The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestations of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convection zone and core. Knowledge of these properties will improve our understanding of the solar cycle and of stellar evolution. Other photospheric observations will contribute to our knowledge of the solar magnetic field and surface motions. The investigation consists of coordinated efforts by several teams pursuing specific scientific objectives. The instrument images the Sun on a 10242 CCD camera through a series of increasingly narrow spectral filters. The final elements, a pair of tunable Michelson interferometers, enable MDI to record filtergrams with a FWHM bandwidth of 94 mA. Normally 20 images centered at 5 wavelengths near the Ni I 6768 spectral line are recorded each minute. MDI calculates velocity and continuum intensity from the filtergrams with a resolution of 4″ over the whole disk. An extensive calibration program has verified the end-to-end performance of the instrument. To provide continuous observations of the longest-lived modes that reveal the internal structure of the Sun, a carefully-selected set of spatial averages are computed and downlinked at all times. About half the time MDI will also be able to downlink complete velocity and intensity images each minute. This high rate telemetry (HRT) coverage is available for at least a continuous 60-day interval each year and for 8 hours each day during the rest of the year. During the 8-hour HRT intervals, 10 of the exposures each minute can be programmed for other observations, such as measurements in MDI's higher resolution (1.25″) field centered about 160″ north of the equator; meanwhile, the continuous structure program proceeds during the other half minute. Several times each day, polarizers will be inserted to measure the line-of-sight magnetic field. MDI operations will be scheduled well in advance and will vary only during the daily 8-hour campaigns. Quick-look and summary data, including magnetograms, will be processed immediately. Most high-rate data will be delivered only by mail to the SOI Science Support Center (SSSC) at Stanford, where a processing pipeline will produce 3 Terabytes of calibrated data products each year. These data products will be analyzed using the SSSC and the distributed resources of the co-investigators. The data will be available for collaborative investigations.

The Solar Oscillations Investigation - Michelson Doppler Imager

The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestations of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convection zone and core. Knowledge of these properties will improve our understanding of the solar cycle and of stellar evolution. Other photospheric observations will contribute to our knowledge of the solar magnetic field and surface motions. The investigation consists of coordinated efforts by several teams pursuing specific scientific objectives.

The Solar Oscillations Investigation — Michelson Doppler Imager

An extension of Joint Phase Diverse Speckle image restoration is presented. Multiple realizations of multiple objects having known wavefront relations with each other can now be restored jointly. As the alignment of the imaging setup does not change, near-perfect alignment can be achieved between different objects, thus greatly reducing false signals in the determination of derived quantities, such as magnetograms, Dopplergrams, etc. The method was implemented in C++ as an image restoration server, to which worker clients can connect and disconnect randomly, so that a large number of CPUs can be used to speed up the restorations. We present a number of examples of applications of the restoration method to observations obtained with the Swedish 1-m Solar Telescope on La Palma.

Solar Image Restoration By Use Of Multi-frame Blind De-convolution With Multiple Objects And Phase Diversity

Observations of very quiet Sun using the Solar Optical Telescope/Spectro-Polarimeter (SOT/SP) aboard the Hinode spacecraft reveal that the quiet internetwork regions are pervaded by horizontal magnetic flux. The spatial average horizontal apparent flux density derived from wavelength-integrated measures of Zeeman-induced linear polarization is -->BTapp = 55 Mx cm −2, as compared to the corresponding average vertical apparent flux density of -->| BLapp| = 11 Mx cm −2. Distributions of apparent flux density are presented. Magnetic fields are organized on mesogranular scales, with both horizontal and vertical fields showing voids of reduced flux density of a few granules spatial extent. The vertical fields are concentrated in the intergranular lanes, whereas the stronger horizontal fields are somewhat separated spatially from the vertical fields and occur most commonly at the edges of the bright granules. High-S/N observations from disk center to the limb help to constrain possible causes of the apparent imbalance between -->| BLapp| and -->BTapp, with unresolved structures of linear dimension on the surface smaller by at least a factor of 2 relative to the SOT/SP angular resolution being one likely cause of this discrepancy. Other scenarios for explaining this imbalance are discussed. The horizontal fields are likely the source of the seething fields of the quiet Sun discovered by Harvey et al. The horizontal fields may also contribute to the hidden turbulent flux suggested by studies involving Hanle effect depolarization of scattered radiation.

https://iopscience.iop.org/article/10.1086/522922/pdf

The Horizontal Magnetic Flux of the Quiet-Sun Internetwork as Observed with the Hinode Spectro-Polarimeter

The flow of energy through the solar atmosphere and the heating of the Sun's outer regions are still not understood. Here, we report the detection of oscillatory phenomena associated with a large bright-point group that is 430,000 square kilometers in area and located near the solar disk center. Wavelet analysis reveals full-width half-maximum oscillations with periodicities ranging from 126 to 700 seconds originating above the bright point and significance levels exceeding 99%. These oscillations, 2.6 kilometers per second in amplitude, are coupled with chromospheric line-of-sight Doppler velocities with an average blue shift of 23 kilometers per second. A lack of cospatial intensity oscillations and transversal displacements rules out the presence of magneto-acoustic wave modes. The oscillations are a signature of Alfven waves produced by a torsional twist of ±22 degrees. A phase shift of 180 degrees across the diameter of the bright point suggests that these torsional Alfven oscillations are induced globally throughout the entire brightening. The energy flux associated with this wave mode is sufficient to heat the solar corona.

/pdf/alfven-waves-in-the-lower-solar-atmosphere-4gzxvudpuz.pdf

Alfvén waves in the lower solar atmosphere.

This article reviews the types and capabilities of birefringent filters. The general operating principles of Lyot (perfect polarizers), partial polarizing, and Solc (no internal polarizers) filters are introduced. Appropriate techniques for tuning each filter type are presented. Field of view of birefringent filters is discussed and is compared to Fabry-Perot and interference filters. The transmission and throughput advantages of birefringent filters are shown. Finally, the current state of the art in practical filters is reviewed.

Tunable birefringent filters

A waveplate is any optical device which retards one polarization component of an incident wave relative to the orthogonal component. This subject is much too general for our purposes so we will restrict the discussion to retardations of one quarter or one half of a wavelength. In particular, we will be concerned with phase retarders which are constant or nearly so over a significant spectral region.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Achromatic Retardation Plates

The field of view characteristics of wide field birefringent (WFB) elements are compared with those of the Fabŕy-Perot (F.P.), Michelson (MI), and wide field Michelson (WFM) interferometers. Throughput gains of 50 to 300 or more with respect to the F.P. or MI are demonstrated. Further, it is shown that by proper choice of material WFB elements can have angular characteristics identical with WFM interferometers. The properties of misaligned and mismatched half-length WFB elements are calculated. It is shown that properly misadjusted WFB elements can exhibit throughput gains with respect to properly adjusted systems. Finally, a catalog of fringe patterns for WFB elements of different materials and differing angular misadjustment is presented.

Improvements in birefringent filters. 5: Field of view effects

As a Japanese National space mission with international collaboration, Solar-B (2005 launch) will carry a spectro- polarimeter (SP) to be operated in visible light to obtain the first high angular resolution, precision measurements of solar vector magnetic fields from space. The SP is part of the Focal Plane Package (FPP) fed by a diffraction-limited 50-cm optical telescope. The SP will be operated exclusively at the photospheric 630 nm Fe I lines. It features a rotating, low-order crystalline quartz retarder for polarization modulation and a reflecting Littrow spectrograph design that is shortened by using diffraction from the 12micrometers wide slit to fill the grating. Polarization analysis is accomplished by a modified Savart plate beam splitter. A custom CCD detector with two active areas, one for each beam from the beam splitter, allows continuous high duty-cycle sampling of polarization. The spectrograph slit will sample a 0.16 x 164 arcsec2 rectangle of the solar image, which may be scanned across the slit by up to +/- 160 arcsec in order to build up vector magnetic field maps of the solar photosphere. Along with simultaneous, co-spatial imaging and polarimetry with the filter imagers of the FPP, the SP will provide a precise view of active and quiet solar magnetic fields that control the structure, dynamics, and energetics of the upper solar atmosphere.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Precision spectro-polarimeter for high-resolution observations of solar magnetic fields

A Solc (1965) filter configuration is presented which is both tunable and spectrally variable, since it possesses an adjustable bandwidth, and which although less efficient than a Lyot filter is attractive because of its spectral versatility. The lossless design, using only an entrance and exit polarizer, improves throughput generally and especially in the IR, where polarizers are less convenient than dichroic sheet polarizers. Attention is given to the transmission profiles of Solc filters with different numbers of elements and split elements, as well as their mechanical design features.

William J. Rosenberg

Papers

Tunable birefringent filters

Achromatic Retardation Plates

Improvements in birefringent filters. 5: Field of view effects

Precision spectro-polarimeter for high-resolution observations of solar magnetic fields

Solc filter engineering