On the Formation Height of the SDO/HMI Fe 6173 Doppler Signal

TL;DR

This study uses simulations to determine the formation height of the SDO/HMI Fe 6173 Doppler signal, revealing it forms low in the solar atmosphere and is affected by instrument resolution and filter uncertainties.

Contribution

It provides a detailed analysis of the formation height of HMI Doppler signals using realistic simulations, improving understanding of the measurement's atmospheric origin.

Findings

HMI Doppler signal forms near 100 km height in the solar atmosphere.

The MDI Doppler signal forms around 125 km height.

Instrument resolution increases apparent formation height by 40-50 km.

Abstract

The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the mag- netic field in the solar photosphere. It observes the full solar disk in the Fe I absorption line at 6173\AA . We use the output of a high-resolution 3D, time- dependent, radiation-hydrodynamic simulation based on the CO5BOLD code to calculate profiles F({\lambda},x,y,t) for the Fe I 6173{\AA} line. The emerging profiles F({\lambda},x,y,t) are multiplied by a representative set of HMI filter transmission profiles R_i({\lambda},1 \leq i \leq 6) and filtergrams I_i(x,y,t;1 \leq i \leq 6) are constructed for six wavelengths. Doppler velocities V_HMI(x,y,t) are determined from these filtergrams using a simplified version of the HMI pipeline. The Doppler velocities are correlated with the original velocities in the simulated atmosphere. The cross- correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal is formed rather low in the solar atmosphere. The same analysis is performed for the SOHO/MDI Ni I line at 6768\AA . The MDI Doppler signal is formed slightly higher at around 125 km. Taking into account the limited spatial resolution of the instruments, the apparent formation height of both the HMI and MDI Doppler signal increases by 40 to 50 km. We also study how uncertainties in the HMI filter-transmission profiles affect the calculated velocities.