arXiv subjects

E. K. J. Kilpua

Publications and source records attributed to E. K. J. Kilpua.

At least 19 recordsLinked to original sources

arXiv2026

Radial and angular evolution of magnetic cloud signatures in the turbulent solar wind: virtual spacecraft analysis

Interplanetary coronal mass ejections (ICMEs) carry magnetic clouds (MCs), large-scale structures with average radial widths about a fifth of an astronomical unit at Earth's orbit. ICMEs display substructures in white light images and reveal rich dynamics across many spatial scales when directly measured by spacecraft. A spacecraft encounter with an ICME can result in smoothly rotating MC intervals or less organised magnetic obstacle (MO) ones. We investigate how the interplay of expansion, turbulence, and internal cloud dynamics affects magnetic cloud properties, which are reflected in the plasma signatures measured by spacecraft. We perform high-resolution 2.5D MHD simulations of a magnetic flux rope cross-section, which is embedded in the turbulent, expanding solar wind with the expanding box model. We probe the local plasma properties, and thus the flux rope signatures and angular coherence, with virtual spacecraft. Our simulations reproduce clear and stable MC signatures when the flux rope core is intercepted by virtual spacecraft. Disordered MO signatures appear at the edges of the flux rope, and are attributed to both expansion and turbulent transport. We vary some key physical parameters of the flux rope and the environment to understand their effect on the observed coherence and signatures. The pace of the expanding flow controls the angular extent of MC signatures, whereas the intensity of interplanetary turbulence controls how asymmetric and distorted the flux rope appears at 1 AU. The geometry of spacecraft encounters determines whether MC or MO signatures are observed. The presence of a magnetic structure which can result in MO signatures is strongly controlled by the flux rope's initial/early magnetic configuration: MO signatures can only be observed when the axial flux rope field is spatially not well confined by the rope's own magnetic tension, and disappear otherwise.

astro-ph.SR↗

arXiv2026

The Correlation Length of Turbulence in Magnetic Clouds

The large-scale limit or outer scale of turbulence in the solar wind is associated with the correlation length of the magnetic field. Determining correlation lengths from magnetic field time series in magnetic clouds is complicated by the presence of the global flux rope: without removal of the flux rope trend, correlation length measurements will be sensitive to the flux rope as well as the turbulence, and give overestimates of the outer scale when turbulence amplitudes at the outer scale are small relative to the flux rope amplitude. We have used force-free flux rope fits to detrend magnetic field time series measured by Parker Solar Probe in two magnetic clouds and calculated the turbulence correlation length in the clouds using the detrended data. The detrended correlation length in terms of the proton inertial length, $d_p$, was $2.7\times10^{4} d_p$ in one cloud (observed at 0.77 au) and $1.6\times10^{4}d_p$ in the other (observed at 0.39 au), significantly smaller than the values obtained without detrending. Increments in the flux rope fits scaled equivalently to a $k^{-3}$ wavenumber power spectrum; this contribution from the flux rope considerably steepened the total spectrum at the largest scales but had a negligible effect in the inertial range, where scaling in both clouds equivalent to $\sim$$k^{-5/3}$ was observed. Finally, we discuss the possible relation of turbulence correlation lengths to mesoscale structure in magnetic clouds.

astro-ph.SR↗

arXiv2025

Residual energy of magnetohydrodynamic shocks

Residual energy quantifies the difference in energy between velocity and magnetic field fluctuations in a plasma. Recent observational evidence highlights that fast-mode interplanetary shock waves have positive residual energy, in sharp contrast to the negative residual energy of the turbulence and magnetic structures that constitute the vast majority of fluctuation power in the solar wind at magnetohydrodynamic (MHD) inertial scales. In this work, we apply the Rankine-Hugoniot conditions to derive an equation for the residual energy of an MHD shock jump as a function of the shock angle, density compression ratio and Alfvén Mach number upstream of the shock. An equation for the cross helicity is similarly derived. The residual energy equation gives only positive values for super-Alfvénic (i.e. fast-mode) shocks. The residual energy and cross helicity of slow-mode shocks and tangential, contact and rotational discontinuities are also determined. A simplified form of the residual energy equation applicable to perpendicular shocks has been verified against residual energy values directly estimated from observations of 141 interplanetary shocks; the equation is found to match well with observations, particularly for shocks with higher density compression ratios and Mach numbers. The use of positive residual energy as a signature for fast-mode shock identification in spacecraft data is briefly considered, and insights from this work relating to compressive fluctuations more generally in the solar wind are discussed.

astro-ph.SR↗

arXiv2025

On the source sizes of type II radio bursts with LOFAR

Solar radio bursts can provide important insights into the underlying physical mechanisms that drive the small and large-scale eruptions on the Sun. Since metric radio observations can give us direct observational access to the inner and middle corona, they are often used as an important tool to monitor and understand the coronal dynamics. While the sizes of the radio sources that can be observed in the solar corona is essential for understanding the nature of density turbulence within the solar corona and its subsequent influence on the angular broadening observed in radio source measurements, the smallest radio sources associated with solar radio bursts have so far been limited by observational techniques and the radio instrument's baselines. We selected three type II bursts that were observed with the LOFAR core and remote stations in the Solar Cycle 24. We estimated the sizes and shapes (ellipticity) of the radio sources between $20-200$ MHz using a two-dimensional Gaussian approximation. Our analysis shows that the smallest radio source size for type II bursts in the solar corona which can be observed in the solar atmosphere at low frequencies is $1.5^\prime \pm 0.5^\prime$ at 150 MHz. However, even though the observations were taken with remote baselines (with a maximum distance of $\sim 85~km$), the effective baselines were much shorter ($\sim 35~km$) likely due to snapshot imaging of the Sun. Our results show that the radio source sizes are less affected by scattering than suggested in previous studies. Our measurements indicate a smaller source sizes at frequencies below 95 MHz compared to previous reports, though some overlap exists with measurements at higher frequencies, using smaller baselines.

astro-ph.SR↗

arXiv2024

Determining the acceleration regions of in situ electrons using remote radio and X-ray observations

Solar energetic particles in the heliosphere are produced by flaring processes on the Sun or shocks driven by coronal mass ejections. These particles are regularly detected remotely as electromagnetic radiation (X-rays or radio emission), which they generate through various processes, or in situ by spacecraft monitoring the Sun and the heliosphere. We aim to combine remote-sensing and in situ observations of energetic electrons to determine the origin and acceleration mechanism of these particles. Here, we investigate the acceleration location, escape, and propagation directions of electron beams producing radio bursts observed with the Low Frequency Array (LOFAR), hard X-ray (HXR) emission and, in situ electrons observed at Solar Orbiter (SolO) on 3 October 2023. These observations are combined with a three-dimensional (3D) representation of the electron acceleration locations and results from a magneto-hydrodynamic (MHD) model of the solar corona in order to investigate the origin and connectivity of electrons observed remotely at the Sun to in situ electrons. We observed a type II radio burst with good connectivity to SolO, where a significant electron event was detected. However, type III radio bursts and Hard X-rays were also observed co-temporally with the elctron event but likely connected to SolO by different far-sided field lines. The injection times of the SolO electrons are simultaneous with both the onset of the type II radio burst, the group of type III bursts and the presence of a second HXR peak, however, the most direct connection to SolO is that of the type II burst location. The in situ electron spectra point to shock acceleration of electrons with a short-term connection to the source region.

astro-ph.SR↗

arXiv2024

An Overview of Solar Orbiter Observations of Interplanetary Shocks in Solar Cycle 25

Interplanetary shocks are fundamental constituents of the heliosphere, where they form as a result of solar activity. We use previously unavailable measurements of interplanetary shocks in the inner heliosphere provided by Solar Orbiter, and present a survey of the first 100 shocks observed in situ at different heliocentric distances during the rising phase of solar cycle 25. The fundamental shock parameters (shock normals, shock normal angles, shock speeds, compression ratios, Mach numbers) have been estimated and studied as a function of heliocentric distance, revealing a rich scenario of configurations. Comparison with large surveys of shocks at 1~au show that shocks in the quasi-parallel regime and with high speed are more commonly observed in the inner heliosphere. The wave environment of the shocks has also been addressed, with about 50\% of the events exhibiting clear shock-induced upstream fluctuations. We characterize energetic particle responses to the passage of IP shocks at different energies, often revealing complex features arising from the interaction between IP shocks and pre-existing fluctuations, including solar wind structures being processed upon shock crossing. Finally, we give details and guidance on the access use of the present survey, available on the EU-project ``solar energetic particle analysis platform for the inner heliosphere'' (SERPENTINE) website. The algorithm used to identify shocks in large datasets, now publicly available, is also described.

astro-ph.SR↗

arXiv2024

Direct Measurements of Synchrotron-Emitting Electrons at Near-Sun Shocks

In this study, we present the first-ever direct measurements of synchrotron-emitting heliospheric traveling shocks, intercepted by the Parker Solar Probe (PSP) during its close encounters. Given that much of our understanding of powerful astrophysical shocks is derived from synchrotron radiation, these observations by PSP provide an unprecedented opportunity to explore how shocks accelerate relativistic electrons and the conditions under which they emit radiation. The probe's unparalleled capabilities to measure both electromagnetic fields and energetic particles with high precision in the near-Sun environment has allowed us to directly correlate the distribution of relativistic electrons with the resulting photon emissions. Our findings reveal that strong quasi-parallel shocks emit radiation at significantly higher intensities than quasi-perpendicular shocks due to the efficient acceleration of ultra-relativistic electrons. These experimental results are consistent with theory and recent observations of supernova remnant shocks and advance our understanding of shock physics across diverse space environments.

physics.space-ph↗

arXiv2024

Observation of a Fully-formed Forward--Reverse Shock Pair Due to the Interaction Between Two Coronal Mass Ejections at 0.5 au

We report direct observations of a fast magnetosonic forward--reverse shock pair observed by Solar Orbiter on March 8, 2022 at the short heliocentric distance of 0.5 au. The structure, sharing some features with fully-formed stream interaction regions (SIRs), is due to the interaction between two successive coronal mass ejections (CMEs), never previously observed to give rise to a forward--reverse shock pair. The scenario is supported by remote observations from the STEREO-A coronographs, where two candidate eruptions compatible with the in-situ signatures have been found. In the interaction region, we find enhanced energetic particle activity, strong non-radial flow deflections and evidence of magnetic reconnection. At 1~au, well radially-aligned \textit{Wind} observations reveal a complex event, with characteristic observational signatures of both SIR and CME--CME interaction, thus demonstrating the importance of investigating the complex dynamics governing solar eruptive phenomena.

astro-ph.SR↗

arXiv2024

Estimating the lateral speed of a fast shock driven by a coronal mass ejection at the location of solar radio emissions

Fast coronal mass ejections (CMEs) can drive shock waves capable of accelerating electrons to high energies. These shock-accelerated electrons act as sources of electromagnetic radiation, often in the form of solar radio bursts. Recent findings suggest that radio imaging of solar radio bursts can provide a means to estimate the lateral expansion of CMEs and associated shocks in the low corona. Our aim is to estimate the expansion speed of a CME-driven shock at the locations of radio emission using 3D reconstructions of the shock wave from multiple viewpoints. We estimated the 3D location of radio emission using radio imaging from the Nançay Radioheliograph and the 3D location of the shock. The 3D shock was reconstructed using white-light and extreme ultraviolet images of the CME from the Solar Terrestrial Relations Observatory, Solar Dynamics Observatory, and the Solar and Heliospheric Observatory. The lateral expansion speed of the CME-driven shock at the electron acceleration locations was then estimated using the approximate 3D locations of the radio emission on the surface of the shock. The radio bursts associated with the CME were found to reside at the flank of the expanding CME-driven shock. We identified two prominent radio sources at two different locations and found that the lateral speed of the shock was in the range of $800-1000\,\mathrm{km\,s^{-1}}$ at these locations. Such a high speed during the early stages of the eruption already indicates the presence of a fast shock in the low corona. We also found a larger ratio between the radial and lateral expansion speed compared to values obtained higher up in the corona. The high shock speed obtained is indicative of a fast acceleration during the initial stage of the eruption. This acceleration is most likely one of the key parameters contributing to the presence of metric radio emissions, such as type II radio bursts.

astro-ph.SR↗

arXiv2023

Connecting remote and in situ observations of shock-accelerated electrons associated with a coronal mass ejection

One of the most prominent sources for energetic particles in our solar system are huge eruptions of magnetised plasma from the Sun called coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism, for example, type II and accompanying herringbone bursts. Here, we investigate the acceleration location, escape, and propagation directions of various electron beams in the solar corona and compare them to the arrival of electrons at spacecraft. To track energetic electron beams, we use a synthesis of remote and direct observations combined with coronal modelling. Remote observations include ground-based radio observations from the Nancay Radioheliograph (NRH) combined with space-based extreme-ultraviolet and white-light observations from the Solar Dynamics Observatory (SDO), the Solar Terrestrial Relations Observatory (STEREO) and Solar Orbiter (SolO). We also use direct observations of energetic electrons from the STEREO and Wind spacecraft. These observations are then combined with a three-dimensional (3D) representation of the electron acceleration locations that combined with results from magneto-hydrodynamic models of the solar corona is used to investigate the origin and link of electrons observed remotely at the Sun to in situ electrons. We observed a type II radio burst followed by herringbone bursts that show single-frequency movement through time in NRH images. The movement of the type II burst and herringbone radio sources seems to be influenced by the regions in the corona where the CME is more capable of driving a shock. We also found similar inferred injection times of near-relativistic electrons at spacecraft to the emission time of the type II and herringbone bursts.

astro-ph.SR↗

arXiv2023

Turbulence Properties of Interplanetary Coronal Mass Ejections in the Inner Heliosphere: Dependence on Proton Beta and Flux Rope Structure

Interplanetary coronal mass ejections (ICMEs) have low proton beta across a broad range of heliocentric distances and a magnetic flux rope structure at large scales, making them a unique environment for studying solar wind fluctuations. Power spectra of magnetic field fluctuations in 28 ICMEs observed between 0.25 and 0.95 au by Solar Orbiter and Parker Solar Probe have been examined. At large scales, the spectra were dominated by power contained in the flux ropes. Subtraction of the background flux rope fields reduced the mean spectral index from $-5/3$ to $-3/2$ at $kd_i \leq 10^{-3}$. Rope subtraction also revealed shorter correlation lengths in the magnetic field. The spectral index was typically near $-5/3$ in the inertial range at all radial distances regardless of rope subtraction, and steepened to values consistently below $-3$ with transition to kinetic scales. The high-frequency break point terminating the inertial range evolved approximately linearly with radial distance and was closer in scale to the proton inertial length than the proton gyroscale, as expected for plasma at low proton beta. Magnetic compressibility at inertial scales did not show any significant correlation with radial distance, in contrast to the solar wind generally. In ICMEs, the distinctive spectral properties at injection scales appear mostly determined by the global flux rope structure while transition-kinetic properties are more influenced by the low proton beta; the intervening inertial range appears independent of both ICME features, indicative of a system-independent scaling of the turbulence.

astro-ph.SR↗

arXiv2023

Type II radio bursts and their association with coronal mass ejections in solar cycles 23 and 24

Metre wavelength type II solar radio bursts are believed to be the signatures of shock-accelerated electrons in the corona. Studying these bursts can give information about the initial kinematics, dynamics and energetics of CMEs in the absence of white-light observations. In this study, we investigate the occurrence of type II bursts in solar cycles 23 and 24 and their association with coronal mass ejections (CMEs). We also explore the possibility of occurrence of type II bursts in the absence of a CME. We performed statistical analysis of type II bursts that occurred between 200 - 25 MHz in solar cycle 23 and 24 and found the temporal association of these radio bursts with CMEs. We categorised the CMEs based on their linear speed and angular width, and studied the distribution of type II bursts with `fast' ($speed ~\geq 500 km/s$), `slow' ($speed ~< 500 km/s$), `wide' ($width ~\geq 60^o$) and `narrow' ($width ~< 60^o$) CMEs. We explored the type II bursts occurrence dependency with solar cycle phases. Our results suggest that type II bursts dominate at heights $\approx 1.7 - 2.3 \pm 0.3 ~R_{\odot}$ with a clear majority having an onset height around 1.7 $\pm 0.3~R_{\odot}$ assuming the four-fold Newkirk model. The results indicate that most of the type II bursts had a white-light CME counterpart, however there were a few type II which did not have a clear CME association. There were more CMEs in cycle 24 than cycle 24. However, the number of type II radio bursts were less in cycle 24 compared to cycle 23. The onset heights of type IIs and their association with wide CMEs reported in this study indicate that the early CME lateral expansion may play a key role in the generation of these radio bursts.

astro-ph.SR↗

arXiv2023

Effects of optimisation parameters on data-driven magnetofrictional modelling of active regions

Data-driven time-dependent magnetofrictional modelling (TMFM) of active region magnetic fields has been proven to be a useful tool to study the corona. The input to the model is the photospheric electric field that is inverted from a time series of the photospheric magnetic field. Constraining the complete electric field, that is, including the non-inductive component, is critical for capturing the eruption dynamics. We present a detailed study of the effects of optimisation of the non-inductive electric field on the TMFM of AR12473. We aim to study the effects of varying the non-inductive electric field on the data-driven coronal simulations, for two alternative parametrisations. By varying parameters controlling the strength of the non-inductive electric field, we wish to explore the changes in flux rope formation and their early evolution and other parameters, for instance, axial flux and magnetic field magnitude.The non-inductive electric field component in the photosphere is critical for energising and introducing twist to the coronal magnetic field, thereby allowing unstable configurations to be formed. We estimated this component using an approach based on optimising the injection of magnetic energy. However, the flux rope formation, evolution and eruption time varies depending on the values of the optimisation parameters. The flux rope is formed and has overall similar evolution and properties with a large range of non-inductive electric fields needed to determine the non-inductive electric field component that is critical for energising and introducing twist to the coronal magnetic field. This study shows that irrespective of non-inductive electric field values, flux ropes are formed and erupted, which indicates that data-driven TMFM can be used to estimate flux rope properties early in their evolution without employing a lengthy optimisation process.

astro-ph.SR↗

arXiv2023

A type II solar radio burst without a coronal mass ejection

The Sun produces the most powerful explosions in the solar system, solar flares, that can also be accompanied by large eruptions of magnetised plasma, coronal mass ejections (CMEs). These processes can accelerate electron beams up to relativistic energies through magnetic reconnection processes during solar flares and CME-driven shocks. Energetic electron beams can in turn generate radio bursts through the plasma emission mechanism. CME shocks, in particular, are usually associated with type II solar radio bursts. However, on a few occasions, type II bursts have been reported to occur either in the absence of CMEs or shown to be more likely related with the flaring process. It is currently an open question how a shock generating type II bursts forms without the occurrence of a CME eruption. Here, we aim to determine the physical mechanism responsible for a type II burst which occurs in the absence a CME. By using radio imaging from the Nan{\c c}ay Radioheliograph, combined with observations from the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory spacecraft, we investigate the origin of a type II radio burst that appears to have no temporal association with a white-light CME. We identify a typical type II radio burst with band-split structure that is associated with a C-class solar flare. The type II burst source is located above the flaring active region and ahead of disturbed coronal loops observed in extreme ultraviolet images. The type II is also preceded by type III radio bursts, some of which are in fact J-bursts indicating that accelerated electron beams do not all escape along open field lines. The type II sources show single-frequency movement towards the flaring active region. The type II is located ahead of a faint extreme-ultraviolet (EUV) front propagating through the corona.

astro-ph.SR↗

arXiv2023

The 17 April 2021 widespread solar energetic particle event

Context. A solar eruption on 17 April 2021 produced a widespread Solar Energetic Particle (SEP) event that was observed by five longitudinally well-separated observers in the inner heliosphere at heliocentric distances of 0.42 to 1 au: BepiColombo, Parker Solar Probe, Solar Orbiter, STEREO A, and near-Earth spacecraft. The event produced relativistic electrons and protons. It was associated with a long-lasting solar hard X-ray flare and a medium fast Coronal Mass Ejection (CME) with a speed of 880 km/s driving a shock, an EUV wave as well as long-lasting radio burst activity showing four distinct type III burst. Methods. A multi-spacecraft analysis of remote-sensing and in-situ observations is applied to attribute the SEP observations at the different locations to the various potential source regions at the Sun. An ENLIL simulation is used to characterize the interplanetary state and its role for the energetic particle transport. The magnetic connection between each spacecraft and the Sun is determined. Based on a reconstruction of the coronal shock front we determine the times when the shock establishes magnetic connections with the different observers. Radio observations are used to characterize the directivity of the four main injection episodes, which are then employed in a 2D SEP transport simulation. Results. Timing analysis of the inferred SEP solar injection suggests different source processes being important for the electron and the proton event. Comparison among the characteristics and timing of the potential particle sources, such as the CME-driven shock or the flare, suggests a stronger shock contribution for the proton event and a more likely flare-related source of the electron event. Conclusions. We find that in this event an important ingredient for the wide SEP spread was the wide longitudinal range of about 110 degrees covered by distinct SEP injections.

astro-ph.SR↗

arXiv2022

Shock-accelerated electrons during the fast expansion of a coronal mass ejection

Context. Some of of the most prominent sources for energetic particles in our Solar System are huge eruptions of magnetised plasma from the Sun called coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism. The main types of bursts associated with CME shocks are type II and herringbone bursts. However, it is currently unknown where early accelerated electrons that produce metric type II bursts and herringbones propagate and when they escape the solar atmosphere. Aims. Here, we investigate the acceleration location, escape, and propagation directions of electron beams during the early evolution of a strongly expanding CME-driven shock wave associated with herrinbgone bursts. Methods. We used ground-based radio observations from the Nançay Radioheliograph combined with space-based extreme-ultraviolet and white-light observations from the Solar Dynamics Observatory and and the Solar Terrestrial Relations Observatory. We produced a three-dimensional (3D) representation of the electron acceleration locations which, combined with results from magneto-hydrodynamic (MHD) models of the solar corona, was used to investigate the origin of the herringbone bursts observed. Results. Multiple herringbone bursts are found close to the CME flank in plane-of-sky images. Some of these herringbone bursts have unusual inverted J shapes and opposite drifting herringbones also show opposite senses of circular polarisation. By using a 3D approach combined with the radio properties of the observed bursts, we find evidence that the first radio emission in the CME eruption most likely originates from electrons that initially propagate in regions of low Alfvén speeds and along closed magnetic field lines forming a coronal streamer.

astro-ph.SR↗

arXiv2022

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.

physics.space-ph↗

arXiv2022

Cross helicity of interplanetary coronal mass ejections at 1 au

Interplanetary coronal mass ejections (ICMEs) contain magnetic field and velocity fluctuations across a wide range of scales. These fluctuations may be interpreted as Alfvénic wave packets propagating parallel or anti-parallel to the background magnetic field, with the difference in power between counter-propagating fluxes quantified by the cross helicity. We have determined the cross helicity of inertial range fluctuations at $10^{-3}-10^{-2}$ Hz in 226 ICME flux ropes and 176 ICME sheaths observed by the Wind spacecraft at 1 au during 1995-2015. The flux ropes and sheaths had mean, normalised cross helicities of 0.18 and 0.24, respectively, with positive values here indicating net anti-sunward fluxes. While still tipped towards the anti-sunward direction on average, fluxes in ICMEs tend to be more balanced than in the solar wind at 1 au, where the mean cross helicity is larger. Superposed epoch profiles show cross helicity falling sharply in the sheath and reaching a minimum inside the flux rope near the leading edge. More imbalanced, solar wind-like cross helicity was found towards the trailing edge and laterally further from the rope axis. The dependence of cross helicity on flux rope orientation and the presence of an upstream shock are considered. Potential origins of the low cross helicity in ICMEs at 1 au include balanced driving of the closed-loop flux rope at the Sun and ICME-solar wind interactions in interplanetary space. We propose that low cross helicity of fluctuations is added to the standard list of ICME signatures.

physics.space-ph↗