What is the Solar Phi Angle

What is the PHI angle seen on ACE?
Why do we watch this data?

We watch the Phi angle at ACE
we can see this here
http://www.swpc.noaa.gov/ace/MAG_6h.html
The Phi angle = Interplanetary magnetic field polar angle
Phi is the angle of the interplanetary magnetic field that is being carried out by the solar wind
http://helios.gsfc.nasa.gov/qa_sun.html#magphi

When we see the Phi angle go southward (into the negative) This shows the increase of the transport of solar winds & mass into the Earth's magnetosphere. When it goes into the positive, this shows a decrease in the transport of solar winds and mass into Earths magnetosphere.

Phi in the negative = higher connection
Phi into the positive = less connection

The phi angle shows the negative (toward) orientation
to a positive (away) orientation


Magnetic Field Phi
Could you explain what Phi is, in terms of the Sun's magnetic field? Is it the angle of the magnetic field? What influences it?

I assume that you are referring to plots of the magnetic field as measured by ACE (located ~1 million miles in front of the Earth (between the Earth and the Sun. These plots appear on the ACE Real Time Solar Wind page.
In these plots Phi is indeed the angle of the interplanetary magnetic field that is being carried out by the solar wind. Phi is measured in the GSM (geocentric solar magnetospheric) coordinate system. In this system the X-axis points from the Earth to the Sun and the Z-axis is pointing along the direction of the Earth's north magnetic pole. This puts the Y-axis roughly pointing to the left as one looks at the Sun from the Earth. Phi is the angle made by the field in the XY plane.
http://helios.gsfc.nasa.gov/qa_sun.html#magphi