N. Balan

Equatorial plasma fountain and its effects over three locations: Evidence for an additional layer, the F 3 layer

The equatorial plasma fountain and equatorial anomaly in the ionospheres over Jicamarca (77°W), Trivandrum (77°E), and Fortaleza (38°W) are presented using the Sheffield University plasmasphere-ionosphere model under magnetically quiet equinoctial conditions at high solar activity. The daytime plasma fountain and its effects in the regions outside the fountain lead to the formation of an additional layer, the F3 layer, at latitudes within about plus or minus 10° of the magnetic equator in each ionosphere. The maximum plasma concentration of the F3 layer, which occurs at about 550 km altitude, becomes greater than that of the F2 layer for a short period of time before noon when the vertical E × B drift is large. Within the F3 layer the plasma temperature decreases by as much as 100 K. The ionograms recorded at Fortaleza on January 15, 1995, provide observational evidence for the development and decay of an F3 layer before noon. The neutral wind, which causes large north–south asymmetries in the plasma fountain in each ionosphere during both daytime and nighttime, becomes least effective during the prereversal strengthening of the upward drift. During this time the plasma fountain is symmetrical with respect to the magnetic equator and rises to over 1200 km altitude at the equator, with accompanying plasma density depletions in the bottomside of the underlying F region. The north–south asymmetries of the equatorial plasma fountain and equatorial anomaly are more strongly dependent upon the displacement of the geomagnetic and geographic equators (Jicamarca and Trivandrum) than on the magnetic declination angle (Fortaleza).

Statistics of geomagnetic storms and ionospheric storms at low and mid latitudes in two solar cycles

[1] The statistics of occurrence of the geomagnetic storms, and ionospheric storms at Kokubunji (35.7°N, 139.5°E; 26.8°N magnetic latitude) in Japan and Boulder (40.0°N, 254.7°E; 47.4°N) in America are presented using the Dst and peak electron density (Nmax) data in 1985–2005 covering two solar cycles (22–23) when 584 geomagnetic storms (Dst ≤− 50 nT) occurred. In addition to the known solar cycle and seasonal dependence of the storms, the statistics reveal some new aspects. (1) The geomagnetic storms show a preference for main phase (MP) onset at around UT midnight especially for major storms (Dst ≤− 100 nT), over 100% excess MP onsets at UT midnight compared to a uniform distribution. (2) The number of positive ionospheric storms at Kokubunji (about 250) is more than double that at Boulder, and (3) the occurrence of the positive storms at both stations shows a preference for the morning‐noon onset of the geomagnetic storms as expected from a physical mechanism of the positive storms. (4) The occurrence of negative ionospheric storms at both stations follows the solar cycle phases (most frequent at solar maximum) better than the occurrence of positive storms, which agrees with the mechanism of the negative storms.

Dependence of ionospheric response on the local time of sudden commencement and the intensity of geomagnetic storms

A study has been designed specifically to investigate the dependence of the ionospheric response on the time of occurrence of sudden commencement (SC) and the intensity of the magnetic storms for a low- and a mid-latitude station by considering total electron content and peak electron density data for more than 60 SC-type geomagnetic storms. The nature of the response, whether positive or negative, is found to be determined largely by the local time of SC, although there is a local time shift of about six hours between low- and mid-latitudes. The time delays associated with the positive responses are low for daytime SCs and high for night-time SCs, whereas the opposite applies for negative responses. The time delays are significantly shorter for mid-latitudes than for low-latitudes and, at both latitudes, are inversely related to the intensity of the storm. There is a positive correlation between the intensity of the ionospheric response and that of the magnetic storm, the correlation being greater at mid-latitudes. The results are discussed in the light of the possible processes which might contribute to the storm-associated ionospheric variations.

Plasmaspheric electron content in the GPS ray paths over Japan under magnetically quiet conditions at high solar activity

Vertical total electron content (GPS-TEC) data obtained from the dual-frequency GPS receiver network (GEONET) in Japan are compared with those calculated using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model is also used to estimate the electron content in the plasmaspheric sections of GPS ray paths for the three seasons of high solar activity (F10.7 = 165) under magnetically quiet conditions. According to the estimates, the plasmaspheric sections of vertical GPS ray paths over Japan at altitudes above the O+ to H+ transition height and above the upper altitude (2500 km) of Faraday rotation contain up to 11 and 9 TEC units (1 TEC unit = 1016 electrons m−2) of free electrons, respectively. The free electrons present above the Faraday rotation altitude can cause propagation errors of up to 4.9 ns in time delay and 1.6 m in range at the GPS L1 (1.57542 GHz) frequency. The plasmaspheric electron content, PEC, changes appreciably with season and latitude and very little with the time of the day. However, the percentage contribution of PEC to GPS-TEC changes most significantly with the time of the day; the contribution varies from a minimum of about 12% during daytime at equinox to a maximum of about 60% at night in winter.

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

Abstract The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density ( N max) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and N max, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes. At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and N max (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F -region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and N max and stronger daytime N max in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.

Plasma temperature variations in the ionosphere over the middle and upper atmosphere radar

The temperature variations in the ionosphere over the middle and upper atmosphere radar at Shigaraki (34.85°N, 136.10°E, magnetic latitude 25°N) in Japan are studied using the electron and ion temperature (Te and Ti, respectively) data measured by the radar during nearly a full solar cycle (1986–1995). A comprehensive picture of the diurnal, seasonal, and solar activity variations of Te and Ti is presented for the altitude range 200–550 km. The temperatures Te and Ti are found to have similar diurnal and altitude variations and different seasonal and solar activity dependence. With season, while daytime Te is highest in summer and lowest in equinox, daytime Ti is highest in equinox and lowest in summer. With solar activity, while daytime Te decreases, the corresponding Ti increases. The diurnal variation of Te is characterized by morning and evening peaks. The occurrence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoelectron heating of the morning and evening electron gas. However, neutral winds play a dominant role in the appearance of the peaks. A poleward wind, which reduces the electron density to a low value before sunset, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in Te are illustrated through model calculations using the Sheffield University plasmasphere-ionosphere model.

Night-time enhancements in TEC at equatorial anomaly latitudes

Abstract The seasonal and solar activity variations of night-time total electron content (TEC) enhancements and their latitudinal and longitudinal dependencies in the northern equatorial anomaly region (11–23° geomagnetic latitude) are studied by using data from three eastern longitude stations and one western longitude station. Two kinds of enhancement are observed: a postsunset enhancement which occurs most frequently in autumn and a postmidnight enhancement which occurs most frequently in summer. The enhancements, especially the postmidnight enhancements, are more frequent and stronger at the eastern longitude stations than at the western longitude station. The enhancements are weak and least frequent at 23° geomagnetic latitude; the seasonal dependence of the postsunset enhancements at this latitude is different from that at lower latitudes confirming that the effect of the prereversal enhancement of the equatorial fountain, which produces the postsunset enhancements in TEC in the equatorial anomaly region, rarely reaches this latitude even under very high levels of solar activity.

Additional stratifications in the equatorial F region at dawn and dusk during geomagnetic storms: Role of electrodynamics

[1] The role of electrodynamics in producing additional stratifications in the equatorial F region (F 3 layer) at dawn and dusk during geomagnetic storms is discussed. Two cases of F 3 layer at dawn (0600-0730 LT on 5 October 2000 and 8 December 2000) and one case of F 3 layer at dusk (1600-1730 LT on 5 October 2000) are observed, for the first time, by the digital ionosonde at the equatorial station Trivandrum (8.5°N; 77°E; dip ∼ 0.5°N) in India. The unusual F 3 layers occurred during the geomagnetic storms and are associated with southward turning of interplanetary magnetic field B z , suggesting that eastward prompt penetration electric field could be the main cause of the F 3 layers. The dawn F 3 layer on 5 October is modeled using the Sheffield University Plasmasphere-Ionosphere Model by using the E x B drift estimated from the real height variation of the ionospheric peak during the morning period. The model qualitatively reproduces the dawn F 3 layer. While the existing F 2 layer rapidly drifts upward and forms the F 3 layer and topside ledge, a new layer forming at lower heights develops into the normal F 2 layer.

Modeling studies of equatorial plasma fountain and equatorial anomaly

Abstract The importance of diffusion, electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earths ionosphere is studied using the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) for the ionosphere above Jicamarca (77°W) under magnetically quiet ( Ap = 4) equinoctial conditions (day 264) at medium solar activity ( F 10.7 = 145). The study also investigates the effects of the fountain, which include the equatorial anomaly. The F -region vertical E × B drift velocity measured at the equatorial station Jicamarca is used to represent the electrodynamic drift. The neutral wind is obtained from the HWM90 thermospheric wind model. As expected, the F -region electrodynamic drift generates the plasma fountain and the anomaly, which are symmetric with respect to the equator. The neutral wind makes the fountain and the anomaly asymmetric, with larger plasma flow (towards the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. The paper also addresses many important (some new) features which are related to the fountain. The features are: (1) the possibility of existence of an additional layer (called the G-layer) in the equatorial ionosphere, (2) the reverse plasma fountain, (3) the equatorial anomaly in vertical ionospheric electron content (IEC), (4) the presence (in Nmax) and absence (in IEC) of noon bite-out, (5) the occurrence of nighttime increase in ionization, and (6) plasma bubbles and spread- F .

Simultaneous mesosphere-lower thermosphere and thermospheric F region observations using middle and upper atmosphere radar

Simultaneous MLT (mesosphere-lower thermosphere) and thermospheric F region (upper thermosphere and ionosphere together) observations conducted using the middle and upper atmosphere (MU) radar (35 degrees N, 136 degrees E) in alternate meteor and incoherent scatter modes in October 2000 and March 2001 are presented. The continuous observations, each lasting more than a week, provide simultaneous zonal and meridional wind velocities at MLT altitudes (80-95 km), meridional wind velocity in the upper thermosphere (220-450 km), and electron density and peak height in the ionosphere with a time resolution of 1.5 hours. The data seem to suggest that the upper atmospheric regions could be dynamically coupled through mean winds, tides, and waves. Diurnal (24-hour) and semidiurnal (12-hour) tides and waves of periods 16-20 hours and 35-55 hours coexist at MLT and upper thermosphere altitudes, and the waves become stronger than tides at mesopause (approximate to 88 km) in both October and March. The data in these equinoctial months also show large differences in mean winds, tides, and waves in the MLT region. The amplitudes and phases of the 24-hour and 12-hour tides at MLT altitudes are compared with those predicted by the global scale wave model (GSWM). The model qualitatively predicts the observed growth of the tides with altitude but does not predict the 12-hour tide becoming stronger than the 24-hour tide at altitudes above mesopause in October.