Evolution in space and time of the quasi static acceleration potential of inverted V aurora and its interaction with Alfvénic boundary processes

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2011ja016537, 2011 Evolution in space and time of the quasi static acceleration potential of inverted V aurora and its interaction with Alfvénic boundary processes G. T. Marklund, 1 S. Sadeghi, 1 J. A. Cumnock, 1 T. Karlsson, 1 P. A. Lindqvist, 1 H. Nilsson, 2 A. Masson, 3 A. Fazakerley, 4 E. Lucek, 5 J. Pickett, 6 and Y. Zhang 7 Received 9 February 2011; revised 17 May 2011; accepted 26 May 2011; published 3 September [1] 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 Alfvénic 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 Alfvénic 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. Citation: Marklund, G. T., et al. (2011), Evolution in space and time of the quasi static acceleration potential of inverted V aurora and its interaction with Alfvénic boundary processes, J. Geophys. Res., 116,, doi: /2011ja Introduction [2] Aurora is a ubiquitous phenomenon occurring on Earth and other solar system planets. The colorful emissions are caused by high energy beams of electrons hitting the upper atmosphere, after being accelerated by quasi static electric fields, aligned with the geomagnetic field and located at altitudes around one Earth radius, or by wave electric fields. The quasi static parallel electric fields and potentials are formed mainly as a result of the requirements 1 Space and Plasma Physics, School of Electrical Engineering, KTH Royal Institute of Technology, Stockholm, Sweden. 2 Swedish Institute of Space Physics, Kiruna, Sweden. 3 European Space Research and Technology Centre, European Space Agency, Noordwijk, Netherlands. Copyright 2011 by the American Geophysical Union /11/2011JA of charge neutrality and current continuity in the high altitude, low density plasma above the aurora. This region is commonly called the auroral acceleration region (AAR), and is typically located between 4000 and km above the polar atmosphere [Paschmann et al., 2003]. The term quasistatic is used to indicate that the structures are stable on a time scale long compared to the transit time of the charged particles. Structures of this kind form and accelerate charged particles producing aurora and plasma outflow not only around Earth but around other solar system planets, such as 4 Mullard Space Science Laboratory, University College London, London, UK. 5 Space and Atmospheric Physics Group, Blackett Laboratory, Imperial College London, London, UK. 6 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA. 7 Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. 1of10

2 Figure 1. Variation of the activity indices AE and AO for a 24 h time period on 5 June The southern auroral oval crossing studied here occurred between 15:30 and 16:00 UT (marked by S) during the early recovery phase of a moderate substorm. Results from another event on the same day but from the northern hemisphere (marked by N), and approximately 100 min later, were presented by Marklund et al. [2011]. Jupiter and Saturn. Thus, the formation and maintenance of such structures are fundamental and ubiquitous processes in space plasmas. [3] The suggestion that electric fields, aligned with Earth s magnetic field, (referred to as parallel electric fields) accelerate particles producing aurora was first made by Alfvén [1958]. Since then it has been confirmed experimentally by numerous spacecraft and rocket measurements. The parallel electric fields often occur together with converging or diverging electric fields, perpendicular to the magnetic field, in U shaped potential structures [Carlqvist and Boström, 1970; Mozer et al., 1980] or with monopolar electric fields in S shaped potential structures [Mizera et al., 1982; Marklund et al., 1997]. Negatively charged potential structures form in the upward current region. These are associated with upward pointing electric fields, accelerating electrons downward, producing intense displays of aurora, and energetic ion beams moving upward, away from Earth. Positively charged potential structures may develop in the adjacent downward current region. These are associated with a downward parallel electric field [Marklund et al., 1994, 1997, 2001], accelerating electrons away from, and ions toward Earth, to energies ranging up to a maximum of a few thousand electron volts. The altitude where this type of acceleration takes place is significantly lower than for the upward current region, and ranges typically between 1000 km and 4000 km [Marklund et al., 1997]. [4] For maintaining the parallel electric fields, various mechanisms have been proposed, such as strong double layers [Block, 1972], weak double layers [Temerin et al., 1982], Alfvén waves [Song and Lysak, 2001], magnetic mirror supported fields [Knight, 1973; Chiu and Schultz, 1978] and anomalous resistivity [Hudson and Mozer, 1978]. Experimental evidence of parallel electric fields has been presented from both sounding rockets [Mozer and Bruston, 1967] and satellite missions, such as Polar [Mozer and Kletzing, 1998] and FAST [Ergun et al., 2000, 2002; Andersson et al., 2002], including the first experimental verification of strong double layers. There are a number of unresolved issues regarding the nature of the parallel electric fields and associated parallel potentials, such as how they are distributed in altitude within the acceleration region and how stable they are in space and time. This is, of course, not possible to determine experimentally from single satellite observations, but require simultaneous multipoint measurements at different altitudes of the acceleration region to be resolved. [5] This became possible when the Cluster orbit perigee was lowered to cover the auroral acceleration region. The altitude distribution of the parallel electric fields has been studied in a numerical simulation of the particle interactions in the upward current region [Ergun et al., 2002]. The parallel electric fields were found to be concentrated in two layers, a relatively stable electron transition layer at lower altitudes, and a more dynamic ion transition layer at higher altitudes, between one and two Earth radii, the latter contributing to the major part of the acceleration potential. Experimental multiprobe studies of the AAR are, however, necessary to reveal its morphology and dynamics. First results from an event study on this topic were presented by Marklund et al. [2011] who used Cluster multiprobe data from the upper and lower part of the AAR to derive a 2 D acceleration potential pattern of a quasi static potential structure, comprising two U shaped and one S shaped structure, which was found to be stable for 5 min or longer. [6] The Cluster data in the Marklund et al. [2011] study were obtained between 16:55 and 17:15 UT on 5 June 2009 above the northern oval (marked by N in Figure 1) in the afternoon magnetic local time sector, roughly 100 min after the event discussed here. [7] The data presented here are from an equatorward Cluster crossing above the southern auroral oval between 15:30 and 16:00 UT (marked by S) around 21 MLT, during the early recovery phase of a moderately intense substorm, as indicated by the geomagnetic disturbance indices AE and AO for 5 June, shown in Figure 1. The Cluster swarm crossed right through the acceleration region of large scale inverted V aurora in the poleward part of a substorm bulge, as verified by imager data from the DMSP F16 satellite, crossing the oval along the dusk to dawn meridian, a few minutes prior to the Cluster crossings. The image, obtained between 15:29 and 15:34 UT, is shown in Figure 2 (left) together with the ionospheric projections of the Cluster and DMSP spacecraft. Note that Cluster crossed above the central, most active part of the bulge and that the DMSP F16 satellite crossed the associated auroral horn, an east west aligned arc extending westward from the bulge, a few minutes prior to the Cluster crossings. Figure 2 (right) gives 2of10

3 Figure 2. (left) UV image (N2 LBH nm) of the evening side aurora taken by the DMSP F16 satellite between 15:29 UT and 15:34 UT. The projected satellite trajectories are indicated by dashed lines (yellow for DMSP F16 at 18 MLT and black for the C1, C3, and C4 spacecraft around 21 MLT). (right) Schematic of the Cluster orbits through the AAR above inverted V aurora between 15:30 and 16:00 UT on 5 June a schematic of the four Cluster trajectories and of the AAR of one inverted V aurora, between 15:30 UT and 16:00 UT on 5 June It can be noted that C3 and C4 had almost equal trajectories, separated in time by roughly two minutes, whereas C1 crossed the bulge roughly 100 km eastward of, and about 4 and 6 min prior to, C4 and C3. [8] The Cluster data presented here were obtained by the EFW electric field instrument [Gustafsson et al., 1997], the FGM fluxgate magnetometer [Balogh et al., 1997], the PEACE electron instrument [Johnstone et al., 1997] and the CIS ion instrument [Rème et al., 1997]. Cluster 2 data are not included in this study. The C2 orbit deviated much from the other three orbits (Figure 1, right) and the C2 observations are thus less likely to be related to the C1, C3 and C4 observations discussed here. 2. Observations [9] Figure 3 presents an overview of the C1 observations above inverted V aurora associated with large scale upward currents within the evening MLT sector of the Southern oval, between 15:30 and 16:00 UT on 5 June The panels show from top to bottom: (1) time energy spectrograms of upgoing electrons; (2) downgoing electrons; (3) upgoing ions; (4) the southward electric field component; (5) the eastward magnetic field component, after subtraction of the background magnetic field; (6) the field aligned current (FAC), calculated from the gradient of the eastward magnetic field component across the current sheet (which from minimum variance analysis was found to be oriented roughly in the magnetic east west direction), using the formula: j k = t n ; (7) the electric potential, derived by integrating the perpendicular (to B) electric field along the satellite orbit; and (8) the negative of the spacecraft potential, indicative of relative plasma density variations. [10] At 15:36 UT, C1 entered the polar cap boundary region (marked by A), characterized by high fluxes of ions and counterstreaming electrons over a broad range of energies peaking at 2 3 kev, interpreted here as Alfvénic particle signatures. The PCB region also reveals quasi static acceleration signatures, such as an upward FAC, enhanced fluxes of accelerated downgoing electrons and an electric potential well extending into the PCB region, but centered outside of it. After this, C1 traversed the acceleration region above a pair of relatively broad inverted V arcs. This is verified by the shapes of the downgoing electron distributions with peak energies of 2 and 3 kev and the shapes of the correlated upgoing ion distributions with peak energies of 8 kev for both structures, coinciding with regions of upward currents and large scale potential wells. In between these structures can be seen a beam of upgoing electrons with a peak energy of 1 kev, carrying a downward current. The electric field is highly structured and irregular, comprising two large scale converging structures, associated with two broad and irregular potential wells or drops, DF?, with minima of 8 kv and 6 kv, coinciding with the centers of the inverted V structures. The estimates of the potential wells were done essentially by eye inspection, by fitting a curve to the ambient potential in the polar cap and subauroral regions, respectively, given that the variation of the fitted ambient potential, over the perpendicular scale size of the structures, is small. The estimates of the potential wells using this approach will be more accurate, the smaller the structures are. For the large scale potential wells presented here, the relative uncertainties are estimated to range between 20% and 30%. The well of the poleward structure is seen to extend into the PCB region, typically by less than 4 kv, whereas the density cavity does not. Summarizing the C1 data, the PCB region is characterized by both Alfvénic and quasi static signatures, and the adjacent large scale Region 1 of upward currents by two very similar inverted V structures on both sides of an upward electron beam, associated with a positive potential structure and a downward current. A major (minor) part of the acceleration potential of 8 kv (2 and 3 kv for the two structures, respectively) was concentrated below (above) the spacecraft orbit. [11] Figure 4 presents an overview of the C3 data using the same format as for the C1 data in Figure 3. The PCB region was entered at 15:42 UT, six minutes lagging behind 3of10

4 Figure 3. Overview of the C1 data between 15:30 and 15:55 UT. Panels show from top to bottom timeenergy spectrograms of upgoing electrons; downgoing electrons; upgoing ions; the southward electric field component; the eastward magnetic field component; the field aligned current (FAC) density; the electric potential, F?, calculated by integrating the perpendicular electric field component along the spacecraft trajectory, the parallel potential drops denoted by DF? ; and the negative of the spacecraft potential, indicative of relative plasma density variations. The polar cap boundary (PCB) region is indicated by the red vertical lines and by the bold letter A (for Alfvénic activity); the Region 1 of upward currents by the vertical red line at 15:37 UT and the vertical black line at 15:45 UT. The large scale variations of the ambient electric potential and spacecraft potential in the regions of interest are represented by the dotted lines. The numbers seen in seventh panel are the potential drops, DF?, in kv from the ambient level. Cluster 1. The C3 observations in the PCB and the inverted V regions are seen to be very different to those of C1. Inverted V distributions of accelerated particles are now seen only in the ion data, implying that the acceleration region is here clearly below the altitude of the C3 spacecraft. The poleward, negative U potential is more intense than that observed by C1, judging from the peak energy of the upgoing ions, reaching 10 kev, and extends more into the Alfvénic PCB region. The 4of10

5 Figure 4. Overview of the data from C3 (entering the PCB at 15:42 UT, 6 min later than C1) using the same format as in Figure 3. equatorward structure, on the other hand, is narrower and less intense, with a peak ion energy of about 4 kev. Also here, the density cavity is seen to coincide with the quasi static upward current region. [12] The acceleration potential estimates inferred from the C3 particle observations are consistent with the estimates of the associated electric potential wells. The poleward potential structure has a minimum well of about 12 kv and the equatorward structure a well of about 4 kv, as compared to 8 kv and 6 kv for C1. The deeper penetration of the potential structure into the PCB region as inferred from the C3 observations is illustrated by the electric potential well which amounts to 8 kv at the PCB interface with the quasistatic region, and by the steep increase in the peak energy of the upgoing ion fluxes, seen close to the interface. The electron beam structure observed by C1 between the two inverted V structures is now gone. [13] An overview of the C4 data is shown in Figure 5. Note that the electric field, magnetic field, and electron data are very similar to the C3 data, which is not surprising considering that the orbits of the C4 and C3 spacecraft were very similar and that the time lag between them was as short as two minutes. The PCB region reveals also here a mixture of Alfvénic and quasi static acceleration signatures. The potential well reaches a maximum value of 13 kv within Region 1 adjacent to the PCB interface, and penetrates deep 5of10

6 Figure 5. Overview of the data from C4 (entering the PCB at 15:40 UT, 4 min later than C1) using the same format as in Figures 3 and 4 but excluding the ion data, which were not considered to be of sufficient quality for this event. into the PCB region, with about 10 kv at the interface. Based on the spacecraft potential data, the density cavity appears to be more pronounced for the C4 crossing than for the C1 and C3 crossings and to coincide with the Region 1 of upward currents. This is more clearly seen in Figure 6, showing plasma density profiles derived from the spacecraft potential data, calibrated to the Cluster Whisper data for each of the C1, C4, and C3 crossings between 15:30 UT and 16:00 UT. Within the PCB region, the density is seen to be locally enhanced, after which it drops abruptly within Region 1. The density cavity coincides with Region 1, the best match obtained for C1 and at the poleward boundary. The equatorward boundary is given by the location of the minimum in the eastward magnetic field component. For C1 this minimum is very well defined at 15:45 UT (see Figure 3.) but for C3 and C4 (compare Figures 4 and 5) the minima in the magnetic field component are much broader, which makes it harder to infer an exact boundary location. Note that the density cavity is the deepest for C4, with a minimum density of 0.1 cm 3, at the same location and time as the deepest potential well of 13 kv, located within Region 1 adjacent to the interface with the PCB region. 3. Discussion [14] Before discussing the results presented here, some brief comments are given on the validity and limitations of the Cluster data for studies of the kind presented here. The Cluster multipoint measurement capability allow temporal and spatial variations to be distinguished and the string ofpearl s/c configuration near perigee allow studies of the temporal evolution of structures, which may provide important clues on their formation and maintenance. After almost ten years of operation of Cluster, it is natural that not all of the instruments are still operating. For example, ion data are no longer available from all spacecraft. Failures on some of the electric field probes certainly imply less accuracy in the electric field data. Despite these limitations, the particle and field data on most of the Cluster spacecraft are still of sufficient quality to enable detailed multipoint studies of the 6of10

7 Figure 6. Plasma density profiles derived from the spacecraft potential and calibrated to the Cluster Whisper data for the C1, C4, and C3 crossings between 15:30 UT and 16:00 UT. The Alfvénic PCB region and Region 1 of upward currents are indicated by the vertical red and black lines, same as in Figures 3, 4, and 5. Note that the density cavity is the deepest for the C4 crossing, reaching a minimum value of 0.1 cm 3, coinciding in space and time with the largest potential well of 13 kv. acceleration region, which has not been done before. The separation between the Cluster spacecraft (minimum separation of 100 km) and the limited resolution of the particle observations do not allow comprehensive studies of the auroral microphysics on small spatial and temporal scales. However, the Cluster data and spacecraft configuration are clearly suitable for studying characteristic properties of largescale aurora with time scales of evolution on the order of several minutes to several ten minutes. [15] As illustrated by the results presented in Figures 3, 4, and 5, the time energy spectrograms of ions and electrons presented here in the PCB region and upward current region are of sufficient quality and resolution to allow identification of quasi static acceleration regions and Alfvénic regions and how these interact, and to derive characteristic energies to infer acceleration potentials. However, the available data and their resolution are not sufficient to reveal which of the mechanism(s) that were responsible for the quasi static parallel potential drops discussed in this study, although the strong candidates are believed to be double layers [Block, 1972] and magnetic mirror supported fields [Knight, 1973]. [16] The C1, C3, and C4 data presented in Figures 3, 4, and 5 are used to infer the acceleration potential pattern of the two inverted V structures. Of particular interest are the variations in space and time of these patterns on the temporal scales given by the spacecraft separations, which ranged between 4 and 2 min. Figure 7 (top) shows the derived acceleration potential patterns for C1, C4, and C3. Figure 7 (middle) shows the time energy ion spectrograms for C1 and C3, and the table in Figure 7 summarizes the parallel potential drops inferred from the downgoing electrons (DF ll above the s/c); the upgoing ions (DF ll below the s/c); and from the electric potential wells (another estimate of DF ll below the s/c); within four different subregions, namely, the PCB region (marked by A), the poleward negative U shaped potential structure (U p), the positive U shaped potential structure (U + ) observed only by C1, and the equatorward negative U potential structure (U E). Figure 7 (top) also illustrates how the distributions of the potential patterns, perpendicular to the geomagnetic field relate: to the PCB region (identified from the Alfvénic particle signatures); the large scale Region 1 of upward currents; the smaller scale currents embedded within Region 1; and the density cavity, inferred from the spacecraft potential data calibrated to Whisper data. Note that the density cavity coincides well with Region 1 for all three spacecraft and that it does not extend into the PCB region, as the potential structures and wells do. The UT time and altitude for entry and exit are indicated at the two ends of the solid black lines, representing the trajectories for each of the spacecraft across the acceleration potential patterns. [17] The acceleration potential patterns shown in Figure 7, reveal two different types of patterns: (1) a symmetric pattern, representative of the C1 observations, characterized by 7of10

8 Figure 7. (top) Schematic summary of the acceleration potentials derived from the C1, C4, and C3 data within the AAR above a pair of inverted V auroras in the large scale upward current region of the southern oval. Figure 7 also indicates how the potential distributions, perpendicular to the geomagnetic field, relate to the PCB region, marked by A (for Alfvénic), the density cavity and the smaller scale FACs within Region 1. (middle) Time energy spectrograms of upgoing ions measured by C1 and C3 but not available by C4 with the same quality. (bottom) The table summarizes the derived acceleration potentials or parallel potential drops, DF ll, (or peak energies/e) in units of kv inferred from the peak energy of the downgoing electrons (DF ll is a measure of the potential drop above the s/c); the electric potential along the spacecraft trajectory ((DF? ) is a measure of the localized drop from the ambient electric potential, which for a U shaped structure should equal the parallel potential drop, DF ll, below the s/c) on C1, C4, and C3; and inferred from the peak energy of the upgoing ions (another estimate of the parallel potential drop, DF ll, below the s/c) on C1 and C3. two roughly equally intense and broad negative potential structures and a positive structure in between, distributed within the large scale Region 1 of upward currents; and (2) an asymmetric distribution, representative of the C4 and C3 observations, where the poleward potential structure is more intense and penetrates deeper into the PCB, and the equatorward structure is less intense and more narrow, as compared to the C1 results. The similarities of the patterns derived for C4 and C3, reflect that the data used for deriving the patterns were very similar, which is not surprising considering that the C4 and C3 orbits were almost identical (Figure 1, left) and the short time lag of 2 min. The only difference is that the total parallel potential was slightly higher for C4 than for C3, 13 kv as compared to 12 kv, and that the associated density cavity was somewhat deeper. [18] At the time of the C1 crossing, the AAR extended both above and below the altitude of the C1 spacecraft. For the two inverted V structures, acceleration potentials of about 2 and 3 kv for the two structures, respectively, was located above and 8 kv below the spacecraft. At the location and time of the C4 and C3 crossings, four and six minutes later, the AAR was totally concentrated below the C4 and C3 orbits, as verified by the absence of accelerated downgoing electrons seen by C4 and C3. [19] The two types of acceleration potential distributions derived from the C1 and C4/C3 data, respectively, presented in Figure 7 (top), could either be explained by an evolution from the symmetric to the asymmetric pattern on the four/six minute time lag between the C1 and the C4/C3 crossings or in terms of two longitudinally separated double arc systems between the C4/C3 and C1 trajectories. The DMSP F16 8of10

9 image shown in Figure 2 suggests that the Cluster swarm passed above a pair of large scale auroral enhancements in the poleward part of the bulge and concentrated within a one hour magnetic local time sector, centered around 21 MLT. The latitudinal and longitudinal scales of the enhancements are somewhat difficult to judge from the image, but they appear to be roughly comparable to, or somewhat larger, than the east west separation between the C1 and C3/C4 trajectories of about 100 km. However, since the image was obtained several minutes prior to the Cluster crossings and since the image resolution is rather limited, we cannot exclude any of the two interpretations as less likely. [20] The positive potential structure associated with the upward electron beam and downward current observed by C1, is likely to be spatially closely tied to the two surrounding inverted V structures. For a structure of this kind, and for the current densities observed here, the life time has been shown to be of the order of a few minutes. This evolution is associated with a broadening of the downward current sheet and a simultaneous reduction of the current density, below the limit where acceleration structures are formed [Marklund et al., 2001]. This scenario applies to the double arc system observed by C1, independent of whether this system extended in longitude beyond the C3/C4 trajectories or not. [21] The most striking difference between the C1 and the C3 data shown in Figures 3 and 5, respectively, is the difference in the peak energy of the upgoing ions, 3 kev (C1) and 10 kev (C3), at the PCB interface with the inverted V region. The poleward potential structure is seen to reach much deeper into the PCB region for C4 and C3, than for C1. This penetration, amounting to 8 kv parallel potential at the interface for C3 and 3 kv for C1, is consistent with the observed peak ion energies at the PCB interface with Region 1, and also with the increasing ion energy between the central and equatorward parts of the PCB region, from about 2 to 3 kev seen by C1, and from 2 to 10 kev seen by C3. [22] Recent results by Hull et al. [2010] indicated that quasistatic field aligned current systems may form out of the dynamic Alfvénic region at the plasma sheet boundary layer. In their event, an equatorward moving PBI became more quasi static with time. In our study, the scenario is quite different. Here the two distinctly different potential patterns and the associated density cavities inferred from the C1 and the C3/C4 data, presented in Figure 7, show that the acceleration potential penetrates into the PCB region, whereas the density cavity does not. For a weak penetration as illustrated by the C1 results, the acceleration acting on the ion population is both Alfvénic and quasi static. For a deeper penetration, as illustrated by the C3/C4 results, the ions will be accelerated up to a maximum energy of 10 kev at the PCB interface with Region 1. The increasing ion energy between the center and the equatorward boundary of the PCB region is consistent with the penetrating acceleration potential. That the density cavity does not extend into the PCB region, but is confined within the quasi static upward current region, illustrates that the depletion is efficient when ions and electrons are accelerated in opposite directions. Within the PCB region, however, the ions and electrons are accelerated both upwards and downward over a broad range of energies, and the associated currents are relatively well balanced over the PCB region (most clearly seen on C3 and C4 and first pointed out by Mende et al. [2003]), which will act so as to prevent cavity formation. 4. Summary and Conclusions [23] Results have been presented from Cluster crossings through the upper acceleration region of auroras, located in the poleward part of the bulge, centered around 21 MLT. The event took place during the early recovery phase of a moderate substorm. The C1, C3, and C4 data all reveal consistent particle and field signatures of two large scale inverted V arcs, distributed within the upward R1 currents of the bulge. The acceleration potentials of the inverted V arcs and their distribution in altitude and geomagnetic latitude relative to the PCB region, Region 1 of upward currents, and the density cavity, are inferred from the Cluster data. The main findings and conclusions are summarized below: [24] 1. Two different kinds of acceleration potential patterns were derived: (1) a symmetric pattern, with two similar negative U potentials and a positive U potential in between, consistent with C1 data; and (2) an asymmetric pattern, with a dominating poleward potential extending deep into the 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. [25] 2. The potential well extended into the PCB region, by about 4 kv for C1 and 8 kv for C3/C4, whereas the density cavity was confined to the Region 1 of upward FACs. This suggests that Alfvénic activity prevents cavity formation, since ions and electrons are accelerated in both directions over a broad energy range, which is consistent with the observation that the associated FACs were roughly balanced in the PCB region. [26] 3. Alfvénic and quasi static acceleration acted jointly in the equatorward part of the PCB region. For C1, the two contributions were about equal, whereas for C3/C4, the quasi static contribution dominated by up to 80% of the maximum ion energy at the interface with Region 1. The distinct increase in the peak ion energy is consistent with the increase of the parallel potential, between the center and the interface with Region 1. [27] 4. The upward electron beam, associated with a positive potential structure and a downward current observed by C1, but not seen by C4/C3, is consistent with the short life time of such a structure, shorter than the time lag between C1 and C4/C3. The evolution is associated with a broadening and a density reduction of the downward current sheet [Marklund et al., 2001], below the critical current density above which parallel electric fields will form. This applies to the double arc system seen by C1 and is independent of the longitudinal extension of the arc system. [28] 5. The maximum (poleward) acceleration potential and the deepest well observed by C4 (C3) of 13 (12) kv was located in Region 1 close to the PCB interface, and coincided with the deepest density cavity, with a minimum value of 0.1 (0.2) cm 3, for C4 (C3). The interface between Region 1 and the PCB region, coinciding with the steep density gradient, appears to be the leading edge of the cavity. 9of10

10 [29] Acknowledgments. The author is grateful to a large number of people who have contributed to the Cluster mission. The Cluster project was supported by the European Space Agency and NASA. Project support has also been provided by a grant from NASA Goddard Space Flight Center to the University of Iowa. This study has been supported by the Swedish National Space Board and by Space and Plasma Physics, School of Electrical Engineering, KTH, Stockholm. [30] Robert Lysak thanks the reviewers for their assistance in evaluating this paper. References Alfvén, H. (1958), On the theory of magnetic storms and auroae, Tellus, 10, , doi: /j tb01991.x. Andersson, L., et al. (2002), Characteristics of parallel electric fields in the downward current region, Phys. Plasmas, 9, , doi: / Balogh, A., et al. (1997), The Cluster magnetic investigation, Space Sci. Rev., 79(1 2), 65 91, doi: /a: Block, L. (1972), Potential double layers in the ionosphere, Cosmic Electrodynamics, 3, Carlqvist, P., and R. 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