Inception of subsequent stepped leaders in negative cloud-to-ground lightning

© 2019, The Author(s). Time-correlated high-speed video and electric field change data for 139 natural, negative cloud-to-ground (CG)-lightning flashes reveal 615 return strokes (RSs) and 29 upward-illumination (UI)-type strokes. Among 121 multi-stroke flashes, 56% visibly connected to more than one ground location for either a RS or UI-type stroke. The number of separate ground-stroke connection locations per CG flash averaged 1.74, with maximum 6. This study examines the 88 subsequent strokes that involved a subsequent stepped leader (SSL), either reaching ground or intercepting a former leader to ground, in 61 flashes. Two basic modes by which these SSLs begin are described and are termed dart-then-stepped leaders herein. One inception mode occurs when a dart leader deflects from the prior main channel and begins propagating as a stepped leader to ground. In these ‘divert’ mode cases, the relevant interstroke time from the prior RS in the channel to the SSL inception from that path is long, ranging from 105 to 204 ms in four visible cases. The alternative mode of SSL inception occurs when a dart leader reaches the end of a prior unsuccessful branch—of an earlier competing dart leader, stepped leader, or initial leader—then begins advancing as a stepped leader toward ground. In this more common ‘branch’ mode (85% of visible cases), there may be no portion of the subsequent RS channel that is shared with a prior RS channel. These two inception modes, and variations among them, can occur in different subsequent strokes of the same flash

Social vs. practical problems in attaining a colonoscopy: Different patient profiles?

Background: Colonoscopy is an effective procedure for identifying precancerous polyps and cancerous lesions, but it is unlike other cancer screening tools in that it requires sedation and thus assistance from at least one other individual. The intent of this paper was to identify logistical problems in completing the colonoscopy and to examine their relationships with sociodemographic characteristics. Methods: All eligible patients (n = 2500) from two academic-affiliated colonoscopy centers (one free standing, one hospital-based) were invited to participate in an onsite, pre-colonoscopy survey; patients agreeing to participate (n = 1841, RR = 73.6%) received a $5.00 gift card. Multiple correspondence analysis (MCA) was used to identify the underlying dimensional structure of the problems. Bivariate statistics were performed to compare demographic variables and health literacy levels among patients reporting problems. Multivariate logistic regression with a backwards conditional solution was used to determine the demographic variables independently associated with problems. Results: Multiple correspondence analyses indicated two dimensions of problems (social and practical). Using logistic regression, social problems (e.g., finding someone to accompany the patient) were associated with not living in the same home as the driver, not working due to disability, and younger age. Practical problems (e.g., making an appointment) were associated with “other” minority race, poorer health, lower health literacy, and younger age. Conclusion: Patients experience different problems completing the colonoscopy based on socio-demographics. Particularly at risk are patients who find it difficult to navigate the system, are of younger age, or who may have smaller social networks

Modeling Initial Breakdown Pulses of Lightning Flashes Using a Matrix Inversion Method

© 2019. American Geophysical Union. All Rights Reserved. This study describes a new method for modeling the radiated electric field (E) of initial breakdown pulses (IBPs) of lightning flashes. Similar to some previous models, it is assumed that E pulses are caused by a current propagating along a vertical path, and an equation based on Maxwell\u27s equations is used to determine E due to the current. A matrix inversion technique is used with the IBP radiation term of E to determine the IBP current waveform directly from far-field E measurements rather than assuming a parameterized current waveform and searching for appropriate parameters. This technique is developed and applied to observations of six previously modeled IBPs. Compared to the prior modeling, this matrix inversion method gives significantly better results, based on calculated IBP goodness of fit to the original E data. In addition, this model can match IBP subpulses along with representing the overall bipolar IBP waveform. This method should be useful for studying IBPs because once the IBP current is known, one can calculate other physical parameters of IBPs, such as charge moment change, total charge moved, and total power radiated. Thus, the more realistic IBP current waveform determined by this technique may offer new clues about the physical mechanism causing IBPs

Modeling initial breakdown pulses of intracloud lightning flashes

In this study 29 initial breakdown pulses (IBPs) from four intracloud (IC) lightning flashes are modeled using data from five or more electric field change (E-change) sites. For each flash the first 5–9 located IBPs are investigated. For each IBP the modeling first extracts the IBP current waveform from the E-change data by matrix inversion and then determines the best channel length and current velocity to match the IBP data. Derived IBP quantities of total charge, charge moment, peak current, peak radiated power, and total energy are calculated. Resulting IBP vertical lengths varied from 27 m to 1300 m; most values were 100–500 m. Current velocities ranged over 4.0–20.0 × 107 m/s, with most values 10–16.5 × 107 m/s. Two of these IC flashes had two “extraordinary” IBPs each with very large E-change amplitude and multiple subpulses; these four extraordinary IBPs had longer current rise times than fall times and charge moments of −3.45 to −20.06C km. Subpulses of classic IBPs were coincident with, and likely caused by, smaller current pulses superimposed on the main IBP current. Overall, most of the 29 IC IBPs had peak current amplitudes \u3c120 kA and total (negative) charge \u3c2C, while the four extraordinary IB pulses had peak currents of 217–359 kA and total charges of −8.4C to −71.7C. The four extraordinary IBPs all have the characteristics of Energetic In-cloud Pulses (EIPs), which are thought to be the radio signals of events producing terrestrial gamma-ray flashes (TGFs). The extraordinary IBPs may have caused double-pulse TGFs and overlapping TGFs