Heliosphere

The heliosphere is a vast, bubble-like region of space surrounding our Sun.

It acts as the Sun's magnetosphere and its outermost atmospheric layer. This structure is essentially a cavity formed within the interstellar medium, which is the matter that fills the space between stars. The heliosphere is vital because it serves as a shield for our Solar System. It protects us from significant amounts of cosmic ionizing radiation. However, this shield cannot stop uncharged gamma rays from passing through.

This massive bubble is continuously inflated by the solar wind. The solar wind is a stream of plasma consisting of ionized atoms from the solar corona.

These particles and magnetic fields stream outward from the Sun in all directions. Because the Sun rotates approximately every 25 days, the magnetic field becomes wrapped into a spiral. This creates a structure called the heliospheric current sheet. This sheet is a ripple that marks the boundary between magnetic regions of opposite polarity. Some scientists describe it as looking like a ballerina's skirt. It may be the largest structure in our Solar System.

The shape of the heliosphere is not a perfect sphere. Instead, it resembles a comet with a long, trailing tail called the heliotail.

This shape is determined by the solar wind, the interstellar medium, and the Sun's motion through the galaxy. The plasma moving "upstream" against the Sun's motion is compressed into a nearly spherical shape. This occurs at a distance of about 100 astronomical units (AU). Conversely, the plasma moving "downstream" flows out for much longer distances. This creates a tail that can stretch for several thousands of AUs. In 2020, researcher Merav Opher suggested the shape might actually be a crescent, like a deflated croissant.

As the solar wind travels outward, it eventually encounters the pressure of the interstellar medium. This interaction happens in several distinct stages. First, the supersonic solar wind reaches the termination shock.

At this point, the wind's motion slows abruptly from supersonic to subsonic speeds. This shock causes the plasma to compress and heat up. The termination shock is believed to be located between 75 and 90 AU from the Sun. Beyond this shock lies the heliosheath. This is a broad transitional region where the wind becomes slow and turbulent.

The heliosheath is a complex and active zone. Scientists have discovered it is not a smooth region. Instead, it is a "foamy zone" filled with magnetic bubbles.

These sausage-shaped bubbles are about 1 AU wide. They are created by magnetic reconnection between different sectors of the solar magnetic field. At about 113 AU, Voyager 1 found a stagnation region where the solar wind slowed to zero. In this area, the magnetic field intensity doubled. High-energy electrons from the galaxy also increased 100-fold. Shortly after, the spacecraft entered a region called the "magnetic highway."

The outermost edge of the heliosphere is the heliopause. This is the theoretical boundary where the solar wind is stopped by the interstellar medium. At the heliopause, the pressures of the solar wind and the interstellar medium reach a balance. Crossing this boundary marks the transition into interstellar space. Voyager 1 encountered the heliopause on 25 August 2012. It measured a sudden forty-fold increase in plasma density. Voyager 2 traversed this boundary later on 5 November 2018. These missions provided the first direct evidence of the edge of our solar bubble.

The study of these processes is known as heliophysics. This field includes the study of space weather and space climate. The heliosphere can change significantly over millions of years. This can happen due to extrasolar effects like nearby supernovas. It can also happen if the Sun moves through different densities of the interstellar medium. Evidence suggests that three million years ago, the heliosphere may have shrunk. This could have exposed Earth to the interstellar medium. Such changes might have impacted Earth's past climate and human evolution.