Within the interplanetary plasma, coronal mass ejections (CMEs) and stream interaction regions (SIRs) constitute very large and intensive transient structures. From their origins in the solar corona, CMEs and SIRs drive space weather on Earth, affecting society (Council, 2008; Eastwood et al., 2017; Oughton et al., 2017). CMEs are most impactful during solar maximum, while SIRs are notable in disrupting the Earth’s geomagnetic space during solar minimum (Zhang et al., 2007; Jian et al., 2015; Gopalswamy, 2016). Geomagnetic storms can bring numerous space weather hazards. Intense particle radiation can cause satellite malfunction and pose a health threat to astronauts. Ionospheric heating and expansion in turn increase drag on satellites, with debris degrading their orbits. Time-varying magnetic fields induce surges in electricity lines. Geomagnetically disturbed ionospheric scintillations can distort the amplitude and phase of traversing radio signals, causing a risk of navigation errors and signal loss. The Committee on the Societal and Economic Impacts of Severe Space Weather Events (2008) estimated a “severe geomagnetic storm scenario” that could incur 1 trillion in societal and economic damage (see references in Jensen et al., 2010 for specific examples).

Understanding and predicting the effects of CMEs and SIRs require sufficient observations of their plasma properties to inform predictive models, including various magnetohydrodynamic (MHD) models. To address these and related questions, the Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006 (Kaiser et al., 2008). To observe solar wind plasma prior to the Earth’s L1 Lagrangian position (Kaiser et al., 2008; Panchenko et al., 2014), each STEREO spacecraft on either side of the Earth–Sun line possesses a pair of coronagraphs, Cor1 and Cor2, and a pair of heliospheric imagers (HIs), HI1 and HI2; combined together, they observe solar wind plasma in Thomson scattered light continuously from the low corona to beyond 1 au. Analysis of these datasets provides dramatic observations of plasma density structures associated with CMEs and SIRs; however, STEREO is unable to remotely observe one of the most important features in transient structures, namely, the magnetic field. In contrast, Faraday rotation (FR) observations have detected clear magnetic structures in several CME events (Kooi et al., 2022b, Kooi et al., 2021; Jensen and Russell, 2008). Numerical simulations of FR measure have demonstrated that for idealized models, the magnetic field orientation and helicity of a flux rope can be determined 2–3 days before it reaches 1 au, providing a valuable space weather forecast (Liu et al., 2007). FR observations can also resolve the portion of CME flux ropes that curves back to the Sun (Liu et al., 2007).

In addition to large-scale structures, studies have shown that FR observations can provide critical information on plasma waves, which are significant to the flow of turbulent energy in interplanetary space (Jensen et al., 2013; Efimov et al., 2019; Kooi et al., 2022c). For example, MHD waves facilitate the transfer of energy between spatial and temporal scale sizes as shown in turbulence studies (Viall and Borovsky, 2020). The waves have a significant role in discontinuities and shocks (Kilpua et al., 2022); they can drive particle heating and acceleration (Suzuki and Inutsuka, 2005), and their presence constrains the characteristics of the plasma that can support them (Hollweg et al., 1982; 2010).

In order to fully exploit FR to probe the magnetic field in Earth-directed structures (e.g., CMEs/SIRs) and other disturbances (e.g., MHD waves), we propose the Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH) for measuring the inner heliospheric magnetic field (Gopalswamy et al., 2022). While photospheric and chromospheric magnetic fields can be readily measured using magnetographs, novel techniques are needed to measure the magnetic field with radio waves, especially at distances far from the Sun. FETCH is one of the proposed instruments on the Multiview Observatory for Solar Terrestrial Science (MOST) (Gopalswamy et al., 2022), for which the goal is to understand the magnetic coupling between the solar interior and the heliosphere and the origin and variability of the solar wind and transient structures. The MOST mission consists of a total of four spacecraft, two pairs of spacecraft distributed in the vicinity of each of the Sun–Earth Lagrange points L4 and L5 and close to the quadrature (see Figure 1). The FETCH-polarized radio transmitters and receivers will be on all four spacecraft to measure the integrated magnetic field along four lines of sight crossing the Sun–Earth line. The frequency of the radio signals is low enough to experience measurable changes in intensity, polarization angle, degree of linear polarization, and apparent Doppler frequency. Between four spacecraft, four propagation paths (aka lines-of-sight) are established for these crucial measurements (Figure 1).

FETCH is an active radio instrument (emitting radio waves for measuring rather than relying on astrophysical sources of radio waves such as pulsars), detecting the modifications of a transmitted signal caused by its traversal through a magnetized plasma. FETCH’s greatest technological challenge is achieving a sufficient signal-to-noise ratio, given the restrictions of power, antenna collecting areas, and distance. It will be the first instrument to pass very high frequency (VHF) radio signals between spacecraft, which are separated by 2 au at positions near L4 and L5. As we detail in the instrument concept later, the signal frequency, phase, polarization, amplitude, and spectral distribution will be combined to measure FR, total electron content (TEC), Faraday rotation fluctuations (FRF), and frequency fluctuations (FF) using established techniques (Efimov et al., 2000; Imamura et al., 2014; Wexler et al., 2017; Jensen et al., 2018; Wexler et al., 2021; Kooi et al., 2022a; Kooi et al., 2022b; Kooi et al., 2022c; Kooi et al., 2022d; Kooi et al., 2022e; Wood et al., 2022).

FR analysis of linearly polarized signals provides information on the LOS-integrated product of electron number density (n _e ) and LOS-aligned component of the magnetic field ( B ⃗ ⋅ d s ⃗ ) . The magnetic field is given by B, and the LOS path is given as s. The integration begins at the transmitter s = 0, and it completes at the receiver s = S. Given the change in the plane of the polarization position angle, FR is expressed as F R = A f o 2 ∫ 0 S d s ⃗ ⋅ n e B ⃗ R M = F R λ 2 , (1)

with A = 2.36 × 10⁴ in mks units rad m ² T ⁻¹ s ⁻² and the frequency of the signal f _o in Hz. A convention that we commonly use to express FR is to normalize it by wavelength λ to calculate the rotation measure (RM).

The total electron content is the measure of the column density of plasma along the LOS. The TEC is critical to deconvolving the average LOS magnetic field strength from the RM, and with the use of tomography, TEC can provide the 3-D plasma density. The TEC can be obtained from both the radio signal’s group and phase velocities. The group velocity translates to an arrival time delay, famously measured with the differenced range versus integrated Doppler (DRVID) (Woo et al., 1976). In contrast, the phase velocity gives the change in TEC and is measured from an apparent Doppler shift in the FF of the narrowband signal after the Doppler shift has been removed (Jensen et al., 2016) [see Kooi et al. (2022a) for more information].

CMEs produce the largest scale and most intense disturbances in the heliosphere (Chen, 2011; Temmer, 2021). As they approach Earth, the magnetic ejecta may extend over 0.25 au, and the CME-driven shock may extend twice as far (Jian et al., 2006; Kilpua et al., 2017a; Manchester et al., 2017; Gopalswamy et al., 2018; Jian et al., 2018). At 1 au, CME velocities can exceed an impressive 2,000 km/s, CME magnetic fields can go over 100 nT, and the plasma densities have been recorded above 50 protons cm⁻³, that is ≈5–10 times larger than the ambient solar wind density values (Gopalswamy et al., 2009; Kilpua et al., 2017b). Previously observed flux ropes in FR had a simple structure within the lower corona (Jensen and Russell, 2008; Jensen et al., 2018). As CMEs expand into the solar wind, their structure becomes increasingly complex, and FETCH enables studying the structures of CMEs when this occurs. Measuring the plasma structure of CMEs and their evolution through space is crucial to predicting space weather as described using MHD models. As discussed earlier, space weather prediction is critical to protecting infrastructure, making this effort a significant safety issue. As we will discuss later, magnetic field measurements with FETCH are enhanced when a structure/disturbance is coherent and comparable in size to the line-of-sight; portions of it have magnetic fields oriented anti-/parallel to the line-of-sight, and it extends radially enough to characterize unique changes as it flows outward.

SIRs lasting more than one Carrington rotation are known as corotating interaction regions (CIRs). For the remainder of the paper, we refer more generally to SIRs. The geoeffectiveness of SIRs is variable; for example, they can cause longer geomagnetic storms than CME-driven events (Gonzalez et al., 1994; Chen et al., 2014), and their inclination could impact substorm intensity (Oliveira and Raeder, 2014). In situ SIR properties are sometimes observed to vary significantly between different spacecraft locations that are near the same heliocentric distance but at different helio-longitudes and helio-latitudes. For instance, while both STEREO A and B were located near 1 au from the Sun (with A and B spacecraft leading and following the Earth, respectively), multiple SIRs observed by the two spacecraft varied in profiles of velocity, magnetic field strength, density, and other parameters (Jian et al., 2019). The fast solar wind speed for the STEREO spacecraft are separated by tens of degrees in orbit that can differ from L1 spacecraft by up to 100 km/s, maximum magnetic field by 8 nT, and the peak proton density by 30 cm⁻³. Observation of such variability over large spatial scales would require either a large number of spacecraft for in situ measurements or the development of a remote-sensing technique as in FETCH.

MHD waves, such as Alfven and magnetosonic waves, have a significant role in the transfer of energy in the plasma of interplanetary space. Generated and modified by shocks and discontinuities, MHD waves provide important information on the plasma properties through which they propagate. Through both the magnetic field and density fluctuations, they are found in FR measurements where the FR plane of polarization angle fluctuates on a wide range of temporal and spatial scales. FR, the change in the plane of polarization along a signal’s propagation path, is due to the combined contribution of the electron density and magnetic field in the plasma through which the signal is passing. Distinguishing the cause of the FRF between the magnetic field and N-wave sources is benefited by comparison with fluctuations in the TEC. For example, frequently, the power in these FRFs is greater than the TEC fluctuations due to their MHD wave source, particularly if the waves are non-compressive (Hollweg et al., 1982 using Helios spacecraft).

The practice of detecting and analyzing MHD plasma waves with FRFs in interplanetary space is limited by the paucity of high time-resolution observations (e.g., MESSENGER spacecraft Jensen et al., 2013 and radio galaxies and quasars Sakurai and Spangler, 1994). With the available data, spectral analyses of FRFs have shown that the greatest amount of wave power is in the lowest frequencies with quasi-periodic fluctuations between 4 and 10 min: Efimov et al. (1993) and Chashei et al. (1999) both use Helios spacecraft and Wexler et al. (2017) uses the MESSENGER spacecraft. It is worth briefly summarizing here examples of the work that FETCH would be capable of with sufficient observation. With the Helios data (f = 2.3 GHz, λ = 13 cm), tantalizing details on mass and energy fluxes in the corona have emerged. Efimov et al. (2019) found little variation in turbulence with the solar cycle, but this contradicts the finding with MESSENGER (Jensen et al., 2013). Efimov et al. (2015) determined that the observed velocities were outward, the sum of the solar wind speed and Alfven velocity. Kooi et al. (2014) found possible evidence of Joule/wave heating of the coronal plasma. Various theoretical calculations suggest the benefits of these observations. For example, Hollweg et al. (2013) found reverse-flow Alfven waves to be weak, whereas oblique Alfven waves could drive MHD turbulence.

The pathway to achieving the objectives of the FETCH instrument can be described by a Science Traceability Matrix (STM). In this section, we explain the details of the technical requirements and project performance columns of the STM (Table 1). To understand the technical requirements, we need to establish the essential characteristics of two plasma observations collected with radio receivers, FR, and TEC. We then explain how interplanetary plasma structures of interest reveal themselves in these observables (STM-CME, STM-SIR, and STM-MHD).

STM-CME: determining the minimum performance parameters for the FETCH instrument, ±8° in Table 1 for the plane of polarization resolution used three uniquely different sources. The first is an MHD model of the 2005 May 13 event (discussed imminently; Manchester et al., 2014). The second is the work by Howard (2011) looking at “typical” CME properties extrapolated from 1 au, which showed that a large ( > 10°) FR response is not unusual. The third is the work with the largest error bars from successful FR experimental data analysis (Jensen, 2007). Examining the FETCH instrument’s performance with the 2005 May 13 CME model, Figure 2 shows that the CME passage could extend for well over 10 h, so observations need to be regularly obtained in order to observe the complete CME structure. This time duration is an issue with ground observations; only the Deep Space Network is capable of maintaining an observation this long. Assuming that simultaneous transmission/reception has an insufficient radio signal-to-noise ratio (worst case scenario), the observing sequence will consist of alternative transmitting and receiving at intervals of 16 min (the length of time for a signal-to-travel 2 au), enabling 32 min of observing every hour among the four spacecraft. The best case scenario is the capability to simultaneously transmit/receive for continuously observing.

STM-SIR: the expected FETCH results from probing stream interaction regions can be estimated by extracting the column density and LOS-FR time series from global heliospheric models. Figure 3 illustrates the L4–L5 LOS placement transverse to an SIR during the Carrington rotation 2109, using the AWSoM model as described by van der Holst et al. (2014). When the LOS is transverse to the SIR, the TEC is low, but TEC reaches a maximum as the SIR rotates across the LOS to approximately tangential orientation within the core of the SIR. From such a time series of TEC results, it is possible to estimate the density compression factor within the SIR. The FR curves show an asymmetrical temporal pattern with an initial steep change in FR accompanied by polarity reversal [see Borovsky (2020)], followed by a more gradual trail-off in the new polarity. These preliminary results suggest that FETCH will be valuable for evaluating inner heliosphere SIRs ahead of their arrival in the local geospace.

STM-MHD: recall that the application of theory for analyzing the plasma waves detected is limited by the paucity of high time-resolution observations (e.g., MESSENGER spacecraft Jensen et al., 2013 and radio galaxies and quasars Sakurai and Spangler, 1994). However, spectral analyses of FRF have shown that the greatest amount of power spectral density is in periods between 4 and 10 min (Efimov et al., 1993; Chashei et al., 1999; both using Helios spacecraft). In analyzing Cassini FR data, Jensen and Russell (2007) found an integration limit of 100 s while minimizing error bars in the plane of polarization. Longer integrations lost coherence in the phase difference between the right- and left-handed polarizations; this is consistent with FRF changing the plane of polarization coherently (not randomly) with periods greater than 200 s. Investigating MHD waves with FR should be undertaken with 30 min of continuous observing to determine the maximum (4–10 min) wave power.