| 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. |
- Vaivads, A., et al.
(författare)
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What high altitude observations tell us about the auroral acceleration : A cluster/DMSP conjunction
- 2003
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 30:3
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Tidskriftsartikel (refereegranskat)abstract
- [1] Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field lines of auroral arc. Cluster is well above the acceleration region and observes upward keV ion beams and bipolar electric structures. The integrated potential at Cluster altitudes shows a dip that is consistent with the keV electron acceleration energy at low altitude. The earthward Poynting flux at Cluster altitudes is comparable to the electron energy flux at low altitudes. Thus, for this event the auroral acceleration can be described as a quasi-stationary potential structure with equipotential lines reaching the Cluster altitudes. The arc forms at the outer edge of the plasma sheet at a density gradient. Multiple Cluster satellite measurements allow us to study the density increase associated with the development of the arc, and to estimate the velocity of the structure. The quasi-potential structure itself may be part of an Alfven wave.
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| 3. |
- Aikio, A. T., et al.
(författare)
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Temporal evolution of two auroral arcs as measured by the Cluster satellite and coordinated ground-based instruments
- 2004
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 22:12, s. 4089-4101
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Tidskriftsartikel (refereegranskat)abstract
- The four Cluster s/c passed over Northern Scandinavia on 6 February 2001 from south-east to north-west at a radial distance of about 4.4 R-E in the post-midnight sector. When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was confined to within 110 km in latitude and 50 km in longitude. This constellation allowed us to study the temporal evolution of plasma with a time scale of a few minutes. Ground-based instrumentation used involved two all-sky cameras, magnetometers and the EISCAT radar. The main findings were as follows. Two auroral arcs were located close to the equatorward and poleward edge of a large-scale density cavity, respectively. These arcs showed a different kind of a temporal evolution. (1) As a response to a pseudo-breakup onset, both the up- and downward field-aligned current (FAC) sheets associated with the equatorward arc widened and the total amount of FAC doubled in a time scale of 1-2 min. (2) In the poleward arc, a density cavity formed in the ionosphere in the return (downward) current region. As a result of ionospheric feedback, a strongly enhanced ionospheric southward electric field developed in the region of decreased Pedersen conductance. Furthermore, the acceleration potential of ionospheric electrons, carrying the return current, increased from 200 to 1000 eV in 70 s, and the return current region widened in order to supply a constant amount of return current to the arc current circuit. Evidence of local acceleration of the electron population by dispersive Alfven waves was obtained in the upward FAC region of the poleward arc. However, the downward accelerated suprathermal electrons must be further energised below Cluster in order to be able to produce the observed visible aurora. Both of the auroral arcs were associated with broad-band ULF/ELF (BBELF) waves, but they were highly localised in space and time. The most intense BBELF waves were confined typically to the return current regions adjacent to the visual arc, but in one case also to a weak upward FAC region. BBELF waves could appear/disappear between s/c crossings of the same arc separated by about 1 min.
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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. |
- Marklund, Göran T., et al.
(författare)
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Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere
- 2001
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 414:6865, s. 724-727
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Tidskriftsartikel (refereegranskat)abstract
- The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field(1-3). On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth(4-11). Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted(12), but how they could be maintained is still a matter for debate(13). The spatial and temporal behaviour of the electric fields-a knowledge of which is crucial to an understanding of their nature-cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.
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| 8. |
- Marklund, Göran T., et al.
(författare)
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Altitude Distribution of the Auroral Acceleration Potential Determined from Cluster Satellite Data at Different Heights
- 2011
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 106:5, s. 055002-
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Tidskriftsartikel (refereegranskat)abstract
- Aurora, commonly seen in the polar sky, is a ubiquitous phenomenon occurring on Earth and other solar system planets. The colorful emissions are caused by electron beams hitting the upper atmosphere, after being accelerated by quasistatic electric fields at 1-2 RE altitudes, or by wave electric fields. Although aurora was studied by many past satellite missions, Cluster is the first to explore the auroral acceleration region with multiprobes. Here, Cluster data are used to determine the acceleration potential above the aurora and to address its stability in space and time. The derived potential comprises two upper, broad U-shaped potentials and a narrower S-shaped potential below, and is stable on a 5 min time scale. The scale size of the electric field relative to that of the current is shown to depend strongly on altitude within the acceleration region. To reveal these features was possible only by combining data from the two satellites.
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| 9. |
- Marklund, Göran T., et al.
(författare)
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Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster
- 2004
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Ingår i: Nonlinear processes in geophysics. - : Copernicus GmbH. - 1023-5809 .- 1607-7946. ; 11:5-6, s. 709-720
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Tidskriftsartikel (refereegranskat)abstract
- Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 cm altitude) peak amplitudes of approximate to 1V/m, bipolar or unipolar profiles, perpendicular scale sizes of approximate to 10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of approximate to 10 muA/m(2), and upward electron beams with characteristic energies of a few hundred eV to a few keV. Two events illustrate he temporal evolution of bipolar, diverging electric field strictures, indicative of positive U-shaped potentials increasing in magnitude from less than 1 kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. The total observation time for these events was typically 10-20 s, too short for monitoring the evolution of the structure, bui yet of interest for revealing their short term stability. The locations of the two bipolar events at the poleward boundary of the central plasma sheet and of the two unipolar events at the polar cap boundary, suggest that the special profile shape depends on whether plasma populations, dense enough to support upward field-aligned currents and closure of the return current, exist on both sides, or on one side only, of the boundary.
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| 10. |
- Sadeghi, Soheil, et al.
(författare)
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Spatiotemporal features of the auroral acceleration region as observed by Cluster
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116:12, s. A00K19-
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Tidskriftsartikel (refereegranskat)abstract
- A pair of negative electric potential structures associated with inverted-V aurora is investigated using electric and magnetic field, ion and electron data from the Cluster spacecraft, crossing the auroral acceleration region (AAR) at different altitudes above the Northern hemisphere midnight auroral oval. The spatial and temporal development of the acceleration structures is studied, given the magnetic conjunction opportunity and the one minute difference between the Cluster spacecraft crossings. The configuration allowed for estimation of characteristic times of development for the two structures and of the parallel electric field and potential drop for the more stable one. The first potential structure had a width of similar to 80 km (projected to the ionosphere) and was relatively short-lived, developing in less than 40 s and decaying in one minute. The parallel potential drop increased between altitudes of 1.13 R(E) and 1.3 R(E), whereas the acceleration potential above 1.3 R(E) remained almost unchanged during that time. This intensification occurred mainly after the time when the associated upward current had reached its maximum value. The second structure had a width of similar to 50 km and was subject to an increase by a factor of 3 of the parallel potential drop below 1.3 R(E), during about 40 s, after which it remained rather stable for one minute or more. Similarly here, the acceleration potential above 1.3 R(E) remained roughly unchanged. For the more stable second structure, an average parallel electric field between 1.13 and 1.3 R(E) could be estimated (similar to 0.56 mV/m). The conductance along the flux tube was also stable for one minute or more.
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