F-region Peak Research Articles

Studies of ionospheric F-region response in the Latin American sector during the geomagnetic storm of 21–22 January 2005

Abstract. In the present investigation, we have studied the response of the ionospheric F-region in the Latin American sector during the intense geomagnetic storm of 21–22 January 2005. This geomagnetic storm has been considered "anomalous" (minimum Dst reached −105 nT at 07:00 UT on 22 January) because the main storm phase occurred during the northward excursion of the Bz component of interplanetary magnetic fields (IMFs). The monthly mean F10.7 solar flux for the month of January 2005 was 99.0 sfu. The F-region parameters observed by ionosondes at Ramey (RAM; 18.5° N, 67.1° W), Puerto Rico, Jicamarca (JIC; 12.0° S, 76.8° W), Peru, Manaus (MAN; 2.9° S, 60.0° W), and São José dos Campos (SJC; 23.2° S, 45.9° W), Brazil, during 21–22 January (geomagnetically disturbed) and 25 January (geomagnetically quiet) have been analyzed. Both JIC and MAN, the equatorial stations, show unusually rapid uplifting of the F-region peak heights (hpF2/hmF2) and a decrease in the NmF2 coincident with the time of storm sudden commencement (SSC). The observed variations in the F-region ionospheric parameters are compared with the TIMEGCM model run for 21–22 January and the model results show both similarities and differences from the observed results. Average GPS-TEC (21, 22 and 25 January) and phase fluctuations (21, 22, 25, 26 January) observed at Belem (BELE; 1.5° S, 48.5° W), Brasilia (BRAZ; 15.9° S, 47.9° W), Presidente Prudente (UEPP; 22.3° S, 51.4° W), and Porto Alegre (POAL; 30.1° S, 51.1° W), Brazil, are also presented. These GPS stations belong to the RBMC/IBGE network of Brazil. A few hours after the onset of the storm, large enhancements in the VTEC and NmF2 between about 20:00 and 24:00 UT on 21 January were observed at all the stations. However, the increase in VTEC was greatest at the near equatorial station (BELE) and enhancements in VTEC decreased with latitude. It should be pointed out that no phase fluctuations or spread-F were observed in the Latin American sector during the post-sunset pre-reversal time in the geomagnetic disturbance (21 January). The disturbance dynamo electric field possibly resulted in downward drift of the F-region plasma and inhibited the formation of spread-F.

Dependence of the F-region peak electron density (foF2) on solar activity observed in the equatorial ionospheric anomaly region in the Brazilian sector

The long-term (solar cycle) changes in the Sun and how it affects the ionospheric F-region observed at São José dos Campos (23.2° S, 45.9° W), Brazil, a location under the southern crest of the equatorial ionospheric anomaly, have been investigated in this paper. The dependence of the F-region peak electron density (foF2) on solar activity during the descending phase of the 23rd solar cycle for the periods of high, medium, and low solar activity has been studied. The ionospheric F-region peak electron densities observed during high and medium solar activity show seasonal variations with maxima close to the equinox periods, whereas during the low solar activity the maxima during the equinox periods is absent. However, during the low solar activity only change observed is a large decrease from summer to winter months. We have further investigated changes in the different ionospheric F-region parameters (minimum virtual height of the F-region (h′F), virtual height at 0.834foF2 (hpF2), and foF2) during summer to winter months in low solar activity periods, 2006–2007 and 2007–2008. Large changes in the two ionospheric parameters (hpF2 and foF2) are observed during summer to winter months in the two low solar activity periods investigated.

Spread F, GPS phase fluctuations, and medium-scale traveling ionospheric disturbances over Wuhan during solar maximum

This study employs a digisonde and a GPS receiver at Wuhan (30.5°N, 114.4°E), China to observe spread F, GPS phase fluctuations, and medium-scale traveling ionospheric disturbances (MSTIDs) at nighttime in 2000. The MSTIDs are derived from the perturbations of total electron content (TEC). In this study, we explored the seasonal and nighttime occurrence rates as well as the one-to-one correspondences for these phenomena. The results show that, for the seasonal variations, three phenomena all are highly active in summer, which confirms that irregularities over Wuhan mainly relate to MSTIDs. Moreover, all spread F types have a minor occurrence peak in winter but none for the MSTIDs and the GPS phase fluctuations. Besides, none of large GPS phase fluctuations event had occurred during observation periods, which indicates that the strength of irregularities related to MSTIDs are weaker than that of equatorial plasma bubbles. For the nighttime variations, the frequency spread F occurs earlier than other spread F types which implies that disturbances start at the altitude near the F-region peak. For the one-to-one correspondences, the occurrence rates of each spread F type and GPS phase fluctuations are low during MSTIDs occurred. This implies that smaller scale irregularities which are observed as spread F or GPS phase fluctuations may only appear in some phase of MSTIDs.

Refractive index effects on the scatter volume location and Doppler velocity estimates of ionospheric HF backscatter echoes

Abstract. Ionospheric E×B plasma drift velocities derived from the Super Dual Auroral Radar Network (SuperDARN) Doppler data exhibit systematically smaller (by 20–30%) magnitudes than those measured by the Defence Meteorological Satellites Program (DMSP) satellites. A part of the disagreement was previously attributed to the change in the E/B ratio due to the altitude difference between the satellite orbit and the location of the effective scatter volume for the radar signals. Another important factor arises from the free-space propagation assumption used in converting the measured Doppler frequency shift into the line-of-sight velocity. In this work, we have applied numerical ray-tracing to identify the location of the effective scattering volume of the ionosphere and to estimate the ionospheric refractive index. The simulations show that the major contribution to the radar echoes should be provided by the Pedersen and/or escaping rays that are scattered in the vicinity of the F-layer maximum. This conclusion is supported by a statistical analysis of the experimental elevation angle data, which have a signature consistent with scattering from the F-region peak. A detailed analysis of the simulations has allowed us to propose a simple velocity correction procedure, which we have successfully tested against the SuperDARN/DMSP comparison data set.

The dependence of plasma density in the topside ionosphere on the solar activity level

Abstract. In this paper, the ten-year (1996–2005) total ion density Ni measurements from the Defense Meteorological Satellite Program (DMSP) spacecraft in the morning and evening (09:30 and 21:30 LT) sectors have been analyzed to explore the dependence of plasma densities in the topside ionosphere at middle and low latitudes on the solar activity level. Results indicate that there is a strong solar activity dependence of DMSP Ni at 848 km altitude, which has latitudinal and seasonal features. The plasma density in the topside ionosphere has an approximately linear dependence on daily F107 and a strongly nonlinear dependence on SEM/SOHO EUV, such that the change rate of Ni becomes greater with increasing solar EUV. This is quite different from the dependence of Ni near the F-Region peak (NmF2), at which the rate of change of NmF2 decreases with increasing solar EUV. The rate of change of Ni at the DMSP altitude is greatest in the latitude range where Ni is greatest during high solar activity. We suggest that this greater rate of change (or amplification effect) of Ni at the DMSP altitude is mainly a consequence of the solar activity variations of the topside scale height. The changes in the height of the F-Region peak (hmF2) and the density NmF2 play a secondary role.

The ionospheric response in the Brazilian sector during the super geomagnetic storm on 20 November 2003

Abstract. A very intense geomagnetic storm (superstorm) began with storm sudden commencement (SSC) at 08:03 UT on 20 November 2003, as a result of the coronal mass ejection (CME) by sunspot 484 hurled into space on 18 November 2003. The geomagnetic storm attained |Dst|max=472 nT at 20:00 UT (20 November). In this paper we present the simultaneous ionospheric sounding observations, using the Canadian Advanced Digital Ionosondes (CADIs), carried out from Palmas (PAL; 10.2° S, 48.2° W; dip latitude 5.5° S; a near equatorial station) and São José dos Campos (SJC; 23.2° S, 45.9° W; dip latitude 17.6° S; station located under the crest of equatorial ionospheric anomaly), Brazil. In addition, total electron content (TEC) measurements from several GPS receiving stations in the Brazilian sector during this storm are presented. The simultaneous ionospheric sounding observations carried out at SJC and PAL, and TEC observations on 3 consecutive days viz., 19 November (quiet), 20 November (disturbed) and 21 November (recovery phase) are presented. Salient features from the ionospheric observations in the Brazilian sector during the superstorm are discussed. The difference in the observed ionospheric response at the two stations (PAL and SJC) is considerable. This is not surprising given that PAL is close to the magnetic equator and SJC is near the crest of the equatorial ionospheric anomaly (EIA). It should be pointed out that soon after the SSC (about 4 h later), the F-region critical frequency (foF2), the F-region peak height (hpF2), and variations of virtual heights at different frequencies (iso-frequency plots) all show wavelike oscillations of the F-region during daytime at both the ionospheric sounding stations. Unusual rapid uplifting of F-region at PAL was observed during both the main and recovery phases of the storm.

RETRACTED: Ionospheric climatology derived from GPS occultation observations made by the ionospheric occultation experiment

Abstract The Ionospheric Occultation Experiment (IOX) is a GPS occultation sensor with an ionospheric mission focus. IOX measurements of GPS L1 and L2 carrier phase during Earth limb views of setting GPS satellites are used together with the Abel transform to determine vertical profiles of electron density from which F-region peak parameters are determined. Data from a four and a half month period beginning in November 2001 are statistically binned and compared with a climatological model. To account for potential errors in interpretation that could arise from violation of the Abel transform assertion of spherical symmetry, the data are compared to both the climatology and to statistics of simulated ionospheric inversions using the climatological model. General characteristics of the climatology are reproduced by the occultation data. However, several significant discrepancies between the model and the data are observed during this near-solar maximum time period. In particular, average mid-latitude daytime densities are shown to be higher than the climatological prediction and the height of F2 layer in the post-sunset equatorial region is underestimated by up to 150 km.

Night−day imprints of ionospheric slab thickness during geomagnetic storm

Spatial maps of the ionosphere–plasmasphere slab thickness ( τ) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 1° spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 35° to 70°N, and longitudes −10° to 40°E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS–TEC and ionosonde observations at solar minimum (1995–1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor ( w) is introduced as a ratio of disturbance to quietness representing area mean TEC, foF2, and τ for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and τ maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but τ enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS–TEC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300 km by night.

Ionospheric climatology derived from gps occultation observations made by the ionospheric occultation experiment

The ionospheric occulta- tion experiment (IOX) is a GPS occultation sensor with an iono- spheric mission focus. IOX mea- surements of GPS L1 and L2 carrier phase during Earth limb views of setting GPS satellites are used to- gether with the Abel transform to determine vertical profiles of elec- tron density from which F-region peak parameters are determined. Data from a four and a half month period beginning in November 2001 are statistically binned and com- pared with a climatological model. To account for potential errors in interpretation that could arise from violation of the Abel transform assertion of spherical symmetry, the data are compared to both the cli- matology and to statistics of simu- lated ionospheric inversions using the climatological model. General characteristics of the climatology are reproduced by the occultation data. However, several significant dis- crepancies between the model and the data are observed during this near-solar maximum time period. In particular, average mid-latitude daytime densities are shown to be higher than the climatological pre- diction and the height of F2 layer in the post-sunset equatorial region is underestimated by up to 150 km.

Climatology of ionospheric slab thickness

Abstract. The ionospheric slab thickness τ defined as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) has been analysed during the solar maximum (1981) and minimum (1985) phases of an intense, the 21st, solar cycle. Hourly values of TEC and NmF2 collected at Hawaii (low-latitude), Boulder (mid-latitude) and Goosebay (high-latitude) are used in the study. Climatology of the slab thickness is described by the diurnal, seasonal, solar and magnetic activity variations of τ for the different latitude zones. It is found that, for magnetically quiet days of solar maximum, increased ionization of NmF2 and TEC during the daytime is accompanied by an increased thickness of the ionosphere compared to the night-time for non-auroral latitudes. However, the reverse is found to be true during the solar minimum compensating TEC against a weak night-time ionization of NmF2. For the high-latitude the night-time slab thickness is higher compared to the daytime for both the solar phases. Ratios of daily peak to minimum values of slab thickness vary from 1.3 to 3.75 with the peaks of τ often observed at pre-sunrise and post-sunset hours. The average night-to-day ratios of τ vary from 0.68 to 2.23. The day-to-day variability of τ, expressed in percentage standard deviation, varies from 10% by day (equinox, high-latitude) to 67% by night (summer, mid-latitude) during solar minimum and from 10% by day (winter and equinox, mid-latitude) to 56% by night (equinox, high-latitude) during solar maximum. A comprehensive review of slab thickness related literature is given in the paper. Key words. Ionospheric physics

Latitudinal variation of ionospheric slab thickness

The ionospheric slab thickness τ, defined as a ratio of the total electron content (TEC) to the F-region peak electron density N mF2, is a first order measure of the shape of the electron density profile. In the present study, we use GPS-derived TEC and vertical electron content estimates ITEC from ground-based ionosonde observations together with the corresponding f oF2 (F2 layer critical frequency) and h mF2 (F2 layer peak height) at 14 world-wide stations lying at different latitude regions from the geomagnetic equator to north pole. The period of study is during 2001–2002, which is close to the solar maximum phase of the 23rd solar cycle. Hourly values of TEC, ITEC, f oF2, h mF2 and τ during the period of study are used to compare τ values from the observation and model predictions using the International Reference Ionosphere extended towards the plasmapause with the plasmasphere option of the Russian standard model of the ionosphere, IRI*. For the three latitudinal zones (high, mid and low latitudes) the IRI* predictions of τ are compared with observed τ values for the bottomside ionosphere (below h mF2), topside ionosphere (between h mF2 and 1000 km), plasmasphere (from 1000 to 20,000 km), and the total height range through the ionosphere and plasmasphere. Significant overestimation of τ has been revealed when using IRI*, particularly for the topside ionosphere at high latitudes. Relation of the topside part of slab thickness with the topside half peak density height above the F2 layer peak provides new characteristic parameter for modeling of the topside shape of electron density profile.

Quantifying spread F by digital ionosondes

The width of frequency-spreading ( Δ f f ) on ionograms was shown (Wright, et al., 1977, by comparison with OGO-6 Satellite observations) to be proportional to Δ N N near and above the F-region peak. Thus, even classical analog ionograms using logarithmic frequency scales were direct reading for this important parameter, although little use has been made of this fact. How can modern digital ionosondes produce (by automated methods) a comparable parameter? This paper provides an answer to the question, as the first of a series of steps aimed at exploiting the full capability of HF sounding to understand the distinctive phenomenon of Spread F.

Comparison of low latitude F region peak densities, heights and equatorial E×B drift from IRI with observational data and the Sheffield University plasmasphere ionosphere model

Observed values of the critical frequency and real height of the ionospheric F-region peak (foF2 and hmF2, respectively) over Fortaleza (4° S, 38° W) and Cachoeira Paulista (22.5° S, 45° W) near solar minimum are compared with their values calculated by the International Reference Ionospheric Model (IRI). A comparison between the equatorial E× B drift velocities represented by IRI and those obtained by a method that uses the Sheffield University Plasmasphere Ionosphere Model (SUPIM) has also been carried out. In general, the values of foF2 and hmF2 calculated by IRI using the International Radio Consultive Committee (CCIR) maps show better agreement with the observed values than do those obtained using the International Union of Radio Science (URSI) maps for both stations, Fortaleza and Cachoeira Paulista. Equatorial E× B drift velocities calculated by IRI/Scherliess and Fejer (1999) have presented values different from those calculated by SUPIM mainly during evening and nighttime for December solstice and equinox conditions.

Storm-time changes in the upper atmosphere at low latitudes

A three-dimensional coupled model of the thermosphere, ionosphere, plasmasphere and electrodynamics has been used to investigate the dynamic and electrodynamic response at low latitudes during a geomagnetic storm. A storm was simulated at equinox and high solar activity, and was characterized by a 12-h enhancement of the high-latitude magnetospheric electric field and auroral precipitation. The deposition of energy at high-latitudes heats the thermosphere and drives equatorward wind surges, and changes the global circulation. The first wave arrives at the equator, 3.5 h after storm onset. The change in the global circulation drives downwelling at low latitudes, which decreases molecular species, and causes a slight positive ionospheric phase. By far the dominant driver of the low latitudes is due to the changes in electrodynamics. The dynamo effect of the altered wind circulation opposes the normal diurnal variation, with downward ion drift during the day and upward drift at night. On the dayside, the equatorial ionization anomaly becomes weaker, the ionospheric F-region peak height is lowered, and the eastward zonal winds are reduced. At night the anomaly is strengthened, the ionosphere is raised, and zonal winds accelerate. The global electrodynamic changes are consistent with earlier results, but the speed of the response was unexpected. The model results showed an equatorial response within 2 h of the storm onset, well before the first gravity waves arrived at the equator. The dynamo action of the mid-latitude wind surges drive an F-region dynamo that can cause charge buildup at the terminators, producing electric fields that immediately leak to the equator. The meridional winds act as the driver of the low-latitude storm response by changing the dynamo action of the winds. In contrast, the zonal winds respond to the redistribution of charge brought about by the electrodynamic changes, rather than acting as a driver of the change.

First observations of an F-region turbulent upwelling coincident with severe E-region plasma and neutral atmosphere perturbations

Highly structured electron densities in the E and F regions over Puerto Rico during the night of February 20/21, 1999 were accompanied by intense coherent VHF radar backscatter from the E region and perturbations in neutral sodium in the mesosphere. Simultaneous observations of the event were made with the VHF Cornell University Portable Radar Interferometer (CUPRI) located near Isabela, PR, the University of Illinois VHF radar located at Salinas, PR, the Arecibo incoherent scatter radar, and the sodium lidar located at the Arecibo Observatory. On this geomagnetically quiet night, regions of very different electron concentrations moved through the region. The F-region peak altitudes of the low density regions differed by about 100 km from the high-density region altitudes. The E region also exhibited an unusual enhancement with a vertical extent of about 6 km and caused intense VHF backscatter. The echoing E regions seen by both VHF radars were highly structured with multiple filaments and Doppler shifts exceeding 300 m/s (directed north and upward) some of the time. The Arecibo incoherent scatter radar recorded a large eastward component of the velocity (∼200 m/s) during the early portion of the event, which then switched to strongly westward (peaking over 500 m/s and averaging perhaps 400 m/s for about half an hour) before returning eastward. The meridional velocity components were also variable. The Arecibo lidar showed an intense sodium layer that maintained a constant altitude until the strongest VHF echoes began. Then the layer fell 2 km over a time span of about half an hour and the lidar echoes intensified. Because (1) the timing of the events at the different locations is well correlated with the F-region drifts as measured with the Arecibo radar, and (2) because the Pedersen conductivity falls precipitiously at the start of the event, we conclude there was strong coupling between the E and F regions, perhaps even reaching the mesosphere, during this event. However, major problems may remain. How can the E-region cloud track the F-region blob when supposedly it is coupled to the neutrals? Does the neutral wind track the F-region ion velocity? We do not think so, but this is the easiest solution.

The TEC and F2 parameters as tracers of the ionosphere and thermosphere

The more common measurements of the ionosphere are the total electron content (TEC) (measured with the positioning systems) and the parameters of the F-region peak, namely its altitude (hmF2) and its density (NmF2). In this paper, we address the question of using these reduced parameters as tracers of the ionosphere and thermosphere. We make use of a 19 h daylight experiment by the EISCAT incoherent scatter radar based in Troms ø, Norway. We reduce the full profile measurement of the electron density (between 80 and 425 km ) to the TEC, hmF2 and NmF2. Using a 1-D fluid/kinetic model, we reproduce these parameters. Then, by inversion, we retrieve the ionospheric profiles of the electron density, electron and ion temperature and, to a smaller extent, of the ion velocity. With this method, we show that it is in principle possible to evaluate a correction factor for the atomic oxygen. This factor evolves with time. We study the different uncertainties (heat flow, molecular densities, limitations of the model). Finally, we evaluate the contribution of the upper ionosphere (up to 3000 km) to the calculation of the TEC and show that 90% of the TEC is reached when the integration is made up to 1200 km .

Comparison of model electron densities and temperatures with millstone hill observations during 5–12 April 1997

Measurements of electron densities, N e, and temperatures, T e, at Millstone Hill are compared with results from the time-dependent mathematical model of the Earth's ionosphere during 5–12 April, 1997. To agree the measured and modelled hmF2 an additional plasma drift is added to compensate inaccuracies of the model input parameters. The height variations in plasma drifts decrease NmF2 up to 24 % in agreement with early results of Schlesier and Buonsanto (1999). The comparison of the measured and modelled altitude profiles of N e shows that anomalously low values of the main F-region peak altitude (< 200 km) seen as a “G condition” on ionograms and observed on April 11 exist in the ionosphere due to a decrease of O + ion production rates resulting from low values of [O] and an increase of molecular ions resulting from high values of [N 2] and [O 2] in comparison with the MSIS-86 neutral densities. The G conditions observed on 7 April can be explained by deviations of [O], [N 2], and [O 2] from the MSIS-86 neutral densities, and the solar flare and post-flare increase of the incident solar flux. The ionospheric storm dusk effect observed on 10 April, 1997, can result from (1) the F2 layer uplifting to more high altitudes due to an enhanced equatorward neutral wind, resulting in an decreased O + recombination rate and an electron density enhancement, and (2) increase by a factor of 1.9 in [O] from 17:00 LT to 18:30 LT on 10 April in comparison with the MSIS-86 [O]. The explanation of the observed negative phases in N e is presented.

Dynasonde observations of electron concentration gradients above Troms ø

The Dynasonde is a digital ionosonde which measures the amplitude and phase of radio echoes reflected from the ionosphere. In addition to the frequency and virtual range of each echo, one parameter of particular interest is the direction of arrival which allows horizontal structures in the ionosphere to be studied. Under suitable ionospheric conditions, namely where structures or strong gradients are present in the electron concentration, echoes may be observed out to horizontal distances of several hundred kilometres. We describe an automated method for latitudinally mapping these gradients in F-region peak electron concentration using the Dynasonde. Good agreement is found when the results are compared with a co-located incoherent scatter radar lending credibility to observations of electron concentration gradients by a Dynasonde alone. This is significant for sites where no supporting instruments exist. Although Dynasondes account for only a small fraction of digital ionosondes in use today, the techniques described in this paper could be extended to other digital ionosondes with similar directional capabilities. The combination of routine soundings, and the worldwide distribution of such instruments, would provide a powerful means to monitor ionospheric structures on a regular basis.

630 nm nightglow observations from 17°N latitude

Regular measurements of [OI] 630 nm nightglow have been carried out at Kolhapur (16.8°N, 74.2°E, dip lat 10.6°N) in India with the help of tilting filter photometers during clear moonless nights. Several characteristic nightglow data sets are presented in the report for geomagnetically quiet and disturbed nights. A comparison has been made between the simultaneous nightglow and the ionospheric parameters of virtual F-layer height and critical frequency of the F-layer at Ahmedabad (23.02°N, 72.6°E). In general, a good correlation (c.c. = 0.8 to 0.9) is found between the measured airglow emission and F-region peak electron densities during quiet periods but for intense magnetic disturbances the airglow fluctuations are mainly controlled by sharp changes in height variations of the h′ F layer. The period of airglow variation generally corresponds to periods of F-region height variation during magnetic disturbances. Using a long series of data between December 1989 and April 1996, the mean seasonal nocturnal variations of 630 nm emission have been compared during high and low solar activity periods.

An empirical ionospheric storm-time correction model

An empirical model for F-region peak ionospheric storm-time changes has been developed based on understanding from theoretical modeling of geomagnetic storms. The model is designed to scale climatology, or monthly-medians, based on the strength of a storm, as a function of geomagnetic latitude, season, and local time. The model is driven by an index derived from the previous thirty hours of auroral or geomagnetic activity, suitably weighted by a filter. The model is particularly effective in capturing the ionospheric storm negative phase in summer mid latitudes, where it reduces the root-mean-square error by more than a factor of two. The winter mid-latitude F region typically experiences a positive phase during a storm accompanied by a high degree of variability. The model does less well in these circumstances but still makes a significant reduction in the variance. The ionospheric storm-time model can be used to scale monthly median values or a quiet time model such as the International Reference Ionosphere.