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Register a new userProperties of the Sheath Regions of Coronal Mass Ejections with or without Shocks from STEREO in situ Observations near 1 AU
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Tarik Mohammad Salman
Publication date
2020
fields
Physics
and research's language is
English
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We examine 188 coronal mass ejections (CMEs) measured by the twin STEREO spacecraft during 2007-2016 to investigate the generic features of the CME sheath and the magnetic ejecta (ME) and dependencies of average physical parameters of the sheath on the ME. We classify the MEs into three categories, focusing on whether a ME drives both a shock and sheath, or only a sheath, or neither, near 1 AU. We also reevaluate our initial classification through an automated algorithm and visual inspection. We observe that even for leading edge speeds greater than 500 km/s, 1 out of 4 MEs do not drive shocks near 1 AU. MEs driving both shocks and sheaths are the fastest and propagate in high magnetosonic solar wind, whereas MEs driving only sheaths are the slowest and propagate in low magnetosonic solar wind. Our statistical and superposed epoch analyses indicate that all physical parameters are more enhanced in the sheath regions following shocks than in sheaths without shocks. However, differences within sheaths become statistically less significant for similar driving MEs. We also find that the radial thickness of ME-driven sheaths apparently has no clear linear correlation with the speed profile and associated Mach numbers of the driver.
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Predicting the Bz magnetic field embedded within ICMEs, also known as the Bz problem, is a key challenge in space weather forecasting. We study the hypothesis that upstream in situ measurements of the sheath region and the first few hours of the magnetic obstacle provide sufficient information for predicting the downstream Bz component. To do so, we develop a predictive tool based on machine learning that is trained and tested on 348 ICMEs from Wind, STEREO-A, and STEREO-B measurements. We train the machine learning models to predict the minimum value of the Bz component and the maximum value of the total magnetic field Bz in the magnetic obstacle. To validate the tool, we let the ICMEs sweep over the spacecraft and assess how continually feeding in situ measurements into the tool improves the Bz prediction. Because the application of the tool in operations needs an automated detection of ICMEs, we implement an existing automated ICME detection algorithm and test its robustness for the time intervals under scrutiny. We find that the predictive tool can predict the minimum value of the Bz component in the magnetic obstacle with a mean absolute error of 3.12 nT and a Pearson correlation coefficient of 0.71 when the sheath region and the first 4 hours of the magnetic obstacle are observed. While the underlying hypothesis is unlikely to solve the Bz problem, the tool shows promise for ICMEs that have a recognizable magnetic flux rope signature. Transitioning the tool to operations could lead to improved space weather forecasting.
We report on a numerical investigation of two coronal mass ejections (CMEs) which interact as they propagate in the inner heliosphere. We focus on the effect of the orientation of the CMEs relative to each other by performing four different simulations with the axis of the second CME rotated by 90 degrees from one simulation to the next. Each magneto-hydrodynamic (MHD) simulation is performed in three dimensions (3-D) with the Space Weather Modeling Framework (SWMF) in an idealized setting reminiscent of solar minimum conditions. We extract synthetic satellite measurements during and after the interaction and compare the different cases. We also analyze the kinematics of the two CMEs, including the evolution of their widths and aspect ratios. We find that the first CME contracts radially as a result of the interaction in all cases, but the amount of subsequent radial expansion depends on the relative orientation of the two CMEs. Reconnection between the two ejecta and between the ejecta and the interplanetary magnetic field (IMF) determines the type of structure resulting from the interaction. When a CME with a high inclination with respect to the ecliptic overtakes one with a low inclination, it is possible to create a compound event with a smooth rotation in the magnetic field vector over more than 180 degrees. Due to reconnection, the second CME only appears as an extended tail, and the event may be mistaken for a glancing encounter with an isolated CME. This configuration differs significantly from the one usually studied of a multiple-magnetic cloud event, which we found to be associated with the interaction of two CMEs with the same orientation.
Coronal mass ejections (CMEs) are the main drivers of geomagnetic disturbances, but the effects of their interaction with Earths magnetic field depend on their magnetic configuration and orientation. Fitting and reconstruction techniques have been developed to determine the important geometrical and physical CME properties. In many instances, there is disagreement between such different methods but also between fitting from in situ measurements and reconstruction based on remote imaging. Here, we compare three methods based on different assumptions for measurements of thirteen CMEs by the Wind spacecraft from 1997 to 2015. These CMEs are selected from the interplanetary coronal mass ejections catalog on https://wind.nasa.gov/ICMEindex.php due to their simplicity in terms of 1) small expansion speed throughout the CME and 2) little asymmetry in the magnetic field profile. This makes these thirteen events ideal candidates to compare codes that do not include expansion nor distortion. We find that, for these simple events, the codes are in relatively good agreement in terms of the CME axis orientation for six out of the 13 events. Using the Grad-Shafranov technique, we can determine the shape of the cross-section, which is assumed to be circular for the other two models, a force-free fitting and a circular-cylindrical non-force-free fitting. Five of the events are found to have a clear circular cross-section, even when this is not a pre-condition of the reconstruction. We make an initial attempt at evaluating the adequacy of the different assumptions for these simple CMEs. The conclusion of this work strongly suggests that attempts at reconciling in situ and remote-sensing views of CMEs must take in consideration the compatibility of the different models with specific CME structures to better reproduce flux ropes.
We present a comprehensive statistical analysis of 106 sheath regions driven by coronal mass ejections (CMEs) and measured near 1 AU. Using data from the STEREO probes, this extended analysis focuses on two discrete categorizations. In the first categorization, we investigate how the generic features of sheaths change with their potential formation mechanisms (propagation and expansion sheaths), namely, their associations with magnetic ejectas (MEs) which are primarily expanding or propagating in the solar wind. We find propagation sheaths to be denser and driven by stronger MEs, whereas expansion sheaths are faster. Exploring the temporal profiles of these sheaths with a superposed epoch technique, we observe that most of the magnetic field and plasma signatures are more elevated in propagation sheaths relative to expansion sheaths. The second categorization is based on speed variations across sheaths. Employing linear least squares regression, we categorize four distinct speed profiles of the sheath plasma. We find that the associated shock properties and solar cycle phase do not impact the occurrence of such variations. Our results also highlight that the properties of the driving MEs are a major source of variability in the sheath properties. Through logistic regression, we conclude that the magnetic field strength and the ME speed in the frame of the solar wind are likely drivers of these speed variations.
We study periods of elevated energetic particle intensities observed by STEREO-A when the partial pressure exerted by energetic ($geq$83 keV) protons ($P_{EP}$) is larger than the pressure exerted by the interplanetary magnetic field ($P_{B}$). In the majority of cases, these periods are associated with the passage of interplanetary shocks. Periods when $P_{EP}$ exceeds $P_{B}$ by more than one order of magnitude are observed in the upstream region of fast interplanetary shocks where depressed magnetic field regions coincide with increases of the energetic particle intensities. When solar wind parameters are available, $P_{EP}$ also exceeds the pressure exerted by the solar wind thermal population ($P_{TH}$). Prolonged periods ($>$12 h) with both $P_{EP}$$>$$P_{B}$ and $P_{EP}$$>$$P_{TH}$ may also occur when energetic particles accelerated by an approaching shock encounter a region well-upstream of the shock characterized by low magnetic field magnitude and tenuous solar wind density. Quasi-exponential increases of the sum $P_{SUM}$=$P_{B}$+$P_{TH}$+$P_{EP}$ are observed in the immediate upstream region of the shocks regardless of individual changes in $P_{EP}$, $P_{B}$ and $P_{TH}$, indicating a coupling between $P_{EP}$ and the pressure of the background medium characterized by $P_{B}$ and $P_{TH}$. The quasi-exponential increase of $P_{SUM}$ implies a convected exponential radial gradient $partial{P_{SUM}}/partial{r}$$>$0 that results in an outward force applied to the plasma upstream of the shock. This force can be maintained by the mobile energetic particles streaming upstream of the shocks that, in the most intense events, drive electric currents able to generate diamagnetic cavities and depressed solar wind density regions.
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