SA13B-01 INVITED 13:40h
Solar XUV and VUV irradiances: Measurement and Modeling Progress
Solar soft X-ray (XUV) and Vacuum Ultraviolet (VUV) spectral irradiances are particularly important for space system engineering, aeronomy, and climate change research since these wavelengths deposit their energy in the thermosphere, mesosphere, and stratosphere as well as create the ionosphere. Because solar spectral irradiances are a foundation for understanding scattering and photoabsorption processes in atmospheres and ionospheres, it is important to provide user communities with accurate, precise and time-resolved solar irradiance products. We present an overview of the solar irradiance community's recent space-based measurements and empirical as well as physics-based modeling for short (minutes to hours), medium (hours to days), and long (weeks to months) time scale irradiance variations as related to energy deposition processes for the thermosphere and ionosphere. In addition, we describe the ISO 21348 standard which provides a common process for determining solar irradiances.
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SA13B-02 13:55h
Flare Irradiance Spectral Model (FISM): a Model of Solar Vacuum Ultraviolet Irradiance Over Time Scales From Flares to Solar Cycles
The Flare Irradiance Spectral Model (FISM) is an empirical model of the solar irradiance spectrum from 0.1 to 195 nm at 1nm resolution and on a 1-minute time cadence. This model is based on the TIMED SEE data and uses the MgII core-to-wing ratio and GOES XRS data as proxies. FISM accounts for the significant irradiances changes due to solar flares, which includes orders of magnitude increases in the X-rays to factors of two increases in the EUV. The need for FISM arises due to the lack of solar irradiance measurements on minute time scales for the entire XUV and EUV spectral region, and currently available daily models and measurements do not represent or include the irradiance changes due to solar flares. Algorithms and current results from FISM will be discussed, as well as the future improvements that are planned for the model.
SA13B-03 INVITED 14:10h
Energy Input to the Magnetosphere and its Dissipation in the Ionosphere
The primary mechanism of energy input to the magnetosphere is dayside magnetic reconnection. The rate at which energy is input depends on the rate at which southward (GSM) magnetic flux is delivered to the dayside magnetopause (VBs) and reconnection efficiency (alpha). In the past efficiency has been assumed to be independent of geometry. Recent studies, however, indicate that it is a maximum when the IMF is antiparallel to the dipole moment (specific universal times) near equinox and a minimum when the dipole is maximally tilted either toward or away from the Sun (solstice). Through the Russell-McPherron effect average IMF Bz is a function of season (DOY) and universal time (UT) as is the average solar wind velocity (heliospheric latitude effect). Reconnection efficiency seems to be a function of both variables as well. Magnetic indices are used as proxies for internal dissipation of the energy input to the magnetosphere. The indices that measure ionospheric currents depend on ionospheric conductivity as well. Conductivity depends on DOY and UT differently than the other variables. Thus we can write that Index = K*Sigma*alpha*V*Bs where K is a constant of proportionality and the remaining quantities are functions of both DOY and UT. The product VBs has been shown to obey the predictions of the Russell-McPherron effect. The reconnection efficiency appears to be described by the Svalgaard function (dependent on the tilt toward and away from Sun) [O'Brien and McPherron, JGR, 107(A11)]. The pattern of conductivity is also complex. We expect that the UT-DOY pattern for an index to be distinctly different from the prediction of the R-M effect since it is the product of the various patterns, each of which have time delays that must also be considered. In this paper we will review the evidence supporting this model and discuss consequences of these geometric effects at different time scales. We will show that there are systematic variations in the behavior of magnetic indices with universal time, season, and solar cycle that are consequences of these geometric effects.
SA13B-04 14:25h
High-Latitude Joule Heating Compared With Observed Global Neutral Density Response
An empirical model of the high-latitude, ionospheric electric potentials has recently undergone substantial revision and improvement. This model, which can predict the electric fields in the ionosphere using only measurements of the solar wind and interplanetary magnetic field (IMF), has a "twin" model that is based on "magnetic Euler potentials." Although the original purpose of the magnetic potentials was to map the distribution of the field-aligned currents into the ionosphere, it can be combined with the electric field predictions in order to calculate the distribution of Joule heating in the ionosphere. Thus, we can calculate the total Joule heat energy input to the ionosphere as a function of time. This energy, which is transferred from the solar wind through the magnetosphere and ionosphere into the thermosphere, increases substantially during geomagnetic storms. Comparing our results with output from the High Accuracy Satellite Drag Model (HASDM), we show that during major storms the Joule heating is a major source of day-long enhancements in the global thermospheric neutral density.
SA13B-05 14:40h
Examing the effects of periodic high latitude forcing on the Joule heating and thermospheric temperature structure
High latitude Joule heating is one of the main causes for the rapid increase of temperature in the thermosphere. This increase in temperature is advected to lower latitudes, and can therefore raise the global mean thermospheric temperature. This rise in temperature can lift the atmosphere, increasing drag on satellites. Many global models have had a difficult time modeling this energy input accuurately. Codrescu[1995] showed that the variability in the high latitude E-field can significantly increase the amount of Joule heating, which has been consistenly underestimated in global models. Using the AMIE procedure, Crowley and Hackert[2001] argued that a significant fraction of this variability arises from oscillations with period less than one hour. While it is understood that the thermosphere responds differently at different frequencies, this effect has not been thoroughly quantified. We use the Global Ionosphere-Thermosphere Model(GITM) to simulate the thermospheric reaction to some simple sin-waves in the highlatitude forcing terms to determine the different frequencies which will optimize the Joule heating in the thermosphere at different altitudes. We then test the idea of variability by adding different levels of random noise to the E-field to check the changes of the Joule heating. So we are able to examine the effects of driving to the thermosphere-ionosphere at different frequencies and quantify the effect of the E-field variability on the Joule heating and the thermospheric temperature structure.
SA13B-06 14:55h
Ionospheric Heating in Aurora: Observations
Small scale aurora has brightness structures with a horizontal scale length of less than 1~km. Currents and electric fields have variations on the same scale, but observations on this scale length are difficult and few observations exist. Using averaged quantities from observation with less resolution for the determination of Joule heating leads to underestimates of the heating rates. We will present case studies of different observations of small scale aurora, including ground based optical images and spectroscopic measurements, FAST particle and current data, SuperDARN, and data from ground based wind and temperature imagers.
SA13B-07 15:10h
Thermospheric Response to High-Latitude Energy Sources at Quiet Times
Recent results from the CHAMP/STAR accelerometer measurements of thermospheric neutral density have brought back to our attention the existence of important energy sources at high latitudes during geomagnetically quiet times. These energy sources produce a large dayside high-latitude density bulge which is more prominent than the sub-solar density bulge. Evidence for this persistent density enhancement during quiet times has accumulated over the past 35 years. We discuss the numerous measurements of the density bulge made by accelerometers, mass spectrometers, pressure gauges, and satellite orbital decay, as well as the correlation with airglow and ionospheric observations. The energy source for this region of increased neutral density is the solar wind, after it has passed through the Earth's bow shock and magnetosphere. The region of increased density appears on the dayside of both the northern and southern hemispheres, and has a geometrical shape similar to a lunette. The central portion of the arc of the lunette coincides with the downward projection of the magnetospheric dayside cusp. Consequently, the density bulge is best described in solar-geomagnetic coordinates. The wings of the lunette extend far beyond the footprint of the dayside cusp, and are most likely energized by particles that come from other parts of the magnetosphere. The arc of the lunette is clearly displayed by airglow observations and is matched by ionospheric measurements. The corresponding neutral density bulge is much broader in geomagnetic latitude, as one might expect from the longer time constants of neutral processes. We show a Mercator projection of the global density distribution at an altitude of 400 km at 12 hours GMT as an example of the neutral density distribution produced by both the UV and corpuscular energy sources at geomagnetically quiet times.
SA13B-08 INVITED 15:25h
Visible Region Dayglow as a Monitor of Thermospheric Solar Energy Deposition
Dayglow is the short name for daytime airglow, the atmospheric emission produced by photochemical processes; i.e. by non-thermal means. It is observed in the ultraviolet and visible regions, as well as in the near-infrared. The nightglow is produced entirely by chemistry, through the recombination of ions and atoms that are formed in the daytime. The dayglow includes the same chemical processes, but as well includes "prompt" emission caused by the absorption of shorter-wavelength solar radiation, and is therefore a valuable monitor of the solar radiation in the corresponding spectral regions. The visible and near-IR dayglow can in principle be detected from the ground, but only with instrumentation capable of distinguishing it from the enormously stronger light scattered from the lower atmosphere. The Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite views visible region dayglow emission from above, at the Earth's limb, with a meter-long baffle that allows it to be measured above the intense light scattered from the lower atmosphere. The atomic oxygen "green line" emission from the O(1S) level, at 557.7 nm occurs in two distinct layers, one peaking near 160 km that is produced almost entirely through the absorption of solar EUV radiation and its subsequent processes, and another peaking near 105 km that is produced mainly by the absorption of solar Lyman-beta and the recombination of atomic oxygen. About one-half million WINDII green line profiles were acquired between 1992 and 1997. The lower layer is found to be enhanced during solar flares, almost certainly by x-rays. Thus the green line responds to at least three different regions of the solar spectrum. The transition from O(1D) at 630.0 nm has a single peak with an average altitude of about 250 km, and is a result mainly of excitation in the EUV. The dayglow response to the solar input is presented and characterized, and its contribution to the understanding of energy deposition in the thermosphere is described.