Solar zenith angle dependence of plasma density and temperature in the polar cap ionosphere and low-altitude magnetosphere during geomagnetically quiet periods at solar maximum

Abstract

[1] We constructed an empirical model of the electron density profile with solar zenith angle (SZA) dependence in the polar cap during geomagnetically quiet periods using 63 months of Akebono satellite observations at solar maximum. The electron density profile exhibits a transition at ∼2000 km altitude only under dark conditions. The electron density and scale height at low altitudes change drastically, by factors of 25 (at 2300 km altitude) and 2.0, respectively, as the SZA increases from 90°to 120°. The SZA dependence of the ion and electron temperatures is also investigated statistically on the basis of data obtained by the Intercosmos satellites and European Incoherent Scatter (EISCAT) Svalbard radar (ESR). A drastic change in the electron temperature occurs near the terminator, similarly to that in the electron density profile obtained by the Akebono satellite. The sum of the ion and electron temperatures obtained by the ESR (∼6500 K at ∼1050 km altitude under sunlit conditions and ∼3000 K at ∼750 km altitude under dark conditions) agrees well with the scale height at low altitudes obtained from the Akebono observations, assuming that the temperature is constant and that O + ions are dominant. Comparisons between the present statistical results (SZA dependence of the electron density and ion and electron temperatures) and modeling studies of the polar wind indicate that the plasma density profile (especially of the O + ion density) in the polar cap is strongly controlled by solar radiation onto the ionosphere by changing ion and electron temperatures in the ionosphere during geomagnetically quiet periods.

Heating of the sunlit polar cap ionosphere by reflected photoelectrons

“…As expected for the sunlit polar cap, O + is the dominant ion at all altitudes below 1 R E [cf. Kitamura et al, 2011;Glocer et al, 2012]. .…”

“…In other seasons, however, the field lines can cross the terminator, possibly multiple times per day depending on the convection pattern. The plasma parameters are expected to be very different on the dayside and nightside of the terminator [Kitamura et al, 2011;Glocer et al, 2012], and every time a field line crosses the terminator in either direction it will exhibit transient behavior as it switches from one equilibrium to another. Nonetheless, RISR-N data from other seasons typically show the electron temperatures cooling down substantially at night.…”

“…Few measurements of electron temperature profiles above 1000 km exist; these altitudes are challenging for incoherent scatter radars to reach given the low electron densities in the polar cap. Kitamura et al [2011] presented electron temperatures from the Interkosmos mission extending to 3000 km. Although these measurements were compiled from multiple satellite passes and thus cannot be regarded as instantaneous temperature profiles, they are nonetheless fairly isothermal.…”

Storm‐time electron density enhancement in the cleft ion fountain

“…In contrast, electron density measurements through plasma waves [e.g., Persoon et al, 1983;Gallagher et al, 1986;Kitamura et al, 2009Kitamura et al, , 2010aKitamura et al, , 2010bKitamura et al, , 2011 and spacecraft potentials [e.g., Laakso et al, 2002;Engwall et al, 2009] are more easily obtainable. Thus, electron densities have been studied intensively in the polar magnetosphere.…”

“…However, since the Akebono observations were limited to the dawn-dusk direction in cases studied by Kitamura et al [2010b], the velocity filter effect could not be detected. Detection of the velocity filter effect is important to distinguish the cleft ion fountain from the polar wind dominated by O + ions, since the polar wind dominated by O + ions was suggested by modeling studies [Tam et al, 1995[Tam et al, , 1998Wilson et al, 1997;Glocer et al, 2012] and statistical electron density observations (field-aligned density profile) under sunlit conditions at solar maximum [Kitamura et al, 2011]. If the ions are strongly related to the polar wind mechanism, the ions would not show the velocity filter effect, since the source region is widespread in the polar cap.…”