A. Interplanetary Space

We already mentioned the solar wind as one aspect of interplanetary space, the "empty" space between the Sun and the planets.  In addition to the solar wind of particles (electrons, protons, helium atoms, and dust), interplanetary space is also filled with very weak magnetic fields.  At about 1 solar radius above the photosphere, the magnetic field is almost radial, but as we go farther out the rotation of the Sun causes the field lines to lag behind and become a spiral.  The spiral form is called an Archimedes' Spiral, which has the simple polar coordinate equation:

r = a q.

(1)

We can get an equation for the field lines by taking the derivative with respect to time

dr/dt = a dq/dt

where dr/dt = v is the velocity that the field lines are being carried away from the Sun (the solar wind velocity) and dq/dt = w is the solar rotation rate.  Thus, we can identify the value for the constant a, which controls the tightness of the spiral (large a means a loose spiral, small a means a tight spiral),

a =v / w.

Starting at r = r_o at an angular position q = q_o, (1) becomes

(r - r_o) = (v / w)(q - q_o)

but this spirals the wrong way, so we need a minus sign.  The final form of the equation is:

(r - ro) = -(v / w)(q - qo).

(2)

This spiral structure can be seen in the path of charged particles accelerated during solar flares.  These particles generate radio emission, so tracking the position of the radio emission allows the tracing of their path:

Types of disturbances that can affect the Earth are

• co-rotating interaction regions. (solar wind "catches up" to slower wind region--see ENLIL model)
• CMEs
• shock waves associated with CMEs

See this link, from U Colorado, for a nice discussion of planetary magnetospheres, and a comparison of magnetospheres of different planets.

The solar wind exerts a pressure radiating outward from the Sun, due to both the gas pressure, P_g, and the magnetic pressure P_mag.  Since the gas is expanding outward into a roughly spherical volume as it leaves the Sun, the gas density drops approximately at 1/R².  An approximate representation for the electron density is

n_e = 5 x 10⁶ (R/R_o)^-2 cm^-3.

Eventually the gas and magnetic pressure from the Sun is overcome by the pressure exerted by the interstellar medium (ISM).  This is made up of the gas and magnetic pressure of material from other stars, or left over from the formation of the galaxy.  We can think of the Sun's influence as forming a "bubble" in the ISM, and this bubble is called the heliosphere.  From my 2001 lecture: We still do not know the exact extent of the heliosphere.  Several spacecraft are slowly making their way outward from the Sun, and looking for the termination shock associated with it.

Since then, both Voyager spacecraft have reached the termination shock. Here is the new picture, based on the Voyager information and from the IBEX (Interstellar Boundary Explorer) spacecraft. The Wikipedia page for heliosphere collects a lot of the new information.

From the Voyager Interstellar Mission webpages, check out this timeline.  Voyager 1 crossed the termination shock at 94 AU in December 2004 and Voyager 2 crossed at 84 AU in August 2007. As of August 2010, Voyager 1 was at a distance of 17.1 Billion Kilometers (114.3 AU) from the sun and Voyager 2 at a distance of 13.9 Billion kilometers (92.9 AU). Voyager 1 is escaping the solar system at a speed of about 3.6 AU per year. Voyager 2 is also escaping the solar system at a speed of about 3.3 AU per year. That puts their current distances (Dec. 2011) at about 119 AU and 97 AU, respectively. It is interesting that Voyager 1 crossed the termination shock once, while Voyage 2 crossed it 5 times! The variable pressure of the heliosphere moved the boundary in and out many times during its crossing.