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  • 1
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    Solar EUV and XUV energy input to thermosphere on solar rotation time scales derived from photoelectron observations (2012)
    Wiley
    Details
    Publication Date: 2012-05-18
    Description: Solar radiation below ∼100 nm produces photoelectrons, a substantial portion of the F region ionization, most of the E region ionization, and drives chemical reactions in the thermosphere. Unquantified uncertainties in thermospheric models exist because of uncertainties in solar irradiance models used to fill spectral and temporal gaps in solar irradiance observations. We investigate uncertainties in solar energy input to the thermosphere on solar rotation time scales using photoelectron observations from the FAST satellite. We compare observed and modeled photoelectron energy spectra using two photoelectron production codes driven by five different solar irradiance models. We observe about 1.7% of the ionizing solar irradiance power in the escaping photoelectron flux. Most of the code/model pairs used reproduce the average escaping photoelectron flux over a 109-day interval in late 2006. The code/model pairs we used do not completely reproduce the observed spectral and solar rotation variations in photoelectron power density. For the interval examined, 30% of the variability in photoelectron power density with equivalent wavelengths between 18 and 45 nm was not captured in the code/model pairs. For equivalent wavelengths below ∼16 nm, most of the variability was missed. This result implies that thermospheric model runs based on the solar irradiance models we tested systematically underestimate the energy input from ionizing radiation on solar rotation time scales.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Seismic monitoring of nuclear explosions (1989)
    Van Nostrand Reinhold
    In:  Bull., Open-File Rept., Encyclopedia of Geophysics, New York, Van Nostrand Reinhold, vol. 78, no. 87-17, pp. 1071-1089, (ISBN 0080419208)
    Details
    Publication Date: 1989
    Keywords: Review article ; Nuclear explosion ; Seismology ; Detectors ; Discrimination
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    BIBLIOGRAPHY SEISMOLOGY
  • 3
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    The importance of the neutral hydrogen for the maintenance of the mid latitude winter nighttime ionosphere: evidence from IS observations at Kharkiv, Ukraine and FLIP simulations (2016)
    Wiley
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    Publication Date: 2016-07-08
    Description: This study investigates the causes of nighttime enhancements in ionospheric density that are observed in winter by the incoherent scatter radar at Kharkiv, Ukraine. Calculations with a comprehensive physical model reveal that large downward ion fluxes from the plasmasphere are the main cause of the enhancements. These large fluxes are enabled by large upward H + fluxes into the plasmasphere from the conjugate summer hemisphere during the daytime. The nighttime downward H + flux at Kharkiv is sensitive to the thermosphere model H density, which had to be increased by factors of 2 to 3 to obtain model-data agreement for the topside H + density. Other studies support the need for increasing the thermosphere model H density for all seasons at solar minimum. It was found that neutral winds are less effective than plasmaspheric fluxes for maintaining the nighttime ionosphere. This is partly because increased equatorward winds simultaneously oppose the downward H + flux. The model calculations also reveal the need for a modest additional heat flow from the plasmasphere in the afternoon. This source could be the quiet time ring current.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    The Collapse of the Midnight Ionosphere and Behavior of Meridional Neutral Winds at Townsville Over a Full Solar Cycle (2015)
    Wiley
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    Publication Date: 2015-10-16
    Description: This paper investigates the causes of the sudden descent (midnight collapse) of the ionosphere at Townsville, Australia during the equinox periods of years between 1970 to 1980. The collapse of h m F 2 at midnight is found to occur on 89% of the 330 equinox nights that are investigated, and the mean magnitude of the midnight collapse is 84 km in the March equinox periods and 99 km in the September equinox periods. Observations of h m F 2 are used to determine equivalent meridional neutral winds using a first-principles physics model. Harmonic analysis of these derived winds reveals the existence of significant diurnal (24-hour), semidiurnal (12-hour), and terdiurnal (8-hour) tidal components. The contribution of wind harmonics to the midnight collapse is determined by bandpass filtering the winds to only allow certain tides and then modeling their effect on h m F 2 near midnight. The results indicate that the diurnal, semidiurnal and terdiurnal components of the meridional neutral wind all play a significant role at various times, but the effect of the 6-hour wind component is minimal. The spectral analysis also reveals that the terdiurnal wind component becomes dominant during solar maximum. Electric fields do not appear to be responsible for the midnight h m F 2 collapse because it is seldom seen at the near-conjugate station of Akita, Japan.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    Solar flux variation of the electron temperature morning overshoot in the equatorial F region (2011)
    Wiley
    Details
    Publication Date: 2011-04-12
    Description: Using 8 years of CHAMP satellite observations of the equatorial electron temperature, Te, we investigate its behavior during the morning overshoot and at ionospheric altitudes below 450 km including its variation with solar activity. The morning Te has a maximum at the dip equator and decreases gradually with increasing latitude, which is due to the increasing importance of heat conduction as the dip angle becomes larger. The amplitude of the equatorial morning overshoot Te decreases with increasing solar flux by about −10°K/solar flux unit depending on season and longitude. Trends of similar magnitude are predicted by the FLIP model. The model calculations confirm that the electron cooling due to enhanced electron-ion collisions increases faster than the heating of thermal electrons through collision with photoelectrons for increasing solar EUV. Both data and model showed that elevated electron temperatures persist to later local times during low solar activity. Obviously, the decreased background plasma density, together with the slower rise of electron density after sunrise under such conditions are responsible for the longer persistence. First investigations of longitudinal aspects revealed that the strength of the anticorrelation between morning Te and solar flux and the seasonal difference of the Te amplitude varies with longitude. The positive correlation between the morning overshoot and solar flux at 600 km as was shown earlier in Hinotori data is consistent with FLIP predictions and radar observations. The solar flux variation of the morning Te reverses sign between 400 and 600 km.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    Investigation of sudden electron density depletions observed in the dusk sector by the Poker Flat, Alaska incoherent scatter radar in summer (2014)
    Wiley
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    Publication Date: 2014-12-07
    Description: This paper investigates unusually deep and sudden electron density depletions (troughs) observed in the Poker Flat (Alaska) Incoherent Scatter Radar (PFISR) data in mid-summer of 2007 and 2008. The troughs were observed in the pre-midnight sector during periods of weak magnetic and solar activity. The density recovered to normal levels around midnight. At the time when the electron density was undergoing its steep decrease, there was usually a surge of the order of 100 to 400°K in the ion temperature that lasted less than one hour. The Ti surges were usually related to similar surges in the AE index, indicating that the high latitude convection pattern was expanding and intensifying at the time of the steep electron density drop. The convection patterns from the Super Dual Auroral Radar Network (SuperDARN) also indicate that the density troughs were associated with the expansion of the convection pattern to Poker Flat. The sudden decreases in the electron density are difficult to explain in summer because the high latitude region remains sunlit for most of the day. This paper suggests that the summer density troughs result from lower latitude plasma that had initially been corotating in darkness for several hours post sunset and brought back toward the sunlit side as the convection pattern expanded. The magnetic declination of ~22 degrees east at 300 km at Poker Flat greatly facilitates the contrast between the plasma convecting from lower latitudes and the plasma that follows the high latitude convection pattern.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Reexamination of ionospheric photochemistry (2011)
    Wiley
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    Publication Date: 2011-08-13
    Description: This paper reexamines the chemistry of N2+, NO+, O2+, and N+ by comparing densities from a photochemical model with data from the Atmosphere Explorer C satellite. These comparisons show that the measurements of N2+, NO+, O2+, and N+ density are well modeled with up-to-date reaction rates, solar EUV irradiances, and photoelectron fluxes. In particular, this study has resolved a long-standing problem wherein the previous investigations overestimated the measured N2+ density by a factor of 2. A new method of determining the important and controversial O+(2D) + N2 reaction rate from the Atmosphere Explorer C data is presented. This reaction rate determination agrees well with the latest laboratory measured reaction rate.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Dawnward shift of the dayside O+ outflow distribution: The importance of field line history in O+ escape from the ionosphere (2012)
    Wiley
    Details
    Publication Date: 2012-12-14
    Description: Observations and modeling of thermal upwelling O+ in the topside ionosphere are used to demonstrate that the well-known dawnward shift of energetic escaping fluxes at higher altitudes is the result of the expansion and contraction of low energy plasma on magnetic field lines and noon focused energization mechanisms. Recent research has shown that magnetospheric impacts of ion outflow are dependent on the local time source location. However, most modelers assume that the peak escaping O+ flux on the dayside is coincident with energy inputs from the magnetosphere associated with the cusp. The dawnward offset has been observed by multiple spacecraft and most recently in a new DMSP database of low altitude upwelling O+. The documented control of the cusp orientation with the Y component of the interplanetary magnetic field is generally a smaller effect than the observed dawnward bias. The dawnward offset is largest for thermal O+ during geomagnetically quiet intervals and smallest for energetic escaping O+ during active intervals. The present study investigates the efficacy of precipitating electrons, solar irradiance and neutral winds on dayside upwelling O+. An ionosphere model is used to show that solar irradiance causes significant upwelling O+ on dawn flux tubes and when combined with noon focused energization mechanisms, establishes the observed dayside escaping energetic O+ flux distribution. Our analysis also reveals that, during geomagnetically quiet intervals, high latitude neutral winds may be as important as electron precipitation in establishing the dayside distribution of upflowing thermal O+.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
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    Dawnward shift of the dayside O+ outflow distribution: The importance of field line history in O+ escape from the ionosphere (2013)
    Wiley
    Details
    Publication Date: 2013-01-05
    Description: [1]  Observations and modeling of thermal upwelling O + in the topside ionosphere are used to demonstrate that the well-known dawnward shift of energetic escaping fluxes at higher altitudes is the result of the expansion and contraction of low energy plasma on magnetic field lines and noon focused energization mechanisms. Recent research has shown that magnetospheric impacts of ion outflow are dependent on the local time source location. However, most modelers assume that the peak escaping O + flux on the dayside is coincident with energy inputs from the magnetosphere associated with the cusp. The dawnward offset has been observed by multiple spacecraft and most recently in a new DMSP database of low altitude upwelling O + . The documented control of the cusp orientation with the Y component of the interplanetary magnetic field is generally a smaller effect than the observed dawnward bias. The dawnward offset is largest for thermal O + during geomagnetically quiet intervals and smallest for energetic escaping O + during active intervals. The present study investigates the efficacy of precipitating electrons, solar irradiance and neutral winds on dayside upwelling O + . An ionosphere model is used to show that solar irradiance causes significant upwelling O + on dawn flux tubes and when combined with noon focused energization mechanisms, establishes the observed dayside escaping energetic O + flux distribution. Our analysis also reveals that, during geomagnetically quiet intervals, high latitude neutral winds may be as important as electron precipitation in establishing the dayside distribution of upflowing thermal O + .
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Investigation of the electron density variation during the August 21, 2017 Solar Eclipse (2018)
    Wiley
    Details
    Publication Date: 2018-01-30
    Description: This paper presents a comparison of modeled and measured electron densities for the August 21, 2017 solar eclipse across the USA. The location of the instrument was (43.81°N, 247.32°E) where the maximum obscuration of 99.6% occurred at 17.53 hours UT on August 21. The solar apparent time was 9.96 hours and the duration of the eclipse was 2.7 hours. It was found that if it is assumed that there are no chromosphere emissions at totality, ~30% coronal emission remaining at totality gave the best fit to the electron density variation at 150 km. The 30% coronal emission estimate has uncertainties associated with respect to uncertainties in the solar spectrum, the measured electron density, and the amount of chromosphere emissions remaining at totality. The agreement between the modeled and measured electron density is excellent at 150 km with the assumed 30% coronal emission at totality. At other altitudes, the agreement is very good, but the altitude profile would be improved if the model peak electron density ( N m F 2 ) decayed more slowly to better match the data. The minimum N m F 2 in the model occurs ~10 minutes after totality when it decreases to 0.55 from its non-eclipse value. The minimum of the N m F 2 data occurs between 6 and 10 minutes after totality but is ~15% larger. The total electron content decreases to 0.65 of its pre-eclipse value. These relative changes agree well with those predicted by others prior to the eclipse.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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