CMEs, the Solar Wind and the Heliosphere Professor Richard Harrison Head of Space Physics & Chief Scientist, RAL Space PI – STEREO/HI (2002-to date) PI – SOHO/CDS (1992-2003)

The Heliosphere We live in a bubble – a volume in space defined by, and dominated by the Sun – we call is the heliosphere

You are here!

The Heliosphere …at its centre – our Sun. An ordinary G-class star, but how does that star influence the space around us?

The Sun:

The ‘engine room’ of

the Solar System:

• Defines the

environment in which

we live

• Generates a range

of natural hazards

which impact human

activity

From the point of view of a planet…

Coupling: Sun to Earth We feel and see heat and light from the Sun; the planets are bathed in the Sun’s EM radiation, but is there a physical coupling? First suggestion of such a coupling: 1 Sept 1859 - the first observation of a solar flare, made by Carrington & Hodgson. Followed by the largest recorded geomagnetic storm ( associated with widespread aurora and major disruption to the telegraph systems; Carrington suspected a solar- terrestrial link.

Coupling: Sun to Earth Once flares discovered - researchers began to propose that flares are coupled to geomagnetic disturbances by beams of electrons from the Sun e.g. Kristian Birkeland, Oliver Heaviside 1916 – Birkeland proposed, from auroral studies and simulation using electron beams and a terrella (small magnetised ‘Earth’), that the Earth was bombarded continuously by a solar wind of ions and electrons – but we can also see comets!

Coupling: Sun to Earth Grotrian (1939) & Edlen (1941) identified emission lines in the coronal spectrum which were due to highly ionised iron (Fe XIV 5303 Å & Fe X 6374 Å – the so-called green & red lines). The corona is hot: ~ 106 K

Movie 1

Movie 2

…The Hot Corona: Recent observations The corona – hot plasma guided & confined by a complex magnetically dominated environment: T ~106 K, N ~10-12 kg m-3 or 10-15 particles m-3. So, what does this star do to the space that surrounds it?

The Solar Wind

So, the idea of a solar wind was established – but we had not measured it! Scene set for Eugene Parker to develop the first model of what he called the solar wind (Parker, E.N., Astrophys. J. 128, 664, 1958) …but it was controversial - his paper was rejected by two reviewers! Basic idea – conservation of mass and momentum in a coronal-type plasma, and the balance between pressure and gravity. A simple but effective model.

A Solar Wind – Parker’s model

Conservation of mass: ∂ρ/ ∂t + .(ρu) = 0 (1) ρ = mass density, u = fluid flow Sum of changes in mass in space and time = 0 Conservation of momentum: ρ∂u/∂t + ρu.u = −p + jxB + ρFg (2) R.H. Terms: Forces due to pressure gradient (fluid moves towards lower density region), magnetic field and gravity. [units: Nm-3]

A Solar Wind – Parker’s model Consider a parcel of fluid moving outwards: Assume a time-independent, spherically symmetric Sun with an isothermal atmosphere i.e. all ∂/∂t terms = 0; flow is radial; so… ρ∂u/∂t + ρu.u = −p + jxB + ρFg

= 0 (dP/dr) er

Fg = - ((GMsun)/r2) er

Note: er is a unit vector pointing radially outwards from Sun-centre; R is

the distance from Sun-centre; Fg is negative because it acts inwards

(ρu(du/dr)) er Ignore!

A Solar Wind – Parker’s model

ρu (du/dr) = −dp/dr - ρ(GMsun /r2) (4) effectively the balance of pressure and gravitational forces along a radial line, and we can consider that balance as a function of r. Need a form of this equation with appropriate parameters to explore the balance...

A Solar Wind – First Observations Luna 1 (USSR) - first detected solar wind directly in January 1959. The discovery was made using hemispherical ion trap instruments led by Konstantin Gringauz. An ion trap amplifies the weak current that results when particles pass through an electric field between an inner grid and collector plate. To obtain information about the energy spectrum, an outer grid could be charged to repel ions below a threshold energy

A Solar Wind – Recent Observations Ulysses, a unique opportunity to ‘map’ the solar wind

A Solar Wind – Recent Observations • High latitudes – fast, ‘smooth’, ~800 kms-1 • Low latitudes (i.e. Earth -impacting) – slow, gusty, ~ 450 kms-1 • Basic associations: fast wind and coronal holes, slow wind and active region belts

• Note the sharp boundary (varies with cycle), the direction of the magnetic field and the lack of a polar enhancement

A Solar Wind – Recent Observations … and with density information: Fast wind, less dense, ~2-3 particles cm-3, compared to ~6-8 particles cm-3 in slow regions

A Solar Wind – What does it all mean? • A basic theory predicted a steady solar wind in 1958

• The solar wind was first measured directly in 1959

• Modern solar wind observations reveal a two-part structure: fast and slow

• The basic theory is sound but it does not explain the high wind speeds and the gusty nature of the slow wind

• However, do note that our solar wind observations are usually single-point in-situ observations and this must be remembered when talking about the global structure

A Solar Wind – Summary

Fast Slow Comments

Speed 700-800 kms-1 300-400 kms-1 Supersonic (slow

wind ~ 4x Cs)

Characteristic Smooth Gusty

Location Higher latitudes,

associated with

coronal holes

Lower latitudes,

associated with

active region

belts

Density 2-3 ions cm-3 or ~ few

x 106 particles m-3

6-8 ions cm-3 or

~107 particles m-3

At 1 AU

He content ~4%, fairly constant ~2%, highly

variable

P temperature 2 x 105 K 3 x 104 K

Av particle flux from Sun ~ 1.3 x 1036 s-1 (6.7 b tonnes per hr)

A Solar Wind – What does it all mean? • The eclipse images are taken at solar minimum (top) and Maximum (bottom) • Minimum: active regions are few, and at low latitudes; overlying these are streamers. These regions correspond to the slow, gusty wind detected at low solar latitudes. • Also at minimum, the polar coronal holes are extended. These are the source of the fast, Smooth wind.

A Solar Wind: ‘Frozen-in’ consequences There is another key feature of the solar wind. Consider Maxwell’s equations: .E = (ρc)/εo (Gauss’ law) .B = 0 x E = - ∂B/ ∂t (Faraday’s law) x B = μoj + μoεo ∂E/∂t (Ampere’s law) Also, j = σ(E + v x B) (Ohm’s Law) ρc = Charge density εo = Permittivity of free space = 8.854 x 10-12 Fm-1 σ = Electrical conductivity μo = Permeability of free space = 4π x 10 -7 VsA-1m-1

A Solar Wind: ‘Frozen-in’ consequences There is another key feature of the solar wind. Consider Maxwell’s equations: .E = (ρc)/εo (Gauss’ law) .B = 0 x E = - ∂B/ ∂t (Faraday’s law) x B = μoj + μoεo ∂E/∂t (Ampere’s law) Also, j = σ(E + v x B) (Ohm’s Law) Combine Ampere’s law, Ohm’s law and Faraday’s law: x B = μoσ(E + v x B) + μoεo ∂E/∂t

i.e. x x B = μoσ( x E + x v x B) assuming displacement current μoεo ∂E/∂t is small

A Solar Wind: ‘Frozen-in’ consequences

We have x x B = μoσ( x E + x v x B) Use the relation x x B = (.B) - 2B and note that .B = 0. So, - 2 B = μoσ( - ∂B/ ∂t + x v x B), or ∂B/ ∂t = x (v x B) + (2 B)/μoσ (5) This is an extremely important equation for characterising the solar wind plasma

A Solar Wind: ‘Frozen-in’ consequences

∂B/ ∂t = x (v x B) + (2 B)/μoσ (5) The balance between the two terms on the RHS is decided by the electrical conductivity. High σ: First term on the RHS dominates. Equ 5 reduces to a convection equation of the magnetic field by the plasma flow – the magnetic flux is frozen to the plasma Low σ: Second term on the RHS dominates. Equ 5 reduces to a diffusion equation of the magnetic field through the plasma In most plasma conditions we consider we have high electrical conductivity – we have a frozen-in condition

A Solar Wind: ‘Frozen-in’ consequences

For the frozen-in condition, there is one further consideration; the field and plasma are frozen into one another – but which dominates? Does the field go with the plasma or the plasma go with the field? The plasma beta – β – the ratio of the plasma and magnetic pressures tells us which dominates: β = p/ (B2/2μo) which is, in effect, derived from a balance of magnetic and pressure forces derived from equation 2 (momentum equation)

A Solar Wind: ‘Frozen-in’ consequences

Solar interior – high β – so the magnetic fields are tied to the plasma motion (convection, turbulence, rotation)

Solar corona – low β – so the magnetic fields dictate the topology of the plasma (loops)

A Solar Wind: ‘Frozen-in’ consequences

A parcel of solar wind takes magnetic field with it, drawing the solar fields into space. Assume a radial outflow, and a rotating Sun. Consider a string of parcels ejected from the same location. The result is a spiral structure to the solar wind magnetic field (crossing Earth’s position at about 45o to the radial)

t0 t1 t2 t3

A Solar Wind: ‘Frozen-in’ consequences

Of course, the spiral is a 3 dimensional structure…

A Solar Wind: Reality! Assumed: (i) Constant outflow, (ii) Uniform solar field. Consider variable outflow speeds & complex fields being carried into space Slower speeds ‘compress’ the spiral; faster speeds ‘expand’ it The complex solar fields complicate the outflow. Each spiral ‘arm’ is associated with inward or outward directed fields.

A Solar Wind: Reality! As the planets witness the rotation of the Parker spiral they move into different magnetic domains – and different domains of plasma flow

A Solar Wind acceleration: Other options

As noted – the Parker solar wind theory is good for a ‘quiet’, slow solar wind For the generation of the fast solar wind, and to investigate the variability of the slow wind, we must call upon other mechanisms … a major debate in solar physics – relevant to all stars! To accelerate plasmas in the solar atmosphere we need to look for an energy source (presumably the thermal or kinetic energy of the plasmas in the body of the Sun) and be able to transfer the energy to the corona where it is dissipated To consider this (qualitatively), let us look at the nature of the solar atmosphere

A Solar Wind acceleration: Other options The magnetic nature of the Sun is revealed using magnetograph; even close-up we see a ‘salt and pepper’ of magnetic polarities. Researchers talk of a magnetic ‘carpet’

A Solar Wind acceleration: Other options Consider the low corona as a ‘wheat-field’ of magnetic field lines rooted in the body of the Sun – subject to plasma motion (high beta) (convection, turbulence, rotation...) This ‘recipe’ can lead to a number of scenarios: • Waves (e.g. Alfvén waves) can propagate along field lines and dissipate energy

• Sites of magnetic reconnection (microflares, jets, explosive events) etc..... can convert energy tied in stressed magnetic fields to heat and acceleration

A Solar Wind acceleration: Other options

SOHO observations show blue-shifted regions, especially in polar regions - evidence for acceleration ‘events’ in the solar atmosphere

…but there is more: Transients in the Solar Wind

The first detected coronal transient – later to be known as a Coronal Mass Ejection.

Movie 3

Solar Activity: Flares Complex magnetic (active) regions can flare as the highly stressed fields break down and restructure, dumping excess energy into the local plasmas Typical parameters: Duration ~ 1000 s Energy 1025 J Temperatures > few million K Frequency Several a day when the Sun is active, and almost none when inactive

Solar Activity: Coronal Mass Ejections Huge magnetic clouds expelled by the Sun which pass through interplanetary space Typical parameters: Speed 300-400 kms-1 i.e. ~ 4-5 days to reach us Mass 1012-13 kg ~ billion tonnes Frequency < 2-3 per day when Sun is active; one every few days when inactive

STEREO-A

(NASA, 2006)

STEREO-B

(NASA, 2006)

SOHO

(ESA, 1995)

SDO

(NASA, 2010)

Hinode

(JAXA, 2006)

Solar Orbiter

(ESA, 2017)

The fleet

© 2010 RAL Space

Understanding the magnetic/plasma structure/processes in the solar atmosphere that lead to heating, acceleration, flares, CMEs etc…..

© 2010 RAL Space

OSO/Skylab – first CME observations did hint that there was NOT a one to one relationship. SMM investigations from 1985 showed more inconsistencies… (e.g. Harrison A&A, 162, 1986)

The CME Onset: Flare-CME Relationship

Standard Flare-CME model - does it fit the bill? Consider geometry, timing, lack of one to one relationship. Too simple minded!

The CME Onset: Flare-CME Relationship

CME outer loop

prominence

Flare arcade

No surprise that magnetic complexities can build up and break down - and responses to this will depend on the local conditions and the magnetic configuration.

• Zhang et al. (2001) used LASCO, EIT & GOES to study a set of particularly well observed events - which included C1 (down to 1.1R).

• Three-phase CME ascent: (i) Initiation phase (pre-flare, slow ascent < 80km/s), (ii) Impulsive acceleration phase (during flare onset/rise, 1.3-4.6 R), (iii) Propagation phase (near constant velocity)

• Few events, but very ‘complete’. Suggests pre-flare onset for CME. Asymmetry consistent with pre-SOHO studies.

Example of a CME Onset

The CME and flare are both consequences of the same magnetic ‘disease’ – neither drives the other.

Their characteristics are the results of local conditions and thus we may witness a spectrum of flare and CME parameters.

This would be my standard model.

The CME Onset: Flare-CME Relationship

The Events of September 23, 2001

2 million K Fe XVI 360 Å line

CME Onsets and Coronal Dimming

12:13 UT 13:03 UT 13:53 UT 14:43 UT 15:33 UT

…. the missing mass?

The NASA STEREO mission: o Launched Oct 2006 o UK hardware & UK science

Door

Radiator

Forward Baffles

Inner Baffles

HI-1

HI-2

Direction of Sunlight

Side & rear

baffles

Door latch

mechanism

Concept: Wide-angle

imager with occultation

& baffle system; light

rejection of ~ one ten

million millionth of the

Solar Brightness.

STEREO Heliospheric Imagers

BigBlast304_fnl_best.mov

Enlil_2012March_2AU_HD1080.mp4

STEREO HI

Apparent Acceleration at large elongations

Ho~1AU

Sun β

A

Time E

lon

ga

tio

n a

ngle

sin( )( ) arctan

cos( )o

vtt

H vt

(Sheeley et al., JGR,1999)

Assuming the CME propagates radially at a constant speed;

Davies et al., 2009, GRL 36, L02102

Davies et al., 2012, ApJ 750, 23

CME* Geometrical modelling

CME Tracking & CME-CME interactions

Illustrates the value of HI observations in understanding what impacts near-Earth space

Space Weather

UK National Risk Register 2013/2014

Courtesy of the

Impacts on business recognised – e.g. Lloyds report

UK space weather expert group (SEIEG; Chair: Prof

Mike Hapgood) advising Cabinet Office

ESA SSA Heliophysics Expert Service Centre at

RAL/Harwell Campus

ESA now has a Space Situational Awareness programme

with a new space weather element in which the UK is fully

engaged

ESA SSA – Application of Coronagraph & HI

instrumentation

Severe space weather on National Risk Register

UK Met Office sets up Space Weather forecasting facility

© 2010 RAL Space

It’s never as simple as you would expect…