Interplanetary Coronal Mass Ejection Sheath Research Articles

Anisotropic Heating and Cooling within Interplanetary Coronal Mass Ejection Sheath Plasma

This study presents the first comprehensive investigation of the relationship between heating and cooling, temperature anisotropy, turbulence level, and collisional age within interplanetary coronal mass ejection (ICME) sheaths, which are highly compressed, heated, and turbulent. Using Wind spacecraft data, we analyze 333 ICME sheaths observed at 1 au from 1995 to 2015. The proton temperature within the ICME sheaths has a log-normal probability distribution. Irrespective of instability growth rates, plasma unstable to proton-cyclotron (PC) and firehose instabilities appear to be statistically hotter, at least by a factor of 5 to 10, compared to stable plasma. We also observe relatively enhanced magnetic fluctuations and low collisional age, especially in regimes unstable to PC and firehose instabilities at low proton betas β p ≤ 2. In the case of high beta β p ≥ 2, we observe high magnetic fluctuations close to the instabilities and less collisional age to the plasma unstable to firehose instability rather than near the mirror mode and PC threshold. Our findings suggest that heating processes dominate over cooling processes in producing proton temperature anisotropy in the ICME sheath region. Moreover, collisional age and magnetic fluctuations are critical in maintaining anisotropic and isotropic conditions.

Interaction of Cosmic Rays With Magnetic Flux Ropes

AbstractThe heliosphere is full of galactic cosmic rays (GCR), high‐energy charged particles coming isotropically from the galaxy. The GCR interact with the solar wind blown by the Sun carrying out plasma, magnetic fields and transient structures such as interplanetary coronal mass ejections (ICMEs) and their associated magnetic flux ropes (MFR). The GCR interaction with ICMEs has been extensively studied particularly the GCR flux attenuation (known as Forbush decreases) as a result of interacting with the ICME sheath and magnetic field. In this work, we investigate the opposite effect: the MFR's ability to generate GCR anisotropies which an observer may detect as an increase in the GCR flux. To achieve this, we simulated a flux of protons with energies in the 10–160 GeV range arriving from all directions to a cylindrical MFR (with and without sheath) with plasma, magnetic field, and spatial dimensions found in average ICMEs observed at 1 au. By following the individual trajectories of the injected particles we found that the MFR deviates the charged particles preferentially in one direction parallel to the MFR–axis. We also found that the peak of this anisotropic GCR flux depends on: the angle between the MFR and ambient magnetic fields; the presence or not of the sheath region; the energy of the incident particles and the observer location inside the MFR.

A Survey of Coronal Mass Ejections Measured In Situ by Parker Solar Probe during 2018–2022

Abstract We present a statistical investigation of the radial evolution of 28 interplanetary coronal mass ejections (ICMEs), measured in situ by the Parker Solar Probe spacecraft from 2018 October to 2022 August. First, by analyzing the radial distribution of ICME classification based on magnetic hodograms, we find that coherent configurations are more likely to be observed close to the Sun. By contrast, more complex configurations are observed farther out. We also notice that the post-ICME magnetic field is more impacted following an ICME passage at larger heliocentric distances. Second, with a multilinear robust regression, we derive a slower magnetic ejecta (ME) expansion rate within 1 au compared to previous statistical estimates. Then, investigating the magnetic field fluctuations within ICME sheaths, we see that these fluctuations are strongly coupled to the relative magnetic field strength gradient from the upstream solar wind to the ME. Third, we identify ME expansion as an important factor in the formation of sheaths. Finally, we determine the distortion parameter (DiP), which is a measure of magnetic field asymmetry in an ME. We discover lower overall asymmetries within MEs. We reveal that even for expanding MEs, the time duration over which an ME is sampled does not correlate with DiP values, indicating that the aging effect is not the sole contributor to the observed ME asymmetries.

Turbulence, Intermittency, and Cross-Scale Energy Transfer in an Interplanetary Coronal Mass Ejection

Solar wind measurements carried out by NASA’s Wind spacecraft before, during, and after the passing of an interplanetary coronal mass ejection (ICME), detected on 12 – 14 September 2014, have been used in order to examine several properties of magnetohydrodynamic (MHD) turbulence. Spectral indices and flatness scaling exponents of magnetic field, velocity, and proton density measurements were obtained, and provided a standard description of the characteristics of turbulence within different sub-regions of the ICME and its surroundings. This analysis was followed by the validation of the third-order moment scaling law for isotropic, incompressible MHD turbulence in the same sub-regions, which confirmed the fully developed nature of turbulence in the ICME plasma. The energy transfer rate was also estimated in each ICME sub-region and in the surrounding solar wind. An exceptionally high value was found within the ICME sheath, accompanied by enhanced intermittency, possibly related to the powerful energy injection associated with the arrival of the ICME.

Small-scale flux ropes in ICME sheaths

Sheath regions of interplanetary coronal mass ejections (ICMEs) are formed when the upstream solar wind is deflected and compressed due to the propagation and expansion of the ICME. Small-scale flux ropes found in the solar wind can thus be swept into ICME-driven sheath regions. They may also be generated locally within the sheaths through a range of processes. This work applies wavelet analysis to obtain the normalized reduced magnetic helicity, normalized cross helicity, and normalized residual energy, and uses them to identify small-scale flux ropes and Alfvén waves in 55 ICME-driven sheath regions observed by the Wind spacecraft in the near-Earth solar wind. Their occurrence is investigated separately for three different frequency ranges between 10–2 − 10–4 Hz. We find that small scale flux ropes are more common in ICME sheaths than in the upstream wind, implying that they are at least to some extent actively generated in the sheath and not just compressed from the upstream wind. Alfvén waves occur more evenly in the upstream wind and in the sheath. This study also reveals that while the highest frequency (smallest scale) flux ropes occur relatively evenly across the sheath, the lower frequency (largest scale) flux ropes peak near the ICME leading edge. This suggests that they could have different physical origins, and that processes near the ICME leading edge are important for generating the larger scale population.

Magnetic Reconnection Within the Boundary Layer of a Magnetic Cloud in the Solar Wind

AbstractThe twisted local magnetic field at the front or rear regions of the magnetic clouds (MCs) associated with interplanetary coronal mass ejections (ICMEs) is often nearly opposite to the direction of the ambient interplanetary magnetic field. There is also observational evidence for magnetic reconnection (MR) outflows occurring within the boundary layers of MCs. In this study, a MR event located at the western flank of the MC occurring on October 3, 2000 is studied in detail. Both the large‐scale geometry of the helical MC and the MR outflow structure are scrutinized in a detailed multipoint study. The ICME sheath is of hybrid propagation‐expansion type. Here, the freshly reconnected open field lines are expected to slip slowly over the MC resulting in plasma mixing at the same time. As for MR, the current sheet geometry and the vertical motion of the outflow channel between ACE‐Geotail‐WIND spacecraft were carefully studied and tested. The main findings on MR include (a) first‐time observation of non‐Petschek‐type slow‐shock‐like discontinuities in the inflow regions; (b) observation of turbulent Hall magnetic field associated with a Lorentz‐force‐deflected electron jet; (c) acceleration of protons by reconnection electric field and their back‐scatter from the slow‐shock‐like discontinuity; (d) observation of relativistic electron near the MC inflow boundary/separatrix; these electron populations can presumably appear as a result of nonadiabatic acceleration, gradient B drift, and via acceleration in the electrostatic potential well associated with the Hall current system; and (e) observation of Doppler‐shifted ion‐acoustic and Langmuir waves in the MC inflow region.

Radial Evolution of Magnetic Field Fluctuations in an Interplanetary Coronal Mass Ejection Sheath

Abstract The sheaths of compressed solar wind that precede interplanetary coronal mass ejections (ICMEs) commonly display large-amplitude magnetic field fluctuations. As ICMEs propagate radially from the Sun, the properties of these fluctuations may evolve significantly. We have analyzed magnetic field fluctuations in an ICME sheath observed by MESSENGER at 0.47 au and subsequently by STEREO-B at 1.08 au while the spacecraft were close to radial alignment. Radial changes in fluctuation amplitude, compressibility, inertial-range spectral slope, permutation entropy, Jensen–Shannon complexity, and planar structuring are characterized. These changes are discussed in relation to the evolving turbulent properties of the upstream solar wind, the shock bounding the front of the sheath changing from a quasi-parallel to quasi-perpendicular geometry, and the development of complex structures in the sheath plasma.

Significant Electric Field Perturbations in Low Latitude Ionosphere due to the Passage of Two Consecutive ICMEs During 6–8 September 2017

AbstractThis investigation shows that the significant electric field disturbances in the dip‐equatorial ionosphere during the geomagnetic storm of 6–8 September 2017 are due to the passage of two consecutive interplanetary coronal mass ejections (ICMEs). During the passage of the first ICME sheath, a long duration (∼10 hr) prompt penetration (PP) event is operational in which 60‐min periodic component is found to be present in vertical drift as well as in equatorial electrojet, but the 45‐min periodicity, though present, is not significant in equatorial electrojet. On 8 September, the shock associated with the second ICME enhances theFregion vertical plasma drift to ∼150 m/s in the evening hours which is one of the highest vertical drift ever measured over Jicamarca. The same PP electric field causes unusually large enhancement of the equatorial electrojet strength to ∼135 nT in the early morning hours over the Philippine sector. The disturbance dynamo (DD) that follows the storm causes an upward vertical drift of ∼55 m/s during postmidnight hours over Jicamarca which is one of the highest observed. These unusually large electric field perturbations cause significant changes in theFregion plasma fountain. It is shown that these electric field perturbations cannot be accounted by PP/DD electric field associated with the geomagnetic storm only and significant contribution from substorm is conspicuous. Therefore, the present investigation highlights the need to evaluate the role of substorm in unusually large electric field perturbations over equatorial ionosphere.

Alfvén Ion Cyclotron Waves in Sheath Regions Driven by Interplanetary Coronal Mass Ejections

AbstractWe report on a statistical analysis of the occurrence and properties of Alfvén ion cyclotron (AIC) waves in sheath regions driven by interplanetary coronal mass ejections (ICMEs). We have developed an automated algorithm to identify AIC wave events from magnetic field data and apply it to investigate 91 ICME sheath regions recorded by the Wind spacecraft. Our analysis focuses on waves generated by the ion cyclotron instability. AIC waves are observed to be frequent structures in ICME‐driven sheaths, and their occurrence is the highest in the vicinity of the shock. Together with previous studies, our results imply that the shock compression has a crucial role in generating wave activity in ICME sheaths. AIC waves tend to have their frequency below the ion cyclotron frequency, and, in general, occur in plasma that is stable with respect to the ion cyclotron instability and has lower ion β‖ than mirror modes. The results suggest that the ion beta anisotropy β⊥/β‖>1 appearing in ICME sheaths is regulated by both ion cyclotron and mirror instabilities.

Plasma and Magnetic-Field Characteristics of Magnetic Decreases in the Solar Wind at 1 AU: Cluster-C1 Observations

Magnetic decreases (MDs) are structures observed in interplanetary space with significant decreases in magnetic-field magnitude. Events with little or no change in the field direction are called linear magnetic decreases (LMDs), the others are called nonlinear MDs (NMDs). In this article we focus on LMD and NMD trains, where LMD trains are defined as at least three LMDs in a row and NMD trains as trains (≥ three MDs in a row) that are not all linear. If the temporal separation between two MDs was shorter than five minutes, they were considered as one train event. A total of 16 273 MD trains (including 897 LMD trains and 15 376 NMD trains) were identified and studied. The details of the background magnetic-field and plasma (e.g. ion-density and velocity) features were examined and compared with the average solar-wind properties. LMD trains are found to occur in regions with relatively low magnetic-field strengths, high ion-number densities, and large plasma βs (ratio of the plasma thermal pressure to the magnetic pressure). In sharp contrast, NMD trains have plasma properties similar to the average solar wind. Forty-three LMD trains are related to interplanetary coronal mass ejections (ICMEs) (including 19 events that occurred in ICME sheaths and 24 in the ICME proper), while 222 LMD trains occurred in corotating interaction regions (CIRs), and the remaining 632 events in the normal solar wind. The LMD trains that occurred in ICME sheaths are thought to be associated with the generation mechanism of the mirror-mode instability. Only 552 of the NMD trains are related to ICMEs (including 236 events in ICME sheaths and 316 in ICMEs proper), while 3889 (25 %) NMD trains occurred in CIRs, and the remaining 71 % occurred in the normal solar wind. Because the NMD trains have various plasma properties that differ from the LMD trains, we suggest that NMD trains may be generated by different mechanisms, for instance by a steepening of Alfven waves.

Revisiting two-step Forbush decreases

[1] Interplanetary coronal mass ejections (ICMEs) and their shocks can sweep out galactic cosmic rays (GCRs), thus creating Forbush decreases (FDs). The traditional model of FDs predicts that an ICME and its shock decrease the GCR intensity in a two-step profile. This model, however, has been the focus of little testing. Thus, our goal is to discover whether a passing ICME and its shock inevitably lead to a two-step FD, as predicted by the model. We use cosmic ray data from 14 neutron monitors and, when possible, high time resolution GCR data from the spacecraft International Gamma Ray Astrophysical Laboratory (INTEGRAL). We analyze 233 ICMEs that should have created two-step FDs. Of these, only 80 created FDs, and only 13 created two-step FDs. FDs are thus less common than predicted by the model. The majority of events indicates that profiles of FDs are more complicated, particularly within the ICME sheath, than predicted by the model. We conclude that the traditional model of FDs as having one or two steps should be discarded. We also conclude that generally ignored small-scale interplanetary magnetic field structure can contribute to the observed variety of FD profiles.

Deflection flows ahead of ICMEs as an indicator of curvature and geoeffectiveness

We examine the upstream meridional deflection flows of interplanetary coronal mass ejections (ICMEs) in an effort to investigate their cross‐sectional shape and the magnetic field orientation in their sheath regions. Eight out of 11 magnetic clouds (MCs) near solar minimum identified for the curvature study are concave outward as indicated by the elevation angle of the MC normal with respect to the solar equatorial plane; an inverse correlation is observed between the meridional deflection flow and the spacecraft latitude for these concave‐outward MCs, which suggests that the upstream plasma is deflected toward the equatorial plane. MHD simulations, however, show that the meridional deflection flow moves poleward for a concave‐outward CME. The poleward flow deflection is observed only ahead of convex‐outward MCs. Possibilities leading to this discrepancy are discussed. The deflection flow speed in sheath regions of ICMEs increases with the ICME speed relative to the ambient solar wind, which together with the coupling between the meridional magnetic field and deflection flow yields a positive linear correlation between the sheath meridional field and the ICME relative speed. This empirical relationship could predict the sheath meridional field based on the observed CME speed, which may be useful for space weather forecasting as ICME sheaths are often geoeffective. Implications of the deflection flows and ICME curvature are also discussed in terms of magnetic reconnection and particle acceleration in ICME sheaths.

Field-Line Draping Around ICMES

Magnetic field orientations in the sheaths of ten fast interplanetary coronal mass ejections (ICMEs) that cover the solar longitude range roughly from 20° East to 33° West (as determined from the associated flare or filament disruption) are overlain on the MHD-computed magnetic field pattern showing draping in Earth’s magnetosheath. The general draping pattern is evident in the ICME sheath orientations including, most importantly, the east flank where draping causes the greatest distortion of the magnetic field away from the general Parker spiral. Deviations from the general draping pattern are also evident which, we suggest, result from the history of accretion of the inhomogeneous interplanetary magnetic field (IMF) into the ICME sheath over a long stretch of solar wind before arriving at one AU. The profiles of magnetic field intensity between the ICME shock and the nose of the ICME deviate significantly from the corresponding profile in Earth’s magnetosheath. The ICME samples are much more irregular and show no general tendency to increase toward the stagnation point. We suggest that again this difference reflects the history of IMF accretion by the ICME sheath. The long stretch of accreted inhomogeneous field (a significant fraction of one AU) can account for the irregularity, and the weakness of the field close to the body possibly reflects a weaker ICME shock closer to the Sun.

Density extremes in the solar wind

The largest density variations in the solar wind occur near the streamer belt, where frequently they are associated with interplanetary coronal mass ejections (ICMEs). They tend to be embedded in larger‐scale regions of high pressure: high densities in ICME sheaths and corotating interaction regions and low densities in structures with distinctive, high‐magnetic‐pressure profiles, sometimes within ICMEs. On average, however, ICME densities are similar to ambient‐wind densities. For a set of 34 ICMEs identified in Wind data as magnetic clouds, the average density was 11 cm−3 in both the clouds and all slow wind during the same period. A set of low‐density structures observed earlier by ISEE 3 recurred for three solar rotations, possibly owing to recurrence of the streamer belt itself, with its frequent transient outflows. Density averages less than 1 cm−3 show a possible solar cycle variation which peaks 1–2 years prior to the peak of ICME signatures.