2015-02-20: Review of Hargreaves [1969]: Auroral Absorption of HF Radio Waves in the Ionosphere

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

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

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]

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/

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/

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

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


Np = Neper


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!

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

Finally …

WHAT IS A RIOMETER?

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)

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

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)

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

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

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

* 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

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!

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

(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

(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

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!

Science

2015-02-20: Review of Hargreaves [1969]: Auroral Absorption of HF Radio Waves in the Ionosphere