LCPFs: Coronal Propagating Fronts

• Large-scale Coronal Propagating Fronts (LCPFs) are expansive solar disturbances observed in EUV images, typically spanning 50–100 megameters and propagating at 100–700 km/s.
• They exhibit dual components—a fast, shock-related wave mimicking type II radio bursts and a slower front connected to magnetic field reconfiguration, highlighting complex dynamics.
• LCPFs serve as crucial tools in coronal seismology by enabling estimation of magnetic field strengths and improving predictive models for solar eruptive events and space weather.

Large-scale Coronal Propagating Fronts (LCPFs), often referred to as EIT waves, are large-scale disturbances observed in the solar corona, typically associated with coronal mass ejections (CMEs). These fronts manifest as bright expansive structures in extreme ultraviolet (EUV) images, propagating with varying speeds across the solar disk and into interplanetary space. Despite their frequent occurrence, the nature and mechanics of LCPFs remain subjects of intense paper and debate due to their complex interactions with solar structures and varying observational characteristics.

Observational Characteristics

LCPFs are identified by their broad, diffuse brightenings in EUV imaging, typically over 50–100 megameters wide, and propagating at velocities ranging from 100 km/s to 700 km/s. High-cadence observations from instruments such as SDO/AIA and STEREO/EUVI have provided detailed kinematic profiles showing both rapid initial acceleration and significant deceleration as these fronts expand. LCPFs are observed to interact with local magnetic field structures, exhibiting stationary fronts or reflections when encountering coronal holes and active regions.

Speed Variability and Wave Nature

Early interpretations likened LCPFs to fast-mode magnetoacoustic waves, but discrepancies in measured speeds have prompted two-wave paradigm considerations. This model distinguishes between a faster, piston-driven shock wave and a slower propagating front akin to the CME frontal loop. The fast wave component fits observations where type II radio bursts, indicative of shock waves, accompany the EUV disturbance. The slower component appears more influenced by gradual magnetic field restructuring rather than true wave characteristics.

Theoretical Models and Mechanisms

Theoretical models propose LCPFs as magnetohydrodynamic (MHD) waves driven by the CME's lateral expansion. The fast-mode MHD wave interpretation suggests that these disturbances can traverse magnetic field lines, a characteristic contributing to their varying speed profiles in different passbands. Conversely, some studies argue for non-wave models, where the apparent propagation results from the continuous opening of magnetic field lines due to the CME.

Interaction with Coronal Structures

LCPFs often interact with various coronal features, such as coronal holes, arcades, and filaments, which modulate their propagation. Fast primary fronts can transmit through polar coronal holes while generating stationary features along equatorial hole boundaries, indicating complex wave reflection and refraction phenomena. Observations of plasma heating and dimming in different EUV channels further substantiate the wave nature in regions where local magnetic configurations guide energy propagation.

Implications for Coronal Seismology

LCPFs serve as valuable tools for coronal seismology, allowing the estimation of magnetic field strengths within the corona. By analyzing the displacement and speed of these fronts, researchers can infer plasma conditions and magnetic topology, offering insights into solar wind acceleration mechanisms and the overall magnetic environment of the corona.

Future Directions and Challenges

Ongoing advancements in high-cadence, multi-wavelength imaging, alongside sophisticated computational models like the Coronal Analysis of SHocks and Waves (CASHeW) framework, continue to refine our understanding of LCPFs. These tools not only improve the accuracy of kinematic and dynamic analyses but also enhance predictive capabilities for solar energetic particle events and space weather forecasting. However, challenges remain in disentangling the two-wave components and correlating observations across diverse wavelengths and observational platforms. Enhanced observational techniques and modeling are essential for resolving these ambiguities and advancing our comprehension of solar eruptive phenomena.

In conclusion, LCPFs represent a crucial aspect of solar physics, illustrating the intricate dynamics of solar eruptions and their impacts on the solar atmosphere. Their paper provides critical insights into the behavior of CMEs and the evolution of magnetic fields in the corona, with broader implications for understanding space weather phenomena that affect Earth.