Solar Variability: Eects on Geospace and the Satellite
Environment
a
a
Kristian Reed , Erlend S. Syrdalen
a Institutt for fysikk, Norges Teknisk-Naturvitenskapelige Universitet, N-7491 Trondheim, Norway.
Abstract
In the following essay an introduction to some of the eects of solar variability on the
satellite environment will be given. Relevant processes on the Sun and in geospace will
be discussed in some detail. In particular the impact of the solar cycle on satellite drag
in low Earth orbit will be discussed.
Contents
1
2
Introduction
2
The Variable Sun
2
2.1
Sunspots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2.2
Active Regions and Prominences
4
2.2.1
2.3
2.4
2.5
3
4
. . . . . . . . . . . . . . . . . . . . . . .
Solar Flares and Coronal Mass Ejections . . . . . . . . . . . . . . .
5
The Solar Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
Measures of variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.4.1
Sunspot Number . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.4.2
10.7 cm Solar Flux . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
The 11-year solar cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Geospace and Space Weather
3.1
Geospace
3.2
Space Weather
9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Impact on Satellites
10
11
12
4.1
Satellite Drag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2
Solar Cosmic Rays
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
15
4.3
Future developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
5
Concluding Remarks
16
6
Acknowledgements
17
Preprint submitted to Vendela Paxal
March 29, 2014
1. Introduction
We live in the extended atmosphere of a magnetic variable star that drives
our solar system and sustains life on Earth. Our Sun varies in every way we
can observe it.
The Sun gives o light in the infrared, visible, ultraviolet,
and at x-ray energies, and it gives o magnetic eld, bulk plasma (the solar
wind) and energetic particles moving up to nearly the speed of light, and all
of these emissions vary.[1]
The above quote is a tting backdroop for the remainder of the text, as it sets the stage for
discussing the eects of some of the above mentioned variations on geospace. Geospace,
the solar-terrestrial environment, can be dened as encompassing the upper parts of
Earths atmosphere, the outer part of the geomagnetic eld, and the solar emissions
which aect them[2].
The goal of this essay is to elucidate the importance of future
research in the related elds of space weather, geospace and the Sun, as well as the
impact of these elds on spaceight. The importance of solar activities on spacecraft is
of vital importance, and as an example consider the following: A typical satellite in a
500 km orbit could last up to 30 years if conditions were constant as they are during
solar cycle minimum, but only around 3 years if the conditions instead were like during
solar maximum[15].
The modern hi-tech society has become ever more vulnerable to disturbances from
outside the closed Earth system. This especially applies to those disturbances inititated
by explosive events on the Sun: Flares lash out radiation heating the atmosphere and
slowing down satellites in low Earth orbit(LEO), making them drop into lower orbits.
Solar energetic particles are accelerated to relativistic velocities and endager astronauts
and spacecraft alike. Coronal Mass Ejections(CMEs) are humoungous clouds of ionized
gass ejected from the Suns upper atmosphere into space, if they are heading in the direction of Earth they may cause large geomagnetic storms. As satellites become ever more
advanced, so too do they become more exposed to solar eects. Proper understanding
of these phenomena is then paramount for the future endeavours in space.
The essay will begin by rst describing the variable Sun and its features, how it eject
matter and radiation earthwards, and how the 11-year solar cycle governs the intensity
of these features. Following this is a discussion of the magnetic eld of the Earth and
the Suns inuence on the Earth environment, the coupling the solar variability and the
Earth manifesting itself as so-called "space weather". After this a discussion of how the
changing geospace enviromnent aects satellites, with a focus on drag in LEO will be
given. To round o, a concluding remark summarizing some of the main points ends the
essay.
2. The Variable Sun
In 1843 the German astronomer Heinrich Schwabe announced that the number of
dark spots visible on the solar disk seemingly varied in a roughly regular cycle.
The
Swiss astronomer Rudolf Wolf picked up on this idea, and he both tracked and compiled
historical data of the sunspot number and found the average period between peaks to
be around 11 years, the so called 11-year solar cycle. The connection of sunspots with
magnetic activity on the Sun were made by American solar astronomer George Ellery
2
Hale and his collaborators, when they discovered evidence of solar magnetic oscillations
in measurements of sunspot spectra. This made him conclude that the 11-year cycle of
sunspot numbers is actually half of a underlying 22-year solar magnetic cycle[13].
2.1. Sunspots
Figre 1 displays a close up photograph of dark features on the Suns surface, and a
superimposed image showing the whole visible sun. The dark areas are called sunspot,
and are areas typically 10,000 km across.
Sunspots are phenomena taking place in
the Suns photosphere, the part of the solar atmosphere where most of the visible light
is emitted.
They appear dark because they are cooler than the surrounding 5800 K
photosphere. The darkest part of the sunspot is called the umbra, and is roughly 4500 K.
The intermediary region between the umbra and the photosphere is called the penumbra,
a slightly lighter colored region at around 5500 K. Sunspots are not steady, and most
change in both size and appearance over their lifetime of anywhere between 1 and 100
days. As seen in gure 1 they can bunch up and form sunspot groups. Sunspots often
occur in magnetically linked pairs, and they tend to form at dierent latitudes during
the sunspot cycle.[6].
Figure 1:
Large sunspot group. Caused by intense magnetic elds emerging from the Suns
interior, the sunspots appear dark only because they are viewed in visible light and contrasted
against the hotter surrounding surface. Credit: NASA/SOHO[23].
3
The most important part of a sunspot however, is their strong magnetic elds at
around 0.1 Teslas, some of which may even be as strong as 0.4 T, around 1000 times
stronger elds than the rest of the solar surface. In addition the eld lines are normal
to the solar surface. The elds hint at the underlying cause of the sunspots, and it is
the magnetic eld lines that block convective transport of energy to the photosphere in
sunspots, and thus make the sunspots cooler and by extension darker than the overall
surface. The complex magnetic structure of the Sun causing the sunspots and believed
to be behind the solar activity cycles is thought to be caused by the convection and
dierential rotation of the Sun, the equator rotates slightly faster than the poles. This
means of creating a magnetic eld is often called dynamo theory[7].
2.2. Active Regions and Prominences
Associated with sunspot groups are vast active regions on the Sun, hundred of thousands of kilometers across.
The magnetic activities on the Sun tends to concentrate
in these active regions of the sun. The region is characterized by explosive and unpredictable surface events, that even though they have little signicance on the Sun's total
luminosity, can have signicant eects on the Earth environment.[6]. When a region of
the sun erupts explosively, it can spew forth massive amounts of energetic particles into
the corona.
Figure 2:
A solar prominence breaking away from the Sun on 17. October 2013. Observed in
UV by the STEREO Ahead spacecraft. Prominences, known to be unstable, are cooler clouds
of particles held above the Sun by strong magnetic forces. Credit: NASA/STEREO.[23].
4
Prominences are sheets or loops of glowing gas ejected from active regions, moving up
into the corona under magnetic inuences. The magnetic elds in and around sunspots
are believed to be causing them, but the underlying causes are still unknown[6]. Figure
2 shows a prominence loop breaking free from the sun. There are two classes of prominences, quiescent and active ones, classied by stability and duration.
The quiescent
prominences can last for days and weeks, while the active ones can come and go in a
couple of hours. Most of the material falls back to the sun due to the gravitational pull
of the massive Sun.
2.2.1. Solar Flares and Coronal Mass Ejections
Even more explosive and violent than the prominences are the solar ares, a massive
one shown in 3. Flares occur low in the Sun's atmosphere, usually near active regions.
They are also believed to be caused by magnetic instabilities. They release staggering
amounts of energy, with observations revealing emissions especially intense in the X-ray
and UV parts of the spectrum. They can be compared to massive bombs going o in
the lower parts of the solar atmosphere, blasting particles into space. They are closely
linked to prominence activity[6].
Coronal mass ejections, such as the one shown in gure 4 are also associated with
both ares and prominences, and by extensions sunspots. These ejections are gigantic
bubbles of electrically neutral ionized and magnetic gas separating from the rest of
the solar atmosphere and escaping into space. They can contain billions of tons of solar
material, and are among the most energetic events in the Solar System yet their cause
remains unkown[7].
Figure 3:
Figure 4:
Solar are visible on right hand
CME on upper left part of im-
side of solar disk. Picture taken on 4. Novem-
age.
ber 2003 of probably one of the most powerful
posed on coronagraph image of the eruption.
ares ever witnessed.
Picture taken on 4.
Image taken in EUV.
Credit: NASA/SOHO[23].
Composite image of sun in UV superim-
NASA/SOHO[23].
5
January 2003.
Credit:
2.3. The Solar Wind
There is a constant stream of electromagnetic radiation and fast-moving particles,
mostly protons and electrons, shedding from the Sun. The corona, the outer atmosphere
of the Sun, a region with temperatures of more than 3 million Kelvin and intense ionization, is too hot too keep the coronal gasses from escaping the Suns gravity. In this
manner, roughly 2 million tons of solar matter is ejected every second. This stream of
escaping solar particles is the solar wind[6].
The solar wind is fully ionized and elec-
trically neutral and carries with it a magnetic eld. Both spatially and temporally the
eld is highly variable, with solar wind velocity depending on the solar cycle[14]. Typical
speeds however are around 450 km/s, meaning the particles hit Earth in about four days.
Some of the wind exits large holes in the corona, where the magnetic eld lines extend
into space instead of looping back onto the surface. Alltough the solar wind transports
less than a millionth of the Sun's electromagnetic energy, it transfers hundreds of billions
of watts a day to the magnetosphere and ionosphere amounts comparable to human
power generation. [11].
2.4. Measures of variability
There are several measures of the dierent ways in which the sun varies, including
total solar irradiance, magnetic eld, ares, CMEs, geomagnetic activity, galactic cosmic
ray uxes and radioisotopes in trees and ice cores[10]. All of the above are intrinsically
linked to the 11-year solar cycle as measured by the sunspot numbe, and in addition the
10.7 cm radio ux is of some importance as a proxy for solar UV ux[17]. Therefore the
details of these two methods will be given in short below:
2.4.1. Sunspot Number
The most obvious manifestation of solar activity is the 11-year sunspot cycle. The
International Sunspot Number(ISU), or Wolf 's relative sunspot number, rst devised by
R. Wolf in 1848, remains even today the key indicator of solar activity.
It is normal
to use the ISU as a standard for the solar cycle, and to compare changes in all other
quantities to the sunspots[14]. This is not nessecarily because it is the best indicator,
but because of the lenght of available record. Wolf himself began recording daily since
1848, and he was able to extend the data all the way back to 1749[10].
His relative sunspot number
Ri
is given as
Ri = k(10g + n),
6
(1)
Figure 5:
Monthly and 13-month smoothed sunspot numbers over the last 6 solar cycles. 11-
year cycle clearly visible, along with the fact that the solar maximum peaks themelves vary in
both magnitude and shape. Credit: SIDC[22].
where
k
is a correction factor depending on the observer conditions,
of sunspot groups, and
n
g
is the number
is the number of individual sunspots. The number takes into
account that it is much easier identifying sunspot groups than individual sunspots. The
correction factor is there to take into account that given a bigger telescope, you can
resolve more individual sunspots and thus obtain a higher sunspot count than if you
do the same trough a pair of binoculars. Today the standard smoothing is a 13-month
running mean, and the solar cycle maxima and minima is usually given as the peaks
and troughs in the smoothed data. See gure 5 for a plot of monthly and smoothed ISU
during the last six cycles.
2.4.2. 10.7 cm Solar Flux
The 10.7 cm Solar Flux,
F10.7 , is the total emisisons from the sun at a radio wavelenght
of 10.7 cm (2800 MHz). The advantages of this method of measuring solar variability is
that it is both more objective and can be made independent of weather conditions compared to the sunspot number. There have been daily measurements of
F10.7
since 1946,
and a plot of this data can be seen in gure 6. Comparing this to the sunspot numbers in
gure 5 clearly demonstrates the connection between the two dierent measures of solar
variables.
2.5. The 11-year solar cycle
The sun varies constantly, but it is the 11-year solar spot cycle and by extension
the 22-year solar magnetic cycle that is of importance. As mentioned above, the sun is
7
Figure 6:
Monthly average of radio emissions from sun at
F10.7
wavelenght.
Credit: Space
Weather Canada[24].
driven by a dynamo, and every second 11-year sunspot cycle the Sun's magnetic polarity
switches, the so called Hale cycle. During the 11-year cycle the total solar brightness only
increases by about 0.1% during activity maxima[11]. The ux of UV radiation however
increases by a factor of two, and is of great importance for heating Earths atmosphere.
As seen previously the number of sunspots and sunspot groups change dramatically, and
during maxima the frequencies of CMEs and solar ares is higher, and the solar wind is
8
faster. Geomagnetic storms, to be discussed later, are much more common during solar
maxima. Figure 7 shows the sun at dierent stages of the cycle, with surface activity
markedly dierent on the three dierent ones.
Figure 7:
A comparison of three images over four years apart illustrates how the level of
solar activity has risen from near minimum to near maximum in the Sun's 11-years solar cycle.
Images captured with hydrogen lter, making the solar corona appearant.
THere are more
sunspots, solar ares, and CMEs occuring during solar maximum. The increase in activity can
be seen in the number of white areas, which are indicators of strong magnetic intensity. Credit:
NASA/SOHO.[23].
3. Geospace and Space Weather
The solar extreme- and far-UV radiation is the primary energy input in the upper
atmosphere.
It creates as well as gets absorbed in the ionosphere.
Extreme ultravio-
let(EUV) radiation exhibits substantial variability of factors of two or more during the
solar cycle[12]. This substantially alters the thermodynamic, chemical an radiative state
of the atmosphere at higher altitudes. The temperatures here also uctuate by a factor
of two, and neutral and electron densities by a order of magnitude.
9
Figure 8:
Illustration of Earths magnetic eld being shaped by solar winds and solar storms.
Credit: NASA/SOHO.[23].
The magnetosphere, as illustrated in gure 8 is a cavity in the solar wind ow caused
by the interaction of Earths near dipole geomagnetic eld and the solar wind.
It de-
ects the bulk of the solar wind, becoming compressed in the sunwards direction and
elongated in the direction facing away from the sun. The boundary region between the
magnetosphere and the solar wind is called the magnetopause. During periods of high
solar activity the magnetopause can be pushed from its normal distance at about 10 earth
radii to well within geostationary orbits, exposing spacecraft there to the solar wind. [8].
3.1. Geospace
Even though they only constitute around 1.5% of the total solar irradiance, solar UV
and x-ray radiation is the primary source of terrestrial atmospheric heating above 10 km.
This intrinsically links the atmosphere of the Sun with that of the Earth[11]. Figure 9
shows how the dierent parts of the Sun connects to the dierent parts of the Earth's
environment.
10
Figure 9:
Energy transfer from the Suns atmosphere to Earths atmosphere. See description
in imagetext. Credit: Lean (2005) [4][11].
At a xed altitude in the thermosphere(ca. 500-1000km), density is highly sensitive to
solar irradiance at extreme ultraviolet(EUV) wavelenghts (0-120 nm). The thermosphere
contracts and expands in response to this radiation, as it is the primary heat source at
these altitudes. Solar EUV irradiance changes by a factor of 2 or more during the solar
cycle, causing the mass density near 400 km height to change by orders of magnitude[18].
Figure 11 shows how variations in UV radiation intensity during the solar cycle massively
impacts the upper atmosphere density.
Figure 12 shows the coupling of solar activity
with geomagnetic activity.
Above ca. 80 km altitude the solar UV radiation ionizes some of the neutral atmospheric gas, and since the atmosphere is relatively thin the ions can exist a while without
recombining, creating what is known as the ionosphere[8].
3.2. Space Weather
A working denition of space weather can be:
11
Figure 10:
Illustration of geospace environment.
Particles and radiation streaming from
the Sun, interacting with Earths magnetosphere and upper atmosphere.
Credit: Hargreaves
(1992)[2].
Conditions on the Sun and in the solar wind, magnetosphere, ionosphere and
thermosphere that can inuence the performance and reliability of spaceborne and ground-based technological systems and can endager human life
or health[8].
4. Impact on Satellites
When the sun burst out a Coronal Mass Ejection (CME) it can result in devastating
and expensive damages to both satellites and humans in space. There is two major consequences from the CME that is of particularity interest for the environment of satellites
in Low Earth Orbit (LEO) which will be discussed in this section. The rst and most
important is the heating and following expansion of the atmosphere caused by Extreme
Ultraviolet Radiation (EUV). This radiation is a lot stronger during sunspot maximum
than normally, causing satellites to be aected by a lot stronger drag from the atmosphere. The second major impact on satellites in LEO is the direct hit of high energetic
particles followed by the coronal mass ejection. These particles might cause large damage
to electrical circuits, which in worst case might lead to a short circuit.
12
Figure 11:
Impact of UV solar cycle variability on upper atmosphere density. See description
in image text. Credit: Lean (2005)[11].
4.1. Satellite Drag
Extreme ultraviolet radiation (EUV) radiation has large impact for increased atmo-
1 dierence between sunspot
spheric temperature between 150 and 1000 km, with largest
maximum and sunspot minimum at 600 km.
During solar minimum the temperature
in the thermosphere is about 700 Degrees Celsius.
than doubled
During Solar maximum it is more
2 to about 1500 degrees Celsius. From the Ideal Gas Law we know that the
temperature is inversely proportional to the density. This means that when temperature
is rising the density gets lower which leads to expansion. This is the reason why satellites
observe a greater drag during sunspot maximum. For instance was the air density at 600
km during solar maxima in 1957-58 about 60 times
3 larger than the following sunspot
minima in 1964.
When the satellite rst is being slowed by friction from atmospheric air, this breaking
force is aected by positive feedback. This means that when the satellite rst is being
1 see
2 see
3 see
[25]page 16
[28]
[25]page 17
13
Figure 12:
Periodicities in solar and geomagnetic activity.
number. Lower left: Geomagnetic
Ap
Upper left:
Monthly sunspot
index. Upper right: solar cycle variation in geomagnetic
and solar record, with peak geomagnetic activity during declining phase of solar cycle. Lower
right: A semiannual variation in geomagnetic data. Credits: Pulkkinen (2007)[8].
slowed a little, it will fall into a lower orbit with even higher satellite drag. Ultimately
this drag get so large that the satellite burns up in the atmosphere.
4
The decelerating force of satellites can be described by the formula
Fd = −0.5pACp V 2
where
p
is the atmospheric density,
to the direction of the motion,
atmosphere,
CD
v
A
(2)
is the satellite's cross-sectional area perpendicular
is the satellite velocity with respect to the corotating
is a dimensionless drag coecient that describes the interaction between
the atmospheric particles and the satellite. As we can see, satellite drag in proportional to
the atmospheric density, which makes a 60 times larger air density causing the breaking
force to be 60 times larger as well.
An example where this got catastrophical consequences is the Skylab 1 space station
sent up by NASA in 1973.
of 10 years.
4 see
5 see
In 1974 NASA predicted
5 it would have a total lifetime
UK scientists on the other hand, which were taking account of the solar
[27]page 2 for derivation of this formula
[25]page 249
14
Figure 13:
Variation of air density with altitude for high and low solar activity. Credit:CIRA
(1972)[25].
maximum in 1979 estimated a lifetime of 6 years.
As we know Skylab 1 burned up
into the atmosphere on July 21th 1979 causing debris littering a
300 km2
large area over
Western Australia.
4.2. Solar Cosmic Rays
6
Normally the solar winds containing mostly protons and electrons reaches a velocity
about 400 km/s with ux
2 × 10
8
2
cm /s of solar winds produced by CME produced
during high solar activity may reach a velocity of more than 3200 km/s and over 20
times as large radiation ux.
This plasma outbreaks from the sun is initially electrical neutral. Because the electrons are moving a lot faster than the protons, it causes the satellite being bombarded
by electrons from all sides, but protons only in the side facing the radiation. This lead to
the satellite being electrical charged. These voltage dierences causes electrical discharge
which is a serious risk for damaging electrical components. These electrical elds surrounding the satellite might also disturb and interfere with sensors and communication
equipment which might be really challenging.
6 see
[26]page 63
15
One of the most common inuence on satellites is something called Single Event Upset
(SEU). This happens when the protons or electrons hitting electrical circuits leads to
changes if bit value of stored data. Normally these errors is corrected by error correcting
algorithms, but sometime these changes might be so large that we get a Single Even
Latchup (SEL) which means that the changes might not be able to repair.
Another problem is the radiation hitting solar panels.
For the rest of the satellite
it's possible to reduce the electrical charges by using conductive materials in the surface.
However, this is not possible for the large solar panels collecting most of the radiation.
Here researchers have developed
7 a method by covering the panels by an almost trans-
parent layer of indiumoxide which slowly will discharge the electrical potential instead
of damaging short circuits.
4.3. Future developments
There is many projects going on around the world on the topic of better understand
variation in the upper atmosphere caused by variations on the sun. Not only for scientic
purposes, but also purely commercial satellites in LEO it is of high interest to predict
how the satellite will be aected by it's environment mainly controlled by the sun.
Communication satellites placed in GEO at 36,000 km is not aected by increased
atmospheric drag during solar maximum but the signal from GPS can be that much
aected that for instance aircraft's are not allowed to use autopilot during periods of
high solar activity [29].
One of the most promising projects is the NASA Living With a Star(LWS) project
which aims to develop a better understand about how the sun variability aects humans in
space and on earth. This projects consists of multiple satellites with one of the rst, Solar
Dynamics Observatory(SDO) sent up in 2010 having a main goal to observe dierences in
the sun's Extreme Ultraviolet Variability(EUV) for making better predictions for future
warnings [30].
5. Concluding Remarks
The Sun varies in a 11-year cycle, and knowledge of the processes taking place on the
Sun is vital for spaceight. There are many open questions concerning the Sun-Earth
connection and features on the Sun. As future tecnology becomes more advanced, it also
become more susceptible to solar inuence. Utilization of space has added a practical
avour to academic research. Knowledge of and ability to predict solar activity levels is
an important goal, but the processes involved might be inherently chaotic and dicult to
model. Only the future will tell whether space weather is as unpredictable at terrestrial
weather, or if the underlying mechanisms can be understood. The authors do hope the
importance of said research has become clear for the reader, and that much of the research
only can be done with satellite platforms. The Living With a Star NASA programme is
directed at answering some of the questions raised in the essay.
7 see
[25]page 63
16
6. Acknowledgements
The authors woulde like to thank Robert Hibbins and Patrick J. Espy for their assistance in choosing the essay topic and nding source material.
Figure 14:
I am Camilla SDO, NASA SDO's Mission Mascot. I help with Education & Public
Outreach and I train to y to Space [31].
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