| 1. |
- Algaba, Juan-Carlos, et al.
(författare)
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Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign
- 2021
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 911:1
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Forskningsöversikt (refereegranskat)abstract
- In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M o˙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
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| 2. |
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| 3. |
- Kitamura, N., et al.
(författare)
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Direct observations of energy transfer from resonant electrons to whistler-mode waves in magnetosheath of Earth
- 2022
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Excitation of whistler-mode waves by cyclotron instability is considered as the likely generation process of the waves. Here, the authors show direct observational evidence for locally ongoing secular energy transfer from the resonant electrons to the whistler-mode waves in Earth's magnetosheath. Electromagnetic whistler-mode waves in space plasmas play critical roles in collisionless energy transfer between the electrons and the electromagnetic field. Although resonant interactions have been considered as the likely generation process of the waves, observational identification has been extremely difficult due to the short time scale of resonant electron dynamics. Here we show strong nongyrotropy, which rotate with the wave, of cyclotron resonant electrons as direct evidence for the locally ongoing secular energy transfer from the resonant electrons to the whistler-mode waves using ultra-high temporal resolution data obtained by NASA's Magnetospheric Multiscale (MMS) mission in the magnetosheath. The nongyrotropic electrons carry a resonant current, which is the energy source of the wave as predicted by the nonlinear wave growth theory. This result proves the nonlinear wave growth theory, and furthermore demonstrates that the degree of nongyrotropy, which cannot be predicted even by that nonlinear theory, can be studied by observations.
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| 4. |
- Kitamura, N., et al.
(författare)
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Energy Transfer Between Hot Protons and Electromagnetic Ion Cyclotron Waves in Compressional Pc5 Ultra-low Frequency Waves
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 126:5
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Tidskriftsartikel (refereegranskat)abstract
- The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of similar to 2-5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror-mode type structures. We apply the Wave-Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst-mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time.
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| 5. |
- Hartley, Philippa, et al.
(författare)
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SKA Science Data Challenge 2: analysis and results
- 2023
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Ingår i: Monthly Notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 523:2, s. 1967-1993
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Tidskriftsartikel (refereegranskat)abstract
- The Square Kilometre Array Observatory (SKAO) will explore the radio sky to new depths in order to conduct transformational science. SKAO data products made available to astronomers will be correspondingly large and complex, requiring the application of advanced analysis techniques to extract key science findings. To this end, SKAO is conducting a series of Science Data Challenges, each designed to familiarize the scientific community with SKAO data and to drive the development of new analysis techniques. We present the results from Science Data Challenge 2 (SDC2), which invited participants to find and characterize 233 245 neutral hydrogen (H i) sources in a simulated data product representing a 2000 h SKA-Mid spectral line observation from redshifts 0.25-0.5. Through the generous support of eight international supercomputing facilities, participants were able to undertake the Challenge using dedicated computational resources. Alongside the main challenge, 'reproducibility awards' were made in recognition of those pipelines which demonstrated Open Science best practice. The Challenge saw over 100 participants develop a range of new and existing techniques, with results that highlight the strengths of multidisciplinary and collaborative effort. The winning strategy - which combined predictions from two independent machine learning techniques to yield a 20 per cent improvement in overall performance - underscores one of the main Challenge outcomes: that of method complementarity. It is likely that the combination of methods in a so-called ensemble approach will be key to exploiting very large astronomical data sets.
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| 6. |
- Huang, S. Y., et al.
(författare)
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Magnetospheric Multiscale Observations of Electron Vortex Magnetic Hole in the Turbulent Magnetosheath Plasma
- 2017
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Ingår i: Astrophysical Journal Letters. - : Institute of Physics Publishing. - 2041-8205 .- 2041-8213. ; 836:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on the observations of an electron vortex magnetic hole corresponding to a new type of coherent structure in the turbulent magnetosheath plasma using the Magnetospheric Multiscale mission data. The magnetic hole is characterized by a magnetic depression, a density peak, a total electron temperature increase (with a parallel temperature decrease but a perpendicular temperature increase), and strong currents carried by the electrons. The current has a dip in the core region and a peak in the outer region of the magnetic hole. The estimated size of the magnetic hole is about 0.23 ρi (∼30 ρe) in the quasi-circular cross-section perpendicular to its axis, where ρi and ρe are respectively the proton and electron gyroradius. There are no clear enhancements seen in high-energy electron fluxes. However, there is an enhancement in the perpendicular electron fluxes at 90° pitch angle inside the magnetic hole, implying that the electrons are trapped within it. The variations of the electron velocity components Vem and Ven suggest that an electron vortex is formed by trapping electrons inside the magnetic hole in the cross-section in the M-N plane. These observations demonstrate the existence of a new type of coherent structures behaving as an electron vortex magnetic hole in turbulent space plasmas as predicted by recent kinetic simulations.
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| 7. |
- Liu, Z-Y, et al.
(författare)
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ULF Waves Modulating and Acting as Mass Spectrometer for Dayside Ionospheric Outflow Ions
- 2019
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Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 46:15, s. 8633-8642
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Tidskriftsartikel (refereegranskat)abstract
- Ionospheric outflow has been shown to be a dominant ion source of Earth's magnetosphere. However, most studies in the literature are about ionospheric outflow injected into the nightside magnetosphere. We still know little about ionospheric outflow injected into the dayside magnetosphere and its further energization after it enters the magnetosphere. Here, with data from Magnetospheric Multiscale mission, we report direct observations of the modulation of dayside ionospheric outflow ions by ultralow frequency (ULF) waves. The observations indicate that the modulation is mass dependent, which demonstrates the possibility of using ULF waves as a mass spectrometer to identify ion species. Moreover, the measurement suggests that polarization drift may play a role in O+ modulation, which may lead to a true acceleration and even nonadiabatic behavior of O+. This interaction scenario can work throughout the whole magnetosphere and impact upon the plasma environment and dynamics.
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| 8. |
- Peng, F. Z., et al.
(författare)
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Quadrupolar pattern of the asymmetric guide-field reconnection
- 2017
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 122:6, s. 6349-6356
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Tidskriftsartikel (refereegranskat)abstract
- With high-resolution data of the recently launched Magnetospheric Multiscale mission, we report a magnetic reconnection event at the dayside magnetopause. This reconnection event, having a density asymmetry N-high/N-low approximate to 2 on the two sides of the reconnecting current sheet and a guide field B-g approximate to 0.4B(0) in the out-of-plane direction, exhibit all the two-fluid features: Alfvenic plasma jets in the outflow region, bipolar Hall electric fields toward the current sheet center, quadrupolar Hall magnetic fields in the out-of-plane direction, and the corresponding Hall currents. Obviously, the density asymmetry N-high/N-low approximate to 2 and the guide field B-g approximate to 0.4B(0) are not sufficient to dismiss the quadrupolar pattern of Hall reconnection. This is different from previous simulations, where the bipolar pattern of Hall reconnection was suggested.
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