1. Flares and Coronal Mass Ejections
CSI 769/ASTR Topics in Space Weather Fall 2005
Lecture Sep. 13, 2005
Solar Eruptions:
Flares and Coronal Mass Ejections
Aschwanden, “Physics of the Solar Corona”
Chap. 16, P
Chap. 17, P
Chap. 10, P
Chap. 11, P

3. :17 17:40 18:03 G XRA 1-8A X E

4. 3-day Solar-Geophysical Forecast issued Sep 12 at 22:00 UTC
Solar Activity Forecast: Solar activity is expected to be moderate to high

5. Magnetogram on Sep Sep. 12

6. What is a solar flare?
A solar flare is a sudden brightening of a part of the chromosphere and/or corona, by traditional definition
Flare shows enhanced emission in almost all wavelength, from long to short, including radio, optical, UV, soft X-rays, hard X-rays, and γ-rays
Flare emissions are caused by thermal plasma heating, and non-thermal particle acceleration
Heating and particle acceleration are believed to be caused by magnetic reconnection in the corona
Flares release ergs energy in a few to tens of minutes.
(Note: one H-bomb: 10 million TNT = 5.0 X 1023 ergs)

7. Flare: Example in optical Hα
Heating: temperature increase in Chromosphere
Structure: Hα ribbons

8. Flare: Example in EUV (~ 195 Å)
TRACE Observation: 2000 July 14 flare
Heating: temperature increase in corona
Structure
EUV ribbons
Loop arcade
Filament eruption

9. Flare: Example in soft X-rays (~ 10 Å)
Heating: temperature increase in Corona
Structure: fat X-ray loops

10. Flare: Example in hard X-ray (< 1 Å)
RHESSI in hard X-rays (red contour, 20 Kev, or 0.6 Å)
and (blue contour, 100 Kev, or 0.1 Å)
Non-thermal emission: due to energetic electron through Bremsstrahlung (braking) emission mechanism

11. Flare: Example in radio (17 Ghz)
Nobeyama Radioheliograph (17 Ghz, or 1.76 cm)
and (34 Ghz, or 0.88 cm)
Non-thermal emission
due to non-thermal energetic electron and magnetic field
emission mechanism: gyro-synchrotron emission

12. Flare Frequency At solar minimum: 1 event/per day At solar Maximum:
10 events/per day
GOES X-ray flare:
11696 events
From 1996 Jan.
To 2001 Dec.

13. Flares: X-ray Classification
Intensity
(erg cm-2 s-1)
I (W m-2) B
10-4
10-7 C
10-3
10-6 M
10-2
10-5 X
10-1

14. Flare: Hα Classification
Area
(millionth of a solar hemisphere)
(in square degree heliocentric)
Size Important
<100
<2.06
S (Sub-flare)
2.06—5.15 1 2 3
>1200
>24.7 4
Brightness Importance
F: faint
N: Normal
B: Brilliant
e.g., 3B means covering >12.4 deg2 and exceptionally bright

15. Flare: Temporal Property
A flare may have three phases:
Preflare phase: e.g., 4 min from 13:50 UT – 13:56 UT
Impulsive phase: e.g., 10 min from 13:56 UT – 14:06 UT
Gradual phase: e.g., many hours after 14:06 UT

16. Flare: Temporal Property (Cont.)
Pre-flare phase: flare trigger phase leading to the major energy release. It shows slow increase of soft X-ray flux
Impulsive phase: the flare main energy release phase. It is most evident in hard X-ray, γ-ray emission and radio microwave emission. The soft X-ray flux rises rapidly during this phase
Gradual phase: no further emission in hard X-ray, and the soft X-ray flux starts to decrease gradually.
Loop arcade (or arch) starts to appear in this phase

17. Flare: Spectrum Property
The emission spectrum during flare’s impulsive phase

18. Flare: Spectrum Property (cont.)
A full flare spectrum may have three components:
Exponential distribution in Soft X-ray energy range (e.g., 1 keV to 10 keV):
thermal Bremsstrahlung emission
Power-law distribution in hard X-ray energy range (e.g., 10 keV to 100 keV):
non-thermal Bremstrahlung emission
dF(E)/dE = AE–γ Photons cm-2 s-1 keV-1
Where γ is the power-law index
Power-law plus spectral line distribution in Gamma-ray energy range (e.g., 100 keV to 100 MeV)
Nuclear reaction

19. Bremsstrahlung Spectrum
Bremsstrahlung emission (German word meaning "braking radiation")
the radiation is produced as the electrons are deflected in the Coulomb field of the ions.
Bremsstrahlung emission

20. Flare Energy Source
Magnetic energy is slowly-built-up and stored in the corona, and then through a sudden release to produce flares, the so called storage-release model
Flare is believed to be caused by magnetic reconnection in the corona
Magnetic reconnection is a process that can rapidly release magnetic energy in the corona

21. Magnetic Reconnection
MHD equations
(Aschwanden , P. 247)
Magnetic field diffusion time τd in the corona
τd = 4πσL2/c2 = L2/η
τd the time scale the magnetic field in size L dissipate away,
σ electric conductivity, η magnetic diffusivity, L the magnetic field scale size
In normal coronal condition, τd ~ s, or 1 million year (assuming L=109 cm, T=106 K, and σ =107T3/2 s-1)
To reduce τd, reduce L to an extremely thin layer

22. Magnetic Reconnection: 2-D (cont.)
quick magnetic energy dissipates at the current sheet (a magnetic neutral sheet) that separates two magnetic regimes, which
have different directions (opposite directions
are forced to push together by a continuous converging motion
Analytic solutions are obtained by
Sweet-Parker model
Petschek model
(Aschwanden book, Chap. 10, p )

23. Magnetic Reconnection: 2D (cont.)
Sweet-Parker reconnection
Petschek reconnection
(Aschwanden Book, Fig. 10.2, P411)

24. Magnetic Reconnection: 3-D (cont.)
(Aschwanden Book, Fig , P423)

25. Magnetic Reconnection: 3-D (cont.)
Separatrix surface: 2D surface divides oppositely directed magnetic field in 3D volume
Separator line: 1D intersection lines of two 2D-separatrix surface
Nullpoint: nullpoint of the intersection of 1D separator lines
Magnetic reconnection occurs in these locations where current is concentrated and magnetic field is zero

26. Flare Model: standard 2-D model
Aschwanden book. Fig , P437

27. Flare Model: standard 2-D model (cont.)
The initial driver of the flare process is a rising flux rope or filament above the neutral line (pre-flare phase)
At a critical point, the magnetic reconnection at the X-type region below the flux rope sets-in (major flare phase)
Newly reconnected field lines beneath the reconnection points have an increasingly larger height and wider footpoint separation (post-flare phase)

28. Flare dynamic Model A cartoon model by Gurman Particle precipitation
Thermal conduction
Chromospheric Evaporation

29. Flare Dynamic Model (cont.)
Loop structure of soft X-ray emission
Compact hard X-ray sources appear at two foot-points of soft X-ray loop
Hard X-ray sources appear at top of soft X-ray loops

30. Coronal Mass Ejection and Filament Eruption
Both are large scale structural eruption in the corona
CMEs are observed in the outer corona by coronagraphs (since 1970s)
Filament Eruption are observed in Hα on the solar disk (since ~1900s)
CME eruptions are often associated with filament eruption

31. Filament Eruption
BBSO Hα
Mt. Wilson Magnetogram

32. Filament Eruption (cont.)
A filament always sits along the magnetic inversion line (magnetic neutral line) that separates regions of different magnetic polarity
A filament is supported by coronal magnetic field in a supporting configuration
Magnetic dip at the top of loop arcade (2-D)
Magnetic flux rope (3-D)
Helical or twisted magnetic structure is seen within filament

33. Filament Eruption (cont.)
One of filament models: filament is supported by twisted magnetic flux rope

34. Filament Eruption (cont.)

35. Filament Eruption TRACE 195 Å, 1999/10/20
Filament eruption and loop arcade
TRACE 195 Å, 2002/05/27
A failed filament eruption
TRACE 195 Å, 1998/07/27
Filament dancing without eruption

36. Filament Eruption Model
model of Martens & Kuin
A unified model of
Filament and
Flare

37. CMEs
A CME is a large scale coronal plasma (and magnetic field structure) structure ejected from the Sun, as seen by a coronagraph
A CME propagates into interplanetary space. Some of them may engulf the earth orbit and cause a series of geo-space activities, such as geomagnetic storms.
A CME disturbs the solar wind, drives shock in interplanetary space, and produce energetic particles at the shock front.

38. Coronagraph Coronagraph
A telescope equipped with an occulting disk that blocks out light from the disk of the Sun, in order to observe faint light from the corona
A coronagraph makes artificial solar eclipse
The earliest CME observation was made in early 1970s

39. Coronagraph: LASCO C1: 1.1 – 3.0 Rs (E corona) (1996 to 1998 only)
C2: 2.0 – 6.0 Rs (white light) (1996 up to date)
C3: 4.0 – 30.0 Rs (white light) (1996 up to date)
C C C3
LASCO uses a set of three overlapping coronagraphs to maximum the total effective field of view. A single coronagraph’s field of view is limited by the instrumental dynamic range.

40. Coronagraph (cont.) White light corona has three components:
K (continuum) corona, caused by scattering of photospheric light of rapidly-moving coronal electrons (so called Thomson scattering); the major component of white-light corona
F (Fraunhofer corona), caused by scattering of photospheric light off dust in interplanetary space between the orbits of Mercury and Earth
E (Emission corona), caused by emission of radiation by highly-ionized species in the actual corona, so called forbidden emission lines, e.g., at 5302 Å from Fe XIV (coronal green line)

41. Coronagraph (cont.) K corona: continuum
F corona: continuum plus absorption
E corona: emission lines

42. Coronagraph (cont.) K corona brightness is less than 10-5 that of disk
Space observation is necessary
Special instrumental design to reduce scattering of light in instrument

43. Several streamers in a typical coronagraph image

44. Streamer (cont.)
A streamer is a stable large-scale structure in the white-light corona.
It has an appearance of extending away from the Sun along the radial direction
It is often associated with active regions and filaments/filament channels underneath.
It overlies the magnetic inversion line in the solar photospheric magnetic fields.
When a CME occurs underneath a streamer, the associated streamer will be blown away
When a CME occurs nearby a streamer, the streamer may be disturbed, but not necessarily disrupted.

45. CME: transient phenomenon (example)
A LASCO C2 movie, showing multiple CMEs

46. CME Property: Measurement
H (height, Rs)
PA (position angle)
AW (angular width)
M (mass)

47. CME Property: velocity
Velocity is derived from a series of CME H-T (height-time) measurement
A CME usually has a near-constant speed in the outer corona (e.g, > 2.0 Rs in C2/C3 field)
Note: such measured velocity is the projected velocity on the plane of the sky; it is not the real velocity in the 3-D space.

48. CME Property: velocity distribution
CME velocity ranges from
50 km/s to 3000 km/s
Average velocity: 400 km/s
Peak velocity: km/s
Median velocity: km/s
6300 LASCO CMEs from 1996 to 2002

49. CME Property: size
AW = 80 degree AW = 360 degree, halo CME

50. CME Property: size Broad distribution of CME apparent angular width
Average width 50 degree
A number of halo CMEs (AW=360 degree), or partial halo CMEs (AW > 120 degree)
Halo CMEs are those likely to impact the Earth orbit

51. CME Property: mass CME mass distribution from 1013 to 1016 gram
Average CME mass about 1015 gram
Based on
2449 LASCO CMEs
From 1996 to 2000

52. CME Morphology

53. CME morphology (cont) Three part CME structure
A bright frontal loop (or leading edge)
Pile-up of surrounding plasma in the front
A dark cavity (surrounded by the frontal loop)
possibly expanding flux rope or filament channel
A bright core (within the cavity)
Composed of densely filament remnant material

54. CMEs and Other Solar Activities
Exp. A CME associated with a filament
EIT movie of 2000/02/27 showing
filament eruption
C2 movie of 2000/02/27 CME

55. CMEs and Other Solar Activities (cont.)
Exp. A CME associated with a (EIT) coronal dimming
Coronal dimming, often seen in EIT 195 Å images (1.5 MK), is caused by mass depletion following CME eruption

56. kinematic Evolution of a CME
A CME is strongly accelerated in the inner corona (<2 Rs); unfortunately, inner corona observations have been very poor.
A CME maintains a more or less constant speed when it travels in the outer corona (>2 Rs); it interacts with background solar wind in the interplanetary space.

57. Geo-effective CMEs: halo CMEs
Whether a CME is able to intercept the Earth depends on its propagation direction in the heliosphere.
A halo CME (360 degree of angular width) is likely to have a component moving along the Sun-Earth connection line
A halo is a projection effect; it happens when a CME is initiated close to the disk center and thus moves along the Sun-Earth connection line.
Therefore, a halo CME is possibly geo-effective.
2000/07/14
C EIT

58. CME models A model shall include many observational elements CME front
cavity
core
Flare
X-ray loop
EUV loop arcade
Hα flare ribbon
Magnetic reconnection
Current sheet
Reconnection inflow
Some Others
filament eruption
coronal dimming
timing relation
Energetic relation

59. CME models (cont.) Lin’s CME eruption model MHD analytic solution
Animation

60. CME models (cont.) Aschwanden book: Fig. 17.3, P710:
Numeric Simulation
By Amari et al

61. CME models (cont.) Antiocs’s CME eruption model MHD numeric solution
Multi-polar
So-called break-out model

62. The End