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Register a new userContribution of the aging effect to the observed asymmetry of interplanetary magnetic clouds
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Large magnetic structures are launched away from the Sun during solar eruptions. They are observed as (interplanetary) coronal mass ejections (ICMEs or CMEs) with coronal and heliospheric imagers. A fraction of them are observed insitu as magnetic clouds (MCs). Fitting these structures properly with a model requires a better understanding of their evolution. In situ measurements are done locally when the spacecraft trajectory crosses the magnetic configuration. These observations are taken for different elements of plasma and at different times, and are therefore biased by the expansion of the magnetic configuration. This aging effect leads to stronger magnetic fields measured at the front than at the rear of MCs, an asymmetry often present in MC data. However, can the observed asymmetry be explained quantitatively only from the expansion? Based on self-similar expansion, we derive a method to estimate the expansion rate from observed plasma velocity. We next correct for the aging effect both the observed magnetic field and the spatial coordinate along the spacecraft trajectory. This provides corrected data as if the MC internal structure was observed at the same time. We apply the method to 90 best observed MCs near Earth (1995-2012). The aging effect is the main source of the observed magnetic asymmetry only for 28% of MCs. After correcting the aging effect, the asymmetry is almost symmetrically distributed between MCs with a stronger magnetic field at the front and those at the rear of MCs. The proposed method can efficiently remove the aging bias within insitu data of MCs, and more generally of ICMEs. This allows one to analyse the data with a spatial coordinate, such as in models or remote sensing observations.
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We analyze the evolution of the interplanetary magnetic field spatial structure by examining the inner heliospheric autocorrelation function, using Helios 1 and Helios 2 in situ observations. We focus on the evolution of the integral length scale (lambda) anisotropy associated with the turbulent magnetic fluctuations, with respect to the aging of fluid parcels traveling away from the Sun, and according to whether the measured lambda is principally parallel (lambda_parallel) or perpendicular (lambda_perp) to the direction of a suitably defined local ensemble average magnetic field B0. We analyze a set of 1065 24-hour long intervals (covering full missions). For each interval, we compute the magnetic autocorrelation function, using classical single-spacecraft techniques, and estimate lambda with help of two different proxies for both Helios datasets. We find that close to the Sun, lambda_parallel < lambda_perp. This supports a slab-like spectral model, where the population of fluctuations having wavevector k parallel to B0 is much larger than the one with k-vector perpendicular. A population favoring perpendicular k-vectors would be considered quasi-two dimensional (2D). Moving towards 1 AU, we find a progressive isotropization of lambda and a trend to reach an inverted abundance, consistent with the well-known result at 1 AU that lambda_parallel > lambda_perp, usually interpreted as a dominant quasi-2D picture over the slab picture. Thus, our results are consistent with driving modes having wavevectors parallel to B0 near Sun, and a progressive dynamical spectral transfer of energy to modes with perpendicular wavevectors as the solar wind parcels age while moving from the Sun to 1 AU.
Magnetic flux ropes (MFRs) are one kind of fundamental structures in the solar physics, and involved in various eruption phenomena. Twist, characterizing how the magnetic field lines wind around a main axis, is an intrinsic property of MFRs, closely related to the magnetic free energy and stableness. So far it is unclear how much amount of twist is carried by MFRs in the solar atmosphere and in heliosphere and what role the twist played in the eruptions of MFRs. Contrasting to the solar MFRs, there are lots of in-situ measurements of magnetic clouds (MCs), the large-scale MFRs in interplanetary space, providing some important information of the twist of MFRs. Thus, starting from MCs, we investigate the twist of interplanetary MFRs with the aid of a velocity-modified uniform-twist force-free flux rope model. It is found that most of MCs can be roughly fitted by the model and nearly half of them can be fitted fairly well though the derived twist is probably over-estimated by a factor of 2.5. By applying the model to 115 MCs observed at 1 AU, we find that (1) the twist angles of interplanetary MFRs generally follow a trend of about $0.6frac{l}{R}$ radians, where $frac{l}{R}$ is the aspect ratio of a MFR, with a cutoff at about $12pi$ radians AU$^{-1}$, (2) most of them are significantly larger than $2.5pi$ radians but well bounded by $2frac{l}{R}$ radians, (3) strongly twisted magnetic field lines probably limit the expansion and size of MFRs, and (4) the magnetic field lines in the legs wind more tightly than those in the leading part of MFRs. These results not only advance our understanding of the properties and behavior of interplanetary MFRs, but also shed light on the formation and eruption of MFRs in the solar atmosphere. A discussion about the twist and stableness of solar MFRs are therefore given.
Solar energetic particles (SEPs), accelerated during solar eruptions, propagate in turbulent solar wind before being observed with in situ instruments. In order to interpret their origin through comparison with remote-sensing observations of the solar eruption, we thus must deconvolve the transport effects due to the turbulent magnetic fields from the SEP observations. Recent research suggests that the SEP propagation is guided by the turbulent meandering of the magnetic fieldlines across the mean magnetic field. However, the lengthening of the distance the SEPs travel, due to the fieldline meandering, has so far not been included in SEP event analysis. This omission can cause significant errors in estimation of the release times of SEPs at the Sun. We investigate the distance travelled by the SEPs by considering them to propagate along fieldlines that meander around closed magnetic islands that are inherent in turbulent plasma. We introduce a fieldline randow walk model which takes into account the physical scales associated to the magnetic islands. Our method remedies the problem of the diffusion equation resulting in unrealistically short pathlengths, and the fractal dependence of the pathlength of random walk on the length of the random-walk step. We find that the pathlength from the Sun to 1 au can be below the nominal Parker spiral length for SEP events taking place at solar longitudes 45E to 60W, whereas the western and behind-the-limb particles can experience pathlengths longer than 2 au due to fieldline meandering.
We use observations from the Interstellar Boundary Explorer (IBEX) and Ulysses to explore the possibility that the interstellar neutral helium flowing through the inner solar system possesses an intrinsic non-Maxwellian velocity distribution. In fitting the IBEX and Ulysses data, we experiment with both a kappa distribution and a bi-Maxwellian, instead of the usual Maxwellian assumption. The kappa distribution does not improve the quality of fit to either the IBEX or Ulysses data, and we find lower limits to the kappa parameter of kappa>12.1 and kappa>6.0 from the IBEX and Ulysses analyses, respectively. In contrast, we do find evidence that a bi-Maxwellian improves fit quality. For IBEX, there is a clear preferred bi-Maxwellian solution with T_perp/T_par=0.62+/-0.11 oriented about an axis direction with ecliptic coordinates (lambda_axis,b_axis)=(57.2+/-8.9 deg,-1.6+/-5.9 deg). The Ulysses data provide support for this result, albeit with lower statistical significance. The axis direction is close to the ISM flow direction, in a heliocentric rest frame, and is therefore unlikely to be indicative of velocity distribution asymmetries intrinsic to the ISM. It is far more likely that these results indicate the presence of asymmetries induced by interactions in the outer heliosphere.
Numerical studies have been performed to interpret the observed shock overtaking magnetic cloud (MC) event by a 2.5 dimensional magnetohydrodynamic (MHD) model in heliospheric meridional plane. Results of an individual MC simulation show that the MC travels with a constant bulk flow speed. The MC is injected with very strong inherent magnetic field over that in the ambient flow and expands rapidly in size initially. Consequently, the diameter of MC increases in an asymptotic speed while its angular width contracts gradually. Meanwhile, simulations of MC-shock interaction are also presented, in which both a typical MC and a strong fast shock emerge from the inner boundary and propagate along heliospheric equator, separated by an appropriate interval. The results show that the shock firstly catches up with the preceding MC, then penetrates through the MC, and finally merges with the MC-driven shock into a stronger compound shock. The morphologies of shock front in interplanetary space and MC body behave as a central concave and a smooth arc respectively. The compression and rotation of magnetic field serve as an efficient mechanism to cause a large geomagnetic storm. The MC is highly compressed by the the overtaking shock. Contrarily, the transport time of incidental shock influenced by the MC depends on the interval between their commencements. Maximum geoeffectiveness results from that when the shock enters the core of preceding MC, which is also substantiated to some extent by a corresponding simplified analytic model. Quantified by $Dst$ index, the specific result gives that the geoeffectiveness of an individual MC is largely enhanced with 80% increment in maximum by an incidental shock.
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