  The tilt of solar active regions described by Joy's law is essential for
converting a toroidal field to a poloidal field in Babcock-Leighton dynamo
models. In thin flux tube models the Coriolis force causes Joy's law, acting on
east-west flows as they rise towards the surface. Our goal is to measure the
evolution of the average tilt angle of hundreds of active regions as they
emerge, so that we can constrain the origins of Joy's law. We measured the tilt
angle of the primary bipoles in 153 emerging active regions in the Solar
Dynamics Observatory Helioseismic Emerging Active Region survey. We used
line-of-sight magnetic field measurements averaged over 6 hours to define the
polarities and measure the tilt angle up to four days after emergence. We find
that at the time of emergence the polarities are on average aligned east-west,
and that neither the separation nor the tilt depends on latitude. We do find,
however, that ARs at higher latitudes have a faster north-south separation
speed than those closer to the equator at the emergence time. After emergence,
the tilt angle increases and Joy's law is evident about two days later. The
scatter in the tilt angle is independent of flux until about one day after
emergence, when higher-flux regions have a smaller scatter in tilt angle than
lower-flux regions. Our finding that active regions emerge with an east-west
alignment is surprising since thin flux tube models predict that tilt angles of
rising flux tubes are generated below the surface. Previously reported tilt
angle relaxation of deeply anchored flux tubes can be largely explained by the
change in east-west separation. We conclude that Joy's law is caused by an
inherent north-south separation speed present when the flux first reaches the
surface, and that the scatter in the tilt angle is consistent with buffeting of
the polarities by supergranulation.
