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Solar Drivers of Space Weather

Steven Hill NOAA/SECJune 14, 2007

Research Experience for Undergraduates

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Energy and Power in Context

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Outline

• Overview of the Sun– Interior– Surface– Corona

• Solar Drivers of Space Weather– Coronal Holes– Flares– Coronal Mass Ejections (CMEs)

• Summary

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Interior

Courtesy UC Berkeley

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Sunspots and Subsurface Structure

Courtesy S. Kosovichev, Stanford University(http://soi.stanford.edu/press/ssu11-01/#movies)

Convection and magnetic fields below the solar surface drive the appearance and physical conditions in sunspots

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Surface and Corona

• The ‘average’ surface and corona are shown

• Heating of the corona is a major unsolved problem in solar physics

• Note comparisons of temperature and density to terrestrial standards

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Sun at Different Wavelengths

Because of the temperature differences at different heights in the corona, different wavelengths highlight features and phenomena occurring at different heights

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Magnetic Fields

Magnetic fields dominate the corona and are responsible for much of its structure, from coronal holes to active regions

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Solar Wind I

• The mass that the Sun continuously loses through the corona is called the solar wind

• It is highly variable spatially and temporally

• Major eruptions of mass are called Coronal Mass Ejections (CMEs)

SOHO coronagraph movie showing solar wind, CMEs, and radiation storm

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Solar Wind II

Courtesy C. Russell

Ulysses near polar passes show the latitudinal structure of the solar wind speed. Open field at high latitudes leads to predominantly high speed winds

A depiction of the process leading to the Parker spiral (Parker, 1958).

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11- year Variability

Courtesy Yohkoh SXT

While the total solar irradiance – the solar constant – is nearly constant over an 11-year cycle, the X-ray irradiance changes by a factor of 100. If flares are included, then this factor is 10000.

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Outline

• Overview of the Sun– Interior– Surface– Corona

• Solar Drivers of Space Weather– Coronal Holes– Flares– Coronal Mass Ejections (CMEs)

• Summary

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Coronal Holes

• Regions of open magnetic field related to high speed solar wind.

• Changes in solar wind speed at the boundaries of coronal holes compresses Earth’s magnetic field.

• Since coronal holes ‘live’ for more than a solar rotation, these storms recur.

One solar rotation shows the persistance of active regions and coronal holes.

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CIRs and Recurring Geomagnetic Storms• Co-rotating Interacting Regions (CIRs) are regions of compressed,

even shocked, solar wind caused by high speed streams from coronal holes

• CIRs lead to the majority of recurring geomagnetic storms as they sweep past Earth

Courtesy C. Russell

Coronal Hole

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Solar Wind Effects

• Stars, comet Encke, and the solar wind

• STEREO spacecraft provides a visual look at solar wind effects near 0.5AU

• Tail disconnections occur at discontinuities in the solar wind

Courtesy D. Biesecker & STEREO

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Flares

• Sudden conversions of magnetic energy into thermal and kinetic energy.

• Closely associated with coronal mass ejections CMEs.

• Rapid changes in solar EUV and X-ray emissions affect thermosphere and ionosphere.

• Large flares can have an extreme effect.

• In addition, flares are related to energetic particle events that result in Solar Radiation Storms.

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Stressed Magnetic Fields

• Active region complexity– Active region magnetic fields become stressed

by convective motions in the photosphere

– As energy builds up it can be released catastrophically

– Particular morphologies, illuminated by emitting plasmas, are more susceptible to eruption

– Region below resulted in flare and CME Non-Potential: Twist

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Simple

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Non-Potential: Shear

Non-potential region (twist / sigmoid) prior to flaring as seen by several instruments (Nitta et al., 2004 – unpublished)

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Solar Eruption Process

• One concept of the relationship between flares and CMEs

• Process of magnetic reconnection is key

• Conceptual models abound

• Computational modeling is more experimental than predictive

Lin & Forbes, 2002

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Forecasting Flare Effects

Flares are a source of ‘directly connected’ energetic particles

(Reames, 1999)

Radiation storm arrival time correlates with flare longitude

(Balch, 2000)

9220 0532 //// 0533 LEA C RSP 035-084 III/1

9230+0537 0546 0558 GO8 5 XRA 1-8A M1.3 1.2E-02 9230 0540 0542 0545 LEA G RBR 2695 16 9230 0540 0542 0546 LEA G RBR 4995 30 9230 0541 0541 0546 LEA G RBR 8800 26

9240 B0618 U0619 0624 LEA 2 FLA S05W04 SF 9718

EarliestPossibleLocation of flares

ImpulsiveFlare Phase on 2 deg grid

WeightedCentroidLocation

2001-12-04 05:36:57.383 -6.500 1.401

SEC ground system, using both XRS and SXI data, provides flare locations within two minutes of initial XRS flare detection (‘XFL’ product)

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Record X28 Flare in False Color

Combined EIT and SXI data.

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Coronal Mass Ejections• Coronal Mass Ejections (CMEs) are massive expulsions

of matter from the Sun’s outer atmosphere.– 400-2000 km/sec speed– 1014 to 1016 g– 1-4 days to reach Earth– Shock generated radiation storms can reach Earth much

sooner– Location of the source region on the solar disk is key to

estimating the trajectory of the CME and its likelihood of hitting Earth

CME generated shock creates energetic particle radiation storm

(Reames, 1999)

Ejected material associated with a CME seen by GOES 12 SXI in 2001 (Hill et al., 2001)

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Coronal Mass Ejection

SOHO Cornograph Observations of a CME.

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IPS Tomography

http://cassfos02.ucsd.edu/solar/tomography/fast_stel_1965.html

•Tomographic reconstruction of CMEs in interplanetary space•Radio sources distant from the solar system are monitored•Signal scintillation occurs when the CME crosses the line-of-sight

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Interplanetary Medium

• Not quite ‘empty’

• Contains ions, elections, currents and magnetic fields

• Solar disturbances propagating toward Earth can be strongly affected by the state of the IPM and the passage of previous disturbances

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Interplanetary Medium

• Contains ions, elections, currents and magnetic fields• Solar disturbances propagating toward Earth can be strongly affected

by the state of the IPM and the passage of previous disturbances

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Summary

• Important, Recurring Themes– Solar activity evolves from the interior, through many layers and

phases until we see it manifested on the surface and in the corona– Magnetic fields thread through every phenomena and dominate

many– Truly predictive physics-based numerical models of solar

phenomena are not yet available– Forecasting relies heavily on observations and empirical models

• Resources:– MSFC - http://solarscience.msfc.nasa.gov/– UCB - http://cse.ssl.berkeley.edu/segwayed/lessons/sunspots/– Flare cartoons -

http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/thepages/Shibata.html