CfA Seminar

Mojtaba Akhavan-Tafti
Magnetic Switchbacks Contribute to Solar Wind Acceleration and Heating
Magnetic switchbacks are reversals (deflection angle θ_B greater than 90°) in the radial magnetic field B_r, accompanied by proton bulk flow v_p enhancement. The switchback generation mechanism(s) is still debated, though switchback evolution is linked to coronal heating. In this study, we provide an exhaustive investigation of magnetic fluctuations in the first 14 Parker Solar Probe (PSP) encounters. Our results show (1) enhanced magnetic fluctuations (θ_B less than or equal to 90°), and (2) clear absence of B_r reversals θ_B greater than 90° in sub-Alfvénic solar corona. It is concluded that switchbacks are either generated locally in super-Alfvénic solar wind (in situ) or that a subpopulation of magnetic fluctuations generated in sub-Alfvénic solar corona (ex situ) evolves into switchbacks once in super-Alfvénic solar wind flow. In other words, in situ mechanisms are necessary for switchback formation. A complimentary study further examines the evolution of a bundle of switchbacks, referred to as a switchback patch, during a PSP-Solar Orbiter radial alignment. Switchback patches are found to evolve into microstreams, defined as bundles of magnetic flux tubes with enhanced v_p, relative to the ambient solar wind. Our studies further confirm increasing evidence that the evolution of magnetic switchbacks contribute to solar wind acceleration and heating.
Casey Brinkman
Rocky Planet Compositions Tend to be Earth-like and Consistent with Host Star Abundances
Super-Earth sized exoplanets, while non-existent in the solar system, are plentiful throughout the galaxy. However, these planets showed such a diversity in densities that some appear to be made entirely of iron, while others appear to have a negligible iron core or host envelopes of high mean molecular weight species. To test this diversity, we conducted a campaign using 124 high precision RVs from Keck/KPF and 154 from Gemini/MAROON-X to present the best masses yet for 6 rocky planets, along with confirmation of the planet TOI-1011 b . All updated planet masses (except TOI-561 b) suggest compositions closer to that of the Earth than previously determined, showing fewer iron-enriched “super-Mercuries.” TOI-561 b, on the other hand, suggests some super-Earth sized planets, even those on ultra-short period orbits, might indeed host gaseous envelopes and cannot be assumed to be purely rocky. We then compare the ratio of iron to rock-building species in the planets to the iron and magnesium abundance ratios in their host stars. These updated planet compositions do not suggest a steep relationship between planet and host star compositions, contradictory to previous results, and suggest a relationship much closer to 1-to-1.