| 1. |
- 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.
|
|
| 2. |
- Huang, S. Y., et al.
(författare)
-
Electron acceleration in the reconnection diffusion region : Cluster observations
- 2012
-
Ingår i: Geophysical Research Letters. - : Blackwell Publishing. - 0094-8276 .- 1944-8007. ; 39:L11103
-
Tidskriftsartikel (refereegranskat)abstract
- We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration.
|
|
| 3. |
- Khotyaintsev, Yuri, et al.
(författare)
-
Plasma Jet Braking : Energy Dissipation and Nonadiabatic Electrons
- 2011
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 106:16
-
Tidskriftsartikel (refereegranskat)abstract
- We report in situ observations by the Cluster spacecraft of wave-particle interactions in a magnetic flux pileup region created by a magnetic reconnection outflow jet in Earth's magnetotail. Two distinct regions of wave activity are identified: lower-hybrid drift waves at the front edge and whistler-mode waves inside the pileup region. The whistler-mode waves are locally generated by the electron temperature anisotropy, and provide evidence for ongoing betatron energization caused by magnetic flux pileup. The whistler-mode waves cause fast pitch-angle scattering of electrons and isotropization of the electron distribution, thus making the flow braking process nonadiabatic. The waves strongly affect the electron dynamics and thus play an important role in the energy conversion chain during plasma jet braking.
|
|
| 4. |
- Khotyaintsev, Yuri V., et al.
(författare)
-
In-flight calibration of double-probe electric field measurements on Cluster
- 2014
-
Ingår i: Geoscientific Instrumentation, Methods and Data Systems. - : European Geosciences Union (EGU). - 2193-0856 .- 2193-0864. ; 3:2, s. 143-151
-
Tidskriftsartikel (refereegranskat)abstract
- Double-probe electric field instrument with long wire booms is one of the most popular techniques for in situ measurement of electric fields in plasmas on spinning spacecraft platforms, which have been employed on a large number of space missions. Here we present an overview of the calibration procedure used for the Electric Field and Wave (EFW) instrument on Cluster, which involves spin fits of the data and correction of several offsets. We also describe the procedure for the offset determination and present results for the long-term evolution of the offsets.
|
|
| 5. |
- Ni, Binbin, et al.
(författare)
-
Global distribution of electrostatic electron cyclotron harmonic waves observed on THEMIS
- 2011
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 38, s. L17105-
-
Tidskriftsartikel (refereegranskat)abstract
- A global, statistical analysis of electrostatic electron cyclotron harmonic (ECH) waves is performed using THEMIS wave data. Our results confirm the high occurrence of <1 mV/m ECH emissions throughout the outer magnetosphere (L > 5). The strongest (>= 1 mV/m) ECH waves are enhanced during geomagnetically disturbed periods, and are mainly confined close to the magnetic equator (|lambda| < 3 degrees) over the region L <= 10 in the night and dawn MLT sector. ECH wave intensities within 3 degrees <= |lambda| < 6 degrees are generally much weaker but not negligible especially for L < similar to 12 on the midnight side. Furthermore, the occurrence rates and variability of moderately intense (>= 0.1 mV/m) ECH emissions suggest that ECH wave scattering could contribute to diffuse auroral precipitation in the outer (L > 8) magnetosphere where chorus emissions are statistically weak.
|
|
| 6. |
- Nishimura, Y., et al.
(författare)
-
Multievent study of the correlation between pulsating aurora and whistler mode chorus emissions
- 2011
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A11221-
-
Tidskriftsartikel (refereegranskat)abstract
- A multievent study was performed using conjugate measurements of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and an all-sky imager during periods of intense lower-band chorus waves. The thirteen identified cases support our previous finding, based on two events, that the intensity modulation of lower-band chorus near the magnetic equator is highly correlated with quasiperiodic pulsating auroral emissions near the spacecraft's magnetic footprint, indicating that lower-band chorus is the driver of the pulsating aurora. Furthermore, we identified a fortuitous measurement made simultaneously by two THEMIS spacecraft with small spatial separation. The two spacecraft were found to be located in a single pulsating chorus patch and the spacecraft footprints were in the same pulsating auroral patch when intense chorus bursts were measured simultaneously, whereas only one of the spacecraft's footprints was in a patch when the other spacecraft did not detect intense chorus. On the basis of this event, we can estimate the pulsating chorus patch size by mapping the pulsating auroral patches from the ionosphere toward the magnetic equator, giving a roughly circular region of similar to 5000 km diameter for corresponding azimuthally elongated patches with similar to 100 km size in the ionosphere. Using a ray-tracing-based calculation of the divergence of chorus raypaths from a point source, together with the corresponding resonant energies, we found that the chorus patch size is most probably not a result of ray divergence but a property of the wave excitation region.
|
|
| 7. |
- Sjögren, Alexander, et al.
(författare)
-
Simulation of Potential Measurements Around a Photoemitting Spacecraft in a Flowing Plasma
- 2012
-
Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 40:4, s. 1257-1261
-
Tidskriftsartikel (refereegranskat)abstract
- Plasma measurements by electrostatic probes are influenced by the spacecraft-plasma interaction, including the photoelectrons emitted by the spacecraft. Such effects get particularly important in tenuous plasmas with large Debye lengths. We have used the particle-in-cell code package SPIS to study the close environment of the Rosetta spacecraft, and the impact of the spacecraft-plasma interaction on the electrostatic potential at the position of the Langmuir probes onboard. The simulations show that in the solar wind, photoemission has a bigger impact than wake formation. Spacecraft potential estimates based on Langmuir probe data in the solar wind need to be compensated for these effects when the spacecraft attitude varies. The SPIS simulations are validated by comparison to an independent code.
|
|
| 8. |
- Tao, J. B., et al.
(författare)
-
A model of electromagnetic electron phase-space holes and its application
- 2011
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A11213-
-
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.
|
|
| 9. |
- Tao, J. B., et al.
(författare)
-
Kinetic instabilities in the lunar wake : ARTEMIS observations
- 2012
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A03106-
-
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.
|
|
| 10. |
- Taubenschuss, Ulrich, et al.
(författare)
-
Wave normal angles of whistler mode chorus rising and falling tones
- 2014
-
Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 119:12
-
Tidskriftsartikel (refereegranskat)abstract
- We present a study of wave normal angles (theta(k)) of whistler mode chorus emission as observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) during the year 2008. The three inner THEMIS satellites THA, THD, and THE usually orbit Earth close to the dipole magnetic equator (+/- 20 degrees), covering a large range of L shells from the plasmasphere out to the magnetopause. Waveform measurements of electric and magnetic fields enable a detailed polarization analysis of chorus below 4 kHz. When displayed in a frequency-theta(k) histogram, four characteristic regions of occurrence are evident. They are separated by gaps at f/f(c,e) approximate to 0.5 (f is the chorus frequency, f(c,e) is the local electron cyclotron frequency) and at theta(k) similar to 40 degrees. Below theta(k) similar to 40 degrees, the average value for theta(k) is predominantly field aligned, but slightly increasing with frequency toward half of f(c,e) (theta(k) up to 20 degrees). Above half of f(c,e), the average theta(k) is again decreasing with frequency. Above theta(k) similar to 40 degrees, wave normal angles are usually close to the resonance cone angle. Furthermore, we present a detailed comparison of electric and magnetic fields of chorus rising and falling tones. Falling tones exhibit peaks in occurrence solely for theta(k) > 40 degrees and are propagating close to the resonance cone angle. Nevertheless, when comparing rising tones to falling tones at theta(k) > 40 degrees, the ratio of magnetic to electric field shows no significant differences. Thus, we conclude that falling tones are generated under similar conditions as rising tones, with common source regions close to the magnetic equatorial plane.
|
|
