We have used the LOw-Frequency ARray (LOFAR) to search for the growing tip of an intra-cloud (IC) positive leader. Even with our most sensitive beamforming method, where we coherently add the signals ...of about 170 antenna pairs, we were not able to detect any emission from the tip. Instead, we put constraints on the emissivity of very-high frequency (VHF) radiation from the tip at 0.5 pJ/MHz at 60 MHz, integrated over 100 ns. The limit is independent on whether this emission is in the form of short pulses or continuously radiating. The non-observation of VHF radiation from intra-cloud positive leaders implies that they proceed in an extremely gradual process, which is in sharp contrast with the observations of other parts of a lightning discharge.
We present the results from the low-frequency (40–78 MHz) extension of the first pulsar census of non-recycled pulsars carried out with the LOw-Frequency ARray (LOFAR). We used the low-band antennas ...of the LOFAR core stations to observe 87 pulsars out of 158 that had been previously detected using high-band antennas. We present flux densities and flux-calibrated profiles for the 43 pulsars we detected. Of this sample, 17 have not, to our knowledge, previously been detected at such low frequencies. Here we recalculate the spectral indices using the new low-frequency flux density measurements from the LOFAR census and discuss the prospects of studying pulsars at very low frequencies using current and upcoming facilities, such as the New Extension in Nançay Upgrading LOFAR (NenuFAR).
We have used the LOw‐Frequency ARray (LOFAR) to image a few lightning flashes during a particularly severe thunderstorm. The images show an exceptional amount of VHF activity at altitudes above ...10 km. Much of this is in the form of small‐scale discharges, not exceeding a few hundred meter, occurring seemingly randomly around the centers of active storm cells. To emphasize the incidental nature of these small‐scale discharges or sparks we refer to them as “sparkles.” A detailed investigation shows evidence that these sparkles are indicative of positive leader channels and that they are equivalent to the needle activity seen around positive leader tracks at lower altitudes.
Plain Language Summary
At the height of the tropopause in very active lightning cells many seemingly unrelated small discharges have been observed. Using the LOw‐Frequency ARray radio telescope, mainly intended for astronomical observations, we have imaged these structures in unprecedented detail and found strong evidence that these are negative discharges that form around a network of positive leaders.
Key Points
In the tops of severe thunderstorms many seemingly isolated sources for VHF radiation are observed
The VHF sources in these tops are small negative discharges, conjectured to be related to an extensive positive leader structure
The charge‐layer structure in these tops is mixed
Abstract
The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of ...kilometers through the atmosphere forming conducting ionized channels called leaders. Since lightning is an electric discharge phenomenon, there are positively and negatively charged leaders. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily build for radio-astronomy observations. It is observed that a negative leader rather suddenly changes, for a few milliseconds, into a mode where it radiates 100 times more VHF power than typical negative leaders after which it spawns a large number of more typical negative leaders. This mode occurs during the initial stage, soon after initiation, of all lightning flashes we have mapped (about 25). For some flashes this mode occurs also well after initiation and we show one case where it is triggered twice, some 100 ms apart. We postulate that this is indicative of a small (order of 5 km
$$^2$$
2
) high charge pocket. Lightning thus appears to be initiated exclusively in the vicinity of such a small but dense charge pocket.
While repeating fast radio bursts (FRBs) remain scarce in number, they provide a unique opportunity for follow-up observations that enhance our knowledge of their sources and potentially of the FRB ...population as a whole. Attaining more burst spectra could lead to a better understanding of the origin of these bright, millisecond-duration radio pulses. We therefore performed ∼20 h of simultaneous observations on FRB 121102 with the Effelsberg 100 m radio telescope and the low frequency array (LOFAR) to constrain the spectral behaviour of bursts from FRB 121102 at 1.4 GHz and 150 MHz. This campaign resulted in the detection of nine new bursts at 1.4 GHz but no simultaneous detections with LOFAR. Assuming that the ratio of the fluence at two frequencies scales as a power law, we placed a lower limit of α > −1.2 ± 0.4 on the spectral index for the fluence of the instantaneous broad band emission of FRB 121102. For the derivation of this limit, a realistic fluence detection threshold for LOFAR was determined empirically assuming a burst would be scattered as predicted by the NE2001 model. A significant variation was observed in the burst repeat rate R at L-band. During observations in September 2016, nine bursts were detected, giving R = 1.1 ± 0.4 h−1, while in November no bursts were detected, yielding R < 0.3 h−1 (95% confidence limit). This variation is consistent with earlier seen episodic emission of FRB 121102. In a blind and targeted search, no bursts were found with LOFAR at 150 MHz, resulting in a repeat rate limit of R < 0.16 h−1 (95% confidence limit). Burst repeat rate ratios of FRB 121102 at 3, 2, 1.4, and 0.15 GHz are consistent within the uncertainties with a flattening of its spectrum below 1 GHz.
Here, we present new radio interferometer beamforming observations of lightning initiation using data from the Low‐Frequency Array (LOFAR). We show that the first lightning source in the flash ...increases exponentially in intensity by two orders of magnitude in 15 μs, while propagating 88 m away from the initiation location at a constant speed of 4.8 ± 0.1 × 106 m/s. A second source replaces the first source at the initiation location, and subsequent propagation of the lightning leader follows. We interpret the first source to be a rapidly propagating and intensifying positive streamer discharge that subsequently produces a hot leader channel near the initiation point. How lightning initiates is one of the greatest unsolved problems in the atmospheric sciences, and these results shed light on this longstanding mystery.
Plain Language Summary
Lightning initiation is poorly understood, in part, due to the difficulty is making detailed observations inside thunderstorms. This research elucidates the initiation process using highly sensitive imaging techniques from data acquired from the Low‐Frequency Array (LOFAR). These data indicate that lightning initiates with a cascading discharge composed of streamers before transitioning into a propagating leader. These streamers appear to propagate at a constant velocity, despite an exponential growth in number. This is an interesting result as it is not clear how it is possible to maintain both an exponential growth and a constant velocity.
Key Points
As seen in VHF, the first lightning signal detectable above background increases exponentially by two orders of magnitude in 15 μs
Initiation is likely caused by branching streamers with constant propagation speed of 4.8 × 106 m/s during the exponential ramp‐up phase
Mechanism is similar to narrow‐bipolar events, but much weaker in VHF power
Ultra-high-energy (UHE) cosmic neutrinos interacting with the Moon’s regolith generate particle showers that emit Askaryan radiation. This radiation can be observed from the Earth using ground-based ...radio telescopes like LOFAR. We simulate the effective detection aperture for UHE neutrinos hitting the Moon. Under the same assumptions, results from this work are in good agreement with previous analytic parameterizations and Monte Carlo codes. The dependence of the effective detection aperture on the observing parameters, such as observing frequency and minimum detection threshold, and lunar characteristics like surface topography have been studied. Using a Monte Carlo simulation, we find that the detectable neutrino energy threshold is lowered when we include a realistic treatment of the inelasticity, transmission coefficient, and surface roughness. Lunar surface roughness at large scales enhances the total aperture for higher observation frequencies (
ν
≥
1
GHz
) but has no significant effect on the LOFAR aperture. However, roughness at scales small compared to the wavelength reduces the aperture at all frequencies.
We report on ultra‐slowly propagating discharge events with speeds in the range 1–13 km/s, much lower than any known lightning process. The propagation speeds of these discharges are orders of ...magnitude slower than leader or streamer speeds, but faster than the ion drift speed. For one particular event, a lightning leader forms about 40 ms later within 50 m of the discharge, likely within the same high field region. A second slow event forms 9 ms prior to the initiation, and leads into the negative leader. Most slow events appear to not be directly involved with lightning initiation. This suggests that the classic streamer cascade model of initiation is not always a definitive process. In this work we describe these discharge events displaying unique behavior, their relation to common lightning discharges, and their implications for lightning initiation.
Plain Language Summary
While lightning is generally a very fast process, here we report on ultra‐slow discharges which may be a new and unexpected method of lightning initiation. These discharges travel at uncharacteristically low speeds and are observed in conjunction with lightning initiation in two cases, while in three different cases they are not. This indicates that these events are also evidence of failed lightning leader formation, which complicates the current understanding of how lightning initiates. Additionally, the velocity of these events is slow enough that in principle the propagation can be observed by the unaided eye ‐ challenging the colloquial notion of “fast as lightning.”
Key Points
The ultra‐slowly propagating events travel at speeds at least an order of magnitude slower than the slowest positive leaders
In one observed case, the slow propagation led directly into the formation of a lightning leader
In most cases, these discharges are not connected with lightning initiation
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