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1. Stellar populations in the transition region of nuclear star cluster and nuclear stellar disc

   https://doi.org/10.1093/mnras/stac1227  

   Feldmeier-Krause, A. ( May 2022 , Monthly Notices of the Royal Astronomical Society)

   The Milky Way nuclear star cluster (NSC) is located within the nuclear stellar disc (NSD) in the Galactic Centre. It is not fully understood if the formation and evolution of these two components are connected, and how they influence each other. We study the stellar populations in the transition region of NSC and NSD. We observed two ∼4.3 pc2 fields with the integral-field spectrograph KMOS (VLT), located at r ∼ 20 pc (>4 Re) to the Galactic East and West of the NSC. We extract and analyse medium-resolution stellar spectra of >200 stars per field. The data contain in total nine young star candidates. We use stellar photometry to estimate the stellar masses, effective temperatures, and spectral types of the young stars. The stars are consistent with an age of 4–6 Myr, they may have formed inside the Quintuplet cluster, but were dispersed in dynamical interactions. Most stars in the two fields are red giant stars, and we measure their stellar metallicities [M/H] using full spectral fitting. We compare our [M/H] distributions to the NSC and NSD, using data from the literature, and find that the overall metallicity decreases from the central NSC, over the transition region, to the NSD. The steep decrease of [M/H] from the NSC to the region dominated by the NSD indicates that the two components have distinct stellar populations and formation histories.

    

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2. Modeled IMF B y Effects on the Polar Ionosphere and Thermosphere Coupling

   https://doi.org/10.1029/2019JA026949  

   Liu, Jing ; Burns, Alan G. ; Wang, Wenbin ; Zhang, Yongliang ( March 2020 , Journal of Geophysical Research: Space Physics)

   There is still an inadequate understanding of how the interplanetary magnetic field (IMF) east‐west component (B_y) affects thermospheric composition, and other ionospheric and thermospheric fields in a systematic way. Utilizing the state‐of‐art first‐principles Coupled Magnetosphere Ionosphere Thermosphere (CMIT) modeling and TIMED/Global Ultraviolet Imager (GUVI)‐observed ΣO/N₂covering an entire solar cycle (year 2002–2016), as well as a neutral parcel trajectory tracing technique, we emphasize that not only the direction ofB_y, but also its strength relative to the IMF north‐south component (B_z) that has important effects on high latitude convection, Joule heating, electron density, neutral winds, and neutral composition patterns in the upper thermosphere. The Northern Hemisphere convection pattern becomes more twisted for positiveB_ycases than negative cases: the dusk cell becomes more rounded compared with the dawn cell. Consequently, equatorward neutral winds are stronger during postmidnight hours in negativeB_ycases than in positiveB_ycases, creating a favorable condition for neutral composition disturbances (characterized by low ΣO/N₂) to expand to lower latitudes. This may lead to a more elongated ΣO/N₂depletion area along the morning‐premidnight direction for negativeB_yconditions compared with the positiveB_yconditions. Backward neutral parcel trajectories indicate that a lower ΣO/N₂parcel in negativeB_ycases comes from lower altitudes, as compared with that for positiveB_ycases, leading to larger enhancements of N₂in the former case.

    

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3. On the Role of Mild Substorms and Enhanced Hall Conductivity in the Plasma Irregularities Onset and Zonal Drift Reversals: Experimental Evidence at Distinct Longitudes Over South America

   https://doi.org/10.1029/2023EA003071  

   Sousasantos, J. ; Rodrigues, F. S. ; Fejer, B. G. ; Abdu, M. A. ; Massoud, A. A. ; Valladares, C. E. ( November 2023 , Earth and Space Science)

   The 14‐panel Advanced Modular Incoherent Scatter Radar (AMISR‐14) system deployed at Jicamarca observed equatorial spread F plumes on two consecutive nights under unfavorable seasonal and solar flux conditions during a period that can be categorized as geomagnetically quiet. The AMISR‐14 capability of observing in multiple pointing directions allowed the characterization of the irregularity zonal drifts revealing that, in addition to their atypical occurrence, the zonal drifts of these plumes/irregularities also presented distinct patterns from one night to another, reversing from east to west on the second night. This work addresses two main subjects: (a) the mechanisms that may have led to the generation of these irregularities, despite the unfavorable conditions, and (b) the mechanisms that possibly led to the reversal (east‐to‐west) in the zonal plasma drift on the second night. To do so a multi‐instrumented and multi‐location investigation was performed. The results indicate the occurrence of simultaneous spread‐F events over the Peruvian and the Brazilian regions, evidencing a non‐local process favoring the development of the irregularities. The results also suggest that, even under very mild geomagnetic perturbation conditions, the recurring penetration of electric fields in the equatorial ionosphere can occur promptly, modifying the equatorial electrodynamics and providing favorable conditions for the plume development. Moreover, the results confirm that the eastward penetration electric fields, combined with the upsurge of Hall conductivity in the nighttime typically associated with the presence of sporadic‐E layers, are likely to be the mechanism leading to the reversal in the irregularity zonal drifts over these regions.

    

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4. Searching for stellar flares from low-mass stars using ASKAP and TESS

   https://doi.org/10.1093/mnras/stac2143  

   Rigney, Jeremy ; Ramsay, Gavin ; Carley, Eoin P. ; Doyle, J. Gerry ; Gallagher, Peter T. ; Wang, Yuanming ; Pritchard, Joshua ; Murphy, Tara ; Lenc, Emil ; Kaplan, David L. ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   Solar radio emission at low frequencies (<1 GHz) can provide valuable information on processes driving flares and coronal mass ejections (CMEs). Radio emission has been detected from active M dwarf stars, suggestive of much higher levels of activity than previously thought. Observations of active M dwarfs at low frequencies can provide information on the emission mechanism for high energy flares and possible stellar CMEs. Here, we conducted two observations with the Australian Square Kilometre Array Pathfinder Telescope totalling 26 h and scheduled to overlap with the Transiting Exoplanet Survey Satellite Sector 36 field, utilizing the wide fields of view of both telescopes to search for multiple M dwarfs. We detected variable radio emission in Stokes I centred at 888 MHz from four known active M dwarfs. Two of these sources were also detected with Stokes V circular polarization. When examining the detected radio emission characteristics, we were not able to distinguish between the models for either electron cyclotron maser or gyrosynchrotron emission. These detections add to the growing number of M dwarfs observed with variable low-frequency emission.

    

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5. Decomposition of the Multimodal Multidirectional M 2 Internal Tide Field

   https://doi.org/10.1175/JTECH-D-19-0022.1  

   Zhao, Zhongxiang ; Wang, Jinbo ; Menemenlis, Dimitris ; Fu, Lee-Lueng ; Chen, Shuiming ; Qiu, Bo ( June 2019 , Journal of Atmospheric and Oceanic Technology)

   The M₂internal tide field contains waves of various baroclinic modes and various horizontal propagation directions. This paper presents a technique for decomposing the sea surface height (SSH) field of the multimodal multidirectional internal tide. The technique consists of two steps: first, different baroclinic modes are decomposed by two-dimensional (2D) spatial filtering, utilizing their different horizontal wavelengths; second, multidirectional waves in each mode are decomposed by 2D plane wave analysis. The decomposition technique is demonstrated using the M₂internal tide field simulated by the MITgcm. This paper focuses on a region lying off the U.S. West Coast ranging 20°–50°N, 220°–245°E. The lowest three baroclinic modes are separately resolved from the internal tide field; each mode is further decomposed into five waves of arbitrary propagation directions in the horizontal. The decomposed fields yield unprecedented details on the internal tide’s generation and propagation, which cannot be observed in the harmonically fitted field. The results reveal that the mode-1 M₂internal tide in the study region is dominantly from the Hawaiian Ridge to the west but also generated locally at the Mendocino Ridge and continental slope. The mode-2 and mode-3 M₂internal tides are generated at isolated seamounts, as well as at the Mendocino Ridge and continental slope. The Mendocino Ridge radiates both southbound and northbound M₂internal tides for all three modes. Their propagation distances decrease with increasing mode number: mode-1 waves can travel over 2000 km, while mode-3 waves can only be tracked for 300 km. The decomposition technique may be extended to other tidal constituents and to the global ocean.

    

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