| 1. |
- Matsui, H., et al.
(författare)
-
Characteristics of storm time electric fields in the inner magnetosphere derived from Cluster data
- 2010
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115, s. A11215-
-
Tidskriftsartikel (refereegranskat)abstract
- Storm-time electric fields in the inner magnetosphere measured by Cluster are reported in this study. First, we show two events around the time when Dst index is at a minimum. The electric field possibly related to subauroral ion drifts and/or undershielding is measured inside the inner edge of the electron plasma sheet in the eveningside. For the second event observed in the nightside, the electric field is partly related to dipolarization and is considered as inductive. An electric field without coincident magnetic signatures is also observed. Spatial coherence of the electric field is not large when we check multispacecraft data. It is inferred that the electric field in the magnetotail penetrates inside the region 1 current, while it is not clear about the electric field within the region 2 current from our data. Then superposed epoch analyses using 71 storms are performed. Electric fields at R = 3.5-6R(E) and less than 25 degrees of magnetic latitudes are enhanced around the minimum Dst at all magnetic local times. Electric fields during the recovery phase decay on a time scale shorter than that of Dst index, which could be interpreted in terms of the relation between electric field and ring current during that storm phase. AC electric fields are generally larger than DC electric fields, indicating that the former component might play some role in accelerating ring current particles. These results will be useful to update our empirical electric field model.
|
|
| 2. |
- Matsui, H., et al.
(författare)
-
Multi-spacecraft observations of small-scale fluctuations in density and fields in plasmaspheric plumes
- 2012
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 30:3, s. 623-637
-
Tidskriftsartikel (refereegranskat)abstract
- In this event study, small-scale fluctuations in plasmaspheric plumes with time scales of similar to 10 s to minutes in the spacecraft frame are examined. In one event, plasmaspheric plumes are observed by Cluster, while IMAGE measured density enhancement at a similar location. Fluctuations in density exist in plumes as detected by Cluster and are accompanied by fluctuations in magnetic fields and electric fields. Magnetic fluctuations are transverse and along the direction of the plumes. The E/B ratio is smaller than the Alfv,n velocity. Another similar event is briefly presented. We then consider physical properties of the fluctuations. Alfv,n mode modulated by the feedback instability is one possibility, although non-local generation is likely. It is hard to show that the fluctuations represent a fast mode. Interchange motion is possible due to the consistency between measurements and expectations. The energy source could be a pressure or density gradient in plasmaspheric plumes. When more events are accumulated so that statistical analysis becomes feasible, this type of study will be useful to understand the time evolution of plumes.
|
|
| 3. |
- Matsui, H., et al.
(författare)
-
Revision of empirical electric field modeling in the inner magnetosphere using Cluster data
- 2013
-
Ingår i: Journal of Geophysical Research-Space Physics. - : American Geophysical Union (AGU). - 2169-9380. ; 118:7, s. 4119-4134
-
Tidskriftsartikel (refereegranskat)abstract
- Using Cluster data from the Electron Drift (EDI) and the Electric Field and Wave (EFW) instruments, we revise our empirically-based, inner-magnetospheric electric field (UNH-IMEF) model at 22.662 mV/m; K-p<1, 1K(p)<2, 2K(p)<3, 3K(p)<4, 4K(p)<5, and K(p)4(+). Patterns consist of one set of data and processing for smaller activities, and another for higher activities. As activity increases, the skewed potential contour related to the partial ring current appears on the nightside. With the revised analysis, we find that the skewed potential contours get clearer and potential contours get denser on the nightside and morningside. Since the fluctuating components are not negligible, standard deviations from the modeled values are included in the model. In this study, we perform validation of the derived model more extensively. We find experimentally that the skewed contours are located close to the last closed equipotential, consistent with previous theories. This gives physical context to our model and serves as one validation effort. As another validation effort, the derived results are compared with other models/measurements. From these comparisons, we conclude that our model has some clear advantages over the others.
|
|
| 4. |
- André, Mats, et al.
(författare)
-
Magnetic reconnection and cold plasma at the magnetopause
- 2010
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37:22, s. L22108-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on detailed observations by the four Cluster spacecraft of magnetic reconnection and a Flux Transfer Event (FTE) at the magnetopause. We detect cold (eV) plasma at the magnetopause with two independent methods. We show that the cold ions can be essential for the electric field normal to the current sheet in the separatrix region at the edge of the FTE and for the associated acceleration of ions from the magnetosphere into the reconnection jet. The cold ions have small enough gyroradii to drift inside the limited separatrix region and the normal electric field can be balanced by this drift, E approximate to -v x B. The separatrix region also includes cold accelerated electrons, as part of the reconnection current circuit.
|
|
| 5. |
- Khotyaintsev, Mykola, et al.
(författare)
-
Radar detection of interplanetary shocks : scattering by anisotropic Langmuir turbulence
- 2010
-
Ingår i: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 45:6, s. 804-811
-
Tidskriftsartikel (refereegranskat)abstract
- Earth-directed interplanetary shocks associated with coronal mass ejections (CMEs) are known to have a severe impact on the magnetosphere, causing strong geomagnetic storms and substorms. Hence, early detection of such shocks is important. Here we study the feasibility of radar detection of interplanetary shocks. We consider a scattering mechanism, which is based on the induced scattering t + l ⇄ t of a radar wave by anisotropic Langmuir turbulence, being generated by the shock-accelerated electrons. The problem is studied for an arbitrary angle between the electron beam and the incident radar wave, vb∧ kt, and special emphasis is put on the study of a dependence of the scattering process on this angle. We obtain and analyze analytical expressions for the frequency shift, scattering cross-section of the turbulence, coefficient of absorption (due to scattering), and the optical depth. We show that the detection of such shocks is possible if the turbulence energy density exceeds W = 10- 5 nT (nT is the thermal energy density of a plasma), which is quite realistic according to our estimations. The altitudes in the solar corona where reflections may occur depend on the angle vb∧ kt. If expressed in local plasma frequencies, ωp, the altitudes span is ωt / 8 ≲ ωp ≤ ωt for vb∧ kt = π and ωt / 120 ≲ ωp ≤ ωt for vb∧ kt = π / 2, where ωt is a frequency of the transmitted radar wave. Thus the scattering occurs much closer to the radar in the second case than in the first. Detection of the scattered signal, in the general case, requires a remote receiver, since the radar wave backscatters only for vb∧ kt = π.
|
|
| 6. |
- Fu, H. S., et al.
(författare)
-
Dipolarization fronts as a consequence of transient reconnection : in situ evidence
- 2013
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union. - 0094-8276 .- 1944-8007. ; 40:23, s. 6023-6027
-
Tidskriftsartikel (refereegranskat)abstract
- Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail from X-GSM=-30 R-E to X-GSM=-7 R-E. How these DFs are formed is still poorly understood. Three possible mechanisms have been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near-Earth region (X-GSM>-10 R-E) are primarily attributed to jet braking, while the DFs detected in the mid- or far-tail region (X-GSM<-15 R-E) are primarily attributed to transient reconnection or spontaneous formation. In the jet-braking mechanism, the high-speed flow pushes the preexisting plasmas to produce the DF so that there is causality between high-speed flow and DF. In the transient-reconnection mechanism, there is no causality between high-speed flow and DF, because the frozen-in condition is violated.
|
|
| 7. |
- Viberg, Henrik, et al.
(författare)
-
Whistler mode waves atmagnetotail dipolarization fronts
- 2014
-
Ingår i: J GEOPHYS RES-SPACE. - 2169-9380. ; 119:4, s. 2605-2611
-
Tidskriftsartikel (refereegranskat)abstract
- We report the statistics of whistler mode waves observed in relation to dipolarization fronts (DFs) in Earth's magnetotail using data from the four Cluster spacecraft spanning a period of 9 years, 2001-2009. We show that whistler mode waves are common in a vicinity of DFs: between 30 and 60% of all DFs are associated with whistlers. Whistlers are about 7 times more likely to be observed near a DF than at any random location in the magnetotail. Therefore, whistlers are a characteristic signature of DFs. We find that whistlers are most often detected in the flux pileup region (FPR) following the DF, close to the center of the current sheet (B-x approximate to 0) and in association with anisotropic electron distributions (T>T-vertical bar). This suggests that we typically observe emissions in the source region where they are generated by the anisotropic electrons produced by the betatron process inside the FPR.
|
|
