Review of the Magnetohydrodynamic Waves and Their Stability in Solar Spicules and X-Ray Jets

One of the most enduring mysteries in solar physics is why the Sun’s outer atmosphere, or corona, is millions of kelvins hotter than its surface. Among suggested theories for coronal heating is one that considers the role of spicules – narrow jets of plasma shooting up from just above the Sun’s surface – in that process (Athay & Holzer, 1982; Athay, 2000). For decades, it was thought that spicules might be sending heat into the corona. However, following observational research in the 1980s, it was found that spicule plasma did not reach coronal temperatures, and so this line of study largely fell out of vogue. Kukhianidze et al. (Kukhianidze et al., 2006) were first to report the observation of kink waves in solar spicules – the wavelength was found to be ∼3500 km, and the period of waves has been estimated to be in the range of 35–70 s. The authors argue that these waves may carry photospheric energy into the corona and therefore can be of importance in coronal heating. Zaqarashvili et al. (Zaqarashvili et al., 2007) analyzed consecutive height series of Hα spectra in solar limb spicules at the heights of 3800–8700 km above the photosphere and detected Doppler-shift oscillations with periods of 20–25 and 75–110 s. According to authors, the oscillations can be caused by waves’ propagation in thin magnetic flux tubes anchored in the photosphere. Moreover, observed waves can be used as a tool for spicule seismology, and the magnetic filed induction in spicules at the height of ∼6000 km above the photosphere is estimated as 12–15 G. De Pontieu et al. (De Pontieu et al., 2007) identified a new class of spicules (see Fig. 1) that moved much faster and were shorter lived than the traditional spicules, which have speeds of between 20 and 40 kms−1 and lifespans of 3 to 7 minutes. These Type II spicules, observed in Ca II 854.2 nm and Hα lines (Sterling et al., 2010), are much more dynamic: they form rapidly (in ∼10 s), are very thin ( 200 km wide), have lifetimes of 10 to 150 s (at any one height), and shoot upwards at high speeds, often in excess of 100–150 kms−1, before disappearing. The rapid disappearance of these jets had suggested that the plasma they carried might get very hot, but direct observational evidence of this process was missing. Both types of spicules are observed to carry Alfvén waves with significant amplitudes of order 20 kms−1. In a recent paper, De Pontieu et al. (De Pontieu et al., 2011) used new observations from the Atmospheric Imaging Assembly on NASA’s recently launched Solar Dynamics Observatory and its Focal Plane Package for the Solar Optical Telescope (SOT) on the Japanese Hinode satellite. Their observations reveal “a ubiquitous coronal mass supply in which chromospheric plasma in fountainlike jets or spicules (see Fig. 2) 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 6