This page provides an overview of online information
resources relevant to the analysis of data from the SOHO/MDI
Use of this page
The primary source of information about access to MDI data and
any special event information or calibration issues is the
MDI home page at http://soi.stanford.edu.
That page, in particular the Data Services and Operations links should be
consulted for up-to-date information and data export requests.
This page is provided as an overview of the use of MDI data.
(Pre-launch: Solar Physics162: 129-188, 1995.)
This paper provides an accurate description of the MDI instrument and
is still the best place to start for an overview of the instrument
and observing modes.
Observables: MDI measures line-of-sight motion (Dopplergrams), magnetic
field (magnetograms), and brightness images in full disk at several resolutions
(4 arc-second to very low resolution) and a fixed selected region in higher resolution (1.2 arc-second).
MAny data products are available in low level and higher level or processing
from raw images to spherical harmonic amplitudes of velocity. Some
high level products available directly on the web include maps of
a proxy for magnetic activity on the farside of the Sun.
For information about planned, current, and past observations please consult
the MDI Operations page. The Documentation section
at the bottom of that page contains links to detailed operations and planning
Calibration information can be found in two places. The
MDI Calibration Notes and Known Problems page contains
data calibration issues that may affect some particular uses of the data.
The importance of these issues varies depending on the application. Please
read this if you discover strange characterists of the Sun to be sure
you have not instead found a known issue with the data. The raw calibration
data that shows variation with time of issues such as focus can be
found on the above mentioned MDI Operations page.
Data file description(s)
Where: All MDI data is available for download without restrictions (see
policy. The entire
data collection is available from the MDI web pages via the
MDI Data Services & Information page. Selected subsets of the
data is also available via the SOHO Archive.
Format: MDI data is stored in FITS
formatted binary data files. When exported from the MDI data system the data is
delivered in sets of FITS files
known as MDI "datasets" grouped by hour, day, or other appropriate interval. Each type
of data is its stored in a "Dataseries" which contains a sequence of "Datasets".
This allows convenient access to long time series of like data as needed for
helioseismic studies. It can be less convenient if e.g. single magnetograms are
desired. As an additional service individual image FITS files can be exported
from the MDI data system or from the SOHO archive (selected products only).
Series: MDI series names contain basic information about the data.
They are formed by concatenating format, observable, cadence, and blocking information.
The format, observable, and blocking tags are nearly always present. The
cadence tag is present only when it is not the normal cadence for the observable.
Two examples are "fd_M_96m_01d" means full-disk, Magnetogram, 96minute cadence,
blocked into 1-day datasets. Or "vw_V_06h" for vector-weighted (medium-l)
Dopplergrams in 6-hour blocks with a 1-minute cadence.
Keywords: For FITS data to be useful, the metadata keywords must be
known. The MDI keywords are derived from a combination of the early SOHO
keyword lins, the GONG keywords, extended with some commonly used WCS
keywords. The list is available at Keywords.
Level: MDI data is available in several processing "levels". For various
historical reasons most users should want the "lev1.8" data. This is calibrated
with our most-up-to-date information. It is an on-the-fly produced update
of the "lev1.5" data that uses fixed calibration values and was for the first
few years the prime science data available. Lev1.8 data is the default
when data es requested from the MDI data system. The various datasets
available from the MDI archive are described
Quality: MDI data is tagged (in the keywords) with per-image
quailty/problem information discovered during processing or during observation
sequences. To see why a particular image looks strange see: Data Quality Bits.
Which: Too many choices? Which data? On the MDI
data export page you will find the link to the automated
data server. On that page most users want to use the "Calibrated Data Products"
for time series of standard products or "Calibrated Campaign Data" for
data observed in support of particular campaigns or SOHO JOPS. When in
doubt about which datasets you want, contact us.
Status: Processing status is updated weekly and available on
the Data Services page here.
How: To export MDI data follow one of the links above to select
the data series, and dates of interest. Then you will be given options to
get the data by one of several methods including ftp and external hard media.
Most requests are handled by ftp.
Acknowledgment of MDI data usage:
If you find MDI data useful and include its use in your research please include some statement like
"Data provided by the SOHO/MDI consortium. SOHO is a project of international
cooperation between ESA and NASA" and send us a copy of the paper as submitted for publication
and/or include the citation to "Scherrer et al., Solar Physics162: 129-188, 1995".
Since the MDI data is stored in standard FITS files there
is no need for MDI-only special purpose software packages to support it.
Most of the helioseismology science analyses are done using code
written in "C" with some "IRAF", MATLAB, etc. The C code developed
at Stanford is available by request.
Alternative software - Many users of MDI magnetograms also
use other SOHO or other solar data and use the "solarsoft" software
package that runs in the "IDL" commercial data analysis program.
Since this package is not used at Stanford a user should examine
the code or consult other users (such as our Lockheed Co-Is) for advice.