Nighttime Enhancements Research Articles

Response of the Midlatitude F2 Layer to Some Strong Geomagnetic Storms during Solar Minimum as Observed at Four Sites of the Globe

In this study, we documented the midlatitude F2-layer response to five strong geomagnetic storms with minimum Dst < -150 nT that occurred in solar minimum years using hourly values of the F2-layer critical frequency (foF2) from four ionosondes located in different hemispheres. The results were very limited, but they illustrated some peculiarities in the behavior of the F2-layer storm. During equinox, the characteristic ionospheric disturbance patterns over the Japanese station Wakkanai in the Northern Hemisphere and the Australian station Mundaring in the Southern Hemisphere were consistent with the well-known scenario by <TEX>$Pr{\ddot{o}}lss$</TEX> (1993); however, during a December solstice magnetic storm, both stations did not observe any noticeable positive ionospheric disturbances. Over the "near-pole" European ionosonde, clear positive ionospheric storms were not observed during the events, but the "far-from-pole" Southern Hemisphere station Port Stanley showed prominent enhancements in F2-layer peak electron density in all magnetic storms except one. No event produced noticeable nighttime enhancements in foF2 over all four ionosondes.

Investigation of TEC variations over the magnetic equatorial and equatorial anomaly regions of the African sector

This paper presents the annual, seasonal and diurnal variations in ionospheric TEC along the African equatorial region. The study also investigated the effects of a geomagnetic storm on ionospheric TEC values. Dual-frequency GPS derived TEC data obtained from four stations within the African equatorial region for the high solar activity year 2012 were used in this study. Annual variations showed TEC having two peaks in the equinoctial months, while minima values were observed in the summer and winter solstices. The diurnal pattern showed a pre-dawn minimum, a steady increase from about sunrise to an afternoon maximum and then a gradual fall after sunset to attain a minimum just before sunrise. Nighttime enhancements of TEC were observed mostly in the equinoctial months. There was comparably higher percentage TEC variability during nighttime than daytime and highest during equinoxes, moderate in winter and least during summer solstice. TEC was observed to exhibit a good correlation with geomagnetic storm indices.

Nighttime electron density enhancements at middle and low latitudes in East Asia

Nighttime enhancements in ionospheric electron density at mid- and low-latitudes are investigated by using the critical frequency of the F2-layer (foF2) data measured from ionosonde stations at Okinawa (26.3°N, 127.8°E, Geomagnetic 15.3°N), Yamagawa (31.2°N, 130.6°E, Geomagnetic 20.4°N), Kokubunji (35.7°N, 139.5°E, Geomagnetic 25.5°N), and Wakkanai (45.4°N, 141.7°E, Geomagnetic 35.4°N) in East Asia during several solar cycles. The results show that there are obvious seasonal and solar activity dependencies of the nighttime electron density enhancements. The enhancements are termed pre-midnight enhancement and post-midnight enhancement, according to the local time when the enhancement appeared. The former has a higher occurrence probability in summer months than in winter months. In contrast, the latter has a larger occurrence probability in winter months than in summer months. Moreover, the nighttime enhancements in electron density are more likely to occur at lower solar activity. These seasonal and solar activity variations of the nighttime enhancements in electron density can be explained in terms of the combined effects of downward plasma flux from the plasmasphere and the neutral winds.

Latitudinal variation of the ionosphere in the African sector using GPS TEC data

Total electron content (TEC) measured by the network of receivers from Global Navigation Satellite Systems (GNSS) at 18 stations in the year 2011 was used to study the ionospheric variations. The stations are chosen along a longitudinal range (27–40° E) covering all the latitudes of the African sector. The latitudinal variations investigated include: annual, seasonal, diurnal, and day-to-day. The annual and seasonal results presented an equinoctial asymmetry in all stations and a winter anomaly during the solstice months for the stations near the equator. The diurnal variation in the same equatorial stations exhibited features like noontime bite-out, night-time enhancements and high day-to-day TEC variability especially in September. The day-to-day variability was larger in the station around geomagnetic latitude 16° S (dodm). Latitudinally, the TEC recorded keep on decreasing as you ascend the latitudes of both hemispheres showing a maximum around the geomagnetic equator with a trough between geomagnetic and geographic equators.

Some features of nighttime enhancements in the electron concentration in the F2-layer maximum of the midlatitude ionosphere

Using the data of vertical sounding of the ionosphere in Alma-Ata (76°55′ E, 43°15′ N) conducted in 2002–2012, the reaction of parameters of the ionospheric F2 layer to various types of nighttime enhancements in the electron concentration in the maximum of the layer (NmF2) was studied, including the height of the maximum and bottom of the layer, its semithickness, and electron concentration at some fixed heights. Examples of recordings of a combination of the enhancements caused by different mechanisms are presented. The similarity of the reaction of the F2-layer parameters to the nighttime enhancements caused by the rise of the layer and plasma flux from the protonosphere and passage of large-scale travelling ionospheric disturbances was found. Difficulties in identifying these two events in the case of their equal duration are noted. The difference in the reaction of the F2-layer parameters to the enhancements caused by the rise of the layer and plasma fluxes from the protonosphere and occurrence of the summer midlatitude ionospheric anomaly is shown.

Modeling study of nighttime enhancements in F region electron density at low latitudes

AbstractIn this paper we carry out case‐control simulations to examine the mechanisms of the nighttime anomalous enhancements in electron density in the ionosphere at low latitudes, which were studied earlier in another research. The results confirm the earlier conclusion that the downward E × B plasma drift due to westward electric field at night is the main driving force for the nighttime enhancement. In addition, the phase of the electric field is found important in forming the enhancement. Delayed westward electric field can produce significant postmidnight enhancement as observed at Sanya (geomagnetic latitude: 8.2°N). In addition, the equatorward neutral wind at night is found to modulate the formation of the nighttime enhancement at geomagnetic latitudes below 15°N. The combined effects of the two drivers cause significant equatorward/downward plasma flux, which results in the enhancement of electron density as well as the drop in ionospheric peak height.

A case study of postmidnight enhancement in F‐layer electron density over Sanya of China

The electron density in the ionospheric F region occasionally stops its decay and rises pronouncedly during night hours, which are termed ionospheric nighttime enhancements. In this case study, we analyzed the manually scaled ionogram records measured by a Lowell DPS‐4D ionosonde operated at Sanya (18.3°N, 109.6°E), China, to explore postmidnight enhancement events occurred in 2012, a year of moderate solar activities. Common features in these cases illustrate that, accompanying nighttime rises in peak electron density of F2 layer (NmF2), the height of F2 layer is depressed significantly, and the ionogram‐derived electron density height profiles become thinner. There are time shifts in the development of electron density enhancements in the F layer; that is, enhancement develops earlier and reaches peaks earlier at higher altitudes than at lower altitudes. Meanwhile, plasma drift is detected downward under such events, revealing the essential role of the westward electric field in forming the postmidnight enhancements in electron density of ionospheric F layer at such low latitudes.

Solar and magnetic control on night-time enhancement in TEC near the crest of the Equatorial Ionization Anomaly

The ionospheric Total Electron Content (TECs), derived by dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Bhopal (23.2°N, 77.4°E, Geomagnetic 14.2°N) were analyzed for the period of January, 2005 to February, 2008. The work deals with monthly, diurnal, solar and magnetic activity variations on night-time enhancement in TEC. From a total of 157 night-time enhancements, 75 occur during pre-midnight and 82 post-midnight hours. The occurrence of night-time enhancement in TEC is utmost during summer months, followed by equinox and winter months. The occurrence of night-time enhancement in TEC decreases with increase in solar and magnetic activities. We observed that peak size and half amplitude duration are positively correlated, while time of occurrence of night-time enhancement in TEC and time of peak enhancement are negatively correlated with solar activity. The peak size, half amplitude duration, time of peak enhancement and time of occurrence of night-time enhancement in TEC shows negative correlation with magnetic activity. The results have been compared with the earlier ones and discussed in terms of possible source mechanism responsible for the enhancement at anomaly crest region.

Total electron content variations in equatorial anomaly region

Diurnal variations in the total electron content (TEC) at Makerere University (00°19′N, 32°40′E, Geo Dip −22°), Uganda, have been investigated using a NovAtel GSV400B GPS receiver for the year 2010. The highest TEC values occurred from 13h00 to 17h00 local time (LT) throughout the year, with the highest values being exhibited during equinoctial months. In addition, there was some correlation between this high TEC and the moderate storms that occurred in 2010. These high TEC values have been attributed to the solar EUV ionization coupled with the upward vertical ExB drift. Nighttime enhancements were also found to be seasonally dependant, attaining maximum values during equinoctial months. These results were also compared with modeled TEC values by the IRI-2007 model. The modeled values were in good agreement with the measured values except for these two points: (1) the model had a short-fall in predicting the nighttime enhancements; and (2) the model’s minimum TEC did not coincide with the measured minimum in most of the months. Observed TEC depletions were found to correlate with an increase in the S4 index and have been identified as a manifestation of the plasma density depletions of the equatorial origin.

Nighttime enhancements in TEC near the crest of northern equatorial ionization anomaly during low solar activity period

Enhancement in nighttime total electron content (TEC) near the crest of equatorial ionization anomaly at Bhopal (Geog. 23.2°N, 77.4°E, and MLAT 14.2°N) has been studied for the solar minimum period March 2005- November 2006. TEC data recorded by GPS Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A receiver is used for the study and results are presented in the paper. More than 10% increase in TEC with respect to background content is considered for analysis. Out of total 138 enhancements, 65 observed during pre midnight hours and 73 during post midnight hours. It is observed that nighttime enhancement in TEC at Bhopal occurs in all seasons; it is more frequent during summer, less during equinox and least during winter months. The enhancement in nighttime TEC can be observed both in geomagnetic disturbed time and in quiet time. We found that mean peak amplitude for pre-midnight TEC enhancement are more in equinox and less in winter, while in post-midnight TEC enhancement it is highest in summer and less in winter. Post-midnight enhancements have smaller peak amplitude as compared to pre-midnight. Also the most probable values for pre- and post-midnights are 4.4 TECU and 2.17 TECU respectively. The percentage occurrence of nighttime TEC enhancement does not show any dependence on solar activity whereas the peak amplitude depends on solar activity. The percentage occurrence of nighttime TEC enhancement decreases as the magnetic activity increases whereas there is no such dependency found with peak amplitude. It is also observed that majority of nighttime TEC enhancements are occurred without scintillation. The localised nighttime TEC enhancements near the crest of equatorial ionization anomaly region have been observed most of the nights during the period of study.

Phase reversal of the diurnal cycle in the midlatitude ionosphere

The typical diurnal cycle of the midlatitude F region electron density consists of a midday maximum and a midnight minimum. However, a phase reversal of this diurnal cycle has been found to occur in three distinct regions on the globe. They are the East Asian (EA) region centered around (53°N, 150°E), the Northern Atlantic (NA) region centered around (45°N, 50°W) and the South Pacific (SP) region centered around (60°S, 110°W). The intensively reported Weddell Sea Anomaly falls inside the SP region. The phase reversal occurs during March–August in EA and NA regions, and between August and March in SP region, being most prominent in local summer. Furthermore, this diurnal anomaly is more pronounced at solar minimum than at solar maximum, and more pronounced in SP region than in NA and EA regions, in terms of larger diurnal magnitude and more months it lasts in a year. It is emphasized that the diurnal anomaly consists of not only a nighttime enhancement, but also a concurrent noontime depletion. Hence, midlatitude summer nighttime enhancements reported in previous studies are just part of this reversed diurnal cycle. The cause for the phase reversal involves several interplaying physical processes. Among these, the neutral wind combined with the geomagnetic field configuration plays a pivotal role. It generates a one‐wave longitudinal pattern at southern middle latitudes and a two‐wave pattern at northern middle latitudes, whose wave peaks correspond to the center of the SP, EA, and NA regions, respectively. The seasonal variation of neutral winds and downward motion of the ionization induced by thermal contraction of the ionosphere at sunset may largely control the occurring local time of the nighttime density enhancement and how long it persists in different months. The phase reversal occurs as a result of close ion‐neutral coupling. It is further noted that winter anomaly in the EA, NA, and SP regions is very weak or missing.

Midlatitude nighttime enhancement in F region electron density from global COSMIC measurements under solar minimum winter condition

Ionospheric electron density profiles retrieved from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites measurements from 6 November 2006 to 5 February 2007 are used to study the ionospheric nighttime electron density enhancements under winter, solar minimum, and geomagnetically quiet conditions. In this work, the peak electron densities of F2 layer (NmF2) derived from COSMIC measurements are found to be in reasonably good agreement with ionosonde observations during the night. Therefore the morphology of nighttime enhancements is investigated at geomagnetic midlatitudes (MLAT 20–60°) in the northern hemisphere using COSMIC observations. The enhancements of electron density are evident near the F2 layer peak at most latitudes and longitudes; however, significant variations in the latitudinal dependence of the occurrence time and net magnitudes of the enhancements are found in different regions. The characteristics of enhancements in the North Atlantic Ocean sector are distinctly different from those in the eastern part of the North American sector and also from those at longitudes from Europe to Asia, in the Pacific Ocean, and in the western part of the North American sectors. The longitudinal variations of the morphology of the electron density enhancements during nighttime are possibly caused by different downward plasma flux, meridional winds, and electric field drifts under the effect of the geomagnetic field.

Ionospheric E-region response to solar-geomagnetic storms observed by TIMED/SABER and application to IRI storm-model development

The large thermospheric infrared radiance enhancements observed from the TIMED/SABER experiment during recent solar storms provide an exciting opportunity to study the influence of solar-geomagnetic disturbances on the upper atmosphere and ionosphere. In particular, nighttime enhancements of 4.3μm emission, due to vibrational excitation and radiative emission by NO+, provide an excellent proxy to study and analyze the response of the ionospheric E-region to auroral electron dosing and storm-time enhancements to the E-region electron density. In this paper, we give a status report of on-going work on model and data analysis methodologies of deriving NO+ 4.3μm volume emission rates, a proxy for the storm-time E-region response, and the approach for deriving an empirical storm-time correction to IRI E-region NO+ and electron densities.

A modeling study of ionospheric F2-region storm effects at low geomagnetic latitudes during 17-22 March 1990

Abstract. We have presented a comparison between the modeled NmF2 and hmF2, and NmF2 and hmF2, which were observed in the low-latitude ionosphere simultaneously by the Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders, by the middle and upper atmosphere (MU) radar during 17-22 March 1990, and by the Arecibo radar for the time period of 20-22 March 1990. A comparison between the electron and ion temperatures measured by the MU and Arecibo radars and those produced by the model of the ionosphere and plasmasphere is presented. The empirical zonal electric field, the meridional neutral wind taken from the HWM90 wind model, and the NRLMSISE-00 neutral temperature and densities are corrected so that the model results agree reasonably with the ionospheric sounder observations, and the MU and Arecibo radar data. It is proved that the nighttime weakening of the equatorial zonal electric field (in comparison with that produced by the empirical model of Fejer and Scherliess (1997) or Scherliess and Fejer (1999)), in combination with the corrected wind-induced plasma drift along magnetic field lines, provides the development of the nighttime enhancements in NmF2 observed over Manila during 17-22 March 1990. As a result, the new physical mechanism of the nighttime NmF2 enhancement formation close to the geomagnetic equator includes the nighttime weakening of the equatorial zonal electric field and equatorward nighttime plasma drift along magnetic field lines caused by neutral wind in the both geomagnetic hemispheres. It is found that the latitudinal positions of the crests depend on the E×B drift velocity and on the neutral wind velocity. The relative role of the main mechanisms of the equatorial anomaly suppression observed during geomagnetic storms is studied for the first time in terms of storm-time variations of the model crest-to-trough ratios of the equatorial anomaly. During most of the studied time period, a total contribution from meridional neutral winds and variations in the zonal electric field to the equatorial anomaly changes is larger than that from geomagnetic storm disturbances in the neutral temperature and densities. Vibrationally excited N2 and O2 promote the equatorial anomaly enhancement during the predominant part of the studied time period, however, the role of vibrationally excited N2 and O2 in the development of the equatorial anomaly is not significant. The asymmetries in the neutral wind and densities relative to the geomagnetic equator are responsible for the north-south asymmetry in NmF2 and hmF2, and for the asymmetry between the values of the crest-to-trough ratios of the Northern and Southern Hemispheres. The model simulations provide evidence in favor of an asymmetry in longitude of the energy input into the auroral region of the Northern Hemisphere on 21 March 1990.

Mechanism of the post-midnight winter night-time enhancements in NmF2 over Millstone Hill during 14–17 January 1986

A time-dependent mid-latitude one-dimensional model of the ionosphere and plasmasphere has been used to study the causes of the post-midnight winter night-time enhancements in NmF2 over Millstone Hill during 14–17 January 1986. The mechanism proposed to explain night-time NmF2 enhancements in the current study is a combination of the plasmaspheric heating and refilling and chemical and plasma transport effects. A magnetic plasma flux tube refilling leads to an increase in the plasmaspheric H + density, and this causes an increase in a flow of H + ions from the plasmasphere to the ionosphere, i.e. the magnitude of [H +] is increased in the topside ionosphere. It leads to an increase in the night-time production rate of O +( 4S) ions due to charge exchanges between O +( 4S) and H and between H + and O. The magnitude of [O +( 4S)] is increased in the topside ionosphere under the influence of an increase in the night-time production rate of O +( 4S) ions, i.e. the absolute value of the night-time ionospheric O +( 4S) diffusion flux from the topside ionosphere to the F2- region is increased, resulting in an NmF2 increase. This implies that part of the night-time NmF2 enhancement is created by interactions between diffusion fluxes of O +( 4S), H +, and He + ions. It is shown that increases in electron and ion temperature created by an additional electron heating of the plasmasphere, Q ad, before midnight produce decreases in the absolute value of the night-time flux of O +( 4S) ions flowing from the topside ionosphere to the F2-region, resulting in an acceleration of the rate of the NmF2 drop before midnight. The rapid electron and ion temperature decay in the ionosphere due to the disappearance of Q ad leads to the rapid decrease in the absolute value of the night-time flux of O +( 4S) ions flowing from the topside ionosphere to the F2-region resulting in the corresponding increase in NmF2. As a result, an appearance of Q ad before midnight helps to create a post-midnight NmF2 enhancement. The model without Q ad produces a mismatch between the measured and modelled night-time electron densities and temperatures, giving an increase in calculated densities and a decrease in calculated electron temperatures by night. An appearance of Q ad inhibits the development of night-time enhancements in NmF2 caused by variations in the wind-induced plasma drift. As a result, night-time equatorward winds were not important drivers in the NmF2 enhancements for most of the nights. However, the neutral wind forces the F2 layer to descend to low altitudes of heavy chemical O +( 4S) ion losses, reducing NmF2 to low values before sunrise on 16 and 17 January forming the peaks in the NmF2 time dependence.

Study of mid-latitude nighttime enhancement in F-region electron density using tomographic images over the UK

Abstract. Nighttime enhancements in ionospheric electron content (IEC)/peak electron density (NmF2) have been studied by various workers in the equatorial anomaly and mid-latitude regions. Such studies give an idea about their enhancement over that location only. In the present study tomographic images over the UK, which give a latitudinal versus height distributions of ionospheric electron density in a much wider area, have been used to study the anomalous increases in nighttime F-region electron density at mid-latitudes. From the analysis of four seasonal representative months (November 1997, March, June and October 1998) data it was noted that the majority of the cases of nighttime enhancements were observed after local midnight, with a maximum between 03:00–04:00 LT in the month of November 1997. Enhancements were observed mostly between 45–50° N latitudes, and their positions are not affected by magnetic activity (Kp ) variations, whereas the separation between the mid-latitude trough and enhancement decreases with increases in magnetic activity. This finding shows that only the trough moves equatorward with the increase in magnetic activity. It is also noted that the electron density gradient from the trough to the enhancement increases with an increase in Kp. Results are discussed in terms of downward plasma transport from the protonosphere to the ionosphere and the nighttime neutral winds.Key words. Ionosphere (mid-latitude ionosphere; modeling and forecasting; instruments and techniques)

Global morphology of night-time &amp;lt;i&amp;gt;Nm&amp;lt;/i&amp;gt;F2 enhancements

Abstract. An overall statistical study of night-time enhancements of NmF2 has been carried out. All available foF2 observations since 1955 at 53 ionosonde stations distributed worldwide in the latitude range fgeom = 15° - 60° were used in the analysis. More than 200 000 station-nights of data were analysed. This large data base allowed us to study seasonal, solar cycle and spatial variations of the NmF2 nighttime enhancements. Both pre-midnight and post-midnight NmF2 peaks demonstrate distinct variations with geophysical conditions, indicating different physical mechanisms responsible for their formation.Key words. Ionosphere (mid-latitude ionosphere, ionosphere-magnetosphere interactions) Radio science (ionospheric physics)

A comparative study of night-time enhancement of TEC at a low latitude station on storm and quiet nights including the local time, seasonal and solar activity dependence

Abstract. The main characteristics of night-time enhancements in TEC during magnetic storms are compared with those during quiet nights for different seasons and solar activity conditions at Palehua, a low latitude station during the period 1980–1989. We find that the mean amplitude has both a seasonal and solar activity dependence: in winter, the values are higher for weak storms as compared to those during quiet nights and increase with an increase in solar activity. In summer, the mean amplitude values during weak storms and quiet nights are almost equal. But during equinox, the mean amplitude values for quiet nights are greater than those during weak storms. The mean half-amplitude duration is higher during weak storms as compared to that during quiet nights in summer. However, during winter and equinox, the durations are almost equal for both quiet and weak storm nights. For the mean half-amplitude duration, the quiet night values for all the seasons and equinoctial weak storm values increase with an increase in solar activity. The occurrence frequency (in percent) of TEC enhancement during weak storms is greater than during quiet nights for all seasons. The mean amplitude, the mean half-amplitude duration and the occurrence frequency (in percent) of TEC enhancement values are higher during major storms as compared to those during quiet nights. The above parameters have their highest values during pre-midnight hours. From the data analysed, this behaviour is true in the case of major storms also.Key words. Ionosphere (ionospheric disturbances; plasma convection) Magnetospheric physics (storms and substorms)

Low latitude ionospheric effects near longitude 120°E during the great geomagnetic storm of july 2000

A great geomagnetic storm occurred on July 15/16, 2000 with a minimum value of about −300 nT in Dst index. Collecting digisonde data from ionospheric stations at Chungli, Wuhan, Kokubunji and Anyang, the ionospheric responses at the low latitudes near longitude 120°E during this storm are analyzed in this paper. There was a strong negative phase storm at low latitudes on July 16. The G-condition in the ionograms was clearly seen on the early first day after the commencement of geomagnetic storm. Those were considered to be caused by the storm-induced increase in the concentration ratios of neutral molecular O2 or N2 to atom O. On July 17 and some days thereafter, a positive phase storm appeared. In addition, anomalous equatorial ionization anomaly (EIA) inhibitions and developments were observed on July 16 and 17. There were also prominent nighttime enhancements in f0F2 during these days, and the diurnal variation of f0F2 was less clear than before.

Positive and negative ionospheric disturbances at middle latitudes during geomagnetic storms

The morphology of middle latitude ionospheric disturbances in response to geomagnetic storms has been investigated to determine the phenomenological differences between positive and negative ionospheric storm effects, using foF2 observations from azimuthal chain of stations. To better organize the disturbance signatures, two ionospheric indices were introduced to describe the maximum positive (Dfu‐index) and negative (Dfl‐index) deviation observed during an ionospheric storm. A systematic appearance of nighttime positive effects was determined with a 24‐hour recurrence. The thermospheric‐ionospheric view associated with positive and negative storm effects proposed by Prolss (1993) was extended to encompass our observations. This test clearly demonstrates that such a model can capture most of the basic aspects of ionospheric storms, nevertheless the prominent feature of large nighttime enhancements in the ionization density have yet to be explained.