Observations of supra-arcade fans: instabilities at the head of reconnection jets

Supra-arcade fans are bright, irregular regions of emission that develop during eruptive flares, above flare arcades. The underlying flare arcades are thought to be a consequence of magnetic reconnection along a current sheet in the corona. At the same time, theory predicts plasma jets from the reconnection site which would be extremely difficult to observe directly because of their low density. It has been suggested that the dark supra-arcade downflows (SADs) seen falling through supra-arcade fans may be low density jet plasma. The head of a low density jet directed towards higher density plasma would be Rayleigh-Taylor unstable, and lead to the development of rapidly growing low and high density fingers along the interface. Using SDO/AIA 131A images, we show details of SADs seen from three different orientations with respect to the flare arcade and current sheet, and highlight features that have been previously unexplained, such as the splitting of SADs at their heads, but are a natural consequence of instabilities above the arcade. Comparison with 3-D magnetohydrodynamic simulations suggests that supra-arcade downflows are the result of secondary instabilities of the Rayleigh-Taylor type in the exhaust of reconnection jets.Comment: 10 pages, 7 figures To be published in ApJ, 796, 27 (2014

Predicting Student Sensitivity to Tuition and Financial Aid

Over the last two decades, a substantial body of research has examined student responsiveness to tuition increases and financial aid offers in postsecondary educational decisions (see, for example, Heller, 1997; Leslie and Brinkman, 1988). Another major research interest in higher education literature is student behavior in choosing a postsecondary educational institution (see, for example, Hossler, Braxton, and Coopersmith, 1989; Paulsen, 1990). As the costs of postsecondary education have risen, policy analysts and scholars have paid increasing attention to the impact of tuition costs and student financial aid on access to postsecondary education, college matriculation decisions, and subsequent student persistence in postsecondary education (McPherson and Shapiro, 1991, 1998; Mumper, 1996; St. John, 1990a, 1990b; St. John, Starkey, Paulsen and Mbaduagha, 1995; Weiler, 1996). Institutional policy-makers are concerned about student recruitment and enrollment on the one hand and institutional financial health on the other, while state and federal policy-makers are worried about the effective use of public funds to meet national interests such as access, choice, and attainment in postsecondary education. Policy analysts and higher education researchers have recently become concerned about whether students attend college and which schools students attend, because the postsecondary destinations of students are related to student educational attainment and career development (Hearn, 1988, 1991; Pascarella and Terenzini, 1991). Thus, from a social equity perspective, college tuition and financial aid have become serious policy issues. It is believed that the influence of perceived college tuition rates and financial aid availability becomes important during student college choice process and reaches the highest level in the senior year of high school (Hossler and Gallagher, 1987; Hossler, Schmit, and Vesper, 1999). However, not until the last few years has research on the impact of college tuition and financial aid been linked with models of student college choice. Savoca (1990) integrated price impact into her research on student application behaviors to college and concluded that this integration would result in estimating student price responsiveness more accurately. Meanwhile, recent research implies that tuition pricing and financial aid offers exert different impacts on student postsecondary participation decisions (St. John and Starkey, 1995). The purpose of this study is to identify the predictors of student sensitivity to college tuition and financial aid and to differentiate the impacts of these predictors on student price sensitivity in the student college choice process

Transverse wave induced Kelvin-Helmholtz rolls in spicules

This research has received funding from the UK Science and Technology Facilities Council (Consolidated Grant ST/K000950/1) and the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214).In addition to their jet-like dynamic behaviour, spicules usually exhibit strong transverse speeds, multi-stranded structure and heating from chromospheric to transition region temperatures. In this work we first analyse Hinode & IRIS observations of spicules and find different behaviours in terms of their Doppler velocity evolution and collective motion of their sub-structure. Some have a Doppler shift sign change that is rather fixed along the spicule axis, and lack coherence in the oscillatory motion of strand-like structure, matching rotation models or long wavelength torsional Alfvén waves. Others exhibit a Doppler shift sign change at maximum displacement and coherent motion of their strands, suggesting a collective MHD wave. By comparing with an idealised 3D MHD simulation combined with radiative transfer modelling, we analyse the role of transverse MHD waves and associated instabilities in spicule-like features. We find that Transverse Wave Induced Kelvin-Helmholtz (TWIKH) rolls lead to coherence of strand-like structure in imaging and spectral maps, as seen in some observations. The rapid transverse dynamics and the density and temperature gradients at the spicule boundary lead to ring-shaped Mg II k and Ca II H source functions in the transverse cross-section, potentially allowing IRIS to capture the KHI dynamics. Twists and currents propagate along the spicule at Alfvénic speeds, and the temperature variations within TWIKH rolls produce sudden appearance/disappearance of strands seen in Doppler velocity and in Ca II H intensity. However, only a mild intensity increase in higher temperature lines is obtained, suggesting there is an additional heating mechanism at work in spicules.PostprintPeer reviewe

Spicules in IRIS Mg II Observations: Automated Identification

We have developed an algorithm to identify solar spicules in the first-ever systematic survey of on-disk spicules using exclusively Mg II spectral observations. Using this algorithm we identify 2219 events in three IRIS datasets with unique solar feature targets spanning a total of 300 minutes: 1) an active region, 2) decayed active region/active network, and 3) a coronal hole. We present event statistics and relate occurrence rates to underlying photospheric magnetic field strength. This method identifies spicule event densities and occurrence rates similar to previous studies performed using H{\alpha} and Ca II observations of active regions. Additionally, this study identifies spicule-like events at very low rates at magnetic field intensities below 20 Gauss and increasing significantly between 100-200 Gauss in active regions and above 20 Gauss in coronal holes, which can be used to inform future observation campaigns. This information can be be used to help characterize spicules over their full lifetime, and compliments existing H-{\alpha} spectral capabilities and upcoming Ly-{\alpha} spectral observations on the SNIFS Sounding Rocket. In total, this study presents a method for detecting solar spicules using exclusively Mg II spectra, and provides statistics for spicule occurrence in Mg II wavelengths with respect to magnetic field strength for the purpose of predicting spicule occurrences.Comment: 17 pages, 9 figures, presented at the AGU Fall 2022 conference, Submitted to AAS Journa

Flux Cancelation: The Key to Solar Eruptions

Solar coronal jets are magnetically channeled eruptions that occur in all types of solar environments (e.g. active regions, quiet-Sun regions and coronal holes). Recent studies show that coronal jets are driven by the eruption of small-scale filaments (minifilaments). Once the eruption is underway magnetic reconnection evidently makes the jet spire and the bright emission in the jet base. However, the triggering mechanism of these eruptions and the formation mechanism of the pre-jet minifilaments are still open questions. In this talk, mainly using SDO/AIA and SDO/HMI data, first I will address the question: what triggers the jet-driving minifilament eruptions in different solar environments (coronal holes, quiet regions, active regions)? Then I will talk about the magnetic field evolution that produces the pre-jet minifilaments. By examining pre-jet evolutionary changes in line-of-sight HMI magnetograms while examining concurrent EUV images of coronal and transition-region emission, we find clear evidence that flux cancellation is the main process that builds pre-jet minifilaments, and is also the main process that triggers the eruptions. I will also present results from our ongoing work indicating that jet-driving minifilament eruptions are analogous to larger-scale filament eruptions that make flares and CMEs. We find that persistent flux cancellation at the neutral line of large-scale filaments often triggers their eruptions. From our observations we infer that flux cancellation is the fundamental process for the buildup and triggering of solar eruptions of all sizes