Multi-Frequency Helioseismic Inversion for Solar Meridional Flows

We have recently introduced a new method for deriving three-dimensional helioseismic sensitivity kernels using simulations of wave propagation in the solar interior through prescribed test perturbations (Hartlep & Zhao, 2021). Here, we report on inversions for meridional flows using these new kernels.

Since the meridional circulation is believed to be closely connected to the transport of momentum and magnetic flux in solar interior, it has been the subject of many studies in the past using various techniques. However, disagreement remains about the nature of the solar meridional flow. Does it consist of a single circulation cell in each hemisphere, as previously believed (Giles, 1999) and again recently reported (Gizon et al, 2020), or is it double or multi cellular (Zhao et al., 2013)?

Among other methods, time-distance helioseismology can be used to infer interior flows by measuring the travel times of helioseismic waves on the solar surface. For this, one needs so-called sensitivity kernels that relate the observed travel-time shifts to flow speeds. Ray- and Born-approximation kernels have been developed and used for this purpose in the past. Recently, we have developed a new class of realistic three-dimensional sensitivity kernels for solar interior flows by using numerical simulations of helioseismic wave propagation through small test perturbations in a simulated solar interior (Hartlep & Zhao, 2021). By measuring the effects of the perturbations on the frequency-dependent travel times of waves, we are able to solve for frequency-dependent sensitivity kernels.

In this paper, we report on the use of these kernels to infer the solar meridional flow. We will present measurements and inversions using numerically simulated wavefields in the presence of single- and multi-cellular meridional flow models for validating our kernels and the inversion technique, and will report on inferring the solar meridional flow using SDO/HMI doppler measurements.