Injections of energetic ions into the inner magnetosphere constitute one of the main sources of ring current enhancements during geomagnetic storm main phases, especially for energies in the range of 10 similar to 200 keV. So far, the majority of investigations of energetic ion injections were performed at L > 4.0 in the inner magnetosphere, while the study of ion injections in L < 4.0 regions is scarcer. In this paper, we have developed a method to identify ion injections for L < 4.0 based on energetic ion fluxes versus L profiles during geomagnetic quiet times. We have selected 120 ion injections with 15 isolated injections and 105 storm-time injections based on the flux ratios between active and quiet periods. Energetic ions can be seldom injected into L < 3.0 during isolated substorms, while they can reach much deeper orbits during storm time. Additionally, we have calculated the correlation coefficients between the adjacent orbits during the geomagnetic active and quiet times in the same orbit categories. The results show that energetic ions with 150 similar to 750 keV are hardly injected into L < 4.0 for both ascending (from low L to high L) and descending (from high L to low L) orbit phases. In contrast, lower energy ions with 50 keV < E < 150 keV are regularly injected into L < 4.0 during geomagnetic storm-times, with the deepest injection depth at L = 2.4.

Abstract Based on high‐resolution measurements from NASA's Magnetospheric Multiscale mission (MMS), we present the first direct observation of an ion diffusion region (IDR) with high number density O+ ions within dayside magnetopause reconnection during the May 2024 superstorm. The O+ ion density reaches a high value of ∼3.3 cm−3. It helps study heavy‐ion dynamics in dayside magnetopause reconnection. In the vicinity of IDR, O+ ions exhibit distinct acceleration to 300 km/s along the normal direction caused by the enhanced Hall electric field (|EN|max ≈ 80 mV/m). The distorted ion velocity distributions reveal the complex energization processes in the IDR. Crucially, these O+ ion dynamics can reduce reconnection rate by ∼10.3%–25.3%, providing the result that heavy‐ion can substantially alter magnetopause reconnection physics during the superstorm. This study advances our understanding of magnetopause reconnection by demonstrating that storm‐enhanced O+ populations modify the structure of diffusion regions, particle energization, and reconnection rate.

We report a Kelvin–Helmholtz vortex (KHV) event observed at the duskside magnetopause boundary layer by the magnetospheric multiscale (MMS) satellites in conjunction with auroral beads detected in the high-latitude ionosphere by the Defense Meteorological Satellite Program (DMSP) on 27 September 2016. During the KHV event, the MMS spacecraft were located in the low-latitude boundary layer and were magnetically mapped to an auroral region adjacent to the DMSP observation footprint. The MMS data revealed small-scale substructures embedded within the KHV; these structures were associated with intense field-aligned currents (FACs) connecting the magnetospheric boundary layer to the ionosphere. The upward FACs are capable of driving aurora precipitations and discrete auroral beads. The characteristic scale ratio between the KHV (∼1,000 km) and auroral electron precipitation region (∼30 km) was consistent with expectations from the mapping of the magnetosphere-ionosphere flux tube. These observations provide evidence that small-scale auroral beads are an ionospheric manifestation of mesoscale KHVs at the low-latitude magnetopause boundary layer. The findings also highlight the importance of the boundary layer instability in regulating magnetosphere-ionosphere coupling.

Using particle and electromagnetic field data from Magnetospheric Multiscale Spacecraft (MMS), we investigate energetic O+ ion characteristics in the strong velocity shear regions in the dusk-side low-latitude boundary layer (LLBL) during the main phase of an intense storm on 13 October 2016. In the large velocity reversal regions, O+ ion number density is very high, No+ similar to 0.3 cm-3. The pitch angle distributions of these energetic O+ ions vary distinctly across different energy ranges. The pitch angles of the lower energetic (3-10 keV) O+ ions are mostly less than 45 degrees and show a quasi-parallel distribution. Conversely, the pitch angles of the higher energetic (20-40 keV) O+ ions are dominantly in the range from 45 to 135 degrees, suggesting a quasi-perpendicular distribution. The quasi-parallel distribution of lower energetic O+ ions implies that these O+ ions are outflow along the magnetic field line from the dayside high-latitude ionosphere. Intense electric fields in the strong shear flow region can accelerate O+ ions to higher energy, altering their motion from along the magnetic field to the transverse direction in the dusk-side LLBL. Our studies present evidence for strong shear flow in the dusk-side LLBL driving energetic O+ ions to traverse the magnetic field motion. The quasi-perpendicular distribution of higher energetic O+ ions, in the inner edge of the dusk-side LLBL, may provide a new source of ring current energetic particles during the main phase of the intense storm.

Previously we found that the inner radiation belt (IRB) shrinks and stretches in solar minimum and maximum. A natural problem comes up that how solar cycle effects the near-Earth space regions including plasmasphere, IRB, ionosphere, mesosphere and lower thermosphere (MLT). We present a thorough analysis of the extent of solar cycle effect on four regions by using mesospheric and thermospheric geopotential height and temperature from SABER on TIMED, ionospheric hmF2 from Chinese Meridian Project, high-energy protons in IRB and electron density in plasmasphere from Van Allen Probes within 2013-2018 intervals. By analyzing evolutions of these quantities, we find that entire IRB, ionosphere and MLT region shrink at solar minimum and stretch at solar maximum by ~103 km, 50~102 km and 1 km scales, respectively, while plasmapause shows an opposite trend. Fourier spectra of these quantities have been investigated by Lomb–Scargle periodogram. The mid-term periodic oscillations (13.5-day, 45-day, and 52-day) have been observed in MLT region, matching well with plasmapause locations and geomagnetic indices, which have not been observed in solar EUV radiation and IRB. This may indicate that those oscillations facilitate energy exchange and mass transportation between MLT region and plasmasphere due to magnetic storms and substorms. The oscillation periods of higher energy (102.6MeV) in IRB have not been observed in MLT region except for annual variations. The impact of higher energy protons on MLT regions may not be significant, although they could penetrate deeper into MLT region. Our results reveal relationships between some quantities and solar cycle multi-scale modulation, which may provide assistance and monitors for mass transportation in the near-Earth space regions.

On 10 May 2024, near-Earth space experienced the most intense geomagnetic storm in nearly two decades, accompanied by extreme substorms and rare low-latitude auroral displays observed in regions such as Beijing and Altay. Geomagnetic storms and substorms are explosive manifestations of solar wind-magnetosphere coupling and are key drivers of space weather. Understanding these phenomena remains a fundamental challenge for space weather forecasting. This article reviews recent progress in storm-substorm research, focusing on internal processes of storm-substorm such as magnetospheric convection, magnetic reconnection, electric current systems, auroras, and energetic particles. Despite significant progress, the way these physical processes are connected across different regions-and the intrinsic relationship between storms and substorms-remains unclear. Inspired by Earth system analogies, such as mantle convection driving plate movement and atmospheric convection causing extreme weather, we propose a new conceptual framework centered on the evolution of magnetospheric convection for future studies of stormsubstorms. The core hypothesis is that magnetospheric convection, through the motion of frozen-in magnetic field lines, reorganizes large-scale current systems and ultimately governs the development of storms and substorms. This main hypothesis is mainly motivated by following considerations. From the perspective of magnetohydrodynamics (MHD), magnetic field lines and plasma are frozen together and convect as a single entity on macroscopic scales, with only a few exceptions at small scales, e.g., diffusion region of magnetic reconnection. As a result, convection inherently drives the global-scale evolution of magnetic field lines. Within the MHD framework, convection is the primary mechanism capable of restructuring magnetic fields, forming thin current sheets, and building up current systems in the magnetosphere. This can occur directly through the stretching of field lines, or indirectly through the injection of energetic particles that enhance current intensity. The framework in which magnetospheric convection drives the space current system is broadly consistent with existing observations and theoretical models. For example, both cross-tail currents and ring currents-key components of substorm and storm current systems-are influenced by magnetospheric convection. This perspective offers a possible way to connect various physical processes involved in storms and substorms, helping to reveal the relation between them within a unified framework. Based on this framework, we outline three key scientific questions: (1) how does magnetospheric convection evolve during substorms, and how do the electric current systems respond? (2) What happens to magnetospheric convection and the corresponding current system during storms? (3) What is the intrinsic link between storm-time and substorm-time convection? This framework can be summarized as "one core hypothesis, three key scientific questions, and eight physical processes of storm-substorm". As an initial application of this framework, we discuss the possible cause of the unusual low-latitude auroras during the May 2024 storm, suggesting that they may result from extreme magnetospheric convection driving large-scale field-aligned currents that expand equatorward in the ionosphere. From the perspective of storm-time convection, the significant equatorward expansion of the auroral oval during the extreme May 2024 geomagnetic storm reflects an overall intensification of magnetospheric convection and its deeper penetration into the inner magnetosphere. Within the magnetosphere, the enhanced convection electric field leads to plasmasphere erosion and drives field-aligned currents deeper into lower L-shell regions. From the ionospheric viewpoint, continuous dayside magnetic reconnection opens previously closed magnetic field lines, pushing the open-closed boundary equatorward. This process causes the entire high-latitude current system to shift toward lower latitudes. Accompanied by these field-aligned currents, aurora-related precipitating electrons also reach lower L-shells, and through magnetic field line mapping, affect the low-latitude ionosphere. Comprehensive studies on the storm and substorm from perspectives of magnetospheric convection are still in an early stage. Forthcoming missions like SMILE (Solar-wind-Magnetosphere-ionosphere Link Explorer) and future multi-scale magnetosphere CubeSat constellation missions like AME (self-Adaptive Multi-scale magnetosphere cubeSat constellation) will provide valuable observations to test this framework. These will help us improve our understanding of how storm and substorm develop.

The occurrence of space weather events, notably geomagnetic storms driven by various solar wind structures, can significantly alter Earth’s electromagnetic environment. In this study, we examined the interplanetary origins and statistical distribution of 384 geomagnetic storms (Dstmin ≤ −50 nT) that occurred from September 1996 to December 2023. We statistically analyzed the correlations between storm intensity and solar wind parameters (SWPs) across different subsets. The results indicate that (1) the solar activity level, indicated by the sunspot number (SSN), and the number of geomagnetic storms during the first four years of the 25th solar cycle were intermediate, compared to the first four years of the 23rd and 24th solar cycles. Specifically, ICME-related structures caused 80% of the strong storms (Dstmin ≤ −100 nT) and 34% of the moderate storms (−100 nT < Dstmin ≤ −50 nT) from 2020 to 2023. (2) The storm intensity correlated with the peak and/or time-integral values of the southward interplanetary magnetic field (IMF Bs), the dawn–dusk electric field (Ey), the Akasofu’s function (ε), and dynamic pressure (Psw) to varying extents. Strong storms exhibited higher correlation levels than moderate ones and ICME-related storms showed larger correlation levels compared to those driven by other sources. (3) Compared with the storms from 1996-09 to 2000-08, the storms that occurred from 2020 to 2023 had lower correlations with the peak values of the IMF Bs and Ey but higher correlations with the peak value of ε and the time-integral values of the IMF Bs, Ey, Psw, and ε. (4) Among the 174 events that featured continuous southward IMF during the storm’s main phase, the duration of southward IMF during about 66.7% of moderate storms and 51.5% of strong storms were less than 13 h. Continuous southward IMF resulted in more direct and efficient energy coupling, enhancing the correlation between the peak values of SWPs and storm intensity but weakening the relationships with the time-integral values of SWPs. Notably, when the southward IMF persisted for a longer duration (e.g., ∆t > 13 h), the continuous energy input further enhanced correlations with both peak and integral values of SWPs, leading to stronger overall correlations with storm intensity. This analysis sheds light on the intricate relationships between geomagnetic storms and their solar wind drivers, emphasizing the significant influence of ICME-related structures and the duration of southward IMF on storm intensity.

The Parker Solar Probe (PSP) provides us the unprecedentedly close approach observation to the Sun, and hence the possibility of directly understanding the "elementary process" which occurs in the kinetic scale of particles collective interactioin in solar coronal plasmas. We reported a kind of weak solar radio bursts (SRBs), which are detected by PSP when it passed a low-density magnetic channel during its second encounter phase. These weak SRBs have low starting frequecny $\sim 20$ MHz and narrow frequency range from a few tens MHz to a few hundres kHz. Their dynamic spectra display a strongly evolving feature of the intermediate relative drift rate decreasing rapidly from above 0.01/s to below 0.01/s. Analyses based on common empirical models of solar coronal plasmas indicate that these weak SRBs originate from the heliocentric distance $\sim 1.1-6.1~R_S$ (the solar radius), a typical solar wind acceleration region with a low-$\beta$ plasma, and indicate that their soruces have a typic motion velociy $\sim v_A$ (Alfv\'en velocity) obviously lower than that of fast electrons required by effectively exciting SRBs. We propose that solitary kinetic Alfv\'en waves with kinetic scales can be responsible for the generation of these small-scalevweak SRBs, called solitary wave radiation (SWR).

The solar cycle includes multi-scale variations in the near-Earth space regions including plasmasphere, inner radiation belt (IRB), ionosphere, mesosphere and lower thermosphere (MLT). We present a thorough analysis of the extent of solar cycle effect on those four regions by using mesospheric and thermospheric geopotential height and temperature from SABER on TIMED, ionospheric hmF2 from Chinese Meridian Project, high-energy protons in IRB and electron density in plasmasphere from Van Allen Probes within 2013-2018 intervals. By analyzing evolutions of these quantities, we find that entire IRB, ionosphere and MLT region shrink at solar minimum and stretch at solar maximum by similar to 103, 50-102, and 1 km scales, respectively, while plasmapause shows an opposite trend. Fourier spectra of these quantities have been investigated by Lomb-Scargle periodogram. The mid-term periodic oscillations (13.5-day, 45-day, and 52-day) have been observed in MLT region, matching well with plasmapause locations and geomagnetic indices, which have not been observed in solar EUV radiation and IRB. This may indicate that those oscillations facilitate energy exchange and mass transportation between MLT region and plasmasphere due to magnetic storms and substorms. The oscillation periods of higher energy (102.6 MeV) in IRB have not been observed in MLT region except for annual variations. The impact of higher energy protons on MLT regions may not be significant, although they could penetrate deeper into MLT region. Our results reveal relationships between some quantities and solar cycle multi-scale modulation, which may provide assistance and monitors for mass transportation in the near-Earth space regions.

Based on current sheet flapping motion on 27 August 2018 in the dusk flank magnetotail,as recorded by instruments aboard Magnetospheric Multiscale(MMS)spacecraft,we present the first study of guide field reconnection observed in the flux rope embedded in kink-like flapping current sheets near the dusk-side flank of the magnetotail.Unlike more common magnetotail reconnections,which are symmetric,these asymmetric small-scale(λi～650 km)reconnections were found in the highly twisted current sheet when the direction normal to the sheet changes from the Z direction into the Y direction.The unique feature of this unusual reconnection is that the reconnection jets are along the Z direction—different from outflow in the X direction,which is the more usual situation.This vertical reconnection jet is parallel or antiparallel to the up-and-down motion of the tail's current sheet.The normalized reconnection rate R is estimated to be～0.1.Our results indicate that such asymmetric reconnections can significantly enlarge current sheet flapping,with large oscillation amplitudes.This letter presents direct evidence of guide field reconnection in a highly twisted current sheet,characterized by enlarged current sheet flapping as a consequence of the reconnection outflow.

This study reports the rare ultralow-frequency (ULF) wave activity associated with the solar wind dynamic pressure enhancement that was successively observed by the GOES-17 (Geostationary Operational Environmental Satellite) in the magnetosphere, the CSES (China Seismo-Electromagnetic Satellite) in the ionosphere, and the THEMIS ground-based observatories (GBO) GAKO and EAGL in the Earth’s polar region during the main phase of an intense storm on 4 November 2021. Along with the enhanced-pressure solar wind moving tailward, the geomagnetic field structure experienced a large-scale change. From dawn/dusk sides to midnight, the GAKO, EAGL, and GOES-17 sequentially observed the ULF waves in a frequency range of 0.04–0.36 Hz at L shells of ∼5.07, 6.29, and 5.67, respectively. CSES also observed the ULF wave event with the same frequency ranges at wide L-shells of 2.52–6.22 in the nightside ionosphere. The analysis results show that the ULF waves at ionospheric altitude were mixed toroidal-poloidal mode waves. Comparing the ULF waves observed in different regions, we infer that the nightside ULF waves were directly or indirectly excited by the solar wind dynamic pressure increase: in the area of L-shells ∼2.52–6.29, the magnetic field line resonances (FLRs) driven by the solar wind dynamic pressure increase is an essential excitation source; on the other hand, around L∼3.29, the ULF waves can also be excited by the outward expansion of the plasmapause owing to the decrease of the magnetospheric convection, and in the region of L-shells ∼5.19–6.29, the ULF waves are also likely excited by the ion cyclotron instabilities driven by the solar wind dynamic pressure increase.

The characteristics of isolated and storm‐time injections are important in the understanding of storm‐substorm relationship. The injection region, flux ratio and energy‐flux spectra have been investigated to find the differences and similarities for isolated and storm‐time injections. A scanning procedure automatically identifies a total of 94 isolated injections and 229 storm‐time injections with the LANL data in 2000 and 2001. A significant difference has been found in longitudinal distribution for isolated and storm‐time injections which have wider injection regions than the former. Based on the trends of characteristic energies and number density from spectra analysis, two types of isolated injections have been found. Type I injections are characterized by characteristic energies increasing with the increase of number density, while Type II has an opposite trend. Further statistical surveys suggest that the flux ratios before and after injections have opposite trends in Type I and II injections. Storm‐time injection is very similar to the Type II injection in characteristic energies‐number density trend and features of flux ratio variations. Our results provide injection regions, flux ratios and spectral features, which may improve our understanding of the isolated and storm‐time injections and their relationship.

In particular during the descending phase of the solar cycle, Alfvén waves in the high-speed solar wind streams are a major form of interplanetary disturbances. The fluctuating southward interplanetary magnetic field (IMF) of Alfvén waves has been suggested to induce geomagnetic activities through intermittent magnetic reconnection at the magnetopause. In this study, we provide in situ observational evidence for dayside magnetopause reconnection induced by such interplanetary Alfvén waves. Using multipoint conjunction observations, we show that the IMF B z from interplanetary Alfvén waves is transmitted through and amplified by the Earth’s bow shock. Associated with the intensified southward B z to the magnetopause, in situ signatures of magnetic reconnection are detected. Repetitively, interplanetary Alfvén waves transmit the intensified B z to the magnetosheath, leading to intervals of large magnetic shear angles across the magnetopause and magnetopause reconnection. Such intervals are promptly followed by hundreds of nanoTesla (nT) increases in the auroral electrojet indices (AE and AU) within 10–20 minutes. These observations are confirmed in multiple events in corotating interaction region-driven geomagnetic storms. To put the observations into context, we propose a phenomenological model of a strongly driven substorm. The substorm electrojet is linked to the enhanced magnetopause reconnection in the short timescale of re-establishing the ionosphere electric field and the two-cell convection. These results provide insights on the temporal patterns of solar wind magnetosphere–ionosphere coupling, especially during the descending phase of the solar cycle.

Long-term and short-term variations of high-energy protons in the inner radiation belt are of major importance in geospace research. The entire inner radiation belt at 1.1 < L < 3.0 have been investigated based on Van Allen Probes Relativistic Electron-Proton Telescope data during the descending phase of the solar cycle (2013-2018). It is found that two belts with peaks near L = 1.5 and L = 2.0 for E < 50 MeV protons and one belt at L = 1.5 for E > 50 MeV protons, once they are formed, last for more than 6 years. In the outer zone of the inner belt (1.8 < L < 2.4), the variations of high-energy proton fluxes with E < 50 MeV are well correlated with SYM-H index, while only weak correlation is found for them in the inner region at 1.3 < L < 1.6. Geomagnetic disturbances do not show clear correlation with variations for E > 50 MeV protons in the whole inner radiation belt. By analyzing the structure evolution of the inner radiation belt during half of the solar cycle, we found that the entire inner radiation belt shrinks at solar minimum and stretches at solar maximum. Our results reveal the relationship between high-energy protons and SYM-H index and solar cycle modulation on the whole configuration of inner radiation belt, which may assist in understanding and modeling of the inner radiation belt protons.

Using multipoint observations over 10 yr near 1 au, we investigate the spectra (5 minutes to 2 hr) of interplanetary Alfvén waves and the responses in the geomagnetic activities. We compute the two-point correlations of the wave magnetic field between the ACE and the THEMIS spacecraft, which are separated by ∼200 Earth radius ( R E ) in the solar wind. Alfvén waves associated with high two-point correlations exhibit steep spectra (spectra index ∼−1.63). Such Alfvén waves occur mostly in slow-speed streams. By contrast, Alfvén waves with low two-point correlations exhibit flatter spectra (spectra index ∼−1.51) with a relative enhancement of power above 2 × 10 −4 Hz. The occurrence of Alfvén waves with low two-point correlations is more equally distributed between high-speed and low-speed streams. In general, interplanetary Alfvén waves show correlations with moderate geomagnetic responses in symmetric ring-current intensity, SuperMAG electrojet (SME), and Kp indices. Statistical analyses indicate that the Alfvén waves with flat spectra correspond to stronger responses in the geomagnetic indices than those with steep spectra, suggesting the importance of the tens of minutes (30–90 minutes) Alfvénic power spectra in the generation of SME/Auroral Electrojets. These observations may shed light on the response of the magnetosphere to fluctuating interplanetary magnetic field B z .

Kinetic Alfvén waves (KAWs) are low-frequency dispersive Alfvén waves with a small perpendicular wavelength and long parallel wavelength to the ambient magnetic field direction. KAWs carry the parallel electric field with a large perpendicular wave vector angle to the ambient magnetic field. It is well known that KAWs play a significant role in the mass and energy transport in the magnetosphere during substorms. The substorm is a 1–3 h short time coupling between the solar wind, magnetosphere, and ionosphere. The mass and energy from the solar wind are transport into the magnetosphere, and then, the energy is dissipative in the magnetotail associated with substorm auroral in the high-latitude ionosphere. The unique feature of kinetic Alfvén waves is that the accompanied parallel electric field which can accelerate electrons and ions along the ambient magnetic field. On the other hand, the KAWs’ large perpendicular electric field can accelerate ions cross the magnetic field and provide large Poynting flux along the magnetic field. The small-scale KAWs can be excited by the surface wave mode conversion on the magnetospheric boundary layer with large density and magnetic field gradient, such as the magnetopause and plasma sheet boundary layer. The finite ion gyroradius effect and electron pressure effect in the high β plasma, such as in the magnetotail plasma sheet and its boundary layer on KAWs, are both consider in this review paper. Under the different β value space plasma, the theory dispersion relation and observations on KAWs in the magnetotail during substorms are presented.

With conjunction observations of electromagnetic fields and plasma from Time History of Events and Macroscale Interactions during Substorm (THEMIS) in the near-Earth magnetotail, we investigate the spatial and temporal properties of substorm dipolarizations in the near-Earth plasma sheet (NEPS) during a substorm at 03:23 UT on 12 February 2008. Substorm dipolarizations with different features are detected by three near-Earth THEMIS probes (THA (P5), THD (P3) and THE (P4)) in the magnetotail. In the current sheet with a large plasma beta value (β > 2, where β is the ratio of the plasma thermal pressure to the magnetic pressure), the dipolarization within the substorm onset region, (−10.4, 2.8, −2.6)RE_gsm, has a large initial magnetic field elevation angle, θ > 60°, θ = arctan (Bz/(Bx2+By2)1/2), and is accompanied by energetic ion (tens to hundred keV) dispersionless injection detected by THD (P3). This substorm onset dipolarization is characterized by Bx and By components around 0 nT with significant fluctuations. The Bz component increases sharply and its subsequent magnitude approaches the total magnetic field, Bt. The maximum value of the elevation angle approaches 85° during the later substorm expansion phase. In the NEPS with β ~ 1, the dipolarization outside the substorm onset region is characterized by a magnetic elevation angle with a small beginning value of θ < 45° and following multi-step enhancements during the substorm expansion phase. The maximum value of the elevation angle approaches to 70° during the later substorm expansion phase. Our observation results indicate that characteristics of dipolarization with a large beginning elevation angle within the substorm onset region provide a new indicator to identify substorm onset location.

The enhancement and loss mechanisms of relativistic electron (known as killer electron) fluxes in the radiation belt have always been hotspots in the research area of space physics and space weather. In this paper, we study a special phenomenon that the relativistic electron flux in the geosynchronous orbit decreased to the background level and lasted for more than 3 days from 2000 to 2016. Firstly, we study the distribution of total 62 events in about 1.5 solar cycles with the sunspot number, the results show that fewer events occurred during the declining phase of a solar cycle. However, in the solar maximum and minimum, the occurrence of events did not seem to be directly related to the sunspot number. Then, in order to analyze and discuss the objective law and generation mechanism of the long-term dropouts of relativistic electrons, a statistical analysis on the beginning, continuation, and end periods of these 62 events is performed. The results show that before the event, the significant increase in solar wind dynamic pressure and density resulting in the inward movement of magnetopause, the maintenance of plasmapause at high L-shell for a long time, the southward IMF B-z component and the magnetic storm process reduced the relativistic electron flux to the background level through adiabatic and non-adiabatic physical mechanisms. When the relativistic electron reached the background flux level, weaker solar wind conditions and geomagnetic activities cannot provide enough sources for relativistic electrons; although sometimes there were some geomagnetic storms, they were small or medium geomagnetic storms. Under these circumstances, the physical mechanisms that led to the flux loss or enhancement during geomagnetic storms could keep a dynamic balance, so the relativistic electron still kept at background level. If long-term substorm activities, high-intensity ULF (Ultra-Low Frequency) wave activities or significant increase in solar wind occurred, these processes could provide enough seed electrons and acceleration mechanisms, so that the relativistic electron flux broke the dropout and presented a significant enhancement.