July 1, 2010 GONG 2010 – SoHO 24

Time-Distance Helioseismology from HMI

Tom Duvall

NASA Goddard Space Flight Center, Laboratory for Solar Physics

July 1, 2010 GONG 2010 – SoHO 24

HMI spectrum of a single 8-hour sequence for a 15 degree area near disk center.

Note: the impressive extension of ridges to high temporal and spatial frequencies.

July 1, 2010 GONG 2010 – SoHO 24

July 1, 2010 GONG 2010 – SoHO 24

a) Spectrum of 8 hours of HMI Doppler data near disk center; b) Spectrum of 8 hours of MDI hi-res data (from 1996); c) spatial power spectra of snapshots of HMI Doppler data and MDI hi-res data

integrated over azimuth.

a)

b)

c)

July 1, 2010 GONG 2010 – SoHO 24

Comparison of simultaneous snapshots of HMI continuum (top) and MDI hi-res continuum (middle) images of the famous sunspot of 29 March 2010. (bottom) shows cuts through the center of the spot

showing larger intensity in the MDI umbra (probably of instrumental origin).

July 1, 2010 GONG 2010 – SoHO 24

One of the first things one notices about HMI data is that it is huge! 4096x4096 images are just a little larger than I have been accustomed to. The HMI data also has better resolution than we have had in the past, say from MDI. Below are examples of 10-minute averages (to reduce the oscillations) of various observables at

disk center in a quiet time. Some are supplied directly by the project (mgtm, doppler, continuum, line depth), while the line core intensity is calculated as the continuum – line depth. One notices immediately that most of the parameters are very sensitive to the solar granulation, while the line core intensity is less so. The line core

intensity does show features correlated with the magnetic field.

Note: using a sqrt lookup table with the magnetogram makes a lot more features visible.

Rms values:

Mgtm 14 Gauss

Doppler 203 m/s

Continuum 0.030

Line depth 0.060

Line core 0.030

July 1, 2010 GONG 2010 – SoHO 24

Average temporal power spectra for several

observables. The temporal spectra are averaged over an area 15 deg on a side and for

8 hours. The top four parameters are supplied by the project and the bottom

two are derived.

Near disk center, the Doppler signal is by far the best in

terms of oscillation signal to background power ratio.

However, once one gets over to 75 deg, the line core has a

larger ratio. To make the best use of HMI data, we may

need to switch observables near the limb.

July 1, 2010 GONG 2010 – SoHO 24

Travel time convergence signals computed for the f mode at disk center and at 70 deg east. The computations are for annulus radius of 10 Mm and thickness 2 Mm.

We see that at disk center that the Doppler yields better s/n ratio, while near the limb the Doppler case is losing sensitivity compared to the line core intensity.

July 1, 2010 GONG 2010 – SoHO 24

A problem we had with MDI data was that

a spatio-temporal

power spectrum of

magnetograms showed p-

mode ridges. This has been fixed with the

new HMI instrument on

SDO.