Periodic Reporting for period 3 - ASTRODUST (The Heliosphere and the Dust: Characterization of the Solar and Interstellar Neighbourhood)
Période du rapport: 2023-01-01 au 2024-06-30
The Solar System currently moves through the outer edges of a local interstellar cloud that contains plasma, gas and dust. It has entered this cloud roughly 60.000 years ago, and will leave it in a few thousand years. As a consequence, the interstellar dust particles move constantly through the solar system, at high speeds of about 26 km/s. The dust flow patterns depend on solar gravitation, solar radiation pressure force, and on Lorentz forces. The latter is because the dust is charged and moves through the magnetic fields entailed in the solar wind, that flows outward from the Sun. Because magnetic fields in interplanetary space change with the solar cycle, the incoming dust particles become focused near the ecliptic plane every 22 years (2029-2030 is expected to be the next dust maximum, which happens near every other solar minimum).
The solar wind shapes a bubble around the Sun and the planets, which is called the “heliosphere”. The heliosphere’s size depends on the solar cycle, and there is no consensus yet about its shape: it could be comet-like, a stretched, roughly symmetric bubble, or a croissant-type of shape. The physics of the heliosphere are also not yet fully understood, in particular in the transition region with interstellar space.
In this project, we study the motion of the interstellar dust particles in the solar system, by building a computer model that includes the inner and outer parts of the heliosphere for calculating the dust trajectories. These theoretical trajectories will be compared with dust measurements from several dust detectors on spacecraft like Ulysses and Cassini, and with indirect measurements by spacecraft antenna that pick up a signal when a dust particle impacts on the spacecraft body. Such data are available from the Wind mission, STEREO and Voyager. We aim to learn about the original dust size distribution in the local cloud (the original distribution that is not altered by the heliosphere), about the dust properties in our local interstellar neighbourhood, and – using the dust as a tracer – about the dynamics and structure of the heliosphere. Also, we will apply what we learned to other astrospheres and to our own heliosphere in a different location in the interstellar medium (for instance, a denser cloud, or a less dense cloud), which is currently being thought to have happened in the history (and future) of the heliosphere, and the solar system, on its journey around the Galaxy. With this work, we perform fundamental science on the interstellar dust, important for, and complementary to astronomers’ work with remote observations, and we contribute to humanity’s first steps in the exploration of our immediate interstellar neighbourhood.
The solar wind shapes a bubble around the Sun and the planets, which is called the “heliosphere”. The heliosphere’s size depends on the solar cycle, and there is no consensus yet about its shape: it could be comet-like, a stretched, roughly symmetric bubble, or a croissant-type of shape. The physics of the heliosphere are also not yet fully understood, in particular in the transition region with interstellar space.
In this project, we study the motion of the interstellar dust particles in the solar system, by building a computer model that includes the inner and outer parts of the heliosphere for calculating the dust trajectories. These theoretical trajectories will be compared with dust measurements from several dust detectors on spacecraft like Ulysses and Cassini, and with indirect measurements by spacecraft antenna that pick up a signal when a dust particle impacts on the spacecraft body. Such data are available from the Wind mission, STEREO and Voyager. We aim to learn about the original dust size distribution in the local cloud (the original distribution that is not altered by the heliosphere), about the dust properties in our local interstellar neighbourhood, and – using the dust as a tracer – about the dynamics and structure of the heliosphere. Also, we will apply what we learned to other astrospheres and to our own heliosphere in a different location in the interstellar medium (for instance, a denser cloud, or a less dense cloud), which is currently being thought to have happened in the history (and future) of the heliosphere, and the solar system, on its journey around the Galaxy. With this work, we perform fundamental science on the interstellar dust, important for, and complementary to astronomers’ work with remote observations, and we contribute to humanity’s first steps in the exploration of our immediate interstellar neighbourhood.
We started building a new model of the heliosphere-dust interaction, at first without the outer parts of the heliosphere, which will later be included. In parallel, we have performed simulations using state-of-the-art software of the inner part of the heliosphere for timely predictions for the Destiny+ mission (DLR-JAXA, launch date in 2024), with different assumptions for the dust properties and incoming dust size distribution. Also, we supported the Interstellar Probe mission concept study with dust simulations in preparation of an interstellar mission that – if it will be selected by NASA – could be launched in 2036 and reach interstellar space about 17 years later. We re-analysed the Ulysses data with an emphasis on the influence of the selection criteria of interstellar dust versus interplanetary dust on the resulting fluxes and mass distributions. We know from Ulysses data that the large interstellar dust particles are likely porous, and also the Stardust sample return mission returned highly porous interstellar material. Because dust detector impact signals depend theoretically on the density of the dust particle with respect to the density of the detector target material, we have investigated calibration data that were available of porous particle impactors, in order to test whether the largest interstellar dust particles measured by Ulysses were “correctly” measured if interstellar dust is indeed porous. The outcome was that the largest particles have at least as much mass as previously derived from earlier measurements – in case of porosity. These results were unexpected and will contribute to the scientific discussion on the physics of impact ionization, as well as to the science of the interstellar medium (largest particle size, cosmic abundances, etc.). Last but not least, the Wind dust impact data are currently thoroughly being examined. Since these are “serendipity data”, taken indirectly by another instrument on the spacecraft, the operations were not optimized for dust measurements, and hence, such an analysis needs extra vigilance. Nevertheless, preliminary results show a clear solar cycle dependence of the interstellar dust flux.
Our work on future missions like the Interstellar Probe and the DOLPHIN mission concept (a new mission concept for measuring cosmic dust and its interaction with the heliosphere), and the close collaborations we set up between dust and heliosphere scientists is pushing both fields forward, in particular because the role of dust for the heliosphere is a fairly new and unexplored topic of research. The heliosphere is in this project not only used for dust dynamics calculations, but also opposite: we aim to use the dust as an extra boundary condition (in addition to plasma, magentic field, etc.) for improving current heliosphere models and understanding better the heliosphere structure and dynamics. This research is important for many missions that will fly (Destiny+, IMAP) or that are conceptualized at the moment (Interstellar Probe, DOLPHIN, etc.). Important are also the links to astrosphere and exoplanet science: once we understand the heliosphere and how it interacts with the dust and the local interstellar environment, these findings will be useful for other stars and their astrospheres, habitability, and the history of the solar system and the Earth in its local interstellar neighbourhood.
Apart from this, the data analysis of the porous dust impacts brought up unexpected results that not only are crucial for understanding the size distribution and dynamics of interstellar dust, but also all other types of (porous) dust (e.g. from comets), and the physics of impact ionization as a concept used in space dust detectors. Last but not least, the Wind data set contains more than 22 years of dust impacts on a large satellite surface. These measurements with a plasma wave instrument are "serendipitous". They are taken by an instrument that was not optimized for dust measurements, but nevertheless these turn out to be very useful because of the large spacecraft surface area and the long time span with respect to the solar cycle. Thoroughly analyzing and understanding these data together with other data from different missions with different vantage points in space, will provide a solid basis for the boundary conditions of the heliosphere-dust dynamics model. Building a new time-variable heliosphere-dust model that includes the heliosphere boundaries will be beyond state-of-the-art.
Apart from this, the data analysis of the porous dust impacts brought up unexpected results that not only are crucial for understanding the size distribution and dynamics of interstellar dust, but also all other types of (porous) dust (e.g. from comets), and the physics of impact ionization as a concept used in space dust detectors. Last but not least, the Wind data set contains more than 22 years of dust impacts on a large satellite surface. These measurements with a plasma wave instrument are "serendipitous". They are taken by an instrument that was not optimized for dust measurements, but nevertheless these turn out to be very useful because of the large spacecraft surface area and the long time span with respect to the solar cycle. Thoroughly analyzing and understanding these data together with other data from different missions with different vantage points in space, will provide a solid basis for the boundary conditions of the heliosphere-dust dynamics model. Building a new time-variable heliosphere-dust model that includes the heliosphere boundaries will be beyond state-of-the-art.