Sigmoid CME source regions at the Sun: some recent results

Identifying coronal mass ejection (CME) precursors in the solar corona would be an important step in space weather forecasting, as well as a vital key to understanding the physics of CMEs. Twisted magnetic field structures are suspected of being the source of at least some CMEs. These features can appear sigmoid (S or inverse-S) shaped in soft X-ray (SXR) images. We review recent observations of these structures and their relation to CMEs, using SXR data from the Soft X-ray Telescope (SXT) on the Yohkoh satellite, and EUV data from the EUV Imaging Telescope (EIT) on the SOHO satellite. These observations indicate that the pre-eruption sigmoid patterns are more prominent in SXRs than in EUV, and that sigmoid precursors are present in over 50% of CMEs. These findings are important for CME research, and may potentially be a major component to space weather forecasting. So far, however, the studies have been subject to restrictions that will have to be relaxed before sigmoid morphology can be used as a reliable predictive tool. Moreover, some CMEs do not display a SXR sigmoid structure prior to eruption, and some others show no prominent SXR signature of any kind before or during eruption.     

Solar large-scale emitting chains: some CME-associated events

Transient large-scale emitting chains and threads, associated with several coronal mass ejections (CMEs), are analyzed by the SOHO/EIT, TRACE, Yohkoh/SXT, Nobeyama Radioheliograph, and some other imaging data. It is illustrated that a pronounced evolution of the chains and threads in the EUV, soft X-ray, microwave, and other ranges can occur many hours both before and after a CME on a considerable part of the solar visible disk, especially near the place of a CME eruption. Such relations between chains and CMEs seem to be plausible due to both phenomena being the consequences of the evolution of large-scale magnetic fields and have often a global character.     

The relationship between prominence eruptions and coronal mass ejections

The SOHO observations with LASCO and EIT present an ideal opportunity to study the relationship between prominence eruptions and coronal mass ejections (CME). High-cadence measurements of prominence eruptions demonstrate that the prominence eruption is not generally the cause of the associated CME, but that it is more probable that the destabilisation of the CME in fact releases the constraints on the prominence, causing it to erupt. We report here selected observations of associated CMEs and prominence eruptions covering the period of SOHO operations from mid-January 1996 to October 1999. In addition to the causality, we find that in general the projected speed of the prominence eruption matches fairly closely the projected speed of the associated CME, but it is always lower. Furthermore, the prominence eruption is generally simply one facet of the coronal transient activity, of which there are often several other discrete parts. The prominence eruption is also generally offset in heliolatitude from the centre of the CME.     

Variations in the large-scale polar solar magnetic flux: The average annual series of the Π index in 1858–2006

A long series of the known Π index of the solar corona structure has been proposed. It seems that this index, which characterizes the limb extension of polar coronal plume systems, is of importance because it is related to the large-scale polar solar magnetic flux. Solar corona photographs and drawings during total solar eclipses, collected for 13 solar activity cycles from different sources (78 eclipses), as well as H-alpha map data on the drift of the high-latitude belt of filaments before polarity reversal of the polar magnetic field have been used. Daily solar corona images, obtained on the SOHO spacecraft (using an EIT ultraviolet telescope), have been additionally used.     

Vla and Spacecraft Observations of Solar Activity

We describe the world's largest synthesis radio telescope, the Very Large Array (VLA), and how it can be used to complement observations with the Solar and Heliospheric Observatory (SOHO) and the Yohkoh solar spacecraft. The VLA provides images with high spatial and temporal resolution, often across the visible solar disk. The VLA also detects nonthermal radiation that is not observed with SOHO and Yohkoh, and provides estimates for the coronal magnetic field strengths that are not directly measured by these spacecraft. The VLA data can be combined with SOHO CDS, SOHO EIT, or Yohkoh SXT observations to provide new insights to the compact, variable sources, called blinkers and bright points, in the solar transition region or low corona. A new 400 cm VLA system provides images of nonthermal burst activity associated with Coronal Mass Ejections (CMEs), and may detect thermal emission from CMEs, that can be compared with SOHO's LASCO and EIT instruments to obtain new information about the origin and evolution of CMEs.     

暗条爆发与日冕物质抛射

本文对太阳活动区ＡＲ６８９１ 中两个暗条爆发的磁场环境、及爆发所引起的日地物理效应进行了比较和分析结果表明, 出现在靠近大尺度单极区的活动区暗条爆发, 可能导致较强烈的日冕物质抛射     

太阳X-EUV成像望远镜

太阳X_EUV成像望远镜用来监测和预报影响空间天气变化的太阳活动,专门服务于空间天气预报研究. 望远镜工作在4～100?的X射线波段和195?极紫外谱段,视场角45′,角度分辨5″,提供全日面、高分辨的成像观测. 文中分析了太阳X、EUV波段的成像观测应用,介绍了X_EUV望远镜的基本设计,分析了望远镜对不同温度日冕等离子体的敏感性、对不同太阳活动现象的响应及反演日冕等离子体参数过滤片的组合利用. 太阳X_EUV成像望远镜集成了掠入射望远镜和正入射望远镜两套系统,扩展了单一X射线望远镜的成像功能,能够观测更多的太阳爆发先兆现象或者伴生现象,是目前国际上同类仪器中最新的太阳成像监测仪器.     

Correlation between the parameters of coronal mass ejections and features of their propagation in the corona with the large-scale structure of the solar magnetic field

Correlation between the parameters of coronal mass ejections (CMEs) that are detected on the LASCO coronographs and are associated with eruptive prominences and the distances of CME axes from the coronal streamer belt has been analyzed. The deviations of CME trajectories from the radial direction have been investigated.     

Method for determining the parameters of full halo coronal mass ejections

A method for determining the parameters of halo-type coronal mass ejections (full halo CMEs)—direction of motion, angular size, CME velocity along the Sun-Earth axis, etc.—has been proposed and tested. The method is based on the found empirical dependence between the angular sizes of CMEs located near the sky plane and angular sizes of associated eruptive prominences or post-eruptive arcades as well as on the relationships between the halo CME parameters derived in a simple geometrical CME model. Using this method and the SOHO/LASCO C3 and SOHO/EIT data, the parameters of 33 full halo CMEs have been determined. It is concluded that (1) the trajectories of all considered full halo CMEs deviate with recession of the CME front to R _F > (2–5)R ₀ toward the Sun-Earth axis; (2) the majority of full halo CMEs recorded by LASCO C3 coronagraphs have relatively large angular sizes, 2α > 60°.     

Magnetic field variations accompanying the filament eruption and the flare related to coronal mass ejections

Field variations in the region of eruptive event of June 7, 2011, associated with the filament eruption (FE), flare, and coronal mass ejection are studied based on vector measurements of the photospheric magnetic field with the SDO/HMI instrument. Variations of the module (B), the radial (Br) and transverse (Bt) components of the magnetic induction, and the inclination angle (α) of field lines to the radial direction from the center of the Sun are analyzed. It is shown that the strongest changes of the field before the event were located near the base of the southeastern leg of the eruptive filament; after the beginning of the event, they were located in the CME flare region. It is suggested that the FE is associated with two episodes of strong and rapid field variations: before the beginning of the slow filament rise and before its sudden acceleration. For the first time, variations of the inclination angles of the field lines over time in different parts of the eruptive event are studied in detail. It was found that the inclination angles of the field lines decrease in the vicinity of its channel during the slow rise of the filament, and the inclination angles of the field lines increase sharply after the beginning of the flare in the flare region in the vicinity of the neutral line.     

Moreton wave, “EIT wave”, and type II radio burst as manifestations of a single wave front

We show that a Moreton wave, an “EIT wave,” and a type II radio burst observed during a solar flare of July 13, 2004, might have been a manifestation of a single front of a decelerating shock wave, which appeared in an active region (AR) during a filament eruption. We propose describing a quasi-spheroidal wave propagating upward and along the solar surface by using relations known from a theory of a point-like explosion in a gas whose density changes along the radius according to a power law. By applying this law to fit the drop in density of the coronal plasma enveloping the solar active region, we first managed to bring the measured positions and velocities of surface Moreton wave and “EIT wave” into correspondence with the observed frequency drift rate of the meter type II radio burst. The exponent of the vertical coronal density falloff is selected by fitting the power law to the Newkirk and Saito empirical distributions in the height range of interest. Formal use of such a dependence in the horizontal direction with a different exponent appears to be reasonable up to distances of less than 200 Mm around the eruption center. It is possible to assume that the near-surface shock wave weakens when leaving this radius and finally the active region, entering the region of the quiet Sun where the coronal plasma density and the fast-mode speed are almost constant along the horizontal.     

CME和太阳耀斑现象

在本文里, 我们对ＣＭＥ 和太阳耀斑现象的各种相互关系进行了讨论希望本文的内容能够引起天文、空间物理和地球物理等人员的兴趣, 促进ＣＭＥ的综合研究     

Seasonal variation of non-migrating semidiurnal tide in the polar MLT region in a general circulation model

Behavior of semidiurnal tides in the north and south polar MLT regions simulated by Middle Atmosphere Circulation Model at Kyushu University is described. Summertime enhancement of westward propagating semidiurnal tide with zonal wavenumber s=1 is found, which is consistent with the observed result at the South Pole (Ann. Geophys. 16 (1998) 828). Additional numerical simulations show that the non-migrating semidiurnal tide is mainly generated by the nonlinear interactions between stationary planetary waves with zonal wavenumber s=1 and the migrating semidiurnal tide in the stratosphere and mesosphere as suggested by Forbes et al. (Geophys. Res. Lett. 22(23) (1995) 3247).     

Episodic tectonic plate reorganizations driven by mantle convection

Periods of relatively uniform plate motion were interrupted several times throughout the Cenozoic and Mesozoic by rapid plate reorganization events [R. Hey, Geol. Soc. Am. Bull. 88 (1977) 1404-1420; P.A. Rona, E.S. Richardson, Earth Planet. Sci. Lett. 40 (1978) 1-11; D.C. Engebretson, A. Cox, R.G. Gordon, Geol. Soc. Am. Spec. Pap. 206 (1985); R.G. Gordon, D.M. Jurdy, J. Geophys. Res. 91 (1986) 12389-12406; D.A. Clague, G.B. Dalrymple, US Geol. Surv. Prof. Pap. 1350 (1987) 5-54; J.M. Stock, P. Molnar, Nature 325 (1987) 495-499; C. Lithgow-Bertelloni, M.A. Richards, Geophys. Res. Lett. 22 (1995) 1317-1320; M.A. Richards, C. Lithgow-Bertelloni, Earth Planet. Sci. Lett. 137 (1996) 19-27; C. Lithgow-Bertelloni, M.A. Richards, Rev. Geophys. 36 (1998) 27-78]. It has been proposed that changes in plate boundary forces are responsible for these events [M.A. Richards, C. Lithgow-Bertelloni, Earth Planet. Sci. Lett. 137 (1996) 19-27; C. Lithgow-Bertelloni, M.A. Richards, Rev. Geophys. 36 (1998) 27-78]. We present an alternative hypothesis: convection-driven plate motions are intrinsically unstable due to a buoyant instability that develops as a result of the influence of plates on an internally heated mantle. This instability, which has not been described before, is responsible for episodic reorganizations of plate motion. Numerical mantle convection experiments demonstrate that high-Rayleigh number convection with internal heating and surface plates is sufficient to induce plate reorganization events, changes in plate boundary forces, or plate geometry, are not required.     

Effects of neutral gas motions on midlatitude E region irregular structure

This paper discusses the physical nature of neutral wind-induced processes. A simple “pictorial” explanation of the neutral-wind-driven gradient drift and thermal instabilities, originally suggested by Kagan and Kelley (Geophys. Res. Lett. 25 (1998) 4141; J. Geophys. Res. 105 (2000) 5291), is developed for the first time. A formula for the growth rate of a neutral-wind-driven gradient drift instability applicable for an arbitrary latitude is presented. The effectiveness of the neutral wind approach to the problem of midlatitude backscatter in the ionospheric E region is discussed. The paper concludes that there are cases when one may experimentally distinguish between electric field- and neutral wind-driven processes and shows that the backscatter character is defined by either the neutral behavior or the structure of ionization clouds but not by the gradient drift process itself.     

The response of the high-latitude ionosphere to IMF variations

The convection of plasma in the high-latitude ionosphere is strongly affected by the interplanetary magnetic field (IMF) carried by the solar wind. From numerous statistical studies, it is known that the plasma circulation conforms to patterns that are characteristic of particular IMF states. Following a change in the IMF, the convection responds by reconfiguring into a pattern that is more consistent with the new IMF. Some early studies reported that the convection first begins to change near noon while on the dawn and dusk flanks and on the nightside it remains relatively unaffected for tens of minutes. Work by Ridley et al. (J. Geophys. Res. 103 (1998) 4023–4039) and Ruohoniemi and Greenwald (Geophys. Res. Lett. 25 (1998) 2913–2916) that was based on measurements with more global sets of instruments challenged this view. A debate ensued as to the true nature of the convection response. We follow the arguments of Lockwood and Cowley (J. Geophys. Res. 104 (1999) 4387–4391) and Ridley et al. (J. Geophys. Res. 104 (1999) 4393–4396) by reviewing recent results on the timing of the onset of the convection response to the changed IMF. We discuss the timing problem from the perspectives of observations and modeling. In our view, the onset of the ionospheric response to changed IMF is globally simultaneous on time scales of a few minutes. A physical basis for the rapid communication of effects in the dayside convection to the nightside has been demonstrated in magnetohydrodynamic simulations. We also offer some cautionary notes on the timing of convection changes and the use of global assimilative techniques to study local behavior.     

Flares Unaccompanied by Interplanetary Coronal Mass Ejections and Solar Proton Events

Geomagnetism and Aeronomy - For a coronal mass ejection (CME) to occur, the magnetic pressure of the overlying layers must be lower than that of the underlying layers, and the corresponding...     

Analysis of the events that occurred on July 30, 2005, according to ground-based observations in the Ca II 8498 ? line and on board spacecraft

The observations of active region (AR) NOAA 10792 in the Ca II 8498 ? line with an ATB-1 solar telescope at the Sternberg State Astronomical Institute, Moscow State University (SSAI MSU) on July 30, 2005, are illustrated, and the events are analyzed using the data obtained on spacecraft. Three flares and accompanying coronal mass ejections (CMEs) are considered. It has been indicated that the beginning of the first compact CME lagged behind the flare onset by 3 min. Plasma ascended with acceleration that reached 0.4 km/s² at the flare maximum. The matter was also apparently accelerated after the flare maximum, since an ejection could only appear at the edge of the occulting C 2 LASCO coronograph disk at 0557 UT when acceleration is about 0.5 km/s². The second CME (of the halo type) leaded the beginning of the corresponding flare.     

A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm

Shear wave splitting parameters represent a useful tool to detail the stress changes occurring in volcanic environments before impending eruptions. In the present paper, we display the parameter estimates obtained through implementation of a semiautomatic algorithm applied to all useful datasets of the following Italian active volcanic areas: Mt. Vesuvius, Campi Flegrei, and Mt. Etna. Most of these datasets have been the object of several studies (Bianco et al., Annali di Geofisica, XXXXIX 2:429–443, 1996, J Volcanol Geotherm Res 82:199–218, 1998a, Geophys Res Lett 25(10):1545–1548, 1998b, Phys Chem Earth 24:977–983, 1999, J Volcanol Geotherm Res 133:229–246, 2004, Geophys J Int 167(2):959–967, 2006; Del Pezzo et al., Bull Seismol Soc Am 94(2):439–452, 2004). Applying the semiautomatic algorithm, we confirmed the results obtained in previous studies, so we do not discuss in much detail each of our findings but give a general overview of the anisotropic features of the investigated Italian volcanoes. In order to make a comparison among the different volcanic areas, we present our results in terms of the main direction of the fast polarization (φ) and percentage of shear wave anisotropy (ξ).     

The millennium eruption of Hekla in February 2000

The 18th historic eruption of Hekla started on 26 February, 2000. It was a short-lived but intense event, emitting basaltic andesitic (55.5 wt% SiO₂) pyroclastic fragments and lava. During the course of the eruption, monitoring was done by both instruments and direct observations, together providing unique insight into the current activity of Hekla. During the 12-day eruption, a total of 0.189 km³ DRE of magma was emitted. The eruptive fissure split into five segments. The segments at the highest altitude were active during the first hours, while the segments at lower altitude continued throughout the eruption. The eruption started in a highly explosive manner giving rise to a Subplinian eruptive column and consequent basaltic pyroclastic flows fed by column collapses. After the explosive phase reached its maximum, the eruption went through three more phases, namely fire-fountaining, Strombolian bursts and lava effusion. In this paper, we describe the course of events of the eruption of Hekla and the origin of its magma, and then show that the discharge rate can be linked to different style of eruptive activity, which are controlled by fissure geometry. We also show that the eruption phases observed at Hekla can be linked with inferred magma chamber overpressure prior to the eruption.