| 1. |
- Berthomier, M., et al.
(författare)
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Alfven : magnetosphere-ionosphere connection explorers
- 2012
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Ingår i: Experimental astronomy. - Dordrecht : Springer. - 0922-6435 .- 1572-9508. ; 33:2-3, s. 445-489
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Tidskriftsartikel (refereegranskat)abstract
- The aurorae are dynamic, luminous displays that grace the night skies of Earth's high latitude regions. The solar wind emanating from the Sun is their ultimate energy source, but the chain of plasma physical processes leading to auroral displays is complex. The special conditions at the interface between the solar wind-driven magnetosphere and the ionospheric environment at the top of Earth's atmosphere play a central role. In this Auroral Acceleration Region (AAR) persistent electric fields directed along the magnetic field accelerate magnetospheric electrons to the high energies needed to excite luminosity when they hit the atmosphere. The "ideal magnetohydrodynamics" description of space plasmas which is useful in much of the magnetosphere cannot be used to understand the AAR. The AAR has been studied by a small number of single spacecraft missions which revealed an environment rich in wave-particle interactions, plasma turbulence, and nonlinear acceleration processes, acting on a variety of spatio-temporal scales. The pioneering 4-spacecraft Cluster magnetospheric research mission is now fortuitously visiting the AAR, but its particle instruments are too slow to allow resolve many of the key plasma physics phenomena. The Alfv,n concept is designed specifically to take the next step in studying the aurora, by making the crucial high-time resolution, multi-scale measurements in the AAR, needed to address the key science questions of auroral plasma physics. The new knowledge that the mission will produce will find application in studies of the Sun, the processes that accelerate the solar wind and that produce aurora on other planets.
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| 2. |
- Andre, M., et al.
(författare)
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Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause
- 2001
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 19:12-okt, s. 1471-1481
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Tidskriftsartikel (refereegranskat)abstract
- Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.
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| 3. |
- Wild, J. A., et al.
(författare)
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Midnight sector observations of auroral omega bands
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A00I30-
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Tidskriftsartikel (refereegranskat)abstract
- We present observations of auroral omega bands on 28 September 2009. Although generally associated with the substorm recovery phase and typically observed in the morning sector, the features presented here occurred just after expansion phase onset and were observed in the midnight sector, dawnward of the onset region. An all-sky imager located in northeastern Iceland revealed that the omega bands were similar to 150 x 200 km in size and propagated eastward at similar to 0.4 km s(-1) while a colocated ground magnetometer recorded the simultaneous occurrence of Ps6 pulsations. Although somewhat smaller and slower moving than the majority of previously reported omega bands, the observed structures are clear examples of this phenomenon, albeit in an atypical location and unusually early in the substorm cycle. The THEMIS C probe provided detailed measurements of the upstream interplanetary environment, while the Cluster satellites were located in the tail plasma sheet conjugate to the ground-based all-sky imager. The Cluster satellites observed bursts of 0.1-3 keV electrons moving parallel to the magnetic field toward the Northern Hemisphere auroral ionosphere; these bursts were associated with increased levels of field-aligned Poynting flux. The in situ measurements are consistent with electron acceleration via shear Alfven waves in the plasma sheet similar to 8 R-E tailward of the Earth. Although a one-to-one association between auroral and magnetospheric features was not found, our observations suggest that Alfven waves in the plasma sheet are responsible for field-aligned currents that cause Ps6 pulsations and auroral brightening in the ionosphere. Our findings agree with the conclusions of earlier studies that auroral omega bands have a source mechanism in the midtail plasma sheet.
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| 4. |
- Forsyth, C., et al.
(författare)
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Temporal evolution and electric potential structure of the auroral acceleration region from multispacecraft measurements
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117:12, s. A12203-
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Tidskriftsartikel (refereegranskat)abstract
- Bright aurorae can be excited by the acceleration of electrons into the atmosphere in violation of ideal magnetohydrodynamics. Modeling studies predict that the accelerating electric potential consists of electric double layers at the boundaries of an acceleration region but observations suggest that particle acceleration occurs throughout this region. Using multispacecraft observations from Cluster, we have examined two upward current regions on 14 December 2009. Our observations show that the potential difference below C4 and C3 changed by up to 1.7 kV between their respective crossings, which were separated by 150 s. The field-aligned current density observed by C3 was also larger than that observed by C4. The potential drop above C3 and C4 was approximately the same in both crossings. Using a novel technique of quantitively comparing the electron spectra measured by Cluster 1 and 3, which were separated in altitude, we determine when these spacecraft made effectively magnetically conjugate observations, and we use these conjugate observations to determine the instantaneous distribution of the potential drop in the AAR. Our observations show that an average of 15% of the potential drop in the AAR was located between C1 at 6235 km and C3 at 4685 km altitude, with a maximum potential drop between the spacecraft of 500 V, and that the majority of the potential drop was below C3. Assuming a spatial invariance along the length of the upward current region, we discuss these observations in terms of temporal changes and the vertical structure of the electrostatic potential drop and in the context of existing models and previous single- and multispacecraft observations.
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| 5. |
- Marklund, Göran T., et al.
(författare)
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Cluster multipoint study of the acceleration potential pattern and electrodynamics of an auroral surge and its associated horn arc
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117:10, s. A10223-
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Tidskriftsartikel (refereegranskat)abstract
- Cluster results are presented from the acceleration region of an auroral surge and connected horn arc, observed during an extended time period of substorm activity. The Cluster spacecraft crossed different magnetic local time (MLT) sectors of the surge and horn, with lag times of 2-10 min. Acceleration potential patterns are derived for the horn arc and for the double arc (surge and horn) at the surge front and deeper into the surge. The parallel potential drop of the horn arc ranged between 4 and 7 kV. At the surge front, two weakly coupled U-potentials with parallel potential drops of 8 (7) kV and 7 (5) kV were derived for the surge and horn, respectively, from the C3 (C4) data. A similar, more coupled pattern was derived for the region deeper into the surge. We also address how the field-aligned currents of the surge and horn system close in the ionosphere. The Cluster data allow almost simultaneous estimates of the latitudinal current closure at various MLT sectors. Significant net upward currents are derived for the horn and surge, whereas the currents at the surge front were found to be balanced. The net upward horn current is proposed to be fed by the zonal divergence of the westward Pedersen current in the horn, consistent with the acceleration potential decrease in the westward horn direction. The net upward surge current is proposed to be fed by the divergence of a westward electrojet and by localized downward currents adjacent to the surge.
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| 6. |
- Marklund, Göran T., et al.
(författare)
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Evolution in space and time of the quasi-static acceleration potential of inverted-V aurora and its interaction with Alfvenic boundary processes
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A00K13-
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Tidskriftsartikel (refereegranskat)abstract
- Results are presented from Cluster crossings of the acceleration region of two inverted-V auroras located in the poleward part of an extensive substorm bulge. The particle and field data are used to infer the acceleration potentials of the arcs and their distribution in altitude and latitude. The C1 data are consistent with a symmetric potential pattern, composed of two negative U potentials and one positive U potential in between, and the C3 and C4 data are consistent with an asymmetric pattern, where the dominating potential structure extends deep into the polar cap boundary (PCB) region. The two patterns may either correspond to different stages of evolution of the same double arc system or represent two longitudinally separated double arc systems. For all spacecraft, the potential well of the poleward arc extends into the PCB region, whereas the density cavity does not but remains confined to R1. This suggests that the Alfvenic activity observed within the PCB region prevents the cavity formation, consistent with the associated FACs being roughly balanced over this region. The results show that Alfvenic and quasi-static acceleration operates jointly in the PCB region, varying from being about equally important (on C1) to being predominantly quasi-static (on C3/C4). The presence (absence) of an upward electron beam, associated with a positive potential structure and a downward current, observed by C1 (C4/C3) is expected from its short life time, shorter than the time lag between the Cluster spacecraft. The evolution involves both a broadening and a density reduction of the associated downward current sheet to below the critical current density above which parallel electric fields will form. The deepest potential well of 13 kV observed by C4 was located in Region 1, adjacent to the PCB region and coinciding with the deepest density cavity, with a minimum density of 0.1 cm(-3). The interface between Region 1 and the PCB region, coinciding with the steep density gradient, appears to be the leading edge of the cavity.
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| 7. |
- Nakamura, R., et al.
(författare)
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Cluster observations of an ion-scale current sheet in the magnetotail under the presence of a guide field
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A7, s. A07S16-
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Tidskriftsartikel (refereegranskat)abstract
- We report on Cluster observations of a thin current sheet interval under the presence of a strong vertical bar B-Y vertical bar during a fast earthward flow interval between 1655 UT and 1703 UT on 17 August 2003. The strong vertical bar B-Y vertical bar in the tail could be associated with a strong IMF vertical bar B-Y vertical bar, but the large fluctuations in B-Y, not seen in the IMF, suggest that a varying reconnection rate causes a varying transport of B-Y-dominated magnetic flux and/or a change in B-Y due to the Hall-current system. During the encounter of the high-speed flow, an intense current layer was observed around 1655: 53 UT with a peak current density of 182 nA/m(2), the largest current density observed by the Cluster four-spacecraft magnetic field measurement in the magnetotail. The half width of this current layer was estimated to be similar to 290 km, which was comparable to the ion-inertia length. Its unique signature is that the strong current is mainly field-aligned current flowing close to the center of the plasma sheet. The event was associated with parallel heating of electrons with asymmetries, which suggests that electrons moving along the field lines can contribute to a strong dawn-to-dusk current when the magnetotail current sheet becomes sufficiently thin and active in a strong guide field case.
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| 8. |
- Runov, A., et al.
(författare)
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Observations of an active thin current sheet
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A7
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Tidskriftsartikel (refereegranskat)abstract
- We analyze observations of magnetotail current sheet dynamics during a substorm between 2330 and 2400 UT on 28 August 2005 when Cluster was in the plasma sheet at [-17.2, -4.49, 0.03] R-E (GSM) with the foot points near the IMAGE ground-based network. Observations from the Cluster spacecraft, ground-based magnetometers, and the IMAGE satellite showed that the substorm started in a localized region near midnight, expanding azimuthally. A thin current sheet with a thickness of less than 900 km and current density of about 30 nA/m(2) was observed during 5 min around the substorm onset. The thinning of the current sheet was accompanied by tailward plasma flow at a velocity of -700 km/s and subsequent reversal to earthward flow at V-x approximate to 500 km/s coinciding with a B-z turning from -5 to + 10 nT. The analysis of magnetic and electric fields behavior and particle distributions reveals signatures of impulsive (with similar to 1 min timescale) activations of the thin current sheet. These observations were interpreted in the framework of transient reconnection, although the data analysis reveals serious disagreements with the classical 2.5-D X line model.
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| 9. |
- Deng, X. H., et al.
(författare)
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Dynamics and waves near multiple magnetic null points in reconnection diffusion region
- 2009
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Ingår i: Journal of Geophysical Research. - : Blackwell Publishing. - 0148-0227 .- 2156-2202. ; 114:7
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Tidskriftsartikel (refereegranskat)abstract
- Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three-dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler-mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for the whistler wave regime of reconnection.
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