from the analysis of VLF whistler wave data. The outer surface of the plasmasphere is generally characterized by a sharp decrease of the plasma density: the plasmapause.

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Plasmasphere and Plasmapause

12.7 Plasmasphere and P lasmapause

Viviane Pierrard*

12. 7.1 Introduction

The plasmasphere is the extension of the ionosphere at higher altitudes. This roughly toroidal region of the mag- netosphere

surro

unds the Earth at low and middle lati- tudes. It contains a relatively dense plasma of low-energy (typically 1 eV) electrons and protons

,

with also a small

amount of helium ions. It was discovered in 1963

from the analysis of VLF whistler wave data. The outer surface of the plasmasphere is generally characterized by a sharp decrease of the plasma density: the plasmapause.

Whistlers and in situ satellite observations show that the plasmapause forms closer to Earth when the geomagnetic activity level is enhanced. Depending on the strength of the level of geomagnetic activity, the equatorial position of the observed plasmapause knee changes from 7

RE

to

2.5 RE.

The position of the plasmapause is due to the interplay between the electric field associated with the Earth's rota- tion (that dominates at small radial distances) and the convection electric field associated with geomagnetic activity (that dominates at larger distance) (Lemaire and Pierrard,

2008).

The depleted region beyond the plasma- pause is called the plasma trough

.

12.7.2 Recent Progress with Observations

Recent spacecraft observations have revolutionized our under- standing of the plasmasphere. Especially, IMAGE/EUV pro- vided from

2000

to

2006

the first global images of the plasmasphere in the equatorial plane when the spacecraft was above the North Pole (Goldstein et al.,

2005;

Darrouzet et al.,

2009).

These images allowed magnetic local time (ML

T)

ana- lyses of the plasmapause evolution with time (Pierrard and Cabrera,

2006).

With sensitive EUV cameras, IMAGE pro- vided global views of the plasmasphere by imaging the distribu- tion of He

+

ions in its

30.4

nm resonance line integrated along the line of sight. An example is illustrated in Figure

12.

7.

I.

New structures like plumes, notches and shoulders have been detected. More recently, the first global meridian images of the plasmasphere were obtained with TEX instrument on board KAGUY A and provided useful information on physical processes of plasmapause formation (Murakami et al.,

2016).

Moreover, many spacecraft determine the plasma density along their orbit and show in situ spatial and temporal structures inside and outside the plasmasphere and the evo- lution of the plasmapause positions. The four CLUSTER spacecraft make multipoint measurements and the observa- tions of the instrument WHISPER (that determines the low-

175

Figure 12.7.1 The global plasmasphere seen by EUV/IMAGE on 22 June 2001 at 0519 UT (when the satellite was above the North Pole and projected in the geomagnetic equatorial plane). The irregular line corresponds to the threshold of 40% of the maximum luminosity and illustrates the plasma pause position. The circles correspond to the Earth's surface and L = 2, 4, 6 and 8. (A black- and-white version of this figure appears in some formats. For the colour version, please refer to the plate section.)

energy plasmaspheric density) could be complemented by measurements of higher energy instruments to analyze the links between the plasmapause and the radiation belts boundaries (Darrouzet et al.,

2013).

Data from Van Allen Probes also showed the influence of the plasmasphere to limit the ultra-relativistic electrons of cosmic rays and solar origin from reaching low altitudes (Baker et al.,

2014).

Simultaneous THEMIS observations are also available to complement the plasmaspheric observations (Bandic et al.,

2017).

Statistical studies have been made to determine the dependence of the plasmapause positions on solar and geo- magnetic indices (Verbanac et al.,

2015,

with Cluster; Bandic et al.,

2016,

with CRRES for instance) and the thickness of the plasmapause boundary layer (Kotova et al.,

2018,

with MAGION). Empirical relations between the plasmapause equatorial distance and a variety of geomagnetic indices like

Kp, Dst or AE have been deduced as well as their dependence

and propagation in ML T.

* Y. Pierrard thanks the Solar-Terrestrial Center of Excellence and the Royal Belgian Institute for Space Aeronomy.

M., Yau, A., and Petrovský, E., doi:10.1017/9781108290135, Cambridge University Press, Cambridge, UK, 2019.

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176 The Magnetosphere

2015-3-16 2015-3-17 2015-3-18

~~~ ~-~

Figure 12. 7 .2 Density of the electrons in the

equatorial (left) and meridian plane (right) obtained with the SPM plasmaspheric model (Pierrard and Voiculescu, 2011) for the date of 17 March 2015 at 1300 UT, during a geomagnetic storm. The plasmasphere corresponds to the dark region with higher density, the plasmapause is illustrated by the diamonds and the low-density plasma beyond the plasma pause corresponds to the plasma trough. The upper panel shows the planetary geomagnetic activity index of Bartels Kp observed from 16 to 18 March 2015. (A black- and-white version of this figure appears in some formats. For the colour version, please refer to the plate section.)

1 2 3 4

10 -10

5

Equatorial plane

5 0 -5

days

-10 -10

-5 10 4 2 O 0 - '0 -21

-4 5 10

2015-3-17 13.0 UT

5 0 -5 -10

4 2 0 -•~2

-4

5 0 -5 -10

Meridian plane 10 ... ~~_,_.

10 5 0

~_,_.~~ ... 10 -5

10

-10

100 Electron Density (cm-3)

12.7.3 Recent Progress in Modeling

In addition to empirical models, more physics-based models have been developed (Pierrard et al.,

2009,

for a review). Recent progress in plasmasphere modeling was obtained by the development of three-dimensional dynamic models taking into account the influence of the geomagnetic activity. The highly dynamic region of the plasmasphere is disturbed during geomagnetic storms and substorms, with formation of a sharp plasmapause closer to the Earth and the generation of a plume in the afternoon MLT sector rotating with the Earth. The position of the plasmapause, the limit of the plasma- sphere, is highly dynamic and depends on the level of geomagnetic activity. During quiet times

,

the plasma- pause is located further from the Earth and the iono- sphere refills the plasmasphere in a few days typically.

Figure

12. 7 .2

illustrates an example of the plasmasphere electron density as obtained from the SWIFF (Space Weather Integrated Forecasting Framework) Plasmasphere Model (SPM) (Pierrard and Voiculescu,

2011)

for the date of

17

March

2015

during a geomagnetic storm. During mag- netic storms, the plasmasphere is eroded and structures like plasma plumes and channels can appear. Such a plume is clearly visible on the figure. This model is available for free run at any chosen date on the space weather portal

1000

(www.spaceweather.eu) and is now provided also in real time on ESA website of SSA (Space Situational Awareness) at http://swe .ssa.esa.int/space-radiation. It provides the time variations of plasmapause position in the geomagnetic and meridian planes, the density of electrons and ions inside and outside the plasmasphere and the temperature of the particles and is coupled with the ionosphere to be able to simulate refilling processes after storms (Pierrard and Voiculescu,

2011).

The density and temperature in the ionosphere are modulated by day/night activity. Different waves circulate inside and outside the plasmasphere and also along the plas- mapause, so that such a model of the plasmasphere is useful also to determine its influence on the radiation belts dynamics.

The results of the simulations have been compared with

various satellite observations to determine the most relevant

physical mechanisms of plasma pause formation and the most

appropriate electric field configurations depending on geo-

magnetic activity (Lemaire and Pierrard,

2008;

Pierrard et al.

, 2008,

Verbanac et al.,

2018).

As predicted by quasi-inter-

change mechanism of plasma pause formation used in SPM

model

,

the plasmapause positions at the post-midnight

observed from the meridian perspective by KAGUYA

clearly show a plasmapause fo1mation first near the equator-

ial region and in the post-midnight sector.

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radiation belts within the same volume of space near Earth (NASA/Troy Benesch).

Figure 12.7.1 The global plasmasphere seen by EUV/IMAGE on 22 June 200 I at 0519 UT (when the satellite was above the North Pole and projected in the geomagnetic equatorial plane). The irregular line corresponds to the threshold of 40% of the maximum luminosity and illustrates the plasma pause position. The circles correspond to the Earth's surface and L = 2, 4, 6 and 8.

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2015-3-16 2015-3-17 2015-3-18

· l ~

¹ ² ³ ⁴

days Equatorial plane

10 5 0 -5 -10

-10f

3

^' ^1-¹⁰

2015-3-17 13.0 UT fiit7

10 5 0 -5

-S r ~ -s r ,__

i-5

4 2

01 •~ •- J ~o1

-2

sr ~ :,.,...·sr

~ i5 -4

10 5 0 -5

Meridian plane

10 10

10 5 0 -5 -10

10 100

Electron Density (cm- 3)

1000 -10

4 2 0 - ~2

-4 -10

Figure 12.7.2 Density of the electrons in the equatorial (left) and meridian plane (right) obtained with the SPM plasmaspheric model (Pierrard and Voiculescu, 2011) for the date of 17 March 2015 at 1300 UT, during a geomagnetic storm. The plasmasphere corresponds to the dark region with higher density, the plasmapause is illustrated by the diamonds and the low-density plasma beyond the plasmapause corresponds to the plasma trough. The upper panel shows the planetary geomagnetic activity index of Bartels Kp observed from 16 to 18 March 2015.

Dst index (blue, nT) and 10.7 cm radio flux (red, 10- 22 Jlstm2/Hz)

•oo- - ~ - - - ~ - - - ~ - - - ~ ~

300 200 100

-400 -500

-600~ - ~ - - - ~ - - - ~ - - - ~ - - - ~ - - - , - ~ ~ 2010

1960 1970

• Ost (blue) since 1957, hourly index based on horizontal field at Hermanus, Kakioka, Honolulu, San Juan

• SYM-H (green) established in 1981 is like Dst, but more observatories (always Honolulu, Memambetsu +4 others), different baseline calculation - evaluated at 1 minute intervals.

1980

1'-

k

A

~

1990 2000

Year

1957-2006 Hourly Dst Index, 1999 SYM-H 10'

10•

10' 10'

10' 10-1 10-2

10-3 10-•

10-• 10-s 10-2 10-1 100 101 102 10s Frequency days-I

Figure 13.1.2 (a) Hourly Dst index variations from 1957 on compared with I 0.7 cm radio flux. (b) the power spectrum of Dst from 1957 to 2006 and of SYM-H from 1999. SYM-H has I min sampling.