| 1. |
- Andersson, L., et al.
(författare)
-
New Features of Electron Phase Space Holes Observed by the THEMIS Mission
- 2009
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:22, s. 225004-
-
Tidskriftsartikel (refereegranskat)abstract
- Observations of electron phase-space holes (EHs) in Earth's plasma sheet by the THEMIS satellites include the first detection of a magnetic perturbation (delta B-parallel to) parallel to the ambient magnetic field (B-0). EHs with a detectable delta B-parallel to have several distinguishing features including large electric field amplitudes, a magnetic perturbation perpendicular to B-0, high speeds (similar to 0.3c) along B-0, and sizes along B-0 of tens of Debye lengths. These EHs have a significant center potential (Phi similar to k(B)T(e)/e), suggesting strongly nonlinear behavior nearby such as double layers or magnetic reconnection.
|
|
| 2. |
- André, Mats, et al.
(författare)
-
Low-energy ions : A previously hidden solar system particle population
- 2012
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L03101-
-
Tidskriftsartikel (refereegranskat)abstract
- Ions with energies less than tens of eV originate from the Terrestrial ionosphere and from several planets and moons in the solar system. The low energy indicates the origin of the plasma but also severely complicates detection of the positive ions onboard sunlit spacecraft at higher altitudes, which often become positively charged to several tens of Volts. We discuss some methods to observe low-energy ions, including a recently developed technique based on the detection of the wake behind a charged spacecraft in a supersonic flow. Recent results from this technique show that low-energy ions typically dominate the density in large regions of the Terrestrial magnetosphere on the nightside and in the polar regions. These ions also often dominate in the dayside magnetosphere, and can change the dynamics of processes like magnetic reconnection. The loss of this low-energy plasma to the solar wind is one of the primary pathways for atmospheric escape from planets in our solar system. We combine several observations to estimate how common low-energy ions are in the Terrestrial magnetosphere and briefly compare with Mars, Venus and Titan.
|
|
| 3. |
- Angelopoulos, V., et al.
(författare)
-
First Results from the THEMIS Mission
- 2008
-
Ingår i: Space Science Reviews. - : Springer Science and Business Media LLC. - 0038-6308 .- 1572-9672. ; 141:1-4, s. 453-476
-
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.
|
|
| 4. |
- Bortnik, J., et al.
(författare)
-
An Observation Linking the Origin of Plasmaspheric Hiss to Discrete Chorus Emissions
- 2009
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 324:5928, s. 775-778
-
Tidskriftsartikel (refereegranskat)abstract
- A long-standing problem in the field of space physics has been the origin of plasmaspheric hiss, a naturally occurring electromagnetic wave in the high-density plasmasphere (roughly within 20,000 kilometers of Earth) that is known to remove the high-energy Van Allen Belt electrons that pose a threat to satellites and astronauts. A recent theory tied the origin of plasmaspheric hiss to a seemingly different wave in the outer magnetosphere, but this theory was difficult to test because of a challenging set of observational requirements. Here we report on the experimental verification of the theory, made with a five-satellite NASA mission. This confirmation will allow modeling of plasmaspheric hiss and its effects on the high-energy radiation environment.
|
|
| 5. |
- Cully, Christopher, et al.
(författare)
-
THEMIS observations of long-lived regions of large-amplitude whistler waves in the inner magnetosphere
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:17, s. L17S16-
-
Tidskriftsartikel (refereegranskat)abstract
- Recent reports of large-amplitude whistler waves (> 100 mV/m) in the radiation belts have intensified interest in the role of whistler waves in accelerating radiation belt electrons to MeV energies. Several critical parameters for addressing this issue have not previously been observed, including the occurrence frequency, spatial extent and longevity of regions of large-amplitude whistlers. The THEMIS mission, with multiple satellites in a near-equatorial orbit, offers an excellent opportunity to study these waves. We use data from the Electric Field Instrument (EFI) to show that in the dawn-side radiation belts, especially near L-shells from 3.5 to 5.5, the probability distribution of wave activity has a significant high-amplitude tail and is hence not well-described by long-term time averages. Regions of enhanced wave activity exhibit four-second averaged wave power above 1 mV/m and sub-second bursts up to several hundred mV/m. These regions are spatially localized to at most several hours of local time azimuthally, but can persist in the same location for several days. With large regions of space persistently covered by bursty, large-amplitude waves, the mechanisms and rates of radiation belt electron acceleration may need to be reconsidered.
|
|
| 6. |
- Eastwood, J. P., et al.
(författare)
-
THEMIS observations of a hot flow anomaly : Solar wind, magnetosheath, and ground-based measurements
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:17, s. L17S03-
-
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.
|
|
| 7. |
- Engwall, Erik, et al.
(författare)
-
Earth’s ionospheric outflow dominated by hidden cold plasma
- 2009
-
Ingår i: Nature Geoscience. - : Nature Publishing Group. - 1752-0894. ; 2:1, s. 24-27
-
Tidskriftsartikel (refereegranskat)abstract
- The Earth constantly loses matter, mostly in the form of H+and O+ ions, through various outflow processes from the upper atmosphere and ionosphere. Most of these ions are cold (below 1 eV in thermal energy), but can still escape and travel farther out along the magnetic field lines into the magnetospheric tail lobes. The outflow has previously beenmeasured close to the Earth. To understand what fraction does not return but instead escapes, the measurements should be conducted at larger geocentric distances. However, at high altitudes the cold ions are normally invisible to spacecraft measurements, because the potential of a sunlit spacecraft exceeds the equivalent energy of the ions. Here we show that cold ions dominate in both flux and density in the distant magnetotail lobes, using a new measurement technique on the Cluster spacecraft. The total loss of cold hydrogen ions from the planet is inferred to be of the order of 1026 s−1, which is larger than the previously observed more energetic outflow. Quantification and insight of the loss processes of the Earth’s atmosphere and ionosphere are also important for understanding the evolution of atmospheres on other celestial bodies.
|
|
| 8. |
- Engwall, Erik, et al.
(författare)
-
Survey of cold ionospheric outflows in the magnetotail
- 2009
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 27:8, s. 3185-3201
-
Tidskriftsartikel (refereegranskat)abstract
- Low-energy ions escape from the ionosphere and constitute a large part of the magnetospheric content, especially in the geomagnetic tail lobes. However, they are normally invisible to spacecraft measurements, since the potential of a sunlit spacecraft in a tenuous plasma in many cases exceeds the energy-per-charge of the ions, and little is therefore known about their outflow properties far from the Earth. Here we present an extensive statistical study of cold ion outflows (0-60 eV) in the geomagnetic tail at geocentric distances from 5 to 19 R-E using the Cluster spacecraft during the period from 2001 to 2005. Our results were obtained by a new method, relying on the detection of a wake behind the spacecraft. We show that the cold ions dominate in both flux and density in large regions of the magnetosphere. Most of the cold ions are found to escape from the Earth, which improves previous estimates of the global outflow. The local outflow in the magnetotail corresponds to a global outflow of the order of 10(26) ions s(-1). The size of the outflow depends on different solar and magnetic activity levels.
|
|
| 9. |
- Ergun, R. E., et al.
(författare)
-
Observations of Double Layers in Earth's Plasma Sheet
- 2009
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:15, s. 155002-
-
Tidskriftsartikel (refereegranskat)abstract
- We report the first direct observations of parallel electric fields (E-parallel to) carried by double layers (DLs) in the plasma sheet of Earth's magnetosphere. The DL observations, made by the THEMIS spacecraft, have E-parallel to signals that are analogous to those reported in the auroral region. DLs are observed during bursty bulk flow events, in the current sheet, and in plasma sheet boundary layer, all during periods of strong magnetic fluctuations. These observations imply that DLs are a universal process and that strongly nonlinear and kinetic behavior is intrinsic to Earth's plasma sheet.
|
|
| 10. |
- Eriksson, S., et al.
(författare)
-
Magnetic island formation between large-scale flow vortices at an undulating postnoon magnetopause for northward interplanetary magnetic field
- 2009
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:2, s. A00C17-
-
Tidskriftsartikel (refereegranskat)abstract
- Time History of Events and Macroscale Interactions during Substorms multispacecraft observations are presented for a similar to 2-h-long postnoon magnetopause event on 8 June 2007 that for the first time indicate that the trailing (sunward) edges of Kelvin-Helmholtz (KH) waves are commonly related to small-scale < 0.56 R-E magnetic islands or flux transfer events (FTE) during the growth phase of these surface waves. The FTEs typically show a characteristic bipolar B-N structure with enhanced total pressure at their center. Most of the small-scale FTEs are not related to any major plasma acceleration. TH-A observations of one small FTE at a transition from the low-latitude boundary layer (LLBL) into a magnetosheath plasma depletion layer were reconstructed using separate techniques that together confirm the presence of a magnetic island within the LLBL adjacent to the magnetopause. The island was associated with a small plasma vortex and both features appeared between two large-scale (similar to 1 R-E long and 2000 km wide) plasma vortices. We propose that the observed magnetic islands may have been generated from a time-varying reconnection process in a low ion plasma beta (beta(i) < 0.2) and low 8.3 degrees field shear environment at the sunward edge of the growing KH waves where the local magnetopause current sheet may be compressed by the converging flow of the large-scale plasma vortices as suggested by numerical simulations of the KH instability.
|
|
