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Energetic Electron Distribution of the Coronal Acceleration Region: First Results from Joint Microwave and Hard X-Ray Imaging SpectroscopyChen, BinBattaglia, MarinaKrucker, SämReeves, Katharine K.Glesener, LindsayDOI: info:10.3847/2041-8213/abe471v. 908L55
Chen, Bin, Battaglia, Marina, Krucker, Säm, Reeves, Katharine K., and Glesener, Lindsay. 2021. "Energetic Electron Distribution of the Coronal Acceleration Region: First Results from Joint Microwave and Hard X-Ray Imaging Spectroscopy." The Astrophysical Journal 908:L55.
ID: 159627
Type: article
Authors: Chen, Bin; Battaglia, Marina; Krucker, Säm; Reeves, Katharine K.; Glesener, Lindsay
Abstract: Nonthermal sources located above bright flare arcades, referred to as the "above-the-loop-top" sources, have been often suggested as the primary electron acceleration site in major solar flares. The X8.2 limb flare on 2017 September 10 features such an above-the-loop-top source, which was observed in both microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array and the Reuven Ramaty High Energy Solar Spectroscopic Imager, respectively. By combining the microwave and HXR imaging spectroscopy observations with multifilter extreme ultraviolet and soft X-ray imaging data, we derive the coronal magnetic field and energetic electron distribution of the source over a broad energy range from 6.0. Temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from >20 to ∼6.0 within 20 s, or less than ∼10 Alfvén crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an ongoing bulk acceleration of energetic electrons is taking place very early in the flare energy release.
Measurement of magnetic field and relativistic electrons along a solar flare current sheetChen, BinShen, ChengcaiGary, Dale E.Reeves, Katharine K.Fleishman, Gregory D.Yu, SijieGuo, FanKrucker, SämLin, JunNita, Gelu M.Kong, XiangliangDOI: info:10.1038/s41550-020-1147-7v. 41140–1147
Chen, Bin, Shen, Chengcai, Gary, Dale E., Reeves, Katharine K., Fleishman, Gregory D., Yu, Sijie, Guo, Fan, Krucker, Säm, Lin, Jun, Nita, Gelu M., and Kong, Xiangliang. 2020. "Measurement of magnetic field and relativistic electrons along a solar flare current sheet." Nature Astronomy 4:1140– 1147.
ID: 158872
Type: article
Authors: Chen, Bin; Shen, Chengcai; Gary, Dale E.; Reeves, Katharine K.; Fleishman, Gregory D.; Yu, Sijie; Guo, Fan; Krucker, Säm; Lin, Jun; Nita, Gelu M.; Kong, Xiangliang
Abstract: In the standard model of solar flares, a large-scale reconnection current sheet is postulated to be the central engine for powering the flare energy release1-3 and accelerating particles4-6. However, where and how the energy release and particle acceleration occur remain unclear owing to the lack of measurements of the magnetic properties of the current sheet. Here we report the measurement of the spatially resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a `magnetic bottle'. This spatial structure agrees with theoretical predictions1,7 and numerical modelling results. A strong reconnection electric field of about 4,000 V m-1 is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons instead concentrate at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is probably the primary site for accelerating and confining the relativistic electrons.
Microwave Spectral Imaging of an Erupting Magnetic Flux Rope: Implications for the Standard Solar Flare Model in Three DimensionsChen, BinYu, SijieReeves, Katharine K.Gary, Dale E.DOI: info:10.3847/2041-8213/ab901av. 895L50
Chen, Bin, Yu, Sijie, Reeves, Katharine K., and Gary, Dale E. 2020. "Microwave Spectral Imaging of an Erupting Magnetic Flux Rope: Implications for the Standard Solar Flare Model in Three Dimensions." The Astrophysical Journal 895:L50.
ID: 156815
Type: article
Authors: Chen, Bin; Yu, Sijie; Reeves, Katharine K.; Gary, Dale E.
Abstract: We report microwave spectral imaging observations of an erupting magnetic flux rope during the early impulsive phase of the X8.2-class limb flare on 2017 September 10, obtained by the Expanded Owens Valley Solar Array. A few days prior to the eruption, when viewed against the disk, the flux rope appeared as a reverse S-shaped dark filament along the magnetic polarity inversion line. During the eruption, the rope exhibited a "hot channel" structure in extreme ultraviolet and soft X-ray passbands sensitive to ∼10 MK plasma. The central portion of the flux rope was nearly aligned with the line of sight, which quickly developed into a teardrop-shaped dark cavity during the early phase of the eruption. A long and thin plasma sheet formed below the cavity, interpreted as the reconnection current sheet viewed edge on. A nonthermal microwave source was present at the location of the central current sheet, which extended upward encompassing the dark cavity. A pair of nonthermal microwave sources were observed for several minutes on both sides of the main flaring region. They shared a similar temporal behavior and spectral property to the central microwave source below the cavity, interpreted as the conjugate footpoints of the erupting flux rope. These observations are broadly consistent with the magnetic topology and the associated energy release scenario suggested in the three-dimensional standard model for eruptive solar flares. In particular, our detection of nonthermal emission at conjugate flux rope footpoints provides solid evidence of particle transport along an erupting magnetic flux rope.
EUV imaging and spectroscopy for improved space weather forecastingGolub, LeonCheimets, PeterDeLuca, Edward E.Madsen, Chad A.Reeves, Katharine K.Samra, JennaSavage, SabrinaWinebarger, AmyBruccoleri, Alexander R.DOI: info:10.1051/swsc/2020040v. 10Article 37
Golub, Leon, Cheimets, Peter, DeLuca, Edward E., Madsen, Chad A., Reeves, Katharine K., Samra, Jenna, Savage, Sabrina, Winebarger, Amy, and Bruccoleri, Alexander R. 2020. "EUV imaging and spectroscopy for improved space weather forecasting." Journal of Space Weather and Space Climate 10:Article 37.
ID: 157469
Type: article
Authors: Golub, Leon; Cheimets, Peter; DeLuca, Edward E.; Madsen, Chad A.; Reeves, Katharine K.; Samra, Jenna; Savage, Sabrina; Winebarger, Amy; Bruccoleri, Alexander R.
Abstract: Accurate predictions of harmful space weather effects are mandatory for the protection of astronauts and other assets in space, whether in Earth or lunar orbit, in transit between solar system objects, or on the surface of other planetary bodies. Because the corona is multithermal (i.e., structured not only in space but also in temperature), wavelength-separated data provide crucial information that is not available to imaging methods that integrate over temperature. The extreme ultraviolet (EUV) wavelengths enable us to focus directly on high temperature coronal plasma associated with solar flares, coronal mass ejections (CMEs), and shocked material without being overwhelmed by intensity from the solar disk. Both wide-field imaging and spectroscopic observations of the solar corona taken from a variety of orbits (e.g., Earth, L1, or L5) using suitably-chosen EUV instrumentation offer the possibility of addressing two major goals to enhance our space weather prediction capability, namely: (1) Improve our understanding of the coronal conditions that control the opening and closing of the corona to the heliosphere and consequent solar wind streams, and (2) Improve our understanding of the physical processes that control the early evolution of CMEs and the formation of shocks, from the solar surface out into the extended corona.
Locating Hot Plasma in Small Flares using Spectroscopic Overlappogram Data from the Hinode EUV Imaging SpectrometerHarra, LouiseMatthews, SarahLong, DavidHasegawa, TakahiroLee, Kyoung-SunReeves, Katharine K.Shimizu, ToshifumiHara, HirohisaWoods, MagnusDOI: info:10.1007/s11207-020-01602-6v. 29534
Harra, Louise, Matthews, Sarah, Long, David, Hasegawa, Takahiro, Lee, Kyoung-Sun, Reeves, Katharine K., Shimizu, Toshifumi, Hara, Hirohisa, and Woods, Magnus. 2020. "Locating Hot Plasma in Small Flares using Spectroscopic Overlappogram Data from the Hinode EUV Imaging Spectrometer." Solar Physics 295:34.
ID: 157471
Type: article
Authors: Harra, Louise; Matthews, Sarah; Long, David; Hasegawa, Takahiro; Lee, Kyoung-Sun; Reeves, Katharine K.; Shimizu, Toshifumi; Hara, Hirohisa; Woods, Magnus
Abstract: One of the key processes associated with the "standard" flare model is chromospheric evaporation, a process during which plasma heated to high temperatures by energy deposition at the flare footpoints is driven upwards into the corona. Despite several decades of study, a number of open questions remain, including the relationship between plasma produced during this process and observations of earlier "superhot" plasma. The Extreme ultraviolet Imaging Spectrometer (EIS) onboard Hinode has a wide slot, which is often used as a flare trigger in the He II emission-line band. Once the intensity passes a threshold level, the study will switch to one focussed on the flaring region. However, when the intensity is not high enough to reach the flare trigger threshold, these datasets are then available during the entire flare period and provide high-cadence spectroscopic observations over a large field of view. We make use of data from two such studies of a C4.7 flare and a C1.6 flare to probe the relationship between hot Fe XXIV plasma and plasmas observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the X-ray Telescope (XRT) to track where the emission comes from and when it begins. The flare trigger slot data used in our analysis has one-minute cadence. Although the spatial and spectral information are merged in the wide-slot data, it is still possible to extract when the hot plasma appears, through the appearance of the Fe XXIV spectral image. It is also possible to derive spectrally pure Fe XXIV light curves from the EIS data, and compare them with those derived from hard X-rays, enabling a full exploration of the evolution of hot emission. The Fe XXIV emission peaks just after the peak in the hard X-ray lightcurve; consistent with an origin in the evaporation of heated plasma following the transfer of energy to the lower atmosphere. A peak was also found for the C4.7 flare in the RHESSI peak temperature, which occurred before the hard X-rays peaked. This suggests that the first peak in hot-plasma emission is likely to be directly related to the energy-release process.
Hot Plasma Flows and Oscillations in the Loop-top Region During the 2017 September 10 X8.2 Solar FlareReeves, Katharine K.Polito, VanessaChen, BinGalan, GiselleYu, SijieLiu, WeiLi, GangDOI: info:10.3847/1538-4357/abc4e0v. 905165
Reeves, Katharine K., Polito, Vanessa, Chen, Bin, Galan, Giselle, Yu, Sijie, Liu, Wei, and Li, Gang. 2020. "Hot Plasma Flows and Oscillations in the Loop-top Region During the 2017 September 10 X8.2 Solar Flare." The Astrophysical Journal 905:165.
ID: 158794
Type: article
Authors: Reeves, Katharine K.; Polito, Vanessa; Chen, Bin; Galan, Giselle; Yu, Sijie; Liu, Wei; Li, Gang
Abstract: In this study, we investigate motions in the hot plasma above the flare loops during the 2017 September 10 X8.2 flare event. We examine the region to the south of the main flare arcade, where there is data from the Interface Region Imaging Spectrograph (IRIS) and the Extreme ultraviolet Imaging Spectrometer (EIS) on Hinode. We find that there are initial blueshifts of 20-60 km s-1 observed in this region in the Fe XXI line in IRIS and the Fe XXIV line in EIS, and that the locations of these blueshifts move southward along the arcade over the course of about 10 minutes. The cadence of IRIS allows us to follow the evolution of these flows, and we find that at each location where there is an initial blueshift in the Fe XXI line, there are damped oscillations in the Doppler velocity with periods of ˜400 s. We conclude that these periods are independent of loop length, ruling out magnetoacoustic standing modes as a possible mechanism. Microwave observations from the Expanded Owens Valley Solar Array (EOVSA) indicate that there are nonthermal emissions in the region where the Doppler shifts are observed, indicating that accelerated particles are present. We suggest that the flows and oscillations are due to motions of the magnetic field that are caused by reconnection outflows disturbing the loop-top region.
Magnetic Reconnection during the Post-impulsive Phase of a Long-duration Solar Flare: Bidirectional Outflows as a Cause of Microwave and X-Ray BurstsYu, SijieChen, BinReeves, Katharine K.Gary, Dale E.Musset, SophieFleishman, Gregory D.Nita, Gelu M.Glesener, LindsayDOI: info:10.3847/1538-4357/aba8a6v. 90017
Yu, Sijie, Chen, Bin, Reeves, Katharine K., Gary, Dale E., Musset, Sophie, Fleishman, Gregory D., Nita, Gelu M., and Glesener, Lindsay. 2020. "Magnetic Reconnection during the Post-impulsive Phase of a Long-duration Solar Flare: Bidirectional Outflows as a Cause of Microwave and X-Ray Bursts." The Astrophysical Journal 900:17.
ID: 157472
Type: article
Authors: Yu, Sijie; Chen, Bin; Reeves, Katharine K.; Gary, Dale E.; Musset, Sophie; Fleishman, Gregory D.; Nita, Gelu M.; Glesener, Lindsay
Abstract: Magnetic reconnection plays a crucial role in powering solar flares, production of energetic particles, and plasma heating. However, where the magnetic reconnections occur, how and where the released magnetic energy is transported, and how it is converted to other forms remain unclear. Here we report recurring bidirectional plasma outflows located within a large-scale plasma sheet observed in extreme-ultraviolet emission and scattered white light during the post-impulsive gradual phase of the X8.2 solar flare on 2017 September 10. Each of the bidirectional outflows originates in the plasma sheet from a discrete site, identified as a magnetic reconnection site. These reconnection sites reside at very low altitudes () above the top of the flare arcade, a distance only ) above the top of the flare arcade, a distance only . Each arrival of sunward outflows at the loop-top region appears to coincide with an impulsive microwave and X-ray burst dominated by a hot source (10-20 MK) at the loop top and a nonthermal microwave burst located in the loop-leg region. We propose that the reconnection outflows transport the magnetic energy released at localized magnetic reconnection sites outward in the form of kinetic energy flux and/or electromagnetic Poynting flux. The sunward-directed energy flux induces particle acceleration and plasma heating in the post-flare arcades, observed as the hot and nonthermal flare emissions.
Multiband Study of a Bidirectional Jet Occurred in the Upper ChromosphereCai, QiangweiShen, ChengcaiNi, LeiReeves, Katharine K.Kang, KaifengLin, JunDOI: info:10.1029/2019JA027017v. 1249824–9846
Cai, Qiangwei, Shen, Chengcai, Ni, Lei, Reeves, Katharine K., Kang, Kaifeng, and Lin, Jun. 2019. "Multiband Study of a Bidirectional Jet Occurred in the Upper Chromosphere." Journal of Geophysical Research (Space Physics) 124:9824– 9846.
ID: 155110
Type: article
Authors: Cai, Qiangwei; Shen, Chengcai; Ni, Lei; Reeves, Katharine K.; Kang, Kaifeng; Lin, Jun
Abstract: We present a study of a jet observed by the Solar Dynamics Observatory (SDO) and the Interface Region Imaging Spectrograph (IRIS), which provide high spatial-temporal resolution observational data of (extreme) ultraviolet images, spectra, and magnetograms. The jet was observed in multiple bands of AIA and manifested clear bidirectional flows in IRIS observations. The emission profiles of the Si IV 1402 Å line of the jet exhibited non-Gaussian features and double-peaked spectra, with the Doppler velocity and the nonthermal velocity up to 100 and 160 km s-1, respectively. The plasma flows of the jet projected on the sky plane and in the line of sight (LOS) are the typical observational evidence of magnetic reconnection. The EM loci curves indicated that the plasma contains multi-temperature components. The result deduced from the DEM method and changes in intensity of several spectral lines imply that the temperature of the plasma in the jet is heated to at least 105.6 K. The electron density is about 1011 cm-3 according to the intensity ratios of the O IV 1399.77/1401.16 Å doublet and Si IV 1402.77/O IV 1401.16 Å lines. Via different approaches, we reached the conclusion that the jet occurred in the upper chromosphere. Investigating the magnetograms in the period when the jet appeared, we suggest that the jet results from the magnetic reconnection between the moving magnetic structure and the magnetic field nearby.
Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-band Feature as Evidence for Shock CompressionChen, BinShen, ChengcaiReeves, Katharine K.Guo, FanYu, SijieDOI: info:10.3847/1538-4357/ab3c58v. 88463
Chen, Bin, Shen, Chengcai, Reeves, Katharine K., Guo, Fan, and Yu, Sijie. 2019. "Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-band Feature as Evidence for Shock Compression." The Astrophysical Journal 884:63.
ID: 154702
Type: article
Authors: Chen, Bin; Shen, Chengcai; Reeves, Katharine K.; Guo, Fan; Yu, Sijie
Abstract: Solar flare termination shocks have been suggested as one of the promising drivers for particle acceleration in solar flares, yet observational evidence remains rare. By utilizing radio dynamic spectroscopic imaging of decimetric stochastic spike bursts in an eruptive flare, Chen et al. found that the bursts form a dynamic surface-like feature located at the ending points of fast plasma downflows above the looptop, interpreted as a flare termination shock. One piece of observational evidence that strongly supports the termination shock interpretation is the occasional split of the emission band into two finer lanes in frequency, similar to the split-band feature seen in fast-coronal-shock-driven type II radio bursts. Here, we perform spatially, spectrally, and temporally resolved analysis of the split-band feature of the flare termination shock event. We find that the ensemble of the radio centroids from the two split-band lanes each outlines a nearly co-spatial surface. The high-frequency lane is located slightly below its low-frequency counterpart by ̃0.8 Mm, which strongly supports the shock-upstream-downstream interpretation. Under this scenario, the density compression ratio across the shock front can be inferred from the frequency split, which implies a shock with a Mach number of up to 2.0. Further, the spatiotemporal evolution of the density compression along the shock front agrees favorably with results from magnetohydrodynamics simulations. We conclude that the detailed variations of the shock compression ratio may be due to the impact of dynamic plasma structures in the reconnection outflows, which results in distortion of the shock front.
Achievements of Hinode in the first eleven yearsHinode Review TeamAl-Janabi, KhalidAntolin, PatrickBaker, DeborahBellot Rubio, Luis R.Bradley, LouisaBrooks, David H.Centeno, RebeccaCulhane, J. LeonardDel Zanna, GiulioDoschek, George A.Fletcher, LyndsayHara, HirohisaHarra, Louise K.Hillier, Andrew S.Imada, ShinsukeKlimchuk, James A.Mariska, John T.Pereira, Tiago M. D.Reeves, Katharine K.Sakao, TaroSakurai, TakashiShimizu, ToshifumiShimojo, MasumiShiota, DaikouSolanki, Sami K.Sterling, Alphonse C.Su, YingnaSuematsu, YoshinoriTarbell, Theodore D.Tiwari, Sanjiv K.Toriumi, ShinUgarte-Urra, IgnacioWarren, Harry P.Watanabe, TetsuyaYoung, Peter R.DOI: info:10.1093/pasj/psz084v. 71R1
Hinode Review Team, Al-Janabi, Khalid, Antolin, Patrick, Baker, Deborah, Bellot Rubio, Luis R., Bradley, Louisa, Brooks, David H., Centeno, Rebecca, Culhane, J. Leonard, Del Zanna, Giulio, Doschek, George A., Fletcher, Lyndsay, Hara, Hirohisa, Harra, Louise K., Hillier, Andrew S., Imada, Shinsuke, Klimchuk, James A., Mariska, John T., Pereira, Tiago M. D., Reeves, Katharine K., Sakao, Taro, Sakurai, Takashi, Shimizu, Toshifumi, Shimojo, Masumi, Shiota, Daikou et al. 2019. "Achievements of Hinode in the first eleven years." Publications of the Astronomical Society of Japan 71:R1.
ID: 154644
Type: article
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick; Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks, David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio; Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra, Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.; Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao, Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota, Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu, Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin; Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young, Peter R.
Abstract: Hinode is Japan's third solar mission following Hinotori (1981-1982) and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in operation currently. Hinode carries three instruments: the Solar Optical Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These instruments were built under international collaboration with the National Aeronautics and Space Administration and the UK Science and Technology Facilities Council, and its operation has been contributed to by the European Space Agency and the Norwegian Space Center. After describing the satellite operations and giving a performance evaluation of the three instruments, reviews are presented on major scientific discoveries by Hinode in the first eleven years (one solar cycle long) of its operation. This review article concludes with future prospects for solar physics research based on the achievements of Hinode.
Nonequilibrium Ionization Effects on Solar EUV and X-Ray Imaging ObservationsLee, Jin-YiRaymond, John C.Reeves, Katharine K.Shen, ChengcaiMoon, Yong-JaeKim, Yeon-HanDOI: info:10.3847/1538-4357/ab24bbv. 879111
Lee, Jin-Yi, Raymond, John C., Reeves, Katharine K., Shen, Chengcai, Moon, Yong-Jae, and Kim, Yeon-Han. 2019. "Nonequilibrium Ionization Effects on Solar EUV and X-Ray Imaging Observations." The Astrophysical Journal 879:111.
ID: 154153
Type: article
Authors: Lee, Jin-Yi; Raymond, John C.; Reeves, Katharine K.; Shen, Chengcai; Moon, Yong-Jae; Kim, Yeon-Han
Abstract: During transient events such as major solar eruptions, the plasma can be far from the equilibrium ionization state because of rapid heating or cooling. Nonequilibrium ionization (NEI) is important in rapidly evolving systems where the thermodynamical timescale is shorter than the ionization or recombination timescales. We investigate the effects of NEI on EUV and X-ray observations by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory and X-ray Telescope (XRT) on board Hinode. Our model assumes that the plasma is initially in ionization equilibrium at low temperature, and it is heated rapidly by a shock or magnetic reconnection. We tabulate the responses of the AIA and XRT passbands as functions of temperature and a characteristic timescale, n e t. We find that most of the ions reach equilibrium at n e t ≤ 1012 cm-3 s. Comparing ratios of the responses between different passbands allows us to determine whether a combination of plasmas at temperatures in ionization equilibrium can account for a given AIA and XRT observation. It also expresses how far the observed plasma is from equilibrium ionization. We apply the ratios to a supra-arcade plasma sheet on 2012 January 27. We find that the closer the plasma is to the arcade, the closer it is to a single-temperature plasma in ionization equilibrium. We also utilize the set of responses to estimate the temperature and density for shocked plasma associated with a coronal mass ejection on 2010 June 13. The temperature and density ranges we obtain are in reasonable agreement with previous works.
Exploring Plasma Heating in the Current Sheet Region in a Three-dimensional Coronal Mass Ejection SimulationReeves, Katharine K.Török, TiborMikić, ZoranLinker, JonMurphy, Nicholas A.DOI: info:10.3847/1538-4357/ab4ce8v. 887103
Reeves, Katharine K., Török, Tibor, Mikić, Zoran, Linker, Jon, and Murphy, Nicholas A. 2019. "Exploring Plasma Heating in the Current Sheet Region in a Three-dimensional Coronal Mass Ejection Simulation." The Astrophysical Journal 887:103.
ID: 154513
Type: article
Authors: Reeves, Katharine K.; Török, Tibor; Mikić, Zoran; Linker, Jon; Murphy, Nicholas A.
Abstract: We simulate a coronal mass ejection using a three-dimensional magnetohydrodynamic code that includes coronal heating, thermal conduction, and radiative cooling in the energy equation. The magnetic flux distribution at 1 R s is produced by a localized subsurface dipole superimposed on a global dipole field, mimicking the presence of an active region within the global corona. Transverse electric fields are applied near the polarity inversion line to introduce a transverse magnetic field, followed by the imposition of a converging flow to form and destabilize a flux rope, producing an eruption. We examine the quantities responsible for plasma heating and cooling during the eruption, including thermal conduction, radiation, adiabatic effects, coronal heating, and ohmic heating. We find that ohmic heating is an important contributor to hot temperatures in the current sheet region early in the eruption, but in the late phase, adiabatic compression plays an important role in heating the plasma there. Thermal conduction also plays an important role in the transport of thermal energy away from the current sheet region throughout the reconnection process, producing a "thermal halo" and widening the region of high temperatures. We simulate emission from solar telescopes for this eruption and find that there is evidence for emission from heated plasma above the flare loops late in the eruption, when the adiabatic heating is the dominant heating term. These results provide an explanation for hot supra-arcade plasma sheets that are often observed in X-rays and extreme ultraviolet wavelengths during the decay phase of large flares.
Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar JetChen, BinYu, SijieBattaglia, MarinaFarid, SamaiyahSavcheva, AntoniaReeves, Katharine K.Krucker, SämBastian, T. S.Guo, FanTassev, SvetlinDOI: info:10.3847/1538-4357/aadb89v. 86662
Chen, Bin, Yu, Sijie, Battaglia, Marina, Farid, Samaiyah, Savcheva, Antonia, Reeves, Katharine K., Krucker, Säm, Bastian, T. S., Guo, Fan, and Tassev, Svetlin. 2018. "Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet." The Astrophysical Journal 866:62.
ID: 149383
Type: article
Authors: Chen, Bin; Yu, Sijie; Battaglia, Marina; Farid, Samaiyah; Savcheva, Antonia; Reeves, Katharine K.; Krucker, Säm; Bastian, T. S.; Guo, Fan; Tassev, Svetlin
Abstract: Magnetic reconnection, the central engine that powers explosive phenomena throughout the universe, is also perceived to be one of the principal mechanisms for accelerating particles to high energies. Although various signatures of magnetic reconnection have been frequently reported, observational evidence that links particle acceleration directly to the reconnection site has been rare, especially for space plasma environments currently inaccessible to in situ measurements. Here we utilize broadband radio dynamic imaging spectroscopy available from the Karl G. Jansky Very Large Array to observe decimetric type III radio bursts in a solar jet with high angular (∼20″), spectral (∼1%), and temporal resolution (50 ms). These observations allow us to derive detailed trajectories of semirelativistic (tens of keV) electron beams in the low solar corona with unprecedentedly high angular precision (2) in the low solar corona. The beam-diverging sites are located behind the erupting jet spire and above the closed arcades, coinciding with the presumed location of magnetic reconnection in the jet eruption picture supported by extreme ultraviolet/X-ray data and magnetic modeling. We interpret each beam-diverging site as a reconnection null point where multitudes of magnetic flux tubes join and reconnect. Our data suggest that the null points likely consist of a high level of density inhomogeneities possibly down to 10 km scales. These results, at least in the present case, strongly favor a reconnection-driven electron-acceleration scenario.
Broad Non-Gaussian Fe XXIV Line Profiles in the Impulsive Phase of the 2017 September 10 X8.3-class Flare Observed by Hinode/EISPolito, VanessaDudík, JaroslavKašparová, JanaDzifčáková, ElenaReeves, Katharine K.Testa, PaolaChen, BinDOI: info:10.3847/1538-4357/aad62dv. 86463
Polito, Vanessa, Dudík, Jaroslav, Kašparová, Jana, Dzifčáková, Elena, Reeves, Katharine K., Testa, Paola, and Chen, Bin. 2018. "Broad Non-Gaussian Fe XXIV Line Profiles in the Impulsive Phase of the 2017 September 10 X8.3-class Flare Observed by Hinode/EIS." The Astrophysical Journal 864:63.
ID: 149174
Type: article
Authors: Polito, Vanessa; Dudík, Jaroslav; Kašparová, Jana; Dzifčáková, Elena; Reeves, Katharine K.; Testa, Paola; Chen, Bin
Abstract: We analyze the spectra of high-temperature Fe XXIV lines observed by the Hinode/Extreme-Ultraviolet Imaging Spectrometer (EIS) during the impulsive phase of the X8.3-class flare on 2017 September 10. The line profiles are broad, show pronounced wings, and clearly depart from a single-Gaussian shape. The lines can be well fitted with κ distributions, with values of κ varying between ≈1.7 and 3. The regions where we observe the non-Gaussian profiles coincide with the location of high-energy (≈100–300 keV) hard X-ray (HXR) sources observed by RHESSI, suggesting the presence of particle acceleration or turbulence, also confirmed by the observations of nonthermal microwave sources with the Expanded Owens Valley Solar Array at and above the HXR loop-top source. We also investigate the effect of taking into account κ distributions in the temperature diagnostics based on the ratio of the Fe XXIII λ263.76 and Fe XXIV λ255.1 EIS lines. We found that these lines can be formed at much higher temperatures than expected (up to log(T[K]) ≈ 7.8) if departures from Maxwellian distributions are taken into account. Although larger line widths are expected because of these higher formation temperatures, the observed line widths still imply nonthermal broadening in excess of 200 km s‑1. The nonthermal broadening related to HXR emission is better interpreted by turbulence than by chromospheric evaporation.
Possible Signatures of a Termination Shock in the 2014 March 29 X-class Flare Observed by IRISPolito, VanessaGalan, GiselleReeves, Katharine K.Musset, SophieDOI: info:10.3847/1538-4357/aadadav. 865161
Polito, Vanessa, Galan, Giselle, Reeves, Katharine K., and Musset, Sophie. 2018. "Possible Signatures of a Termination Shock in the 2014 March 29 X-class Flare Observed by IRIS." The Astrophysical Journal 865:161.
ID: 149391
Type: article
Authors: Polito, Vanessa; Galan, Giselle; Reeves, Katharine K.; Musset, Sophie
Abstract: The standard model of flares predicts the existence of a fast-mode magnetohydrodynamic shock above the looptops, also known as termination shock (TS), as the result of the downward-directed outflow reconnection jets colliding with the closed magnetic loops. A crucial spectral signature of a TS is the presence of large Doppler shifts in the spectra of high-temperature lines (≥10 MK), which has been rarely observed so far. Using high-resolution observations of the Fe XXI line with the Interface Region Imaging Spectrograph (IRIS), we detect large redshifts (≈200 km s‑1) at the top of the bright looptop arcade of the X1-class flare on 2014 March 29. In some cases, the redshifts are accompanied by faint simultaneous Fe XXI blueshifts of about ‑250 km s‑1. The values of red and blueshifts are in agreement with recent modeling of Fe XXI spectra downflow of the reconnection site and previous spectroscopic observations with higher temperature lines. The locations where we observe the Fe XXI shifts are co-spatial with 30–70 keV hard X-ray sources detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), indicating that nonthermal electrons are located above the flare loops. We speculate that our results are consistent with the presence of a TS in flare reconnection models.
Solar Flare Termination Shock and Synthetic Emission Line Profiles of the Fe xxi 1354.08 Å LineGuo, LijiaLi, GangReeves, KathyRaymond, JohnDOI: info:10.3847/2041-8213/aa866av. 846L12
Guo, Lijia, Li, Gang, Reeves, Kathy, and Raymond, John. 2017. "Solar Flare Termination Shock and Synthetic Emission Line Profiles of the Fe xxi 1354.08 Å Line." Astrophysical Journal Letters 846:L12.
ID: 144709
Type: article
Authors: Guo, Lijia; Li, Gang; Reeves, Kathy; Raymond, John
Abstract: Solar flares are among the most energetic phenomena that occur in the solar system. In the standard solar flare model, a fast mode shock, often referred to as the flare termination shock (TS), can exist above the loop-top source of hard X-ray emissions. The existence of the TS has been recently related to spectral hardening of a flare’s hard X-ray spectra at energies >300 keV. Observations of the Fe xxi 1354.08 Å line during solar flares by the Interface Region Imaging Spectrograph (IRIS) spacecraft have found significant redshifts with >100 km s-1, which is consistent with a reconnection downflow. The ability to detect such a redshift with IRIS suggests that one may be able to use IRIS observations to identify flare TSs. Using a magnetohydrodynamic simulation to model magnetic reconnection of a solar flare and assuming the existence of a TS in the downflow of the reconnection plasma, we model the synthetic emission of the Fe xxi 1354.08 line in this work. We show that the existence of the TS in the solar flare may manifest itself in the Fe xxi 1354.08 Å line.
Heating of an Erupting Prominence Associated with a Solar Coronal Mass Ejection on 2012 January 27Lee, Jin-YiRaymond, John C.Reeves, Katharine K.Moon, Yong-JaeKim, Kap-SungDOI: info:10.3847/1538-4357/aa79a4v. 8443
Lee, Jin-Yi, Raymond, John C., Reeves, Katharine K., Moon, Yong-Jae, and Kim, Kap-Sung. 2017. "Heating of an Erupting Prominence Associated with a Solar Coronal Mass Ejection on 2012 January 27." The Astrophysical Journal 844:3.
ID: 143805
Type: article
Authors: Lee, Jin-Yi; Raymond, John C.; Reeves, Katharine K.; Moon, Yong-Jae; Kim, Kap-Sung
Abstract: We investigate the heating of an erupting prominence and loops associated with a coronal mass ejection and X-class flare. The prominence is seen as absorption in EUV at the beginning of its eruption. Later, the prominence changes to emission, which indicates heating of the erupting plasma. We find the densities of the erupting prominence using the absorption properties of hydrogen and helium in different passbands. We estimate the temperatures and densities of the erupting prominence and loops seen as emission features using the differential emission measure method, which uses both EUV and X-ray observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory and the X-ray Telescope on board Hinode. We consider synthetic spectra using both photospheric and coronal abundances in these calculations. We verify the methods for the estimation of temperatures and densities for the erupting plasmas. Then, we estimate the thermal, kinetic, radiative loss, thermal conduction, and heating energies of the erupting prominence and loops. We find that the heating of the erupting prominence and loop occurs strongly at early times in the eruption. This event shows a writhing motion of the erupting prominence, which may indicate a hot flux rope heated by thermal energy release during magnetic reconnection.
An Exploration of Heating Mechanisms in a Supra-arcade Plasma Sheet Formed after a Coronal Mass EjectionReeves, Katharine K.Freed, Michael S.McKenzie, David E.Savage, Sabrina L.DOI: info:10.3847/1538-4357/836/1/55v. 83655
Reeves, Katharine K., Freed, Michael S., McKenzie, David E., and Savage, Sabrina L. 2017. "An Exploration of Heating Mechanisms in a Supra-arcade Plasma Sheet Formed after a Coronal Mass Ejection." The Astrophysical Journal 836:55.
ID: 142800
Type: article
Authors: Reeves, Katharine K.; Freed, Michael S.; McKenzie, David E.; Savage, Sabrina L.
Abstract: We perform a detailed analysis of the thermal structure of the region above the post-eruption arcade for a flare that occurred on 2011 October 22. During this event, a sheet of hot plasma is visible above the flare loops in the 131 Å bandpass of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory. Supra-arcade downflows (SADs) are observed traveling sunward through the post-eruption plasma sheet. We calculate differential emission measures using the AIA data and derive an emission measure weighted average temperature in the supra-arcade region. In areas where many SADs occur, the temperature of the supra-arcade plasma tends to increase, while in areas where no SADs are observed, the temperature tends to decrease. We calculate the plane-of-sky velocities in the supra-arcade plasma and use them to determine the potential heating due to adiabatic compression and viscous heating. Of the 13 SADs studied, 10 have noticeable signatures in both the adiabatic and the viscous terms. The adiabatic heating due to compression of plasma in front of the SADs is on the order of 0.1-0.2 MK/s, which is similar in magnitude to the estimated conductive cooling rate. This result supports the notion that SADs contribute locally to the heating of plasma in the supra-arcade region. We also find that in the region without SADs, the plasma cools at a rate that is slower than the estimated conductive cooling, indicating that additional heating mechanisms may act globally to keep the plasma temperature high.
Time-dependent Ionization in a Steady Flow in an MHD Model of the Solar Corona and WindShen, ChengcaiRaymond, John C.Mikic, ZoranLinker, Jon A.Reeves, Katharine K.Murphy, Nicholas A.DOI: info:10.3847/1538-4357/aa93f3v. 85026
Shen, Chengcai, Raymond, John C., Mikic, Zoran, Linker, Jon A., Reeves, Katharine K., and Murphy, Nicholas A. 2017. "Time-dependent Ionization in a Steady Flow in an MHD Model of the Solar Corona and Wind." The Astrophysical Journal 850:26.
ID: 144807
Type: article
Authors: Shen, Chengcai; Raymond, John C.; Mikic, Zoran; Linker, Jon A.; Reeves, Katharine K.; Murphy, Nicholas A.
Abstract: Time-dependent ionization is important for diagnostics of coronal streamers and pseudostreamers. We describe time-dependent ionization calculations for a three-dimensional magnetohydrodynamic (MHD) model of the solar corona and inner heliosphere. We analyze how non-equilibrium ionization (NEI) influences emission from a pseudostreamer during the Whole Sun Month interval (Carrington rotation CR1913, 1996 August 22 to September 18). We use a time-dependent code to calculate NEI states, based on the plasma temperature, density, velocity, and magnetic field in the MHD model, to obtain the synthetic emissivities and predict the intensities of the Ly?, O VI, Mg x, and Si xii emission lines observed by the SOHO/Ultraviolet Coronagraph Spectrometer (UVCS). At low coronal heights, the predicted intensity profiles of both Ly? and O VI lines match UVCS observations well, but the Mg x and Si xii emission are predicted to be too bright. At larger heights, the O VI and Mg x lines are predicted to be brighter for NEI than equilibrium ionization around this pseudostreamer, and Si xii is predicted to be fainter for NEI cases. The differences of predicted UVCS intensities between NEI and equilibrium ionization are around a factor of 2, but neither matches the observed intensity distributions along the full length of the UVCS slit. Variations in elemental abundances in closed field regions due to the gravitational settling and the FIP effect may significantly contribute to the predicted uncertainty. The assumption of Maxwellian electron distributions and errors in the magnetic field on the solar surface may also have notable effects on the mismatch between observations and model predictions.
Density diagnostics derived from the O iv and S iv intercombination lines observed by IRISPolito, V.Del Zanna, G.Dudík, J.Mason, H. E.Giunta, A.Reeves, Katharine K.DOI: info:10.1051/0004-6361/201628965v. 594A64
Polito, V., Del Zanna, G., Dudík, J., Mason, H. E., Giunta, A., and Reeves, Katharine K. 2016. "Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS." Astronomy and Astrophysics 594:A64.
ID: 142027
Type: article
Authors: Polito, V.; Del Zanna, G.; Dudík, J.; Mason, H. E.; Giunta, A.; Reeves, Katharine K.
Abstract: The intensity of the O iv 2s2 2p 2P-2s2p24P and S iv 3 s2 3p 2P-3s 3p24 P intercombination lines around 1400 Å observed with the Interface Region Imaging Spectrograph (IRIS) provide a useful tool to diagnose the electron number density (Ne) in the solar transition region plasma. We measure the electron number density in a variety of solar features observed by IRIS, including an active region (AR) loop, plage and brightening, and the ribbon of the 22-June-2015 M 6.5 class flare. By using the emissivity ratios of O iv and S iv lines, we find that our observations are consistent with the emitting plasma being near isothermal (logT[K] ? 5) and iso-density (Ne ? 1010.6 cm-3) in the AR loop. Moreover, high electron number densities (Ne ? 1013 cm-3) are obtained during the impulsive phase of the flare by using the S iv line ratio. We note that the S iv lines provide a higher range of density sensitivity than the O iv lines. Finally, we investigate the effects of high densities (Ne ? 1011 cm-3) on the ionization balance. In particular, the fractional ion abundances are found to be shifted towards lower temperatures for high densities compared to the low density case. We also explored the effects of a non-Maxwellian electron distribution on our diagnostic method. The movie associated to Fig. 3 is available at