M. David

Storm-Time Density Enhancements in the Middle Latitude Dayside Ionosphere

[1] Enhancements of the total electron content (TEC) in the middle-latitude dayside ionosphere have often been observed during geomagnetic storms. The enhancements can be as large as a factor of 2 or more, and many sightings of such structures have occurred over the United States. Here we investigate the effectiveness of an expanded convection electric field as a mechanism for producing such ionospheric enhancements. As a test case, we examine the storm period of 5-7 November 2001, for which observations from the DMSP F 13 are used to drive the Time Dependent Ionospheric Model (TDIM). Our findings indicate that at favorable universal times, the presence of the expanded electric field is sufficient to create dayside TEC enhancements of a factor of 2 or more. The modeled enhancements consist of locally produced plasma; we do not find it necessary to transport high-density plasma northward from low latitudes.

A modeling study of the longitudinal dependence of storm time midlatitude dayside total electron content enhancements

[1]xa0Large-scale storm time enhancements of total electron content (TEC) have often been observed at Millstone Hill and other locations in the United States and have been less frequently reported from other stations around the globe. This has raised the question of whether the formation of such enhancements may have a longitudinal dependence and whether the North American continent might occupy a favored longitude sector for the appearance of such effects. We examine two mechanisms for producing storm time dayside TEC enhancements. Heelis et al. (2009) showed that a high-latitude electric field that has expanded to midlatitudes can enhance the dayside TEC by as much as 300 units. We use such an electric field as a driver for the TDIM ionospheric model, studying its effect across a range of longitudes, and we find that there is indeed a longitudinal asymmetry that favors the enhancement of TEC in the American sector. Second, we examine the role of the thermospheric wind during storm conditions and find that it has potentially an equally large effect, with a longitudinal dependence of its own that may either enhance or counteract the effect of the expanded electric field. In both cases, the effect is of the order of a 10% to 20% change in TEC.

Structuring of high latitude plasma patches with variable drive

[1]xa0Three-dimensional nonlinear simulations of the structuring of high-latitude patches with variable drive demonstrate the role of the gradient drift instability as the primary operative structuring mechanism. In this letter we show that by introducing a variable convective E × B drive obtained from the ionosphere module of a global MHD code simulation of a real substorm [Sojka et al., 1997], the nature of the structuring is strongly influenced by the drive. In fact, due to occasional reversal of the direction of the convection, the structuring from the gradient-drift instability takes place on both edges of the patch. Using different ion-neutral collisionality to account for the seasonal or solar cycle variability of the ionosphere, the four categories of structured patches identified by Kivanc and Heelis [1997] have been simulated.

Dayside midlatitude ionospheric response to storm time electric fields: A case study for 7 September 2002

[1]xa0With the storm of 7–8 September 2002 as a study case, we demonstrate that an ionospheric model driven by a suitable storm time convection electric field can reproduce the F region dayside density enhancements associated with the ionospheric storm positive phase. The ionospheric model in this case is the Utah State University Time Dependent Ionospheric Model (TDIM); the electric field model is the University of Michigans Hot Electron and Ion Drift Integrator (HEIDI). Extensive ground truth is available throughout the study period from two independent sources: ground-based vertical TEC and ionosonde stations; our simulation results are in good agreement with these observations. We address the question of what is the source of the high-density plasma that is seen during the positive storm phase and show that in this case a magnetospheric electric field with an eastward component that penetrates to midlatitudes increases local production on the dayside to a degree that is sufficient to account for the storm time density increases that have been observed.

Single‐day dayside density enhancements over Europe: A survey of a half‐century of ionosonde data

[1]xa0With the National Geophysical Data Centers Space Physics Interactive Data Resource (SPIDR), we have collected ionosonde data from stations around the world from 1958 to 2006. The database consists of F-layer peak frequency values (foF2) at hourly intervals (when data are available). We limit our study to midlatitude dayside density peak enhancements; we find that when such features appear, they are usually confined in longitude, being not wider than about 60–90 degrees, or 4–6 hours of local time. In this paper, we look only at enhancements over Europe, as data coverage is superior there. We define a “single-day enhancement” and search the European sector of the database for such occurrences, finding 890 events during the 49-year period. The frequency of occurrence of these events is seen to be anticorrelated with the solar activity cycle. There is also a seasonal distribution, with more events during the equinox months and the fewest during winter. We examine the geomagnetic activity levels on the days of the enhancements and find that a majority of the events are associated with small or moderate geomagnetic disturbances. Most of these disturbances are not large enough to be called “storms”; in many cases, there is a Dst drop of just 20–40 units or a Kp increase to a level of 3 or 4. These disturbances are not intense enough to cause a storm negative phase, but in many cases they are sufficient to bring about a dayside ionospheric density enhancement of 50% or more.

A mid-latitude space weather hazard driven directly by the magnetosphere

Abstract This study revisits and attempts to quantify the effects of high-latitude electric field penetration on the mid-latitude ionosphere. These penetration electric fields (PEFs) are strongest during geomagnetically dynamic and disturbed conditions. The consequences of PEFs arise principally from the induced vertical drift of the F-layer from the eastward electric field component. Both positive and negative storm phases are associated with the PEF. Although no readily available description of the PEF exists, observational and modeling results are combined to provide a crude model. The largest uncertainty arises from a lack of knowledge of whether PEFs are always short lived (less than 1 h ) or are a persisting feature of disturbed conditions. According to simulations with the Utah State University time dependent ionospheric model (TDIM), under K p =3 conditions the PEF readily generates a factor of 2 increase in the pre-midnight ionosphere F-layer density (positive storm phase). For K p =5 conditions this positive phase is further enhanced to produce almost an order of magnitude increase in F-region density. Negative storm phases, F-layer density decreases, are also present in the pre-dawn and pre-noon sectors. The pre-dawn negative storm phase can reach a factor of 10 for K p =5 conditions while the pre-noon depletions are a few 10s of percent. These large density changes have operational impact on systems using coordinate registration based upon Global Positioning Satellite (GPS) measurements. GPS satellite-to-ground radio paths pass through the ionosphere at different angles relative to the zenith and hence have different propagation corrections dependent upon the paths total ionospheric electron content. Factors of 2 change in electron density corresponds to tens of centimeters to meters correction errors in coordinate registration position finding. Corrections this large are a potentially insurmountable obstacle for the GPS based wide area augmentation system (WAAS) designed to provide the airline industry over the USA position accuracy of only a few centimeters. Examples of mid-latitude ray tracing at 5 and 9 MHz are used to demonstrate the frequency sensitivity and coordinate registration dependence upon these factors of 2 ionospheric density changes. Operational systems such as the over-the-horizon (OTH) radar is very sensitive to such dependences and hence upon the mid-latitude PEF.

Model Study of Ionospheric Dynamics During a Substorm

A global substorm electrodynamic model and a global ionospheric model were coupled in order to study ionospheric dynamics during substorms, with the focus on small-scale substorm electrodynamic and plasma structures. The simulation results show that in the expansion phase, structured precipitation and channeled field-aligned currents quickly develop in the substorm onset region. The Hall and Pedersen conductance ratio in the region increases significantly, and the magnetospheric field-aligned currents are mainly closed by highly structured Hall currents. Correspondingly, the plasma in the ionosphere also undergoes significant changes during a substorm and is highly structured in both the horizontal and vertical directions. In the substorm onset region, there are spatially separated small-scale Ti and Te hot spots, downward ExB drifts, decreased total electron contents, and a lowered ionosphere. Also, there is a significant O+ → NO+ conversion, leading to a great increase of NO+ and a lowering of the O+ peak height. These small-scale electrodynamic and plasma structures are very important for more realistically simulating the ionospheric dynamics during substorms. These results not only help to elucidate the multiscale ionospheric responses to substorms but also provide a theoretical guidance and cautions for the interpretation of various substorm observational data.

Relative solar and auroral contribution to the polar F region: Implications for National Space Weather Program

[1]xa0When plasma in the polar cap F region becomes highly structured, patches, irregularities, and scintillations of HF signals may be observed. The topic of this paper is not the mechanism for structuring or distributing the plasma but rather the source of the plasma. By understanding the plasma source we gain insight into the specification and forecasting of ionospheric structures and irregularities as required for space weather applications. The two major sources of polar cap F region plasma are the solar EUV radiation and the auroral precipitation. The region over which solar EUV production occurs is readily modeled. In contrast, the auroral precipitation is not subject to diurnal or seasonal dependences in the same predictable manner; the auroral precipitation can almost be viewed as stochastic within certain geomagnetic coordinate constraints. In this study we use a physical model to separate the effects of solar EUV and auroral precipitation. We find that the auroral contribution does provide a far-from-negligible “baseline” level of polar cap F region plasma, upon which is superimposed the UT and seasonally dependent TOI. This baseline level of ionization is very difficult to predict or forecast since it is determined by plasma flux tube histories through extended regions of the auroral oval over several hours. This result raises the need for more advanced auroral precipitation modeling in order to obtain improved space weather specification. The inclusion of soft auroral precipitation is especially important since it can be a significant source of F region plasma.

Ionospheric model‐observation comparisons: E layer at Arecibo Incorporation of SDO‐EVE solar irradiances

This study evaluates how the new irradiance observations from the NASA Solar Dynamics Observatory (SDO) Extreme Ultraviolet Variability Experiment (EVE) can, with its high spectral resolution and 10u2009s cadence, improve the modeling of the E region. To demonstrate this a campaign combining EVE observations with that of the NSF Arecibo incoherent scatter radar (ISR) was conducted. The ISR provides E region electron density observations with high-altitude resolution, 300u2009m, and absolute densities using the plasma line technique. Two independent ionospheric models were used, the Utah State University Time-Dependent Ionospheric Model (TDIM) and Space Environment Corporations Data-Driven D Region (DDDR) model. Each used the same EVE irradiance spectrum binned at 1u2009nm resolution from 0.1 to 106u2009nm. At the E region peak the modeled TDIM density is 20% lower and that of the DDDR is 6% higher than observed. These differences could correspond to a 36% lower (TDIM) and 12% higher (DDDR) production rate if the differences were entirely attributed to the solar irradiance source. The detailed profile shapes that included the E region altitude and that of the valley region were only qualitatively similar to observations. Differences on the order of a neutral-scale height were present. Neither model captured a distinct dawn to dusk tilt in the E region peak altitude. A model sensitivity study demonstrated how future improved spectral resolution of the 0.1 to 7u2009nm irradiance could account for some of these model shortcomings although other relevant processes are also poorly modeled.

A Theoretical Model Study of F-Region Response to High Latitude Neutral Wind Upwelling Events

Abstract Neutral wind upwelling events are well documented in both the northern and southern high latitude ionosphere. The vertical winds in the F-region frequently exceed 100 m / s , and winds in excess of 200 m / s have been observed. These upwelling events occur in a latitudinal band of 4°–6° width that extends in local time in the midnight-morning sector; this band always lies poleward of the auroral precipitation. Using the time dependent ionospheric model (TDIM), a series of sensitivity simulations are carried out, based on observational constraints provided by the spectacular upwelling event seen at both the Mawson and Davis stations in the Antarctic on 08 June 1997 (Innis et al., 1999). The model simulations indicate that the F-layer density at any given point may either increase or decrease during an upwelling event, depending upon the past history of the plasma flux tube. Because this past history of the F-layer convection is unknown for the specific upwelling events a detailed case study cannot be undertaken. Instead a series of sensitivity simulations based upon a range of possible convection histories will be studied to determine the relative effect of the upwelling. The absolute density is not dependent upon solar EUV production because of winter conditions, but is sensitive to the auroral electron precipitation. The best F-layer indicator of the upwelling is the height of the layer, hmF2. For upwelling events with vertical drifts of 100 m / s h m F 2 can be increased by 100 km in 10 min . Upon leaving an upwelling region, the hmF2 almost as rapidly decreases to its normal height. Resulting from this lifting of the O+ layer is the reduction in O+ recombination and 630-nm emission; this latter consequence is observed as a standard feature of the upwelling events. In the topside ionosphere the electron density is responsive to the upwelling. The total electron content (TEC) is not, in general, sensitive to the uplifting events, however, low elevation slant path GPS TEC measurements might well detect the rapid uplifting of the F-layer. The upwelling event observations are insufficient to constrain our understanding of their impact upon the ionosphere. This model study does imply that upwelling events can modify the F-layer height severely. Such layer height modification can have measurable effects on radio frequency ray paths through the ionosphere. To quantify such effects a fuller description of the upwelling events as well as the past history of ionospheric plasma is needed. Experiments with higher time resolution of both the neutral parameters and F-region at multiple locations are necessary to unravel these complex events.