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 Xrays 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
X-ray Flux
GOES
Energetic
Proton Flux
Flare in last movie
GOES X-ray and Energetic Proton Fluxes
25 October – 6 November 2003
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
flows
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
Simple flare reconnection model
flows
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
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.
Corona:
B2/2m0 >> nkT
Magnetic forces
control field
shape, gas flow
constrained by
field.
Photosphere:
nkT >> B2/2m0, field lines are advected with the
motion of the convecting photospheric gas.
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
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
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
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?
The sun exhibits three different transient
phenomena that are often related:
● Solar Flares
● Coronal Mass Ejections
● Erupting Prominences
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.
Flare ribbons above a sunspot
Hinode/SOT high-resolution observations of 12/13/06 flare
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 Phenomenon
Solar Flares
Effect at Earth
Enhanced X-ray and EUV fluxes
Energetic Particles
Coronal Mass Ejections
Great Magnetic Storms
From Jack Gosling, The Solar Flare Myth, JGR, November 1993.
Capital letters indicate observable phenomena