Sunrise Effect Research Articles

The asymmetric sunrise effect on Thales’ alleged measurement of the Sun angular size

Abstract Reports from the second and third centuries AD attribute the first measurement of the angular size of the Sun to Thales of Miletus, in the sixth century BC. Cleomedes, also in the second century AD, described a method to perform the measurement based on timing the duration of the sunrise. Several modern authors have suggested that Thales used Cleomedes’ method, but others are skeptical of the connection. Here I present an objection that has not been discussed in the literature, namely, that the proportionality between the size of the Sun and the duration of sunrise is not constant, but changes with latitude and the time of the year, due to what I call the “asymmetric sunrise effect.” I show that this effect is large enough to have prevented Thales from obtaining the roughly accurate recorded value.

Understanding the Diurnal Cycle of Midlatitude Sporadic E. The Role of Metal Atoms

AbstractMidlatitude ionosonde observations show that there is a sporadic E (Es) diurnal cycle that starts in higher altitudes at sunrise. This property is labeled as the “Sporadic E sunrise effect.” Given that sporadic E layers are composed of metal ions, the sunrise effect implies that metal atom solar photoionization could play a role in the diurnal variability of sporadic E occurrence. This possibility is endorsed by Arecibo's incoherent scatter radar observations, showing that weak ion layers at lowest E and uppermost D region heights appear at sunrise to live during the daytime, apparently coming out of nighttime metal atom mesospheric layers. The solar photoionization of metal atoms increases the abundance of metal ions available for Es layer generation during the daytime, whereas this effect is absent at nighttime. This can explain why sporadic E layers start or intensify at sunrise all year round and why Es activity maximizes during sunlit hours, as has been reported in many ionosonde and satellite radio occultation studies. The significance of metal atom solar photoionization on the regular diurnal variation of Es went unnoticed, despite existing evidence for a long time. The present paper provides a base for a better physical understanding of the pronounced 24‐hr periodicity in Es layer intensity and places a step toward the improvement of simulation models for the predictions of sporadic E layer characteristics.

First observation of atmospheric electric field at Kashmir valley North Western Himalayas, Srinagar (India)

Preliminary results from the measurement of potential gradient (PG) for the period of March 2018–February 2019 at the University of Kashmir, Srinagar, (34° 03′ N, 74° 51’ E, 1585 m amsl) India is reported. The annual mean diurnal variation of PG represents typical double-maxima pattern with the primary maximum at 0900–1000 LT and the secondary maximum at 1900–2000 LT which is in accordance with the measurements at different continental stations. The primary maximum of PG and its extended duration is explained by the sunrise effect due to increasing surface temperature, the development of surface convection and hence the boundary layer probably favour the uplifting of the surface aerosols. The higher PG values measured at Kashmir valley were explained with the height of the measurement from the surface and the role of mountain-valley winds circulation. Monthly mean PG showed the higher value during December and minimum in June and the amplitude variations were attributed to the local PM concentrations. PG is found to be highly influenced by PM2.5 compared to PM10, due to the high suspension duration in the ambient air. Measurement of PM2.5 to Srinagar showed higher values in December followed by the November and January under the strong inversion layer formation in Kashmir valley with the westerly winds apart from an increase in the local tourist activity. The measured PG values and its observed variations are explained considering the role of aerosol concentration, atmospheric mixing and mountain-valley wind circulations. The present results are comparable with measurements of PG at different continental stations. The level of aerosols and site location shows variation in amplitude as well as temporal shift.

Comparative analysis of local and global atmospheric electric field at the Northern Pakistan

Daily variations of the Atmospheric Electric Field (AEF) allow to observe separately the atmospheric electricity due to global and local factors. Observations of the AEF are made for three different sites of Pakistan during fair weather conditions. These three sites are Islamabad (ISB), Muzaffarabad (MZF) and Balakot (BKT). Analyses of the diurnal variations of the atmospheric electric field for these sites are discussed in this paper with respect to annual and seasonal behavior. Muzaffarabad and Balakot sites show two daily maxima and Islamabad site shows single maxima, characteristic of local influences, such as the aerosol, sunrise effect and “Austausch effect” or exchange layer effect. Later, these results are compared globally with land based measurements available in the literature and with the Carnegie curve. Local effects found to be dominant among these sites with respect to one another due to geographical location, climate and different meteorological parameters. This is the first different creation of the ISB-MZF-BKT database which will be very effective to improve the information of atmospheric electricity in the complex processes due to weather and climate. This database from Pakistan will be useful for the scientific community for further understating and investigation of the Global Electric Circuit (GEC).

A global atmospheric electricity monitoring network for climate and geophysical research

The Global atmospheric Electric Circuit (GEC) is a fundamental coupling network of the climate system connecting electrically disturbed weather regions with fair weather regions across the planet. The GEC sustains the fair weather electric field (or potential gradient, PG) which is present globally and can be measured routinely at the surface using durable instrumentation such as modern electric field mills, which are now widely deployed internationally. In contrast to lightning or magnetic fields, fair weather PG cannot be measured remotely. Despite the existence of many PG datasets (both contemporary and historical), few attempts have been made to coordinate and integrate these fragmented surface measurements within a global framework. Such a synthesis is important in order to fully study major influences on the GEC such as climate variations and space weather effects, as well as more local atmospheric electrical processes such as cloud electrification, lightning initiation, and dust and aerosol charging.The GloCAEM (Global Coordination of Atmospheric Electricity Measurements) project has brought together experts in atmospheric electricity to make the first steps towards an effective global network for atmospheric electricity monitoring, which will provide data in near real time. Data from all sites are available in identically-formatted files, at both 1 s and 1 min temporal resolution, along with meteorological data (wherever available) for ease of interpretation of electrical measurements. This work describes the details of the GloCAEM database and presents what is likely to be the largest single analysis of PG data performed from multiple datasets at geographically distinct locations. Analysis of the diurnal variation in PG from all 17 GloCAEM sites demonstrates that the majority of sites show two daily maxima, characteristic of local influences on the PG, such as the sunrise effect. Data analysis methods to minimise such effects are presented and recommendations provided on the most suitable GloCAEM sites for the study of various scientific phenomena. The use of the dataset for further understanding of the GEC is also demonstrated, in particular for more detailed characterization of day-to-day global circuit variability. Such coordinated effort enables deeper insight into PG phenomenology which goes beyond single-location PG measurements, providing a simple measurement of global thunderstorm variability on a day-to-day timescale. The creation of the GloCAEM database is likely to enable much more effective study of atmospheric electricity variables than has ever been possible before, which will improve our understanding of the role of atmospheric electricity in the complex processes underlying weather and climate.

Global and Regional Electricity Components in Undisturbed Midlatitude Lower Atmosphere

The physical mechanisms determining the variability of the vertical profiles of electrical conductivity, space charge density, and electric field in the undisturbed midlatitude lower atmosphere are discussed. The influence of the global and local mesoscale processes on the variability of electrical conductivity and the main component of the atmospheric electric field is estimated. The sunrise effect is studied, estimates are obtained for the charge accumulation rate in the column of the lower atmosphere and the corresponding growth rate of the field strength close to the ground. It is shown that the increase in the average charge density is mainly due to the breakdown of the stable stratification of the atmospheric boundary layer and transformation of the vertical profile of electrical conductivity following the convective mixing of a radon and its daughter products.

Sunrise effect on 40 kHz signal amplitude and its characteristic variation with respect to geomagnetic storms

The sunrise effect is a characteristic feature of very low frequency and low frequency radio waves propagated over a large distance. The 40 kHz signal level, transmitted from Miyakoji station (37.4° N, 140.9° E), Japan, and received at Tripura University (23° N, 91.4° E), is found to be attenuated during sunrise with an enhancement before the decrease in the signal level. On the basis of the nature of attenuation of the observed records from 2005 to 2006, those are classified into four different types, namely, type A (three step attenuation), type B (two step attenuation), type C (one step attenuation), and type D (no attenuation). During geomagnetically active days, 84% of type D cases and 31% of type C cases are observed, whereas only 0.9% of type A cases and 7% of type B cases are observed during geomagnetically active days. The fade amplitude of type C fade is also found to maintain a good negative correlation of 77.3% with the geomagnetic Ap indices over the period of 2 years. From the model calculation it is found that in the altitude range from 65 to 80 km, on average the electron density increases by a factor of 5.22 times during geomagnetically active days versus normal days.

Nature of the sunrise effect in daily electric field variations at Kamchatka: 2. Electric field frequency variations

The power spectra of time variations in the electric field strength in the near-Earth’s atmosphere and in the geomagnetic field horizontal component, which were simultaneously observed at the Paratunka observatory (φ = 52°58.3′ N; λ = 158°14.9′ E) in September 1999, have been studied. The periods of the day (including sunrise, sunset, and night) have been considered. It has been indicated that oscillations with periods T ∼ 2.0–2.5 h are present in the power spectra of these parameters during the day. The intensity of these oscillations increases noticeably and the oscillations in the band of periods T < 1 h increase simultaneously in the field strength power spectra at sunrise. The variations in the argument of the cross-spectrum of these parameters indicated that oscillations in the 2.0–2.5 h period band are caused by sources that are located above the ionospheric dynamo region; at the same time, oscillations in the 0.5–1 h period band are caused by sources in the lower atmosphere. A possible mechanism by which these oscillations are generated, related to the vortex motion of convective cells that originate at sunrise in the boundary atmospheric layer, is proposed.

On the sunrise oscillation of the F region in the equatorial ionosphere

This paper shows that, contrary to previous explanations, the apparent undulating motion of the equatorial F region peak at sunrise is produced by photochemistry rather than dynamics. Our study is based on an investigation of the behavior of the early morning ionosphere observed by a Digital Ionosonde at Trivandrum, India. The phenomenon is rooted in the production of new plasma at the upper F region altitudes soon after sunrise. As the peak photoproduction rate moves down in altitude and increases in magnitude the newly formed plasma follows a similar trend. Once the density becomes large enough to be detected by an ionosonde, a jump is observed in the F region peak altitude. The jump is followed by a quick downward motion of the increasingly strong F peak. Chemistry causes the downward motion of the F peak to end near 250 km. Electrodynamics is not responsible for the sunrise undulation, but plays an indirect role in the detection of the sunrise effect by simultaneously lowering during the night the peak height and decreasing the density. When detectable, the remnant plasma introduces a lower peak height that facilitates the observation of the initial increase in peak height, while the lower background density allows the relatively small initial density increase from photoionization to be observed.

Problem of the nature of the sunrise effect in diurnal variations in the electric field in Kamchatka: 1. Time variations in the electric field

The effect of sunrise in time variations in the electric field in the near-Earth atmosphere at the Kamchatka Paratunka observatory has been studied. Twenty-nine records under fair-weather conditions have been selected. It has been indicated that the estimated effect parameters—the times of the effect’s onset and field strength maximum relative to the sunrise time, as well as the ratio of the strength maximum to its value before sunrise and the effect duration—coincide with the previously published data. Thereby, the conclusion is confirmed that the sunrise effect in diurnal variations in the electric field in the near-Earth atmosphere is related to the turbulence and convection processes in the atmospheric boundary layer at a change in atmospheric temperature.

Ionosperic anomaly due to seismic activities – Part 1: Calibration of the VLF signal of VTX 18.2 KHz station from Kolkata and deviation during seismic events

Abstract. VLF signals are long thought to give away important information about the lithosphere-ionosphere coupling. In order to establish co-relations, if any, between the ionospheric activities and the earthquakes, we need to understand what the reference signals are, throughout the year. The best opportunity to do this is during the period of solar minimum where the number of flares and sunspots are negligible and the data would be primarily affected by the sun and variation would be due to normal sunset and sunrise effects. In this paper, we present the result of the sunrise and sunset terminators as a function of the day of the year for a period of four years, viz, 2005–2008 when the solar activity was very low. The terminators are for the 18.2 KHz VTX signal of the Indian Navy as observed from Indian Centre for Space Physics receiving station located in Kolkata. A total of 624 days of data have been used to obtain the mean plot. Any deviation of observations from this so-called the standardized calibration curve would point to influences by terrestrial (such as earthquakes) and extra-terrestrial events (such as solar activities). We present examples of deviations which occur in a period of 16 months and show that the correlation with seismic events is significant and typically the highest deviation takes place up to a couple of days prior to the seismic event. Simultaneous observations of such deviations from more than one station could improve the predictability of earthquakes.

Relationship between effective solar radiation and sap flow process during an entire growing season in Western Mountains of Beijing

In order to explore the relationship between the time processes of solar radiation and sap flow, sap flow velocity (SFV) of Platycladus orientalis and Pinus tabulaeformis, effective solar radiation (ESR) and other environmental factors were synchronously monitored for one year in the Beijing Western Mountains by using a thermal dissipation probe (TDP) system and an automatic weather station. Results showed significant differences between changes in diurnal characteristics of ESR and sap flow in sunny days during three seasons. Starting times of sap flow occurred generally 1.5–3 hours later than those of solar radiation and there were small differences between Platycladus orientalis and Pinus tabulaeformis. But peak times and stopping times of sap flow varied considerably with large contrasts in ESR. The duration of sap flow showed clear differences among the seasons owing to the variable rhythms of climate factors in Beijing. Fluctuation amplitude in the duration of sap flow remained relatively stable during the autumn but changed greatly during spring and summer. Changes in diurnal sap flow velocity of both Platycladus orientalis and Pinus tabulaeformis were about 0–3 hours later than those of ESR but with the same configuration. The start of sap flow was mainly induced by the sudden intensification of ESR (sunrise effect). Seasonal models of SFV indicated that a cubic equation had the best fit. It was more practical to simulate seasonal water consumption models of trees with ESR. In further investigations, these models should be optimized.

Diurnal variation of atmospheric Maxwell current over the low-latitude continental station, Tirunelveli, India (8.7°N, 77.8°E)

Abstract Observations of atmospheric Maxwell current from the low-latitude continental station, Tirunelveli (8.7°N, 77.8°E), over a period of 8 years are presented in this study. The horizontal long-wire antenna is used as a sensor for picking up charges from the atmosphere. The objectives of the present work have been to understand the antenna system in response to different meteorological conditions and on fairweather days. We define a fairweather day as a day when there is no snowfall/rainfall at the measuring site, when there are high clouds less than 3 octas throughout the day, and when the wind speed is less than 10 m s−1. A sunrise effect is observed in the measured Maxwell current during fairweather days in all seasons. The measured current exhibits an increase that commences about 30 min before sunrise and lasts for nearly 1.5 h. The sunrise effect is found to be inhibited on days when there is pre-dawn convection and during the presence of rain, haze, or cloud cover. Barring the sunrise effect, the rest of the variations on fairweather day appear to follow a trend typical of the Carnegie curve. On clear cloudless conditions, reasonably good data are obtained during equinox and winter months but not in summer when strong winds associated with the onset of the southwest monsoon make the measurement of Maxwell current difficult and limited data are available.

A comparative study of atmospheric Maxwell current and electric field from a low latitude station, Tirunelveli

Simultaneous measurements of atmospheric Maxwell current and electric field, using horizontal long wire antenna and passive horizontal wire antenna system at 1 m above the Earth’s surface, were carried out at Tirunelveli (8.7°N, 77.8°E), India, during January/February 2002. The objectives of the present work have been to understand the nature of the measured atmospheric electrical parameters and explore the possibility of detecting the signature associated with the global thunderstorm activity. As the measurements indicate, the atmospheric electrical parameters at Tirunelveli are severely masked by the locally induced current components during disturbed weather conditions, for example, severe convective activity. During the selected fair weather conditions, the hourly averaged diurnal variation curves of Maxwell current and electric field are characterized by two peaks: The first peak is the local “sunrise effect” and the second peak occurs at times close to 1900 UT, the time of maximum global thunderstorm activity as noted in the famous “Carniege curve”. The correlation coefficient between the measured Maxwell current and electric field has a high value (more than 0.8) for all the fair weather days. These results support our view that this site is free from local pollution during the fair weather conditions and is suitable for long-term measurements of atmospheric electrical parameters.

Polarization effects of the finite-size low-altitude ionosphere

We use two-fluid or Hall effect MHD description of weakly-ionized stratified atmosphere to describe several polarization features of the MHD disturbance penetration. We employ a pair of functions for the electric and magnetic field components ratio which can be treated analytically. As an example we derive an approximation to the case of the MHD waves in the Earth's Hall ionosphere and demonstrate its different polarization responses (ellipticity and rotation) for Alfven and fast magnetosonic modes depending on the Hall region thickness. Neglecting the Hall thickness effect we derive previously obtained, well-known results for the rotation of the polarization plane of the MHD waves (Dungey, 1963; Nishida, 1964; Inoue, 1973; Hughes, 1974; Hughes and Southwood, 1976). The ionospheric effects are more essential for the polarization of the fast magnetosonic waves. The polarization changes of the magnetosonic waves are expressed as a function of i) the ratio (R) of the height-integrated Hall (ΣH) and Pedersen (Σp) conductivities (conductances) in the Hall region (85-125 km) and ii) a wave/magnetospheric parameter (A m) and the ratio A m/Σp. The wave/magnetospheric parameter Am depends on the wave frequency and the horizontal scale of the ULF waves. Using standard models IRI 90 and MSIS 86, responses of ULF magnetosonic waves to seasonal/diurnal ionospheric variations at subauroral/middle latitudes are illustrated for arbitrary, but reasonable values of the wave/magnetospheric parameter A m. The polarization plane rotation for the ULF compressional waves ranges between 0 and π/2 and reaches the classical π/2 degree only for special cases. Along with the rotation effect an ellipticity effect has also local time course. These findings suggest a new dissipative mechanism (non-resonant) of transformation of magnetosonic waves into Alfven modes in the ionosphere. In addition we suggest a physical insight for the MHD wave transformation effects by the ionosphere. These findings should be taken into account for the analysis of various polarization features of the geomagnetic pulsations observed on the ground. Sunrise effect on the polarization of the Pc 3-4 pulsations (Saka et al., 1982), the effect of transformation of pure compressional ULF disturbance in the magnetosphere into transverse wave on ground (Lanzerotti and Tartaglia, 1972) proved to be explained in terms of both the polarization rotation and the ellipticity mechanism by the ionosphere. Simultaneous measurements of the electric and magnetic field of ULF waves at ground and balloon heights have revealed polarizations of opposite handedness (Bering et al., 1995). It is shown that the polarization changes of the magnetosonic wave through a horizontally homogeneous high-latitude ionosphere continue further through the atmosphere and would result in different polarization states for the electric and magnetic fields. The northern (southern) hemisphere ionosphere causes an additional left(right)-hand polarization effect in the ionosphere/atmosphere produced mostly on the ULF wave magnetic field. The opposite handedness of the Pc5 wave polarization recorded at the South Pole by measurements of the ULF electric and magnetic field components (Bering et al., 1995) might be explained as a result of an influence of the ionosphere on the ULF waves of an initially left-hand polarization.

A study of the forenoon ionospheric F2 layer behavior over the middle and upper atmospheric radar

The middle and upper atmosphere (MU) radar incoherent scatter observations of electron density Ne profiles for 1986–1995 at Shigaraki (34.85°N, 136.10°E) show a forenoon density depression (bite‐out) in summer both on individual days and in statistical averages. The depression takes place generally between 0700 and 1200 LT, above the F2 peak for low solar activity and around the F2 peak for high solar activity. The maximum electron density NmF2 shows, in both percentage and time duration, a greater depression for low solar activity than for high solar activity. With increasing altitude, the forenoon Ne depression develops earlier for low solar activity. In combination with the MU radar observations of ion drifts and plasma temperatures, our modeling indicates the following: (1) Transport processes such as the enhancement of electron temperature Te, the poleward neutral wind, and the reduced upper boundary ionization density help to reduce Ne and result in a negative ∂Ne/∂t in the morning hours in summer. Among those processes the morning enhancement of Te is found to be most important for the presence of the forenoon bite‐out, since it greatly increases the O+ diffusion such that ionization outflow is significantly intensified. (2) The forenoon bite‐out occurs only in summer, not in other seasons, because the smaller atmospheric composition ratio [O]/[N2] in summer leads to a smaller daytime background Ne and thus to an enhanced Te, and the after‐sunrise photochemical contributions (mainly the O+ production) are so small that ∂Ne/∂t also tends to be smaller. These two conditions give prominence to the transport contributions. (3) The solar activity dependences of the forenoon bite‐out can be explained basically in terms of the low‐electron‐density background. (4) The height of formation of the forenoon bite‐out is also related to the sunrise effect on photoionization.

A study of enhanced fair‐weather electric fields occurring soon after sunrise

In this paper we describe several series of electric field soundings made in the lowest few hundred meters above the ground on 6 days at Kennedy Space Center in Florida. These soundings are used to determine the charge density and thickness of the charged electrode layer just above the surface of the Earth both before and after sunrise during fair weather. On most of the days considered, there was an anomalous enhancement in the ground‐level electric field that was probably associated with the “sunrise effect” previously described by others. At our tether site we found that the electrode‐layer charge density began increasing at about the same time as the local enhancement in the electric field magnitude at the ground. Shortly before the peak in the local E enhancement, the electrode‐layer charge density decreased while the charge thickness increased; these changes were coincident with a decrease in relative humidity, a shift in the average wind direction, and increases in the fluctuations in relative humidity, wind speed, and wind direction. The typical decrease in charge density was from 0.2 to 0.05 nC m−3, while the charge layer thickness increased from less than 20 m to almost 200 m. Our measurements suggest that enhanced positive electrode layers accumulate before sunrise very close to the surface because there is relatively little radioactivity in the soil or air. Local, upward mixing of the denser, low‐lying, electrode‐layer charge may account for the observed sunrise enhancement in electric field. The larger enhancements observed at some sites may indicate that upward convection is supplemented by advection of denser charge from above water surfaces a few tens of meters (or less) from the measurement sites.

Sunrise effects on VLF signals propagating over a long north‐south path

We present a detailed study of the times of amplitude minima observed on the 12‐Mm path from NAA (24 kHz, 1 MW, Cutler, Maine) to Faraday, Antarctica, during the period 1990–1995. (NAA is a naval transmitter call sign.) This study represents the first account of the effect of the sunrise terminator when it is parallel to a propagation path at some times of the year. Since the NAA‐Faraday path is within 3° of the north‐south meridian, parallel orientation happens close to the equinoxes, while the maximum angle of incidence occurs during the solstices. During the solstices the terminator takes a significant length of time to cross the entire propagation path, so modal conversion effects are observed over a range of hours. During the equinoxes, however, the leading edge of the night‐day transition region crosses the whole propagation path within 20 min. The interpretation of the timing of minima is consistent with modal conversion taking place as the sunrise terminator crosses the NAA‐Faraday transmission path at specific, consistent locations. The timing of minima is remarkably consistent from year to year. Long wave propagation modeling is used to show that the location of nightside minima at an altitude of 45–75 km in the subionospheric waveguide represents the location of the sunrise terminator on the great circle path when dayside minima occur.

Global Pc5 geomagnetic pulsations of March 24, 1991, as observed along the American Sector

Analysis of the globally coherent Pc5 geomagnetic pulsation event of March 24, 1991, from a chain of stations extending from the auroral oval to the equatorial region along the American sector, has shown unequivocal evidence on the enhancement of pulsation amplitude in the narrow equatorial band centered at the dip equator. The fine spatial structure of this equatorial enhancement, documented observationally for the first time, emphasizes the importance of an ionospheric component associated with enhanced Cowling conductivity. The presence of an ionospheric component is also indicated at mid‐latitudes by the sharp increase in the amplitude of pulsations, more dominantly in the Y than in the X component, on the sunward side of the dawn terminator. The sunrise effect also causes a reversal of the sense of rotation of the wave polarization across the dawn terminator. Propagation of the magnetospheric inducted polar electric field to low and equatorial latitudes through the ionosphere is invoked to account for the equatorial enhancement and the sunrise effect seen in these Pc5 pulsations.

Sunrise effects in the equatorial F-layer

Abstract In the sunrise period, under certain conditions, a cavity of a low electron density arises in the equatorial ionosphere at altitudes between the peaks of the night F-layer and the morning F-layer. The dependence of the cavity parameters on the initial conditions, solar activity, season, photochemistry, geomagnetic disturbance, diffusion, and vertical drift of the ionospheric plasma is studied theoretically. It is shown that temporal variations of the electron density, as well as of the cavity parameters, depend mainly on the initial profile parameters, ionizing solar radiation flux, ion formation rate, diffusion, and geomagnetic activity. It is concluded that reduced diffusion can be one of the major physical mechanisms which can assist the preservation, at large heights in the pre-sunrise F-layer, of the electron density at the day-time level.