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Solène Lejosne and Forrest MozerUniversity of California, Berkeley

What Can Be Learned from the Van Allen Probe Measurements of the Electric Drift in the Inner

Magnetosphere?

Acknowledgments: Jack Vernetti & UCB team, RBSP team

solene@ssl.berkeley.edu

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What I’m going to talk about…

2. Ground–truth measurements (!)

1. Van Allen Probes = accurate enough to

characterize electric drift inside L~3 (!)

3. Indication of a small but persistent subcorotation (?!)

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The Van Allen Probes (RBSP A and RBSP B)

Apogee: 5.8 Perigee: 600+ kmPeriod: 9 hInclination: Full Local Time coverage: 2- years

5.8

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Electric field and electric drift measurements

Pictures from the 1st double probe DC electric field measurements, in 1966 [Mozer, 2016]

Electric field measurements = challenging

That is particularly true closest to Earth:

𝐄 (measured )=𝐄 ( inertial )+𝐕𝐒𝐂×𝐁

Electric Field Measured

SpacecraftVelocity

Electric Field of Interest

Magnetic Field

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First Results

1) Database of Electric Drift L < 3

• 2+ years (Oct12 - Dec14)

• 2 probes

• Spin-period averaged data (~12 s)

2) Maps of Typical Electric Drifts L < 3

• Function of different parameters (L,MLT, geographic longitude, UT, ...)

(km

.s-1)

Azimuthal Electric Drifts

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Electric Drift Shell Splitting in the Inner Belt (1/2)The electron intensity at L=1.3 suggests a distortion of the drift shell by a dawn-dusk electric field

Equa

toria

l e- i

nten

sity

MLT∈ [ 4 ;6 ]

MLT∈ [ 20 ;22 ]

L~1.3

(adapted from [Selesnick et al., 2016])

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Electric Drift Shell Splitting in the Inner Belt (2/2)The measurements confirm the existence of an electric drift shell splitting, of a few tens of km (!)

From Selesnick et al. [2016]From measurements

Tracking drift trajectories of guiding centers

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Subcorotation of the Plasmasphere (1/2)The tracking of plasmaspheric notches (IMAGE-EUV) indicates the existence of a corotational lag at L~2.5

(adapted from [Galvan et al., 2010])

L~2.5C

orot

atio

n Fa

ctor

Van Allen Probe Medians

Quartiles

Medians

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Subcorotation of the Plasmasphere (2/2)The measurements show that the electric drift is typically smaller than corotation inside L ~ 2.5+

L

Supercorotation

Subcorotation

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Mapping along field lines to the ionosphereThe subcorotation of the plasmasphere leads to difficulties when mapping the electric drift up to the ionosphere

Similar to average plasma drifts over

Arecibo

Simple Mapping

L~1.4 Mapping with Subcorotation

East

war

d Ve

loci

ty

(m.s

-1)

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Thank you for your attention!

Database of electric drift measurements inside L~3

Reference for modeling

Reference for case studies

Reference for ionosphere magnetospherestudies