(Meier et al. 2021). Overall, the preponderance of evidence suggests that our target planets are unlikely to have an ionosphere that can form an induced magnetosphere similar to Io’s. However, if they have a strong extended magnetosphere above the planet’s surface similar to Ganymede, then strong SPI interaction can occur without an ionosphere (Saur et al. 2013). All of our target stars are slowly rotating early-to-mid M dwarfs with weak stellar magnetic activity and low flare rates. Slow rotators minimize the risk of contamination by non-SPI flaring (Pope et al.

SPI Radio Search 5 [Table: Table 2. Observations
Telescope & Band Project Epochs Porb
Orbital
(Duration in hr) (hr) Coverage
LTT 3780 VLA 2-4 GHz 19B-274 2020-01-09 (6.3), 01-10 (6.3), 01-11 (6.3) 18.4 96.5% LHS 3844 ATCA 1.1-3.1 GHz C3303 2019-06-21 (11.8), 06-26 (11.4) 11.1 100% GJ 367 ATCA 1.1-3.1 GHz C1726, 2022-01-19 (9.5), 09-13 (7.3), 7.7 100% C3303 09-14 (8.2), 09-15 (8.0), 09-24 (7.6) LHS 1678 ATCA 1.1-3.1 GHz C3303 2022-09-23 (7.6), 09-26 (9.5), 09-27 (10.2) 20.6 81.4% GJ 1252 ATCA 1.1-3.1 GHz C3303 2022-09-28 (12.9), 09-29 (12.7) 12.4 98.8%
Note—The final column lists the percentage of orbital phase that was observed at least once.]
2021) and rotationally-powered aurorae (Vedantham et al. 2020). The rotation periods in Table 1 are photometric. Studies have found weak activity indicators for all of our targets consistent with slow rotation: log R′ HK = −5.6, non-detection in Hα, and no flares in one TESS sector for LTT 3780 (Cloutier et al. 2020; Jeffers et al. 2018; Pope et al. 2021); undetected or unsaturated Hα, CaII, and HeI D for LHS 3844 with only one TESS flare (Medina et al. 2020); log R′ HK = −5.2 for GJ 367 (Lam et al. 2021); absorption in Hα for LHS 1678 (Silverstein et al. 2022); and log R′ HK =-5.4 and Hα absorption for GJ 1252 (Astudillo-Defru et al. 2017; Shporer et al. 2020). Perhaps the most notably quiet target is LHS 1678, which showed no spot modulation or flares in 4 months of TESS (Silverstein et al. 2024), and which Kar et al. (2024) identify as the most photometrically stable of 32 M dwarfs monitored for multiple years. Compared to younger M dwarfs, the predicted SPI power from slow rotators is reduced due to their weakened stellar magnetic fields and winds. However, compared to FGK stars, even slowlyrotating M dwarfs can have relatively strong global magnetic fields, increasing both SPI power and the emitted cyclotron frequency (see Section 7). For example, the Sun and Prox Cen, respectively, have large-scale magnetic fields of ∼10 G (Vidotto 2016) and 200 G (Klein et al. 2021).
3. OBSERVATIONS [Table: Table 2 summarizes the observations. Four of the targets were observed with the Australia Telescope Compact Array (ATCA) at 1.1-3.1 GHz, in projects C1726 (PI: Hollow) and C3303 (PI: Pineda). LTT 3780 was observed with the Karl G. Jansky Very Large Array (VLA) at 2-4 GHz, in project 19B-274 (PI: Villadsen). The arrays had 5-6 antennas per observation date (ATCA) or 26-27 (VLA). The observation durations in the table count from the beginning of the first to the end of the last target scan. These durations slightly exceed the true time on source due to calibrator observations, which occurred for 1-3 minutes every 20-60 minutes. Each system was observed across 80-100% of orbital phases at least once, with multi-date observations sampling across different orbits in order to achieve near-complete phase coverage.]