Simultaneous middle and upper atmosphere radar and ionospheric sounder observations of midlatitude <b><i>E</i></b> region irregularities and sporadic <b><i>E</i></b> layer

<jats:p>We made middle and upper atmosphere (MU) radar observations of midlatitude <jats:italic>E</jats:italic> region field‐aligned irregularities (FAIs) in the summer of 1999 and 2000. Sporadic <jats:italic>E</jats:italic> (<jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>) layer was monitored with a routine ionosonde, and its altitude was measured with an FM‐CW sounder (FCS). In this paper we draw attention to two findings. First, we show that quasiperiodic (QP) radar echoes appearing before 0200 LT are more enhanced with increasing <jats:italic>f</jats:italic><jats:sub><jats:italic>o</jats:italic></jats:sub><jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> − <jats:italic>f</jats:italic><jats:sub><jats:italic>b</jats:italic></jats:sub><jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>, which means that the FAI generation is closely related to localized density gradients within <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>, and extend from 100 to 130 km in altitude, while <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> altitudes determined from the FCS soundings are between 100 and 110 km. The latter fact suggests that existing models for the QP echo generation, which require a deep modulation of <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> altitude, are not applicable to our observational results. We propose a new working model for generating QP echoes in which polarization electric fields originated from high‐density plasma clouds within <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> are mapped upward along the geomagnetic field to produce relatively weak irregularities above the <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> layer. Second, we show new findings obtained from the current observations, namely, two types of QP echoes that occur below 100 km in the morning: one is the morning QP (MQP) echoes with periods of 4‐8 min, and the other is the QP echoes with periods of ∼1 min. The latter type can be categorized as low‐altitude QP echoes that were found from previous nighttime MU radar observations. Until now the MU radar QP echoes have been believed to occur above 100 km for the period from sunset to midnight. Although we do not know the generation mechanisms of the low‐altitude MQP echoes, we suppose that these echoes might be caused by a weak <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> that exists below 100 km.</jats:p>