Monthly averages of diurnal temperature variation from meteor radar at Cachoeira Paulista (22.7°S, 45°W), Brazil

Highlights

• Meteor decay time has been used to study the diurnal fluctuations in monthly temperatures.

• Diurnal amplitudes and phases in temperatures at 22.7ºS are comparable to those from SABER.

• Semidiurnal amplitudes of the radar temperatures at 22.7ºS are comparable to those from SABER.

Abstract

Monthly averages of the superposed values of decay times of meteor echoes from underdense meteor trails obtained by meteor radar observations at Cachoeira Paulista (22.7°S, 45°W), Brazil, have been employed to estimate temperature and to study the diurnal temperature fluctuations in the upper mesosphere and lower thermosphere region. The monthly diurnal amplitudes and phases in meteor radar temperatures at Cachoeira Paulista are comparable to those of the westward migrating diurnal tide, with zonal wave number 1, found in the reported temperatures obtained from SABER data, when the observations were obtained during the same period. The diurnal amplitudes in the temperature do not show the semiannual oscillation present in those of the diurnal tide in zonal and meridional winds. The diurnal temperature peaks 4 to 5 h ahead of diurnal tide in the zonal winds and is in anti-phase with the diurnal tide in the meridional winds. The semidiurnal amplitudes of the radar temperature fluctuations also are comparable with SABER westward migrating semidiurnal tide, with zonal wave number 2, and do not present a defined seasonal behavior, while their phases undergo small changes. They lead the semidiurnal tide in the zonal winds by about 4 h during austral summer, but it remains in-phase for the other months, and lead in meridional winds by about 5 to 8 h.

Introduction

Meteoroids are space fragments originated from asteroids andcomets. When are passing the upper Earth’s atmosphere meteoroids interact with the air molecules, loses mass due to ablation process and produce spatially well-defined ionization trails, which contains enough concentration of free electrons to be able to reflect radio waves in the high frequency (HF), very high frequency (VHF) and ultra high frequency (UHF) bands.

Meteor radars make use of the reflection of radio waves at meteor trails and provide observations to deduce the atmospheric structure and dynamics at altitudes from 80 to 100 km, which surrounds the mesopause and is called upper mesosphere and lower thermosphere (MLT) region. The Doppler shifts of meteor trails are used to deduce MLT winds (Elford, 1959, Batista et al., 2004, Nakamura et al., 1991), whilst the mesopause region temperature has been estimated by measuring the ambipolar diffusion from the meteor trail echo fading times (Hocking et al., 1997, Hocking, 1999, Holdsworth et al., 2006, Lima et al., 2018).

Understanding the structure and dynamics of the MLT region is of great interest since besides registering the coldest temperatures, the lowest ionized part of the atmosphere is formed there, which disappears overnight, and where temperature and dynamics are mainly influenced by atmospheric waves and solar variability. It is well known that MLT winds and temperatures show considerable variability and these variations are used to study atmospheric waves and tides to better understand both horizontal and vertical dynamic coupling processes.

In general, the mesopause temperatures from meteor radar measurements have been estimated to validate with those obtained by other techniques (Meek et al., 2013, Yi et al., 2016, Liu et al., 2017). However, some studies have focused on mesospause temperature fluctuations from meteor radar observations. For example, Tsutsumi et al., 1994, Tsutsumi et al., 1996 have investigated short period and 2-day fluctuations in mesopause temperature using their relationship with ambipolar diffusion variations, whilst (Stober et al., 2012) pointed out the possibility of such measurements exhibit signatures of planetary wave oscillations. Other studies have explored seasonal behavior at northern high latitudes (Hall et al., 2006) as well as at southern low latitude (Lima et al., 2018), and the possible influence of the 11-year solar cycle on mesopause temperature (Hall et al., 2012, Holmen et al., 2016).

A procedure for extracting tidal parameters at temperature by meteor radar, obtained by the gradient method, has been described by Hocking and Hocking (2002), which requires the vertical wavelength of the tidal modes, extracted from the wind data, to solve the amplitude and phase of the mesopause temperature tides. Monthly diurnal temperature variations derived from meteor radar ambipolar diffusion have also been used to improve the calibration of mesopause temperature (Dyrland et al., 2010, Holmen et al., 2016). The temperature tides determined with meteor radar measurements has been briefly analyzed by Meek et al. (2013) and has been argued that if a fixed daily pressure is adopted, the temperature tide parameters may have noticeable distortions.

Despite the numerous studies on improving the estimation of the mesopause temperature from meteor radar measurements, diurnal variations in these temperatures have been little explored, except in the lower latitudes in the southern hemisphere. Therefore, in this study we estimate the temperatures at 90 km in height from the decay time observations of the VHF all-sky meteor radar at Cachoeira Paulista, Brazil, in order to investigate the diurnal fluctuations in monthly temperature averages.