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1.
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
2.
The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by interplanetary coronal mass ejections
(ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic
flux and helicity, which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled by
neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We
present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 – 10 November 2004. This MC was embedded in an ICME. After determining an approximate orientation for
the flux rope using the minimum variance method, we obtain a precise orientation of the cloud axis by relating its front and
rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic
flux between the inbound and outbound branches and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). The MC is also studied using dynamic
models with isotropic expansion. We have found (6.2±1.5)×1020 Mx for the axial flux and (78±18)×1020 Mx for the azimuthal flux. Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed
by an extended coherent magnetic region. These observations are interpreted by considering the existence of a previously larger
flux rope, which partially reconnected with its environment in the front. We estimate that the reconnection process started
close to the Sun. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed
to the observed ICME (with a remnant flux rope in the front part). 相似文献
3.
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) which exhibit signatures consistent with
a magnetic flux rope structure. Techniques for reconstructing flux rope orientation from single-point in situ observations typically assume the flux rope is locally cylindrical, e.g., minimum variance analysis (MVA) and force-free flux rope (FFFR) fitting. In this study, we outline a non-cylindrical magnetic
flux rope model, in which the flux rope radius and axial curvature can both vary along the length of the axis. This model
is not necessarily intended to represent the global structure of MCs, but it can be used to quantify the error in MC reconstruction
resulting from the cylindrical approximation. When the local flux rope axis is approximately perpendicular to the heliocentric
radial direction, which is also the effective spacecraft trajectory through a magnetic cloud, the error in using cylindrical
reconstruction methods is relatively small (≈ 10∘). However, as the local axis orientation becomes increasingly aligned with the radial direction, the spacecraft trajectory
may pass close to the axis at two separate locations. This results in a magnetic field time series which deviates significantly
from encounters with a force-free flux rope, and consequently the error in the axis orientation derived from cylindrical reconstructions
can be as much as 90∘. Such two-axis encounters can result in an apparent ‘double flux rope’ signature in the magnetic field time series, sometimes
observed in spacecraft data. Analysing each axis encounter independently produces reasonably accurate axis orientations with
MVA, but larger errors with FFFR fitting. 相似文献
4.
磁云因其独特的磁场结构经常是重大灾害性空间天气的驱动源. 近来从磁云的边界层结构、环向通量、大尺度结构等方面关于磁云传播的动力学演化过程的研究取得了一些进展. 在磁云边界存在一个由于磁场重联而形成的边界层结构. 在磁云传播过程中, 这种发生在边界处的磁场重联可能会把磁云的磁场剥蚀掉, 进而引起其磁通量绳结构环向通量的减少以及不对称. 在磁云内部, 经常会观测到多个子通量绳结构. 这些特性各异的子通量绳可以通过磁场重联而合并, 进而引起磁云磁结构的改变. 关于磁云大尺度磁场拓扑位形的演化机制, 除了较早提出的交换重联外, 目前的研究表明在行星际空间中, 磁云边界处的重联过程也可以将磁云闭合或半开放的磁场线打开或断开. 尽管在相关研究中已经取得了较大进展, 但关于磁云传播的动力学演化过程还有许多问题尚不清楚. 在行星际小尺度磁通量绳边界也发现了边界层结构, 那么磁云是否会因剥蚀而成为小尺度通量绳? 磁云内子通量绳结构在相互作用中会不会引起某些不稳定性而导致整个通量绳系统的崩溃? 这些问题的解决还有待于进一步的理论、观测和数值模拟研究. 相似文献
5.
Data observed during spacecraft encounters with magnetic clouds have been extensively analyzed in the literature. Moreover, several models have been proposed for the magnetic topology of these events, and fitted to the observations. Although these interplanetary events present well-defined plasma features, none of those models have included a simultaneous analysis of magnetic field and plasma data. Using as a starting point a non-force-free model that we have developed previously, we present a global study of MCs that include both the magnetic field topology and the plasma pressure. In this paper we obtain the governing equations for both magnitudes inside a MC. The expressions deduced are fitted simultaneously to the measurements of plasma pressure and magnetic field vector. We perform an analysis of magnetic field and plasma WIND observations within several MCs from 1995 to 1998. The analysis is confined to four of these events that have high-quality data. Only in one fitting procedure we obtain the orientation of the magnetic cloud relative to the ecliptic plane and the current density of the plasma inside the cloud. We find that the equations proposed reproduce the experimental data quite well. 相似文献
6.
Although the dynamical evolution of magnetic clouds (MCs) has been one of the foci of interplanetary physics for decades, only few studies focus on the internal properties of large-scale MCs. Recent work by Wang et al. (J. Geophys. Res. 120, 1543, 2015) suggested the existence of the poloidal plasma motion in MCs. However, the main cause of this motion is not clear. In order to find it, we identify and reconstruct the MC observed by the Solar Terrestrial Relations Observatory (STEREO)-A, Wind, and STEREO-B spacecraft during 19?–?20 November 2007 with the aid of the velocity-modified cylindrical force-free flux-rope model. We analyze the plasma velocity in the plane perpendicular to the MC axis. It is found that there was evident poloidal motion at Wind and STEREO-B, but this was not clear at STEREO-A, which suggests a local cause rather than a global cause for the poloidal plasma motion inside the MC. The rotational directions of the solar wind and MC plasma at the two sides of the MC boundary are found to be consistent, and the values of the rotational speeds of the solar wind and MC plasma at the three spacecraft show a rough correlation. All of these results illustrate that the interaction with ambient solar wind through viscosity might be one of the local causes of the poloidal motion. Additionally, we propose another possible local cause: the existence of a pressure gradient in the MC. The significant difference in the total pressure at the three spacecraft suggests that this speculation is perhaps correct. 相似文献
7.
An interplanetary shock and a magnetic cloud(MC)reached the Earth on 2012 July 14 and 15 one after another.The shock sheath and the MC triggered an intense geom... 相似文献
8.
Lepping R.P. Berdichevsky D.B. Burlaga L.F. Lazarus A.J. Kasper J. Desch M.D. Wu C.-C. Reames D.V. Singer H.J. Smith C.W. Ackerson K.L. 《Solar physics》2001,204(1-2):285-303
The energetic charged particle, interplanetary magnetic field, and plasma characteristics of the `Bastille Day' shock and
ejecta/magnetic cloud events at 1 AU occurring over the days 14–16 July 2000 are described. Profiles of MeV (WIND/LEMT) energetic
ions help to organize the overall sequence of events from the solar source to 1 AU. Stressed are analyses of an outstanding
magnetic cloud (MC2) starting late on 15 July and its upstream shock about 4 hours earlier in WIND magnetic field and plasma
data. Also analyzed is a less certain, but likely, magnetic cloud (MC1) occurring early on 15 July; this was separated from
MC2 by its upstream shock and many heliospheric current sheet (HCS) crossings. Other HCS crossings occurred throughout the
3-day period. Overall this dramatic series of interplanetary events caused a large multi-phase magnetic storm with min Dst lower than −300 nT. The very fast solar wind speed (≥ 1100 km s−1) in and around the front of MC2 (for near average densities) was responsible for a very high solar wind ram pressure driving
in the front of the magnetosphere to geocentric distances estimated to be as low as ≈ 5 R
E, much lower than the geosynchronous orbit radius. This was consistent with magnetic field observations from two GOES satellites
which indicated they were in the magnetosheath for extended times. A static force-free field model is used to fit the two
magnetic cloud profiles providing estimates of the clouds' physical and geometrical properties. MC2 was much larger than MC1,
but their axes were nearly antiparallel, and their magnetic fields had the same left-handed helicity. MC2's axis and its upstream
shock normal were very close to being perpendicular to each other, as might be expected if the cloud were driving the shock
at the time of observation. The estimated axial magnetic flux carried by MC2 was 52×1020 Mx, which is about 5 times the typical magnetic flux estimated for other magnetic clouds in the WIND data over its first
4 years and is 17 times the flux of MC1. This large flux is due to both the strong axially-directed field of MC2 (46.8 nT
on the axis) and the large radius (R
0=0.189 AU) of the flux tube. MC2's average speed is consistent with the expected transit time from a halo-CME to which it
is apparently related. 相似文献
9.
The behavior of solar energetic particles (SEPs) in a shock – magnetic cloud interacting complex structure observed by the
Advanced Composition Explorer (ACE) spacecraft on 5 November 2001 is analyzed. A strong shock causing magnetic field strength and solar wind speed increases
of about 41 nT and 300 km s−1, respectively, propagated within a preceding magnetic cloud (MC). It is found that an extraordinary SEP enhancement appeared
at the high-energy (≥10 MeV) proton intensities and extended over and only over the entire period of the shock – MC structure
passing through the spacecraft. Such SEP behavior is much different from the usual picture that the SEPs are depressed in
MCs. The comparison of this event with other top SEP events of solar cycle 23 (2000 Bastille Day and 2003 Halloween events)
shows that such an enhancement resulted from the effects of the shock – MC complex structure leading to the highest ≥10 MeV
proton intensity of solar cycle 23. Our analysis suggests that the relatively isolated magnetic field configuration of MCs
combined with an embedded strong shock could significantly enhance the SEP intensity; SEPs are accelerated by the shock and
confined into the MC. Further, we find that the SEP enhancement at lower energies happened not only within the shock – MC
structure but also after it, probably owing to the presence of a following MC-like structure. This is consistent with the
picture that SEP fluxes could be enhanced in the magnetic topology between two MCs, which was proposed based on numerical
simulations by Kallenrode and Cliver (Proc. 27th ICRC
8, 3318, 2001b). 相似文献
10.
The geoeffective magnetic cloud (MC) of 20 November 2003 was associated with the 18 November 2003 solar active events in previous
studies. In some of these, it was estimated that the magnetic helicity carried by the MC had a positive sign, as did its solar
source, active region (AR) NOAA 10501. In this article we show that the large-scale magnetic field of AR 10501 has a negative
helicity sign. Since coronal mass ejections (CMEs) are one of the means by which the Sun ejects magnetic helicity excess into
interplanetary space, the signs of magnetic helicity in the AR and MC must agree. Therefore, this finding contradicts what
is expected from magnetic helicity conservation. However, using, for the first time, correct helicity density maps to determine
the spatial distribution of magnetic helicity injections, we show the existence of a localized flux of positive helicity in
the southern part of AR 10501. We conclude that positive helicity was ejected from this portion of the AR leading to the observed
positive helicity MC. 相似文献
11.
J. J. Blanco E. Catalán M. A. Hidalgo J. Medina O. García J. Rodríguez-Pacheco 《Solar physics》2013,284(1):167-178
In this work, non-recurrent Forbush decreases (FDs) triggered by the passage of shock-driving interplanetary coronal mass ejections (ICMEs) have been analyzed. Fifty-nine ICMEs have been studied, but only 25 % of them were associated to a FD. We find that shock-driving magnetic clouds (MCs) produce deeper FDs than shock-driving ejecta. This fact can be explained regarding the observed growing trends between decreases in neutron monitor (NM) count rate and MC/ejecta speed and its associated rigidity. MCs are faster and have higher associated rigidities than ejecta. Also the deceleration of ICMEs seems to be a cause for producing FDs, as can be inferred from the decreasing trend between NM count rate and deceleration. This probably implies that the interaction between the ICME traveling from the corona to the Earth and the solar wind can play an important role in producing deeper FDs. Finally, we conclude that ejecta without flux rope topology are the ones less effective in unchaining FDs. 相似文献
12.
13.
Magnetic clouds (MCs) observed by Wind during the 1995-2003 years and listed in Lepping et al. [2006. A summary of WIND magnetic clouds for years 1995-2003: model-fitted parameters, associated errors and classifications. Ann. Geophys. 24(1), 215-245] are fitted using force-free cylindrical flux rope models. The cloud parameters yielded by a static model are compared with those obtained from a modified model that includes magnetic cloud expansion. The deviations of these fits from observations are quantified using 1 h averages of measured parameters by Wind. In the case of the static MC model, the deviations between fitted and observed magnetic fields increase with the magnetic strength and with the MC radius. The comparison of both the static and the dynamic models reveals that the modified model with expansion provides better fits in the range of low expansion speeds , whereas the results are not conclusive for larger expansion speeds. The dynamic model provides better fits than static one in 70% of investigated MCs. We conclude that the expansion of magnetic clouds plays an important role in the MC formation and propagation but a further progress requires determination of cloud parameters from a model with expansion self-consistently involved. 相似文献
14.
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
15.
Using nine years of solar wind plasma and magnetic field data from the Wind mission, we investigated the characteristics of both magnetic clouds (MCs) and magnetic cloud-like structures (MCLs) during
1995 – 2003. A MCL structure is an event that is identified by an automatic scheme (Lepping, Wu, and Berdichevsky, Ann. Geophys.
23, 2687, 2005) with the same criteria as for a MC, but it is not usually identifiable as a flux rope by using the MC (Burlaga et al., J. Geophys. Res.
86, 6673, 1981) fitting model developed by Lepping, Jones, and Burlaga (Geophys. Res. Lett.
95(11), 957, 1990). The average occurrence rate is 9.5 for MCs and 13.6 for MCLs per year for the overall period of interest, and there were
82 MCs and 122 MCLs identified during this period. The characteristics of MCs and MCL structures are as follows: (1) The average
duration, Δt, of MCs is 21.1 h, which is 40% longer than that for MCLs (Δt=15 h); (2) the average
(minimum B
z
found in MC/MCL measured in geocentric solar ecliptic coordinates) is −10.2 nT for MCs and −6 nT for MCLs; (3) the average
Dstmin (minimum Dst caused by MCs/MCLs) is −82 nT for MCs and −37 nT for MCLs; (4) the average solar wind velocity is 453 km s−1 for MCs and 413 km s−1 for MCLs; (5) the average thermal speed is 24.6 km s−1 for MCs and 27.7 km s−1 for MCLs; (6) the average magnetic field intensity is 12.7 nT for MCs and 9.8 nT for MCLs; (7) the average solar wind density
is 9.4 cm−3 for MCs and 6.3 cm−3 for MCLs; and (8) a MC is one of the most important interplanetary structures capable of causing severe geomagnetic storms.
The longer duration, more intense magnetic field and higher solar wind speed of MCs, compared to those properties of the MCLs,
are very likely the major reasons for MCs generally causing more severe geomagnetic storms than MCLs. But the fact that a
MC is an important interplanetary structure with respect to geomagnetic storms is not new (e.g., Zhang and Burlaga, J. Geophys. Res.
93, 2511, 1988; Bothmer, ESA SP-535, 419, 2003). 相似文献
16.
Multiple magnetic clouds in interplanetary space 总被引：7，自引：0，他引：7
An interplanetary magnetic cloud (MC) is usually considered the byproduct of a coronal mass ejection (CME). Due to the frequent occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one MC catches up with another, should be a relatively common phenomenon. A simple flux rope model is used to get the primary magnetic field features of multi-MCs. Results indicate that the magnetic field configuration of multi-MCs mainly depends on the magnetic field characteristics of each member of multi-MCs. It may be entirely different in another situation. Moreover, we fit the data from the Wind spacecraft by using this model. Comparing the model with the observations, we verify the existence of multi-MCs, and propose some suggestions for further work. 相似文献
17.
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
18.
C. H. Mandrini M. S. Nakwacki G. Attrill L. van Driel-Gesztelyi P. Démoulin S. Dasso H. Elliott 《Solar physics》2007,244(1-2):25-43
Coronal dimmings are often present on both sides of erupting magnetic configurations. It has been suggested that dimmings
mark the location of the footpoints of ejected flux ropes and, thus, their magnetic flux can be used as a proxy for the flux
involved in the ejection. If so, this quantity can be compared to the flux in the associated interplanetary magnetic cloud
to find clues about the origin of the ejected flux rope. In the context of this physical interpretation, we analyze the event,
flare, and coronal mass ejection (CME) that occurred in active region 10486 on 28 October 2003. The CME on this day is associated
with large-scale dimmings, located on either side of the main flaring region. We combine SOHO/Extreme Ultraviolet Imaging
Telescope data and Michelson Doppler Imager magnetic maps to identify and measure the flux in the dimming regions. We model
the associated cloud and compute its magnetic flux using in situ observations from the Magnetometer Instrument and the Solar Wind Electron Proton Alpha Monitor aboard the Advance Composition Explorer. We find that the magnetic fluxes of the dimmings and magnetic cloud are incompatible, in contrast to what has been found
in previous studies. We conclude that, in certain cases, especially in large-scale events and eruptions that occur in regions
that are not isolated from other flux concentrations, the interpretation of dimmings requires a deeper analysis of the global
magnetic configuration, since at least a fraction of the dimmed regions is formed by reconnection between the erupting field
and the surrounding magnetic structures. 相似文献
19.
We investigated a set of 54 interplanetary coronal mass ejection (ICME) events whose solar sources are very close to the disk center (within ±?15° from the central meridian). The ICMEs consisted of 23 magnetic-cloud (MC) events and 31 non-MC events. Our analyses suggest that the MC and non-MC ICMEs have more or less the same eruption characteristics at the Sun in terms of soft X-ray flares and CMEs. Both types have significant enhancements in ion charge states, although the non-MC structures have slightly lower levels of enhancement. The overall duration of charge-state enhancement is also considerably smaller than that in MCs as derived from solar wind plasma and magnetic signatures. We find very good correlation between the Fe and O charge-state measurements and the flare properties such as soft X-ray flare intensity and flare temperature for both MCs and non-MCs. These observations suggest that both MC and non-MC ICMEs are likely to have a flux-rope structure and the unfavorable observational geometry may be responsible for the appearance of non-MC structures at 1 AU. We do not find any evidence for an active region expansion resulting in ICMEs lacking a flux-rope structure because the mechanism of producing high charge states and the flux-rope structure at the Sun is the same for MC and non-MC events. 相似文献
20.
A Comparative Study of Coronal Mass Ejections with and Without Magnetic Cloud Structure near the Earth: Are All Interplanetary CMEs Flux Ropes? 总被引：1，自引：0，他引：1
An outstanding question concerning interplanetary coronal mass ejections (ICMEs) is whether all ICMEs have a magnetic flux rope structure. We test this question by studying two different ICMEs, one having a magnetic cloud (MC) showing smooth rotation of magnetic field lines and the other not. The two ICMEs are chosen in such a way that their progenitor CMEs are very similar in remote sensing observations. Both CMEs originated from close to the central meridian directly facing the Earth. Both CMEs were associated with a long-lasting post-eruption loop arcade and appeared as an elliptical halo in coronagraph images, indicating a flux rope origin. We conclude that the difference in the in-situ observation is caused by the geometric selection effect, contributed by the deflection of flux ropes in the inner corona and interplanetary space. The first event had its nose pass through the observing spacecraft; thus, the intrinsic flux rope structure of the CME appeared as a magnetic cloud. On the other hand, the second event had the flank of the flux rope intercept the spacecraft, and it thus did not appear as a magnetic cloud. We further argue that a conspicuous long period of weak magnetic field, low plasma temperature, and density in the second event should correspond to the extended leg portion of the embedded magnetic flux rope, thus validating the scenario of the flank-passing. These observations support the idea that all CMEs arriving at the Earth include flux rope drivers. 相似文献