June 19, 2012 GEM - Cusp Tutorial - R. J. Strangeway 1

The Magnetospheric Cusp: Solar Wind – Magnetosphere –

Ionosphere – Thermosphere Coupling

R. J. StrangewayIGPP & ESS /UCLA

Special Acknowledgement to J. Raeder, UNH, and the Community Coordinated Modeling Center.

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Outline

• Introduction – why talk about the cusp?

• Reconnection topology and the cusp – where is the cusp?

• Cusp ion dispersion at middle altitudes – where is the reconnection?

• Cusp ion dispersion at low altitudes – multiple cusps?

• Field-aligned currents – Force balance and cusp dynamics

• Ion outflows – Joule dissipation and electron precipitation

• Neutral upwelling – Important for satellite drag

• Summary

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What is so Special About the Cusp?

• The cusp is the region that provides the most direct path from the solar wind to the ionosphere and thermosphere

• The cusp requires an understanding of several processes: Reconnection topology and Interplanetary Magnetic Field direction

effects (IMF By)

Steady-state, multi-point, and time-varying reconnection

Particle kinematics (time-of-flight, velocity dispersion)

Field-aligned current generation

Joule Dissipation – ion and neutral upwelling and outflows

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Outline

• Introduction – why talk about the cusp?

• Reconnection topology and the cusp – where is the cusp?

• Cusp ion dispersion at middle altitudes – where is the reconnection?

• Cusp ion dispersion at low altitudes – multiple cusps?

• Field-aligned currents – Force balance and cusp dynamics

• Ion outflows – Joule dissipation and electron precipitation

• Neutral upwelling – Important for satellite drag

• Summary

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Anti-parallel merging [Luhmann, 1984]

Luhmann et al. [1984] used Spreiter gas dynamic model to map IMF to the magnetopause

Contours show regions of anti-parallel merging (up to 90˚)

Crooker [1985] used superposition of IMF and Chapman-Ferraro field at magnetopause, considered component merging

Cooling [2001] extended Luhmann et al. [1984] to allow for component merging

B = (0,0,1)(northward)

B = (0,1,0)(By only)

B = (0,0,-1)(southward)

B = (0,-1,-1)

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Cooling Model – Component Merging

Luhmann et al. [1984] Cooling et al. [2001]

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Outline

• Introduction – why talk about the cusp?

• Reconnection topology and the cusp – where is the cusp?

• Cusp ion dispersion at middle altitudes – where is the reconnection?

• Cusp ion dispersion at low altitudes – multiple cusps?

• Field-aligned currents – Force balance and cusp dynamics

• Ion outflows – Joule dissipation and electron precipitation

• Neutral upwelling – Important for satellite drag

• Summary

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Trattner et al. [2012] component and anti-parallel merging

Characteristic signature of cusp is energy dispersed ions

Plot shows Polar data at ~ 5.5 Re

Mapping to magnetopause (source region) uses time-of-flight energy dispersion comparing downgoing to reflected [Onsager et al., 1990]

Multiple dispersion events could be because of:•Multiple reconnection sites•Time-varying reconnection•High latitude and low-latitude reconnection (different history)

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Trattner et al. [2012], different topology

By-dominated – nearly anti-parallel, multiple X-lines?

Southward Bz – anti-parallel, multiple injections from same point

Northward Bz – component merging, extended region

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IMF By – Hemispherical Asymmetry

Østgaard et al. [2005] investigate cusp proton precipitation asymmetry using IMAGE (viewing north) and Polar (viewing south)

Cusp precipitation consistent with locus of anti-parallel merging

Bx contributes to hemi-spherical differences

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Outline

• Introduction – why talk about the cusp?

• Reconnection topology and the cusp – where is the cusp?

• Cusp ion dispersion at middle altitudes – where is the reconnection?

• Cusp ion dispersion at low altitudes – multiple cusps?

• Field-aligned currents – Force balance and cusp dynamics

• Ion outflows – Joule dissipation and electron precipitation

• Neutral upwelling – Important for satellite drag

• Summary

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Newell et al. [2007] – Forward Dispersion

DMSP “FAST”

Note: At low altitudes the spacecraft moves through structures

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Newell et al. [2007] – Reverse Dispersion

DMSP “FAST”

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Newell et al. [2007] – Double Cusp

DMSP “FAST”

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Double Cusp – Wing et al. [2001]

Wing et al. [2001] argue that double cusps are because the dispersing ions come from two different reconnection sites

DMSP data from Newell et al. [2007] (not the same event)

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Outline

• Introduction – why talk about the cusp?

• Reconnection topology and the cusp – where is the cusp?

• Cusp ion dispersion at middle altitudes – where is the reconnection?

• Cusp ion dispersion at low altitudes – multiple cusps?

• Field-aligned currents – Force balance and cusp dynamics

• Ion outflows – Joule dissipation and electron precipitation

• Neutral upwelling – Important for satellite drag

• Summary

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Force Balance

For simplicity assume neutrals at rest (frame of reference)

Ionospheric flow:

Frozen-in electrons:

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IMF By-Dependent Convection

Burch et al. [1985]

By > 0 By < 0

Currents Convection

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Weimer [2001] FAC morphology

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FAST Orbit 8276 – Strong IMF By

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MHD FAC Predictions

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Field Topology – Shock Passage

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FAST Orbit 8284 – Double Cusp?

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Outline

• Introduction – why talk about the cusp?

• Reconnection topology and the cusp – where is the cusp?

• Cusp ion dispersion at middle altitudes – where is the reconnection?

• Cusp ion dispersion at low altitudes – multiple cusps?

• Field-aligned currents – Force balance and cusp dynamics

• Ion outflows – Joule dissipation and electron precipitation

• Neutral upwelling – Important for satellite drag

• Summary

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Ion Outflows

Type 1 Type 2

Type 1 and Type 2 defined by Wahlund et al. [1992]

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Ion Outflows – Including Alfvén Waves

Type 1 Type 2

Type 1 and Type 2 defined by Wahlund et al. [1992]

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Joule Dissipation and Heating RatesPlasma frame:

Neutral frame:

Rate of temperature increase:

Minority species temperature increases more rapidly

See [Strangeway, JGR, 2012]

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Outline

• Introduction – why talk about the cusp?

• Reconnection topology and the cusp – where is the cusp?

• Cusp ion dispersion at middle altitudes – where is the reconnection?

• Cusp ion dispersion at low altitudes – multiple cusps?

• Field-aligned currents – Force balance and cusp dynamics

• Ion outflows – Joule dissipation and electron precipitation

• Neutral upwelling – Important for satellite drag

• Summary

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Cusp Statistics – Knipp et al. [2011]

Poynting flux statistics from DMSP

Southern hemisphere is mirror-reflected about noon as a function of IMF By

Events required |By| > 10 nT

(a) N:By < 0, Bz < 0; (b) N:By > 0, Bz < 0(c) N:By < 0, Bz > 0; (d) N:By > 0, Bz > 0

Red is 100 µW/m2

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Thermospheric Response – Crowley et al. [2011]

CHAMP sees strongly neutral density modulation near the cusp

Modified TIME-GCM using real-time AMIE data shows enhanced densities too

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Poynting Flux Versus Neutral Density – Crowley et al. [2011]

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Small scale FACs – Lühr et al. [2004]

CHAMP also sees large-amplitude small-scale FACs in the Cusp

Lühr et al [2004] argue that these can significantly enhance the heating

Heating rate depends on E2,

But there is an issue with time scales, neutrals heat much more slowly than ions

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Summary

• Reconnection topology controls the location of the cusp

• There is both a high latitude (anti-parallel merging) and low latitude (component merging) source of dispersing ions – depends on IMF orientation

• Field-aligned currents and Joule dissipation in the ionosphere strongly affected by IMF By – I suggest this is high latitude merging

• Ion outflows and neutral upwelling both appear to be associated with Joule dissipation – question of timescales for the neutrals