses were located at a trough-like Alfven speed profile in the northern plasma sheet at a distance of 10-11R(E) from Earth. The dominant wave components consist of a southward E-z toward the neutral plane and a +B-y toward the dusk. The |E-z|/|B-y| ratio was in the range of a few times the local Alfven speed, a strong indication of KAW nature. The wave Poynting flux was earthward and nearly parallel to the background magnetic field. The pulse was associated with an earthward field-aligned current carried by electrons. These observational facts strongly indicate a KAW eigenmode that is confined by the plasma sheet but propagates earthward along the field line. The KAW eigenmode was accompanied by short timescale (1min) dipolarizations likely generated by transient magnetotail reconnection. The observed polarity of the KAW field/current is consistent with that of the Hall field/current in magnetic reconnection, supporting the scenario that the Hall fields/current propagate out from reconnection site as KAW eigenmodes. Aurora images on the footprint of THEMIS spacecraft suggest that KAW eigenmode may power aurora brightening during substorm expansion phase.