首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Highly variable conditions prevail in the geospace environment due to the variations in Solar activity. The characteristics of the magnetic clouds (MCs) and their effects on geosphere, which have occurred during the period January 1996 to December 2006; have been investigated. No systematic trend has been observed between MCs and Solar activity cycle which is analyzed on the basis of maximum Sunspot number in that particular year. 85% MCs are found to be geoeffective. MCs are divided into two major classes: unipolar and bipolar. Unipolar MCs are of south (S) or north (N) type while bipolar MCs are of south-north (SN) or north-south (NS) type. During Solar cycle 23, SN-type MCs dominated over NS-type MCs. Highly intense geomagnetic storms (GMSs) of Dst <−300 nT follow from SN or S-type MCs. No preference is observed for right handed (RH) or left handed (LH) clouds for being geoeffective. MCs of very high speed lead to intense GMSs. The correlation coefficient (r) of southward component of magnetic field (Bz), total magnetic field (B) and their products with plasma flow speed (VB and VBz) with Dst are observed to be r=0.78, −0.81, −0.79 and 0.82, respectively, which suggests that these parameters are reliable indicators of the strength of GMS. SN clouds do not always lead to more fall in Dst value (or lead to high strength of GMS) than NS clouds for similar value of Bz minimum associated with both type of MCs.  相似文献   

2.
Magnetic clouds (MCs) are transient magnetic structures giving the strongest southward magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry a southward magnetic field. The southward magnetic field is responsible for space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions for 1995 to 2017. 85% of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. MCs of the strongest magnetic magnitude and Bz south occur in the declining phase of the solar cycle. Bipolar MCs depend on the solar cycle in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency, but not in their polarity. MCs with the highest speeds, the largest total-\(B\) magnitudes, and sheath Bz south originate from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. Combinations of a south Bz in the sheath and south-first MCs in close succession have caused the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017 and now spans 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs that originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has a mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.  相似文献   

3.
Magnetic clouds (MCs) belong to an important subset of interplanetary coronal mass ejections. The identification of their boundaries is always a problem in the studies of MCs. This paper discusses a method to identify the boundaries of MCs by coordinate transformation. Instead of the conventional GSE (Geocentric Solar Ecliptic) coordinate system, the interplanetary magnetic field data are converted into a cloud natural coordinate system, in which the profile of the MC as a magnetic flux tube is clearly displayed. Then, combining with the plasma properties of the MC, the boundary of the cloud can be identified easily. Six observed MCs are analyzed using this method, and the results show that this method is feasible and can reduce the uncertainty in the identification of MC boundaries.  相似文献   

4.
Recent studies have revealed that there are a large number of low frequency electromagnetic cyclotron waves (ECWs) occurring in and around magnetic clouds (MCs) that are common magnetic structures in interplanetary space. Using magnetic field data from the STEREO spacecraft, this paper investigates polarization properties of ECWs associated with 120 MCs. Results show that the ECWs are highly transverse, strongly polarized waves with large ellipticities. Specifically, almost all of the waves take place with the ratios of transverse power to total power higher than 0.94, polarization degrees greater than 0.85, and ellipticities larger than 0.5. The mean values of these quantities can be up to 0.99, 0.96, 0.85, respectively. In particular, there is a tendency of ellipticities decreasing with respect to the wave normal angles for ECWs with left handed polarization. The decreasing tendency is consistent with the recent theory and simulation results.  相似文献   

5.
The expected lifetimes for molecular clouds has become a topic of considerable debate as numerical simulations have shown that MHD turbulence, the nominal means of support for clouds against self-gravity, will decay on short timescales. Thus it appears that either molecular clouds are transient features or they are resupplied with turbulent energy through some means. Jets and molecular outflows are recognized as a ubiquitous phenomena associated with star formation. Stars however form not isolation but in clusters of different density and composion. The ubiquity and high density of outflows from young stars in clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we present new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their abilityto drive turbulent flows in molecular clouds. Our studies focus on scales associated with young star forming clusters. In particular we first show that direct collisions between active outflows are not effective at stirring the ambient medium. We then show that fossil cavities from “extinct” outflows may provide the missing link in terms of transferring momentum and energy to the cloud.  相似文献   

6.
We compare the number and characteristics of interplanetary coronal mass ejections (ICMEs) to those of magnetic clouds (MCs) by using in-situ solar wind plasma and magnetic field observations made at 1 AU during solar cycle 23. We found that ≈ 28% of ICMEs appear to contain MCs, since 103 magnetic clouds (MCs) occurred during 1995  – 2006, and 307 ICMEs occurred during 1996 – 2006. For the period between 1996 and 2006, 85 MCs are identified as part of ICMEs, and six MCs are not associated with ICMEs, which conflicts with the idea that MCs are usually a subset of ICMEs. It was also found that solar wind conditions inside MCs and ICMEs are usually similar, but the linear correlation between geomagnetic storm intensity (Dst min ) and relevant solar wind parameters is better for MCs than for ICMEs. The differences between average event duration (Δt) and average proton plasma β (〈β〉) are two of the major differences between MCs and ICMEs: i) the average duration of ICMEs (29.6 h) is 44% longer than for MCs (20.6 hours), and ii) the average of 〈β〉 is 0.01 for MCs and 0.24 for ICMEs. The difference between the definition of a MC and that for an ICME is one of the major reasons for these average characteristics being different (i.e., listed above as items i) and ii)), and it is the reason for the frequency of their occurrences being different.  相似文献   

7.
Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. Observations of magnetic fields can distinguish between these two models because of different predictions in three areas: (1) magnetic field morphology, (2) the scaling of field strength with density and non-thermal velocities, and (3) the mass to magnetic flux ratio, M/Φ. We first discuss the techniques and limitations of methods for observing magnetic fields in star formation regions, then describe results for the L1544 prestellar core as an exemplar of the observational results. Application of the three tests leads to the following conclusions. The observational data show that both magnetic fields and turbulence are important in molecular cloud physics. Field lines are generally regular rather than chaotic, implying strong field strengths. But fields are not aligned with the minor axes of oblate spheroidal clouds, suggesting that turbulence is important. Field strengths appear to scale with non-thermal velocity widths, suggesting a significant turbulent support of clouds. Giant Molecular Clouds (GMCs) require mass accumulation over sufficiently large volumes that they would likely have an approximately critical M/Φ. Yet H I clouds are observed to be highly subcritical. If self-gravitating (molecular) clouds form with the subcritical M/Φ of H I clouds, the molecular clouds will be subcritical. However, the observations of molecular cloud cores suggest that they are approximately critical, with no direct evidence for subcritical molecular clouds or cloud envelopes. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.  相似文献   

8.
A subset of CMEs, called interplanetary magnetic clouds (MCs), are observed to have systematic rotation [northward to southward (NS) or southward to northward (SN)] in their field structures. These MCs identified in the heliospheric plasma and field data at 1 AU may have different features associated with them. These structures (NS/SN) may be isolated MC moving with the ambient solar wind. MCs (NS/SN) may also be associated with shock/sheath region, formed due to compression of the ambient plasma/field ahead of them. A fraction from each of these four types of MCs have additional features, being ‘pushed’ by fast solar wind streams from coronal holes, forming interaction region (IR) between MCs and high-speed solar wind streams (HSS). Using these different sets of MCs, we have done a detailed study of the geoeffectiveness of NS and SN turning MCs and their associated features (shock/sheath, IR and HSS). To study the process that produces the geomagnetic disturbances and influences its amplitude/duration, we have utilized the interplanetary plasma and field parameters, namely, plasma velocity, density, temperature, pressure, field strength and its north-south component, during the passage of these structures with different associated properties. Differences in the geoeffectiveness of MCs with different structural and dynamical properties have been identified. The possible role of high-speed stream in influencing the recovery time (and hence duration) of geomagnetic disturbance has also been investigated. A best-fit equation representing the relation between level of the geomagnetic activity (due to MCs) and interplanetary plasma/field parameter has been obtained.  相似文献   

9.
Interplanetary coronal mass ejections (ICMEs) and their subset, magnetic clouds (MCs), are important manifestations of solar activity which have substantial impact on the geomagnetic field. We re-analyze events already identified in Wind and Voyager 2 data and estimate changes of their geometry along the path from the Sun. The analysis is based on the thickness of the sheath between a shock and a particular ICME or MC which is proportional to the apparent curvature radius of ICMEs/MCs. We have found that this apparent radius of curvature increases with the Mach number and this effect is attributed to the larger deformation of the fast ICME/MC. Further, the relative sheath thickness that is proportional to the flux rope oblateness decreases with the magnetic field intensity inside the ICME/MC and increases with the heliospheric distance.  相似文献   

10.
Li  Y.  Luhmann  J. G.  Lynch  B. J.  Kilpua  E. K. J. 《Solar physics》2011,270(1):331-346
Coronal mass ejections (CMEs) carry magnetic structure from the low corona into the heliosphere. The interplanetary CMEs (ICMEs) that exhibit the topology of helical magnetic fluxropes are traditionally called magnetic clouds (MCs). MC fluxropes with axis of low (high) inclination with respect to the ecliptic plane have been referred to as bipolar (unipolar) MCs. The poloidal field of bipolar MCs has a solar cycle dependence. We report a cyclic reversal of the poloidal field of low inclination MC fluxropes during 1976 to 2009. The MC poloidal field cyclic reversal on the same time scale of the solar magnetic cycle is evident over three sunspot cycles. Approximately 48% of ICMEs are MCs, and 40% of IMCs are bipolar MCs during solar cycle 23. The speed of the bipolar MCs has essentially the same distribution as all ICMEs, which implies that they are not from any special type of CMEs in terms of the solar origin. Although CME fluxropes may undergo a number of complications during the eruption and propagation, a significant group of MCs retains sufficient similarity to the source region magnetic field to posses the same cyclic periodicity in polarity reversal. The poloidal field of bipolar MCs gives the out-of-ecliptic-plane field or B z component in the IMF time series. MCs with southward B z field are particularly effective in causing geomagnetic disturbances. During the solar minima, the B z field IMF sequence within MCs at the leading portion of a bipolar MC is the same with the solar global dipole field. Our finding shows that MCs preferentially remove the like polarity of the solar dipole field, and it supports the participation of CMEs in the solar magnetic cycle.  相似文献   

11.
In-situ measurements of interplanetary coronal mass ejections (ICMEs) display a wide range of properties. A distinct subset, “magnetic clouds” (MCs), are readily identifiable by a smooth rotation in an enhanced magnetic field, together with an unusually low solar wind proton temperature. In this study, we analyze Ulysses spacecraft measurements to systematically investigate five possible explanations for why some ICMEs are observed to be MCs and others are not: i) An observational selection effect; that is, all ICMEs do in fact contain MCs, but the trajectory of the spacecraft through the ICME determines whether the MC is actually encountered; ii) interactions of an erupting flux rope (FR) with itself or between neighboring FRs, which produce complex structures in which the coherent magnetic structure has been destroyed; iii) an evolutionary process, such as relaxation to a low plasma-β state that leads to the formation of an MC; iv) the existence of two (or more) intrinsic initiation mechanisms, some of which produce MCs and some that do not; or v) MCs are just an easily identifiable limit in an otherwise continuous spectrum of structures. We apply quantitative statistical models to assess these ideas. In particular, we use the Akaike information criterion (AIC) to rank the candidate models and a Gaussian mixture model (GMM) to uncover any intrinsic clustering of the data. Using a logistic regression, we find that plasma-β, CME width, and the ratio O 7/O 6 are the most significant predictor variables for the presence of an MC. Moreover, the propensity for an event to be identified as an MC decreases with heliocentric distance. These results tend to refute ideas ii) and iii). GMM clustering analysis further identifies three distinct groups of ICMEs; two of which match (at the 86 % level) with events independently identified as MCs, and a third that matches with non-MCs (68 % overlap). Thus, idea v) is not supported. Choosing between ideas i) and iv) is more challenging, since they may effectively be indistinguishable from one another by a single in-situ spacecraft. We offer some suggestions on how future studies may address this.  相似文献   

12.
In situ data provide only a one-dimensional sample of the plasma velocity along the spacecraft trajectory crossing an interplanetary coronal mass ejection (ICME). Then, to understand the dynamics of ICMEs it is necessary to consider some models to describe it. We derive a series of equations in a hierarchical order, from more general to more specific cases, to provide a general theoretical basis for the interpretation of in situ observations, extending and generalizing previous studies. The main hypothesis is a self-similar expansion, but with the freedom of possible different expansion rates in three orthogonal directions. The most detailed application of the equations is though for a subset of ICMEs, magnetic clouds (MCs), where a magnetic flux rope can be identified. The main conclusions are the following ones. First, we obtain theoretical expressions showing that the observed velocity gradient within an ICME is not a direct characteristic of its expansion, but that it depends also on other physical quantities such as its global velocity and acceleration. The derived equations quantify these dependencies for the three components of the velocity. Second, using three different types of data we show that the global acceleration of ICMEs has, at most, a small contribution to the in situ measurements of the velocity. This eliminates practically one contribution to the observed velocity gradient within ICMEs. Third, we provide a method to quantify the expansion rate from velocity data. We apply it to a set of 26 MCs observed by Wind or ACE spacecrafts. They are typical MCs, and their main physical parameters cover the typical range observed in MCs in previous statistical studies. Though the velocity difference between their front and back includes a broad range of values, we find a narrow range for the determined dimensionless expansion rate. This implies that MCs are expanding at a comparable rate, independently of their size or field strength, despite very different magnitudes in their velocity profiles. Furthermore, the equations derived provide a base to further analyze the dynamics of MCs/ICMEs.  相似文献   

13.
  相似文献   

14.
During solar cycle 23, 82 interplanetary magnetic clouds (MCs) were identified by the Magnetic Field Investigation (MFI) team using Wind (1995 – 2003) solar wind plasma and magnetic field data from solar minimum through the maximum of cycle 23. The average occurrence rate is 9.5 MCs per year for the overall period. It is found that some of the anomalies in the frequency of occurrence were during the early part of solar cycle 23: (i) only four MCs were observed in 1999, and (ii) an unusually large number of MCs (17 events) were observed in 1997, just after solar minimum. We also discuss the relationship between MCs, coronal mass ejections (CMEs), and geomagnetic storms. During the period 1996 – 2003, almost 8000 CMEs were observed by SOHO-LASCO. The occurrence frequency of MCs appears to be related neither to the occurrence of CMEs as observed by SOHO LASCO nor to the sunspot number. When we included “magnetic cloud-like structures” (MCLs, defined by Lepping, Wu, and Berdichevsky, 2005), we found that the occurrence of the joint set (MCs + MCLs) is correlated with both sunspot number and the occurrence rate of CMEs. The average duration of the MCL structures is ~40% shorter than that of the MCs. The MCs are typically more geoeffective than the MCLs, because the average southward field component is generally stronger and longer lasting in MCs than in MCLs. In addition, most severe storms caused by MCs/MCLs with Dst min≤ −100 nT occurred in the active solar period.  相似文献   

15.
The Grad–Shafranov reconstruction is a method of estimating the orientation (invariant axis) and cross section of magnetic flux ropes using the data from a single spacecraft. It can be applied to various magnetic structures such as magnetic clouds (MCs) and flux ropes embedded in the magnetopause and in the solar wind. We develop a number of improvements of this technique and show some examples of the reconstruction procedure of interplanetary coronal mass ejections (ICMEs) observed at 1 AU by the STEREO, Wind, and ACE spacecraft during the minimum following Solar Cycle 23. The analysis is conducted not only for ideal localized ICME events but also for non-trivial cases of magnetic clouds in fast solar wind. The Grad–Shafranov reconstruction gives reasonable results for the sample events, although it possesses certain limitations, which need to be taken into account during the interpretation of the model results.  相似文献   

16.
There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are `illuminated' due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds.  相似文献   

17.
Two distinct regions of shock-associated magnetic clouds, (i) magnetically turbulent regions formed due to interaction between magnetic cloud and ambient magnetic field i.e. turbulent interaction region (TIR), and magnetically quiet region called magnetic cloud have been considered separately and correlation of interplanetary plasma and field parameters, magnetic field strength (B) and solar wind speed (V), with cosmic ray intensity (I) have been studied during the passage of these two regions. A good correlation between B and I and between V and I has been obtained during the passage of sheath when the magnetic field is high and turbulent, while these correlation have been found to be poor during the passage of magnetic clouds when the field is strong and smooth. Further, there is a positive correlation between enhancement in field strength and its variance in the sheath region. These results strongly support the hypothesis that most Forbush decreases are due to scattering of particles by region of enhanced magnetic turbulence. These results also suggest that it will provide a better insight if not the magnetic field enhancement alone but in addition, the nature of magnetic field enhancement is also considered while correlating the field enhancements with depressions in cosmic rays. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

18.
We present a three-dimensional, fully parallelized, efficient implementation of ionizing ultraviolet (UV) radiation for smoothed particle hydrodynamics ( sph ) including self-gravity. Our method is based on the sph / tree code vine . We therefore call it iVINE (for Ionization + VINE). This approach allows detailed high-resolution studies of the effects of ionizing radiation from, for example, young massive stars on their turbulent parental molecular clouds. In this paper, we describe the concept and the numerical implementation of the radiative transfer for a plane-parallel geometry and we discuss several test cases demonstrating the efficiency and accuracy of the new method. As a first application, we study the radiatively driven implosion of marginally stable molecular clouds at various distances of a strong UV source and show that they are driven into gravitational collapse. The resulting cores are very compact and dense exactly as it is observed in clustered environments. Our simulations indicate that the time of triggered collapse depends on the distance of the core from the UV source. Clouds closer to the source collapse several 105 yr earlier than more distant clouds. This effect can explain the observed age spread in OB associations where stars closer to the source are found to be younger. We discuss possible uncertainties in the observational derivation of shock front velocities due to early stripping of protostellar envelopes by ionizing radiation.  相似文献   

19.
In this work we have performed a systematic study of all the magnetic clouds identified in the time interval 2000–2003. The study shows that the non force-free model of Hidalgo is a good approximation to the magnetic topology of the MCs in the interplanetary medium. This conclusion is reached based on the good fits obtained with the model for most of the clouds, in spite of the variety of profiles found in the magnetic field strength and in every of its components. The model incorporates the distortion and expansion of the cross-section of the MCs. We have compared, when available, the results obtained with those in literature. The unique published global study of the MCs at the same time interval has been provided by Lepping using the circular cross-section model of Burlaga, and the results are available in his web page. From all the parameters he obtained, only the longitude, φ, the latitude, θ, and the distance of maximum approach of the spacecraft to the cloud axis, y0, may be compared with those obtained by Hidalgo's model. As we show, the main discrepancy between both models refers to the longitude values. Concerning the comparison with other models of literature, only the Bastille day and October 2003 magnetic clouds have been studied by other authors.  相似文献   

20.
From a comparative study between stellar and gas data it is seen that turbulent and hydrodynamic motions in the Galaxy are common to both types of materials:
  1. Galactic clusters have sizes and intrinsic dispersions compatible with the modified form of the Kolmogorov law seen in molecular clouds: undimensional velocities σ(km s?1)=0.54d 0.38 (pc). This indicates that ‘typic’ clusters were born from ‘typic’ dark clouds as these of the Lynds's catalogue (diametersd<10 pc, dispersions σ<1.5 km s?1 hydrogen densitiesn H>200 atom cm?3). These clouds have mass enough to form galactic clusters (1000–3000M ⊙).
  2. The cluster formation is related to the supersonic range of the Kolmogorov relationship σ(d>1 pc) while the AFGKM stars are related to the subsonic range of the same relationship σ(d<0.3 pc), the intermediate transition zone is probably related to OB stars and/or trapezia.
  3. The effects of the magnetic fields in the clouds are also discussed. It seems to be that in the clouds the magnetic energy does not exceed the kinetic energy (proportional toσ 2(d)) and that this determinates the freezing criteria. The hypotheses introduced here can be checked with 21 cm Zeeman splitting.
  4. Low-density globular clusters are also coherent with the Kolmogorov relationship. Some hypotheses about their origin and the type of clouds where they were born are discussed. This last part of the study lets open the possibility of further studies about evolution of globular clusters.
  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号