LeandraMerola

8/2/2019 LeandraMerola

1/19

A Study of the Effects of Sunrise and Sunset on the Ionosphere

as Observed by VLF Wave Behavior

By Leandra Merola

South Side High School

Rockville Centre, New York


2/19

Merola, Leandra

2

Abstract

The purpose of this study was to observe solar activity and sudden ionospheric

disturbances (SIDs) by monitoring the strength of VLF (very low frequency) radio waves.

Specifically I studied how sunrise and sunset affected VLF radio waves. To accomplish this it

was necessary to accurately record ionospheric disturbances and relate them to solar activity.

The sun releases electromagnetic radiation, which is absorbed by the atmosphere around the

Earth. This radiation has the potential to disturb or ionize the thermosphere, the outer most layer

of the atmosphere, and this then affects radio waves, including VLF waves, that are reflected by

the ionosphere. To conduct my study, I built an antenna that captured very low frequency radio

waves that are reflected by the ionosphere. The antenna was attached to a VLF radio reciever,

obtained from the Solar Center at Stanford University in California. The specific VLF waves

observed in this study are used to communicate with U.S. submarines. The data that I collected

was recorded by a computer and converted into graphs that illustrate disturbances in the

ionosphere. The results of this study showed that radiation from the sun disturbs VLF waves that

are reflected by the ionosphere. I was then able to conclude that the ionosphere was also affected

by solar radiation. I also found that sunrise and sunset drastically change the signal strength of

VLF radio waves because of the dramatic variation in the ionizing of the ionosphere.


3/19

Merola, Leandra

3

Introduction

The purpose of this project was to investigate the effects of sun-induced ionospheric

disturbances on Very Low Frequency (VLF) radio waves. The ionosphere reflects transmitted

VLF radio waves back to Earth. The strength of these radio waves depends on how much or

how little the ionosphere is ionized. Solar radiation contributes to ionization of the atmosphere,

which can alter the strength of the transmitted VLF radio waves. This project explores how the

ionosphere and VLF radio waves react to the solar radiation, especially in

different geographic locations on Earth. The project also looks specifically at

the impact of solar radiation on the ionosphere at local sunrise and sunset.

The sun emits electromagnetic radiation that ranges from gamma rays to

radio waves. This energy can be directly from the sun in the form of thermal

energy or the energy can be in the form of solar flares. A solar flare is an

explosion on the sun that happens when energy stored in twisted magnetic fields

is suddenly released (http://www.spaceweather.com/, 2006). According to

Davies (1990) solar flares emit radiation in various ranges of wavelengths from

long radio waves to short X-rays and can last from a few minutes to several

hours. This radiation, depending on the strength of the solar flare and the angle

to the Earth, affects the Earths atmosphere.

The ionosphere is located approximately 80 km and higher above the

Earth and is the outer-most layer of the atmosphere (Fig 1,

http://www.haarp.alaska.edu/haarp/ion1.html). The ionosphere is made up of

many gases including nitrogen and oxygen. It is called the ionosphere

Figure 1:

Layers of

the

Ionosphere


4/19

Merola, Leandra

4

because the small fraction of the solar energy at UV and smaller wavelengths is energetic enough

to strip electrons off molecules in the air. When this ionizing radiation enters the atmosphere, it

collides, primarily, with oxygen molecules. The shortest energy waves that come from the sun

are partly absorbed by the oxygen molecules; it is these waves that ionize the particles

(http://www.haarp.alaska.edu/haarp/ion1.html, 2006). The ionosphere is broken up into several

layers, all of which are ionized at all times except for the D layer, the layer closest to the Earth

(Fig 1), which disappears at night because the neutral density below about 200 km is high

enough that electrons recombine with oxygen in a few hours or less (Khanal, 2004). Ionization

in the E region decreases a little during the night, but the D layer does not reappear until the sun

rises at that altitude again.

VLF (very low frequency) radio waves are used primarily by the military to communicate

with submarines, but they are also used as avalanche beacons and for wireless heart rate

monitors. VLF waves have a frequency somewhere between 3 and 30 kHz and a wavelength

between 100 and 10 km. These radio waves are used to communicate with submarines because

at this frequency, they can penetrate water roughly 10 to 40 meters (Radio Waves, 2006). VLF

waves are useful because not only do they travel along the ground but the ionosphere also

reflects them. Because of this quality they are ideal for communicating around the world. They

Figure 2: Radio Waves are Reflected by the Ionosphere

(http://www.haarp.alaska.edu/haarp/ion1.html)


5/19

Merola, Leandra

5

are also ideal for monitoring the ionosphere. VLF waves bounce off the ionosphere, so if

anything happens to the ionosphere the radio wave will also be affected.

Solar activity, such as solar flares, the ionosphere and VLF radio waves are all

interrelated. Solar flares affect the ionosphere, which then affects the VLF radio waves that

bounce off the ionosphere. There are two major types of radiation that affect the ionosphere,

solar flares and simple background cosmic radiation. The cosmic radiation is not as strong as the

solar flares but it is constantly there whereas solar flares are only discrete events during the

daytime when the sun is facing a certain location, but they come in powerful bursts. During the

night VLF waves have to travel 90 km, to the E layer in the ionosphere before returning to Earth.

During the day the D layer is partially ionized so the VLF radio wave must first travel through

the D layer, losing some of its energy along the way. The signal strength of the VLF wave will

be stronger at night than in the daytime (Khanal, 2004). However, if there is any kind of solar

activity the radio waves will not have to travel as far because the increased radiation will ionize

the D layer and the D layer, instead of the E layer, will reflect the VLF radio waves. Therefore

the signal strength fluctuates as the reflection fluctuates because of the amount of absorption.

More than ever, people rely on radio wave technology such as GPS systems and

communication devices. However, there are many studies that show that radio wave technology

is subject to disruption. In a study cited by Contreira et al. (2004) showed that AM radio waves

faded when there was a solar flare. Contreira et al. (2004) also showed that the AM radio signal

was not as strong during the day because the energy from the sun was ionizing the ionosphere,

which results in the absorption of some of the AM radio waves. Although VLF waves respond

differently to solar activity, my study shows that radio wave technology is affected by

disturbances in the ionosphere and this disturbance can affect everyday life. When using VLF


6/19

Merola, Leandra

6

radio waves for communication, solar activity will affect the strength of the receiving signal.

Therefore the purpose of this study was to investigate how VLF radio waves are impacted by the

radiation given off by the sun at various times.

To monitor solar flare activity using VLF radio waves, Stanford University set up a

program that distributes inexpensive ionospheric monitors to students around the world

(http://solar-center.stanford.edu/SID/, 2006). These monitors, with the help of an antenna,

record VLF radio waves that are emitted from a specific VLF transmitter station. By tracking

the strength of the radio waves over a period of time it is possible to draw conclusions about the

characteristics of the ionosphere, because the radio waves and the ionosphere are so closely

related. When there is a fluctuation in the signal strength during the day, it is known as a Sudden

Ionospheric Disturbance or a SID. A SID is a change in the level of ionized particles in the

ionosphere, which cause the VLF waves to be reflected either more or less. To find out what

caused the SID, there is data from the GOES satellite that monitors the sun directly and records

solar flares. This data can be found on

http://www.sec.noaa.gov/ftpdir/warehouse/2006/2006_plots/xray/. If the SID occurs at the same

time as a solar flare, which can be found by using data from the GOES satellite, then there is a

direct correlation between the sun, the ionosphere and VLF radio waves. Therefore the sun and

solar flares affect VLF radio waves. The SID monitor can be useful because it allows indirect

observation of the ionosphere and the Sun. The SID monitor also allows data to be collected

from many different locations. This can be useful not only in understanding the ionosphere but

also in understanding the impact on radio wave technology that relies on the ionosphere. It is

important to study the ionosphere and radio wave technology that relies on the ionosphere

because the ionosphere will not remain constant in its level of ionization for a long time. When


7/19

Merola, Leandra

7

the ionization does change we should be aware of the fact that radio waves will also change

because the ionization level in the atmosphere impacts them.

Presently, there has been a very low level of SID activity in the ionosphere because the

sun is relatively calm. If the characteristics of the ionosphere change because solar activity

increases then VLF radio waves will no longer work the way they were supposed to. Therefore

today VLF waves work fine; however, if there is a solar flare tomorrow then the VLF radio

waves will work in unexpected ways. Although there is a known 11-year cycle for solar activity

corresponding to the suns magnetic field reversal, it is not possible to predict, with a high level

of accuracy, when individual solar flares will occur. This makes us vulnerable to potential

unexpected negative effects on important technological tools that rely on the ionosphere and the

level of ionization. Currently the sun is at its lowest point of the 11-year cycle so there are not a

great number of solar flares, or SIDs. However, it is predicted that in the next five or six years

there will be a 30 to 50 percent increase in solar activity compared to the last solar maximum

(Leary, 2006). It is important to take advantage of the present lull in the solar activity to explore

the suns baseline effect on the ionosphere and VLF waves when it is not highly active. If we

can understand what is happening in the ionosphere when there is no solar activity it could be

easier to solve the issues of the accuracy of technology so that we will be prepared when the

solar activity is more frequent. For example, more than ever we are using GPS or global

positioning systems, however, one study showed that disturbances in the ionosphere cause GPS

satellite signals to fade (Peter, 2003). Aguire, (2005) confirms that more and more scientists and

amateurs from around the world are recording SIDs and noticing how they affect radio

communications. In the near future, because of solar disturbances, there will be a decrease in the

accuracy of technology that is dependant on radio waves that are reflected by the ionosphere.


8/19

Merola, Leandra

8

For this project a SID monitor was used in a suburban area on Long Island, New York to

record VLF radio waves from a US Navy transmitter in North Dakota and track changes in the

ionosphere. Data was collected and compared to data recorded in Boston, Massachusetts and

Logan, Utah by the same type of device monitoring the same transmitter. Logan is south of the

transmitter, but at nearly the same latitude as Long Island. I have examined the variation of VLF

intensity at my receiver nearly every day since April 2006. The signal varies in a characteristic

way at each station at sunrise and sunset due to variations in the ionosphere. In particular,

variations at sunrise and sunset are not mirror images of each other. A typical signal over 24

hours is shown in Fig 4, in which the characteristic events during sunrise and sunset are

identified. I tabulated these times during sunrise and sunset at the location of my receiver and

compared them to the times of sunrise and sunset at the transmitter and to the times of the sunrise

and sunset spikes and dips. This data is shown in Figures 5 and Table 1. By studying what is

happening in the ionosphere at a specific moment, the conclusion is that the ionosphere is

affected by the sun and specifically by the angle of the sun in relationship to a certain location.

The sun similarly affects the VLF radio waves that are reflected by the ionosphere.

Procedure

A Sudden Ionospheric Disturbance (SID) monitor was obtained from Stanford

Universitys Solar Center (http://solar-center.stanford.edu/SID/). The monitor was made

available to educators, high schools and research institutions. The programs goals are to

distribute SID monitors to people around the world, have them track SIDs and share the data

collected through their website.


9/19

Merola, Leandra

9

Monitor

The particular monitor used, which is connected to an antenna which I made, records

VLF radio waves transmitted from a military radio station in LaMoure, North Dakota (Latitude

46.37N, Longitude 98.33). The monitor was programmed by Stanford University, to receive

radio waves with a wavelength of 25.2 kHz, which LaMoure transmits.

The device was connected to a computer in Rockville Centre where it runs continuously,

measuring the signal strength every 5 seconds. The setup was located in Rockville Centre, New

York (Latitude 40.67N, Longitude 73.64W). The VLF radio waves were recorded by the

computer, which transferred the raw analog data into digital values. The program measures the

radio waves strength in voltage and takes a point of data every five seconds.

Antenna Construction (Figure 3, A-C)

The monitor was connected to an antenna, which I altered from the original plans

included with the kit from the Stanford Solar Center. I used a square structure for the base og the

Figure 3: Construction of Antenna

A B C


10/19

Merola, Leandra

10

antenna that I built which was sturdier than the one recommended by Stanford University. The

base of the antenna was constructed of PVC pipe (diameter = ) (Fig 3, A). Eight, 4 pieces of

PVC were connected into a square using elbows and Ts. I then used 8 pieces that were 4 932 "

long to divide the square into quadrants using Ts and a cross joint. Two inch pieces were used

to build vertically and 4 932 " pieces to build parallel to the ground. This raised the base and

added another dimension. To make the base sturdier I added another level by building up with 6

pieces and across with 4 932 " pieces. To attach the two appendages I used a 2 piece of PVC.

I then attached the antenna, which was made in the shape of an X, of 1x 2 pine attached to

the base using another piece of PVC 25 long (Fig 3, C). After the frame of the antenna was

built, with help from my classmates, I wrapped 24-gauge wire around the mast 50 times (Fig 3,

B). Once the coil antenna was built I placed it outside and positioned it so that the plane of the

wire was perpendicular to the expected transmission source (Fig 3, C). I connected the antenna

to the SID monitor and then calibrated the monitor.

Connection with the Computer

The monitor was attached to my computer using an available serial port. The computer

recorded the data that the monitor collected and stored the information in a file. The computer

recorded a data point every 5 seconds in 24 hour blocks.

Excel Graphs and comparing Data

The raw data was recorded and stored as voltage versus time values in a .csv formatted

file. I then convert the raw data into graphs, which were easier to use to make comparisons. To

change a csv (comma separated value) file, which is the format the program automatically uses,

into a excel file I clicked on the file and used Excels chart wizard. Once the data was

represented as a line graph (Fig 4), I looked for the phenomenon called the sunrise/sunset effect.


11/19

Merola, Leandra

11

Around sunrise and sunset the graphs have large spikes because of the sudden change in

ionization. The phenomenon is a good indicator of accurate data because it is a phenomenon that

has shown up on previous data from SID monitors. After I was sure I could see the

sunrise/sunset effect, I looked for SIDs, which are changes in the voltage over time on the

daytime part of my graph (Fig 4). If I had any sort of spike, like the one in Fig. 4, then I would

check it against solar flare activity using graphs from the GOES satellite, which can be found on

http://www.sec.noaa.gov/ftpdir/warehouse/2006/2006_plots/xray/, to see if there was a

correlation between a spike on the graph and the spikes on the graphs from the GOES satellite

(Fig 5). The GOES satellite orbits the Earth outside of the atmosphere and captures data from

the sun. The satellite records solar flare activity. This includes the time at which solar flares

occur, how long they last and the intensity of the solar flare. By comparing this information with

the graphs from the SID monitor, I was able to determine whether or not the SID is induced by a

solar flare. I also studied the sunrise/sunset effect to find out what was happening that was

different during sunrise and sunset. Finally, I used my graphs to compare them with other data

from scientists from two locations in the United States. I compared data with a scientist in Utah

and a professor in Boston, Massachusetts and I looked for any similarities and differences

between our graphs, especially those differences during sunrise and sunset.


12/19

Merola, Leandra

12

Results

The graphs from my SID monitor created a definite daily pattern. Each graph showed

that after local sunset there was an increase in the signal strength or spike (A) as seen in Fig. 4.

Figure 4: Sample Data from July 4th

Figure 5: Solar Flare Data from GOES Satellite

SID Project NML_S-0021 SSHS 2006-07-04

[email protected]

-5

-4

-3

-2

-1

0

1

2

3

4

5

7/4/2006

0:00

7/4/2006

0:40

7/4/2006

1:21

7/4/2006

2:02

7/4/2006

2:43

7/4/2006

3:24

7/4/2006

4:05

7/4/2006

4:46

7/4/2006

5:27

7/4/2006

6:08

7/4/2006

6:49

7/4/2006

7:30

7/4/2006

8:11

7/4/2006

8:52

7/4/2006

9:32

7/4/2006

10:13

7/4/2006

10:54

7/4/2006

11:35

7/4/2006

12:16

7/4/2006

12:57

7/4/2006

13:38

7/4/2006

14:19

7/4/2006

15:00

7/4/2006

15:41

7/4/2006

16:22

7/4/2006

17:03

7/4/2006

17:43

7/4/2006

18:24

7/4/2006

19:05

7/4/2006

19:46

7/4/2006

20:27

7/4/2006

21:08

7/4/2006

21:49

7/4/2006

22:30

7/4/2006

23:11

7/4/2006

23:52

Hours (UTC)

Sunset

Spike (A)

Sunrise Dip

(B)

Recovery (C)

SID (D)

Solar Flare


13/19

Merola, Leandra

13

During the nighttime the signal was high, but the signal strength also varied from night to night.

There was, however, a definite dip, or decrease in signal strength, before sunrise (B), which is

followed by a recovery (C) and a more subtle decrease in signal strength as seen in Fig. 4. After

local sunrise the graph leveled out and remained at a relative constant unless there was a SID

(D). These trends were repeated almost everyday and an example of an everyday graph can be

found in Fig. 4. Because there were so few SIDs I focused on sunrise and sunset and how they

affect the ionosphere.

To find patterns in my data, during sunrise and sunset, I took a week of data in July and

compared the local sunrise time, the sunrise time by the transmitter, the times of the sunrise

DateLocal sunrise

(UTC) NML sunrise First spike Recovery Local sunset NML sunset Spike

7/2/2006 10:27 11:44 8:59 9:52 1:29 3:29 2:59

7/3/2006 10:27 11:45 9:02 9:52 1:29 3:29 3:01

7/4/2006 10:28 11:46 9:07 9:44 1:29 3:28 2:59

7/5/2006 10:28 11:46 8:59 9:52 1:28 3:28 2:55

7/6/2006 10:29 11:47 8:57 9:54 1:28 3:28 2:53

7/7/2006 10:30 11:48 9:10 9:44 1:28 3:27 2:56

7/8/2006 10:30 11:49 9:08 9:55 1:28 3:27 2:57

Average 10:28 11:46 9:03 9:50 1:28 3:28 2:57

Table 1: Sunrise and Sunset Times


14/19

Merola, Leandra

14

spike and recovery, the local sunset time, the sunset time by the transmitter and the time of the

sunset spike. By using Table 1 and Fig. 6 we can clearly see the order of the events. Fig. 6

shows that the sunrise spike on my graphs happen before the sun rises over the location of the

receiver or the transmitter. Fig. 6 also shows that the sunset spike occurs after sunset at the

location of the receiver and before sunset at the location of the transmitter. This means that the

ionosphere is ionized before the sun rises and after the sun sets because there is a change in

signal strength which means that there is a change in the level of ionization.

Once I knew what types of patterns my graphs formed, I compared my own graphs with

graphs from other receivers using the same type of transmitter. I found that my graphs were

Figure 6: Sunrise and Sunset Effect Graph

Sunrise and Sunset Effect

0:00

2:24

4:48

7:12

9:36

12:00

14:24

0 1 2 3 4 5 6 7 8 9

Days

Time(UTC)

NML sunrise

Local sunrise

Recovery

Sunrise spike

NML sunset

Sunset spike

Local sunset

Deleted:


15/19

Merola, Leandra

15

similar to those graphs created from receivers at a similar longitude. On the other hand, my

graphs were different from those graphs created from receivers with a very different longitude.

These similarities and differences can be seen in Fig. 6.

Stanford University has hosted this project along with many other projects using SID monitors

(Fig 7). In order to look at data more carefully, Stanford set up a website where people with SID

monitors can share their data. This graph, from the Stanford University website, shows how

similar the Rockville Centre and the Boston graphs are. They have similar sunrise and sunset

spikes and the daytime values are almost exactly the same. The Logan graph is different because

there is a decrease in the signal strength before sunset and the sunrise spike and recovery are

larger and more spread out over a period of time.

Figure 7: Graph of Data from Rockville Centre, NY; Logan, UT; Boston MA for July 10th

2006

__ = Rockville

Centre

__ = Logan

__ = Boston

Local

Sunset

for

Rockville

Centre

and

Boston

Sunset Spike for

Rockville Centre

and Boston

Sunrise Dip for

Rockville Centre

and Boston

Rockville Centre

Lo an

Local Sunrise forRockville Centre

and Boston

Boston

Deleted:


16/19

Merola, Leandra

16

Discussion

The goal of this project was to use VLF radio waves as a tool to monitor the ionosphere

and to observe how radiation from the sun affects the strength of VLF radio waves. Because

there was not a lot of solar activity in the form of solar flares, I concentrated my analysis on how

sunrise and sunset affect the ionosphere and VLF radio waves that are reflected by the

ionosphere.

By graphing the sunrise and sunset times and the times of the various peaks on a graph, I

was able to determine the order of events that affect the ionosphere and VLF radio waves. I

found that the sun rises in the ionosphere first. This causes the ionosphere above the receiver

and transmitter to be ionized before the sun even rises at the location of the receiver, assuming

that, like my set up, the receiver is east of the transmitter. When radiation from the sun comes

around the Earth, which acts like a shield during the night, the radiation again ionizes the

ionosphere. This sudden ionization causes the signal strength of the VLF radio waves to

decrease in a short period of time. As seen in Fig. 7 when the sun first starts to ionize the part of

Earth

SunIonosphere

Figure 7: Diagram of the Suns Radiation and the Ionosphere

Not Drawn to Scale

Rockville Centre

Transmitter

Logan


17/19

Merola, Leandra

17

the ionosphere observation, it is from the underside of the atmosphere. This was an interesting

find because for the most part the ionosphere is always ionized from above. With this piece of

information I concluded that the second sunrise spike or the recovery is due to the fact that the

suns rays of radiation have moved past the ionosphere directly above the transmitter and

receiver and the sun is slowly starting to ionize the ionosphere from above. Once the sun is

ionizing the ionosphere from above, the curve on the graph is relatively constant due to the

constant bombardment of radiation from the sun.

The sunset effect can be similarly explained. The sun sets at the location of the receiver

first. The Earth then blocks the rays of radiation as the sun sets on the ionosphere. As soon as

the radiation cannot reach the ionosphere, the signal strength increases drastically in a short

period of time. The sun then sets at the location of the transmitter because the transmitter is west

of the receiver.

When comparing my data with other data from across the United States, I noticed that

while all the graphs were similar in shape they had some differences. One major cause of these

differences could be the calibration of the individual monitor. When setting up a monitor you

must calibrate it for +- 5V and even the slightest difference in calibration can cause differences

in the results. For example, in Fig. 6, the Logan graph has a much higher daytime reading. This

could be due to differences in calibration or the variation in local wave strength because Logan is

west of the transmitter and the wave propagation from east to west could be different from the

west to east observations. This difference in signal strength, while noticeable, is unimportant

because it is the change in signal strength, not the actual reading, that matters.

I also noticed, when comparing the graphs, that latitudinal differences do not affect the

results as much as longitudinal differences do. Boston and Rockville Centre have similar


18/19

Merola, Leandra

18

longitudes but different latitudes and the graphs are very similar, in fact the sunrise and sunset

spikes shown in Fig. 6 match up almost perfectly. However, because Rockville Centre and

Logan have very different longitudes, even though their latitudes are similar, their graphs are

very different. For example, Fig. 6 shows that the Logan graph has an entirely different sunset

spike and a delayed sunrise spike. This is because Logan is west of the transmitter and the

sunset effect is the same as the sunrise effect except backwards. In Rockville Centre and Boston

the sun sets over the receiver and then in the ionosphere and finally at the transmitter. However,

in Logan, the sun sets at the transmitter first. The sun then ionizes the ionosphere from the

underside one last time, which causes the signal strength to decrease. The sun then sets in the

ionosphere over Logan and causes the signal strength to increase rapidly and finally the sun sets

in Logan. Therefore, longitudinal differences play a major role in the differences of the VLF

radio wave strength, especially because of the relationship to the transmitter. After analyzing

data from these multiple sources, I can conclude that the angle of the sun has an effect on VLF

radio waves strength and the ionosphere.

My project has shown how many of the interesting characteristics of VLF radio waves

depend on the behaviors of the ionosphere, which rely on the suns radiation. However, there is

still a lot out there that is not known about VLF radio waves and their relationship with the

ionosphere. As further research it would be interesting to study why the signal strength at night

is not constant. It would also be interesting to study how solar flares affect VLF radio waves in

different locations around the world, especially since the solar activity will soon be increasing. I

hope that his project can be the jumping off point for other fascinating studies about radio wave

technology and their relationship with the ionosphere and the sun.


19/19

Merola, Leandra

19

References

(2006). Index of /ftpdir/warehouse/2006/2006_plots/xray . Retrieved November 13, 2006, from

GOES Solar Flare Graphs Web site:

http://www.sec.noaa.gov/ftpdir/warehouse/2006/2006_plots/xray/

About the Ionosphere, 21 Jan. 2003. HAARP. (Nov 22, 2005) URL:

http://www.haarp.alaska.edu/haarp/ion1.html

Aguirre, E. L. (2005 May). Amateurs detect effects on earth. Sky and Telescope, 109,

Retrieved August 8 2006, from http://proquest.umi.com/pqdweb

Contreira, D., Rodrigues F. S., Makita K., and Brum, C. G. (2004). An experiment to

study solar flare effects on radio-communication signals.Advances in Space Research,36(12), 2455-2459.

Davies, Kenneth (1990).Ionospheric Radio. London: Peter Peregrinus Ltd.

Khanal, S. (2004). Retrieved October 17, 2006, from The Effect of Solar Flares on the

VLF Radio Waves Transmitted in the Ionosphere Web site:

http://solar-center.stanford.edu/SID/docs/effect-of-flares.ppt

Leary, W. E. (2006, March 6). Scientists say next solar cycle will be stronger but

delayed.New York Times.

Peter N., (2003, March 3). In Iraq, solar storms play havoc with communication.Christian Science Monitor, p. 15.

Radio Waves. (2006). In Wikipedia [Web]. Wikipedia Foundation, Inc.. Retrieved Oct.

14, 2006, from http://en.wikipedia.org/wiki/Radio_spectrum

Space Weather Monitors Stanford SOLAR Center. (2006). Retrieved December 5, 2005,

from http://solar-center.stanford.edu/SID/

LeandraMerola

Documents