| 1. |
- Martin-Torres, Javier, et al.
(författare)
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Modelling of the non-LTE populations of the nitric acid and methane vibrational states in the middle atmosphere
- 1998
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - 1364-6826 .- 1879-1824. ; 60:17, s. 1631-
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Tidskriftsartikel (refereegranskat)abstract
- A modelling of the non-LTE populations of the HNO3 and CH4 vibrational levels in the middle atmosphere has been carried out and the results are presented. The work is oriented to assess the potential impact of non-LTE effects on the remote sensing of these gases. The models developed for this purpose include a complete set of radiative and collisional processes. In order to cover typical and extreme remote sensing scenarios, the models have been applied to different atmospheric and solar illumination conditions. The vibrational levels responsible for the major emissions of HNO3 are found to be in LTE up to the lower mesosphere, driven by the dominant V-T processes with the air molecules. In the non-LTE region, the absorption from the warmer tropospheric layers and solar direct excitation produce small enhancements over the equilibrium populations. The mesospheric CH4 vibrational temperatures are mainly determined by two mechanisms: the radiative absorption of the upcoming radiation emitted by the lower layers of the atmosphere, and the near-resonant vibrational coupling between the CH4 levels and the first vibrationally excited level of O2. By day, non-LTE is significantly enhanced as a consequence of the collisional relaxation of overtone and combinational states excited by the solar radiation at 3.3 μm. The effects derived from the uncertainties in the parameters of the models have been studied.
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| 2. |
- Saito, A, et al.
(författare)
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Conjugate observations of the mid-latitude electric field fluctuations with the MU radar and the Freja satellite
- 1998
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - 1364-6826 .- 1879-1824. ; 60:1, s. 129-140
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Tidskriftsartikel (refereegranskat)abstract
- Conjugate Freja satellite and MU radar observations were conducted from 1993 to 1995 to clarify the generation mechanism of mid-latitude electric field fluctuations (MEFs) which were found from DE-2 satellite observations. The detection of MEFs at higher altitudes than 1000 km by the Freja satellite confirms that the MEFs are transmitted along the geomagnetic held line without any significant damping. On one night of such conjugate observations, MEFs were observed in a traveling ionospheric disturbance (TID). On the other nights when the MEFs were observed by Freja, large scale density structures in the ionosphere were observed with the MU radar. Spread-F phenomena were also observed by ionosondes associated with these structures. The large scale modulation of the ionosphere, such as TIDs, might cause small scale modulations which grow through ionospheric instabilities. The linear growth rate is evaluated using a set of equations describing the growth of the mid-latitude ionospheric irregularities. The growth rate is shown to be too small under average conditions; however, when the neutral wind in TIDs is included, it is large enough to generate MEFs. (C) 1998 Elsevier Science Ltd. All rights reserved.
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| 3. |
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| 4. |
- Ahmad, Noor Azlinda, et al.
(författare)
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Characteristics of narrow bipolar pulses observed in Malaysia
- 2010
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 72:5-6, s. 534-540
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Tidskriftsartikel (refereegranskat)abstract
- Narrow bipolar pulses (NBPs) are considered as isolated intracloud events with higher peak amplitude and strong high frequency emission compared to the first return strokes and other intracloud discharges. From 182 NBPs recorded in Malaysia in the tropic, 75 were narrow negative bipolar pulses (NNBPs) while 107 were narrow positive bipolar pulses (NPBPs). The mean duration of NNBPs was 24.6 +/- 17.1 mu s, while 30.2 +/- 12.3 mu s was observed for NPBPs. The mean full-width at half-maximum (FVVHM) was 2.2 +/- 0.7 and 2.4 +/- 1.4 mu s for NNBPs and NPBPs, respectively. The mean peak amplitude of NPBPs normalized to 100 km was 22.7 V/m, a factor of 1.3 higher than that of NNBPs which is 17.6 V/m. In contrast to the previous studies, it was observed that the electric field change was characterized by a bipolar pulse with a significant amount of fine structures separated by a few tens of nanoseconds intervals, embedded on it. (C) 2010 Elsevier Ltd. All rights reserved.
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| 5. |
- Ahmad, Noor Azlinda, et al.
(författare)
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The first electric field pulse of cloud and cloud-to-ground lightning discharges
- 2010
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 72:2-3, s. 143-150
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the first electric field pulse of cloud and cloud-to-ground discharges were analyzed and compared with other pulses of cloud discharges. Thirty eight cloud discharges and 101 cloud-to-ground discharges have been studied in this analysis. Pulses in cloud discharges were classified as [`]small', [`]medium' and [`]large', depending upon the value of their relative amplitude with respect to that of the average amplitude of the five largest pulses in the flash. We found that parameters, such as pulse duration, rise time, zero crossing time and full-width at half-maximum (FWHMs) of the first pulse of cloud and cloud-to-ground discharges are similar to small pulses that appear in the later stage of cloud discharges. Hence, we suggest that the mechanism of the first pulse of cloud and cloud-to-ground discharges and the mechanism of pulses at the later stage of cloud discharges could be the same.
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| 6. |
- Amyx, K., et al.
(författare)
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In-situ measurement of smoke particles in the wintertime polar mesosphere between 80 and 85 km altitude
- 2008
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826. ; 70, s. 61-70
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Tidskriftsartikel (refereegranskat)abstract
- The MAGIC sounding rocket, launched in January 2005 into the polar mesosphere, carried two detectors for charged aerosol particles. The detectors are graphite patch collectors mounted flush with the skin of the payload and are connected to sensitive electrometers. The measured signal is the net current deposited on the detectors by heavy aerosol particles. The collection of electrons and ions is prevented by magnetic shielding and a small positive bias, respectively. Both instruments detected a layer of heavy aerosol particles between 80 and 85 km with a number density approximately 103 cm−3. Aerodynamic flow simulations imply that the collected particles are larger than 1 nm in radius. The particles are detected as a net positive charge deposited on the graphite collectors. It is suggested that the measured positive polarity is due to the electrification of the smoke particles upon impact on the graphite collectors.
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| 7. |
- Asmus, H., et al.
(författare)
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Charge balance for the mesosphere with meteoric dust particles
- 2015
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 127, s. 137-149
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Tidskriftsartikel (refereegranskat)abstract
- An aerosol particle charging model initially developed for noctilucent cloud ice particles has been extended in several steps in order to better explain the data for charged meteoric smoke particles (MSPs) obtained by the nighttime and daytime CHAMPS rockets launched from Andoya, Norway, in October 2011. Addition of photodetachment to the model shows that this process reduces the number density of positively charged MSPs as well as the number density of negatively charged MSPs as a consequence of the photodetached electrons neutralizing the positively charged MSPs. In addition, the model shows that the ionization rate can be deduced from the electron number density and the electron-ion recombination rate only at the highest altitudes (those with ionization rates above 20 cm(-3) s(-1)) as a consequence of recombination on the MSPs being dominant at lower altitudes. The differences between the daytime and the nighttime rocket data suggest a photodetachment rate between 0.1 and 0.01 s(-1). A further extension of the model to include the formation of negative ions and their destruction helps explain the ledge seen in the number density of the lightest negatively charged particles. The MSP number densities that are the inputs to the charging model are taken from the CARMA/CHEM2D model. The CHAMPS data are more consistent with number densities generated with an assumed input flux from ablation of 4 t d(-1) than with 44 t d(-1) assumed previously.
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| 8. |
- Baharin, Shamsul Ammar Shamsul, et al.
(författare)
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Microwave radiation associated with positive narrow bipolar events
- 2023
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier. - 1364-6826 .- 1879-1824. ; 242
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we examined seven isolated positive Narrow Bipolar Events (NBEs), one positive NBE that initiated an IC flash progressed to a single-stroke Cloud-to-Ground (CG) flash, and one positive NBE that initiated an IC flash. Seven NBEs have been accompanied by significant Very-High Frequency (VHF) and microwave radiation pulses. We recorded all NBEs from two measurement stations (ST1 and ST2) separated at 13.3 km apart which consisted of fast antenna (FA) and slow antenna (SA) sensors, a magnetic field (B-field) sensor, a VHF sensor (60 MHz), and a microwave sensor (0.97 GHz). The waveforms were sampled at 2.5 GHz (400 ps). The key finding is that all microwave radiation pulses have been found to precede both the VHF radiation pulses and NBEs with average lead time of 63 +/- 39 ns and 122 +/- 143 ns, respectively. In comparison to stepped leader pulses or SLPs (conventional breakdown), the average lead time of microwave to VHF for NBEs (fast breakdown) was 88% faster compared to the average lead time of microwave to VHF for SLPs. Moreover, the average lead time of VHF to NBEs was 56% faster when compared to the average lead time of VHF to SLPs. The VHF interferometer map for an isolated NBE (NBE6) showed upward propagation of VHF radiation sources (fast negative breakdown) with initiation altitude, total length of the VHF radiation sources propagation, and estimated velocity were 10.2 +/- 0.3 km, 2.9 +/- 0.6 km, and 1.8 x 108 and 2.8 x 108 ms-1, respectively. On the other hand, interferometer map for an NBE that initiated a single-stroke CG (NBE3) showed bidirectional fast streamers propagation with initiation altitude, total length of the VHF radiation sources propagation, and estimated velocity were 14.0 +/- 0.4 km, 2.9 +/- 0.82 km, and 1.6 x 108 and 2.8 x 108 ms-1, respectively. Clearly, the microwave and VHF radiation pulses associated with positive NBEs have been emitted by different processes of fast breakdown mechanism. Therefore, it can be suggested that the microwave radiation is emitted by electron avalanches/corona while the VHF ra-diation is emitted by fast propagating streamers.
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| 9. |
- Baharin, Shamsul Ammar Shamsul, et al.
(författare)
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Very high frequency radiation emitted by negative narrow bipolar events occurred over malacca strait
- 2024
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier. - 1364-6826 .- 1879-1824. ; 259
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, Very -High Frequency (VHF) radiation pulses associated with 11 negative Narrow Bipolar Events (NBEs) produced by a tropical storm over Malacca Strait are examined. The lightning data were recorded from a measurement station (ST) which consisted of a fast antenna (FA) and three VHF sensors (two 5 m perpendicular baselines interferometer). The average rise time (RT), average zero -crossing time (ZCT), average pulse duration (PD), and range of peak currents of the negative NBEs were 1.4 +/- 0.4 mu s, 2.7 +/- 1.0 mu s, 12.0 +/- 6.9 mu s, and -10 to -64 kA, respectively. The key finding is that all VHF radiation pulses have been found to precede the negative NBEs with an average lead time of 0.7 +/- 0.3 mu s. An interferometer map for one negative NBE (labelled as NBE10) detected at 35.7 km from ST has shown a characteristic of mixed propagation direction of fast streamers. The first VHF radiation source was detected at 12.4 +/- 0.4 km above sea level. The total length and estimated velocity of the main propagation of the VHF radiation sources were 2.2 +/- 0.7 km and between 1.4 x 10 8 and 2.8 x 10 8 ms -1 , respectively. Moreover, based on the Himawari satellite image, the maximum extent of the cloud top height was estimated to be around 20.9 km over sea level (over Malacca Strait). All the VHF radiation sources associated with NBE10 were suggested to be detected above the main negative charge region (6 km altitude that corresponds to -10 degrees C). Thus, it could be suggested that NBE10 was initiated most likely in the environment of the ice crystals alone, based on the first altitude of the VHF radiation source and maximum extent of cloud top height.
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| 10. |
- Baharudin, Zikri A., et al.
(författare)
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Electric field changes generated by the preliminary breakdown for the negative cloud-to-ground lightning flashes in Malaysia and Sweden
- 2012
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 84-85, s. 15-24
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Tidskriftsartikel (refereegranskat)abstract
- We present the study of the electric field changes generated by the preliminary breakdown for negative cloud-to-ground lightning flashes in Malaysia and Sweden concerning the association of slow field changes associated in preliminary breakdown process. We examined the total of 1685 negative cloud-to-ground lightning flashes from the total of 39 thunderstorms by recording the slow electric field, fast electric field and narrowband radiation field at 3 and 30 MHz signals simultaneously. Our results show that there is a pre-starting time, i.e. the duration between the first preliminary breakdown pulse and slow field changes starting point, which is found to be after the first preliminary breakdown pulse. The pre-starting time has the arithmetic and geometric mean range from 1.4-6.47 and 1-3.36 ms, respectively. The mean values of pre-starting time in Malaysia are greater than the values observed in Sweden by more than a factor of 3. From the two data sets it shows that the slow field changes never start before the preliminary breakdown. Furthermore, the use of single-station electric fields measurement with high resolutions of 12 bits transient recorder with several nanosecond accuracy allow one to distinguish the slow field changes generated by preliminary breakdown, which preceded the negative first return stroke, between tens to hundreds of milliseconds of pre-return stroke duration.
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