Periodic Reporting for period 4 - ASTRODUST (The Heliosphere and the Dust: Characterization of the Solar and Interstellar Neighbourhood)
Reporting period: 2024-07-01 to 2025-12-31
The Solar System moves through the interstellar medium that contains plasma, gas and dust. Therefore, interstellar dust (ISD) particles move through the solar system and their motion depends on solar gravitation, radiation pressure force, and Lorentz forces due to charged dust moving through the solar wind the magnetic fields. The incoming dust particles are focused near the ecliptic plane every 22 years following the solar magnetic cycle. The next dust flux maximum is expected between ca. 2029 and 2035.
The solar wind shapes a bubble around the Sun and the planets: the “heliosphere” that interacts with its local interstellar neighbourhood. Its dynamical size, shape and physics are not yet fully understood, particularly in the transition region with interstellar space.
In this project, we studied the dynamics and flux of the ISD particles in the heliosphere, by relying on computer models and in situ dust data. Timely predictions for future missions supported planned missions with a dust detector, and future concepts, particularly for in the early 2030s. In situ data were (re-) analyzed from dust detectors on spacecraft like Ulysses, and from indirect measurements by spacecraft antenna from Wind, STEREO and Voyager, that pick up a signal when a dust particle impacts on the spacecraft body. The aim is to learn about the original dust size distribution (before entering the heliosphere) and about the dust properties in the local interstellar environment (e.g. maximum dust size). Ultimately, we can use simulations and measurements for learning bout the dynamics and structure of the heliosphere by using the small dust as a tracer. Also, what we learn “here” can be applied to other astrospheres. With this work, we perform fundamental science on the ISD, 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: the “heliosphere” that interacts with its local interstellar neighbourhood. Its dynamical size, shape and physics are not yet fully understood, particularly in the transition region with interstellar space.
In this project, we studied the dynamics and flux of the ISD particles in the heliosphere, by relying on computer models and in situ dust data. Timely predictions for future missions supported planned missions with a dust detector, and future concepts, particularly for in the early 2030s. In situ data were (re-) analyzed from dust detectors on spacecraft like Ulysses, and from indirect measurements by spacecraft antenna from Wind, STEREO and Voyager, that pick up a signal when a dust particle impacts on the spacecraft body. The aim is to learn about the original dust size distribution (before entering the heliosphere) and about the dust properties in the local interstellar environment (e.g. maximum dust size). Ultimately, we can use simulations and measurements for learning bout the dynamics and structure of the heliosphere by using the small dust as a tracer. Also, what we learn “here” can be applied to other astrospheres. With this work, we perform fundamental science on the ISD, 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 expanded simulations of interstellar dust (ISD) in the solar system to include the dust material properties, and we investigated the influence of the “starting distance” on the simulated fluxes and flow directions. They turned out to be relevant, meaning the ISD can help to constrain the dynamic boundaries of the heliosphere, and the dust properties, provided sufficient data is available [Baalmann24]. Timely predictions of ISD fluxes and flow directions were made in support of existing or planned missions like the JAXA/DLR mission Destiny+ [Simolka24, Krüger24] with launch in 2028 (originally 2022), New Horizons [Bernardoni22], JUICE, and Voyager. Dust predictions, science questions and possible instrumentation were provided for mission or instrument concepts for in the early 2030s, led by our group, like our proposed DOLPHIN mission concept, and the Lunar Gateway dust package. We played a key role in defining the (interstellar) dust science for the Interstellar Probe study [Sterken23], including trajectory trade-offs, instrument options, and emphasizing synergies between dust and heliospheric science [Sterken23]. Finally, we also led the dust science concept for the Korean L4 mission [Posner21, Cho23]. Some of these mission concepts may be revisited and resubmitted (e.g. Interstellar Probe, DOLPHIN), while others (e.g. Gateway) are expected to provide a basis for alternative missions in the same timeframe (e.g. lunar missions, a science mission to Mars).
We investigated the influence of ISD selection criteria in the Ulysses data on the inferred ISD flux and flow direction, which was moderate (except for the mass distribitions). Most important is how the impacting dust particle’s mass is determined, from which we concluded that a dust analyzers with trajectory grids are necessary to reliably infer the size distribution and gas-to-dust mass ratio in the local interstellar environment [Baalmann24]. The largest ISD particles in the dataset were re-analyzed to constrain the gas-to-dust mass ratio outside the heliosphere, because these contribute the most to the total mass, but most of them are likely not interstellar [Baalmann24]. In this context we also contributed to the discussions on interstellar meteors [Hajduková 2020, Hajduková2024]. The time-dependent ISD size distribution after crossing the heliosheath was inferred from simulations and the data. The heliosheath not only diverts particles, but can also focus them on their way through [Baalmann24].
Plasma wave antennas pick up indirectly a signal when dust impacts on a spacecraft. Such data from the Wind mission span more than one magnetic solar cycle [Hervig22] and contain good statistics of about 10-20 impacts per day. The data clearly show the solar magnetic cycle imprint on the dust flux, and the yearly variation due to Earth’s motion w.r.t. ISD. A solar rotation frequency was unexpectedly discovered in the data, that – after extensive investigation – were correlated with Corotational Interaction Regions in the solar wind (compression regions of fast solar wind overtaking slow solar wind, that can rotate with the Sun) [Baalmann24]. These were also found in the STEREO data [Chadda24].
We simulated dust charging in the heliosphere (equilibrium charges and charging times) and made an inventory of dust charging in the entire heliosphere, including time-variability (i.e. solar events) and high latitudes. Uncertainties in the charging model are important and were emphasized, for instance when the dust composition is unknown. This work yields insights for ISD near the heliosphere boundary, where slow charging times for the smallest particles can cause them to enter a few AU before their decreasing gyroradius limits their path inwards. The dust charging was implemented in the trajectory model at the heliosphere boundary, as was the non-radial plasma flow and the magnetic fields. These findings are also relevant for small interplanetary dust that leaves the solar system.
Ulysses data and simluations, and Stardust sample return indicated that the larger ISD particles are likely porous. This can influence the dust detector response and thus the inferred mass distribution and its upper limit. Therefore we investigated available laboratory calibration data of porous dust, and concluded that the largest particles in the Ulysses dataset have at least as much mass as previously derived from earlier calibrations if they are moderately porous [Hunziker22]. These results were somewhat unexpected and contribute to the scientific discussion on the physics of impact ionization, and concerning the dust masses in the interstellar medium.
We investigated the influence of ISD selection criteria in the Ulysses data on the inferred ISD flux and flow direction, which was moderate (except for the mass distribitions). Most important is how the impacting dust particle’s mass is determined, from which we concluded that a dust analyzers with trajectory grids are necessary to reliably infer the size distribution and gas-to-dust mass ratio in the local interstellar environment [Baalmann24]. The largest ISD particles in the dataset were re-analyzed to constrain the gas-to-dust mass ratio outside the heliosphere, because these contribute the most to the total mass, but most of them are likely not interstellar [Baalmann24]. In this context we also contributed to the discussions on interstellar meteors [Hajduková 2020, Hajduková2024]. The time-dependent ISD size distribution after crossing the heliosheath was inferred from simulations and the data. The heliosheath not only diverts particles, but can also focus them on their way through [Baalmann24].
Plasma wave antennas pick up indirectly a signal when dust impacts on a spacecraft. Such data from the Wind mission span more than one magnetic solar cycle [Hervig22] and contain good statistics of about 10-20 impacts per day. The data clearly show the solar magnetic cycle imprint on the dust flux, and the yearly variation due to Earth’s motion w.r.t. ISD. A solar rotation frequency was unexpectedly discovered in the data, that – after extensive investigation – were correlated with Corotational Interaction Regions in the solar wind (compression regions of fast solar wind overtaking slow solar wind, that can rotate with the Sun) [Baalmann24]. These were also found in the STEREO data [Chadda24].
We simulated dust charging in the heliosphere (equilibrium charges and charging times) and made an inventory of dust charging in the entire heliosphere, including time-variability (i.e. solar events) and high latitudes. Uncertainties in the charging model are important and were emphasized, for instance when the dust composition is unknown. This work yields insights for ISD near the heliosphere boundary, where slow charging times for the smallest particles can cause them to enter a few AU before their decreasing gyroradius limits their path inwards. The dust charging was implemented in the trajectory model at the heliosphere boundary, as was the non-radial plasma flow and the magnetic fields. These findings are also relevant for small interplanetary dust that leaves the solar system.
Ulysses data and simluations, and Stardust sample return indicated that the larger ISD particles are likely porous. This can influence the dust detector response and thus the inferred mass distribution and its upper limit. Therefore we investigated available laboratory calibration data of porous dust, and concluded that the largest particles in the Ulysses dataset have at least as much mass as previously derived from earlier calibrations if they are moderately porous [Hunziker22]. These results were somewhat unexpected and contribute to the scientific discussion on the physics of impact ionization, and concerning the dust masses in the interstellar medium.
Our leadership in the work on future missions and the close collaborations set up between dust and heliospheric scientists for mission concepts and instruments is pushing the fields forward and will be used in future such work. Our studies of trajectories in the outer heliosphere, dust charging under different conditions and assumptions throughout the heliosphere, and new discoveries like the solar rotation frequency in the Wind data show that dust and plasma are coupled in an interesting way. The data analysis of the porous dust impacts brought up unexpected results that not only are crucial for understanding the size distribution (and thus dynamics) of ISD, but also all other types of (porous) dust (e.g. from comets), and the physics of impact ionization as used in space dust detectors.