The origins of hot plasma in the solar corona.

The Sun's outer atmosphere, or corona, is heated to millions of degrees, considerably hotter than its surface or photosphere. Explanations for this enigma typically invoke the deposition in the corona of nonthermal energy generated by magnetoconvection. However, the coronal heating mechanism remains unknown. We used observations from the Solar Dynamics Observatory and the Hinode solar physics mission to reveal a ubiquitous coronal mass supply in which chromospheric plasma in fountainlike jets or spicules is accelerated upward into the corona, with much of the plasma heated to temperatures between ~0.02 and 0.1 million kelvin (MK) and a small but sufficient fraction to temperatures above 1 MK. These observations provide constraints on the coronal heating mechanism(s) and highlight the importance of the interface region between photosphere and corona.

Chromospheric alfvenic waves strong enough to power the solar wind.

Alfvén waves have been invoked as a possible mechanism for the heating of the Sun's outer atmosphere, or corona, to millions of degrees and for the acceleration of the solar wind to hundreds of kilometers per second. However, Alfvén waves of sufficient strength have not been unambiguously observed in the solar atmosphere. We used images of high temporal and spatial resolution obtained with the Solar Optical Telescope onboard the Japanese Hinode satellite to reveal that the chromosphere, the region sandwiched between the solar surface and the corona, is permeated by Alfvén waves with strong amplitudes on the order of 10 to 25 kilometers per second and periods of 100 to 500 seconds. Estimates of the energy flux carried by these waves and comparisons with advanced radiative magnetohydrodynamic simulations indicate that such Alfvén waves are energetic enough to accelerate the solar wind and possibly to heat the quiet corona.

Coronal transverse magnetohydrodynamic waves in a solar prominence.

Solar prominences are cool 10(4) kelvin plasma clouds supported in the surrounding 10(6) kelvin coronal plasma by as-yet-undetermined mechanisms. Observations from Hinode show fine-scale threadlike structures oscillating in the plane of the sky with periods of several minutes. We suggest that these represent Alfvén waves propagating on coronal magnetic field lines and that these may play a role in heating the corona.

Small-scale jetlike features in penumbral chromospheres.

We observed fine-scale jetlike features, referred to as penumbral microjets, in chromospheres of sunspot penumbrae. The microjets were identified in image sequences of a sunspot taken through a Ca II H-line filter on the Solar Optical Telescope on board the Japanese solar physics satellite Hinode. The microjets' small width of 400 kilometers and short duration of less than 1 minute make them difficult to identify in existing observations. The microjets are possibly caused by magnetic reconnection in the complex magnetic configuration in penumbrae and have the potential to heat the corona above a sunspot.

Twisting motions of sunspot penumbral filaments.

The penumbra of a sunspot is composed of numerous thin, radially extended, bright and dark filaments carrying outward gas flows (the Evershed flow). Using high-resolution images obtained by the Solar Optical Telescope aboard the solar physics satellite Hinode, we discovered a number of penumbral bright filaments revealing twisting motions about their axes. These twisting motions are observed only in penumbrae located in the direction perpendicular to the symmetry line connecting the sunspot center and the solar disk center, and the direction of the twist (that is, lateral motions of intensity fluctuation across filaments) is always from limb side to disk-center side. Thus, the twisting feature is not an actual twist or turn of filaments but a manifestation of dynamics of penumbral filaments with three-dimensional radiative transfer effects.

White light sunspot observations from the solar optical universal polarimeter on spacelab-2.

The flight of the Solar Optical Universal Polarimeter on Spacelab-2 provided the opportunity for the collection of time sequences of diffraction-limited (0.5 are second) solar images with excellent pointing stability (0.003 are second) and with freedom from the distortion that plagues ground-based images. A series of white-light images of active region 4682 were obtained on 5 August 1985, and the area containing the sunspot has been analyzed. These data have been digitally processed to remove noise and to separate waves from low-velocity material motions. The results include (i) proper motion measurements of a radial outflow in the photospheric granulation pattern just outside the penumbra; (ii) discovery of occasional bright structures ("streakers") that appear to be ejected outward from the penumbra; (iii) broad dark "clouds" moving outward in the penumbra, in addition to the well-known bright penumbral grains moving inward; (iv) apparent extensions and contractions of penumbral filaments over the photosphere; and (v) observation of a faint bubble or looplike structure that seems to expand from two bright penumbral filaments into the photosphere.

Improvements in birefringent filters. 6: Analog birefringent elements.

A 100-mA analog birefringent element has been built from a polarizing wide field solid Michelson interferometer. The analog element behaves on-axis identically to a crystal element but has the field characteristics of a wide field Michelson. It is demonstrated that by proper choice of glasses for the arms of the Michelson the wide field condition can hold over a large spectral and large temperature range. Most important, proper choice of arm materials allows minimization of the wavelength sensitivity to temperature. A reduction of a factor of 10(2)-10(3) compared to birefringent crystals can be achieved.

Improvements in birefringent filters. 5: Field of view effects.

The field of view characteristics of wide field birefringent (WFB) elements are compared with those of the Fabry-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.

Improvement in birefringent filters. 4: The alternate partial polarizer filter.

A design for a birefringent filter is proposed in which alternate polarizers are partial polarizers. Calculated performance characteristics of alternate partial polarizer filters (APP) are compared with those of Lyot and contrast element Lyot filters. These calculations show that the APP design has significant advantages in both transmission and profile shape. Using pulse techniques, partial polarizer systems are shown to be a natural evolution from the standard Lyot and contrast element Lyot systems. The APP filter using achromatic waveplates discussed in earlier papers of this series has been used to construct a universal alternate partial polarizer filter. This filter has a measured full width at half-maximum (FWHM) of 0.09 A at 5500 A and a transmission in polarized light of 38%. It is tunable from 4500 A to 8500 A. The measured characteristics of the filter agree well with theoretical predictions.

Improvement of birefringent filters. 2:achromatic waveplates.

By use of Jones' matrix techniques, nine-element achromatic waveplates are developed. These combination plates are achromatic to within 1 degrees throughout the visible (3,500-10,000 A). In addition, a two-section general retarder rotator is demonstrated. The retardation of the combination is twice the complement of the angle between the halves, while the rotation is equal to the angle between the halves. For a 90 degrees retardation, the dual five-element combination is also achromatic to within 1 degrees throughout the visible.

Improvement of birefringent filters. 3: effect of errors on wide field elements.

The properties of nontunable and tunable Lyot wide field elements are examined when the components of the elements deviate from their proper values. Special emphasis is put on determining what variations cause light to be transmitted at the transmission minima. The analysis shows that the nine- and ten-element plastic waveplates described in Paper 2 of this series can be used to make a Lyot filter that is tunable from 3500 A to 10,000 A.

Tilt tunable ultra narrow-band filters for high resolution infrared photometry.

The design and operating characteristics of several recently fabricated solid-spaced Fabry-Perot interference filters having bandwidths from 0.3 A to 8 A in the 1-5-microm ir wavelength range are discussed. The 10(4) achievable resolving power of these devices, particularly at the higher wavelengths, represents several orders of magnitude improvement over conventional ir filters, and the implications of this very high resolution along with the tilt-tuning properties of narrow-band filters are discussed in relation to atmospheric remote sensing.

Total reduction of distorted echelle spectrograms: an automatic procedure.

We describe a semiautomatic procedure for the reduction of high-dispersion echelle spectra recorded with an image tube. The spectra are traced with a computer-controlled microdensitometer that scans along the curved spectral orders. The curvature of each order is calculated approximately by a FORTRAN program from known grating and distortion parameters. A typical spectrum includes 25 orders (covering 1500 A) and is traced with a slit 0.012 A wide. To produce an atlas of intensity vs wavelength and to determine the equivalent widths of 300 lines currently require a day. We discuss the reduction procedures and time requirements in detail.

Drift in interference filters. Part 1.

Studies of narrow-band interference filters have shown that two mechanisms exist that cause drift to shorter wavelengths. One is dependent on the thermal history of the filter, and the other depends on the radiation history. The present paper presents experimental results on thermal effects; and it is shown that by a proper bake cycle, ZnS-cryolite filters are stable for years if stored at less than 38 degrees C.

Drift in interference filters. 2: radiation effects.

Studies of peak transmission drift in narrow-band interference filters have shown that there exist two mechanisms that cause drift toward shorter wavelengths. One is dependent on the thermal history of the filter and is discussed in Part 1 of this paper. The other is dependent on the exposure of the filter to radiation. For ZnS-cryolite filters of the design [(HL)(4)H(8)(LH)(4)L](3)L(-1), it is experimentally demonstrated that the filters are most sensitive to radiation in a 100-A band centered at approximately 3900 A. The drift rate in the focal plane of an f/20 solar image is approximately 3 A/100 h of exposure. Further, it is also shown by model calculations that the observed radiation-induced drift is consistent with the hypothesis that the optical thickness of ZnS decreases in proportion to the radiant energy absorbed.