arXiv subjects

Lars Berger

Publications and source records attributed to Lars Berger.

18 recordsLinked to original sources

arXiv2026

Observations of stable pickup He$^+$ tori in a magnetic flux rope at 0.85 au

Interstellar pickup ions originate from the neutral interstellar medium, are ionized in the heliosphere, and picked up by the solar wind. They initially form a torus-shaped velocity distribution function, which is generally believed to be transformed rapidly into an isotropic shell distribution by pitch-angle scattering. With the SupraThermal Electron Proton onboard Solar Orbiter we observe clear torus-shaped velocity distribution functions at an unprecedented one minute resolution. While these tori are variable on a time scale of one minute, they remain stable for over ten hours without signs of significant scattering. We conclude that they are populated by a huge fraction of the expected total number of pick-up ions injected in the past of the same solar wind stream.

astro-ph.SR↗

arXiv2026

Anomalous cosmic rays within the inner heliosphere: Observations of helium by the High Energy Telescope onboard Solar Orbiter

Radial gradients of cosmic rays are key parameters for understanding the transport of particles in space. Solar Orbiter, launched on 2020 February 10, approaches the Sun approximately every half year, with a closest perihelion distance of 0.29 au after the end of 2022 during the nominal mission phase. The two double-ended high energy telescopes(HET)onboard the Solar Orbiter measure energetic particles in the energy range between a few MeV/nuc and a few hundred MeV/nuc, which are dominated by anomalous cosmic rays (ACRs) and galactic cosmic rays (GCRs) during solar quiet times. By obtaining the radial gradient of the ACR helium in the inner heliosphere, we advance our understanding of how the transport of the cosmic rays is affected by the particle drift effect and the large-scale magnetic field. The helium observations at Solar Orbiter/HET between 11.1 and 49 MeV/nuc are analyzed. Since we focus on quiet time measurements, we remove the periods of solar energetic particle (SEP) events. The intensities are averaged over the Carrington rotation period. The helium observations from the Proton and Helium Instrument(EPHIN)onboard SOHO were utilized as the baseline to correct the long-term variation caused by the solar modulations. We present the first observation of ACR helium at Solar Orbiter/HET between 2020 February and 2022 July in the inner heliosphere before the sun became fully active. We derive the radial gradient of the ACR helium between 0.3 and 1 au. The averaged radial gradient between 11.1 and 49MeV/nuc is about 22$\pm$4%/au and the averaged value between 11.1 and 41.2MeV/nuc is raised to 32$\pm$8%/au after removing the GCR contribution, which is estimated by a GCR model. In addition, the temporal variation of radial gradients indicates that the gradients are increasing with the enhancement of the solar modulation and the increased tilt angle of the heliospheric current sheet.

physics.space-ph↗

arXiv2025

Bidirectional anisotropic solar energetic particle events observed by Solar Orbiter

Solar Energetic Particle (SEP) events are critical for understanding particle acceleration and transport in the heliosphere. While most SEP events involve outward streaming particles along open magnetic field lines, bidirectional events characterized by simultaneous sunward and anti-sunward particle flows offer unique insights into magnetic field topology and the interplay of multiple acceleration sources. We aim to investigate the origin and transport of energetic particles in two rare bidirectional anisotropic SEP events observed by Solar Orbiter. Both events showed two clear velocity dispersion signatures with opposite particle anisotropies during their onset phase. The sunward streaming protons, characterized by delayed release time, harder spectral index, and higher intensities, may be attributed to coronal mass ejection-driven shock acceleration, while the promptly released anti-sunward streaming protons are likely linked to flare acceleration. Notably, in both cases, small-scale flux ropes were identified in situ during the time intervals corresponding to the bidirectional particle streaming. Path lengths derived for sunward and anti-sunward injections were substantially greater than nominal values of the Parker field lines, further supporting the role of the flux rope in shaping particle trajectories. These observations demonstrate that magnetic flux rope could significantly affect magnetic connectivity to the source region and SEP propagation in the inner heliosphere, while simultaneous velocity dispersion from two distinct particle sources allows for direct constraints on the topology of the flux rope. Our results highlight the value of combining particle anisotropy, release time, source spectra, and magnetic structure diagnostics to unravel SEP transport in complex transient magnetic structures, and also present new challenges for the current SEP transport model.

astro-ph.SR↗

arXiv2025

A Tale of Two Shocks

Energetic particles in interplanetary space are normally measured at time scales that are long compared to the ion gyroperiod. Such observations by necessity average out the microphysics associated with the acceleration and transport of 10s - 100s keV particles. We investigate previously unseen non-equilibrium features that only become observable at very high time resolution, and discuss possible explanations of these features. We use unprecedentedly high-time-resolution data that were acquired by the in situ instruments on Solar Orbiter in the vicinity of two interplanetary shocks observed on 2023-11-29 07:51:17 UTC and 2023-11-30 10:47:26 UTC at $\sim 0.83$ astronomical units from the Sun. The solar-wind proton beam population follows the magnetic field instantaneously, on time scales which are significantly shorter than a gyro-period. Energetic particles, despite sampling large volumes of space, vary on remarkably short time scales, typically on the order of the convection time of their gyro-radius. Non-equilibrium features such as bump-on-tail distributions of energetic particles are formed by small-scale magnetic structures in the IMF. High-time-resolution observations show previously unobserved microphysics in the vicinity of two traveling interplanetary shocks, including ion reflection at a current sheet, which may explain where ions are reflected in shock acceleration.

astro-ph.SR↗

arXiv2025

Investigation of Inverse Velocity Dispersion in a Solar Energetic Particle Event Observed by Solar Orbiter

Inverse velocity dispersion (IVD) events, characterized by higher-energy particles arriving later than lower-energy particles, challenge the classical understanding of SEP events and are increasingly observed by spacecraft, such as Parker Solar Probe (PSP) and Solar Orbiter (SolO). However, the mechanisms underlying IVD events remain poorly understood. This study aims to investigate the physical processes responsible for long-duration IVD events by analyzing the SEP event observed by SolO on 2022 June 7. We explore the role of evolving shock connectivity, particle acceleration at interplanetary (IP) shocks, and cross-field transport in shaping the observed particle profiles.We utilize data from Energetic Particle Detector (EPD) suite onboard SolO to analyze the characteristics of the IVD, and model the event using the Heliospheric Energetic Particle Acceleration and Transport (HEPAT) model. The IVD event exhibited a distinct and long-duration IVD signature, across proton energies from 1 to 20 MeV and lasting for approximately 10 hours. Simulations suggest that evolving shock connectivity and the evolution of shock play a primary role in the IVD signature, with SolO transitioning from shock flank to nose over time, resulting in a gradual increase in maximum particle energy along the field line. Furthermore, model results show that limited cross-field diffusion can influence both the nose energy and the duration of the IVD event. This study demonstrates that long-duration IVD events are primarily driven by evolving magnetic connectivity along a non-uniform shock that evolves over time, where the connection moves to more efficient acceleration sites as the shock propagates farther from the Sun. Other mechanisms, such as acceleration time at the shock, may also contribute to the observed IVD features.

astro-ph.SR↗

arXiv2025

Three dimensional He+ pickup ion velocity distribution functions observed with STEREO-A PLASTIC

Freshly injected interstellar Pickup Ions (PUIs) are expected to exhibit a simple, torus-shaped velocity distribution function. The PUI velocity in the solar wind frame depends on the velocity of the interstellar neutral (ISN) population at the pick-up position. In this study, we compare PUI velocity distributions measured by the PLasma And SupraThermal Ion Composition (PLASTIC) instrument over the full orbit of Solar TErestrial RElations Observatory-Ahead (STEREO-A) directly. We define a new position-independent velocity measure for PUIs that takes the local direction of the interstellar neutral inflow into account. The resulting new PUI velocity measure corrects thereby for the position-dependent contribution of the ISN velocity. Pitch-angle distributions are then analysed depending on the magnetic-field azimuthal angle for different orbital positions and different values of the PUI velocity measure. The new PUI velocity measure shows an approximately constant cut-off over the complete orbit of STEREO-A. A torus signature is visible everywhere. Therein, a broadening of the torus signature outside the focusing cone and crescent regions and for lower velocity measure observed. In addition, we illustrate the symmetry between the primary and secondary ISN trajectory in the vicinity of the focusing cone. A torus signature associated with freshly injected PUIs is visible over the complete orbit of STEREO-A with increased density in the focusing cone. At least remnants of a torus signature remain for smaller values of the PUI velocity measure. The new velocity measure also prepares for PUI studies with Solar Orbiter.

astro-ph.SR↗

arXiv2025

Challenges in identifying the coronal hole wind

Solar wind is frequently categorized based on its respective solar source region. Two well-established categorizations of the coronal hole wind, the scheme based on the charge-state composition, and the scheme based on proton plasma, identify a very different fraction of solar wind in the data from the Advanced Composition Explorer (ACE) as coronal hole wind during the solar activity minimum at the end of solar cycle 24. We investigate possible explanations for the different identifications of the coronal wind in 2009 in the scheme based on the charge-state composition (almost only coronal hole wind) and in the scheme based on the proton plasma (almost no coronal hole wind at the same time). We compared the properties of the respective coronal hole wind types and their changes with solar activity cycle in 2001- 2010. As a comparison reference, we included the coronal hole wind as identified by an unsupervised machine-learning approach, k-means, in our analysis. We find that the scheme based on charge-state composition likely misidentifies some slow solar wind as coronal hole wind during the solar activity minimum. The k-means classification we considered includes two types of coronal hole wind, the first of which is dominant during the solar activity maximum, whereas the second is dominant during the solar activity minimum. A low fraction of coronal hole wind from low-latitude coronal holes observed by ACE in 2009 is plausible because during this time period, a very small number of low-latitude coronal holes was observed. The results imply that the origin-oriented solar wind classification needs to be revisited, and they also suggest that an explicit inclusion of the phase of the solar activity cycle can be expected to improve the classification of the solar wind.

astro-ph.SR↗

arXiv2024

Intelligence Education made in Europe

Global conflicts and trouble spots have thrown the world into turmoil. Intelligence services have never been as necessary as they are today when it comes to providing political decision-makers with concrete, accurate, and up-to-date decision-making knowledge. This requires a common co-operation, a common working language and a common understanding of each other. The best way to create this "intelligence community" is through a harmonized intelligence education. In this paper, we show how joint intelligence education can succeed. We draw on the experience of Germany, where all intelligence services and the Bundeswehr are academically educated together in a single degree program that lays the foundations for a common working language. We also show how these experiences have been successfully transferred to a European level, namely to ICE, the Intelligence College in Europe. Our experience has shown that three aspects are particularly important: firstly, interdisciplinarity or better, transdisciplinarity, secondly, the integration of IT knowhow and thirdly, the development and learning of methodological skills. Using the example of the cyber intelligence module with a special focus on data-driven decision support, additionally with its many points of reference to numerous other academic modules, we show how the specific analytic methodology presented is embedded in our specific European teaching context.

cs.CY↗

arXiv2023

The Intelligence College in Europe (ICE): An Effort to Create a European Intelligence Community

In fulfilling the European security commitment, the actors of the so-called "Intelligence Community" play a central role. They provide political and military decision-makers with important analyses and information. The Intelligence College in Europe (ICE) is the first entity to offer professional intelligence training as well as postgraduate level academic education in intelligence and security studies at a pan-European level. In developing its postgraduate provision, ICE has benefited from the experience of the German Master of Intelligence and Security Studies (MISS), which is a joint effort of the University of the Bundeswehr Munich and the Department of Intelligence at the Federal University of Administrative Sciences in Berlin. As a main contribution of this paper, the module Counterterrorism (adapted from the MISS) is examined in more detail as a case study of how postgraduate modules can be modified to speak to a pan-European audience of intelligence professionals.

cs.CY↗

arXiv2023

Irregular proton injection to high energies at interplanetary shocks

How thermal particles are accelerated to suprathermal energies is an unsolved issue, crucial for many astrophysical systems. We report novel observations of irregular, dispersive enhancements of the suprathermal particle population upstream of a high-Mach number interplanetary shock. We interpret the observed behavior as irregular "injections" of suprathermal particles resulting from shock front irregularities. Our findings, directly compared to self-consistent simulation results, provide important insights for the study of remote astrophysical systems where shock structuring is often neglected.

physics.space-ph↗

arXiv2023

Scope and limitations of ad hoc neural network reconstructions of solar wind parameters

Solar wind properties are determined by the conditions of their solar source region and transport history. Solar wind parameters, such as proton speed, proton density, proton temperature, magnetic field strength, and the charge state composition of oxygen, are used as proxies to investigate the solar source region of the solar wind. The transport and conditions in the solar source region affect several solar wind parameters simultaneously. The observed redundancy could be caused by a set of hidden variables. We test this assumption by determining how well a function of four of the selected solar wind parameters can model the fifth solar wind parameter. If such a function provided a perfect model, then this solar wind parameter would be uniquely determined from hidden variables of the other four parameters. We used a neural network as a function approximator to model unknown relations between the considered solar wind parameters. This approach is applied to solar wind data from the Advanced Composition Explorer (ACE). The neural network reconstructions are evaluated in comparison to observations. Within the limits defined by the measurement uncertainties, the proton density and proton temperature can be reconstructed well. We also found that the reconstruction is most difficult for solar wind streams preceding and following stream interfaces. For all considered solar wind parameters, but in particular the proton density, temperature, and the oxygen charge-state ratio, parameter reconstruction is hindered by measurement uncertainties. The reconstruction accuracy of sector reversal plasma is noticeably lower than that of streamer belt or coronal hole plasma. The fact that the oxygen charge-state ratio, a non-transport-affected property, is difficult to reconstruct may imply that recovering source-specific information from the transport-affected proton plasma properties is challenging.

astro-ph.SR↗

arXiv2022

In-situ Measurement of the Energy Fraction in Supra-thermal and Energetic Particles at ACE, Wind, and PSP Interplanetary Shocks

The acceleration of charged particles by interplanetary shocks (IPs) can drain a non-negligible fraction of the plasma pressure. In this study, we have selected 17 IPs observed in-situ at $1\,\text{au}$ by the Advanced Composition Explorer (ACE) and the Wind spacecraft, and 1 shock at $0.8\,\text{au}$ observed by Parker Solar Probe (PSP). We have calculated the time-dependent partial pressure of supra-thermal and energetic particles (smaller and greater than $50\,\text{keV}$ for protons and $30\,\text{keV}$ for electrons, respectively) in both the upstream and downstream regions. The particle fluxes were averaged for 1 hour before and 1 hour after the shock time to remove short time scale effects. Using the MHD Rankine-Hugoniot jump conditions, we find that the fraction of the total upstream energy flux transferred to supra-thermal and energetic downstream particles is typically $\lesssim\!16\%$, in agreement with previous observations and simulations. Notably, by accounting for errors on all measured shock parameters, we have found that for any given fast magnetosonic Mach number, $M_{f}\!<7$, the angle between the shock normal and average upstream magnetic field, $\theta_{Bn}$, is not correlated with the energetic particle pressure; in particular, the partial pressure of energized particles does not decrease for $\theta_{Bn} \gtrsim 45^\circ$. The downstream electron-to-proton energy ratio in the range $\gtrsim\!140\,\text{eV}$ for electrons and $\gtrsim\!70\,\text{keV}$ for protons exceeds the expected $\sim\!1\%$ and nears equipartition ($>\!0.1$) for the Wind events.

astro-ph.SR↗

arXiv2021

Energy Balance at Interplanetary Shocks: In-situ Measurement of the Fraction in Supra-thermal and Energetic Particles with ACE and Wind

The acceleration of charged particles by interplanetary shocks can drain a non-negligible fraction of the upstream ram pressure. For a sample of shocks observed in-situ at 1 AU by the ACE and Wind spacecraft, time-series of the non-Maxwellian components (supra-thermal and higher-energy) of the ion and electron energy spectra were acquired for each event. These were averaged for one hour before and after the time of the shock passage to determine their partial pressure. Using the MHD Rankine-Hugoniot jump conditions, we find that the fraction of the total upstream energy flux transferred to non-Maxwellian downstream particles is typically about 2-16%. Notably, our sample shows that neither the fast magnetosonic Mach number nor the angle between the shock normal and average upstream magnetic field are correlated with non-Maxwellian particle pressure.

physics.space-ph↗

arXiv2021

Radial Evolution of the April 2020 Stealth Coronal Mass Ejection between 0.8 and 1 AU -- A Comparison of Forbush Decreases at Solar Orbiter and Earth

Aims. We present observations of the first coronal mass ejection (CME) observed at the Solar Orbiter spacecraft on April 19, 2020, and the associated Forbush decrease (FD) measured by its High Energy Telescope (HET). This CME is a multispacecraft event also seen near Earth the next day. Methods. We highlight the capabilities of HET for observing small short-term variations of the galactic cosmic ray count rate using its single detector counters. The analytical ForbMod model is applied to the FD measurements to reproduce the Forbush decrease at both locations. Input parameters for the model are derived from both in situ and remote-sensing observations of the CME. Results. The very slow (~350 km/s) stealth CME caused a FD with an amplitude of 3 % in the low-energy cosmic ray measurements at HET and 2 % in a comparable channel of the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter, as well as a 1 % decrease in neutron monitor measurements. Significant differences are observed in the expansion behavior of the CME at different locations, which may be related to influence of the following high speed solar wind stream. Under certain assumptions, ForbMod is able to reproduce the observed FDs in low-energy cosmic ray measurements from HET as well as CRaTER, but with the same input parameters, the results do not agree with the FD amplitudes at higher energies measured by neutron monitors on Earth. We study these discrepancies and provide possible explanations. Conclusions. This study highlights that the novel measurements of the Solar Orbiter can be coordinated with other spacecraft to improve our understanding of space weather in the inner heliosphere. Multi-spacecraft observations combined with data-based modeling are also essential to understand the propagation and evolution of CMEs as well as their space weather impacts.

astro-ph.SR↗

arXiv2020

Proton-proton collisional age to order solar wind types

The properties of a solar wind stream are determined by its source region and by transport effects. Independently of the solar wind type, the solar wind measured in situ is always affected by both. We consider the proton-proton collisional age as an ordering parameter for the solar wind at 1AU and explore its relation to the solar wind classification scheme developed by Xu & Borovsky (2015). We use this to show that explicit magnetic field information is not required for this solar wind classification. Based on the observation that the three basic solar wind types from this categorization cover different regimes in terms of proton-proton collisional age $a_{col,p-p}$ , we propose a simplified solar wind classification scheme that is only based on the proton-proton collisional age. The resulting so-called PAC solar wind classifier is an alternative to the full Xu & Borovsky (2015) solar wind classification scheme and leads to a classification that is very similar to the full Xu & Borovsky (2015) scheme. The solar wind is well ordered by the proton-proton collisional age. This implies underlying intrinsic relationships between the plasma properties, in particular, proton temperature and magnetic field strength in each plasma regime. We argue that sector-reversal plasma is a combination of particularly slow and dense solar wind and most stream interaction boundaries. Most solar wind parameters (e.g., the magnetic field strength, B, and the oxygen charge state ratio $n_{O^{7+}}$ /$n_{O^{6+}}$) change with the solar activity cycle. Thus, all solar wind categorization schemes based on threshold values need to be adapted to the solar activity cycle as well.

astro-ph.SR↗

arXiv2018

Disparity among low first ionization potential elements

The elemental composition of the solar wind differs from the solar photospheric composition. Elements with low first ionization potential (FIP) appear enhanced compared to O in the solar wind relative to the respective photospheric abundances. This so-called FIP effect is different in the slow solar wind and the coronal hole wind. However, under the same plasma conditions, for elements with similar FIPs such as Mg, Si, and Fe, comparable enhancements are expected. We scrutinize the assumption that the FIP effect is always similar for different low FIP elements, namely Mg, Si, and Fe. We investigate the dependency of the FIP effect of low FIP elements on the O7+/O6+ charge state ratio depending on time and solar wind type. We order the observed FIP ratios with respect to the O7+/O6+ charge state ratio into bins and analyze separately the respective distributions of the FIP ratio of Mg, Si, and Fe for each O7+/O6+ charge state ratio bin. We observe that the FIP effect shows the same qualitative yearly behavior for Mg and Si, while Fe shows significant differences during the solar activity maximum and its declining phase. In each year, the FIP effect for Mg and Si always increases with increasing O7+/O6+ charge state ratio, but for high O7+/O6+ charge state ratios the FIP effect for Fe shows a qualitatively different behavior. During the years 2001-2006, instead of increasing with the O7+/O6+ charge state ratio, the Fe FIP ratio exhibits a broad peak. Also, the FIP distribution per O7+/O6+ charge state bin is significantly broader for Fe than for Mg and Si. These observations support the conclusion that the elemental fractionation is only partly determined by FIP. In particular, the qualitative difference behavior with increasing O7+/O6+ charge state ratio between Fe on the one hand and Mg and Si on the other hand is not yet well explained by models of fractionation.

astro-ph.SR↗

arXiv2018

Observations of high and low Fe charge states in individual solar wind streams with coronal-hole origin

The solar wind originating from coronal holes is comparatively well-understood and is characterized by lower densities and average charge states compared to the so-called slow solar wind. Except for wave perturbations, the average properties of the coronal-hole solar wind are passably constant. In this case study, we {ocus on observations of the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) of individual streams of coronal-hole solar wind that illustrate that although the O and C charge states are low in coronal-hole wind, the Fe charge distribution is more variable. In particular, \markmeold{we illustrate that the Fe charge states in coronal-hole solar wind are frequently as high as in slow solar wind. We selected individual coronal-hole solar wind} streams based on their collisional age as well as their respective O and C charge states and analyzed their Fe charge-state distributions. Additionally, with a combination of simple ballistic back-mapping and the potential field source surface model, transitions between streams with high and low Fe charge states were mapped back to the photosphere. The relative frequency of high and low Fe charge-state streams is compared for the years 2004 and 2006. We found several otherwise typical coronal-hole streams that include Fe charge states either as high as or lower than in slow solar wind. Eight such transitions in 2006 were mapped back to equatorial coronal holes that were either isolated or connected to the northern coronal-hole. Attempts to identify coronal structures associated with the transitions were so far inconclusive.

physics.space-ph↗

arXiv2018

Evolution of an equatorial coronal hole structure and the released coronal hole wind stream: Carrington rotations 2039 to 2050

The Sun is a highly dynamic environment that exhibits dynamic behavior on many different timescales. In particular, coronal holes exhibit temporal and spatial variability. Signatures of these coronal dynamics are inherited by the coronal hole wind streams that originate in these regions and can effect the Earth's magnetosphere. Both the cause of the observed variabilities and how these translate to fluctuations in the in situ observed solar wind is not yet fully understood. During solar activity minimum the structure of the magnetic field typically remains stable over several Carrington rotations (CRs). But how stable is the solar magnetic field? Here, we address this question by analyzing the evolution of a coronal hole structure and the corresponding coronal hole wind stream emitted from this source region over 12 consecutive CRs in 2006. To this end, we link in situ observations of Solar Wind Ion Composition Spectrometer (SWICS) onboard the Advanced Composition Explorer (ACE) with synoptic maps of Michelson Doppler imager (MDI) on the Solar and Heliospheric Observatory (SOHO) at the photospheric level through a combination of ballistic back-mapping and a potential field source surface (PFSS) approach. Together, these track the evolution of the open field line region that is identified as the source region of a recurring coronal hole wind stream. We find that the shape of the open field line region and to some extent also the solar wind properties are influenced by surrounding more dynamic closed loop regions. We show that the freeze-in order can change within a coronal hole wind stream on small timescales and illustrate a mechanism that can cause changes in the freeze-in order. The inferred minimal temperature profile is variable even within coronal hole wind and is in particular most variable in the outer corona.

physics.space-ph↗