SW 101: Lecture 4: Active Sun I: Solar Flares

The Great Flare of 1859, observed by Richard Carrington

•  First solar flare ever reported. •  “White Light Flare” – only the very largest flares can be seen in white light. •  No flare approaching this intensity has been observed since. •  Signatures frozen into Arctic/Antarctic ice cores indicate that flares of similar

intensity occur on the century time scale.

Richard Carrington (1826-1875) was an English gentleman astronomer, who made careful meticulous observations. He particularly famous for the first continuous detailed observation of sunspots. He established that the sun rotates differentially (so must be a fluid not a solid body) and began the series of solar rotations now known as Carrington Rotations.

Monthly Notices of the Royal Astronomical Society, Volume 20, November 11, 1859

Magnetic Observations at Kew

Two responses seen in the new photographic recordings of magnetic variations being made at Kew (London)

•  Prompt response (due to X-rays increasing ionospheric ionization)

•  Great Magnetic Storm begins 18 hours later (due to associated CME reaching Earth, with average speed of 2300 km/s )

History of Flare Observations ● Carrington’s observation of a white light flare (1859)

● Ground-based Ha spectrohelioscopes provide images of the solar disk (chromosphere) in the red hydrogen line. (~1900)

●  Full disk X-ray monitors (e.g. GOES) (~1970)

● X-ray and EUV imagers (SOHO, TRACE) (~1990)

April 21, 2002: GOES X-ray fluxes – integrated over the visible disk of the sun.

GOES Energetic Proton Flux

GOES X-ray Flux

GOES X-ray and Energetic Proton Fluxes 25 October – 6 November 2003

Flare in last movie

An Erupting Prominence

SOHO EUV image at 195 Å 12-18 Oct 2002

GOES X-ray and proton fluxes for 12-18 Oct 2002

Understanding Solar Flares – Magnetic Reconnection •  Solar flares are a sudden burst of radiation lasting minutes –

hours at wavelengths that can include: •  Hard X-rays and g-rays (bremsstrahlung) •  Soft (thermal) X-rays and EUV (multi-million degree K gas) •  Ha (hot chromosphere emissions) •  Radio bursts (energetic electrons in magnetic fields)

•  A large quantity of energy is released from a small volume in a short period of time.

•  This requires: •  Either a large amount of energy stored in that small volume

that can be quickly transformed and released as energetic electrons and photons.

•  Or very efficient transport of energy into that volume where it is then converted into the observed forms.

•  The only viable energy source is intense solar magnetic fields. •  Thus we need a very rapid means of converting stored magnetic

energy into particle energy and heat – magnetic reconnection.

Magnetic Reconnection

● Occurs when antiparallel magnetic field lines break (disconnect) at an X point and reconnect with new partners

● Converts magnetic energy into kinetic energy

● Results in new topological configurations

●  Solar flares provided the first observational evidence for the reconnection process

●  Plays a critical role in solar flares, in CME release from the Sun, and in geomagnetic storms at Earth.

Simple flare reconnection model

Cusped magnetic loops Loops move downward cooling as they fall. Loops are visible at a given wavelength only as they pass through a narrow temperature range Flare ribbons in chromosphere

Chromosphere is heated by gas flowing down from reconnection site

flows

Simple flare reconnection model

Cusped magnetic loops Loops move downward cooling as they fall. Loops are visible at a given wavelength only as they pass through a narrow temperature range Flare ribbons in chromosphere

Chromosphere is heated by gas flowing down from reconnection site

flows

Stop and Think ● Solar flares are

1. The cause of magnetic reconnection on the Sun

2. Explosions of material from the Sun. 3. A brand of roadside warning device. 4. Intense, short lived, photon emissions 5. The drivers of CMEs.

Storing energy in the coronal magnetic field.

Photosphere: nkT >> B2/2m0, field lines are advected with the motion of the convecting photospheric gas.

Corona: B2/2m0 >> nkT Magnetic forces control field shape, gas flow constrained by field.

Stressing the coronal field through shear flow in the photosphere (idealized MHD model)

Occurrence of major (X-class) flares over the solar cycle

Drivers of Space Weather

Solar X-Rays and EUV (Flares): • Arrive in 8 Minutes • Last minutes to hours • Increases ionosphere density • Increases neutral density • Systems Affected:

• Radio Communications • Navigation • Satellite Orbits

Coronal Mass Ejections (CMEs): •  Arrive 1- 4 Days later •  Last a day or two •  Produce Geomagnetic

Storms at Earth •  Systems Affected

•  Radio Communications •  Navigations •  Electric Power Grids •  Pipelines

Solar Energetic Particles: • Arrive in 30 Minutes to 24 hours • Last several days • Systems Affected:

• Astronauts • Spacecraft • Airlines • Radio Communications

High-Speed Solar Wind: •  Common During Solar Minimum •  Enhances Radiation Belts •  Systems Affected

•  Satellite Charging •  Astronauts

How are space weather drivers related to solar activity?

● Solar Flares ● Coronal Mass Ejections ● Erupting Prominences

The sun exhibits three different transient phenomena that are often related:

How are these related to each other? How are they related to space weather?

Relationship between Flares and Sunspots

● Sunspots are photospheric phenomena indicative of strong magnetic fields emerging through the photosphere.

● Sunspots are associated with magnetic active regions, regions of strong fields in the corona often with complex geometries/topologies.

●  Flares usually occur in the active regions above sunspots; they reconfigure the coronal magnetic field.

●  The sunspots underlying the active region usually don’t change.

Hinode/SOT high-resolution observations of 12/13/06 flare

Flare ribbons above a sunspot

Solar Energetic Particles

  The source of Solar Energetic Particles:   Magnetic reconnection   Shock acceleration via a multistep process

  Both flares and CME’s can generate shocks in the corona. Fast CME’s generate shocks in the solar wind.

  Both flares and CME’s involve magnetic reconnection.

  SEP’s travel easily along magnetic fields – they also scatter (diffuse) across field lines, but more slowly.

SEP propagation to Earth   SEP’s arrive promptly at Earth when Earth is connected

magnetically to the source region.   Earth is usually magnetically connected to the western side

of the visible disk of the Sun because if the Parker spiral.

Relationship between Flares and CME’s ●  Both flares and CMEs are associated with the strong magnetic

fields with complex topologies present in active regions. ●  Flares are the electromagnetic radiation (X-ray, EUV, etc)

generated by the energetic electrons and flows arising from rapid magnetic reconnection.

●  CMEs are the release of previously confined magnetic field and plasma into interplanetary space.

●  All CME models include magnetic reconfiguration via magnetic reconnection to affect the release of the CME.

●  CMEs are occasionally released without a recognizable flare – usually they are slow moving ones. Presumably the reconnection can occur slowly enough for the intense electromagnetic radiation not to be generated.

●  Most flares are not associated with CMEs. Presumably the field can reconfigure without releasing a plasma cloud.

●  Large CME’s are invariably associated with a flare and large flares are invariably associated with a CME.

Relationship between solar transients and space weather

Solar Flares

Great Magnetic Storms

Energetic Particles

Enhanced X-ray and EUV fluxes

Solar Phenomenon Effect at Earth

Coronal Mass Ejections

From Jack Gosling, The Solar Flare Myth, JGR, November 1993. Capital letters indicate observable phenomena