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<jats:title>Abstract</jats:title> <jats:p>A rare reconnection outflow reversal in the Earth's midtail observed by the two Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes, is presented. During the event, two ion-scale magnetic flux ropes were separately observed by THC in the earthward and tailward reconnection outflows that were adjacent and accompanied, respectively, by the positive and negative normal magnetic field components to the current sheet. The two flux ropes were separated by ∼2.75 minutes and at the center of the flux ropes, the magnetic field strength was enhanced with a large core field. Comparison results of the convection and measured electric fields reveal that the ions and the magnetic fields were decoupled in the regions surrounding the two flux ropes. The two-dimensional magnetic field maps from the Grad–Shafranov reconstruction show that the diameters of the two flux ropes were similar, being ∼7.1 and ∼7.9 ion inertial lengths, but the aspect ratios of the width to the length were different, being ∼0.35 and ∼0.47. Moreover, one of the reconstructed field maps suggests that multiple <jats:italic>x</jats:italic>-lines may exist in the midtail reconnection and that the traveling compression region of the flux rope was seen at THB. The angle between the axial orientations of the two flux ropes was large, being ∼55°, and their axes were tilted away from the direction of the reconnection guide field, in agreement with the earlier studies of the magnetotail flux ropes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>One of the most promising mechanisms for producing radio halos (RHs) in galaxy clusters is the reacceleration of cosmic-ray electrons by turbulence. However, the origin of the seed electrons for reacceleration is still poorly constrained. In the secondary scenario, most of the seed electrons are injected via collision of proton cosmic-rays, while nonthermal electrons are directly injected in the primary scenario. In this paper, we examine the two scenarios for seed electrons with the observed statistical properties of RHs by combining two methods: by following the temporal evolutions of the electron energy and the radial distributions in a cluster, as well as the merger history of clusters. We find that the RH lifetime largely depends on the seed origin, as it could be longer than the cosmological timescale in the secondary scenario. We study the condition for the onset of RHs with the observed RH fraction and the RH lifetime we obtained and find that long-lived RHs in the secondary scenario should originate from major mergers with a mass ratio of <jats:italic>ξ</jats:italic> ∼ 0.1, while the short lifetime in the primary scenario requires more frequent onsets from minor mergers with <jats:italic>ξ</jats:italic> ∼ 0.01. Our simple model of the turbulence acceleration can reproduce the observed radio luminosity–mass relation. The RH luminosity functions we obtained suggest that the expected RH number count with the ASKAP survey will detect ≈10<jats:sup>3</jats:sup> RHs in both scenarios.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Proton cyclotron waves (PCWs) upstream from Mars are usually interpreted as waves generated by ion/ion instabilities due to the interaction between the solar wind plasma and the pickup protons, originating from the extended hydrogen (H) exosphere of Mars. Their generation mainly depends on the solar wind properties and the relative density of the newborn protons with respect to the background solar wind. Under stable solar wind conditions, a higher solar irradiance leads to both increased exospheric H density and ionization rate of H atoms, and therefore a higher relative density, which tends to increase the linear wave growth rate. Here we show that the solar irradiance is likely to contribute significantly to PCW generation. Specifically, we present observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft indicating that, around the peak of the X8.2 flare on 2017 September 10, the increased solar irradiance gave rise to higher pickup H<jats:sup>+</jats:sup> fluxes, which in turn excited PCWs. This result has implications for inferring the loss of hydrogen to space in early Martian history with more intense and frequent X-class flares as well as their contributions to the total loss.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>For about half a century, the radio pulsar population was observed to spin in the ∼0.002–12 s range, with different pulsar classes having a spin-period evolution that differs substantially depending on their magnetic fields or past accretion history. The recent detection of several slowly rotating pulsars has reopened the long-standing question of the exact physics, and observational biases, driving the upper bound of the period range of the pulsar population. In this work, we perform a parameter study of the spin-period evolution of pulsars interacting with supernova fallback matter and specifically look at the fallback accretion disk scenario. Depending on the initial conditions at formation, this evolution can differ substantially from the typical dipolar spin-down, resulting in pulsars that show spin periods longer than their coeval peers. By using general assumptions for the pulsar spin period and magnetic field at birth, initial fallback accretion rates, and including magnetic field decay, we find that very long spin periods (≳100 s) can be reached in the presence of strong, magnetar-like magnetic fields (≳10<jats:sup>14</jats:sup> G) and moderate initial fallback accretion rates (∼10<jats:sup>22</jats:sup>−10<jats:sup>27</jats:sup> g s<jats:sup>−1</jats:sup>). In addition, we study the cases of two recently discovered periodic radio sources, the pulsar PSR J0901–4046 (<jats:italic>P</jats:italic> = 75.9 s) and the radio transient GLEAM-X J162759.5–523504.3 (<jats:italic>P</jats:italic> = 1091 s), in light of our model. We conclude that the supernova fallback scenario could represent a viable channel to produce a population of long-period isolated pulsars that only recent observation campaigns are starting to unveil.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The current Cepheid-calibrated distance ladder measurement of <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> is reported to be in tension with the values inferred from the cosmic microwave background (CMB), assuming standard cosmology. However, some tip of the red giant branch (TRGB) estimates report <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> in better agreement with the CMB. Hence, it is critical to reduce systematic uncertainties in local measurements to understand the Hubble tension. In this paper, we propose a uniform distance ladder between the second and third rungs, combining Type Ia supernovae (SNe Ia) observed by the Zwicky Transient Facility (ZTF) with a TRGB calibration of their absolute luminosity. A large, volume-limited sample of both calibrator and Hubble flow SNe Ia from the <jats:italic>same</jats:italic> survey minimizes two of the largest sources of systematics: host-galaxy bias and nonuniform photometric calibration. We present results from a pilot study using the existing TRGB distance to the host galaxy of ZTF SN Ia SN 2021rhu (aka ZTF21abiuvdk) in NGC7814. Combining the ZTF calibrator with a volume-limited sample from the first data release of ZTF Hubble flow SNe Ia, we infer <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 76.94 ± 6.4 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>, an 8.3% measurement. The error budget is dominated by the single object calibrating the SN Ia luminosity in this pilot study. However, the ZTF sample includes already five other SNe Ia within ∼20 Mpc for which TRGB distances can be obtained with the Hubble Space Telescope. Finally, we present the prospects of building this distance ladder out to 80 Mpc with James Webb Space Telescope observations of more than 100 ZTF SNe Ia.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present extensive radio monitoring of a Type IIb supernova (SN IIb), SN 2016gkg during <jats:italic>t</jats:italic> ∼ 8−1429 days postexplosion at frequencies <jats:italic>ν</jats:italic> ∼ 0.33−25 GHz. The detailed radio light curves and spectra are broadly consistent with self-absorbed synchrotron emission due to the interaction of the SN shock with the circumstellar medium. The model underpredicts the flux densities at <jats:italic>t</jats:italic> ∼ 299 days postexplosion by a factor of 2, possibly indicating a density enhancement in the circumstellar medium due to a nonuniform mass loss from the progenitor. Assuming a wind velocity <jats:italic>v</jats:italic> <jats:sub>w</jats:sub> ∼ 200 km s<jats:sup>−1</jats:sup>, we estimate the mass-loss rate to be <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}\sim $?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>∼</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7c1eieqn1.gif" xlink:type="simple" /> </jats:inline-formula> (2.2, 3.6, 3.8, 12.6, 3.7, and 5.0) ×10<jats:sup>−6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> during ∼8, 15, 25, 48, 87, and 115 yr, respectively, before the explosion. The shock wave from SN 2016gkg is expanding from <jats:italic>R</jats:italic> ∼ 0.5 × 10<jats:sup>16</jats:sup> to 7 × 10<jats:sup>16</jats:sup> cm during <jats:italic>t</jats:italic> ∼ 24−492 days postexplosion indicating a shock deceleration index, <jats:italic>m</jats:italic> ∼ 0.8 (<jats:italic>R</jats:italic> ∝ <jats:italic>t</jats:italic> <jats:sup> <jats:italic>m</jats:italic> </jats:sup>), and mean shock velocity <jats:italic>v</jats:italic> ∼ 0.1<jats:italic>c</jats:italic>. The radio data are inconsistent with a free–free absorption model and higher shock velocities are in support of a relatively compact progenitor for SN 2016gkg.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Following an established protocol of science—that results must be reproducible—we examine the Gaussian fits to Galactic <jats:italic>λ</jats:italic>21 cm (H <jats:sc>i</jats:sc>) emission profiles obtained by two seemingly complementary methods using data from the Leiden–Argentine–Bonn all-sky survey. One is based on the method used by Verschuur, the other by Nidever et al. (2008). The comparisons led to the identification of four problems that might arise when an algorithm is applied to huge databases without close monitoring: (1) different methods of calculating <jats:inline-formula> <jats:tex-math> <?CDATA ${\tilde{\chi }}^{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>˜</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7c14ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> measuring the goodness of fit; (2) an ultra-broad component found to imperfectly bridge the gap between low- and intermediate-velocity gas; (3) the lack of an imposed spatial coherence allowing different components to appear and disappear in profiles separated by a fraction of a beamwidth; and (4) multiple, fundamentally different solutions for profiles at both the north and south Galactic poles. Confirming evidence emerges from this study of an underlying component with a line width of an order 34 km s<jats:sup>−1</jats:sup>. If this feature is the result of the critical ionization velocity effect acting on interstellar helium, it can be used to calculate its interstellar abundance. Analysis of H <jats:sc>i</jats:sc> profiles in an area in the southern Galactic hemisphere using multitelescope data gives a helium abundance of 0.094 ± 0.035, in excellent agreement with the accepted cosmic abundance of 0.085.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Fermi Large Area Telescope (Fermi-LAT) Collaboration reported the Second Gamma-ray Burst Catalog (2FLGC), which comprises a subset of 29 bursts with photon energies above 10 GeV. Although the standard synchrotron forward-shock model has successfully explained the gamma-ray burst (GRB) afterglow observations, energetic photons higher than 10 GeV from these transient events can hardly be described in this scenario. We present the closure relations (CRs) of the synchrotron self-Compton (SSC) afterglow model in the adiabatic and radiative scenario, and when the central engine injects continuous energy into the blast wave to study the evolution of the spectral and temporal indexes of those bursts reported in 2FLGC. We consider the SSC afterglow model evolving in stellar-wind and the interstellar medium (ISM), and the CRs as a function of the radiative parameter, the energy injection index, and the electron spectral index for 1 &lt; <jats:italic>p</jats:italic> &lt; 2 and 2 ≤ <jats:italic>p</jats:italic>. We select all GRBs that have been modeled with both a simple or a broken power law in the 2FLGC. We found that the CRs of the SSC model can satisfy a significant fraction of the burst that cannot be interpreted in the synchrotron scenario, even though those that require an intermediate density profile (e.g., GRB 130427A) or an atypical fraction of total energy given to amplify the magnetic field (<jats:italic>ε</jats:italic> <jats:sub> <jats:italic>B</jats:italic> </jats:sub>). The value of this parameter in the SSC model ranges (<jats:italic>ε</jats:italic> <jats:sub> <jats:italic>B</jats:italic> </jats:sub> ≈ 10<jats:sup>−5</jats:sup> − 10<jats:sup>−4</jats:sup>) when the cooling spectral break corresponds to the Fermi-LAT band for typical values of GRB afterglow. The analysis shows that the ISM is preferred for the scenario without energy injection and the stellar-wind medium for an energy injection scenario.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We simulate the space environment around AU Microscopii b and the interaction between the magnetized stellar wind and a planetary atmospheric outflow for ambient stellar wind conditions and coronal mass ejection (CME) conditions. We also calculate synthetic Ly<jats:italic>α</jats:italic> absorption due to neutral hydrogen in the ambient and the escaping planetary atmosphere affected by this interaction. We find that the Ly<jats:italic>α</jats:italic> absorption is highly variable owing to the highly varying stellar wind conditions. A strong Doppler blueshift component is observed in the Ly<jats:italic>α</jats:italic> profile, in contradiction to the actual escape velocity observed in the simulations themselves. This result suggests that the strong Doppler blueshift is likely attributed to the stellar wind, not the escaping neutral atmosphere, either through its advection of neutral planetary gas or through the creation of a fast neutral flow via charge exchange between the stellar wind ions and the planetary neutrals. Indeed, our CME simulations indicate a strong stripping of magnetospheric material from the planet, including some of the neutral escaping atmosphere. Our simulations show that the pressure around close-in exoplanets is not much lower, and may be even higher, than the pressure at the top of the planetary atmosphere. Thus, the neutral atmosphere is hydrodynamically escaping with a very small velocity (&lt;15 km s<jats:sup>−1</jats:sup>). Moreover, our simulations show that an MHD treatment is essential in order to properly capture the coupled magnetized stellar wind and the escaping atmosphere, despite the atmosphere being neutral. This coupling should be considered when interpreting Ly<jats:italic>α</jats:italic> observations in the context of exoplanets’ atmospheric escape.</jats:p>