Introduction
Polar mesospheric clouds (PMCs) or noctilucent clouds (NLCs) consist of
water-ice particles, which can be produced in summer at the mesopause region,
mainly at high latitudes. The first report on PMCs was made by
. Since then, various methods have been used
to perform PMC observations. Optical observations by ground-based cameras,
imagers, or lidars are often limited by weather conditions because a clear
sky is required for such observations. Hence, satellite observations from
space are valuable for more continuous observations, which enable significant
systematic data coverage. Such systematic data coverage would be of benefit,
for example, for monitoring long-term PMC activity, which may be related to
global change cf. because
water-ice particle production can be enhanced by CO2 cooling and
H2O increase, which may be induced by CO2 and CH4 increases
cf..
A comprehensive review of PMC observations from satellites can be found in
. In addition, the Aeronomy of Ice in the
Mesosphere (AIM) satellite has been in operation, making PMC observations
since 2007 . These observations include
both limb- and nadir-viewing from low-Earth-orbit (LEO) satellites. By
contrast, there are only few reports of PMC observations by limb-viewing
Geostationary Earth Orbit (GEO) satellites
.
The first PMC observations from a GEO satellite were reported using images by
Meteosat First Generation (MFG)
, and their
PMC images had ∼2.5 km spatial resolution in a single visible band.
Subsequently, extended such observations to
Meteosat Second Generation (MSG), and he reported PMC observations using
∼1 km spatial resolution images in a single visible band. This kind of
GEO satellite can produce full-disk images including the Earth's limb, which
would provide valuable opportunities for PMC observations by continuous
limb-viewing from its almost fixed location relative to the Earth.
(a) An original true-color image (composite of the three visible
bands) around northern high latitudes at 21:00 UT on 9 July 2016. (b) Same as
(a), but the color scale is 50× enhanced. (c) Same
as (b), but latitudes and heights of the tangential points
are overlaid.
In the present paper, we make an initial report on PMC observations from
Himawari-8, the Japanese GEO meteorological satellite. Our PMC images from
Himawari-8 have ∼1 km spatial resolution in three visible bands.
Japanese GEO meteorological satellites have a long history from 1977
(Himawari-1) to the present (Himawari-8). However, there was no PMC report
from Japanese GEO satellite observations before this work. Therefore, in the
present paper we examine basic features in PMC emissions observed by
Himawari-8 and compare these with typical PMC characteristics, as a first
step for our PMC research using Himawari-8 data.
Data
Himawari-8 is the Japanese GEO meteorological satellite
, which was successfully launched in October 2014.
It has 16 observation bands, including three visible bands: blue (0.47 µm),
green (0.51 µm), and red (0.64 µm). In the
initial survey for PMCs, we used full-disk images in Portable Network
Graphics (PNG) format, generated from the level-1a data, Himawari Standard
Data (HSD). The PNG full-disk image is a true-color image, i.e., a composite
of the three visible bands. Each color has an 8 bit resolution (i.e., values
ranging from 0 to 255), describing emission intensities from 0 to 641.5092 Wm-2sr-1µm-1
for the blue band, from 0 to 601.9766 Wm-2sr-1µm-1 for the green band, and from 0 to
519.3457 Wm-2sr-1µm-1 for the red band. The color
value has a linear relation with the emission intensity for each band. The
PNG full-disk image has a spatial resolution of ∼1 km and is obtained
every 10 min. The geometric accuracy of the images is typically less than 0.6 km,
i.e., less than the ∼1 km spatial resolution. More detailed
information for the PNG images can be found in .
For the present survey, we collected a year of PNG images for 2016, and
focused our attention on the Earth's limb region, namely the middle and upper
atmospheric regions.
(a) Height–latitude distribution of emission intensity in the blue
band at 21:00 UT on 9 July 2016. (b) Same as (a), but in
the green band. (c) Same as (a), but in the red band.
Results and discussion
Figure shows an example of PMC emissions observed at 21:00 UT
on 9 July 2016 in the northern high latitudes. It is difficult to see any
emissions in the limb region in the original true-color image (see Fig. a),
but the 50× enhanced image makes it obvious that
clear emissions exist in the limb region (see Fig. b). The
appearance of these emissions is similar to that of PMC emissions in previous
reports
.
Here, we calculated tangential points in each line-of-sight (LOS) direction
(i.e., for each pixel in the image). The image is described by the normalized
geostationary projection, so the pixel corresponds to the LOS angle. From
each LOS angle, we derived each vector along each LOS direction. Then, we
considered intersections between each vector and the Earth-like ellipsoid of
eccentricity defined by the World Geodetic System 1984 (WGS84). The equation for
the intersections is expressed in quadratic form. Hence, if there is
only a single solution of the equation, there is only a single intersection
that corresponds to the tangential point. Thus, we solved the equation by
changing the radius of the ellipsoid to produce only a single solution. Thus,
we obtained information for the tangential points for each pixel in the
image. The heights and latitudes of tangential points are overlaid in Fig. c.
As shown in Fig. c, the typical height of the
emissions was about 80 km.
For further details, Fig. shows height–latitude distributions
of emission intensities in the three visible bands. The emissions were mainly
located at 80–82 km height at latitudes of 78–81∘ N. This emission
height is consistent with typical reported PMC heights of 82–83 km
cf.. It should be noted that the tangential
height may be an underestimation of the actual emission height because
emissions may be coming from not only the tangential point but also
foreground or background points. Our observations also show oblique,
step-like structures with a spatial scale of ∼1 km. These artifactual
structures are because of the height resolution and the mapping calculation
of the tangential point. Before application of the mapping calculation,
images show step-like structures that are not oblique but horizontal, with a
spatial scale of ∼1 km because of ∼1 km height or spatial
resolution. These artifactual structures are because of a limitation of the
height resolution. Such horizontal, step-like structures then become oblique,
step-like structures through the mapping calculation from the pixel
coordinate in the original images to the height–latitude coordinate in the
tangential points.
(a) Arctic map showing footprints (red line) of the tangential
points at tangential heights of 80–85 km. (b) Same as (a),
but for an Antarctic map. It should be noted that the center longitude is the
sub-satellite longitude of Himawari-8: 140.7∘ E.
We confirm that the PMC-like emission layer has a wavelike structure in the
height–latitude cross section. For example, the heights of the layer were
80–81 km at ∼81∘ N, 81–82 km at ∼80∘ N, ∼81 km at ∼79∘ N, and 81–82 km at ∼78∘ N. We attribute
these fluctuations to atmospheric waves. Because the latitude range of
79–81∘ N corresponds to about a distance of ∼700 km over the
Arctic Ocean (see Fig. ), the wavelength of the wavelike
structure can be estimated to be ∼700 km. Such PMC structures can be
observed by several methods using observations from LEO satellites such as
the Cloud Imaging and Particle Size experiment (CIPS) onboard AIM and the
Optical Spectrograph and InfraRed Imager System (OSIRIS) onboard Odin. For
example, AIM/CIPS can provide PMC nadir imaging, which is a powerful tool to
observe horizontal information for PMC structures
e.g.,. In addition,
tomographic techniques using AIM/CIPS and
Odin/OSIRIS can provide horizontal and
vertical information for PMC structures. By contrast, Himawari-8 observation features high cadence and wide limb-viewing PMC observation from its
almost fixed location relative to the Earth. The Himawari-8 full-disk image
can be obtained every 10 min, and its field-of-view (FOV) coverage can be several thousands
of kilometers (see Fig. ). These features will provide
valuable data, which can be complementary to PMC data from the LEO
satellites.
(a) Yearly variation in the total emission intensities for a region
at heights of 70–90 km and latitudes of 60–90∘ N in 2016. Blue,
green, and red lines correspond to the blue, green, and red visible bands,
respectively. It should be noted that the upper limit of the latitude range
is actually ∼81∘, that is, the highest latitude of the
tangential points when the height range is set to 70–90 km. Vertical black
lines indicate the northern summer solstice in 2016 (i.e., 20 June 2016), 20 days
before the solstice (i.e., 31 May 2016), and 60 days after the solstice
(i.e., 19 August 2016), in reference to the typical PMC period. (b) Same as
(a), but for a region at heights of 70–90 km and latitudes
of 60–90∘ S. Vertical black lines indicate the southern summer
solstice in 2016 (i.e., 21 December 2016), 20 days before the solstice (i.e.,
1 December 2016), and 60 days after the 2015 southern summer solstice (i.e.,
20 February 2016), in reference to the typical PMC period.
Emission intensity was the strongest in the blue band, and the weakest in the
red band. This result can be explained by Rayleigh or Mie scattering of
sunlight by water-ice particles (i.e., PMCs). According to the Mie theory
cf., Rayleigh scattering is Mie scattering in the
limit where particle size is much smaller than the wavelength. In the
Rayleigh scattering region, the scattering cross section at a fixed
wavelength decreases with decreasing particle size, and the scattering cross
section at a fixed particle size decreases with increasing wavelength. Most
PMC particles are understood to have radii of 20–60 nm, along with a very
small number of ∼200 nm particles cf..
Such particle size is smaller than the visible observation wavelengths: blue
(0.47 µm), green (0.51 µm), and red (0.64 µm).
Hence, the scattering would be close to Rayleigh scattering, though it may
not be pure Rayleigh scattering. Therefore, the predominance of such
small particles (20–60 nm) would account for the higher scattering
intensities or stronger emission intensities at shorter wavelengths. Although
additional quantitative evaluation would be helpful, the inferential
framework described above would imply that the three visible images obtained
by Himawari-8 may contain information on the size of PMC particles. In
addition, Himawari-8 observation can cover an entire day of local time (i.e.,
00:00–24:00 JST), which means that emission data at almost all scattering
angles (i.e., 0–360∘) are available. Such information may be useful
for particle size investigation, considering the wavelength dependence and
the angle distribution in the Rayleigh to Mie scattering region. However,
this concern is beyond the scope of the present paper.
To investigate seasonal variation of PMC emissions in 2016, we calculated
total pixel values in specific regions (i.e., sum of pixel values in
localized pixels). We set two regions; one is a region at heights of 70–90 km
and latitudes of 60–90∘ N for the northern polar region (see
Fig. a), whereas the other is a region at heights of
70–90 km
and latitudes of 60–90∘ S for the southern polar region (see Fig. b).
The height range of 70–90 km covers typical PMC occurrence
heights, and the latitude range of 60–90∘ covers typical PMC
occurrence latitudes. It should be noted that the upper limit of the latitude
range is actually ∼81∘ (see Fig. ), that is, the
highest latitude of the tangential points when the height range is set to
70–90 km. Seasonal variation in the total pixel values is shown in
Fig. . To remove within-day variation, data in Fig.
include only data at a single local time each day, 06:00 JST (21:00 UT). At
that local time, 06:00 JST, the LOS of Himawari-8 is almost perpendicular to
the sunward direction. This configuration would be beneficial, providing
solar illumination to some extent while minimizing sun-induced noise, such as
the stray light of direct sunlight, which can be a problem close to local
midnight (00:00 JST) cf.. As shown in Fig. a and b, PMC-like emissions, expressed as total
pixel values, were active only during local summer months. The observed
active periods would be similar to the typical PMC active period, from
∼20 days before summer solstice to ∼60 days after summer solstice
cf..
As discussed in the above text, the heights and seasonal variations of the
PMC emissions are consistent with the general characteristics of PMCs. These
results suggest that the peculiar emissions observed by Himawari-8 are indeed
PMCs. We further suggest that the availability of imaging in three visible
bands constitutes a particular advantage of Himawari-8 for PMC study. This
capability may provide valuable opportunities, for example, for obtaining
information on the size of PMC particles. In addition, collaborations between
Himawari-8 and LEO satellites such as AIM would allow a synergy of
complementary capabilities. In particular, high time resolution data (imaging
every 10 min) from Himawari-8, when combined with data from LEO satellites,
can contribute to PMC research in the near future, e.g., diurnal PMC
variation.