arXiv subjects

A. Niemela

Publications and source records attributed to A. Niemela.

3 recordsLinked to original sources

arXiv2026

Deformation of the CME and CME-driven shock due to interaction with the ambient solar wind: I. Modelling with Cone Model

Context. The near-Earth environment is continuously impacted by the solar wind and the transients embedded in it. The largest eruptions of plasma and magnetic field at the Sun are Coronal Mass Ejections (CMEs), which, upon reaching Earth, can induce geomagnetic storms and disrupt our technologically advanced societies. Aims. We aim to improve our understanding of how CMEs and CME-driven shock waves interact with the ambient solar wind, and how the resulting deformations in time and space affect the accuracy of space weather forecasting. Methods. We studied two Earth-directed CMEs, observed on December 7, 2020 and October 28, 2021. To model the solar wind and CME propagation in the inner heliosphere, we used the state-of-the-art 3D MHD model EUHFORIA with the cone CME model, focusing on the regions along the main direction of propagation (MDP) and along the Sun-Earth line. Results. The deformations of the CME and the CME-driven shock can be significant. Both CMEs propagate in a variable background wind and are structured differently even at very close angular distances. The first, in a mildly structured wind, shows its stronger deformation predominantly away from the MDP; the second, in a more complex environment, develops a strongly structured shock at very different locations, including regions close to the MDP. Conclusions. These interactions also affect the characteristics of the CME-driven shock, such as the gas compression ratio across it. CMEs observed as flank encounters at Earth can be strongly affected by the ambient solar wind, with the expected differences in the arrival time at Earth reaching up to 16 hours.

astro-ph.SR↗

arXiv2026

Assessing VBz variations during CME propagation: a preparatory study for the HENON mission using EUHFORIA

Coronal mass ejections (CMEs) are among the main drivers of space weather hazards. In this context, HENON is a new space mission designed to carry out observations in the solar wind upstream of the Earth, aiming to provide timely alerts for hazardous perturbations propagating towards the Earth. HENON will orbit Earth on a distant retrograde orbit, approximately 0.082 AU upstream of the Earth when it is on the Sun-Earth line. The measurements taken by HENON will allow us to determine plasma and magnetic field parameters with a lead time of several hours with respect to the Lagrangian point L1. We assess the VB_z parameter variations (the product of solar wind speed V and southward magnetic field B_z) along the HENON orbit. Given its role as a primary driver of geomagnetic activity, we analyse how these measurements change with respect to Earth's position to evaluate HENON's forecasting potential. We used the FRi3D CME model of the EUHFORIA simulation code to characterize the initial properties of the CME. FRi3D allows us to set the CME magnetic field as a magnetic flux rope. From the simulation results, we evaluated the VB_z parameter at nine virtual spacecraft positions along the planned HENON orbit. The heliocentric longitudes of the virtual spacecraft range from about -6.9° to 6.9°, while the geocentric longitudes vary from -60° to +60° in steps of 15°. The initial direction of propagation of the CME central apex is either along the Sun-Earth line or at heliocentric longitudes of {\pm}30°. We find that with the proposed orbital parameters, the values of the VBz parameter along the HENON orbit are sufficiently similar to those measured in the vicinity of the Earth to be useful for space weather forecasts. HENON enables reliable VB_z estimates 2-8 hours in advance, improving space weather forecasting and protection of critical infrastructure and satellites.

astro-ph.SR↗

arXiv2024

Validation of EUHFORIA cone and spheromak Coronal Mass Ejection Models

Aims. We present the validation results for arrival times and geomagnetic impact of Coronal Mass Ejections (CMEs), using the cone and spheromak CME models implemented in EUropean Heliospheric FORecasting Information Asset (EUHFORIA). Validating numerical models is crucial in ensuring their accuracy and performance with respect to real data. Methods. We compare CME plasma and magnetic field signatures, measured in situ by satellites at the L1 point, with the simulation output of EUHFORIA. The validation of this model was carried out by using two datasets in order to ensure a comprehensive evaluation. The first dataset focuses on 16 CMEs that arrived at the Earth, offering specific insights into the model's accuracy in predicting arrival time and geomagnetic impact. Meanwhile, the second dataset encompasses all CMEs observed over eight months within Solar Cycle 24, regardless of whether they arrived at Earth, covering periods of both solar minimum and maximum activity. This second dataset enables a more comprehensive evaluation of the model's predictive precision in term of CME arrivals and misses. Results. Our results show that EUHFORIA provides good estimates in terms of arrival times, with root mean square errors (RMSE) values of 9 hours. Regarding the number of correctly predicted ICME arrivals and misses, we find a 75% probability of detection in a 12 hours time window and 100% probability of detection in a 24 hours time window. The geomagnetic impact forecasts, measured by the $K_p$ index, provide different degrees of accuracy, ranging from 31% to 69%. These results validate the use of cone and spheromak CMEs for real-time space weather forecasting.

astro-ph.SR↗