Solar-Ionosphere Connection: Physics with the 20 MHz Antenna

INTRODUCTION & SUMMARY

Radio astronomy with a 20 MHz system is used as a probe for both solar physics and geophysics. Antenna signals originating from the sun (or Jupiter) are simultaneously received at several different locations and show similarity in gross features. In attempts to reconcile fine details, numerous resources are consulted (see previous web page: SOHO, GOES satellite data, GLOBE weather displays, planetarium software like Starry Night Backyard, etc.). Differences may be attributable to many things. Local weather conditions, distant lightning, antenna sensitivity and integration times, man-made interferences are just some of them. Often dismissed is the possible effect of a concurrent Jovian storm in the antenna beam while a solar burst is being recorded. It is argued that Jovian emissions are too weak compared to solar emissions. Typically the sun is observed during the day and Jupiter, at night, when visible. However, it will be shown below that in some cases, this may be a credible complication of a solar burst spectrum.

In addition to the more familiar temporal comparisons, there is another technique involving frequency analysis. In a typical application, a broadband antenna sweeps various frequencies and records their intensities (in false color or sound) as a function of time. Such an instrument is called a spectrograph. But in this work, digital signal processing techniques (FFT) extract the legacy of the radio wave's "flight." From the coronal out-reaches of the sun, the interplanetary plasma, the magnetosphere, the ionosphere, and even the troposphere, there are many opportunities for amplitude and phase modulation of the radio wave. The power spectrum reveals these modulations and something about the physics of the encounter. Preliminary studies show that a simple system may be able to detect twinkling of the radio wave as it passes through the turbulent regions of fluctuating charge particle density. Several types of twinkle are examined: the tropospheric, ionospheric and the interplanetary scintillation. Motivated by their importance in communication, navigation, and pulsar research, these scintillation studies are often relegated to more sophisticated systems working at much higher frequencies. Consequently, this is a fertile ground for new exploration.

Because of the simplicity of the systems used, all interested amateur radio astronomers can obtain measurements or access archived samples, and successfully analyze them with simple tools, within the limitations of receiver bandwidth, sound card characteristics, Radio SkyPipe Pro software, and Microsoft Excel.

The reader is refered to the "RA Web Tools" page on this web site for the data and analysis presented below (also, The Radio Jove Newsletter, Oct 2003, briefly introduces this work- http://radiojove.gsfc.nasa.gov/library/newsletters/2003Oct/).

Adapted from the NANCAY Decametric Array-Scientific Results
(http://www.obs-nancay.fr/html_an/a_scidam.htm#Soleil) and Radio Sky Publishing (http://www.radiosky.com/suncentral.html):

Type I - Continuum emission, plus a succession of bursts shorter than 1 second, associated with sunspots of strong magnetic field. They last from a few hours till up to a few days and represent the most common radio emission activity of the Sun. Their production mechanism has not yet been understood.

Type II - Shock waves caused by chromospheric eruptions, traveling from the solar corona into the interplanetary medium about 1000 km/s. They move slowly from high to low radio frequencies (order of 10 MHz/min or less); caused by plasma oscillations induced by the passage of the shock wave.

Type III - Most common type observed. Relativistic electrons beams (10,000 km/s) are ejected into the solar corona and into interplanetary space. They change quickly from high to low radio frequencies (order of 100 MHz/min). May exhibit harmonics. Often accompany the flash phase of large flares. Also ascribed to plasma waves caused by the passage of the electron beams.

Type IV - Flare-related broad-band continua. Synchrotron emission by electrons at the top of a magnetic loop in the corona, or in a plasma bubble moving at a speed of about 100 km/s. The only highly polarized radio bursts. They last from a few minutes to a few hours.

Type V - Attributed to high-energy electrons enclosed in coronal magnetic arcs. Broad-band continua which may appear with III bursts. Last 1 to 2 minutes, with duration increasing as frequency decreases.

Morphology Classification: radio continua (noise storms, type IV, and V bursts), fast (type III and RS), slow (type II) drift bursts and spectral fine structures (e.g. pulsations, fiber bursts, spikes, type I bursts).

TYPE III SOLAR BURST RECORDED AT FIVE LOCATIONS

Solar burst data from five 20 MHz antenna systems located around the country was captured and reported to the Radio Jove archive. Since Radio SkyPipe is a PC-only software, the wave files were converted to exportable text files so that the signals could be reconstructed in Microsoft Excel on Macintosh platform.

The five locations to be compared, the equipment set-up there, and their operators are listed below (proceeding westerly):

Orangeburg, SC 29115 33.29N 80.5W (-5); RJ 1.1; Dual Dipole (coutesy of Jim Brown)

Saginaw, MI, USA 43.4N, 84.0W (-5); RF-2001C+DEMI 144-28RX; 9-element M2 Yagi (courtesy of Thomas Schopp)

Lamy, NM, USA 35.29 N 105.53 W (-7); Sangean ATS-803 (AGC disabled); half-wave dipoles (courtesy of Tom Ashcraft)

Sula, MT, USA 45.9N 114.0W (-7); RJ 1.1; double vertical Moxon (courtesy of Dusty Samouce)

Oahu, Hawaii 21.43N 158.0W (-10); Icom 8500; 17-30 MHz Log Periodic (tuned to 20 MHz) (courtesy of Richard Flagg)

The solar burst captured at Sula, MT March 26, 2002 around 20:23Z is shown below:

But before these radio waves penetrated Earth's atmosphere, they were intercepted by the WIND spacecraft a million miles out with the WAVES instruments at lower radio frequencies (scale <1 to 13,800 KHz). Notice that the 14 MHz part of the spectrogram intersects the timeline at 20:24Z.

A Type III solar burst is confirmed by the steep frequency sweep in the spectrogram. These radio emissions, associated with gyrating electrons during a solar flare, may be coincident with or preceded by X-ray emission. The GOES 8 satellite data validates the flare-related emission (see far right of the X-ray flux figure):

Below is an incomplete list of general considerations to aid in assessing and identifying differences among stations receiving simultaneous 20 MHz signals. (The list may should be modified for other frequencies):

I Receivers
......A Design
......B Electronic Noise
......C Calibration

II Antennas
......A Frequency
......B Antenna Pattern
......C Physical Location
............c1 Local Obstructions
............c2 Earth's Curvature/Target's Perspective
......D Ground Capacitance
............d1 Distance Above Ground
............d2 Soil Type
......E Calibration

III Transmission Lines
......A Impedance Mismatch
......B Microphonic Cable/Wind Loading

IV Man-made Interferences
......A Power Lines
......B Cycling Electrical Equipment (motors)
......C Transmitters (Radio, TV, proposed digital phone lines, etc.)

V Natural Interferences and Phenomena
......A Atmosphere
............a1 Lightning (Tropospheric and Stratospheric)
............a2 Weather (Clouds, Precipitation)
............a3 Upper Level Turbulence (Tropospheric Scintillation)
......B Ionosphere
............b1 Classic Radio Twinkling (Ionospheric Scintillation)
......C Magnetopause
............c1 Shocks
............c2 Schumann Resonances (VLF Earth Cavity)
......D Solar Wind
............d1 Interplanetary Scintillation
......E Corona
............e1 Coronal Loop Oscillations
............e2 Plasma Instabilities
......F Photosphere/Flares/Prominences
............f1 Bunching/Stretching Magnetic Field Lines
......G Solar Cavity
............g1 Resonant "Acoustics" (Pulse Modes)
......H Other Radio Targets
............h1 Effect of Jupiter Storms on Solar Signals
............h2 Cassiopeia-A and Cygnus-A
............h3 Gamma Ray Bursters

Now, lets compare the five stations and observe the differences in the time traces. Reserve your attention to the left-hand figures for now. Note the gross features are comparable, but the fine details are different. Most notably are the presence or absence of spikes, the variation in smoothness of the signal floor, and the presence of more "structure" in some traces. (The heavy green lines are for alignment of the figures).

Since lightning spikes may be a consideration, the GLOBE Program was queried for the multi-satellite precipitation estimate ending at local solar noon for March 26, 2002. From the time zone, we know that 20:00-20:30Z corresponds to 3:00-3:30 PM EST in SC and MI, 1:00-1:30 PM MST in NM and MT, and 10:00-10:30 AM local standard Hawaiian time in Oahu. With a little knowledge of weather systems, the general conditions at the time of the measurements can be estimated. The correlation here is the heavy precipitation is probably associated with thunderstorms and therefore lightning.

Broadband radio emissions from lightning can travel for hundreds of miles. So, it should not be surprising to see their imprint on SC and MI in the vicinity of what appears like an advancing cold front. Conceivably, the activity was far more intense a few hours later when the solar burst data was recorded.

Similarly, there is some activity near Hawaii, but is very inconclusive since the radio data was taken a couple of hours earlier. There may have been more or less thunderstorm activity closer to Oahu. The single spike could be lightning related, but is indeterminate without more information.

Montana seems to be relatively clear. On the other hand, the apparent lack of lightning anywhere near New Mexico motivated a search for another cause of the spikes in the time trace.

The Jovian Daily Ephemeris (Imai) for March 26, 2002 predicts Jovian Io-A storms at 21 through 22Z (with non-Io-C storms following from 23-24Z). A cursory examination of some Jupiter data shows that the actual storm may occur an hour before the predicted time. Therefore, it is possible, though not independently confirmed, that a Jovian storm occurred during the recording of the solar burst at Lamy, NM at 20:23Z.

Another key issue is whether the Jovian storm could be seen with the same receiver sensitivity settings used for recording solar bursts. Popular opinion dismisses this option more out of presumption than of analysis. At least for "A" storms (whether Io related or not), the comparison of calibrated Jovian signals with calibrated Type III solar bursts show peak values in the same order of magnitude. For example, examine the Jovian March 28, 2003 data from Sula, MT collected between 0600 and 0700Z (compare with the prediction times) and the solar data collected later in the day (18:12-18:20Z) with the same calibrated equipment. One will note (1) the predictions can be an hour off, (2) calibrated spiked peaks are comparable in magnitude to calibrated solar burst peaks, (3) the spread of spikes is not inconsistent in what has been observed in the NM data presented in the paper, and (4) coicidence with three stations increases the likelihood of a common event. It is a reasonable speculation that some spikes may be attributable to Jupiter storms.

This now warrants a closer inspection of the alledged Jovian Io-A (L-burst emissions). Comparison of Jovian spikes with typical lightning spikes reveal differences in the pulse shape and width.

Radio bursts from some electrical interference, like switching transients, will have a rectangular waveform with very serious ripple.

Radio bursts from lightning have a more complex shape. This is anticipated in cloud-to-ground when the main leader splits into several branches. Each plasma branch capable of producing radio emission. The predominant radio is ELF and VLF. However, HF, like 20 MHz, comes from the smaller branches and twigs.

Jovian bursts seem to be decidedly Gaussian, often negatively cusped (one or both sides).

The peaks attributable to Jovian storms seem to be deformed by superimposed solar emissions. Nonetheless, the Lamy, NM spikes hypothesized to be Jovian in origin, seem to be well supported.

A statistical analysis of pulse shape and width is necessary to quantify the probability that suspect spike in a solar burst time series is indeed Jovian (given the possibility of a concurrent Jovian storm).

FREQUENCY DOMAIN ANALYSIS

As mentioned in the Introduction, a radio wave can be modulated by the fluctuating medium it propagates through. Digital signal processing of the time series record is for now kept as simple as possible. A text file of the antenna signal trace (time, signal strength) is imported into Microsoft Excel. Here, it is recreated and processed with the FFT available under the Data Analysis Tools. The magnitude and phase spectra are easily generated. There are two treatments of the data which yield different information.

The first involves the Power Spectrum (square of the magnitude function vs. frequency). It is here that discrete resonances, combs, and bands are exposed. The comparative spectra above clearly show this.

The second examines the same data in a log-log format. This approach takes advantage of looking at the bigger picture. The galactic radio background is subject to condition of the intervening media. The 20 MHz radio telescope used in these studies is very much like a very crude Riometer (Relative Ionospheric Opacity meter). Normally, this sensitive instrument (typically 30 MHz) is examined for just its temporal output. Since our telescopes are relatively insensitive, a time series analysis will not reveal much about the ionosphere, but a frequency domain analysis, especially over a particular range, will reveal some very interesting features interpretable in terms of radio twinkling.

As noted above, but repeated with more detail here, familiar digital signal processing techniques are applied in a somewhat different way here for amateur radio astronomy. Radio waves will be amplitude modulated in the course of their "radio flight". Before reaching the antennas, radio waves may be affected by the coronal out-reaches of the sun, the interplanetary plasma, the magnetosphere, the ionosphere, the stratosphere, and even the troposphere. The only requirement is the medium imposes an oscillatory modulation. The antenna signal is digitally processed with a Fast Fourier Transform (FFT), which should reveal this modulation and possibly elucidate on the underlying physics.

For example, radio light can twinkle (scintillate) by the same fundamental mechanism that causes starlight to optically twinkle- a fluctuation in the refractive index. The familiar reason for twinkling stars is that light from a star, considered a point light source, is "randomly" refracted by the turbulent upper atmosphere. The refractive index depends on air density. A radio wave may similarly experience refraction in the upper neutral atmosphere by the same turbulent process. However, it can also experience both refraction and diffraction by a fluctuating electron number density in the interplanetary medium and in the ionosphere. The solar wind and solar storms often cause this.

Example time traces and the corresponding power spectra are shown above. When the power spectrum of a time series of the antenna signal is examined as a log-log plot, the slope(s), after the roll-off (between 0.05 and 1 Hz), will betray any radio scintillation present (note 1). For a normal radio sky background (note 2), this slope, called the spectral slope p, is around -0.6. Radio scintillation caused by ionospheric scintillation will have typical p of -2.6 and those due to plasma bubbles will have p values from -2 to -8, with an average of -4 (notes 1 and 3). Other scintillations will have a decided negative slope as well (see discussion below).

The March 26, 2002 data set (note 4) from SC, MI, NM, MT, and HI was analyzed with encouraging results displaying some kind of radio twinkling. Below are the log-log power spectra these signals as well as for receiver noise (antenna disconnected). The sky background during a solar burst is probed between 0.1 and 1 Hz.

LOCATION	SLOPE OF FIRST SEGMENT	SLOPE OF SECOND SEGMENT	CORNER FREQUENCY, mHz	IONOSPHERE
Receiver Noise	0	-	-	No bias
SC	0	-0.59	224	Normal radio sky
MI	0	-1.05	363	Off-normal radio sky
NM	-1.25	0	227	Weak scintillation
MT	-2.48	0	413	Classic scintillation
HI	-4.67	-0.98	130	Strong scintillation

Note that daytime scintillation seems to have been observed 20 MHz. However, ionospheric scintillation is generally expected only at night and mostly between sunset and midnight, and then only significantly in the polar and equatorial regions.

Most research on ionospheric scintillation has been limited to the much higher frequencies of communication and GPS satellites. However, one ionosphere scintillation modeling and prediction expert voiced the possibility of exploring some new physics at the 20 MHz regime (see note 6).

On the other hand, irregularities in the solar wind plasma region, also can produce "twinkling". It is called interplanetary scintillation (IPS) and it is not restricted to the nocturnal rearrangements of the ionosphere (http://radio.astro.gla.ac.uk/ips/ips.htm ). The predicted spectral slope by Kolmogorov (see note 7) for IPS (p ~ -3.67) is somewhat steeper than for ionospheric scintillation (-2.5). The June 18, 2001 solar burst data (not shown) shows similar behaviour. Furthermore, many fluctuations (lines and bands) observed in the power spectra are consistent with the reported fluctuations of the solar wind (4.8 mHz and 13.3 Hz) (see note 8).

The two slopes sometimes seen in the log-log power spectra may be indicative a combination of modulating effects of both plasma sources, the heliosphere and the ionosphere, as well as the neutral troposphere.

What else can be learned from examining the power spectra (see the exploded view above)? Because of the turbulent nature of all the media through which the radio wave traverses, sharp resonances are not generally expected, but rather bands or combs. The NM data is very interesting and under investigation. There are currently two explanations. The first is mechanical resonance of the antenna and cables with a strong wind exciting the oscillation. A microphonic cable could confer this oscillation to the signal. Though winds can be strong in the New Mexico desert, I believe its effect should be visible on the time trace. It is not. A second explanation involves more atmospheric electrodynamics. Stratospheric lightning, in the form of "red sprites," is known to cause significant effects on the F-layers, which can scatter radio waves (see note 9).

Though the findings are provocative, further study is required to prove which kind of scintillation has been detected with the 20 MHz antennas. Several experiments are under development at the Tamke-Allan Observatory, e.g., simultaneous optical and radio twinkle measurements helpful in understanding tropospheric scintillation. Another key experiment concerning ionospheric scintillation and plasma bubbles is being organized with our associates in Puerto Rico.

With a virtual global antenna farm, Radio Jove amateur radio astronomers could potentially make a significant contribution to ionospheric physics.

REFERENCES

1. Ionospheric Plasma by VHF Waves, Pramana- Journal of Physics, Indian Academy of Sciences, Patel, Singh, and Singh, Vol. 55, Nos. 5 & 6, Nov/Dec 2000, p. 699-705.

2. Value determined from two sources: Planetary Lightning, Philippe Zarka, Observatoire de Paris, Figure 1: Spectra of Solar System Low frequency Radio Emission shows the sky background (1-100 MHz) with a spectral index estimated at –0.572 and, an NRAO reference Figure 6.2: Atmospheric Window and Sky Brightness shows the sky background (20-10,000 MHz) with a spectral index estimated at –0.639. The average value of –0.606 is taken for 20 MHz.

3. Patel, et al (reference 1) show a wider range at 244 MHz presumably (my unsubstantiated guess) because of the more complex phenomenology in the equatorial regions concerning plasma bubbles. A private communication with Northwest Research Associates, Jim Secan, cited values of p between -2.3 to –2.5 at 137 MHz. Care should be taken not to confuse ionospheric scintillation with tropospheric scintillation, which predicts similar values for p derived from weak scattering theory. The prediction of –2.67 is typically computed between 100 and 1000 millihertz after the roll-off beyond the 300 millihertz corner frequency (e.g., A Dynamic Model of Tropospheric Scintillation, C. Kassianides and I. E. Otung, University of Glamorgan, UK).

4. SPD files of solar burst data has been acquired from the Radio Jove Archive and was kindly supplied by Leonard Garcia. Jim Brown (SC), Thomas Schopp (MI), Tom Ashcraft (NM), Dusty Samouce (MT), and Dick Flagg (HI) originally collected the data. Receiver noise signatures are courtesy of David Fields (TAO TN).

5. Study of Cosmic Noise Absorption In Ionosphere Using Riometer, Rajiv S. Vhatkar, Space Science Laboratory, Department of Physics, Shivaji University, Kolhapur, Maharashtra State, India.

6. In a private communications with NWRA, Inc. on August 20 and 21, 2003, Jim Secan comments: “The data from Oahu look like the best possible case for scintillation, but at 2030 UT Oahu is in later morning when we don’t typically see much scintillation. Again, this is based on data collected at 137 MHz and above. There is anecdotal evidence of daytime scintillation-like variations at mid- and equatorial latitudes at lower frequencies, but not enough qualitative data for us to use in our modeling work.” Again on the Hawaiian data, “Given the time of day, I’d be a bit surprised if this was “classical” ionospheric scintillation, but at 20 MHz you might see ionospheric effects that are below the noise level of observations made at 137MHz. I’ve long suspected that we’ve missed some interesting phenomenology that could be seen at lower frequencies…”

7. Solar Corona Amplitude Scintillation Modeling and Comparison to Measurements at X-Band and Ka-Band, D. Morabito, IPN Progress Report 42-153, Jet Propulsion Laboratories, Pasadena, CA, p. 7, March 15, 2003.

8. Direct measurements of Solar Wind Fluctuations Between 0.0048 and 13.3 Hz, T. W. J. Unti, et al, Astrophysical Journal, Vol. 180, No. 2, pp. 591-598, 1973.

9. Private communication (August 6, 2002) with Martin Fuellekrug, Institut fuer Meteorologie und Geophysik, Feldbergstrasse 47, Universitaet Frankfurt/Main, D-60323 Frankfurt/Main, Germany

“ Could sprites and other similar phenomena have a transient oscillatory effect on radio wave amplitude?” (My question)

“Perhaps. About 20 % of all sprites are associated with radiation and hence strong conductivity in the mesosphere, which may scatter VLF waves (Rodger, Reviews of Geophysics, 37, 317, 1999) and be detectable with radar (Roussel-Dupre and Blanc, Journal of Geophysical Research, 102, 4613, 1997)” (Dr. Fullekrug’s response).

IONOSPHERE