Rocket underflights for SOHO cross-calibration and solar atmosphere investigations is a multi-year program to provide the absolute solar flux in the extreme ultraviolet (EUV) wavelength region. The data obtained will provide verification of the Solar Heliospheric Observatory (SOHO) Solar EUV Monitor (SEM) calibration and will be distributed to the SOHO investigators on a priority basis. The data will be analyzed and compared with appropriate solar models and will be published in appropriate scientifi c journals. The specific instrumentation to be flown is presented within the body of this document.
The scientific objectives of the proposed 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 primary objective of the proposed series of sounding rocket missions is to 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 spe ctroscopic 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. T he SEM is included in the SOHO package to address this particular requirement. Figure 1 shows a representative solar EUV spectrum with the wavelength regions of the instrumentation aboard SOHO. 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. Once in operatio n at L1, any possible changes in spectral responsivity of the above complement of instruments aboard will be monitored by the SEM in order not to lose their extensive pre-flight calibration. The presently proposed underflights avoid the long term calibration problems that have existed in previous Federally funded solar EUV missions (e.g. the Atmospheric Explorer series of missions). To verify the absolute intensity measurements we intend to incorporate a flight proven rare gas ionization cell using neon as a working gas (absolute flux shortward of 575 Å will be obtained). Further, to supplement these data, and the SOHO inter-calibration procedure discussed above, the spectral emissions from key solar EUV lines and continuum will be measured on the proposed underflights by an Optics Free Spectrometer (OFS). Our science objectives will thus be met in two ways. We will first assure the correct calibration of the SOHO data by comparing it with the underflight data. We will then distribute it to the relevant personnel so that the SOHO instruments calibration c an be adjusted as necessary. Secondly, the underflight data and available SOHO data will then be compared with an appropriate global semi-empirical collisional model (Shemansky and Smith, 1981) to improve our understanding of the physics, and the variabi lity 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 opted to study other solar models in order to obtain a reliable method which can be used to investigate the mechanisms for solar EUV variability.
A schematic diagram of the proposed EUV transmission grating spectrometer using three isolated silicon photodiodes as detectors is shown in Figure 2. A description of the optical design of the SEM and the preliminary test results (performed at NIST Ga ithersburg Md.) have been described by Ogawa et al., 1992. A brief description is given below. The photodiodes to be used are of the type flown successfully by us aboard a sounding rocket at the White Sands Missile Range in which we have obtained the int egrated absolute solar EUV flux in the Al band pass (Ogawa et al. 1990). To achieve spectral resolution as well, a free standing X-ray transmission grating (5000 grooves/mm) is placed on the optic axis between two highly stable (1500 Å thick each) alumin um filters. The filters not only limit the radiation that enters the spectrometer to the Al transmission band pass (170 - 700 Å), but when placed in series with each other as shown will also serve to sensibly eliminate the effects due to any pinholes wh ich may develop.
The rare gas ionization cell provides the absolute integral solar flux in the ionization region of the working gas (l £ 575 Å for neon). The cell has been previously described by Carlson et al. (1984) and by Ogawa and Judge (1986) and a schematic diag ram of the cell is shown in Figure 3. This cell is operated in the optically thick mode. The neon gas is periodically introduced and exhausted through an open (windowless) aperture in order to avoid any time dependent sensitivity changes associated with an accumulation of contaminant gas and/or time dependent window transmission. The absolute flux is proportional to the extrapolated current at zero density. Since the quantum yield is one for this instrument, and since no window is utilized, the rare g as cell used is a radiometric absolute detector. A similar cell (double ionization cell) has been previously used by the National Institute of Standards and Technology (NIST) as the standard reference detector for VUV radiation, prior to the recently ad opted NIST SURF II electron synchrotron storage ring as the standard. The electron-ion pairs formed in the gas cell by the absorption of ionizing EUV photons are collected and the ion current is measured using a highly stable electrometer. Knowledge of the absolute gas density is not required since the cell is optically th ick at all wavelengths of interest. Aside from its intrinsic interest, the absolute integral flux obtained is used to normalize the spectral data obtained by a solar EUV spectrometer. The rare gas cell is ~ 130 cm in length and completely absorbs all photons shortward of the ionization limit of the working gas. The diameter of the cell is 5 cm. The length and diameter of the cell were selected to provide an optically thick cell for E UV photons, and optically thin cell for the secondary electrons produced in the photoionization process. The power consumption amounts to ~ 5 watts during continuous operation with a 12% duty cycle (28 V @ 1.5 A peak power required to open the gas valve) . The gas consumption rate is ~ 10-5 moles/s during operation.
A schematic diagram of the Helium Double-Ionization Cell is shown in Figure 4. It is similar in operation to the RGIC as described above, except that the Helium Cell is operated in an optically thin mode. The helium gas is cycled into the chamber per iodically, and exhausted through the windowless aperture. The ionization current is measured by two ion collectors, each approximately half the length of the cell, one in front of the other. Since the cell is operated optically thin, knowledge of the ab solute gas density is required to complete the data analysis. This is accomplished with an ionizing radiation pressure gauge. The Radiation Pressure Gauge (RPG) is of the same design flown successfully by us on flights 21.065, 27.076, 27.079, 36.041, 36 .059, and 36.072. Optics Free Spectrometer (OFS) A prototype OFS has been developed for space application and has been fully described by Daybell et al. (1991, 1993), and by Judge et al. (1993). In this recently developed spectrometer, the energy spectrum of the incoming photons is transformed directly into an electron energy spectrum by taking advantage of the photoelectric effect in one or several rare gases at low pressure (1 to 10 mTorr). A schematic diagram of the rare gas cell, electron lens, and photoelectric energy analyzer is shown in Figure 5. This instrument has no degradable optical elements, a problem common to conventional optical spectrometers, and it also eliminates the problem of multiple order separation. The OFS technique has been successfully demonstrated on rocket flight 36.131U S, and is presently being enhanced. A schematic view of the OFS is shown in Figure 4. Photons enter from the right through a collimator filled with the rare gas of interest. The photoelectron produced in the small ionization volume (gas box) located above an electron lens are collected an d energy analyzed. Behind the gas box on the left is a light trap. The ion coincidence channel shown above the gas box is an enhancement (option) which lets us sweep out and detect the ion created in the same photoionization event which produced the ele ctron. This optional feature allows us to eliminate background events, and to monitor the absolute sensitivity of the OFS. One enhancement not shown in the schematic, but already implemented, is the magnetic shielding required to screen out the Earth's field. The absence of such shielding is known to restrict the wavelength region of the OFS operation to wavelengths shortwar d of 400 Å. We hold the field in our spectrometer to about one milligauss, thus permitting high count rate spectral data to be obtained throughout the rare gas ionization region (l£1022 Å). The flow of target gas out of the gas box region is constrained by the aperture and collimator along the incoming photon beam, and by the small slit on the bottom of the gas box connecting it with the electron lens. The five element electron lens allows a reduction of escaping target gas in the path of the electrons being analyzed.
The German GEOSOLLY experiment is shown in Fig. x. The hydrogen cell shown contains H2 gas at low pressure (a few Torr) and is sealed by MgF2-windows. A hot filament dissociates the molecules into hydrogen atoms. By measuring the scattered Lyman Alpha fluorescence, the core region of the solar Lyman Alpha line is studied and hence its line shape can be determined. The intensity of the solar Lyman Alpha is also obtained from the transmission through the hydrogen cell. With the help of a mirror system in front of the cell the geocoronal emissions can be observed and analyzed, instead.
In addition to the above instrumentation, the experimental payload will contain two video cameras to monitor payload attitude and onboard experiment functions. One camera selected is a SONY model XC-77 high shock industrial CCD camera that operates a t 12V, 6.2W with an auto iris lens. The other camera is a 9V, 118mA B&W board camera with a pinhole lens.