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The large-scale magnetic structure of interplanetary coronal mass ejections (ICMEs) has been shown to affect the galactic cosmic ray (GCR) flux measured in situ by spacecraft, causing temporary decreases known as Forbush decreases (Fds). In some ICMEs, the magnetic ejecta exhibits a magnetic flux rope (FR) structure; the strong magnetic field strength and closed field line geometry of such ICME FRs has been proposed to act as a shield to GCR transport. In this study, we identify four ICMEs near Earth that drove Fds with similar mean magnetic field strengths (20 - 25 nT); two ICMEs with more typical mean speeds (~400 km/s), and two fast (~750 km/s) ICMEs. Within each speed pairing, we identify an ICME that exhibited an open magnetic field line topology and compare its effect on the GCR flux to that which exhibited a mostly closed topology. We investigate the different mechanisms that contribute to the resulting ICME-related Fds and their recovery, and determine which properties, if any, play a more important role than others in driving Fds. We find that much of the GCR response to the ICME events in this study is independent of the open or closed magnetic field line topology of the flux rope, and that features such as the fluctuations in speed, magnetic field structure, and expansion within the FR may play more of a role in determining the smaller-scale structure of the Fd profile.

The efficacy of coronal mass ejection (CME) observations as a key input to space weather forecasting is explored by comparing on and off Sun-Earth line observations from the ESA/NASA SOHO and NASA STEREO spacecraft. A comparison is made of CME catalogues based on L1 coronagraph imagery and off Sun-Earth line coronagraph and heliospheric imager (HI) observations, for the year 2011. Analysis reveals inconsistencies in the identification of a number of potentially Earth-directed CMEs. The catalogues reflect our ability to identify and characterise CMEs, so any discrepancies can impact our prediction of Earth-directed CMEs. We show that 15 CMEs, which were observed by STEREO, that had estimated directions compatible with Earth-directed events, had no identified halo/partial halo counterpart listed in the L1 coronagraph CME catalogue. In-situ data confirms that for 9 of these there was a consistent L1 Interplanetary CME (ICME). The number of such "discrepant" events is significant compared to the number of ICMEs recorded at L1 in 2011, stressing the need to address space weather monitoring capabilities, particularly with the inclusion of off Sun-Earth line observation. While the study provides evidence that some halo CMEs are simply not visible in near-Earth coronagraph imagery, there is evidence that some halo CMEs viewed from L1 are compromised by preceding CME remnants or the presence of multiple-CMEs. This underlines (1) the value of multiple vantage point CME observation, and (2) the benefit of off Sun-Earth line platform heliospheric imaging, and coronagraph imaging, for the efficient identification and tracking of Earth-directed events., Comment: 36 pages, 6 figures, in press at AGU Space Weather, 2023

Coronal mass ejections (CMEs) are large-scale eruptions with a typical radial size at 1 au of 0.21 au but their angular width in interplanetary space is still mostly unknown, especially for the magnetic ejecta (ME) part of the CME. We take advantage of STEREO-A angular separation of 20$^\circ$-60$^\circ$ from the Sun-Earth line from October 2020 to August 2022, and perform a two-part study to constrain the angular width of MEs in the ecliptic plane: a) we study all CMEs that are observed remotely to propagate between the Sun-STEREO-A and the Sun-Earth lines and determine how many impact one or both spacecraft in situ, and b) we investigate all in situ measurements at STEREO-A or at L1 of CMEs during the same time period to quantify how many are measured by the two spacecraft. A key finding is that, out of 21 CMEs propagating within 30$^\circ$ of either spacecraft, only four impacted both spacecraft and none provided clean magnetic cloud-like signatures at both spacecraft. Combining the two approaches, we conclude that the typical angular width of a ME at 1 au is $\sim$ 20$^\circ$-30$^\circ$, or 2-3 times less than often assumed and consistent with a 2:1 elliptical cross-section of an ellipsoidal ME. We discuss the consequences of this finding for future multi-spacecraft mission designs and for the coherence of CMEs., Comment: 21 pages, revision submitted to ApJ

Predictions of coronal mass ejections (CMEs) and solar energetic particles (SEPs) are a central issue in space weather forecasting. In recent years, interest in space weather predictions has expanded to include impacts at other planets beyond Earth as well as spacecraft scattered throughout the heliosphere. In this sense, the scope of space weather science now encompasses the whole heliospheric system, and multi-point measurements of solar transients can provide useful insights and validations for prediction models. In this work, we aim to analyse the whole inner heliospheric context between two eruptive flares that took place in late 2020, i.e. the M4.4 flare of November 29 and the C7.4 flare of December 7. This period is especially interesting because the STEREO-A spacecraft was located ∼60° east of the Sun–Earth line, giving us the opportunity to test the capabilities of “predictions at 360°” using remote-sensing observations from the Lagrange L1 and L5 points as input. We simulate the CMEs that were ejected during our period of interest and the SEPs accelerated by their shocks using the WSA–Enlil–SEPMOD modelling chain and four sets of input parameters, forming a “mini-ensemble”. We validate our results using in-situ observations at six locations, including Earth and Mars. We find that, despite some limitations arising from the models’ architecture and assumptions, CMEs and shock-accelerated SEPs can be reasonably studied and forecast in real time at least out to several tens of degrees away from the eruption site using the prediction tools employed here.

We present an analysis of {\it in situ} and remote-sensing measurements of a coronal mass ejection (CME) that erupted on 2021 February 20 and impacted both the Solar TErrestrial RElations Observatory (STEREO)-A and the {\it Wind} spacecraft, which were separated longitudinally by 55$^\circ$. Measurements on 2021 February 24 at both spacecraft are consistent with the passage of a magnetic ejecta (ME), making this one of the widest reported multi-spacecraft ME detections. The CME is associated with a low-inclined and wide filament eruption from the Sun's southern hemisphere, which propagates between STEREO-A and {\it Wind} around E34. At STEREO-A, the measurements indicate the passage of a moderately fast ($\sim 425$~km\,s$^{-1}$) shock-driving ME, occurring 2--3 days after the end of a high speed stream (HSS). At {\it Wind}, the measurements show a faster ($\sim 490$~km\,s$^{-1}$) and much shorter ME, not preceded by a shock nor a sheath, and occurring inside the back portion of the HSS. The ME orientation measured at both spacecraft is consistent with a passage close to the legs of a curved flux rope. The short duration of the ME observed at {\it Wind} and the difference in the suprathermal electron pitch-angle data between the two spacecraft are the only results that do not satisfy common expectations. We discuss the consequence of these measurements on our understanding of the CME shape and extent and the lack of clear signatures of the interaction between the CME and the HSS., Comment: accepted to ApJ

We present a new analytical approach with the aim to describe generally curved and twisted magnetic flux ropestructures, that are embedded within interplanetary coronal mass ejections, under the constraint of invariant axialflux. In this paper we showcase the simplest case of a generally curved flux rope with a circular cross-sectionwhich can be described in terms of the curvature and the torsion of the Frenet-Serret equations. The magneticfield configuration, for the axial and poloidal field components, are described in terms of a radial expansion usinga Legendre basis. We further derive equations that allow us to configure our model for any arbitrary magnetictwist and also evaluate the force distribution. We show the effects and differences of our proposed modelcompared to a purely cylindrical or toroidal geometry using an arbitrarily twisted exemplary flux rope structurewith an uniformly twisted magnetic field configuration. In order to indirectly compare our model with realin-situ measurements we generate two synthetic in-situ profiles using virtual spacecraft trajectories, realisticallysimulating apex and flank encounters of an interplanetary coronal mass ejection. This proposed model presentsan intermediate steps towards describing more complex flux rope structures with arbitrary cross-section shapes., Comment: 14 pages, 6 figures

We present fast algorithms to automatically estimate the statistical position of the bow shock from spacecraft data, using existing analytical two-dimensional (2D) and three-dimensional (3D) models of the shock surface. We derive expressions of the standoff distances in 2D and 3D and of the normal to the bow shock at any given point on it. Two simple bow shock detection algorithms are constructed, one solely based on a geometrical predictor from existing models, the other using this predicted position to further refine it with the help of magnetometer data, an instrument flown on many planetary missions. Both empirical techniques are applicable to any planetary environment with a defined shock structure. Applied to the Martian environment and the NASA/MAVEN mission, the predicted shock position is on average within 0.15 planetary radius Rp of the bow shock crossing. Using the predictor-corrector algorithm, this estimate is further refined to within a few minutes of the true crossing (≈0.05Rp). Between 2014 and 2021, we detect 14,929 clear bow shock crossings, predominantly quasi-perpendicular. Thanks to 2D conic and 3D quadratic fits, we investigate the variability of the shock surface with respect to Mars Years (MY), solar longitude (Ls), and solar EUV flux levels. Although asymmetry in Y and Z Mars Solar Orbital coordinates is on average small, we show that for MY32 and MY35, Ls = [135°−225°] and high solar flux, it can become particularly noticeable, and is superimposed to the usual North-South asymmetry due in part to the presence of crustal magnetic fields.

Observations of magnetic clouds, within interplanetary coronal mass ejections (ICMEs), are often well described by flux rope models. Most of these assume either a cylindrical or toroidal geometry. In some cases, these models are also capable of accounting for non-axisymmetric crosssections but they generally all assume axial invariance. It can be expected that any ICME, and its flux rope, will be deformed along its axis due to influences such as the solar wind. In this work, we aim to develop a writhed analytical magnetic flux rope model which would allow us to analytically describe a flux rope structure with varying curvature and torsion so that we are no longer constrained to a cylindrical or toroidal geometry. In this first iteration of our model we will solely focus on a circular cross-section of constant size. We describe our flux rope geometry in terms of a parametrized flux rope axis and a parallel transport frame. We derive expressions for the axial and poloidal magnetic field components under the assumption that the total axial magnetic flux is conserved. We find an entire class of possible solutions, which differ by the choice of integration constants, and present the results for a specific example. In general, we find that the twist of the magnetic field locally changes when the geometry deviates from a cylinder or torus. This new approach also allows us to generate completely new types of in situ magnetic field profiles which strongly deviate from those generated by cylindrical or toroidal models., Comment: 19 pages, 4 figures

Interplanetary coronal mass ejections (ICMEs) are one of the main drivers for space weather disturbances. In the past, different approaches have been used to automatically detect events in existing time series resulting from solar wind in situ observations. However, accurate and fast detection still remains a challenge when facing the large amount of data from different instruments. For the automatic detection of ICMEs we propose a pipeline using a method that has recently proven successful in medical image segmentation. Comparing it to an existing method, we find that while achieving similar results, our model outperforms the baseline regarding training time by a factor of approximately 20, thus making it more applicable for other datasets. The method has been tested on in situ data from the Wind spacecraft between 1997 and 2015 with a True Skill Statistic (TSS) of 0.64. Out of the 640 ICMEs, 466 were detected correctly by our algorithm, producing a total of 254 False Positives. Additionally, it produced reasonable results on datasets with fewer features and smaller training sets from Wind, STEREO-A and STEREO-B with True Skill Statistics of 0.56, 0.57 and 0.53, respectively. Our pipeline manages to find the start of an ICME with a mean absolute error (MAE) of around 2 hours and 56 minutes, and the end time with a MAE of 3 hours and 20 minutes. The relatively fast training allows straightforward tuning of hyperparameters and could therefore easily be used to detect other structures and phenomena in solar wind data, such as corotating interaction regions.