The Ionospheric Photometer (IPM) is carried on the Feng
Yun 3-D (FY3D) meteorological satellite, which allows for the measurement of
far-ultraviolet (FUV) airglow radiation in the thermosphere. IPM is a
compact and high-sensitivity nadir-viewing FUV remote sensing instrument. It
monitors 135.6 nm emission in the nightside thermosphere and 135.6 nm and
N2 Lyman–Birge–Hopfield (LBH) emissions in the dayside thermosphere that can be used to
invert the peak electron density of the F2 layer (NmF2) at night
and the O/N2 ratio in the daytime, respectively. Preliminary observations
show that the IPM could monitor the global structure of the equatorial
ionization anomaly (EIA) structure around 02:00 LT using atomic oxygen (OI) 135.6 nm
nightglow. It could also identify the reduction of O/N2 in the
high-latitude region during the geomagnetic storm of 26 August 2018. The
IPM-derived NmF2 agrees well with that observed by four ionosonde stations
along 120∘ E with a standard deviation of 26.67 %. Initial
results demonstrate that the performance of IPM meets the design
requirements and therefore can be used to study the thermosphere and
ionosphere in the future.
Introduction
The Earth's far-ultraviolet (FUV) airglow radiation from the thermosphere
includes the emission of H, O, and N2 and the absorption of O2
(Meier, 1991). The atomic oxygen (OI) 135.6 nm nightglow emission, which is mainly produced
by the recombination of ionospheric O+ and electrons, represents the
spatial and temporal variations of the ionosphere in the nighttime. The
135.6 nm and N2 Lyman–Birge–Hopfield (LBH) dayglow emissions, which are produced by energetic
photon-electron impact excitation of the neutral atmosphere, are used to
derive the column O/N2 in the sunlit disk. The Earth's atmosphere is
opaque to the FUV radiation due to the lower atmosphere absorption. The
background emission of FUV airglow from the Earth's surface is absent. Thus,
FUV airglow radiation is particularly well suited to space-based remote
sensing (Paxton et al., 2003; Budzien et al., 2019). In past decades, FUV
spectrography has been used extensively in studying the thermosphere and
ionosphere from satellites, such as GUVI (the Global Ultra-Violet Imager) on
the NASA TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and
Dynamics) satellite (Christensen et al., 2003) and the Far Ultraviolet
Imager (FUV) on the NASA IMAGE (Imager for Magnetopause-to-Aurora Global
Exploration) satellite (Sagawa et al., 2005). The other useful instrument is
ionospheric photometer, which is compact and highly sensitive. The photometer
on the polar-orbiting Department of Defense satellite S3-4 was used to
measure the airglow, aurora, and solar scatter radiance of the Earth's
atmosphere (Huffman et al., 1980). The US Naval Research Laboratory gave
the concept for a new class of ionospheric photometer 20 years ago. It
was supplied in the Tiny Ionospheric Photometer (TIP) on the Constellation
Observing System for Meteorology, Ionosphere, and Climate satellites (Anthes
et al., 2008; Dymond et al., 2016), complemented and upgraded in TIP as part of the GPS Radio Occultation and
Ultraviolet Photometry – Colocated (GROUP-C) experience on the
International Space Station (Budzien et al., 2019, 2017),
and notably improved in the Triple Tiny Ionospheric Photometer (Tri-TIP) in the
Coordinated Ionospheric Reconstruction CubeSat Experience (Dymond et al.,
2017; Stephan et al., 2018).
The compact and high-sensitivity nadir-viewing FUV Ionospheric Photometer
(IPM) is one of 10 scientific payloads aboard the Feng
Yun 3-D (FY3D) meteorological
satellite. IPM monitors 135.6 nm emission in the nightside thermosphere and
135.6 nm and N2 LBH emissions in the dayside thermosphere by employing
a filter wheel that adds two red-leak signal channels for daytime and
nighttime red leaks, respectively. Red leaks refer to weak residual
sensitivity of the sensor to detect unwanted wavelengths including visible
light that is “redder” than ultraviolet (Budzien et al., 2019). The main
scientific objectives of IPM are follows: (1) measure 135.6 nm emission in
the nightside thermosphere to capture the large-scale structure of the low-
and mid-latitude ionosphere. (2) Measure 135.6 nm and N2 LBH
emissions in the dayside thermosphere to capture global variations
O/N2 ratio and evolutions of the thermosphere and ionosphere during
extreme space weather events. The FY3D is an afternoon Sun-synchronous
satellite with an orbit altitude of 830 km, an inclination of
98.75∘, and orbit period of ∼ 102 min, and
is designed for weather forecast, atmospheric chemistry, climate change
monitoring, and space weather monitoring. The FY3D satellite was launched at
18:35 UTC on 14 November 2017 from the Taiyuan Satellite Base, Shanxi
province, China. This paper presents instrumental descriptions and initial
observations by IPM.
According to the two main scientific objectives mentioned above, the IPM
instrument requirements are summarized in the Table 1. In the design of the
ionospheric photometer, there are two important problems to be solved. One
problem is red leak. It is a major challenge to ionospheric photometers that
visible light radiation from the Sun is about 109 times more than FUV
radiation. The other problem is that ionospheric photometers need to
eliminate 130.44 nm and shorter wavelengths airglow and collect 135.6 nm
airglow emissions with high sensitivity.
FY3D IPM instrument requirements.
ParametervalueWavelength135.6 nm (night mode)135.6 nm and 145–180 nm (day mode)Field of view∼ 3.5∘ (along orbit) × 1.6∘ (cross orbit)Sensitivityday mode: ≥ 1 counts/s/Rayleigh at 135.6 nmnight mode: ≥ 150 counts/s/Rayleigh at 135.6 nmSpatial resolution∼ 30 km at ionosphere (300 km)Time resolution2 s (day mode)10 s (night mode)Composition, channel, and mode
The IPM instrument is shown in Fig. 1 and includes a telescope, a filter
wheel, a detector system, and control electronics cabinet. The telescope has
a field of view of 3.5∘ (along orbit) × 1.6∘
(cross orbit). An off-axis aluminum mirror coating MgF2 is used to
collected airglow emission in the telescope. To suppress the longer
wavelength radiance, a sunblind photomultiplier tube (PMT)
(R10825, Hamamatsu) with cesium iodide (CsI)
photocathode is used in the detector system (Fu et al., 2015). The quantum
efficiency of the PMT with an effective area of 4 × 9.5 mm and is
about 26 % at a wavelength of 135.6 nm, 6.17×10-5 at 254 nm,
and 4.06×10-8 at 514 nm. The PMT has better than
10-4 rejection at wavelengths longer than 200 nm.
IPM instrument.
IPM monitors 135.6 nm emissions in the nighttime and 135.6 nm and N2
LBH emissions in the daytime by employing a filter wheel. There are six
spots in the filter wheel (Fig. 1c) corresponding to six channels of IPM:
dark count channel, 135.6 nm nightside channel, red-leak nightside channel,
red-leak dayside channel, N2 LBH dayside channel, and 135.6 nm
dayside channel. The channel information of IPM is shown in Table 2. In
order to suppress the longer wavelength radiance further, the bandpass
filter centered on 135.6 nm is used in the 135.6 nm dayside channel, and the
bandpass filter centered on 160 nm is used in the N2 LBH channel.
Besides, IPM specifically adds two red-leak signal channels for daytime and
nighttime red leaks, respectively. Based on the design of dayside or nightside
channel, a SiO2 filter is added in the red-leak channels in order to
eliminate longer than 180 nm wavelengths. By differencing the measurements of dayglow
channels and red-leak dayside channel, dayglow radiations can be detected.
And by differencing the measurements of 135.6 nm nightside channel and
red-leak nightside channel, 135.6 nm radiation in the nighttime can be
detected. To exclude radiation shorter than 135.6 nm completely, a 0.5 mm
thin vacuum ultraviolet (VUV)-grade barium fluoride (BaF2) flat filter is used, and the transmittance at 135.6 nm
at room temperature is 0.5 (Fu et al., 2015). The emission of wavelengths
shorter than 132 nm cannot pass the 0.5 mm thick BaF2 filter over a
temperature range of 5 to 35 ∘C.
IPM has two observation modes: day mode and night mode. The day mode
includes four observations of the 135.6 nm dayside channel, four observations of
the N2 LBH channel, two observations of the red-leak dayside channel,
and one dark count observation in each frame. The night mode includes eight
observations of the 135.6 nm night channel, one observation of the red-leak
nightside channel, and one dark count observation.
Laboratory calibration
The IPM was calibrated at ground laboratory prior to flight. The optical
calibration facility at ground has a deuterium lamp, a monochromator, a
collimator, a diffuser, a National Institute of Standards and Technology (NIST) standard detector, and a vacuum chamber
assembled in a modular pattern (Fig. 2). The deuterium lamp (L11798) with a
MgF2 window has 150 W power and provides a bright, stable source of FUV
radiation. The source of FUV radiation is wavelength selected by the
monochromator (234/302) which has a f/4.5 0.2 m Czerny–Turner with a
1200 grooves mm-1 grating. A collimator ensures that the beam consists of parallel
rays. The NIST standard detector (AXUV-100G) traced from NIST provides a
reference for calibrating IPM.
The optical calibration facility at ground.
The processes of calibration are as follows: first, the FUV light at 125–200 nm from
the deuterium lamp is selected by the monochromator. Second, the wavelength
selected reaches the NIST standard detector through the collimator, and the
NIST standard detector obtains the irradiance of the wavelength selected.
And then, by using a rotating platform, the wavelength selected reaches the
diffuser board through the collimator and enters IPM. IPM obtains the signal
for the wavelength selected. Finally, the count and irradiance of the
wavelength selected are used in calculating the responsivity to the
wavelength selected. The uncertainty of the ground calibration comes from
the stability of the FUV light source, the error of the standard detector,
the bi-directional reflection distribution function (BRDF) uncertainty of
the diffuser board, the non-uniformity of the light source, and so on. The
uncertainty of the ground calibration is estimated to reach 11.25 %. As a
function of wavelength, the responsivity of the 135.6 nm nightside channel
from 130 to 200 nm is shown in Fig. 3. The responsivity to 135.6 nm
radiation at night is about 266.9 counts/s/R near the peak of the
responsivity function distribution and reaches the design requirement of
the 135.6 nm nightside channel. The responsivity to 135.6 nm radiation at
night provides high sensitivity in observations of OI 135.6 nm radiation at
night.
The IPM responsivity of the 135.6 nm nightside channel in
counts/s/R.
The IPM responsivity of the 135.6 nm dayside channel in
counts/s/R.
As a function of wavelength, the responsivity of the 135.6 nm dayside
channel from 130 to 200 nm is shown in Fig. 4. The responsivity to the
135.6 nm radiation in daytime is about 23.2 counts/s/R and also reaches the
design requirement of the 135.6 nm dayside channel. The responsivity is much
less than the one on the nightside due to the bandpass used in the 135.6 nm
dayside channel, which is designed to obtain the radiation of 135.6 nm in
daytime and suppress the radiation at wavelengths shorter than 135.6 nm,
N2 LBH, and red-leak contributions in daytime. The other bandpass is
used in the N2 LBH day channel in order to obtain the radiation of
N2 LBH and suppress the radiation of 135.6 nm and red-leak
contributions in daytime. The responsivity of N2 LBH channel is shown
in Fig. 5.
The IPM responsivity of the N2 LBH channel in counts/s/R.
Observation resultsOI 135.6 nm emission on the nightside
After the FY3D satellite was launched at 18:35 UTC on 14 November 2017, IPM
started operation at 10:20 UTC on 25 November 2017. In IPM data processing,
dark count is used to confirm the working status of IPM. Generally, the dark
count of IPM is less than 10 counts per second. When the FY3D satellite
passes by the South Atlantic Anomaly (SAA), the dark count of IPM increases
rapidly and reaches about 2000 counts per second due to the energetic
particles in the SAA.
The count of the 135.6 nm nightside channel with red leak
(a, d), without red leak (c, f), and the count of the red-leak nightside
channel (b, e). 17 March 2018 is the new moon day, and 31 March 2018 is full
moon day.
The count of the 135.6 nm nightside channel is presented in Fig. 6. The
count with red leak on 17 March 2018 (new moon) and on 31 March 2018 (full
moon) are shown in panels (a) and (d), respectively. The counts without red leak on
17 and 3 March 2018 are shown in panels (c) and (f), respectively. The
count of the 135.6 nm nightside channel in panel (d) is several times the count of
the 135.6 nm nightside channel in panel (a) due to moonlight reflecting into the
135.6 nm nightside channel from cloud tops, while the count levels in panels (c)
and (f) are very similar. We found that the red-leak nightside channel is
effective to eliminate the contamination of moonlight on the 135.6 nm
nightside channel.
An example of the global count of the 135.6 nm nightside channel is
presented in Fig. 7a. The solid red line indicates the magnetic dip
Equator. The data in Fig. 7 are from 7 to 11 December 2017. From 7 to 11
December 2017, Kp index was not more than 4, and the geomagnetic conditions
were relatively quiet. As shown in Fig. 7a, there is a high-count area
near the magnetic dip Equator in South America, which shows the
contamination in SAA associated with particles impacting the instrument. An
example of global brightness of the 135.6 nm nightside channel without
red leak and the effect of dark count is presented in Fig. 7b. As shown in
Fig. 7b, there are some brighter areas located on either side of the
magnetic dip Equator in South America and Africa, which are the so-called
equatorial ionization anomaly (EIA) structure. The EIA has
been studied extensively by using data from ground-based ionosondes (Moffett
and Hanson, 1965; Walker, 1981) and ground-based optical observations
(Thuillier et al., 1976). The OI 135.6 nm emission data from GUVI aboard the
TIMED satellite, FUV aboard the IMAGE satellite, and the TIP aboard the
COSMIC satellites have also been used in study of the EIA phenomenon
(Christensen et al., 2003; Sagawa et al., 2005; Immel et al., 2006 and Coker
et al., 2009). The local time of the IPM orbit on the nightside is 02:00 LT.
The EIA structure which we found at 02:00 LT is later than other
results mentioned earlier, and it needs to be studied further.
The global count (a) and brightness (b) of the
135.6 nm nightside channel from 7 to 11 December 2017. The brightness is
without red leak and the effect of dark count. The solid red line indicates
the magnetic dip Equator.
NmF2 and total electron content
OI 135.6 nm emission is one of the strongest lines in the FUV nightglow at
low latitudes and has relatively high transparency in the upper atmosphere.
In the nightside ionosphere, there are two primary production mechanisms of
OI 135.6 nm emission: (1) atomic oxygen is excited through the recombination
of atomic oxygen ions with electrons and produces OI 135.6 nm emission; (2)
atomic oxygen is excited through the mutual neutralization of O+ with
O- and produces OI 135.6 nm emission (Meier, 1991). The mutual
neutralization has a relatively smaller contribution. The brightness of OI
135.6 nm emission varies with the electron density and the oxygen ion
concentration basically. Equivalently, OI 135.6 nm emission is approximately
proportional to the square of the electron density in the F region.
The algorithm of deriving NmF2 from the nighttime OI 135.6 nm emission
is provided by Rajesh et al. (2011) and Jiang et al. (2014, 2018). The
nighttime OI 135.6 nm emission is calculated based on a nighttime OI 135.6 nm
airglow radiative and emissive model. The electron density profile, the
O+ density profile, and the electron temperature profile are calculated
using the IRI2000 model, and the neutral components are calculated using the
MSISE90 model. The OI 135.6 nm emission is fitted to the square of NmF2
linearly. The ratio of the square of NmF2 to the OI 135.6 nm emission
is obtained. Finally, NmF2 is retrieved based on the observed OI 135.6
nm emission and the ratio. We selected the IPM-derived NmF2 data which
were near four Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS)
ionosonde stations (Sanya (18.3∘ N, 109.6∘ E), Wuhan
(30.5∘ N, 114.4∘ E), Beijing (40.3∘ N, 116.2∘ E),
and Mohe (50.2∘ N, 122.5∘ E)) from 25 November 2017 to 8 May 2018 (shown in
Fig. 8). Their difference in longitude was less than 12∘ and
in latitude was less than 5∘. There is a standard deviation
of 26.67 % between IPM NmF2 and IGGCAS ionosonde NmF2 (shown in
Fig. 9).
IPM-derived NmF2 and IGGCAS
ionosonde NmF2 from 25 November 2017 to 8 May
2018. (The longitude difference between the IPM substellar point and
ionosonde stations is less than 12∘, and the latitude
difference is less than 5∘.)
The relative difference distribution between IPM
NmF2 and IGGCAS ionosonde
NmF2.
The algorithm of deriving total electron content (TEC) from the nighttime OI 135.6 nm emission is
provided by Rajesh et al. (2011) and Jiang et al. (2014). The process of
deriving TEC based on the ratio between TEC and the nighttime OI 135.6 nm
emission intensity is similar to that of deriving NmF2. We further
calculated TEC from IPM results and compared with
that of MIT TEC data from 25 November 2017 to 8 April 2018. The MIT TEC
data (Rideout and Coster, 2006) was obtained from the MIT Haystack
Observatory Madrigal database (http://www.openmadrigal.org, last access: 29 June 2021). There is a
standard deviation of 39.41 % between IPM TEC (total electron content
unit, TECu) and MIT TEC (TECu) (shown in Fig. 10). The standard deviation
between IPM TEC (TECu) and MIT TEC (TECu) is more than the one between IPM
NmF2 and IGGCAS ionosonde NmF2. MIT TEC is integrated from ground
to 20 200 km. It includes plasmasphere contribution and ionosphere
contribution. IPM TEC is integrated from ground to 830 km; it only includes
ionosphere contribution. There is diurnal interchange between the ionosphere
and the plasmasphere, the downward diffusion from the plasmasphere helps to
maintain the nighttime F2 layer. The results of Jason-1, MetOp-A, and
TerraSAR-X (Yizengawa et al., 2008; Zakharenkova and Cherniak, 2015;
Klimenko et al., 2015) show that the plasmasphere contribution at
night cannot be neglected.
IPM TEC and MIT TEC (TECu) from 25 November 2017 to
8 April 2018.
O/N2
Energetic photon–electron impact excitation of the neutral atmosphere
produces 135.6 nm emission and N2 LBH emission, which are
proportional to the concentration of O and N2, respectively (Meier,
1991). The 135.6 nm emission and N2 LBH emission can be used to derive
column O/N2. The derivation of O/N2 from disk 135.6 nm
and N2 LBH dayglow observations was first addressed by Strickland et al. (1995),
and the topic of O/N2 from 135.6 nm emission
and N2 LBH emission has been studied extensively (Christensen et al.,
2003; Strickland et al., 2004; Zhang et al., 2014). During geomagnetic
storms, enhanced joule and particle heating in the high-latitude ionosphere
produces upwelling of the oxygen-depleted or nitrogen-rich air. The
upwelling rises from much lower in the thermosphere into the F region. The
heating also leads to enhanced horizontal equatorward neutral winds that
can change the distribution of the nitrogen-rich/oxygen-depleted air.
Column O/N2 from
IPM around the magnetic storm of 26 August 2018.
Column O/N2 from GUVI around the
magnetic storm of 26 August 2018.
Giving an N2 depth of 1017 cm-2, column O and N2 ratio
is derived from the value at a given solar zenith angle (SZA) by
two-dimensional interpolation. The retrieval algorithm was described by
Strickland et al. (1995) and Zhang et al. (2014). The brightness of the
135.6 nm emission and the N2 LBH emission on the dayside was derived
from observations of the 135.6 nm dayside channel and the N2 LBH
dayside channel, respectively. In order to further deduct the red leak from
the cloud tops, we used a Butterworth filter in the data processing. The
input parameters of the filter were optimized according to the red leak from
the cloud tops. The improved AURIC model (Wang and Wang, 2016) was used to
produce a simulation. The simulation provided the coefficient for deriving
O/N2 from a measured pair of 135.6 nm and LBH. The column O/N2
ratio during the magnetic storm of 26 August 2018 is presented in Fig. 11. On
24 August and most of 25 August 2018, Kp index was not more than 3. It
abruptly rose to 7 in 26 August 2018. From 29 to 31 August 2018, Kp index
was not more than 3. The column O/N2 on 24 and 25 August was relatively
quiet, and significant changes in column O/N2 occurred on 26 and 27
August. The reduction of O/N2 extended from the high-latitude region to
mid- and low-latitude regions in the Northern Hemisphere and Southern Hemisphere. On 30
and 31 August, column O/N2 returned to a quiet state.
The column O and N2 ratio derived from GUVI during the magnetic storm
of 26 August 2018 is presented in Fig. 12. The GUVI column O/N2 data
(Strickland et al., 2004) was obtained from the GUVI website
(http://guvitimed.jhuapl.edu/data_fetch_l3_on2_idlsave, last access: 30 December 2021). The column O/N2 from
GUVI on 24 and 25 August was relatively quiet, and significant
changes in column O/N2 occurred on 26 and 27 August. The reduction of
O/N2 also extended from the high-latitude region to mid- and low-latitude
regions in the Northern and Southern Hemisphere. On 30 and 31
August, the column O/N2 of GUVI also returned to a quiet state. The features of
column O/N2 of IPM and GUVI during the magnetic storm of 26 August 2018
were similar. These results showed that the IPM data could provide a good
monitoring of O/N2 changes during the magnetic storm.
Conclusions
The FY3D meteorological satellite was launched at 18:35 UTC on
14 November 2017 from the Taiyuan Satellite Base, Shanxi province, China.
The Ionospheric Photometer instrument carried aboard the FY3D meteorological
satellite measures the spectral radiance of the Earth far-ultraviolet
airglow in the spectral region from 133 to 180 nm. IPM is a tiny, highly
sensitive, and robust remote sensing instrument. Preliminary observations
show that the IPM could monitor the global structure of the equatorial
ionization anomaly structure around 02:00 LT using OI 135.6 nm
nightglow properly. It could also identify the reduction of O/N2 in the
high-latitude region during the geomagnetic storm of 26 August 2018. The
IPM-derived NmF2 agrees well with that observed by four ionosonde stations
along 120∘ E with a standard deviation of 26.67 %. Initial
results demonstrate that the performance of IPM meets the design
requirements and therefore can be used to study the thermosphere and
ionosphere in the future.
Data availability
Data are available at http://satellite.nsmc.org.cn/PortalSite/Default.aspx (FENGYUN Satellite Data Center, 2018).
Data can be searched and downloaded from the website after registration.
Author contributions
YW and TM performed the data validation and prepared the
paper and most of the plots; LF and FJ designed the IPM and
provided laboratory calibration data; XH, CL, XZ, JL, LS, ZY, PZ, and JW
participated in instrument parameter requirements, judging of instrument
design, and data validation; ZR, FH, and LS
participated in validation and intercomparisons.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Financial support
This research has been supported by the Natural Science Foundation of China (grant nos. 41874187, 41774195, and 41931073) and the Fengyun Satellite Ground
Application System.
Review statement
This paper was edited by Jörg Gumbel and reviewed by two anonymous referees.
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