The Phi angle refers to the orientation of the Interplanetary Magnetic Field (IMF) relative to Earth's magnetic field.
When the Phi angle is negative (especially near -90°), the solar wind's magnetic field is southward, which facilitates energy transfer into Earth's magnetosphere, leading to geomagnetic disturbances that can disrupt HF radio communications.
When the Phi angle is positive, the solar wind's magnetic field is northward, resulting in more stable geomagnetic conditions and better HF propagation.