If my character is flying, is there anything I can do to change the orientation of the hemisphere, such as flying sideways, directing the spell to take a different position, etc.?
No; the orientation of the "Y" axis is fixed in relationship to gravity.
Wall of Force says:
A wall of force creates an invisible wall of pure force. The wall cannot move... The caster can form the wall into a flat, vertical plane....
Emergency Force Sphere says:
As wall of force, except you create a hemispherical dome... The bottom edge of the dome forms a relatively watertight space if you are standing on a reasonably flat surface.... The dome shape means that falling debris (such as rocks from a collapsing ceiling) tend to tumble to the side and pile up around the base of the dome.
The descriptor of the Wall of Force spell states the orientation of the spell is vertical along it's Y axis in relationship to the ground which is the X axis. This orientation is reliant upon orientation with the ground, and not the caster. No other alternative formation is given, so this is the only X-Y manifestation of the spell that can occur.
We know this through the other 6 wall spells like wall of Wind, Iron, Stone, Ice, etc... either reference the ground, or give specific formation or orientation exceptions to non-vertical formations in relationship to the ground.
The Emergency Sphere spell also gives a reference orientation that the dome extends overhead with the height opposite relative gravity. This is seen with the both the wording of falling rocks, resulting dome and the caster standing on a "relatively flat" or horizontal surface.
With Emergency Sphere saying "As Wall of Force" and then failing to contradict or specifically address the vertical orientation, means that the sphere can only have 1 such formation, with the apex being top, and the circle base being bottom, in relationship to the ground or gravity.
This would mean that if you (could) cast it in the air, the dome would orient itself with the dome's apex opposite gravity and not in relation to the caster. A wall of force does not become horizontal in relationship to the ground if a caster is prone.
Argument against casting walls in mid-air
Many of the wall spells explicitly reference the ground as a supporting structure for the base of a wall. (Emergency Force Sphere, Wall of Ice, Wind Wall, Wall of Iron, Stone Wall). No wall spell ever gives reference to or suggest that you can cast a wall in mid air.
Wind Wall - the one wall you think could be created in midair - specifically states:
While the wall must be vertical, you can shape it in any continuous path along the ground that you like. It is possible to create cylindrical or square wind walls to enclose specific points.
Wall of Iron and wall of stone also specifically state that you must have a surface to support the walls created.
That said, if you could create a flying surface you could cast Force Sphere onto (A flying carpet) you could create one in mid air, which would stay in place and not move, even if you were to fly your carpet down and away.
Unfortunately the only argument in favor of casting a wall mid air is that the spell descriptors do not explicitly state that you cannot do it, but the references and assumptions of a ground deployment are overwhelming.
Yes, you can
The sphere does, unfortunately, inherit Wall of Force's vertical orientation, but needing to be vertically oriented isn't an insurmountable obstacle. Here are some ways of rotating the hemisphere:
- Use Reverse Gravity to reverse gravity. Cast the spell a second time afterward.
- Travel to any plane with the 'subjective gravity' gravity trait, such as the Plane of Air. Then you can just decide what direction down is at the time of casting.
- Alter your surroundings to have the 'subjective gravity' gravity trait, for example via the 3.5 spell Precipitate Breach.
Note that altering the way gravity works requires significantly higher-level magics than Emergency Force Sphere.