Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Fall, 2009 Copyright © The Sun: Flares and CMEs Oct. 01, 2009

Roadmap Part 1: The Sun Part 2: The Heliosphere Part 3: The Magnetosphere Part 4: The Ionsophere Part 5: Space Weather Effects Part 1: The Sun 1.The Structure of the Sun: Interior and Atmosphere 2.Solar Magnetism: Sunspots, Solar Cycle, and Solar Dynamo 3.Solar Corona: Magnetic Structure, Active Regions, Coronal Heating 4.Major Solar Activities: Flares and Coronal Mass Ejections

Major Solar Activities: Flares and Coronal Mass Ejections CSI 662 / PHYS 660 October References: Kallenrode: Chap. 3, Chap. 6.7 Aschwanden: Chap.16, Chap. 17

Plasma Physics 1. Magneto-hydrodynamic Equations –Kallenrode: Chap 3.1, Chap Magnetic Reconnection –Kallenrode: Chap 3.5

Solar Flare A solar flare is a phenomenon of sudden brightening in solar atmospheres, including photosphere, chromosphere and corona Flares release ergs energy in minutes. (Note: one H-bomb: 10 million tons of TNT = 5.0 X ergs) (Note: worldwide energy consumption/year = ergs) A flare produces enhanced emission in all wavelengths across the electro-magnetic spectrum, including radio, optical, UV, soft X-rays, hard X-rays, and γ-rays Flare EM emissions are caused by 1.hot plasma (through heating mechanisms): radio, visible, UV, soft X-ray 2.non-thermal energetic particles (through particle acceleration mechanisms): radio, hard X-ray, γ-rays

Carrington Flare Diagram of the Flare of 1859 and associated sunspot group, drawn by Richard C. Carrington. The flare is the items labeled A-D.

Carrington Flare On September 2 nd, 1859, the Earth went mad. Auroras lit up the sky over Australia, Japan, Colorado, and even as close to the equator as Venezuela. The worldwide telegraph system, which had gone from a laboratory curiosity to the wonder of the age in the previous twenty years, went haywire—sparking operators, scorching paper tapes, and mysteriously still transmitting messages between Boston, Massachusetts and Portland, Maine although the batteries that ran the system had been disconnected out of self-defense. At Kew Gardens in London, a set of magnetometers designed to study the Earth’s magnetic field started showing “disturbances of unusual violence and very wide extent”

Flare: Hα Heating: temperature increases in Chromosphere. Transient Structures: ribbons

Flare: in EUV (~ 195 Å) TRACE Observation: 2000 July 14 flare Heating: temperature and density increase in corona Transient Structures Ribbons Post-eruption loop arcade Filament eruption

Flare: in soft X-rays (~ 10 Å) Heating: temperature increase in Corona (~ 10 MK) Transient Structures: fat X-ray loops

Flare: 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: 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 emission mechanism: gyro-synchrotron emission

Flares: X-ray Classification ClassIntensity (erg cm -2 s -1 ) I (W m -2 ) B C M X Based on intensities recorded by NOAA GOES Satellite Soft X-ray instrument (1 – 8 )

Flare: Temporal Evolution 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 Evolution 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 Flare ribbons starts to appear in 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 The emission spectrum during flare’s impulsive phase

Flare: Spectrum 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

Question? What is the physical mechanism of a solar flare? How possible is the sudden energy release in the corona, given large conductivity?

Magnetic Reconnection Magnetic reconnection is believed to be the physical process that explosively dissipate, or “annihilate”, magnetic energy stored in magnetic field Magnetic reconnection causes violent solar activities, such as flares and CMEs, which in turn drive severe space weather

Magnetic Reconnection magnetic field diffusion time τ d in the corona τ d = L 2 /η τ d: the time scale the magnetic field in size L dissipates away, η magnetic diffusivity, L the magnetic field scale size In normal coronal condition, τ d ~ s, or 1 million year (assuming L=10 9 cm, T=10 6 K, and σ =10 7 T 3/2 s -1 ) To reduce τ d, reduce L to an extremely thin layer, and/or reduce the conductivity (increase resistivity, e.g., anomalous resistivity due to plasma turbulence)

Magnetic Reconnection Magnetic fields with opposite polarities are pushed together At the boundary, B  0, forming a high-β region. Called diffusion region, since plasma V could cross B Since E= -(V × B)/c, it induces strong electric current in the diffusion region, also called current sheet Outside the diffusion region, plasma remains low β Strong energy dissipation in the current sheet, because of high current and enhanced resistivity

Magnetic Reconnection Sweet-Parker Reconnection (1958) Plasma Inflow Plasma Outflow Diffusion Region Magnetic Reconnection Rate M = V i /V O (in-speed/out-speed)

(Continued) Flares and CMEs October 8, 2009

Flare Models

Flare Model 1.Magnetic reconnection occurs at the top of the magnetic loop 2.Energetic particles are accelerated at the reconnection site 3.Particles precipitates along the magnetic loop, giving radio emission 4.Energetic particles hit the chromosphere footpoints, giving hard X-ray emission, γ-ray emission, Hα emission and ribbons 5. Heated chromspheric plasma evaporates into the corona (chromospheric evaporation), filling up the loops with hot plams, giving soft X-ray emission. 5. Post-eruption loop arcade appears successively high, because of the reconnection site rises with time 6. The ribbon separates with time because of the increasing distance between footpoints due to higher loop arcades

CME (Coronal Mass Ejection) A CME is a large scale coronal plasma and magnetic field structure ejected from the Sun Flares are observed low in the corona close to the chromosphere A CME propagates into interplanetary space. Some of them may intercept the Earth orbit if it moves toward the direction of the Earth CME eruptions are often associated with filament eruption and flares

A LASCO C2 movie, showing multiple CMEs CME

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 photons seen in white light are from Thomson-scattering of photospheric photons by free elections in the corona Therefore, a coronagraph maps the line-of-sight column density of coronal electrons.

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.

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 (in active regions) and filaments/filament channels (outside active regions) underneath. It overlies the magnetic polarity inversion line in the solar photospheric magnetic field Streamer

Polarity Inversion Line BBSO HαMt. Wilson Magnetogram Filaments always ride along the inversion line Flares and CMEs always occur along the inversion line The prime location for magnetic reconnection

Magnetic configuration Open field with opposite polarity centered on the current sheet Extends above the cusp of a coronal helmet Closed magnetic structure underneath the cusp Streamer is associated with the heliospheric current sheet Streamer Structure

CME Properties H (height, Rs) PA (position angle) AW (angular width) M (mass)

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 deviates from the real velocity in the 3-D space. CME Properties

Velocity: 10 km/s to 2500 km/s Mass: to g Kinetic energy: – ergs Width: between 10 to 120 degree, average 50 degree CME Properties

Whether a CME is able to intercept the Earth depends on its propagation direction in the heliosphere. A halo CME (360 degree of apparent 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 Halo CME

Twisted magnetic flux rope forms above the polarity inversion line due to the shearing motion of photospheric magnetic field and/or emergence Flux rope carries strong electric current (Ampere’s Law), thus carries a large amount of free energy Keep an expanding magnetic flux rope structure post the eruption CME Structure

CME is caused by the eruption of the twisted flux rope above the magnetic inversion line When the twist is strong enough, it undergoes kink instability, causing the eruption CME model

Unified CME-Flare model Magnetic reconnection occurs underneath the rising flux rope, causing flares down below Magnetic reconnection imposes tether-cutting effect, removing the line-tying magnetic field, increasing the helical flux, thus further accelerating the flux rope Plasma evacuation strengthens the inflow velocity of the surrounding magnetic field, further feeding the reconnection

Unified CME-Flare model Lin’s 2-D CME eruption model

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 CME Eruption

Antiocs’s 3-D CME eruption model So-called break-out model There is no need of a twisted flux rope existing prior the eruption Flux rope forms during the eruption Multi-polar MHD numeric solution Other CME Models

MHD Equations Magneto-hydrodynamic Equations –Kallenrode: Chap 3.1, Chap 3.2

The End