Surprisingly Large Doppler Shifts in Hinode EUV Imaging Spectrometer (EIS) Solar Spectra, Resulting from an Inconspicuous Small-scale Jet in EUV Images

TL;DR

This study investigates unexpectedly large EUV Doppler blueshifts detected by Hinode/EIS in regions without flaring activity. By combining EIS spectroscopy with SDO/AIA imaging and HMI magnetograms, the authors identify an inconspicuous coronal jet at the edge of AR 12824 as the likely source, quantified by a plane-of-sky speed of $159\pm 29$ km s$^{-1}$ and a line-of-sight Doppler velocity around $200$ km s$^{-1}$. They also document a brighter, earlier jet at roughly the same location with comparable speeds, supporting a minifilament eruption scenario triggered by flux cancelation. The findings imply that EUV Doppler maps can reveal small-scale eruptions invisible in EUV images and that such jets may contribute to solar wind acceleration; future missions with broader FOV and higher cadence spectroscopy promise to uncover many more of these events.

Abstract

Strong EUV lineshifts in solar spectra are generally indicative of highly dynamic and explosive events that are easily detected in comparable-wavelength EUV images, with the strongest such line shifts (several 100 km/s) occurring in solar flares. Here we present observations of exceptionally strong lineshifts detected in Hinode/EUV Imaging Spectrometer (EIS) spectra outside the time of a flare-like brightening, with 195 Ang blueshifts of ~200 km/s. Although the likely culprit is too weak to register in GOES Soft X-ray fluxes, EIS pinpoints the source at the edge of an active region. Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) images and Helioseismic and Magnetic Imager (HMI) magnetograms show a nondescript small-scale eruptive event at this location. We find this event likely to be an inconspicuous coronal jet, apparently triggered by converging/canceling magnetic flux patches, with plane-of-sky velocity ~159+-29 km/s. AIA and HMI observations of this faint transient feature, together with observations of a slightly brighter jetting event near the same location an hour earlier, suggest that the strong EIS Doppler shifts are indeed due to a coronal jet that is hard to detect in AIA images. These observations, together with other recent studies, show that EUV Doppler maps are a much more sensitive tool for detecting small-scale eruptions than are EUV images, and those eruptions are frequently triggered by magnetic flux cancelation episodes. Such-detected small-scale eruptions, that often produce small-scale coronal-jet-like features, might propagate into and help drive the solar wind.

Figures (5)

• Figure 1: (a) Hinode/EIS raster image showing the observed active region, NOAA AR 12824. The raster started on the west edge of the field of view (FOV) at 20:32:10 UT on 2021 May 21, and finished at the east edge at 21:32:55 UT, and the arrow points to the source of the strong Doppler-blueshifted spectral line shown in (b). Blue lines show the FOV of panels (c)--(e). (b) The EIS 195.12 Å spectrum from 20:39:51 UT on the same day (histogram) with a two-Gaussian fit. The peak on the left is strongly blueshifted compared to the peak on the right near the rest wavelength. Panels (c---f) show images from SDO/AIA and a magnetogram from HMI of the region of the EIS strong-Doppler-shifted non-flare event, at the times given in the tops of the respective panels: (c) AIA 1600 Å, (d) HMI line-of-sight magnetogram, (e) AIA 304 Å, and (f) AIA 193 Å. The blue boxes show the FOV of Fig. \ref{['big_blue_zoom2_zu']}. To within a minute, all four images in (d---f) are at the same time, and at the same time as the spectrum in (b). Solar images in all figures have north upwards and west to the right. AIA and HMI images in all figures are displayed rotated to 20:40 UT on 2021 May 21.
• Figure 2: SDO/AIA images of the zoomed-in region of the EIS strong-Doppler-shifted non-flare event, of the blue-boxed FOV of Figs. \ref{['bb_zu1']}(c)---(f), at wavelengths: (a) 304 Å, (b) 171 Å, (c) 193 Å, and (d) 94 Å. To within a minute, these panels are at the same time as those in Figs. \ref{['bb_zu1']}(c)---(f). In (b), the green rectangle shows the $y$-range covered in Fig. \ref{['ronbun_zu3']}(d) (the time-distance map in Fig. \ref{['ronbun_zu3']}(d) is produced by integrating over the green box in the $x$ dirction). In (d), the red box shows the FOV over which the AIA 94 Å lightcurve is calculated and plotted in Fig. \ref{['ronbun_zu3']}(b). An animation accompanies the figure, running for 10 s.
• Figure 3: (a) GOES soft X-ray fluxes with time in the 1---8 Å (red) and the 0.5---4 Å (green) channels. (b) Variation of AIA 94 Å intensity with time integrated over the red box of Fig. \ref{['big_blue_zoom2_zu']}(d), through the time of the AIA images in Fig. \ref{['big_blue_zoom2_zu']}. (c) Variation of positive magnetic flux with time in the positive-polarity flux patch in the yellow box of Fig. \ref{['bb_hmi_zu']}(g). The red dashed line in (b) and (c) shows the time of the strong EIS Doppler shift. (d) A time-distance plot from AIA 171 Å images, from the green rectangle in Fig. \ref{['big_blue_zoom2_zu']}(b), where the ordinate of that rectangle is the y-axis here, where we integrated over the width of that green rectangle to increase the signal in this time-distance plot. The resulting southward-directed flows in the Fig. \ref{['big_blue_zoom2_zu']}(b) green rectangle show up as bright tracks moving southward in time that are pointed out by the arrows; the red arrow is at the time of the strong Doppler feature of Fig. \ref{['bb_zu1']}, and the green arrows show other south-flowing events. Each of the arrowed south-flowing events can be identified in the animation accompanying Fig. \ref{['big_blue_zoom2_zu']}, near the times indicated by the arrows. (e--g) A sequence of images from the Fig. \ref{['big_blue_zoom2_zu']}(b) AIA 171 Å animation, showing a faint south-directed flow corresponding to the feature pointed to by the red arrow in (d). The green arrows track a location near the front of the moving feature, which is used in estimating the flow speed given in the text.
• Figure 4: HMI magnetograms of the strong-Dopper-shift region. Panels (a)---(c) show the magnetic evolution of the region with the same FOV as in Fig. \ref{['big_blue_zoom2_zu']}, at three different times: 20:17, 20:41, and 20:58 UT on 2021 May 21, with white and black denoting positive and negative polarities, respectively. Panels (d)---(f) show AIA 193 Å images at the same times, with contours of the magnetograms overlaid, where the image times are at the top of the panels without parentheses and the magnetogram times are at the tops of the panels in parentheses. Red and green contours denote positive and negative polarities, respectively, and the contour levels are at plus and minus 30, 100, 300, and 1000 G. Panels (g)---(i) show the magnetograms of (a)---(c), with the FOV of the blue boxes of panels (a)---(f). Panel (g) shows a positive-polarity magnetic-flux patch (red arrow) that progressively becomes canceled with a negative-polarity magnetic-flux patch (green arrow) with time over the course of the displayed time period. It is plausible that this flux cancelation triggers the dynamics resulting in the strong Doppler shifts of Fig. \ref{['bb_zu1']}. An animation accompanies the figure, showing the FOV of panels (a)---(c), covering the time span of 19---21 UT on 2021 May 21, and running for 6 s.
• Figure 5: AIA 193 Å images of the same location on the Sun as for the 20:40 UT event in previous figures, but showing an event near 19:35 UT that was stronger in AIA images than that 20:40 UT event. In (a), HMI magnetogram contours as those in Fig. \ref{['bb_hmi_zu']} are overlayed onto the 193 Å image. This 19:35 UT event much more obviously appears as a coronal jet, and emanates from about the same location as the 20:40 UT event. This jet extending outward toward the south, in the same direction as the weaker extensions from the 20:40 UT event shown in the green box of Fig. \ref{['big_blue_zoom2_zu']}. This 19:35 UT coronal jet-like feature originates near a mixed-polarity location, near the neutral lines indicated by the dashed line segments in (a) and/or from the location where a weak negative polarity element is growing at the location of the arrow in (a). An intensity clump of the jet tracked by the arrows in (b) and (c) extends outward with a velocity of $\sim$200 km s$^{-1}$. This 19:35 UT event may be a stronger version of the 20:40 UT event, where in that 20:40 UT event EIS was able to discern the strong blueshifts of Fig. \ref{['bb_zu1']}(b) even though any outflowing jet-like material from that 20:40 UT event was near the detection capabilities of AIA in the images and accompanying video of Fig. \ref{['big_blue_zoom2_zu']}. An animation accompanies this figure, running for 2 s.