| 51. |
- Alqeeq, S. W., et al.
(författare)
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Investigation of the homogeneity of energy conversion processes at dipolarization fronts from MMS measurements
- 2022
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 29:1
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Tidskriftsartikel (refereegranskat)abstract
- We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during a substorm event on 23 July 2017. We analyzed Ohm's law for each event and found that ions are mostly decoupled from the magnetic field by Hall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electron decoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from the electromagnetic field to the plasma (J & BULL; E > 0) ahead or at the DF, whereas it is the opposite (J & BULL; E < 0) behind the front. This reversal is mainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy is mostly transferred from the plasma to the electromagnetic field (J & BULL; E & PRIME; < 0) and should contribute to the deceleration of the fast flow. However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely related to lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.
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| 52. |
- Breuillard, H., et al.
(författare)
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New Insights into the Nature of Turbulence in the Earth's Magnetosheath Using Magnetospheric MultiScale Mission Data
- 2018
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 859:2
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Tidskriftsartikel (refereegranskat)abstract
- The Earth's magnetosheath, which is characterized by highly turbulent fluctuations, is usually divided into two regions of different properties as a function of the angle between the interplanetary magnetic field and the shock normal. In this study, we make use of high-time resolution instruments on board the Magnetospheric MultiScale spacecraft to determine and compare the properties of subsolar magnetosheath turbulence in both regions, i. e., downstream of the quasi-parallel and quasi-perpendicular bow shocks. In particular, we take advantage of the unprecedented temporal resolution of the Fast Plasma Investigation instrument to show the density fluctuations down to sub-ion scales for the first time. We show that the nature of turbulence is highly compressible down to electron scales, particularly in the quasi-parallel magnetosheath. In this region, the magnetic turbulence also shows an inertial (Kolmogorov-like) range, indicating that the fluctuations are not formed locally, in contrast with the quasi-perpendicular magnetosheath. We also show that the electromagnetic turbulence is dominated by electric fluctuations at sub-ion scales (f > 1Hz) and that magnetic and electric spectra steepen at the largest-electron scale. The latter indicates a change in the nature of turbulence at electron scales. Finally, we show that the electric fluctuations around the electron gyrofrequency are mostly parallel in the quasi-perpendicular magnetosheath, where intense whistlers are observed. This result suggests that energy dissipation, plasma heating, and acceleration might be driven by intense electrostatic parallel structures/waves, which can be linked to whistler waves.
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| 53. |
- Broderick, Avery E., et al.
(författare)
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Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
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Tidskriftsartikel (refereegranskat)abstract
- The extraordinary physical resolution afforded by the Event Horizon Telescope has opened a window onto the astrophysical phenomena unfolding on horizon scales in two known black holes, M87* and Sgr A*. However, with this leap in resolution has come a new set of practical complications. Sgr A* exhibits intraday variability that violates the assumptions underlying Earth aperture synthesis, limiting traditional image reconstruction methods to short timescales and data sets with very sparse (u, v) coverage. We present a new set of tools to detect and mitigate this variability. We develop a data-driven, model-agnostic procedure to detect and characterize the spatial structure of intraday variability. This method is calibrated against a large set of mock data sets, producing an empirical estimator of the spatial power spectrum of the brightness fluctuations. We present a novel Bayesian noise modeling algorithm that simultaneously reconstructs an average image and statistical measure of the fluctuations about it using a parameterized form for the excess variance in the complex visibilities not otherwise explained by the statistical errors. These methods are validated using a variety of simulated data, including general relativistic magnetohydrodynamic simulations appropriate for Sgr A* and M87*. We find that the reconstructed source structure and variability are robust to changes in the underlying image model. We apply these methods to the 2017 EHT observations of M87*, finding evidence for variability across the EHT observing campaign. The variability mitigation strategies presented are widely applicable to very long baseline interferometry observations of variable sources generally, for which they provide a data-informed averaging procedure and natural characterization of inter-epoch image consistency.
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| 54. |
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| 55. |
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| 56. |
- Eastwood, J. P., et al.
(författare)
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Ion-scale secondary flux ropes generated by magnetopause reconnection as resolved by MMS
- 2016
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:10, s. 4716-4724
-
Tidskriftsartikel (refereegranskat)abstract
- New Magnetospheric Multiscale (MMS) observations of small-scale (similar to 7 ion inertial length radius) flux transfer events (FTEs) at the dayside magnetopause are reported. The 10 km MMS tetrahedron size enables their structure and properties to be calculated using a variety of multispacecraft techniques, allowing them to be identified as flux ropes, whose flux content is small (similar to 22 kWb). The current density, calculated using plasma and magnetic field measurements independently, is found to be filamentary. Intercomparison of the plasma moments with electric and magnetic field measurements reveals structured non-frozen-in ion behavior. The data are further compared with a particle-in-cell simulation. It is concluded that these small-scale flux ropes, which are not seen to be growing, represent a distinct class of FTE which is generated on the magnetopause by secondary reconnection.
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| 57. |
- Ergun, R. E., et al.
(författare)
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Magnetic Reconnection, Turbulence, and Particle Acceleration : Observations in the Earth's Magnetotail
- 2018
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing Ltd. - 0094-8276 .- 1944-8007. ; 45:8, s. 3338-3347
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Tidskriftsartikel (refereegranskat)abstract
- We report observations of turbulent dissipation and particle acceleration from large-amplitude electric fields (E) associated with strong magnetic field (B) fluctuations in the Earth's plasma sheet. The turbulence occurs in a region of depleted density with anti-earthward flows followed by earthward flows suggesting ongoing magnetic reconnection. In the turbulent region, ions and electrons have a significant increase in energy, occasionally >100 keV, and strong variation. There are numerous occurrences of |E| >100 mV/m including occurrences of large potentials (>1 kV) parallel to B and occurrences with extraordinarily large J · E (J is current density). In this event, we find that the perpendicular contribution of J · E with frequencies near or below the ion cyclotron frequency (fci) provide the majority net positive J · E. Large-amplitude parallel E events with frequencies above fci to several times the lower hybrid frequency provide significant dissipation and can result in energetic electron acceleration.
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| 58. |
- Ergun, R. E., et al.
(författare)
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Magnetospheric Multiscale observations of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the magnetopause
- 2016
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 43:11, s. 5626-5634
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Tidskriftsartikel (refereegranskat)abstract
- We report observations from the Magnetospheric Multiscale satellites of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the Earth's magnetopause. The observed waves have parallel electric fields (E-||) with amplitudes on the order of 100mV/m and display nonlinear characteristics that suggest a possible net E-||. These waves are observed within the ion diffusion region and adjacent to (within several electron skin depths) the electron diffusion region. They are in or near the magnetosphere side current layer. Simulation results support that the strong electrostatic linear and nonlinear wave activities appear to be driven by a two stream instability, which is a consequence of mixing cold (<10eV) plasma in the magnetosphere with warm (similar to 100eV) plasma from the magnetosheath on a freshly reconnected magnetic field line. The frequent observation of these waves suggests that cold plasma is often present near the magnetopause.
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| 59. |
- Ergun, R. E., et al.
(författare)
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Magnetospheric Multiscale Satellites Observations of Parallel Electric Fields Associated with Magnetic Reconnection
- 2016
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 116:23
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Tidskriftsartikel (refereegranskat)abstract
- We report observations from the Magnetospheric Multiscale satellites of parallel electric fields (E-vertical bar vertical bar) associated with magnetic reconnection in the subsolar region of the Earth's magnetopause. E-vertical bar vertical bar events near the electron diffusion region have amplitudes on the order of 100 mV/m, which are significantly larger than those predicted for an antiparallel reconnection electric field. This Letter addresses specific types of E-vertical bar vertical bar events, which appear as large-amplitude, near unipolar spikes that are associated with tangled, reconnected magnetic fields. These E-vertical bar vertical bar events are primarily in or near a current layer near the separatrix and are interpreted to be double layers that may be responsible for secondary reconnection in tangled magnetic fields or flux ropes. These results are telling of the three-dimensional nature of magnetopause reconnection and indicate that magnetopause reconnection may be often patchy and/or drive turbulence along the separatrix that results in flux ropes and/or tangled magnetic fields.
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| 60. |
- Eriksson, S., et al.
(författare)
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Magnetospheric Multiscale Observations of the Electron Diffusion Region of Large Guide Field Magnetic Reconnection
- 2016
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 117:1
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Tidskriftsartikel (refereegranskat)abstract
- We report observations from the Magnetospheric Multiscale (MMS) satellites of a large guide field magnetic reconnection event. The observations suggest that two of the four MMS spacecraft sampled the electron diffusion region, whereas the other two spacecraft detected the exhaust jet from the event. The guide magnetic field amplitude is approximately 4 times that of the reconnecting field. The event is accompanied by a significant parallel electric field (E-parallel to)that is larger than predicted by simulations. The high-speed (similar to 300 km/s) crossing of the electron diffusion region limited the data set to one complete electron distribution inside of the electron diffusion region, which shows significant parallel heating. The data suggest that E-parallel to is balanced by a combination of electron inertia and a parallel gradient of the gyrotropic electron pressure.
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