Mihail Codrescu

Response of the thermosphere and ionosphere to geomagnetic storms

Four numerical simulations have been performed, at equinox, using a coupled thermosphere-ionosphere model, to illustrate the response of the upper atmosphere to geomagnetic storms. The storms are characterized by an increase in magnetospheric energy input at high latitude for a 12-hour period; each storm commences at a different universal time (UT). The initial response at high latitude is that Joule heating raises the temperature of the upper thermosphere and ion drag drives high-velocity neutral winds. The heat source drives a global wind surge, from both polar regions, which propagates to low latitudes and into the opposite hemisphere. The surge has the character of a large-scale gravity wave with a phase speed of about 600 m s−1. Behind the surge a global circulation of magnitude 100 m s −1 is established at middle latitudes, indicating that the wave and the onset of global circulation are manifestations of the same phenomena. A dominant feature of the response is the penetration of the surge into the opposite hemisphere where it drives poleward winds for a few hours. The global wind surge has a preference for the night sector and for the longitude of the magnetic pole and therefore depends on the UT start time of the storm. A second phase of the meridional circulation develops after the wave interaction but is also restricted, in this case by the buildup of zonal winds via the Coriolis interaction. Conservation of angular momentum may limit the buildup of zonal wind in extreme cases. The divergent wind field drives upwelling and composition change on both height and pressure surfaces. The composition bulge responds to both the background and the storm-induced horizontal winds; it does not simply rotate with Earth. During the storm the disturbance wind modulates the location of the bulge; during the recovery the background winds induce a diurnal variation in its position. Equatorward winds in sunlight produce positive ionospheric changes during the main driving phase of the storm. Negative ionospheric phases are caused by increases of molecular nitrogen in regions of sunlight, the strength of which depends on longitude and the local time of the sector during the storm input. Regions of positive phase in the ionosphere persist in the recovery period due to decreases in mean molecular mass in regions of previous downwelling. Ion density changes, expressed as a ratio of disturbed to quiet values, exhibit a diurnal variation that is driven by the location of the composition bulge; this variation explains the ac component of the local time variation of the observed negative storm phase.

On the seasonal response of the thermosphere and ionosphere to geomagnetic storms

Ionosonde observations have provided the data to build a picture of the response of the midlatitude ionosphere to a geomagnetic storm. The particular characteristic of interest is the preference for “negative storms” (decrease in the peak electron density, NmF2) in summer and “positive storms” (increase in NmF2) in winter. A three-dimensional, time-dependent model of the coupled thermosphere and ionosphere is used to explain this dependence. During the driven phase of a geomagnetic storm the two main magnetospheric energy sources to the upper atmosphere (auroral precipitation and convective electric field) increase dramatically. Auroral precipitation increases the ion density and conductivity of the upper atmosphere; the electric field drives the ionosphere and, through collisions, forces the thermosphere into motion and then deposits heat via Joule dissipation. The global wind response is divergent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and carry molecule-rich air to higher levels. Once created, the “composition bulge” of increased mean molecular mass is transported by both the storm-induced and background wind fields. The storm winds imposed on the background circulation do not have a strong seasonal dependence, and this is not necessary to explain the observations. Numerical computations suggest that the prevailing summer-to-winter circulation at solstice transports the molecule-rich gas to mid and low latitudes in the summer hemisphere over the day or two following the storm. In the winter hemisphere, poleward winds restrict the equatorward movement of composition. The altered neutral-chemical environment in summer subsequently depletes the F region midlatitude ionosphere to produce a “negative storm”. In winter midlatitudes a decrease in molecular species, associated with downwelling, persists and produces the characteristic “positive storm”.

Global Assimilation of Ionospheric Measurements (GAIM)

Abstract : Our primary goal is to construct a real-time data assimilation model for the ionosphere-plasmasphere system that will provide reliable specifications and forecasts. A secondary goal is to validate the model for a wide range of geophysical conditions, including different solar cycle, seasonal, storm, and substorm conditions.

Stratospheric effects of energetic particle precipitation in 2003-2004

Upper stratospheric enhancements in NOx (NO and NO2) were observed at high northern latitudes from March through at least July of 2004. Multi-satellite data analysis is used to examine the temporal evolution of the enhancements, to place them in historical context, and to investigate their origin. The enhancements were a factor of 4 higher than nominal at some locations, and are unprecedented in the northern hemisphere since at least 1985. They were accompanied by reductions in O-3 of more than 60% in some cases. The analysis suggests that energetic particle precipitation led to substantial NOx production in the upper atmosphere beginning with the remarkable solar storms in late October 2003 and possibly persisting through January. Downward transport of the excess NOx, facilitated by unique meteorological conditions in 2004 that led to an unusually strong upper stratospheric vortex from late January through March, caused the enhancements.

Interaction between direct penetration and disturbance dynamo electric fields in the storm-time equatorial ionosphere

[1] The direct penetration of the high-latitude electric field to lower latitudes, and the disturbance dynamo, both play a significant role in restructuring the storm-time equatorial ionosphere and thermosphere. Although the fundamental mechanisms generating each component of the disturbance electric field are well understood, it is difficult to identify the contribution from each source in a particular observation. In order to investigate the relative contributions of the two processes, their interactions, and their impact on the equatorial ionosphere and thermosphere, the response to the March 31, 2001, storm has been modeled using the Rice Convection Model (RCM) and the Coupled Thermosphere-Ionosphere-Plasmasphere-Electrodynamics (CTIPe) model. The mid- and low-latitude electric fields from RCM have been imposed as a driver of CTIPe, in addition to the high latitude magnetospheric sources of ion convection and auroral precipitation. The high latitude sources force the global storm-time wind fields, which act as the driver of the disturbance dynamo electric fields. The magnitudes of the two sources of storm-time equatorial electric field are compared for the March 2001 storm period. During daytime, and at the early stage of the storm, the penetration electric field is dominant; while at night, the penetration and disturbance dynamo effects are comparable. Both sources are sufficient to cause significant restructuring of the low latitude ionosphere. Our results also demonstrate that the mid- and low-latitude conductivity and neutral wind changes initiated by the direct penetration electric field preferentially at night are sufficient to alter the subsequent development of the disturbance dynamo.

Storm-time changes in the upper atmosphere at low latitudes

Abstract 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.

On the importance of E‐field variability for Joule heating in the high‐latitude thermosphere

Joule heating is known to be one of the major energy sources of the upper atmosphere. Knowledge of the magnitude of this source is fundamentally important to a thorough understanding of the regions physics. However, Joule heating is currently one of the largest sources of uncertainty in the thermospheres energy budget. In numerical models the distribution of Joule heating is generally computed using mean or average convection patterns, which evolve on a relatively long time scale in response to changes in solar wind conditions. The convection patterns represent average electric potential distributions, and thus the resulting amount of Joule heating is proportional to the square of the average E-field. That method ignores the important component of Joule heating due to rapid or small-scale fluctuations in E-field or ion drifts. However, E-field fluctuations are known to exist on a variety of temporal and spatial scales, and the actual amount of Joule heating in the thermosphere is proportional to the average of the square of the E-field. The computation of the average of the square of the E-field requires knowledge of the statistical characteristics of E-field variability; thus knowledge not available at present. In this paper we assess, on the bases of theoretical considerations, the importance of E-field variability as an upper-atmosphere energy source. We show that the inclusion of E-field variability in the high-latitude convection model can significantly increase the amount of Joule heating for a given pattern.

Medium energy particle precipitation influences on the mesosphere and lower thermosphere

Data accumulated for more than a solar cycle by the TIROS/NOAA polar orbiting satellites are used to extract patterns of particle precipitation in several medium energy bands for varying geomagnetic conditions. While the atmospheric effects of auroral particles measured by these satellites (300 eV-20 keV) have been widely studied, the effects of medium energy (30 keV-2.5 MeV) particles have received less attention; in this paper we present average particle precipitation patterns for medium energy protons and electrons as a function of geomagnetic activity. A magnetic coordinate system is used to bin data at high latitudes to produce patterns that are then used as additional inputs to the National Center for Atmospheric Research thermosphere ionosphere mesosphere electrodynamic general circulation model (TIME-GCM) to study the effects of medium energy particles on the mesosphere and lower thermosphere. Model results obtained with these additional inputs are used to assess the significance of medium energy particles on the composition and dynamics of the region. Zonally averaged model results from the run that includes the medium energy particles show significant increases in ne, HOX, NO2, and NO and a 27 percent decrease in O3 between 70 and 80 km in the polar regions when compared to a similar run without the particles. The temperature and wind differences are small.

An overview of the early November 1993 geomagnetic storm

This paper describes the development of a major space storm during November 2-11, 1993. We discuss the history of the contributing high-speed stream, the powerful combination of solar wind transients and a corotating interaction region which initiated the storm, the high-speed flow which prolonged the storm and the near-Earth manifestations of the storm. The 8-day storm period was unusually long; the result of a high-speed stream (maximum speed 800 km/s) emanating from a distended coronal hole. Storm onset was accompanied by a compression of the entire dayside magnetopause to within geosynchronous Earth orbit (GEO). For nearly 12 hours the near-Earth environment was in a state of tumult. A super-dense plasma sheet was observed at GEO, and severe spacecraft charging was reported. The effects of electrons precipitating into the atmosphere penetrated into the stratosphere. Subauroral electron content varied by 100% and F layer heights oscillated by 200 km. Equatorial plasma irregularities extended in plumes to heights of 1400 km. Later, energetic particle fluxes at GEO recovered and rose by more than an order of magnitude. A satellite anomaly was reported during the interval of high energetic electron flux. Model results indicate an upper atmospheric temperature increase of 200°K within 24 hours of storm onset. Joule heating for the first 24 hours of the storm was more than 3 times that for typical active geomagnetic conditions. We estimate that total global ionospheric heating for the full storm interval was ∼190 PJ, with 30% of that generated within 24 hours of storm onset.

Electric field variability associated with the Millstone Hill electric field model

Joule heating that is generated at high latitudes in the thermosphere because of the magnetospherically imposed electric potential is proportional to the average of the square of the electric field (E field). Most theoretical Joule heating computations use only average electric fields, resulting in heating that is proportional to the square of the average E field. The computation of the average of the square of the E field requires knowledge about the statistical characteristics of E field variability associated with the average electric field model. In this paper we present the variability associated with the Millstone Hill bin-averaged empirical E field model [Foster et al. 1986] and discuss the implications of variability as an upper atmosphere energy source. We rebinned the radar plasma drift measurements from Millstone Hill, Massachusetts, in magnetic latitude and local time as a function of auroral activity and calculated the average electric fields and the variability associated with them as reflected in the bin standard deviations. We present the E field patterns and the associated variability for both quiet and disturbed geomagnetic conditions for the four seasons. We show that for an electric field model with a Gaussian distribution of small-scale variability around the mean, the average field and the variability have equal contributions to Joule heating generation.