A review of the first decade of riometry with an eye towards modern implementations (e.g., the IRIS riometer and the Automated Geophysical Observatories in Antarctica).

1. Hargreeves [1969]: Part 1
A Review of Results from the
First Decade of Riometry
Auroral Absorption of HF Radio
Waves in the Ionosphere:
Kevin
Urban,
NJIT,
2015-­‐Feb-­‐20
Riometer
Paper
Reviews,
Spring
2015

2. MoAvaAon:
What
is
a
Riometer
used
to
study?
Riometer_RF
=
20-­‐50
MHz
à
Riometer_λ
=
6
–
15
m
Directly:
Short-­‐:me
varia:ons
in
cosmic
radio
noise
intensity
Indirectly:
Ionospheric
electron
density,
conduc:vity
Indirectly:
Par:cle
precipita:on
Causes
of
this
cosmic
noise
varia:on
differ
at
equatorial
(diurnal
solar-­‐control)
and
polar
la:tudes
(geomagne:c
and
solar
events)
*
With
a
riometer,
we
measure
the
absorp:on
of
cosmic
radio
waves,
but
we
do
so
as
a
means
to
infer
ionospheric
parameters,
such
as
electron
density
*
In
other
words:
absorp:on
is
a
quan:ta:ve
observable
(it
can
be
measured!)
that
can
then
be
used
to
deduce
informa:on
about
other
physical
parameters
of
interest
that
might
not
be
directly
or
easily
measurable,
at
least
from
the
ground

3. Riometers
at
High
LaAtudes
Picture
it:
You’re
on
a
navy
vessel
in
the
1930s,
rounding
the
:p
of
Antarc:ca;
you’re
closing
in
on
the
enemy
and
-­‐-­‐-­‐
suddenly,
your
communica:ons
are
wiped
out.
Two
major
types
of
ionospheric
radio
wave
absorp:on
events:
1. Auroral
Absorp:on
[AA]
2. Polar
Cap
Absorp:on
[PCA]
Both
can
produce
over
10
dB
of
absorp:on
on
a
30MHz
riometer,
so
before
satellite
era
the
categories
were
dis:nguished
by
:me
and
geographic
signatures.
*
Time:
PCAs
lasted
several
days
while
AA
was
reserved
for
rela:vely
frequent,
shorter-­‐lived
and
irregular
absorp:on
events.
*
Geography:
PCA
covers
the
en:re
polar
cap,
while
AA
is
limited
to
auroral
zone.
A
recommended
3rd
category
by
Hargreaves
and
others
circa
1969:
3. Sudden
Commencement
Absorp:on
[SCA]

4. Auroral
AbsorpAon
*
Most
frequent
and
most
complex
high-­‐la:tude
type
of
absorp:on
event
Sporadic
and
non-­‐obvious:
grows
and
decays
with
auroral
and
magne:c
ac:vity,
yet
does
so
without
any
exact
correspondence
*
First
iden:fied
in
Appleton
et
al
[1933]:
“Ionospheric
inves:ga:ons
in
high
la:tudes”
They
no:ced
that
reflected
radio
waves
were
weakened
or
wiped
out
during
periods
of
auroral
and
magne:c
ac:vity
*
Produced
by
the
entry
of
auroral
electrons
Appleton
[1933]
inferred
the
cause
to
be
"ionizing
charged
par:cles
[which]
produce
electrifica:on
below
the
normal
lower
region
[i.e.,
the
E
region].”
hfp://spears.lancs.ac.uk/data/summary/interpret/

5. Polar
Cap
AbsorpAon
*
Due
to
abnormal
ioniza:on
produced
by
the
incidence
of
solar
protons
ajer
an
intense
solar
flare.
*
Hargreeves
[1969]
says
PCA
was
prefy
well
understood
(solar
energe:c
protons
from
solar
flares),
unlike
AA
hfp://www-­‐istp.gsfc.nasa.gov/istp/outreach/workshop/img/nicky/slide14.jpg
hfp://spears.lancs.ac.uk/data/summary/interpret/

6. To
understand
riometers,
one
must
understand
the
physics
they
purport
to
study!
Radio
Wave
PropagaAon
in
the
Ionosphere
Higher
ioniza:on
rate
in
atmosphere
à
higher
electron
density
à
more
electrons
to
steal
energy
from
radio
waves
à
less
radio
wave
energy
at
riometer
site
30
MHz

7. Radio
Wave
PropagaAon:
Appleton-­‐Hartree
Quasi-­‐Longitudinal
ApproximaAon

8. Radio
Wave
PropagaAon:
Appleton-­‐Hartree
Quasi-­‐
Longitudinal
ApproximaAon
Np
=
Neper

9. Radio
Wave
PropagaAon:
Appleton-­‐Hartree
Quasi-­‐Longitudinal
ApproximaAon
If
one
sets
µ=1
for
the
lower
ionosphere,
one
can
compute
the
"total
absorp:on"
over
a
path
for
both
E-­‐mode
(-­‐)
and
O-­‐mode
radio
waves
(+):
IntuiAve.
Easily
interpreted.
For
the
general
case:
Overly
simplisAc!

10. Radio
Wave
PropagaAon:
Sen-­‐Wyller
FormulaAon
At
low
al:tudes,
where
ν≫ω,
the
generalized
Sen-­‐Wyller
formula
recovers
the
Appleton-­‐
Hartree
approxima:on
by
sesng
At
high
al:tudes,
where
ν≪ω,
the
Appleton-­‐
Hartree
formula
is
recovered
from
the
generalized
(Sen-­‐Wyller)
formula
by
sesng

11. Finally …
WHAT IS A
RIOMETER?

12. What
advantage
did
the
riometer
have
over
other
popular
techniques
at
the
Ame?
Riometer
• Before
the
riometer,
auroral
absorp:on
was
studied
mainly
by
radio
reflec:on
methods:
(i)
pulse-­‐amplitude
methods
(ii)
polar
communica:on
circuit
monitoring
(iii)
“blackout”
records
from
ionosondes
• These
methods
were
too
sensi:ve:
the
amount
of
absorp:on
at
high
la:tudes
leads
to
“blackouts”
all
too
readily
-­‐-­‐-­‐
blackouts
are
essen:ally
instrument
satura:on,
so
measurements
ceased
to
be
quan:ta:ve
More
popular
circa
1969
for
absorp:on
studies
were:
(i)
the
cosmic-­‐noise
method
(ii)
the
riometer
technique
(a
type
of
cosmic-­‐noise
method)

13. 1.
The
apparent
intensity
of
the
cosmic
radio
emission
is
monitored
con:nuously
on
a
stable
receiver.
2.
The
gala:c
radio
flux
is
contant
over
long
periods
of
:me,
so
presumably
any
changes
in
the
apparent
intensity
from
one
day
to
the
next
at
the
same
sidereal
:me
represent
corresponding
varia:ons
of
ionospheric
absorp:on.
3.
Since
this
method
depends
on
wave
propaga:on
through
the
ionosphere,
the
frequency
must
be
comfortably
above
f0F2.
In
the
mid-­‐la:tudes,
the
amount
of
absorp:on
at
these
frequencies
is
small
and
varies
slowly
throughout
the
day
(it
is
"solar
controlled");
given
that
there
ojen
exists
``receiver
drij,''
it
is
fairly
tough
to
parse
out
what
the
cosmic-­‐noise
intensity
is,
versus
the
drij,
versus
ionospheric
absorp:on.
At
high
la:tudes,
however,
this
is
not
the
case:
the
absorp:on
is
strong
and
structured.
This
allows
one
to
determine
the
background
level
(ojen
called
the
"quiet-­‐day
curve").
Using
a
regular
receiver
and
frequent
calibra:ons,
researchers
were
able
to
use
this
technique…however,
this
technique
became
extremely
powerful
when
the
riometer
was
developed.
Pre-­‐Cursor
to
the
Riometer:
the
Cosmic
Noise
Method

14. What
is
a
Riometer?
Rela:ve
Ionospheric
Opac:city
Meter
1. The
riometer
achieves
high
gain
stability
by
switching
rapidly
between
the
antenna
and
a
local
noise
source.
2. The
local
noise
source
is
con:nuously
adjusted
so
that
its
power
output
equals
that
received
by
the
antenna.
3. Thus
the
receiver
acts
as
a
sensi:ve
null
indicator,
in
which
gain
varia:ons
are
unimportant.
4. Ul:mately,
a
recording
is
made
of
the
current
through
the
noise
source,
the
current
being
linearly
related
to
the
power
output.
See:
Block
Diagram
(Fig.1)

15. Yagi
Antennas
Yagis
are
those
antennas
you
see
on
roojops
that
get
people
their
TV
channels…
The
crazier
ones
on
roojops
are
log-­‐periodic
antennas…
Never
seen
an
antenna
on
a
roojop,
you
say?
You
callin’
me
old?!
Riometer
antennas
were
ojen
of
simple
design,
e.g.,
a
Yagi
or
a
simple
broadside
array
over
a
ground
plane.
PRO:
At
typical
frequencies
of
~30MHz,
these
antennas
are
conveniently
small
CON:
they
have
rather
broad
beamwidths
(~
+/-­‐
30*
between
half-­‐power
points)
3-­‐element
Yagi
4-­‐element
Yagi
Riometer
Design
Circa
1969
Log-­‐Periodic
Antenna
Broadside
Array
Antenna

16. Improvements
upon
the
classical
riometer
technique
circa
1969
1.
Narrow
beam
antenna
systems
vs
broadbeam
When
a
broadbeam
antenna
is
used,
the
noise
power
is
from
a
large
area
of
the
sky,
and
so
if
an
absorp:on
event
occurs,
one
can
only
say
“it
happened
somewhere
in
this
huge
region
of
the
sky.”
So
Just
around
this
:me,
some
larger
antenna
arrays
were
being
used
to
try
to
study
the
finer
structure
in
absorp:on:
For
example:
Ansari
[1965]
used
a
36MHz,
narrow-­‐beam
(7*
beamwidth,
symmetrical)
antenna
system
comprised
of
a
6x8
(Mag
EW
x
Mag
NS)
array
of
3-­‐element
Yagi
antennas
to
study
absorp:on
in
two
direc:ons
(6*MS
and
6*MN
from
the
site
zenith).
Such
a
system
allowed
them
to
make
ini:al
es:mates
of
the
absorp:on
distribu:on
across
the
sky.
Prior
to,
most
narrow-­‐beam
antennas
were
narrow
only
in
the
magne:c
NS
plane,
and
fairly
broad
in
the
magne:c
EW
plane.
To
measure
auroral-­‐ionospheric
absorp:on
in
the
two
chosen
direc:ons,
they
swung
the
main
beam
of
the
array
every
10
seconds.
Their
primary
goal
was
to
study
thin
auroral
arcs.
Ansari,
1965:
A
Narrow-­‐Beam
Antenna
Array
for
Radio
Wave
Absorp:on
Studies
in
the
Auroral
Zone
Riometer
Design
Circa
1969:
Room
for
Improvement

17. Why
narrow
beams
are
becer
*
For
an
antenna
w/
finite
beamwidth,
i.e.
for
any
antenna
whose
beamwidth
is
not
a
3D
dirac
impulse,
i.e.,
for
any
antenna
in
real
life,
the
measured
absorp:on
is
called
the
“apparent
absorpAon”
*
Due
to
oblique
waves,
the
apparent
absorp:on
is
greater
than
the
value
that
we
actually
want,
which
is
called
the
“zenithal
absorpAon”
-­‐-­‐
that
is,
we
want
to
determine
the
absorp:on
of
a
plane
wave
passing
ver:cally
through
a
horizontally-­‐
stra:fied
absorp:on
region

18. *
To
compute
the
zenithal
absorp:on,
some
assump:ons
must
be
made,
and
a
correc:on
must
be
computed
and
applied
to
t h e
m e a s u r e d
( a p p a r e n t )
absorp:on
*
The
typical
assump:ons
(at
least
circa
1969)
are:
(i)
if
spa:ally-­‐distributed
observa:ons
are
NOT
available,
then
the
absorp:on
layer
is
assumed
horizontally
uniform
(ii)
if
spa:ally-­‐distributed
observa:ons
are
available,
then
it
possible
to
take
large-­‐scale
horizontal
gradients
into
account
Fig.
3:
NORMALIZATION
FACTORS:
ZENITHAL
ANTENNA
Curves
for
correc:ng
apparent
absorp:on
to
zenithal
absorp:on.
These
were
computed
for
an
antenna
pointed
ver:cally
and
having
beamwidth
+/-­‐
32
to
half-­‐power
points
in
both
planes.
CompuAng
the
Zenithal
AbsorpAon

19. Why
narrow
beams
are
becer:
Conclusion
Why
narrow
beams
are
worse
A
broad-­‐beam
zenithal
absorp:on
:me
series
represents
the
actual
zenithal
absorp:on
very
poorly
in
that
the
broadbeam
includes
events
from
a
wide
patch
of
the
sky!
A
narrow-­‐beam
zenithal
absorp:on
:me
series
represents
the
actual
zenithal
absorp:on
much
befer,
however
you
only
know
about
a
fairly
small
patch
of
the
sky!

20. AddiAonal
improvements
upon
the
classical
riometer
technique
(e.g.,
that
used
in
late
1950s,
early
1960s)
circa
1969:
1. Groups
of
closely-­‐spaced
riometer
sites
2. Groups
of
closely-­‐spaced
riometers
at
one
site
3. Use
of
mulAple
frequencies
Closely-­‐spaced
riometers
at
one
site
greatly
eases
logis:cs.
The
small
setback
is
one
needs
to
know
the
height
of
the
absorp:on
before
horizontal
separa:on
can
be
es:mated.
When
absorp:on
is
to
be
measured
on
mul:ple
frequencies,
Hargreaves
recommends
allosng
one
riometer
per
frequency,
making
sure
to
scale
the
antennas
so
that
each
one
has
the
same
beam
pafern.
(Swept-­‐frequency
and
stepped-­‐frequency
riometers
proved
to
not
be
very
successful
riometer
designs.)
Riometer
Design
Circa
1969:
Room
for
Improvement
SPA
MCM

21. (1)
Automated,
unmanned
riometer
staAons:
“Riometers
which
can
operate
una2ended
for
long
periods
of
7me
at
deserted
sites
without
mains
power
are
needed
but
have
not
yet
been
developed
as
far
as
the
author
is
aware.”
Riometers
Circa
1969:
Further
Goals
38.2
MHz
imaging
riometers
are
housed
at
several
Automated
Geophysical
Observatories
[AGOs]
and
at
SPA
and
MCM.
For
more
info:
(1) hfp://www.sienageospace.dreamhosters.com/
(2) hfp://www.polar.umd.edu/instruments.html)
Mission
Complete!
SPA
MCM

22. (2)
Becer
data
products:
There
existed
a
need
to
“simplify
the
data
processing
by
which
the
nega:ve
deflec:on
on
a
chart
that
is
nonlinear
in
decibels
(see
Fig.
2)
is
converted
to
a
linear
scale
of
decibels…a
means
of
removing
the
quiet-­‐day
curve
at
the
instrument
and
of
producing
on
the
spot
a
record
[that
is]
linear
in
absorp:on
against
:me
would
be
[AWESOME!]”
Riometers
Circa
1969:
Goals
Two
methods
had
already
been
put
forward
circa
1969:
(i)
Con:nuous
es:mates
of
quiet-­‐day
curve
given
la:tude
and
eleva:on
of
the
antenna
beam
(ii)
Es:mates
of
the
quiet-­‐day
curve
by
comparing
O-­‐
and
E-­‐modes
of
the
received
signal
[2]
[1]
Chivers
and
Prescof,
1967:
Applica:ons
of
a
new
technique
for
the
detec:on
of
absorp:on
events
using
a
riometer
[2]
Benediktov,
1959:
On
a
radioastronomical
method
for
determina:on
of
the
absorp:on
of
radio
waves
in
the
ionosphere

23. In
the
future:
Forget
single-­‐beam
or
single-­‐
frequency
riometers.
Check
out
this
sweet
riometer
(a
la
Detrick,
Rosenberg,
Weatherwax,
Lutz)
hfp://www.polar.umd.edu/haarp/riometer_paper/haarp.html
The
IRIS
Riometer!