Waveforms of vertical electric field observed at distances of 27, 57, and 101km from the 634-m tall Tokyo Skytree (Japan) struck by lightning have been reproduced using the 3-D finite difference time domain method. Optically observed 3-D lightning channels and currents directly measured in the tower were used. Distribution of current along the lightning channel was represented using the transmission line model, assuming a constant propagation speed v=0.5c. Simulations with commonly assumed vertical channel did not allow us to reproduce the observed early (within 5 s or so) field zero crossing when the 1-D speed was set to be constant and equal to the 3-D speed (v=0.5c) used for the actual-channel-geometry case. It was, however, possible to achieve a good match for a vertical channel when the vertical component of 3-D speed was evaluated for each nonvertical channel segment and used in the corresponding range of heights. Plain Language Summary Since tall towers are often struck by lightning, direct lightning current measurements are usually performed on such towers. Also, lightning strikes to tall towers have been used to test lightning locating systems in terms of their detection efficiency, location accuracy, and current estimation errors. Hence, it is important to study the interaction of lightning with towers and associated electromagnetic fields. In this letter, waveforms of vertical electric field observed at distances of 27, 57, and 101km from the 634-m tall Tokyo Skytree struck by lightning have been reproduced using the 3-D finite difference time domain method. Optically observed 3-D lightning channels and currents directly measured in the tower were used. Distribution of current along the lightning channel was represented using the transmission line model, assuming a constant propagation speed v=0.5c. Simulations with commonly assumed vertical channel did not allow us to reproduce the observed field zero crossing when the 1-D speed was set to be constant and equal to the 3-D speed (v=0.5c) used for the actual-channel-geometry case. It was, however, possible to achieve a good match for a vertical channel when the vertical component of 3-D speed was evaluated for each nonvertical channel segment and used in the corresponding range of heights.
Using the high-speed optical and electric field records, in conjunction with Earth Networks Total Lightning Network and radar data, we examined in detail the morphology and evolution of an upward negative flash containing six downward leader/upward return stroke sequences terminated on a 257-m tower in Florida. The upward flash was induced (triggered) by a single-stroke 50-kA +CG that occurred about 45 km from the tower. The in-cloud part of +CG was optically detected to extend toward the tower and appeared to stop at a height of about 3 km above the tower top. The six leader-return-stroke sequences were each initiated by a bidirectional leader utilizing the remnants of branches created during the initial stage. Electric field signatures of bidirectional leaders were similar to K-changes. The upper end of the return-stroke channel in all six cases exhibited branching and appeared to extend to higher altitudes or/and move closer to Lightning Observatory in Gainesville with increasing stroke order.
Using high-speed video cameras operating with framing rates of 20 and 525 kfps, we imaged the attachment process of a natural negative cloud-to-ground flash, occurring at a distance of 490 m. Nine upward leaders were observed. A total of 12 space stems/leaders in 47 steps of the downward negative stepped leader were captured. The two-dimensional length of them was between 2.0 and 5.9 m, with an average of 3.0 m. The average interstep interval, step length, and two-dimensional speed of the downward negative leader and that of upward positive leader were statistically analyzed. The last step of the downward negative leader making contact with the upward connecting leader was recorded. The two-dimensional length of the final imaged gap between the tips of opposite-polarity leaders was estimated to be about 13 m.
High-speed video and electric field change data have been used to examine the initiation and propagation of 21 recoil leaders, 7 of which evolved into dart (or dart-stepped) leaders (DLs) initiating return strokes and 14 were attempted leaders (ALs), in a Canton-Tower upward flash. Three DLs and two ALs clearly exhibited bidirectional extension. Each DL was preceded by one or more ALs and initiated near the extremity of the positive end of the preceding AL. The positive end of each bidirectional DL generally appeared to be inactive (stationary) or intermittently propagated along the positive part of the preceding AL channel and extended into the virgin air. A sequence of two floating channel segments was formed ahead of the approaching positive end of one DL, causing its abrupt elongation. Plain Language Summary In this paper, we present high-speed video observations of recoil leaders (RLs) which were not obscured by the cloud. We examined the initial positions, morphology, and dynamics of RLs in a Canton-Tower upward flash. Out of a total of 21 RLs, 7 evolved into dart (or dart-stepped) leaders, attaching to the Tower and producing return strokes, and 14 failed to reach the Tower, that is, were attempted dart (or dart-stepped) leaders. We infer that all the RLs were bidirectional with the negative end steadily extending toward the Tower and the positive end generally being stationary or showing intermittent propagation. Each dart (or dart-stepped) leader initiated near the extremity of the positive end of the preceding bidirectional attempted leader. The positive end of one dart (or dart-stepped) leader extended abruptly via connection of at least one floating channel to the positive channel tip.
This work presents the first results of measurements of artificial plasma disturbance characteristics using the low-orbit NorSat-1 satellite, which are excited when the ionospheric F2 layer is modified by powerful high-frequency (HF) waves emitted by the SURA heating facility. NorSat-1 carries the multineedle Langmuir probe instrument, which is capable of sampling the electron density at a nominal rate up to 1 kHz. The uniqueness of this experiment lies in the fact that the satellite passes very close to the center of the HF-perturbed magnetic flux tube and in situ observations are first carried out in winter when the absorption is still small in the morning as the Sun is low above the horizon. There are HF-induced plasma temperature and density variations at satellite altitudes of about 580 km. Plasma irregularities are detected by in situ measurements down to 200 m at the southern border of the SURA heating region.
We use data from the 2013–2014 Cluster Inner Magnetosphere Campaign, with its uniquely small spacecraft separations (less than or equal to electron inertia length, λe), to study multiscale magnetic structures in 14 substorm‐related prolonged dipolarizations in the near‐Earth magnetotail. Three time scales of dipolarization are identified: (i) a prolonged growth of the BZ component with duration ≤20 min; (ii) BZ pulses with durations ≤1 min during the BZ growth; and (iii) strong magnetic field gradients with durations ≤2 s during the dipolarization growth. The values of these gradients observed at electron scales are several dozen times larger than the corresponding values of magnetic gradients simultaneously detected at ion scales. These nonlinear features in magnetic field gradients denote the formation of intense and localized (approximately a few λe) current structures during the dipolarization and substorm current wedge formation. These observations highlight the importance of electron scale processes in the formation of a 3‐D substorm current system.
We have observed unusual plasma formations (UPFs) in artificial clouds of charged water droplets using a high-speed infrared camera operating in conjunction with a high-speed visible-range camera. Inferred plasma parameters were close to those of long-spark leaders observed in the same experiments, while the channel morphology was distinctly different from that of leaders, so that UPFs can be viewed as a new type of in-cloud discharge. These formations can occur in the absence of spark leaders and appear to be manifestations of collective processes building, essentially from scratch, a complex hierarchical network of interacting channels at different stages of development (some of which are hot and live for milliseconds). We believe that the phenomenon should commonly occur in thunderclouds and might give insights on the missing link in the still poorly understood lightning initiation process.
Mallick et al. (2012, https://doi.org/10.1029/2012JD017555) discovered that subsequent-stroke leaders in natural negative lightning could be more prolific producers of hard X-rays and gamma rays than the first-stroke leader in the same flash. However, they had no optical records to confirm that their subsequent leaders followed the same path to ground as the first leader, as opposed to forging a new path to ground through cold air. In this paper, we present new observations, including optical data, showing that a second stroke produced more detectable X-ray pulses than the first stroke, with both strokes following the same channel to ground. Additionally, we present data for the fifth stroke from a different flash, which show the occurrence of X-ray emission at the onset of the common streamer zone between the hot channels of the downward negative dart-stepped leader and upward positive connecting leader. However, there were no detectable X-rays associated with negative leader steps. Plain Language Summary: Cloud-to-ground lightning flashes are each typically composed of 3 to 5 strokes. First stroke necessarily develops in virgin (cold) air, while subsequent strokes often retrace the remnants of the channel(s) of preceding stroke(s). Lightning is known to produce hard X-rays during the initial (leader) stage of each of its strokes. Traditionally, first-stroke leaders were thought to be the main producers of X-rays, and subsequent-stroke leaders (developing in warm, low-density air) were thought to be less active X-ray producers. Mallick et al. (2012) observed subsequent-stroke leaders that were more prolific producers of X-rays than the first-stroke leader in the same flash. However, they had no high-speed video images to confirm that their subsequent leaders followed the same path to ground as the first leader, as opposed to forging a new path to ground through cold air. We present new observations, including high-speed video images, showing that a second stroke produced more detectable X-ray pulses than the first stroke, with both strokes following the same channel to ground. Additionally, we present data for a subsequent stroke from a different flash, which show the rarely observed occurrence of significant X-ray emission at the time of attachment of that stroke to the ground.
Current sheets (CSs) play a crucial role in the storage and conversion of magnetic energy in planetary magnetotails. Using high‐resolution magnetic field data from MAVEN spacecraft, we report the existence of super thin current sheets (STCSs) in the Martian magnetotail. The typical half‐thickness of the STCSs is ~5 km, and it is much less than the gyroradius of thermal protons (ρp). The STCSs are embedded into a thicker sheet with L ≥ ρp forming a multiscale current configuration. The formation of STCS does not depend on ion composition, but it is controlled by the small value of the normal component of the magnetic field at the neutral plane (BN). A number of the observed multiscale CSs are located in the parametric map close to the tearing‐unstable domain, and thus, the inner STCS can provide an additional free energy to excite ion tearing mode in the Martian magnetotail.
Thin current sheets (TCSs) with thicknesses about ion Larmor radii are widespread in space. It is important to describe their equilibrium structure allowing them to store and then explosively release the accumulated free energy. When ions are moving along quasi‐adiabatic trajectories while magnetized electrons follow guiding center drift orbits, TCSs can be described within the framework of a hybrid approach. The thickness of the embedded electron sheet remains uncertain because of the scale‐free character of electron motion. In this work, we propose a novel analytical model of the multilayer TCS that provides a universal expression describing the inner (embedded) electron sheet in dependence of TCS characteristics. An unusual property of the embedded electron layer revealed in this analysis is the nonlinear profile of the magnetic field in the inner layer: B(z) ~ z1/3, which conforms excellently with MAVEN observations of 43 TCSs in the Martian magnetotail.