Solar Flare

CSI 769-001/PHYS 590-001 Solar Atmosphere Fall 2004 Lecture 10 Nov. 03, 2004

Solar Flare

What is solar flare?

•Solar flare is a rapid heating of a part of the solar corona and chromosphere, by traditional definition

•A solar flare is an enormous explosion in the solar atmosphere, involving sudden bursts of plasma heating, particle acceleration, and bulk mass motion.

•A flare can release 1027 - 1032 ergs or more energy in a few minutes to a few tens of minutes.

•Flare emission covers a wide range (if not full) of electromagnetic spectrum, from long to short wavelength, including radio, optical, soft X-rays, hard X-rays, Gamma-rays (and in between IR, UV, and EUV)

Example: Flare in optical Hα (6563 Å)

Heating: temperature increase in ChromosphereStructure: Hα ribbons

Example: Flare in EUV (~ 195 Å)

TRACE Observation: 2000 July 14 flare

•Heating: temperature increase in corona•Structure

• EUV ribbons• Loop arcade• Filament eruption

Example: Flare in soft X-ray (~ 10 Å)

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

Example: Flare 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

•Structure:

Example: Flare in hard X-ray (< 1 Å)

RHESSI in hard X-ray (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 Light Curves

Also see Figure 9.2 at P. 279 in the textbook

Flare Light Curves (cont.)

GOES X-ray flare is frequent

Flare Light Curves (cont.)

Strong flares in 2004 Oct.

Flare Frequency

GOES X-ray flare:11696 events From 1996 Jan. To 2001 Dec.

At solar minimum:1 event/per day

At solar Maximum:10 event/per day

X-ray Classification of Solar Flares

•See textbook Table 9.3 at P. 281

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

Geo-effect of X-ray radiation from flares•X-ray emission from flares cause sudden ionosphere disturbance (SID) by increasing ionization of Earth’s atmosphere•This causes radio blackouts.•NOAA Space Weather Scale for Radio Blackouts

•R5—R1: Extreme, Severe, Strong, Moderate and Minor•R5: Extreme X20 flare, less than 1 per cycle

•HF Radio:Complete HF (high frequency**) radio blackout on the entire sunlit side of the Earth lasting for a number of hours. This results in no HF radio contact with mariners and en route aviators in this sector. •Navigation: Low-frequency navigation signals used by maritime and general aviation systems experience outages on the sunlit side of the Earth for many hours, causing loss in positioning. Increased satellite navigation errors in positioning for several hours on the sunlit side of Earth, which may spread into the night side.

•R1: Minor M1 flare, 2000 per cycle•HF Radio: Weak or minor degradation of HF radio communication on sunlit side, occasional loss of radio contact.•Navigation: Low-frequency navigation signals degraded for brief intervals.

Hα Classification of Solar Flares•See textbook Table 9.2 at P. 281•Size Importance

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

Magnetic Reconnection•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 field diffusion time τd in the corona (Eq. 9.1 at P 284 and Eq. 7.34 at P. 217)

• τd = 4πσL2/c2 = L2/η•τd the time scale the magnetic field in size L dissipate awayWhere 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)

Magnetic Reconnection (cont.)• Magnetic reconnection

• Quick magnetic energy dissipation at the current sheet (a neutral sheet) that separates two magnetic regimes, which1. have different directions (opposite directions in a 2-D

space)2. are forced to push together by a continuous

converging motion• The current sheet can be very thin, which makes the scale

size (L) of magnetic field cross the current sheet very small. The smaller the L, the fast the dissipation

τd = 4πσL2/c2 = L2/η• Ohmic dissipation rate ~ ηJ2, ~ ηB2/L2

Magnetic Reconnection (cont.)

See text book P. 284, Figure 9.4

Sweet-Parker reconnection Petschek reconnection

Magnetic Reconnection (cont.)• Sweet and Parker Reconnection (Fig. 9.4 left, P. 284-286)

• A steady state magnetic reconnection• Y-shape magnetic configuration• Oppositely-directed field lines, frozen to the plasma, are

carried towards one another at a speed by a converging flow

• Magnetic energy dissipated in the current sheet. • Within the current sheet, magnetic field is not in frozen-in

state.• Plasma ejected out of the diffusion region• Much faster energy dissipation, but still not fast enough to

account for the flare energy release in time scale of minutes

Magnetic Reconnection (cont.)

• Petschek Reconnection (Fig 9.4, right, P. 284-286)• Similar to Sweet-Parker geometry in the large scale, but it

has a smaller diffusion region• X-shape magnetic configuration instead of Y-shape in the

current sheet• True magnetic reconnection at the X-point• Ejection of plasma as well as magnetic field out of the

diffusion region• Allowing faster inflow• Magnetic energy dissipation is fast enough to account for

the flare