Jiuhou Lei

Variations of electron density based on long‐term incoherent scatter radar and ionosonde measurements over Millstone Hill

[1] Measurements from the incoherent scatter radar (ISR) and ionosonde over Millstone Hill (42.6 D N, 288.5 D E) are analyzed to explore ionospheric temporal variations. The F-2 layer peak density NmF2, peak height h(m)F(2), and scale height H are derived from a Chapman a layer fitting to observed ISR electron density profiles. Diurnal, seasonal, and solar activity variations of the ionospheric characteristics are presented. Our study on the solar activity dependence of NmF2, h(m)F(2), and H indicates that the peak parameters (NmF2 and h(m)F(2)) of the F-2 layer increase with daily F-10.7 index and saturate ( or increase with a much lower rate) for very high F-10.7; however, they show almost linear dependence with the solar proxy index F-10.7p = (F-10.7 + F-10.7A)/2, where F-10.7A is the 81-day running mean of daily F-10.7. This suggests that the overall effect of solar EUV and neutral atmosphere changes on the solar activity variation of ionospheric ionization is linear with F-10.7p. The rate of change in the ionospheric characteristics with solar activity exhibits a seasonal and local time variation. Over Millstone Hill, NmF2 in summer is characterized by the evening peak in its diurnal variation, and NmF2 exhibits winter anomaly under low and high solar activity levels. The temporal variations of the topside effective scale height H-0 can be explained in terms of those in the slab thickness. The IRI model overestimates the N-e effective topside scale height over Millstone Hill; therefore our analysis for the effective topside scale height from the Millstone Hill measurements might help to improve the IRI topside profiles at middle latitudes.

An analysis of the scale heights in the lower topside ionosphere based on the Arecibo incoherent scatter radar measurements

[1] We statistically analyze the ionospheric scale heights in the lower topside ionosphere based on the electron density (Ne) and temperature profiles observed from the incoherent scatter radar (ISR) at Arecibo (293.2E, 18.3N), Puerto Rico. In this study, a database containing the Arecibo ISR observations from 1966 to 2002 has been used in order to investigate the diurnal and seasonal variations and solar activity dependences of the vertical scale height (VSH), which is deduced from the electron concentration profiles

Ionosphere response to recurrent geomagnetic activity: Local time dependency

[1] Observations of global positioning system total electron content (TEC) and in situ electron densities at altitudes of ~350-370 km from the CHAMP satellite are used to illustrate the local time and latitude dependence of 9 day periodicities in the ionosphere due to recurring high-speed solar wind streams and geomagnetic activity during 2005. A local time dependence is found, with nighttime TEC oscillations concentrated at high latitudes and close to ±40% of background levels. The largest oscillations in daytime TEC occur at midlatitudes and are ±25% of background levels. Furthermore, the daytime response is generally symmetric about the geomagnetic equator with anticorrelation between high and low latitudes, whereas at night the high-latitude Northern Hemisphere is generally in-phase with low latitudes and anticorrelated with the high-latitude Southern Hemisphere. A combination of enhanced equatorward neutral winds and changes in neutral composition are thought to be the primary mechanisms responsible for the observed ionospheric response. Although similar mechanisms are driving the response, the local time dependency arises because of the presence (lack) of photoionization during the daytime (nighttime). Similar trends are observed in CHAMP in situ electron densities; however, the oscillations at a near-constant altitude are ~10-15% larger than the TEC oscillations. Additionally, the CHAMP observations reveal possible variations in the strength of the equatorial ionization anomaly, indicating that disturbance dynamo electric fields may also contribute to the ionospheric response to recurrent geomagnetic activity. The results presented are the first to reveal the significant differences between the daytime and nighttime response of the ionosphere to periodic forcing from solar wind high-speed streams.

Solar activity variations of equivalent winds derived from global ionosonde data

equivalentwindsarefoundofnonlinearlydecreaseddiurnalamplitudesinallseasonsatmost stations. This implies that the increase in ion drag more than compensates for pressure gradients and thus restrains the diurnal amplitude at high solar activity. The diurnal phase of the derived equivalent winds generally shifts later at higher solar activity. It is the first time to explicitly report this striking feature that emerged at so many stations. Another pronounced feature is that the diurnal phase has a summer-winter difference. The diurnal phases at most stations in the Northern Hemisphere are later in winter than in summer at higher solar activity. Furthermore, a decrease in the semidiurnal amplitudes of equivalent winds with increasing solar activity is evident in winter over most stations considered and in other seasons at stations with a lower dip, but the decrease trend becomes weak in other seasonsatstationswithalargerdip.However,complicateddependencesonsolaractivitycan be found in the diurnal mean and the semidiurnal phases of equivalent winds at stations considered. INDEX TERMS: 2427 Ionosphere: Ionosphere/atmosphere interactions (0335); 3369 MeteorologyandAtmosphericDynamics:Thermosphericdynamics(0358);3309MeteorologyandAtmospheric Dynamics: Climatology (1620); 2162 Interplanetary Physics: Solar cycle variations (7536); KEYWORDS: ionosphere, climatology, solar activity variation

Ionosphere variability during the 2009 SSW: Influence of the lunar semidiurnal tide and mechanisms producing electron density variability

To investigate ionosphere variability during the 2009 sudden stratosphere warming (SSW), we present simulation results that combine the Whole Atmosphere Community Climate Model Extended version and the thermosphere-ionosphere-mesosphere electrodynamics general circulation model (TIME-GCM). The simulations reveal notable enhancements in both the migrating semidiurnal solar (SW2) and lunar (M2) tides during the SSW. The SW2 and M2 amplitudes reach ∼50 m s−1 and ∼40 m s−1, respectively, in zonal wind at E region altitudes. The dramatic increase in the M2 at these altitudes influences the dynamo generation of electric fields, and the importance of the M2 on the ionosphere variability during the 2009 SSW is demonstrated by comparing simulations with and without the M2. TIME-GCM simulations that incorporate the M2 are found to be in good agreement with Jicamarca Incoherent Scatter Radar vertical plasma drifts and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) observations of the maximum F region electron density. The agreement with observations is worse if the M2 is not included in the simulation, demonstrating that the lunar tide is an important contributor to the ionosphere variability during the 2009 SSW. We additionally investigate sources of the F region electron density variability during the SSW. The primary driver of the electron density variability is changes in electric fields. Changes in meridional neutral winds and thermosphere composition are found to also contribute to the electron density variability during the 2009 SSW. The electron density variability for the 2009 SSW is therefore not solely due to variability in electric fields as previously thought.

A statistical study of ionospheric profile parameters derived from Millstone Hill incoherent scatter radar measurements

Diurnal, seasonal, and solar activity variations of the bottomside electron density profile parameters B0 and B1, representing the F2 layer thickness and shape, are studied using a large incoherent scatter radar dataset for Millstone Hill covering the period 1976 - 2002. These results are compared with the latest IRI model. Our statistical study is characterized by morning and afternoon falls in the diurnal variation of B0 for seasons other than summer and a similar to 15% change in B1 over a solar cycle, features not fully well represented by the standard IRI model. The standard IRI B1, however, is very close to observations in terms of the diurnal variation.

Annual and semiannual variations of thermospheric density: EOF analysis of CHAMP and GRACE data

[1] In this paper, observations from CHAMP and GRACE during 2002–2010 are used to study the seasonal variations of thermospheric density by characterizing the dominant modes of thermospheric density variability as empirical orthogonal functions (EOFs). Our results showed that the first three EOFs captured most of the density variability, which can be as large as 98% of total density variability. Subsequently, the obtained mean field, first three EOFs and the corresponding amplitudes of three EOFs are applied to construct a thermospheric density model at 400 km to study seasonal variations of thermospheric density under geomagnetically quiet conditions. Thermospheric density shows strong latitudinal dependence in seasonal variation, although it usually has maxima near the equinoxes and minimum in the local winter at middle and high latitudes. Semiannual variations imbedded in the annual variations are seen at all latitudes; annual variations however become dominant in the southern hemisphere. Specifically, the observations show that the annual amplitude can reach as large as 40–50% of the annual mean at high latitudes in the southern hemisphere and it decreases gradually from the southern to northern hemisphere. The semiannual component to the annual mean is about 15–20% without significant latitudinal dependence. Additionally, the relative amplitudes of annual and semiannual variations in the MSISE00 density agree fairly well with the observations, albeit the MSISE00 gives an opposite solar activity dependence for the annual and semiannual variations compared with the positive F107 dependence seen in the observations.

Spatial and temporal variations in carbon and sulfur isotopic compositions of Sinian sedimentary rocks in the Yangtze platform, South China

Abstract Eight stratigraphic sections from the Yangtze Platform, South China were selected for a study of spatial and temporal variations in carbon and sulfur isotope compositions of Sinian sedimentary rocks. Carbon isotope compositions of carbonate from lower Sinian strata deposited between pulses of glacial diamictite range from −3.9 to −9.9‰; the more negative values recorded from Mn carbonates suggest that, in part, the carbon was derived from the oxidation of organic matter during early diagenesis. In the upper Sinian strata δ13C varies with the facies. They are positive for the platform carbonates, whereas they are negative from about −3 to −12‰ for the carbonates formed in or near basin environments. It is evident that post-depositional alteration accounts for the strongly negative δ13C values. However, some information on primary isotopic compositions can be obtained by evaluating sample quality. The primary δ13C values of carbonates from the Doushantuo Formation are ca −3 to −4‰, and from the Dengying Formation as low as −6‰. δ34S values for pyrite from the lower Sinian rocks are highly positive, and range from values near the coeval seawater sulfate up to ca +60‰. In contrast, δ34S values for pyrite from the upper Sinina rocks are negative down to −27 to −30‰. The feature of sulfur isotopic composition for the lower Sinian rocks is consistent with the late Proterozoic sulfur isotopic record. It might be explained by the ‘supercontinent model’: many basins on the Yangtze block in a supercontinent tectonic background had limited or no access to the open ocean and the basin waters contained sulfate with considerably higher δ34S values than the coeval seawater for a geologically long period. An alternative explanation is based on the sulfate-minimum zone (SMZ) modal. Because of the slow sinking of organic matter bacterial sulfate reduction would be important in the lower Sinian shallow water environment. Particularly in the SMZ, the residual sulfate might have been significantly enriched in 34S, so that pyrite produced in closed diagenetic system would yield very positive δ34S values.

Longitudinal and geomagnetic activity modulation of the equatorial thermosphere anomaly

[1] In this paper we examine the detailed similarities and differences between the equatorial thermosphere anomaly (ETA) and the equatorial ionization anomaly (EIA) from 20 March to 6 April 2002, when both the ETA and the EIA are distinct in the Challenging Minisatellite Payload (CHAMP) observations. The characteristics of the ETA and the EIA are obtained from the CHAMP accelerometer, in situ electron density measurements, and total electron content (TEC) above the CHAMP satellite. Our results show that the trough locations of the ETA and the EIA in latitude show a good agreement, and both correspond well with the dip magnetic equator, while the ETA crests are usually located poleward of the EIA. Meanwhile, the latitudinal locations of the ETA crests exhibit strong hemispheric asymmetry and large variability during our study interval. The longitudinal variations between the EIA and the ETA show significant differences. The EIA crests from the CHAMP observations show strong wave 4 structures, but the primary component in the ETA is wave 1. Moreover, the ETA densities show strong variations in response to geomagnetic activity, whereas CHAMP in situ electron densities and TEC at the EIA do not reflect such large day-to-day variability. Therefore, a simple EIA-ETA relationship cannot explain the dependence of the longitudinal and geomagnetic activity modulation of the ETA and the EIA. The meridional ion drag, which is significantly modulated by enhanced equatorward winds during elevated geomagnetic activity, is probably responsible for some of the observed features in the ETA, although no unambiguous explanation for ETA formation yet exists.

Long-duration depletion in the topside ionospheric total electron content during the recovery phase of the March 2015 strong storm

Topside ionospheric total electron content (TEC) observations from multiple low-Earth orbit (LEO) satellites have been used to investigate the local time, altitudinal, and longitudinal dependence of the topside ionospheric storm effect during both the main and recovery phases of the March 2015 geomagnetic storm. The results of this study show, for the first time, that there was a persistent topside TEC depletion that lasted for more than 3 days after the storm main phase at most longitudes, except in the Pacific Ocean region, where the topside TECs during the storm recovery phase were comparable to the quiet time ones. The observed depletion in the topside ionospheric TEC was relatively larger at higher altitudes in the evening sector and greater at local times closer to midnight. Moreover, the topside TEC patterns observed by MetOp-A (832 km) were different from those seen by other LEO satellites with lower orbital altitudes during the storm main phase and at the beginning of the recovery phase, especially in the evening sector. This suggests that the physical processes that control the storm time behavior of topside ionospheric response to storms are altitude-dependent.