1. HMI Investigation Overview
Philip Scherrer
HMI Principal Investigator

2. HMI Investigation Overview – Agenda
HMI – Investigation Overview - What HMI measures and why.
Investigation Overview
Science Objectives
Science Team and Institutional Roles
Data Product Examples
Data Products and Objectives
How HMI works at the topmost level
Helioseismology – What is it?
Local HS example
Vector Field example
HMI observing needs summary

3. Investigation Overview - 1
The primary goal of the Helioseismic and Magnetic Imager (HMI) investigation is to study the origin of solar variability and to characterize and understand the Sun’s interior and the various components of magnetic activity.
The HMI investigation is based on measurements obtained with the HMI instrument as part of the Solar Dynamics Observatory (SDO) mission.
HMI makes measurements of the motion of the solar photosphere to study solar oscillations and measurements of the polarization in a spectral line to study all three components of the photospheric magnetic field.

4. Investigation Overview - 2
The basic HMI measurements must be processed into higher level data products before analysis can proceed
HMI produces data products suitable to determine the interior sources and mechanisms of solar variability and how the physical processes inside the Sun are related to surface magnetic field and activity.
It also produces data products to enable model estimates of the low and far coronal magnetic field for studies of variability in the extended solar atmosphere.

5. Investigation Overview - 3
The production of HMI high level data products and analysis of HMI data requires the participation of the HMI Team with active collaboration with other SDO instrument teams and the LWS community.
HMI observations will enable establishing the relationships between the internal dynamics and magnetic activity in order to understand solar variability and its effects. This is a prerequisite to understanding possible solar activity predictability.
HMI data and results will be made available to the scientific community and the public at large through data export, publications, and an Education and Public Outreach program.

6. HMI Science Objectives – Top Level
HMI science objectives are grouped into five broad categories:
Convection-zone dynamics and the solar dynamo;
How does the solar cycle work?
Origin and evolution of sunspots, active regions and complexes of activity;
What drives the evolution of spots and active regions?
Sources and drivers of solar activity and disturbances;
How and why is magnetic complexity expressed as activity?
Links between the internal processes and dynamics of the corona and
heliosphere;
What are the large scale links between the important domains?
Precursors of solar disturbances for space-weather forecasts.
What are the prospects for predictions?
These objectives are divided into 18 sub-objectives each of which needs data from multiple HMI data products.
Progress will require a science team with experience in multiple disciplines.

7. HMI Co-Investigator Science Team

8. HMI Institutional Roles
LWS Science
SDO Science
HMI Instrument
HMI Science Team
Stanford
HMI SOC
HMI
E/PO
LMSAL

9. HMI Data Product Examples
B – Rotation Variations
C – Global Circulation
D – Irradiance Sources
H – Far-side Imaging
F – Solar Subsurface Weather
E – Coronal Magnetic Field
I – Magnetic Connectivity
J – Subsurface flows
G – Magnetic Fields
A – Interior Structure
Sound speed variations relative to a standard solar model.
Solar cycle variations in the sub-photospheric rotation rate.
Solar meridional circulation and differential rotation.
Sunspots and plage contribute to solar irradiance variation.
MHD model of the magnetic structure of the corona.
Synoptic map of the subsurface flows at a depth of 7 Mm.
EIT image and magnetic field lines computed from the photospheric field.
Active regions on the far side of the sun detected with helioseismology.
Vector field image showing the magnetic connectivity in sunspots.
Sound speed variations and flows in an emerging active region.

10. HMI Data Products and Objectives
Global
Helioseismology
Processing
Local
Filtergrams
Line-of-sight
Magnetograms
Vector
Doppler
Velocity
Continuum
Brightness
Brightness Images
Line-of-Sight
Magnetic Field Maps
Coronal magnetic
Field Extrapolations
Coronal and
Solar wind models
Far-side activity index
Deep-focus v and cs
maps (0-200Mm)
High-resolution v and cs
maps (0-30Mm)
Carrington synoptic v and cs
Full-disk velocity, v(r,Θ,Φ),
And sound speed, cs(r,Θ,Φ),
Maps (0-30Mm)
Internal sound speed,
cs(r,Θ) (0<r<R)
Internal rotation Ω(r,Θ)
(0<r<R)
Vector Magnetic
Field Maps
Tachocline
Differential Rotation
Meridional Circulation
Near-Surface Shear Layer
Activity Complexes
Active Regions
Sunspots
Irradiance Variations
Magnetic Shear
Flare Magnetic Config.
Flux Emergence
Magnetic Carpet
Coronal energetics
Large-scale Coronal Fields
Solar Wind
Far-side Activity Evolution
Predicting A-R Emergence
IMF Bs Events
Science Objective
Data Product
Observables
HMI Data
Version 1.0

11. HMI – How It Works
Measure Here
HMI consists of a telescope, tunable filter, camera, and necessary electronics.
HMI images the Sun in four polarizations at five wavelengths across a spectral line.
The position of the line tells us the velocity while the shape changes of the line in different polarizations tell us the magnetic field direction and strength in the part of the Sun’s surface seen by each pixel.
Long gap-free sequences of velocity measurements are needed to use the techniques of helioseismology.

12. Magnetic Field Sample Profile
HMI measures magnetic fields by sampling the Zeeman split line in multiple polarizations.
The figure shows the five sample positions for a sunspot umbral field (about 3000G) with a 1000 m/s offset.
The green and red curves are Left and Right circular polarized components and allow measurement of the line-of-sight projection of the field.
Analysis of both polarizations is required to infer the Doppler velocity and line-of-sight magnetic flux.
For vector fields two directions of linear polarization are added to infer the field direction.

13. Helioseismology – What Is It?
Helioseismology is the study of solar interior structure and dynamics via analysis of the propagation of waves through the Sun’s interior.
The Sun is filled with acoustic waves with periods near five minutes.
These waves are refracted upward by the temperature gradient and reflected inward by the drop in density at the surface
The travel times of these waves depends on the temperature, composition, motion, and magnetic fields in the interior.
The visible surface moves when the waves are reflected enabling their frequency, phase, and amplitude to be measured.
Analysis of travel times over a multitude of paths enables inference of internal conditions.

14. Helioseismology - 2
The wave reflections result in oscillations of the surface.
These motions are a few hundred m/s and are superimposed on the 1500 m/s granulation, 400 m/s supergranulation, 2000 m/s solar rotation and 3500 m/s SDO orbit.
The dynamic range of HMI must accommodate all these motions in addition to the line splitting equivalent to 3000 m/s from sunspot magnetic fields.
Measurements must be often enough to resolve the oscillations (c. 45 seconds).
Sequences must be long enough to resolve phase and frequency yet short enough to sample the evolving structures.

15. Solar Domain of HMI Helioseismology2 3 4 5 6 7
Sun
Log Size (km)
Zonal flow AR
spot SG
dynamo
P-modes
Time-Distance
Rings
Global HS 1 8 9
min
5min
hour
day
rotation
year
cycle
Log Time (s) 10
polar field
Earth
HMI resolution
granule

16. Time-Distance Helioseismology Example
Waves going in all directions are reflected at each point on the surface.
Cross-correlations of the time series observed at pairs of points (A,B) reveal the integrated travel-time along the interior path that “connects” A with B.
Differences between the A→B and B→A directions arise from bulk motion along the path.
Analyses of travel-time maps provide maps of flows and temperatures beneath the surface.

17. Vector Magnetic Field
Traditional solar magnetic measurements provide only the line-of-sight magnetic flux.
Experience has shown that the full vector field is necessary to understand the connectivity in and between active regions.
Inversions of polarization measurements provide all three components of the field as well as the filling-factor of the unresolved magnetic elements.
Long sequences of vector field data have yet to be measured.
We expect to learn a lot.

18. HMI “Level 1” Requirements
To enable accomplishment of the science objectives of the investigation, the HMI instrument will produce measurements in the form of filtergrams in a set of polarizations and spectral line positions at a regular cadence for the duration of the mission that meet these basic requirements:
Full-disk Doppler velocity and line-of-sight magnetic flux images with 1.5 arc-sec* resolution at least every 50 seconds.
Full-disk vector magnetic images of the solar magnetic field with 1.5 arc-sec* resolution at least every 10 minutes.
The HMI data completeness and continuity requirement is to capture 99.99% of the HMI data 95% of the time.
The purpose of rest of this PDR is to describe how these requirements will be met.
*(1.0 arc-sec goal)