ASEN5335- Aerospace Environments -- The Solar Wind 1 THE INTERPLANETARY MEDIUM AND IMF Consequently, the "spiral" pattern formed by particles spewing.

ASEN5335- Aerospace Environments -- The Solar Wind 1

THE INTERPLANETARY MEDIUM AND IMF

Consequently, the "spiral" pattern formed by particles spewing from a rotating sun is also manifested in the IMF. The field winds up becauseof the rotation of the sun. Fields in a low speed wind will be more wound up than those in high speed wind.

The solar wind is a “high-” plasma, so the IMF is "frozen in”; the IMF goes where the plasma goes.

Intermixed with the streaming solar wind is a weak magnetic field, the IMF.

ASEN5335- Aerospace Environments -- The Solar Wind 2

Loci of a succession of fluid particles emitted at constant speed from a source fixed on the rotating Sun.

Loci of a successionof fluid parcels (eightof them in this sketch)emitted at a constantspeed from a sourcefixed on the rotatingSun.

rdt

vSWdt

tan = r/vSW

ASEN5335- Aerospace Environments -- The Solar Wind 3

• Plasma leaves the sun predominantly at high latitudes and flows out and towards the equator where a current sheet is formed corresponding to the change in magnetic field polarity.

• The Sun’s magnetic field is dragged out by the high-beta solar wind. The current sheet prevents the oppositely-directed fields from reconnecting.

• The current sheet is tilted with respect to the ecliptic (about 7°), ensuring that earth will intersect the current sheet at least twice during each solar rotation. This gives the appearance of "magnetic sectors".

The Sun’s magnetic field has a 22-year cycle. Near solar minimum, the

Sun has a well-defined dipole field, with helmet streamers around the equator and

coronal holes near the poles. Near solar maximum, this situation is destroyed, with

coronal streamers and coronal holes everywhere. This takes place in the midst of a reversal of the solar magnetic field.

Pre-2001configuration

3

2

12

3

1

j

€

j =1

μ0

∇ × B

€

μ0 j1 =∇ × B = −∂B2

∂x3

ˆ i 1

current sheet

current sheet

j

ASEN5335- Aerospace Environments -- The Solar Wind 4

SUN

+

+

+++++

++++++

-------

-------

---

At Earth, the IMF can be directed either inward or outward with respect to the Sun, forming a pattern of “magnetic sectors” thatappear to rotate with the Sun.

ASEN5335- Aerospace Environments -- The Solar Wind 5

Wavy Structure of the Interplanetary Current Sheet

Where Earth’s orbit intersects this current sheet determines whether Earth “sees” a positive or negative magnetic sector.

ASEN5335- Aerospace Environments -- The Solar Wind 6

Co-rotating Interaction Regions (CIRs)

The fast streams “catch up” with slower streams.

The leading edge of a fast stream compresses the plasma and produces a high-pressure region that prevents actual overlap between fast and slow solar wind regions.

Since these structures usually persist over several solar rotations, a conventionally-used name is co-rotating interaction region or CIR.

ASEN5335- Aerospace Environments -- The Solar Wind 7

3 CMEs in the Solar Wind during April 2002

Density

Temperature

Absolute B

Bz (B-south)

Nonrecurring Disturbances in the Solar wind

Outward propagating CMEs generate interplanetary shock waves. The slower, “quiet” solar wind is “snowplowed” by the fast material, forming a traveling shock ahead of the ejecta.

shock

compressedheatedplasma

earth

ASEN5335- Aerospace Environments -- The Solar Wind 8

The Heliosphere and its Interaction with the Interstellar Medium

Interstellar MediumHeliopause

Heliosphere

Termination Shock

Heliospheric Bow Shock ?

The heliosphere and heliopause represent the region of space influenced by the Sun and its expanding corona, and in some respects encompass the true extent of the solar system.

Where does the Solar Wind Terminate?

Heliosheath

• The solar wind becomes increasingly supersonic away from the Sun:

‘Class-2” solution, r >> rC:

€

u2 ≈ 8kT

mln

r

rC

or u(r)∝kT

mln

r

rC

⇒ M =u(r)

Cs

∝ ln r

• The solar wind density decreases as 1/r2 from the Sun:

€

⇒ ρu2 → 0 as r → ∞

• However, the interstellar density is ~10-13 Pa, so at some point u2 will be less than this; one might expect the solar wind to slow down as it reacts to this external pressure force.

• It is anticipated that a shock will form, so that the solar wind slows down abruptly, and comes to equilibrium with the pressure presented by the interstellar medium.

This is called the termination shock, more like 100 AU

• The shock should form where u2 ~ 10-13 Pa.

Behavior of the Solar Wind far From the Sun

€

rEarth

rshock

⎛

⎝ ⎜

⎞

⎠ ⎟

2

ρu2[ ]

Earth≈10−12 dyne cm-2

or

rshock

rEarth

≈160 ⇒ rshock ≈160 AU (outside solar system)

• Knowing u2 at 1 AU to be ~2.6 x 10-8 dyne cm-2, and if u does not vary much, then

ASEN5335- Aerospace Environments -- The Solar Wind 10

The radially-expanding supersonic solar wind must be somehow diverted to the downstream direction to merge with the flow of the interstellar medium. This diversion can only take place in subsonic flow, and therefore the supersonic expansion of the solar wind must be terminated by an “inner shock” or “termination shock”.

Flow lines of the interstellar plasma do not penetrate into the region dominated by the solar wind flow but flow around a “contact surface” called the heliopause, which is considered to be the outer boundary of the heliosphere.

The interstellar medium (ISM) will form a heliospheric bow shock if it is supersonic withrespect to theheliopause

26 km/sec

ASEN5335- Aerospace Environments -- The Solar Wind 11

ASEN5335- Aerospace Environments -- The Solar Wind 12

http://voyager.jpl.nasa.gov/multimedia/JPLvoyagerModule/voyagerModule.swf

Voyager Interstellar Mission (VIM)The VIM is an extension of the Voyager primary mission that was completed in 1989 with the close flyby of Neptune by Voyager 2

As of July 2007, Voyager 1 was at a distance of 103 AU from the sun and Voyager 2 at a distance of 83 AU.

Both Voyager 1 and Voyager 2 were launched

in 1977.At the start of the VIM,

Voyager 1 was at ≈ 40 AU from the Sun, and

Voyager 2 was at ≈ 31 AU.

ASEN5335- Aerospace Environments -- The Solar Wind 13

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QuickTime™ and a decompressor

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QuickTime™ and a decompressor

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1AU ≈ 1.50 x 108 km

(108 AU) (88 AU)

ASEN5335- Aerospace Environments -- The Solar Wind 14

Both Voyagers are headed towards the outer boundary of the solar system in search of the heliopause. The heliopause has never been reached by any spacecraft.

In December 2004 Voyager 1 crossed the termination shock at 94 AU.

Next is the heliosheath phase. The heliosheath could be tens of AU thick.

The Voyagers have enough electrical power (nuclear batteries) and thruster fuel to operate at least until 2020.

Voyager 2 is also escaping the solar system at a speed of about 3.3 AU per year, 48 degrees out of the ecliptic plane to the south.

Voyager 2 is currently (2007) observing pre-shock phenomena, indicating that it is close to the termination shock.

Deflected downward by Neptune

Voyager 1 is escaping the solar system at a speed of about 3.6 AU per year, 35 degrees out of the ecliptic plane to the north, in the general direction of the Solar Apex (the direction of the Sun's motion relative to nearby stars).

Deflected upward by Saturn

ASEN5335- Aerospace Environments -- The Solar Wind 15

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Anomalous Cosmic Rays are Produced at the Termination Shock

Interstellar atoms are swept into the heliosphere by the motion of the Sun through the interstellar medium.

At 1-3 AU these neutral atoms (H, He, C, N, O) are ionized either by UV photo-ionization or charge exchange with protons.

The solar wind “picks up” these ions and carries them to the termination shock.

The pick-up ions repeatedly interact with the termination shock, and are energized to several tens of MeV. Some ions undergo multiple ionization (+2, +3, +4, etc.), and wind up in our radiation belts. ACRs are studied because they provide insight into the interstellar medium, and the motion of particles in the heliosphere.

ASEN5335- Aerospace Environments -- The Solar Wind 16

A Bow Shock has been Observed Around the Star R-Hydra