living in a star sarah gibson high altitude observatory / ncar

Living in a Star

Sarah GibsonHigh Altitude Observatory / NCAR

3D real-time astronomy

• We are observing a stellar system from within• What we see may impact us in near-real time• Unescapably three-dimensional • Fundamentally magnetic

Talk outline:

– Coupled sun-earth system/space weather– 3D models for understanding/predicting solar eruptions

Solar eruption (CME)

Weather as we usually think of it

All Weather Originates at the SunSolar Structure:

CoreWhere the energy is created. Nuclear reactions burn every second about 700 million tons of hydrogen into helium.

Radiation ZoneWhere energy is transported by radiation.Although the photons travel at the speed of light, they bounce so many times through the dense material that they use about a million years to escape the Sun.

Convection ZoneEnergy transported by convection (just like boiling soup) where heat is transported to the photosphere.

The Solar Corona

Million Degrees

Magnetically Dominated

Expands to fill interplanetary space

EIT 195 Angstroms May 2, 1998TRACE 171 Angstroms

The Magnetic Sun

Magnetic fields organize solar structures, and drive solar dynamics

Solar Activity - Flares

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Space Weather - Flares

Flares represent transfer of magnetic energy to thermal/radiative energy.

Flares affect the ionosphere and radio communication at the Earth.

Flares also release energetic particles into space.

Flares are not the whole story, though!

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Solar Activity - Mass Ejections

Not just hot stuff involved in solar dynamics

(Relatively) cool/dense “prominence” mass may erupt

Coronal mass ejections: CMEs

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Space Weather - CMEs

CMEs represent transfer of magnetic energy into kinetic energy

The interaction of CME and Earth magnetic fields may drive geomagnetic storms

CMEs drive shock waves where energetic particles are accelerated

CMEs are often associated with flares -- double whammy!

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Propagate through Solar System

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Impact Earth’s Magnetic Field

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Negative Space Weather Impacts

• Radiation hazards for astronauts• Damage to and/or disabling of satellites• Communication blackouts • Power grid damage• Introduction of errors in GPS systems

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Frequency of Large Events

Between 1978 through May 2005, (which includes the last 3 solar maxima) there were 41 severe storms (DsT < -200)

Severe Storm Rates:

Activity maximum ~ 4 per year

Activity minimum ~ 0.2 per year

CME Rates: Activity maximum: ~ 4 per day (avg.)

Activity minimum: ~ 0.4 per day (avg.)

Expect only a small percentage (~10%) of CMEs to intercept earth and only a small percentage of those will cause severe activity

Magnetic Fields are Key!

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(Illing and Hundhausen, 1986)

• plasma and field are “frozen together”

• plasma structures trace out field

Most importantly -

• magnetic energy the driver of eruptions

• understanding the magnetic structure key to understanding what makes the CME go

Modeling Magnetic Fields

A range of observations suggest a “twisted flux rope” configuration for the CME

Magnetic Flux Ropes

What’s a magnetic flux rope?

Suggested definition: A set of magnetic field lines that winds

about an axial field line

Roussev (2003) (after Titov and Demoulin (1999))

After Gibson and Low (1998)

Amari et al. (2003)

Loosely woundTightly wound

Spheromak-based

Magnetic Flux Ropes: Magnetic Clouds

Are there flux ropes in interplanetary space?

Widely used as models for magnetic clouds

(interplanetary manifestation of CMEs)

Lepping et al. (1995)

Manchester et al. (2004a; 2004b)

Magnetic Flux Ropes: CMEs

Are there flux ropes in the corona during the eruption of a coronal mass ejection (CME)?

Also quite well accepted

Amari et al., (2003 )

Examples of models where rope forms during eruption

Antiochos et al. (1999); Lynch et al. (2004); MacNeice et al. (2004) Tokman and Bellan (2002)

Do flux ropes exist quiescently in the corona prior to the CME, or are they formed in eruption?

Controversial!

Erupting flux rope model

Gibson and Fan (2006)

A pre-CME equilibrium coronal magnetic flux rope

• Can store magnetic energy sufficient to drive a CME

• Can trigger the CME by its loss of equilibrium

(Lin et al. 1998; Amari et al, 2000; 2003, 2004, 2005; Linker et al., 2003; Toeroek and Kliem, 2003; Fan 2005)

Equilibrium flux rope model

Fan and Gibson(2006)

Magnetic Flux Ropes: Pre-CME

Fan and Gibson (2006)

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Simulation 1: Flux Rope Emerges and Finds Equilibrium

Simulation 2: Loss of Equilibrium and Eruption of Flux Rope

Fan (2005)

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Alternatively, if the rope changes its axis orientation it can “rupture” through the overlying arcade (e.g., Sturrock et al., 2001). This can occur if the twist threshold for the kink instability is crossed.

Simulation 3: Partial Ejection of a Magnetic Flux Rope

Gibson and Fan (2006)

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Simulation 3: Partial Ejection of a Magnetic Flux Rope

Gibson, S. E., and Y. Fan, The partial expulsion of a magnetic flux rope, Astrophys. Journ. Lett., 637L, 65, 2006.

Partially-Ejected Flux Rope: Partial-Mass Eruption

CME partial eruption of prominence and cavity: HAO/MLSO Mk4 coronagraph, November 19, 1999

CME partial eruption of prominence and cavity. Isocontours show density; cool, dense prominence mass in brown.

Part of the flux rope is carried away, part remains behind.Part of the (prominence) mass is carried away, part remains behind.Magnetic twist (and energy) also remains behind — likely to erupt again.

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End state: X-ray cusp over reformed sigmoid with non-erupting prominence below

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Partially-Ejected Flux Rope: Flares

Partially-ejected flux rope: change

in topology

Writhing Motion During Eruption

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Partially-Ejected Flux Fope: Forming Tori

• Writhing plus reconnection -- created toroidal topology

• Grey field lines -- poloidal axes • Red field lines -- many times toroidally winding

Implications for Magnetic Clouds

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Need to get topology right.

Conclusions

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"If the sun didn't have a magnetic field, then it would be as boring a star as most astronomers think it is.” (Leighton)

“Magnetic fields are to astrophysics what sex is to psychoanalysis” (van de Hulst)

We live in the outer atmosphere of a 3D, magnetic star

The sun uniquely affects us, but we have unique views (up-close and 3D) of it

Magnetic fields are the key to understanding, predicting, and mitigating space weather

We need coronal magnetic field observations!

The Sun at different temperatures

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Activity Maximum 1980

Activity Minimum 1994

11-year cycle changes in the corona

Solar cycle evolution of corona

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Writhing prominence

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Reconnected Field Lines Over Surviving Part of the Prominence Mass

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Create the Aurora

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