Abstract
Development of Kelvin–Helmholtz (KH) instability in solar coronal jets can trigger the wave turbulence considered as one of the main mechanisms of coronal heating. In this review, we have investigated the propagation of normal MHD modes running on three X-ray jets modeling them as untwisted and slightly twisted moving cylindrical flux tubes. The basic physical parameters of the jets are temperatures in the range of 5.2–8.2 MK, particle number densities of the order of109 cm−3, and speeds of 385, 437, and 532 km s−1, respectively. For small density contrast between the environment and a given jet, as well as at ambient coronal temperature of 2.0 MK and magnetic field around 7 G, we have obtained that the kink (m=1) mode propagating on moving untwisted flux tubes can become unstable in the first and second jets at flow speeds of≅348 and 429 km s−1, respectively. The KH instability onset in the third jet requires a speed of≅826 km s−1, higher than the observed one. The same mode, propagating in weakly twisted flux tubes, becomes unstable at flow speeds of≅361 km s−1for the first and of 443 km s−1for the second jet. Except the kink mode, the twisted moving flux tube supports the propagation of higher (m>1) MHD modes that can become unstable at accessible jets’ speeds.
Highlights
Jets are considered to be ubiquitous confined plasma ejecta in the solar atmosphere. They have been extensively observed in the solar atmosphere in various wavebands, such as Hα [1, 2], Ca ii H [3,4,5], EUV [6], and soft X-ray [7] in order to understand their multitemperature characteristics
X-ray jets were discovered by the Soft X-Ray Telescope (SXT) on board Yohkoh [8], as transient X-ray energy release and enhancement with apparent collimated ballistic motions of the plasma associated with the flares in X-ray bright points, emerging flux regions, or active regions
We study the propagation of kink and higher MHD modes in standard active region soft X-ray jets, notably jets #8, #11, and #16 of Shimojo and Shibata’s set of sixteen observed flares and jets [10], and have shown that with one order higher electron densities, ∼109 cm−3, and moderate magnetic field, ∼7 G, MHD modes in high-speed jets, like these ones, can become unstable against the KH instability at accessible jets speeds, except for jet #16 which requires a higher flow velocity
Summary
Jets are considered to be ubiquitous confined plasma ejecta in the solar atmosphere. They have been extensively observed in the solar atmosphere in various wavebands, such as Hα [1, 2], Ca ii H [3,4,5], EUV [6], and soft X-ray [7] in order to understand their multitemperature characteristics. The lowest critical jet speed of 4.025VA = 3220 km s−1 was derived by numerically solving the wave dispersion relation without any approximations, that is, treating both media as compressible plasmas Even the latter critical jet speed is still too high for the KH instability to be detected/observed in coronal hole soft X-ray jets. We study the propagation of kink and higher MHD modes in standard active region soft X-ray jets, notably jets #8, #11, and #16 of Shimojo and Shibata’s set of sixteen observed flares and jets [10], and have shown that with one order higher electron densities, ∼109 cm−3, and moderate magnetic field, ∼7 G, MHD modes in high-speed jets, like these ones, can become unstable against the KH instability at accessible jets speeds, except for jet #16 which requires a higher flow velocity.
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