Non-magnetic aspect sensitive auroral echoes from the lower E region observed at 50 MHz

Backscatter from E-region irregularities was observed at aspect angles close to 90° (almost parallel to the direction of the magnetic field) using the ALOMAR SOUSY radar at Andoya/Norway. Strong electric fields and increased E-region electron temperatures simultaneously measured with the incoherent scatter facility EISCAT proved that the Farley-Buneman plasma instability was excited. In addition, strong particle precipitation was present as inferred from EISCAT electron densities indicating that the gradient drift instability may have been active, too. Backscatter at such large aspect angles was not expected and has not been observed before. The characteristics of the observed echoes, however, are in many aspects completely different from usual auroral radar results: the Doppler velocities are only of the order of 10 m/s, the half-width of the spectra is around 5 m/s, the echoes originate at altitudes well below 100 km, and they seem to be not aspect-sensitive with respect to the magnetic field direction. We, therefore, conclude that the corresponding irregularities are not caused by the mentioned instabilities and that other mechanism have to be invoked.     

ULF wave occurrence statistics in a high-latitude HF Doppler sounder

Ultra low frequency (ULF) wave activity in the high-latitude ionosphere has been observed by a high frequency (HF) Doppler sounder located at Tromsø, Norway (69.71°N, 19.2°E geographic coordinates). A statistical study of the occurrence of these waves has been undertaken from data collected between 1979 and 1984. The diurnal, seasonal, solar cycle and geomagnetic activity variations in occurrence have been investigated. The findings demonstrate that the ability of the sounder to detect ULF wave signatures maximises at the equinoxes and that there is a peak in occurrence in the morning sector. The occurrence rate is fairly insensitive to changes associated with the solar cycle but increases with the level of geomagnetic activity. As a result, it has been possible to characterise the way in which prevailing ionospheric and magnetospheric conditions affect such observations of ULF waves.     

Sausage mode instability of thin current sheets as a cause of magnetospheric substorms

Observations have shown that, prior to substorm explosions, thin current sheets are formed in the plasma sheet of the Earth’s magnetotail. This provokes the question, to what extent current-sheet thinning and substorm onsets are physically, maybe even causally, related. To answer this question, one has to understand the plasma stability of thin current sheets. Kinetic effects must be taken into account since particle scales are reached in the course of tail current-sheet thinning. We present the results of theoretical investigations of the stability of thin current sheets and about the most unstable mode of their decay. Our conclusions are based upon a non-local linear dispersion analysis of a cross-magnetic field instability of Harris-type current sheets. We found that a sausage-mode bulk current instability starts after a sheet has thinned down to the ion inertial length. We also present the results of three-dimensional electromagnetic PIC-code simulations carried out for mass ratios up to M_i/m_e = 64. They verify the linearly predicted properties of the sausage mode decay of thin current sheets in the parameter range of interest.     

Upstream perturbation and floodplain formation effects on chute-cutoff-dominated meandering river pattern and dynamics

A sustained dynamic inflow perturbation and bar–floodplain conversion are considered crucial to dynamic meandering. Past experiments, one-dimensional modelling and linear theory have demonstrated that the initiation and persistence of dynamic meandering require a periodic transverse motion of the inflow. However, it remains unknown whether the period of the inflow perturbation affects self-formed meander dynamics. Here, we numerically study the effect of the inflow perturbation period on the development and meander dynamics of a chute-cutoff-dominated river, which requires two-dimensional modelling with vegetation forming floodplain on bars. We extended the morphodynamic model Nays2D with growth and mortality rules of vegetation to allow for meandering. We tested the effect of a transversely migrating inflow boundary by varying the perturbation period between runs over an order of magnitude around typical modelled meander periods. Following the cutoff cascade after initial meander formation from a straight channel, all runs with sufficient vegetation show series of growing meanders terminated by chute cutoffs. This generates an intricate channel belt topography with point bar complexes truncated by chutes, oxbow lakes, and scroll-bar-related vegetation age patterns. The sinuosity, braiding index and meander period, which emerge from the inherent biomorphological feedback loops, are unrelated to the inflow perturbation period, although the spin-up to dynamic equilibrium takes a longer time and distance for weak and absent inflow perturbations. This explains why, in previous experimental studies, dynamic meandering was only accomplished with a sustained upstream perturbation in flumes that were short relative to the meander wavelength. Our modelling of self-formed meander patterns is evidence that scroll-bar-dominated and chute-cutoff-dominated meanders develop from downstream convecting instabilities. This insight extends to many more fluvial, estuarine and coastal systems in morphological models and experiments, which require sustained dynamic perturbations to form complex patterns and develop natural dynamics. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

宁静期间双峰电流片的观测研究

2001年8月19日2011～2030UT,AE指数相对较小(40～130 nT),Cluster 卫星穿越磁尾电流片.利用Cluster 观测资料分析,发现宁静期间有双峰电流片存在,这期间没有明显的高速流,没有明显的电流片振荡. 同时,进一步分析双峰电流片中的离子特性发现:质子数密度Np在中性线（<Bx> = 0）附近相对较大,呈非对称性分布;质子温度分布比较均匀;在中性线两侧,质子y方向上的流速Vy方向相反. 最后讨论LHDI（低混杂漂移不稳定性）的非线性演化可能是形成宁静期间双峰电流片的主要机制. 这些对进一步完善电流片形成机制可能有很重要的意义.     

Detecting ionized bubbles in redshifted 21-cm maps

Linear transient phenomena induced by flow non-normality in thin self-gravitating astrophysical discs are studied using the shearing sheet approximation. The considered system includes two modes of perturbations: vortex and (spiral density) wave. It is shown that self-gravity considerably alters the vortex mode dynamics; its transient (swing) growth may be several orders of magnitude stronger than in the non-self-gravitating case and two to three times larger than the transient growth of the wave mode. Based on this finding, we comment on the role of vortex mode perturbations in a gravitoturbulent state. We also describe the linear coupling of the perturbation modes, caused by the differential character of disc rotation. The coupling is asymmetric: vortex mode perturbations are able to excite wave mode perturbations, but not vice versa. This asymmetric coupling lends additional significance to the vortex mode as a participant in spiral density waves and shock manifestations in astrophysical discs.     

Tailored Blast Wave Production Pertaining to Supernova Remnants

We report on the first production of “tailored” blast waves in cluster media using a 1 ps laser pulse focused to 2×10¹⁶ W/cm². This new technique allows cylindrical blast waves to be produced with a strong axial modulation of variable spatial frequency, as a seed for instability growth. Energy deposition is modified by changing the cluster density whilst keeping the atomic density of the target constant. Electron density maps show the production of strongly modulated blast waves and the development of a thin shell structure in H at late times, and the trajectories show blast waves forming in H, and Ar. In Xe, a blast wave does not form on the timescales studied. Analysis of astrophysical similarity parameters suggests that a hydrodynamically similar situation is created in H, and that further evolution would create a regime where radiative effects may be influential in Ar and Xe.     

Instabilities of galactic discs in the presence of star formation

We discuss the stability of galactic discs in which the energy of interstellar clouds is gained in encounters with expanding supernova (SN) remnants and lost in inelastic collisions. Energy gain and loss processes introduce a phase difference between the pressure and density perturbations, making discs unstable on small scales for several recipes of star formation. This is in contrast to the standard stability analysis in which small-scale perturbations are stabilized by pressure. In the limit of small scales, the dispersion relation for the growth rate reduces to that of thermal instabilities in a fluid without gravity. If instabilities lead to star formation, then our results imply a secondary mode of star formation that operates on small scales and feeds on the existence of a primary mode on intermediate scales. This may be interpreted as positive feedback. Further, the standard stability criterion on intermediate scales is significantly modified.     

Features of discrete VLF emissions observed at Gulmarg,India during the magnetic storm of 6–7 March, 1986

During the analysis of archived VLF data from Indian low latitude ground stations, some discrete VLF emissions recorded at the low latitude ground station Gulmarg (geomagnetic latitude 24°26′N; geomagnetic longitude 147°09′E, L = 1.28) during moderate magnetic storm activity (Σ K _P ⁻ = 32, K _P index varies from 4 to 6 during the observation period) on 6/7 March, 1986 are presented in this paper. The dynamic spectra of these discrete VLF emissions were observed along with tweeks and its harmonics, which is interesting and complex to explain. In most of the events the harmonic frequency of tweeks correlates with the starting frequency of harmonics of discrete emissions. In order to explain the observed features of discrete VLF emissions, we propose cyclotron resonance interaction between whistler mode wave and energetic electrons of inner radiation belt as possible generation mechanism. An attempt is also made to determine parallel energy, anisotropy and wave growth relevant to the generation process of VLF emissions.