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Proc. R. Soc. A (2010) 466, 3409–3419
doi:10.1098/rspa.2010.0077
Published online 26 May 2010
Short-term periodic features observed
in the infrared cooling of the thermosphere
and in solar and geomagnetic indexes from
2002 to 2009
BY MARTIN G. MLYNCZAK1, *, LINDA A. HUNT2 , JANET U. KOZYRA3
AND JAMES M. R USSELL III4
1 Science
Directorate, Climate Science Branch, NASA Langley Research
Center, Hampton, VA 23681-2199, USA
2 Science Systems and Applications Inc., 1 Enterprise Parkway,
Hampton, VA 23666, USA
3 University of Michigan, Department of Atmospheric, Oceanic and Space
Sciences, 2455 Hayward Drive, Ann Arbor, MI 48109, USA
4 Center for Atmospheric Sciences, Hampton University,
Hampton, VA 23668, USA
We report derivations of short-term periodic features observed in time series of the
radiative cooling of the Earth’s thermosphere. In particular, we diagnose observations
of the infrared emission from nitric oxide (NO) at 5.3 mm to reveal periodicities equal to
the solar rotation period (27 days) and its next three harmonics. From 2002 to 2009 we
observe 27 day, 13.5 day, 9 day and (occasionally) 6.75 day periods in the thermospheric
NO cooling, the solar wind speed and the Kp geomagnetic index. Periodic features shorter
than 27 days are absent in the time series of the 10.7 cm radio flux (F10.7) over this same
time period. The periodic features in the NO cooling are found to occur throughout the
depth of the thermosphere and are strongest at high latitudes. These results confirm the
persistent coupling between the solar corona, the solar wind and the energy budget of
the thermosphere.
Keywords: thermosphere; energy balance; solar–terrestrial coupling; infrared cooling
1. Introduction
In the last few years, short-term periodic features of 9 days have been observed
in a number of parameters in the thermosphere, including density (Lei et al.
2008), composition (Crowley et al. 2008) and energy budget (Mlynczak et al.
2008). The physical cause of these periodic features has been attributed to
the recurrence of high-speed solar wind streams emanating from coronal holes
on the Sun. The 9 day periodicity is the third harmonic of the solar rotation
*Author for correspondence ([email protected]).
One contribution of 8 to a Special feature ‘Geospace effects of high-speed solar wind streams’.
Received 9 February 2010
Accepted 23 April 2010
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M. G. Mlynczak et al.
period of 27 days. In this paper, we further examine the occurrence of shortterm periodicities (27 days and shorter) in the infrared radiative cooling of
the thermosphere over a timespan of 8 years. The radiative cooling data are
derived from observations made by the sounding of the atmosphere using
the broadband emission radiometry (SABER) instrument launched on the
NASA thermosphere–ionosphere–mesosphere energetics and dynamics (TIMED)
satellite in December 2001.
We examine the entire SABER dataset from January 2002 to November 2009
to determine the occurrence of the 27 day periodicity and its higher harmonics
in the radiative cooling rates owing to emission from nitric oxide (NO) at 5.3 mm.
Our prior studies have focused on periodic features in the globally averaged
power (watts) radiated by NO. In this present study, we not only examine the
global power on an annual basis but we also search for periodic features as a
function of altitude and latitude. As will be shown later, we find that, while
the periodic features exist throughout the thermosphere, there exists a definite
latitude dependence in the periodic features. In addition, periodic features as
short as 6.75 days (27/4) are shown to occur as well.
In the next section, we briefly review the processes by which the cooling
owing to NO emission is derived. We follow this with an examination of periodic
features in the solar wind speed, the 10.7 cm radio flux index (F10.7) and
the Kp geomagnetic index. These are compared, on an annual basis, with the
periodic features found in the NO radiative cooling. The occurrence of periodic
features as a function of latitude and altitude is then given. A summary section
concludes the paper.
2. Data preparation
The SABER instrument is a limb-scanning radiometer that measures spectrally
integrated radiance (W cm−2 sr) in 10 discrete spectral channels ranging from
1.27 mm to approximately 16 mm (Russell et al. 1999). A fundamental scientific
objective of the instrument is to make measurements to quantify the radiative
energy budget of the thermosphere and mesosphere (Mlynczak 1996, 1997). The
instrument was launched in December 2001 on the TIMED satellite into an
orbit inclined 74◦ to the equator. Routine scientific operations began in January
2002. The instrument has operated nearly continuously since commencing science
operations, recording in excess of 97 per cent of available data.
SABER views the atmosphere at 90◦ to the spacecraft velocity vector. This fact,
combined with the satellite inclination, results in latitude coverage from 55◦ north
(south) to 83◦ south (north). The TIMED spacecraft undergoes a yaw manoeuvre
every 60 days to prevent the Sun from illuminating the SABER telescope radiator.
This manoeuvre results in a shifting of the latitude coverage from north (south)
to south (north). Thus, the atmosphere poleward of 55◦ is observed in only one
hemisphere at a time and for a 60 day period.
The SABER instrument continuously scans the Earth’s limb recording profiles
of infrared radiance. A single limb scan, from approximately 400 km to the hard
Earth surface, requires approximately 50 s. One of the SABER channels records
NO infrared emission at 5.3 mm. NO is the dominant infrared emitter responsible
for cooling the atmosphere above about 120 km (Kockarts 1980).
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Periodic features in thermosphere
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In this paper, we examine the power (watts) radiated by the NO molecule
from the thermosphere (100–250 km). The process by which the SABER radiance
measurements are converted to profiles of radiative cooling, radiative fluxes and
radiated power is described in Mlynczak et al. (2008). Briefly, the measured
limb radiance profile is inverted assuming the emission is in the weak-line
radiative transfer limit. This yields, after a correction for the limited SABER
bandpass, a vertical profile of radiative cooling rate in watts per cubic metre.
These cooling rates are then integrated vertically to yield values of fluxes
of infrared radiation that exit the thermosphere (W m−2 ). Integration around
latitude circles with respect to area yields the power (watts) radiated by the
thermosphere. A final integration from pole-to-pole yields the total global power
radiated by NO.
In these studies, we also use time series of the solar wind speed, the F10.7
index and the Kp geomagnetic index. The Kp and F10.7 data are obtained from
the Space Physics Interactive Data Resource at the National Geophysical Data
Center, NOAA Satellite and Information Service. The solar wind speed data are
obtained from measurements made by the Solar Wind Electron Proton and Alpha
Monitor (SWEPAM) instrument on the Advanced Composition Explorer (ACE)
satellite (McComas et al. 1998).
3. Results
Shown in figure 1 are four time series comprising, from top to bottom, the daily
global power (watts) radiated by NO from January 2002 to October 2009, as
derived from the SABER instrument; the daily solar wind speed obtained from
the SWEPAM instrument on the ACE satellite; the daily average Kp geomagnetic
index; and the daily average F10.7 solar radio flux. As discussed in Mlynczak et al.
(2010), the NO time series shows the long-term decline in the NO cooling owing to
the 11 year solar cycle. The effects of the prolonged solar minimum are manifest
in the lower levels of NO power observed from 2007 onward. The ‘spikes’ observed
in the data are due to the ‘thermostat’ effect (Mlynczak et al. 2003) caused by
the impact of coronal mass ejections (CMEs) and other geomagnetic phenomena.
There is substantial high-frequency content visually evident in the data.
The solar wind speed and Kp indexes do not exhibit a dramatic decline with
time as does the NO power. They do, however, exhibit substantial persistent
short-term variability. In particular, both Kp and the solar wind speed appear
more uniform with similar short-term structure from mid-2005 onward. In
contrast, the F10.7 index in the bottom panel in figure 1 exhibits a clear shortterm variability that will be shown to be associated with the 27 day solar rotation
period. F10.7 exhibits a decrease with time since 2002 that is associated with the
declining phase of the current solar cycle. From 2008 onwards, there is an absence
of short-term periodic features in the F10.7, which is associated with the absence
of sunspots during this time.
Figure 2 shows the lomb normalized periodogram (LNP) for the solar wind
speed, for each year 2002–2009 to date. The 27 day periodicity appears in all
years but 2005. The first harmonic of the solar rotation period (13.5 days) is
evident, with different strengths, in all years. The second harmonic (9 days) is
strongest in 2005, 2006, 2007 and 2008. The fourth harmonic (6.75 days or 27/4)
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F10.7 cm flux
(W m–2 Hz–1)
Kp index
speed (km s–1)
power (1011 W)
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M. G. Mlynczak et al.
10
8
6
4
2
0
1000
800
SABER global NO power
ACE/SWEPAM solar wind speed
600
400
200
8
6
Kp index
4
2
0
300
250
200
150
100
50
F10.7 cm solar radio flux
Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct JanApr Jul Oct JanApr Jul Oct JanApr Jul Oct JanApr Jul Oct JanApr Jul Oct
2003
2004
2005
2006
2007
2008
2009
Figure 1. From top to bottom, daily global power emitted by nitric oxide (NO); solar wind speed;
the Kp geomagnetic index; and the 10.7 cm solar radio flux, from January 2002 to October 2009.
is evident in 2006, 2007 and 2009. Occasionally, other higher harmonics appear,
such as the fifth harmonic (27/5 or 5.4 days) in 2004 and 2007. These features
are typically not statistically significant owing to their small magnitude.
Short-term periodic features in the Kp index are shown in figure 3. The 27 day
and 13.5 day periods are evident in most years. However, the 13.5 day period is
strongest in the years when the 27 day period is weakest. The 9 day period is
prominent in 2004–2008. The 6.75 day period is strongest in 2006. In figure 4, the
LNP for the F10.7 cm solar radio flux exhibits periodic features of approximately
27 days. No higher harmonics of the solar rotation period are evident in the LNP
for this parameter in any year from 2002 to 2009.
In summary, the solar wind and Kp indexes exhibit, depending on year,
periodicities equal to the 27 day solar rotation period. The second, third and
fourth harmonics are also evident at times. The most prominent of all the
harmonics is the third harmonic at 9 days followed by the second harmonic
at 13.5 days. Both Kp and the solar wind speed exhibit a period of 6.75 days
(fourth harmonic) in 2003, 2006 and 2007. In contrast, the F10.7 exhibits only
the fundamental 27 day first harmonic.
The LNP for the NO cooling is provided for each year 2002–2009 (to date)
in figure 5. The 27 day solar rotation period is evident in most years, but not
all. The occurrence of the 27 day period in the NO power is correlated most
strongly in the years in which it is also strong in the solar wind speed and the
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Periodic features in thermosphere
lomb normalized periodogram magnitude
2009
2008
2007
2006
2005
2004
2003
2002
27.0
13.5
9.0
6.75 5.4 4.5
2.0
periodicity (days)
Figure 2. Lomb normalized periodogram (LNP) of the solar wind speed by year, 2002–2009, for
periods between 30 and 2 days. The solar rotation period (27 days) and the next five harmonics
(13.5, 9.0, 6.75, 5.4 and 4.5 days) are indicated.
lomb normalized periodogram magnitude
2009
2008
2007
2006
2005
2004
2003
2002
27.0
13.5
9.0
6.75 5.4 4.5
2.0
periodicity (days)
Figure 3. LNP of the Kp geomagnetic index by year, 2002–2009, for periods between 30 and 2 days.
The solar rotation period (27 days) and the next five harmonics (13.5, 9.0, 6.75, 5.4 and 4.5 days)
are indicated.
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M. G. Mlynczak et al.
lomb normalized periodogram magnitude
2009
2008
2007
2006
2005
2004
2003
2002
27.0
13.5
9.0
6.75 5.4 4.5
2.0
periodicity (days)
Figure 4. LNP of the 10.7 cm solar radio flux (F10.7) by year, 2002–2009, for periods between 30
and 2 days. The solar rotation period (27 days) is present but higher harmonics are absent.
lomb normalized periodogram magnitude
2009
2008
2007
2006
2005
2004
2003
2002
27.0
13.5
9.0
6.75 5.4 4.5
2.0
periodicity (days)
Figure 5. LNP of the global NO power between 100 and 250 km by year, 2002–2009, for periods
between 30 and 2 days. The solar rotation period (27 days) and the next five harmonics (13.5, 9.0,
6.75, 5.4 and 4.5 days) are indicated.
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Periodic features in thermosphere
Table 1. Years in which periods of 27, 13.5, 9 and 6.75 days occurred in the Kp index, the NO
global annual power and the solar wind speed, at the 90% confidence level.
27 day
13.5 day
9 day
6.75 day
year
Kp
NO
Vsw
Kp
NO
Vsw
Kp
NO
Vsw
Kp
NO
Vsw
2009
2008
2007
2006
2005
2004
2003
2002
—
x
x
x
—
x
x
x
x
x
x
x
x
x
—
x
—
x
x
x
x
x
x
x
x
x
—
—
—
x
x
—
—
x
—
—
—
—
—
—
x
x
x
—
—
x
x
x
—
x
x
x
x
x
—
—
—
x
x
x
x
—
—
—
—
x
x
x
x
x
—
—
—
—
x
x
—
—
x
—
—
—
—
x
—
—
—
—
—
—
x
x
—
—
x
—
Kp index. The second harmonic 13.5 day period is evident in the NO cooling
most strongly in 2008 although it appears present, but weaker, in other years. It
appears well correlated with the occurrence of the 13.5 day period in Kp and the
solar wind speed.
The most prominent harmonic in the NO power is the 9 day period, occurring
strongly in 2005, 2006, 2007 and 2008. The fourth harmonic of the solar rotation
period at 6.75 days is most evident in 2006 and 2007. The occurrence of the 9
day periodicity was tied to the occurrence of coronal holes on the Sun that were
spaced approximately 120◦ apart in solar longitude (e.g. Temmer et al. 2005).
New work by Lei et al. (submitted) links similar behaviour in the distribution of
coronal holes in 2008 with the occurrence of thermospheric density variations with
periods of 13.5 and 9 days. This result strongly implies that the periodic features
observed in NO in 2008 are due to the distribution of coronal holes, as in 2005.
We suggest that, given the prolonged solar minimum from 2005 to 2008, the 13.5,
9 and 6.75 day periods observed in the infrared cooling by NO are associated
with the occurrence of high-speed streams associated with coronal holes on the
Sun in these years.
An advantage of the Lomb technique is that it affords a test of the significance
of the various periodic features in the data. Shown in table 1, as indicated by an
‘x’, are the occurrences of the various periodicities in solar wind speed (Vsw ), the
Kp index and the NO infrared power in the years 2002–2009, at the 90 per cent
confidence level. The 9 day periodicity in the NO emission is significant at this
level in 2005–2008, while the 6.75 day periodicity is significant only in 2006.
The LNP results shown in figure 5 are for the daily, global, vertical integrated
(from 100 to 250 km) power radiated by NO. To examine whether there is any
variation in the occurrence or strength of the periodic features as a function of
altitude or latitude we compute time series of the global power, but integrating
only over 10 km bins in the vertical. We then compute the LNP for the global
time series in each of these bins. This approach will elucidate whether or not there
is any altitude variation in the occurrence of the periodicities. Shown in figure 6
is the LNP for the daily global time series of NO power in five different 10 km
deep altitude bins. The length of the record assessed here is nearly 8 years in each
LNP. The results in figure 6 show that the 27 day, 13.5 day and 9 day features
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lomb normalized periodogram magnitude
M. G. Mlynczak et al.
180–189 km
160–169 km
140–149 km
120–129 km
100–109 km
27.0
13.5
9.0
6.75 5.4 4.5
2.0
periodicity (days)
Figure 6. LNP of the global NO power in 10 km altitude bins, from 2002 to 2009. The results
demonstrate the presence of periodic features throughout the depth of the thermosphere.
occur throughout the depth of the thermosphere—that is, they are not limited to
a certain altitude range. This result is fundamental as it demonstrates that the
geomagnetic coupling responsible for the periodicities in radiative cooling exists
throughout the entire depth of the thermosphere.
Finally, we examine the latitude dependence of the periodicity in the radiated
NO power. To do so, we compute the NO power in eight latitude bins spanning
the equator to the pole in each hemisphere. Seven of the bins are 11◦ wide,
covering the equator to 77◦ latitude, and the eighth bin covers 77◦ to the
pole. Although SABER data exist only to approximately ±83◦ latitude, we
assume the SABER-derived flux (W m−2 ) is the same between 83◦ and 90◦
latitudes as between 77◦ and 83◦ latitudes. To obtain the power between 77◦
and the pole we multiply the flux measured between 77◦ and 83◦ by the area
between 77◦ and 90◦ latitudes. Furthermore, as discussed above, the polar
regions are observed in alternate periods of 60 days owing to the SABER
viewing geometry and the spacecraft orbital inclination. Thus, we examine time
series of 60 day length, which is sufficient to see the short-term periodicities of
27 days or less.
Shown in figures 7 and 8 are the LNP for the NO power in several latitude bins
for two successive 60 day ‘yaw periods’ in 2005–2006. Period 23 (figure 7) is from
September to November 2005 and period 24 (figure 8) is from November 2005 to
January 2006. The prominent 9 day periodicity is apparent most strongly in the
polar regions and mid-latitudes while it is very weak but still visually apparent
in the tropics. This latitude dependence is consistent with the theory that the
periodicity is geomagnetic in origin (Mlynczak et al. 2008).
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Periodic features in thermosphere
global NO power 100–250 km for yaw period 23 by latitude bin
lomb normalized periodogram magnitude
77 N to 90 N
66 N to 77 N
55 N to 66 N
44 N to 55 N
33 N to 44 N
22 N to 33 N
11 N to 22 N
0 N to 11 N
11 S to 0 N
22 S to 11 S
33 S to 22 S
44 S to 33 S
55 S to 44 S
27.0
13.5
9.0
6.75 5.4 4.5
2.0
periodicity (days)
Figure 7. LNP of the global NO power as a function of latitude from September to November 2005.
Note the decrease in the strength of the 9 day periodicity from high latitudes to the equator.
global NO power 100–250 km for yaw period 24 by latitude bin
lomb normalized periodogram magnitude
44 N to 55 N
33 N to 44 N
22 N to 33 N
11 N to 22 N
0 N to 11 N
11 S to 0 N
22 S to 11 S
33 S to 22 S
44 S to 33 S
55 S to 44 S
66 S to 55 S
77 S to 66 S
90 S to 77 S
27.0
13.5
9.0
6.75 5.4 4.5
2.0
periodicity (days)
Figure 8. Same as figure 7 except for the different latitude range covered between November 2005
and January 2006.
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4. Summary
We have presented studies of the occurrence of short-term periodic features
in the radiative cooling of the thermosphere associated with infrared emission
by the NO molecule. The infrared emission measurements are taken by the
SABER instrument on the TIMED spacecraft and now span 8 years. We
specifically look for periodic features associated with the solar rotation period
(27 days) and its higher harmonics. Over the 8 years, we observe periods
of 27 days, 13.5 days, 9 days and, in 2006 and 2007, 6.75 days. These
periods correspond to the solar rotation period and its subsequent three
harmonics.
The origin of the periodic features in the radiative cooling data is confirmed
to be from coupling of the solar wind to the thermosphere. In particular,
the 9 day and 6.75 day periods, when they occur in the NO cooling, are
simultaneously present in the solar wind speed and the Kp geomagnetic index.
Periods shorter than 27 days are absent in the time series of the 10.7 cm radio
flux, thus excluding solar ultraviolet radiation as the cause of the periodic
features.
The occurrence of the short-term periodic features was also examined as a
function of altitude and latitude. We observe the periodic features throughout the
depth of the thermosphere (defined herein as 100–250 km) over the duration of the
dataset to date. We also find a distinct latitude dependence of the periodicities,
with the periodic features being strongest at the poles and weakest in the tropics.
These data and related analyses provide insight into the persistent and pervasive
coupling between processes in the solar corona, the solar wind and the terrestrial
thermosphere.
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