Scott E. Palo

Variability of the ionosphere

Abstract Hourly foF2 data from over 100 ionosonde stations during 1967–89 are examined to quantify F-region ionospheric variability, and to assess to what degree the observed variability may be attributed to various sources, i.e., solar ionizing flux, meteorological influences, and changing solar wind conditions. Our findings are as follows. Under quiet geomagnetic conditions ( K p σ ( σ is the standard deviation) variability of N max about the mean is approx. ±25–35% at ‘high frequencies’ (periods of a few hours to 1–2 days) and approx. ±15–20% at ‘low frequencies’ (periods approx. 2–30 days), at all latitudes. These values provide a reasonable average estimate of ionospheric variability mainly due to “meteorological influences” at these frequencies. Changes in N max due to variations in solar photon flux, are, on the average, small in comparison at these frequencies. Under quiet conditions for high-frequency oscillations, N max is most variable at anomaly peak latitudes. This may reflect the sensitivity of anomaly peak densities to day-to-day variations in F-region winds and electric fields driven by the E-region wind dynamo. Ionospheric variability increases with magnetic activity at all latitudes and for both low and high frequency ranges, and the slopes of all curves increase with latitude. Thus, the responsiveness of the ionosphere to increased magnetic activity increases as one progresses from lower to higher latitudes. For the 25% most disturbed conditions ( K p >4), the average 1- σ variability of N max about the mean ranges from approx. ±35% (equator) to approx. ±45% (anomaly peak) to approx. ±55% (high-latitudes) for high frequencies, and from approx. ±25% (equator) to approx. ±45% (high-latitudes) at low frequencies. Some estimates are also provided on N max variability connected with annual, semiannual and 11-year solar cycle variations.

Middle atmosphere effects of the quasi-two-day wave determined from a General Circulation Model

A set of numerical experiments have been conducted using the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (NCAR TIME-GCM) to understand the effects of the quasi-two-day wave (QTDW) on the middle atmosphere horizontal wind and temperature fields. A zonal wavenumber three perturbation with a period of 48 hours and a latitudinal structure identical to the (3, 0) Rossby-gravity mode has been included at the lower-boundary of the model. A response in the middle atmosphere horizontal wind fields is observed with a structure qualitatively similar to observations and other model results. There is also some evidence to suggest an increase in the lower-thermosphere QTDW response due to the interaction with gravity waves. Changes are observed in the zonal mean wind and temperature fields that are clearly related to the QTDW, however it is unclear if these changes are the direct result of wave driving due to the QTDW or are from another source. Evidence for nonlinear interactions between the QTDW and the migrating tides is presented. This includes significant (40–50%) decreases in the amplitude of the migrating tides when the QTDW is present and the generation of wave components which can be tracked back to an interaction between the QTDW and the migrating tides. Clear evidence for the existence of a westward propagating zonal wavenumber six nonmigrating diurnal tidal component which results from the nonlinear interaction between the QTDW and the migrating tides is also presented.

Polar mesospheric cloud structures observed from the cloud imaging and particle size experiment on the Aeronomy of Ice in the Mesosphere spacecraft: Atmospheric gravity waves as drivers for longitudinal variability in polar mesospheric cloud occurrence

patterns and structures in polar mesospheric clouds (PMCs), around the summertime mesopause region, which are qualitatively similar to structures seen in noctilucent clouds (NLCs) from ground‐based photographs. The structures in PMC are generally considered to be manifestations ofupward propagating atmospheric gravity waves (AGWs). Variability of AGW effects on PMC reported at several lidar sites has led to the notion of longitudinal differences in this relationship. This studycompares thelongitudinal variability in theCIPS‐ observed wave occurrence frequency with CIPS‐measured PMC occurrence frequency and albedo along with mesospheric temperatures measured by the sounding of the atmosphere using broadband emission radiometry instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics spacecraft. Our results for the latitude ranges between 70° and 80° show a distinct anticorrelation of wave structures with cloud occurrence frequency and correlations with temperature perturbations for at least two of the four seasons analyzed, supporting the idea of gravity wave‐induced cloud sublimation. The locations of the observed wave events show regions of high wave activity in both hemispheres. In the Northern Hemisphere, while the longitudinal variability in observed wave structures show changes from the 2007–2008 seasons, there exist regions of both low and high wave activities common to the two seasons. These persistent features may explain some of the observed differences in PMC activity reported by ground‐based lidar instruments distributed at different longitudes. The statistical distribution of horizontal scales increases with wavelength up to at least 250 km. We also discuss the possibility of atmospheric tides, especially the nonmigrating semidiurnal tide, aliasing our observations and affecting the results presented in this analysis.

Hemispheric properties of the two-day wave from mesosphere-lower-thermosphere radar observations

Abstract Measurements of the pseudo 2-day wave have been made at many of the mesosphere-lower-thermosphere radars. Comparisons are made here between measurements taken at Saskatoon MF radar (52°N, 107°W) and two meteor radars, one at Christmas Island (2°N, 157°W) and the other at Durham (43°N, 71°W). Although results averaged for 10 days or longer agree with previous measurements (i.e. larger amplitudes and more phase stability in late summer), when the wave is analyzed over 2–4 days as a 48 h component, interesting phase properties emerge and the wave is seen over more of the year. The wave is amplitude and phase modulated, making the interpretation of results obtained over long time frames (20 days or more) difficult. There is strong evidence of solar influence on the 2-day wave.

More total electron content climatology from TOPEX/Poseidon measurements

We have used the TOPEX satellite data set to compare climatologies of total electron content (TEC) measurements from the dual-frequency altimeter with the Doppler orbitography and radiopositioning integrated by satellite (DORIS) TEC results and with the international reference ionosphere (IRI) and Bent model predictions corresponding to the satellite measurements. We have used the TOPEX measurements from launch in 1992 through 1997 to build a database that includes time, geographic and geomagnetic coordinates of the measurement, geomagnetic indices (Kp, previous Kp, hemispheric power, and integral of hemispheric power over the previous 36 hours), solar index (F 10.7), TOPEX and DORIS TEC measurements, and empirical model (IRI and Bent) results corresponding to the TOPEX measurements. We have binned the measurements in a magnetic local time, magnetic latitude coordinate system to produce global maps of TEC. We present climatological differences between TOPEX TEC and DORIS measurements as well as between TOPEX and the two empirical models. The maps were constructed using all appropriate TOPEX TEC data, DORIS TEC measurements, and the corresponding Bent and IRI model results for solar minimum conditions.

Tec climatology derived fromtopex/poseidonmeasurements

Abstract We have used the TOPEX data sets available from JPL to create a customizeddatabase of the TOPEX TEC measurements that contain data from 1992 through part of 1996.Thedata base includes time, geographic and geomagnetic coordinates of themeasurement,geomagnetic indices ( Kp , previous K p , HemisphericPower, andintegral of hemispheric power over the previous 36 h), solar index ( F 10.7),andInternational Reference Ionosphere (IRI) results corresponding to the TOPEX measurements.Inthis paper we present global maps of TEC for low solar activity conditions ( F 10.7 ⩽ 120) for quiet (integral of hemispheric power less than 800 GWh, roughly correspondingto K p = 2), moderately disturbed (integral of hemispheric power greaterthan 800GWh but less than 1200 GWh, roughly corresponding to K p = 3),anddisturbed (integral of hemispheric power greater than 1200 GWh, roughly corresponding to K p = 4) geomagnetic conditions, derived by binning all appropriateTOPEX TECdata from 1992 to 1996. The analysis is performed in a Magnetic-Local-Time,Magnetic-Latitudecoordinate system. The most prominent feature of the global TEC maps is thefeaturecorresponding to the equatorial anomaly. The feature becomes wider in magnetic latitudeandmore pronounced in amplitude as the activity level increases. The equatorward shift of thecrests,with increased magnetic activity, can produce apparent decreases in TEC at their quiettimelocation for individual storms as evident in the conflicting conclusions of TECgeomagneticdependence studies of the 1960s. For the same activity level, TEC values in theequatorialanomaly are higher during equinox compared to solstice.

Observations of the quasi‐two‐day wave in the middle and lower atmosphere over Christmas Island

We present analyses of data collected at Christmas Island in the mesosphere and lower thermosphere using a meteor echo detection and collection system (MEDAC) in addition to data collected in the troposphere and lower stratosphere using a clear air stratosphere/troposphere (ST) radar. These data, which provide 5 years of semicontinuous horizontal wind velocity estimates, are analyzed to determine the temporal structure of the quasi-two-day wave in the middle and lower atmosphere. Our results show the presence of strong wave activity in the middle atmosphere during late January and early February as well as a secondary maxima in June/August for each of the years we have data. However, the magnitude and duration of the wave events observed in late January and early February exhibit interannual variability.

Dynamics of the lower thermosphere over South Pole from meteor radar wind measurements

A meteor radar was operated at Amundsen-Scott Station, South Pole, from January 19, 1995 through January 26, 1996 and from November 21, 1996 through January 27, 1997. Hourly wind measurements were obtained nearly continuously over these time periods, at an approximate altitude of 95 km and at about 2° latitude from South Pole along the longitude meridians 0°, 90°E, 90°W, and 180°. The scientific advances achieved to date through analyses of these data are presented, including updates to several of our previously published works. The findings addressed herein include the following: (1) Strong divergences of zonal-mean meridional winds occasionally occur over South Pole, implying extreme vertical winds; (2) The monthly mean zonally asymmetric (zonal wavenumber s = 1) wind component varies during the year in a manner consistent with migration of the center of the polar vortex with respect to the geographic (rotational) pole; (3) Strong (>15 m/s) westward-propagating migrating diurnal (s = 1) and non-migrating semidiurnal (s = 1) oscillations exist except during winter months; (4) Long-period (∼2–10 days) waves exist during winter months which are primarily eastward-propagating; (5) Intradiurnal (periods ∼6–11.5 hours) westward-propagating oscillations exist, which are thought to be gravitational normal modes, or “Lamb” waves.

Lamb waves in the lower thermosphere: Observational evidence and global consequences

Meteor radar observations of hourly neutral meridional winds at 95 km altitude near 88°S and along the four meridians 0°, 90°E, 90°W, and 180°, were made from Amundsen-Scott Station at South Pole from January 19, 1995, through January 26, 1996, and from November 21, 1996, through January 27, 1997. These data reveal the existence of ±5-15 m s -1 oscillations with periods between about 7.5 and 10.5 h, propagating to the west with zonal wavenumber S = 1. These oscillations are interpreted as the atmospheric manifestations of gravitational normal modes or Lamb waves. Barring significant Doppler-shifting effects, the second symmetric mode with period near 8.6 hours, and the first asymmetric mode with period near 10.4 hours, appear to dominate. At middle latitudes, for limited duration time series, it would be easy to confuse these waves with terdiurnal (8 hours) and semidiurnal (12 hours) solar tides. The Global Scale Wave Model (GSWM) is used to simulate the global perturbation temperature and wind fields consistent with these observations. Wind and temperature oscillations exceeding 12 m s -1 and 12 K, and 80 m s -1 and 40 K, are predicted to occur for the 10.4-hour and 8.6-hour waves, respectively, above 110 km. Such perturbations may be observable by incoherent scatter radars now in existence.

New perspectives on thermosphere tides: 1. Lower thermosphere spectra and seasonal-latitudinal structures

Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) temperature measurements at 110 km and between ±50° latitude extending from 2002 through 2010 are analyzed to reveal the tidal spectrum entering the ionosphere-thermosphere (IT) system. Seasonal-latitudinal structures are presented for the most prominent spectral components which include DE3, DE2, D0, DW1, DW2, SE1 to SE4, SW1 to SW4, SW6, TE1, TW4, TW5, and TW7. Referring to recent calculations of lower atmosphere heat sources as well as vertical structure characteristics of these waves anticipated from classical tidal theory, we analyze the likely origins of these waves and the nature of their seasonal-latitudinal structures. Several waves are likely to arise through nonlinear wave-wave interactions, and in some cases, this appears to be the sole viable mechanism leading to their existence. The tidal spectrum quantified here is especially relevant to the dynamo generation of electric fields which then impose the tidal variability on the overlying F-region ionosphere. Part 2 of this 2-part study examines penetration of the tidal spectrum to the upper thermosphere.Please see related article: http://www.earth-planets-space.com/content/66/1/122