Geomagnetic Activity Dependence Research Articles

The Influence of the Inner Magnetospheric Electric Field on Plasma Sheet Access

AbstractThis study presents observations of the large‐scale electric field and H+ fluxes over the entire Van Allen Probes mission duration. These observations are used to determine the spatial distributions of the azimuthal component of the ExB drift velocity and the plasma sheet H+ pressure, and their geomagnetic activity dependence. To investigate the influence of the inner magnetospheric electric field on H+ plasma sheet access, the boundary of zero azimuthal ExB drift velocity (BZEB) is identified as a proxy of the inner extent of the convection electric field and its characteristics are compared with that of the inner edge of the H+ plasma sheet (H+ IEPS). We find that the large‐scale convection electric field has a dominant role on H+ plasma sheet access. This is evidenced by the good correlation between the BZEB and the H+ IEPS. Both boundaries present overall deeper access with increasing Kp. Moreover, both boundaries exhibit a similar deeper access on the evening sector compared to the afternoon sector, and both boundaries are more closely collocated in L for moderate and high Kp, while for low Kp the H+ IEPS is located at lower L than the BZEB. Furthermore, the collocation of both boundaries in L is better in the evening sector than in the afternoon sector, where the H+ IEPS lies farther inward in the afternoon sector for all Kp levels. This implies that the afternoon sector is the preferred region of the overlap between the hot ion plasma sheet and the cold plasmasphere.

Geomagnetic activity dependence and dawn-dusk asymmetry of thermospheric winds from 9-year measurements with a Fabry–Perot interferometer in Tromsø, Norway

Ion drag associated with the ionospheric plasma convection plays an important role in the high-latitude thermospheric dynamics, yet changes in the thermospheric wind with geomagnetic activity are not fully understood. We performed a statistical analysis of the thermospheric wind measurements with a Fabry–Perot interferometer (FPI; 630 nm wavelength) in Tromsø, Norway, in the winter months for 9 years. The measurements were sorted by a SuperMAG (SME) index, and a quiet-time wind pattern was defined as an hourly mean under SME ≤ 40 nT. The quiet-time wind pattern can be expected to be represented by a pressure gradient associated with the solar radiation and a geostrophic force balance. With an increase in the geomagnetic activity level, the thermospheric wind turned over from eastward to westward at dusk and increased the equatorward magnitude from midnight to dawn. Deviations from the quiet-time wind presented similar patterns in the direction with the ionospheric plasma convection but were larger in magnitude at dusk than at dawn. This is the first study to report a dawn-dusk asymmetry of the thermospheric wind acceleration feature and signatures of the eastward wind acceleration at dawn by ion drag.Graphical

Study of the Average Ion Mass of the Dayside Magnetospheric Plasma

AbstractThe investigation of heavy ions dynamics and properties in the Earth's magnetosphere is still an important field of research as they play an important role in several space weather aspects. We present a statistical survey of the average ion mass in the dayside magnetosphere made comparing plasma mass density with electron number density measurements and focusing on both spatial and geomagnetic activity dependence. Field line resonance frequency observations across the European quasi‐Meridional Magnetometer Array, are used to infer the equatorial plasma mass density in the range of magnetic L‐shells 1.6–6.2. The electron number density is derived from local electric field measurements made on Van Allen Probes using the Neural‐network‐based Upper‐hybrid Resonance Determination algorithm. The analysis is conducted separately for the plasmasphere and the plasmatrough during favorable periods for which both the plasma parameters are observed simultaneously. We found that throughout the plasmasphere the average ion mass is ≃1 amu for a wide range of geomagnetic activity conditions, suggesting that the plasma mainly consist of hydrogen ions, without regard to the level of geomagnetic activity. Conversely, the plasmatrough is characterized by a variable composition, highlighting a heavy ion mass loading that increases with increasing levels of geomagnetic disturbance. During the most disturbed conditions, the average radial structure shows a broad maximum around 3–4 Earth radii, probably correlated with the accumulation of oxygen ions near the plasmapause. Those ions are mostly observed in the post‐dawn and pre‐dusk longitudinal sectors.

Climatology of Nighttime Upper Thermospheric Winds From Fabry‐Perot Interferometer 2011–2019 Measurements Over Kelan (38.7°N, 111.6°E), China: Local Time, Seasonal, Solar Cycle, and Geomagnetic Activity Dependence

AbstractFor the first time, we develop a local time and day of year dependent climatological model in different geomagnetic activity based on 630.0 nm Fabry‐Perot Interferometer (FPI) measurements over Kelan (38.7°N, 111.6°E) during 2011–2019. With this model, we analyze the regional climatological characteristics of nighttime thermospheric neutral winds, and a comparison is made with two recent versions of the Horizontal Wind Model series (HWMs), HWM07 and HWM14. Due to the location of Kelan far from the northern magnetic pole longitude at middle latitude, the statistical characterizations of meridional and zonal winds show some unique local time, seasonal, solar cycle and geomagnetic activity dependence. There are predominantly annual variations of horizontal winds characterized by rapid transitions from eastward December winds to westward or less eastward June winds and largest equatorward meridional winds during summer months. To some degree, the wind patterns show casual solar activity dependence especially for the zonal component. The maximum southward winds show negative relationship with increasing solar activity in summer months, while the maximum eastward winds present negative trend in winter months. The Kp dependencies are characterized by larger westward perturbation winds for all night and enhanced northward winds in the premidnight sector and equatorward surges in the postmidnight sector during strongly active conditions. With increasing solar flux, the westward perturbation winds become weaker at nights. Results also reveal that HWM14 predictions are in better agreement with Kelan FPI winds in the shape and magnitude than HWM07, and the consistency improves with increasing Kp.

HL‐TWiM Empirical Model of High‐Latitude Upper Thermospheric Winds

AbstractWe present an empirical model of thermospheric winds (High‐latitude Thermospheric Wind Model [HL‐TWiM]) that specifies F region high‐latitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HL‐TWiM represents the large‐scale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical high‐latitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 ground‐based stations (Fabry‐Perot Interferometers and Scanning Doppler Imaging Fabry‐Perot Interferometers) located at various northern and southern high latitudes and two space‐based instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HL‐TWiM are represented using vector spherical harmonics, day of year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B splines. In this paper, we describe the HL‐TWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HL‐TWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Model‐data intra‐annual comparisons in this study show semiannual oscillation‐like behavior of GOCE winds, rarely observed before in wind data.

The latitudinal structure of nighttime ionospheric TEC and its empirical orthogonal functions model over North American sector

AbstractWe collected total electron content (TEC) data in the longitudinal sector of 60°W–90°W during 1999–2015 to investigate the latitudinal variation of nighttime middle‐ and high‐latitude ionosphere. The midlatitude trough is one of the important features of the nighttime ionosphere. The statistical analysis provides unprecedented detail of the local time, seasonal, solar activity, and geomagnetic activity variations of the total electron content in the latitude range of 40°N–75°N, focusing on the variation of midlatitude trough. The results show that the trough minimum position has significant local time, seasonal, and geomagnetic activity dependences but slight solar activity dependence. In addition, an empirical model of the TEC in the middle to high latitudes was constructed by empirical orthogonal function analysis. The empirical model can reconstruct latitudinal profiles of TEC and well reproduce the dependence of the midlatitude trough on local time, seasonal, solar cycle, and geomagnetic activity. In addition, we also analyzed the geomagnetic activity dependence of TEC at different latitudes and different local time sectors.

Geomagnetic and solar activity dependence of ionospheric upflowing O+: FAST observations

This paper investigates the dependence of the occurrence frequency of ionospheric upflowing oxygen (O+) ions on the sunspot cycle and geomagnetic activity. We examine the upflows response to the geomagnetic disturbances as well as the influence of the ion energy factor in controlling the magnitude of the occurrence frequency and the net energy flux. We discuss the spatial distribution of the upflow occurrence frequency and construct a regression model as a function of the magnetic latitude. The results show an overall enhancement of the upflow occurrence frequency during magnetically disturbed periods and indicate that the high-occurrence area spreads out from the source regions during magnetically quiet periods. The high-occurrence areas are located at 70° magnetic latitude (mLat) in the dayside auroral oval zone and between 76–80° mLat in the dayside polar cusp region. In the nightside auroral oval zone, these areas are near 60° mLat, penetrating further equatorward to 55° mLat during magnetically disturbed periods. High energy (≥1 keV) upflowing ions are common in the nightside auroral oval zone while low energy (<1 keV) upflowing ions are found escaping from the high latitude dayside cusp region. A Gaussian function is shown to be a good fit to the occurrence frequency over the magnetic latitude. For high energy upflowing O+ ions, the occurrence frequency exhibits a single peak located at about 60° mLat in the nightside auroral oval zone while for low energy upflowing O+ ions, it exhibits two peaks, one near 60° mLat in the auroral oval zone and the other near 78° mLat in the cusp region. We study the solar activity dependence by analyzing the relationship between the upflow occurrence frequency and the sunspot number (\(R_{Z}\)). The statistical result shows that the frequency decreases with declining solar activity level, from ∼30 % at solar maximum to ∼5 % at solar minimum. In addition, the correlation coefficient between the occurrence frequency and \(R_{Z}\) is 0.9.

Statistical analysis of ionospheric mid-latitude trough over the Northern Hemisphere derived from GPS total electron content data

This study statistically investigated the seasonal variation, magnetic local time (MLT) variation, geomagnetic activity dependence, and solar activity dependence of the mid-latitude trough using GPS total electron content (TEC) data from 2000 to 2014. The daily median Kp index was used to characterize the daily geomagnetic activity level. The results showed that the trough minimum position depended primarily on the geomagnetic activity, MLT, and the season. The trough depth depended primarily on the solar flux index (F107) and, to a lesser degree, on MLT. The trough depth increased as F107 increased and as the incidence angle of solar flux decreased. The trough equatorward half-width decreased as the geomagnetic activity increased. These variations in the GPS-TEC trough minimum position were compared with the variations in the TEC trough derived from the International Reference Ionosphere (IRI)-2007 model. The GPS-TEC trough minimum position changed little with respect to F107, whereas the IRI-TEC trough minimum position showed a strong F107 dependence.

Altitude and solar activity dependence of 1967–2005 thermospheric density trends derived from orbital drag

AbstractWe examine 1967–2005 thermospheric mass density trends (as well as 1967–2013 trends) derived from satellite orbit data, as a function of altitude, solar flux, and geomagnetic activity. At 400 km altitude, the estimated 1967–2005 trend is −2.0 ± 0.5% per decade. The estimated trends become increasingly negative with increasing height between 250 and 575 km, suggesting an exospheric temperature trend of −1 to −2 K per decade, which is much smaller than temperature trends that have been inferred from ground‐based incoherent scatter radar measurements. The orbit‐derived trend height profiles are in good agreement with model simulations of the enhanced cooling that results from increasing concentration of CO2 in the mesosphere and lower thermosphere. In contrast to earlier results, the solar flux dependence of the estimated trends is weak, relative to the trend uncertainty. There is some indication that the trends may be stronger during very low geomagnetic activity conditions. Estimation of the solar flux and geomagnetic activity dependence of the trends is complicated by monotonic decreases in these drivers over the past four solar minima together with the CO2 increase, all of which drive interminima decreases in density.

ULF wave electromagnetic energy flux into the ionosphere: Joule heating implications

AbstractUltralow‐frequency (ULF) waves—in particular, Alfvén waves–transfer energy into the Earth's ionosphere via Joule heating, but it is unclear how much they contribute to global and local heating rates relative to other energy sources. In this study we use Time History of Events and Macroscale Interactions during Substorms satellite data to investigate the spatial, frequency, and geomagnetic activity dependence of the ULF wave Poynting vector (electromagnetic energy flux) mapped to the ionosphere. We use these measurements to estimate Joule heating rates, covering latitudes at or below the nominal auroral oval and below the open/closed field line boundary. We find ULF wave Joule heating rates (integrated over 3–30 mHz frequency band) typically range from 0.001 to 1 mW/m2. We compare these rates to empirical models of Joule heating associated with large‐scale, static (on ULF wave timescales) current systems, finding that ULF waves nominally contribute little to the global, integrated Joule heating rate. However, there are extreme cases with ULF wave Joule heating rates of ≥10 mW/m2—in these cases, which are more likely to occur when Kp ≥ 3, ULF waves make significant contributions to the global Joule heating rate. We also find ULF waves routinely make significant contributions to local Joule heating rates near the noon and midnight local time sectors, where static current systems nominally contribute less to Joule heating; the most important contributions come from lower frequency (&lt;7 mHz) waves.

Variation of the ionospheric scintillation index with elevation angle of the transmitter

We have analyzed GPS data from 2007–2011 to determine the nature of variation of scintillation index with elevation of the direction of propagation at an observing point Warsaw, Poland, and Hornsund, Svalbard. To compare with the theory, the intensity scintillation index is simulated as a function of elevation angle, azimuth, magnetic field inclination, and shape of irregularities, using the phase screen model of scintillation as formulated by Rino (1979). Data analysis has been done for the seasonal as well as geomagnetic activity dependence of ionospheric scintillation. Scintillation index is a power-law function of the cosecant of the elevation angle. Results show that the power law strongly depends on the form of irregularities, being larger than in isotropic case for irregularities with dimension along the magnetic field direction smaller than those across the magnetic field. The present work also shows the need to use experimentally derived dependence on elevation.

Variance of the vertical ion speed measured with the European Incoherent Scatter (EISCAT) UHF radar in the polar lower ionosphere at Tromsø, Norway

[1] The vertical component of the ion velocity measured with the European Incoherent Scatter (EISCAT) Tromso UHF radar (69.6°N, 19.2°E) in the lower ionosphere (from 95 to 130 km) was found to be characterized by notably large variances at oscillation periods of 2–8 min. Of particular interest was the geomagnetic activity dependence above ∼106 km, which showed larger variances (100–500 m2 s−2) during periods of geomagnetic disturbance than during quiet periods. Below 106 km, the variance was less sensitive to the geomagnetic activity. Height profiles of the variance during the disturbed period showed steep increases with heights above 106 km, then reaching a peak at 120 km, where the ion gyrofrequency is equal to the modeled ion-neutral collision frequency. A theoretical prediction well reproduced the height profile for the geomagnetically disturbed period by assuming oscillations in the meridional component of the electric field. The theoretical study suggested that the electric field oscillation is a possible cause of large variances of the vertical ion speed in the polar lower ionosphere. However, below 106 km, other mechanisms were necessary.

Validation of the STORM model in IRI-2001 at a high latitude station

The International Reference Ionosphere IRI-2001 model contains geomagnetic activity dependence based on an empirical storm time ionospheric correction (STORM model). An extensive validation of the STORM model for the middle latitude region has been performed. In this paper the ability of the STORM model to predict foF2 values at high latitudes is analyzed. For this, ionosonde data obtained at Base Gral. San Martin (68.1°S, 293°E) are compared with those obtained by the IRI-2001 model with or without storm correction during four geomagnetic storms that occurred in 2000 (Rz 12 = 117) and 2001 (Rz 12 = 111). The results show that predicted values with the STORM model follow the behaviour of foF2 experimental data better than without the STORM model. The relative deviation between measured and predicted foF2 reaches values of up to 24% and 43% with and without the STORM model in IRI-2001, during the main phase of the storms. In order to explain increases of electron density that occurred prior to the storm onset and also decreases of electron density observed during the first part of the recovery of the storm, possible physical mechanisms are discussed.

Geomagnetic activity dependence of O + in transit from the ionosphere

Energetic O + ions have important dynamic effects on the ring current. Insights into the effects of O + on ring current dynamics have come primarily from models, not observations. Here, we discuss observations of O + populations escaping from the ionosphere and their access to the plasma sheet and ring current. We review data establishing that a significant flux of O + escapes the ionosphere during geomagnetically quiet intervals. We then estimate the relative magnitude of the O + population in transit between the ionosphere and ring current during quiet intervals before geomagnetic storms. Our analysis suggests that dynamic reconfigurations of the magnetosphere during geomagnetic storms significantly alter the O + transport paths from the ionosphere to the ring current. During these reconfigurations some of the pre-existing, quiet time, in-transit O + populations are captured on magnetic field lines leading to the ring current. The prompt appearance of this O + population in the ring current could modify the evolution of the ring current in the storm growth phase. Our analysis suggests that the consequences of an activity-dependent O + transport path to the ring current should be systematically investigated.

Longitudinal variability of low‐latitude total electron content: Tidal influences

Recently, nighttime ultraviolet (UV) observations obtained by IMAGE FUV and TIMED GUVI instruments have revealed a longitudinal wave number four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions. In the present study, we have extended this work and determined the longitudinal variability of the low‐latitude total electron content (TEC) climatology during different geophysical conditions with a special emphasis on the longitudinal wave number four structure in the low‐latitude ionosphere. We have used more than 5 million low‐latitude TOPEX TEC observations covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data set was used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes, and in particular, to address the existence and evolution of the wave number four longitudinal pattern under these conditions. Our study shows that the wave number four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during December solstice. During equinox the wave number four pattern is created around noon with well‐defined longitudinal enhancements in the low‐latitude TEC. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wave number four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The variations between the well‐defined longitudinal maxima and minima are of the order of 20%. During June solstice, the wave number four pattern is also observed in the afternoon hours but, in contrast to the equinox cases, it exhibits a strong hemispheric asymmetry and is not observed during the night. The low‐latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the east Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic Ocean, but a clear wave number four pattern is not observed during this season. Although the equatorial and low‐latitude TEC values exhibit clear longitudinal patterns during all seasons, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle, the season, or the local time or by the longitudinal variability. This remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40%.

Evaluation of International Reference Ionosphere 2000 at a midlatitude station

To check the validity of the latest version of the International Reference Ionosphere (IRI‐2000), which contains geomagnetic activity dependence based on an empirical storm time ionospheric correction (STORM model), comparison of measured foF2 values with those obtained from the model was made. To quantify the degree of accuracy of the IRI‐2000 model during disturbed conditions, the relative difference between the model outputs (with STORM model turned on and turned off options) and experimental data was calculated. The ionosonde foF2 data used were obtained at Ebro Observatory (40.8°N, 0.49°E; geomagnetic latitude 43.2°N) during intense geomagnetic storms occurring in the years 2000 and 2001 (high solar activity). Although only a few case study comparisons have been made, the results show that during storm conditions, predicted values with the STORM model included in IRI‐2000 follow the variations of foF2 data better than IRI‐2000 without the STORM model. In general, IRI‐2000 with the STORM model has almost 30–40% improvement over IRI‐2000 without the STORM model. The greater deviations between model outputs and observations are observed during the end of the main phase and early stage of the recovery phase. For this station, in general, IRI predictions with the STORM model underestimate foF2 data in February–March and significantly overestimate them in April–May.

Geomagnetic activity dependence of the inner magnetospheric proton distribution: An empirical approach for the 21–25 April 2001 storm

The empirical approach in global modeling the proton distributions of the inner magnetosphere (Orsini et al., 2004) is here applied for a detailed analysis of the 21–25 April 2001 storm. This method, based on comparison of the Milillo et al. (2001) model with local proton spectra, results as a simple and powerful tool for inner magnetospheric studies. In this work we use the Los Alamos National Laboratory proton spectra in order to obtain the time‐evolving model parameters at different magnetic local times (MLT). We compare the model parameters with the geomagnetic indexes as well as with the solar wind data. The final outcome of this study is the analysis of the evolution of the magnetospheric populations during storms. This analysis shows that specific local characteristics of the proton fluxes, as the width of proton spectra, can be interpreted as global plasma distributions tracers. Furthermore, their temporal evolution is recurrent during any storm development, depending on solar wind characteristics. In particular, this study suggests the existence of a MLT‐dependent time delay between the solar wind disturbance arrival and the broadening of the energy spectra of proton fluxes in the inner magnetosphere. Finally, the model shows how the particles convected or injected from the nightside during storm main phase quickly decay or diffuse gaining energy so that the diffused particles play a major role during the recovery phase.

Acceleration sites of energetic ions upstream of the Earth's bow shock and in the magnetosheath: Statistical study on charge states of heavy ions

In order to identify acceleration sites of energetic ions upstream of the Earth's bow shock and in the dayside magnetosheath, we investigated energetic ion events observed by the Energetic Particles and Ion Composition (EPIC) instrument on board the Geotail spacecraft from 1995 to 2001. Energetic ion data obtained by the Ion Composition System (ICS) sensor were used to select “events,” that is, intervals when the 77–107 keV ion flux increased by more than two orders of magnitude within 10 min, and examine their spatial distribution and its geomagnetic activity dependence. Heavy ions with energy range of 9–210 keV/e detected by the Suprathermal Ion Composition Spectrometer (STICS) sensor were chosen to calculate the percentage of low‐charge‐state heavy ions (PLCS = (O+ + N+)/(O+ + N+ + O6+ + O7+)) in each event, and the events were classified into three groups: high‐charge‐state (HCS) events (PLCS ≤ 25%), intermediate (IM) events (25% &lt; PLCS &lt; 75%), and low‐charge‐state (LCS) events (PLCS ≥ 75%). The HCS and LCS events occurred at almost the same rate in geomagnetically quiet conditions (SYM‐H ≥ 0 nT), while the LCS events were observed more frequently than the HCS events in geomagnetically disturbed conditions (SYM‐H ≤ −30 nT). The percentage of the IM and LCS events summed was higher in disturbed conditions than in quiet conditions. The HCS and LCS events were frequently observed in the dawnside upstream region and in the duskside magnetosheath, respectively. The IM events occurred both in the dawnside upstream region and in the magnetosheath around noon. We conclude that energetic ion events including ions accelerated in the magnetosphere are dominant. Ions accelerated in the magnetosphere leak out from the duskside magnetopause, in particular when geomagnetic activity is high, and then most likely mix with ions accelerated at the bow shock on the way to the dawnside upstream region. The acceleration site of the HCS events is also discussed.

Time Empirical Ionospheric Correction Model (STORM) response in IRI2000 and challenges for empirical modeling in the future

IRI2000 [Bilitza, 2001] now contains a geomagnetic activity dependence based on the Time Empirical Ionospheric Correction Model (STORM) [Araujo‐Pradere and Fuller‐Rowell, 2002; Araujo‐Pradere et al., 2002]. The storm correction is driven by the previous time history of ap and is designed to scale the quiet time F layer critical frequency (foF2) to account for storm‐time changes in the ionosphere. The quality of the storm‐time correction was recently evaluated by comparing the model with the observed ionospheric response during all the significant geomagnetic storms in 2000 and 2001. The model output was compared with the actual ionospheric response at 15 stations for each storm. These quantitative comparisons using statistical metrics showed that the model captures the decreases in electron density particularly well in summer and equinox conditions, but is not so good during winter conditions. To further assess the capabilities of the model, STORM has been compared in detail with observations during the Bastille Day storm in July 2000. This storm, considered to be on the extreme end of the statistical scale of storm magnitude, highlights two main areas were challenges remain for the empirical storm‐time ionospheric model. The first is the rapid onset of the positive storm phase; the second is the regional composition changes that can affect one longitude sector at the expense of another for a particular storm. Both these challenges, although appreciated during the development of STORM, remain to be addressed.

Evaluation and prospects for storm-time corrections in the International Reference Ionosphere

IRI2000 [Radio Sci. 36 (2) (2001) 261] now contains a geomagnetic activity dependence based on an empirical storm-time ionospheric correction model STORM [Geof. Int. 39 (1) (2000) 29; Radio Sci. 37 (5) (2002) 1070] The storm correction is driven by the previous time-history of a p , and is designed to scale the quiet-time F-layer critical frequency ( f oF2) to account for storm-time changes in the ionosphere. The quality of the storm-time correction has been evaluated by comparing the model with the observed ionospheric response during all the significant geomagnetic storms in 2000 and 2001. The model output was compared with the actual ionospheric response at 15 stations for each storm. The comparisons show the model captures the decreases in electron density particularly well in summer and equinox conditions, but has less quality during winter conditions. The value of the model has been quantified by comparing the data with the IRI2000 model, which contains the storm correction, and with IRI95, which has no geomagnetic activity dependence. The results of this study illustrate that the root mean square error (RMSE) with IRI2000 has a 28% improvement over IRI95 during the storm days, a significant improvement. A similar model for TEC is currently being developed based on data from ground-based dual frequency GPS observations. During the Bastille Day storm the GPS data over Florida showed dramatic changes. Data such this will be used to develop an empirical storm-time TEC correction model adopting a similar approach, as was done for the F-region model.