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Global magnetic field models use as input synoptic data, which usually show "aging effects" as the longitudinal 360 ∘ information is not obtained simultaneously. Especially during times of increased solar activity, the evolution of the magnetic field may yield large uncertainties. A significant source of uncertainty is the Sun's magnetic field on the side of the Sun invisible to the observer. Various methods have been used to complete the picture: synoptic charts, flux-transport models, and far side helioseismology. In this study, we present a new method to estimate the far-side open flux within coronal holes using STEREO EUV observations. First, we correlate the structure of the photospheric magnetic field as observed with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (HMI/SDO) with features in the transition region. From the 304 Å intensity distribution, which we found to be specific to coronal holes, we derive an empirical estimate for the open flux. Then we use a large sample of 313 SDO coronal hole observations to verify this relation. Finally, we perform a cross-instrument calibration from SDO to STEREO data to enable the estimation of the open flux at solar longitudes not visible from Earth. We find that the properties of strong unipolar magnetic elements in the photosphere, which determine the coronal hole's open flux, can be approximated by open fields in the transition region. We find that structures below a threshold of 78 % (STEREO) or 94 % (SDO) of the solar disk median intensity as seen in 304 Å filtergrams are reasonably well correlated with the mean magnetic flux density of coronal holes (cc = sp 0.59 ). Using the area covered by these structures ( A OF ) and the area of the coronal hole ( A CH ), we model the open magnetic flux of a coronal hole as | Φ CH | = 0.25 A CH exp (0.032 A OF) with an estimated uncertainty of 40 to 60 % . [ABSTRACT FROM AUTHOR]

The Sun, as an active star, is the driver of energetic phenomena that structure interplanetary space and affect planetary atmospheres. The effects of Space Weather on Earth and the solar system is of increasing importance as human spaceflight is preparing for lunar and Mars missions. This review is focusing on the solar perspective of the Space Weather relevant phenomena, coronal mass ejections (CMEs), flares, solar energetic particles (SEPs), and solar wind stream interaction regions (SIR). With the advent of the STEREO mission (launched in 2006), literally, new perspectives were provided that enabled for the first time to study coronal structures and the evolution of activity phenomena in three dimensions. New imaging capabilities, covering the entire Sun-Earth distance range, allowed to seamlessly connect CMEs and their interplanetary counterparts measured in-situ (so called ICMEs). This vastly increased our knowledge and understanding of the dynamics of interplanetary space due to solar activity and fostered the development of Space Weather forecasting models. Moreover, we are facing challenging times gathering new data from two extraordinary missions, NASA's Parker Solar Probe (launched in 2018) and ESA's Solar Orbiter (launched in 2020), that will in the near future provide more detailed insight into the solar wind evolution and image CMEs from view points never approached before. The current review builds upon the Living Reviews article by Schwenn from 2006, updating on the Space Weather relevant CME-flare-SEP phenomena from the solar perspective, as observed from multiple viewpoints and their concomitant solar surface signatures. [ABSTRACT FROM AUTHOR]

We analyze the well-observed flare and coronal mass ejection (CME) from 1 October 2011 (SOL2011-10-01T09:18) covering the complete chain of effects - from Sun to Earth - to better understand the dynamic evolution of the CME and its embedded magnetic field. We study in detail the solar surface and atmosphere associated with the flare and CME using the Solar Dynamics Observatory (SDO) and ground-based instruments. We also track the CME signature off-limb with combined extreme ultraviolet (EUV) and white-light data from the Solar Terrestrial Relations Observatory (STEREO). By applying the graduated cylindrical shell (GCS) reconstruction method and total mass to stereoscopic STEREOSOHO (Solar and Heliospheric Observatory) coronagraph data, we track the temporal and spatial evolution of the CME in the interplanetary space and derive its geometry and 3D mass. We combine the GCS and Lundquist model results to derive the axial flux and helicity of the magnetic cloud (MC) from in situ measurements from Wind. This is compared to nonlinear force-free (NLFF) model results, as well as to the reconnected magnetic flux derived from the flare ribbons (flare reconnection flux) and the magnetic flux encompassed by the associated dimming (dimming flux). We find that magnetic reconnection processes were already ongoing before the start of the impulsive flare phase, adding magnetic flux to the flux rope before its final eruption. The dimming flux increases by more than 25% after the end of the flare, indicating that magnetic flux is still added to the flux rope after eruption. Hence, the derived flare reconnection flux is most probably a lower limit for estimating the magnetic flux within the flux rope. We find that the magnetic helicity and axial magnetic flux are lower in the interplanetary space by ~50% and 75%, respectively, possibly indicating an erosion process. A CME mass increase of 10% is observed over a range of ~4-20 R. The temporal evolution of the CME-associated core-dimming regions supports the scenario that fast outflows might supply additional mass to the rear part of the CME. [ABSTRACT FROM AUTHOR]

We present a review of the different aspects associated with the interaction of successive coronal mass ejections (CMEs) in the corona and inner heliosphere, focusing on the initiation of series of CMEs, their interaction in the heliosphere, the particle acceleration associated with successive CMEs, and the effect of compound events on Earth’s magnetosphere. The two main mechanisms resulting in the eruption of series of CMEs are sympathetic eruptions, when one eruption triggers another, and homologous eruptions, when a series of similar eruptions originates from one active region. CME – CME interaction may also be associated with two unrelated eruptions. The interaction of successive CMEs has been observed remotely in coronagraphs (with the Large Angle and Spectrometric Coronagraph Experiment – LASCO – since the early 2000s) and heliospheric imagers (since the late 2000s), and inferred from in situ measurements, starting with early measurements in the 1970s. The interaction of two or more CMEs is associated with complex phenomena, including magnetic reconnection, momentum exchange, the propagation of a fast magnetosonic shock through a magnetic ejecta, and changes in the CME expansion. The presence of a preceding CME a few hours before a fast eruption has been found to be connected with higher fluxes of solar energetic particles (SEPs), while CME – CME interaction occurring in the corona is often associated with unusual radio bursts, indicating electron acceleration. Higher suprathermal population, enhanced turbulence and wave activity, stronger shocks, and shock – shock or shock – CME interaction have been proposed as potential physical mechanisms to explain the observed associated SEP events. When measured in situ, CME – CME interaction may be associated with relatively well organized multiple-magnetic cloud events, instances of shocks propagating through a previous magnetic ejecta or more complex ejecta, when the characteristics of the individual eruptions cannot be easily distinguished. CME – CME interaction is associated with some of the most intense recorded geomagnetic storms. The compression of a CME by another and the propagation of a shock inside a magnetic ejecta can lead to extreme values of the southward magnetic field component, sometimes associated with high values of the dynamic pressure. This can result in intense geomagnetic storms, but can also trigger substorms and large earthward motions of the magnetopause, potentially associated with changes in the outer radiation belts. Future in situ measurements in the inner heliosphere by Solar Probe+ and Solar Orbiter may shed light on the evolution of CMEs as they interact, by providing opportunities for conjunction and evolutionary studies. [ABSTRACT FROM AUTHOR]

We study the interaction of two successive coronal mass ejections (CMEs) during the 2010 August 1 events using STEREO/SECCHI COR and heliospheric imager (HI) data. We obtain the direction of motion for both CMEs by applying several independent reconstruction methods and find that the CMEs head in similar directions. This provides evidence that a full interaction takes place between the two CMEs that can be observed in the HI1 field of view. The full de-projected kinematics of the faster CME from Sun to Earth is derived by combining remote observations with in situ measurements of the CME at 1 AU. The speed profile of the faster CME (CME2; ~1200 km s-1) shows a strong deceleration over the distance range at which it reaches the slower, preceding CME (CME1; ~700 km s-1). By applying a drag-based model we are able to reproduce the kinematical profile of CME2, suggesting that CME1 represents a magnetohydrodynamic obstacle for CME2 and that, after the interaction, the merged entity propagates as a single structure in an ambient flow of speed and density typical for quiet solar wind conditions. Observational facts show that magnetic forces may contribute to the enhanced deceleration of CME2. We speculate that the increase in magnetic tension and pressure, when CME2 bends and compresses the magnetic field lines of CME1, increases the efficiency of drag. [ABSTRACT FROM AUTHOR]

We study three coronal mass ejection (CME)/interplanetary coronal mass ejection (ICME) events (2008 June 1-6, 2009 February 13-18, and 2010 April 3-5) tracked from Sun to 1 AU in remote-sensing observations of Solar Terrestrial Relations Observatory Heliospheric Imagers and in situ plasma and magnetic field measurements. We focus on the ICME propagation in interplanetary (IP) space that is governed by two forces: the propelling Lorentz force and the drag force. We address the question: which heliospheric distance range does the drag become dominant and the CME adjust to the solar wind flow. To this end, we analyze speed differences between ICMEs and the ambient solar wind flow as a function of distance. The evolution of the ambient solar wind flow is derived from ENLIL three-dimensional MHD model runs using different solar wind models, namely, Wang-Sheeley-Arge and MHD--Around--A--Sphere. Comparing the measured CME kinematics with the solar wind models, we find that the CME speed becomes adjusted to the solar wind speed at very different heliospheric distances in the three events under study: from below 30 R⊙, to beyond 1 AU, depending on the CME and ambient solar wind characteristics. ENLIL can be used to derive important information about the overall structure of the background solar wind, providing more reliable results during times of low solar activity than during times of high solar activity. The results from this study enable us to obtain greater insight into the forces acting on CMEs over the IP space distance range, which is an important prerequisite for predicting their 1 AU transit times. [ABSTRACT FROM AUTHOR]

We compared the variability of coronal hole (CH) areas (determined from daily GOES/SXI images) with solar wind (daily ACE data) and geomagnetic parameters for the time span 25 January 2005 until 11 September 2005 (late declining phase of solar cycle 23). Applying wavelet spectral analysis, a clear 9-day period is found in the CH time series. The GOES/SXI image sequence suggests that this periodic variation is caused by a mutual triangular distribution of CHs ∼120° apart in longitude. From solar wind parameters a 9-day periodicity was obtained as well, simultaneously with the 9-day period in the CH area time series. These findings provide strong evidence that the 9-day period in solar wind parameters, showing up as higher harmonic of the solar rotation frequency, is caused by the “periodic” longitudinal distribution of CHs on the Sun recurring for several solar rotations. The shape of the wavelet spectrum from the Dst index matches only weakly with that from the CH areas and is more similar to the wavelet spectrum of the solar wind magnetic field magnitude. The distinct 9-day period does not show up in sunspot group areas which gives further evidence that the solar wind modulation is strongly related to CH areas but not to active region complexes. The wavelet power spectra for the whole ACE data range (∼1998 – 2006) suggest that the 9-day period is not a singular phenomenon occurring only during a specific time range close to solar minimum but is occasionally also present during the maximum and decay phase of solar cycle 23. The main periods correspond to the solar rotation (27d) as well as to the second (13.5d) and third (9d) harmonic. [ABSTRACT FROM AUTHOR]

We analyze the relationship between the coronal hole (CH) area/position and physical characteristics of the associated corotating high-speed stream (HSS) in the solar wind at 1 AU. For the analysis we utilize the data in the period DOY 25 – 125 of 2005, characterized by a very low coronal mass ejection (CME) activity. Distinct correlations between the daily averaged CH parameters and the solar wind characteristics are found, which allows us to forecast the solar wind velocity v, proton temperature T, proton density n, and magnetic field strength B, several days in advance in periods of low CME activity. The forecast is based on monitoring fractional areas A, covered by CHs in the meridional slices embracing the central meridian distance ranges [−40°,−20°], [−10°,10°], and [20°,40°]. On average, the peaks in the daily values of n, B, T, and v appear delayed by 1, 2, 3, and 4 days, respectively, after the area A attains its maximum in the central-meridian slice. The peak values of the solar wind parameters are correlated to the peak values of A, which provides also forecasting of the peak values of n, B, T, and v. The most accurate prediction can be obtained for the solar wind velocity, for which the average relative difference between the calculated and the observed peak values amounts to $\overline{\vert\delta\vert}\approx10$ %. The forecast reliability is somewhat lower in the case of T, B, and n ( $\overline{\vert\delta\vert}\approx20$ , 30, and 40%, respectively). The space weather implications are discussed, including the perspectives for advancing the real-time calculation of the Sun – Earth transit times of coronal mass ejections and interplanetary shocks, by including more realistic real-time estimates of the solar wind characteristics. [ABSTRACT FROM AUTHOR]

We present a simple method of forecasting the geomagnetic storms caused by high-speed streams (HSSs) in the solar wind. The method is based on the empirical correlation between the coronal hole area/position and the value of the Dst index, which is established in a period of low interplanetary coronal mass ejection (ICME) activity. On average, the highest geomagnetic activity, i.e., the minimum in Dst, occurs four days after a low-latitude coronal hole (CH) crosses the central meridian. The amplitude of the Dst dip is correlated with the CH area and depends on the magnetic polarity of the CH due to the Russell – McPherron effect. The Dst variation may be predicted by employing the expression Dst( t)=(−65±25×cos λ)[ A( t *)]0.5, where A( t *) is the fractional CH area measured in the central-meridian slice [−10°,10°] of the solar disc, λ is the ecliptic longitude of the Earth, ± stands for positive/negative CH polarity, and t− t *=4 days. In periods of low ICME activity, the proposed expression provides forecasting of the amplitude of the HSS-associated Dst dip to an accuracy of ≈30%. However, the time of occurrence of the Dst minimum cannot be predicted to better than ±2 days, and consequently, the overall mean relative difference between the observed and calculated daily values of Dst ranges around 50%. [ABSTRACT FROM AUTHOR]

We analyze the evolution of the flare/postflare-loop system in the two-ribbon flare of November 3, 2003, utilizing multi-wavelength observations that cover the temperature range from several tens of MK down to 104 K. A non-uniform growth of the loop system enables us to identify analogous patterns in the height–time, h( t), curves measured at different temperatures. The “knees,” “plateaus,” and “bends” in a higher-temperature curve appear after a certain time delay at lower heights in a lower-temperature curve. We interpret such a shifted replication as a track of a given set of loops (reconnected field lines) while shrinking and cooling after being released from the reconnection site. Measurements of the height/time shifts between h( t) curves of different temperatures provide a simultaneous estimate of the shrinkage speed and cooling rate in a given temperature domain, for a period of almost ten hours after the flare impulsive phase. From the analysis we find the following: (a) Loop shrinkage is faster at higher temperatures – in the first hour of the loop-system growth, the shrinkage velocity at 5 MK is 20 – 30 km s−1, whereas at 1 MK it amounts to 5 km s−1; (b) Shrinking becomes slower as the flare decays – ten hours after the impulsive phase, the shrinkage velocity at 5 MK becomes 5 km s−1; (c) The cooling rate decreases as the flare decays – in the 5 MK range it is 1 MK min−1 in the first hour of the loop-system growth, whereas ten hours later it decreases to 0.2 MK min−1; (d) During the initial phase of the loop-system growth, the cooling rate is larger at higher temperatures, whereas in the late phases the cooling rate apparently does not depend on the temperature; (e) A more detailed analysis of shrinking/cooling around one hour after the impulsive phase reveals a deceleration of the loop shrinkage, amounting to ā ≈ 10 m s−2 in the T < 5 MK range; (f) In the same interval, conductive cooling dominates down to T ≈ 3 MK, whereas radiation becomes dominant below T ≈ 2 MK; (g) A few hours after the impulsive phase, radiation becomes dominant across the whole T < 5 MK range. These findings are compared with results of previous studies and discussed in the framework of relevant models. [ABSTRACT FROM AUTHOR]

The soft X-ray background flux (XBF) based on GOES 1–8 Å measurements for the period 1975–2003 is studied. There is strong evidence that in the XBF the flare contribution is not eliminated but the XBF is dominated by flare and post-flare emission of intense events. The significant delay (∼ 2 years) of the peak of the X-ray background flux with regard to sunspot numbers reported for cycle 21 recurs in the present cycle 23. The relation between monthly XBF and sunspot numbers can be well represented by a power law. For cycles 21 and 23 the derived fit values are the same within the uncertainties, whereas the values for cycle 22 are significantly different. It is suggested that the lag of the XBF in cycles 21 and 23 is a secondary effect related to the substantial contribution of energetic flares which is not fully subtracted out by the actual XBF definition. [ABSTRACT FROM AUTHOR]

The important role played by magnetic reconnection in the early acceleration of coronal mass ejections (CMEs) has been widely discussed. However, as CMEs may have expansion speeds comparable to their propagation speeds in the corona, it is not clear whether and how reconnection contributes to the true acceleration and expansion separately. To address this question, we analyze the dynamics of a moderately fast CME on 2013 February 27, associated with a continuous acceleration of its front into the high corona, even though its speed had reached ∼700 km sâˆ'1, which is faster than the solar wind. The apparent acceleration of the CME is found to be due to its expansion in the radial direction. The true acceleration of the CME, i.e., the acceleration of its center, is then estimated by taking into account the expected deceleration caused by the drag force of the solar wind acting on a fast CME. It is found that the true acceleration and the radial expansion have similar magnitudes. We find that magnetic reconnection occurs after the eruption of the CME and continues during its propagation in the high corona, which contributes to its dynamic evolution. Comparison between the apparent acceleration related to the expansion and the true acceleration that compensates the drag shows that, for this case, magnetic reconnection contributes almost equally to the expansion and to the acceleration of the CME. The consequences of these measurements for the evolution of CMEs as they transit from the corona to the heliosphere are discussed. [ABSTRACT FROM AUTHOR]

We investigate the deflection and rotation behaviour of 49 Earth-directed coronal mass ejections (CMEs) spanning the period from 2010 to 2020 aiming to understand the potential influence of coronal holes (CHs) on their trajectories. Our analysis incorporates data from coronagraphic observations captured from multiple vantage points, as well as extreme ultraviolet (EUV) observations utilised to identify associated coronal signatures such as solar flares and filament eruptions. For each CME, we perform a 3D reconstruction using the Graduated Cylindrical Shell (GCS) model. We perform the GCS reconstruction in multiple time steps, from the time at which the CME enters the field of view (FOV) of the coronagraphs to the time it exits. We analyse the difference in the longitude, latitude, and inclination between the first and last GCS reconstructions as possible signatures of deflection/rotation. Furthermore, we examine the presence of nearby CHs at the time of eruption and employ the Collection of Analysis Tools for Coronal Holes (CATCH) to estimate relevant CH parameters, including magnetic-field strength, centre of mass, and area. To assess the potential influence of CHs on the deflection and rotation of CMEs, we calculate the Coronal Hole Influence Parameter (CHIP) for each event and analyse its relationship with their trajectories. A statistically significant difference is observed between CHIP force and the overall change in a CME's direction in the lower corona. The overall change in a CME's direction accounts cumulatively for the change in latitude, longitude, and rotation. This suggests that the CHIP force in the low corona has a significant influence on the overall change in the direction of Earth-directed CMEs. However, as the CME evolves outward, the CHIP force becomes less effective in causing deflection or rotation at greater distances. Additionally, we observe a negative correlation between the deflection rate of the CMEs and their velocity, suggesting that higher velocities are associated with lower deflection rates. Hence, the velocity of a CME, along with the magnetic field from CHs, appears to play a significant role in the deflection of CMEs. By conducting this comprehensive analysis, we aim to enhance our understanding of the complex interplay between CHs, CME trajectories, and relevant factors such as velocity and magnetic-field strength. [ABSTRACT FROM AUTHOR]

Many scientists use coronal hole (CH) detections to infer open magnetic flux. Detection techniques differ in the areas that they assign as open, and may obtain different values for the open magnetic flux. We characterize the uncertainties of these methods, by applying six different detection methods to deduce the area and open flux of a near-disk center CH observed on 2010 September 19, and applying a single method to five different EUV filtergrams for this CH. Open flux was calculated using five different magnetic maps. The standard deviation (interpreted as the uncertainty) in the open flux estimate for this CH ≈ 26%. However, including the variability of different magnetic data sources, this uncertainty almost doubles to 45%. We use two of the methods to characterize the area and open flux for all CHs in this time period. We find that the open flux is greatly underestimated compared to values inferred from in situ measurements (by 2.2–4 times). We also test our detection techniques on simulated emission images from a thermodynamic MHD model of the solar corona. We find that the methods overestimate the area and open flux in the simulated CH, but the average error in the flux is only about 7%. The full-Sun detections on the simulated corona underestimate the model open flux, but by factors well below what is needed to account for the missing flux in the observations. Under-detection of open flux in coronal holes likely contributes to the recognized deficit in solar open flux, but is unlikely to resolve it. [ABSTRACT FROM AUTHOR]

Coronal holes are usually defined as dark structures seen in the extreme ultraviolet and X-ray spectrum which are generally associated with open magnetic fields. Deriving reliably the coronal hole boundary is of high interest, as its area, underlying magnetic field, and other properties give important hints as regards high speed solar wind acceleration processes and compression regions arriving at Earth. In this study we present a new threshold-based extraction method, which incorporates the intensity gradient along the coronal hole boundary, which is implemented as a user-friendly SSW-IDL GUI. The Collection of Analysis Tools for Coronal Holes (CATCH) enables the user to download data, perform guided coronal hole extraction and analyze the underlying photospheric magnetic field. We use CATCH to analyze non-polar coronal holes during the SDO-era, based on 193 Å filtergrams taken by the Atmospheric Imaging Assembly (AIA) and magnetograms taken by the Heliospheric and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory (SDO). Between 2010 and 2019 we investigate 707 coronal holes that are located close to the central meridian. We find coronal holes distributed across latitudes of about ± 60 ∘ , for which we derive sizes between 1.6 × 10 9 and 1.8 × 10 11 km 2 . The absolute value of the mean signed magnetic field strength tends towards an average of 2.9 ± 1.9 G. As far as the abundance and size of coronal holes is concerned, we find no distinct trend towards the northern or southern hemisphere. We find that variations in local and global conditions may significantly change the threshold needed for reliable coronal hole extraction and thus, we can highlight the importance of individually assessing and extracting coronal holes. [ABSTRACT FROM AUTHOR]

In a thorough study, we investigate the origin of a remarkable plasma and magnetic field configuration observed in situ on June 22, 2011, near L1, which appears to be a magnetic ejecta (ME) and a shock signature engulfed by a solar wind high-speed stream (HSS). We identify the signatures as an Earth-directed coronal mass ejection (CME), associated with a C7.7 flare on June 21, 2011, and its interaction with a HSS, which emanates from a coronal hole (CH) close to the launch site of the CME. The results indicate that the major interaction between the CME and the HSS starts at a height of 1.3 R ⊙ up to 3 R ⊙ . Over that distance range, the CME undergoes a strong north-eastward deflection of at least 30 ∘ due to the open magnetic field configuration of the CH. We perform a comprehensive analysis for the CME–HSS event using multi-viewpoint data (from the Solar TErrestrial RElations Observatories, the Solar and Heliospheric Observatory and the Solar Dynamics Observatory), and combined modeling efforts (nonlinear force-free field modeling, Graduated Cylindrical Shell CME modeling, and the Forecasting a CME's Altered Trajectory – ForeCAT model). We aim at better understanding its early evolution and interaction process as well as its interplanetary propagation and related in situ signatures, and finally the resulting impact on the Earth's magnetosphere. [ABSTRACT FROM AUTHOR]

Coronal mass ejections (CMEs) propagating in the heliosphere are exposed to a drag force due to the ambient solar wind. Mass pile-up in interplanetary space can have strong effects on the drag force, and with that on the CME propagation time and energy input to the magnetosphere. For a sample of well observed events, we determine the de-projected 3D mass and its evolution up to a distance range of about 15Rs using combined STEREO-SECCHI COR1 and COR2 data, for which no pile-up at the CME front is found (see also Bein et al., 2013). Applying the GCS forward fitting model (Thernisien et al., 2006, 2009) on COR2 data, we obtain the volume of the CMEs. Working under the assumption that the CME mass is constant beyond 15Rs and that the CME undergoes self-similar expansion, we estimate the CME density at the distance of 1AU. The results are compared to in-situ proton density data measured for the associated ICME's sheath and magnetic structure for which we derive a trend towards a mass increase at the CME front. [ABSTRACT FROM AUTHOR]

The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have been observing the in situ solar wind properties since its arrival to Mars at the end of 2014. Together with the Galactic Cosmic Ray (GCR) observation continuously monitored by the Radiation Assessment Detector (RAD) on the Martian ground, we are able to analyze the correlation of the solar wind evolution and the modulated GCR variations at Mars. The transient variations (mostly observed as short-term decreases) in these in situ observations are usually related to either the impact of Coronal Mass Ejections (CMEs) erupted from Solar active regions or the pass-by of High Speed Streams (HSS) in the solar wind arising from Coronal Holes (CHs) on the Sun. During the opposition phase in 2016 when Earth and Mars were radially aligned on the same side of the Sun, we observe the stable evolution of a few CHs on the solar surface over several solar rotations and analyze the re-current in situ solar wind and GCR signatures at both Earth and Mars. [ABSTRACT FROM AUTHOR]

We present a refined statistical analysis based on ICMEs as well as CIRs for the time period 2003-2015 to estimate the impact of different solar wind types on the geomagnetic activity and the neutral density in the Earth's thermosphere. For the time-based delimitation of the events, we rely on the catalog maintained by Richardson and Cane and the corotating interaction region lists provided by S. Vennerstrom and L.K. Jian. These archives are based on in situ measurements from the ACE and/or the Wind spacecraft. On this basis, we will thoroughly investigate about 400 Earth-directed ICME and CIR events. To verify the impact on the Earths thermosphere we determine neutral mass densities by using accelerometer measurements collected by the low-Earth-orbiting satellites GRACE and CHAMP. Subsequently, the atmospheric densities will be to characteristic ICME parameters and since increased solar activity may lead to a decrease of the satellites orbital altitude we additionally assess the orbital decay for each of the events and satellites. [ABSTRACT FROM AUTHOR]

The rotation of the solar corona is not a fully resolved issue. Coronal holes (CHs) reveal sometimes more rigid, but in other cases more differential rotation profiles. We used two datasets of coronal bright points (CBPs), one within CHs, and one outside of CHs. We analyzed rotation profiles in the two datasets of CBPs and compared them to check if there is any difference between rotation profiles for CBPs within CHs and those outside of CHs. The reported rigidity of the CHs rotational profiles implies that Reynolds stresses, which are considered to be the main drivers of the solar differential profile, should also be different between the two datasets. Therefore, we analyzed the horizontal Reynolds stress for the two datasets as well. We also compared the meridional motion of the two datasets. In all cases the results between the two datasets were the same within the observational error. In both datasets the solar rotation profile is significantly differential and similar to the photospheric profile. [ABSTRACT FROM AUTHOR]

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. We present the 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 Advanced Composition Explorer (ACE) in-situ measurements. For the purposes 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 high-activity time interval considered. The quality of the modeled solar-wind parameters is found to be rather variable. Therefore, to better understand the results obtained we also qualitatively inspected characteristics of coronal holes, i.e. the 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 improvement of the model. [ABSTRACT FROM AUTHOR]

Current efforts in space weather forecasting of CMEs have been focused on predicting their arrival time and magnetic structure. To make these predictions, methods have been developed to derive the true CME speed, size, position, and mass, among others. Difficulties in determining the input parameters for CME forecasting models arise from the lack of direct measurements of the coronal magnetic fields and uncertainties in estimating the CME 3D geometric and kinematic parameters after eruption. White-light coronagraph images are usually employed by a variety of CME reconstruction techniques that assume more or less complex geometries. This is the first study from our International Space Science Institute (ISSI) team "Understanding Our Capabilities in Observing and Modeling Coronal Mass Ejections", in which we explore how subjectivity affects the 3D CME parameters that are obtained from the Graduated Cylindrical Shell (GCS) reconstruction technique, which is widely used in CME research. To be able to quantify such uncertainties, the "true" values that are being fitted should be known, which are impossible to derive from observational data. We have designed two different synthetic scenarios where the "true" geometric parameters are known in order to quantify such uncertainties for the first time. We explore this by using two sets of synthetic data: 1) Using the ray-tracing option from the GCS model software itself, and 2) Using 3D magnetohydrodynamic (MHD) simulation data from the Magnetohydrodynamic Algorithm outside a Sphere code. Our experiment includes different viewing configurations using single and multiple viewpoints. CME reconstructions using a single viewpoint had the largest errors and error ranges overall for both synthetic GCS and simulated MHD white-light data. As the number of viewpoints increased from one to two, the errors decreased by approximately 4 ° in latitude, 22 ° in longitude, 14 ° in tilt, and 10 ° in half-angle. Our results quantitatively show the critical need for at least two viewpoints to be able to reduce the uncertainty in deriving CME parameters. We did not find a significant decrease in errors when going from two to three viewpoints for our specific hypothetical three spacecraft scenario using synthetic GCS white-light data. As we expected, considering all configurations and numbers of viewpoints, the mean absolute errors in the measured CME parameters are generally significantly higher in the case of the simulated MHD white-light data compared to those from the synthetic white-light images generated by the GCS model. We found the following CME parameter error bars as a starting point for quantifying the minimum error in CME parameters from white-light reconstructions: Δ θ (latitude)= 6 ° - 3 ° + 2 ° , Δ ϕ (longitude)= 11 ° - 6 ° + 18 ° , Δ γ (tilt)= 25 ° - 7 ° + 8 ° , Δ α (half-angle)= 10 ° - 6 ° + 12 ° , Δ h (height)= 0.6 - 0.4 + 1.2 R ⊙ , and Δ κ (ratio)= 0.1 - 0.02 + 0.03 . [ABSTRACT FROM AUTHOR]

Progress in space weather research and awareness needs community-wide strategies and procedures to evaluate our modeling assets. Here we present the activities of the Ambient Solar Wind Validation Team embedded in the COSPAR ISWAT initiative. We aim to bridge the gap between model developers and end-users to provide the community with an assessment of the state-of-the-art in solar wind forecasting. To this end, we develop an open online platform for validating solar wind models by comparing their solutions with in situ spacecraft measurements. The online platform will allow the space weather community to test the quality of state-of-the-art solar wind models with unified metrics providing an unbiased assessment of progress over time. In this study, we propose a metadata architecture and recommend community-wide forecasting goals and validation metrics. We conclude with a status update of the online platform and outline future perspectives. [ABSTRACT FROM AUTHOR]

The Kanzelhöhe Observatory (KSO) was founded during World War II by the Deutsche Luftwaffe (German Airforce) as one station of a network of observatories that were set up to provide information on solar activity in order to better assess the actual conditions of the Earth's ionosphere in terms of radio-wave propagation. Solar observations began in 1943 with photographs of the photosphere and drawings of sunspots, plage regions, and faculae, as well as patrol observations of the solar corona. At the beginning, all data were sent to Freiburg (Germany). After WW II, international cooperation was established and the data were sent to Zurich, Paris, Moscow, and Greenwich. Relative sunspot numbers have been derived since 1944. The agreement between relative sunspot numbers derived at KSO and the new International Sunspot Number (ISN) (SILSO World Data Center in International Sunspot Number Monthly Bulletin and online catalogue, 1945 - 2015) lies within ${\approx}\,10~\%$ . However, revisiting the historical data, we also find periods with larger deviations. The reasons for the deviations were twofold: On the one hand, a major instrumental change took place during which the instrument was relocated and modified. On the other hand, a period of frequent replacements of personnel caused significant deviations; this clearly shows the importance of experienced observers. In the long term, the instrumental improvements led to better image quality. Additionally, we find a long-term trend towards better seeing conditions that began in 2000. [ABSTRACT FROM AUTHOR]

In this study, we give correlations between the geomagnetic storm (GS) intensity and parameters of solar and interplanetary (IP) phenomena. We also perform 3D geometry reconstructions of geo-effective coronal mass ejections (CMEs) using the recently developed PyThea framework and compare on-sky and de-projected parameter values, focusing on the reliability of the de-projection capabilities. We utilize spheroid, ellipsoid and graduated cylindrical shell models. In addition, we collected a number of parameters of the GS-associated phenomena. A large variation in 3D de-projections is obtained for the CME speeds depending on the selected model for CME reconstruction and observer subjectivity. A combination of fast speed and frontal orientation of the magnetic structure upon its arrival at the terrestrial magnetosphere proves to be the best indicator for the GS strength. More reliable estimations of geometry and directivity, in addition to de-projected speeds, are important for GS forecasting in operational space weather schemes. [ABSTRACT FROM AUTHOR]

Abstract: We analyze the relationship between some space weather indices (Dst, Ap, F10.7) and geomagnetic effects on the regional (European) scale, over the period 1960–2001. The remaining external field signal (RES) detected in the Northward magnetic component of the European observatory annual means are used as an indicator of the regional geomagnetic activity. Relationship RES-F10.7 suggests correction factors for getting the geomagnetic annual means of the Northern component less affected by the external sources. We have found some time lags among investigated parameters. These delays may suggest that the Ap responds to the solar activity in a differently than Dst and RES, Ap being more sensitive to the high-speed streams (HSS) and the Alfvenic waves present in HSS, while Dst and RES being more influenced by the coronal mass ejections activity (CME). [Copyright &y& Elsevier]

We consider an expanding three-dimensional (3-D) piston as a driver of an MHD shock wave. It is assumed that the source-region surface accelerates over a certain time interval to achieve a particular maximum velocity. Such an expansion creates a large-amplitude wave in the ambient plasma. Owing to the nonlinear evolution of the wavefront, its profile steepens and after a certain time and distance a discontinuity forms, marking the onset of the shock formation. We investigate how the formation time and distance depend on the acceleration phase duration, the maximum expansion velocity (defining also acceleration), the Alfvén velocity (defining also Mach number), and the initial size of the piston. The model differs from the 1-D case, since in the 3-D evolution, a decrease of the wave amplitude with distance must be taken into account. We present basic results, focusing on the timing of the shock formation in the low- and high-plasma-beta environment. We find that the shock-formation time and the shock-formation distance are (1) approximately proportional to the acceleration phase duration; (2) shorter for a higher expansion velocity; (3) larger in a higher Alfvén speed environment; (4) only weakly dependent on the initial source size; (5) shorter for a stronger acceleration; and (6) shorter for a larger Alfvén Mach number of the source surface expansion. To create a shock causing a high-frequency type II burst and the Moreton wave, the source region expansion should, according to our results, achieve a velocity on the order of 1000 km s−1 within a few minutes, in a low Alfvén velocity environment. [ABSTRACT FROM AUTHOR]

The Drag-based Model (DBM) is a 2D analytical model for heliospheric propagation of Coronal Mass Ejections (CMEs) in ecliptic plane predicting the CME arrival time and speed at Earth or any other given target in the solar system. It is based on the equation of motion and depends on initial CME parameters, background solar wind speed, w and the drag parameter γ . A very short computational time of DBM (< 0.01 s) allowed us to develop the Drag-Based Ensemble Model (DBEM) that takes into account the variability of model input parameters by making an ensemble of n different input parameters to calculate the distribution and significance of the DBM results. Thus the DBEM is able to calculate the most likely CME arrival times and speeds, quantify the prediction uncertainties and determine the confidence intervals. A new DBEMv3 version is described in detail and evaluated for the first time determining the DBEMv3 performance and errors by using various CME–ICME lists and it is compared with previous DBEM versions, ICME being a short-hand for interplanetary CME. The analysis to find the optimal drag parameter γ and ambient solar wind speed w showed that somewhat higher values (γ ≈ 0.3 × 10 − 7 km−1, w ≈ 425 km s−1) for both of these DBEM input parameters should be used for the evaluation than the previously employed ones. Based on the evaluation performed for 146 CME–ICME pairs, the DBEMv3 performance with mean error (ME) of −11.3 h, mean absolute error (MAE) of 17.3 h was obtained. There is a clear bias towards the negative prediction errors where the fast CMEs are predicted to arrive too early, probably due to the model physical limitations and input errors (e.g. CME launch speed). This can be partially reduced by using larger values for γ resulting in smaller prediction errors (ME = − 3.9 h, MAE = 14.5 h) but at the cost of larger prediction errors for single fast CMEs as well as larger CME arrival speed prediction errors. DBEMv3 showed also slight improvement in the performance for all calculated output parameters compared to the previous DBEM versions. [ABSTRACT FROM AUTHOR]

Coronal holes are large-scale structures in the solar atmosphere that feature a reduced temperature and density in comparison to the surrounding quiet Sun and are usually associated with open magnetic fields. We perform a differential emission measure analysis on the 707 non-polar coronal holes in the Collection of Analysis Tools for Coronal Holes (CATCH) catalog to derive and statistically analyze their plasma properties (i.e. temperature, electron density, and emission measure). We use intensity filtergrams of the six coronal EUV filters from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, which cover a temperature range from ≈ 10 5.5 to 10 7.5 K . Correcting the data for stray and scattered light, we find that all coronal holes have very similar plasma properties with an average temperature of 0.94 ± 0.18 MK , a mean electron density of (2.4 ± 0.7) × 10 8 cm − 3 , and a mean emission measure of (2.8 ± 1.6) × 10 26 cm − 5 . The temperature distribution within the coronal holes was found to be largely uniform, whereas the electron density shows a 30 to 40% linear decrease from the boundary towards the inside of the coronal hole. At distances greater than 20″ (≈ 15 Mm ) from the nearest coronal hole boundary, the density also becomes statistically uniform. The coronal hole temperature may show a weak solar-cycle dependency, but no statistically significant correlation of plasma properties with solar-cycle variations could be determined throughout the observed period between 2010 and 2019. [ABSTRACT FROM AUTHOR]

A joint campaign of various space-borne and ground-based observatories, comprising the Japanese Hinode mission (Hinode Observing Plan 338, 20 – 30 September 2017), the GREGOR solar telescope, and the Vacuum Tower Telescope (VTT), investigated numerous targets such as pores, sunspots, and coronal holes. In this study, we focus on the coronal hole region target. On 24 September 2017, a very extended non-polar coronal hole developed patches of flux emergence, which contributed to the decrease of the overall area of the coronal hole. These flux emergence patches erode the coronal hole and transform the area into a more quiet-Sun-like area, whereby bipolar magnetic structures play an important role. Conversely, flux cancellation leads to the reduction of opposite-polarity magnetic fields and to an increase in the area of the coronal hole. Other global coronal hole characteristics, including the evolution of the associated magnetic flux and the aforementioned area evolution in the EUV, are studied using data of the Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The interplanetary medium parameters of the solar wind display values compatible with the presence of the coronal hole. Furthermore, a particular transient is found in those parameters. [ABSTRACT FROM AUTHOR]

We use Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) data to reconstruct the plasma properties from differential emission measure (DEM) analysis for a previously studied long-lived, low-latitude coronal hole (CH) over its lifetime of ten solar rotations. We initially obtain a non-isothermal DEM distribution with a dominant component centered around 0.9 MK and a secondary smaller component at 1.5 – 2.0 MK. We find that deconvolving the data with the instrument point spread function (PSF) to account for long-range scattered light reduces the secondary hot component. Using the 2012 Venus transit and a 2013 lunar eclipse to test the efficiency of this deconvolution, significant amounts of residual stray light are found for the occulted areas. Accounting for this stray light in the error budget of the different AIA filters further reduces the secondary hot emission, yielding CH DEM distributions that are close to isothermal with the main contribution centered around 0.9 MK. Based on these DEMs, we analyze the evolution of the emission measure (EM), density, and averaged temperature during the CH's lifetime. We find that once the CH is clearly observed in EUV images, the bulk of the CH plasma reveals a quite constant state, i.e. temperature and density reveal no major changes, whereas the total CH area and the photospheric magnetic fine structure inside the CH show a distinct evolutionary pattern. These findings suggest that CH plasma properties are mostly "set" at the CH formation or/and that all CHs have similar plasma properties. [ABSTRACT FROM AUTHOR]

Large-scale solar coronal compressive waves are often observed in extreme UV (EUV) and white light to precede nascent coronal mass ejections (CMEs), which previous work has shown develop most dynamically (expansion, acceleration) in the low and middle solar corona (below 5-8 solar radii). Multiple studies in the last ten years have suggested that these waves may be manifestations of driven coronal shock waves, and may accelerate ions to solar energetic particle (SEP) energies. A commonly invoked condition for the generation of EUV waves and their capability to produce energetic particles is the presence of rapid lateral expansion of the front and driver behind it. As a step to characterizing this capability, we study the radial and lateral kinematics of a number of EUV off-limb waves in the low corona, and their departure from spherical expansion. We characterize their time-dependent angular extent. We compare the results with the later-stage CME angular sizes and radial kinematics deduced from SOHO/LASCO observations of the events. [ABSTRACT FROM AUTHOR]

We present a statistical study on CME arrival prediction using ELEvoHI (ELlipse Evolution model based on Heliospheric Imager observations) ensemble modeling. ELEvoHI is the current state-of-the-art HI elongation fitting method that utilizes heliospheric imager data obtained by the STEREO (Solar TErrestrial RElations Observatory) twin spacecraft assuming that the drag force exerted by the ambient solar wind is the dominant force influencing the CME propagation in the IP-space. The HI time-elongation profiles needed by ELEvoHI as well as the in-situ data for validating the results are taken from the FP7 HELCATS project. GCS (Graduated Cylindrical Shell) fitting is applied for each CME separately. We perform a cut in the ecliptic plane, based on the GCS fit, to derive the initial values needed for the ELEvoHI ensemble modeling. In this study, we select CMEs in a 12-month interval (June 2009 to June 2010) corresponding to a location of STEREO-B close to Lagrange point 5 (60° trailing Earth), making the model results valuable for future studies (STEREO-A near L5 in mid 2020) and for a planned L5 mission. Our analysis contains two parts: First, a contingency table (hit/false alarms/misses) with the corresponding skill scores and second the times and speeds for the predicted and observed Earth arriving events. The statistical results are compared to other studies and will serve as benchmark for future enhanced ELEvoHI versions. [ABSTRACT FROM AUTHOR]

Coronal Mass Ejections (CMEs) and their Interplanetary counterparts (ICMEs) are the primary source of strong space weather disturbances at Earth and other places in the heliosphere. Key parameters determining the geo-effectiveness of CMEs are their plasma dynamic pressure and internal magnetic field intensity and orientation. In addition, phenomena such as the interaction with other CME structures along the way, or the pre-conditioning of interplanetary (IP) space due to the passage of previous CMEs, can significantly modify the properties of single CME events and influence their geo-effectiveness. Therefore, investigating and modeling such phenomena via physics-based heliospheric models is crucial in order to assess and improve our space weather prediction capability in relation to complex CME events.In this regard, we present a comprehensive analysis of the CME events that erupted from AR 12673 during the unusually active week of September 4-10, 2017, with the aim of validating for the first time the prediction capabilities of the EUHFORIA model in the case of complex CME events. As AR 12673 rotated along with the solar disk, CMEs were launched over a wide range of longitudes, interacting with each other and paving the way for the propagation of the following CMEs. Following the eruptions, ICME-related signatures were observed at both Earth and Mars, while associated particle events were reported at Earth, Mars, and STEREO-A. In terms of impact on Earth, an intense geomagnetic storm, triggered by a strong southward magnetic field associated to an ICME sheath, was recorded on September 8, 2017.In order to study these CME-CME interactions and their influence on the geo-effectiveness of single CMEs, we simulate the events using the EUHFORIA model. With the intent of preserving a predictive approach, we use kinematic, geometric and magnetic input parameters for the CMEs as derived from remote-sensing and multi-spacecraft observations of the CMEs and their source regions. We model CMEs first using an over-simplified cone model, and then a more realistic flux- rope model so to quantify the improvement in the prediction of the interplanetary magnetic field and CME geo-effectiveness at Earth in the latter case. Furthermore, we investigate the modelling of CME-CME interactions considering the spatial and temporal evolution of ICMEs in terms of their shocks, sheaths and ejecta structures in the heliosphere, and we quantify the impact of such phenomena on the propagation and evolution of single CME events. Results from this study will not only benchmark our current prediction capabilities in the case of complex CME events, but will also provide better insights on the large-scale evolution and interaction of complex CME events in the inner heliosphere. [ABSTRACT FROM AUTHOR]

On July 25 2017 a multi-step Forbush decrease (FD) with the total amplitude of more than 15% was observed by MSL/RAD at Mars and this is one of the biggest FDs ever detected on Mars. We find that these particle signatures are related to very pronounced plasma and magnetic field signatures detected in situ by STEREO-A on July 24 2017, with a higher than average total magnetic field strength reaching more than 60 nT. In the observed time period STEREO-A was longitudinally close to Mars and both were located at the back side of Sun as viewed from Earth. Using multi-spacecraft and multi-instrument (both in situ and remote-sensing) observations, as well as modelling, we find that the solar sources of these in situ signatures are 2 CMEs which erupted on July 23 2017 from the same source region on the back side of the Sun as viewed from Earth and interacted in the interplanetary space, inhibiting the expansion of one of the CMEs. We present a detailed investigation on this complex interaction event on its way from Sun to Mars. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 745782. [ABSTRACT FROM AUTHOR]

In this study, we investigate in detail the photospheric magnetic structure of 98 coronal holes using line-of-sight magnetograms of SDO/HMI, and for a subset of 42 coronal holes using HINODE/SOT G-band filtergrams. We divided the magnetic field maps into magnetic elements and quiet coronal hole regions by applying a threshold at ±25 G. We find that the number of magnetic bright points in magnetic elements is well correlated with the area of the magnetic elements (cc = 0.83 ± 0.01). Further, the magnetic flux of the individual magnetic elements inside coronal holes is related to their area by a power law with an exponent of 1.261 ± 0.004 (cc = 0.984 ± 0.001). Relating the magnetic elements to the overall structure of coronal holes, we find that on average (69 ± 8)% of the overall unbalanced magnetic flux of the coronal holes arises from long-lived magnetic elements with lifetimes > 40 h. About (22 ± 4)% of the unbalanced magnetic flux arises from a very weak background magnetic field in the quiet coronal hole regions with a mean magnetic field density of about 0.2−1.2 G. This background magnetic field is correlated to the flux of the magnetic elements with lifetimes of > 40 h (cc = 0.88 ± 0.02). The remaining flux arises from magnetic elements with lifetimes < 40 h. By relating the properties of the magnetic elements to the overall properties of the coronal holes, we find that the unbalanced magnetic flux of the coronal holes is completely determined by the total area that the long-lived magnetic elements cover (cc = 0.994 ± 0.001). [ABSTRACT FROM AUTHOR]

Magnetic-field reconnection is believed to play a fundamental role in magnetized plasma systems throughout the Universe, including planetary magnetospheres, magnetars and accretion disks around black holes. This letter presents extreme ultraviolet and X-ray observations of a solar flare showing magnetic reconnection with a level of clarity not previously achieved. The multi-wavelength extreme ultraviolet observations from SDO/AIA show inflowing cool loops and newly formed, outflowing hot loops, as predicted. RHESSI X-ray spectra and images simultaneously show the appearance of plasma heated to >10 MK at the expected locations. These two data sets provide solid visual evidence of magnetic reconnection producing a solar flare, validating the basic physical mechanism of popular flare models. However, new features are also observed that need to be included in reconnection and flare studies, such as three-dimensional non-uniform, non-steady and asymmetric evolution. [ABSTRACT FROM AUTHOR]

We analyze the relationship between the acceleration of coronal mass ejections (CMEs) and the energy release in associated flares, employing a sample of 22 events in which the CME kinematics were measured from the pre-eruption stage up to the post-acceleration phase. The data show a distinct correlation between the duration of the acceleration phase and the duration of the associated soft X-ray (SXR) burst rise, whereas the CME peak acceleration and velocity are related to the SXR peak flux. In the majority of events the acceleration started earlier than the SXR burst, and it is usually prolonged after the SXR burst maximum. In about one half of the events the acceleration phase is very closely synchronized with the fastest growth of the SXR burst. An additional one quarter of the events may be still considered as relatively well-synchronized, whereas in the remaining quarter of the events there is a considerable mismatch. The results are interpreted in terms of the feedback relationship between the CME dynamics and the reconnection process in the wake of the CME. [ABSTRACT FROM AUTHOR]

We study kinematics of 22 coronal mass ejections (CMEs) whose motion was traced from the gradual pre-acceleration phase up to the post-acceleration stage. The peak accelerations in the studied sample range from 40, up to 7000 m s−2, and are inversely proportional to the acceleration phase duration and the height range involved. Accelerations and velocities are, on average, larger in CMEs launched from a compact source region. The acceleration phase duration is proportional to the source region dimensions; i.e., compact CMEs are accelerated more impulsively. Such behavior is interpreted as a consequence of stronger Lorentz force and shorter Alfvén time scales involved in compact CMEs (with stronger magnetic field and larger Alfvén speed being involved at lower heights). CMEs with larger accelerations and velocities are on average wider, whereas the widths are not related to the source region dimensions. Such behavior is explained in terms of the field pile-up ahead of the erupting structure, which is more effective in the case of a strongly accelerated structure. [ABSTRACT FROM AUTHOR]

One of the very common in situ signatures of interplanetary coronal mass ejections (ICMEs), as well as other interplanetary transients, are Forbush decreases (FDs), i.e. short-term reductions in the galactic cosmic ray (GCR) flux. A two-step FD is often regarded as a textbook example, which presumably owes its specific morphology to the fact that the measuring instrument passed through the ICME head on, encountering first the shock front (if developed), then the sheath, and finally the CME magnetic structure. The interaction of GCRs and the shock/sheath region, as well as the CME magnetic structure, occurs all the way from Sun to Earth, therefore, FDs are expected to reflect the evolutionary properties of CMEs and their sheaths. We apply modeling to different ICME regions in order to obtain a generic two-step FD profile, which qualitatively agrees with our current observation-based understanding of FDs. We next adapt the models for energy dependence to enable comparison with different GCR measurement instruments (as they measure in different particle energy ranges). We test these modeling efforts against a set of multi-spacecraft observations of the same event, using the Forbush decrease model for the expanding flux rope (ForbMod). We find a reasonable agreement of the ForbMod model for the GCR depression in the CME magnetic structure with multi-spacecraft measurements, indicating that modeled FDs reflect well the CME evolution. [ABSTRACT FROM AUTHOR]

EUV waves are large-scale propagating disturbances in the solar corona initiated by coronal mass ejections. We investigate the multiple EUV wave reflections at a coronal hole boundary, as observed by SDO/AIA on 1 April 2017. The EUV wave originates from Active Region (AR) 12645 close to the disk center and propagates toward the south polar coronal hole with an average velocity of 430 km/s. The interaction of the EUV wave with the coronal hole, which represents a region of high Alfven speed, is observed as a splitting into two wave components: one continues propagation inside the coronal hole with an increased velocity of 850 km/s (transmitted wave), while the other one moves back toward the AR, also with an increased velocity of 600 km/s (reflected wave). The reflected EUV wave is subsequently reflected again from the AR and propagates toward the coronal hole with an average velocity of 350 km/s, where it is reflected for the second time at the coronal hole boundary and propagates again toward the AR with a velocity of 300 km/s. These events are observed over an interval of 40 minutes. The high cadence SDO imagery allows us to study in detail the kinematics of the direct and multiple times reflected EUV wave. In addition, its multi-wavelength EUV imagery allows us to derive the plasma properties of the corona and the EUV wave pulse via Differential Emission Measure analysis. These results are used to compare the observed characteristics of the wave interaction with the coronal hole with simulations. [ABSTRACT FROM AUTHOR]

The extreme X8.2 event of 2017 September 10 provides unique observations to study thegenesis, magnetic morphology, impulsive dynamics and shock formation in a very fastcoronal mass ejection (CME). Combining GOES-16/SUVI and SDO/AIA EUVimagery, we identify a hot (T ≈ 10–15 MK) bright rim around a quickly expandingcavity, embedded inside a much larger CME shell (T ≈ 1–2 MK). The CME shelldevelops from a dense set of large AR loops (&#x003E;0.5Rs) and seamlessly evolves into theCME front observed in LASCO C2. The strong lateral overexpansion of the CMEshell acts as a piston initiating the fast EUV shock wave. The hot cavity rim isdemonstrated to be a manifestation of the dominantly poloidal flux and frozen-inplasma added to the rising flux rope by magnetic reconnection in the current sheetbeneath. The same structure is later observed as the core of the white-light CME,challenging the traditional interpretation of the CME three-part morphology. The largeamount of added magnetic flux suggested by these observations explains the extremeaccelerations of the radial and lateral expansion of the CME shell and cavity, allreaching values up to 5–10 km s−2. The acceleration peaks occur simultaneously withthe first RHESSI 100–300 keV hard X-ray burst of the associated flare, furtherunderlining the importance of the reconnection process for the impulsive CME evolution.Finally, the much higher radial propagation speed of the flux rope in relation to theCME shell causes a distinct deformation of the white-light CME front and shock. [ABSTRACT FROM AUTHOR]

The Radiation Assessment Detector (RAD) onboard the Mars Science Laboratory (MSL) mission's Curiosity rover has been measuring galactic cosmic rays (GCR) as well as solar energetic particles (SEP) on the surface of Mars for more than 6 years since its landing in August 2012 and in interplanetary space during its 8-month cruise to Mars between 2011 and 2012. The observations include a large number of Forbush decreases (FD) caused by interplanetary coronal mass ejections (ICMEs) and their associated shocks passing MSL.Our previous work (Freiherr von Forstner et al. 2018, JGR: Space Physics) studied 15 ICME events close to oppositions of Mars as seen from Earth or the STEREO A and B spacecraft, where in situ Forbush decrease observations at both locations could be used to derive the propagation time of the ICME from 1 AU to Mars. We found that on average, ICMEs in our sample continued to decelerate beyond 1 AU.We now investigate a different constellation where MSL/RAD Forbush decrease measurements are combined with remote tracking of ICMEs using the STEREO Heliospheric Imager (HI) telescopes. A large catalog of such remote observations was created by the HELCATS project (Möstl et al. 2017, Space Weather), not only including ICMEs propagating towards Earth, but also some that passed Mars.This allows to enlarge our sample for a statistical study of ICMEs at Mars. We associate STEREO-HI observations from the catalog with corresponding FDs at MSL/RAD and study the accuracy when predicting the arrival of an ICME at Mars using common models applied to HI data.Based on the catalogue of events built using this method, we also investigate the properties of the corresponding Forbush decreases at RAD, such as their magnitude, steepness and duration. We find both correlations between the parameters themselves as well as possible relations to the ICME properties (derived from HI data). These data are also compared to findings from previous studies using Earth-based observations. [ABSTRACT FROM AUTHOR]

Modelling accurately the ambient solar wind is important for space weather forecasting. EUHFORIA (European Heliospheric Forecasting Information Asset) model employs an empirical solar wind model that is based on the Wang-Sheeley-Arge model. It combines the Potential Field Source Surface (PFSS) and the Schatten Current Sheet (SCS) models. In previous studies it was shown that placing the inner boundary of the SCS model at a radius, Ri, lower than that of the outer boundary of the PFSS model, Rii, improves the simulation output. Here, we investigate the capability of the empirical solar wind model adopted in EUHFORIA to recreate the geometry and size of coronal holes for a large set of pairs of PFSS and SCS radii. We vary Rii within the interval [1.4, 3.0]Rs with a step of 0.1Rs, and the Ri within the interval [1.3, 2.8]Rs with the same step size. The study is repeated for 12 coronal holes of different latitudinal position and geometry. We compare the modelled coronal holes with boundaries obtained by remote sensing EUV observations using the CATCH tool (Collection of Analysis Tools for Coronal Holes). Preliminary results of the study indicate that a previously defined pair of PFSS and SCS radii results in underestimated coronal hole sizes. It also indicates that different radii sets give better results for different types of coronal holes. [ABSTRACT FROM AUTHOR]

Operational CME arrival prediction is mainly conducted using magnetohydrodynamic models based on coronagraph observations and magnetograms. Although the Solar TErrestrial RElations Observatory with its heliospheric imagers (HI) provides the possibility to trace a CME's propagation along its path from the Sun to 1 AU, these data can hardly be used to predict CME arrivals in real time (except for a few events in an early phase of the mission). One of the main reasons for that is a large number of data gaps in beacon data, which is available in near real time (in contrast to the complete science data), impeding a proper measurement of the CME front.With regard to a possible future L5 mission carrying HIs we investigate the most suitable way of extracting the time-elongation track of CMEs from HI observations leading to a prediction with the highest possible accuracy. As a first step to reach this goal, we use time-elongation tracks measured from STEREO/HI science data and provided by the FP7 HELCATS team as well as tracks derived using time-elongation maps and tracks measured directly in an HI image time series. These time-elongation tracks are further used as input to our CME ensemble prediction tool ELEvoHI (ELlipse Evolution model based on HI data), which assumes a drag-based interplanetary CME propagation and an elliptical CME frontal shape. ELEvoHI produces post-event predictions of arrival times and speeds at 1 AU for all tracks of each CME under study. By comparing the prediction results from several ways of tracking we attempt to deduce a preferable approach for future studies, e.g. when using data from Parker Solar Probe's Wide-Field Imager for Solar Probe (WISPR), and maybe for future real time predictions when STEREO-A approaches the L5 point. [ABSTRACT FROM AUTHOR]

Stellar activity is mainly characterized by the high energy phenomena such as outbreaks of radiation (flares) and sporadic expulsions of particles into the astrosphere termed coronal mass ejections (CMEs). Both phenomena are known to cause space weather in our solar system. On stars, flares and their parameters are well determined, in contrast to CMEs; their parameters are still not determined statistically. Both phenomena may have severe effects on planetary atmospheres and, in addition, stellar CMEs may play an important role in stellar mass and angular momentum loss and therefore in stellar evolution. Flares are directly detectable from photometric observations, whereas the detection of CMEs requires different observational methods. CMEs have different signatures in different wavelength regimes. Most of the stellar CMEs were detected so far using the method of Doppler-shifted Balmer flux which is accessible via spectroscopic measurements. Several observational programs have been carried out and are planned for the future, including new observations and archival data. Also several attempts to detect stellar CMEs via radio emission have been carried out in the past decades. Finally, the X-ray regime may provide a valuable data pool to look for so-called dimmings, well-known from the Sun, which are closely related to CMEs and which are detectable in X-ray light curves. So far stellar CMEs have been detected rarely and only a handful of distinct events is known, mainly for dMe stars. We report on past, ongoing and future campaigns of stellar CMEs on F-, G-, K-, and M-type pre- and main-sequence stars. [ABSTRACT FROM AUTHOR]

Coronal mass ejections and flares are solar eruptive phenomena responsible for space weather activities near Earth. They can accelerate particles, and generate shock waves which are a threat to our technologies at Earth and in space. Therefore, predicting shock arrival at Earth has been an important goal for space weather. Space based radio observations provide the unique opportunity to track shock waves in the inner heliosphere. We present the study of CME/flare event on September 27/28, 2012. The GOES C3.1 flare that originated from NOAA AR 1577 was associated with a full-halo CME (first seen in SOHO/LASCO C2 field of view at 23:47) and white light shock wave observed by all three spacecraft STEREO A, STEREO B, and SOHO. The associated radio event shows a group of type III bursts and two somewhat unusual type II bursts with significantly different starting frequencies. To understand the origin of the two shock waves we performed multi-wavelength study, and perform radio triangulation to get their source position in the 3D space. For the radio triangulation study, we used goniopolarimetric measurements from STEREO/WAVES and WIND/WAVES instruments. We also did data-driven modelling of the CME propagation using EUHFORIA cone model (EUropean Heliospheric FORecasting Information Asset) and validate the results by comparison with in-situ data. Results of this study indicate that, although temporal association between the shock and the CME is good, the low frequency type II burst occurs significantly higher in the solar corona than the associated CME and has therefore unclear origin. To understand the origin of the low frequency type II burst we studied preceding event at 10:20 UT (STEREO A/COR2) on September 27, 2012. The radio triangulation study shows that the type II source positions are in the southern solar hemisphere and thus may be associated to the type II emissions in the radio event succeeding it. We therefore discuss different possibilities for the origin of two type II bursts. [ABSTRACT FROM AUTHOR]