Despite its name, the heliosphere's shape is not a perfect sphere.
Its shape is determined by three factors: the interstellar medium (ISM), the solar wind, and the overall motion of the Sun and heliosphere as it passes through the ISM.
Because the solar wind and the ISM are both fluids, the heliosphere's shape and size are also fluid.
Changes in the solar wind, however, more strongly alter the fluctuating position of the boundaries on short timescales (hours to a few years).
The solar wind's pressure varies far more rapidly than the outside pressure of the ISM at any given location.
In particular, the effect of the 11-year solar cycle, which sees a distinct maximum and minimum of solar wind activity, is thought to be significant.

On a broader scale, the motion of the heliosphere through the fluid medium of the ISM results in an overall comet-like shape.
The solar wind plasma which is moving roughly "upstream" (in the same direction as the Sun's motion through the galaxy) is compressed into a nearly-spherical form, whereas the plasma moving "downstream" (opposite the Sun's motion) flows out for a much greater distance before giving way to the ISM, defining the long, trailing shape of the heliotail.

The limited data available and unexplored nature of these structures have resulted in many theories as to their form.
Despite its name, the heliosphere's shape is not a perfect sphere. Its shape is determined by three factors: the interstellar medium (ISM), the solar wind, and the overall motion of the Sun and heliosphere as it passes through the ISM. Because the solar wind and the ISM are both fluids, the heliosphere's shape and size are also fluid. Changes in the solar wind, however, more strongly alter the fluctuating position of the boundaries on short timescales (hours to a few years). The solar wind's pressure varies far more rapidly than the outside pressure of the ISM at any given location. In particular, the effect of the 11-year solar cycle, which sees a distinct maximum and minimum of solar wind activity, is thought to be significant.
On a broader scale, the motion of the heliosphere through the fluid medium of the ISM results in an overall comet-like shape. The solar wind plasma which is moving roughly "upstream" (in the same direction as the Sun's motion through the galaxy) is compressed into a nearly-spherical form, whereas the plasma moving "downstream" (opposite the Sun's motion) flows out for a much greater distance before giving way to the ISM, defining the long, trailing shape of the heliotail.
The limited data available and unexplored nature of these structures have resulted in many theories as to their form.
Heliosphere