Local Time Occurrence Research Articles

Westward PPEF Plays Important Role in the Suppression of Post‐Midnight Plasma Irregularities: A Case Study of the November 2021 Geomagnetic Storm

AbstractWe use multiple instruments data to investigate the behavior of the equatorial and low‐latitude ionosphere during the geomagnetically active and quiet period of November 1–6, 2021. In this context, total electron content (TEC) data obtained from the Global Positioning System (GPS) receivers in the equatorial and low‐latitude regions of Asia, Africa, and America are used to assess variations in plasma density during the storm. The storm‐time ionization levels were found to vary significantly in the crests of the Equatorial Ionization Anomaly (EIA) region over the 3 longitudes. The Rate of Change TEC Index (ROTI) derived from GPS receiver measurements, is used to study the equatorial/low‐latitude ionospheric plasma irregularities at various longitudes under geomagnetically quiet and disturbed conditions. Observations showed longitudinal variations in the ionospheric irregularities under both quiet and disturbed conditions. Some days exhibit a decrease in the strength of the midnight plasma irregularities toward the East, that is, the irregularities are more pronounced in West America, less common in East America, and almost non‐existent in Africa and Asia. Our investigations show this storm prevented the occurrence of plasma irregularities at the equatorial/low‐latitude region in the American sector during the night following the main phase. In general, no significant storm effects were observed at the target locations in Africa and Asia. The existence of westward Prompt Penetration Electric Field (PPEF) and the Equatorial Electrojet (EEJ) during the main phase, from midnight to noon, is clearly related with the constriction of plasma diffusion and the consequent suppression of plasma irregularities. Thus, the longitudinal dependence for the generation of midnight plasma irregularities during this storm is mainly influenced by local time occurrence of maximum ring current, and the ionospheric electric fields.

Ground and Space-based response of the ionosphere during the geomagnetic storm of 02–06 November 2021 over the low-latitudes across different longitudes

The strong geomagnetic storm of class G3 that occurred on 03 November 2021 (Dst min = −118 nT, Kp = 8) was the first of this class recorded during the ascending phase of solar cycle 25. In this study, we examined the response of the ionosphere during the geomagnetic storm period from 02–06 November 2021 using the Total Electron Content (TEC) observations from a chain of Global Navigation Satellite System (GNSS) receiver stations in five longitudinal sectors i.e, Asian, East-African, West-African, South-American, and Pacific-west sectors, in the low-latitudes and space-based Swarm satellite electron density, Ne and TEC measurements. The Rate of Change of TEC Index (ROTI) was derived from the ground and space based TEC measurements to examine the occurrence of ionospheric irregularities. The Rate of change of electron density index (RODI) was also derived from Swarm Ne measurements to identify the occurrence of ionospheric irregularities at Swarm altitudes. A positive ionospheric storm effect was observed in all the longitudinal sectors considered in this study. A clear hemispherical asymmetry, with higher VTEC in the northern hemisphere was observed. The determining factors for ionospheric responses to this storm are; local time of the storm’s onset, local time of storm’s minimum SYM-H, and changes in thermospheric O/N2 ratio. These geomagnetic storm effects were discussed in terms of the Prompt Penetration Electric Field (PPEF) storm induced wind lifting effect and Disturbance Dynamo Electric Field (DDEF). The geomagnetic storm inhibited the occurrence of ionospheric irregularities in all the sectors, during the main phase, except for three IGS receiver stations in South American sector. Overall, a longitudinal dependence of the enhancement/inhibition of ionospheric irregularities was observed. The generation of post-sunset irregularities was attributed to the local time occurrence of maximum ring current, PPEF and DDEF.

Effect of Solar Wind Pressure and Substorm Linked Particle Injection on Local Time Distribution of Electromagnetic Ion Cyclotron Waves

Several observations of electromagnetic ion cyclotron (EMIC) waves have been reported from the Earth’s magnetosphere. It is an important component of space weather research due to its efficient role in precipitating highly energetic electrons from the Earth’s radiation belt. In general, the interplanetary solar wind conditions and geomagnetic activity control the generation of EMIC waves by modifying the ambient plasma conditions in the generation region. Therefore, EMIC wave occurrence patterns observed both in space and on the ground are influenced by the variations in the parameters like auroral electrojet (AE index), solar wind dynamic pressure, and Dst, which represent the magnetospheric conditions. We present a detailed local time distribution of EMIC waves observed at the Antarctic station, Maitri (geographic 70.7oS, 11.8oE, L = 5), for 2011–2017. We have cataloged the EMIC wave occurrence in four frequency ranges (between 0.12–2 Hz) and examined their occurrence pattern for different levels of AE index and solar wind pressure. The present analysis reveals that at the ground station, Maitri, the occurrence of EMIC waves is dominated in the lower frequency range (0.12–1 Hz) with lower occurrence in the higher frequency range (>1Hz). It is found that the EMIC waves having frequencies > 1 Hz occur dominantly in the early morning hours with a peak close to 5.7 LT hours, and they are linked to the magnetic activity that occurred in the preceding days, whereas the effect of solar wind dynamic pressure and AE index on the local time occurrence of EMIC waves is unambiguously seen for the waves with <1 Hz. The study revealed the larger AE index shifts occurrence of EMIC waves in the dusk sector.

Influence of Nonmigrating Tides and Geomagnetic Field Geometry on the Diurnal and Longitudinal Variations of the Equatorial Electrojet

AbstractIn this study, equatorial electrojets (EEJs) simulated by the Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIE‐GCM) were compared with those observed by ground‐based magnetometers and the Challenging Minisatellite Payload (CHAMP) satellite. Simulations were performed with or without nonmigrating tidal forcing at the lower boundary and in the frame of the International Geomagnetic Reference Field (IGRF) or dipole field. The model forced by the nonmigrating tides could qualitatively reproduce the local time and longitudinal variation of an EEJ in the frame of the IGRF with accuracy. The tides were important in causing the later local time occurrence of the EEJ peak in the Peruvian sector, and they favored the occurrence of the morning counterelectrojet (CEJ) in the frame of the IGRF. The difference in the EEJ peak intensity between the Peruvian and Indian stations was enhanced due to the tidal effect. The net effect of the tides on the two‐station difference was almost doubled in the dipole field compared with that in the IGRF. The TIE‐GCM with a tidal input could reproduce a Wavenumber‐4 longitudinal structure of the noontime EEJ, but it underestimated the peak EEJ in the East Asia sector. The magnetically eastward electric field and the neutral zonal wind at noontime in the E region in the TIE‐GCM were both weak in the East Asia sector. This model‐data discrepancy was at least partly due to insufficient lower boundary forcing in this longitudinal sector.

Longitudinal variability of occurrence of ionospheric irregularities over the American, African and Indian regions during geomagnetic storms

This study presents the longitudinal variabilities of occurrence of ionospheric irregularities over the equatorial/low-latitude region of America, Africa and Indian sectors. Five major geomagnetic storms of the equinoctial months of the year 2012–2013 were analyzed for this study. Total Electron Content (TEC) data obtained from Global Navigation Satellite Systems (GNSS) over American, African and Indian longitudinal sectors were used to derive, the rate of change of TEC index (ROTIave), the ionospheric irregularities proxy. Longitudinal variability of occurrence of irregularities were discussed with respect to the local time when the maximum negative of SYM-H index occurs and polarity of IMF Bz in the dusk hours. Almost all of the storms under investigation inhibited the occurrence of irregularities in Indian sector. Out of five storms, two and three storm periods revealed inhibition in the generation of irregularities over the American and African sectors, respectively. Most of the generation of ionospheric irregularities observed in this study were related to post-midnight local time occurrence of the maximum negative excursion of SYM-H index. There were also cases where irregularities are observed when the minimum value of SYM-H occurred in the pre-midnight and daytime period. It appears that the local time at which minimum SYM-H occur is not a sufficient condition to explain the effect of storms on generation/inhibition of ionospheric irregularities. Significant longitudinal variability in the generation of irregularities were also observed over wide longitudinal ranges (South America, Africa and India) during 09 March 2012 and 17 March 2013 storm events. Difference in longitudinal variation of storm-time electric field were a suggested reason.

Interchange Injections at Saturn: Statistical Survey of Energetic H+ Sudden Flux Intensifications

AbstractWe present a statistical study of interchange injections in Saturn's inner and middle magnetosphere focusing on the dependence of occurrence rate and properties on radial distance, partial pressure, and local time distribution. Events are evaluated from over the entirety of the Cassini mission's equatorial orbits between 2005 and 2016. We identified interchange events from CHarge Energy Mass Spectrometer (CHEMS) H+ data using a trained and tested automated algorithm, which has been compared with manual event identification for optimization. We provide estimates of interchange based on intensity, which we use to investigate current inconsistencies in local time occurrence rates. This represents the first automated detection method of interchange, estimation of injection event intensity, and comparison between interchange injection survey results. We find that the peak rates of interchange occur between 7 and 9 Saturn radii and that this range coincides with the most intense events as defined by H+ partial particle pressure. We determine that nightside occurrence dominates as compared to the dayside injection rate, supporting the hypothesis of an inversely dependent instability growth rate on local Pedersen ionospheric conductivity. Additionally, we observe a slight preference for intense events on the dawnside, supporting a triggering mechanism related to large‐scale injections from downtail reconnection. Our observed local time dependence paints a dynamic picture of interchange triggering due to both the large‐scale injection‐driven process and ionospheric conductivity.

Limb clouds and dust on Mars from images obtained by the Visual Monitoring Camera (VMC) onboard Mars Express

The Visual Monitoring Camera (VMC) onboard the Mars Express (MEx) spacecraft is a simple camera aimed to monitor the release of the Beagle-2 lander on Mars Express and later used for public outreach. Here, we employ VMC as a scientific instrument to study and characterize high altitude aerosols events (dust and condensates) observed at the Martian limb. More than 21,000 images taken between 2007 and 2016 have been examined to detect and characterize elevated layers of dust in the limb, dust storms and clouds. We report a total of 18 events for which we give their main properties (areographic location, maximum altitude, limb projected size, Martian solar longitude and local time of occurrence). The top altitudes of these phenomena ranged from 40 to 85 km and their horizontal extent at the limb ranged from 120 to 2000 km. They mostly occurred at Equatorial and Tropical latitudes (between ∼30°N and 30°S) at morning and afternoon local times in the southern fall and northern winter seasons. None of them are related to the orographic clouds that typically form around volcanoes. Three of these events have been studied in detail using simultaneous images taken by the MARCI instrument onboard Mars Reconnaissance Orbiter (MRO) and studying the properties of the atmosphere using the predictions from the Mars Climate Database (MCD) General Circulation Model. This has allowed us to determine the three-dimensional structure and nature of these events, with one of them being a regional dust storm and the two others water ice clouds. Analyses based on MCD and/or MARCI images for the other cases studied indicate that the rest of the events correspond most probably to water ice clouds.

Peculiar features of ionospheric F3 layer during prolonged solar minimum (2007–2009)

AbstractWe present the seasonal and local time occurrence of ionospheric F3 layer over Tirunelveli (geographic longitude 77.8°E, geographic latitude 8.7°N, dip 0.7°) during extremely low and prolonged solar activity period (2007–2009). Canadian Advanced Digital Ionosonde observations from this station are used in the present study. We find that the occurrence of F3 layer is nearly 3 times higher during 2009 (∼ 48%) as compared to that during 2007 (∼16%). The increase of this order just within the low solar activity period is unusual. In earlier studies similar increase in F3 occurrence has been reported when solar activity changes from high (F10.7=182) to low (F10.7=72). The other important feature is the presence of postnoon F3 layers which are observed dominantly during summer solstice of 2009. Such occurrence of postnoon F3 layers was nearly absent during summer solstice of the previous solar minimum (1996) over nearby dip equatorial station Trivandrum. We take equatorial electrojet (EEJ) as a proxy for eastward electric field. It is noticed that the EEJ strength and the maximum rate of change of EEJ are higher for F3 days as compared to those on non‐F3 days. We find that the peak occurrence of prenoon F3 layer closely coincides with the time of maximum rate of change of EEJ. It is in general accordance with the theory proposed by Balan et al. (1998) that suggests the formation of F3 through vertically upward E × B drift in presence of equatorward neutral wind. The present study reveals that the rate of change of eastward electric field (dE/dt) as well plays an important role in the formation of F3 layer.

Occurrence of blanketing E<sub>s</sub> layer (E<sub>sb</sub>) over the equatorial region during the peculiar minimum of solar cycle 24

Abstract. A thin and highly dense sporadic E layer, which can occasionally block the upper ionospheric layers, is called blanketing sporadic E (Esb). We present the statistical seasonal local time occurrence pattern of Esb at equatorial station Tirunelveli (8.7° N, 77.8° E, dip latitude 0.7° N) during the extended minimum of solar cycle 24 (2007–2009). In spite of nearly the same average solar activity during both 2007 and 2009, considerable differences are noticed in the seasonal occurrence of Esb during this period. The percentage of Esb occurrence is found to be the highest during the summer solstice (≥ 50%) for both 2007 and 2009, which is in general accordance with the earlier studies. The occurrences of Esb during the vernal equinox (~ 33%) and January–February (~ 28%) are substantial in 2009 as compared to those during the same seasons in 2007. We find that, during winter (January–February), ~ 75% of Esb occurred during or just after the period of sudden stratospheric warming (SSW). We suggest that enhanced Esb occurrence during winter (January–February) and the vernal equinox of 2009 could be associated with SSW-driven changes in the E region ambient conditions. Furthermore, the close association of Esb with counter equatorial electrojet (CEEJ) suggested by earlier studies is re-examined carefully using the scenario of Esb occurrence on non-CEEJ days. Such an exercise is crucial as we are unaware whether the physical mechanisms driving Esb and CEEJ are linked or not. We find that, of all the seasons, the association of Esb and CEEJ is strongest during winter (November–December).

Magnetopause surface oscillation frequencies at different solar wind conditions

Abstract. Statistical analyses of the magnetopause (MP) motion observed by THEMIS suggested that the MP oscillates preferably at some prominent (sometimes called "magic") frequencies, which were found to stand out also in ground-based and ionospheric measurements of geomagnetic ultra-low frequency pulsations. In this paper we present an extension to these statistical analyses of the observed MP oscillations examining their dependence on the prevalent interplanetary magnetic field (IMF), solar wind (SW) flow speed and cone angle conditions as well as their local time of occurrence. Our results show enhanced oscillation activity at these frequencies in the noon local time sector during periods of northward IMF, slow or moderate SW speed and low SW cone angle. This combination of conditions supports an interpretation in terms of standing Alfvénic Kruskal-Schwarzschild surface modes on the MP.

First Southern Hemisphere common-volume measurements of PMC and PMSE

[1] Using Rayleigh lidar and Mesosphere-Stratosphere-Troposphere radar at Davis Antarctica (68.6°S) during the 2005–2006 austral summer, we obtained the first Southern Hemisphere common-volume measurements of Polar Mesospheric Clouds (PMC) and Polar Mesosphere Summer Echoes (PMSE). PMC were observed with 6% probability during 349 hours of lidar observations. PMSE were simultaneously observed during 70% of the PMC events. The mean centroid altitudes of simultaneous PMC and PMSE were 83.97 ± 0.10 km and 85.31 ± 0.14 km, respectively. The altitude difference (∼1.3 km) is comparable to earlier boreal common-volume measurements. Both phenomena show clear diurnal variations in altitude and occurrence. PMC had a unimodal local time occurrence distribution which peaked 4–6 hours after solar midnight. PMSE were most prevalent at this time, but had a secondary occurrence peak ∼6 hours after solar noon when PMC were absent. We explain the altitude-time distributions of PMC and PMSE in terms of the thermal structure of the mesopause region determined using measurements from the Aura satellite.

Generation of transient dayside subauroral proton precipitation

The Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft obtained the first global images of the proton aurora. One of the discoveries from these images was a region of proton precipitation equatorward of the nominal auroral oval. This precipitation can be observed for ∼10 min immediately following a large solar wind pressure pulse. Various mechanisms have been proposed for producing this precipitation. This study focuses on one of the proposed mechanisms, the scattering and precipitation of protons due to interaction with electromagnetic ion cyclotron (EMIC) waves. Using data from the IMAGE spacecraft FUV and EUV imagers, in situ ring current data from the Los Alamos geosynchronous spacecraft, and in situ solar wind data from the ACE spacecraft, two features of the precipitation, the local time occurrence and limited latitudinal extent, are explained. The local time occurrence of the precipitation is correlated with the orientation of the pressure pulse front in the solar wind. The limited latitudinal extent of the emissions appears to be related to the interaction between the hot ring current protons and the cold plasmaspheric ions that gives rise to EMIC wave growth.

Local time dependence of the equatorial counter electrojet effect in a narrow longitudinal belt

Irregular occurrence characteristics of the equatorial counter electrojet phenomena at different longitudinal zones are investigated using the magnetic data of the horizontal component ‘H’ at the two equatorial stations Trivandrum and Addis Ababa, separated in longitude by about 40°. The limited longitudinal extent of the counter electrojet (CEJ) events are examined on the basis of the occurrence of the phenomenon at different local time intervals. Importance of the solar semi diurnal tides in modifying the equatorial counter electrojet phenomena at Trivandrum and Addis Ababa is discussed during these local time groups. Dominance of semi-diurnal tidal modes of varying magnitude is found to influence the local time occurrence behaviour of CEJ events over the equator. Peculiarities of both the change in amplitude and phase pattern of the diurnal variation of the ‘H’ component within the narrow longitude sector, especially during the noon hours, indicate the modulation of the diurnal variation of the equatorial electrojet currents by vertical wind shears of highly localised nature.

Angular distributions of suprathermal electrons observed at geosynchronous orbit

Six months of low‐energy electron plasma data collected using the Los Alamos National Laboratory magnetospheric plasma analyzer (MPA) onboard the geosynchronous satellite 1989–046 have been surveyed. The MPA instrument measures the three‐dimensional energy per unit charge distributions of cold ions and electrons, allowing for the simultaneous determination of the angular distribution and ambient plasma regime. Suprathermal electrons in the energy range 15 to 200 eV were characterized by local time of occurrence, angular distribution, and ambient plasma regime. Results indicate a local time dependence in angular distributions, with trapped distributions, i.e., enhanced fluxes of particles with pitch angles near 90° and 270°, primarily being observed in the morning, coincident trapped and field‐aligned angular distributions (enhanced fluxes of particles with pitch angles near 90° and 270° coincident with enhanced fluxes of particles with pitch angles near 0° and 180°) occurring around noon, a lack of detectable low‐energy electron fluxes near dusk, and a complex combination of angular distributions on the nightside. When both trapped and field‐aligned angular distributions are present, the field‐aligned component generally has lower energy than the trapped component. Dayside field‐aligned angular distributions are interpreted as being photoelectrons from the ionosphere, while trapped angular distributions are from the low‐energy tail of the plasma sheet distribution. A plasma regime dependence in angular distributions was also observed. Plasma sheet angular distributions are generally isotropic or trapped. The plasma trough angular distributions are trapped or coincident trapped and field‐aligned. Plasmaspheric electrons commonly have energies below our 15 eV threshold.

A reexamination of the local time asymmetry of lobe encounters at geosynchronous orbit: CRRES, ATS 5, and LANL observations

The local time of occurrence of a geosynchronous plasma flux dropout or lobe encounter is dependent on the longitude of the observing spacecraft. Satellites located on or west of the International Geomagnetic Reference Field (IGRF) model magnetic dip prime meridian (magnetic dip meridian) observe almost exclusively (77%) dawnside lobe encounters, while satellites located east of the magnetic dip meridian observe mostly duskside lobe encounters (67%). However, the transition from observing dawnside or duskside lobe encounters is not symmetric about the magnetic dip meridian. Three satellites (ATS 5, 1991‐0180, and 1990‐095) located essentially at the magnetic dip meridian observed a majority (73%) of dawnside encounters. This study therefore concludes that there is a dawn‐dusk asymmetry of the synchronous orbit altitude magnetic equatorial plane during active geomagnetic periods with the dawnside magnetosphere experiencing more radical distortions than the duskside. A case study of near‐simultaneous lobe observations from CRRES and 1989‐046 supports this conclusion. Note that it is found that the IGRF model dip prime meridian better organizes the local time occurrence frequency than does the magnetic dipole magnetic meridian, which has been shown to organize the seasonal dependence of geosynchronous lobe encounters in previous studies (i.e., Thomsen et al., 1994). Hence this study emphasizes the importance of considering the relative position of satellites with respect to both the magnetic dipole equator and the geomagnetic equator when utilizing geosynchronous plasma, magnetic field, and energetic particle data as diagnostics of the energy state of the magnetosphere.

Substorm associated radar auroral surges: a statistical study and possible generation model

Abstract. Substorm-associated radar auroral surges (SARAS) are a short lived (15–90 minutes) and spatially localised (~5° of latitude) perturbation of the plasma convection pattern observed within the auroral E-region. The understanding of such phenomena has important ramifications for the investigation of the larger scale plasma convection and ultimately the coupling of the solar wind, magnetosphere and ionosphere system. A statistical investigation is undertaken of SARAS, observed by the Sweden And Britain Radar Experiment (SABRE), in order to provide a more extensive examination of the local time occurrence and propagation characteristics of the events. The statistical analysis has determined a local time occurrence of observations between 1420 MLT and 2200 MLT with a maximum occurrence centred around 1700 MLT. The propagation velocity of the SARAS feature through the SABRE field of view was found to be predominately L-shell aligned with a velocity centred around 1750 m s–1 and within the range 500 m s–1 and 3500 m s–1. This comprehensive examination of the SARAS provides the opportunity to discuss, qualitatively, a possible generation mechanism for SARAS based on a proposed model for the production of a similar phenomenon referred to as sub-auroral ion drifts (SAIDs). The results of the comparison suggests that SARAS may result from a similar geophysical mechanism to that which produces SAID events, but probably occurs at a different time in the evolution of the event.Key words. Substorms · Auroral surges · Plasma con-vection · Sub-auroral ion drifts

Substorm associated radar auroral surges: a statistical study and possible generation model

Substorm-associated radar auroral surges (SARAS) are a short lived (15–90 minutes) and spatially localised (~5° of latitude) perturbation of the plasma convection pattern observed within the auroral E-region. The understanding of such phenomena has important ramifications for the investigation of the larger scale plasma convection and ultimately the coupling of the solar wind, magnetosphere and ionosphere system. A statistical investigation is undertaken of SARAS, observed by the Sweden And Britain Radar Experiment (SABRE), in order to provide a more extensive examination of the local time occurrence and propagation characteristics of the events. The statistical analysis has determined a local time occurrence of observations between 1420 MLT and 2200 MLT with a maximum occurrence centred around 1700 MLT. The propagation velocity of the SARAS feature through the SABRE field of view was found to be predominately L-shell aligned with a velocity centred around 1750 m s–1 and within the range 500 m s–1 and 3500 m s–1. This comprehensive examination of the SARAS provides the opportunity to discuss, qualitatively, a possible generation mechanism for SARAS based on a proposed model for the production of a similar phenomenon referred to as sub-auroral ion drifts (SAIDs). The results of the comparison suggests that SARAS may result from a similar geophysical mechanism to that which produces SAID events, but probably occurs at a different time in the evolution of the event.Key words. Substorms · Auroral surges · Plasma con-vection · Sub-auroral ion drifts

Cloud‐to‐ground lightning flash characteristics in southeastern Brazil for the 1992–1993 summer season

A lightning positioning and tracking system (LPATS) has been used for the first time to obtain the cloud‐to‐ground flash characteristics in southeastern Brazil during the 1992–1993 summer season. About 300,000 flashes to ground were recorded. The lightning data are presented in terms of polarity, multiplicity, first‐stroke peak current, stroke of maximum peak current, and local time occurrence. The preliminary results are compared with lightning data obtained by several authors in other parts of the world.

Local time occurrence frequency of energetic ions in the Earth's magnetosheath

The occurrence frequency of energetic ions in the energy range from 8 to 40 keV/e as a function of local time in the magnetosheath is presented. Energetic ions are observed in the magnetosheath with a minimum probability of 25% for all local times. The occurrence frequency for the energetic ions is higher on the dawnside than on the duskside but shows a relative maximum post‐noon in the local time range from 12 to 15 hours. The post‐noon relative maximum is attributed to a magnetospheric source for the energetic ions while the dawn‐dusk asymmetry is attributed to a quasi‐parallel bow shock source.

The Ashen Light

The Ashen Light first reported in 1643 is one of the oldest unsolved mysteries of the solar system. It consists of a faint luminosity on the nightside of the planet, similar in appearance to “earthshine” on the Moon but generally not as bright. Ashen Light is most often reported when Venus is in the evening sky at which time the evening terminator of Venus is toward the Earth. While some of this local time asymmetry may be associated with a tendency of observers to view Venus more in the evening hours than in the morning hours, we believe the asymmetry is too great to be explained by terrestrial effects alone. Thus, it appears that Ashen Light is principally a dusk side phenomenon on Venus. To our knowledge there is only one phenomenon that shows a marked dawn-dusk asymmetry on Venus and that could also explain the occurrence of Ashen Light. That phenomenon is Venus lightning whose local time of occurrence has been inferred from impulsive VLF waves observed by the Pioneer Venus Orbiter in the night ionosphere. The following factors appear to affect the visibility of Ashen Light from Earth: the distance of Venus from the Earth: the length of time Venus is above the horizon and the local time distribution of the Ashen Light source on Venus. Presently an Ashen Light campaign undertaken by amateur observers worldwide is underway. This campaign should help resolve whether the reported local time asymmetry is indeed intrinsic to the planet.