Equatorial North‐South Difference of Noontime Electron Density Bite‐Out in the F2 Layer
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Abstract Noontime bite‐out (depletion) of N m F 2 (peak electron density of the F 2 layer) often occurs in the equatorial ionosphere; it is characterized by a noontime minimum sandwiched by a prenoon peak and a postnoon peak of N m F 2 . The fountain effect was suggested to be responsible for this abnormal diurnal variation pattern. Noontime bite‐out should occur not only at the dip equator but also concurrently at both equatorial north and south according to the fountain effect mechanism. In this study, ionosonde measurements in the equatorial ionization anomaly trough region were selected to investigate the climatology of noontime bite‐out. North‐south difference of noontime bite‐out was presented for the first time. Noontime bite‐out occurs near the dip equator in all months. However, N m F 2 enhances continuously since early morning till late afternoon at equatorial north (south) in northern summer (winter) months; nevertheless, noontime bite‐out still occurs in other months. This cannot be understood only in terms of the fountain effect mechanism. We suggested the reason is seasonally dependent transequator plasma transport induced by neutral meridional winds. Transequator transport works since early morning to modulate N m F 2 variation pattern in daytime. Plasma outflow induced by the transequator transport results in significant N m F 2 depletion at equatorial north or south before the fountain effect induced bite‐out; thus, N m F 2 prenoon peak is restrained and noontime bite‐out variation pattern is absent.Keywords:
Ionosonde
Local time
Diurnal temperature variation
WT5BZ] Two induction magnetometers have been installed at Zhongshan and Davis Stations, Antarctica respectively. We adopt with cross spectral analysis technique to analyze the data of the two induction magnetometers, in June, September, December 1996 and March 1997, to investigate the Pc3 frequency range pulsation occurrence and propagation characteristics in the cusp latitudes. The results are summarized as following: At Zhongshan Davis Station, the Pc3 frequency range pulsation occurs mainly around the local noon/ local magnetic noon and local magnetic midnight respectively. In daytime, the pulsation has a seasonal variation in amplitude, occurrence and temporal range, all of them are smallest in winter. But in nighttime, the pulsation has no such a variation. The pulsation amplitude is much larger in nighttime than in daytime all over the year. The pulsation propagating direction is mainly western in daytime and irregularly in nighttime. It can be thought that the different sources of the pulsation and the ionospheric electric conductivity are mainly responsible for these characteristics.
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Abstract Spread F on ionograms has been considered to be a phenomenon mainly occurred at nighttime. This study presented a case study of daytime spread F observed by the ionosonde installed at Puer (PUR; 22.7°N, 101.05°E; dip latitude 12.9°N), where daytime spread F that lasted for more than 2 h (about 08:30 LT~10:45 LT) was observed on 14 November 2015. To investigate the possible mechanism, ionograms recorded at PUR and Chiang Mai (18.76°N, 98.93°E; dip latitude 9.04°N) were used in this study. We found that traveling ionospheric disturbances were observed before the occurrence of daytime spread F . Meanwhile, the movement of the peak height of the ionosphere was downward. We suggested that downward vertical neutral winds excited by traveling atmospheric disturbances/atmospheric gravity waves might play a significant role in forming daytime spread F over PUR during geomagnetic storms.
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The previous studies show that scintillations under 150 km height shown by radio occultation (RO) of COSMIC are considered to be associated with ionospheric sporadic E (Es) layer. In this paper, we perform a statistical study on daytime Es observed by ground-ionosonde at five stations in magnetic Equator region in 2010, and make a comparison between foEs and S4 index from RO-COSMIC within 6 degree of each station. The results show that the foEs and S4 index had similar distributions. The agreement between scintillation occurrence and extreme Es cases showed that the Es had a significant contribution to the S4 index in daytime. It implies the satellite RO data could be used to study ionospheric structures/irregularities. The results also showed that the Es had more contribution at Sao Luis/Ilorin than that at Jicamarca/Kwajalein. However, the Es contribution to the S4 index during daytime need to be further studied in detail.
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Peculiarities of daytime and nighttime Pi 2 pulsations at the dip equator are examined by using multipoint measurements from the 210° magnetic meridian (MM) magnetometer network. We found that during daytime the amplitude of Pi 2 pulsations at the dip equator is enhanced, and the phase lags ∼ 34° behind those at low‐latitude (magnetic latitude Φ = 19.5‐46.2°) stations. On the other hand, during nighttime the amplitude of Pi 2 pulsations at the dip equator is depressed, and the phase lags ∼ 18° behind those at the lower latitudes. Because the zonal ionospheric conductivity at the dip equator is much higher than that at the off‐dip equator region, Pi 2 signals are expected to be distorted more effectively at the dip equator. The observations imply that the daytime and nighttime Pi 2 pulsations in the equatorial and low‐latitude regions can be explained by invoking an instantaneous penetration of electric field variations from the nightside polar ionosphere to the dayside equatorial ionosphere, and a direct incidence of compressional oscillations from the nightside inner magnetosphere, respectively.
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Plasmasphere
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Two induction magnetometers have been installed at Zhongshan Station and Davis Station, Antarctica respectively. We adopt with cross-spectral analysis technique to analyze the data of the two induction magnetometers, in June, September, December 1996 and March 1997. to investigate the Pc3 frequency range pulsation occurrence and propagation characteristics in the cusp latitudes. The results are summarized as following: At Zhongshan-Davis Stations, the Pc3 frequency range pulsations occur mainly around the local noon/ local magnetic noon and local magnetic midnight respectively. In daytime, the pulsations have a seasonal variation in amplitude, occurrence and temporal range, all of them are sma1lest in winter. But in nighttime, the pulsations have no such a variation. The pu1sation amplitude in nighttime is much larger than the one in daytime all oveI the year. The pulsation propagating direction is mainly western in daytime and irregularly in nighttime. It can be thought that the different sources of the pulsation and the ionospheric electric conductivity are mainly responsible for these characteristics.
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In this reseach, the changes in the low latitude critical frequencies (foF2) was investigated during the geomagnetically active hours (geomagnetic storms). The critical frequency (foF2) data obtained for 1991 over Manila that is low latitude ionosonde station and the planetary geomagnetic indices (Kp) were used. The superposed epoch analysis was used as a statistical method to investigate the change in the critical frequencies during geomagnetically active hours. The analyzes were conducted separately for the night hours, day hours and all hours of 1991 and they were compared with each other. The results from this research show that the highest change (increase or decrease) in foF2 values during geomagnetically active hours occurs at the event time (zero time) and the local time (day or night) has a significant impact on this change of the foF2 values. The foF2 values increase at local day hours, while the foF2 values decrease at local night hours.
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