The Midlatitude Summer Night Anomaly as observed by CHAMP and GRACE: Interpreted as tidal features

Abstract

This paper presents a description of the Midlatitude Summer Night Anomaly (MSNA) in terms of solar tidal signatures, based on in situ observations from CHAMP (CHAllenging Minisatellite Payload) and GRACE (Gravity Recovery and Climate Experiment) during the solar minimum years 2008 and 2009. Our analysis is focusing on 40• to 60• magnetic latitude ranges in both hemispheres, where the reversed diurnal variations of the electron density are strongest. The results revealed that in the Southern Hemisphere the longitudinally symmetric tide D0 is particularly strong during December solstice. The well-known Weddell Sea Anomaly is caused by a simultaneous constructive interference of three components D0, DW2, and SPW1. During June solstice the eastward propagating tide DE1 is the strongest in the Northern Hemisphere, which causes a wave-2 longitudinal pattern. The two crests of the wave-2 pattern at nighttime correspond well with the MSNA feature in the Northern Hemisphere. The MSNA feature over the USA continent is particularly strong, which can be explained by the combined contributions of the components DE1, D0, and DW2. The diurnally varying difference in electron density between the USA East and West Coast can also be explained by the phase propagation of the DE1. A similar effect has also been observed in the Asian region.The peak electron densities of the tidal component D0 appear around 0700 LT and 2000 LT in the Southern and Northern Hemispheres, respectively. The time shift suggests that the two hemispheres move in antiphase up and down. The planetary wave SPW1 exhibits an electron density crest near longitude sectors where the dip equator reaches far into the summer hemisphere. IntroductionIn recent years, more and more studies are focusing on the longitudinal modulation of the ionosphere by tidal effects originating from the lower atmosphere. These tides are often excited by latent heat release in tropospheric deep convective tropical clouds and propagate vertically upward. Therefore, tidal effects in the ionosphere have attained more attention in the equatorial and low-latitude regions, where prominent longitudinal wave patterns have been found in ionospheric quantities such as the equatorial ionization anomaly (EIA), Equatorial Electrojet, vertical plasma drift, and total electron content (TEC) Lühr et al., 2008;Kil et al., 2007;Scherliess et al., 2008;Lühr et al., 2012]. The magnitudes of these tidal components vary differently with season, causing the ionospheric quantities to show longitudinal patterns with varying wave numbers over the course of a year. Best known are the wave number-4 (WN4) patterns during the months around August and wave number-3 (WN3) pattern around solstice seasons, corresponding to the diurnal DE3 and DE2 nonmigrating tides, respectively Wan et al., 2010;Lühr et al., 2008Lühr et al., , 2012. For the labeling of the tidal components we use the common convention. The first letter D, S, or T stands for diurnal, semidiurnal or terdiurnal; the second letter E or W for eastwar...

Tidal signatures of the thermospheric mass density and zonal wind at midlatitude: CHAMP and GRACE observations

“…The most prominent tidal components in mass density and zonal wind are the diurnal tides D0 and DW2 and the semidiurnal tides SW1 and SW3. This is consistent with the tidal signatures in the F region electron density at midlatitudes as reported by Xiong and Lühr (2014). These same tidal components are observed both in the thermospheric and ionospheric quantities, supporting a mechanism that the non-migrating tides in the upper atmosphere are excited in situ by ion-neutral interactions at midlatitudes, consistent with the modeling results of Jones Jr.…”

“…They further explained that the standing diurnal tide (D0) dominates the vertical neutral wind causing the southern MSNA feature, and the tide manifests itself as a diurnal eastward-propagating wave-1 pattern. Our recent study confirmed the eastward movement of the MSNA in both hemispheres from in situ measurements of the CHAMP and GRACE missions (Xiong and Lühr, 2014). From the tidal perspective, the prominent MSNA features have further been interpreted as the simultaneous constructive interferences of the tidal components D0, DW2, and stationary planetary wave SPW1 in the Southern Hemisphere, and DE1, D0, and DW2 in the Northern Hemisphere.…”

“…In order to compare the tidal features of the topside ionosphere that are related to MSNA, as reported by Xiong and Lühr (2014), the same magnetic latitude (MLAT) range (±40 to ±60 • ) has been selected for the thermospheric mass density and zonal wind measurements. As the background thermospheric mass density is largely controlled by solar activity and magnetic storms, data from solar minimum years (from 2007 to 2009, mean P10.7 = 71 sfu) have been selected for the mass density to avoid their influence on our tidal signatures analysis.…”

“…At midlatitudes, in our recent work Xiong and Lühr (2014), we have reported prominent longitudinal wave-1 and wave-2 patterns in electron density of the southern and northern midlatitude summer nighttime anomaly (MSNA) features, respectively. The MSNA refers to the phenomena of nighttime electron density anomalous enhancements at midlatitudes during local summer seasons.…”

“…The purpose of the study is to compare the tidal effects on the thermospheric mass density and zonal wind with that of the electron density; therefore, the same method for deriving the tidal components has been used as described in Xiong and Lühr (2014). The general mathematical formulation of the relationship between longitudinal patterns in satellite observations and the non-migrating tidal description in the Earth-fixed frame is given by Forbes et al (2006) and Häusler and Lühr (2009).…”