This paper addresses the need to investigate the fundamental solar and heliospheric magnetic structures known as flux ropes (FRs). FRs are commonly associated with coronal mass ejections (CMEs, Webb and Howard, 2012), streamer blow-outs (SBOs, Vourlidas and Webb, 2018; Nitta et al., 2021), density blobs generated due to magnetic reconnection at the tip of helmet streamers within the heliospheric plasma sheet (HPS, Lavraud et al., 2020; Réville et al., 2022), small structures called “plasmoids” or “blobs” observed in 2D by heliospheric imagers (e.g., Khabarova et al., 2021; Pezzi et al., 2021), solar flares (e.g., Kumar and Cho, 2013), small magnetic structures observed by in situ instrumentation (e.g., Moldwin et al., 2000; Cartwright and Moldwin, 2010a; Chen et al., 2020; Liu et al., 2021; Chen and Hu, 2022), as substructures of a larger structure (see for instance, Chen et al., 2023), and magnetospheric flux transfer events (FTEs, Russell and Elphic, 1978; Slavin et al., 2012; Murphy et al., 2020). FRs contribute greatly to the transport of energy, mass, and helicity from the Sun through the heliosphere and from the heliosphere to the planets’ local environments. They are characterized by an organized bundle of magnetic field lines, twisting around a common axis, confining plasma, and dragging away a large part of the Sun’s or a planet’s atmosphere (e.g., Linton and Moldwin, 2009). Considering the diversity of FRs described above, a question still remains: are all these structures alike in terms of morphology, magnetic and plasma properties, and dynamics?.

The FR concept was borrowed from the laboratory plasma physics experiments in the 1950–60s to confine and reach a stable plasma equilibrium to produce thermonuclear fusion power (e.g., Lundquist, 1950). Helical magnetic field structures were produced by induced toroidal current densities in laboratory devices, such as Tokamaks, to determine their stability. However, as the Heliophysics discipline has matured, the idealized FR concept (i.e., that of a circularly-symmetric, force-free, twisted flux tube) has become insufficient to accurately describe the structures, which are not always static or in equilibrium but ubiquitous in Heliophysics.

In this paper, we will discuss some of the issues that prevent us from advancing our understanding of the origin of these structures and the physical processes associated with their evolution. For example, the interpretation of remote-sensing and in situ observations often suggests complex distortions of FRs that are ambiguous and open to debate, and current models are not equipped to reproduce and simulate such complexities. In our opinion, the challenges that we present here range from data returned by space-based observatories to more theoretical approaches, but also encompass the development of more robust plasma physics laboratory experiments. On the basis of current challenges in FR research, we envision strategies and future venues to be addressed in the upcoming years.

Despite countless observations, both remote and in situ, that account for the existence of FRs, we have only a vague idea of their formation. Most models that are focused on CME eruption include a FR as an essential part of the process. However, there is a long-standing debate about whether these FRs exist in the corona before the eruption and later become unstable (ideal or magnetohydrodynamic instability, e.g., Török et al., 2004) or whether the FR forms as a consequence of the take-off of an unstable sheared arcade that triggers magnetic reconnection in its wake (resistive magnetohydrodynamic instability, e.g., Antiochos et al., 1999). The nature of the pre-eruptive configuration of solar eruptions has been extensively debated (see the reviews of Klimchuk, 2001; Forbes et al., 2006; Green et al., 2018; Patsourakos et al., 2020). Episodes of magnetic flux emergence can be regarded as the manifestation of twisted magnetic flux tubes rising through the solar surface, which result from the buoyant rise of magnetic plasma from the convection zone into the overlying atmosphere (e.g., Lites, 2009; Cheung and Isobe, 2014; Pontin and Priest, 2022). It is currently believed that the combination of photospheric plasma flows and magnetic reconnection above polarity inversion lines (see for instance, van Ballegooijen and Martens, 1989; Jiang et al., 2021) leading to FR formation, also during flux emergence, is the most common mechanism.

In light of observations of SBOs, it has been proposed that FRs can also be created later in the corona through reconnection processes (Lynch et al., 2016). The same mechanism seems to be responsible for the formation of small FRs or blobs and plasmoids (e.g., Sheeley et al., 2009; Sanchez-Diaz et al., 2017; Khabarova et al., 2021). Although there is supporting evidence in favor of each of the different aforementioned mechanisms, there are no conclusive findings, and this prevents us from fully understanding the formation mechanisms of different FRs.

The FRs originating further away from the Sun in the heliosphere mainly result from the solar wind’s evolution. This corresponds to magnetic reconnection in the heliospheric current sheet (HCS, e.g., Moldwin et al., 2000; Eastwood et al., 2002; Lavraud et al., 2020) and discontinuities produced by the action of turbulence in the solar wind (e.g., Zheng and Hu, 2018). Daughton et al. (2011) showed that for the most common type of reconnection layer with a finite guide field, the 3D evolution is dominated by the formation and interactions of FRs.

Several studies have correlated small FRs with interplanetary shock waves, particle energization, and stream interaction regions (SIRs, e.g., Feng et al., 2007; Cartwright and Moldwin, 2010b; Zank et al., 2014; le Roux et al., 2015). Thus, although the origin of large-scale FRs possesses well-defined observational signatures and unambiguously corresponds to CMEs and similar solar events, identification of the procedures involved in small-scale FR generation is still inconclusive.

In the ideal FR built in the laboratory, an axial current density induces the helical magnetic field topology. However, a non-idealized and more realistic heliospheric FR could be described by more complex internal current density distributions that, perhaps, impact the way the structure evolves. Therefore, does the formation mechanism determine the internal magnetic structure and impact the subsequent evolutionary processes?

In the heliosphere, FRs are not static. They may continuously evolve through expansion, rotation, deflection, erosion, and distortion (e.g., Manchester et al., 2017; Kilpua et al., 2019; Luhmann et al., 2020). The physical processes associated with these effects are clearly related to the interaction with the local environment, but disentangling them is not an easy task. Most of the processes are coupled; for instance, the erosion with the distortion (Good et al., 2019; Nieves-Chinchilla et al., 2022a; Rodríguez-García et al., 2022), the expansion with the deflection (Nieves-Chinchilla et al., 2012, 2013), and they result in local significant changes within the global structures (Owens, 2020). Studies on the early evolution of FRs originating from the Sun estimate that the expansion and acceleration are probably due to the Lorentz force (e.g., Vršnak, 2008; Kay and Nieves-Chinchilla, 2021), but the range of influence of the different forces are not yet well defined.

In the interplanetary medium, the evolution of FRs is mostly dominated by interactions with the ambient solar wind. The magnetohydrodynamic (MHD) and/or aerodynamic drag affects FR kinematics and overall dynamics. It is also believed that with increasing heliocentric distance (e.g., Leitner et al., 2007; Gulisano et al., 2012) the FR radial expansion weakens, leading to FR deformations such as the “pancaking effect” (e.g., Cargill et al., 1996; Owens et al., 2006; Savani et al., 2010; Davies et al., 2021). However, the question of whether the global structure of FRs can be distorted or not is still open in the Heliophysics community. The interpretation of the remote-sensing and in situ observations that suggest complex distortions are ambiguous and open to debate (Owens, 2020). It is also important to highlight the importance of varied solar wind background structures that can distort longitudinally the coherent flux rope and significantly affect its local parameters probed at different places. Also, the interaction between structures can temporally change, even relatively quickly the FR properties (Kilpua et al., 2019) but, there are just a few physics-driven FR models flexible enough to advance such investigations (Hidalgo, 2003; Hidalgo and Nieves-Chinchilla, 2012; Nieves-Chinchilla et al., 2022a; Vršnak et al., 2004; 2008; 2013; Weiss et al., 2022).

The deflection or rotation effects are related to the change of the global orientation of a FR in the heliosphere, but their physical cause may be completely different. (e.g., Vourlidas et al., 2011; Nieves-Chinchilla et al., 2012). While the deflection is mostly driven by the force imbalance with the solar wind (Wang et al., 2004; Kay et al., 2017; Sahade et al., 2020), the rotation appears to be an internal magnetic instability (see for instance, Lynch et al., 2009; Florido-Llinas et al., 2020). Currently, running MHD simulations can be computationally expensive in time and resources and prevent us from testing different assumptions and conditions.

Finally, the erosion effect might significantly contribute to CME evolution. This well-known observed effect at the front, and sometimes also at the back, of in situ observations of FRs is due to the magnetic reconnection of the FR magnetic field with the ambient interplanetary magnetic field. This may impact the FR’s magnetic flux, twist, helicity, and cross-sectional area by “peeling off” its outer layers (Ruffenach et al., 2012; Pal et al., 2020, 2021; Pezzi et al., 2021; Pal et al., 2022a; Pal, 2022; Rodríguez-García et al., 2022). Magnetic reconnection is also associated with the internal changes of the FR, e.g., impacting the complexity of the in situ magnetic field profiles and/or the FR boundary layers (see, e.g., Feng et al., 2011; Hwang et al., 2020).

In this section, we have focused on the open challenges of large-scale FRs in the heliosphere associated with CMEs. However, all of these challenges can be extrapolated to other FRs in the heliosphere such as small-scale FRs or FTEs, for instance. In any case, we lack of a current effort to understand the physical characteristics of the FR internal structures, the changes as they evolve in the heliosphere, and the way the innate FR features connect to the matured structure’s features. Above all, there is a need to investigate how the temporal and spatial evolution impacts the stability, equilibrium, morphology, and entity of FRs.

To study the FRs’ internal structure and evolution at any point of the heliosphere, it is customary to assemble observations from different assets in space, connect them with different models and data-analysis techniques, and elaborate on a scenario that reasonably describes their source region and the impact of the evolution on their structure. Figure 1 illustrates an exercise of connecting the remote and local in situ measurements of a FR at its source and in the inner heliosphere (see also Palmerio et al., 2018).

However, the unavailability of enough multi-point observations often misleads us in interpreting the global structure of FRs. We use different models and data-analysis techniques to bridge the gap resulting from the lack of observations with the caveat that these models can differ significantly from each other and can lead to different conclusions. For example, most models that use white-light observations (coronagraphs and heliospheric imagers) to study FR evolution fit static geometrical structures to match the morphology of a CME in simultaneous images (Thernisien et al., 2006; Rodríguez-García et al., 2022). These models do not include magnetic field information, and require multi-view points to (very often poorly) reproduce the 3D structure of the FR (see the discussion in Nieves-Chinchilla et al., 2022a). Furthermore, they do not provide thorough information about the evolutionary physical processes.

On the other hand, physical models that include magnetic field estimations (i.e., FR fitting models) are designed to match local in situ measurements and rely on, in the best scenarios, on single/few-point observations with relatively small spatial and varied temporal separations (e.g., Palmerio et al., 2021; Weiss et al., 2021; Pal et al., 2022b). Contemporary FR fitting approaches are not necessarily guaranteed to work well on larger scale separations (e.g., Weiss et al., 2021) as the simplifications in these models can break down. However, it is not well understood if by increasing the number of local FR measurement points, the FR reconstruction capabilities will improve unless the appropriate modeling techniques are developed in lockstep.

The aforementioned aspects prevent us from reaching a comprehensive understanding of FRs in the heliosphere. The ultimate challenge is to develop a model that is able to consistently respond to the wealth of observations and the evolution of these structures. From our perspective, to address this challenge, in addition to increasing space-based observations, the community should also make an effort to develop fundamental physics to explore the diversity of FRs in the heliosphere as well as to develop new techniques and approaches to further investigate their stability, dynamics, and interaction with the surrounding environment.

Here we summarize the challenges that result from the discussion in the previous sections and strategies to address those challenges. The goal of this perpective article is to raise awareness in the scientific community of the importance of magnetic FRs as a fundamental and ubiquitous magnetic structure in Heliophysics.