Introduction
    1.1  The HESSI Mission
    1.2  HESSI Instrument Data
    1.3  References
    1.4  Contacts
    1.5  Acknowledgements


1  Introduction

This is the HESSI Analysis Guide (HAG). The Guide is in two main parts, the HESSI Users Guide (Section 2) and the HESSI Instrument Guide (Section 3). The User Guide describes how to analyse HESSI data and the Instrument Guide discusses aspect of the HESSI instrument. Additional information is provided in the Appendices.

1.1  The HESSI Mission

The HESSI mission consists of a single spin-stabilized spacecraft in a 600km orbit inclined at 38° to the Earth's equator. The only instrument on board is an imaging spectrometer with the ability to obtain high fidelity movies of solar flares in X-rays and g-rays. It uses two new complementary technologies: fine grids to modulate the solar radiation, and germanium detectors to measure the energy of each photon very precisely.

HESSI's full Sun imaging capability is achieved with grids that modulate the solar X-ray flux as the spacecraft rotates at ~15 rpm; up to 20 images can be obtained per second. The high-resolution spectroscopy uses germanium crystals that detect the X-ray and g-ray photons transmitted through the grids over the broad energy range from 3 keV to 17 MeV.

HESSI will be launched in June 2001 on a Pegasus (SELVS-II) launch vehicle. The hoped for baseline mission duration is 3 years. The low-altitude equatorial orbit was chosen to minimize damage to the germanium detectors from the charged particles in the Earth's radiation belts. The orbit's inclination was chosen so that the ground-station at University of California at Berkeley (UCB) can command the spacecraft and receive telemetry for ~5 orbits/day.

1.2   HESSI Instrument Data

Telemetry packets downlinked from the HESSI spacecraft are saved in a number of files with names that correspond to the time of the contact initiation. There are three types of files: spacecraft housekeeping (VC1), real time science (VC2), and event list (VC3). The VC1 and VC3 packets are used to produce the reformatted HESSI instrument data - VC2 packets, which should be a subset of VC3, are only used during the real-time contacts.

The HESSI archive data are reformatted into files corresponding to one orbit's worth of data. The files are in FITS format and have names of the form ``hsi_yyyymmdd_hhmmss_nnn.fits'', where the time yyyymmdd_hhmmss refers to date and UT for the first packet of the file, and nnn is a version number (used if the file is regenerated).

HESSI data largely consists of event lists; these include the energy and arrival time of each event, together with information about the spacecraft aspect and rotation. The size of an orbit file depends on the detectors countrates, but they are typically a few tens of megabytes in size. Files which are larger than 50 Mb are broken into smaller files of 50 Mb or less - this should only happen for very large flares. Normally, unless there are multiple files in an orbit, the start time of a file is the middle of spacecraft night - for multiple files, a new file is created for every 30 thousand packets, each with a corresponding start time. Approximately 1 Gb of data are produced by HESSI each day. At 4 byte/photon, this equates to 200 M photons/day, most of which are background photons, except during very large flares.

After reformatting, the archived data are used to generate the HESSI Data Catalog - this includes the Observing Summary, Flare List and Instrument Log. The Data Catalog is in the form of files that cover a whole day and have names of the form hessi_catalog_yyyymmdd.fits. The part of the data catalog for the time interval covered by the FITS file is included in the file. The complete catalog for the entire mission is distributed within the SSWDB (SSW DataBase) tree; it is also kept online on an SOC workstation at UCB and is available for browsing through the Web.

The whole-mission flare list (hessi_flare_list.fits), and a file database (hsi_filedb.fits) that contains information relating to all files created in reformatting, are distributed within the HESSI branch of the SolarSoft (SSW) tree.

The archived data will be available through the SDAC (Solar Data Analysis Center) at NASA GSFC, and through the HEDC (HESSI Experimental Data Center) at ETH, Zurich.

1.3  References

The High Energy Solar Spectroscopic Imager (HESSI); 15 June 1997, proposal in response to NASA AO 97-OSS-03 (UCBSSL 1925/97)
The High Energy Solar Spectroscopic Imager (HESSI) - As Built; 6 Feb 2001, Brian Dennis, GSFC.
Behind, Beneath, and Before HESSI Spectroscopy; 4 Feb 2001, David Smith, UCB.

1.4  Contacts

Bob Lin              (SSL, UCB)
Gordon Hurford       (SSL, UCB)   ghurford@apollo.ssl.berkeley.edu
David Smith          (SSL, UCB)   dsmith@ssl.berkeley.edu
Jim McTiernan        (SSL, UCB)   jimm@ssl.berkeley.edu
Andre Csillaghy      (SSL, UCB)   csillag@ssl.berkeley.edu
Chris Johns-Krull    (SSL, UCB)   cmj@ssl.berkeley.edu

Brian Dennis         (GSFC)       Brian.R.Dennis@gsfc.nasa.gov
Richard Schwartz     (GSFC)       richard.schwartz@gsfc.nasa.gov
Kim Tolbert          (GSFC)       Anne.K.Tolbert@gsfc.nasa.gov
Ed Schmahl           (GSFC)       schmahl@hesperia.gsfc.nasa.gov
Also see the HESSI Team web page.

1.5  Acknowledgements

The HESSI Analysis Guide (HAG) was prepared by Bob Bentley (rdb@mssl.ucl.ac.uk) of the Mullard Space Science Laboratory, with significant help and input from Richard Schwartz, Andre Csillaghy, Kim Tolbert, Dominic Zarro, Chris Johns-Krull, and many other members of the HESSI Team.




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