Browsing by Author "Tobiska, W. K."
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- Global Variations in the Time Delays Between Polar Ionospheric Heating and the Neutral Density ResponseWeimer, Daniel R.; Mehta, Piyush M.; Licata, R. J.; Tobiska, W. K. (American Geophysical Union, 2023-04)We present results from a study of the time lags between changes in the energy flow into the polar regions and the response of the thermosphere to the heating. Measurements of the neutral density from the Challenging Mini-satellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) missions are used, along with calculations of the total Poynting flux entering the poles. During two major geomagnetic storms in 2003, these data show increased densities are first seen on the dayside edge of the auroral ovals after a surge in the energy input. At lower latitudes, the densities reach their peak values on the dayside earlier than on the night side. A puzzling response seen in the CHAMP measurements during the November 2003 storm was that the density at a fixed location near the "Harang discontinuity" remained at unusually low levels during three sequential orbit passes, while elsewhere the density increased. The entire database of measurements from the CHAMP and GRACE missions were used to derive maps of the density time lags across the globe. The maps show a large gradient between short and long time delays between 60 degrees and 30 degrees geographic latitude. They confirm the findings from the two storm periods, that near the equator, the density on the dayside responds earlier than on the nightside. The time lags are longest near 18-20 hr local time. The time lag maps could be applied to improve the accuracy of empirical thermosphere models, and developers of numerical models may find these results useful for comparisons with their calculations. The interaction of the solar wind with the Earth's magnetosphere causes varying levels of heating in the ionosphere. This heating is produced by auroral currents at high latitudes, which in tur n causes the density of the upper atmosphere to change. A topic of importance is to determine how rapidly the density can increase at different locations around the globe following a surge in the heating, which can be calculated from measurements of the solar wind velocity and embedded magnetic fields. This study used measurements of the atmospheric density on two satellite missions known as Challenging Mini-satellite Payload and Gravity Recovery and Climate Experiment. The results show that the density increases first near the poles, and much longer at lower latitudes, as expected. The time lags between changes in the energy input and the density response have been determined for the first time on a global scale. Maps of the time lags are derived. Near the equator the lags are shorter near local noon, and longer before local midnight. The time lag maps can used to improve empirical and numerical models of the thermosphere. More accurate models are needed for more precise predictions of the drag that satellites will encounter, and the subsequent changes in their orbits.
- High correlations between temperature and nitric oxide in the thermosphereWeimer, Daniel R.; Mlynczak, M. G.; Hunt, L. A.; Tobiska, W. K. (American Geophysical Union, 2015-07-01)Obtaining accurate predictions of the neutral density in the thermosphere has been a long-standing problem. During geomagnetic storms the auroral heating in the polar ionospheres quickly raises the temperature of the thermosphere, resulting in higher neutral densities that exert a greater drag force on objects in low Earth orbit. Rapid increases and decreases in the temperature and density may occur within a couple days. A key parameter in the thermosphere is the total amount of nitric oxide (NO). The production of NO is accelerated by the auroral heating, and since NO is an efficient radiator of thermal energy, higher concentrations of this molecule accelerate the rate at which the thermosphere cools. This paper describes an improved technique that calculates changes in the global temperature of the thermosphere. Starting from an empirical model of the Poynting flux into the ionosphere, a set of differential equations derives the minimum, global value of the exospheric temperature, which can be used in a neutral density model to calculate the global values. The relative variations in NO content are used to obtain more accurate cooling rates. Comparisons with the global rate of NO emissions that are measured with the Sounding of the Atmosphere using Broadband Emission Radiometry instrument show that there is very good agreement with the predicted values. The NO emissions correlate highly with the total auroral heating that has been integrated over time. We also show that the NO emissions are highly correlated with thermospheric temperature, as well as indices of solar extreme ultraviolet radiation.
- Improving Neutral Density Predictions Using Exospheric Temperatures Calculated on a Geodesic, Polyhedral GridWeimer, Daniel R.; Mehta, P. M.; Tobiska, W. K.; Doornbos, E.; Mlynczak, M. G.; Drob, Douglas P.; Emmert, J. T. (2019-12-10)A new model of exospheric temperatures has been developed, with the objective of predicting global values with greater spatial and temporal accuracy. From these temperatures, the neutral densities in the thermosphere can be calculated, through use of the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended (NRLMSISE-00) model. The exospheric temperature model is derived from measurements of the neutral densities on several satellites. These data were sorted into triangular cells on a geodesic grid, based on location. Prediction equations are derived for each grid cell using least error fits. Several versions of the model equations have been tested, using parameters such as the date, time, solar radiation, and nitric oxide emissions, as measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. Accuracy is improved with the addition of the total Poynting flux flowing into the polar regions, from an empirical model that uses the solar wind velocity and interplanetary magnetic field. Given such inputs, the model can produce global maps of the exospheric temperature. These maps show variations in the polar regions that are strongly modulated by the time of day, due to the daily rotation of the magnetic poles. For convenience the new model is referred to with the acronym EXTEMPLAR (EXospheric TEMperatures on a PoLyhedrAl gRid). Neutral densities computed from the EXTEMPLAR-NRLMSISE-00 models combined are found to produce very good results when compared with measured values.
- New solar extreme-ultraviolet irradiance observations during flaresWoods, T. N.; Hock, R.; Eparvier, F. G.; Jones, A. R.; Chamberlin, P. C.; Klimchuk, J. A.; Didkovsky, L.; Judge, D.; Mariska, J.; Warren, H.; Schrijver, C. J.; Webb, D. F.; Bailey, S.; Tobiska, W. K. (IOP Publishing, 2011-10-01)New solar extreme-ultraviolet (EUV) irradiance observations from the NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide full coverage in the EUV range from 0.1 to 106 nm and continuously at a cadence of 10 s for spectra at 0.1 nm resolution and even faster, 0.25 s, for six EUV bands. These observations can be decomposed into four distinct characteristics during flares. First, the emissions that dominate during the flare's impulsive phase are the transition region emissions, such as the He II 30.4 nm. Second, the hot coronal emissions above 5 MK dominate during the gradual phase and are highly correlated with the GOES X-ray. A third flare characteristic in the EUV is coronal dimming, seen best in the cool corona, such as the Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of the EUV coronal emissions peak a few minutes after the GOES X-ray peak. One interesting variation of the post-eruptive loop reconnection is that warm coronal emissions (e. g., Fe XVI 33.5 nm) sometimes exhibit a second large peak separated from the primary flare event by many minutes to hours, with EUV emission originating not from the original flare site and its immediate vicinity, but rather from a volume of higher loops. We refer to this second peak as the EUV late phase. The characterization of many flares during the SDO mission is provided, including quantification of the spectral irradiance from the EUV late phase that cannot be inferred from GOES X-ray diagnostics.