分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： We present high-resolution observations of a quiescent solar prominence that consists of a vertical and a horizontal foot encircled by an overlying spine and has ubiquitous counter-streaming mass flows. While the horizontal foot and the spine were connected to the solar surface, the vertical foot was suspended above the solar surface and was supported by a semicircular bubble structure. The bubble first collapsed, then reformed at a similar height, and finally started to oscillate for a long time. We find that the collapse and oscillation of the bubble boundary were tightly associated with a flare-like feature located at the bottom of the bubble. Based on the observational results, we propose that the prominence should be composed of an overlying horizontal spine encircling a low-lying horizontal and vertical foot, in which the horizontal foot consists of shorter field lines running partially along the spine and has ends connected to the solar surface, while the vertical foot consists of piling-up dips due to the sagging of the spine fields and is supported by a bipolar magnetic system formed by parasitic polarities (i.e., the bubble). The upflows in the vertical foot were possibly caused by the magnetic reconnection at the separator between the bubble and the overlying dips, which intruded into the persistent downflow field and formed the picture of counter-streaming mass flows. In addition, the counter-streaming flows in the horizontal foot were possibly caused by the imbalanced pressure at the both ends.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： The superthermal ions at a quasi-parallel collisionless shock are considered to be generated during the reformation of the shock. Recently, hybrid simulations of a quasi-parallel shock have shown that during the reformation of a quasi-parallel shock the large-amplitude upstream low-frequency waves can trap the reflected ions at the shock front when they try to move upstream, and then these reflected ions can be accelerated several times to become superthermal ions. In this paper, with the Cluster observations of a quasi-parallel shock event, the relevance between the large-amplitude upstream low-frequency waves and the superthermal ions (about several keV) have been studied. The observations clearly show that the differential energy flux of superthermal ions in the upstream region is modulated by the upstream low-frequency waves, and the maxima of the differential energy flux are usually located between the peaks of these waves (including the shock front and the peak of the upstream wave just in front of the shock front). These superthermal ions are considered to originate from the reflected ions at the shock front, and the modulation is caused due to the trapping of the reflected ions between the upstream waves or the upstream waves and the shock front when these reflected ions try to travel upstream. It verifies the results from hybrid simulations, where the upstream waves play an important role in the generation of superthermal ions in a quasi-parallel shock.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： The solar wind is observed to display high speeds in high heliolatitude coronal holes and low speeds near the ecliptic plane. The heliospheric magnetic field associated with the solar wind plays a very important role in the transport and modulation of charged energetic particles, including galactic cosmic rays (GCRs) and solar energetic particles (SEPs), in the three-dimensional heliosphere. In previous studies, a constant solar wind speed, which is independent of heliolatitude, was assumed and commonly used in modulation modeling of cosmic rays. In this work, we investigate the realistic latitudinally dependent solar wind speed and utilize the theoretical models in hyperbolic and piecewise polynomial forms to explore the important effects on the heliospheric magnetic field and the diffusion coefficients (parallel, perpendicular, and drift scale) of cosmic rays in the presence of adiabatic focusing. Comparisons of the diffusion coefficients derived from standard Parker field and modified magnetic fields are presented. Since the structures and properties of different solar wind sources (coronal streamer belt, polar coronal hole, and transition region between them) differ from each other in essence, we suggest that the latitudinally dependent solar wind speed and the corresponding heliospheric magnetic field and diffusion coefficients with adiabatic focusing should be employed in the global modeling studies of GCRs and SEPs in the heliosphere.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Two-ribbon brightenings are one of the most remarkable characteristics of an eruptive solar flare and are often used to predict the occurrence of coronal mass ejections (CMEs). Nevertheless, it was recently called into question whether all two-ribbon flares are eruptive. In this paper, we investigate a two-ribbon-like white-light (WL) flare that is associated with a failed magnetic flux rope (MFR) eruption on 2015 January 13, which has no accompanying CME in the WL coronagraph. Observations by the. Optical and Near-infrared Solar Eruption Tracer and the Solar Dynamics Observatory reveal that with the increase of the flare emission and the acceleration of the unsuccessfully erupting MFR, two isolated kernels appear at the WL 3600 angstrom passband and quickly develop into two elongated ribbon-like structures. The evolution of the WL continuum enhancement is completely coincident in time with the variation of Fermi hard X-ray 26-50 keV flux. An increase of continuum emission is also clearly visible at the whole FUV and NUV passbands observed by the. Interface Region Imaging Spectrograph. Moreover, in one WL kernel, the Si IV, C II, and Mg II h/k lines display significant enhancement and non-thermal broadening. However, their Doppler velocity pattern is location-dependent. At the strongly bright pixels, these lines exhibit a blueshift, while at moderately bright ones, the lines are generally redshifted. These results show that the failed MFR eruption is also able to produce a two-ribbon flare and high-energy electrons that heat the lower atmosphere, causing the enhancement of the WL and FUV/NUV continuum emissions and chromospheric evaporation.
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