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In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge.

Forsyth C, Fazakerley AN, Rae IJ, J Watt CE, Murphy K, Wild JA, Karlsson T, Mutel R, Owen CJ, Ergun R, Masson A, Berthomier M, Donovan E, Frey HU, Matzka J, Stolle C, Zhang Y - J Geophys Res Space Phys (2014)

Bottom Line: We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 4000-7000 km.We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora.The substorm current wedge (SCW) has significant azimuthal structureCurrent sheets within the SCW are north-south alignedThe substructure of the SCW raises questions for the proposed wedgelet scenario.

View Article: PubMed Central - PubMed

Affiliation: Mullard Space Science Laboratory, UCL Dorking, UK.

ABSTRACT

: The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 4000-7000 km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120-240 s after Cluster 4 at 1300-2000 km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven "wedgelets." Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the SCW.

Key points: The substorm current wedge (SCW) has significant azimuthal structureCurrent sheets within the SCW are north-south alignedThe substructure of the SCW raises questions for the proposed wedgelet scenario.

No MeSH data available.


Related in: MedlinePlus

Figure showing the locations of Cluster 1 (black), Cluster 4 (blue), and the ground-based instrumentation used in this study. (a) The spacecraft foot points in geographic latitude and longitude. The crosses and diamonds show the spacecraft locations at 02:30 and 02:45 UT, respectively. The underlying map shows the locations of the ground magnetometer stations (yellow dots) and the fields of view of the RANK and NRSQ all-sky imagers at 02:30 UT. (b and c) The locations of Cluster 1 and Cluster 4 projected on the YZ and YX GSE planes, respectively. (d–g) The spacecraft foot point tracks from Cluster 1, Cluster 2, and Cluster 4 up to 02:28, 02:30, 02:35, and 02:50 UT. Overlaid on the spacecraft track are the magnetic field gradients perpendicular to the spacecraft track and the Tsyganenko and Stern [1996] magnetic field (negative gradients in red, positive gradients in blue). Gradients from Cluster 1 point down the page, and gradients form Cluster 2 and Cluster 4 point up the page. The dayside auroral data are from a single image compiled from DMSP SSUSI data from around 02:40 UT. Also plotted are auroral data from the THEMIS ASI at RANK projected to 110 km altitude. Auroral data are plotted in gray scale with darker colors indicating brighter aurora. An animation incorporating Figures 3d–g is provided in the supporting information.
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fig03: Figure showing the locations of Cluster 1 (black), Cluster 4 (blue), and the ground-based instrumentation used in this study. (a) The spacecraft foot points in geographic latitude and longitude. The crosses and diamonds show the spacecraft locations at 02:30 and 02:45 UT, respectively. The underlying map shows the locations of the ground magnetometer stations (yellow dots) and the fields of view of the RANK and NRSQ all-sky imagers at 02:30 UT. (b and c) The locations of Cluster 1 and Cluster 4 projected on the YZ and YX GSE planes, respectively. (d–g) The spacecraft foot point tracks from Cluster 1, Cluster 2, and Cluster 4 up to 02:28, 02:30, 02:35, and 02:50 UT. Overlaid on the spacecraft track are the magnetic field gradients perpendicular to the spacecraft track and the Tsyganenko and Stern [1996] magnetic field (negative gradients in red, positive gradients in blue). Gradients from Cluster 1 point down the page, and gradients form Cluster 2 and Cluster 4 point up the page. The dayside auroral data are from a single image compiled from DMSP SSUSI data from around 02:40 UT. Also plotted are auroral data from the THEMIS ASI at RANK projected to 110 km altitude. Auroral data are plotted in gray scale with darker colors indicating brighter aurora. An animation incorporating Figures 3d–g is provided in the supporting information.

Mentions: Figure 3a shows the magnetic foot points in geographic latitude and longitude of Cluster 1 (black) and Cluster 4 (blue) along with the locations of magnetometer stations in Canada [Engebretson et al., 1995; Mann et al., 2008; Russell et al., 2008; Peticolas et al., 2008] and Greenland (http://www.space.dtu.dk/English/Research/Scientific_data_and_models/Magnetic_Ground_Stations) along with the fields of view of the THEMIS ASIs at RANK and NRSQ [Mende et al., 2008]. Figures 3b and 3c show the spacecraft locations projected on the GSE YZ and YX planes, respectively. In order to put the spacecraft locations in context, Figures 3d–3g show the spacecraft foot points of Cluster 1, 2, and 4 up to 02:28, 02:30, 02:35, and 02:50 UT, respectively, in MLT and ILAT coordinates. Projecting from the spacecraft foot point paths are the magnitudes of the gradients in the magnetic field perpendicular to the background (T96) [Tsyganenko and Stern, 1996] field and the spacecraft trajectory, corresponding to upward (red) and downward (blue) field-aligned currents. The gradients are calculated from data from the fluxgate magnetometers (FGM) [Balogh et al., 2001] on the spacecraft. Magnetic field gradients shown in Figures 3d–3g from Cluster 2 and Cluster 4 point up the page and magnetic field gradients from Cluster 1 point down the page. The dayside auroral data were obtained around 02:40 UT from a single FUV (N2 Lyman-Birge-Hopfield band short: 140–150 nm) image compiled from F16 Defense Meteorological Satellite Program (DMSP) Special Sensor Ultraviolet Scanning Imager (SSUSI) [Paxton et al., 2002; Zhang et al., 2008]. Also shown is the white light aurora observed by the THEMIS ASI at RANK projected to an altitude of 110 km. Auroral brightness is grey scaled such that darker grey corresponds to more intense auroral emission.


In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge.

Forsyth C, Fazakerley AN, Rae IJ, J Watt CE, Murphy K, Wild JA, Karlsson T, Mutel R, Owen CJ, Ergun R, Masson A, Berthomier M, Donovan E, Frey HU, Matzka J, Stolle C, Zhang Y - J Geophys Res Space Phys (2014)

Figure showing the locations of Cluster 1 (black), Cluster 4 (blue), and the ground-based instrumentation used in this study. (a) The spacecraft foot points in geographic latitude and longitude. The crosses and diamonds show the spacecraft locations at 02:30 and 02:45 UT, respectively. The underlying map shows the locations of the ground magnetometer stations (yellow dots) and the fields of view of the RANK and NRSQ all-sky imagers at 02:30 UT. (b and c) The locations of Cluster 1 and Cluster 4 projected on the YZ and YX GSE planes, respectively. (d–g) The spacecraft foot point tracks from Cluster 1, Cluster 2, and Cluster 4 up to 02:28, 02:30, 02:35, and 02:50 UT. Overlaid on the spacecraft track are the magnetic field gradients perpendicular to the spacecraft track and the Tsyganenko and Stern [1996] magnetic field (negative gradients in red, positive gradients in blue). Gradients from Cluster 1 point down the page, and gradients form Cluster 2 and Cluster 4 point up the page. The dayside auroral data are from a single image compiled from DMSP SSUSI data from around 02:40 UT. Also plotted are auroral data from the THEMIS ASI at RANK projected to 110 km altitude. Auroral data are plotted in gray scale with darker colors indicating brighter aurora. An animation incorporating Figures 3d–g is provided in the supporting information.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4497475&req=5

fig03: Figure showing the locations of Cluster 1 (black), Cluster 4 (blue), and the ground-based instrumentation used in this study. (a) The spacecraft foot points in geographic latitude and longitude. The crosses and diamonds show the spacecraft locations at 02:30 and 02:45 UT, respectively. The underlying map shows the locations of the ground magnetometer stations (yellow dots) and the fields of view of the RANK and NRSQ all-sky imagers at 02:30 UT. (b and c) The locations of Cluster 1 and Cluster 4 projected on the YZ and YX GSE planes, respectively. (d–g) The spacecraft foot point tracks from Cluster 1, Cluster 2, and Cluster 4 up to 02:28, 02:30, 02:35, and 02:50 UT. Overlaid on the spacecraft track are the magnetic field gradients perpendicular to the spacecraft track and the Tsyganenko and Stern [1996] magnetic field (negative gradients in red, positive gradients in blue). Gradients from Cluster 1 point down the page, and gradients form Cluster 2 and Cluster 4 point up the page. The dayside auroral data are from a single image compiled from DMSP SSUSI data from around 02:40 UT. Also plotted are auroral data from the THEMIS ASI at RANK projected to 110 km altitude. Auroral data are plotted in gray scale with darker colors indicating brighter aurora. An animation incorporating Figures 3d–g is provided in the supporting information.
Mentions: Figure 3a shows the magnetic foot points in geographic latitude and longitude of Cluster 1 (black) and Cluster 4 (blue) along with the locations of magnetometer stations in Canada [Engebretson et al., 1995; Mann et al., 2008; Russell et al., 2008; Peticolas et al., 2008] and Greenland (http://www.space.dtu.dk/English/Research/Scientific_data_and_models/Magnetic_Ground_Stations) along with the fields of view of the THEMIS ASIs at RANK and NRSQ [Mende et al., 2008]. Figures 3b and 3c show the spacecraft locations projected on the GSE YZ and YX planes, respectively. In order to put the spacecraft locations in context, Figures 3d–3g show the spacecraft foot points of Cluster 1, 2, and 4 up to 02:28, 02:30, 02:35, and 02:50 UT, respectively, in MLT and ILAT coordinates. Projecting from the spacecraft foot point paths are the magnitudes of the gradients in the magnetic field perpendicular to the background (T96) [Tsyganenko and Stern, 1996] field and the spacecraft trajectory, corresponding to upward (red) and downward (blue) field-aligned currents. The gradients are calculated from data from the fluxgate magnetometers (FGM) [Balogh et al., 2001] on the spacecraft. Magnetic field gradients shown in Figures 3d–3g from Cluster 2 and Cluster 4 point up the page and magnetic field gradients from Cluster 1 point down the page. The dayside auroral data were obtained around 02:40 UT from a single FUV (N2 Lyman-Birge-Hopfield band short: 140–150 nm) image compiled from F16 Defense Meteorological Satellite Program (DMSP) Special Sensor Ultraviolet Scanning Imager (SSUSI) [Paxton et al., 2002; Zhang et al., 2008]. Also shown is the white light aurora observed by the THEMIS ASI at RANK projected to an altitude of 110 km. Auroral brightness is grey scaled such that darker grey corresponds to more intense auroral emission.

Bottom Line: We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 4000-7000 km.We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora.The substorm current wedge (SCW) has significant azimuthal structureCurrent sheets within the SCW are north-south alignedThe substructure of the SCW raises questions for the proposed wedgelet scenario.

View Article: PubMed Central - PubMed

Affiliation: Mullard Space Science Laboratory, UCL Dorking, UK.

ABSTRACT

: The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 4000-7000 km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120-240 s after Cluster 4 at 1300-2000 km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven "wedgelets." Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the SCW.

Key points: The substorm current wedge (SCW) has significant azimuthal structureCurrent sheets within the SCW are north-south alignedThe substructure of the SCW raises questions for the proposed wedgelet scenario.

No MeSH data available.


Related in: MedlinePlus