Heliophysics Group home
Welcome to the Heliophysics group website. Information relevant to the activity of this science discussion group can be found here.
The Heliophysics Group consists of members from 2 ESA sites: ESTEC (Noordwijk, The Netherlands) and ESAC (near Madrid, Spain).
- Andrew Walsh, ESAC
- Anik De Groof, ESAC
- Arnaud Masson, ESAC
- Christophe Escoubet, ESTEC
- Claire Vallat, ESAC
- Daniel Mueller, ESTEC
- David Williams, ESAC
- Helen Middleton, ESAC
- Japhet Yates, ESAC (Research Fellow)
- Joe Zender, ESTEC
- Matt Taylor, ESTEC
- Olivier Witasse, ESTEC
- Owen Roberts, ESTEC (Research Fellow)
- Pedro Osuna, ESAC
- Sergio Toledo Redondo, ESAC (Research Fellow)
- Yannis Zouganelis, ESAC
The meetings are held every week, Fridays between 10:00 - 11:00 CET, both at ESTEC and ESAC (videoconference).
Each week it is the responsibility of a different person to lead the discussions. They can either give a seminar about some of their almost-finished work, ask questions about something they're having problems with to see if anyone can help, talk about their in progress work to get ideas from the rest of the group, or ideas for something completely new, review some interesting papers, whatever you like.
We agreed that the discussion leader will be a research fellow or YGT one week, and a staff member or contractor the week after, so those us doing full time research have the opportunity to lead discussions more often. Where possible we'll try and alternate between ESTEC and ESAC leaders as well.
We aim to present a poster at the SSW each year summarising the group's achievements.
List of lectures:
2017/11/02: Benoit Lavraud: First results from the Magnetospheric Multiscale mission
Since its launch in March 2015, NASA’s Magnetospheric Multiscale mission (MMS) provides a wealth of unprecedented high resolution measurements of space plasma properties and dynamics in the near-Earth environment. MMS was designed in the first place to study the fundamental process of collision-less magnetic reconnection. The two first results reviewed here pertain to this topic and highlight how the extremely high resolution MMS data (electrons, in particular, with full three dimensional measurements at 30 ms in burst mode) have permitted to tackle electron dynamics associated with magnetic reconnection in unprecedented details. Aside from magnetic reconnection, we also show how MMS contributes to topics such as wave properties and their interaction with particles. Thanks again to extremely high resolution measurements, the lossless and periodical energy exchange between wave electromagnetic fields and particles, as expected in the case of kinetic Alfvén waves, is unveiled. We quickly discuss how MMS has the potential to solve many other outstanding issues in collision-less plasma physics, for example regarding shock or turbulence acceleration, with obvious broader impacts for astrophysics in general.
2017/10/06: Owen Roberts: Studying solar wind turbulence with the Alfven ratio
Typically fluctuations in the magnetic field are observed to have low phase speeds in the plasma frame. In the linear wave picture this has been interpreted as a superposition of quasi perpendicular kinetic Alfven waves. However it is also possible that the kinetic slow wave also plays a role. This has often been dismissed since the damping of KSWs is significant but they share many of the properties of kinetic Alfven waves (anti-correlation of B and n) similar dispersion relations at kinetic scales. In the study of Zhao 2014 APJ, three properties of KAWs and KSWs were found to be significantly different from one another. The magnetic helicity and polarization, and the Alfven ratio. Determining the Alfven ratio requires high time resolution plasma data as is possible with the MMS spacecraft. In this talk the plasma data and the issues surrounding it in the solar wind will be discussed, and some preliminary results will be presented. This is a work in progress and discussion and comments will be most welcome.
2017/10/05: Adam Masters: How does the solar wind interact with each of the giant planets?
Of all the planets in the Solar System the giant planets have the strongest magnetic fields, which produces enormous magnetospheric cavities in the continuous flow of solar wind plasma from the Sun. However, changes in the solar wind regularly perturb these natural magnetic shields, and processes that can operate at the edge of each planetary magnetic field can transport solar wind energy in to each system. Here we give an overview of what we know about each giant planet magnetosphere, with particular focus on what we know about the interaction with the solar wind and its consequences. Key open questions and relevance for ongoing and future missions will be discussed.
2017/09/29: Yana G. Maneva: Hybrid modeling of heating and acceleration processes in the solar corona and the solar wind
Plasma heating and cooling in the solar wind is a dynamic process, which can occur sporadically at various heliocentric distances. Furthermore the solar wind is composed of multiple ion populations, which move relative to each other with observed different bulk velocities. Observational data from various spacecraft shows compelling evidence for in situion heating via wave-particle interactions, as well as plasma cooling regulated by linear or non-linear kinetic micro-instabilities. In the vicinity of the solar corona the heavy ions, such as alpha particles and oxygen ions for example, stream much faster than the protons and this relative drift speed can decrease with heliospheric distance as we approach the Earth. The two main scenarios of exploring the non-thermal heating and acceleration of minor ions in the solar wind are based on currents sheets and waves. This talk will mainly focus on the wave-based heating and acceleration mechanisms through linear and nonlinear wave-particle interactions and related energy exchange. We have performed hybrid numerical simulations (with fluid electrons and kinetic protons and minor ions) to study the effect of the different types of fluctuations and the related plasma instabilities. Next we investigate which ones are most efficient in scattering the ions and might be responsible for their subsequent relaxation. The results are applicable for small-scale heating and acceleration processes in collisionless coronal holes and the solar wind.
2017/09/08: Sergio Toledo: Energy budget and mechanisms of cold ion heating in asymmetric magnetic reconnection
Cold ions (few tens of eV) of ionospheric origin are commonly observed on the magnetospheric side of the Earth's dayside magnetopause. As a result, they can participate in magnetic reconnection, changing locally the reconnection rate and being accelerated and heated. We present four events where cold ion heating was observed by the Magnetospheric Multiscale mission, associated with the magnetospheric Hall E field region of magnetic reconnection. For two of the events the cold ion density was small compared to the magnetosheath density, and the cold ions were heated roughly to the same temperature as magnetosheath ions inside the exhaust. On the other hand, for the other two events the cold ion density was comparable to the magnetosheath density and the cold ion heating observed was significantly smaller. Magnetic reconnection converts magnetic energy into particle energy, and ion heating is known to dominate the energy partition. We find that at least 10 - 25% of the energy spent by reconnection into ion heating, went into magnetospheric cold ion heating. The total energy budget for cold ions may be even higher when properly accounting for the heavier species, namely Helium and Oxygen. Large E field fluctuations are observed in this cold ion heating region, i.e., gradients and waves, that are likely the source of particle energization.
2017/07/13: Roberto Bruno: Exploring the solar wind turbulence spectrum from fluid to kinetic scales
About five decades of in-situ observations by spacecrafts unraveled the complex nature of the fluctuations of solar wind magnetic field and plasma parameters. The frequency range of variability of these fluctuations extends from scales of the order of the solar rotation period to the smallest scales of the order of the ion and electron characteristic lengths. Some of these fluctuations are due to coronal transients, others to propagating modes and others simply derive from inhomogeneities and structures advected by the wind across the observer. I will provide a short overview of the state of art of our current interpretation of the complex phenomenology observed so far, also in view of the next solar missions, namely Solar Orbiter and SPP. I will start from the large-scale structure of the solar wind and its connection to the low corona. I will continue through the MHD regime, where turbulence energy is non-linearly transferred to smaller and smaller scales towards the kinetic scales where, eventually, it is dissipated. I will focus on the different turbulent character of the fluctuations within different wind speed regimes and their associated radial dependence. Particular attention will be dedicated to describe the nature of fluctuations at kinetic scales taking into account the observational limitations of available data sets.
2017/06/29: Sergio Servidio: Magnetic reconnection in turbulence: from MHD to Vlasov models
2017/06/01: Luca Sorriso: Gone with the Solar Wind: Turbulence, Intermittency, and Energy Dissipation in Space Plasmas
Space and astrophysical plasmas often show turbulent fluctuations on a broad range of scales. The associated dissipative and/or dispersive processes resulting in both thermal and nonthermal energization of particles are still not fully understood. Decades of spacecraft observations have shown that the solar wind provides a great opportunity to study in-situ plasma turbulence, representing a natural 'wind tunnel'. Measurements indicate that solar wind plasma is constantly heated during its expansion in the heliosphere, but at the same time solar wind electrons, protons and alpha particles show clear non-Maxwellian features of distribution functions (e.g. beams and energetic tails), suggesting that both heating and acceleration are at work. In order to understand which mechanisms are responsible for dispersion and dissipation of the energy carried by the turbulent fluctuations, the properties of solar wind turbulence must be known in great detail. One of the main features of turbulent flows is intermittency, namely the nonlinear generation of highly energetic small-scale structures, such as vortexes and current sheets. These are often associated with localized heating or particle accelerations, as evidenced by numerical simulations. We will show here how to describe the properties of intermittency in solar wind turbulence and how to connect them to energy dissipation.Finally, we will outline the forthcoming challenges in the identification of the energy dissipation mechanisms, also highlighting the compelling need for a dedicated new space mission.
2017/05/19: Andrew Walsh: solar wind strahl / core electron distributions
2017/04/21: Sergio Toledo-Redondo: Cold ions at the dayside magnetopause: implications for magnetic reconnection
Magnetic reconnection is a key plasma process that couples the shocked solar wind (magnetosheath) to the Earth’s magnetosphere. The magnetospheric side of the subsolar magnetopause is often populated by cold (10 eV) plasma of ionospheric origin, in addition to the common hot (10 keV) magnetospheric plasma. The presence of cold plasma mass loads the subsolar region up to several particles per cc. In addition, the ion gyroradius of cold plasma is much smaller than the hot ion gyroradius and introduces a new length-scale into magnetic reconnection and its associated processes. Finally, the cold plasma is heated inside the separatrix region of magnetic reconnection, although this mechanism is not always present. We present MMS observations of magnetic reconnection with the presence of ionospheric cold plasma and investigate the heating mechanisms as well as their implications for the global energy budget.
2017/03/10: Japhet yates: Response of Jupiter's thermosphere to a transient solar wind pulse
The importance of the thermosphere in regards to magnetosphere-ionosphere coupling is often neglected in magnetospheric physics. We simulate the response of the Jovian thermosphere and aurorae to transient variations in solar wind pressure by coupling a magnetosphere model with an azimuthally symmetric global circulation model. The Jovian magnetosphere is compressed from an expanded steady-state over a period of 90 minutes. We present the response of thermospheric flows and energy terms and the resulting auroral signatures to this transient event. We find significant changes in ion drag and advection (of momentum) as well as a significant brightening of the main aurora.
2017/02/24: Owen Roberts: Variability of the magnetic field power spectrum in the solar wind at electron scales
Solar wind magnetic fluctuations show the presence of several power laws. At the electron scales the power spectrum can be highly variable and the dissipation mechanisms of the magnetic field energy into the various particle species remain under debate. In this paper we investigate the morphology of the power spectrum at electron scales using data from the Cluster missions Search coil magnetometer when the Cluster spacings were ~ 10000km. By using wavelet coherence on the magnetic field data, times when the magnetic field signals are characteristic of certain phenomena such as whistler waves and coherent structures can be identified. Several different morphologies of the power spectrum are seen including: (1) two power laws separated by a break (2) an exponential cutoff near the Taylor shifted electron scales (3) strong spectral knees at the Taylor shifted electron scales .The different morphologies of the power spectrum are investigated by using wavelet coherence and show that in this interval a clear break and strong spectral knees are features which are associated with sporadic quasi parallel propagating whistler waves. Meanwhile when no signatures of whistler waves are present a clear break is difficult to find and the spectrum is often more characteristic of a power law with an exponential cutoff . The presence of these waves even for short times in an interval can affect the spectral shape drastically, and cause pitch angle scattering of electrons. The electron temperature and the electron heat flux measurements do not show unstable conditions to generate whistlers., however in several cases they are seen to be related to discontinuities in the large scale magnetic field. While large scale discontinuities are often seen at all spacecraft, not all spacecraft are accompanied with whistler waves which we interpret as being related to the geometry of the discontinuity and the path of the spacecraft through the plasma. It is possible that in the vicinity of these discontinuities, electron distribution functions are distorted and so unstable, but we cannot capture these distortions because of insufficient time, energy and angular resolution of the particle measurements.
2017/02/10: Lucile Turc: Geoeffectivity of magnetic clouds
Magnetic clouds are a subset of coronal mass ejections characterised by a well-defined magnetic structure, often modelled by a flux rope. Specifically, their magnetic field is stronger than the ambient interplanetary magnetic field and rotates smoothly over periods of the order of one day. Magnetic clouds play a central role in space weather at Earth because they can induce very strong disturbances in the terrestrial magnetosphere, the so-called geomagnetic storms. To better understand and predict their effects in the geospace, many studies look for possible relationships (coupling functions) between the magnetic clouds' parameters and the level of geomagnetic activity, as measured from the ground. In this talk, I will review the most widely used coupling functions for magnetic cloud events. I will then present some preliminary results based on a large catalogue of magnetic clouds, covering 15 years of observations, in order to test these previously established functions with a new data set and identify those which provide the best correlations. I will show the importance of separating the events as a function of the main driver of the storm (sheath or magnetic cloud proper) and whether they are part of a complex series of events. Finally, I will focus on a few events which appear not to follow the general trend and discuss the possible reasons for that.
2017/01/27: Jérémy Dargent: Space Weather : The role of cold ions on magnetic reconnection at the terrestrial magnetopause.
A key point of space weather sciences is the interaction between Sun and Earth magnetic fields. The boundary between magnetic environments of the Sun (the solar wind) and the Earth (the magnetosphere) is called magnetopause. The amount of solar wind particles able to penetrate the terrestrial magnetosphere determines the impact of the solar activity on the Earth. There are few phenomena allowing particles to cross the magnetopause. The main one is the magnetic reconnection.This phenomenon, ubiquitous in astrophysics, is very sensitive to the plasma properties. On the other hand, the magnetosphere plasma is composed of different populations, whose density and temperature can greatly vary with time. In particular, low temperature ions coming from the ionosphere are hard to study, being located at the limit of detection of spacecraft instruments. This study uses simulation to look at the effects of these cold ions on the magnetic reconnection.
2017/01/13: Andrew Walsh: Update on Solar wind Core Electron Study.
For the first heliophysics group of 2017 I'll give a brief update on the progress of our solar wind electron study and the feedback I got at AGU. This shouldn't take the full amount of time so afterwards we could think about any changes we want to make to how the group operates.
2016/12/02: Olivier Witasse: New information on Mars' ionosphere (work in progress) from the NGIMS instrument aboard the MAVEN spacecraft
MAVEN is NASA satellite in orbit around Mars since September 2014. NGIMS is an ion and neutral mass spectrometer devoted
to the study of the structure and composition of the Mars' upper atmosphere.
I will show a general overview about the Mars' ionosphere (my favorite topic) and present some analysis of the NGIMS data, downloaded from a public archive.
I will focus on ion data, and show why this data set is interesting.
In particular, I'll discuss:
- the detection of a new ion (prediction made in my PhD in... 2000! fun...)
- the response of the ionosphere to ICMEs
- how symmetric/dissymetric the ionosphere is.
2016/11/18: Japhet Yates: Quasiperiodic MHD waves in Saturn’s magnetosphere
Quasiperiodic ∼1 h fluctuations have been recently reported by numerous instruments on board the Cassini spacecraft. The interpretation of the sources of these fluctuations has remained elusive to date. Here we provide an explanation for the origin of these fluctuations using magnetometer observations. We find that magnetic field fluctuations at high northern latitudes are Alfvenic, with small amplitudes (∼0.4 nT), and are concentrated in wave packets similar to those observed in Kleindienst et al. (2009). The wave packets recur periodically at the northern magnetic oscillation period. We use a magnetospheric box model to provide an interpretation of the wave periods. Our model results suggest that the observed magnetic fluctuations are second harmonic Alfven waves standing between the northern and southern ionospheres in Saturn’s outer magnetosphere.
2016/11/04: Jack Carlyle: Mass and Magnetic Field of Eruptive Solar Filaments
Erupting filaments are not only beautiful, captivating solar phenomena, but also a major source of extreme space-weather events. The more we understand how and why these events occur, the better chance we have at predicting and preparing for such events. In this presentation I will explain how we can determine the density of filament material, and demonstrate the application of this method to two events, especially focussing on eruptions which failed to carry away at least some of the mass involved. The method employed is a quasi-spectroscopic technique, which utilises co-temporal multiple passband SDO/AIA EUV images to determine column density. I then go on to examine the dynamics and morphology of the erupted material from one event and investigate the effect of magnetic field strength, density and mass configuration on the possibilities of structural formation (specifically the Rayleigh-Taylor instability). MHD simulations of observed instabilities are run and the evolution of these simulations (growth rates and length scales, specifically) are compared with the observations.
2016/10/21: Denise Perrone: Kinetic physics of minor ions in turbulent solar wind
Spacecraft measurements of solar-wind plasma generally reveal that the electromagnetic field fluctuations are in a state of fully developed turbulence and both cross-scale couplings and strong modifications of the particle distribution function are involved. The general picture of plasma turbulence becomes more complicated because of the multi-component nature of the solar wind. The interplanetary medium, although predominantly constituted of protons, is also made up of a finite amount of alpha particles, together with a few percent of heavier ions. ‘In situ’ observations have shown that heavy ions (alpha particles in particular) seem to be preferentially heated and accelerated with respect to protons. However, due to very scarce measurements of heavy ions at time resolutions comparable with their kinetic scales, energy partition between species in turbulent plasma dissipation is basically unexplored. For the moment, most of the information comes from numerical simulations and a crucial support is given by self-consistent, fully nonlinear Vlasov models.
Here, hybrid Vlasov-Maxwell simulations are used to investigate the kinetic behavior of protons and alpha particles in a two-dimensional turbulent plasma. The results show that the response of the two ion species to the fluctuating electromagnetic fields is different. In particular, a significant differential heating of alpha particles with respect to protons is observed, localized nearby the peaks of ion vorticity and where strong deviations from thermodynamic equilibrium are recovered. Moreover, by using a simulator of a top-hat ion spectrometer, planned on board the Turbulence Heating ObserveR (THOR mission, a candidate for the next M4 space mission of ESA), the details of the three-dimensional ion velocity distributions can be solved, highlighting important non-Maxwellian features, that are clearly crucial ingredients for the understanding of the complex process of the particle heating.
2016/10/07: David Williams: Observing the internal dynamics of a solar filament eruption
Solar filament eruptions are phenomena of interest to our understanding of the Sun and its influence on the heliosphere. For the Sun, they represent an important mechanism for removing highly non-potential magnetic flux from active regions. In the wider heliosphere, they represent the eruption of a flux rope, creating a strong transient disturbance to the magnetic field and plasma dynamic pressure, and driving energetic particles at the shock that precedes it.
Recreations of the path taken by the shock front have demonstrated that these are not necessarily radial, but instead experience feedback from heliospheric environment as they pass through it. One of the most challenging things to do in astronomy, however, is to simultaneously measure 3 spatial dimensions or velocity components while using 2-dimensional detectors typically found in cameras. Almost always, one dimension is sacrificed or two are mixed, compromising our ability to get the full 3D velocity, for example, of a target of interest. Since 2010, though, the Solar Dynamics Observatory has recorded high-sensitivity images of two ion species also observed through the slit of Hinode’s EUV Imaging Spectrometer (EIS). We have analysed a rare dataset of a filament eruption that was captured by both observatories, with a view to constraining the eruption’s full velocity vector. In the process, we have uncovered some interesting dynamics, not only in the apex of the eruption, but also along its flanks, and discuss how these might fit our current picture of coronal mass ejections.
The Kelvin-Helmholtz (KH) instability at the Earth's magnetopause is predominantly excited during northward interplanetary magnetic field (IMF). The generated waves may transfer particle, momentum, and energy from solar wind into magnetosphere, known from numerous observational and simulation works. Magnetic reconnection due to KH waves has been suggested as one of the mechanisms to transfer solar wind plasma into the magnetosphere. NASA’s Magnetospheric Multiscale (MMS) mission was launched in March 2015, and aims to study electron-scale physics of magnetic reconnection. The magnetic reconnection due to KHW is firstly studied in detail by using the MMS observations on September 8th 2015. The topics include asymmetric electric and magnetic fields, kinetic evidence, and electron diffusion region (EDR) in the reconnection region due to KHWs.
2016/09/02: Olivier Witasse: Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en-route to Pluto.
I will discuss observations of a large coronal mass ejection (CME) ejected on 14 October 2014. It hit Mars on 17 October 2014, as observed by Mars Express, MAVEN, Mars Odyssey and MSL instruments, about 44 hours before the close encounter with the Siding Spring comet. Interestingly, comet 67P/Churyumov-Gerasimenko was perfectly aligned with the Sun and Mars at 3.1 AU heliocentric distance. Rosetta measured the event on 22 October 2014. The CME was also detected by Stereo-A on 16 October, and by Cassini on 12 November, which indicates that its angular extension was at least 115 degrees. Fortuitously, the New Horizons spacecraft was also along the propagation direction of the CME, which can take 3-5 months to reach the distance of 31.7 AU. By the time the solar wind travels that far from the Sun, the fast solar wind parcels have interacted with slower wind parcels emitted at an earlier time along the same radial line. We investigate if the CME observed at Stereo-A, Mars, Rosetta and Saturn has a unique and non-ambiguous signature at New Horizons. This presents a challenge since many solar structures can either be worn down as they propagate, or they can merge into larger ones. I will present 3D WSA-ENLIL simulations out to 40 AU showing the evolution of the CME, including other CMEs during this period. Predictions for a possible detection by Voyager 2 at 110 AU are also made. I compare the Forbush effect -a transient decrease followed by a gradual recovery in the observed galactic cosmic ray intensity- due to this CME, as observed at Mars, comet 67P and at Saturn.
2016/07/01 Andrew Walsh: Update on Solar wind Core Electron Study.
I'll discuss progress on the solar wind core electron study, covering our new way of correcting spacecraft potential and, depending on how much gets done between now and then, show some initial fitting results. We should also spend a bit of time discussing any changes we want to make to the heliophysics group before it restarts in September.
2016/06/10 Owen Roberts: Understanding compressible turbulence with multi-point and wavelet techniques. From Cluster to Thor.
Compressive fluctuations in the solar wind make up a non-negligible component of solar wind turbulence. However the lack of data with high time resolution especially for density measurements make this challenging. One novel method is to use the spacecraft potential measurement as a proxy for the electron number density, which has a much higher sampling rate on the Cluster spacecraft than the density measurements. Using this high resolution density data both single and multi-point techniques can be used to determine properties of the compressible turbulence. Using the multipoint signal resonator technique the dispersion plot of the incompressible and compressible fluctuations can be obtained. Plasma frame frequencies for the compressive component are shown to vary more than the incompressible component. The majority of both compressible and incompressible points have lower frequencies than the proton cyclotron frequency. To complement this analysis wavelet techniques are also used to obtain several scale dependent quantities e.g. cross correlation, reduced magnetic helicity. The compressive component of the turbulence is largely dominated by regions of anti-correlation between density and magnetic field, with small regions of positive correlation. Since both kinetic Alfven waves (KAW), Kinetic slow waves (KSW), and pressure balanced structures (PBS) can produce such an anti-correlation, reduced magnetic helicity is also investigated however this cannot differentiate between the two since a sunward KSW has the same signature as an anti-sunward KAW. At the scales studied KAWs and KSWs share many properties making distinguishing between them difficult. It has often been assumed that slow waves cannot exist in the solar wind since the fluctuations would be heavily damped. The excellent time resolution offered by Thor for plasma and magnetic field measurements will allow determination of the scale dependant Alf\én ratio, which can differentiate definitively between KSWs and KAWs.
2016/05/13 Denise Perrone: Compressible coherent structures in the solar wind at ion scales.
Understanding the physical mechanisms of dissipation, and the related heating, in turbulent collisionless plasmas (such as the solar wind) represents nowadays one of the key issues of plasma physics. Although the complex behavior of the solar wind has been matter of investigation of many years, some of the primary problems still remain a puzzle for the scientific community.
Here, the nature of the turbulent fluctuations close to the ion scales, in both slow and fast solar wind streams, is investigated using high-time resolution magnetic field data of multi-point measurements of Cluster spacecraft.
In the slow solar wind stream, about 40% of the analyzed time interval is characterized by the presence of coherent structures, that have a strong wave-vector anisotropy in the perpendicular direction with respect to the local magnetic field and typical scales around ion characteristic scales. Moreover, although most of the structures are merely convected by the wind, the 25% propagate in plasma frame. Furthermore, it has been shown for the first time that different families of coherent structures participate to the intermittency at ion scales in slow solar wind, such as compressible structures, i.e. magnetic holes, solitons and shock; and alfvénic structures in form of current sheets and vortices. These last ones can have an important compressible part and they are the most frequently observed during the present interval of slow solar wind.
On the other hand, different results are obtained if a stream of fast solar wind is considered. In this case, the ion scales are dominated by Alfvén vortices with small and/or finite compressible part and by several current sheets aligned with the magnetic field, almost convected by the wind. No compressive structures, such as solitons or magnetic holes, are found.
2016/04/29 Anik De Groof: The coronal magnetic field structure between 1.3 and 2.5 solar radii: SWAP coronal fans. MOVIE
The EUV telescope PROBA2/SWAP has been observing the solar corona in a bandpass near 17.4 nm since February 2010. SWAP's wide field-of-view provides a unique and continuous view of the extended EUV corona up to 2-3 solar radii. By carefully processing and combining multiple SWAP images, low-noise composites were produced that reveal large-scale, EUV-emitting, coronal structures. These extended structures appear mainly above or at the edges of active regions and typically curve towards the poles. As they trace out the 1-million-degree solar corona and persist for multiple Carrington rotations, they give an interesting view on how the coronal magnetic field is structured between 1.3 and 2-3 solar radii, in the gap between SDO/AIA’s FOV and typical lower boundaries of coronagraph FOVs. With the help of magnetic field models, we analyse the geometry of the extended EUV structures in more detail and compare with EUV coronagraph measurements up to as close as 1.5Rs. The long-term evolution of the SWAP coronal fans over the rising phase and maximum of Solar Cycle 24 is analysed to study their persistence, their rotation rate and what makes them appear or disappear.
Understanding the magnetic complexity of this coronal region will be necessary to link SPPs and Solar Orbiter’s in-situ measurements back to their sources. In addition we explore how Solar Orbiter’s future remote-sensing observations can be used to further constrain the magnetic structure of the mid-corona.
2016/04/15 Sandy Cardnell: A photochemical model of the dust-loaded ionosphere of Mars.
The ionisation of the Martian atmosphere and the presence of charged species are fundamental in the understanding of atmospheric electricity phenomena, such as electric discharges and large scale electric currents. Micro-ARES, the electric filed and conductivity sensor on board the Schiaparelli lander of ExoMars 2016, will conduct the very first measurement and characterization of Martian atmospheric electricity. In the framework of the data interpretation and analysis of Micro-ARES, a photochemical model of the lower ionosphere of Mars (0-70km) has been developed to characterise the atmospheric plasma. It accounts for the most abundant charged species and the photochemical reactions that interrelate them. Suspended dust has an important effect on the resulting concentration of ions and free electrons. Therefore, several dust scenarios have been studied, as well as the day-night variability. The results of the model are also relevant to the study of Schumann resonances in the Martian atmosphere.
2016/04/08 Andreas Otto and Isabella Kraus: Detecting and tracking solar disk bright points in the EUV.
In a recent work here at ESTEC, we have developed a software and analysis that automatically segments the features of the solar disk using data from several wavelength provided from several missions and instruments currently in orbit (PROBA2/SWAP, SDO/AIA).
The research group (basically collaborators in India, Japan, Austria and ESA) would like to extend the analysis by two activities: first, to extend the analysis to the solar magnetosphere, and secondly, to include EUV Bright Points in our analysis of the coronal plasma.
EUV Bright Points are associated with small, bipolar magnetic fields in the photosphere and sites of local magnetic reconnection in the corona.
Andreas and Isabella will give an introduction to the EUV Bright Points and why they might be important to understand the heating of the solar corona. The their current work they try to track these EUV Bright Points over hours and days, and they will give us an overview of the algorithm and the results so far.
2016/04/01 Lucile Turc: Solar Wind - magnetospheric coupling: why should we care about the magnetosheath?
When the solar wind arrives in the vicinity of Earth, it first encounters the bow shock which slows down the flow and deflects it around the magnetosphere. The region of shocked plasma between the bow shock and the magnetopause is called the magnetosheath. This region operates as a natural interface between the solar wind and the magnetosphere. However, its role in the solar wind-magnetospheric coupling is generally overlooked, and many studies simply assume that the properties of the solar wind impinging on the magnetopause are the same as those observed in the upstream medium.
I will present an overview of the magnetosheath and of its large-scale properties, and discuss how they can influence the solar wind-magnetospheric coupling.
2016/03/18 Sergio Toledo-Redondo: Cold ion demagnetization near the X-line of magnetic reconnection.
The effects of magnetic reconnection in magnetospheres can be observed at planetary scales but reconnection is initiated at electron scales of the plasma, where it becomes diffusive and magnetic field changes its topology. Around the electron diffusion region there is an ion decoupling region governed by the ion length-scales, i.e. the ion skin depth and the ion gyroradius. Reconnection occurring at the dayside magnetopause of Earth often includes cold ions (few tens of eV) of ionospheric origin. They have a smaller gyroradius than the hot magnetospheric ions (∼10 keV), and they introduce a new length-scale into reconnection. We report observations close to the X-line of a layer of the size of the cold ion gyroradius (∼10 km) where the cold ion population is demagnetized. Outside this layer the cold ions follow the E×B motion together with electrons, while the hot ions are already demagnetized.
2016/03/11 Arnaud Masson: Magnetic reconnection vs. Kelvin-Helmholtz instability: is the debate really over?
Magnetic reconnection is widely considered as the main mechanism for plasma entry into the Earth’s magnetosphere. Another plasma entry mechanism called Kelvin-Helmholtz instability (KHI) is also known for decades to occur, especially when the Interplanetary Magnetic Field is oriented Northward. Over the past 15 years, the Cluster mission has brought a wealth of new results and shed a total new light on the KHI mechanism. Very recent statistics based on the Themis mission data tend to confirm the importance of the KHI. A summary of the key results found by Cluster and Themis will first be presented. Now the question is: what is the next step? Shall we continue this sterile debate of my mechanism is better than yours? In other words, how do we get from a qualitative picture to a quantitative one? How to quantify plasma entry in the magnetosphere for these two mechanisms at the same time, both dependent on solar wind input? Which observations would be then needed? An attempt of the new observations needed will be proposed.
2016/03/04 Andrew Walsh: Solar wind core electrons: Maxwellian or Kappa?
Solar wind core electrons are typically considered to have a Maxwellian velocity distribution function. However, most measurements made of them to date don’t have sufficient energy resolution to distinguish between a Maxwellian and a kappa distribution at low energies. Here we present a survey of solar wind electron velocity distribution functions observed by Cluster PEACE in its highest energy resolution mode, which is sufficient to distinguish between Maxwellian and kappa distributions for energies below 15eV. Initial results suggest that a kappa distribution better fits the data than a Maxwellian in all cases; in the majority of cases the difference in goodness of fit between a kappa and Maxwellian is small but in some cases, a kappa distribution fits the data significantly better.
2016/02/26 Owen Roberts: Multipoint investigations of plasma turbulence in the solar wind.
In neutral fluids, turbulence yields eddies from large to ever smaller scales until the turbulent energy is eventually dissipated by viscosity. In plasmas, the magnetic field brings complications that not only eddies but waves and current sheets are also commonplace, and all these contribute to the dissipation of the turbulence power. In order to understand the inherently three dimensional processes associated with plasma turbulence, multiple spacecraft such as Cluster are needed to enable unambiguous differentiation between changes that occur in space and those that occur in time. This is essential when determining whether a certain fluctuation (such as in the magnetic field) corresponds to a propagating wave or an advected structure. Previous observations of the incompressible turbulence in the solar wind will be presented, where fluctuations typically have low frequencies in the plasma frame, corresponding to either Kinetic Alfven waves, or advected structures such as the Alfven vortex. While minority of intervals also show the presence of Ion cyclotron waves. Preliminary results investigating compressible turbulence will also be presented by analysing the magnitude of the magnetic field and density derived from the spacecraft potential.
The interaction between the Sun and the Earth through the solar wind causes the Earth’s magnetosphere to undergo sporadic bursts of activity that release peta-Joules of energy into the Earth’s ionosphere. These events, known as substorms, have been closely studied since the 1960s, but are still a source of controversy in space plasma physics. After 15 years of operations, ESA’s Cluster mission is still providing new insights into the phenomena and physical processes behind substorms. In this talk, we will explore how multi-spacecraft observations at low altitude allow us to determine the spatial structure of the substorm current wedge, and how nearly a decade’s worth of observations of the magnetotail are helping us to understand how the magnetotail plasma sheet reacts to the solar wind energy input prior to its release in a substorm.
I am planning to present the paper which I am currently writing, entitled "Cone angle control of the interaction of magnetic clouds with the Earth's bow shock". I will start with an introduction about magnetic clouds and their effects on the Earth's environment. Then I will present statistical results about the orientation of the interplanetary magnetic field during magnetic cloud events, based on an extensive catalogue of magnetic clouds. I will show how these properties in the solar wind, combined with a bow shock model, can give us information about the shock configuration which will be encountered upon reaching the near-Earth space. I will then discuss the implications of these results in the framework of space weather.
2016/02/05 Yannis Zouganelis: Solar wind electrons: observations and theory.
I will present an overview of what we know about solar wind electrons, especially their velocity distribution functions, how they look like (core, halo, strahl, superhalo), why (recent theories that could explain their form) and what are the implications to the physics of the solar wind. I will also talk about the electron heat conduction problem that seems to be similar with other semi-collisional astrophysical plasmas like the hot interstellar medium in galaxies or some accretion disks around neutron stars and black holes. All this in less than one hour.
2016/01/29 Denise Perrone: An interesting problem: the turbulence.
Turbulence is a highly non-linear process ubiquitous in Nature. The nonlinearity is responsible for the coupling of many degrees of freedom leading to an unpredictable dynamical evolution of a turbulent system. In both hydrodynamics (HD) and magnetohydrodynamics (MHD), fluctuations of bulk quantities that describe turbulent flows exhibit the property of statistical scale invariance, which is a form of self-similarity.
In plasma physics, the turbulence represents one of the most spectacular and unsolved problems, where both cross-scale couplings and kinetic effects are present. The energy, injected at large scales, progressively decays towards smaller scales, where kinetic effects dominate the plasma dynamics (heating, particle acceleration and so on).
Living on Earth, and thanks to the support of many space missions, we have the unique opportunity to analyze directly the features of the dynamical behavior of a natural plasma: the solar wind. ‘In situ’ measurements revel that kinetic effects control solar-wind dynamics. In this scenario, the use of both kinetic numerical simulations and spacecraft measurements becomes crucial.
2016/01/22 Daniel Mueller: 3D Visualization of Solar Data: Getting ready for Solar Orbiter and discovering new things while doing so.
ESA’s next heliophysics mission, Solar Orbiter, will focus on exploring the linkage between the Sun and the heliosphere. It will collect unique data that will allow us to study, e.g., the coupling between macroscopic physical processes to those on kinetic scales, the generation of solar energetic particles and their propagation into the heliosphere and the origin and acceleration of solar wind plasma. Combined with the several petabytes of data from NASA's Solar Dynamics Observatory, the scientific community will soon have access to multidimensional remotesensing and complex in-situ observations from different vantage points, complemented by petabytes of simulation data.
Answering overarching science questions like “How do solar transients drive heliospheric variability and space weather?” will only be possible if the community has the necessary tools at hand. As of today, there is an obvious lack of capability to both visualise these data and assimilate them into sophisticated models to advance our knowledge. A key piece needed to bridge the gap between observables, derived quantities like magnetic field extrapolations and model output is a tool to routinely and intuitively visualise large heterogeneous, multidimensional, time-dependent data sets.
In this contribution, I will present recent progress in visualising the Sun and its magnetic field in 3D using the open-source JHelioviewer framework, which is part of the ESA/NASA Helioviewer Project. In addition, I would like to solicit feedback about functionality that would help scientists, i.e. YOU, to make new discoveries.
2016/01/15 Sergio Toledo-Redondo: Observations of cold ions at the Earth's magnetopause: implications for magnetic reconnection.
Cold ions of ionospheric origin are known to be present in the magnetospheric side of the Earth's magnetopause. They can be very abundant, with densities up to 100 cm−3. These cold ions can mass load the magnetosphere, changing global parameters of magnetic reconnection, like the Alfvén speed or the reconnection rate. In addition they introduce a new length-scale related to their gyroradius and kinetic effects which must be accounted for. We report in-situ observations of cold ion heating in the separatrix owing to time and space fluctuations of the electric field. When this occurs, the cold ions are pre-heated before crossing the Hall electric field barrier. However, when this mechanism is not present cold ions can be observed well inside the reconnection exhaust. Our observations suggest that the perpendicular cold ion heating is stronger close to the X-line owing to waves and electric field gradients linked to the reconnection process.