The Heliosphere

Elena Moise Institute for Astronomy, University of Hawai’i

Importance

  Heliosphere is the first defense in the three-layer shield system of the Earth against high-energy galactic cosmic rays, which consists of:

1.  Heliosphere with the Sun's magnetic field carried out by the solar wind

2.  Earth's magnetic field (magnetosphere) 3.  Earth's atmosphere.

What is the Heliosphere?

  The solar wind, which expands radially from the Sun at supersonic speed, blows a huge cavity - heliosphere - into the surrounding interstellar cloud, filling it with solar material and magnetic field.

  The size of the heliosphere is determined by a balance between the dynamic pressure of the solar wind and the pressure of the interstellar medium.

How Would Our Heliosphere Look from Outside?

  Our sun is moving through the local interstellar gas cloud in approximately the direction of the constellation Scorpio, with a speed of about 25 km/s (900 m/h).

If We Could See Our Heliosphere from Outside

Heliosphere and Local Interstellar Medium (LISM) Interaction

  Termination Shock (TS) •  Boundary where solar wind becomes

subsonic •  Standing shock wave •  Passed by Voyager 1 on Dec 2004 at

94 AU

  Heliopause (HP) •  Separates the heliosphere from the

LISM plasma

  Bow Shock (BS) •  If supersonic relative motion between

heliosphere and LISM

26km/s

Heliosphere and Local Interstellar Medium (LISM) Interaction

LIC and Heliosphere Interaction

  Outflowing solar wind contributes vast majority of Ram Pressure

 Time-dependence through Merged Interaction Regions and Solar Cycle and longer term variations

 Heliospheric Magnetic Field

  The Local Interstellar Cloud  Inflowing neutral atoms give rise to a hydrogen wall

 Pickup Ions carried out by the solar wind (leading to ACRs)

 Interstellar Magnetic Field

  Energetic Particles (Cosmic Rays)  Galactic Cosmic Rays

 Anomalous Cosmic Rays  Suprathermal Tails

  Dust, Grains  From interstellar medium

 Kuiper Belt grains  Outer Source for Pickup Ions (heavy composition)

Hydrogen Wall

  Most neutrals stream in unperturbed, except neutral hydrogen, which due to charge exchange reactions, is heated and decelerated forming “Hydrogen Wall”

  Charge exchange between incoming IS neutral H and solar wind protons → energetic neutral atoms (ENAs) and pick-up ions (PUIs)

Müller (2004)

Interstellar Neutrals in the Heliosphere: He

  He is focused by the Sun’s gravitational field on the downwind side   Interstellar neutrals are ionized -> pickup ions (PUI)   PUI are accelerated at TS -> anomalous cosmic rays (ACRs)

26km/s   LISM He velocity 26km/s T=6500K, n=0.013 cm-3

  Photoionization of He lifetime ~100d at 1AU

Acceleration of Pickup Ions

Interplanetary Magnetic Field

  Magnetic field is “frozen-in” to the solar wind plasma

  For a given blob of plasma: uSW

|B|

uSW

Increased B field strength Reduced B field strength

Parker’s spiral

€

|r B | uSW

Equatorial plane of the Sun |B|

is conserved

Interplanetary Magnetic Field Beyond Earth’s Orbit

  Here we have a sketch of the full heliosphere; it’s complicated.

  Magnetic fields get wrapped up to become almost circumferential as the solar wind carries them outward.

  The fields wrap up differently at different latitudes.

  Eventually the solar wind runs into the the plasmas and magnetic fields in the local interstellar medium and is slowed down.

  Since the wind is supersonic, it slows down by forming a standing shock wave, the Termination Shock.

Solar Wind

❂  Fast, tenuous wind: 700-800km/s "  Originates from coronal holes

❂  Slow, denser wind: 350-400km/s "  Originates from coronal streamers "  Highly variable

Ballerina Skirt

The Size of the Heliosphere   Sun, solar wind, solar activity

•  At 400 km/s, it takes the solar wind a little over 14 months to reach 100 AU.

  Interstellar medium •  At 25 km/s, it takes the heliosphere about 38 years to move 200

AU, its own diameter, in the Interstellar Medium.

CME Impact on the Heliosphere

  In a period of about two weeks starting at the end of 17 October 2004 major flares erupted on the Sun. One was the most powerful ever recorded.

  The series of events and their effects were observed by spacecraft throughout the heliosphere.

  Ulysses was very near Jupiter, Cassini was near Saturn, and Voyager 1 and 2 were approaching the Termination Shock when the events occurred.

The CME Interaction with the Heliopause

  Even beyond the Termination Shock, the CME blast waves will continue to have effects.

  The CMEs may push the heliopause outwards by about 400 million miles (~3AU).

  It will likely take a year or two for the heliopause to settle back to it’s normal position.

ISM Effects on Planets   About 98% of diffuse material in heliosphere is interstellar gas

  Solar wind and interstellar gas densities are equal near Jupiter, or at ~6AU

  At 1AU (at Earth) in 10cm3 there is 1 atom of interstellar gas and 10 ions of solar wind

  Inner versus outer planets

  Cosmic rays: •  Anomalous Cosmic Rays (ACRs) require neutral ISM •  Galactic Cosmic Rays (GCRs) are sensitive to the heliospheric

magnetic field

  In principle, core samples on inner versus outer planets would sort solar variations from interstellar variations

Interstellar Medium Impact on the Heliosphere

  How large of a density increase in the ISM is needed to significantly alter the structure of the heliosphere?

  Increase the density of the surrounding LISM by only a factor of 50 (nH from 0.2 to 10 cm-3) and the termination shock (TS) shrinks from 100 AU to 10 AU.

Zank & Frisch (1999)

nH(LISM) = 10 cm-3

nH(LISM) = 0.2 cm-3

Müller (2004)

What Are the Consequences of a Compressed

Heliosphere?

  Galactic Cosmic Ray flux modulated by magnetized solar wind •  GCR ionization in lower atmosphere, leads to cloud nucleation, increase in

planetary albedo •  Lightning triggered by GCR secondaries

•  GCR increase NO and NO2 production, alters ozone layer chemistry •  GCR source DNA mutation

  Direct deposition of ISM material onto planetary atmosphere •  Dust deposition through GMC could trigger “snowball” Earth episode •  Create mesospheric ice clouds, increase in planetary albedo

Entry of LISM into Heliosphere

• 1912 - Victor Hess reaches 5350 m altitude in a balloon and shows conclusively that the rate of charged particles increases significantly with height: There is an extraterrestrial source of radiation !

• 1930 – Pierre Auger discovers particle showers.

• 1936 - Hess gets Nobel Prize for discovery of cosmic rays.

But not: Where are they coming from ? How do they get their momentum ?

Questions that have been answered by now: - What are these particles made of ? - What different kinds are there ?

Some Cosmic Ray Facts

The highest energy cosmic rays measured to date have energy of about 1020 eV, equivalent to the kinetic energy of a baseball traveling at about 100 mph!

Cosmic Rays continually bombard the Earth.

In fact, about 100,000 cosmic rays will pass through a person every hour!

Cosmic Ray Composition

  They include essentially all of the elements in the periodic table. About 89% of GCRs are hydrogen (single protons), 10% helium nuclei, and about 1% heavier nuclei - in fact, all of the elements in the periodic table.

  GCRs for the most part are fully ionized atoms, i.e. bare nuclei.

0

20

40

60

80

100

Protons 89% Helium Nuclei10%

Heavy Elements1%

Cosmic ray energy spectrum

•  Fairly similar shapes •  Maximum around a few 100 MeV/nucleon •  Steeply falling spectra

Where Are Cosmic Rays Coming From? ���

  1- Galactic Cosmic Rays (GCRs)   These Cosmic Rays originate in sources outside

the solar system but inside the Milky Way Galaxy.

  Most GCRs are probably accelerated in the blast waves of supernovae remnants.

  This doesn’t mean that a supernova explosion itself gets the particles up to these speeds, but the remnants of the explosions, i.e. expanding clouds of gas and associated magnetic fields can last for thousands of years, and can accelerate Cosmic Rays.

Most GCRs have energies between 100 MeV (0.43 c) and 10 GeV (0.996 c).

Bouncing back and fourth in the magnetic field of the remnant lets particles gain energy and become Cosmic Rays. Eventually they build up enough speed to escape to the Galaxy.

Where Are Cosmic Rays Coming From?

  2- Solar Energetic Particles (SEPs)   The sun is one of the sources of Cosmic Rays. Nuclei and

electrons are accelerated by shock waves traveling through the Corona and by magnetic energy released in CMEs.

  The Solar Wind contains roughly equal numbers of electrons and protons along with heavier ions and blows continuously from the Sun at an average speed of 400 km/sec. This leads to a mass loss of 10 million tons of material from the sun every year.

Where Are Cosmic Rays Coming From?

  They are produced by neutral atoms from the interstellar medium which leak into the heliosphere and become ionized by either solar UV –radiation or charge exchange with the solar wind. They (pickup ions) are then carried back by the solar wind to the outer heliosphere.

  Then these particles are accelerated by the solar wind termination shock and drift back to the inner heliosphere as Anomalous Cosmic Rays.

3- Anomalous Cosmic Rays (ACR)

Where Are Cosmic Rays Coming From?

  Cosmic Ray Particles coming from outside galaxies passing through the Milky Way Galaxy.

4- Extra Galactic Cosmic Rays

Galactic Cosmic Ray Shielding

Galactic Cosmic Rays (GCRs)

  Most GCRs have energies between 100Mev (0.43 c) and 10Gev (0.996 c)

  GCRs originates in sources outside the solar system, but inside the Milky Way

  Most GCRs are probably accelerated in the blast waves of supernova remnants •  Once a supernova explodes, its remnants (expanding clouds of gas and

associated magnetic fields) can last for thousands of years and can accelerate cosmic rays by bouncing them back and fourth in the magnetic field of the remnant

•  Eventually GCRs build up enough speed to escape to the Galaxy

GCR Intensities Measured at Earth

  GCRs arrive at Earth after traveling through the Galaxy for several million years

The Present Solar Activity   Heliospheric magnetic field at 1AU is at the lowest since

spacecraft records began…

  Heliosphere is shrinking! Owens et al (2008)

Heliospheric Spacecrafts   The first observations of the solar wind were made in the vicinity of the Earth in the

early 1960s.

  Pioneer 10 and 11, launched in 1972 and 1973 were the first spacecraft to explore beyond 1 AU. Contact with these spacecraft have now been lost although Pioneer 10 was tracked to nearly 80 AU.

  Voyager 1 and 2 were launched in 1977. Both have scientific instruments still operating. Voyager 1 crossed the termination shock in 2004 at 94.5 AU, has now passed 100 AU and continues out towards the heliopause. Voyager 2, following behind, crossed the termination shock at 84 AU in 2007.

  Ulysses, launched in 1990 into a ~6 year period orbit of the Sun inclined at 80.2° to the solar equator, with perihelion at 1.3 AU and aphelion at 5.4 AU. It was thus the first spacecraft to explore the 3D structure of the heliosphere over a large latitude range. Operations ceased in 2009 after nearly 3 orbits.

  At 1AU: SOHO, ACE, WIND, STEREO, etc.

  IBEX launched in June 2008 to make the first global observations of the outer heliosphere, discovering the global interaction between the solar wind and the interstellar medium on a low-cost budget. Detects ENAs.

IBEX and Energetic Neutral Atoms (ENAs)

  ENA imaging is the only way to globally observe the interaction between the solar wind and the interstellar medium (structures, dynamics, energetic particle acceleration and charged particle propagation) in the complex region that separates our solar system from the galactic environment.