1. Low-latitude plasma blobs in the topside ionosphere as observed by the Swarm satellites
Jaeheung Park1, Hermann Lühr1, Claudia Stolle1,
Stephan Buchert2, Reine Gill2, Jose Merayo3, Peter Brauer3
1Helmholtz Centre Potsdam, GFZ, German Research Centre for Geosciences, Potsdam, Germany
2Swedish Institute of Space Physics, Uppsala, Sweden
3Technical University of Denmark, Copenhagen, Denmark
The 3rd International Swarm Science Meeting in Copenhagen
(17:10-17:30 on 19 June 2014)

2. I. Introduction 1: What can we get from Swarm observations?

3. Until now, plasma density measurements onboard satellites were mostly one-dimensional.
CHAMP

4. Using plasma density observations during the early commissioning phase of Swarm, we can get horizontal 2D images of plasma irregularities.
Swarm-C
Swarm-A
blob
horizontal plane
Swarm-B

5. In addition, we can get more information about the (nearly) vertical structures from the magnetic field perturbation. o
FAC sheet
FAC sheet x db o x
blob α
Swarm
blob
horizontal plane β o x
magnetic east
Higher L-shell
blob o x
plane perpendicular to B

6. Ionospheric currents diverge at plasma irregularity walls, and the resultant current circuit generates magnetic perturbation.
For example, if a plasma irregularity near the equator has a vertically elongated structure, the magnetic field perturbation due to the irregularity also points to the vertical directions.
Vertical magnetic deflection
Gravity-driven/Pedersen eastward current
BUBBLE /BLOB
Field-aligned current
Ambient B-field X

7. If low-latitude plasma density irregularity is tilted,
the magnetic field perturbation due to the irregularity is also tilted.
Hence, from the magnetic field data we can get information on the vertical (not horizontal) tilt of plasma density irregularities near the equator.
magnetic deflection tilted from the meridian
Ambient B-field X
BUBBLE / BLOB
Gravity-driven/Pedersen eastward current
Gravity-driven/Pedersen eastward current
Field-aligned current

8. By combining the plasma density and magnetic field data (i. e
By combining the plasma density and magnetic field data (i.e., using β and α) we can deduce the three-dimensional structure of plasma irregularities. o
FAC sheet
FAC sheet x db o x
blob α
Swarm
blob
horizontal plane β o x
magnetic east
Higher L-shell
blob o x
plane perpendicular to B

9. II. Introduction 2: bubbles and blobs

10. (NOTE) in this presentation we only focus on the low-latitude ionosphere.

11. There are two kinds of plasma irregularity in the low-latitude ionosphere.
Bubbles represent plasma density depletion.
Blobs represent plasma density enhancement.

12. Relatively many things are known about the 3D structure of bubbles, thanks to optical and radio imaging from space and from ground.

13. Bubble shell structure
(horizontal plane)
(vertical plane)
(3D structure) = +
Plasma depletion shell model
Inverted-C structure as observed by the TIMED/GUVI
Westward tilt of bubbles in equatorial radar images
Kil et al. [JGR, 2009]
Hysell et al. [GRL, 2005]
Kil et al. [JGR, 2009]

14. Motivation of this study - blob shell structure?
(horizontal plane)
(quasi-vertical plane)
(3D structure)
There has been no report on blob signatures in the TIMED/GUVI images: possibly due to smaller spatial scale and weaker amplitude of blobs than of bubbles.
Ground-based nm blob images were reported by Pimenta et al. [2004]. However, they did not discuss the blob tilt in detail.
Field-aligned structure of blobs was suggested based on the satellite conjunction events [Park et al., 2008].
Westward tilt of blobs (on the perpendicular plane) was suggested based on FAC sheet tilt angles, but only for 2 events [Park et al., 2010]. = +
Plasma blob shell model???
Are blobs tilted westward or eastward?
Kil et al. [JGR, 2009]

15. III. Swarm observations

16. Blob tilt in the horizontal plane
Westernmost satellite (Charlie)
Blob tilt in the horizontal plane
The three Swarm satellites observed bubbles within 2.5 minutes in NH.
The cross-correlation among the three blobs is very high.
The whole blob structure seems to be elongated from SE to NW.  The tilt angle is about 57o (in the direction from east towards south)
Center satellite (Alpha)
Easternmost satellite (Bravo)
LOWER-LATITUDE
WEST
HIGHER-LATITUDE
EAST

17. Blob tilt in the perpendicular plane
The Swarm-C satellite observed perpendicular deflection of geomagnetic field around the blob.
The zonal and meridional components are highly anti-correlated with comparable magnitudes.
The anti-correlation implies that the FAC sheets are elongated from in-E to out-W.  The tilt angle is about 41o (in the direction from east to inwards)

18. Hence, the three-dimensional structure of plasma blobs may be summarized by the following cartoon.
FAC sheet
FAC sheet x db o x
blob α
Swarm
blob
horizontal plane β o x
magnetic east
Higher L-shell
blob o x
plane perpendicular to B

19. Conclusion
From the early commissioning phase data of Swarm we can deduce three-dimensional structure of plasma irregularities.
Plasma density  2D image on the horizontal plane
Magnetic field  tilt angle on the vertical plane
A typical example of Swarm observations shows that the blobs are tilted westward on both the horizontal and vertical planes, just as bubbles are.