Dr. Lennart R. Baalmann

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From April 2022 until June 2024, I was employed as a postdoctoral researcher in the Astrodust group of Dr. Veerle J. Sterken at ETH Zürich, Switzerland. The group researches cosmic dust in the heliosphere; my focus was on numerical simulations and data analysis of spacecraft measurements.

Before coming to Zürich, I graduated in early 2022 with a PhD 'summa cum laude' in theoretical astrophysics at Ruhr-Universität Bochum (RUB), Germany, researching and studying in the heliophysics group under Dr. Klaus Scherer and Dr. Horst Fichtner. My research at RUB was centred on magnetohydrodynamic models of astrospheres, which translated well to my later focus on cosmic dust the heliosphere. In parallel to my PhD I also completed a Bachelor's degree (B.Sc.) in mathematics.

I greatly enjoy doing scientific outreach activities, for example for Lecturers Without Borders (LeWiBo). From 2023 until 2024, I was a co-coordinator of the OH-VLISM online seminar series.

Outside of academia, I'm a lifeguard and swim instructor for the German Life Saving Association (DLRG), I play the piano, and enjoy all kinds of Science Fiction media, particularly novels and video games. I'm greatly interested in the current advances of artificial intelligence and the dangers that go with it, and enjoy informing myself about cognition and rationality.

If you would like to contact me for professional or academic purposes, please find my contact information on my ETHZ page. For other concerns, contact information is located in the legal notices and impressum

Space is not empty but filled with low-density plasma, the so-called interstellar medium (ISM). Most stars – such as the Sun – move at high speed through their surrounding ISM, like a plane flying through air. If the star moves fast enough, a paraboloid structure called a bow wave forms ahead of the star, similar to the bow wave in front of a moving boat. If the star moves even faster, with a speed that is higher than the local speed of sound, the bow waves becomes discontinuous, which is called a stellar bow shock (BS). Across the bow shock, the properties of the ISM (density, flow velocity, magnetic field strength, etc.) abruptly change; in a bow wave, these changed are gradual. It has not yet been fully ascertained whether the Sun has a bow shock or bow wave in front of it.

The star also continuously blows out plasma in all directions, which we call the stellar wind; our Sun's stellar wind is called the solar wind. The stellar wind is typically supersonic as well; therefore, before it encounters the inflowing ISM, it is shocked to subsonic speeds as well. This shock is called the termination shock (TS); it surrounds the star like a drop-shaped bubble. Across the termination shock, the plasma parameters of the stellar wind change as well.

So now we have the star with its supersonic outflow, the stellar wind, inside the bubble-like termination shock; and much father out we have the paraboloid bow shock, outside of which the ISM resides. The region in-between is called the astrosheath (or, for the Sun, the heliosheath). Outside the termination shock, we have the subsonic stellar wind; and inside the bow shock, we have the subsonic ISM inflow.

And yet, these two plasma flows don't mix in the astrosheath but are separated by another structure, the astropause (AP; or, for the Sun, the heliopause, HP). For the plasma, the astropause acts like a wall; neither the stellar wind can flow outwards into the subsonic ISM, nor the ISM can flow inwards into the subsonic stellar wind. This is also true for the magnetic field: neither the magnetic field of the ISM nor the magnetic field of the stellar wind can cross the astropause; they wind around it. The region between the termination shock and the astropause is commonly called the inner astrosheath, and the region between the astropause and the bow shock the outer astrosheath. Confusingly, for the Sun, some people call the region between the termination shock and the heliopause the heliosheath, and the region between the heliopause and the (potential) bow shock the very local interstellar medium (VLISM).

All these structures together are typically called an astrosphere, or, for the Sun, the heliosphere. An astrosphere can act a bit like a protective shield for the planetary system inside; the heliosphere, for example, acts like a barrier for some cosmic rays. Because cosmic rays can interact with planetary atmospheres in a way that creates biosignatures, it is important to know the surrounding astrosphere and its effects on cosmic rays. Otherwise, the cosmic-ray-induced biosignatures may be misidentified as signs of extraterrestrial life.

The Sun moves through the local interstellar medium (LISM) with about 26 km/s. There are many microscopic particles in the LISM, which we call interstellar dust (ISD). Because the Sun moves through this dust, within the rest frame of the Sun it looks as if this ISD were coming towards us with 26 km/s.

The smallest dust particles follow the magnetic field lines of the LISM and the VLISM. Because magnetic fields cannot penetrate the heliopause, these smallest dust particles cannot enter the inner heliosphere. Intermediate-sized and large particles, however, can penetrate into the solar system more or less unhindered.

What happens to the ISD particles inside the solar system depends mainly on three forces: solar gravity attracts the ISD to the Sun, whereas solar radiation pressure pushes particles away from the Sun. These two forces oppose each other; their ratio is called the β-ratio. If β > 1, solar radiation pressure is dominant and pushes the particle away from the Sun; if β < 1, solar gravity is dominant and attracts the particle towards the Sun. The β-ratio depends only on the properties of the particle itself, predominantly on its size: large particles have β ≈ 0; they are not influenced by solar radiation pressure. The smallest particles typically have β < 1; they are attracted to the Sun. Intermediate particles, however, can have β > 1; they are pushed away from the Sun.

The third important force that acts on the ISD particles is the Lorentz force, which depends on the magnetic field. However, the magnetic field of the solar wind is not constant but changes with the solar magnetic cycle: every eleven years it flips its orientation. Depending on the orientation, the dust particles are either focused towards the plane in which the planets and most spacecraft are located, which is called the ecliptic plane, or defocused away from it. Currently, we are in the defocusing phase of the solar magnetic cycle, so many ISD particles do not reach the ecliptic plane and cannot be measured by the spacecraft located there. The next focusing phase will be in the 2030s; this will be our greatest chance of measuring interstellar dust within our solar system in the near future. It will take another 22 years until the successive focusing phase begins.

If the dust particles manage to penetrate deep enough into the solar system, they can impact on spacecraft. Some of these spacecraft, for example Ulysses, are equipped with dust detectors that can determine the properties of impacting dust particles. Other spacecraft, such as Wind, do not have dust detectors; nevertheless, some of these spacecraft are still able to detect dust impacts through secondary effects.

My proudest academic achievement to date is my PhD thesis. You can download it from the website of the RUB university library.

I greatly enjoy giving talks in seminars, workshops, or at conferences! My talk on A multi-mission study of interstellar dust in the heliosphere: observations and simulations in the online seminar series of Outer Heliosphere & VLISM (OH-VLISM) is available in their archive.

Publications related to my research on cosmic dust in the Astrodust group at ETHZ (sorted by newest):

• L.R. Baalmann, S. Hunziker, A. Péronne, et al. – A solar rotation signature in cosmic dust: Frequency analysis of dust particle impacts on the Wind spacecraft (A&A 689 A329, Sept. 2024)
  [ADS] [A&A] [arXiv]
• V.J. Sterken, S. Hunziker, K. Dialynas, et al. – Synergies between interstellar dust and heliospheric science with an Interstellar Probe (white paper; RASTI 2 1 pp. 532-547, Aug. 2023)
  [ADS] [RASTI] [arXiv]
• H.-W. Hsu, A. Poppe, J. Szalay, et al. – In Situ Cosmic Dust Detection for Heliophysics (white paper; BAAS 55 176, Jul. 2023)
  [BAAS]
• V.J. Sterken, L.R. Baalmann, E. Godenko, et al. – Dust in and around the heliosphere and astrospheres (SSR 218 71, Dec. 2022)
  [ADS] [SSR]

Publications related to my research on astrospheres at RUB (sorted by newest):

• L.R. Baalmann, K. Scherer, J. Kleimann, et al. – Modelling O-star astrospheres with different relative speeds between the ISM and the star: 2D and 3D MHD model comparison (A&A 663 A10, July 2022)
  [ADS] [A&A] [arXiv]
• K. Herbst, L.R. Baalmann, A. Bykov, et al. – Astrospheres of Planet-Hosting Cool Stars and Beyond: When Modeling Meets Observations (SSR 218 4, June 2022)
  [ADS] [SSR]
• L.R. Baalmann, K. Scherer, J. Kleimann, et al. – Simulating observable structures due to a perturbed interstellar medium in front of astrospheric bow shocks in 3D MHD (A&A 650 A36, June 2021)
  [ADS] [A&A] [arXiv]
• K. Herbst, K. Scherer, S.E.S. Ferreira, et al. – On the Diversity of M-star Astrospheres and the Role of Galactic Cosmic Rays Within (ApJL 897 L27, July 2020)
  [ADS] [APJL] [arXiv]
• K. Scherer, L.R. Baalmann, H. Fichtner, et al. – MHD-shock structures of astrospheres: λ-Cephei-like astrospheres (MNRAS 493 4172S, Apr. 2020)
  [ADS] [MNRAS] [arXiv]
• L.R. Baalmann, K. Scherer, H. Fichtner, et al. – Skymaps of observables of three-dimensional MHD astrosphere models (A&A 634 A67, Feb. 2020)
  [ADS] [A&A] [arXiv]