Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Plasma density undulations in the dayside low‐latitude/midlatitude ionospheric <jats:italic>F</jats:italic> region were often attributed to thermospheric gravity waves (TGWs). However, the relationship between the former and the latter has been at best indirectly evidenced. In this study we investigate daytime fluctuations in neutral mass density (<jats:italic>ρ</jats:italic>) and plasma density (<jats:italic>n</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub>) measured onboard CHAMP from 2001 to 2010. A significant amount of daytime fluctuations in <jats:italic>n</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub> is strongly correlated with in situ fluctuations of <jats:italic>ρ</jats:italic>, which we term “TGW‐related <jats:italic>n</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub> fluctuations.” The TGW‐related <jats:italic>n</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub> fluctuations are (1) stronger in the winter hemisphere than in the summer hemisphere and (2) strongest in the South American sector during June solstice months. These climatological features are in general agreement with those of TGWs reported previously, especially at midlatitudes. On the other hand, the relative amplitude of TGW‐related <jats:italic>n</jats:italic><jats:sub><jats:italic>e</jats:italic></jats:sub> fluctuations does not depend strongly on solar activity.</jats:p>