MULTI-WAVELENGTH SPECTROSCOPIC OBSERVATION OF EXTREME-ULTRAVIOLET JET IN AR 10960

The jet occurred in the active region NOAA 10960 (S03, E25) around 04:18 UT, 2007 June 5. Figure 1 shows the images observed with TRACE, XRT, and EIS. Figures 1(a)–(e) show EUV images observed with TRACE 171 Å. An EUV jet J1 is shown in Figure 1(b). After J1 was observed, a second EUV jet J2 was observed (Figure 1(d)). Figures 1(f), (h), (i), and (j) show the X-ray images observed with the XRT Ti_poly filter. Figure 1(f) shows bright loops L1' and L2' which appear before J1. The X-ray image of J1 is unavailable due to the data gap of the XRT observation. After J2, there are new loops (Figure 1(h)). After J1 occurred, L2 changes into L2' and L3 appears. Figure 1(i) shows J2 in X-ray at the same time of Figure 1. EIS observed J1 with a raster scan. Figure 1(g) shows the EIS intensity map in Fe xii 195 Å. In this paper, we analyze the multi-wavelength spectroscopic observation of J1.

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The three-dimensional structure can be obtained from the STEREO-A and STEREO-B images taken from the different viewing angles. The right (left) panel of Figure 2 shows J1 observed with STEREO-A (STEREO-B). Figure 3 is a polar coordinate system view of the jet. In this coordinate system, the x-axis is toward the observer and the z-axis is parallel to the direction of the solar north pole. The origin of this system is the footpoint of the jet. The direction of the x-axis depends on the position of the observer as in the case of STEREO-A and STEREO-B. The origin and z-axis do not change for any observers. The inclination and azimuth of the jet are θ and ϕ, respectively. The observed jet in Figure 3 is the projection on the yz-plane. The apparent inclination of the observed jet is ψ in this figure. This angle is expressed using θ and ϕ,

$$\begin{eqnarray} &&\tan \psi = \tan \theta \sin \phi. \end{eqnarray} \tag{ 1 }$$

From the position of STEREO-A and STEREO-B, the azimuths are ϕ_A = ϕ_E − α and ϕ_B = ϕ_E + β, respectively, where ϕ_E is the azimuth for observers at the Earth. α (β) is the angle between STEREO-A (STEREO-B) and the Earth around the sun. The inclination θ is independent of the position of the considering observers. The apparent inclinations observed with STEREO-A and STEREO-B are ψ_A = 45° and ψ_B = 49°, respectively, and are overplotted in Figure 2. ψ_A and ψ_B are described using Equation (1),

$$\begin{eqnarray} &&\frac{ \tan \psi _{\rm {A}}}{ \tan \psi _{\rm {B}}} = \frac{ \sin (\phi _{\rm {E}} - \alpha)}{ \sin (\phi _{\rm {E}} + \beta)}. \end{eqnarray} \tag{ 2 }$$

When STEREO-A and STEREO-B observed the jet, their locations are at α = 724 and β = 407. The azimuth and inclination are derived from Equation (2) and ϕ_E = 563 and θ = 529, respectively.

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Figure 4 is the intensity distribution along J1 observed with TRACE 171 Å at 04:16 UT in solid line, 04:17 UT in dashed line, and 04:18 UT in dash-dotted line. The distributions of the intensity in each exposure time are well described by an exponential function, that is consistent with Shimojo et al. (1996, 2001). The temporal offsets of the intensity profile have a speed of 155 km s⁻¹. We interpret this as a projection velocity of the plasma motion along J1 at the temperature of this filter range.

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EIS observed J1 with many emission lines. The observed emission lines and their formation temperatures are summarized in Table 1. Figure 5 shows the intensity images observed with (a) He ii 256 Å (log₁₀T_e[K] = 4.9), (b) Fe viii 185 Å (log₁₀T_e[K] = 5.6), (c) Fe xii 195 Å (log₁₀T_e[K] = 6.12), and (d) Fe xvi 262 Å (log₁₀T_e[K] = 6.45), respectively. The field of view (hereafter FOV) of Figure 5 is part of the EIS raster scan and corresponds to that of the TRACE and XRT images in Figure 1. The scan of Figure 5 started at 04:16 UT and ended at 04:21UT. Figures 5(a) and (b) are examples of the cool lines and Figures 5(c) and (d) are examples of the hot lines. The jet is clearly identified in the cool lines while it is not clear in the hot lines. The Doppler velocity images are shown in Figures 6(a)–(d). Doppler velocities are obtained by fitting the spectrum with a single Gaussian function and calculated from the displacement of the center of the fitting Gaussian function using the eis_auto_fit in SolarSoftWare. The blueshift of the jet is clearly shown in Figures 6(a)–(c) but is not clear in Figure 6(d) because of the weak intensity and large noise of Fe xvi 262 Å.

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We determine the jet region shown by the black lines in Figures 5 and 6 by a visual inspection. In this region, the intensity of He ii is bright. He ii, Fe viii and Fe xii show the strong blueshift in this region. Figure 7 shows the line profiles averaged in the jet region. All lines show strong blueshift even in Fe xvi. The blending effect of the neighboring Si x 256.37 Å (log₁₀T_e[K] = 6.2) with the He ii blueshifted component is weak because it can be estimated by using the ratio of the blueshifted component to the rest component of Fe xii. The effect of the hotter Fe xvii 262.8 Å line around the blueshift component of Fe xvi is negligible because the expected intensity from the CHIANTI atomic database (Landi et al. 2006; Dere et al. 1997) and Fe xvii 254.9 Å observed with EIS is much smaller than that of the blueshift component.

The spectrum of each line (summarized in Table 1) is averaged in the jet region and fitted with a double Gaussian function to derive the Doppler velocities of the blueshift component. The true velocity of the jet is derived from the Doppler velocity and projection velocity,

$$\begin{eqnarray} &&V_{\rm {jet}} = \frac{V_{\rm {Doppler}}}{\sin \theta \cos \phi _{\rm {E}}}\hspace{52pt} \end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray} &&= \frac{V_{\rm {projection}}}{\sqrt{1 - \sin ^2\theta \cos ^2\phi _{\rm {E}}}}, \end{eqnarray} \tag{ 4 }$$

where V_jet, V_Doppler, and V_projection are the jet velocity, Doppler velocity, and projection velocity, respectively. The azimuth ϕ_E and inclination θ are derived by Equation (2). The relationship of the temperature and thus derived true jet velocity is shown in Figure 8. The temperature of the jet is defined by the line formation temperature (Table 1) and its error is estimated by the FWHM of the contribution function based on the CHIANTI atomic database. The error bar of the velocity indicates the error of the double Gaussian fitting. The velocity derived from the projection velocity of TRACE 171 Å is indicated by the triangle symbol. The temperature of this component is defined by the temperature response of TRACE 171 Å filter (Handy et al. 1999). The FWHM of the temperature response is shown as an error bar on the triangle symbol in Figure 8. The solid line in Figure 8 shows the sound speed and the dashed line shows a theoretical upper limit to the chromospheric evaporation velocities derived in Fisher et al. (1984).

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Figure 9(a) shows the magnetic field observed with SOHO/MDI. The red and blue contours represent the positive and negative line-of-sight magnetic fields, respectively, and the background shows the TRACE 171 Å intensity at the J1 occurrence time. It is found that J1 occurred near the large negative sunspot and positive polarities (P1 and P2). From the overlaid plot on the X-ray image in Figure 9(b), we find that the eastern (L2' in Figure 1(h)) and western (L3) loops connect P1 and P2 with the sunspot, respectively. Figures 9(c) and (d) show the Hα and 1600 Å intensities at the J2 occurrence time (see also Figures 1(e) and (j)). In both panels, we see two ribbons, each of which corresponds to each magnetic polarity. One of the ribbons is located at P1's position, suggesting that they are footpoints of loop L2'. We will discuss the geometrical magnetic structure driving the jets in Section 4.1 based on these observations.

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We consider the global magnetic field structure (Figure 10) consistent with the observed jets, loops (Figure 1), magnetic fields, and flare ribbons (Figure 9). The red and blue lines are the positive and negative magnetic fields on the photosphere observed with MDI, respectively, and the black lines represent the interpreted magnetic field lines. Before the onset of the jet (Figure 10(a)), an open magnetic field F1 is connected with the sunspot. There are also closed magnetic loops (L1', L2') between the positive magnetic field P1 and the sunspot. Another positive magnetic field P2 is also connected with the sunspot by a closed loop (L3). The first jet J1 is produced by the reconnection which occurred between the closed loop L1 and open field F1 in Figure 10(a). This scenario is a kind of a standard jet (Shibata et al. 1994). As a result of the reconnection between these two fields, field lines F1' and L1' are produced. F1' is greatly bent because of L3 in Figure 10(b). F1'and L3 reconnect again and L3 becomes bright in Figure 1(g) around y = 80''. L3 seems to be filled with the dense plasma caused by the chromospheric evaporation. L3 also shows redshift in Figure 6(c), which might be the shrinking of the reconnected field lines with plasma. Flare ribbons in Figures 9(c) and (d) around the footpoints of L1' and L2' are thought to be due to another reconnection between L1' and L2. The second jet J2 is considered to be related to another open magnetic field, F2, shown in Figure 10(b) as a dashed line, which is not seen in the observations. Another possibility for J2 is a blowout eruption of the sheared core field around L2 (Moore et al. 2010). In this interpretation, L2'' in Figure 10(b) is the erupted loop. This eruption is triggered by the reconnection that produced J1. In this case, the first standard jet creates the second blowout jet.

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EIS observed strong blueshifts in the multiple lines covering the wide temperature range as shown in Figure 8. The profiles in Figure 7 show the blueshifts of the observed lines. At the temperature (Fe xii and Fe xvi), the intensities of the blueshift components are weaker than the rest one. At the low temperature line (He ii and Fe viii), on the other hand, the intensities of the blueshift components are stronger than the rest one. This implies that the density of the cool plasma in the jet is larger than that of background cool plasma, which probably relates to the difference of the acceleration of the cool and hot plasma though the reason therefore is not revealed clearly in this study.

Finally, we discuss the difference of the acceleration of the cool and hot plasma. We consider the two types of acceleration, i.e., magnetic acceleration due to magnetic reconnection and thermal acceleration due to chromospheric evaporation. The thermal accelerations like the chromospheric evaporation (Milligan & Dennis 2009) and dimming flows (Imada et al. 2007, 2011) show a dependence of the velocity on the temperature. If the jet is accelerated by chromospheric evaporation, the velocity of the jet increases with increasing temperature along with the sound speed. Its theoretical upper limit is approximately 2.35 C_s (Fisher et al. 1984). The velocities of the hot plasma described in Figure 8 show a tendency to change along with the sound speed within this upper limit, which is consistent with the chromospheric evaporation. On the other hand, the velocity of He ii is much faster than this upper limit. This probably suggests that He ii is accelerated by magnetic acceleration. We interpret that the magnetic acceleration also occurs in the jet along with the chromospheric evaporation. In other words, hot plasma is accelerated by chromospheric evaporation while cool plasma is accelerated by magnetic force. This magnetic acceleration of the cool plasma is consistent with a blowout jet (Moore et al. 2010). In this jet, two types of accelerations, i.e., the magnetic and thermal accelerations, occur at the same time. This result is consistent with the magnetic reconnection model.