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1.
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
2.
Interplanetary magnetic clouds (MCs) are one of the main sources of large non-recurrent geomagnetic storms. With the aid of
a force-free flux rope model, the dependence of the intensity of geomagnetic activity (indicated by Dst index) on the axial orientation (denoted by θ and φ in GSE coordinates) of the magnetic cloud is analyzed theoretically. The distribution of the Dst values in the (θ, φ) plane is calculated by changing the axial orientation for various cases. It is concluded that (i) geomagnetic storms tend
to occur in the region of θ<0°, especially in the region of θ≲−45°, where larger geomagnetic activity could be created; (ii) the intensity of geomagnetic activity varies more strongly
with θ than with φ; (iii) when the parameters B
0 (the magnetic field strength at the flux rope axis), R
0 (the radius of the flux rope), or V (the bulk speed) increase, or |D| (the shortest distance between the flux rope axis and the x-axis in GSE coordinates) decreases, a flux rope not only can increase the intensity of geomagnetic activity, but also is
more likely to create a storm, however the variation of n (the density) only has a little effect on the intensity; (iv) the most efficient orientation (MEO) in which a flux rope can
cause the largest geomagnetic activity appears at φ∼0° or ∼ 180°, and some value of θ which depends mainly on D; (v) the minimum Dst value that could be caused by a flux rope is the most sensitive to changes in B
0 and V of the flux rope, and for a stronger and/or faster MC, a wider range of orientations will be geoeffective. Further, through
analyzing 20 MC-caused moderate to large geomagnetic storms during 1998 – 2003, a long-term prediction of MC-caused geomagnetic
storms on the basis of the flux rope model is proposed and assessed. The comparison between the theoretical results and the
observations shows that there is a close linear correlation between the estimated and observed minimum Dst values. This suggests that using the ideal flux rope to predict practical MC-caused geomagnetic storms is applicable. The
possibility of the long-term prediction of MC-caused geomagnetic storms is discussed briefly. 相似文献
3.
As demonstrated by many previous studies, a system consisting of an isolated coronal flux rope and a surrounding background
magnetic field exhibits a catastrophic behavior. In particular, if the magnetic field of the system is force-free and axisymmetric
in spherical geometry, the magnetic energy at the catastrophic point, referred to as the catastrophic energy threshold, is
found to be larger than the corresponding partly or fully open field energy. This paper takes an octapole field as the background
and introduces a flux rope within the central arcade of the octapole field. A relaxation method based on time-dependent ideal
magnetohydrodynamic (MHD) simulations is used to find axisymmetric force-free field solutions in spherical geometry associated
with the flux rope system. With respect to an increase of either the annular flux Φp or the axial flux Φϕ of the rope, the system exhibits a catastrophic behavior as expected, and the catastrophic energy threshold is larger than
that of the corresponding partly open field, in which the central arcade is opened up, but the remainder remains closed. For
a given octapole field, the energy threshold depends on either Φp or Φϕ at the catastrophic point, and it increases with increasing Φp or decreasing Φϕ. On the other hand, the extent to which the central bipolar component of the octapole field is open also affects the energy
threshold. These results differ from those for the bipolar background field case, in which the catastrophic energy threshold
is almost independent of the magnetic properties of the flux rope at the catastrophic points and the extent to which the background
field is open. The reason for such a difference is briefly discussed. 相似文献
4.
We studied the soft X-ray solar events that could be associated with the interplanetary magnetic flux ropes observed by the WIND satellite during 1995 through 1998. The timings of the launches of the magnetic flux ropes from the Sun were estimated using flux rope speeds derived by the fitting of a cylindrical model. In the reasonable time window, soft X-ray solar signatures were found in approximately 70% of the flux ropes. Parameters (e.g., axis direction, strength of magnetic field, radius, and helicity) of the magnetic flux ropes obtained by the model fitting were compared with the characteristics of the corresponding soft X-ray events observed by Yohkoh. According to the result of the comparison, the magnetic flux ropes with strong magnetic fields or high speeds were observed in association with higher soft X-ray solar activities. 相似文献
5.
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
6.
We investigate the solar wind structure for 11 cases that were selected for the campaign study promoted by the International Study of Earth-affecting Solar Transients (ISEST) MiniMax24 Working Group 4. We can identify clear flux rope signatures in nine cases. The geometries of the nine interplanetary magnetic flux ropes (IFRs) are examined with a model-fitting analysis with cylindrical and toroidal force-free flux rope models. For seven cases in which magnetic fields in the solar source regions were observed, we compare the IFR geometries with magnetic structures in their solar source regions. As a result, we can confirm the coincidence between the IFR orientation and the orientation of the magnetic polarity inversion line (PIL) for six cases, as well as the so-called helicity rule as regards the handedness of the magnetic chirality of the IFR, depending on which hemisphere of the Sun the IFR originated from, the northern or southern hemisphere; namely, the IFR has right-handed (left-handed) magnetic chirality when it is formed in the southern (northern) hemisphere of the Sun. The relationship between the orientation of IFRs and PILs can be taken as evidence that the flux rope structure created in the corona is in most cases carried through interplanetary space with its orientation maintained. In order to predict magnetic field variations on Earth from observations of solar eruptions, further studies are needed about the propagation of IFRs because magnetic fields observed at Earth significantly change depending on which part of the IFR hits the Earth. 相似文献
7.
Based on the observations of the Sun and the interplanetary medium, a series of solar activities in late October 2003 and their consequences are studied comprehensively. Thirteen X-ray flares with importance greater than M-class, six frontside halo coronal mass ejections (CMEs) with span angle larger than 100 and three associated eruptions of filament materials are identified by examining lots of solar observations from October 26 to 29. All these flares were associated with type III radio bursts, all the frontside halo CMEs were accompanied by type II or type II-like radio bursts. Particularly, among these activities, two major solar events caused two extraordinary enhancements (exceeding 1000 particles/(cm2s–1sterMev–1) of solar energetic particle (SEP) flux intensity near the Earth, two large ejecta with fast shocks preceding, and two great geomagnetic storms with Dst peak value of –363 and –401 nT, respectively. By using a cross correlation technique and a force-free cylindrical flux rope model, the October 29 magnetic cloud associated with the largest CME are analyzed, including its orientation and the sign of its helicity. It is found that the helicity of the cloud is negative, contrary to the regular statistical pattern that negative- and positive-helical interplanetary magnetic clouds would be expected to come from northern and southern solar hemisphere. Moreover, the relationship between the orientation of magnetic cloud and associated filament is discussed. In addition, some discussion concerning multiple-magnetic-cloud structures and SEP events is also given. 相似文献
8.
This study aims to quantify characteristic features of the bipolar flux appearance of solar intranetwork (IN) magnetic elements.
To attack this problem, we use the Narrowband Filter Imager (NFI) magnetograms from the Solar Optical Telescope (SOT) on board Hinode; these data are from quiet and enhanced network areas. Cluster emergence of mixed polarities and IN ephemeral regions (ERs)
are the most conspicuous forms of bipolar flux appearance within the network. Each of the clusters is characterized by a few
well-developed ERs that are partially or fully coaligned in magnetic axis orientation. On average, the sampled IN ERs have
a total maximum unsigned flux of several 1017 Mx, a separation of 3 – 4 arcsec, and a lifetime of 10 – 15 minutes. The smallest IN ERs have a maximum unsigned flux of
several 1016 Mx, separations of less than 1 arcsec, and lifetimes as short as 5 minutes. Most IN ERs exhibit a rotation of their magnetic
axis of more than 10 degrees during flux emergence. Peculiar flux appearance, e.g., bipole shrinkage followed by growth or the reverse, is not unusual. A few examples show repeated shrinkage–growth or growth–shrinkage,
like magnetic floats in the dynamic photosphere. The observed bipolar behavior seems to carry rich information on magnetoconvection
in the subphotospheric layer. 相似文献
9.
Pankaj Kumar Ablishek K. Srivastava B. Filippov R. Erdélyi Wahab Uddin 《Solar physics》2011,272(2):301-317
We present the multiwavelength observations of a flux rope that was trying to erupt from NOAA AR 11045 and the associated
M-class solar flare on 12 February 2010 using space-based and ground-based observations from TRACE, STEREO, SOHO/MDI, Hinode/XRT, and BBSO. While the flux rope was rising from the active region, an M1.1/2F class flare was triggered near one of its
footpoints. We suggest that the flare triggering was due to the reconnection of a rising flux rope with the surrounding low-lying
magnetic loops. The flux rope reached a projected height of ≈0.15R
⊙ with a speed of ≈90 km s−1 while the soft X-ray flux enhanced gradually during its rise. The flux rope was suppressed by an overlying field, and the
filled plasma moved towards the negative polarity field to the west of its activation site. We found the first observational
evidence of the initial suppression of a flux rope due to a remnant filament visible both at chromospheric and coronal temperatures
that evolved a couple of days earlier at the same location in the active region. SOHO/MDI magnetograms show the emergence
of a bipole ≈12 h prior to the flare initiation. The emerged negative polarity moved towards the flux rope activation site,
and flare triggering near the photospheric polarity inversion line (PIL) took place. The motion of the negative polarity region
towards the PIL helped in the build-up of magnetic energy at the flare and flux rope activation site. This study provides
unique observational evidence of a rising flux rope that failed to erupt due to a remnant filament and overlying magnetic
field, as well as associated triggering of an M-class flare. 相似文献
10.
To model irregularities in the magnetic structure of solar flux ropes or in interplanetary magnetic clouds, we propose the following approach. A local irregularity in the form of a compact toroid is added into a cylindrical linear force-free magnetic structure. The radius of the cylinder and the small radius of the toroid are the same, since the force-free parameter α is constant, that is, we have in total a linear force-free configuration, too. Meanwhile, the large radius of the toroid can be smaller. The effect of such modeling depends on the aspect ratio of the compact toroid, its location and orientation, and on its magnetic field magnitude in comparison with that of the cylinder. 相似文献
11.
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
12.
To model and study local magnetic-field enhancements in a solar flux rope we consider the magnetic field in its interior as
a superposition of two linear (constant α) force-free magnetic-field distributions, viz. a global one, which is locally similar to a part of the cylinder, and a local torus-shaped magnetic distribution. The newly
derived solution for a toroid with an aspect ratio close to unity is applied. The symmetry axis of the toroid and that of
the cylinder may or may not coincide. Both the large and small radii of the toroid are set equal to the cylinder’s radius.
The total magnetic field distribution yields a flux tube which has a variable diameter with local minima and maxima. In principle,
this approach can be used for the interpretation and analysis of solar-limb observations of coronal loops. 相似文献
13.
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
14.
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
15.
Analysis of the Interball-1 spacecraft data (1995 – 2000) has shown that the solar wind ion flux sometimes increases or decreases abruptly by more than
20% over a time period of several seconds or minutes. Typically, the amplitude of such sharp changes in the solar wind ion
flux (SCIFs) is larger than 0.5×108 cm−2 s−1. These sudden changes of the ion flux were also observed by the Solar Wind Experiment (SWE), on board the Wind spacecraft, as the solar wind density increases and decreases with negligible changes in the solar wind velocity. SCIFs occur
irregularly at 1 AU, when plasma flows with specific properties come to the Earth’s orbit. SCIFs are usually observed in slow,
turbulent solar wind with increased density and interplanetary magnetic field strength. The number of times SCIFs occur during
a day is simulated using the solar wind density, magnetic field, and their standard deviations as input parameters for a period
of five years. A correlation coefficient of ∼0.7 is obtained between the modelled and the experimental data. It is found that
SCIFs are not associated with coronal mass ejections (CMEs), corotating interaction regions (CIRs), or interplanetary shocks;
however, 85% of the sector boundaries are surrounded by SCIFs. The properties of the solar wind plasma for days with five
or more SCIF observations are the same as those of the solar wind plasma at the sector boundaries. One possible explanation
for the occurrence of SCIFs (near sector boundaries) is magnetic reconnection at the heliospheric current sheet or local current
sheets. Other probable causes of SCIFs (inside sectors) are turbulent processes in the slow solar wind and at the crossings
of flux tubes. 相似文献
16.
E. Palmerio E. K. J. Kilpua A. W. James L. M. Green J. Pomoell A. Isavnin G. Valori 《Solar physics》2017,292(2):39
A key aim in space weather research is to be able to use remote-sensing observations of the solar atmosphere to extend the lead time of predicting the geoeffectiveness of a coronal mass ejection (CME). In order to achieve this, the magnetic structure of the CME as it leaves the Sun must be known. In this article we address this issue by developing a method to determine the intrinsic flux rope type of a CME solely from solar disk observations. We use several well-known proxies for the magnetic helicity sign, the axis orientation, and the axial magnetic field direction to predict the magnetic structure of the interplanetary flux rope. We present two case studies: the 2 June 2011 and the 14 June 2012 CMEs. Both of these events erupted from an active region, and despite having clear in situ counterparts, their eruption characteristics were relatively complex. The first event was associated with an active region filament that erupted in two stages, while for the other event the eruption originated from a relatively high coronal altitude and the source region did not feature a filament. Our magnetic helicity sign proxies include the analysis of magnetic tongues, soft X-ray and/or extreme-ultraviolet sigmoids, coronal arcade skew, filament emission and absorption threads, and filament rotation. Since the inclination of the post-eruption arcades was not clear, we use the tilt of the polarity inversion line to determine the flux rope axis orientation and coronal dimmings to determine the flux rope footpoints, and therefore, the direction of the axial magnetic field. The comparison of the estimated intrinsic flux rope structure to in situ observations at the Lagrangian point L1 indicated a good agreement with the predictions. Our results highlight the flux rope type determination techniques that are particularly useful for active region eruptions, where most geoeffective CMEs originate. 相似文献
17.
The singularities of an irrotational magnetic field are lines of electric current. This property derives from the relationship between vector fields and the topology of the underlying three-space and allows for a definition of cosmic field flux tubes and flux ropes as cores (in the sense of the physics of defects) of helical singularities. When applied to force-free flux ropes, and assuming current conservation, an interesting feature is the quantization of the radii, pitches, and helicities. One expects similar quantization effects in the general case. In the special case when the total electric current vanishes, a force-free rope embedded in a medium devoid of magnetic field is nonetheless topologically stable, because it is the core of a singularity of the vector potential. Magnetic merging is also briefly discussed in the same framework. 相似文献
18.
19.
Axially symmetric constant-alpha force-free magnetic fields in toroidal flux ropes with elliptical cross sections are constructed in order to investigate how their alphas and magnetic helicities depend on parameters of the flux ropes. Magnetic configurations are found numerically using a general solution of a constant-alpha force-free field with an axial symmetry in cylindrical coordinates for a wide range of oblatenesses and aspect ratios. Resulting alphas and magnetic helicities are approximated by polynomial expansions in parameters related to oblateness and aspect ratio. These approximations hold for toroidal as well as cylindrical flux ropes with an accuracy better than or of about 1%. Using these formulae, we calculate relative helicities per unit length of two (probably very oblate) magnetic clouds and show that they are very sensitive to the assumed magnetic cloud shapes (circular versus elliptical cross sections). 相似文献
20.
A major solar active event called Bastille Day Event occurred in AR 9077 on July 14, 2000. Simultaneous occurrence of a filament
eruption, a flare and a coronal mass ejection was observed in this event. Previous analyses of this event show that before
the event, there existed an activation and eruption of a huge trans-equatorial filament, which might play a crucial role in
triggering the Bastille Day event. This implies that independent flux systems are closely related to and affect each other,
which has encouraged us to investigate the catastrophic behavior of a multiple coronal flux rope system with the use of a
2.5-D time-dependent MHD model. A force-free field that contains three separate coronal flux ropes is taken to be the initial
state. Starting from this state, we increase either the annular or the axial flux of a certain flux rope to examine the catastrophic
behavior of the system in two regimes, the ideal MHD regime and the resistive MHD regime. It is found that a catastrophe occurs
if the flux exceeds a certain critical value, or the magnetic energy of the system exceeds a certain threshold: the rope of
interest breaks away from the base and escapes to infinity, leaving a current sheet below. Moreover, the destiny of the remainder
flux ropes relies on whether reconnection takes place across the current sheet. In the ideal MHD regime, i.e., in the absence of reconnection, these ropes remain to be attached to the base in equilibrium, whereas in the resistive MHD
regime they abruptly erupt upward during reconnection and escape to infinity. Reconnection causes the field lines to close
back to the base and thus changes the background field outside the attached flux ropes in such a way that the constraint on
these ropes is substantially relaxed and the corresponding catastrophic energy threshold is reduced accordingly, leading to
a catastrophic eruption of these ropes. Since magnetic reconnection is generally inevitable when a current sheet forms and
develops through an eruption of one flux rope, the eruption of this flux rope must lead to an eruption of the others. This
provides an example to demonstrate the interaction between several independent magnetic flux systems in different regions,
as implied by the Bastille Day event, and may serve as a possible mechanism for sympathetic events occurring on the Sun. 相似文献