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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.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.

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 the ground magnetic field perturbations due to the SCW calculated using the technique of Murphy et al. [2012] in MLT and invariant latitude coordinates. (a and b) The deflection of the H component of the magnetic field at 02:34 and 02:36:30 UT. (c and d) show the deflection in the D component of the magnetic field at 02:34 and 02:36:30 UT. The asterisks show the magnetic foot point of Cluster 4 at 02:34 UT, and the crosses show the magnetic foot point of Cluster 1 at 02:36:30 UT. (e) A summary of the interpretation of the currents from the magnetic data in terms of a simple line current model of the substorm current wedge. The foot point paths of Cluster 1 and 4 are shown in black and green, respectively. During this interval, the spacecraft foot points moved along the westward electrojet.
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fig02: Figure showing the locations of the ground magnetic field perturbations due to the SCW calculated using the technique of Murphy et al. [2012] in MLT and invariant latitude coordinates. (a and b) The deflection of the H component of the magnetic field at 02:34 and 02:36:30 UT. (c and d) show the deflection in the D component of the magnetic field at 02:34 and 02:36:30 UT. The asterisks show the magnetic foot point of Cluster 4 at 02:34 UT, and the crosses show the magnetic foot point of Cluster 1 at 02:36:30 UT. (e) A summary of the interpretation of the currents from the magnetic data in terms of a simple line current model of the substorm current wedge. The foot point paths of Cluster 1 and 4 are shown in black and green, respectively. During this interval, the spacecraft foot points moved along the westward electrojet.

Mentions: A two-dimensional map of the magnetic field perturbations due to the SCW ionospheric currents can only be estimated using a network of latitudinally and longitudinally spaced ground-based magnetometers. For an accurate determination of the SCW currents, a baseline quiet time preceding the interval in question is found. Deviations from this time are assumed to represent deviations due to new currents systems or enhancements to existing current systems. For this event we use a baseline time of 02:15 UT. The three components of the magnetometer data set are interpolated onto a constant spatial grid to generate a map of magnetic field perturbations as a function of time [Murphy et al., 2012]. In this study we use data from the following magnetometer arrays: Canadian Array for Realtime Investigations of Magnetic Activity (CARISMA) [Mann et al., 2008], THEMIS ground-based observatories and education and outreach program [Russell et al., 2008; Peticolas et al., 2008], midcontinent magnetoseismic chain [Chi et al., 2013], Magnetometer Array for Cusp and Cleft Studies [Engebretson et al., 1995], INTERMAGNET (http:\www.intermanget.org), and Greenland magnetometer chain (http://www.space.dtu.dk/English/Research/Scientific_data_and_models/Magnetic_Ground_Stations.aspx). Figures 2a and 2b show maps of the deflection of the H components of the magnetic field, and Figures 2c and 2d show the D components of the magnetic field at 02:34 (Figures 2a and 2c) and 02:36:30 UT (Figures 2b and 2d). These maps are presented on a grid of MLT and invariant latitude. Figure 2(green crosses) shows the foot points of C1, and Figure 2(green asterisks) show the foot points of C3 at the times of the plots, showing that the spacecraft foot points moved along the westward electrojet.


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 the ground magnetic field perturbations due to the SCW calculated using the technique of Murphy et al. [2012] in MLT and invariant latitude coordinates. (a and b) The deflection of the H component of the magnetic field at 02:34 and 02:36:30 UT. (c and d) show the deflection in the D component of the magnetic field at 02:34 and 02:36:30 UT. The asterisks show the magnetic foot point of Cluster 4 at 02:34 UT, and the crosses show the magnetic foot point of Cluster 1 at 02:36:30 UT. (e) A summary of the interpretation of the currents from the magnetic data in terms of a simple line current model of the substorm current wedge. The foot point paths of Cluster 1 and 4 are shown in black and green, respectively. During this interval, the spacecraft foot points moved along the westward electrojet.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig02: Figure showing the locations of the ground magnetic field perturbations due to the SCW calculated using the technique of Murphy et al. [2012] in MLT and invariant latitude coordinates. (a and b) The deflection of the H component of the magnetic field at 02:34 and 02:36:30 UT. (c and d) show the deflection in the D component of the magnetic field at 02:34 and 02:36:30 UT. The asterisks show the magnetic foot point of Cluster 4 at 02:34 UT, and the crosses show the magnetic foot point of Cluster 1 at 02:36:30 UT. (e) A summary of the interpretation of the currents from the magnetic data in terms of a simple line current model of the substorm current wedge. The foot point paths of Cluster 1 and 4 are shown in black and green, respectively. During this interval, the spacecraft foot points moved along the westward electrojet.
Mentions: A two-dimensional map of the magnetic field perturbations due to the SCW ionospheric currents can only be estimated using a network of latitudinally and longitudinally spaced ground-based magnetometers. For an accurate determination of the SCW currents, a baseline quiet time preceding the interval in question is found. Deviations from this time are assumed to represent deviations due to new currents systems or enhancements to existing current systems. For this event we use a baseline time of 02:15 UT. The three components of the magnetometer data set are interpolated onto a constant spatial grid to generate a map of magnetic field perturbations as a function of time [Murphy et al., 2012]. In this study we use data from the following magnetometer arrays: Canadian Array for Realtime Investigations of Magnetic Activity (CARISMA) [Mann et al., 2008], THEMIS ground-based observatories and education and outreach program [Russell et al., 2008; Peticolas et al., 2008], midcontinent magnetoseismic chain [Chi et al., 2013], Magnetometer Array for Cusp and Cleft Studies [Engebretson et al., 1995], INTERMAGNET (http:\www.intermanget.org), and Greenland magnetometer chain (http://www.space.dtu.dk/English/Research/Scientific_data_and_models/Magnetic_Ground_Stations.aspx). Figures 2a and 2b show maps of the deflection of the H components of the magnetic field, and Figures 2c and 2d show the D components of the magnetic field at 02:34 (Figures 2a and 2c) and 02:36:30 UT (Figures 2b and 2d). These maps are presented on a grid of MLT and invariant latitude. Figure 2(green crosses) shows the foot points of C1, and Figure 2(green asterisks) show the foot points of C3 at the times of the plots, showing that the spacecraft foot points moved along the westward electrojet.

Bottom Line: We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 4000-7000 km.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.

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