Relativistic electron enhancements,magnetic storms,and substorm activity

The relativistic electron fluxes of the Earth's outer radiation belt are subjected to strong temporal variations. The most prominent changes are initiated by fast solar wind streams impinging upon the magnetosphere, which often also cause enhanced substorm activity and magnetic storms. Using 4 years of data from the particle detector REM aboard the UK satellite Strv-1b in a GTO, we investigated the relation between these different appearances of geomagnetic activity. A typical sequence is that there is a drop in the relativistic electron intensity during the main phase of the magnetic storm and a successive enhancement during the recovery phase which sometimes leads to much higher than pre-storm fluxes. Whereas the flux drop is well correlated with the magnetic storm intensity and is mainly due to the deceleration and loss of particles caused by the ring-current-induced magnetic field changes, there is only a bad correlation between the post-storm electron flux and Dst. As we show here, it is much more the level of substorm activity during the whole event which determines the size of the flux enhancements.     

中低纬地区电离层对CIR和CME响应的统计分析

本文利用中低纬日本地区(131°E, 35°N)GPS-TEC格点化数据,分析了2001—2009年间109个共转相互作用区(CIR)事件、45个日冕物质抛射 (CME)事件引起的地磁扰动期间电离层的响应.结果表明,电离层暴的类型随太阳活动的变化而有不同的变化,CIR事件引发的电离层正相暴、正负双相暴多发生在太阳活动下降年,负相暴多发生在高年,负正双相暴多发生在低年;CME事件引发的电离层正相暴和负相暴多发生在高年.CIR和CME引发的不同类型的电离层暴的季节性差异不大,在夏季多发生正负双相暴.电离层暴发生时间相对地磁暴的时延大部分在-6~6 h之间,但CIR引发的电离层暴时延范围更广,在-12~24 h之间,而CME引发的电离层暴时延主要在-6~6 h之间.中低纬的电离层暴多发生在主相阶段,其中CIR引发的双相暴也会发生在初相阶段.电离层负暴多发生在AE最大值为800~1200 nT之间.CIR引起的电离层扰动持续时间较长,一般在1~6天左右,而CME引起的电离层扰动持续时间一般在1~4天左右.     

磁暴期间极尖区场向电子事件研究

根据Cluster卫星在中高度极尖区的观测数据,分析研究了两次连续磁暴期间极尖区场向电子事件的持续时间以及与Dst值和Dst时间变化率之间的关系.结果表明,磁暴期间场向电子事件的持续时间的范围为6~54 s,大多数场向事件的持续时间小于34 s;极尖区场向电子事件的最大密度和最大场向通量与Dst值没有明显的相关关系;而随着Dst变化率的增加,场向电子最大密度和最大通量也随之增加,场向电子最大密度与Dst变化率之间的相关系数为0.81,场向电子最大通量与Dst变化率之间的相关系数为0.56,下行电子最大通量与Dst变化率之间的相关系数为0.85.经讨论认为行星际磁场持续南向、太阳风速度和动压的急剧增加是引起场向电子通量增加的主要原因.     

ULYSSES observations of energetic particle acceleration and the superposed CME and CIR events of November 1992

During November 1992, a series of forward and reverse shocks passed the ULYSSES spacecraft. Spectral and anisotropy measurements are reported for protons and alpha particles between 0.28 and 6 MeV observed by the Energetic Particle Composition Experiment, data recorded by the Magnetometer Experiment and the high-energy (2.7-300 MeV) proton data from the Kiel Electron Telescope. An analysis of energetic particle, plasma and magnetometer data from ULYSSES has allowed a unique study of the corresponding arrival of fare particles, particles within a corotating interaction region and particles transported with a coronal mass ejection. We present an analysis of these data in terms of possible diffusive shock acceleration but conclude that this is likely to be incompatible with the short transit time of the particles. Shock drift acceleration of particles with energies 0.3 MeV/nucleon or solar acceleration followed by particle trapping behind the shock front are alternative possibilities.     

Total ozone response to major geomagnetic storms during non-winter period

Summary This paper is a continuation of the recent studies of ozone response to major geomagnetic storms, now for the non-winter period. No significant response was found under different solar cycle/QBO conditions due to a rather homogeneous distribution of ozone during this period. Nevertheless, locations of the largest ozone variability are the same as the centers of meteorological activity and closely related to the centers of quasi-persistent extremes in the geographical distribution of total ozone along latitudinal circle 50 N.     

Variability of total electron content near the crest of the equatorial anomaly during moderate geomagnetic storms

The ionospheric responses to a large number (116) of moderate (?50≥D_st>?100 nT) geomagnetic storms distributed over the period (1980–1990) are investigated using total electron content (TEC) data recorded at Calcutta (88.38°E, 22.58°N geographic, dip: 32°N). TEC perturbations exhibit a prominent dependence on the local times of main phase occurrence (MPO). The storms with MPO during daytime hours are more effective in producing larger deviations and smaller time delays for maximum positive deviations compared to those with nighttime MPO. Though the perturbations in the equinoctial and winter solstitial months more or less follow the reported climatology, remarkable deviations are detected for the summer solstitial storms. Depending on the local times of MPO, the sunrise enhancement in TEC is greatly perturbed. The TEC variability patterns are interpreted in terms of the storm time modifications of equatorial electric field, wind system and neutral composition.     

Relativistic electron flux enhancement at synchronous orbit during SEP event on July 14, 2000

Relativistic (E >1.6 MeV) electron flux enhancements during Solar Energetic Particle (SEP) events as observed by the synchronous FY-2 satellite at orbit located at 105°E are investigated. Energetic protons during SEP events heavily contaminate relativistic electron flux measurements. The ratio of the contamination in the original measurement of relativistic electron flux was over 30% during most of the SEP event on July 14, 2000. A method has been developed to eliminate the contamination caused by the energetic protons, and a "corrected" relativistic electron flux has been obtained. The "cleaned-up" relativistic electron flux measurement shows that relativistic electron flux enhancement at synchronous orbit is well correlated with SEP events during which the IMF Bz has some southward periods. The enhancement could arise as the transport of relativistic electrons from the upstream solar wind into synchronous orbit via the magnetotail.     

Interrelation between physical processes during the main phases of geomagnetic storms related to the IMF B Z component according to a cluster analysis

The main causes of the main phases of geomagnetospheric storms (D _st ^min = ?(37?226) nT) have been studied using a cluster analysis in the form of the nearest neighbor method. Weak, moderate, strong and severe storms (samples) related to the IMF B _Z component have been distinguished based on the two-dimensional (with respect to the IMF B _Z component and D _st index) scale cluster classification of storm main phases. The correlation clustering of 32 interrelated physical processes characterizing each main phase made it possible to determine that interrelated physical processes included the common part of the internal structure for all samples. The studied samples of storm main phases are characterized by different physical development levels, depending on the event scale. The presence of a common part indicates that magnetospheric activity mostly depends on the IMF B _Z and B _Y components and the coupling functions between them, as well as on the total IMF B value during the main phases of storms of all D _st index scales. It has been established that the closest relationships are typical of D _st (V ² B _S ) and D _st (VB _S ), where B _S is the IMF southward component, and V is the solar wind velocity. Substorm activity (AE) generated by V ² B _S and VB _S is only substantial during the main phases of weak and moderate storms, whereas grouping with respect to the velocity V only shows substantial activity during severe magnetic storms. The role of the Akasofu parameter (?) proved to be less pronounced. It has been indicated that, in a first approximation, it is preferred to use the V ² B _S and VB _S coupling functions in order to predict the D _st index and estimate the injection function Q during the main phases of geomagnetospheric storms.     

A new instability mechanism related to high-angle waves

Waves with a large incidence angle in deep water can drive a morphodynamic instability on a sandy coast whereby shoreline sand waves, cuspate forelands, and spits can emerge. This instability is related to bathymetric perturbations extending offshore in the shoaling zone. Here, we explore a different mechanism where the large incidence angle is supposed to occur at breaking and the bathymetric perturbations occur only in the surf zone. For wave incidence angles at breaking above ≈?45^°, the one-line approximation of coastal dynamics predicts an unstable shoreline. This instability (EHAWI) is scale-free and the growth rate increases without bound for decreasing wavelength. Here we use a 2DH morphodynamic model resolving surf zone instabilities to investigate whether EHAWI could approximate a real instability in nature with a characteristic length scale. Assuming very idealized conditions on the bathymetric profile and sediment transport, we find a 2DH instability mode consisting of shore-oblique up-current bars coupled to a meandering of the longshore current. This mode grows for high-angle waves, above about 30^° (offshore) and the maximum growth rate occurs for the angle maximizing the angle at breaking, about 70^° (offshore). The dominant wavelength is of the order of the surf zone width. Interestingly, for long sand waves, the growth rate never becomes negative and it matches very well the anti-diffusive behavior of EHAWI. This distinguishes the present instability mode from other modes found in previous studies for other bathymetric and sediment transport conditions. Thus, we conclude that EHAWI approximates a real morphodynamic instability only for quite particular conditions. In such case, a characteristic length scale of the instability emerges thanks to surf zone processes that damp short wavelengths.     

Dust storms and related phenomena measured from meteorological records in Australia

Using meteorological records (1957–1984) in Australia, formal definitions are proposed of four types of dust event: dust storms, blowing dust, dust whirls, and dust haze. Australia-wide maps of the extent of these four dust event types are presented.     

Equatorial and low latitude ionosphere during intense geomagnetic storms

An investigation is made in order to analyse the role of neutral gas composition in the equatorial and low latitude ionosphere during intense geomagnetic storms. To this end data taken by the Dynamic Explorer 2 satellite at 280–300 km (molecular nitrogen N₂ and atomic oxygen O concentrations, electron density and vertical plasma drifts) are used. The sudden commencements of the events considered occurred at 11:38 UT on March 1, 1982, 18:41 UT on November 20, 1982 and 16:14 UT on February 4, 1983. Vertical plasma drifts are the most important contributor to the initial storm time response of the equatorial F region. Neutral composition changes (expressed as an increase in the molecular species, mainly N₂) possibly play a predominant role in the equatorial and low latitude (10–20°) decreases of electron density at heights near F2-region maximum during the main and recovery phases of intense geomagnetic storms. Delayed increases of electron density observed at daytime during the recovery phase may be also attributed to increases in atomic oxygen. At low latitudes possibly a combined effect of O increase and upward plasma drift due to enhanced equatorward winds is the responsible mechanism for the maintenance of enhanced electron density values.     

Contribution of global Pc5 oscillations to magnetic disturbance during geomagnetic storms

The contribution of global magnetospheric oscillations to magnetic disturbance during magnetospheric storms is studied. The bases of magnetic data from the INTERMAGNET global network in combination with the interplanetary and intramagnetospheric measurements of the magnetic field and plasma and the sets of the Kp, Dst, and AE indices are used for this purpose. The most favorable conditions in the solar wind and magnetosphere for generation of global Pc5 have been revealed. The contribution of these oscillations to the variations in the magnetic disturbance level, characterized by the AE index, has been estimated. The findings confirm that magnetospheric MHD oscillations participate in the processes of energy income from the solar wind and energy dissipation in the magnetosphere.     

Energetic proton precipitation related to ion–cyclotron waves

This brief review summarizes recent findings related to particle precipitation associated with electromagnetic ion–cyclotron (EMIC) waves seen on the ground as geomagnetic Pc1 and IPDP pulsations.Particle precipitation signatures of ion–cyclotron interaction are described as revealed from low-altitude satellite measurements of the energetic proton fluxes as well as from observations of the proton aurora. As a result, localized proton precipitation patterns situated equatorward of the isotropy boundary are disclosed. One of the patterns is a proton precipitation spot in the morning sector, presumably mapped onto plasmapause; another one is an elongated region of the precipitation, presumably mapped onto the plasmaspheric plume.Clear evidence of the pitch-angle scattering associated with the ion–cyclotron wave activity is found near the equatorial plane in the region conjugated with the localized proton precipitation at low altitude.Thus, the revealed precipitation patterns determine the location of the region of intense pitch-angle scattering of energetic protons, and, therefore, their observations can be used to monitor the region of the ion–cyclotron interaction and to study its origin and properties. Some examples of such application of the low-altitude observations of energetic particles are described.     

Supply of energy to the magnetosphere during the main phase of magnetic storms

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Barrier response to sea level rise and storms

This commentary brings together, as a Virtual Special Issue, several recent papers in Earth Surface Processes and Landforms that improve our understanding of coastal barrier response to relative sea level rise and a change in the frequency and/or magnitude of storm events. The ability to predict barrier response depends on the ability to quantify the spatial and temporal scales of sediment exchange amongst the nearshore, beach and dune. This exchange controls the height of the dune, which in turn determines the transfer of sediment to the backbarrier through washover and/or blowouts. The papers in this issue provide new insight on beach–dune interaction and the importance of this interaction to long‐term barrier evolution across a range of sites and scales, and how active management can influence this interaction and alter barrier response. Copyright © 2018 John Wiley & Sons, Ltd.     

Afternoon mid-latitude current system and low-latitude geomagnetic field asymmetry during geomagnetic storms

For four geomagnetic storms of middle intensity the relationship between the low-latitude magnetic field asymmetry using ASY indices and the intensity of the auroral eastward and westward electro-jet was considered. It was asked whether there exists a connection between ASY and the eastward electrojet. To answer this question equivalent current systems were estimated in mid-latitudes. It was found that the observations obviously show no correlative relationship between the low-latitude magnetic-field asymmetry and the eastward electrojet, whereas one exists between ASY and the westward electrojet. To explain the generally accepted common three-dimensional current system between the partial ring current and the eastward electrojet, a condensor model of the three-dimensional current system was developed. It could be shown that the short periodic variations of the partial ring current are shielded by the condensor and cannot influence the eastward-electrojet current.     

经验和抛物面模式模拟暴时磁场的误差特性研究

利用两种不同的磁场模式,协同GOES10/12、Polar及Geotail的实测资料,本文对3种不同强度磁暴的磁层磁场进行了详细分析,从而检验了两种不同磁场模式的可靠性,给出了模式间的性能对比.我们采用Tsyganenko经验模式及Alexeev抛物面模式,分别模拟了2004年4月、7月和11月的磁暴,3个磁暴过程中最强Dst指数分别达到-110 nT,-150 nT以及-289 nT.结果发现:T01模式对于中等磁暴时磁场模拟准确,而由于建立模式的数据库中强磁暴资料少,模式中不包含亚暴效应,T01对于强磁暴磁场模拟的误差增大,磁场分量远远偏离观测值;A2000模式缺少部分环电流及Ⅱ区场向电流,模式对于近地点Polar轨道上的磁场扰动模拟不足,对强磁暴时亚暴效应的过高模拟使其模拟的磁场分量偏高;T01模式对于中等磁暴时磁场的模拟效果明显好于A2000模式,随着磁层扰动的增强,T01与A2000模式的总体性能相当.     

Coherence of river and ocean conditions along the US West Coast during storms

The majority of water and sediment discharge from the small, mountainous watersheds of the US West Coast occurs during and immediately following winter storms. The physical conditions (waves, currents, and winds) within and acting upon the proximal coastal ocean during these winter storms strongly influence dispersal patterns. We examined this river–ocean temporal coherence for four coastal river–shelf systems of the US West Coast (Umpqua, Eel, Salinas, and Santa Clara) to evaluate whether specific ocean conditions occur during floods that may influence coastal dispersal of sediment. Eleven years of corresponding river discharge, wind, and wave data were obtained for each river–shelf system from USGS and NOAA historical records, and each record was evaluated for seasonal and event-based patterns. Because near-bed shear stresses due to waves influence sediment resuspension and transport, we used spectral wave data to compute and evaluate wave-generated bottom-orbital velocities. The highest values of wave energy and discharge for all four systems were consistently observed between October 15 and March 15, and there were strong latitudinal patterns observed in these data with lower discharge and wave energies in the southernmost systems. During floods we observed patterns of river–ocean coherence that differed from the overall seasonal patterns. For example, downwelling winds generally prevailed during floods in the northern two systems (Umpqua and Eel), whereas winds in the southern systems (Salinas and Santa Clara) were generally downwelling before peak discharge and upwelling after peak discharge. Winds not associated with floods were generally upwelling on all four river–shelf systems. Although there are seasonal variations in river–ocean coherence, waves generally led floods in the three northern systems, while they lagged floods in the Santa Clara. Combined, these observations suggest that there are consistent river–ocean coherence patterns along the US West Coast during winter storms and that these patterns vary substantially with latitude. These results should assist with future evaluations of flood plume formation and sediment fate along this coast.