We describe the scientific objectives and instrument design of the ASPIICS coronagraph launched aboard the Proba-3 mission of the European Space Agency (ESA) on 5 December 2024. Proba-3 consists of two spacecraft in a highly elliptical orbit around the Earth. One spacecraft carries the telescope, and the external occulter is mounted on the second spacecraft. The two spacecraft fly in a precise formation during 6 hours out of 19.63 hour orbit, together forming a giant solar coronagraph called ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun). Very long distance between the external occulter and the telescope (around 144 m) represents an increase of two orders of magnitude compared to classical externally occulted solar coronagraphs. This allows us to observe the inner corona in eclipse-like conditions, i.e. close to the solar limb (down to 1.099 Rs) and with very low straylight. ASPIICS will provide a new perspective on the inner solar corona that will help solve several outstanding problems in solar physics, such as the origin of the slow solar wind and physical mechanism of coronal mass ejections.

A. N. Zhukov C. Thizy D. Galano B. Bourgoignie L. Dolla C. Jean B. Nicula S. Shestov C. Galy R. Rougeot J. Versluys J. Zender P. Lamy S. Fineschi S. Gunar B. Inhester M. Mierla P. Rudawy K. Tsinganos S. Koutchmy R. Howard H. Peter S. Vives L. Abbo C. Aime K. Aleksiejuk J. Baran U. Bak-Steslicka A. Bemporad D. Berghmans D. Besliu-Ionescu S. Buckley O. Buiu G. Capobianco I. Cimoch E. DHuys M. Dziezyc K. Fleury-Frenette S. E. Gibson S. Giordano L. Golub K. Grochowski P. Heinzel A. Hermans J. Jacobs S. Jejcic N. Kranitis F. Landini D. Loreggia J. Magdalenic D. Maia C. Marque R. Melich M. Morawski M. Mosdorf V. Noce P. Orleanski A. Paschalis R. Peresty L. Rodriguez D. B. Seaton L. Short J. -F. Simar M. Steslicki R. Sorensen G. Terrasa N. Van Vooren F. Verstringe L. Zangrilli

Context. Coronal mass ejections (CMEs) are large eruptions of magnetised plasma from the Sun that are often accompanied by solar radio bursts produced by accelerated electrons. Aims. A powerful source for accelerating electron beams are CME-driven shocks, however, there are other mechanisms capable of accelerating electrons during a CME eruption. So far, studies have relied on the traditional classification of solar radio bursts into five groups (Type I-V) based mainly on their shapes and characteristics in dynamic spectra. Here, we aim to determine the origin of moving radio bursts associated with a CME that do not fit into the present classification of the solar radio emission. Methods. By using radio imaging from the Nan\c{c}ay Radioheliograph, combined with observations from the Solar Dynamics Observatory, Solar and Heliospheric Observatory, and Solar Terrestrial Relations Observatory spacecraft, we investigate the moving radio bursts accompanying two subsequent CMEs on 22 May 2013. We use three-dimensional reconstructions of the two associated CME eruptions to show the possible origin of the observed radio emission. Results. We identified three moving radio bursts at unusually high altitudes in the corona that are located at the northern CME flank and move outwards synchronously with the CME. The radio bursts correspond to fine-structured emission in dynamic spectra with durations of ~1 s, and they may show forward or reverse frequency drifts. Since the CME expands closely following an earlier CME, a low coronal CME-CME interaction is likely responsible for the observed radio emission.

D. E. Morosan E. Palmerio J. E. Räsänen E. K. J. Kilpua J. Magdalenić B. J. Lynch A. Kumari J. Pomoell M. Palmroth

We present a detailed analysis of an eruptive event that occurred on early 2019 March 8 in active region AR 12734, to which we refer as the International Women's day event. The event under study is intriguing in several aspects: 1) low-coronal eruptive signatures come in ''pairs'' (a double-peak flare, two coronal dimmings, and two EUV waves); 2) although the event is characterized by a complete chain of eruptive signatures, the corresponding coronagraphic signatures are weak; 3) although the source region of the eruption is located close to the center of the solar disc and the eruption is thus presumably Earth-directed, heliospheric signatures are very weak with little Earth-impact. We analyze a number of multi-spacecraft and multi-instrument (both remote-sensing and in situ) observations, including Soft X-ray, (extreme-) ultraviolet (E)UV), radio and white-light emission, as well as plasma, magnetic field and particle measurements. We employ 3D NLFF modeling to investigate the coronal magnetic field configuration in and around the active region, the GCS model to make a 3D reconstruction of the CME geometry and the 3D MHD numerical model EUHFORIA to model the background state of the heliosphere. Our results indicate two subsequent eruptions of two systems of sheared and twisted magnetic fields, which merge already in the upper corona and start to evolve further out as a single entity. The large-scale magnetic field significantly influences both, the early and the interplanetary evolution of the structure. During the first eruption the stability of the overlying field was disrupted which enabled the second eruption. We find that during the propagation in the interplanetary space the large-scale magnetic field, i.e. , the location of heliospheric current sheet between the AR and the Earth likely influences propagation and the evolution of the erupted structure(s).

Dumbovic M. Veronig A. M. Podladchikova T. Thalmann J. K. Chikunova G. Dissauer K. Magdalenic J. Temmer M. Guo J. Samara E

In order to address the growing need for more accurate space weather predictions, a new model named EUHFORIA (EUropean Heliospheric FORecasting Information Asset) was recently developed (Pomoell and Poedts, 2018). We present first results of the performance assessment for the solar wind modeling with EUHFORIA and identify possible limitations of its present setup. Using the basic EUHFORIA 1.0.4. model setup with the default input parameters, we modeled background solar wind (no coronal mass ejections) and compared the obtained results with ACE, in situ measurements. For the need of statistical study we developed a technique of combining daily EUHFORIA runs into continuous time series. The combined time series were derived for the years 2008 (low solar activity) and 2012 (high solar activity) from which in situ speed and density profiles were extracted. We find for the low activity phase a better match between model results and observations compared to the considered high activity time interval. The quality of the modeled solar wind parameters is found to be rather variable. Therefore, to better understand the obtained results we also qualitatively inspected characteristics of coronal holes, sources of the studied fast streams. We discuss how different characteristics of the coronal holes and input parameters to EUHFORIA influence the modeled fast solar wind, and suggest possibilities for the improvements of the model.

J. Hinterreiter J. Magdalenic M. Temmer C. Verbeke I. C. Jeberaj E. Samara E. Asvestari S. Poedts J. Pomoell E. Kilpua L. Rodriguez C. Scolini A. Isavnin

We analyse in this work the propagation and geoeffectiveness of four successive coronal mass ejections (CMEs) that erupted from the Sun during 21--23 May 2013 and that were detected in interplanetary space by the Wind and/or STEREO-A spacecraft. All these CMEs featured critical aspects for understanding so-called "problem space weather storms" at Earth. In the first three events a limb CMEs resulted in moderately geoeffective in-situ structures at their target location in terms of the disturbance storm time (Dst) index (either measured or estimated). The fourth CME, which also caused a moderate geomagnetic response, erupted from close to the disc centre as seen from Earth, but it was not visible in coronagraph images from the spacecraft along the Sun--Earth line and appeared narrow and faint from off-angle viewpoints. Making the correct connection between CMEs at the Sun and their in-situ counterparts is often difficult for problem storms. We investigate these four CMEs using multiwavelength and multipoint remote-sensing observations (extreme ultraviolet, white light, and radio), aided by 3D heliospheric modelling, in order to follow their propagation in the corona and in interplanetary space and to assess their impact at 1 AU. Finally, we emphasise the difficulties in forecasting moderate space weather effects provoked by problematic and ambiguous events and the importance of multispacecraft data for observing and modelling problem storms.

Erika Palmerio Camilla Scolini David Barnes Jasmina Magdalenić Matthew J. West Andrei N. Zhukov Luciano Rodriguez Marilena Mierla Simon W. Good Diana E. Morosan Emilia K. J. Kilpua Jens Pomoell Stefaan Poedts

Coronal mass ejections (CMEs) are a manifestation of the Sun's eruptive nature. They can have a great impact on Earth, but also on human activity in space and on the ground. Therefore, modelling their evolution as they propagate through interplanetary space is essential. EUropean Heliospheric FORecasting Information Asset (EUHFORIA) is a data-driven, physics-based model, tracing the evolution of CMEs through background solar wind conditions. It employs a spheromak flux rope, which provides it with the advantage of reconstructing the internal magnetic field configuration of CMEs. This is something that is not included in the simpler cone CME model used so far for space weather forecasting. This work aims at assessing the spheromak CME model included in EUHFORIA. We employed the spheromak CME model to reconstruct a well observed CME and compare model output to in situ observations. We focus on an eruption from 6 January 2013 encountered by two radially aligned spacecraft, Venus Express and STEREO-A. We first analysed the observed properties of the source of this CME eruption and we extracted the CME properties as it lifted off from the Sun. Using this information, we set up EUHFORIA runs to model the event. The model predicts arrival times from half to a full day ahead of the in situ observed ones, but within errors established from similar studies. In the modelling domain, the CME appears to be propagating primarily southward, which is in accordance with white-light images of the CME eruption close to the Sun. In order to get the observed magnetic field topology, we aimed at selecting a spheromak rotation angle for which the axis of symmetry of the spheromak is perpendicular to the direction of the polarity inversion line (PIL). The modelled magnetic field profiles, their amplitude, arrival times, and sheath region length are all affected by the choice of radius of the modelled spheromak.

E. Asvestari J. Pomoell E. Kilpua S. Good T. Chatzistergos M. Temmer E. Palmerio S. Poedts J. Magdalenic

We report observations of a unique, large prominence eruption that was observed in the He II 304 {\AA} passband of the the Extreme Ultraviolet Imager/Full Sun Imager telescope aboard Solar Orbiter on 15-16 February 2022. Observations from several vantage points (Solar Orbiter, the Solar-Terrestrial Relations Observatory, the Solar and Heliospheric Observatory, and Earth-orbiting satellites) were used to measure the kinematics of the erupting prominence and the associated coronal mass ejection. Three-dimensional reconstruction was used to calculate the deprojected positions and speeds of different parts of the prominence. Observations in several passbands allowed us to analyse the radiative properties of the erupting prominence. The leading parts of the erupting prominence and the leading edge of the corresponding coronal mass ejection propagate at speeds of around 1700 km/s and 2200 km/s, respectively, while the trailing parts of the prominence are significantly slower (around 500 km/s). Parts of the prominence are tracked up to heights of over $6 R_\sun$. The He II emission is probably produced via collisional excitation rather than scattering. Surprisingly, the brightness of a trailing feature increases with height. The reported prominence is the first observed in He II 304 {\AA} emission at such a great height (above 6 $R_\sun$).

M. Mierla A. N. Zhukov D. Berghmans S. Parenti F. Auchere P. Heinzel D. B. Seaton E. Palmerio S. Jejcic J. Janssens E. Kraaikamp B. Nicula D. M. Long L. A. Hayes I. C. Jebaraj D. -C. Talpeanu E. D'Huys L. Dolla S. Gissot J. Magdalenic L. Rodriguez S. Shestov K. Stegen C. Verbeeck C. Sasso M. Romoli V. Andretta

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.

L. Rodriguez D. Shukhobodskaia A. Niemela A. Maharana E. Samara C. Verbeke J. Magdalenic R. Vansintjan M. Mierla C. Scolini R. Sarkar E. Kilpua E. Asvestari K. Herbst G. Lapenta A. D. Chaduteau J. Pomoell S. Poedts

Solar radio bursts can provide important insights into the underlying physical mechanisms that drive the small and large-scale eruptions on the Sun. Since metric radio observations can give us direct observational access to the inner and middle corona, they are often used as an important tool to monitor and understand the coronal dynamics. While the sizes of the radio sources that can be observed in the solar corona is essential for understanding the nature of density turbulence within the solar corona and its subsequent influence on the angular broadening observed in radio source measurements, the smallest radio sources associated with solar radio bursts have so far been limited by observational techniques and the radio instrument's baselines. We selected three type II bursts that were observed with the LOFAR core and remote stations in the Solar Cycle 24. We estimated the sizes and shapes (ellipticity) of the radio sources between $20-200$ MHz using a two-dimensional Gaussian approximation. Our analysis shows that the smallest radio source size for type II bursts in the solar corona which can be observed in the solar atmosphere at low frequencies is $1.5^\prime \pm 0.5^\prime$ at 150 MHz. However, even though the observations were taken with remote baselines (with a maximum distance of $\sim 85~km$), the effective baselines were much shorter ($\sim 35~km$) likely due to snapshot imaging of the Sun. Our results show that the radio source sizes are less affected by scattering than suggested in previous studies. Our measurements indicate a smaller source sizes at frequencies below 95 MHz compared to previous reports, though some overlap exists with measurements at higher frequencies, using smaller baselines.

A. Kumari D. E. Morosan V. Mugundhan P. Zhang J. Magdalenic P. Zucca E. K. J. Kilpua F. Daei

Based on a sample of 1114 flares observed simultaneously in hard X-rays (HXR) by the BATSE instrument and in soft X-rays (SXR) by GOES, we studied several aspects of the Neupert effect and its interpretation in the frame of the electron-beam-driven evaporation model. In particular, we investigated the time differences ($\Delta t$) between the maximum of the SXR emission and the end of the HXR emission, which are expected to occur at almost the same time. Furthermore, we performed a detailed analysis of the SXR peak flux -- HXR fluence relationship for the complete set of events, as well as separately for subsets of events which are likely compatible/incompatibe with the timing expectations of the Neupert effect. The distribution of the time differences reveals a pronounced peak at $\Delta t = 0$. About half of the events show a timing behavior which can be considered to be consistent with the expectations from the Neupert effect. For these events, a high correlation between the SXR peak flux and the HXR fluence is obtained, indicative of electron-beam-driven evaporation. However, there is also a significant fraction of flares (about one fourth), which show strong deviations from $\Delta t = 0$, with a prolonged increase of the SXR emission distinctly beyond the end of the HXR emission. These results suggest that electron-beam-driven evaporation plays an important role in solar flares. Yet, in a significant fraction of events, there is also clear evidence for the presence of an additional energy transport mechanism other than the nonthermal electron beams, where the relative contribution is found to vary with the flare importance.

A. Veronig B. Vrsnak B. R. Dennis M. Temmer A. Hanslmeier J. Magdalenic