Introduction to Space Weather Jie Zhang CSI 662 / PHYS 660 Spring, 2012 Copyright © The Sun: Solar Eruptions Feb. 23, 2012

Roadmap Part 1: Sun Part 2: Heliosphere Part 3: Magnetosphere Part 4: Ionosphere Part 5: Space Weather Effects CH1: Structure CH2: Magnetism and Dynamo CH3: Magnetic Structure CH4: Solar Eruptions

CSI 662 / PHYS 660 Feb. 07, Flares 4.2. Models of Flares 4.3. CMEs 4.4. Models of CMEs Plasma-7: Magnetic Reconnection

CH4: Solar Eruptions References and Reading Assignment: KAL CH 3.5 (on Magnetic Reconnection) KAL CH 6.7 (on Flares and CMEs)

CH 4.1 Flares Solar flare are a phenomenon of sudden brightening in solar atmospheres, including in corona, chromosphere and even photosphere A flare releases Joules of energy in several minutes. (Note: one H-bomb: 10 million tons of TNT = 5.0 X Joules) (Note: worldwide energy consumption/year = Joules) Flares produce enhanced emission in all wavelengths across the electro-magnetic spectrum, including radio, optical, UV, soft X- rays, hard X-rays, and γ-rays Flare electro-magnetic emissions are caused by 1.plasma heating (up to 20 million K): radio, visible, UV, EUV and soft X-ray 2.non-thermal energetic particles (electrons, tens – hundreds of keV; protons ): radio, hard X-ray, γ-rays

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

The 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: in Hα Hα line is the best spectral line of observing the Chromosphere What you see is the heating of the Chromosphere Transient features: flare ribbons The Seahorse Flare

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

Flare: in soft X-rays (~ 10 Å) Heating: temperature increases in Corona (~ 20 MK) Transient features: 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 electrons through Bremsstrahlung (braking) emission mechanism

Flare: in microwave Nobeyama Radioheliograph (17 Ghz, or 1.76 cm) and (34 Ghz, or 0.88 cm) Non-thermal emission due to non-thermal energetic electrons (~100 keV) emission mechanism: gyro-synchrotron emission

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

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 preparation phase. It shows small increase of emission in soft X-ray and Hα, indicating heating Impulsive phase: the flare main energy release phase. It is most evident in hard X-ray, γ-ray and microwave emission. The soft X-ray flux rises rapidly during this phase Flare ribbons in Hα start to appear in this phase Gradual phase: no further emission in hard X-rays, and the soft X-ray flux starts to decrease gradually. Loop arcade (in EUV) starts to appear in this phase

Flare: Spectrum The emission spectrum during a 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 emission through Bremsstrahlung 2.Power-law distribution in hard X-ray energy range (e.g., 10 keV to 100 keV): non-thermal emission through Bremstrahlung 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 emission through Bremstrahlung Nuclear reaction

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

CH 4.2 Models of Flares How possible is the sudden release of magnetic energy in minutes in the corona, given large conductivity? The big question?

Plasma-7: Magnetic Reconnection Magnetic field lines with opposite polarities are pushed together in an X configuration At the X point, B  0 Called the diffusion region It contains strong electric current. Outside the diffusion region, plasma remains frozen-in In the diffusion region, B field decouples from the V field, allowing reconnection Extremely small characteristic scale L, resulting in small dissipation time

Sweet-Parker Reconnection (1958) Plasma Inflow Plasma Outflow Diffusion Region Reconnection in the current sheet Plasma-7: Magnetic Reconnection

1.Rearrange the magnetic field, thus change the magnetic topology 2.Heating plasma in the diffusion region, through Joule heating 3.Accelerating energetic particles in the diffusion region Consequences: Plasma-7: Magnetic Reconnection

Models of Flares Schematic Models

Models of Flares 1.Magnetic reconnection occurs at the top of the magnetic loop 2.Energetic particles are accelerated at the reconnection site 3.Particles precipitate along the magnetic loop, giving microwave 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 plamas, giving soft X-ray emission. 6. Post-eruption loop arcade appears successively high, because of the reconnection site rises with time 7. The ribbon separates with time because of the increasing distance between the footpoints due to higher loop arcades

CH 4.3 CMEs (Coronal Mass Ejections) A CME is a large scale coronal structure ejected from the Sun Flares are observed low in the corona close to surface CMEs are observed high in the corona A CME propagates into the interplanetary space. Some of them may intercept the Earth orbit if it moves toward the direction of the Earth, causing the space weather CME eruptions are often associated with flares and filament eruptions.

LASCO C2 movie CMEs

Coronagraphs A coronagraph is a telescope equipped with an occulting disk that blocks out the sunlight from the disk of the Sun, in order to observe the faint light from the corona A coronagraph makes artificial solar eclipses The photons seen in white light images 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, emission) (1996 to 1998 only) C2: 2.0 – 6.0 Rs (K corona, scattering) (1996 up to date) C3: 4.0 – 30.0 Rs (K corona, scattering) (1996 up to date) C1 C2 C3 The LASCO uses a set of three overlapping coronagraphs to maximize the effective fields 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 outer 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 (outside active regions) underneath. It overlies the magnetic polarity inversion line Streamers

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

Magnetic configuration Open field lines with opposite polarity Current sheet in between Extends above the cusp of a coronal helmet Closed magnetic structure underneath the cusp Streamer is associated with the heliospheric current sheet Streamers

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 CMEs

CH 4.4 Models of CMEs How CMEs formed? What is the magnetic structure of CMEs prior to the eruption? What triggers the CME? How is a CME accelerated? The big question?

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 CH 4.4 Models of CMEs

CMEs are caused by the eruption of the twisted flux rope When the twist is strong enough, it undergoes Torus Instability or Kink instability, causing the eruption CH 4.4 Models of CMEs

Unified CME-Flare model Magnetic reconnection occurs underneath the rising flux rope, causing flares down below Magnetic reconnection imposes a tether-cutting effect, removing the overlying magnetic field Magnetic reconnection produces the helical field, thus further accelerating the flux rope Plasma and magnetic 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

Antiochos’s 3-D CME eruption model So-called break-out model There is no need of a twisted flux rope prior to the eruption Flux rope forms during the eruption Unified CME-Flare model

Zhang, Cheng & Ding, 2012 Evidence of Flux Ropes

The End