Quasi-persistent feature of highly
structured field-aligned currents in theduskside auroral oval:
Conjugate observation by Swarmsatellites and ground all-sky imager
Yoshihiro Yokoyama1, Satoshi Taguchi1, 2,
Toshihiko Iyemori2 and Keisuke Hosokawa3, 4
1Department of Geophysics, Graduate School of Science, Kyoto University
2Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science,

Kyoto University,
3Department of Communication Engineering and Informatics, University of Electro-Communications

4Center for Space Science and Radio Engineering, University of Electro-Communications

Introduction: FAC in duskside aurora oval

[Iijima et al.,1978][Iijima and Potemra, 1976b]

Statistical distribution of FAC

Strongest upward current density is
located near 1500 MLT
150eV Electrons precipitations also
have peakIntroduction: small-scale magnetic perturbations
Prolonged quiet times→Region 1 almost diminishes
except its near-noon part
Small-scale magnetic perturbations existat latitudes where Region 1 disappeared
Still uncertain
those small-scale magnetic perturbations
reflect a simple remnant of the Region 1orphenomena that can be prominent especiallyduring strongly northward IMF,.

Introduction: Multiple discreate arc
557.7nmDayside distribution ofauroral occurrence/emissions
shows a peak at around 1500 MLT
Multiple discrete arcs found to be
typically displayed
between 1430 and 1800 MLT[Hu et al., 2009][Moen et al ., 1994]

Multiple auroral arc on sunward convection
life time: 2~5 min
width : 10~20km
MLT1400-1500
1.Magnetosheath plasma injection
2.Kinetic Alfven wave
Magnetopause surface wave(KHI, Pressure wave)

Purpose
Using Swarm’s observation data:Focus on spatial structure of mesoscale field aligned currents
in associate with aurora arc.
Try to reveal features of meso-scale FACs
in the dusk sector where large-scale Region 1/2 current system diminishes .
DataOrbit : polar orbit
Swarm satellites(Swarm A and C)
Altitude : about 460km
Magnetic data : 1Hz→E-W component (subtracted IGRF)
Current density : 1Hz→calculated from magnetic perturbations
For simultaneous observation events
The All Sky Imager(Longyearbyen, Svalbard)
630.0nm
DMSP
Energy flux data of electron and ion

Swarm A and C have been flying side byside with about 10s differenceMethodEvent selection
Target events:
Mesoscale currents are prominent and multiple
Large-scale R1 currents are diminishedMeso-scale field aligned current
Spatial scale ~10 to 100km (Hasunuma et al., 2008)
Magnetic perturbation
Period with several seconds to tens of seconds
Event search
November 2014 ~ January 2015
Dusk part of auroral oval : 1400MLT~1800MLTIMF Bz is positive and stable(at least 30 minutes before events).
→2 simultaneous observation events

(a)

(b)

Event 1: 17 January 2015

C

Event 2: 22 December 20147DDD

1414

1D'(. − '*+,5D26'( − '*+,- D L
5D265!"⁄#$ ]7DCF77D2)(32 14

14:43:20

14:43:40

14:44:00'(. − '*+,- D L
5D26$]⁄#5!"'( −'*+,- D L
5D265!"⁄#$ ]((:)

A- D L5!"⁄#$ ]((UT(hh:mm:ss)14:43:20
Mlat(SW-A) 79.42
MLT(SW-A) 16.85
0 7 1478.84
0 2 1416.85

14:43:40
78.18
16.70
77.59
16.72

14:44:00
76.93
16.57
76.33
16.60

14:44:20
75.65
16.46
75.05
16.49

14:44:40
74.37
16.38
73.80
16.40

14:44:20

14:44:40

Event 2: Precipitation particle data from DMSP satellite
22 December 2014

magnetic field lines along which upward FAC flows connect to LLBL

Results: Quasi-persistent feature of multiple arcs0()

1)

57 68

 234(mesoscale aurora continued to exist at least during approximately 30 minResults: Quasi-persistent feature of multiple arcs)0( (13(42025 2







2(( (Arc 1 was defined well during 14:07:32 – 15:38:05 UT(more than 90 min).
(Results : horizontal width of mesoscale upward FACs


2015/01/17"#⁄$ 2&'(()*+ ,-.+ℎ= 123)(45+-1* +-$) ×[35+)88-+)3 4)813-+9]×;13<Conceptual figure of multiple mesoscale FACs

αangle between MLT meridian and orbit

6 1 72

Results : horizontal width of mesoscale upward FACs((())

-8 3 5 0 51 46

Typical size of the upward FAC region is 20 – 30 km.

Discussion: Magnetospheric footprints of auroral regionsTsyganenko-96
IGRF-12.20 80 85 013.2

13 -((1 61 619AC6)(4 0)()( (()Discussion: Possible configuration ofthe source for the mesoscale FAC))! ≈ #$%&! ≈ '$%&

Conclusion 

1. Small-scale magnetic perturbations are
due to the mesoscale structure of quasi-static FACs, not dynamic Alfven waves.
Typical horizontal scale of the mesoscale upward FACs is 20 – 30 km.
2. Each region of the mesoscale upward FACs
correspond to relatively strong 630-nm aurora emissions.
Aurora structures showed quasi-persistence;existed stably during at least 30 min (17 January 2014) ;~90 min(22 December 2015).
→highly-structured FACs have quasi-persistent features.
3. Precipitating particle data from DMSP satelliteindicate indicatesthat the source of the precipitating particles is LLBL.
Tsyganenko magnetic field line model also suggeststhat ionospheric regions of the enhanced auroras are connected to duskside LLBL.
Quasi-persistent multiple FACs arephenomena that are pertinent to the magnetosphere for strongly northward IMF condition.