N. J. Schuch

Rare measurements of a sprite with halo event driven by a negative lightning discharge over Argentina

[1]xa0As part of a collaborative campaign to investigate Transient Luminous Events (TLEs) over South America, coordinated optical, ELF/VLF, and lightning measurements were made of a mesoscale thunderstorm observed on February 22–23, 2006 over northern Argentina that produced 445 TLEs within a ∼6 hour period. Here, we report comprehensive measurements of one of these events, a sprite with halo that was unambiguously associated with a large negative cloud-to-ground (CG) lightning discharge with an impulsive vertical charge moment change (ΔMQv) of −503 C.km. This event was similar in its location, morphology and duration to other positive TLEs observed from this storm. However, the downward extent of the negative streamers was limited to 25 km, and their apparent brightness was lower than that of a comparable positive event. Observations of negative CG events are rare, and these measurements provide further evidence that sprites can be driven by upward as well as downward electric fields, as predicted by the conventional breakdown mechanism.

Conjugate Point Equatorial Experiment (COPEX) Campaign in Brazil: Electrodynamics highlights on spread F development conditions and day-to-day variability

[1]xa0A Conjugate Point Equatorial Experiment (COPEX) campaign was conducted during the October–December 2002 period in Brazil, with the objective to investigate the equatorial spread F/plasma bubble irregularity (ESF) development conditions in terms of the electrodynamical state of the ionosphere along the magnetic flux tubes in which they occur. A network of instruments, including Digisondes, optical imagers, and GPS receivers, was deployed at magnetic conjugate and dip equatorial locations in a geometry that permitted field line mapping of the conjugate E layers to dip equatorial F layer bottomside. We analyze in this paper the extensive Digisonde data from the COPEX stations, complemented by limited all-sky imager conjugate point observations. The Sheffield University Plasmasphere-Ionosphere Model (SUPIM) is used to assess the transequatorial winds (TEW) as inferred from the observed difference of hmF2 at the conjugate sites. New results and evidence on the ESF development conditions and the related ambient electrodynamic processes from this study can be highlighted as follows: (1) large-scale bottomside wave structures/satellite traces at the equator followed by their simultaneous appearance at conjugate sites are shown to be indicative of the ESF instability initiation; (2) the evening prereversal electric field enhancement (PRE)/vertical drift presents systematic control on the time delay in SF onset at off-equatorial sites indicative of the vertical bubble growth, under weak transequatorial wind; (3) the PRE presents a large latitude/height gradient in the Brazilian sector; (4) conjugate point symmetry/asymmetry of large-scale plasma depletions versus smaller-scale structures is revealed; and (5) while transequatorial winds seem to suppress ESF development in a case study, the medium-term trend in the ESF seems to be controlled more by the variation in the PRE than in the TEW during the COPEX period. Competing influences of the evening vertical plasma drift in favoring the ESF development and that of the TEW in suppressing its growth are discussed, presenting a perspective on the ESF day-to-day and medium-term variabilities.

Ionospheric zonal velocities at conjugate points over Brazil during the COPEX campaign: Experimental observations and theoretical validations

[1]xa0We analyze in detail the zonal velocities of large-scale ionospheric plasma depletions over two conjugate stations inferred from OI 630 nm airglow all-sky images obtained during the Conjugate Point Equatorial Experiment (COPEX) campaign carried out in Brazil between October and November 2002. The conjugate stations were Boa Vista (BV) (geogr. 2.8N, 60.7W, dip angle 22.0°N) and Campo Grande (CG) (geogr. 20.5S, 54.7W, dip angle 22.32°S). Over Campo Grande, the zonal velocities were measured also by a system of spaced GPS scintillation receivers. The airglow zonal velocities at the conjugate sites were seen to agree very closely, except for a slightly increased velocity over CG which we attribute to the presence of the geomagnetic anomaly. The results show a high degree of alignment of the bubbles along the geomagnetic field lines during the bubble development phase and as the bubbles travel eastward, thereby suggesting that the neutral zonal wind effect in the zonal plasma motion is an integrated effect along the flux tube. The zonal velocities obtained from the GPS technique were always larger than those calculated by the airglow technique, which permitted observation of zonal plasma velocity shear between the altitudes of the airglow emitting layer and of the GPS scintillation. Theoretical ambient plasma zonal velocities calculated using the formulations by Haerendel et al. (1992) and Eccles (1998) are compared with the experimental results. Our results also reveal some degree of dependence of the zonal velocities on the solar flux (F10.7) and magnetic (Kp) indices during the COPEX period.

Observations of prolific transient luminous event production above a mesoscale convective system in Argentina during the Sprite2006 Campaign in Brazil

[1]xa0On the night of 22–23 February 2006, 444 transient luminous events (TLEs), 86% sprites, were observed above a prolific mesoscale convective system (MCS) over Argentina, as part of the third sprite campaign in Brazil. GOES infrared (IR) cloud top temperatures (Tc) and Tropical Rainfall Measuring Mission (TRMM) radar (PR) and microwave (TMI) data were used to investigate the MCS convective characteristics and their relationship with World Wide Location Network (WWLLN) detected cloud-to-ground (CG) lightning and TLE activity. The MCS had a minimum lifetime of 20 hours, 8.5 as a MCS, a maximum extent of ∼430,000 km2, and gusty winds of ∼39–50 km/h. It had several distinctive characteristics: exceptionally high TLE rate, multicellular structure with 19 distinguishable convective regions, and cloud tops temperatures (Tc) ∼10–20 °C higher than regular TLE-producing MCSs over the central USA and South America. Most TLEs occurred above “individual stratiform regions”, where Tc varied from −45 °C to −53 °C from the beginning to the end of the night, surrounding the areas of strong convections, with convective cores at Tc −59 °C to −74 °C, which did not extend up to or overshoot the tropopause, estimated at −75 °C (∼17.1 km) as normally observed for TLE-producing MCS in these regions. The moderated convection is contrary to the expectation that large charge production is accompanied by vigorous updrafts within deep convection that give rise to cold cloud overshooting tops, thus prompting a detailed study of this prolific TLE-producing thunderstorm. On the basis of a charge moment change threshold of 350 Ckm and estimated 5 km charge removal altitude, a lower threshold of ∼4,300 C/h was estimated for the hourly charge transfer rate necessary for the observed sprite production (383 events), which is twice the rate for an average TLE-producing MCS (70 events), also estimated. TMI/TRMM data for the storm at early development showed a low brightness temperature of 84 K, indicative of significant ice content, which is important for cloud electrification processes. We suggest that the unusually high incidence of TLEs in this moderately convective MCS may be related to other local geophysical phenomena such as a large tropospheric aerosol concentration due to smoke from forest fires. Satellite fire count data showed that there were ∼200 fires between 20 and 22 February immediately north of the MCS initiation region and a transport simulation with the Coupled Aerosol-Tracer Transport model from the Brazilian developments on Regional Atmospheric Modeling System (CATT/BRAMS) model showed a large PM2.5 aerosol concentration, 10,000 μg/m2 (column integrated), at the region where the MCS developed. The aerosols present in the smoke may have been a source of ice nuclei affecting the production of ice particles that get positively charged, accounting for the charge transfer rate necessary to originate the observed TLE production.

COMPARISON BETWEEN HALO CME EXPANSION SPEEDS OBSERVED ON THE SUN, THE RELATED SHOCK TRANSIT SPEEDS TO EARTH AND CORRESPONDING EJECTA SPEEDS AT 1 AU

We have compared characteristics of 38 halo coronal mass ejections observed on the Sun by the Large Angle and Spectrometric Coronagraph onboard SOHO with their corresponding counterparts observed near Earth by the magnetic field and plasma instruments onboard the ACE, WIND and SOHO satellites, in the period from January 1997 to April 2001. We only have selected events that have some associated interplanetary ejecta structure at 1xa0AU and we have compared the lateral expansion speeds of these halo CMEs and the corresponding ejecta speeds near Earth. We found that there is a high correlation between these two speeds. The results are very similar to the study done by Lindsay etxa0al. (1999) using observations made by Solwind and SMM coronagraphs, and Helios-1 and PVO plasma and interplanetary field data from the period of 1979 to 1988. Also, we reviewed the relation between the CME-related shock transit speed to Earth and the ejecta speeds near Earth. This kind of relation is very important to estimate ejecta speeds of events for which no interplanetary observations are available.

Great geomagnetic storms in the rise and maximum of solar cycle 23

Geomagnetic storms are intervals of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial injection of energy into the magnetosphere-ionosphere system, to an intensified ring current, strong enough to exceed some key threshold of the quantifying storm time Dst index. We have studied all the 9 great magnetic storms (peak Dst < -200 nT) observed during the rise and maximum of solar cycle 23 (from 1997 to early 2001), in order to identify their solar and interplanetary causes. Apart of one storm occurred during the period without observations from the Solar and Heliospheric Observatory (SOHO), all of them were related to coronal mass ejections observed by the Large Angle and Spectroscopic Coronagraph (LASCO). The sources of interplanetary southward magnetic field, Bs, responsible for the occurrence of the storms were related to the intensified shock/sheath field, interplanetary magnetic clouds field, or the combination of sheath-cloud or sheath-ejecta field. It called our attention the fact that one of the events was related to a slow CME, with CME expansion speed not greater than 550 km/s. The purpose of this paper is to address the main sources of large geomagnetic disturbances using the current satellite capability available. As a general conclusion, we found that shock/sheath compressed fields are the most important interplanetary causes of great magnetic storms during this period.

A Very Active Sprite-Producing Storm Observed Over Argentina

During the night of 22–23 February 2006, more than 400 middle-atmospheric optical discharges were observed above one large thunderstorm system over northeastern Argentina. These transient luminous events (TLEs) were imaged during the Southern Brazil Sprite Campaign, the first campaign to focus on TLEs over southern Brazil, northeastern Argentina, and Uruguay. All of the TLEs were imaged from the Brazilian Southern Space Observatory (SSO) near Santa Maria, which is nearly in the center of the southernmost Brazilian state of Rio Grande do Sul. Although the fields of view of the imaging cameras were too narrow to view the entire storm, the more than 400 confirmed TLEs imaged indicate that this storm ranks as the third most active TLE producer ever reported. Hence, storms in this region of South America might be some of the leading TLE generators on Earth.

E region electric field dependence of the solar activity

We have being studying the zonal and vertical E region electric field components inferred from the Doppler shifts of type 2 echoes (gradient drift irregularities) detected with the 50u2009MHz backscatter coherent radar set at Sao Luis, Brazil (SLZ, 2.3°S, 44.2°W) during the solar cycle 24. In this report we present the dependence of the vertical and zonal components of this electric field with the solar activity, based on the solar flux F10.7. For this study we consider the geomagnetically quiet days only (Kpu2009≤u20093+). A magnetic field-aligned-integrated conductivity model was developed for proving the conductivities, using the IRI-2007, the MISIS-2000, and the IGRF-11 models as input parameters for ionosphere, neutral atmosphere, and Earth magnetic field, respectively. The ion-neutron collision frequencies of all the species are combined through the momentum transfer collision frequency equation. The mean zonal component of the electric field, which normally ranged from 0.19 to 0.35u2009mV/m between the 8 and 18u2009h (LT) in the Brazilian sector, show a small dependency with the solar activity. Whereas the mean vertical component of the electric field, which normally ranges from 4.65 to 10.12u2009mV/m, highlights the more pronounced dependency of the solar flux.

Thermospheric F-region travelling disturbances detected at low latitude by an OI 630 nm digital imager system

Abstract Gravity wave effects in the nocturnal thermospheric F -region domain are seldom detected in the intertropical region by optical (airglow) techniques, especially during geomagnetically quiet times, in part because the low inclination of the magnetic field, as opposed to the case of the mid-latitude region, does not favor significant vertical excursions of ionospheric plasma in response to meridional winds. Such difficulty in detecting gravity wave signatures in the F -region by means of optical techniques tends to increase in the absence of geomagnetic storms because of the lack of strong forcing mechanisms necessary to generate high intensity gravity waves. The purpose of this work is to show that during the quiet day of 9 August 1999, the Terminator may have been a source region of wave-like disturbances in the nocturnal F -region at the low-latitude station Cachoeira Paulista (22°41S; 45°00W, dip 30°). A digital all-sky OI 630nm imager system located at that station has shown propagating wave-like spatial structures in the airglow intensity near the Terminator. This observation supports a previous study on the evidence of the presence of gravity waves during the post-sunset period at the same location by means of a scanning photometer system (1997, Sobral, J. Atmos. Terr. Phys. 59 , 1611–1623). The absence of range-type spread- F as monitored by a local digisonde and the absence of radio wave scintillation as monitored by a local GPS receiver, excludes the hypothesis that the wave-like airglow structures are associated with the occurrence of the ionospheric plasma bubbles. Downwards motion of the iso-density real height contours at 22.0 ms −1 and 33.1 ms −1 are observed. The wave detection by the imager system is reported and discussed here.

Equatorial E region electric fields at the dip equator: 1. Variabilities in eastern Brazil and Peru

The equatorial electrojet (EEJ) is an intense eastward ionospheric electric current centered at about 105u2009km of altitude along the dip equator, set up by the global neutral wind dynamo that generates the eastward zonal (Ey) and the daytime vertical (Ez) electric fields. The temporal variation of the EEJ is believed to be well understood. However, the longitudinal variability of the Ey and Ez between 100 and 110u2009km is still quite scarce. Due to their importance overall phenomenology of the equatorial ionosphere, we investigate the variabilities of the Ey and Ez inferred from measurements of the Doppler frequency of Type II echoes provided by coherent backscatter radars installed in locations close to the magnetic equator in the eastern Brazil (2.33°S, 44.20°W) and Peru (11.95°S, 76.87°W). This study is based on long-term (609u2009days for both systems) radar soundings collected from 2001 to 2010. The variabilities of the electric fields are studied in terms of the position of the soundings with respect to the dip equator and the magnetic declination angle. Among the results, Ey and Ez show longitudinal dependence, being higher in Peru than east Brazil. Under quiet geomagnetic activity, the mean diurnal variations of Ey ranged from 0.21 to 0.35u2009mV/m between 8 and 18u2009h (LT) in Brazil and from 0.23u2009mV/m to 0.45u2009mV/m in Peru, while the mean diurnal variations of the Ez ranges from 7.09 to 8.80u2009mV/m in Brazil and from 9.00 to 11.18u2009mV/m in Peru.