Space weather refers to the "weather" in
space. These short term variations of the near-Earth environment
are due to changes in solar activity on all time scales from the
11-year solar cycle (more like space climate), to individaul
storms, solar flares, coronal mass ejections (CMEs), geomagnetic
storms, etc. that can occur on time scales of minutes to days.
When the sun is active, the Earth can experience increases in both
electromagnetic radiation from the sun, as well as increases in
particles (electrons, protons) arriving at the Earth. This
increase in radiation can impact the Earth's magnetosphere, ionosphere
and atmosphere, and the entire near-Earth environment.
These solar storms and space weather can have major implications on our
everyday lives. Below we can see some of the areas impacted by
space weather, from satellites and GPS navigation systems, to power
outages, pipeline corrosion, and communication blackouts. With
the increased number of space technologies used today in our lives
(from cell phones, GPS, TV, internet, etc.) space storms can result in
tremendous damage in a very short time. If we are aware of these
storms ahead of time (similar to regular weather on Earth) we could
better prepare and take precautions before it is too late. Hence,
a new field is developing to try and better understand and predict
these solar storms, and their impacts on the Earth's systems.
One of the major impacts of solar
variability is on the Earth's ionosphere and magnetosphere. On
the ground we can study these interactions and changes using our
sensors in the ULF,
frequncy range. Below we can see two examples of space
weather interactions with the Earth, detected with our ULF sensors in
Israel. The first (left) is related to the changes in solar
radiation impacting the ionosphere during day and night. The left
image shows the long term ULF amplitude measurments (0.01Hz) in Israel
(y-axis) that show the "terminator effect" at sunrise and less at
sunset (x-axis is time of day). The Kp index of geomagnetic
activity is also shown in red. Note that due to the changing
length of the day during the year, we can a wavy pattern in the ULF
data around sunrise and sunset. The black curved line shows the
time of sunrise and sunset above our station throughout the year.
Another interesting phenomenon is what we know as the Ionospheric
Alfven Resonances (IAR) that can be seen as diagonal lines between
0.5-4 Hz during nighttime only, in the right hand plot below.
These resonant lines represent the "wiggling" of the magnetic field
lines that enter the Earth at our latitude. The sources of these
wiggling field lines is still uncertain. However, the frequency
of these resonance lines increases during the nighttime hours, and they
can tell us about the properties of the ionosphere and magnetosphere
from the ground.
VLF range we can also study the lower ionosphere by tracking the
background levels of atmospheric noise in this frequency band,
primarily from lightning. These radio waves are reflected off the
lower ionosphere, and therefore any changes in the lower ionosphere due
to solar storms or solar activity will be detected in our ground-based
observations of VLF radiation here in Israel. One interesting
recent finding was that the solar rotation rate of 27 days can be
nicely detected in our VLF measurements in Israel. The sun
rotates once every 27 Earth days, and since the sun's surface is
not uniform, there are small variations in the amount of energy leaving
the sun from different regions, arriving at the Earth with a
periodicity of 27 days. These small changes impact the
ionosphere, which impacts the propagation of VLF waves within the
Earth-ionosphere waveguide. Below we can see the variations in
our VLF measurements (representing the background noise from
lightning), and various solar indices that often show the 27-day
variability of the sun. Note that this was close to the recent
deep solar minimum, and therefore not all indices show clearly the
27-day changes. However, when all indices agree and are in phase,
there is a clear anti-correlation with the VLF noise
measurements. Furthermore, when often the solar indices show no
27-day signal, the VLF data still show clearly the solar variability at
27-days, implying that monitoring VLF measurements continuously may
supply more reliable information on the solar rotation, than the solar
indices themselves! The physical meaning of this anti-correlation
is that the 27-day solar rotation modulates the height of the lower
ionosphere (D-region) and the reflection height moves up and down every
27 days. As the reflection height moves down into the denser
atmosphere, more VLF waves from lightning are absorbed, and as the
reflection height rises, less absorption occurs. In this way we
receive at our station less or more background radiation from distance
storms. Our calcualtions show that the modulation is of the order
of 4-5 km in height of the reflection layer in the D-region of the
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L. Alperovich, M. Finkelstein and M. Merzer, 2008: ULF amplitude
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Reuveni, Y., and C.
2009: A new approach for monitoring the 27-day solar rotation
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Y., C. Price, E. Greenberg and A. Shuval, 2010: Atmospheric
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