Lecture 4 - Solar Wind and Planetary Magnetospheres ... · oCharged particles, such as electrons...
9
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres Lecture Lecture 4 - Solar Wind and Planetary Magnetospheres 4 - Solar Wind and Planetary Magnetospheres o Today’s topics: o Planetary magnetospheres. o Interaction of solar wind with solar system objects. February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres Pressure due to the Solar Wind Pressure due to the Solar Wind o Solar wind exerts magnetic and dynamic pressure on objects (comets, planets, etc. ) in the solar system. o Magnetic pressure: P B = B 2 / 8 ! o Dynamic pressure: P D = 1/2 " v 2 o Sun’s field is a dipole: B = M / r 3 , where M is the dipole moment => P B = M 2 / 8 ! r 6 o The solar wind density ~r -2 . o As P B ~ r -6 and P D ~ r --2 => P D >>P B at large distances from the Sun. => only consider dynamic pressure of solar wind on objects.
Transcript of Lecture 4 - Solar Wind and Planetary Magnetospheres ... · oCharged particles, such as electrons...
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
LectureLecture 4 - Solar Wind and Planetary Magnetospheres4 - Solar Wind and Planetary Magnetospheres
o Today’s topics:
o Planetary magnetospheres.
o Interaction of solar wind with solar system objects.
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Pressure due to the Solar WindPressure due to the Solar Wind
o Solar wind exerts magnetic and dynamicpressure on objects (comets, planets, etc. ) inthe solar system.
o Magnetic pressure: PB = B2 / 8 !
o Dynamic pressure: PD = 1/2 " v2
o Sun’s field is a dipole: B = M / r3, where Mis the dipole moment
=> PB = M2 / 8 ! r6
o The solar wind density ~r-2.
o As PB ~ r-6 and PD ~ r--2 => PD>>PB atlarge distances from the Sun.
=> only consider dynamic pressure of solarwind on objects.
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Parker SpiralParker Spiral
o Solar wind propagates outward fromthe Sun carrying a magnetic field.
o Poduces an archimedean spiral by"frozen-in" magnetic field beingcarried radially outward while theSun continues to rotate.
o Charged particles, such as electronsand protons, propagate along theParker spiral.
o Flares and CMEs that occur close tofield lines that connect the Sun tothe Earth are the most geoeffective.
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Solar Wind EffectsSolar Wind Effects
o Depends on magnetic field and atmosphericproperties of object.
o Planets with magnetic fields essentially have adipole field (B(r) = M / r3).
o Consider two types of magnetosphere:
1. Induced magnetosphere - solar windinteraction creates a magnetosphere.
2. Intrinsic magnetosphere - object generatesits own magnetic field.
Magnetic Field and Atmosphere
No Magnetic Field but Atmosphere
No Magnetic Field or Atmosphere
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Induced MagnetospheresInduced Magnetospheres
o No field, no atmosphere
o Solar particles encounter surface of body and areabsorbed or bounce back.
o Can lead to evaporation and outgassing ofmaterial (e.g., comet nucleus and coma).
o Pressure due to outgassing reaches balance withsolar wind: 1/2 "g vg
2 = 1/2 "sw vsw2
o No field, but atmosphere present
o Atmosphere has gas pressure which balances thesolar wind pressure:
Ppa = Psw
o This occurs at ionopause.
o Occurs on Venus and Mars, neither of whichhave significant intrinsic magnetic fields.
No Magnetic Field but Atmosphere
No Magnetic Field or Atmosphere
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Induced Magnetospheres (cont.)Induced Magnetospheres (cont.)
o Solar EUV radiation ionizes upperatmospheres of planets.
o If thermal pressure of ionosphere equalssolar wind dynamic pressure, thenionosphere can balance the solar windpressure.
o Magnetosheath forms above theionosphere and deflects the solar wind.
o Ionopause separates the ionospherefrom the magnetospheath.
o Solar wind is supersonic and thus formsa detached bow shock.
ionopause
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
What is the height of the What is the height of the ionopauseionopause??
o Assuming hydrostatic equilibrium in the planetary atmosphere:
o As " = n m and P = n k T => " = P / k T, and we can write:
o Rearranging and integrating:
o Letting, H =kT / mg and P = Ppa (H is the scale height):
o This is the pressure as a function of height from surface of a planet’s atmosphere.
!
dP
dr= "#g
!
dP
dr= "
Pmg
kT
!
dP
PP0
P
" = #mg
kTdr
r0
r
"
lnP
P0
$
% &
'
( ) = #
mg
kT(r # r0)
=> P = P0e#mg
kT(r#r0 )
!
Ppa = P0e"(r"r0 ) / H
r0 = radius of planet
P0 = pressure at surface
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
What is the height of theWhat is the height of the ionopause ionopause? (cont.)? (cont.)
o The ionopause occurs at a height where the pressure due to the solar wind equals
the pressure of the planetary atmosphere, i.e., Psw = Ppa
o The dynamic pressure due to the solar wind is Psw = 1/2 "sw vsw2 = 1/2 nnm msw vsw
2.
o Therefore,
o At Mars, nsw = 1 x 106 m-3, vsw = 330 km s-1, T = 200 K (planet surface
temperature), npa = 3 x 1010 m-3, r0 = 3393 km. What is r-r0?
o On Mars, r is so small that solar wind particles reach the surface. What are the
implications for humans on Mars?
!
1
2nswm
swvsw
2= P0e
"(r"r0 ) / H
# r " r0 = H ln2P0
nswm
swv2sw
$
% &
'
( )
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Magnetospheres of MarsMagnetospheres of Mars
o Martian atmosphere diverts the solar
wind, because Mars lacks a significant
planetary magnetic field (it’s internal
dynamo shut off).
o Mars is an “induced obstacle”; the
ionosphere interacts with the solar wind.
o Very unlike Earth, which is encapsulated
within an intrinsic magnetosphere. This
magnetosphere buffers us from charged
particles in the solar wind.
Mars in the solar wind
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Induced Magnetospheres of PlanetsInduced Magnetospheres of Planets
o Venus: The magnetic moment is less than one hundred thousandths that ofEarth. Plays no role in the solar wind interaction with the planet. Still donot know how much atmosphere is being lost to the solar wind.
o Mars: Precise size of the magnetic field is not known but its strength ismuch less than one ten thousandths of Earth. Like Venus, the intrinsicmagnetic field is not significant for the solar wind interaction. Theionosphere is thought to be magnetized because the solar wind dynamicpressure exceeds the thermal pressure of the ionosphere. Other features,such as the bow shock and magnetotail, are very similar to those of Venus.
o Comets: Comets are much smaller objects than planets if only their nucleiare considered. The size over which the cometary gas can spread in thesolar wind is thus controlled by the speed of expansion of the cometary gas(about one km/s) and the ionization time (about a day at 1 AU from theSun).
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Intrinsic MagnetospheresIntrinsic Magnetospheres
o Geomagnetic field of manyplanets can be approximated by adipole. The forcing by the solarwind modifies this field, creating acavity called the magnetosphere.
o Magnetosphere shelters surfacefrom high energy solar wind.
o Outer boundary of magnetosphereis called the magnetopause.
o In front of dayside magnetopauseanother boundary called the bowshock is formed because solarwind is supersonic. Regionbetween bow shock andmagnetopause is magnetosheath.
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
What is the height of the magnetopause?What is the height of the magnetopause?
o Magnetopause located where Earth’s magnetic field pressure balances pressure dueto the solar wind: PE = Psw.
o Magnetic pressure is PE = B2 / 8 !.
o If dipole moment of the Earth is M, the field along the equator varies as M/r3, so
PE = M2 / 8 ! r6
o We can therefore write, Psw = 1/2 "sw vsw2 and 1/2 nnm msw vsw
2 = M2 / 8 ! r6.
=> height of magnetopause varies as r ~ Psw-1/6
o Height of magnetopause is therefore:
!
r =M
2
4"nmvsw
2
#
$ %
&
' (
1/ 6
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Shape of the Bow ShockShape of the Bow Shock
o Solar wind is both supersonic and super-Alfvenic at large distances from Sun.
i.e., vsw >> vs and >> vA ( )
or MA = vsw / vA = Ms = vsw / vs ~ 8.
o Changes in shock shape can be understoodusing Mach cone:
o Thus, the shock shape becomes blunter forsmaller MA and more swept back for larger MA.
!
= B / 4"#
!
"A
= sin#1vA
vsw
$
% &
'
( ) = sin
#1M
#1( )
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Intrinsic Magnetospheres of the PlanetsIntrinsic Magnetospheres of the Planets
o Mercury: Magnetic moment is ~1/3000th of
Earth’s. Equatorial surface magnetic field strength
is ~250 nT.
o Earth: Surface field is ~31,000 nT.
o Jupiter: Magnetic moment is largest of planets at
~10,000 times Earth’s. Strength of field combined
with weakness of wind at Jupiter produces
enormous magnetosphere.
o Saturn: Since Saturn is smaller planet, its core in
which the planetary magnetic field is generated is
smaller => so is magnetic field. Magnetic moment
of Saturn is 580 times that of Earth.
o Uranus and Neptune: Magnetic fields are irregular and not be well represented by a simple
dipole. Magnetic moments are ~40 times < Earth’s. Reason weakness and irregularity may be
that the magnetic field is generated in salty ice/water oceans closer to the surface.
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
EarthEarth’’s Intrinsic Magnetospheres Intrinsic Magnetosphere
o Pressure of Earth's magnetic field is sufficient tobalance the solar wind pressure. Upwind a bow-shock forms as the plasma slows from supersonicvelocities (~400 km s-1) .
o As solar activity changes, pressure will vary andthe magnetosphere will, in response, expand andcontract accordingly.
o Magnitude of solar wind dynamic pressure is:Psw = 1/2 nsw msw vsw
2
o At Earth, n ~107 m-3, m =1.66 x 10-27 kg, vsw ~400 km s-1. (To give an answer in dyne cm-2 thenthese quantites must be expressed in C.G.S units.Ans = 1.328 x 10-8 dyn cm-2).
o What must the strength of Earth’s field (B) tobalance this pressure?
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
EarthEarth’’s Intrinsic Magnetospheres Intrinsic Magnetosphere
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Aurorae Aurorae of Jupiter and Saturnof Jupiter and Saturn
February 9, 2006 Lecture 4 - Solar Wind and Planetary Magnetospheres
Further ReadingFurther Reading
o Iver Cairs (University of Sydney)
o http://www.physics.usyd.edu.au/~cairns/teaching/lecture14/node1.html
o C. T. Russell (UCLA)
o http://www-spc.igpp.ucla.edu/ssc/tutorial.html
