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Electron Precipitation & the Aurora

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Aurora from the Ground

Visible Wavelengths

video

video

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Aurora from Space

UltravioletWavelengths

video video

video

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Aurora from POLAR Satellite during Bastille-day Storm

video

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Auroral Potential Structure

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Diffuse Aurora

•Broad, nondescript auroral emission•Extended 1000’s of kilometers in longitude•Extended 100-1000km in latitude•Pitch-angle diffusion of plasma sheet particles•Ee < 1 keV

Discrete Aurora

•Structured emission in arcs, bands, rays•Extended 100 to 1000 km in longitude•Extended 100m to 1km in latitude•Acceleration process •Ee > 1 keV and typically 10 keV

Geomagnetic Substorm

Themis launched Feb 17, 2007!

http://www.nasa.gov/mission_pages/themis/main/index.html

• Mission Characteristics:• Five probes, 2 year

lifetime baseline mission.• PI Institution: UC

Berkeley• During tail-phase

(winter):– Probes line up linearly

down tail every ~4 days– Apogees align over

North America.

• Ground-based determination of auroral onset.

• Launched: February 17th, 2007

THEMIS Space-Based Element

THEMIS Ground-Based Element

Scale Sizes of the Aurora

High resolution camera (Josh Semeter)Ground Allsky Imager (Rick Doe)

Geomagnetic Coordinates

COUNTS

1000

IMAGE Spacecraft (Stephen Mende)

Geographic Coordinates

Ionosphere responseA

ltit

ud

e (

km)

Universal Time Incoherent Scatter Radar

Electron Density

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HST Images of Aurora

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NASA and Prof. John Clarke (Boston University) HST-STIS

Jupiter Aurora

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Current Systems of the Magnetosphere

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MAGNETOSPHERIC CURRENTS

The combination of plasma and electric fields in the magnetosphere allows electric current to flow. Several current systems have been identified:

• magnetopause current (A) • tail current (B) • Birkeland (field-aligned) currents • ring current (C)

The strong deviations of the magnetosphere from a dipole shape are in fact due to the first three (A, B, C) of the above current systems. These are now discussed in turn.

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MAGNETOPAUSE CURRENT

If we ignore any magnetic or electric field in the solar wind, the origin of the magnetopause current and the corresponding modifications of the magnetic field can be grossly understood as follows:

Consider a small section of the dayside magnetopause with the solar wind normal to it (see following figure). The ions are deflected one way and the electrons the other, causing a current to flow (consisting mostly of ions due to their greater penetration depth). The current flow at the magnetopause is such that its magnetic field cancels the geomagnetic field outside the boundary.

Similarly, earthward of the boundary the field strength is doubled -- this is essentially the "compression" of the dayside magnetosphere that we have alluded to before.

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Currents and Fields at the Magnetopause Boundary

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It turns out that the force produced by this current (the so-called Lorentz or J X B force) balances the momentum force of the solar wind, which is another way of stating the "dynamic pressure" vs. "magnetic pressure" balance as we discussed before.

When the solar wind intensifies, the magnetopause current is increased:

-- This further "compresses" the dayside magnetosphere;

-- The ground magnetic signature of this sudden currentincrease associated with the compression is called a "sudden impulse" (SI), or if it is connected with the beginning of a storm, it is called a "sudden storm commencement" (SSC).

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TAIL CURRENT

The down-wind extension of the magnetosphere into a tail indicates the presence of a current system as follows:

View from Earth

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This is what keeps the dark-side magnetic field from assuming its dipolar form. The energy to do this comes from the solar wind. The magnetic flux in a current loop is

Since BT ~ 20 nT, then iT ~ .016 A/m. Assuming something reasonable for the tail length, iTail ~ 108 A. For a cross-tail potential of 60 kV, the power extracted from the solar wind ~ 6,000 GW !!

BT oiT

The neutral sheet separates the northern lobe from the southern lobe.

"neutral sheet" here refers to the magnetic field, and the region where currents flow so that reconnection is inhibited, similar to the heliospheric current sheet.

• • • • • •

• Current out of page

View of Tail from Earth

Particle motion along a neutral sheet

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Currents and Magnetic Fields in the Geotail

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RING CURRENT

Under magnetic storm conditions the magnetic field of the earth at low latitudes may be depressed ~ 1-2% for a day or two (main phase). This is due to a westward ring current which we have already discussed in relation to particle drift (gradient-curvature drift) on curved field lines with the magnitude of B increasing towards the earth.

-- hence gradient drift is not important for "cold"particles like those populating the ionosphere and plasmasphere; these particles co-rotate.

vg B B

eB2

1

2

mv2

B

Recall that the gradient drift depends on the particle "magnetic dipole moment"

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However, it is also true that it is not the energetic Van Allen particles that are the main contributors to the ring current. The fluxes of these particles are too small. The main ring current particles are protons of 20 - 100 keV (see figure below).

The ring current is located between 4 and 6 RE, close to the inner edge of the plasma sheet and and outer edge of the trapping zone.

Energy and Number of Protons in the Ring Current at L = 4 during a Magnetic Storm

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Particles are energized and injected into the ring current, as reflected in the Dst index

(100’s keV)

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BIRKELAND (FIELD-ALIGNED) CURRENTS

• energizing particles which give up their • dispelling blobs of plasma out energy to the neutral atmosphere; the magnetotail;

• developing a current system capable of • transferring momentum dissipating energy through ohmic losses; to the neutral atmosphere.

This dissipation occurs in the form of:

The solar wind/magnetosphere interaction provides energy and momentum to the magnetosphere system; the magnetospheric circulation is determined by redistributing its plasma and fields in a way that allows for dissipation of this energy.

The following figure shows the average locations of these currents as determined from measurements on the TRIAD satellite.

Magnetometers carried on satellites have detected persistent magnetic perturbations over the auroral zones which can only be interpreted as resulting from currents flowing into and out of the ionosphere.