CSI 769/ASTR 769 Topics in Space Weather Fall 2005Lecture 03 Sep. 13, 2005

Solar Eruptions: Flares and Coronal Mass Ejections

•Aschwanden, “Physics of the Solar Corona”•Chap. 16, P671-702, Flare Plasma Dynamics•Chap. 17, P703-737, Coronal Mass Ejections (CMEs)•Chap. 10, P407-463, Magnetic Reconnection

8010 + 17:17 17:40 18:03 G12 5 XRA 1-8A X17.0 2.6E00 0808

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

http://www.sec.noaa.gov

Magnetogram on Sep. 08 Sep. 12

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 1027 - 1032 ergs energy in a few to tens of minutes. (Note: one H-bomb: 10 million TNT = 5.0 X 1023 ergs)

Flare: Example in optical Hα

Heating: temperature increase in ChromosphereStructure: Hα ribbons

Flare: Example in EUV (~ 195 Å)

TRACE Observation: 2000 July 14 flare

•Heating: temperature increase in corona•Structure

• EUV ribbons• Loop arcade• Filament eruption

Flare: Example in soft X-rays (~ 10 Å)

Heating: temperature increase in CoronaStructure: fat X-ray loops

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

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

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.

Flares: X-ray Classification

Class 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 10-4

Flare: Hα Classification

Area

(millionth of a solar hemisphere)

Area

(in square degree heliocentric)

Size Important

<100 <2.06 S (Sub-flare)

100---250 2.06—5.15 1

250--600 5.15---12.4 2

600---1200 12.4---24.7 3

>1200 >24.7 4

•Brightness Importance•F: faint•N: Normal •B: Brilliant

•e.g., 3B means covering >12.4 deg2 and exceptionally bright

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

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

Flare: Spectrum Property

• The emission spectrum during flare’s impulsive phase

Flare: Spectrum Property (cont.)• A full flare spectrum may have three components:

1. Exponential distribution in Soft X-ray energy range (e.g., 1 keV to 10 keV): • thermal Bremsstrahlung emission

2. 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

3. Power-law plus spectral line distribution in Gamma-ray energy range (e.g., 100 keV to 100 MeV)• non-thermal Bremstrahlung emission• Nuclear reaction

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

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

Magnetic Reconnection

•MHD equations

(Aschwanden 6.1.36, 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 ~ 1014 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

BcBVtB

)(4

2)(

Magnetic Reconnection: 2-D (cont.)

• quick magnetic energy dissipates at the current sheet (a magnetic neutral sheet) that separates two magnetic regimes, which1. have different directions (opposite directions2. 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.408-412)

Magnetic Reconnection: 2D (cont.)

Sweet-Parker reconnection

Petschek reconnection

(Aschwanden Book, Fig. 10.2, P411)

Magnetic Reconnection: 3-D (cont.)

(Aschwanden Book, Fig. 10.10, P423)

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

Flare Model: standard 2-D model

•Aschwanden book. Fig. 10.20, P437

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)

Flare dynamic Model

A cartoon model by Gurman

•Particle precipitation

•Thermal conduction

•Chromospheric Evaporation

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

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

Filament Eruption

BBSO Hα Mt. Wilson Magnetogram

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

Filament Eruption (cont.)

•One of filament models: filament is supported by twisted magnetic flux rope

Filament Eruption (cont.)

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

Filament Eruption Model

•model of Martens & Kuin

•A unified model of•Filament and•Flare

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.

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

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)

C1 C2 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.

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)

Coronagraph (cont.)

• K corona: continuum• F corona: continuum plus absorption• E corona: emission lines

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

Streamer

Several streamers in a typical coronagraph image

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.

CME: transient phenomenon (example)

A LASCO C2 movie, showing multiple CMEs

CME Property: Measurement

H (height, Rs)

PA (position angle)

AW (angular width)

M (mass)

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.

CME Property: velocity distribution

6300 LASCO CMEs from 1996 to 2002

•CME velocity50 km/s to 3000 km/s

•Average vel.: 400 km/s

•Peak vel.: 300 km/s

•Median vel.: 350 km/s

CME Property: size

AW = 80 degree AW = 360 degree, halo CME

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

CME Property: mass

•CME mass distribution from 1013 to 1016 gram

•Average CME mass about 1015 gram

Based on2449 LASCO CMEsFrom 1996 to 2000

CME Morphology

CME morphology (cont)

• Three part CME structure1. A bright frontal loop (or leading edge)

• Pile-up of surrounding plasma in the front2. A dark cavity (surrounded by the frontal loop)

• possibly expanding flux rope or filament channel3. A bright core (within the cavity)

• Composed of densely filament remnant material

kinematic Evolution of a CME

• A CME is strongly accelerated in the inner corona (<2 Rs) (Zhang et al., 2001, 2004)

• A CME maintains a more or less constant speed when it travels in the outer corona (>2 Rs)

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

C2 EIT

CME modelsA 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

CME models (cont.)

Lin’s CME eruption model• MHD analytic solution•Animation

CME models (cont.)

Aschwanden book:Fig. 17.3, P710:

Numeric SimulationBy Amari et al

CME models (cont.)

Antiocs’s CME eruption model•MHD numeric solution •Multi-polar•So-called break-out model

The End