General Background
The Sun's corona is millions of degrees hotter than the photosphere. For years, this heating problem has been addressed by a variety of ideas/methods, e.g. magnetic reconnection, nano-flaring, Alfven waves, etc. Despite these efforts it remains unclear how and where coronal heating occurs and how the corona is filled with hot plasma. In all likelihood, more than one mechanism is responsible.
Spicules were first reported in 1877. However, despite knowing of their existence for over 130 years, rather little of their overall contribution is known. Several years ago, chromospheric spicules were considered a candidate for coupling between the chromosphere and corona by providing mass to the corona and/or solar wind (Beckers 1968, Sol Phys 3, 367). These jet-likefeatures, with lifetimes of the order of 3-10 minutes, expel cool matter upwards to the corona with velocities of 20-30 km/s. With the launch of SoHO, spicules can now be observed at temperatures from 5,000 to 500,000 K (Xia
et al 2005, A&A 438, 1115). A paper published more than 3 decades ago by Pneuman & Kopp (1978, Sol Phys 57, 49), estimated that spicules carry upward a mass flux 100 times larger than that of the solar wind. However, no signatures of these ejecta at coronal temperatures have been reported. Consequently, a direct role of spicules in the coronal mass/energy balance has been dismissed as unlikely (Withbroe 1983, ApJ 267, 825).
With the launch of the Hinode satellite, spicules are once again being investigated using Ca II H images from the Solar Optical Telescope (SOT). De Pontieu et al. (2007, PASJ 59, 655) reported a second type of spicule that these authors considered as a candidate for establishing a link between the corona and chromosphere. These ``type II'' spicules have much shorter lifetimes (10-100 s), have very fast rise-times (upflows of order 50-150 km/s), and often fade rapidly from the chromospheric Ca II H 3968 A SOT passband. This may suggest rapid heating. Magnetic reconnection has been suggested as the driving mechanism of these phenomena.
Recently, De Pontieu et al. (2009, ApJ 701, L1) claimed to detect the coronal component of the type-II spicule in the form of a weak component (2-5 % of the peak line emission) in the blue wings of three emission lines observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer onboard SoHO and the Extreme-ultraviolet Imaging Spectrometer (EIS) onboard Hinode, formed in the transition region and corona (SUMER C IV 1548 A at 150,000K, Ne VIII 770 A at 600,000K, and EIS Fe XIV 274 A at 2MK). Note that this work does not present any direct evidence that these blue-shifts (if any, see below our discussion) correspond to features seen in the SOT/Ca II H or any other similar observations where spicules are usually detected.
One of the main uncertainties of the above mentioned study is not the lack of direct correspondence but line blends of all three lines used. The SUMER C IV 1548 A line is blended at the 3-5% level by several weak chromospheric lines in the blue wing. These can be seen in high spectral resolution data (higher than any solar data ever taken) of alpha Cen, a G V star with similar spectral signatures as the Sun (Pagano et al. 2004, A&A 415, 331) taken with the Space Telescope Imaging Spectrograph (STIS). Also, these lines can be seen in solar Skylab and HRTS spectra presented by Sandlin et al. (1986, ApJS 61, 801). The SUMER Ne VIII line is observed in second order and also has the same problem. Note that SUMER sensitivity drops very sharp at these wavelengths which brings the next problem, i.e. the 2-5% of peak radiance flux constraint, which is highly demanding and requires very high S/N data. The Hinode Fe XIV 274 line is blended in the blue wing by a Si VII line. Despite these problems, we should not dismiss the ideas by De Pointieu and co-authors, instead we suggest further investigations, e.g. do the type-II spicules ``merge'' (as suggested by the above authors) to form blobs which can be observed by the TRACE satellite and X-ray Telescope onboard Hinode, i.e at coronal temperatures? Can we detect them in un-blended coronal/upper transition region lines? What is the correlation between the ``classical'' H alpha spicule and the ``SOT'' Ca II H spicule, as one of the problems with the SOT Ca II H filter is its width, which therefore has both a photospheric and chromospheric contribution.
We would like to address these issues via a coordinated ground-based/space-based program involving ROSA and IBIS (observing in the Ca II infrared triplet) on the Dunn Solar Telescope (DST), plus SOT (Ca II H filter), XRT (combination of filters) plus EIS (observing both upper transition region and coronal lines). With simultaneous data taken in the Ca II core with IBIS, plus narrowband H alpha data from ROSA (also on the DST), we will be able to investigate the structure of Ca II versus H alpha spicules. With good seeing, cadences of at most a few seconds will enable us to obtain many images over the 50--100s lifetime of these high velocity events. Also, having simultaneous data of upper transition region/coronal lines will allow us to investigate whether there is a signature of activity in the blue wings of these lines, or whether the data is being pushed beyond its limits.
SOT Ca II H images show the formation of a spicule following small-scale Ca II brightenings. Perhaps these type-II spicules are eruptions, similar to coronal mass ejection's (CME), but occurring on a much smaller scale (mini CMEs) and more frequently. Mini-CMEs have being reported in EUVI/STEREO data (Innes et al. 2009, A&A 495, 319). With Hinode instruments (and hopefully SDO instruments if the January 2010 launch date holds), we have much improved spatial and temporal resolution which would enable an investigation of this possibility. |
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