Interplanetary Medium

The interior of the bubble of space created by the Sun's magnetic field — the heliosphere. Extends from the Sun outward to the heliopause at 110–160 AU, well beyond Neptune's orbit.

Solar Wind

The Sun emits a continuous stream of charged particles called the solar wind, which creates a tenuous atmosphere (the heliosphere) for billions of kilometres into space:

• Particle density: 5–10 protons/cm³
• Velocity: 350–400 km/s
• The Sun's magnetic field combined with solar wind fields deflects low-energy galactic cosmic rays; deflection varies with solar activity

Contents

Despite being a near-total vacuum (mean free path ~1 AU at Earth's orbital distance), interplanetary space is not empty. It contains:

• Cosmic rays (ionized atomic nuclei and subatomic particles)
• Gas, plasma, and dust
• Small meteors
• Dozens of organic molecules (detected via microwave spectroscopy)
• Zodiacal light — a faint band visible at night caused by a cloud of interplanetary dust

Planetary Magnetospheres

Planets with magnetic fields (Jupiter, Saturn, Mercury, Earth) have magnetospheres shaped by the solar wind into a teardrop form, with a long magnetotail trailing behind. These trap solar wind particles in belts of charged particles (e.g., van-allen-radiation-belts).

Planets without magnetic fields — such as Mars — have their atmospheres gradually eroded by the solar wind.