Using HMI to Understand Flux Cancellation by Brian Welsch 1, George Fisher 1, Yan Li 1, and Xudong Sun 2 1 Space Sciences Lab, UC-Berkeley, 2 Stanford University Cancellation of magnetic flux in magnetograms has been defined in observational terms as "the mutual apparent loss of magnetic flux in closely spaced features of opposite polarity." Physically, this removal of flux could correspond to one of three mechanisms: (i) the emergence of U-shaped magnetic loops, (ii) the submergence of Omega-shaped loops, or (iii) reconnection in the magnetogram layer. Evidence has been reported for all three of these mechanisms, but does one predominate? We can investigate cancellation mechanisms at work in an active region's magnetic fields using time- averaged Doppler shifts along polarity inversion lines (PILs) of the line-of-sight (LOS) magnetic field near disk center. Along these PILs, the LOS component of the magnetic field vanishes, so LOS flows inferred from Doppler shifts are perpendicular to the magnetic field. If the evolution is ideal, such flows imply the transport of magnetic flux across the atmospheric layer imaged in the magnetogram. As a preliminary step in our study, we present an innovative method to remove biases in the measured Doppler velocities due to offset in the line-center position, which might arise from a well- known correlation between brightness and blueshifts in the convecting photospheric plasma

Q: But what is cancellation? Cancellation could be any of three processes: Doppler shifts on PILs can distinguish between these!

Q: Which process predominates in active regions? Which model more accurately describes the Sun?

We have solved for v 0 using data from > 100 PILs in three successive magnetogram triplets from Fig. 3. Incorporating error bars on estimates of ΔΦ ’s is still a work in progress; ideally, we’d estimate v 0 using total least squares.

Conclusions We have demonstrated a method to correct the bias velocity v 0 in HMI’s Doppler velocities from convective-blueshifts. Using v 0 -corrected Doppler data, we can study flux cancellation in active regions, to determine which physical process underlies cancellation.

Future Work Incorporate error bars on estimates of ΔΦ ’s and magnetic lengths, and investigate estimation of v 0 via total least squares. – Errors are present in both magnetic lengths and ΔΦ ’s. Actually investigate cancellation! – Investigate statistics of cancellation mechanisms in both active regions (and quiet sun). – Extrapolate flux submergence rates over solar cycle (and quiet Sun flux turnover time) to constrain flux recycling in solar cycle (and quiet sun) dynamo.