  The ground-state Hanle effect (GSHE) in alkali-metal atomic vapors using a
single circularly polarized wave underlies one of the most robust and simplest
techniques in atomic magnetometry. This effect causes a narrow
(subnatural-width) resonance in the light wave intensity transmitted through a
vapor cell. Usually, GSHE-based sensors operate in the
spin-exchange-relaxation-free (SERF) regime. However, this regime requires a
relatively high temperature of vapors (150 C or higher), leading to a
relatively large heat release and power consumption of the sensor head.
Besides, without applying special measures, SERF regime significantly limits a
dynamic range of measurements. Here, we study a pump-probe scheme involving a
single elliptically polarized wave and a polarimetric detection technique. The
wave is in resonance with two adjacent optical transitions in the cesium D1
line (894.6 nm) owing to their overlapping in presence of a buffer gas (Ne, 130
Torr). Using a small (0.1 cm$^3$) glass vapor cell, we demonstrate a
possibility of observing subnatural-width resonances with a high
contrast-to-width ratio (up to 45 %/mG) under a low-temperature (60 C) regime
of operation thanks to a strong light-induced circular dichroism. Basing on a
$\Lambda$ scheme of atomic energy levels, we obtain explicit analytical
expressions for the line shape. The model reveals a linewidth narrowing effect
due to openness of the scheme. This result is unusual for magneto-optical
atomic spectroscopy because the openness is commonly considered as a
undesirable effect, degrading the resonance characteristics. We estimate a
sensitivity of 1.8 pT/$\surd$Hz with a 60 fT/$\surd$Hz sensitivity in the
photon-shot-noise limit. The results contribute to the theory of GSHE
resonances and can be applied to development of a low-temperature
high-sensitivity miniaturized magnetic field sensor with an extended dynamic
range.
