The Helioseismic and Magnetic Imager (HMI) investigation (Solar Phys.

The ultraviolet photometer of the University of Iowa spin scan auroral imaging instrumentation on board the Dynamics Explorer 1 satellite has returned numerous images of the geocorona from altitudes of 570 km to 23,300 km. The geocoronal observations from 1981 through 1985 are compared to a spherically symmetric isothermal Chamberlain model of the exospheric density distribution. Model parameters are varied to obtain an acceptable fit. The radiative transfer equation is solved numerically. Stellar intensities are monitored for an independent calibration of the DE 1 instrument in flight. The solar Ly • flux is estimated through concurrent measurements made by the Solar Mesosphere Explorer satellite, supplemented by published values of ground-observable solar indices. Extraterrestrial background intensities are adopted from earlier OGO 5 high-altitude measurements. The optimum fit for 1981 imaging data utilizes a Chamberlain model of temperature T = 1050 K and exobase density n½ = 44,000 atoms cm-3. The exobase is taken as r e ---1.08 Rg (500 km altitude), and a critical radius for satellite atoms of r•s = 3.0 r e is adopted. This model continues to compare well with the DE 1 measurements over the entire 4-year period studied, even though the exobase conditions are expected to have changed appreciably during this interval of declining solar activity. It is concluded that the apparently constant hydrogen density and scale height observed by DE 1 are not directly indicative of the exobase conditions through the classical Chamberlain model but rather show the effects of charge exchange with thermal ions in the plasmasphere. A readily observable departure from spherical symmetry is the geotail, an enhancement in the atomic hydrogen column densities in the antisunward direction. atmosphere of the earth and the northern auroral oval are visible mainly in the emission lines of atomic oxygen (primarily 1304 •), which are transmitted with approximately equal sensitivities by the two filters represented on this plate. The sensitivity to Lyman • (1216 •) is lesser by a factor of • 12 for the filter used to obtain the image in the right-hand panel. These are consecutive images, each requiring 12 min to collect and telemeter. The spacecraft motion during this interval is revealed by the slight change in location of the back-ground star field. Emission rates represented by the color code for this and the following plates range from ~ 25 kR in white at the dayside limb down to ~ 2.5 kR in red; lesser values are black. The 6.8-hour DE 1 orbit, with an apogee altitude of 23,300 km and a perigee altitude of 570 km, is ideally suited to return images from a useful range of depths within the geocorona. The views in Plate 1, obtained on October 20, 1981, are from a near-apogee altitude of approximately 22,300 km above the north pole. In Plate 2 a sequence of six consecutive images collected on November 20, 1981, shows a rapidly changing aspect as the spacecraft climbs from 8300 km to 17,900 km in altitude. Three months later, the orbit...

The Helioseismic and Magnetic Imager (HMI) investigation (Solar Phys. doi:10. 1007/s11207-011-9834-2, 2011) will study the solar interior using helioseismic techniques as well as the magnetic field near the solar surface. The HMI instrument is part of the Solar Dynamics Observatory (SDO) that was launched on 11 February 2010. The instrument is designed to measure the Doppler shift, intensity, and vector magnetic field at the solar pho- tosphere using the 6173 Å Fe I absorption line. The instrument consists of a front-window filter, a telescope, a set of waveplates for polarimetry, an image-stabilization system, a blocking filter, a five-stage Lyot filter with one tunable element, two wide-field tunable Michelson interferometers, a pair of 4096 2 pixel cameras with independent shutters, and associated electronics. Each camera takes a full-disk image roughly every 3.75 seconds giving an overall cadence of 45 seconds for the Doppler, intensity, and line-of-sight magnetic-field measurements and a slower cadence for the full vector magnetic field. This article describes the design of the HMI instrument and provides an overview of the pre-launch calibration efforts. Overviews of the investigation, details of the calibrations, data handling, and the science analysis are provided in accompanying articles.

The Helioseismic and Magnetic Imager (HMI) instrument will produce Dopplervelocity and vector-magnetic-field maps of the solar surface, whose accuracy is dependent on a thorough knowledge of the transmission profiles of the components of the HMI opticalfilter system. Here we present a series of wavelength-dependence calibration tests, performed on the instrument from 2005 onwards, to obtain these profiles. We obtained the transmittances as a function of wavelength for the tunable and non-tunable filter elements, as well as the variation of these transmittances with temperature and the angle of incidence of rays of light. We also established the presence of fringe patterns produced by interferences inside the blocking filter and the front window, as well as a change in transmitted intensity with the tuning position. This thorough characterization of the HMI-filter system confirmed the very high quality of the instrument, and showed that its properties are well within the required specifications to produce superior data with high spatial and temporal resolution.

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