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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Comparison of Enhanced Absorption in He I 10830 in Observations and Modeling During the Early Phase of a Solar Flare  

Nengyi Huang   Submitted: 2020-07-30 11:47

The He I 10830 triplet is a very informative indicator of chromospheric activities as the helium is the second most abundant element in the solar atmosphere. Taking advantage of the high resolution of the 1.6 m Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO), previous observations have shown clear evidence of the enhanced absorption, instead of typically-observed emission, for two M-class flares. In this study, we analyze the evolution of the He I 10830 emission in numerical models and compare it with observations. The models represent the RADYN simulation results obtained from the F-CHROMA database. We consider the models with the injected electron spectra parameters close to observational estimates for the 2013-August-17 flare event (δ =8, Ec=15,20 keV, F = 1e11, 3e11 erg cm-2) in detail, as well as other available models. The modeling results agree well with observations, in the sense of both the maximum intensity decrease (-17.1%, compared to the observed value of -13.7%) and the trend of temporal variation (initial absorption phase followed by the emission). All models demonstrate the increased number densities and decreased ratio of the upper and lower level populations of He I 10830 transition in the initial phase, which enhances the opacity and forms an absorption feature. Models suggest that the temperatures and free-electron densities at heights of 1.3-1.5 Mm should be larger than ~10,000 K and 6e11 cm-3 thresholds for the line to start being in emission.

Authors: Nengyi Huang, Viacheslav M. Sadykov, Yan Xu, Ju Jing, Haimin Wang
Projects: None

Publication Status: ApJ Letter (published)
Last Modified: 2020-08-03 19:21
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Relationship between Intensity of White-Light Flares and Proton Flux of Solar Energetic Particles  

Nengyi Huang   Submitted: 2018-01-12 13:54

Solar energetic particles (SEPs), including protons and heavy ions, are believed to be accelerated either by CME shock or by magnetic reconnection. The latter also produces solar flares, in which the white light flares (WLFs) are among the most energetic ones. Magnetic reconnections occur in the corona and the accelerated particles propagate both downward and upward along the magnetic loops. The former is the source of flare emission and the latter is thought to be SEPs. Therefore, a comparison between flare emission and SEP events provides valuable constraint in determining the acceleration site of SEPs. We collect 43 SEP events, observed from 2010 to 2017, and investigate their correlation with WL emission and SXR flus, observed by SDO/HMI and GOES, respectively. Our preliminary results show: 1) Among 47 SEP events, 39 of which do not have detectable flare emissions in white light and SXR. 2) Most strong WLFs are not associated with SEPs. 3) No clear correlation is found between the proton flux and the equivalent area, a quantity that measures the magnitude of WLF emission. A straightforward speculation is that the acceleration process could be different for SEPs and the energetic electrons powering WLFs in the events analyzed.

Authors: Nengyi Huang, Yan Xu, Haimin Wang
Projects: None

Publication Status: accepted
Last Modified: 2018-01-14 00:00
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The Energetics of White-light Flares Observed by SDO/HMI and RHESSI  

Nengyi Huang   Submitted: 2016-08-24 09:28

White-light (WL) flares have been observed and studied more than a century since the first discovery. However, some fundamental physics behind the brilliant emission remains highly controversial. One of the important facts in addressing the flare energetics is the spatial-temporal correlation between the white-light emission and the hard X-ray radiation, presumably suggesting that the energetic electrons are the energy sources. In this study, we present a statistical analysis of 25 strong flares (greater than and equal to M5) observed simultaneously by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Among these events, WL emission was detected by SDO/HMI in 13 flares, associated with HXR emission. To quantitatively describe the strength of WL emission, equivalent area (EA) is defined as the integrated contrast enhancement over the entire flaring area. Our results show that the equivalent area is inversely proportional to the HXR power index, indicating that stronger WL emission tends to be associated with larger population of high energy electrons. However, no obvious correlation is found between WL emission and flux of non-thermal electrons at 50 keV. For the other group of 13 flares without detectable WL emission, the HXR spectra are softer (larger power index) than those flares with WL emission, especially for the X-class flares in this group.

Authors: Huang, N., Xu, Y., Wang, H.
Projects: RHESSI,SDO-HMI

Publication Status: accepted
Last Modified: 2016-08-24 12:01
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Abstracts by Author
Comparison of Enhanced Absorption in He I 10830 in Observations and Modeling During the Early Phase of a Solar Flare
Relationship between Intensity of White-Light Flares and Proton Flux of Solar Energetic Particles
The Energetics of White-light Flares Observed by SDO/HMI and RHESSI

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