Rapid variations of Si IV spectra in a flare observed by IRIS at a sub-second cadence

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	The emission lines formed in the		The emission lines formed in the
	[https://en.wikipedia.org/wiki/Solar_transition_region solar transition region]		[https://en.wikipedia.org/wiki/Solar_transition_region solar transition region]
-	(TR))	+	(TR) prove to be very informative as probes for analyzing how the energy is
-	prove to be very informative as probes for analyzing how the energy is	+
	released during solar flares.		released during solar flares.
	Observationally we define the TR in terms of electron temperature		Observationally we define the TR in terms of electron temperature
Line 22:		Line 21:
	An example of a line often employed to analyze plasmas of		An example of a line often employed to analyze plasmas of
	[https://est-east.eu/?option=com_content&view=article&id=836&Itemid=622&lang=en flare ribbons]		[https://est-east.eu/?option=com_content&view=article&id=836&Itemid=622&lang=en flare ribbons]
-	is the Si IV 1402.77 Â line (nominally formed at	+	is the Si IV 1402.77 Å line (nominally formed at
	T<sub>e</sub> ~ 70,000 K).		T<sub>e</sub> ~ 70,000 K).
	This is one of the strongest lines routinely observed by the		This is one of the strongest lines routinely observed by the
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	[https://en.wikipedia.org/wiki/Doppler_effect redshifted]		[https://en.wikipedia.org/wiki/Doppler_effect redshifted]
	in solar flares, and also broadened as a consequence of		in solar flares, and also broadened as a consequence of
-	[https://www.google.com/search?client=safari&rls=en&q=plasma+ionizaation+state&ie=UTF-8&oe=UTF-8 magnetohydrodynamic (MHD) turbulence].	+	[https://en.wikipedia.org/wiki/Magnetohydrodynamic_turbulence magnetohydrodynamic (MHD) turbulence].
	Some observational studies of Si IV line-broadening		Some observational studies of Si IV line-broadening
	have reported high-frequency oscillations with periods of		have reported high-frequency oscillations with periods of
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	In this Nugget we describe rapid variability of the		In this Nugget we describe rapid variability of the
-	Si IV 1402.77 &Acirc; line in observations of the first major	+	Si IV 1402.77 &Aring; line in observations of the first major
	solar flare captured in a newly-designed IRIS observing mode		solar flare captured in a newly-designed IRIS observing mode
	enabling high time resolution (Ref. [1]).		enabling high time resolution (Ref. [1]).
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	The IRIS spectroscopy slit captured a bright kernel (panel (B))		The IRIS spectroscopy slit captured a bright kernel (panel (B))
	in the southern ribbon corresponding to flare loop footpoints		in the southern ribbon corresponding to flare loop footpoints
-	observed in SDO/AIA 131 &Acirc; (panel (C)).	+	observed in SDO/AIA 131 &Aring;; (panel (C)).
-	The Si IV 1402.77 &Acirc; line spectra observed in this	+	The Si IV 1402.77 &Aring; line spectra observed in this
	kernel were double-peaked, consisting of the primary component		kernel were double-peaked, consisting of the primary component
	consistently located close to the line's rest wavelength and a		consistently located close to the line's rest wavelength and a
	well-resolved redshifted secondary component.		well-resolved redshifted secondary component.
	In addition to the		In addition to the
-	[https://en.wikipedia.org/wiki/Method_of_moments_(statistics) moments analysis].	+	[https://en.wikipedia.org/wiki/Method_of_moments_(statistics) moments analysis]
-	typically used for this purpose, the properties of the Si	+	typically used for this purpose, the properties of each of the Si
	IV profiles (intensity, Doppler velocity, non-thermal broadening)		IV profiles (intensity, Doppler velocity, non-thermal broadening)
-	were therefore also be determined by fitting two independent	+	were therefore also determined by fitting two independent
	[https://mathworld.wolfram.com/GaussianFunction.html Gaussian] profiles.		[https://mathworld.wolfram.com/GaussianFunction.html Gaussian] profiles.
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	[[File:440f1.png|700px|thumb|center|<b>Figure 1:</b>		[[File:440f1.png|700px|thumb|center|<b>Figure 1:</b>
	<i> First row: context observations of the 2022 January 18		<i> First row: context observations of the 2022 January 18
-	flare in AIA 304 &Acirc; (A), SJI 2796 &Acirc; (B), and AIA	+	flare in AIA 304 &Aring; (A), SJI 2796 &Aring; (B), and AIA
-	131 &Acirc; (C).	+	131 &Aring; (C).
	The white arrow points to a bright kernel in the southern		The white arrow points to a bright kernel in the southern
	ribbon whose spectra we analyze, and the white dashed line marks		ribbon whose spectra we analyze, and the white dashed line marks
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	The redshifts and the		The redshifts and the
	relatively-larger non-thermal broadening of the secondary component		relatively-larger non-thermal broadening of the secondary component
-	(c.f. panels (E) and (F)) were suggestive of an ongoing energization	+	(c.f. panels (E) and (F)) suggest ongoing energization by magnetic reconnection,
-	by models involving	+	reflecting he downflows existed due to the "chromospheric condensation" required by models of
-	[https://en.wikipedia.org/wiki/Magnetic_reconnection magnetic reconnection],	+	[https://en.wikipedia.org/wiki/Solar_flare solar flares].
-	indicating that the downflows existed due to the "chromospheric	+
-	condensation" required by such models.	+
	Upon estimating the speeds of		Upon estimating the speeds of
	field-aligned flows along flare loops emanating from the analyzed		field-aligned flows along flare loops emanating from the analyzed
-	kernel we found that the observed Doppler shifts indeed do correspond to	+	kernel we found that the observed Doppler shifts indeed do indeed correspond to
	strong condensation flows recently predicted in radiative-hydrodynamic		strong condensation flows recently predicted in radiative-hydrodynamic
	models of flares (Ref. [3]].		models of flares (Ref. [3]].
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	above flare loop arcade (blue). Panel (B) shows fit to the brightness		above flare loop arcade (blue). Panel (B) shows fit to the brightness
	temperature spectrum of EOVSA. The curves in panel (C) are time		temperature spectrum of EOVSA. The curves in panel (C) are time
-	derivatives of SXR flux in the 1 - 8 &Acirc; channel of GOES (blue) and	+	derivatives of SXR flux in the 1 - 8 &Aring; channel of GOES (blue) and
	EOVSA radio flux averaged in the frequency range between 2.4 and 5 GHz		EOVSA radio flux averaged in the frequency range between 2.4 and 5 GHz
	(orange). The blue arrows indicate quasi-periodic pulsations (QPPs)		(orange). The blue arrows indicate quasi-periodic pulsations (QPPs)
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	acquired at a sub-second cadence.		acquired at a sub-second cadence.
	We found that oscillations in the nonthermal broadening of the Si IV		We found that oscillations in the nonthermal broadening of the Si IV
-	1402.77 &\Acirc;line profiles resulting	+	1402.77 &Aring; line profiles resulting
	from the moments analysis were more likely to be signatures of		from the moments analysis were more likely to be signatures of
	reconnection-driven condensation downflows rather than MHD turbulence.		reconnection-driven condensation downflows rather than MHD turbulence.

---

Latest revision as of 09:49, 17 November 2022

Nugget
Number:	440
1st Author:	Juraj LÖRINČÍK
2nd Author:
Published:	November 14, 2022
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Contents 1 Introduction 2 Variability of Si IV line properties 3 Combining IRIS, radio, and SXR data 4 Conclusions

Introduction

Solar flares emit radiation in most wavelength ranges. The emission lines formed in the solar transition region (TR) prove to be very informative as probes for analyzing how the energy is released during solar flares. Observationally we define the TR in terms of electron temperature T_e in the range .01-1 MK (or by ionization state). An example of a line often employed to analyze plasmas of flare ribbons is the Si IV 1402.77 Å line (nominally formed at T_e ~ 70,000 K). This is one of the strongest lines routinely observed by the IRIS satellite. Its profiles are typically redshifted in solar flares, and also broadened as a consequence of magnetohydrodynamic (MHD) turbulence. Some observational studies of Si IV line-broadening have reported high-frequency oscillations with periods of 10 s, in some cases accompanied by oscillations of other spectral properties of this line. This rapid variability in the line broadening presents a difficulty in studying the manifestations of the turbulence, highlighting the need for flare spectral observations carried-out at a very high cadence.

In this Nugget we describe rapid variability of the Si IV 1402.77 Å line in observations of the first major solar flare captured in a newly-designed IRIS observing mode enabling high time resolution (Ref. [1]).

Variability of Si IV line properties

We analyzed observations of the M1.5-class flare SOL2022-01-18 (Figure 1A) observed by IRIS at an unprecedented 0.8 s cadence. The IRIS spectroscopy slit captured a bright kernel (panel (B)) in the southern ribbon corresponding to flare loop footpoints observed in SDO/AIA 131 Å; (panel (C)). The Si IV 1402.77 Å line spectra observed in this kernel were double-peaked, consisting of the primary component consistently located close to the line's rest wavelength and a well-resolved redshifted secondary component. In addition to the moments analysis typically used for this purpose, the properties of each of the Si IV profiles (intensity, Doppler velocity, non-thermal broadening) were therefore also determined by fitting two independent Gaussian profiles.

In agreement with previous studies (e.g., Ref. [2]), the time evolution of the quantities determined using moments exhibited high-frequency oscillations (Figure 1(D)). The oscillations of the non-thermal broadening with periods of roughly 7 s (green curve) that we primarily focused on in our work were found to be well-correlated with enhancements of the Doppler velocity of the secondary component of the line determined from the Gaussian fits (magenta curve in panel (F)). This means that the enhancements of the broadening of the entire profile determined via moment analysis were in fact dictated by varying redshifts of the secondary component, similar to what has been reported in spectral observations of active-region loops.

Our next task was to investigate what mechanism led to the formation of the secondary component of the line. The redshifts and the relatively-larger non-thermal broadening of the secondary component (c.f. panels (E) and (F)) suggest ongoing energization by magnetic reconnection, reflecting he downflows existed due to the "chromospheric condensation" required by models of solar flares. Upon estimating the speeds of field-aligned flows along flare loops emanating from the analyzed kernel we found that the observed Doppler shifts indeed do indeed correspond to strong condensation flows recently predicted in radiative-hydrodynamic models of flares (Ref. [3]].

Combining IRIS, radio, and SXR data

Evidence for an ongoing magnetic reconnection was found in observations of microwave flux from the EOVSA radio telescope. The dominant source of EOVSA emission was located above the arcade of flare loops (Figure 2(A)) and its brightness temperature spectrum was consistent with a non-thermal electron source (panel (B)). Moreover, the time derivative of the EOVSA radio flux (orange curve, panel (C)) showed several enhancements co-temporal with 1 - 3 minute quasi-periodic pulsations (QPPs) detected in the time derivative of the GOES SXR flux (blue arrows in the same panel). The lightcurve of the prominent QPP showed in panel (D) exhibited trends similar to the intensity and the Doppler velocity of the secondary component averaged in the ribbon, suggesting that some of these QPPs were likely driven by magnetic reconnection.

Conclusions

We have analyzed observations of the flare SOL2022-01-18 acquired at a sub-second cadence. We found that oscillations in the nonthermal broadening of the Si IV 1402.77 Å line profiles resulting from the moments analysis were more likely to be signatures of reconnection-driven condensation downflows rather than MHD turbulence. Magnetic reconnection leading to these condensation downflows was also a likely driver of QPPs observed in microwave radio and SXR spectra.

For further details see Ref. [1].

[1] "Rapid variations of Si IV spectra in a flare observed by interface region imaging spectrograph at a sub-second cadence"

[2] "The development of lower-atmosphere turbulence early in a solar flare"

[3] "The Atmospheric Response to High Nonthermal Electron-beam Fluxes in Solar Flares. II."