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University of Southern California


SOHO spacecraft - SOHO

Initiated in 1995 following the launch of SOHO, the sounding rocket underflights for SOHO cross-calibration and solar atmosphere investigations is an ongoing multi-year program that provides the absolute solar flux in the extreme ultraviolet (EUV) wavelength region. Leveraging its well established sounding rocket program, USC has conducted a number of sounding rocket underflights to obtain accurate solar EUV measurments with a suite of solar EUV instrumentation.
These EUV measurments provide verification of the Solar Heliospheric Observatory (SOHO) Solar EUV Monitor (SEM) calibration and are distributed to the SOHO investigators on a priority basis. The data are analyzed and compared with appropriate solar models and are published in various scientific journals. A summary of the sounding rockets missions to date can be found on the sounding rocket missions page.
Science Objectives

The scientific objectives of the series of sounding rocket missions are: to measure, distribute, and publish accurate absolute solar EUV Irradiance data in support of the SOHO mission; to analyze and interpret the solar EUV data for the purpose of improving global solar atmospheric models and hence our understanding of solar variability.

The sounding rocket missions provide the absolute solar EUV data base required for the calibration of the solar instrumentation aboard SOHO. The joint ESA/NASA SOHO mission is aimed at quantitative spectroscopic observation of the sun and its corona, and these measurements are performed at the Sun-Earth L1 Lagrange point. Therefore, a careful calibration, cross-calibration, and in-operation performance monitoring of the instrumentation is required. The SEM is included in the SOHO package to address this particular requirement.

Cross-calibration is effected by comparing the response in the overlapping spectral regions. The SEM is calibrated using the SURF II synchrotron storage ring as a primary standard, and primarily monitors the prominent solar HeII 304 Å emission line and the wavelength band between 170 and 700 Å. Changes in the spectral responsivity of the CELIAS instrumentation are monitored by the SEM in order to maintain their extensive pre-flight calibrations. A flight proven rare gas ionization cell using neon as the working gas which obtains absolute intensity measurements (absolute flux shortward of 575 Å is obtained) is also integrated into the payload. Spectral emissions from key solar EUV lines and continuum are also measured with an Optics Free Spectrometer (OFS). Our science objectives has thus be met in two ways.

Raw data obtained in channels 1 and three, the first order of the SEM and ch2, the 0th order, from calibration rocket underflgiht US 36.227

solar EUV spectrum with the wavelength regions of the instrumentation aboard SOHO showing calibration data points obtained from rocket underflights. In the near future, this and other sem data graphs will be updated with the most current SOHO/SEM data.

(1) We first assure the correct calibration of the SOHO data by comparing it with the underflight data, which is then distribute to the relevant personnel so that the SOHO instruments calibration can be adjusted as necessary. (2) Secondly, the underflight data and available SOHO data is compared with an appropriate global semi-empirical collisional model (Shemansky and Smith, 1981) to improve our understanding of the physics, and the variability of the global solar atmosphere. This model differs substantially from existing proxy models that correlate ground based solar activity indices (such as solar microwave emissions) with the solar EUV emissions. Recently, there have been some evidence indicating that the 10.7 and at 21 cm emissions, widely used as proxies, are poorly correlated with both chromospheric, and transition region full disk solar EUV emissions (Neupert, 1993). We have therefore study other solar models in order to obtain a reliable method which can be used to investigate the mechanisms for solar EUV variability.
Instrumentation Description

The Center has developed a suite of EUV isntrumentation ( listed below) that are flown routinely on the Centers sounding rocket to meet the science objectives of the CAL-SO program. Follow the links to find out more information about each instrument.

To more accurately and efficiently meet the science objectives, the Center is continually developing new instrumentation that is subsequently tested by leveraging the Centers sounding rocket program. Information about instruments that are under development can be found on the "instruments" page.

    website last updated 5/31/2007