arXiv subjects

M. Temmer

Publications and source records attributed to M. Temmer.

At least 19 recordsLinked to original sources

arXiv2026

A view of the evolution of a CME and the associated wave-trains at high spatial and temporal resolution

Context. Studying the kinematic and dynamic evolution of fast eruptive events from the middle to high solar corona is a primary objective of the Metis coronagraph on Solar Orbiter. During perihelion, Metis acquires visible light images at a 20s cadence, reaching a spatial resolution of around 2000 km at 0.28 au. This enables capturing coronal mass ejections (CMEs) and transient structures with unprecedented spatial and temporal resolution. Aims. On October 8-9, 2022, an extensive CME was observed by Metis at 0.3 au (resolution: 4.4 $10^{3}$ km/pixel). We aim to exploit this high resolution to resolve multiple substructures within the CME front, revealing plasma elements with distinct trajectories and speeds to provide a detailed kinematic characterization of the eruption. Methods. A normalization-based running difference algorithm was applied to enhance the complex morphology. Height-time diagrams were used to estimate propagation speeds and frequency variations. A 3D flux rope reconstruction, combined with multi-spacecraft coronagraphs and disk imagers, enabled tracking the CME from its initiation in the lower corona to approximately 5 solar radii. Joint observations with Solar Orbiter EUI-FSI provided insights into the eruption's onset, while Metis captured its development into the middle corona. Results. Metis observations resolve the CME's fine structure and internal plasma motions. The detection of circular, fast-propagating wavefronts (500 km/s, 3 minute period) at the western flank suggests wave excitation and magnetic reconfiguration processes. Multiple interpretations are proposed for these coronal wave trains, including quasi periodic propagating fast modes, offering new insights into wave generation and energy transport in the solar corona.

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arXiv2025

Solar cycle evolution of ICME sheath regions at 1 AU

We investigate the evolution of interplanetary coronal mass ejection (ICME) sheath regions at 1 AU across solar cycles 23, 24, and the rising phase of 25, focusing on their variability and turbulence in relation to upstream solar wind conditions and the global heliospheric state. Using a dataset of over 900 ICME sheath events, we apply statistical metrics such as the interquartile range (IQR) and the Turbulence Index (TI) to quantify variability and turbulence. The analysis compares full and rising phases of solar cycles and examines both local ICME parameters (e.g., sheath total pressure, non-radial flows) and global interplanetary indicators such as open solar flux (OSF). From SC23 to SC24, sheath total pressure and magnetic field strength decreased by over 40% and 25%, respectively, accompanied by reduced turbulence and variability. In contrast, the rising phase of SC25 shows increased magnetic complexity, particularly in non-radial field components, despite stable bulk parameters. Non-radial flow patterns also shift from tangentially dominated in SC23-SC24 to normal-dominated in SC25, suggesting changes in ICME orientation and sheath formation mechanisms. No significant correlation is found between OSF and sheath properties, indicating that local solar wind and ICME specific factors are the primary drivers of sheath evolution. The study reinforces the importance of upstream solar wind dynamics in relation to variations in plasma and magnetic field measured components of ICME sheaths. The derived trends in turbulence, magnetic orientation, and flow geometry suggest that sheath regions are sensitive indicators of solar cycle phase and should be considered as distinct, structured components in ICME modeling.

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arXiv2025

Comparative analysis of two episodes of strongly geoeffective CME events in November and December 2023

In autumn 2023, a series of closely timed eruptive events were observed remotely and measured in situ. We studied analogous solar events, where several CMEs were launched partly from the same (active) regions near a CH. These events occurred in two episodes, separated by a full solar rotation, covering October 31-November 3 and November 27-28, 2023. Both episodes are linked to strong geomagnetic storms on November 4-5 and December 1-2, 2023. We aim to understand the complexity of these events and how the global magnetic field, solar wind conditions, and structural interactions relate to the observed geomagnetic effects. Using the GCS 3D reconstruction method, we derived each CME's motion direction and speed. These results were input into the DBM with enhanced latitudinal information (3D DBM), aiding in connecting in-situ measurements with solar surface structures for integrated interpretation. The first episode caused SAR arcs, with a three-step Dst index drop to -163 nT on November 5, 2023. Two CME-related shocks arrived close in time, separated by a SBC, followed by a short-duration flux rope-like structure. The second episode saw auroral lights and a two-step Dst index drop to -108 nT on December 1, 2023. A shock from one CME interacted with the magnetic structure of a preceding CME, again combined with an SBC. A clear flux rope structure from the shock-producing CME was detected. Both events showed distinct magnetic field 'ripples' and fluctuations in density and temperature following the SBC. This study compares two episodes of multiple eruptive events in November and December 2023. Interacting CME structures and SBC-related magnetic modulations contributed to the stronger geomagnetic impacts, particularly in the November 4-5, 2023 event. The highly tilted heliospheric current sheet may have further influenced the CMEs' impact at Earth.

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arXiv2024

The influence of different coronal hole geometries on simulations of coronal wave -- coronal hole interaction

The geometry of a coronal hole (CH) affects the density profile of the reflected part of an incoming global coronal wave (CW). In this study, we perform for the first time magnetohydrodynamic (MHD) simulations of fast-mode MHD waves interacting with CHs of different geometries, such as circular, elliptic, convex, and concave shapes. We analyse the influence these geometries have on the density profiles of the reflected waves and we generate the corresponding simulation-based time-distance plots. Within these time-distance plots we determine regions that exhibit specific density features, such as large reflected density amplitudes. In a further step, these interaction features can be compared to actual observed CW-CH interaction events which makes it possible to explain interaction parameters from the observed interaction events, such as the density structure of the reflected wave, which are usually difficult to comprehensively understand by only analysing the measurements. Moreover, we show that the interaction between a concave shaped CH and CWs, whose density profile include an enhanced as well as a depleted wave, can lead to reflected density amplitudes that are more than two times larger than the incoming ones. Another effect of the interplay between the constructive and destructive interference of the reflected wave parts is a strongly depleted region in the middle of the CW-CH interaction process. In addition, we show how important the choice of the path is that is used to generate the time-distance plots and how this choice affects the interpretation of the CW-CH interaction results.

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arXiv2024

Evolution of coronal mass ejections with and without sheaths from the inner to the outer heliosphere -- statistical investigation for 1975-2022

This study covers a thorough statistical investigation of the evolution of interplanetary coronal mass ejections (ICMEs) with and without sheaths, through a broad heliocentric distance and temporal range. The analysis treats the sheath and magnetic obstacle (MO) separately to gain more insight about their physical properties. In detail, we aim to unravel different characteristics of these structures occurring over the inner and outer heliosphere. The method is based on a large statistical sample of ICMEs probed over different distances in the heliosphere. For this, information about detection times for sheath and MO from 13 individual ICME catalogs were collected and cross-checked. The time information was then combined into a main catalog used as basis for the statistical investigation. The data analysis based on that covers a wealth of spacecraft missions enabling in-situ solar wind measurements from 1975--2022. This allows to study differences between solar cycles. All the structures under study (sheath, MO with and without sheath) show the biggest increase in size together with the largest decrease in density at a distance 0.75 AU. At 1 AU we find different sizes for MOs with and without sheath, with the former being larger. Up to 1 AU, the upstream solar wind shows the strongest pile-up close to the interface with the sheath. For larger distances the pile-up region seems to shift and recedes from that interface further into the upstream solar wind. This might refer to a change in the sheath formation mechanism (driven versus non-driven) with heliocentric distance, suggesting the relevance of the CME propagation and expansion behavior in the outer heliosphere. Comparison to previous studies shows inconsistencies over the solar cycle, which makes more detailed studies necessary to fully understand the evolution of ICME structures.

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arXiv2024

Internal magnetic field structures observed by PSP/WISPR in a filament related coronal mass ejection

We track and investigate from white-light data taken with the Wide-field Instrument for Solar PRobe (WISPR) aboard Parker Solar Probe (PSP), localized density enhancements, reflecting small-scale magnetic structures belonging to a filament-related coronal mass ejection (CME). We aim to investigate the 3D location, morphology, and evolution of the internal magnetic fine structures of CMEs. Specifically, we ask: what is their relationship with the filament/source region and the flux rope? The fast tangential motion of the PSP spacecraft during its perihelion permits viewing the same event from multiple angles in short times relative to the event's evolution. Hence, we can derive the three-dimensional information of selected CME features from a single spacecraft using triangulation techniques. We group small-scale structures with roughly similar speeds, longitude and latitude, into three distinct morphological groups. We find twisted magnetic field patterns close to the eastern leg of the CME that may be related to 'horns' outlining the edges of the flux-rope cavity. Aligned thread-like bundles are identified close to the western leg. They may be related to confined density enhancements evolving during the filament eruption. High density blob-like features (magnetic islands) are widely spread in longitude ($\sim$40{\deg}) close to the flanks and rear part of the CME. We demonstrate that CME flux ropes may comprise different morphological groups with a cluster behavior, apart from the blobs which instead span a wide range of longitudes. This may hint either to the three-dimensionality of the post-CME current sheet (CS) or to the influence of the ambient corona in the evolutionary behavior of the CS. Importantly, we show that the global appearance of the CME can be very different in WISPR (0.11--0.16~AU) and instruments near 1~AU because of shorter line-of-sight integration of WISPR.

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arXiv2024

Unveiling the Journey of a Highly Inclined CME: Insights from the March 13, 2012 Event with 110$^\circ$ Longitudinal Separation

A fast and wide Coronal Mass Ejection (CME) erupted from the Sun on 2012-03-13. Its interplanetary counterpart was detected in situ two days later by STEREO-A and near-Earth spacecraft. We suggest that at 1 au the CME extended at least 110$^\circ$ in longitude, with Earth crossing its east flank and STEREO-A crossing its west flank. Despite their separation, measurements from both positions showed very similar in situ CME signatures. The solar source region where the CME erupted was surrounded by three coronal holes (CHs). Their locations with respect to the CME launch site were east (negative polarity), southwest (positive polarity) and west (positive polarity). The solar magnetic field polarity of the area covered by each CH matches that observed at 1 au in situ. Suprathermal electrons at each location showed mixed signatures with only some intervals presenting clear counterstreaming flows as the CME transits both locations. The strahl population coming from the shortest magnetic connection of the structure to the Sun showed more intensity. The study presents important findings regarding the in situ measured CME on 2012-03-15, detected at a longitudinal separation of 110$^\circ$ in the ecliptic plane despite its initial inclination being around 45$^\circ$ when erupted. This suggests that the CME may have deformed and/or rotated, allowing it to be observed near its legs with spacecraft at a separation angle greater than 100$^\circ$. The CME structure interacted with high-speed streams generated by the surrounding CHs. The piled-up plasma in the sheath region exhibited an unexpected correlation in magnetic field strength despite the large separation in longitude. In situ observations reveal that at both locations there was a flank encounter, where the spacecraft crossed the first part of the CME, then encountered ambient solar wind, and finally passed near the legs of the structure.

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arXiv2023

Effects of coronal mass ejection orientation on its propagation in the heliosphere

Context. In the scope of space weather forecasting, it is crucial to be able to more reliably predict the arrival time, speed, and magnetic field configuration of coronal mass ejections (CMEs). From the time a CME is launched, the dominant factor influencing all of the above is the interaction of the interplanetary CME (ICME) with the ambient plasma and interplanetary magnetic field. Aims. Due to a generally anisotropic heliosphere, differently oriented ICMEs may interact differently with the ambient plasma and interplanetary magnetic field, even when the initial eruption conditions are similar. For this, we examined the possible link between the orientation of an ICME and its propagation in the heliosphere (up to 1 AU). Methods. We investigated 31 CME-ICME associations in the period from 1997 to 2018. The CME orientation in the near-Sun environment was determined using an ellipse-fitting technique applied to single-spacecraft data from SOHO/LASCO C2 and C3 coronagraphs. In the near-Earth environment, we obtained the orientation of the corresponding ICME using in situ plasma and magnetic field data. The shock orientation and nonradial flows in the sheath region for differently oriented ICMEs were investigated. In addition, we calculated the ICME transit time to Earth and drag parameter to probe the overall drag force for differently oriented ICMEs. The drag parameter was calculated using the reverse modeling procedure with the drag-based model. Results. We found a significant difference in nonradial flows for differently oriented ICMEs, whereas a significant difference in drag for differently oriented ICMEs was not found.

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arXiv2023

The role of initial density profiles in simulations of coronal wave - coronal hole interaction

Interactions between global coronal waves (CWs) and coronal holes (CHs) reveal many interesting features of reflected waves and coronal hole boundaries (CHB) but have fairly been studied so far. Magnetohydrodynamic (MHD) simulations can help us to better understand what is happening during these interaction events, and therefore, to achieve a broader understanding of the parameters involved. In this study, we perform for the first time 2D MHD simulations of a CW-CH interaction including a realistic initial wave density profile that consists of an enhanced as well as a depleted wave part. We vary several initial parameters, such as the initial density amplitudes of the incoming wave, the CH density, and the CHB width, which are all based on actual measurements. We analyse the effects of different incident angles on the interaction features and we use the corresponding time-distance plots to detect specific features of the incoming and the reflected wave. We found that a particular combination of a small CH density, a realistic initial density profile and a sufficiently small incident angle leads to remarkable interaction features, such as a large density amplitude of the reflected wave with respect to the incoming one. The parameter studies in this paper provide a tool to compare time-distance plots based on observational measurements to those created from simulations and therefore enable us to derive interaction parameters from observed CW-CH interaction events that usually cannot be obtained directly. The simulation results in this study are augmented by analytical expressions for the reflection coefficient of the CW-CH interaction which allows us to verify the simulations results in an additional way. This work is the first of a series of studies aiming to finally reconstruct actual observed CW-CH interaction events by means of MHD-simulations.

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arXiv2023

CME Propagation Through the Heliosphere: Status and Future of Observations and Model Development

The ISWAT clusters H1+H2 have a focus on interplanetary space and its characteristics, especially on the large-scale co-rotating and transient structures impacting Earth. SIRs, generated by the interaction between high-speed solar wind originating in large-scale open coronal magnetic fields and slower solar wind from closed magnetic fields, are regions of compressed plasma and magnetic field followed by high-speed streams that recur at the ca. 27 day solar rotation period. Short-term reconfigurations of the lower coronal magnetic field generate flare emissions and provide the energy to accelerate enormous amounts of magnetised plasma and particles in the form of CMEs into interplanetary space. The dynamic interplay between these phenomena changes the configuration of interplanetary space on various temporal and spatial scales which in turn influences the propagation of individual structures. While considerable efforts have been made to model the solar wind, we outline the limitations arising from the rather large uncertainties in parameters inferred from observations that make reliable predictions of the structures impacting Earth difficult. Moreover, the increased complexity of interplanetary space as solar activity rises in cycle 25 is likely to pose a challenge to these models. Combining observational and modeling expertise will extend our knowledge of the relationship between these different phenomena and the underlying physical processes, leading to improved models and scientific understanding and more-reliable space-weather forecasting. The current paper summarizes the efforts and progress achieved in recent years, identifies open questions, and gives an outlook for the next 5-10 years. It acts as basis for updating the existing COSPAR roadmap by Schrijver+ (2015), as well as providing a useful and practical guide for peer-users and the next generation of space weather scientists.

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arXiv2022

Differences in physical properties of coronal bright points and their ALMA counterparts within and outside coronal holes

This study investigates and compares brightness and area of coronal bright points (CBPs) inside and outside of coronal holes (CHs) using the single-dish Band 6 observations by the Atacama Large Millimeter/submillimeter Array (ALMA), combined with extreme-ultraviolet (EUV) 193 $\overset{\circ}{\mathrm{A}}$ filtergrams obtained by the Atmospheric Imaging Assembly (AIA) and magnetograms obtained by the Helioseismic and Magnetic Imager (HMI), both on board Solar Dynamics Observatory (SDO). The CH boundaries were extracted from the SDO/AIA images using the Collection of Analysis Tools for Coronal Holes (CATCH) and CBPs were identified in the SDO/AIA, SDO/HMI, and ALMA data. Measurements of brightness and areas in both ALMA and SDO/AIA images were conducted for CBPs within CHs and quiet Sun regions outside CHs. A statistical analysis of the measured physical properties resulted in a lower average CBP brightness in both ALMA and SDO/AIA data for CBPs within the CHs. Depending on the CBP sample size, the difference in intensity for the SDO/AIA data, and brightness temperature for the ALMA data, between the CBPs inside and outside CHs ranged from between 2$\sigma$ and 4.5$\sigma$, showing a statistically significant difference between those two CBP groups. For CBP areas, CBPs within the CH boundaries showed smaller areas on average, with the observed difference between the two CBP groups between 1$\sigma$ and 2$\sigma$ for the SDO/AIA data, and up to 3.5$\sigma$ for the ALMA data, indicating that CBP areas are also significantly different. Given the measured properties, we conclude that the CBPs inside CHs tend to be less bright on average, but also smaller in comparison to those outside of CHs. This conclusion might point to the specific physical conditions and properties of the local CH region around a CBP limiting the maximum achievable intensity (temperature) and size of a CBP.

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arXiv2022

Tracking magnetic flux and helicity from Sun to Earth -- Multi-spacecraft analysis of a magnetic cloud and its solar source

We analyze the complete chain of effects caused by a solar eruptive event in order to better understand the dynamic evolution of magnetic-field related quantities in interplanetary space, in particular that of magnetic flux and helicity. We study a series of connected events (a confined C4.5 flare, a flare-less filament eruption and a double-peak M-class flare) that originated in NOAA active region (AR) 12891 on 2021 November 1 and November 2. We deduce the magnetic structure of AR 12891 using stereoscopy and nonlinear force-free (NLFF) magnetic field modeling, allowing us to identify a coronal flux rope and to estimate its axial flux and helicity. Additionally, we compute reconnection fluxes based on flare ribbon and coronal dimming signatures from remote sensing imagery. Comparison to corresponding quantities of the associated magnetic cloud (MC), deduced from in-situ measurements from Solar Orbiter and near-Earth spacecraft, allows us to draw conclusions on the evolution of the associated interplanetary coronal mass ejection (ICME). The latter are aided through the application of geometric fitting techniques (graduated cylindrical shell modeling; GCS) and interplanetary propagation models (drag based ensemble modeling; DBEM) to the ICME. NLFF modeling suggests the host AR's magnetic structure in the form of a left-handed (negative-helicity) sheared arcade/flux rope reaching to altitudes of 8-10 Mm above photospheric levels, in close agreement with the corresponding stereoscopic estimate. Revealed from GCS and DBEM modeling, the ejected flux rope propagated in a self-similar expanding manner through interplanetary space. Comparison of magnetic fluxes and helicities processed by magnetic reconnection in the solar source region and the respective budgets of the MC indicate a considerable contribution from the eruptive process, though the pre-eruptive budgets appear of relevance too.

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arXiv2021

Validation scheme for solar coronal models -- constraints from multi-perspective observations in EUV and white-light

Context: In this paper we present a validation scheme to investigate the quality of coronal magnetic field models, which is based upon comparisons with observational data from multiple sources. Aims: Many of these coronal models may use a range of initial parameters that produce a large number of physically reasonable field configurations. However, that does not mean that these results are reliable and comply with the observations. With an appropriate validation scheme the quality of a coronal model can be assessed. Methods: The validation scheme is developed on the example of the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) coronal model. For observational comparison we use EUV and white-light data to detect coronal features on the surface (open magnetic field areas) and off-limb (streamer and loop) structures from multiple perspectives (Earth view and the Solar Terrestrial Relations Observatory - STEREO). The validation scheme can be applied to any coronal model that produces magnetic field line topology. Results: We show its applicability by using that validation scheme on a large set of model configurations, which can be efficiently reduced to an ideal set of parameters that matches best with observational data. Conclusions: We conclude that by using a combined empirical visual classification with a mathematical scheme of topology metrics a very efficient and rather objective quality assessment for coronal models can be performed.

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arXiv2021

Drag-based CME modeling with heliospheric images incorporating frontal deformation: ELEvoHI 2.0

The evolution and propagation of coronal mass ejections (CMEs) in interplanetary space is still not well understood. As a consequence, accurate arrival time and arrival speed forecasts are an unsolved problem in space weather research. In this study, we present the ELlipse Evolution model based on HI observations (ELEvoHI) and introduce a deformable front to this model. ELEvoHI relies on heliospheric imagers (HI) observations to obtain the kinematics of a CME. With the newly developed deformable front, the model is able to react to the ambient solar wind conditions during the entire propagation and along the whole front of the CME. To get an estimate of the ambient solar wind conditions, we make use of three different models: Heliospheric Upwind eXtrapolation model (HUX), Heliospheric Upwind eXtrapolation with time dependence model (HUXt), and EUropean Heliospheric FORecasting Information Asset (EUHFORIA). We test the deformable front on a CME first observed in STEREO-A/HI on February 3, 2010 14:49 UT. For this case study, the deformable front provides better estimates of the arrival time and arrival speed than the original version of ELEvoHI using an elliptical front. The new implementation enables us to study the parameters influencing the propagation of the CME not only for the apex, but for the entire front. The evolution of the CME front, especially at the flanks, is highly dependent on the ambient solar wind model used. An additional advantage of the new implementation is given by the possibility to provide estimates of the CME mass.

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arXiv2021

Modelling a multi-spacecraft coronal mass ejection encounter with EUHFORIA

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.

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arXiv2021

Multi-channel coronal hole detection with convolutional neural networks

We develop a reliable, fully automatic method for the detection of coronal holes, that provides consistent full-disk segmentation maps over the full solar cycle and can perform in real-time. We use a convolutional neural network to identify the boundaries of coronal holes from the seven EUV channels of the Atmospheric Imaging Assembly (AIA) as well as from line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). For our primary model (Coronal Hole RecOgnition Neural Network Over multi-Spectral-data; CHRONNOS) we use a progressively growing network approach that allows for efficient training, provides detailed segmentation maps and takes relations across the full solar-disk into account. We provide a thorough evaluation for performance, reliability and consistency by comparing the model results to an independent manually curated test set. Our model shows good agreement to the manual labels with an intersection-over-union (IoU) of 0.63. From the total of 261 coronal holes with an area $>1.5\cdot10^{10}$ km$^2$ identified during the time range 11/2010 - 12/2016, 98.1% were correctly detected by our model. The evaluation over almost the full solar cycle no. 24 shows that our model provides reliable coronal hole detections, independent of the level of solar activity. From the direct comparison over short time scales of days to weeks, we find that our model exceeds human performance in terms of consistency and reliability. In addition, we train our model to identify coronal holes from each channel separately and show that the neural network provides the best performance with the combined channel information, but that coronal hole segmentation maps can be also obtained solely from line-of-sight magnetograms.

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arXiv2021

Coronal Hole Detection and Open Magnetic Flux

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 9/19/2010, 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 was about 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.

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arXiv2020

Deriving CME density from remote sensing data and comparison to in-situ measurements

We determine the 3D geometry and deprojected mass of 29 well-observed coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) using combined STEREO-SOHO white-light data. From the geometry parameters we calculate the volume of the CME for the magnetic ejecta (flux-rope type geometry) and sheath structure (shell-like geometry resembling the (I)CME frontal rim). Working under the assumption that the CME mass is roughly equally distributed within a specific volume, we expand the CME self-similarly and calculate the CME density for distances close to the Sun (15-30 Rs) and at 1AU. Specific trends are derived comparing calculated and in-situ measured proton densities at 1AU, though large uncertainties are revealed due to the unknown mass and geometry evolution: i) a moderate correlation for the magnetic structure having a mass that stays rather constant (~0.56-0.59), and ii) a weak correlation for the sheath density (~0.26) by assuming the sheath region is an extra mass - as expected for a mass pile-up process - that is in its amount comparable to the initial CME deprojected mass. High correlations are derived between in-situ measured sheath density and the solar wind density (~ -0.73) and solar wind speed (~0.56) as measured 24 hours ahead of the arrival of the disturbance. This gives additional confirmation that the sheath-plasma indeed stems from piled-up solar wind material. While the CME interplanetary propagation speed is not related to the sheath density, the size of the CME may play some role in how much material could be piled up.

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