| 1. |
- Angelopoulos, V., et al.
(författare)
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First Results from the THEMIS Mission
- 2008
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Ingår i: Space Science Reviews. - : Springer Science and Business Media LLC. - 0038-6308 .- 1572-9672. ; 141:1-4, s. 453-476
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Forskningsöversikt (refereegranskat)abstract
- THEMIS was launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. During the first seven months of the mission the five satellites coasted near their injection orbit to avoid differential precession in anticipation of orbit placement, which started in September 2007 and led to a commencement of the baseline mission in December 2007. During the coast phase the probes were put into a string-of-pearls configuration at 100 s of km to 2 R-E along-track separations, which provided a unique view of the magnetosphere and enabled an unprecedented dataset in anticipation of the first tail season. In this paper we describe the first THEMIS substorm observations, captured during instrument commissioning on March 23, 2007. THEMIS measured the rapid expansion of the plasma sheet at a speed that is commensurate with the simultaneous expansion of the auroras on the ground. These are the first unequivocal observations of the rapid westward expansion process in space and on the ground. Aided by the remote sensing technique at energetic particle boundaries and combined with ancillary measurements and MHD simulations, they allow determination and mapping of space currents. These measurements show the power of the THEMIS instrumentation in the tail and the radiation belts. We also present THEMIS Flux Transfer Events (FTE) observations at the magnetopause, which demonstrate the importance of multi-point observations there and the quality of the THEMIS instrumentation in that region of space.
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| 2. |
- Richter, I., et al.
(författare)
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Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko
- 2015
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 33:8, s. 1031-1036
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Tidskriftsartikel (refereegranskat)abstract
- We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low-activity state. Quasi-coherent, large-amplitude (delta B/B similar to 1), compressional magnetic field oscillations at similar to 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pickup-ion-driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.
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| 3. |
- Tao, J. B., et al.
(författare)
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Kinetic instabilities in the lunar wake : ARTEMIS observations
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A03106-
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Tidskriftsartikel (refereegranskat)abstract
- The Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission is a new two-probe lunar mission derived from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. On 13 February 2010, one of the two probes, ARTEMIS P1 (formerly THEMIS-B), made the first lunar wake flyby of the mission. We present detailed analysis of the electrostatic waves observed on the outbound side of the flyby that were associated with electron beams. Halekas et al. (2011) derived a net potential across the lunar wake from observations and suggested that the net potential generated the observed electron beams and the electron beams in turn excited the observed electrostatic waves due to kinetic instabilities. The wavelengths and velocities of the electrostatic waves are estimated, using high-resolution electric field instrument data with cross-spectrum analysis and cross-correlation analysis. In general, the estimated wavelengths vary from a few hundred meters to a couple of thousand meters. The estimated phase velocities are on the order of 1000 km s(-1). In addition, we perform 1-D Vlasov simulations to help identify the mode of the observed electrostatic waves. We conclude that the observed electrostatic waves are likely on the electron beam mode branch.
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| 4. |
- De Keyser, J., et al.
(författare)
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In situ plasma and neutral gas observation time windows during a comet flyby : Application to the Comet Interceptor mission
- 2024
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Ingår i: Planetary and Space Science. - : Elsevier. - 0032-0633 .- 1873-5088. ; 244
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Tidskriftsartikel (refereegranskat)abstract
- A comet flyby, like the one planned for ESA's Comet Interceptor mission, places stringent requirements on spacecraft resources. To plan the time line of in situ plasma and neutral gas observations during the flyby, the size of the comet magnetosphere and neutral coma must be estimated well. For given solar irradiance and solar wind conditions, comet composition, and neutral gas expansion speed, the size of gas coma and magnetosphere during the flyby can be estimated from the gas production rate and the flyby geometry. Combined with flyby velocity, the time spent in these regions can be inferred and a data acquisition plan can be elaborated for each instrument, compatible with the limited data storage capacity. The sizes of magnetosphere and gas coma are found from a statistical analysis based on the probability distributions of gas production rate, flyby velocity, and solar wind conditions. The size of the magnetosphere as measured by bow shock standoff distance is 105-106 km near 1 au in the unlikely case of a Halley-type target comet, down to a nonexistent bow shock for targets with low activity. This translates into durations up to 103-104 seconds. These estimates can be narrowed down when a target is identified far from the Sun, and even more so as its activity can be predicted more reliably closer to the Sun. Plasma and neutral gas instruments on the Comet Interceptor main spacecraft can monitor the entire flyby by using an adaptive data acquisition strategy in the context of a record-and-playback scenario. For probes released from the main spacecraft, the inter-satellite communication link limits the data return. For a slow flyby of an active comet, the probes may not yet be released during the inbound bow shock crossing.
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| 5. |
- Eastwood, J. P., et al.
(författare)
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THEMIS observations of a hot flow anomaly : Solar wind, magnetosheath, and ground-based measurements
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:17, s. L17S03-
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Tidskriftsartikel (refereegranskat)abstract
- The THEMIS spacecraft encountered a Hot Flow Anomaly ( HFA) on the dusk flank of the Earth's bow shock on 4 July 2007, observing it on both sides of the shock. Meanwhile, the THEMIS ground magnetometers traced the progress of the associated Magnetic Impulse Event along the dawn flank of the magnetosphere, providing a unique opportunity to study the transmission of the HFA through the shock and the subsequent downstream response. THEMIS-A, in the solar wind, observed classic HFA signatures. Isotropic electron distributions inside the upstream HFA are attributed to the action of the electron firehose instability. THEMIS-E, just downstream, observed a much more complex disturbance with the pressure perturbation decoupled from the underlying discontinuity. Simple calculations show that the pressure perturbation would be capable of significantly changing the magnetopause location, which is confirmed by the ground-based observations.
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| 6. |
- Roux, A., et al.
(författare)
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A mechanism for heating electrons in the magnetopause current layer and adjacent regions
- 2011
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 29:12, s. 2305-2316
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Tidskriftsartikel (refereegranskat)abstract
- Taking advantage of the string-of-pearls configuration of the five THEMIS spacecraft during the early phase of their mission, we analyze observations taken simultaneously in the magnetosheath, the magnetopause current layer and the magnetosphere. We find that electron heating coincides with ultra low frequency waves. It seems unlikely that electrons are heated by these waves because the electron thermal velocity is much larger than the Alfven velocity (V-a). In the short transverse scale (k (perpendicular to) rho(i) >> 1) regime, however, short scale Alfven waves (SSAWs) have parallel phase velocities much larger than V-a and are shown to interact, via Landau damping, with electrons thereby heating them. The origin of these waves is also addressed. THEMIS data give evidence for sharp spatial gradients in the magnetopause current layer where the highest amplitude waves have a large component delta B perpendicular to the magnetopause and k azimuthal. We suggest that SSAWs are drift waves generated by temperature gradients in a high beta, large T-i/T-e magnetopause current layer. Therefore these waves are called SSDAWs, where D stands for drift. SSDAWs have large k(perpendicular to) and therefore a large Doppler shift that can exceed their frequencies in the plasma frame. Because they have a small but finite parallel electric field and a magnetic component perpendicular to the magnetopause, they could play a key role at reconnecting magnetic field lines. The growth rate depends strongly on the scale of the gradients; it becomes very large when the scale of the electron temperature gradient gets below 400 km. Therefore SSDAW's are expected to limit the sharpness of the gradients, which might explain why Berchem and Russell (1982) found that the average magnetopause current sheet thickness to be similar to 400-1000 km (similar to 500 km in the near equatorial region).
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| 7. |
- Tao, J. B., et al.
(författare)
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A model of electromagnetic electron phase-space holes and its application
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A11213-
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Tidskriftsartikel (refereegranskat)abstract
- Electron phase-space holes (EHs) are indicators of nonlinear activities in space plasmas. Most often they are observed as electrostatic signals, but recently Andersson et al. [2009] reported electromagnetic EHs observed by the THEMIS mission in the Earth's plasma sheet. As a follow-up to Andersson et al. [2009], this paper presents a model of electromagnetic EHs where the delta E x B(0) drift of electrons creates a net current. The model is examined with test-particle simulations and compared to the electromagnetic EHs reported by Andersson et al. [2009]. As an application of the model, we introduce a more accurate method than the simplified Lorentz transformation of Andersson et al. [2009] to derive EH velocity (v(EH)). The sizes and potentials of EHs are derived from v(EH), so an accurate derivation of v(EH) is important in analyzing EHs. In general, our results are qualitatively consistent with those of Andersson et al. [2009] but generally with smaller velocities and sizes.
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| 8. |
- Lu, S. W., et al.
(författare)
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Prolonged Kelvin-Helmholtz Waves at Dawn and Dusk Flank Magnetopause : Simultaneous Observations by MMS and THEMIS
- 2019
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 875:1
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Tidskriftsartikel (refereegranskat)abstract
- The Kelvin-Helmholtz (K-H) waves predominantly excited at the Earth's low-latitude magnetopause were suggested to be dawn-dusk asymmetric. We report a prolonged simultaneous observations of the K-H waves on the dawn and dusk magnetopause by Magnetospheric Multiscale (MMS) and THEMIS-A (THA) spacecraft, respectively. The quasi-periodic K-H waves on both flanks have unambiguous low-density and high-speed patterns. The wave periods vary gradually on both flanks, with similar average periods (303 ± 107 s for MMS and 266 ± 102 s for THA). The lag time between the variations of the wave periods is close to the wave propagation time from THA to MMS, which suggests that the K-H waves generate and propagate quasi-symmetrically on both flanks. Larger local magnetic shear angles are observed on the trailing edges by MMS than by THA, which is probably due to the strong magnetic field distortion during the tailward propagation. The increased magnetic shear may excite magnetic reconnection, thus contributing to the formation of the low-latitude boundary layer.
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| 9. |
- Rojo, M., et al.
(författare)
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Electron moments derived from the Mercury Electron Analyzer during the cruise phase of BepiColombo
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 683
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We derive electron density and temperature from observations obtained by the Mercury Electron Analyzer on board Mio during the cruise phase of BepiColombo while the spacecraft is in a stacked configuration. Methods. In order to remove the secondary electron emission contribution, we first fit the core electron population of the solar wind with a Maxwellian distribution. We then subtract the resulting distribution from the complete electron spectrum, and suppress the residual count rates observed at low energies. Hence, our corrected count rates consist of the sum of the fitted Maxwellian core electron population with a contribution at higher energies. We finally estimate the electron density and temperature from the corrected count rates using a classical integration method. We illustrate the results of our derivation for two case studies, including the second Venus flyby of BepiColombo when the Solar Orbiter spacecraft was located nearby, and for a statistical study using observations obtained to date for distances to the Sun ranging from 0.3 to 0.9 AU. Results. When compared either to measurements of Solar Orbiter or to measurements obtained by HELIOS and Parker Solar Probe, our method leads to a good estimation of the electron density and temperature. Hence, despite the strong limitations arising from the stacked configuration of BepiColombo during its cruise phase, we illustrate how we can retrieve reasonable estimates for the electron density and temperature for timescales from days down to several seconds.
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| 10. |
- Hartill, E. C., et al.
(författare)
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Deep coral habitats of Glacier Bay National Park and Preserve, Alaska
- 2020
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Ingår i: PLoS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 15:8
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Tidskriftsartikel (refereegranskat)abstract
- Glacier Bay National Park and Preserve (GBNPP) in Southeast Alaska is a system of glaciated fjords with a unique and recent history of deglaciation. As such, it can serve as a natural laboratory for studying patterns of distribution in marine communities with proximity to glacial influence. In order to examine the changes in fjord-based coral communities, underwater photo-quadrats were collected during multipurpose dives with a remotely operated vehicle (ROV) in March of 2016. Ten sites were chosen to represent the geochronological and oceanographic gradients present in GBNPP. Each site was surveyed vertically between 100 and 420 meters depth and photo-quadrats were extracted from the video strip transects for analysis. The ROV was equipped with onboard CTD which recorded environmental data (temperature and salinity), in order to confirm the uniformity of these characteristics at depth across the fjords. The percent cover and diversity of species were lowest near the glaciated heads of the fjords and highest in the Central Channel and at the mouths of the fjords. Diversity is highest where characteristics such as low sedimentation and increased tidal currents are predominant. The diverse communities at the mouths of the fjords and in the Central Channel were dominated by large colonies of the Red Tree Coral,Primnoa pacifica, as well as sponges, brachiopods, multiple species of cnidarians, echinoderms, molluscs and arthropods. The communities at the heads of the fjords were heavily dominated by pioneering species such as brachiopoda, hydrozoan turf, the encrusting stoloniferan coralSarcodyction incrustans, and smaller colonies ofP.pacifica. This research documents a gradient of species dominance from the Central Channel to the heads of the glaciated fjords, which is hypothesized to be driven by a combination of physical and biological factors such as glacial sedimentation, nutrient availability, larval dispersal, and competition.
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