Barbara A. Emery

Coexistence of ionospheric positive and negative storm phases under northern winter conditions: A case study

The response of the thermosphere and ionosphere to the famous January 10, 1997, geomagnetic storm is simulated using the thermosphere-ionosphere-electrodynamics general circulation model with realistic, time-dependent distributions of ionospheric convection and auroral precipitation as inputs. The simulation results show a dominant positive storm phase of increased F layer electron density over much of the northern winter hemisphere, but a negative storm phase with reduced electron density at middle and low latitudes is also evident in the simulation. The coexistence of both positive and negative storm phases is a result of the complex dynamical and chemical interactions between charged particles and neutral gases. The impulsive magnetospheric energy inputs via auroral precipitation and Joule heating generate traveling atmospheric and ionospheric disturbances (TADs and TIDs) which propagate from the northern auroral zone to lower latitudes and penetrate well into the Southern Hemisphere. The simulation results demonstrate that positive storm phases are caused primarily by enhanced auroral precipitation over high latitudes and by TIDs at middle and low latitudes. Globally speaking, composition changes in terms of enhancements in the N2/O ratio are mainly responsible for negative storm effects. However, although there is some correlation between increases in N2/O and decreases in the F layer critical frequency ƒoF2 in the winter hemisphere during the storm main phase and early recovery phase, the overall changes in ƒoF2 are also determined by other processes, such as the ionization production associated with enhanced auroral precipitation and the variations associated with TIDs. In the low to middle-latitude region changes in ƒoF2 approximately anticorrelate with changes at the height of the F layer electron density peak (e.g., hmF2) at 70°W during the storm main phase as well as its early recovery phase. This is attributed in part to the relation that exists between meridional wind velocity and vertical shear of that velocity for aurorally produced TADs.

The high latitude circulation and temperature structure of the thermosphere near solstice

Abstract The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCARs thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F -region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E -region ionosphere. The results of several TGCM runs are used to show that at F -region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid. The F -region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer ( DE −2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s −1 ) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s −1 ). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE −2 satellite measurements.

On the global mean temperature of the thermosphere

Abstract The solar extreme ultraviolet (e.u.v.) flux and solar ultraviolet (u.v.) flux in the Schumann-Runge continuum region have been measured by spectrometers on board the Atmosphere Explorer satellites from about 1974 to 1981. The solar flux spectra measured on 23 April 1974 (a day the Atmosphere Explorer satellite reference spectrum was obtained), 13–28 July 1976 (a period of spotless conditions near solar cycle minimum), and 19 February 1979 (a day near solar cycle maximum) are used to examine the global mean temperature structure of the thermosphere above 120 km. The results show that for solar cycle minimum the calculated global mean exospheric temperature is in agreement with empirical model predictions, indicating that the energy absorbed by the thermosphere is balanced by downward molecular thermal conduction. For solar cycle maximum the energy absorbed by the thermosphere is not balanced by downward thermal conduction but agreement between the calculated and observed temperature is obtained with the inclusion of 5.3μm radiational cooling by nitric oxide. Model calculations of the minor neutral constituents in the thermosphere show that about three times more nitric oxide is produced during solar cycle maximum than solar cycle minimum conditions. The results suggest that nitric oxide cooling is small during solar cycle minimum, because of low nitric oxide densities and low thermospheric temperatures, but it becomes significantly larger during solar cycle maximum, when nitric oxide densities and thermospheric temperatures are larger. 23 April 1974 was a moderately disturbed day and the results of the global mean temperature calculation indicate that it is necessary to consider a high latitude heat source associated with the geomagnetic activity to obtain agreement between the calculated and observed global mean temperature structure.

Polar cap index as a proxy for hemispheric Joule heating

The polar cap (PC) index measures the level of geomagnetic activity in the polar cap based on magnetic perturbations from overhead ionospheric currents and distant field-aligned currents on the poleward edge of the nightside auroral oval. Because PC essentially measures the main sources of energy input into the polar cap, we propose to use PC as a proxy for the hemispheric Joule heat production rate (JH). In this study, JH is estimated from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. We fit hourly PC values to hourly averages of JH. Using a data base approximately three times larger than studies, we find a quadratic relationship between JH and PC, differentiated by season. A comparison during the November 1993 storm interval with earlier reported methods using the AE index and the cross polar cap potential, shows that the PC-based Joule heating estimate is as equally accurate. Thus the single station PC index appears to provide a quick estimate of, and is an appropriate proxy for, the hemispheric Joule heating rate.

Ionospheric convection response to changing IMF direction

By combining ground-based and satellite-based measurements of ionospheric electric fields, conductivities and magnetic perturbations, we are able to examine the characteristics of instantaneous, ionospheric convection patterns associated with changing directions of the Interplanetary Magnetic Field (IMF). In response to a rapid southward-to-northward turning of the IMF on 23 July 1983, the ionospheric convection reconfigured over a period of 40 minutes. The configuration changed from a conventional two-cell pattern to a contracted four-cell pattern, with reversed convection cells in the high-latitude dayside, associated with a strong potential drop of about 75 kV. Later, in response to a gradual rotation of the IMF from the +Z through the −Y. toward the −Z direction, the nightside cells disappeared and the dawn cell in the reversed pair wrapped around and displaced the dusk cell until a conventional two-cell pattern was reestablished, largely in accord with the qualitative model of Crooker [1988]. Our results suggest that multiple cells can arise as a result of strong southward to northward transitions in the IMF. They appear to persist for sometime thereafter.

TIME DEPENDENT THERMOSPHERIC NEUTRAL RESPONSE TO THE 2-11 NOVEMBER 1993 STORM PERIOD

Abstract Many satellite and ground-based observations from 2–11 November 1993 werecombined in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure toderive realistic time dependent global distributions of the auroral precipitation and ionosphericconvection. These were then used as inputs to the Thermosphere–Ionosphere–ElectrodynamicsGeneral Circulation Model (TIEGCM) to simulate the thermospheric and ionospheric responseduring the storm period. The November 1993 storm was an unusually strong storm associatedwith a recurring high speed stream of solar plasma velocity in the declining phase of the solarcycle. Significant gravity waves with phase speeds of about 700 m/s caused by Joule heating werepresent in the upper thermosphere as perturbations to the neutral temperature and wind fields,especially on 4 November. The observed gravity waves in the meridional wind and in the height ofthe electron density peak at several southern hemisphere stations were generally reproduced bythe model using the AMIE high latitude inputs. Both model and observed equatorward windswere enhanced during the peak of the storm at Millstone Hill and at Australian ionosondestations. The observed neutral temperature at Millstone Hill increased about 400 K during thenight on 4 November, returning to normal on 9 November, while the model increased 300 K thefirst night at that location but was still elevated on 11 November. Enhanced westward windsduring the storm were evident in the UARS WIND Imaging Interferometer (WINDII) data. Theenhanced westward winds in the model were largest around 40–45° magnetic latitude at night,and also tended to be largest in the longitudes containing the magnetic poles. The peak westwardwind enhancements at 0 LT reached about 250 m/s at 300 km, and about 100 m/s at 125 km thefirst day of the storm at 40° magnetic latitude. At 20° magnetic latitude, the maximum westwardwind enhancements at 125 km at 0 LT appeared 2–4 days after the major part of the storm,indicating very long time constants in the lower thermosphere. The model showed global averageneutral temperature enhancements of 188 K after the peak of the storm that decayed with time,and which correlated with variations 8 h earlier in the Dst index and in the electric potential dropinput from AMIE. The global average temperature enhancement of 188 K corresponded to apotential drop increase of only about 105 kV. The results showed that the TIEGCM usingrealistic AMIE auroral forcings were able to reproduce many of the observed time dependentfeatures of this long-lived geomagnetic storm. The overall global average exospheric temperaturevariation correlated well with the time variation of the cross-tail potential drop and the Dst indexduring the storm period. However, the enhanced westward winds at mid-latitudes were stronglyrelated to the corrected Joule heating defined by the time dependent AMIE inputs.

Modeling studies of the impact of high-speed streams and co- rotating interaction regions on the thermosphere-ionosphere

Received 28 November 2011; revised 14 June 2012; accepted 14 June 2012; published 1 August 2012. [1] Changes in the thermosphere-ionosphere system caused by high-speed streams in the solar wind, and the co-rotating interaction regions they engender, are studied using a combination of model simulations and data analysis. The magnetospheric responses to these structures and consequent ionospheric drivers are simulated using the numerical Coupled Magnetosphere-Ionosphere-Thermosphere model and the empirical Weimer 2005 model, finding that the interplanetary magnetic field (IMF) is more important than solar wind speed and density per se in controlling magnetosphere-ionosphere coupling. The NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model is then employed to calculate neutral density, nitric oxide cooling, and electron density, for comparison to space-based measurements from the STAR instrument on the CHAMP satellite, the SABER instrument on the TIMED satellite, and GPS occultations from the COSMIC mission, respectively. The recurrent, periodic changes observed under solar minimum conditions during 2008, and particularly during the Whole Heliospheric Interval (March–April of 2008), are simulated by the model and compared to these measurements. Numerical experiments were conducted to elucidate the mechanisms of solar wind and IMF forcing, setting the solar wind speed and density to nominal values, smoothing the IMF, and also setting it to zero. The results confirm the importance of IMF variations, particularly its north-south component (Bz), but also show that when the average Bz values are negative (southward), the interaction with increased solar wind speed amplifies the magnetosphere-ionosphere-thermosphere response. Conversely, during events when Bz is on average positive (northward), even large increases in solar wind speed have small effects on the system.

Energetics of Magnetic Storms Driven by Corotating Interaction Regions: A Study of Geoeffectiveness

We investigate the energetics of magnetic storms associated with corotating interaction regions (CIRs). We analyze 24 storms driven by CIRs and compare to 18 driven by ejecta-related events to determine how they differ in overall properties and in particular in their distribution of energy. To compare these different types of events, we look at events with comparable input parameters such as the epsilon parameter and note the properties of the resulting storms. We estimate the energy output by looking at the ring current energy along with ionospheric Joule heating derived from the PC and Dst indices. We also include the energy of auroral precipitation, estimated from NOAA/TIROS and DMSP observations. In general, ejecta-driven storms produce more intense events, as parameterized by Dst*, but they are usually not as long lasting, and in most cases deposit less energy. This is observed even for events that have similar input quantities, such as epsilon. This may be related to the high speed of the solar wind, in that an increased magnetosonic Mach number may influence the reconnection rate and therefore the coupling. Additionally, we find the efficiency of the coupling varies greatly from CIR-driven to ejecta-driven storms, with the CIR-driven storms coupling substantially more efficiently, particularly in the recovery phase. The efficiency of coupling (output energy divided by input energy) for CIR-driven storms in recovery phase was double that of ejecta-driven storms.

Energy transfer during intense geomagnetic storms driven by interplanetary coronal mass ejections and their sheath regions

The interaction of the solar wind and Earths magnetosphere is complex, and the phenomenology of the interaction is very different for interplanetary coronal mass ejections (ICMEs) compared to their sheath regions. In this paper, a total of 71 intense (Dst <= -100 nT) geomagnetic storm events in 1996-2006, of which 51 are driven by ICMEs and 20 by sheath regions, are examined to demonstrate similarities and differences in the energy transfer. Using superposed epoch analysis, the evolution of solar wind energy input and dissipation is investigated. The solar wind-magnetosphere coupling functions and geomagnetic indices show a more gradual increase and recovery during the ICME-driven storms than they do during the sheath-driven storms. However, the sheath-driven storms have larger peak values. In general, solar wind energy input (the epsilon parameter) and dissipation show similar trends as the coupling functions. The trends of ion precipitation and the ratio of ion precipitation to the total (ion and electron) are quite different for both classes of events. There are more precipitating ions during the peak of sheath-driven storms. However, a quantitative assessment of the relative importance of the different energy dissipation branches shows that the means of input energy and auroral precipitation are significantly different for both classes of events, whereas Joule heating, ring current, and total output energy display no distinguishable differences. The means of electron precipitation are significantly different for both classes of events. However, ion precipitation exhibits no distinguishable differences. The energy efficiency bears no distinguishable difference between these two classes of events. Ionospheric processes account for the vast majority of the energy, with the ring current only being 12%-14% of the total. Moreover, the energy partitioning for both classes of events is similar.

CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge for systematic assessment of ionosphere/thermosphere models: NmF2, hmF2, and vertical drift using ground‐based observations

[1] Objective quantification of model performance based on metrics helps us evaluate the current state of space physics modeling capability, address differences among various modeling approaches, and track model improvements over time. The Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Electrodynamics Thermosphere Ionosphere (ETI) Challenge was initiated in 2009 to assess accuracy of various ionosphere/thermosphere models in reproducing ionosphere and thermosphere parameters. A total of nine events and five physical parameters were selected to compare between model outputs and observations. The nine events included two strong and one moderate geomagnetic storm events from GEM Challenge events and three moderate storms and three quiet periods from the first half of the International Polar Year (IPY) campaign, which lasted for 2 years, from March 2007 to March 2009. The five physical parameters selected were NmF2 and hmF2 from ISRs and LEO satellites such as CHAMP and COSMIC, vertical drifts at Jicamarca, and electron and neutral densities along the track of the CHAMP satellite. For this study, four different metrics and up to 10 models were used. In this paper, we focus on preliminary results of the study using ground-based measurements, which include NmF2 and hmF2 from Incoherent Scatter Radars (ISRs), and vertical drifts at Jicamarca. The results show that the model performance strongly depends on the type of metrics used, and thus no model is ranked top for all used metrics. The analysis further indicates that performance of the model also varies with latitude and geomagnetic activity level.