ASEN 5335 Aerospace Environments -- Magnetospheres 1 PARTICLES IN THE MAGNETOSPHERE The plasmasphere represents the relatively cold ionospheric plasma.

ASEN 5335 Aerospace Environments -- Magnetospheres 1

PARTICLES IN THE MAGNETOSPHERE

The plasmasphere represents the relatively cold ionospheric plasma (~ .3 eV or T ~ 2000 K) which is co-rotating with the earth (frictional coupling).

We have discussed the radiation belts extensively, and the plasma sheet to some extent. We will return to the plasma sheet when discussing magnetic storms.

-- plasmasphere -- ring current

-- radiation belts -- plasma sheet

-- boundary layers (magnetosheath, mantle)

-- polar wind

The main particle populations are:


Principal Plasma Populations in Earth’s Magnetosphere


The Plasmapause

However, the plasmapause boundary is very dynamic, and varies between about 3 to 6 RE, sometimes getting as low as 2 RE.

Note that the plasmasphere overlaps a considerable part of the radiation belt region as well as the ring current. However, these represent different particle energy populations.

The outer boundary of the plasmasphere, at about 4 RE, is where the plasma density undergoes a sudden drop. This is the plasmapause.

Ring Current

Radiation Belts


Plasmapause Boundary

Now, the co-rotating plasmasphere sets up a "co-rotation" electric field (to a stationary observer):

E R B

where BE = equatorial magnetic flux density at the surface, L = distance in RE, and RE = radius of earth.

ET ~BE

L3LRE

Essentially, the plasmapause represents the boundary where these two electric fields are of the same order:

Outside the plasmapause the plasma is not co-rotating, and the circulation there is determined by the cross-tail potential.

Dawn-DuskE-field

Co-rotation E-field


Putting in numbers,

mVm-1

~ 1 mVm-1 at 4 RE

ET ~14.4

L2

In fact, it is thought that the intensified outer circulation leads to a peeling off of outer layers of the plasmasphere, which are then lost as detached plasma chunks in the magnetotail and solar wind.

Viewed this way, one expects intensification of the outer magnetospheric circulation to lead to a contraction of the plasmasphere (inward movement of the plasmapause). This indeed happens (see subsequent figures).

Put another way, the plasmapause represents the boundary between the "inner magnetosphere" and "outer magnetosphere" plasma circulation patterns. The former is co-rotating, and the latter is strongly influenced by the solar wind interaction (see following figure):

Plasmasphere = corotating ionospheric plasma

Plasmapause = boundary between corotating plasma and convecting plasma


Daily variation of the plasmapause in relation to plasma convection in the magnetospheric

equatorial plane

SolarWind

EARTHSolar Wind Driven Convection

Side View

Polar View Connected to solar wind

Closed magnetic field

Equatorial Plane

Open field region Closed field region

Dissecting the Magnetosphere


Earth's plasmasphere at 30.4 nm (He+ resonant emission). This image from the Extreme Ultraviolet Imager was taken at 07:34 UTC on 24 May 2000, at a range of 6.0 Earth radii from the center of Earth and a magnetic latitude of 73 N. The Sun is to the lower right, and Earth's shadow extends through the plasmasphere toward the upper left. The bright ring near the center is an aurora, and includes emissions at wavelengths other than 30.4 nm. (From Sandel, B. R., et al., Space Sci. Rev., 109, 25, 2003.)

The EUV Imager on the IMAGE satellite is able to provide information on theplasmasphere distribution, boundary, aurora and other geospace properties.

QuickTime™ and aCinepak decompressor

are needed to see this picture.


Satellite observations of ion density, showing the plasmapause at several Kp levels

L


Flow patterns for cross-tail fields of 0.2 and 0.6 mV/m

For 0.6 mVm-1, theouter magnetospherecirculation “intrudes”upon the plasmasphere.


Detaching of plasma due to changing flow patterns during a magnetic storm


With the decay of magnetic activity, the magnetospheric circulation and electric fields return to their previous state but now the outer tubes of magnetic flux are devoid of plasma.

“Filling In” of Plasmasphere

O H H O

Observations of the filling are shown in the following figure. Since active periods may recurr every few days there will be times when the outer tubes are never full and the plasmasphere has some degree of depletion.

The rate of filling is determined by the diffusion speed of protons (formed in the upper ionosphere by charge exchange between hydrogen atoms and oxygen ions) coming up along the field, and by the volume of the flux tube which varies as L4. It therefore takes much longer to refill tubes originating at higher latitude.

These gradually refill from the ionosphere over a period of days.


“Filling In” of Plasmasphere



BOUNDARY LAYERS AND PARTICLE TRANSFER TO THE PLASMA SHEET

Solar wind particles find their way from the magnetosheath into the cusp region. There is experimental evidence for this entry, in that particles with characteristic "magnetosheath energy" (i.e., less than 1 keV) have been observed over a limited region centered around 77° magnetic latitude and noon (see following figures).

Such particles on newly-merged field lines flow down towards the earth, mirror there, and then return to find themselves on a field line sweeping back towards the tail. These particles form a particle population known as the "plasma mantle" (see following figures).

At many (~100) RE, these particles are swept into the plasma sheet. Another closer (~ 50 RE) source of plasma sheet particles is the polar wind emanating from the ionosphere at high latitudes (see following figures).


Details of the Cusp Region

Plasma MantleMagnetosheath

EntryLayer

Low-Latitude Boundary Layer


Proton precipitation patterns:

Cusp & plasma tail footprint

24 June 2000~0200 UT

Cusp signatures from the IMAGE satellite


Note: since ~1028-1029 particles/s impact the dayside magnetopause, and ~ 1026 particles/s are estimated to enter the plasma sheet, only 1% efficiency of this process is required.


Particle Flow in the Merging - Reconnection - Convection Process

As particles convect towards the earth, B increases, therefore the particle energies increase. The energy comes from the E-field.

During the return flow, the particles are also energized in their attempt to satisfy the first adiabatic invariant,

= const

1

2

mv2

B

“Dipolarization” of the B-field

dB/dt ≠ 0, inducesE-field, energize particles


Some of the sunward-convecting particles precipitate into the upper atmosphere and produce the aurora.

video


Numerical Simulation of the Solar Wind - Magnetosphere Interaction

video

ASEN 5335 Aerospace Environments -- Magnetospheres 1 PARTICLES IN THE MAGNETOSPHERE The plasmasphere represents the relatively cold ionospheric plasma.

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