Four Traveling Foreshocks Demonstrate Novel Transient Phenomena at Edges

Scientists are increasingly focused on understanding the complex boundaries surrounding travelling foreshocks (TFs), regions forming upstream of planetary bow shocks. Primoz Kajdic, Xóchitl Blanco-Cano (Departamento de Ciencias Espaciales, Instituto de Geofísica, Universidad Nacional Autónoma de México), and Diana Rojas-Castllo, alongside Nojan Omidi et al., detail previously unobserved transient phenomena at the edges of four TFs, challenging the established view of boundaries being solely foreshock compressional boundaries or bubbles. Their analysis of data from the Cluster and MMS spacecraft reveals structures resembling hot flow anomalies , but crucially, lacking the expected solar wind beam heating, and instead dominated by suprathermal particles. This discovery suggests a link between these ‘HFA-like FCBs’ and the structure of magnetic field discontinuities, potentially reshaping our understanding of plasma behaviour and the combined impact of multiple upstream structures on the space environment downstream of the bow shock.

Detailed analysis demonstrates that instead of beam heating, the beam almost vanishes within these events, with plasma moments significantly influenced by suprathermal particles. This finding suggests a fundamental link between magnetic field geometry and the formation of these boundary structures. Scientists observed that the Cluster spacecraft detected an HFA on one side of a TF, while the other three probes registered an FCB, highlighting the spatial variability of these phenomena.

Moreover, the MMS spacecraft identified structures exhibiting HFA-like signatures, but with a distinct absence of solar wind beam heating, indicating a different underlying physical mechanism. This work opens new avenues for investigating the intricate dynamics of the near-Earth space environment and the transfer of solar wind energy into the magnetosphere. The team suggests this occurs when the motional electric field aligns with the IMF directional discontinuities bounding the foreshock. The0.461 UT (C3), 10:49:54.517 UT (C2), 10:49:54.265 UT (C1), and 10:49:55.902 UT (C4). This timing data enabled the calculation of the normal vector in GSE coordinates as n = (−0.81, 0.11, 0.57), visually represented with a brown arrow in accompanying figures.

The team then classified the discontinuities as rotational by calculating Bn/B = 0.46 and ∆B/B = 0.024, utilising established criteria. Further analysis involved evaluating the convection electric field (−V × B) on both sides of the discontinuity, using C1 data averaged over one-minute intervals. Inside the TF, the angle between the normal and the convection electric field was 106°, while outside it was 79°, confirming a condition for HFA formation. The study then harnessed ion fluxes and velocity distribution functions (VDFs) to characterise plasma behaviour within the TFs. Magnetic field magnitude, ion spectra, and energy fluxes were examined across various energy ranges, revealing a strong peak in ion flux below SW beam energy inside the HFA, alongside a less prominent peak within the SW beam energy range.

Detailed VDF analysis, projected onto (Vper1, Vpar) and (Vper1, Vper2) planes, revealed the evolution of ion populations. Before TF arrival, two dominant populations were observed: a SW wind beam and a field-aligned ion beam (FAB). Just prior to the HFA, a partial ring formed around the SW beam, transitioning to strong heating of both populations within the HFA core, and finally, a gyrophase-bunched suprathermal population at the rear edge of the TF. A subsequent event on 12 January 2005, also exhibiting a trailing HFA, showed a B-magnitude decrease to 1.6 nT and a plasma density drop to near zero, with temperatures increasing from 0.8 MK to over 23 MK and significant SW deceleration.

Traveling Foreshock Edges Show HFA and FCB Hybrids

In two instances observed by Cluster, one TF edge was bounded by a distinct HFA, while the other three probes detected an FCB, demonstrating the variability of these boundaries. However, detailed inspection revealed a crucial difference: the absence of heating in the solar wind beam, a feature typically associated with HFAs. Instead, the beam almost entirely disappeared within these events, with plasma moments strongly influenced by suprathermal particles. Experiments utilising kinetic simulations corroborated these observations, demonstrating that FCBs forming at the leading edge of a TF exhibit similar characteristics to those observed in spacecraft data.

Specifically, simulations showed deeper dips in temperature and density profiles, coupled with higher temperatures within the FCB core, compared to the rest of the TF. Measurements confirm that the formation of FCBs is not directly tied to RD properties, but rather to plasma conditions like solar wind Mach number and cone angle. Furthermore, the study demonstrated that the interaction of RDs and backstreaming ions results in substantial changes to RD properties, including significant decreases in density and magnetic field strength, ultimately impacting the overall structure of the TFs.

HFA-like Boundaries Define Travelling Foreshocks and Their Evolution

Acknowledging limitations, the authors note that discerning the precise relationship between HFA-like FCBs and standard HFAs requires further investigation. Future research should focus on exploring the combined effects of these different upstream structures on the magnetosphere and refining models to account for the influence of suprathermal ions. These findings contribute to a more nuanced understanding of the complex interactions occurring in the near-Earth space environment and could improve our ability to predict and mitigate the effects of space weather on technological systems.