Saku Tsuneta

Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind

Alfvén waves have been invoked as a possible mechanism for the heating of the Suns 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.

Hot-Plasma Ejections Associated with Compact-Loop Solar Flares

Masuda et al. found a hard X-ray source well above a soft X-ray loop in impulsive compact-loop flares near the limb. This indicates that main energy release is going on above the soft X-ray loop, and suggests magnetic reconnection occurring above the loop, similar to the classical model for two ribbon flares. If the reconnection hypothesis is correct, a hot plasma (or plasmoid) ejection is expected to be associated with these flares. Using the images taken by the soft X-ray telescope aboard Yohkoh, we searched for such plasma ejections in eight impulsive compact-loop flares near the limb, which are selected in an unbiased manner and include also the Masuda flare, 1992 January 13 flare. We found that all these flares were associated with X-ray plasma ejections high above the soft X-ray loop and the velocity of ejections is within the range of 50-400 km s-1. This result gives further support for magnetic reconnection hypothesis of these impulsive compact-loop flares.

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

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.

Evidence for Alfvén Waves in Solar X-ray Jets

Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed (∼800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 × 103 to 2 × 104 kilometers wide and 1 × 105 kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind.

The SOLAR-A mission : an overview

The SOLAR-A spacecraft is to be launched by the Institute of Space and Astronautical Science, Japan (ISAS) in August, 1991. As a successor of HINOTORI, this mission is dedicated principally to the study of solar flares, especially of high-energy phenomena observed in the X- and gamma-ray ranges. The SOLAR-A will be the unique space solar observatory during the current activity maximum period (1989–1992). With a coordinated set of instruments including hard X-ray and soft X-ray imaging telescopes as well as spectrometers with advanced capabilities, it will reveal many new aspects of flares and help better understand their physics, supporting international collaborations with ground-based observatories as well as theoretical investigations. An overview of this mission, including the satellite, its scientific instruments, and its operation, is given in this paper. Also the scientific objectives are briefly discussed.

Coronal Transverse Magnetohydrodynamic Waves in a Solar Prominence

Solar prominences are cool 104 kelvin plasma clouds supported in the surrounding 106 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.

Continuous plasma outflows from the edge of a solar active region as a possible source of solar wind

The Sun continuously expels a huge amount of ionized material into interplanetary space as the solar wind. Despite its influence on the heliospheric environment, the origin of the solar wind has yet to be well identified. In this paper, we report Hinode X-ray Telescope observations of a solar active region. At the edge of the active region, located adjacent to a coronal hole, a pattern of continuous outflow of soft-x-ray–emitting plasmas was identified emanating along apparently open magnetic field lines and into the upper corona. Estimates of temperature and density for the outflowing plasmas suggest a mass loss rate that amounts to ∼1/4 of the total mass loss rate of the solar wind. These outflows may be indicative of one of the solar wind sources at the Sun.

The Magnetic Landscape of the Sun's Polar Region

We present observations of the magnetic landscape of the polar region of the Sun that are unprecedented in terms of spatial resolution, field of view, and polarimetric precision. They were carried out with the Solar Optical Telescope aboard Hinode. Using a Milne-Eddington inversion, we find many vertically oriented magnetic flux tubes with field strengths as strong as 1 kG scattered in latitude between 70° and 90°. They all have the same polarity, consistent with the global polarity of the polar region. The field vectors are observed to diverge from the centers of the flux elements, consistent with a view of magnetic fields that are expanding and fanning out with height. The polar region is also found to have ubiquitous horizontal fields. The polar regions are the source of the fast solar wind, which is channeled along unipolar coronal magnetic fields whose photospheric source is evidently rooted in the strong-field, vertical patches of flux. We conjecture that vertical flux tubes with large expansion around the photospheric-coronal boundary serve as efficient chimneys for Alfven waves that accelerate the solar wind.

Fermi Acceleration at the Fast Shock in a Solar Flare and the Impulsive Loop-Top Hard X-Ray Source

Because of its high injection energy, Fermi acceleration has not been considered to be viable to explain nonthermal electrons (20-100 keV) produced in solar flares. Here we propose that nonthermal electrons are efficiently accelerated by the first-order Fermi process at the fast shock, as a natural consequence of the new magnetohydrodynamic picture of the flaring region revealed with Yohkoh. An oblique fast shock is naturally formed below the reconnection site and boosts the acceleration to significantly decrease the injection energy. The slow shocks attached to the reconnection X-point heat the plasma up to 10-20 MK, exceeding the injection energy. The combination of the oblique shock configuration and the preheating by the slow shock allows bulk electron acceleration from the thermal pool. The accelerated electrons are trapped between the two slow shocks due to the magnetic mirror downstream of the fast shock, thus explaining the impulsive loop-top hard X-ray source discovered with Yohkoh. The acceleration timescale is ~0.3-0.6 s, which is consistent with the timescale of impulsive bursts. When these electrons stream away from the region enclosed by the fast shock and the slow shocks, they are released toward the footpoints and may form the simultaneous double-source hard X-ray structure at the footpoints of the reconnected field lines.

Hot and Superhot Plasmas above an Impulsive Flare Loop

We report the discovery of a high-temperature source above the soft X-ray loop of the impulsive limb flare on 1992 January 13 with the Yohkoh Soft X-ray Telescope (SXT). The hot source coincides in position with the loop-top impulsive hard X-ray source and continues to sit above the soft X-ray flare loop throughout the flare. The single high-temperature source in the initial phase evolves to the two high-temperature (15-20 MK) ridge structures in the peak and decay phases, and the compact hard X-ray source appears to be located in between the high-temperature ridges. We assume that the loop-top hard X-ray source is of thermal origin (superhot source). The loop-top hard X-ray source has an effective temperature of 100-150 MK and a total emission measure of ~10-4 of the hot source. The geometrical relationship between the superhot and the hot sources indicates that (1) the hot source is heated by the slow shocks associated with magnetic reconnection and that (2) the superhot source is heated with the fast bow shock owing to the collision of the supersonic downward outflow with the reconnected flux tube. The small emission measure of the superhot source indicates a narrow outflow jet in between the slow shocks. These observations suggest that there is no essential difference between the compact-loop (impulsive) and long-duration event (LDE) flares. The time coincidence of the loop-top hard X-ray source with the footpoint sources indicates that nonthermal electron acceleration is causally related to the formation of the fast shock rather than the slow shocks.