Phillip Hess

Context. White-light observations of the solar corona contain contributions from both the electron-scattered K-corona and the dust-scattered F-corona. Because the diffuse F-corona dominates the measured intensity at altitudes of approximately 4 R _⊙ and above, background subtraction techniques are routinely employed to isolate the electron corona in coronagraph observations. Aims. Since no direct observational ‘ground truth’ for the K-corona exists, the amount of coronal signal that is inadvertently removed by background subtraction methods remains unknown. We aim to quantify, for the first time, the impact of background subtraction on the inferred large-scale structure and brightness of the solar corona. Methods. We used a 32-day simulation based on a novel near-real-time, data-assimilative, time-evolving magnetohydrodynamic model to generate synthetic white-light observables. Because these synthetic observations are free from dust and instrumental stray-light contributions, they provide direct access to the ‘true’ K-corona and enable a quantitative assessment of the effects of background subtraction. Results. We find that background subtraction generally preserves the large-scale morphology of the electron corona, including the locations and shapes of major coronal structures. However, the procedure systematically modifies the relative brightness of different features and removes a non-negligible fraction of the K-coronal signal. Conclusions. Although standard background subtraction techniques successfully recover the overall appearance of the electron corona, they can alter its quantitative photometric properties. These findings should be considered when interpreting coronal brightness measurements and deriving physical properties from processed white-light observations.

We present a case study of a coronal mass ejection (CME) that was associated with in situ signatures at Mars and erupted on 2025 March 28. We estimate the time of arrival (ToA) of its associated shock at Mars, focusing on the role of remote sensing observations in improving forecast accuracy. The CME shock is tracked from the middle corona to approximately 150 R _⊙ using all available viewpoints. This CME was observed by multiple instruments on board three spacecraft, including LASCO/C2 and C3, STEREO-A/HI1A and HI2A, and SolO/SoloHI. A semiautomatic, minimization-based fitting technique, adapted to work with heliospheric imager data for the first time, is employed to reconstruct the 3D shock geometry and to quantify event-specific fitting uncertainties. These reconstructions are then used as input to a data-driven two-phase kinematics model to predict the ToA at Mars. Predictions are validated using in situ measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. By including heliospheric imager data in our analysis, we are able to determine the shock arrival at Mars within 3.1 hr of the observed shock in MAVEN. We also test different combinations of remote sensing datasets to assess how data coverage influences arrival-time estimates. We find that in this case observations from heliospheric imagers at larger heliocentric distances are essential for capturing the CME deceleration and avoiding early-arrival predictions. Finally, we highlight the importance of realistic, event-specific uncertainty estimates for space weather forecasting at multiple locations, particularly in the context of ongoing space exploration and future human missions.

Late on 2025 June 17, near Parker Solar Probe’s perihelion of 10 R _⊙ , the Wide-field Imager for Solar Probe (WISPR) observed a slow, narrow mass ejection with the properties of a pseudostreamer coronal mass ejection (CME). Using tracking and fitting techniques that utilize Parker Solar Probe (PSP)'s rapid speed at perihelion, the locations of various features associated with the CME were determined. These locations, when compared with coronal models, indicated that the CME source region was a large, equatorial pseudostreamer that was later traversed by PSP on June 19. WISPR’s high-resolution near-Sun images revealed the internal structure of the CME in the corona. It was seen to be a twisted flux rope with legs initially connecting back to the corona. For about 36 hr, from late on June 17 until midday on June 19, WISPR continued to observe the pseudostreamer as the CME disconnected from the corona. Transient density enhancements (“blobs”) and bright rays were visible behind the CME, indicating magnetic reconnection. The blobs were found to be embedded in the bright rays, suggesting a common origin for the rays and blobs. Near perihelion, as PSP approached the longitude of the pseudostreamer, several bright streamer rays showed a rapid increase in projected latitude, indicating that PSP was passing very close, then under the rays. Previously, WISPR had only observed rays showing such a rapid change in projected latitude as PSP traveled through helmet streamer-associated solar wind surrounding the heliospheric current sheet.

We perform a detailed study of the energetics for a well-observed solar eruption and flare that occurred on 2021 October 28. This event included a GOES class X1.0 flare, a global extreme-UV (EUV) wave, and a coronal mass ejection (CME) that reached speeds of >2000 km s ⁻¹ . The event was observed from a variety of spacecraft in NASA’s Heliophysics System Observatory, including multiple missions near Earth, STEREO-A off the Sun-Earth line, and Solar Orbiter, near the Sun-Earth line at about 0.8 au. Using remote sensing, in situ observations, and in some cases scaling laws based on previous observations, we characterize the following quantities: free magnetic energy, energy in nonthermal electrons, energy in nonthermal ions, bolometric energy, energy deposited in the chromosphere, thermal energy radiated in the flare loops, energy dissipated by the EUV wave, CME kinetic and gravitational potential energy, CME energy flux in the heliosphere, and the energy partition in the CME shock. We find that the total energy released during the event is consistent with estimates of the pre-event stored magnetic energy, and the CME kinetic + potential energy dominates the energy partition.

Comet C/2023 P1 (Nishimura) was observed by the Solar Orbiter Heliospheric Imager (SoloHI), on board the Solar Orbiter spacecraft, from 2023 September 1 to 14. During this period, the ion tail of the comet exhibited continual fluctuations and four tail disconnection events (TDEs), each coinciding with the passage of a coronal mass ejection (CME). In this work, we report on the ion tail dynamics of the best observed TDE, which occurred on September 11. The SoloHI white-light images reveal an abrupt bending, subsequent kinks, and severing of a downstream portion of the preexisting ion tail. The onset of disconnection occurred ∼6.5 hr after the projected passage of the CME leading edge in the images, consistent with a CME flank encounter. After the disconnection, the ion tail reformed in over ∼24 hr, with a regrowth rate of ∼86 ± 7 km s ⁻¹ , indicating the rate at which newly ionized material forms along the magnetic field draped around the comet’s coma. After the TDE, the detached tail drifted anti-sunward at an estimated speed of ∼295 ± 20 km s ⁻¹ , comparable to the local CME flank’s speed, suggesting that the severed plasma was most likely carried away from the comet by the CME. This study provides the first direct, quantitative characterization of comet–CME interactions and the subsequent regrowth phase of a cometary TDE. These measurements were achievable by SoloHI’s unique inner-heliospheric coverage, thanks to a combination of high photometric sensitivity, short exposure times, and a wide field of view that preserves the fine-scale tail dynamics.

During the 22nd perihelion of Parker Solar Probe at 9.86 solar radii on 2024 December 24, the WISPR telescope captured a detailed sequence of events marking the return of open magnetic flux back to the Sun via the heliospheric current sheet (HCS) reconfiguration. In this Letter, we present a preliminary analysis and interpretation of three intriguing events: inflow swarms, HCS rearrangement, and in/out pair generation. Although all three have been reported from past 1 au coronagraphic observations, the WISPR images reveal new insights into the nature of the reconnection driving these events. We propose that inflowing “tadpoles” around a streamer boundary are likely due to interchange reconnection, but that similarly looking tadpoles forming higher up are the result of tearing-mode instabilities in the HCS. We also capture, for the first time in detail, the role of pinch-off reconnection in the creation of in/out pairs. Although based on only 24 hr of WISPR observations, these results showcase the remarkable power of local imaging and highlight the exciting scientific opportunities from Parker Solar Probe’s future coronal flybys.

Coronal mass ejections (CMEs), as seen in white-light (WL) coronagraphs, often exhibit a classic three-part structure consisting of a bright front, a dark cavity, and a bright core. With the launch of Solar Orbiter, cospatial imaging of solar eruptions in multiwavelengths of extreme-ultraviolet (EUV) and WL has become available. We present a CME that erupted on 2022 September 23, observed under a uniquely favorable viewing geometry. The CME bright core and its eruptive prominence can be cospatially observed up to a coronal height of 3.5 R _⊙ in the middle corona, in WL using COR1 on board STEREO-A and in EUV using the Full Sun Imager on board Solar Orbiter. Cospatial, multiwavelength observations indicate that the CME bright core observed in WL was almost entirely composed of the prominence material, which was heated during the CME eruption. EUV emissions in 174 and 304 Å of the prominence were largely cospatial when the CME propagated to the middle corona, though subtle differences remained. We further discuss the potential temperature in the bright core region and find that the core was heated as it rose, likely reaching temperatures of about 0.1–0.8 MK.

Understanding the location and evolution of the cool dense prominence in relation to the large-scale structure of coronal mass ejections (CMEs) is critical to distinguish between different CME initiation mechanisms and to further deepen our understanding of CME evolution through the heliosphere. Combining remote observations of extreme-ultraviolet images and white-light coronagraphs and heliospheric imagers (HIs) obtained from the Solar Dynamics Observatory, Solar and Heliospheric Observatory, STEREO-A, and Solar Orbiter, we present an analysis of the continuous tracking from the corona to interplanetary space of the substructures of a CME associated with a prominence that erupted on 2022 September 23. The prominence is found to remain bright and compact during the CME propagation for more than three days. We investigate the kinematic evolution of the CME substructures as the CME propagated to around 0.5 au. We find that for the first 0.28 au, both the CME front and prominence propagated coherently, indicating that the prominence was tied to the CME magnetic structure. Beyond 0.28 au, the CME bright front was seen to be distorted. However, the prominence continued to propagate at a nearly constant velocity up to at least 0.5 au. STEREO-A/HI images further show a dark ridge-like feature trailing the CME that passed over the prominence, and the prominence appeared tilted. We deduce that the prominence propagated independently of the CME at larger distances from the Sun. Overall, this study shows that both previously proposed hypotheses—namely, that the prominence is tied to or propagates independently of the CME—are valid but within different distance ranges.